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What Actually Happens When Two Atoms Touch · Night Science Stories

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  Speaker 2: Good evening and welcome to the Sleepy Scientist. It's lovely
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  Speaker 2: a great deal. Now, I'll do my best not to
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  Speaker 2: make tonight's subject so interesting that it keeps you up.
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  Speaker 2: But if it does, at least you'll drift off knowing
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  Speaker 2: something rather strange about the supposedly solid world around you.
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  Speaker 2: Because when you rest your hand on a table …
• 1:56
  Speaker 2: The atoms in your skin do not meet the atoms
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  Speaker 2: in the wood in quite the way two ordinary objects
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  Speaker 2: seem to meet. There are no tiny hard shells neatly
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  Speaker 2: pressing against one another. Instead, there are electron distributions, electromagnetic interactions,
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  Speaker 2: quantum rules, and an increasingly energetic resistance to being pushed
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  Speaker 2: any closer, which raises a wonderfully simple question with a
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  Speaker 2: surprisingly deep answer. So, shall we begin? What actually happens
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  Speaker 2: when two atoms touch? Press the tip of one finger
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  Speaker 2: gently against a table. Nothing about the experience seems particularly puzzling.
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  Speaker 2: Your finger moves downward, reaches the surface, and stops. Push
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  Speaker 2: a little harder, and the table pushes back. The boundary
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  Speaker 2: seems perfectly obvious. Your skin occupies one region of space,
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  Speaker 2: the table occupies another. and somewhere between them there appears
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  Speaker 2: to be a precise surface where the two objects meet.
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  Speaker 2: At the scale of atoms, however, that simple picture begins
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  Speaker 2: to dissolve. Neither your fingertip nor the table possesses the
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  Speaker 2: kind of perfectly defined microscopic edge that our everyday senses suggest.
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  Speaker 2: Matter is not built from tiny solid beads stacked together
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  Speaker 2: like an unimaginably complicated pile of marbles. And when atoms
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  Speaker 2: come close enough for us to describe them as touching,
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  Speaker 2: There is no instant when two hard atomic shells click
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  Speaker 2: neatly against one another. Instead, something much more interesting happens.
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  Speaker 2: The electrons associated with neighbouring atoms begin to interact strongly.
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  Speaker 2: Their quantum states are affected by one another. Electrical attractions
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  Speaker 2: and repulsions change. The possible arrangements available to the electrons
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  Speaker 2: become constrained. Their energies change, and if the atoms are
• 3:49
  Speaker 2: suitable for it, Their electronic structures may even reorganise to
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  Speaker 2: form a chemical bond. That collection of effects is much
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  Speaker 2: closer to what atomic contact actually means. It helps first
• 4:00
  Speaker 2: to let go of the picture of an atom as
• 4:02
  Speaker 2: a miniature billiard ball. A billiard ball has a clear surface.
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  Speaker 2: On one side of that surface there is material, and
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  Speaker 2: on the other side there is not. You can meaningfully
• 4:13
  Speaker 2: ask for the precise moment when two billiard balls first
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  Speaker 2: make contact. An atom is not built that way. Near
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  Speaker 2: its centre is an atomic nucleus containing positively charged protons and,
• 4:25
  Speaker 2: in most atoms, electrically neutral neutrons. Surrounding the nucleus are
• 4:30
  Speaker 2: electrons whose behaviour must be described using quantum mechanics. The
• 4:35
  Speaker 2: nucleus is extraordinarily small compared with the size normally assigned
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  Speaker 2: to the atom as a whole. A typical atom has
• 4:42
  Speaker 2: dimensions of roughly a tenth of a nanometre. A nanometre
• 4:45
  Speaker 2: is one billionth of a metre. Atomic nuclei, by contrast,
• 4:50
  Speaker 2: have characteristic dimensions measured in femtometers, which are millionths of
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  Speaker 2: a nanometer. So even when two atoms are sitting beside
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  Speaker 2: one another in a solid object, their nuclei are nowhere
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  Speaker 2: near touching. Imagine enlarging an atom until its overall size
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  Speaker 2: became something familiar. If the atom were expanded until it
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  Speaker 2: filled a large room, the nucleus would still occupy only
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  Speaker 2: a tiny region near the centre. The exact comparison depends
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  Speaker 2: on the atom. But the central point is simple. Nuclear
• 5:20
  Speaker 2: matter occupies a minuscule fraction of the dimensions we associate
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  Speaker 2: with an ordinary atom. The size of the atom is
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  Speaker 2: instead largely connected with its electrons, and even here the
• 5:31
  Speaker 2: word size needs care. Electrons are not little balls orbiting
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  Speaker 2: the nucleus along fixed tracks like planets travelling around the Sun.
• 5:40
  Speaker 2: Quantum mechanics describes them using quantum states. with spatial distributions
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  Speaker 2: that tell us about the likelihood of detecting an electron
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  Speaker 2: in different regions. Those distributions do not usually end at
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  Speaker 2: one perfectly sharp radius. They fade. That means there is
• 5:56
  Speaker 2: no universal microscopic line around an atom where you can
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  Speaker 2: confidently say that everything inside is atom and everything beyond
• 6:03
  Speaker 2: is not atom. Scientists can define useful atomic radii for
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  Speaker 2: particular purposes. But these are practical descriptions of characteristic size
• 6:13
  Speaker 2: rather than measurements of a rigid external shell. So when
• 6:17
  Speaker 2: two atoms approach each other, their outer electronic distributions can
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  Speaker 2: begin influencing one another long before anything resembling two solid
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  Speaker 2: surfaces meeting could occur. At relatively large separations the interactions
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  Speaker 2: may be weak, move the atoms closer and the details
• 6:35
  Speaker 2: become increasingly important. The positively charged nucleus of one atom
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  Speaker 2: interacts electrically with the electrons of the other. The electrons
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  Speaker 2: repel other electrons because they possess the same negative electric charge.
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  Speaker 2: The two positively charged nuclei repel one another as well.
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  Speaker 2: At the same time, each nucleus attracts negatively charged electrons.
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  Speaker 2: These electrical effects are part of electromagnetism, one of the
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  Speaker 2: fundamental interactions of nature. But electromagnetism alone is not enough
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  Speaker 2: to understand ordinary atomic contact. Electrons are quantum particles, and
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  Speaker 2: identical electrons obey the Pauli exclusion principle. Wolfgang Pauli introduced
• 7:17
  Speaker 2: the principle during the development of quantum mechanics in the
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  Speaker 2: 20th century. In its most familiar atomic form, it means
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  Speaker 2: that identical electrons cannot simply pile into exactly the same
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  Speaker 2: complete quantum state. As neighbouring atoms are pushed closer together,
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  Speaker 2: and their electronic states overlap more strongly. This quantum restriction
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  Speaker 2: matters enormously. The electrons must reorganise into allowed states. Their
• 7:44
  Speaker 2: spatial behaviour changes. Their kinetic energy can increase. Electrical interactions
• 7:50
  Speaker 2: change at the same time. The combined energy of the
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  Speaker 2: system begins rising very rapidly if the atoms are forced
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  Speaker 2: substantially closer than their preferred separations. This is why the
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  Speaker 2: atoms in your fingertip do not simply pass through the
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  Speaker 2: atoms in the table. It is sometimes said that the
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  Speaker 2: Pauli exclusion principle is the reason solid objects cannot pass
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  Speaker 2: through one another. There is an important truth hidden inside
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  Speaker 2: that statement, but the complete explanation involves both quantum mechanics
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  Speaker 2: and electromagnetism. Pauli exclusion is crucial, but so are the
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  Speaker 2: electrical interactions among nuclei and electrons and the changing kinetic
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  Speaker 2: energy associated with rearranging the electrons. There is no single
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  Speaker 2: invisible wall surrounding every atom. There is instead a rapidly
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  Speaker 2: growing energetic resistance to forcing neighbouring atoms into configurations they
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  Speaker 2: do not naturally occupy. That resistance becomes something remarkably familiar
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  Speaker 2: when enormous numbers of atoms participate at once. It becomes
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  Speaker 2: the force you feel when the table supports your hand.
• 8:57
  Speaker 2: Physicists call the force exerted perpendicular to a surface the
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  Speaker 2: normal force. It can sound rather disappointingly ordinary for something
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  Speaker 2: built from quantum mechanics, but the name simply means that
• 9:08
  Speaker 2: the force acts normal or perpendicular to the surface. When
• 9:12
  Speaker 2: your finger presses down, atoms within your skin and atoms
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  Speaker 2: within the table are displaced slightly from their preferred arrangements.
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  Speaker 2: Their electronic states respond. The energy of the combined system rises.
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  Speaker 2: The material resists further deformation, producing an upward force on
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  Speaker 2: your finger. Nothing needs to strike a microscopic solid wall.
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  Speaker 2: The sensation of solidity emerges from the collective behaviour of
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  Speaker 2: enormous numbers of interacting atoms. The relevant distances are astonishingly small.
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  Speaker 2: In ordinary solids, neighbouring atomic nuclei are commonly separated by
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  Speaker 2: only a few tenths of a nanometre. Their nuclei themselves
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  Speaker 2: have dimensions closer to a few millionths of a nanometre.
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  Speaker 2: So when your hand rests on a table, the nuclei
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  Speaker 2: on opposite sides of the contact remain separated by distances
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  Speaker 2: enormously greater than their own sizes, and yet the contact
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  Speaker 2: is completely real. This is an important point, because you
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  Speaker 2: may occasionally hear the dramatic claim that nothing ever truly
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  Speaker 2: touches anything. That depends entirely on what the word touch
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  Speaker 2: is supposed to mean, If touching requires two perfectly hard
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  Speaker 2: microscopic surfaces to collide, then atoms do not behave that
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  Speaker 2: way because atoms do not possess those surfaces in the
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  Speaker 2: first place. But if touching means two pieces of matter
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  Speaker 2: approaching closely enough for their interactions to produce the forces, deformation,
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  Speaker 2: pressure and resistance that we recognise as physical contact, then
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  Speaker 2: your hand absolutely does touch the table. The physics is
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  Speaker 2: not telling us that everyday contact is an illusion. It
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  Speaker 2: is telling us that the mechanism behind contact is stranger
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  Speaker 2: than our senses reveal. The more precise question, then, is
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  Speaker 2: not where one atom ends and another begins. It is
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  Speaker 2: what happens to the energy and quantum state of two
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  Speaker 2: atoms as the distance between their nuclei becomes smaller. And
• 11:08
  Speaker 2: once that question is asked, the familiar solidity of the
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  Speaker 2: world begins to look less like tiny objects bumping together
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  Speaker 2: and more like an intricate negotiation between electric charge, quantum states,
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  Speaker 2: and the rules governing electrons. For a long time, one
• 11:25
  Speaker 2: of the easiest ways to picture an atom was to
• 11:27
  Speaker 2: imagine a miniature solar system. At the centre sat the nucleus,
• 11:32
  Speaker 2: rather like a tiny sun, while electrons travelled around it
• 11:35
  Speaker 2: like planets moving along neat paths. It was a useful picture,
• 11:40
  Speaker 2: especially during the early development of atomic theory, and it
• 11:43
  Speaker 2: still appears in Logos diagrams and schoolbooks, because it is
• 11:47
  Speaker 2: wonderfully easy to recognise. The difficulty is that nature does
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  Speaker 2: not actually build atoms that way. A modern atom begins
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  Speaker 2: with a remarkably compact central nucleus. Inside that nucleus are
• 12:00
  Speaker 2: positively charged protons and in almost every naturally occurring atom,
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  Speaker 2: other than the most common form of hydrogen, electrically neutral neutrons.
• 12:09
  Speaker 2: Surrounding the nucleus are negatively charged electrons. Already, however, the
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  Speaker 2: word surrounding can tempt us into the wrong picture. Electrons
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  Speaker 2: do not simply circle the nucleus along precise little tracks.
• 12:21
  Speaker 2: Their behaviour is described by quantum mechanics, and that changes
• 12:25
  Speaker 2: almost everything about what it means to say where an
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  Speaker 2: electron is. The first major clue that atoms were not
• 12:32
  Speaker 2: solid little spheres came from experiments carried out in the
• 12:35
  Speaker 2: early 20th century. Ernest Rutherford was working with Hans Geiger
• 12:40
  Speaker 2: and Ernest Marsden on experiments in which fast-moving alpha particles
• 12:44
  Speaker 2: were directed towards extremely thin sheets of gold. Alpha particles
• 12:49
  Speaker 2: are positively charged particles emitted during certain forms of radioactive decay.
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  Speaker 2: If atoms had been filled fairly evenly with positive charge
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  Speaker 2: and matter, the alpha particles should mostly have passed through
• 13:02
  Speaker 2: the gold while being deflected only slightly. Most of them
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  Speaker 2: did pass through, but a very small number changed direction dramatically.
• 13:11
  Speaker 2: Some were scattered through large angles. and a tiny fraction
• 13:14
  Speaker 2: were effectively sent back towards the direction from which they
• 13:18
  Speaker 2: had arrived. Rutherford realised that this could only make sense
• 13:22
  Speaker 2: if almost all of the atom's positive charge and most
• 13:25
  Speaker 2: of its mass were concentrated into an extremely small central region.
• 13:31
  Speaker 2: That region became known as the atomic nucleus. The result
• 13:35
  Speaker 2: transformed the scientific picture of matter. Instead of being a
• 13:39
  Speaker 2: roughly uniform blob, an atom was revealed to contain an
• 13:43
  Speaker 2: extraordinarily tiny massive centre surrounded by a much larger region
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  Speaker 2: associated with its electrons. This immediately produces one of the
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  Speaker 2: strangest scale differences in nature. A typical atom has a
• 13:56
  Speaker 2: characteristic diameter of roughly a tenth of a nanometre, although
• 14:00
  Speaker 2: the exact value depends on which element and which definition
• 14:03
  Speaker 2: of atomic size is being used. A nanometre is one
• 14:07
  Speaker 2: billionth of a metre. An atomic nucleus, by comparison, has
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  Speaker 2: dimensions of only a few femtometers. A femtometer is one
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  Speaker 2: millionth of a nanometre, so the nucleus is tens of
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  Speaker 2: thousands of times smaller in diameter than the atom as
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  Speaker 2: a whole. If an atom were somehow enlarged until its
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  Speaker 2: overall size became comparable with a large building, the nucleus
• 14:33
  Speaker 2: would still be astonishingly tiny by comparison. Most of the
• 14:38
  Speaker 2: atom's familiar dimensions come not from the nucleus, but from
• 14:42
  Speaker 2: the quantum behaviour of its electrons. Early atomic models tried
• 14:46
  Speaker 2: to describe those electrons using more familiar ideas. Niels Bohr,
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  Speaker 2: for example, proposed a model in which electrons occupied particular,
• 14:55
  Speaker 2: allowed orbits around the nucleus. That model was extremely important
• 15:00
  Speaker 2: historically and successfully explained several features of hydrogen, but it
• 15:04
  Speaker 2: still retained something resembling a planetary picture. As quantum mechanics
• 15:10
  Speaker 2: developed during the 1920s, that picture had to change. Erwin
• 15:15
  Speaker 2: Schrödinger developed a mathematical equation describing how quantum states evolve.
• 15:20
  Speaker 2: The Schrödinger equation became one of the central foundations of
• 15:23
  Speaker 2: non-relativistic quantum mechanics. When applied to electrons in atoms, the
• 15:29
  Speaker 2: equation does not tell us that an electron travels around
• 15:32
  Speaker 2: the nucleus along one definite little road. Instead, it produces
• 15:37
  Speaker 2: a wave function. A wavefunction is a mathematical object that
• 15:42
  Speaker 2: contains information about the quantum state of a system. The
• 15:46
  Speaker 2: wavefunction itself is not simply a little physical cloud floating
• 15:50
  Speaker 2: around the nucleus. Its meaning became clearer through work by
• 15:54
  Speaker 2: Max Born, who proposed that the wavefunction could be connected
• 15:58
  Speaker 2: with probabilities. More precisely, the squared magnitude of the wavefunction
• 16:04
  Speaker 2: gives a probability density for detecting the particle in different regions.
• 16:09
  Speaker 2: That distinction matters. Quantum mechanics does not generally tell us
• 16:13
  Speaker 2: that an electron secretly occupies one precise classical location while
• 16:18
  Speaker 2: we simply fail to know where it is. Instead, the
• 16:22
  Speaker 2: electron is described by a quantum state whose spatial properties
• 16:26
  Speaker 2: are fundamentally different from those of an ordinary miniature ball.
• 16:30
  Speaker 2: This is where atomic orbitals enter the picture. An atomic
• 16:34
  Speaker 2: orbital is a quantum state associated with an electron in
• 16:38
  Speaker 2: an atom. It has a characteristic spatial distribution determined by
• 16:42
  Speaker 2: the wave function. Some orbitals are concentrated in roughly spherical regions.
• 16:48
  Speaker 2: Others have more complicated shapes, including lobes and nodal regions
• 16:53
  Speaker 2: where the probability of detecting an electron becomes very small
• 16:56
  Speaker 2: or vanishes. None of these orbitals should be imagined as
• 17:00
  Speaker 2: physical tracks. The electron is not racing around the outline
• 17:03
  Speaker 2: of an orbital as though following a wire. The orbital
• 17:07
  Speaker 2: describes the quantum state itself. And crucially, for the question
• 17:11
  Speaker 2: of atomic contact, these spatial distributions usually do not end abruptly.
• 17:16
  Speaker 2: Electron probability density can become very small as distance from
• 17:21
  Speaker 2: the nucleus increases, but there is not normally one perfectly
• 17:24
  Speaker 2: sharp radius beyond which the electron's quantum state suddenly disappears.
• 17:29
  Speaker 2: That means the atom does not possess a rigid outer skin.
• 17:33
  Speaker 2: Its characteristic size is real and measurable. but its boundary
• 17:37
  Speaker 2: is not like the painted surface of a cricket ball.
• 17:40
  Speaker 2: This is also why the familiar statement that atoms are
• 17:43
  Speaker 2: mostly empty space needs a little care. There is a
• 17:47
  Speaker 2: useful truth behind it. The nucleus occupies only a tiny
• 17:51
  Speaker 2: fraction of the volume associated with an atom. If we
• 17:54
  Speaker 2: compare nuclear size with atomic size, there really is an
• 17:57
  Speaker 2: enormous amount of spatial separation. But saying that the rest
• 18:02
  Speaker 2: of the atom is simply empty can create the wrong impression.
• 18:06
  Speaker 2: It may suggest that an electron is a tiny speck
• 18:09
  Speaker 2: occasionally appearing somewhere inside a vast hollow shell, with absolutely
• 18:13
  Speaker 2: nothing physically relevant happening between those appearances. Quantum mechanics gives
• 18:19
  Speaker 2: a richer picture. The electron's state extends through space. The
• 18:24
  Speaker 2: wave function carries information about where measurements may find the electron,
• 18:28
  Speaker 2: and how it can interact with other particles. Those extended
• 18:33
  Speaker 2: quantum states are exactly what begin to matter when another
• 18:36
  Speaker 2: atom approaches. Two atoms therefore do not behave like two
• 18:40
  Speaker 2: pebbles rolling towards one another. Their nuclei may remain far
• 18:45
  Speaker 2: apart compared with nuclear dimensions, while the quantum states of
• 18:49
  Speaker 2: their electrons already overlap significantly and alter one another. Electrical
• 18:53
  Speaker 2: interactions change. Allowed electronic arrangements change. Energies shift. That is
• 19:00
  Speaker 2: why the structure of an atom has to be understood
• 19:02
  Speaker 2: before the idea of atoms touching can make sense. If
• 19:06
  Speaker 2: an atom had a hard shell, contact would simply mean
• 19:08
  Speaker 2: the moment one shell met another. But an atom has
• 19:12
  Speaker 2: a tiny nucleus, surrounded by electrons described by extended quantum
• 19:17
  Speaker 2: states with no universal hard edge. So atomic contact cannot
• 19:22
  Speaker 2: be defined by looking for a microscopic surface. It has
• 19:25
  Speaker 2: to be understood by asking when the quantum state's and
• 19:29
  Speaker 2: interactions of neighbouring atoms begin to affect one another strongly
• 19:33
  Speaker 2: enough that treating them as completely separate objects no longer works.
• 19:38
  Speaker 2: If atoms do not have hard outer shells, then a
• 19:41
  Speaker 2: surprisingly awkward question appears almost immediately. Where exactly does an
• 19:48
  Speaker 2: atom end? For an ordinary object such as a marble,
• 19:51
  Speaker 2: the answer seems obvious. There is a surface. Inside that
• 19:55
  Speaker 2: surface there is glass, and outside it there is air.
• 20:00
  Speaker 2: The boundary may look slightly rough under a microscope, but
• 20:04
  Speaker 2: in principle we can still point to a region where
• 20:06
  Speaker 2: the material stops. An isolated atom does not offer us
• 20:10
  Speaker 2: such a convenient edge. The electrons associated with an atom
• 20:14
  Speaker 2: are described by quantum states and those states possess spatial distributions.
• 20:20
  Speaker 2: For many atomic orbitals, the probability density is greatest within
• 20:23
  Speaker 2: certain characteristic regions around the nucleus and then becomes smaller
• 20:28
  Speaker 2: as the distance increases. But smaller is not the same
• 20:32
  Speaker 2: as zero. In many cases, the mathematical distribution fades gradually outward.
• 20:37
  Speaker 2: There is no universal distance at which nature suddenly draws
• 20:41
  Speaker 2: a line and declares that the atom has finished. This
• 20:44
  Speaker 2: creates an interesting problem whenever someone asks how large an
• 20:48
  Speaker 2: atom is. Scientists routinely quote atomic sizes, and those numbers
• 20:53
  Speaker 2: are perfectly useful. But they are not measurements of a
• 20:57
  Speaker 2: tiny solid shell. Instead, an atomic radius is usually an
• 21:01
  Speaker 2: effective size defined according to a particular physical situation. and
• 21:06
  Speaker 2: there is more than one way to define it. Consider
• 21:09
  Speaker 2: two atoms joined together by a covalent bond. A covalent
• 21:13
  Speaker 2: bond involves electrons occupying molecular quantum states associated with more
• 21:19
  Speaker 2: than one nucleus. If the distance between the two nuclei
• 21:23
  Speaker 2: can be measured, one useful approximation is to divide that
• 21:27
  Speaker 2: separation between the atoms. This leads to the idea of
• 21:30
  Speaker 2: a covalent radius. For two identical bonded atoms, The covalent
• 21:35
  Speaker 2: radius can be thought of approximately as half the distance
• 21:39
  Speaker 2: between their nuclei. For different atoms, chemists use collections of
• 21:43
  Speaker 2: measured bond lengths to estimate characteristic covalent radii for different elements.
• 21:49
  Speaker 2: These values tell us something meaningful about atomic size in molecules.
• 21:54
  Speaker 2: But place the same element in a metal and another
• 21:56
  Speaker 2: definition becomes more useful. In a metallic crystal, atoms are
• 22:02
  Speaker 2: arranged in repeating structures while some of their electrons occupy
• 22:06
  Speaker 2: states extending through the material rather than belonging neatly to
• 22:11
  Speaker 2: one isolated atomic pair. The metallic radius is commonly related
• 22:16
  Speaker 2: to the distance between neighbouring atomic nuclei within that structure. Again,
• 22:21
  Speaker 2: the radius is real in the sense that it describes
• 22:23
  Speaker 2: measurable spacing. It is simply not the radius of a
• 22:27
  Speaker 2: microscopic hard sphere. There is another important measure known as
• 22:31
  Speaker 2: the van der Waals radius. The name comes from Johannes
• 22:35
  Speaker 2: Diederik van der Waals, whose work on gases and intermolecular
• 22:39
  Speaker 2: interactions helped reveal that molecules cannot always be treated as
• 22:44
  Speaker 2: perfectly non-interacting points. The van der Waals radius is particularly
• 22:49
  Speaker 2: useful when atoms are close to one another, but are
• 22:52
  Speaker 2: not connected by an ordinary covalent bond. Two neutral atoms
• 22:56
  Speaker 2: can still attract one another through weak intermolecular effects, while
• 23:01
  Speaker 2: strongly resisting further approach once they become sufficiently close. Measurements
• 23:06
  Speaker 2: of distances between non-bonded atoms in molecules and crystals allow
• 23:11
  Speaker 2: scientists to assign characteristic van der Waals radii. These are
• 23:15
  Speaker 2: often larger than covalent radii for the same element. That
• 23:19
  Speaker 2: is not because the atom has somehow swollen. The difference
• 23:23
  Speaker 2: comes from what we are choosing to measure. A carbon
• 23:27
  Speaker 2: atom participating in a covalent bond may have one useful
• 23:31
  Speaker 2: effective radius. A carbon atom approaching another atom without forming
• 23:36
  Speaker 2: such a bond may be described by a different one.
• 23:39
  Speaker 2: The atom has not changed into two different sized balls. Instead,
• 23:44
  Speaker 2: different definitions capture different aspects of how its electron distribution
• 23:48
  Speaker 2: interacts with its surroundings. This is one reason tables of
• 23:52
  Speaker 2: atomic radii do not always contain exactly the same values.
• 23:56
  Speaker 2: The radius depends on the definition, the bonding environment, the
• 23:59
  Speaker 2: electronic state, and sometimes the particular structure being studied. At
• 24:05
  Speaker 2: first this may sound inconvenient. It would certainly be tidier
• 24:09
  Speaker 2: if every element arrived with one permanent radius, perhaps stamped
• 24:13
  Speaker 2: somewhere on the side. Nature has declined to provide that service,
• 24:17
  Speaker 2: but the different definitions are not arbitrary guesses. They are
• 24:21
  Speaker 2: grounded in measurements. Techniques such as diffraction can reveal the
• 24:26
  Speaker 2: arrangements of atoms within crystals and molecules with remarkable precision.
• 24:31
  Speaker 2: From those measurements, researchers can determine distances between atomic nuclei.
• 24:37
  Speaker 2: Those nuclear separations then provide operational ways of discussing atomic dimensions.
• 24:42
  Speaker 2: So even though an atom has no knife-edge boundary, its
• 24:44
  Speaker 2: characteristic size is not vague or unknowable. We can measure
• 24:49
  Speaker 2: how closely atoms sit in solids, We can measure bond
• 24:52
  Speaker 2: lengths in molecules. We can determine the separations at which
• 24:56
  Speaker 2: non-bonded atoms tend to remain. We can study the energies
• 25:00
  Speaker 2: required to push atoms closer together. All of these tell
• 25:03
  Speaker 2: us something about size. This also explains why textbook illustrations
• 25:08
  Speaker 2: nearly always draw atoms as circles or spheres. The drawings
• 25:12
  Speaker 2: are useful. A sphere gives us an immediate visual sense
• 25:16
  Speaker 2: that an atom occupies a characteristic region around its nucleus.
• 25:20
  Speaker 2: In molecular models, different size spheres can communicate relative atomic
• 25:25
  Speaker 2: dimensions very efficiently. But the line around the sphere should
• 25:29
  Speaker 2: not be mistaken for a physical shell. No tiny camera
• 25:33
  Speaker 2: could zoom in and photograph that line. The illustration is
• 25:37
  Speaker 2: a map of characteristic extent, not a portrait of a
• 25:40
  Speaker 2: literal surface. Once that distinction is clear, the meaning of
• 25:44
  Speaker 2: atomic contact becomes much easier to handle. Imagine two atoms
• 25:48
  Speaker 2: moving slowly towards one another. If each possessed a hard
• 25:52
  Speaker 2: radius there would be one exact separation at which the
• 25:55
  Speaker 2: two surfaces first touched. One moment they would be apart,
• 26:00
  Speaker 2: the next moment they would meet. Real atoms interact more gradually.
• 26:04
  Speaker 2: Even while their nuclei are relatively far apart, their electric
• 26:08
  Speaker 2: fields and quantum states can influence one another weakly. As
• 26:12
  Speaker 2: the separation decreases, those interactions generally become stronger. Electron distributions
• 26:19
  Speaker 2: may distort. Attractive effects may become important. Eventually, the overlap
• 26:24
  Speaker 2: between electronic quantum states can no longer be ignored. If
• 26:28
  Speaker 2: the atoms are brought still closer, the energetic cost of
• 26:32
  Speaker 2: further compression can rise very steeply. At what exact instant
• 26:36
  Speaker 2: did touching begin? There is no universal answer. In one experiment,
• 26:41
  Speaker 2: contact might be defined by the distance at which a
• 26:43
  Speaker 2: measurable force becomes significant. In another, it might mean the
• 26:47
  Speaker 2: equilibrium distance between neighbouring atoms. In atomic collision physics, researchers
• 26:53
  Speaker 2: may describe atoms as colliding according to their scattering behaviour
• 26:57
  Speaker 2: and the interaction that changes their motion. In quantum chemistry,
• 27:01
  Speaker 2: attention may instead fall on the overlap of electron density
• 27:05
  Speaker 2: or the formation of a chemical bond. Each definition is
• 27:09
  Speaker 2: answering a slightly different physical question. This is not a
• 27:13
  Speaker 2: weakness in the science. It is a consequence of the
• 27:16
  Speaker 2: fact that the everyday word touch is trying to describe
• 27:19
  Speaker 2: a quantum system that does not possess everyday boundaries. Two
• 27:24
  Speaker 2: atoms can begin affecting one another without crossing a rigid line.
• 27:28
  Speaker 2: Their interaction strengthens continuously as their separation changes. Their electron
• 27:33
  Speaker 2: distributions respond to one another, their energies shift, and the
• 27:37
  Speaker 2: quantum state of the combined system can become increasingly different
• 27:42
  Speaker 2: from the states of two isolated atoms. So asking whether
• 27:45
  Speaker 2: two atoms are touching is often less useful than asking
• 27:48
  Speaker 2: what their separation is. how strongly they are interacting, what
• 27:52
  Speaker 2: their total energy is, and what their electrons are doing.
• 27:56
  Speaker 2: At the atomic scale, those quantities provide the meaningful boundaries,
• 28:01
  Speaker 2: not a thin outer skin, not two invisible shells meeting,
• 28:05
  Speaker 2: but forces, energies, electron density and quantum states changing as
• 28:10
  Speaker 2: matter moves closer together. Imagine two atoms separated by a
• 28:15
  Speaker 2: comfortable distance, drifting slowly towards one another. At first, depending
• 28:21
  Speaker 2: on the atoms and their surroundings, the interaction between them
• 28:24
  Speaker 2: may be extraordinarily weak. There is no sudden alarm that
• 28:28
  Speaker 2: sounds when they cross some invisible border. No switch changes
• 28:32
  Speaker 2: from not touching to touching. Instead, the influence of one
• 28:36
  Speaker 2: atom on the other changes continuously as the distance between
• 28:40
  Speaker 2: their nuclei becomes smaller. Much of what happens begins with
• 28:44
  Speaker 2: the electric charge. During the 18th century, the French physicist
• 28:49
  Speaker 2: Charles-Augustin de Coulomb investigated the forces between electrically charged objects.
• 28:55
  Speaker 2: The relationship now called Coulomb's Law describes how the electrical
• 28:59
  Speaker 2: force between two point charges depends on their charges and
• 29:03
  Speaker 2: the distance separating them. Charges with the same sign repel
• 29:07
  Speaker 2: one another while charges with opposite signs attract. Atoms contain both.
• 29:13
  Speaker 2: A typical neutral atom has a positively charged nucleus surrounded
• 29:17
  Speaker 2: by negatively charged electrons. The positive and negative charges balance overall,
• 29:22
  Speaker 2: so from sufficiently far away the atom can appear electrically neutral.
• 29:27
  Speaker 2: But electrically neutral does not mean electrically featureless. As two
• 29:32
  Speaker 2: atoms approach one another, several electrical interactions are occurring at
• 29:36
  Speaker 2: the same time. The positively charged nucleus of one one
• 29:40
  Speaker 2: atom repels the positively charged nucleus of the other. The
• 29:44
  Speaker 2: negatively charged electrons associated with one atom repel the electrons
• 29:48
  Speaker 2: associated with the other. Meanwhile, the nucleus of each atom
• 29:52
  Speaker 2: attracts the electrons of its neighbour So even a meeting
• 29:55
  Speaker 2: between two perfectly neutral atoms contains a complicated collection of
• 29:59
  Speaker 2: attractions and repulsions. If the atoms are far apart, much
• 30:04
  Speaker 2: of this can nearly cancel. At long enough distances, the
• 30:08
  Speaker 2: detailed positive and negative structure of a neutral atom becomes
• 30:11
  Speaker 2: difficult for another atom to distinguish. Move them closer, however,
• 30:15
  Speaker 2: and the internal distributions of charge begin to matter. One
• 30:20
  Speaker 2: important effect is electric polarisation. An atom's electron distribution is
• 30:25
  Speaker 2: not a rigid shell bolted into position around its nucleus.
• 30:29
  Speaker 2: An external electric influence can distort that distribution slightly. The
• 30:34
  Speaker 2: negatively charged electron density can shift relative to the positively
• 30:38
  Speaker 2: charged nucleus, giving the atom an uneven distribution of charge.
• 30:43
  Speaker 2: One side can become slightly more negative while the other
• 30:45
  Speaker 2: becomes slightly more positive. This does not necessarily give the
• 30:50
  Speaker 2: whole atom a net charge. It creates what physicists call
• 30:53
  Speaker 2: an electric dipole, or in many cases an induced dipole.
• 30:58
  Speaker 2: A nearby atom can therefore alter the charge distribution of
• 31:01
  Speaker 2: its neighbour. That altered distribution can then affect the first
• 31:05
  Speaker 2: atom in return. Already, two apparently neutral objects are engaged
• 31:10
  Speaker 2: in quite a subtle conversation. But even atoms with no
• 31:14
  Speaker 2: permanent electric dipole can attract one another. This is where
• 31:18
  Speaker 2: quantum mechanics becomes essential. During the 1930s, the physicist Fritz
• 31:24
  Speaker 2: London developed the quantum mechanical explanation for an attractive interaction
• 31:29
  Speaker 2: now known as the London dispersion force. Electrons in an
• 31:33
  Speaker 2: atom are not frozen into a perfectly static arrangement. Their
• 31:37
  Speaker 2: quantum state allows fluctuations in the distribution of electronic charge.
• 31:42
  Speaker 2: At any particular moment, the electron distribution can be uneven
• 31:45
  Speaker 2: in a way that produces a temporary electric dipole. A
• 31:49
  Speaker 2: neighbouring atom responds to that changing distribution. Its electrons become
• 31:53
  Speaker 2: correlated with those of the first atom, producing arrangements in
• 31:57
  Speaker 2: which the two atoms experience a small net attraction. These
• 32:02
  Speaker 2: London dispersion forces occur even between atoms that are neutral
• 32:05
  Speaker 2: and have no permanent dipole. Helium atoms, for example, can
• 32:10
  Speaker 2: attract one another through dispersion interactions despite being about as
• 32:14
  Speaker 2: reluctant to form ordinary chemical bonds as atoms can reasonably be.
• 32:18
  Speaker 2: The attraction is weak, but weak does not mean irrelevant.
• 32:22
  Speaker 2: Across large numbers of atoms and molecules, dispersion forces help
• 32:27
  Speaker 2: influence the properties of liquids, molecular solids, biological structures and surfaces.
• 32:34
  Speaker 2: They form part of the wider family of interactions commonly
• 32:37
  Speaker 2: described as Van der Waals forces. So as two neutral
• 32:41
  Speaker 2: atoms approach from relatively large separation, weak attractive effects may
• 32:45
  Speaker 2: begin lowering the energy of the pair. bring them closer,
• 32:49
  Speaker 2: and the attraction usually becomes stronger for a while. This
• 32:52
  Speaker 2: can make it tempting to imagine that the atoms should
• 32:55
  Speaker 2: simply continue falling towards one another until their nuclei meet.
• 33:00
  Speaker 2: They do not. At shorter distances, a very different regime appears.
• 33:05
  Speaker 2: The electron distributions begin overlapping significantly. Electron-electron repulsion becomes increasingly important.
• 33:14
  Speaker 2: The positively charged nuclei repel one another more strongly as
• 33:17
  Speaker 2: their separation falls. The available quantum states of the electrons
• 33:21
  Speaker 2: also have to change. At this point, simply adding together
• 33:25
  Speaker 2: a few classical Coulomb forces no longer gives a complete description.
• 33:30
  Speaker 2: The electrons are quantum particles. When the wave functions associated
• 33:34
  Speaker 2: with neighbouring atoms overlap, the electronic system has to be
• 33:38
  Speaker 2: treated as a combined quantum system. The electrons cannot always
• 33:42
  Speaker 2: be assigned permanently to atom 1 or atom 2, as
• 33:46
  Speaker 2: though each were carrying a private collection of tiny orbiting beads.
• 33:50
  Speaker 2: Their allowed quantum states depend on the arrangement of both
• 33:53
  Speaker 2: nuclei and on the presence of the other electrons. And
• 33:57
  Speaker 2: because electrons are identical fermions, the Pauli exclusion principle places
• 34:02
  Speaker 2: fundamental restrictions on the states available to them. As the
• 34:05
  Speaker 2: atoms are pushed into increasingly overlapping configurations, The electronic wave
• 34:10
  Speaker 2: functions must rearrange. That rearrangement can increase electron kinetic energy,
• 34:16
  Speaker 2: alter electrostatic energies and force electrons into different allowed states.
• 34:21
  Speaker 2: The energetic cost can rise very rapidly. This produces the
• 34:25
  Speaker 2: strong short-range resistance that prevents ordinary atoms from simply collapsing
• 34:30
  Speaker 2: through one another. It is worth being careful with the
• 34:33
  Speaker 2: word repulsion here. There is no single mysterious repulsive force
• 34:38
  Speaker 2: that suddenly activates at a fixed atomic radius. Instead, the
• 34:43
  Speaker 2: total energy of the system reflects several effects at once.
• 34:46
  Speaker 2: There is electrostatic repulsion between like charges. There is electrostatic
• 34:51
  Speaker 2: attraction between opposite charges. There is the quantum behaviour of
• 34:55
  Speaker 2: the electrons. There is the Pauli exclusion principle. There are
• 34:59
  Speaker 2: changes in kinetic energy. Depending on the atoms, there may
• 35:03
  Speaker 2: also be the possibility of forming chemical bonds. The balance
• 35:06
  Speaker 2: changes continuously with separation. At one distance, weaker traction may dominate.
• 35:12
  Speaker 2: At another, the atoms may reach a stable separation, where
• 35:16
  Speaker 2: moving noticeably closer or farther apart would raise the energy.
• 35:20
  Speaker 2: Push them closer still, and the energy may climb extremely steeply.
• 35:26
  Speaker 2: This smooth transition is a much better picture of atomic
• 35:28
  Speaker 2: contact than two hard shells striking one another. There need
• 35:32
  Speaker 2: not be a precise instant when two surfaces touch because
• 35:36
  Speaker 2: there are no such surfaces to begin with. Instead, there
• 35:39
  Speaker 2: is a changing relationship between two quantum systems. From far away,
• 35:44
  Speaker 2: they can behave almost independently. As they approach, subtle attractions appear.
• 35:50
  Speaker 2: Their electronic distributions begin responding to one another. Their quantum
• 35:55
  Speaker 2: states increasingly overlap and reorganize, eventually attempting to reduce the
• 36:00
  Speaker 2: distance further becomes energetically expensive. So if we could somehow
• 36:05
  Speaker 2: follow two atoms on their slow journey towards one another,
• 36:08
  Speaker 2: we would not see a collision with an invisible wall.
• 36:11
  Speaker 2: We would see interaction gradually becoming impossible to ignore. The
• 36:15
  Speaker 2: atoms do not suddenly discover that they have touched. They
• 36:18
  Speaker 2: simply reach a point where each can no longer be
• 36:20
  Speaker 2: understood without taking the other seriously. When physicists say that
• 36:25
  Speaker 2: the electron clouds of two atoms begin to overlap, The
• 36:28
  Speaker 2: phrase can create an image that is far more physical
• 36:31
  Speaker 2: than the underlying reality. It is easy to imagine two
• 36:35
  Speaker 2: faint fuzzy balls drifting into one another like coloured smoke.
• 36:40
  Speaker 2: But an electron cloud is not a mist of material
• 36:43
  Speaker 2: surrounding the nucleus. The phrase is shorthand for the spatial
• 36:47
  Speaker 2: behaviour of electrons described by quantum mechanics. For an electron
• 36:51
  Speaker 2: in an atom, that behaviour is represented by a wavefunction.
• 36:55
  Speaker 2: The wavefunction is a mathematical description of the electron's quantum state,
• 37:00
  Speaker 2: and from it we can determine the probability density for
• 37:03
  Speaker 2: detecting the electron in different regions of space. Around an
• 37:07
  Speaker 2: isolated atom, that probability density may be concentrated strongly in
• 37:12
  Speaker 2: some regions and become much smaller farther away, but it
• 37:15
  Speaker 2: does not normally end at one sharp boundary, so when
• 37:18
  Speaker 2: another atom approaches, there eventually comes a point where the
• 37:22
  Speaker 2: wavefunctions associated with electrons in one atom have significant values
• 37:27
  Speaker 2: in regions where the wave functions associated with the other
• 37:30
  Speaker 2: atom also have significant values. That is what physicists mean
• 37:35
  Speaker 2: by orbital overlap. Nothing has to burst. No fuzzy shell
• 37:39
  Speaker 2: has been punctured. The mathematical descriptions of the electronic state
• 37:43
  Speaker 2: simply extend into some of the same regions of space.
• 37:47
  Speaker 2: Quantum chemistry even has a way of describing how significant
• 37:51
  Speaker 2: this overlap is. One useful mathematical quantity is called the
• 37:56
  Speaker 2: overlap integral. Without getting buried in the formal calculation, the
• 38:00
  Speaker 2: idea is straightforward. The overlap integral compares two orbital wave
• 38:04
  Speaker 2: functions and measures how strongly they occupy corresponding regions of space.
• 38:09
  Speaker 2: If two orbitals are centered very far apart, their overlap
• 38:12
  Speaker 2: may be extremely small. Bring the atoms closer and the
• 38:16
  Speaker 2: overlap can become larger. The precise value depends not only
• 38:20
  Speaker 2: on distance, but also on the shapes and orientations of
• 38:23
  Speaker 2: the orbitals involved. That detail becomes important in chemistry, because
• 38:28
  Speaker 2: orbitals that overlap strongly in a suitable arrangement can contribute
• 38:32
  Speaker 2: to chemical bonding, while other combinations may produce very different behaviour.
• 38:37
  Speaker 2: But even before a bond is considered, overlap changes how
• 38:41
  Speaker 2: we should think about the electrons themselves. Suppose we begin
• 38:45
  Speaker 2: with two hydrogen atoms far apart. Each contains one proton
• 38:50
  Speaker 2: and one electron. When the atoms are separated by a
• 38:53
  Speaker 2: very large distance, it is perfectly convenient to talk about
• 38:57
  Speaker 2: the electron associated with the first atom and the electron
• 39:00
  Speaker 2: associated with the second. As the atoms approach, that language
• 39:05
  Speaker 2: becomes less fundamental. Electrons are identical quantum particles. One electron
• 39:11
  Speaker 2: does not carry a hidden name badge saying that it
• 39:14
  Speaker 2: permanently belongs to a particular atom. There is no physically
• 39:17
  Speaker 2: meaningful internal serial number distinguishing one electron from another electron
• 39:22
  Speaker 2: of the same kind. This is known as the indistinguishability
• 39:26
  Speaker 2: of identical particles. If two electrons have the same intrinsic
• 39:30
  Speaker 2: properties such as electric charge, mass and spin, quantum mechanics
• 39:34
  Speaker 2: requires us to describe them as fundamentally indistinguishable. That means
• 39:39
  Speaker 2: the combined quantum state cannot simply be built by saying
• 39:42
  Speaker 2: electron 1 belongs here and electron 2 belongs there forever. Instead,
• 39:47
  Speaker 2: The quantum state must describe the entire system of electrons together.
• 39:52
  Speaker 2: This becomes particularly important once their wave functions overlap. The
• 39:56
  Speaker 2: electrons are now part of one combined quantum problem, involving
• 40:00
  Speaker 2: both nuclei, all relevant electrons, and the interactions among them.
• 40:05
  Speaker 2: An electron may have significant probability density near one nucleus,
• 40:10
  Speaker 2: near the other nucleus, or across regions associated with both.
• 40:14
  Speaker 2: the labels that seemed perfectly natural while the atoms were
• 40:18
  Speaker 2: far apart begin losing their physical usefulness. That does not
• 40:22
  Speaker 2: mean the electrons dissolve into one another, nor does it
• 40:25
  Speaker 2: mean that twice as much negative material can simply be
• 40:28
  Speaker 2: packed into exactly the same place without consequence. Several effects
• 40:33
  Speaker 2: become increasingly important as overlap grows. First, the electrons repel
• 40:38
  Speaker 2: one another electrically because they carry the same negative charge.
• 40:42
  Speaker 2: At the same time, Each electron is attracted to both
• 40:45
  Speaker 2: positively charged nuclei. The balance between these attractions and repulsions
• 40:50
  Speaker 2: changes as the nuclear separation changes. The electron density itself
• 40:55
  Speaker 2: can redistribute. Regions that contained relatively little electron density when
• 41:00
  Speaker 2: the atoms were isolated may become more populated, while density
• 41:04
  Speaker 2: elsewhere may decrease. The allowed electronic quantum states also change.
• 41:09
  Speaker 2: What were once approximately atomic orbitals can combine into states
• 41:13
  Speaker 2: belonging to the pair of atoms, or more generally to
• 41:16
  Speaker 2: an entire molecule or solid. The kinetic energy associated with
• 41:20
  Speaker 2: the electrons can change as well. That point is especially
• 41:24
  Speaker 2: important because an electron's energy is not determined only by
• 41:28
  Speaker 2: electrostatic attraction and repulsion. The spatial form of its wavefunction matters.
• 41:34
  Speaker 2: Forcing electronic states into more restricted or rapidly varying spatial arrangements,
• 41:39
  Speaker 2: can increase their kinetic energy. So when atoms are pushed
• 41:43
  Speaker 2: closer together, there is a complicated rearrangement involving electron density,
• 41:48
  Speaker 2: electrostatic energy, quantum state restrictions and kinetic energy. The result
• 41:55
  Speaker 2: depends enormously on which atoms are involved. Sometimes the atoms approach,
• 42:00
  Speaker 2: interact briefly and then move apart again. This is common
• 42:04
  Speaker 2: in atomic collisions. Their wave functions overlap for a short
• 42:07
  Speaker 2: time Their motion changes and afterwards the atoms may emerge
• 42:11
  Speaker 2: still essentially intact. In another case, the overlap may contribute
• 42:16
  Speaker 2: to the formation of a stable chemical bond. The electronic
• 42:20
  Speaker 2: states of the two atoms reorganise into molecular states whose
• 42:23
  Speaker 2: total energy is lower than that of the separated atoms.
• 42:27
  Speaker 2: Now the atoms remain together. Elsewhere, particularly in solids, changing
• 42:32
  Speaker 2: the overlap between neighbouring atomic orbitals can alter the structure
• 42:35
  Speaker 2: and properties of the entire material. Electrical conductivity, chemical reactivity,
• 42:41
  Speaker 2: mechanical strength and optical behaviour can all depend on the
• 42:45
  Speaker 2: way electronic states overlap across neighbouring atoms. And if the
• 42:49
  Speaker 2: atoms are forced much too close, the combined configuration becomes
• 42:53
  Speaker 2: energetically very unfavourable. Electron-electron repulsion increases. The nuclei repel one
• 42:59
  Speaker 2: another more strongly. The available quantum states become more constrained.
• 43:04
  Speaker 2: Electron-kinetic energy may rise sharply. The result is the steep,
• 43:09
  Speaker 2: short-range resistance that helps prevent ordinary matter from collapsing into itself.
• 43:14
  Speaker 2: But one more ingredient is essential before that resistance is
• 43:18
  Speaker 2: fully understood. Electrons do not merely have charge and wave functions.
• 43:23
  Speaker 2: They also belong to a class of quantum particles called fermions,
• 43:27
  Speaker 2: and identical fermions obey a profound restriction on how their
• 43:31
  Speaker 2: quantum states can be arranged. That restriction is the Pauli
• 43:35
  Speaker 2: exclusion principle. It is not a simple additional force acting
• 43:39
  Speaker 2: between two electron clouds. It is a rule built into
• 43:42
  Speaker 2: the quantum description of identical electrons themselves. Once electron wavefunctions
• 43:48
  Speaker 2: overlap significantly, that rule becomes impossible to ignore. So the
• 43:52
  Speaker 2: overlap of electron clouds is not the meeting of two
• 43:55
  Speaker 2: fuzzy substances. It is the point at which two previously
• 43:59
  Speaker 2: convenient atomic descriptions begin merging into one larger quantum problem.
• 44:04
  Speaker 2: The electrons become increasingly governed by the combined environment of
• 44:08
  Speaker 2: both nuclei. Their probability distributions shift, their energies change, their
• 44:14
  Speaker 2: allowed states reorganise, and the distinction between this electron and
• 44:17
  Speaker 2: that electron becomes less meaningful than the structure of the
• 44:20
  Speaker 2: quantum state they all share. One of the deepest reasons
• 44:24
  Speaker 2: ordinary matter behaves as though it has substance comes from
• 44:27
  Speaker 2: a rule that is not really a force at all.
• 44:30
  Speaker 2: It is the Pauli Exclusion Principle. Wolfgang Pauli introduced the
• 44:34
  Speaker 2: principle during the development of quantum mechanics in the 1920s,
• 44:39
  Speaker 2: initially to explain the structure of atomic spectra and the
• 44:42
  Speaker 2: way electrons arrange themselves inside atoms. Its consequences turned out
• 44:47
  Speaker 2: to reach much further. Electrons belong to a family of
• 44:51
  Speaker 2: quantum particles called fermions. All electrons are fundamentally identical. They
• 44:57
  Speaker 2: have the same electric charge, the same mass, the same
• 45:01
  Speaker 2: intrinsic spin, and no hidden label distinguishes one electron from another.
• 45:07
  Speaker 2: For identical fermions, quantum mechanics imposes a remarkable restriction. Two
• 45:12
  Speaker 2: identical fermions cannot occupy exactly the same complete quantum state
• 45:17
  Speaker 2: within the same quantum system. In atoms, this has an
• 45:21
  Speaker 2: immediate consequence. Electrons are arranged into quantum states associated with
• 45:26
  Speaker 2: different energies, orbital structures and spin states. A single spatial
• 45:32
  Speaker 2: orbital can accommodate two electrons, but only because the two
• 45:35
  Speaker 2: electrons can have opposite spin states. Electron spin is an
• 45:39
  Speaker 2: intrinsic quantum property. It should not be imagined too literally
• 45:43
  Speaker 2: as a tiny ball physically spinning around an axis. The
• 45:47
  Speaker 2: word survives because some of its mathematical behaviour resembles angular momentum,
• 45:52
  Speaker 2: but spin is fundamentally quantum mechanical. For electrons, measurements of
• 45:56
  Speaker 2: spin along a chosen direction produce one of two allowed outcomes.
• 46:01
  Speaker 2: So two electrons can occupy the same spatial orbital, provided
• 46:05
  Speaker 2: their spin states differ appropriately. A third electron cannot simply
• 46:09
  Speaker 2: squeeze into that same complete state. It must occupy another
• 46:14
  Speaker 2: available quantum state. This is one of the reasons atoms
• 46:17
  Speaker 2: develop their familiar shell structure. As electrons are added to
• 46:22
  Speaker 2: an atom, they cannot all collapse into the lowest available state. Instead,
• 46:27
  Speaker 2: they progressively occupy different quantum states. that organization is deeply
• 46:32
  Speaker 2: connected with the structure of the periodic table. The recurring
• 46:37
  Speaker 2: chemical behavior of elements, the distinction between noble gases and
• 46:41
  Speaker 2: reactive metals, and the arrangements of electrons responsible for chemical
• 46:45
  Speaker 2: bonding all depend in part on the Pauli exclusion principle.
• 46:51
  Speaker 2: But the principle becomes especially important when neighboring atoms are
• 46:54
  Speaker 2: pushed towards one another. When two atoms are far apart,
• 46:59
  Speaker 2: Their electrons can often be described approximately using quantum states
• 47:02
  Speaker 2: centred on their own nuclei. As the atoms approach and
• 47:07
  Speaker 2: their wavefunctions overlap more strongly, that simple separation becomes less useful.
• 47:13
  Speaker 2: The electrons have to be described as members of one
• 47:16
  Speaker 2: combined quantum system, and that combined state must obey the
• 47:20
  Speaker 2: purely exclusion principle. The deeper mathematical statement is that the
• 47:25
  Speaker 2: total wavefunction describing identical fermions must be anti-symmetric under exchange.
• 47:31
  Speaker 2: This means that if two identical fermions are mathematically swapped,
• 47:36
  Speaker 2: the total wave function changes sign. That may sound like
• 47:40
  Speaker 2: a small technical detail, but it has enormous physical consequences.
• 47:45
  Speaker 2: One immediate result is that if two identical fermions were
• 47:48
  Speaker 2: placed in exactly the same complete quantum state, the anti-symmetric
• 47:53
  Speaker 2: wave function would cancel to zero. In other words, that
• 47:57
  Speaker 2: arrangement is not an allowed physical state. So when atoms
• 48:01
  Speaker 2: are pushed closer together, electrons cannot simply continue occupying their
• 48:05
  Speaker 2: original arrangements as though the neighbouring electrons were not there.
• 48:10
  Speaker 2: The available quantum states of the combined system change. Some
• 48:14
  Speaker 2: electrons must occupy different spatial arrangements. Their wave functions can
• 48:18
  Speaker 2: develop additional structure. Their characteristic momenta can change. And all
• 48:24
  Speaker 2: of this alters the total energy. This is sometimes described
• 48:28
  Speaker 2: informally as poorly repulsion. The phrase is useful, but it
• 48:32
  Speaker 2: can also be misleading. Poorly repulsion is not a separate
• 48:35
  Speaker 2: fundamental force in the way electromagnetism is. There is no
• 48:39
  Speaker 2: poorly forced particle flying back and forth between two atoms. Instead,
• 48:44
  Speaker 2: the apparent repulsive effect emerges from the requirement that the
• 48:48
  Speaker 2: quantum state of identical fermions obey anti-symmetry. and from the
• 48:52
  Speaker 2: energetic consequences of rearranging electrons into allowed states. As two
• 48:57
  Speaker 2: closed-shell atoms are pushed closer together, maintaining the required fermionic
• 49:03
  Speaker 2: quantum structure can force electron wave functions into configurations with
• 49:07
  Speaker 2: greater spatial variation. Greater spatial variation is generally associated with
• 49:12
  Speaker 2: higher electron kinetic energy. The total energy of the system
• 49:16
  Speaker 2: therefore rises. This rising energy produced an effective resistance to
• 49:21
  Speaker 2: further compression. But Pauli exclusion is not acting alone. That
• 49:26
  Speaker 2: distinction is important. The electrons also repel one another through
• 49:31
  Speaker 2: their negative electric charges. The positively charged nuclei repel each other.
• 49:37
  Speaker 2: At the same time, electrons are attracted to both nuclei.
• 49:41
  Speaker 2: All of these electrostatic interactions change as the atoms move closer.
• 49:46
  Speaker 2: The final short-range behaviour comes from the full quantum mechanical
• 49:51
  Speaker 2: balance of these effects, together with the restrictions imposed by
• 49:54
  Speaker 2: fermionic statistics. So the familiar statement that your hand cannot
• 49:59
  Speaker 2: pass through a table because of the Pauli exclusion principle
• 50:03
  Speaker 2: is only partly correct. Poorly exclusion is crucial. Without it,
• 50:08
  Speaker 2: ordinary matter would have dramatically different properties. But the resistance
• 50:12
  Speaker 2: between the atoms in your hand and the atoms in
• 50:15
  Speaker 2: the table is not produced by exclusion alone. Electromagnetism is
• 50:20
  Speaker 2: essential as well. The electric interactions among electrons and nuclei
• 50:25
  Speaker 2: help determine how the energy changes as atomic structures are compressed.
• 50:30
  Speaker 2: Changes in electron kinetic energy also contribute substantially. The solidity
• 50:35
  Speaker 2: we experience is therefore not the result of one microscopic
• 50:39
  Speaker 2: force standing guard at the boundary of every atom. It
• 50:42
  Speaker 2: is an emergent consequence of quantum mechanics and electromagnetism operating together.
• 50:49
  Speaker 2: The same principle that helps stop your hand passing through
• 50:52
  Speaker 2: a tabletop also helps explain why atoms have the sizes
• 50:56
  Speaker 2: they do. Because electrons cannot all occupy the same lowest state,
• 51:01
  Speaker 2: Successive electrons fill different quantum states around the nucleus. Those
• 51:06
  Speaker 2: states determine the spatial extent and chemical character of atoms,
• 51:11
  Speaker 2: place many atoms together, and the restrictions continue to matter.
• 51:15
  Speaker 2: In solids, enormous numbers of electrons collectively occupy allowed states
• 51:21
  Speaker 2: across the material. Their quantum organization influences electrical conductivity, mechanical stiffness,
• 51:29
  Speaker 2: chemical stability, and the structure of matter itself. The exclusion
• 51:34
  Speaker 2: principle even becomes important in environments far removed from an
• 51:38
  Speaker 2: ordinary table. In white dwarf stars, for example, matter is
• 51:43
  Speaker 2: compressed so strongly that electron degeneracy pressure becomes essential. This
• 51:49
  Speaker 2: pressure is another consequence of fermionic quantum statistics. Electrons are
• 51:55
  Speaker 2: forced into progressively higher momentum states because the lower states
• 51:59
  Speaker 2: are already occupied. That is an extreme astronomical version of
• 52:04
  Speaker 2: the same underlying rule. On a tabletop, of course, nothing
• 52:09
  Speaker 2: nearly so dramatic is occurring. The atoms need only be
• 52:12
  Speaker 2: displaced slightly from their preferred separations before their total energy
• 52:16
  Speaker 2: begins rising strongly. Push harder and the electronic arrangements become
• 52:21
  Speaker 2: increasingly unfavourable. The energy rises steeply so the material pushes back.
• 52:27
  Speaker 2: That steep rise is what matters for ordinary contact. It
• 52:30
  Speaker 2: gives matter an effective resistance to compression long before atomic
• 52:34
  Speaker 2: nuclei could ever come close to one another. So when
• 52:38
  Speaker 2: two atoms approach, the Pauli exclusion principle does not place
• 52:42
  Speaker 2: an invisible wall between them. Instead, it restricts the quantum
• 52:47
  Speaker 2: arrangements their electrons are allowed to occupy. Those restrictions reshape
• 52:52
  Speaker 2: the wave functions, alter the electron kinetic energy, and work
• 52:56
  Speaker 2: alongside electromagnetic interactions to make further compression increasingly costly. A
• 53:03
  Speaker 2: table feels solid, not because its atoms are tiny hard balls.
• 53:08
  Speaker 2: It feels solid because the quantum states of matter have rules.
• 53:12
  Speaker 2: And those rules leave remarkably little freedom for enormous numbers
• 53:16
  Speaker 2: of electrons to be pushed carelessly into the same place.
• 53:20
  Speaker 2: Once two atoms are close enough for their electron states
• 53:23
  Speaker 2: to overlap strongly, The question of what happens next becomes,
• 53:28
  Speaker 2: in large part, a question of energy. Two non-bonding atoms
• 53:32
  Speaker 2: do not behave like hard spheres that simply hit one
• 53:36
  Speaker 2: another and bounce away. Instead, as their nuclei are pushed
• 53:40
  Speaker 2: closer than the separation they naturally prefer, the total quantum
• 53:44
  Speaker 2: mechanical energy of the combined system begins to rise. That
• 53:49
  Speaker 2: rising energy is what makes further compression difficult. Part of
• 53:53
  Speaker 2: the reason comes from the way electron wave functions must change.
• 53:57
  Speaker 2: Electrons are described by quantum states that extend through space.
• 54:02
  Speaker 2: If those states are forced into more restricted spatial arrangements,
• 54:05
  Speaker 2: the wave functions generally become more sharply structured. This has
• 54:10
  Speaker 2: consequences for momentum. Werner Heisenberg formulated the uncertainty principle during
• 54:16
  Speaker 2: the early development of quantum mechanics. In its familiar form,
• 54:20
  Speaker 2: the principle tells us that position and momentum cannot both
• 54:23
  Speaker 2: be specified with unlimited precision. The more tightly a particle's
• 54:28
  Speaker 2: position is confined, the broader the range of momentum values
• 54:32
  Speaker 2: associated with its quantum state must become. This does not
• 54:36
  Speaker 2: mean that an electron literally bounces around faster, because it
• 54:40
  Speaker 2: has realised it is trapped. It means that stronger spatial
• 54:44
  Speaker 2: confinement changes the form of the quantum state itself. A
• 54:48
  Speaker 2: more confined wave function generally involves a wider spread of
• 54:52
  Speaker 2: possible momenta, and this often raises the characteristic kinetic energy
• 54:56
  Speaker 2: of the electron. That increase in kinetic energy becomes important
• 55:01
  Speaker 2: when atoms are compressed. Suppose two closed-shell atoms are pushed
• 55:05
  Speaker 2: closer together. Their electron distributions begin to overlap more strongly.
• 55:11
  Speaker 2: The Pauli exclusion principle restricts the quantum states the electrons
• 55:15
  Speaker 2: are allowed to occupy. the combined wavefunction must reorganise in
• 55:19
  Speaker 2: a way that preserves the anti-symmetry required for identical fermions.
• 55:24
  Speaker 2: That rearrangement can force electronic states into more rapidly varying
• 55:28
  Speaker 2: spatial patterns, and rapidly varying wavefunctions are associated with higher
• 55:33
  Speaker 2: momentum components. So part of the energetic cost of forcing
• 55:37
  Speaker 2: atoms too close comes from electron kinetic energy increasing, as
• 55:42
  Speaker 2: the allowed quantum states become more confined and more structured.
• 55:47
  Speaker 2: The uncertainty principle helps us understand why this happens. But
• 55:51
  Speaker 2: it would be misleading to say that atomic repulsion is
• 55:54
  Speaker 2: simply caused by the uncertainty principle. Real interatomic behaviour is
• 55:59
  Speaker 2: more complicated. Electrostatic interactions are changing at the same time.
• 56:04
  Speaker 2: The negatively charged electrons repel one another. The positively charged
• 56:09
  Speaker 2: nuclei repel one another. Each electron is also attracted to
• 56:13
  Speaker 2: both nuclei. As the nuclear separation decreases, All of these
• 56:17
  Speaker 2: contributions to the total energy shift together. Some favour closer approach.
• 56:24
  Speaker 2: Others oppose it. The final result depends on the complete
• 56:28
  Speaker 2: quantum state. This is why it is usually better to
• 56:31
  Speaker 2: think in terms of a total interaction energy rather than
• 56:35
  Speaker 2: trying to identify one single microscopic force responsible for contact.
• 56:41
  Speaker 2: At sufficiently short separation, that interaction energy usually rises very
• 56:46
  Speaker 2: steeply This steep rise is often described as short-range repulsion.
• 56:52
  Speaker 2: The phrase is useful, but once again it should not
• 56:54
  Speaker 2: be imagined as a rigid invisible shell wrapped around every atom.
• 56:59
  Speaker 2: There is no sharply defined radius where a repulsive wall
• 57:02
  Speaker 2: suddenly switches on. The word wall refers to the shape
• 57:06
  Speaker 2: of the energy curve. As the distance between the nuclei
• 57:09
  Speaker 2: becomes smaller than the preferred separation, the potential energy begins rising.
• 57:15
  Speaker 2: Push a little closer, and the energy rises more. Push
• 57:19
  Speaker 2: closer again, and it may rise extremely rapidly. On a
• 57:22
  Speaker 2: graph of interaction energy against nuclear separation, the short-range side
• 57:27
  Speaker 2: can become so steep that it looks almost like a wall.
• 57:31
  Speaker 2: This is sometimes called a repulsive wall. The phrase Born
• 57:35
  Speaker 2: repulsion is also used in models of inter-atomic interactions. The
• 57:40
  Speaker 2: name is associated with Max Born and the development of
• 57:43
  Speaker 2: early quantum descriptions of solids where short-range repulsion had to
• 57:47
  Speaker 2: be included to explain why ionic crystals did not simply
• 57:50
  Speaker 2: collapse under electrostatic attraction. In simplified models, this repulsive behaviour
• 57:57
  Speaker 2: may be represented by a mathematical term that increases dramatically
• 58:01
  Speaker 2: as atoms approach. Such models are extremely useful, but the
• 58:05
  Speaker 2: mathematical repulsive term should not be mistaken for a new
• 58:09
  Speaker 2: fundamental interaction. It is a convenient representation of the complicated
• 58:14
  Speaker 2: quantum behaviour of overlapping electron states, the Pauli exclusion principle,
• 58:19
  Speaker 2: electrostatic effects and changing kinetic energy. The underlying physics is
• 58:24
  Speaker 2: richer than the simple curve used to approximate it. The
• 58:28
  Speaker 2: steepness of this energy increase explains a great deal about
• 58:31
  Speaker 2: ordinary matter. Consider a solid block of metal. The atoms
• 58:36
  Speaker 2: inside it occupy positions near preferred separations determined by their
• 58:41
  Speaker 2: bonding and crystal structure. Apply a modest pressure and the
• 58:45
  Speaker 2: atoms move slightly closer together. Their electron states adjust. The
• 58:51
  Speaker 2: total energy rises a little. The material pushes back. If
• 58:56
  Speaker 2: the pressure is removed, the atoms can often return to
• 58:59
  Speaker 2: nearly their original spacing. This is part of what gives
• 59:02
  Speaker 2: materials their elasticity. but if you try to compress the
• 59:06
  Speaker 2: same solid dramatically, the situation becomes very different. Reducing atomic
• 59:12
  Speaker 2: spacing by a large fraction requires forcing electron distributions into
• 59:17
  Speaker 2: increasingly unfavourable configurations. The energy cost climbs rapidly. That is
• 59:24
  Speaker 2: why ordinary condensed matter is so difficult to compress significantly.
• 59:30
  Speaker 2: Solids are not perfectly incompressible. Their volume does change under pressure.
• 59:35
  Speaker 2: But reducing that volume substantially can require enormous pressures. Even liquids,
• 59:41
  Speaker 2: which can easily change shape, are often surprisingly resistant to
• 59:45
  Speaker 2: being compressed because their molecules already sit at separations, where
• 59:49
  Speaker 2: pushing them much closer carries a steep energetic cost. This
• 59:54
  Speaker 2: is an important distinction. A liquid may flow because its
• 59:58
  Speaker 2: molecules can rearrange. That does not mean those molecules are
• 1:00:02
  Speaker 2: easy to squeeze into much less space. The short-range part
• 1:00:05
  Speaker 2: of their interaction still resists strong compression. This same basic
• 1:00:10
  Speaker 2: idea appears throughout condensed matter. The mechanical stiffness of a
• 1:00:14
  Speaker 2: material depends partly on how rapidly its energy changes when
• 1:00:18
  Speaker 2: neighbouring atoms are displaced from their preferred separations. A shallow
• 1:00:23
  Speaker 2: energy curve allows larger changes in spacing for a given force.
• 1:00:28
  Speaker 2: A steep curve resists displacement much more strongly. So macroscopic
• 1:00:33
  Speaker 2: properties such as compressibility and stiffness have roots in the
• 1:00:37
  Speaker 2: quantum mechanical energy landscape between atoms. This also gives a
• 1:00:41
  Speaker 2: clearer picture of what an atom experiences as another atom approaches.
• 1:00:46
  Speaker 2: It does not encounter a sudden geometric collision. There is
• 1:00:49
  Speaker 2: no final empty gap followed by an instant of impact. Instead,
• 1:00:54
  Speaker 2: the combined state becomes gradually less favourable. Electron densities distort.
• 1:01:00
  Speaker 2: Quantum states reorganise. Electrostatic energies change. The constraints imposed by
• 1:01:06
  Speaker 2: fermionic statistics become more important. Electron kinetic energy rises. The
• 1:01:12
  Speaker 2: closer the nuclei are forced beyond their preferred separation, the
• 1:01:16
  Speaker 2: more expensive the arrangement becomes in energy. And because physical
• 1:01:20
  Speaker 2: systems tend to respond in ways that reduce unfavourable increases
• 1:01:24
  Speaker 2: in energy, the atoms resist being pushed closer. The force
• 1:01:28
  Speaker 2: we describe as repulsive is the mechanical expression of that
• 1:01:31
  Speaker 2: rising energy. So the solidity of ordinary matter can be
• 1:01:35
  Speaker 2: understood in a rather elegant way. Atoms do not need
• 1:01:39
  Speaker 2: hard surfaces in order to resist compression. They only need
• 1:01:42
  Speaker 2: quantum states whose energy rises sharply. When those atoms are
• 1:01:46
  Speaker 2: forced into arrangements they do not naturally favour. At the
• 1:01:51
  Speaker 2: atomic scale, a wall does not need to be made
• 1:01:54
  Speaker 2: from something solid. Sometimes, an energy curve is enough. One
• 1:01:59
  Speaker 2: of the clearest ways to understand what happens when two
• 1:02:02
  Speaker 2: atoms approach each other is to stop thinking in terms
• 1:02:05
  Speaker 2: of surfaces and start thinking in terms of energy. Imagine
• 1:02:09
  Speaker 2: the distance between the two atomic nuclei slowly changing. When
• 1:02:14
  Speaker 2: the atoms are very far apart, their interaction may be
• 1:02:17
  Speaker 2: so weak that their total energy is almost the same
• 1:02:21
  Speaker 2: as if each atom existed on its own. As the
• 1:02:24
  Speaker 2: separation decreases, attractive effects can begin to matter. For suitable
• 1:02:30
  Speaker 2: atoms or molecules, the interaction energy falls. In other words,
• 1:02:34
  Speaker 2: the combined system moves into a lower energy arrangement. That
• 1:02:38
  Speaker 2: is what attraction looks like on an interatomic potential energy curve.
• 1:02:43
  Speaker 2: The horizontal direction represents the separation between the nuclei. The
• 1:02:48
  Speaker 2: vertical direction represents the potential energy associated with their interaction.
• 1:02:53
  Speaker 2: Far apart, the curve is relatively flat because the atoms
• 1:02:57
  Speaker 2: barely influence one another. Move them closer and the energy
• 1:03:01
  Speaker 2: may begin to fall, but the decline does not continue forever. Eventually,
• 1:03:06
  Speaker 2: as the electron distributions overlap more strongly and the atoms
• 1:03:10
  Speaker 2: are pushed into increasingly unfavourable quantum configurations, the energy reaches
• 1:03:15
  Speaker 2: a lowest point. This lowest point corresponds to an equilibrium separation.
• 1:03:21
  Speaker 2: At that distance, the atoms have no net tendency, in
• 1:03:24
  Speaker 2: the idealised picture, either to move closer together or farther apart.
• 1:03:29
  Speaker 2: Move them slightly farther apart and attractive effects tend to
• 1:03:33
  Speaker 2: pull them back. Push them slightly closer and short-range repulsive
• 1:03:38
  Speaker 2: effects tend to drive them apart again. The equilibrium separation
• 1:03:42
  Speaker 2: is therefore like the bottom of a valley in the
• 1:03:44
  Speaker 2: energy landscape. It is one of the most useful ways
• 1:03:48
  Speaker 2: of describing the preferred spacing between neighbouring atoms. For many
• 1:03:53
  Speaker 2: atomic systems, these separations are measured in tenths of a nanometre.
• 1:03:57
  Speaker 2: A typical nearest neighbour distance in a solid might be
• 1:04:01
  Speaker 2: somewhere around a few tenths of a nanometre, depending on
• 1:04:04
  Speaker 2: the material and type of bonding involved. That is still
• 1:04:08
  Speaker 2: enormously larger than the dimensions of an atomic nucleus. A
• 1:04:12
  Speaker 2: nucleus measures only a few femtometres across. while neighbouring nuclei
• 1:04:17
  Speaker 2: in ordinary matter may be separated by hundreds of thousands
• 1:04:20
  Speaker 2: of femtometers. So even at their preferred distance, neighbouring atoms
• 1:04:25
  Speaker 2: keep their nuclei extremely far apart compared with nuclear dimensions.
• 1:04:30
  Speaker 2: The minimum in the energy curve is not a place
• 1:04:32
  Speaker 2: where two nuclear surfaces meet. It is simply the separation
• 1:04:36
  Speaker 2: where the competing contributions to the total interaction produce the
• 1:04:40
  Speaker 2: lowest energy. There is also a close relationship between this
• 1:04:44
  Speaker 2: energy curve and force. If the potential energy changes with
• 1:04:49
  Speaker 2: distance then a force is associated with that change. Very
• 1:04:52
  Speaker 2: loosely the steeper the energy curve the stronger the tendency
• 1:04:56
  Speaker 2: for the system to move towards lower energy. On the
• 1:04:59
  Speaker 2: attractive side of the minimum the atoms tend to move closer.
• 1:05:03
  Speaker 2: At the exact minimum the net force vanishes in the
• 1:05:07
  Speaker 2: idealised static case. On the short range side the atoms
• 1:05:10
  Speaker 2: tend to move apart. This allows attraction, equilibrium and repulsion
• 1:05:15
  Speaker 2: to be described within one continuous picture. There is no
• 1:05:19
  Speaker 2: need for a separate rule suddenly announcing that the atoms
• 1:05:22
  Speaker 2: have begun touching. The entire transition appears naturally in the
• 1:05:26
  Speaker 2: shape of the energy landscape. A particularly famous simplified example
• 1:05:31
  Speaker 2: is the Leonard Jones potential, associated with the British physicist
• 1:05:36
  Speaker 2: John Leonard Jones. It is often used to describe interactions
• 1:05:40
  Speaker 2: between neutral atoms or molecules when they are not forming
• 1:05:43
  Speaker 2: strong chemical bonds. The model combines a weak attractive region
• 1:05:48
  Speaker 2: at somewhat larger separations with a very steep repulsive region
• 1:05:52
  Speaker 2: at short range. The resulting curve has the familiar valley shape.
• 1:05:56
  Speaker 2: At large separation, the interaction is almost negligible. Moving inward,
• 1:06:02
  Speaker 2: attraction lowers the energy. At the minimum, the preferred separation
• 1:06:06
  Speaker 2: is reached. push farther inward and the energy climbs sharply.
• 1:06:11
  Speaker 2: The Leonard Jones model is useful because it captures the
• 1:06:14
  Speaker 2: overall behaviour in a simple form. It is especially helpful
• 1:06:18
  Speaker 2: when thinking about noble gases and some molecular systems, where
• 1:06:22
  Speaker 2: weak intermolecular forces are important. But it's not a universal
• 1:06:26
  Speaker 2: law of atomic interaction. Real atoms are more complicated. Different
• 1:06:31
  Speaker 2: elements have different electron structures. Chemical bonding can strongly alter
• 1:06:35
  Speaker 2: the shape of the interaction. Ionic systems, metals, covalent molecules
• 1:06:40
  Speaker 2: and weakly bound atoms all have different energy landscapes. Accurate
• 1:06:44
  Speaker 2: quantum mechanical calculations can therefore produce potential curves that differ
• 1:06:49
  Speaker 2: considerably from the simple Leonard Jones shape. The model is
• 1:06:53
  Speaker 2: best treated as a useful illustration, not as nature's one-size-fits-all
• 1:06:57
  Speaker 2: atomic rulebook. The bottom of the potential well is also
• 1:07:01
  Speaker 2: not perfectly still. Even if the temperature could be reduced
• 1:07:06
  Speaker 2: extremely close to absolute zero, quantum mechanics does not generally
• 1:07:10
  Speaker 2: allow the nuclei to sit completely motionless at one exact separation.
• 1:07:15
  Speaker 2: The atoms retain what is called zero-point motion. Their positions
• 1:07:20
  Speaker 2: fluctuate around the equilibrium configuration because the lowest energy vibrational
• 1:07:24
  Speaker 2: state still possesses a finite amount of quantum motion. At
• 1:07:28
  Speaker 2: ordinary temperatures, thermal energy adds further vibrations. the nuclei jiggle
• 1:07:34
  Speaker 2: around their average positions. As temperature rises, the amplitude of
• 1:07:38
  Speaker 2: that motion generally increases. So an equilibrium separation is best
• 1:07:42
  Speaker 2: understood as a preferred average distance, not as a rigid
• 1:07:46
  Speaker 2: spacing maintained with absolute precision. This vibration also reveals something
• 1:07:51
  Speaker 2: important about the shape of the energy curve. Near the minimum,
• 1:07:54
  Speaker 2: the curve may be fairly shallow or quite steep. If
• 1:07:58
  Speaker 2: it is shallow, the atoms can move relatively far from
• 1:08:01
  Speaker 2: their equilibrium separation. without a large increase in energy. If
• 1:08:05
  Speaker 2: it is steep, even a small displacement costs considerably more energy.
• 1:08:10
  Speaker 2: That difference helps influence how stiff or soft a material is.
• 1:08:15
  Speaker 2: In a crystal, vast numbers of neighbouring atoms sit in
• 1:08:18
  Speaker 2: energy landscapes determined by their collective interactions. When the material
• 1:08:23
  Speaker 2: is stretched or compressed slightly, those atoms are displaced from
• 1:08:27
  Speaker 2: their preferred positions, the energy rises, restoring forces appear. The
• 1:08:32
  Speaker 2: microscopic shape of the interatomic energy curve therefore contributes to
• 1:08:36
  Speaker 2: macroscopic properties such as elasticity, stiffness and compressibility. A material
• 1:08:42
  Speaker 2: that seems solid and unremarkable in the hand is, at
• 1:08:45
  Speaker 2: the atomic scale, a huge network of nuclei vibrating around
• 1:08:49
  Speaker 2: preferred separations defined by quantum and electromagnetic interactions. This gives
• 1:08:55
  Speaker 2: us one useful answer to the question of when atoms
• 1:08:58
  Speaker 2: are touching. If the neighbouring atoms settle near their equilibrium separation,
• 1:09:03
  Speaker 2: we can reasonably say they are in close atomic contact.
• 1:09:07
  Speaker 2: Their quantum states interact strongly. Their electron distributions influence one another.
• 1:09:12
  Speaker 2: Their energy depends strongly on their separation. And moving them
• 1:09:16
  Speaker 2: either closer or farther apart changes that energy. But this
• 1:09:20
  Speaker 2: is still not a universal touching radius. Different atoms have
• 1:09:24
  Speaker 2: different equilibrium distances. Different types of bonding produce different preferred separations.
• 1:09:30
  Speaker 2: Atoms in molecules, metals, crystals and weakly bound systems do
• 1:09:35
  Speaker 2: not all follow the same rule. So the equilibrium distance
• 1:09:39
  Speaker 2: is not the final definition of touch. It is one
• 1:09:42
  Speaker 2: physically meaningful version of it. The important shift is that
• 1:09:46
  Speaker 2: the question is no longer about two tiny surfaces meeting.
• 1:09:49
  Speaker 2: It is about where the total energy of the interacting
• 1:09:52
  Speaker 2: atoms prefers them to be. At the atomic scale, distance
• 1:09:57
  Speaker 2: is negotiated through energy. Too far apart, an attraction may
• 1:10:01
  Speaker 2: draw the atoms inward. Too close, and the energy rises sharply.
• 1:10:06
  Speaker 2: Between those extremes lies a preferred spacing, where the competing
• 1:10:10
  Speaker 2: effects balance. And for much of ordinary matter, that quiet
• 1:10:14
  Speaker 2: minimum in an energy curve is about as close as
• 1:10:17
  Speaker 2: nature comes to deciding where one atom should sit beside another.
• 1:10:21
  Speaker 2: So far, it has been useful to imagine neighbouring atoms
• 1:10:24
  Speaker 2: sitting close to a preferred separation, perhaps as part of
• 1:10:28
  Speaker 2: a molecule, liquid or solid. But atoms are not always
• 1:10:32
  Speaker 2: held neatly in place. In gases, atoms and molecules travel
• 1:10:37
  Speaker 2: freely through space, constantly encountering one another. In laboratory molecular beams,
• 1:10:43
  Speaker 2: carefully prepared streams of particles can be directed towards other particles,
• 1:10:48
  Speaker 2: so that individual collisions can be studied. In plasmas, where
• 1:10:53
  Speaker 2: atoms may be partly or completely ionised, collisions between electrons,
• 1:10:58
  Speaker 2: ions and remaining neutral atoms continually redistribute energy. Physicists routinely
• 1:11:03
  Speaker 2: describe these encounters as collisions, but once again, The word
• 1:11:08
  Speaker 2: should not make us imagine two tiny hard balls striking
• 1:11:11
  Speaker 2: one another. An atomic collision is fundamentally an interaction. Imagine
• 1:11:16
  Speaker 2: two neutral atoms moving towards each other through otherwise empty space.
• 1:11:20
  Speaker 2: Far apart, their mutual interaction may be extremely weak. Most
• 1:11:25
  Speaker 2: of their energy is associated with their motion, which means
• 1:11:28
  Speaker 2: they possess relative kinetic energy. As the distance between their
• 1:11:32
  Speaker 2: nuclei decreases, the interaction potential begins to matter. If there
• 1:11:37
  Speaker 2: is an attractive region in the potential, the atoms can
• 1:11:40
  Speaker 2: accelerate towards one another. Potential energy decreases while kinetic energy increases.
• 1:11:46
  Speaker 2: If they continue into much shorter separations, however, they eventually
• 1:11:50
  Speaker 2: encounter the steeply rising part of the interaction energy. Now
• 1:11:55
  Speaker 2: the process reverses. As the atoms move farther into this
• 1:11:58
  Speaker 2: energetically unfavourable region, some of their kinetic energy is converted
• 1:12:03
  Speaker 2: into potential energy. their inward relative motion slows. In a
• 1:12:08
  Speaker 2: simplified classical picture, there may be a particular separation at
• 1:12:12
  Speaker 2: which that inward motion temporarily stops. This is called a
• 1:12:16
  Speaker 2: turning point. At the turning point, the atoms have reached
• 1:12:20
  Speaker 2: their distance of closest approach for that particular collision. The
• 1:12:24
  Speaker 2: strongly rising interaction prevents them moving farther inward, and they
• 1:12:28
  Speaker 2: begin separating again. As they move apart, potential energy is
• 1:12:33
  Speaker 2: converted back into kinetic energy. from a distance, the event
• 1:12:37
  Speaker 2: can look rather like two objects bouncing. But no rigid
• 1:12:41
  Speaker 2: atomic surfaces need to have collided. The turning point is
• 1:12:45
  Speaker 2: determined by energy and interaction strength, rather than by two
• 1:12:49
  Speaker 2: hard radii meeting. For atomic particles, even this picture eventually
• 1:12:54
  Speaker 2: becomes an approximation. The more complete description comes from quantum
• 1:12:58
  Speaker 2: scattering theory. Quantum mechanics does not generally assign every atom
• 1:13:03
  Speaker 2: an exact microscopic trajectory that exists independently of measurement. Instead,
• 1:13:09
  Speaker 2: a collision can be described using an incoming quantum state,
• 1:13:13
  Speaker 2: an interaction between the particles, and an outgoing quantum state.
• 1:13:18
  Speaker 2: The incoming state contains information about quantities such as momentum
• 1:13:22
  Speaker 2: and energy. The interaction changes that state. Afterwards, quantum mechanics
• 1:13:27
  Speaker 2: predicts probabilities for different possible outcomes, including the directions and
• 1:13:32
  Speaker 2: energies with which particles may emerge. This is quantum scattering.
• 1:13:37
  Speaker 2: It is one of the fundamental tools used to study atomic,
• 1:13:41
  Speaker 2: molecular and subatomic interactions. Instead of asking precisely which tiny
• 1:13:46
  Speaker 2: root an atom followed as it curved around another atom,
• 1:13:50
  Speaker 2: researchers can ask measurable questions. At what angles are particles scattered?
• 1:13:55
  Speaker 2: How much energy is transferred? How frequently does a particular
• 1:13:59
  Speaker 2: reaction occur? What internal states do the particles occupy afterwards?
• 1:14:05
  Speaker 2: These measurements reveal the underlying interaction. One especially useful quantity
• 1:14:10
  Speaker 2: is the collision cross-section. The name can make it sound
• 1:14:14
  Speaker 2: as though physicists have measured the literal circular face of
• 1:14:17
  Speaker 2: an atom, but that is not what it means. A
• 1:14:20
  Speaker 2: cross-section is an effective measure of the probability that a
• 1:14:23
  Speaker 2: particular interaction will occur. It has units of area, which
• 1:14:27
  Speaker 2: makes it useful to imagine each particle presenting an effective
• 1:14:31
  Speaker 2: target to an incoming particle, but the size of that
• 1:14:34
  Speaker 2: effective target depends on the process being considered and on
• 1:14:37
  Speaker 2: the collision energy. An atom might have one cross section
• 1:14:41
  Speaker 2: for elastic scattering and another for ionisation. Both describe the
• 1:14:45
  Speaker 2: same atom, so the collision cross section is another example
• 1:14:48
  Speaker 2: of atomic size being defined through physical behaviour rather than
• 1:14:52
  Speaker 2: through a rigid outer boundary. Collisions themselves can also have
• 1:14:56
  Speaker 2: very different outcomes. In an elastic collision, the total kinetic
• 1:15:01
  Speaker 2: energy associated with the relative motion is conserved, and the
• 1:15:04
  Speaker 2: internal states of the colliding particles remain unchanged. The atoms approach, interact,
• 1:15:12
  Speaker 2: and separate again without being internally excited. Their directions and
• 1:15:16
  Speaker 2: individual speeds may change, but there is no lasting transfer
• 1:15:20
  Speaker 2: of energy into their internal structure. In an inelastic collision
• 1:15:26
  Speaker 2: some kinetic energy can instead be transferred into internal degrees
• 1:15:30
  Speaker 2: of freedom. An atom may be left in an electronically
• 1:15:33
  Speaker 2: excited state with one of its electrons occupying a higher
• 1:15:37
  Speaker 2: energy configuration. If molecules are involved energy can be transferred
• 1:15:43
  Speaker 2: into molecular rotation or vibration. At sufficiently high collision energies
• 1:15:49
  Speaker 2: an electron can even be removed completely from an atom
• 1:15:51
  Speaker 2: or molecule. producing ionisation. The collision energy therefore matters enormously.
• 1:15:58
  Speaker 2: At low relative energy, two atoms may only explore the outer,
• 1:16:02
  Speaker 2: relatively gentle region of their interaction potential. They approach, feel
• 1:16:07
  Speaker 2: one another's influence and separate, without reaching the strongly repulsive
• 1:16:12
  Speaker 2: region very deeply. Increase their relative kinetic energy, and they
• 1:16:17
  Speaker 2: can reach smaller nuclear separations before the rising potential energy
• 1:16:21
  Speaker 2: halts their inward motions. In this sense, faster collisions can
• 1:16:25
  Speaker 2: probe deeper into the interaction. The distance of closest approach
• 1:16:29
  Speaker 2: becomes smaller, and the collision begins sampling regions where electron overlap.
• 1:16:34
  Speaker 2: Electrostatic interactions and quantum state rearrangements become increasingly intense. At
• 1:16:41
  Speaker 2: high enough energies, entirely new processes may become possible. Electrons
• 1:16:46
  Speaker 2: can be excited or removed. Chemical bonds can break. In
• 1:16:51
  Speaker 2: sufficiently energetic environments, the familiar electronic structure of the atom
• 1:16:55
  Speaker 2: can be disrupted dramatically. There is also a more constructive possibility.
• 1:17:01
  Speaker 2: Some collisions can create molecules. Suppose two compatible atoms approach
• 1:17:06
  Speaker 2: one another, and their electronic states can reorganise into a
• 1:17:09
  Speaker 2: bound molecular configuration with lower energy. Simply reaching that configuration
• 1:17:14
  Speaker 2: is not always enough. Energy and momentum still have to
• 1:17:18
  Speaker 2: be conserved. If two isolated atoms collide in empty space,
• 1:17:23
  Speaker 2: and are to remain permanently bound, the excess energy must
• 1:17:26
  Speaker 2: usually go somewhere. It might be carried away by a photon,
• 1:17:30
  Speaker 2: transferred to a third particle, or redistributed through some other
• 1:17:35
  Speaker 2: allowed process. If that happens, a brief encounter can become
• 1:17:39
  Speaker 2: a stable bound state. Two particles that arrived independently can
• 1:17:43
  Speaker 2: leave the collision no longer independent at all. This is
• 1:17:47
  Speaker 2: one reason collisions are so important in chemistry, planetary atmospheres
• 1:17:51
  Speaker 2: interstellar clouds and laboratory experiments. They are opportunities for matter
• 1:17:57
  Speaker 2: to exchange energy, change internal state, scatter, ionise or form
• 1:18:03
  Speaker 2: new structures. And throughout all of this, the word collision
• 1:18:07
  Speaker 2: remains perfectly useful. It simply does not mean what it
• 1:18:11
  Speaker 2: means for two snooker balls. Atomic particles do not require
• 1:18:15
  Speaker 2: geometrically sharp surfaces in order to collide. Their quantum states interact.
• 1:18:21
  Speaker 2: Momentum is exchanged. Energy moves between different forms. Their outgoing
• 1:18:26
  Speaker 2: motion differs from their incoming motion. That is enough. So
• 1:18:31
  Speaker 2: when two freely moving atoms meet, there may never be
• 1:18:34
  Speaker 2: a single microscopic instant when one surface strikes another. There
• 1:18:40
  Speaker 2: may instead be a gradual increase in interaction, a conversion
• 1:18:43
  Speaker 2: of kinetic energy into potential energy, a closest approach, and
• 1:18:48
  Speaker 2: then separation. Or there may be excitation, ionisation, chemical rearrangement
• 1:18:54
  Speaker 2: or bonding. At the atomic scale a collision is not
• 1:18:59
  Speaker 2: defined by the soundless impact of two tiny shells, it
• 1:19:02
  Speaker 2: is defined by what the encounter does When two atoms
• 1:19:06
  Speaker 2: move close together, repulsion is not always the final result.
• 1:19:10
  Speaker 2: For certain combinations of atoms, approaching one another can actually
• 1:19:14
  Speaker 2: lower the total energy of the system. Instead of meeting
• 1:19:17
  Speaker 2: briefly and separating again, the atoms can settle into a
• 1:19:21
  Speaker 2: stable arrangement and remain together. That is the beginning of
• 1:19:25
  Speaker 2: a chemical bond. A chemical bond is not a tiny
• 1:19:28
  Speaker 2: hook connecting one atom to another, and it is not
• 1:19:32
  Speaker 2: a rigid rod joining two solid spheres. It emerges from
• 1:19:36
  Speaker 2: the quantum behaviour of electrons and nuclei, and from the
• 1:19:39
  Speaker 2: way the total energy changes when those particles are arranged
• 1:19:43
  Speaker 2: at different separations. One of the most influential early pictures
• 1:19:48
  Speaker 2: of chemical bonding was developed by Gilbert Newton Lewis. During
• 1:19:52
  Speaker 2: the early 20th century, Lewis proposed that many chemical bonds
• 1:19:56
  Speaker 2: could be understood in terms of pairs of electrons shared
• 1:19:59
  Speaker 2: between atoms. His electron pair picture became enormously important in chemistry.
• 1:20:05
  Speaker 2: because it offered a simple way to understand why atoms
• 1:20:08
  Speaker 2: combine in particular ratios and why certain arrangements are especially stable.
• 1:20:13
  Speaker 2: That picture remains useful, but quantum mechanics later revealed what
• 1:20:18
  Speaker 2: electron sharing actually means at a deeper level. One of
• 1:20:22
  Speaker 2: the earliest major steps came in 1927, when Walter Heitler
• 1:20:26
  Speaker 2: and Fritz London applied the new quantum mechanics to the
• 1:20:30
  Speaker 2: hydrogen molecule. A hydrogen molecule is wonderfully simple by chemical standards.
• 1:20:36
  Speaker 2: It contains two protons and two electrons. Even this small
• 1:20:41
  Speaker 2: system cannot be understood properly by imagining one electron orbiting
• 1:20:45
  Speaker 2: one proton while another electron orbits the other. When the
• 1:20:49
  Speaker 2: two hydrogen atoms approach, their electron wave functions overlap. The
• 1:20:54
  Speaker 2: electrons become part of a combined quantum system involving both nuclei.
• 1:20:59
  Speaker 2: Certain combined electronic states have lower energy than the two
• 1:21:03
  Speaker 2: widely separated atoms. If the atoms enter one of these
• 1:21:07
  Speaker 2: lower energy arrangements, a stable molecule can form. This was
• 1:21:11
  Speaker 2: a profound change in the understanding of chemistry. A chemical
• 1:21:15
  Speaker 2: bond was no longer simply a useful line drawn between
• 1:21:18
  Speaker 2: two element symbols. It could be understood as a consequence
• 1:21:22
  Speaker 2: of the quantum state of the electrons. In a covalent bond,
• 1:21:26
  Speaker 2: electron density can be distributed across more than one nucleus.
• 1:21:30
  Speaker 2: The electrons are attracted electrically to the positively charged nuclei
• 1:21:35
  Speaker 2: and a suitable redistribution of electron density can reduce the
• 1:21:38
  Speaker 2: total energy of the system. In a simple bonding state,
• 1:21:43
  Speaker 2: increased electron density between the nuclei can help hold the
• 1:21:46
  Speaker 2: nuclei together. Each nucleus attracts that negative electron density while
• 1:21:52
  Speaker 2: the complete quantum state settles into an arrangement that is
• 1:21:55
  Speaker 2: energetically more favourable than the separated atoms. This does not
• 1:22:00
  Speaker 2: mean that all orbital overlap automatically creates a bond. That
• 1:22:04
  Speaker 2: distinction is important. When atomic orbitals combine, different molecular quantum
• 1:22:10
  Speaker 2: states can result. Some combinations are bonding states. Occupying a
• 1:22:15
  Speaker 2: bonding state tends to lower the energy and stabilise the
• 1:22:18
  Speaker 2: atoms at a particular separation. Other combinations are antibonding states.
• 1:22:24
  Speaker 2: Occupying an antibonding state can raise the energy and work
• 1:22:28
  Speaker 2: against the formation of a stable bond. Which outcome occurs
• 1:22:32
  Speaker 2: depends on several things, including the symmetry of the orbitals,
• 1:22:36
  Speaker 2: the spin arrangement of the electrons and how many electrons
• 1:22:40
  Speaker 2: occupy the available molecular states. So overlap itself is neither
• 1:22:44
  Speaker 2: purely attractive nor purely repulsive. What matters is the complete
• 1:22:49
  Speaker 2: quantum arrangement produced by that overlap. Two atoms can therefore
• 1:22:53
  Speaker 2: approach one another and discover, in the language of energy
• 1:22:57
  Speaker 2: rather than intention, that remaining together is more favourable than separating.
• 1:23:03
  Speaker 2: If that happens, they settle near a characteristic bond length.
• 1:23:07
  Speaker 2: The distance is determined by a balance. At relatively large separation,
• 1:23:12
  Speaker 2: attractive contributions can lower the energy as the atoms approach.
• 1:23:16
  Speaker 2: At very short separation, nucleus-nucleus repulsion, electron-electron interactions, the Pauli
• 1:23:23
  Speaker 2: exclusion principle and changes in electron-kinetic energy make further compression
• 1:23:28
  Speaker 2: increasingly expensive. Between those extremes lies a minimum in the
• 1:23:33
  Speaker 2: total energy. That minimum gives the bonded atoms their preferred
• 1:23:37
  Speaker 2: nuclear separation. For many ordinary covalent bonds, these distances are
• 1:23:42
  Speaker 2: measured in tenths of a nanometre. The nuclei are therefore
• 1:23:45
  Speaker 2: still enormously farther apart than their own nuclear dimensions. The
• 1:23:49
  Speaker 2: bond is overwhelmingly an electronic phenomenon. Covalent bonding is not
• 1:23:54
  Speaker 2: the only way atoms can remain together. In ionic bonding,
• 1:23:58
  Speaker 2: electrons are redistributed much more strongly. One atom may lose
• 1:24:03
  Speaker 2: one or more electrons and become a positively charged ion,
• 1:24:07
  Speaker 2: while another gains electrons and becomes negatively charged. The resulting
• 1:24:12
  Speaker 2: oppositely charged ions attract one another through the electromagnetic interaction.
• 1:24:17
  Speaker 2: Sodium chloride provides a familiar example. In a crystal of
• 1:24:21
  Speaker 2: sodium chloride, however, it is misleading to imagine isolated pairs
• 1:24:26
  Speaker 2: of sodium and chlorine ions behaving independently. Each ion interacts
• 1:24:31
  Speaker 2: with many neighbours in the repeating crystal structure. The stability
• 1:24:35
  Speaker 2: of the solid emerges from the collective arrangement of the
• 1:24:38
  Speaker 2: entire lattice. metals provide another type of bonding again. In
• 1:24:43
  Speaker 2: metallic bonding, some electronic states extend across large regions of
• 1:24:47
  Speaker 2: the crystal. The electrons cannot always be described as belonging
• 1:24:51
  Speaker 2: to particular pairs of neighbouring atoms. Instead, the material behaves
• 1:24:56
  Speaker 2: as a many-atom quantum system in which electronic states can
• 1:25:00
  Speaker 2: spread through the lattice. This delocalised electronic structure helps explain
• 1:25:05
  Speaker 2: several familiar properties of metals, including electrical conductivity, and their
• 1:25:10
  Speaker 2: ability to deform without immediately breaking every bond in a
• 1:25:14
  Speaker 2: localised molecular sense. So even the word bond covers several
• 1:25:18
  Speaker 2: different kinds of quantum organisation, and all of them complicate
• 1:25:22
  Speaker 2: the everyday idea of atoms touching. If two atoms form
• 1:25:26
  Speaker 2: a stable covalent bond, are they touching? In an ordinary sense,
• 1:25:32
  Speaker 2: it would be perfectly reasonable to say yes. Their electron
• 1:25:35
  Speaker 2: states strongly overlap, their energies depend intimately on one another,
• 1:25:40
  Speaker 2: The electrons can occupy molecular states extending across both nuclei.
• 1:25:45
  Speaker 2: The atoms are no longer behaving as isolated systems, yet
• 1:25:49
  Speaker 2: there is still no hard surface where one atom ends
• 1:25:52
  Speaker 2: and the other begins. In fact, bonding makes that boundary
• 1:25:56
  Speaker 2: even less distinct. The electron density associated with the molecule
• 1:26:01
  Speaker 2: may extend continuously across the region containing both nuclei. Trying
• 1:26:07
  Speaker 2: to draw... A perfectly sharp line dividing the electrons into
• 1:26:11
  Speaker 2: those belonging to one atom and those belonging to the
• 1:26:13
  Speaker 2: other can become physically artificial. The two atoms have not
• 1:26:17
  Speaker 2: merely bumped together. They have entered a shared quantum structure.
• 1:26:22
  Speaker 2: It is also important to separate this kind of contact
• 1:26:25
  Speaker 2: from anything happening inside the nuclei. Chemical bond energies are
• 1:26:30
  Speaker 2: typically measured in electron volts per bond. Nuclear processes involve
• 1:26:35
  Speaker 2: energy scales commonly millions of times larger per event. Breaking
• 1:26:39
  Speaker 2: a chemical bond rearranges electrons. A nuclear reaction changes the
• 1:26:44
  Speaker 2: atomic nucleus itself. These are profoundly different physical regimes. When
• 1:26:49
  Speaker 2: hydrogen and oxygen form water, for example, the nuclei remain
• 1:26:53
  Speaker 2: nuclei of hydrogen and oxygen. The chemistry involves their electrons
• 1:26:58
  Speaker 2: and the quantum states those electrons occupy. No proton has
• 1:27:02
  Speaker 2: to collide directly with another nucleus for the bond to exist,
• 1:27:06
  Speaker 2: so bringing atoms together can lead to several possibilities. They
• 1:27:10
  Speaker 2: may repel and separate. They may collide and exchange energy.
• 1:27:15
  Speaker 2: Or their electronic states may reorganise into a new, lower-energy
• 1:27:18
  Speaker 2: configuration and hold the nuclei at a preferred separation. In
• 1:27:23
  Speaker 2: that case, what began as two approaching atoms becomes something new,
• 1:27:27
  Speaker 2: a molecule. and the idea of touching becomes less like
• 1:27:32
  Speaker 2: two objects meeting at their edges, and more like two
• 1:27:35
  Speaker 2: quantum systems discovering that the lowest energy way to exist
• 1:27:39
  Speaker 2: is together. Rest your hand on a table again, and
• 1:27:43
  Speaker 2: the whole problem suddenly becomes wonderfully ordinary. Your skin presses
• 1:27:48
  Speaker 2: against the surface. The table supports the weight. You feel resistance.
• 1:27:54
  Speaker 2: If you push harder, the resistance increases. Nothing about the
• 1:27:58
  Speaker 2: experience suggests quantum mechanics, and yet that familiar sensation depends
• 1:28:03
  Speaker 2: on quantum mechanics everywhere. A fingertip contains an enormous number
• 1:28:08
  Speaker 2: of atoms, so does even a tiny patch of the
• 1:28:11
  Speaker 2: tabletop beneath it. Those atoms are already interacting strongly with
• 1:28:15
  Speaker 2: their neighbours long before the two surfaces meet. Inside your skin,
• 1:28:20
  Speaker 2: atoms are bound into molecules, proteins, water, and complex biological structures.
• 1:28:27
  Speaker 2: Inside the table, atoms may be arranged in cellulose fibres, polymers, minerals, metals,
• 1:28:34
  Speaker 2: or whatever material happens to form the surface. None of
• 1:28:38
  Speaker 2: these atoms are sitting alone. Each is already part of
• 1:28:42
  Speaker 2: a complicated many-body system, held in a preferred range of
• 1:28:46
  Speaker 2: positions by chemical bonds and electromagnetic interactions. When your hand
• 1:28:52
  Speaker 2: comes down onto the table, the atoms in one surface
• 1:28:55
  Speaker 2: begin interacting with those in the other, They do not
• 1:28:58
  Speaker 2: simply stop at an invisible boundary. Both surfaces deform. The
• 1:29:04
  Speaker 2: deformation may be far too small to see, but at
• 1:29:07
  Speaker 2: the microscopic scale the atomic positions shift. Atoms in your
• 1:29:12
  Speaker 2: fingertip are displaced slightly from their preferred arrangements. Atoms in
• 1:29:16
  Speaker 2: the table move slightly as well. Their electron distributions respond.
• 1:29:21
  Speaker 2: Their quantum states adjust. The total energy changes. That microscopic
• 1:29:27
  Speaker 2: rearrangement is what gives rise to the familiar contact force.
• 1:29:31
  Speaker 2: Physicists often call the force acting perpendicular to a surface
• 1:29:35
  Speaker 2: the normal force. Despite the name, it is not a
• 1:29:39
  Speaker 2: separate fundamental interaction. There is no special normal force particle.
• 1:29:45
  Speaker 2: The normal force is a macroscopic description of the combined
• 1:29:48
  Speaker 2: electromagnetic and quantum mechanical response of enormous numbers of atoms.
• 1:29:54
  Speaker 2: When the atoms in the opposing surfaces are pushed slightly
• 1:29:57
  Speaker 2: closer than their preferred arrangements, the relevant interaction energies begin
• 1:30:01
  Speaker 2: to rise. Electron wave functions overlap more strongly. The Pauli
• 1:30:07
  Speaker 2: exclusion principle restricts how identical electrons can occupy the available
• 1:30:12
  Speaker 2: quantum states. Electron kinetic energy changes. Electrostatic repulsion between electrons
• 1:30:19
  Speaker 2: and between nuclei also changes. while electron-nucleus attractions shift at
• 1:30:24
  Speaker 2: the same time. The full quantum system resists being compressed.
• 1:30:29
  Speaker 2: Push your hand down a little harder and the atoms
• 1:30:32
  Speaker 2: are displaced farther from their preferred configurations. They move deeper
• 1:30:36
  Speaker 2: into the steeply rising part of their inter-atomic energy landscapes.
• 1:30:41
  Speaker 2: That larger increase in energy produces a larger restoring force.
• 1:30:45
  Speaker 2: The table pushes back more strongly. This is why the
• 1:30:49
  Speaker 2: normal force is not simply a fixed property of the table,
• 1:30:52
  Speaker 2: it adjusts. Rest a light object on the surface and
• 1:30:56
  Speaker 2: only a modest supporting force is required. Place something heavier
• 1:31:00
  Speaker 2: on the same surface and the atomic distortions become slightly greater,
• 1:31:04
  Speaker 2: producing a larger upward force. The changes in atomic spacing
• 1:31:08
  Speaker 2: can remain incredibly small, while the macroscopic force changes substantially.
• 1:31:15
  Speaker 2: This ability of matter to deform slightly and then return
• 1:31:18
  Speaker 2: towards its original shape is closely connected with the elasticity.
• 1:31:23
  Speaker 2: Elastic deformation occurs when atoms or molecules are displaced from
• 1:31:27
  Speaker 2: their preferred arrangements, but not so dramatically that the structure
• 1:31:31
  Speaker 2: is permanently rearranged. Imagine atoms sitting near the bottom of
• 1:31:36
  Speaker 2: their potential energy valleys. A small external force nudges them
• 1:31:41
  Speaker 2: away from those minimum energy positions. The total energy rises,
• 1:31:46
  Speaker 2: When the external force is removed, the restoring forces tend
• 1:31:50
  Speaker 2: to bring the atoms back towards their earlier arrangement. That
• 1:31:54
  Speaker 2: is the microscopic root of much everyday elasticity. The details
• 1:32:00
  Speaker 2: vary enormously from one material to another. Diamond is extremely stiff.
• 1:32:06
  Speaker 2: Its carbon atoms are connected through a strong three-dimensional network
• 1:32:10
  Speaker 2: of covalent bonds. Changing the distances and angles within that
• 1:32:14
  Speaker 2: network requires significant energy, so diamonds strongly resist deformation. Rubber
• 1:32:21
  Speaker 2: behaves very differently. Its long polymer molecules can change shape
• 1:32:25
  Speaker 2: and orientation in ways that allow the material to stretch
• 1:32:29
  Speaker 2: much farther before returning towards its original form. The individual
• 1:32:35
  Speaker 2: chemical bonds are not necessarily stretching by anything like the
• 1:32:38
  Speaker 2: same proportion as the entire piece of rubber. Much of
• 1:32:43
  Speaker 2: the macroscopic deformation comes from the rearrangement of long molecular chains.
• 1:32:48
  Speaker 2: Glass is different again. Its atoms are linked in a
• 1:32:51
  Speaker 2: disordered solid network rather than a regular crystal, giving it
• 1:32:55
  Speaker 2: a characteristic combination of stiffness and brittleness. Wood adds another
• 1:33:00
  Speaker 2: level of complexity. Its mechanical behaviour depends on cellulose fibres, lignin,
• 1:33:07
  Speaker 2: moisture content and the orientation of its biological structure. so diamond, rubber,
• 1:33:12
  Speaker 2: glass and wood can all feel solid while responding very
• 1:33:16
  Speaker 2: differently to pressure. The fundamental physics governing electrons and nuclei
• 1:33:21
  Speaker 2: is the same. What changes is the arrangement of atoms,
• 1:33:26
  Speaker 2: the nature of their bonds and the collective structure produced
• 1:33:29
  Speaker 2: by enormous numbers of particles. This is why ordinary contact
• 1:33:34
  Speaker 2: is a many-body problem. The atoms in your fingertip do
• 1:33:38
  Speaker 2: not arrive as independent little projectiles each searching for one
• 1:33:42
  Speaker 2: matching table atom to bump into. They are already connected
• 1:33:47
  Speaker 2: to their neighbours. The atoms in the table are connected
• 1:33:50
  Speaker 2: to theirs. Pressing the two surfaces together changes enormous networks
• 1:33:56
  Speaker 2: of interactions at once. Forces propagate through those networks, bonds distort,
• 1:34:02
  Speaker 2: molecules shift, lattice positions move, electron states adjust. The macroscopic
• 1:34:09
  Speaker 2: sensation of resistance emerges from all of these microscopic changes
• 1:34:14
  Speaker 2: acting together. This also helps clear up one of the
• 1:34:17
  Speaker 2: most popular statements about atomic physics. You may sometimes hear
• 1:34:21
  Speaker 2: that your hand never really touches the table. It is
• 1:34:25
  Speaker 2: an entertaining sentence, but taken literally, it is misleading. If
• 1:34:30
  Speaker 2: touching is defined as two rigid atomic shells physically striking
• 1:34:34
  Speaker 2: one another, then yes, that picture does not occur. But
• 1:34:39
  Speaker 2: rigid atomic shells do not exist in the first place.
• 1:34:42
  Speaker 2: Physical contact in ordinary matter is precisely the situation in
• 1:34:46
  Speaker 2: which the quantum and electromagnetic interactions between two bodies become
• 1:34:51
  Speaker 2: strong enough to produce forces, deformation and mechanical support. By
• 1:34:57
  Speaker 2: that physically useful definition, your hand really does touch the table.
• 1:35:02
  Speaker 2: There is no need for atomic nuclei to meet. The
• 1:35:05
  Speaker 2: nuclei remain separated by distances enormously greater than their own sizes.
• 1:35:11
  Speaker 2: That does not make the interaction unreal. Two planets can
• 1:35:14
  Speaker 2: gravitationally affect one another while remaining millions of kilometres apart.
• 1:35:19
  Speaker 2: We do not conclude that gravity is imaginary because their
• 1:35:22
  Speaker 2: surfaces have not collided. In the same way, contact between
• 1:35:27
  Speaker 2: pieces of ordinary matter is not made less real because
• 1:35:30
  Speaker 2: the underlying interaction acts through electronic quantum states rather than
• 1:35:35
  Speaker 2: through hard microscopic shells. The deeper lesson is that solidity
• 1:35:40
  Speaker 2: is not a substance hidden inside matter. It is a response.
• 1:35:45
  Speaker 2: Matter feels solid because its preferred quantum and electromagnetic structure
• 1:35:50
  Speaker 2: resists being changed. Atoms settle into arrangements where their total
• 1:35:55
  Speaker 2: energy is relatively low. Push them away from those arrangements
• 1:35:59
  Speaker 2: and the energy rises. Push harder and it rises more steeply.
• 1:36:03
  Speaker 2: The material responds with forces that oppose the deformation. That
• 1:36:08
  Speaker 2: resistance travels upward through layers of organisation, from electrons and
• 1:36:13
  Speaker 2: nuclei to atoms, molecules, fibres, crystals and finally the object
• 1:36:17
  Speaker 2: beneath your hand. So the simple experience of touching a
• 1:36:21
  Speaker 2: table is not evidence that matter is made from tiny
• 1:36:24
  Speaker 2: hard things. It is evidence that enormous collections of quantum
• 1:36:29
  Speaker 2: particles strongly resist being forced out of the arrangements they prefer.
• 1:36:33
  Speaker 2: The solidity you feel is the macroscopic signature of that resistance.
• 1:36:38
  Speaker 2: Two objects can look perfectly smooth and flat, while barely
• 1:36:42
  Speaker 2: touching each other at all. A polished metal plate may
• 1:36:45
  Speaker 2: seem almost mirror-like. A sheet of glass may appear perfectly even.
• 1:36:50
  Speaker 2: A kitchen worktop may feel smooth beneath a fingertip. But
• 1:36:55
  Speaker 2: under a microscope, and especially at atomic scales, every one
• 1:36:59
  Speaker 2: of those surfaces becomes a landscape. There are rises and dips,
• 1:37:03
  Speaker 2: tiny ridges, microscopic scratches and irregular patches. These small high
• 1:37:09
  Speaker 2: points are known as asperities. When two apparently flat objects
• 1:37:14
  Speaker 2: are brought together, the asperities usually make contact first. That
• 1:37:19
  Speaker 2: means the real area over which atoms from one object
• 1:37:22
  Speaker 2: are interacting closely with atoms from the other can be
• 1:37:25
  Speaker 2: far smaller than the apparent contact area that we see.
• 1:37:29
  Speaker 2: Imagine placing a large metal block on a flat table.
• 1:37:32
  Speaker 2: From across the room, it looks as though the entire
• 1:37:36
  Speaker 2: lower face of the block is resting against the table. Microscopically,
• 1:37:41
  Speaker 2: that is not quite true. At first, much of the
• 1:37:45
  Speaker 2: load can be carried through a scattered collection of tiny
• 1:37:47
  Speaker 2: contact regions, where asperities on one surface meet asperities on
• 1:37:52
  Speaker 2: the other, The rest of the apparent surface may still
• 1:37:55
  Speaker 2: be separated by microscopic gaps. This difference between apparent contact
• 1:38:01
  Speaker 2: area and real contact area is one of the most
• 1:38:04
  Speaker 2: important ideas in surface physics. The apparent area is simply
• 1:38:08
  Speaker 2: the visible geometric area over which the two objects seem
• 1:38:12
  Speaker 2: to meet. The real contact area is the combined area
• 1:38:16
  Speaker 2: of all the microscopic regions where the surfaces are actually
• 1:38:20
  Speaker 2: close enough for strong atomic interactions to occur. At light loads,
• 1:38:24
  Speaker 2: the real contact area can be surprisingly small. Increase the pressure, however,
• 1:38:29
  Speaker 2: and the situation changes. The asperities begin to deform, some
• 1:38:34
  Speaker 2: flatten slightly, others bend, new microscopic contact regions appear, the
• 1:38:39
  Speaker 2: real contact area increases. This helps explain why the force
• 1:38:43
  Speaker 2: between two surfaces can change so dramatically even though from
• 1:38:46
  Speaker 2: a distance. They do not appear to have changed shape
• 1:38:49
  Speaker 2: at all. Their microscopic geometry has changed. Atoms near the
• 1:38:54
  Speaker 2: surface are being displaced. Local stresses rise. More regions move
• 1:38:59
  Speaker 2: into close enough proximity for strong electromagnetic and quantum interactions
• 1:39:03
  Speaker 2: to matter. The atoms at a surface also live in
• 1:39:07
  Speaker 2: a different environment from atoms deeper inside a material. An
• 1:39:11
  Speaker 2: atom buried within a solid is surrounded by neighbouring atoms
• 1:39:15
  Speaker 2: in several directions. Its electronic structure is shaped by that
• 1:39:20
  Speaker 2: local environment. A surface atom has fewer neighbours. Some of
• 1:39:24
  Speaker 2: the bonds or interactions that would normally continue into the
• 1:39:28
  Speaker 2: material are simply absent because the solid ends. That difference
• 1:39:32
  Speaker 2: can change the energy and electronic structure of the surface.
• 1:39:36
  Speaker 2: Physicists often describe this in terms of surface energy. Creating
• 1:39:40
  Speaker 2: a new surface usually costs energy because atoms at the
• 1:39:44
  Speaker 2: boundary are left in a different, and often less energetically
• 1:39:47
  Speaker 2: favourable environment than atoms in the interior. This helps make
• 1:39:52
  Speaker 2: surfaces chemically and mechanically interesting places. Bring two surfaces close
• 1:39:58
  Speaker 2: together and attractive interactions can appear between them. This is adhesion.
• 1:40:05
  Speaker 2: Adhesion is the tendency of two surfaces to stick together
• 1:40:09
  Speaker 2: because of forces acting across their interface. Several mechanisms can contribute.
• 1:40:15
  Speaker 2: Van der Waals forces can create attraction between atoms and
• 1:40:18
  Speaker 2: molecules on opposing surfaces even when no permanent chemical bond forms.
• 1:40:24
  Speaker 2: If the surfaces are sufficiently clean and chemically compatible, atoms
• 1:40:29
  Speaker 2: across the interface may come close enough for actual chemical
• 1:40:32
  Speaker 2: bonds to form. At that point, distinguishing which atom belongs
• 1:40:36
  Speaker 2: to which original object can become less meaningful. The interface
• 1:40:40
  Speaker 2: becomes a connected structure. In everyday conditions, however, surfaces are
• 1:40:44
  Speaker 2: almost never perfectly clean. Even something that looks spotless can
• 1:40:49
  Speaker 2: be covered by microscopic layers of water molecules, oxygen, hydrocarbons
• 1:40:54
  Speaker 2: and other contaminants from the surrounding environment. Metals often form
• 1:40:59
  Speaker 2: thin oxide layers. Water from the atmosphere can coat many surfaces.
• 1:41:04
  Speaker 2: Organic molecules from the air, fingerprints or manufacturing processes can
• 1:41:09
  Speaker 2: remain behind in extremely thin films. These layers matter. They
• 1:41:14
  Speaker 2: alter the distance between the underlying atoms. They change the
• 1:41:17
  Speaker 2: forces acting across the interface. They can reduce or increase adhesion.
• 1:41:22
  Speaker 2: They can influence friction. They can change whether chemical bonds
• 1:41:26
  Speaker 2: form when two materials are pressed together. So when you
• 1:41:29
  Speaker 2: place two pieces of metal against one another, the atoms
• 1:41:32
  Speaker 2: in one metal are often not interacting directly with bare
• 1:41:36
  Speaker 2: atoms in the other. There may be oxide layers and
• 1:41:39
  Speaker 2: adsorbed molecules between them, Remove those layers and surface behaviour
• 1:41:45
  Speaker 2: can become dramatically different. One striking example is cold welding.
• 1:41:51
  Speaker 2: Cold welding can occur when sufficiently clean metallic surfaces are
• 1:41:55
  Speaker 2: brought into close contact without being heated to the temperatures
• 1:41:59
  Speaker 2: normally associated with welding. If oxide layers and other contaminants
• 1:42:03
  Speaker 2: are absent, atoms on the opposing surfaces can approach closely
• 1:42:08
  Speaker 2: enough for metallic bonding to develop across the interface. boundary
• 1:42:12
  Speaker 2: between the two pieces can begin to disappear. Atoms that
• 1:42:16
  Speaker 2: were originally part of one object become electronically connected with
• 1:42:19
  Speaker 2: atoms in the other. The two surfaces effectively become one
• 1:42:23
  Speaker 2: continuous metallic structure. This effect is especially relevant in vacuum environments,
• 1:42:29
  Speaker 2: where freshly exposed metal surfaces may remain cleaner because there
• 1:42:33
  Speaker 2: is less surrounding gas available to coat them. Cold welding
• 1:42:37
  Speaker 2: has therefore been an engineering concern for components operating in
• 1:42:41
  Speaker 2: spacecraft and other high vacuum systems. It is a remarkable
• 1:42:45
  Speaker 2: reminder that two separate pieces of metal are not separated
• 1:42:48
  Speaker 2: by some fundamental rule. In ordinary conditions, surface contamination often
• 1:42:54
  Speaker 2: helps preserve that separation. Take it away, and atomic bonding
• 1:42:58
  Speaker 2: can sometimes do the rest. The same microscopic surface physics
• 1:43:03
  Speaker 2: appears in much more familiar situations. Sticking depends partly on
• 1:43:08
  Speaker 2: how surfaces deform and how strongly they adhere, where occurs
• 1:43:12
  Speaker 2: when contact stresses and repeated motion rearrange or remove material.
• 1:43:17
  Speaker 2: Polishing changes surface roughness, altering the distribution and size of asperities.
• 1:43:24
  Speaker 2: Even setting a mug quietly onto a table creates a
• 1:43:27
  Speaker 2: complicated network of microscopic contacts carrying the load. The apparent
• 1:43:32
  Speaker 2: simplicity is deceptive. Every contact point contains atoms shifting slightly
• 1:43:37
  Speaker 2: from equilibrium. Electron states respond. Local regions deform. Attractive and
• 1:43:45
  Speaker 2: repulsive interactions balance one another, and those small regions collectively
• 1:43:49
  Speaker 2: create the macroscopic force that supports the object. This also
• 1:43:54
  Speaker 2: explains why contact is not one perfectly defined flat plane.
• 1:43:58
  Speaker 2: The boundary seen by the eye is an average description.
• 1:44:02
  Speaker 2: At smaller scales, there may be isolated islands of close
• 1:44:05
  Speaker 2: interaction separated by gaps. As pressure changes, those islands grow, disappear,
• 1:44:11
  Speaker 2: merge and rearrange. If the surfaces slide, the entire network
• 1:44:15
  Speaker 2: continually changes. Some atomic contacts break while others form. The
• 1:44:20
  Speaker 2: interface is dynamic even when the object seems perfectly still.
• 1:44:25
  Speaker 2: So when two ordinary objects touch, there is no single
• 1:44:28
  Speaker 2: sheet-like boundary. where every atom on one side simultaneously meets
• 1:44:33
  Speaker 2: an atom on the other. There is a scattered and
• 1:44:36
  Speaker 2: evolving collection of microscopic contact regions. At each one, atoms
• 1:44:41
  Speaker 2: approach closely enough for their quantum states and electromagnetic interactions
• 1:44:47
  Speaker 2: to respond strongly. Seen from far away, we call the
• 1:44:51
  Speaker 2: objects touchings. Seen closely enough, the same event becomes a
• 1:44:55
  Speaker 2: landscape of asperities, deformation, adhesion, contamination, and atomic interaction. The
• 1:45:01
  Speaker 2: flat surface was never quite flat, and the contact was
• 1:45:05
  Speaker 2: never quite where it seemed to be. Now imagine that
• 1:45:09
  Speaker 2: the two surfaces are no longer simply resting against one another.
• 1:45:13
  Speaker 2: One begins to slide. At first glance, friction seems easy
• 1:45:17
  Speaker 2: to understand. Two rough surfaces rub together, Tiny bumps catch,
• 1:45:23
  Speaker 2: and motion becomes difficult. That picture is not completely wrong,
• 1:45:27
  Speaker 2: but at the atomic scale it is far too simple.
• 1:45:31
  Speaker 2: Friction does not come from one universal microscopic mechanism. Different
• 1:45:36
  Speaker 2: materials produce friction in different ways, and several effects can
• 1:45:40
  Speaker 2: act at the same time. Surface roughness matters. Adhesion matters.
• 1:45:46
  Speaker 2: Local deformation matters. Chemical bonds can sometimes form and break
• 1:45:51
  Speaker 2: across the interface. Atoms can be rearranged. Energy can be
• 1:45:56
  Speaker 2: transferred into vibrations inside the material. And small fragments of
• 1:46:00
  Speaker 2: matter can even be pulled from one surface and transferred
• 1:46:04
  Speaker 2: to the other. So when two objects slide, the interface
• 1:46:08
  Speaker 2: between them becomes a constantly changing atomic landscape. Consider two
• 1:46:14
  Speaker 2: crystalline surfaces pressed lightly together. Their atoms occupy repeating arrangements,
• 1:46:20
  Speaker 2: and the total energy depends on how those arrangements line up.
• 1:46:24
  Speaker 2: As one surface begins to move relative to the other,
• 1:46:27
  Speaker 2: the atoms near the interface experience a changing potential energy landscape.
• 1:46:33
  Speaker 2: Some relative positions are energetically favourable, others are less favourable.
• 1:46:38
  Speaker 2: The atoms are therefore not simply gliding through a perfectly
• 1:46:41
  Speaker 2: smooth environment. They're continually moving through a pattern of microscopic
• 1:46:46
  Speaker 2: hills and valleys in energy. If the applied force is small,
• 1:46:51
  Speaker 2: atoms or groups of atoms at the interface can temporarily
• 1:46:54
  Speaker 2: settle into one of those lower energy arrangements. The surfaces
• 1:46:58
  Speaker 2: resist further movement, increase the force, and eventually that local
• 1:47:03
  Speaker 2: arrangement becomes unstable. The atoms shift suddenly into a new configuration.
• 1:47:09
  Speaker 2: The surfaces move forward slightly, Then another temporary arrangement may form.
• 1:47:15
  Speaker 2: This repeated pattern is known as stick-slip motion. At macroscopic scales,
• 1:47:20
  Speaker 2: stick-slip behaviour can sometimes produce the familiar jerking motion of
• 1:47:24
  Speaker 2: an object that refuses to slide smoothly. At much smaller scales,
• 1:47:29
  Speaker 2: similar behaviour can occur between tiny contact regions containing relatively
• 1:47:33
  Speaker 2: small numbers of atoms. These surface sticks, stress builds, the
• 1:47:38
  Speaker 2: configuration gives way. then it sticks again somewhere else. This
• 1:47:43
  Speaker 2: is one reason friction can exist even between surfaces that
• 1:47:47
  Speaker 2: appear extremely smooth. The resistance does not require miniature mountains
• 1:47:52
  Speaker 2: physically locking together like the teeth of two gears. It
• 1:47:56
  Speaker 2: can arise from the changing energy of the atoms at
• 1:47:59
  Speaker 2: the interface. Adhesion can add another contribution. Atoms and molecules
• 1:48:04
  Speaker 2: across the two surfaces may attract one another. Van der
• 1:48:08
  Speaker 2: Waals interactions can pull them together. In some materials, stronger
• 1:48:13
  Speaker 2: chemical bonds can form across microscopic contact regions. Sliding then
• 1:48:18
  Speaker 2: requires those interactions to be weakened or broken. New ones
• 1:48:22
  Speaker 2: may form immediately afterwards. The interface can become a continual
• 1:48:26
  Speaker 2: sequence of atomic connections being created and destroyed. That process
• 1:48:31
  Speaker 2: consumes organised mechanical energy. The energy does not vanish. It
• 1:48:36
  Speaker 2: changes form. One important destination is vibration within the materials themselves.
• 1:48:43
  Speaker 2: Atoms in a solid are not perfectly stationary, even when
• 1:48:46
  Speaker 2: the object appears completely still. They vibrate around their average positions.
• 1:48:51
  Speaker 2: In quantum mechanics, collective vibrations of atoms within a crystal
• 1:48:56
  Speaker 2: can be described using quasi-particles called phonons. A phonon is
• 1:49:01
  Speaker 2: not a separate fundamental particle flying through the solid in
• 1:49:05
  Speaker 2: the same sense as an electron. It is a quantized
• 1:49:09
  Speaker 2: excitation of collective vibration within the lattice. When sliding surfaces interact,
• 1:49:14
  Speaker 2: some of the mechanical energy associated with their organized motion
• 1:49:18
  Speaker 2: can be transferred into these vibrational modes. The surfaces slow slightly.
• 1:49:23
  Speaker 2: The atoms vibrate more energetically. Energy spreads through the material. Eventually,
• 1:49:29
  Speaker 2: what began as coherent motion in one direction becomes increasingly
• 1:49:33
  Speaker 2: disordered microscopic motion. Macroscopically, We recognise that as heat. Rub
• 1:49:40
  Speaker 2: your hands together and they warm. At the atomic level,
• 1:49:44
  Speaker 2: organised motion has been redistributed into many microscopic degrees of freedom.
• 1:49:49
  Speaker 2: The energy has not been destroyed by friction. It has
• 1:49:52
  Speaker 2: been dispersed. This link between friction and heating is therefore
• 1:49:57
  Speaker 2: not a mysterious conversion caused by surfaces being generally unhappy
• 1:50:01
  Speaker 2: about movement. It is energy transfer. The moving interface excites vibrations.
• 1:50:07
  Speaker 2: electronic processes and local structural rearrangements. These forms of microscopic
• 1:50:13
  Speaker 2: motion gradually spread through the material. Friction can also alter
• 1:50:18
  Speaker 2: the surfaces permanently. Repeated sliding may deform asperities. Atoms can
• 1:50:23
  Speaker 2: be displaced from their original lattice sites. Small groups of
• 1:50:27
  Speaker 2: atoms can be detached. Material can be transferred from one
• 1:50:31
  Speaker 2: surface to the other. This is where. At sufficiently small scales,
• 1:50:37
  Speaker 2: Wear can involve only tiny volumes of material, but repeated
• 1:50:41
  Speaker 2: millions of times it becomes visible damage. A polished surface
• 1:50:45
  Speaker 2: may become scratched. A machine component may slowly lose material.
• 1:50:50
  Speaker 2: A frequently used mechanical contact can eventually change shape, enough
• 1:50:55
  Speaker 2: to fail. The science devoted to these processes is called tribology.
• 1:51:01
  Speaker 2: Tribology is the study of friction, wear and lubrication. It
• 1:51:06
  Speaker 2: may sound like an unusually specialised corner of physics and engineering,
• 1:51:10
  Speaker 2: but it affects almost every machine containing moving parts. Bearings, engines, gears,
• 1:51:17
  Speaker 2: artificial joints, manufacturing tools, computer storage devices and microscopic mechanical
• 1:51:23
  Speaker 2: systems all depend on controlling what happens where surfaces meet
• 1:51:28
  Speaker 2: and move. At very small scales, tribology becomes especially interesting
• 1:51:33
  Speaker 2: because individual atomic interactions can dominate behaviour that would be
• 1:51:37
  Speaker 2: averaged out in larger systems. Lubrication is one way of
• 1:51:41
  Speaker 2: changing those interactions. A lubricant places another material between the
• 1:51:46
  Speaker 2: two surfaces. Instead of the atoms in one solid repeatedly
• 1:51:50
  Speaker 2: interacting directly with atoms in the other, they interact through
• 1:51:54
  Speaker 2: a layer of molecules designed to move more easily. That
• 1:51:57
  Speaker 2: layer can reduce adhesion between the original surfaces. It can
• 1:52:01
  Speaker 2: help prevent strong chemical bonds from forming across the interface.
• 1:52:06
  Speaker 2: It can alter the way load is distributed, and it
• 1:52:09
  Speaker 2: can provide lower energy pathways for sliding. Some lubricants form
• 1:52:14
  Speaker 2: extremely thin molecular films. Others create much thicker fluid layers
• 1:52:18
  Speaker 2: that keep the solid surfaces largely separated. Either way, the
• 1:52:23
  Speaker 2: basic goal is not simply to make the surface slippery
• 1:52:25
  Speaker 2: in some vague sense. It is to change which atoms interact,
• 1:52:30
  Speaker 2: how strongly they interact, and where the mechanical energy goes
• 1:52:34
  Speaker 2: during motion. This makes friction a particularly useful example of
• 1:52:39
  Speaker 2: what atomic contact really means. At the everyday scale, two
• 1:52:43
  Speaker 2: objects appear simply to rub against each other. At the
• 1:52:47
  Speaker 2: microscopic scale, nothing is so simple. Small regions of contact
• 1:52:52
  Speaker 2: form and disappear. Electron distributions respond. Atoms move through changing
• 1:52:58
  Speaker 2: potential energy landscapes. Temporary bonds can form and break. Momentum
• 1:53:03
  Speaker 2: is transferred. Lattice vibrations are excited. Heat spreads through the material.
• 1:53:10
  Speaker 2: Atoms can even be torn from one surface and deposited
• 1:53:13
  Speaker 2: onto the other. All of this can happen without any
• 1:53:17
  Speaker 2: tiny hard shells scraping together like grains of sand. The
• 1:53:21
  Speaker 2: surfaces interact because their atoms and electrons occupy quantum states.
• 1:53:26
  Speaker 2: whose energies depend on how closely they approach and how
• 1:53:29
  Speaker 2: they move past one another. Touch, once motion is added,
• 1:53:33
  Speaker 2: becomes a continuous exchange. Energy moves, momentum moves, sometimes matter
• 1:53:40
  Speaker 2: moves as well. And friction is the large-scale trace left
• 1:53:44
  Speaker 2: behind by that restless atomic activity. For most of human history,
• 1:53:50
  Speaker 2: the forces acting between individual atoms were far beyond anything
• 1:53:54
  Speaker 2: we could directly probe. Atoms revealed themselves through chemistry, spectroscopy
• 1:54:00
  Speaker 2: and the behaviour of matter, but no human hand could
• 1:54:03
  Speaker 2: move gently enough to explore a surface one atom at
• 1:54:06
  Speaker 2: a time. Modern instruments have changed that. One of the
• 1:54:11
  Speaker 2: most remarkable is atomic force microscopy, a technique developed during
• 1:54:15
  Speaker 2: the 1980s by Gerd Binnig, Calvin Quate and Christoph Gerber.
• 1:54:20
  Speaker 2: In a sense, an atomic force microscope gives scientists an
• 1:54:24
  Speaker 2: extraordinarily delicate artificial fingertip. At its heart is a tiny
• 1:54:29
  Speaker 2: flexible structure called a cantilever. Near the end of that
• 1:54:32
  Speaker 2: cantilever is an extremely sharp tip, sometimes narrowing towards a
• 1:54:37
  Speaker 2: region only a few atoms across. The tip is brought
• 1:54:40
  Speaker 2: incredibly close to a sample surface. As it approaches, the
• 1:54:45
  Speaker 2: atoms near the tip begin interacting with atoms in the
• 1:54:47
  Speaker 2: material beneath it. Those interactions are extremely weak by everyday standards,
• 1:54:53
  Speaker 2: but the cantilever is sensitive enough for them to alter
• 1:54:56
  Speaker 2: its position or motion. By measuring that response while the
• 1:55:01
  Speaker 2: tip moves across a surface, researchers can reconstruct information about
• 1:55:05
  Speaker 2: the material on scales approaching individual atoms and molecules. The
• 1:55:10
  Speaker 2: instrument is not feeling a row of hard little spheres.
• 1:55:14
  Speaker 2: What it detects is interaction. When the tip is still
• 1:55:17
  Speaker 2: relatively far from the surface, Attractive forces can become measurable.
• 1:55:22
  Speaker 2: Depending on the materials and environment, these can include van
• 1:55:26
  Speaker 2: der Waals interactions and electrostatic forces. As the separation becomes smaller,
• 1:55:32
  Speaker 2: the balance changes. The electronic states of atoms in the
• 1:55:36
  Speaker 2: tip and surface begin interacting increasingly strongly. Electron density overlap
• 1:55:42
  Speaker 2: becomes more significant, and the energetic cost of bringing the
• 1:55:45
  Speaker 2: atoms closer starts to rise. Eventually, short-range repulsive behaviour becomes dominant.
• 1:55:52
  Speaker 2: This is essentially the same progression involved whenever two pieces
• 1:55:56
  Speaker 2: of matter approach one another. Far apart, interaction is small.
• 1:56:00
  Speaker 2: Closer in, attraction may become important. Closer still, strong electronic
• 1:56:06
  Speaker 2: overlap produces a rapidly increasing resistance to further approach. Atomic
• 1:56:11
  Speaker 2: force microscopy allows scientists to explore that change experimentally. There
• 1:56:16
  Speaker 2: are several ways to operate such an instrument. In what
• 1:56:19
  Speaker 2: is known as contact mode, the tip remains in extremely
• 1:56:23
  Speaker 2: close interaction with the surface while it moves across it.
• 1:56:27
  Speaker 2: The cantilever bends in response to the forces it experiences
• 1:56:31
  Speaker 2: and those changes can be used to construct a map
• 1:56:33
  Speaker 2: of the surface. There are also dynamic approaches in which
• 1:56:37
  Speaker 2: the cantilever is made to oscillate. Instead of simply measuring
• 1:56:41
  Speaker 2: how far it bends, Researchers monitor how the interaction with
• 1:56:45
  Speaker 2: the surface changes properties of that oscillation. The frequency, amplitude
• 1:56:51
  Speaker 2: or phase of the motion can respond to tiny variations
• 1:56:55
  Speaker 2: in force. Some versions operate with the tip extremely close
• 1:56:59
  Speaker 2: to the surface without maintaining continuous mechanical contact in the
• 1:57:03
  Speaker 2: ordinary sense. This can reduce disturbance to delicate samples and
• 1:57:08
  Speaker 2: reveal subtle variations in atomic scale interactions. The details of
• 1:57:13
  Speaker 2: the machinery can become wonderfully elaborate, but the central principle
• 1:57:17
  Speaker 2: remains simple. Bring a tiny probe near a surface and
• 1:57:21
  Speaker 2: measure how the interaction changes. That makes atomic force microscopy
• 1:57:26
  Speaker 2: particularly useful for the question of what touching actually means.
• 1:57:31
  Speaker 2: Researchers can measure what is often called a force distance curve.
• 1:57:36
  Speaker 2: The tip begins farther from the surface and is gradually
• 1:57:39
  Speaker 2: moved closer. At first, the measured interaction may be almost negligible. Then,
• 1:57:46
  Speaker 2: attraction appears. The force changes as the separation decreases. At
• 1:57:51
  Speaker 2: very close distances, the interaction can pass into the strongly
• 1:57:54
  Speaker 2: repulsive regime. Moving the tip away again reveals how the
• 1:57:58
  Speaker 2: force changes in the opposite direction, and it can sometimes
• 1:58:02
  Speaker 2: expose adhesion between tip and surface. There is no universal
• 1:58:06
  Speaker 2: point where the instrument suddenly announces that contact has occurred. Instead,
• 1:58:11
  Speaker 2: there is a measurable progression from weak interaction to strong interaction.
• 1:58:17
  Speaker 2: The atomic boundary behaves exactly as quantum mechanics leads us
• 1:58:21
  Speaker 2: to expect. It is not a hard edge. It is
• 1:58:25
  Speaker 2: an energy landscape. This also helps explain what the beautiful
• 1:58:29
  Speaker 2: atomic scale images produced by these instruments really represent. An
• 1:58:33
  Speaker 2: atomic force microscope is not taking an ordinary photograph. There
• 1:58:38
  Speaker 2: is no microscopic camera looking down and seeing little balls
• 1:58:41
  Speaker 2: arranged across the surface. The image is reconstructed from measured
• 1:58:45
  Speaker 2: interactions as the tip moves from one position to another.
• 1:58:49
  Speaker 2: Bright and dark regions or high and low regions in
• 1:58:53
  Speaker 2: a rendered image represent changes in quantities derived from the motion,
• 1:58:58
  Speaker 2: or force, experienced by the probe. Under suitable conditions, the
• 1:59:03
  Speaker 2: repeating atomic structure of a surface can become visible with
• 1:59:06
  Speaker 2: extraordinary clarity. But the apparent atoms are features in a
• 1:59:10
  Speaker 2: map of physical measurements. They are not coloured spherical shells
• 1:59:14
  Speaker 2: photographed directly. A related instrument makes this point even more clearly.
• 1:59:20
  Speaker 2: Before the atomic force microscope was developed, Gerd Binig, Heinrich
• 1:59:24
  Speaker 2: Aurora developed the scanning tunnelling microscope. This instrument also uses
• 1:59:30
  Speaker 2: a very sharp tip positioned extraordinarily close to a conducting surface,
• 1:59:35
  Speaker 2: but the physical quantity it measures is different. Instead of
• 1:59:39
  Speaker 2: primarily measuring mechanical force, scanning tunnelling microscopy relies on quantum tunnelling.
• 1:59:46
  Speaker 2: Quantum mechanics allows electrons to have a probability of crossing
• 1:59:49
  Speaker 2: a narrow region that would be forbidden in a simple
• 1:59:52
  Speaker 2: classical description. When a conducting tip is placed extremely close
• 1:59:58
  Speaker 2: to a conducting or semi-conducting sample and an appropriate electrical
• 2:00:02
  Speaker 2: potential is applied, electrons can tunnel across the tiny gap.
• 2:00:08
  Speaker 2: This creates a tunnelling current. The strength of that current
• 2:00:12
  Speaker 2: is extraordinarily sensitive to the distance between the tip and
• 2:00:15
  Speaker 2: the surface. Even very small changes in separation can produce
• 2:00:20
  Speaker 2: large changes in the measured current. By moving the tip
• 2:00:23
  Speaker 2: across the sample and monitoring that current, researchers can map
• 2:00:27
  Speaker 2: atomic scale features of the electronic structure of the surface. Again,
• 2:00:32
  Speaker 2: what appears in the final image is not an ordinary photograph.
• 2:00:36
  Speaker 2: A scanning tunnelling microscope is particularly sensitive to the electronic
• 2:00:40
  Speaker 2: states available near the surface. So what may look like
• 2:00:44
  Speaker 2: individual atoms is really a map influenced by electron density
• 2:00:48
  Speaker 2: and the quantum states participating in tunnelling. Atomic force microscopy
• 2:00:54
  Speaker 2: and scanning tunnelling microscopy therefore reveal atoms in different ways.
• 2:01:00
  Speaker 2: One measures forces between a probe and a sample. The
• 2:01:03
  Speaker 2: other measures an electrical current produced by quantum tunnelling. Both
• 2:01:08
  Speaker 2: can reach astonishing spatial resolution, but neither discovers tiny hard
• 2:01:13
  Speaker 2: shells waiting beneath the surface. Instead, they reveal exactly the
• 2:01:17
  Speaker 2: kind of atomic world suggested by quantum mechanics. Atoms have
• 2:01:22
  Speaker 2: characteristic positions and structures, but the experimentally accessible boundary between
• 2:01:28
  Speaker 2: them is defined through interactions. Scientists can approach one atom
• 2:01:33
  Speaker 2: towards another and watch attraction grow. They can detect the
• 2:01:37
  Speaker 2: region where a stable separation becomes favourable. They can move
• 2:01:40
  Speaker 2: closer and measure the sharply increasing resistance associated with short-range interactions.
• 2:01:47
  Speaker 2: They can even manipulate individual atoms under carefully controlled conditions.
• 2:01:52
  Speaker 2: Yet there is still no microscopic click announcing the universal
• 2:01:55
  Speaker 2: instant when one atom has touched another. Nature gives something
• 2:02:00
  Speaker 2: more informative. It gives a continuously changing force, a changing energy,
• 2:02:06
  Speaker 2: a changing electron distribution, and a changing quantum state. and
• 2:02:11
  Speaker 2: with instruments sensitive enough, scientists can quite literally measure that
• 2:02:15
  Speaker 2: transition one atom at a time. Ordinary touch only squeezes
• 2:02:20
  Speaker 2: atoms by a very small amount. Even when you press
• 2:02:24
  Speaker 2: firmly against a solid object, the average spacing between neighbouring
• 2:02:28
  Speaker 2: atoms changes only slightly. The energetic resistance rises quickly enough
• 2:02:34
  Speaker 2: that familiar materials feel stiff long before their atoms have
• 2:02:38
  Speaker 2: been compressed by anything approaching a dramatic fraction of their
• 2:02:41
  Speaker 2: normal separation. But nature is capable of applying much greater pressure.
• 2:02:47
  Speaker 2: Deep inside planets, during powerful impacts, and inside specialised laboratory equipment,
• 2:02:53
  Speaker 2: matter can be compressed so strongly that the usual arrangements
• 2:02:57
  Speaker 2: of atoms begin to change. At first, compression does something
• 2:03:01
  Speaker 2: fairly straightforward– The average distance between neighbouring nuclei becomes smaller.
• 2:03:07
  Speaker 2: Electron distributions overlap more strongly. The electronic states reorganise. Short-range
• 2:03:14
  Speaker 2: electromagnetic interactions become increasingly important and the restrictions imposed by
• 2:03:19
  Speaker 2: quantum mechanics become harder to accommodate without increasing the energy
• 2:03:24
  Speaker 2: of the system. But if enough pressure is applied, matter
• 2:03:28
  Speaker 2: does not simply continue behaving like a slightly squashed version
• 2:03:31
  Speaker 2: of itself. It can transform. A crystal that is stable
• 2:03:36
  Speaker 2: at ordinary pressure may rearrange into a completely different crystal
• 2:03:39
  Speaker 2: structure because the new arrangement packs atoms more efficiently. Pressure
• 2:03:45
  Speaker 2: changes the competition between different possible structures. At ordinary conditions,
• 2:03:50
  Speaker 2: one arrangement may have the lowest energy. Under extreme compression,
• 2:03:55
  Speaker 2: the importance of volume increases and another arrangement can become
• 2:03:59
  Speaker 2: more favourable. Atoms may acquire different numbers of nearest neighbours.
• 2:04:04
  Speaker 2: Bond lengths and bond angles can change. Entire crystal lattices
• 2:04:08
  Speaker 2: can reconstruct themselves. These are pressure-induced phase transitions. They are
• 2:04:13
  Speaker 2: a reminder that the familiar form of a material is
• 2:04:16
  Speaker 2: not always permanent. It is simply one arrangement that happens
• 2:04:20
  Speaker 2: to be stable under a particular range of temperature and pressure.
• 2:04:25
  Speaker 2: Electronic behaviour can change just as dramatically. A material that
• 2:04:30
  Speaker 2: barely conducts electricity at ordinary pressure can become much more
• 2:04:34
  Speaker 2: conductive when compressed. This happens because decreasing the distances between
• 2:04:39
  Speaker 2: atoms changes the overlap of their electronic wave functions. Electronic
• 2:04:44
  Speaker 2: energy levels that were relatively separate can broaden into bands.
• 2:04:48
  Speaker 2: Energy gaps can shrink. Eventually, electrons that were previously confined
• 2:04:53
  Speaker 2: to particular states may become able to move much more
• 2:04:57
  Speaker 2: freely through the material. A substance that behaved as an
• 2:05:00
  Speaker 2: electrical insulator can therefore undergo a transition towards metallic behaviour.
• 2:05:06
  Speaker 2: Nothing has been added to the material. The atoms have
• 2:05:09
  Speaker 2: simply been forced close enough together that the quantum states
• 2:05:13
  Speaker 2: available to the electrons have changed. Researchers can explore these
• 2:05:17
  Speaker 2: conditions in the laboratory using an instrument called a diamond
• 2:05:21
  Speaker 2: anvil cell. The principle is beautifully simple. A tiny sample
• 2:05:25
  Speaker 2: is placed between the flattened tips of two diamonds. Diamond
• 2:05:29
  Speaker 2: is extremely hard and can withstand enormous stresses. By applying
• 2:05:34
  Speaker 2: force to the backs of the diamonds, researchers concentrate that
• 2:05:38
  Speaker 2: force onto a microscopic area containing the sample. Because pressure
• 2:05:42
  Speaker 2: is force distributed over area, concentrating the force onto such
• 2:05:47
  Speaker 2: a tiny region produces extraordinary pressures. Conventional diamond anvil cells
• 2:05:52
  Speaker 2: can reach pressures of hundreds of gigapascals, meaning millions of
• 2:05:57
  Speaker 2: times atmospheric pressure. More specialised designs have pushed static laboratory
• 2:06:02
  Speaker 2: pressures even higher. Researchers can combine this compression with lasers,
• 2:06:07
  Speaker 2: spectroscopy and X-ray measurements to examine how materials behave while
• 2:06:12
  Speaker 2: they are still under pressure. The samples may be microscopic,
• 2:06:16
  Speaker 2: but the conditions can resemble those found deep inside planets.
• 2:06:21
  Speaker 2: One of the most famous substances studied this way is
• 2:06:24
  Speaker 2: also the simplest element in the universe– hydrogen. At ordinary conditions,
• 2:06:30
  Speaker 2: hydrogen exists as molecules containing pairs of hydrogen atoms and
• 2:06:34
  Speaker 2: behaves as a gas. But nearly a century ago, physicists
• 2:06:38
  Speaker 2: predicted that sufficiently strong compression should eventually transform dense hydrogen
• 2:06:44
  Speaker 2: into a metallic state. Under those conditions, the electron should
• 2:06:48
  Speaker 2: become mobile enough for the material to conduct electricity rather
• 2:06:52
  Speaker 2: like a metal. Achieving and identifying that state has proved
• 2:06:56
  Speaker 2: exceptionally difficult. Hydrogen is tiny, highly mobile and challenging to
• 2:07:02
  Speaker 2: confine at enormous pressures. Experiments have found strong evidence for
• 2:07:07
  Speaker 2: metallic behaviour in extremely dense hydrogen and deuterium under particular
• 2:07:13
  Speaker 2: high pressure and high temperature conditions, especially in fluid states.
• 2:07:18
  Speaker 2: Claims of solid metallic hydrogen at lower temperatures have been
• 2:07:22
  Speaker 2: far more controversial. As of the present state of high
• 2:07:26
  Speaker 2: pressure research, An unambiguous, independently established sample of stable, solid
• 2:07:32
  Speaker 2: metallic hydrogen has not become an ordinary laboratory reality. That
• 2:07:38
  Speaker 2: distinction matters because metallic hydrogen is not one simple condition
• 2:07:43
  Speaker 2: that scientists can identify merely by squeezing until the sample
• 2:07:48
  Speaker 2: changes colour. Whether hydrogen is molecular or atomic, solid or fluid,
• 2:07:55
  Speaker 2: Electrically insulating or conducting depends on both pressure and temperature.
• 2:08:00
  Speaker 2: Dense hydrogen therefore possesses a complicated phase diagram. Nature, however,
• 2:08:06
  Speaker 2: has had rather more practice at producing extreme pressure than
• 2:08:10
  Speaker 2: laboratories have. Jupiter provides an extraordinary example. The planet is
• 2:08:15
  Speaker 2: made mostly from hydrogen and helium. Near the visible cloud tops,
• 2:08:20
  Speaker 2: hydrogen behaves broadly like the familiar molecular substance we know.
• 2:08:25
  Speaker 2: Travel deeper and the pressure rises enormously. The hydrogen becomes denser,
• 2:08:31
  Speaker 2: hotter and eventually fluid. Farther inward, pressures of millions of
• 2:08:36
  Speaker 2: times Earth's atmospheric pressure alter its electronic behaviour so dramatically
• 2:08:41
  Speaker 2: that hydrogen enters a metallic conducting state. Inside Jupiter, Metallic
• 2:08:47
  Speaker 2: hydrogen is thought to occupy a vast region. This electrically
• 2:08:53
  Speaker 2: conducting fluid is important to models of Jupiter's powerful magnetic field.
• 2:08:58
  Speaker 2: The material there is not simply ordinary hydrogen gas that
• 2:09:01
  Speaker 2: has been pressed into a slightly smaller container. Its electronic
• 2:09:05
  Speaker 2: structure has changed fundamentally. Under still more extreme conditions, even
• 2:09:11
  Speaker 2: the familiar idea of intact neutral atoms can cease to
• 2:09:15
  Speaker 2: be useful. If sufficient energy is available, electrons can be
• 2:09:19
  Speaker 2: removed from atoms. The material becomes ionized. Instead of neutral
• 2:09:25
  Speaker 2: atoms retaining their ordinary electronic structures, the system may contain
• 2:09:30
  Speaker 2: positively charged ions and mobile electrons. At high enough temperatures
• 2:09:35
  Speaker 2: and pressures, matter can enter plasma states or other strongly
• 2:09:38
  Speaker 2: coupled forms in which the tidy picture of individual neutral
• 2:09:42
  Speaker 2: atoms becomes increasingly inappropriate. It is tempting to describe this
• 2:09:47
  Speaker 2: by saying that the electron clouds have finally been crushed
• 2:09:50
  Speaker 2: or broken. But that returns us to the misleading picture
• 2:09:54
  Speaker 2: of atoms as objects with physical shells. There was never
• 2:09:58
  Speaker 2: a rigid electron cloud waiting to crack. What changes under
• 2:10:03
  Speaker 2: pressure is the quantum state. Electron distributions reorganise continuously as
• 2:10:08
  Speaker 2: the distances between nuclei decrease. Orbitals change. Energy bands change.
• 2:10:15
  Speaker 2: Chemical bonds can disappear and new ones can form. Electrons
• 2:10:19
  Speaker 2: may become delocalised across many nuclei. Eventually, some may no
• 2:10:24
  Speaker 2: longer be usefully associated with individual neutral atoms at all.
• 2:10:28
  Speaker 2: Matter changes its description because the most useful quantum states
• 2:10:32
  Speaker 2: have changed. And even after all of this extraordinary compression,
• 2:10:37
  Speaker 2: another enormous scale remains. Atomic nuclei are tiny compared with
• 2:10:42
  Speaker 2: ordinary interatomic distances. under pressures capable of transforming crystals, metalising
• 2:10:48
  Speaker 2: materials and profoundly rearranging electronic structure, neighbouring nuclei can still
• 2:10:54
  Speaker 2: remain far apart compared with a few femtometers characteristic of
• 2:10:58
  Speaker 2: nuclear dimensions. Electronic matter can therefore undergo astonishing transformations without
• 2:11:05
  Speaker 2: individual nuclei coming into anything resembling ordinary nuclear contact. The
• 2:11:10
  Speaker 2: strong nuclear interaction becomes dominant only when nucleons approach one
• 2:11:15
  Speaker 2: another at distances comparable with nuclear dimensions. That requires a
• 2:11:19
  Speaker 2: completely different regime, so forcing atoms closer does not eventually
• 2:11:24
  Speaker 2: reveal a hidden hard surface underneath their electrons. Instead, it
• 2:11:29
  Speaker 2: reveals how adaptable quantum matter can be. First the atoms
• 2:11:33
  Speaker 2: compress slightly, then their electron distributions change. Crystal structures reorganise,
• 2:11:40
  Speaker 2: electrical properties transform, chemical identities can become less distinct. Neutral
• 2:11:46
  Speaker 2: atoms may give way to ions and delocalised electrons, and
• 2:11:51
  Speaker 2: all the while the nuclei can remain comparatively tiny islands
• 2:11:55
  Speaker 2: separated by distances far greater than their own size. Extreme
• 2:11:59
  Speaker 2: pressure does not finally make atoms behave like hard balls,
• 2:12:03
  Speaker 2: it does almost the opposite. It shows just how profoundly
• 2:12:07
  Speaker 2: matter can reinvent itself when the space available to its
• 2:12:11
  Speaker 2: quantum states is taken away. If we take the idea
• 2:12:14
  Speaker 2: of touching to its most extreme possible meaning, one final
• 2:12:18
  Speaker 2: question appears. Can two atomic nuclei ever get close enough
• 2:12:23
  Speaker 2: to interact directly? The answer is yes, but by the
• 2:12:27
  Speaker 2: time that happens we have left ordinary contact far behind.
• 2:12:31
  Speaker 2: In everyday matter, neighbouring atomic nuclei are typically separated by
• 2:12:35
  Speaker 2: distances of a few tenths of a nanometre. A nanometre
• 2:12:39
  Speaker 2: is one billionth of a metre. Atomic nuclei themselves are
• 2:12:43
  Speaker 2: far smaller. Their dimensions are usually measured in femtometers, with
• 2:12:48
  Speaker 2: one femtometre equal to one millionth of a nanometre. So
• 2:12:52
  Speaker 2: a typical nucleus is tens of thousands of times smaller
• 2:12:56
  Speaker 2: in linear size than the distance separating neighbouring nuclei in
• 2:12:59
  Speaker 2: ordinary matter. That difference is enormous. When your hand rests
• 2:13:04
  Speaker 2: on a table, the atoms in your skin interact strongly
• 2:13:08
  Speaker 2: with the atoms in the surface. But their nuclei remain
• 2:13:11
  Speaker 2: extraordinarily far apart compared with their own dimensions. To bring
• 2:13:16
  Speaker 2: two nuclei close enough for nuclear physics to dominate requires
• 2:13:20
  Speaker 2: very different conditions. The first obstacle is electrical repulsion. Atomic
• 2:13:26
  Speaker 2: nuclei contain positively charged protons. Two positively charged nuclei therefore
• 2:13:31
  Speaker 2: repel one another through the electromagnetic interaction. As the nuclei approach,
• 2:13:37
  Speaker 2: this repulsion becomes stronger. The energetic obstacle created by that
• 2:13:41
  Speaker 2: repulsion is called the Coulomb barrier. The name comes from
• 2:13:45
  Speaker 2: the same electrostatic physics described by Charles-Augustin de Coulomb. For
• 2:13:51
  Speaker 2: ordinary atoms sitting peacefully in a solid, there is nowhere
• 2:13:54
  Speaker 2: near enough energy to drive their nuclei deeply into this barrier.
• 2:13:58
  Speaker 2: Their electrons can overlap, rearrange and form bonds while the
• 2:14:02
  Speaker 2: nuclei remain safely separated. Nuclear contact requires pushing much farther.
• 2:14:08
  Speaker 2: And once the separation falls to only a few femtometers,
• 2:14:12
  Speaker 2: another interaction becomes important. The strong nuclear interaction. The strong
• 2:14:18
  Speaker 2: interaction is fundamentally described by quantum chromodynamics. But within atomic nuclei,
• 2:14:25
  Speaker 2: its residual effects bind protons and neutrons together. At nuclear distances,
• 2:14:30
  Speaker 2: this interaction can be powerful enough to overcome the electrical
• 2:14:33
  Speaker 2: repulsion between positively charged protons and hold a nucleus together.
• 2:14:39
  Speaker 2: That is why nuclei containing several protons can exist at all.
• 2:14:44
  Speaker 2: The electromagnetic interaction tries to push those positive charges apart.
• 2:14:49
  Speaker 2: The nuclear interaction, acting over extremely short distances, can bind
• 2:14:55
  Speaker 2: the nucleons into a stable or temporarily stable structure. If
• 2:14:59
  Speaker 2: two suitable light nuclei can approach closely enough, the nuclear
• 2:15:03
  Speaker 2: interaction may rearrange them into a new nucleus. This is
• 2:15:07
  Speaker 2: nuclear fusion. Fusion does not simply mean that two rigid
• 2:15:11
  Speaker 2: nuclear balls touch and stick together. It is a quantum
• 2:15:15
  Speaker 2: process in which the states of the participating nuclei reorganise
• 2:15:19
  Speaker 2: into a new configuration. If the resulting nucleus is more
• 2:15:23
  Speaker 2: tightly bound, some of the difference in energy can be released.
• 2:15:27
  Speaker 2: This is the basic source of the energy produced by stars.
• 2:15:31
  Speaker 2: The problem is getting the nuclei close enough in the
• 2:15:33
  Speaker 2: first place. Inside a star such as the Sun, temperatures
• 2:15:38
  Speaker 2: are enormous by everyday standards. The core temperature is around
• 2:15:42
  Speaker 2: 15 million Kelvin. Hydrogen nuclei move rapidly through an extremely
• 2:15:48
  Speaker 2: dense plasma. They collide constantly. Even so, a simple classical
• 2:15:53
  Speaker 2: calculation creates an apparent difficulty. Most protons in the solar
• 2:15:58
  Speaker 2: core do not possess enough kinetic energy to climb completely
• 2:16:01
  Speaker 2: over the Coulomb barrier and reach the short distances where
• 2:16:05
  Speaker 2: nuclear reactions become likely. If classical mechanics were the whole story,
• 2:16:11
  Speaker 2: fusion in the Sun would be much more difficult than
• 2:16:13
  Speaker 2: observation tells us it is. Quantum mechanics provides the missing root.
• 2:16:19
  Speaker 2: A quantum particle can have a non-zero probability of passing
• 2:16:22
  Speaker 2: through an energy barrier even when it does not possess
• 2:16:26
  Speaker 2: enough classical energy to cross over the top. This is
• 2:16:29
  Speaker 2: quantum tunnelling. During the late 1920s, George Gamow, along with
• 2:16:34
  Speaker 2: Ronald Gurney and Edward Condon in closely related work, helped
• 2:16:38
  Speaker 2: establish quantum tunnelling as a physical mechanism in nuclear processes.
• 2:16:43
  Speaker 2: Gurney and Condon used the new quantum mechanics to explain
• 2:16:46
  Speaker 2: alpha decay, while Gamow independently developed a tunnelling treatment of
• 2:16:51
  Speaker 2: nuclear decay and later applied related ideas to nuclear reactions.
• 2:16:56
  Speaker 2: The same general principle is essential to understanding stellar fusion.
• 2:17:01
  Speaker 2: Two positively charged nuclei approach one another. Classically, their electrical
• 2:17:06
  Speaker 2: repulsion would turn them away before they reached nuclear distances.
• 2:17:10
  Speaker 2: Quantum mechanically, however, their wave functions extend into the classically
• 2:17:15
  Speaker 2: forbidden region. There is a small probability that the nuclei
• 2:17:19
  Speaker 2: can effectively penetrate the Coulomb barrier. and reach separations where
• 2:17:23
  Speaker 2: the strong nuclear interaction becomes important. That probability can be
• 2:17:27
  Speaker 2: tiny for any individual encounter. But the Sun contains an
• 2:17:32
  Speaker 2: almost absurd number of particles. Its core provides enormous densities,
• 2:17:37
  Speaker 2: high temperatures and continuous collisions over billions of years. A
• 2:17:42
  Speaker 2: very small probability multiplied across an enormous population can still
• 2:17:47
  Speaker 2: produce a steady rate of nuclear reactions. The rate of
• 2:17:50
  Speaker 2: fusion depends on much more than temperature alone. Density matters
• 2:17:55
  Speaker 2: because it affects how often nuclei encounter one another. The
• 2:17:59
  Speaker 2: nuclear cross-section matters because different reactions have different probabilities. The
• 2:18:04
  Speaker 2: distribution of particle energies matters because faster moving nuclei penetrate
• 2:18:09
  Speaker 2: the Coulomb barrier more effectively. The quantum tunnelling probability also
• 2:18:14
  Speaker 2: changes rapidly with energy. Together, these ingredients determine how frequently
• 2:18:19
  Speaker 2: fusion occurs. In the Sun, the main process begins with
• 2:18:24
  Speaker 2: hydrogen nuclei. Through a sequence known as the proton-proton chain,
• 2:18:29
  Speaker 2: four protons are ultimately transformed into a helium nucleus. with
• 2:18:33
  Speaker 2: other particles and energy released along the way. The process
• 2:18:37
  Speaker 2: is slow on the scale of individual protons. That slowness
• 2:18:41
  Speaker 2: is one of the reasons the Sun can shine steadily
• 2:18:44
  Speaker 2: for billions of years, rather than burning through its fuel
• 2:18:48
  Speaker 2: almost immediately. This environment could hardly be more different from
• 2:18:53
  Speaker 2: the atomic contact occurring when you rest a hand on
• 2:18:55
  Speaker 2: a desk. In the desk, neighbouring nuclei remain separated by
• 2:19:00
  Speaker 2: roughly tenths of a nanometre. The important physics involves electrons,
• 2:19:05
  Speaker 2: chemical bonds, electromagnetic interactions and changes in electron-kinetic energy. In
• 2:19:14
  Speaker 2: the Sun, some nuclei eventually approach to distances measured in femtometers.
• 2:19:20
  Speaker 2: Only then does the strong nuclear interaction begin to dominate
• 2:19:23
  Speaker 2: the final stage of the encounter. Even here, however, the
• 2:19:27
  Speaker 2: word touch should still be used carefully. A proton is
• 2:19:30
  Speaker 2: not a tiny hard sphere with a crisp surface. Neither
• 2:19:34
  Speaker 2: is a neutron, neither is an atomic nucleus. Protons and
• 2:19:38
  Speaker 2: neutrons are quantum systems composed of quarks and gluons, and
• 2:19:42
  Speaker 2: nuclei themselves are described by quantum states. Their matter and
• 2:19:47
  Speaker 2: charge distributions have characteristic sizes, but not perfectly rigid geometric skins.
• 2:19:53
  Speaker 2: So when two nuclei come close enough for nuclear interactions
• 2:19:56
  Speaker 2: to become strong, There is still no simple microscopic moment
• 2:20:00
  Speaker 2: resembling two marbles clicking together. Their quantum states begin interacting intensely.
• 2:20:07
  Speaker 2: The particles can scatter, they can fuse, they can exchange
• 2:20:11
  Speaker 2: energy and momentum, they can rearrange into new nuclear configurations.
• 2:20:16
  Speaker 2: But the language of solid surfaces remains misleading even at
• 2:20:20
  Speaker 2: this scale. This completes the distinction that ordinary touch tends
• 2:20:24
  Speaker 2: to hide. Atoms can collide without their nuclei touching. They
• 2:20:28
  Speaker 2: can form molecules without their nuclei touching. They can resist
• 2:20:32
  Speaker 2: compression without their nuclei touching. Two pieces of matter can
• 2:20:37
  Speaker 2: support one another, rub against one another and behave as
• 2:20:40
  Speaker 2: unquestionably solid objects, while the nuclei on opposite sides remain
• 2:20:45
  Speaker 2: enormous distances apart by nuclear standards. Only under far more
• 2:20:50
  Speaker 2: extreme conditions do nuclei approach closely enough for the strong
• 2:20:54
  Speaker 2: nuclear interaction to take over. And when that finally happens,
• 2:20:58
  Speaker 2: the event is no longer ordinary contact. It belongs to
• 2:21:02
  Speaker 2: nuclear physics. So two atoms do not need their nuclei
• 2:21:06
  Speaker 2: to meet in order to touch in any everyday sense.
• 2:21:10
  Speaker 2: Ordinary touch happens much earlier. It happens when electron distributions overlap,
• 2:21:15
  Speaker 2: quantum states reorganise, electromagnetic interactions become strong and the total
• 2:21:21
  Speaker 2: energy begins strongly resisting further approach. Nuclear contact sits far
• 2:21:27
  Speaker 2: beyond that familiar boundary. By the time matter reaches it,
• 2:21:31
  Speaker 2: the gentle physics of a hand resting on a table
• 2:21:34
  Speaker 2: has become the physics that powers stars. Rest a fingertip
• 2:21:39
  Speaker 2: on a table one last time. The experience still feels simple.
• 2:21:43
  Speaker 2: Your skin reaches the surface. The table supports your hand.
• 2:21:47
  Speaker 2: You feel pressure, resistance and solidity. But beneath that ordinary moment,
• 2:21:53
  Speaker 2: the idea of two objects physically meeting has become much stranger.
• 2:21:57
  Speaker 2: When atoms touch in ordinary matter, their nuclei do not collide.
• 2:22:01
  Speaker 2: Tiny solid shells do not strike one another. Instead, the
• 2:22:06
  Speaker 2: electron distributions and quantum states associated with neighbouring atoms begin
• 2:22:10
  Speaker 2: interacting strongly enough to change the energy of the combined systems.
• 2:22:14
  Speaker 2: Depending on the atoms involved, that interaction may produce attraction,
• 2:22:19
  Speaker 2: chemical bonding, repulsion, or the restoring force that prevents one
• 2:22:23
  Speaker 2: object from passing through another. That is what atomic contact
• 2:22:27
  Speaker 2: really is. The difficulty comes from trying to apply an
• 2:22:31
  Speaker 2: everyday picture of surfaces to objects that do not possess
• 2:22:35
  Speaker 2: ordinary surfaces. An atom has no perfectly sharp outer edge.
• 2:22:40
  Speaker 2: There is no universal radius beyond which the atom suddenly
• 2:22:43
  Speaker 2: stops existing. and empty space begins. The quantum states associated
• 2:22:48
  Speaker 2: with its electrons extend through space and gradually become less
• 2:22:51
  Speaker 2: significant with distance. That means there is no single geometric
• 2:22:56
  Speaker 2: instant when two rigid atomic boundaries must meet. The familiar
• 2:23:00
  Speaker 2: image of atoms as tiny spheres is useful for diagrams,
• 2:23:04
  Speaker 2: but it should not be mistaken for literal structure. Ernest
• 2:23:07
  Speaker 2: Rutherford's work revealed just how misleading the solid sphere picture
• 2:23:11
  Speaker 2: could be. The atom contains an extraordinarily small nucleus holding
• 2:23:15
  Speaker 2: most of its mass and all of its positive charge.
• 2:23:19
  Speaker 2: The overall dimensions associated with the atom are tens of
• 2:23:22
  Speaker 2: thousands of times larger than the dimensions of that nucleus.
• 2:23:27
  Speaker 2: So when two atoms sit beside each other in ordinary matter,
• 2:23:30
  Speaker 2: their nuclei remain separated by enormous distances compared with nuclear size.
• 2:23:36
  Speaker 2: Every touch of a hand, every footstep, every book resting
• 2:23:40
  Speaker 2: on a shelf, happens without neighbouring nuclei coming remotely close
• 2:23:44
  Speaker 2: to nuclear contact. The space associated with the rest of
• 2:23:48
  Speaker 2: the atom is governed largely by electrons. Erwin Schrödinger' s quantum
• 2:23:53
  Speaker 2: mechanics replaced the idea of definite classical electron paths with
• 2:23:57
  Speaker 2: quantum states described by wavefunctions. Max Born connected those wavefunctions
• 2:24:02
  Speaker 2: with probability, showing that their mathematical structure tells us about
• 2:24:06
  Speaker 2: the likelihood of detecting electrons in different regions. So an
• 2:24:10
  Speaker 2: electron is not simply a tiny bead following a hidden
• 2:24:13
  Speaker 2: orbit around the nucleus. Its quantum state can extend across
• 2:24:17
  Speaker 2: a region. When another atom approaches, those extended states begin
• 2:24:22
  Speaker 2: affecting one another. Electromagnetism then supplies much of the underlying interaction.
• 2:24:28
  Speaker 2: Charles-Augustin de Coulombe's work describes the basic electrical attraction between
• 2:24:34
  Speaker 2: opposite charges and repulsion between like charges. Two approaching atoms
• 2:24:39
  Speaker 2: contain several of these effects at once. Nuclei repel nuclei.
• 2:24:44
  Speaker 2: Electrons repel electrons. Electrons are attracted towards positively charged nuclei.
• 2:24:50
  Speaker 2: Neutral atoms can also polarize one another and experience weaker
• 2:24:54
  Speaker 2: attractions through correlated fluctuations in their electron distributions. The final
• 2:25:00
  Speaker 2: behavior comes from the balance of all these contributions. Quantum
• 2:25:04
  Speaker 2: mechanics adds another crucial restriction. Wolfgang Pauli's exclusion principle tells
• 2:25:10
  Speaker 2: us that identical fermions such as electrons cannot occupy exactly
• 2:25:14
  Speaker 2: the same complete quantum state. As atomic electron states overlap
• 2:25:19
  Speaker 2: more strongly, the combined many-electron wave function must obey the
• 2:25:24
  Speaker 2: anti-symmetry required for identical fermions. That restriction changes which electronic
• 2:25:29
  Speaker 2: states are available. Wave functions reorganise. Electron kinetic energy can rise.
• 2:25:35
  Speaker 2: electrostatic energies change at the same time. This is why
• 2:25:39
  Speaker 2: it is too simple to say that the pearly exclusion
• 2:25:42
  Speaker 2: principle alone stops your hand passing through a table. Poorly
• 2:25:46
  Speaker 2: exclusion matters enormously, but ordinary short-range resistance emerges from quantum statistics,
• 2:25:53
  Speaker 2: changing electron-kinetic energy and electromagnetic interactions working together. One of
• 2:25:59
  Speaker 2: the clearest ways to gather all of this physics into
• 2:26:02
  Speaker 2: one picture is the interatomic potential energy curve. Far apart,
• 2:26:07
  Speaker 2: two atoms may interact only weakly. As they approach, attraction
• 2:26:12
  Speaker 2: can lower the energy. At some separation, the system may
• 2:26:15
  Speaker 2: reach an energy minimum. That minimum represents a preferred distance.
• 2:26:20
  Speaker 2: Move the atoms farther apart and attraction may pull them
• 2:26:23
  Speaker 2: back together. Push them closer and the energy begins rising.
• 2:26:28
  Speaker 2: Push farther still and it can rise extremely steeply. That
• 2:26:32
  Speaker 2: steep increase is what gives ordinary matter much of its
• 2:26:35
  Speaker 2: resistance to compression. But overlap does not always mean repulsion.
• 2:26:40
  Speaker 2: Some atoms approach one another and discover in energetic terms
• 2:26:43
  Speaker 2: that the lower energy arrangement is to stay together. Their
• 2:26:47
  Speaker 2: electronic states reorganise into molecular states. Electron density can become
• 2:26:52
  Speaker 2: shared across more than one nucleus. A chemical bond forms.
• 2:26:57
  Speaker 2: So the same basic act of bringing atoms close can
• 2:27:00
  Speaker 2: produce temporary scattering in one case, strong repulsion in another,
• 2:27:05
  Speaker 2: and stable bonding in another. Even atomic collisions fit naturally
• 2:27:09
  Speaker 2: into this picture. Physicists routinely describe atoms as colliding without
• 2:27:14
  Speaker 2: requiring hard surfaces. The incoming particles interact. Their kinetic and
• 2:27:20
  Speaker 2: potential energies change. They may reach a distance of closest approach.
• 2:27:25
  Speaker 2: Momentum can be exchanged. Internal states may change. The atoms
• 2:27:30
  Speaker 2: may then separate again along different directions. Collision is therefore
• 2:27:34
  Speaker 2: defined through interaction and measurable outcome, not through the impact
• 2:27:39
  Speaker 2: of too much shells. At larger scales, these same processes
• 2:27:44
  Speaker 2: become the forces we feel. When your hand rests on
• 2:27:47
  Speaker 2: the table, huge numbers of atoms in the two surfaces
• 2:27:50
  Speaker 2: are displaced slightly from their preferred arrangements. Their electronic states respond.
• 2:27:56
  Speaker 2: The energy rises, The material resists further deformation. That resistance
• 2:28:01
  Speaker 2: appears macroscopically as the normal force supporting your hand. And
• 2:28:06
  Speaker 2: even that contact is not spread evenly across a perfectly
• 2:28:09
  Speaker 2: flat plane. Real surfaces contain microscopic asperities. Tiny regions make
• 2:28:15
  Speaker 2: closer contact than others. Those regions deform. Adhesive forces can appear.
• 2:28:21
  Speaker 2: Atoms can form and break local bonds. If the surfaces
• 2:28:26
  Speaker 2: move … Friction transfers mechanical energy into vibrations, heat, deformation
• 2:28:31
  Speaker 2: and sometimes wear. Macroscopic touch is therefore not one clean boundary.
• 2:28:37
  Speaker 2: It is an evolving network of atomic interactions distributed across
• 2:28:41
  Speaker 2: countless microscopic regions. Modern instruments can measure these effects directly.
• 2:28:47
  Speaker 2: Atomic force microscopy can bring an extremely sharp tip towards
• 2:28:51
  Speaker 2: a surface and measure how the interaction changes with distance.
• 2:28:56
  Speaker 2: At larger separations, attractive forces may dominate. Closer in, the
• 2:29:00
  Speaker 2: interaction changes. At very short separation, strong repulsive behaviour appears.
• 2:29:06
  Speaker 2: There is no universal instant when nature announces that contact
• 2:29:10
  Speaker 2: has begun. The force simply evolves continuously. Only under conditions
• 2:29:15
  Speaker 2: vastly more extreme than ordinary touch does a different regime appear.
• 2:29:20
  Speaker 2: Intense compression can alter electronic structure, destroy familiar atomic behaviour,
• 2:29:26
  Speaker 2: and eventually create conditions where nuclei themselves approach nuclear distances.
• 2:29:31
  Speaker 2: Inside stars, quantum tunnelling helps some light nuclei penetrate the
• 2:29:35
  Speaker 2: Coulomb barrier, far enough for the strong nuclear interaction to
• 2:29:39
  Speaker 2: become important. Then fusion becomes possible. But that is no
• 2:29:44
  Speaker 2: longer the physics of a fingertip on a table. It
• 2:29:47
  Speaker 2: is the physics of stellar interiors. So the answer to
• 2:29:50
  Speaker 2: the original question is both simpler and stranger than it
• 2:29:54
  Speaker 2: first appeared. When two atoms touch, they do not meet
• 2:29:59
  Speaker 2: like tiny marbles. Their electron distributions overlap. Their quantum states
• 2:30:04
  Speaker 2: respond to one another. Electromagnetic interactions change. The allowed electronic
• 2:30:10
  Speaker 2: arrangements reorganise. The total energy may fall into a bonded
• 2:30:14
  Speaker 2: state or rise steeply against further compression. And from those
• 2:30:18
  Speaker 2: microscopic changes comes everything we recognise as contact. Atoms do
• 2:30:23
  Speaker 2: not need hard surfaces in order to touch. At the
• 2:30:27
  Speaker 2: quantum scale, touching is what happens when neighbouring pieces of
• 2:30:31
  Speaker 2: matter begin interacting strongly enough that neither can be understood
• 2:30:35
  Speaker 2: as though the other were not there.