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What Actually Happens When Two Atoms Touch · Night Science Stories

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Speaker 2: a great deal. Now, I'll do my best not to
1:43
Speaker 2: make tonight's subject so interesting that it keeps you up.
1:47
Speaker 2: But if it does, at least you'll drift off knowing
1:50
Speaker 2: something rather strange about the supposedly solid world around you.
1:54
Speaker 2: Because when you rest your hand on a table... The
1:57
Speaker 2: atoms in your skin do not meet the atoms in
1:59
Speaker 2: the wood in quite the way two ordinary objects seem
2:02
Speaker 2: to meet. There are no tiny hard shells neatly pressing
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Speaker 2: 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
2:28
Speaker 2: when two atoms touch? Press the tip of one finger
2:31
Speaker 2: gently against a table. Nothing about the experience seems particularly puzzling.
2:36
Speaker 2: Your finger moves downward, reaches the surface, and stops. Push
2:42
Speaker 2: a little harder, and the table pushes back. The boundary
2:45
Speaker 2: seems perfectly obvious. Your skin occupies one region of space,
2:50
Speaker 2: the table occupies another. and somewhere between them there appears
2:54
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
3:01
Speaker 2: to dissolve. Neither your fingertip nor the table possesses the
3:05
Speaker 2: kind of perfectly defined microscopic edge that our everyday senses suggest.
3:11
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
3:19
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
3:26
Speaker 2: neatly against one another. Instead, something much more interesting happens.
3:31
Speaker 2: The electrons associated with neighbouring atoms begin to interact strongly.
3:36
Speaker 2: Their quantum states are affected by one another. Electrical attractions
3:41
Speaker 2: and repulsions change. The possible arrangements available to the electrons
3:45
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
3:53
Speaker 2: form a chemical bond. That collection of effects is much
3:56
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.
4:06
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
4:16
Speaker 2: make contact. An atom is not built that way. Near
4:20
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
4:39
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
4:54
Speaker 2: a nanometer. So even when two atoms are sitting beside
4:58
Speaker 2: one another in a solid object, their nuclei are nowhere
5:01
Speaker 2: near touching. Imagine enlarging an atom until its overall size
5:05
Speaker 2: became something familiar. If the atom were expanded until it
5:09
Speaker 2: filled a large room, the nucleus would still occupy only
5:12
Speaker 2: a tiny region near the centre. The exact comparison depends
5:16
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
5:27
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.
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Speaker 2: Quantum mechanics describes them using quantum states. with spatial distributions
5:45
Speaker 2: that tell us about the likelihood of detecting an electron
5:48
Speaker 2: in different regions. Those distributions do not usually end at
5:52
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
5:59
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
6:21
Speaker 2: begin influencing one another long before anything resembling two solid
6:25
Speaker 2: surfaces meeting could occur. At relatively large separations, the interactions
6:31
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
6:40
Speaker 2: interacts electrically with the electrons of the other. The electrons
6:43
Speaker 2: repel other electrons because they possess the same negative electric charge.
6:48
Speaker 2: The two positively charged nuclei repel one another as well.
6:52
Speaker 2: At the same time, each nucleus attracts negatively charged electrons.
6:58
Speaker 2: These electrical effects are part of electromagnetism, one of the
7:01
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
7:25
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,
7:34
Speaker 2: and their electronic states overlap more strongly. This quantum restriction
7:38
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
7:53
Speaker 2: system begins rising very rapidly if the atoms are forced
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Speaker 2: substantially closer than their preferred separation. This is why the
8:01
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
8:07
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
8:15
Speaker 2: that statement, but the complete explanation involves both quantum mechanics
8:20
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
9:01
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
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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
9:16
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.
9:35
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.
9:45
Speaker 2: In ordinary solids, neighbouring atomic nuclei are commonly separated by
9:49
Speaker 2: only a few tenths of a nanometre. Their nuclei themselves
9:53
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
10:00
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
10:07
Speaker 2: is completely real. This is an important point, because you
10:11
Speaker 2: may occasionally hear the dramatic claim that nothing ever truly
10:14
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
10:22
Speaker 2: microscopic surfaces to collide, then atoms do not behave that
10:26
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 school books, because it
11:47
Speaker 2: is wonderfully easy to recognise. The difficulty is that nature
11:51
Speaker 2: does not actually build atoms that way. A modern atom
11:55
Speaker 2: begins with a remarkably compact central nucleus. Inside that nucleus
12:00
Speaker 2: are 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.
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Speaker 2: Surrounding the nucleus are negatively charged electrons. Already, however, the
12:14
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.
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Speaker 2: Their behaviour is described by quantum mechanics, and that changes
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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
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Speaker 2: solid little spheres came from experiments carried out in the
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Speaker 2: early 20th century. Ernest Rutherford was working with Hans Geiger
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Speaker 2: and Ernest Marsden on experiments in which fast-moving alpha particles
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Speaker 2: were directed towards extremely thin sheets of gold. Alpha particles
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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
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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.
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Speaker 2: Some were scattered through large angles, and a tiny fraction
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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
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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.
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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
13:52
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
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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
14:29
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
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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,
14:51
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
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Speaker 2: the nucleus along one definite little road. Instead, it produces
15:37
Speaker 2: a wave function. A wave function is a mathematical object
15:42
Speaker 2: that contains information about the quantum state of a system.
15:46
Speaker 2: The wave function itself is not simply a little physical
15:49
Speaker 2: cloud floating around the nucleus. Its meaning became clearer through
15:54
Speaker 2: work by Max Born. who proposed that the wavefunction could
15:57
Speaker 2: be connected with probabilities. More precisely, the squared magnitude of
16:02
Speaker 2: the wavefunction gives a probability density for detecting the particle
16:07
Speaker 2: in different regions. That distinction matters. Quantum mechanics does not
16:12
Speaker 2: generally tell us that an electron secretly occupies one precise
16:16
Speaker 2: classical location while we simply fail to know where it is. Instead,
16:22
Speaker 2: the electron is described by a quantum state whose spatial
16:25
Speaker 2: properties 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 wavefunction. 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 state extends through space. The
18:24
Speaker 2: wave function carries information about where measurements may find the
18:28
Speaker 2: electron and how it can interact with other particles. Those
18:32
Speaker 2: extended quantum states are exactly what begin to matter when
18:36
Speaker 2: another atom approaches. Two atoms therefore do not behave like
18:40
Speaker 2: two pebbles rolling towards one another. Their nuclei may remain
18:44
Speaker 2: far apart compared with nuclear dimensions. While the quantum states
18:49
Speaker 2: of their electrons already overlap significantly and alter one another,
18:53
Speaker 2: electrical interactions change, allowed electronic arrangements change, energies shift. That
19:00
Speaker 2: is why the structure of an atom has to be
19:02
Speaker 2: understood before the idea of atoms touching can make sense.
19:06
Speaker 2: If an atom had a hard shell, contact would simply
19:08
Speaker 2: mean the moment one shell met another. But an atom
19:12
Speaker 2: has a tiny nucleus, surrounded by electrons described by extended
19:16
Speaker 2: quantum states, with no universal hard edge. So atomic contact
19:21
Speaker 2: cannot be defined by looking for a microscopic surface. It
19:25
Speaker 2: has to be understood by asking when the quantum states
19:29
Speaker 2: and interactions of neighbouring atoms begin to affect one another
19:33
Speaker 2: strongly enough that treating them as completely separate objects no
19:36
Speaker 2: longer works. If atoms do not have hard outer shells,
19:41
Speaker 2: then a surprisingly awkward question appears almost immediately. Where exactly
19:47
Speaker 2: does an atom end? For an ordinary object such as
19:51
Speaker 2: a marble, the answer seems obvious. There is a surface.
19:55
Speaker 2: Inside that surface there is glass, and outside it there
19:59
Speaker 2: is air. The boundary may look slightly rough under a microscope,
20:04
Speaker 2: but in principle we can still point to a region
20:06
Speaker 2: where the material stops. An isolated atom does not offer
20:10
Speaker 2: us such a convenient edge. The electrons associated with an
20:14
Speaker 2: atom are described by quantum states and those states possess
20:18
Speaker 2: spatial distributions. For many atomic orbitals the probability density is
20:23
Speaker 2: greatest within certain characteristic regions around the nucleus and then
20:27
Speaker 2: becomes smaller as the distance increases. But smaller is not
20:31
Speaker 2: the same as zero. In many cases the mathematical distribution
20:36
Speaker 2: fades gradually outward There is no universal distance at which
20:40
Speaker 2: nature suddenly draws a line and declares that the atom
20:43
Speaker 2: has finished. This creates an interesting problem whenever someone asks
20:47
Speaker 2: how large an atom is. Scientists routinely quote atomic sizes,
20:52
Speaker 2: and those numbers are perfectly useful. But they are not
20:56
Speaker 2: measurements of a tiny solid shell. Instead, an atomic radius
21:01
Speaker 2: is usually an effective size defined according to a particular
21:04
Speaker 2: physical situation. and there is more than one way to
21:07
Speaker 2: define it. Consider two atoms joined together by a covalent bond.
21:13
Speaker 2: A covalent bond involves electrons occupying molecular quantum states associated
21:18
Speaker 2: with more than one nucleus. If the distance between the
21:22
Speaker 2: two nuclei can be measured, one useful approximation is to
21:26
Speaker 2: divide that separation between the atoms. This leads to the
21:30
Speaker 2: idea of a covalent radius. For two identical bonded atoms,
21:35
Speaker 2: The covalent radius can be thought of approximately as half
21:38
Speaker 2: the distance between their nuclei. For different atoms, chemists use
21:43
Speaker 2: collections of measured bond lengths to estimate characteristic covalent radii
21:48
Speaker 2: for different elements. These values tell us something meaningful about
21:51
Speaker 2: atomic size in molecules. But place the same element in
21:55
Speaker 2: a metal and another definition becomes more useful. In a
22:00
Speaker 2: metallic crystal, atoms are arranged in repeating structures while some
22:04
Speaker 2: of their electrons occupy states extending through the material rather
22:09
Speaker 2: than belonging neatly to one isolated atomic pair. The metallic
22:14
Speaker 2: radius is commonly related to the distance between neighbouring atomic
22:18
Speaker 2: nuclei within that structure. Again, the radius is real in
22:22
Speaker 2: the sense that it describes measurable spacing. It is simply
22:26
Speaker 2: not the radius of a microscopic hard sphere. There is
22:30
Speaker 2: another important measure known as the van der Waals radius.
22:34
Speaker 2: The name comes from Johannes Diederik van der Waals, whose
22:37
Speaker 2: work on gases and intermolecular interactions helped reveal that molecules
22:42
Speaker 2: cannot always be treated as perfectly non-interacting points. The van
22:47
Speaker 2: der Waals radius is particularly useful when atoms are close
22:50
Speaker 2: to one another, but are not connected by an ordinary
22:53
Speaker 2: covalent bond. Two neutral atoms can still attract one another
22:58
Speaker 2: through weak intermolecular effects, while strongly resisting further approach once
23:03
Speaker 2: they become sufficiently close. Measurements of distances between non-bonded atoms
23:08
Speaker 2: in molecules and crystals allow scientists to assign characteristic van
23:13
Speaker 2: der Waals radii. These are often larger than covalent radii
23:17
Speaker 2: for the same element. That is not because the atom
23:21
Speaker 2: has somehow swollen. The difference comes from what we are
23:24
Speaker 2: choosing to measure. A carbon atom participating in a covalent
23:29
Speaker 2: bond may have one useful effective radius. A carbon atom
23:34
Speaker 2: approaching another atom without forming such a bond may be
23:37
Speaker 2: described by a different one. The atom has not changed
23:40
Speaker 2: into two different sized balls. Instead, different definitions capture different
23:46
Speaker 2: aspects of how its electron distribution interacts with its surroundings.
23:50
Speaker 2: This is one reason tables of atomic radii do not
23:53
Speaker 2: always contain exactly the same values. The radius depends on
23:57
Speaker 2: the definition, the bonding environment, the electronic state, and sometimes
24:02
Speaker 2: the particular structure being studied. At first this may sound inconvenient.
24:07
Speaker 2: It would certainly be tidier if every element arrived with
24:10
Speaker 2: one permanent radius, perhaps stamped somewhere on the side. Nature
24:15
Speaker 2: has declined to provide that service, but the different definitions
24:19
Speaker 2: are not arbitrary guesses. They are grounded in measurements. Techniques
24:24
Speaker 2: such as diffraction can reveal the arrangements of atoms within
24:27
Speaker 2: crystals and molecules with remarkable precision. From those measurements, researchers
24:33
Speaker 2: can determine distances between atomic nuclei. Those nuclear separations then
24:38
Speaker 2: provide operational ways of discussing atomic dimensions. So even though
24:43
Speaker 2: an atom has no knife edge boundary, its characteristic size
24:46
Speaker 2: is not vague or unknowable. We can measure how closely
24:49
Speaker 2: atoms sit in solids, We can measure bond lengths in molecules.
24:54
Speaker 2: We can determine the separations at which non-bonded atoms tend
24:57
Speaker 2: to remain. We can study the energies required to push
25:01
Speaker 2: atoms closer together. All of these tell us something about size.
25:05
Speaker 2: This also explains why textbook illustrations nearly always draw atoms
25:10
Speaker 2: as circles or spheres. The drawings are useful. A sphere
25:14
Speaker 2: gives us an immediate visual sense that an atom occupies
25:17
Speaker 2: a characteristic region around its nucleus. In molecular models, different
25:22
Speaker 2: size spheres can communicate relative atomic dimensions very efficiently. But
25:27
Speaker 2: the line around the sphere should not be mistaken for
25:30
Speaker 2: a physical shell. No tiny camera could zoom in and
25:34
Speaker 2: photograph that line. The illustration is a map of characteristic extent,
25:39
Speaker 2: not a portrait of a literal surface. Once that distinction
25:42
Speaker 2: is clear, the meaning of atomic contact becomes much easier
25:46
Speaker 2: to handle. Imagine two atoms moving slowly towards one another.
25:51
Speaker 2: If each possessed a hard radius there would be one
25:53
Speaker 2: exact separation at which the two surfaces first touched. One
25:58
Speaker 2: moment they would be apart, the next moment they would meet.
26:02
Speaker 2: Real atoms interact more gradually. Even while their nuclei are
26:06
Speaker 2: relatively far apart, their electric fields and quantum states can
26:10
Speaker 2: influence one another weakly. As the separation decreases, those interactions
26:16
Speaker 2: generally become stronger. Electron distributions may distort. Attractive effects may
26:22
Speaker 2: become important. Eventually, the overlap between electronic quantum states can
26:26
Speaker 2: no longer be ignored. If the atoms are brought still closer,
26:30
Speaker 2: the energetic cost of further compression can rise very steeply.
26:35
Speaker 2: At what exact instant did touching begin? There is no
26:38
Speaker 2: universal answer. In one experiment, contact might be defined by
26:42
Speaker 2: the distance at which a measurable force becomes significant. In another,
26:47
Speaker 2: it might mean the equilibrium distance between neighbouring atoms. In
26:51
Speaker 2: atomic collision physics, researchers may describe atoms as colliding according
26:55
Speaker 2: to their scattering behaviour and the interaction that changes their motion.
27:00
Speaker 2: In quantum chemistry, attention may instead fall on the overlap
27:04
Speaker 2: of electron density or the formation of a chemical bond.
27:08
Speaker 2: Each definition is answering a slightly different physical question. This
27:12
Speaker 2: is not a weakness in the science. It is a
27:15
Speaker 2: consequence of the fact that the everyday word touch is
27:18
Speaker 2: trying to describe a quantum system that does not possess
27:22
Speaker 2: everyday boundaries. Two atoms can begin affecting one another without
27:26
Speaker 2: crossing a rigid line. Their interaction strengthens continuously as their
27:30
Speaker 2: separation changes. Their electron distributions respond to one another, their
27:36
Speaker 2: energies shift, and the quantum state of the combined system
27:39
Speaker 2: can become increasingly different from the states of two isolated atoms.
27:44
Speaker 2: So asking whether two atoms are touching is often less
27:48
Speaker 2: useful than asking what their separation is. how strongly they
27:51
Speaker 2: are interacting, what their total energy is, and what their
27:54
Speaker 2: electrons are doing. At the atomic scale, those quantities provide
27:59
Speaker 2: the meaningful boundaries, not a thin outer skin, not two
28:03
Speaker 2: invisible shells meeting, but forces, energies, electron density and quantum
28:09
Speaker 2: states changing as matter moves closer together. Imagine two atoms
28:14
Speaker 2: separated by a comfortable distance, drifting slowly towards one another.
28:19
Speaker 2: At first, depending on the atoms and their surroundings, the
28:23
Speaker 2: interaction between them may be extraordinarily weak. There is no
28:27
Speaker 2: sudden alarm that sounds when they cross some invisible border.
28:31
Speaker 2: No switch changes from not touching to touching. Instead, the
28:35
Speaker 2: influence of one atom on the other changes continuously as
28:39
Speaker 2: the distance between their nuclei becomes smaller. Much of what
28:43
Speaker 2: happens begins with the electric charge. During the 18th century,
28:47
Speaker 2: the French physicist Charles-Augustin de Coulomb investigated the forces between
28:53
Speaker 2: electrically charged objects. The relationship now called Coulomb's Law describes
28:58
Speaker 2: how the electrical force between two point charges depends on
29:02
Speaker 2: their charges and the distance separating them. Charges with the
29:06
Speaker 2: same sign repel one another while charges with opposite signs attract.
29:11
Speaker 2: Atoms contain both. A typical neutral atom has a positively
29:15
Speaker 2: charged nucleus surrounded by negatively charged electrons. The positive and
29:20
Speaker 2: negative charges balance overall, so from sufficiently far away the
29:24
Speaker 2: atom can appear electrically neutral. But electrically neutral does not
29:29
Speaker 2: mean electrically featureless. As two atoms approach one another, several
29:34
Speaker 2: electrical interactions are occurring at the same time. The positively
29:38
Speaker 2: charged nucleus of one atom repels the positively charged nucleus
29:42
Speaker 2: of the other. The negatively charged electrons associated with one
29:47
Speaker 2: atom repel the electrons associated with the other. Meanwhile, the
29:51
Speaker 2: nucleus of each atom attracts the electrons of its neighbour
29:54
Speaker 2: So even a meeting between two perfectly neutral atoms contains
29:58
Speaker 2: a complicated collection of attractions and repulsions. If the atoms
30:03
Speaker 2: are far apart, much of this can nearly cancel. At
30:06
Speaker 2: long enough distances, the detailed positive and negative structure of
30:10
Speaker 2: a neutral atom becomes difficult for another atom to distinguish.
30:14
Speaker 2: Move them closer, however, and the internal distributions of charge
30:18
Speaker 2: begin to matter. One important effect is electric polarisation. An
30:23
Speaker 2: atom's electron distribution is not a rigid shell bolted into
30:26
Speaker 2: position around its nucleus. An external electric influence can distort
30:31
Speaker 2: that distribution slightly. The negatively charged electron density can shift
30:36
Speaker 2: relative to the positively charged nucleus, giving the atom an
30:40
Speaker 2: uneven distribution of charge. One side can become slightly more
30:44
Speaker 2: negative while the other becomes slightly more positive. This does
30:48
Speaker 2: not necessarily give the whole atom a net charge. It
30:52
Speaker 2: creates what physicists call an electric dipole, or in many
30:56
Speaker 2: cases an induced dipole. A nearby atom can therefore alter
31:00
Speaker 2: the charge distribution of its neighbour. That altered distribution can
31:04
Speaker 2: then affect the first atom in return. Already, two apparently
31:09
Speaker 2: neutral objects are engaged in quite a subtle conversation. But
31:13
Speaker 2: even atoms with no permanent electric dipole can attract one another.
31:18
Speaker 2: This is where quantum mechanics becomes essential. During the 1930s,
31:23
Speaker 2: the physicist Fritz London developed the quantum mechanical explanation for
31:27
Speaker 2: an attractive interaction now known as the London dispersion force.
31:32
Speaker 2: Electrons in an atom are not frozen into a perfectly
31:35
Speaker 2: static arrangement. Their quantum state allows fluctuations in the distribution
31:40
Speaker 2: of electronic charge. At any particular moment, the electron distribution
31:44
Speaker 2: can be uneven in a way that produces a temporary
31:47
Speaker 2: electric dipole. The neighbouring atom responds to that changing distribution.
31:52
Speaker 2: Its electrons become correlated with those of the first atom,
31:56
Speaker 2: producing arrangements in which the two atoms experience a small
31:59
Speaker 2: net attraction. These London dispersion forces occur even between atoms
32:04
Speaker 2: that are neutral and have no permanent dipole. Helium atoms,
32:08
Speaker 2: for example, can attract one another through dispersion interactions despite
32:13
Speaker 2: being about as reluctant to form ordinary chemical bonds as
32:16
Speaker 2: atoms can reasonably be. The attraction is weak, but weak
32:20
Speaker 2: does not mean irrelevant. Across large numbers of atoms and molecules,
32:25
Speaker 2: dispersion forces help influence the properties of liquids, molecular solids,
32:31
Speaker 2: biological structures and surfaces. They form part of the wider
32:35
Speaker 2: family of interactions commonly described as Van der Waals forces.
32:40
Speaker 2: So as two neutral atoms approach from relatively large separation,
32:44
Speaker 2: weak attractive effects may begin lowering the energy of the pair.
32:48
Speaker 2: bring them closer, and the attraction usually becomes stronger for
32:51
Speaker 2: a while. This can make it tempting to imagine that
32:54
Speaker 2: the atoms should simply continue falling towards one another until
32:58
Speaker 2: their nuclei meet. They do not. At shorter distances, a
33:03
Speaker 2: very different regime appears. The electron distributions begin overlapping significantly.
33:09
Speaker 2: Electron-electron repulsion becomes increasingly important. The positively charged nuclei repel
33:16
Speaker 2: one another more strongly as their separation falls. The available
33:20
Speaker 2: quantum states of the electrons also have to change. At
33:23
Speaker 2: this point, simply adding together a few classical Coulomb forces
33:27
Speaker 2: no longer gives a complete description. The electrons are quantum particles.
33:32
Speaker 2: When the wave functions associated with neighbouring atoms overlap, the
33:36
Speaker 2: electronic system has to be treated as a combined quantum system.
33:41
Speaker 2: The electrons cannot always be assigned permanently to atom 1
33:44
Speaker 2: or atom 2, as though each were carrying a private
33:47
Speaker 2: collection of tiny orbiting beads. Their allowed quantum states depend
33:52
Speaker 2: on the arrangement of both nuclei and on the presence
33:55
Speaker 2: of the other electrons. And because electrons are identical fermions,
33:59
Speaker 2: the Pauli exclusion principle places fundamental restrictions on the states
34:04
Speaker 2: available to them. As the atoms are pushed into increasingly
34:07
Speaker 2: overlapping configurations, The electronic wave functions must rearrange. That rearrangement
34:13
Speaker 2: can increase electron kinetic energy, alter electrostatic energies and force
34:19
Speaker 2: electrons into different allowed states. The energetic cost can rise
34:23
Speaker 2: very rapidly. This produces the strong short-range resistance that prevents
34:28
Speaker 2: ordinary atoms from simply collapsing through one another. It is
34:32
Speaker 2: worth being careful with the word repulsion here. There is
34:36
Speaker 2: no single mysterious repulsive force that suddenly activates at a
34:39
Speaker 2: fixed atomic radius. Instead, the total energy of the system
34:44
Speaker 2: reflects several effects at once. There is electrostatic repulsion between
34:49
Speaker 2: like charges. There is electrostatic attraction between opposite charges. There
34:54
Speaker 2: is the quantum behaviour of the electrons. There is the
34:57
Speaker 2: Pauli exclusion principle. There are changes in kinetic energy. Depending
35:01
Speaker 2: on the atoms, there may also be the possibility of
35:04
Speaker 2: forming chemical bonds. The balance changes continuously with separation. At
35:09
Speaker 2: one distance, weaker traction may dominate. At another, the atoms
35:14
Speaker 2: may reach a stable separation, where moving noticeably closer or
35:18
Speaker 2: farther apart would raise the energy. Push them closer still,
35:22
Speaker 2: and the energy may climb extremely steeply. This smooth transition
35:27
Speaker 2: is a much better picture of atomic contact than two
35:30
Speaker 2: hard shells striking one another. There need not be a
35:33
Speaker 2: precise instant when two surfaces touch because there are no
35:36
Speaker 2: such surfaces to begin with. Instead, there is a changing
35:40
Speaker 2: relationship between two quantum systems. From far away, they can
35:45
Speaker 2: behave almost independently. As they approach, subtle attractions appear. Their
35:51
Speaker 2: electronic distributions begin responding to one another. Their quantum states
35:55
Speaker 2: increasingly overlap and reorganise, eventually attempting to reduce the distance
36:00
Speaker 2: further becomes energetically expensive. So if we could somehow follow
36:05
Speaker 2: two atoms on their slow journey towards one another, we
36:08
Speaker 2: would not see a collision with an invisible wall. We
36:11
Speaker 2: would see interaction gradually becoming impossible to ignore. The atoms
36:15
Speaker 2: do not suddenly discover that they have touched. They simply
36:19
Speaker 2: reach a point where each can no longer be understood
36:21
Speaker 2: without taking the other seriously. When physicists say that the
36:25
Speaker 2: electron clouds of two atoms begin to overlap, The phrase
36:29
Speaker 2: can create an image that is far more physical than
36:32
Speaker 2: the underlying reality. It is easy to imagine two faint
36:36
Speaker 2: fuzzy balls drifting into one another like coloured smoke. But
36:40
Speaker 2: an electron cloud is not a mist of material surrounding
36:43
Speaker 2: the nucleus. The phrase is shorthand for the spatial behaviour
36:47
Speaker 2: of electrons described by quantum mechanics. For an electron in
36:51
Speaker 2: an atom, that behaviour is represented by a wave function.
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
37:44
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
38:13
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 seem 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
41:38
Speaker 2: arrangements can increase their kinetic energy. So when atoms are
41:43
Speaker 2: pushed 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
44:18
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 organisation is deeply
46:32
Speaker 2: connected with the structure of the periodic table. The recurring
46:37
Speaker 2: chemical behaviour 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 neighbouring 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
47:03
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 wavefunction changes sign. That may sound like a
47:40
Speaker 2: small technical detail, but it has enormous physical consequences. One
47:45
Speaker 2: immediate result is that if two identical fermions were placed
47:49
Speaker 2: in exactly the same complete quantum state, the anti-symmetric wavefunction
47:54
Speaker 2: would cancel to zero. In other words, that arrangement is
47:58
Speaker 2: not an allowed physical state. So when atoms are pushed
48:01
Speaker 2: closer together, electrons cannot simply continue occupying their original arrangements
48:07
Speaker 2: as though the neighbouring electrons were not there. The available
48:10
Speaker 2: quantum states of the combined system change. Some electrons must
48:15
Speaker 2: occupy different spatial arrangements. Their wave functions can develop additional structure.
48:20
Speaker 2: Their characteristic momenta can change. And all of this alters
48:24
Speaker 2: the total energy. This is sometimes described informally as Pauli repulsion.
48:30
Speaker 2: The phrase is useful, but it can also be misleading.
48:33
Speaker 2: Pauli repulsion is not a separate fundamental force in the
48:37
Speaker 2: way electromagnetism is. There is no Pauli force particle flying
48:41
Speaker 2: back and forth between two atoms. Instead, the apparent repulsive
48:45
Speaker 2: effect emerges from the requirement that the quantum state of
48:49
Speaker 2: identical fermions obey anti-symmetry. and from the energetic consequences of
48:54
Speaker 2: rearranging electrons into allowed states. As two closed-shell atoms are
48:59
Speaker 2: pushed closer together, maintaining the required fermionic quantum structure can
49:04
Speaker 2: force electron wave functions into configurations with greater spatial variation.
49:09
Speaker 2: Greater spatial variation is generally associated with higher electron kinetic energy.
49:15
Speaker 2: The total energy of the system therefore rises. This rising
49:19
Speaker 2: energy produced an effective resistance to further compression, but Pauli
49:23
Speaker 2: exclusion is not acting alone. That distinction is important. The
49:29
Speaker 2: electrons also repel one another through their negative electric charges.
49:33
Speaker 2: The positively charged nuclei repel each other. At the same time,
49:38
Speaker 2: electrons are attracted to both nuclei. All of these electrostatic
49:42
Speaker 2: interactions change as the atoms move closer. The final short-range
49:48
Speaker 2: behaviour comes from the full quantum mechanical balance of these effects,
49:52
Speaker 2: together with the restrictions imposed by fermionic statistics. So the
49:57
Speaker 2: familiar statement that your hand cannot pass through a table
50:01
Speaker 2: because of the Pauli exclusion principle is only partly correct.
50:05
Speaker 2: Poorly exclusion is crucial. Without it, ordinary matter would have
50:09
Speaker 2: dramatically different properties. But the resistance between the atoms in
50:13
Speaker 2: your hand and the atoms in the table is not
50:16
Speaker 2: produced by exclusion alone. Electromagnetism is essential as well. The
50:22
Speaker 2: electric interactions among electrons and nuclei help determine how the
50:26
Speaker 2: energy changes as atomic structures are compressed. Changes in electron
50:31
Speaker 2: kinetic energy also contribute substantially. The solidity we experience is
50:36
Speaker 2: therefore not the result of one microscopic force standing guard
50:40
Speaker 2: at the boundary of every atom. It is an emergent
50:43
Speaker 2: consequence of quantum mechanics and electromagnetism operating together. The same
50:50
Speaker 2: principle that helps stop your hand passing through a tabletop
50:53
Speaker 2: also helps explain why atoms have the sizes they do.
50:57
Speaker 2: Because electrons cannot all occupy the same lowest state, Successive
51:02
Speaker 2: electrons fill different quantum states around the nucleus. Those states
51:06
Speaker 2: determine the spatial extent and chemical character of atoms, place
51:11
Speaker 2: many atoms together, and the restrictions continue to matter. In solids,
51:17
Speaker 2: enormous numbers of electrons collectively occupy allowed states across the material.
51:23
Speaker 2: Their quantum organisation influences electrical conductivity, mechanical stiffness, chemical stability,
51:30
Speaker 2: and the structure of matter itself. The exclusion principle even
51:35
Speaker 2: becomes important in environments far removed from an ordinary table.
51:40
Speaker 2: In white dwarf stars, for example, matter is compressed so
51:44
Speaker 2: strongly that electron degeneracy pressure becomes essential. This pressure is
51:50
Speaker 2: another consequence of fermionic quantum statistics. Electrons are forced into
51:55
Speaker 2: progressively higher momentum states because the lower states are already occupied,
52:01
Speaker 2: That is an extreme astronomical version of the same underlying rule.
52:07
Speaker 2: On a tabletop, of course, nothing nearly so dramatic is occurring.
52:11
Speaker 2: The atoms need only be displaced slightly from their preferred
52:14
Speaker 2: separations before their total energy begins rising strongly. Push harder
52:19
Speaker 2: and the electronic arrangements become increasingly unfavourable. The energy rises
52:24
Speaker 2: steeply so the material pushes back. That steep rise is
52:28
Speaker 2: what matters for ordinary contact. It gives matter an effective
52:32
Speaker 2: resistance to compression long before atomic nuclei could ever come
52:36
Speaker 2: close to one another. So when two atoms approach, the
52:40
Speaker 2: Pauli exclusion principle does not place an invisible wall between them. Instead,
52:45
Speaker 2: it restricts the quantum arrangements their electrons are allowed to occupy.
52:50
Speaker 2: Those restrictions reshape the wave functions, alter the electron kinetic energy,
52:56
Speaker 2: and work alongside electromagnetic interactions to make further compression increasingly costly.
53:01
Speaker 2: A table feels solid, not because its atoms are tiny
53:06
Speaker 2: hard balls. It feels solid because the quantum states of
53:10
Speaker 2: matter have rules. And those rules leave remarkably little freedom
53:15
Speaker 2: for enormous numbers of electrons to be pushed carelessly into
53:18
Speaker 2: the same place. Once two atoms are close enough for
53:22
Speaker 2: their electron states to overlap strongly, The question of what
53:26
Speaker 2: happens next becomes, in large part, a question of energy.
53:31
Speaker 2: Two non-bonding atoms do not behave like hard spheres that
53:35
Speaker 2: simply hit one another and bounce away. Instead, as their
53:39
Speaker 2: nuclei are pushed closer than the separation they naturally prefer,
53:42
Speaker 2: the total quantum mechanical energy of the combined system begins
53:47
Speaker 2: to rise. That rising energy is what makes further compression difficult.
53:53
Speaker 2: Part of the reason comes from the way electron wave
53:55
Speaker 2: functions must change. Electrons are described by quantum states that
54:00
Speaker 2: extend through space. If those states are forced into more
54:03
Speaker 2: restricted spatial arrangements, the wave functions generally become more sharply structured.
54:09
Speaker 2: This has consequences for momentum. Werner Heisenberg formulated the uncertainty
54:15
Speaker 2: principle during the early development of quantum mechanics. In its
54:19
Speaker 2: familiar form, the principle tells us that position and momentum
54:23
Speaker 2: cannot both be specified with unlimited precision. The more tightly
54:27
Speaker 2: a particle's position is confined, the broader the range of
54:31
Speaker 2: momentum values associated with its quantum state must become. This
54:36
Speaker 2: does not mean that an electron literally bounces around faster,
54:40
Speaker 2: because it has realised it is trapped. It means that
54:43
Speaker 2: stronger spatial confinement changes the form of the quantum state itself.
54:48
Speaker 2: A more confined wave function generally involves a wider spread
54:52
Speaker 2: of possible momenta, and this often raises the characteristic kinetic
54:56
Speaker 2: energy of the electron. That increase in kinetic energy becomes
55:00
Speaker 2: important when atoms are compressed. Suppose two closed-shell atoms are
55:05
Speaker 2: pushed 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
59:03
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
1:00:24
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
1:02:39
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 distance,
1:04:49
Speaker 2: then a force is associated with that change. Very loosely,
1:04:53
Speaker 2: the steeper the energy curve, the stronger the tendency for
1:04:56
Speaker 2: the system to move towards lower energy. On the attractive
1:05:00
Speaker 2: 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 tend
1:05:11
Speaker 2: to move apart. This allows attraction, equilibrium and repulsion to
1:05:15
Speaker 2: be described within one continuous picture. There is no need
1:05:19
Speaker 2: for a separate rule suddenly announcing that the atoms have
1:05:22
Speaker 2: begun touching. The entire transition appears naturally in the shape
1:05:27
Speaker 2: of the energy landscape. A particularly famous simplified example is
1:05:31
Speaker 2: the Leonard Jones potential, associated with the British physicist John
1:05:36
Speaker 2: Leonard Jones. It is often used to describe interactions between
1:05:40
Speaker 2: neutral atoms or molecules when they are not forming strong
1:05:44
Speaker 2: chemical bonds. The model combines a weak attractive region at
1:05:48
Speaker 2: somewhat larger separations with a very steep repulsive region at
1:05:52
Speaker 2: 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 two 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 and 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 for
1:14:41
Speaker 2: elastic scattering and another for ionisation. Both describe the same atom,
1:14:46
Speaker 2: so the collision cross-section is another example of atomic size
1:14:49
Speaker 2: being defined through physical behaviour rather than through a rigid
1:14:53
Speaker 2: outer boundary. Collisions themselves can also have very different outcomes.
1:14:58
Speaker 2: In an elastic collision, the total kinetic energy associated with
1:15:02
Speaker 2: the relative motion is conserved, and the internal states of
1:15:05
Speaker 2: the colliding particles remain unchanged. The atoms approach, interact, and
1:15:12
Speaker 2: separate again without being internally excited. Their directions and individual
1:15:17
Speaker 2: speeds may change, but there is no lasting transfer of
1:15:20
Speaker 2: energy into their internal structure. In an inelastic collision, some
1:15:26
Speaker 2: kinetic energy can instead be transferred into internal degrees of freedom.
1:15:31
Speaker 2: An atom may be left in an electronically excited state
1:15:35
Speaker 2: with one of its electrons occupying a higher energy configuration.
1:15:40
Speaker 2: If molecules are involved, energy can be transferred into molecular
1:15:44
Speaker 2: rotation or vibration. At sufficiently high collision energies, an electron
1:15:49
Speaker 2: can even be removed completely from an atom or molecule
1:15:53
Speaker 2: producing ionisation. The collision energy therefore matters enormously. At low
1:15:59
Speaker 2: relative energy, two atoms may only explore the outer, relatively
1:16:03
Speaker 2: gentle region of their interaction potential. They approach, feel one
1:16:08
Speaker 2: another's influence and separate, without reaching the strongly repulsive region
1:16:13
Speaker 2: very deeply. Increase their relative kinetic energy and they can
1:16:17
Speaker 2: reach smaller nuclear separations before the rising potential energy halts
1:16:21
Speaker 2: their inward motions. In this sense, faster collisions can probe
1:16:25
Speaker 2: deeper into the interaction. The distance of closest approach becomes smaller,
1:16:30
Speaker 2: and the collision begins sampling regions where electron overlap. Electrostatic
1:16:35
Speaker 2: interactions and quantum state rearrangements become increasingly intense. At high
1:16:41
Speaker 2: enough energies, entirely new processes may become possible. Electrons can
1:16:46
Speaker 2: be excited or removed. Chemical bonds can break. In sufficiently
1:16:51
Speaker 2: energetic environments, The familiar electronic structure of the atom can
1:16:56
Speaker 2: 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
1:19:29
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.
1:20:14
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,
1:27:28
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 abound 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
1:29:36
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
1:30:02
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.
1:30:46
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
1:33:22
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
1:33:30
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,
1:37:04
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
1:37:48
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
1:39:25
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 where the chemical
1:41:25
Speaker 2: bonds form when two materials are pressed together. So when
1:41:29
Speaker 2: you place two pieces of metal against one another, the
1:41:32
Speaker 2: atoms in one metal are often not interacting directly with
1:41:36
Speaker 2: bare atoms in the other. There may be oxide layers
1:41:39
Speaker 2: and adsorbed molecules between them. Remove those layers and surface
1:41:44
Speaker 2: behaviour 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
1:44:21
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 Touching. Seen closely enough, the same event becomes a
1:44:55
Speaker 2: landscape of asperities, deformation, adhesion, contamination and atomic interaction. The
1:45:02
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,
1:45:28
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
1:46:59
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
1:50:24
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
1:55:18
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.
1:56:01
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
1:57:14
Speaker 2: the machinery can become wonderfully elaborate, but the central principle
1:57:18
Speaker 2: remains simple. Bring a tiny probe near a surface and
1:57:22
Speaker 2: measure how the interaction changes. That makes atomic force microscopy
1:57:27
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.
1:58:18
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
2:01:56
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
2:03:02
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
2:05:01
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
2:07:53
Speaker 2: fluid … Electrically insulating or conducting depends on both pressure
2:07:59
Speaker 2: and temperature. 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 ionised. 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
2:15:34
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
2:16:02
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 wave functions. Max Born connected those
2:24:02
Speaker 2: wave functions with probability, showing that their mathematical structure tells
2:24:06
Speaker 2: us about the likelihood of detecting electrons in different regions.
2:24:10
Speaker 2: So an electron is not simply a tiny bead following
2:24:13
Speaker 2: a hidden orbit around the nucleus. Its quantum state can
2:24:16
Speaker 2: extend across a region. When another atom approaches, those extended
2:24:21
Speaker 2: states begin affecting one another. Electromagnetism then supplies much of
2:24:26
Speaker 2: the underlying interaction. Charles-Augustin de Coulombe's work describes the basic
2:24:32
Speaker 2: electrical attraction between opposite charges and repulsion between like charges.
2:24:38
Speaker 2: Two approaching atoms contain several of these effects at once.
2:24:42
Speaker 2: Nuclei repel nuclei. Electrons repel electrons. Electrons are attracted towards
2:24:48
Speaker 2: positively charged nuclei. Neutral atoms can also polarise one another
2:24:53
Speaker 2: and experience weaker attractions through correlated fluctuations in their electron distributions.
2:24:59
Speaker 2: The final behaviour comes from the balance of all these contributions.
2:25:04
Speaker 2: Quantum mechanics adds another crucial restriction. Wolfgang Pauli's exclusion principle
2:25:10
Speaker 2: tells us that identical fermions such as electrons cannot occupy
2:25:14
Speaker 2: exactly the same complete quantum state. As atomic electron states
2:25:19
Speaker 2: overlap more strongly, the combined many-electron wave function must obey
2:25:24
Speaker 2: the anti-symmetry required for identical fermions. That restriction changes which
2:25:29
Speaker 2: electronic states are available. Wave functions reorganise. Electron kinetic energy
2:25:34
Speaker 2: can rise. electrostatic energies change at the same time. This
2:25:39
Speaker 2: is why it is too simple to say that the
2:25:41
Speaker 2: Perle exclusion principle alone stops your hand passing through a table.
2:25:45
Speaker 2: Perle exclusion matters enormously, but ordinary short-range resistance emerges from
2:25:51
Speaker 2: quantum statistics, changing electron-kinetic energy and electromagnetic interactions working together.
2:25:59
Speaker 2: One of the clearest ways to gather all of this
2:26:01
Speaker 2: physics into one picture is the interatomic potential energy curve.
2:26:06
Speaker 2: Far apart, two atoms may interact only weakly. As they approach,
2:26:11
Speaker 2: attraction can lower the energy. At some separation, the system
2:26:15
Speaker 2: may 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 move,
2:28:26
Speaker 2: Friction transfers mechanical energy into vibrations, heat, deformation and sometimes wear.
2:28:33
Speaker 2: Macroscopic touch is therefore not one clean boundary. It is
2:28:37
Speaker 2: an evolving network of atomic interactions distributed across countless microscopic regions.
2:28:44
Speaker 2: Modern instruments can measure these effects directly. Atomic force microscopy
2:28:49
Speaker 2: can bring an extremely sharp tip towards a surface and
2:28:52
Speaker 2: measure how the interaction changes with distance. At larger separations,
2:28:57
Speaker 2: attractive forces may dominate. Closer in, the interaction changes. At
2:29:02
Speaker 2: very short separation, strong repulsive behaviour appears. There is no
2:29:07
Speaker 2: universal instant when nature announces that contact has begun. The
2:29:11
Speaker 2: force simply evolves continuously. Only under conditions vastly more extreme
2:29:16
Speaker 2: than ordinary touch does a different regime appear. Intense compression
2:29:21
Speaker 2: can alter electronic structure, destroy familiar atomic behaviour, and eventually
2:29:27
Speaker 2: create conditions where nuclei themselves approach nuclear distances. Inside stars,
2:29:32
Speaker 2: quantum tunnelling helps some light nuclei penetrate the Coulomb barrier,
2:29:37
Speaker 2: far enough for the strong nuclear interaction to become important.
2:29:41
Speaker 2: Then fusion becomes possible. But that is no longer the
2:29:44
Speaker 2: physics of a fingertip on a table. It is the
2:29:47
Speaker 2: physics of stellar interiors. So the answer to the original
2:29:51
Speaker 2: question is both simpler and stranger than it first appeared.
2:29:57
Speaker 2: When two atoms touch, they do not meet like tiny marbles.
2:30:01
Speaker 2: Their electron distributions overlap. Their quantum states respond to one another.
2:30:06
Speaker 2: Electromagnetic interactions change. The allowed electronic arrangements reorganise. The total
2:30:13
Speaker 2: energy may fall into a bonded state or rise steeply
2:30:16
Speaker 2: against further compression. And from those microscopic changes comes everything
2:30:20
Speaker 2: we recognise as contact. Atoms do not need hard surfaces
2:30:25
Speaker 2: in order to touch. At the quantum scale, touching is
2:30:29
Speaker 2: what happens when neighbouring pieces of matter begin interacting strongly
2:30:33
Speaker 2: enough that neither can be understood as though the other
2:30:36
Speaker 2: were not there.