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Einstein’s Gravity Just Entered the Quantum World · The Quark Side - Quantum Physics Podcast

• 0:01
  Speaker 1: Welcome to the QuarkSide Quantum Physics Podcast, an exploration of
• 0:05
  Speaker 1: the fundamental structure of reality, where quantum laws govern matter, energy,
• 0:11
  Speaker 1: and information. Here, uncertainty is a feature, not a flaw,
• 0:16
  Speaker 1: and understanding begins at the smallest scales.
• 0:32
  Speaker 2: Suddenly the cables just snap.
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  Speaker 3: A terrifying thought, honestly. Right.
• 0:36
  Speaker 2: Absolute nightmare. You're plummeting down an empty shaft. But ignore
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  Speaker 2: the obvious panic for a second. Let's say you're holding
• 0:43
  Speaker 2: your keys. If you let go of your keys right
• 0:46
  Speaker 2: at that exact moment, what happens?
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  Speaker 3: Well, they wouldn't fall to the floor.
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  Speaker 2: Exactly. Einstein tells us those keys won't drop to your feet.
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  Speaker 2: They will just, you know, hover right there next to
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  Speaker 2: your hand. From your perspective, inside that falling box, gravity
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  Speaker 2: has essentially just.
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  Speaker 3: Yeah, it's a perfect illusion of weightlessness.
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  Speaker 2: Right, and this is the equivalence principle. It's the absolute
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  Speaker 2: bedrock of Einstein's theory of general relativity. It basically says
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  Speaker 2: that free fall and weightlessness are effectively the exact same thing.
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  Speaker 1: What?
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  Speaker 3: Fundamentally indistinguishable.
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  Speaker 2: But now, imagine those keys aren't made of solid brass.
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  Speaker 2: Imagine those keys are a single quantum wave, like existing
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  Speaker 2: in a state of superposition.
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  Speaker 3: Where they haven't actually decided where they are yet.
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  Speaker 2: Exactly. They haven't picked a location. So the question is,
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  Speaker 2: how on earth do you test gravity on an object
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  Speaker 2: that is technically in two places at once?
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  Speaker 3: And that right there is the paradox that has haunted
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  Speaker 3: physics for, well, basically a century.
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  Speaker 2: It's a huge problem.
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  Speaker 3: Huge. I mean, we have these two phenomenal rule books
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  Speaker 3: for the universe. On one hand, general relativity perfectly maps
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  Speaker 3: out gravity's And, you know, falling apples and orbiting planets,
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  Speaker 3: the macro cosmos.
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  Speaker 2: Right.
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  Speaker 3: And then on the other hand, quantum mechanics beautifully dictates
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  Speaker 3: the bizarre jittery probabilities of the microscopic world.
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  Speaker 2: But they just do not get along.
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  Speaker 3: No, they refuse to cooperate. If you try to apply
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  Speaker 3: Einstein's smooth rules of gravity to a chunky probabilistic quantum system.
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  Speaker 3: the mathematics just completely breaks down.
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  Speaker 2: Just spits out nonsense.
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  Speaker 3: Exactly. Nonsense or infinities. And until now, we haven't really
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  Speaker 3: had a way to physically force them into the same
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  Speaker 3: room to see what happens. But a brand new experiment
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  Speaker 3: has finally managed to do just that. They managed to
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  Speaker 3: trap a quantum wave, hold it, drop it, and basically
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  Speaker 3: force these two distinct realities to shake hands in the
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  Speaker 3: exact same space.
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  Speaker 2: And that is exactly what our mission is today. We
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  Speaker 2: are exploring this groundbreaking, highly intricate experiment Where a team
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  Speaker 2: actually figured out a way to take a quantum probability
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  Speaker 2: wave and basically drop it down an elevator shaft.
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  Speaker 3: It's genuinely at the very edge of our understanding of reality.
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  Speaker 2: It really is. Okay, let's unpack this. Because taking a
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  Speaker 2: solid object, right, taking a solid object and dropping it
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  Speaker 2: is one thing. You can see it. You can measure
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  Speaker 2: its trajectory with a camera.
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  Speaker 3: Sure, it's classical physics.
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  Speaker 2: But a quantum object doesn't have a single defined location
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  Speaker 2: until you measure it. That makes sense for a person
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  Speaker 2: in an elevator. But how on earth do you test
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  Speaker 2: that on a quantum object? Quantum objects act like waves
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  Speaker 2: and travel multiple paths at once.
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  Speaker 3: What's fascinating here is that you're hitting on the exact
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  Speaker 3: monumental hurdle the researchers faced. Testing a solid object is easy,
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  Speaker 3: but testing a smear of potential locations that is essentially
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  Speaker 3: in two places at once, that requires something entirely new.
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  Speaker 2: Because if you just look at it too hard, it breaks. Exactly.
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  Speaker 3: Exactly. If you interact with the wave too heavily to
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  Speaker 3: try and see what it's doing, you collapse the superposition.
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  Speaker 2: You force it to pick a spot.
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  Speaker 3: Right. You destroy the very quantum state you are trying
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  Speaker 3: to test. So you can't just toss it in the
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  Speaker 3: air and watch it fall. You need to subject that
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  Speaker 3: delicate probability wave to gravity and somehow see if the
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  Speaker 3: whole smear behaves the way a falling rock would. Without
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  Speaker 3: actually looking at it while it falls.
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  Speaker 2: Which sounds completely impossible. So they had to invent a
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  Speaker 2: whole new machine to do this. I know it was
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  Speaker 2: an international team of researchers from Ben-Gurion University, the University
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  Speaker 2: of Ulm in Oxford. And they built this incredibly sci-fi
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  Speaker 2: sounding device. Yeah, the name is great. The Quantum Galileo Interferometer.
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  Speaker 3: It's a fantastic name. Galileo dropping spheres from the leaning
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  Speaker 3: tower of Pisa, but, you know, for the quantum age.
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  Speaker 2: But they aren't dropping brass spheres. They're using a specialized
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  Speaker 2: atom chip. And clouds of rubidium atoms.
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  Speaker 3: Right, rubidium. And the first step is preparation. They have
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  Speaker 3: to cool these rubidium atoms down to just above absolute zero.
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  Speaker 2: Like fraction of a degree above stopping all motion entirely.
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  Speaker 3: Exactly. And you have to get them that cold just
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  Speaker 3: to quiet things down. I mean, at room temperature, atoms
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  Speaker 3: are basically zipping around like crazy, crashing into each other.
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  Speaker 2: Just total chaos.
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  Speaker 3: Total chaos. Their thermal energy completely masks their quantum properties.
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  Speaker 1: Yeah.
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  Speaker 3: But when you chill them down to near absolute zero,
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  Speaker 3: all that thermal jittering just freezes out.
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  Speaker 2: They calm down.
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  Speaker 3: Right. They slow down so much that their physical size,
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  Speaker 3: as like classical particles, becomes way less important than their
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  Speaker 3: quantum wavelength. They stop acting like tiny billiard balls and
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  Speaker 3: start acting more like... Ripples. Yeah, like spreading ripples in
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  Speaker 3: a pond.
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  Speaker 2: Okay, so we have this ultra cold cloud of rippling
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  Speaker 2: rubidium hovering over this atom chip. And this chip, It's
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  Speaker 2: laced with these incredibly fine gold wires, right?
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  Speaker 3: Microscopic, perfectly patterned gold wires, yeah. By running very specific
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  Speaker 3: electrical currents through those wires, the team can generate these
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  Speaker 3: localized magnetic fields.
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  Speaker 2: Which act like tweezers.
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  Speaker 3: Invisible magnetic tweezers. They can hold the ultra-cold atoms in
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  Speaker 3: a magnetic trap, just perfectly suspended over the surface of
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  Speaker 3: the chip.
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  Speaker 2: But holding them still isn't the experiment. They have to
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  Speaker 2: split them. And they do this by hitting the atoms with, like,
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  Speaker 2: A very precise microwave pulse, right?
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  Speaker 3: Yes, a highly calibrated microwave pulse.
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  Speaker 2: Wait, so when you say superposition, you mean each individual
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  Speaker 2: atom is effectively traveling along two different paths at the
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  Speaker 2: exact same time. It's like taking a literal fork in
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  Speaker 2: the road and walking down both paths simultaneously.
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  Speaker 3: That is a phenomenal analogy, yes. Let me break down
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  Speaker 3: how the microwave pulse actually does that. Every atom has
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  Speaker 3: this internal property called spin.
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  Speaker 2: Like a little internal compass needle. Exactly.
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  Speaker 3: Exactly. Now, the microwave pulse is tuned to interact with
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  Speaker 3: that compass. But because this is the quantum world, it
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  Speaker 3: doesn't just flip the needle from north to south. It
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  Speaker 3: puts the atom's spin into a superposition of both.
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  Speaker 2: So it's literally pointing north and south simultaneously.
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  Speaker 1: Yes.
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  Speaker 3: Now, here is the trick. Those two different spin states,
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  Speaker 3: the north and the south, they react differently to the
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  Speaker 3: magnetic fields coming from the chip.
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  Speaker 2: Oh, I see.
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  Speaker 3: Right. So one spin state might feel a strong push
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  Speaker 3: from the magnetic field. while the other spin state feels
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  Speaker 3: a weak push.
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  Speaker 2: So the magnetic field acts almost like a prism. It
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  Speaker 2: physically pushes the atom in two different physical directions at
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  Speaker 2: the exact same time because the atom is in two
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  Speaker 2: states at the same time.
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  Speaker 3: Exactly. The internal superposition forces a physical separation. You now
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  Speaker 3: have a single atom whose wave function has been split.
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  Speaker 3: It is exploring two distinct paths.
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  Speaker 2: So now we have our quantum keys. We have a
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  Speaker 2: single atom. split into two spatial realities. Now we have
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  Speaker 2: to drop it.
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  Speaker 3: And this is where the experimental choreography is just a masterpiece.
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  Speaker 3: We have these two paths. Let's call them path A
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  Speaker 3: and path B.
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  Speaker 2: Okay.
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  Speaker 3: Path A is the control. The researchers used the magnetic
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  Speaker 3: trap on the chip to generate an upward push on
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  Speaker 3: Path A that perfectly, and I mean exactly, counteracts the
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  Speaker 3: downward pull of Earth's gravity.
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  Speaker 2: So Path A is locked in place. It's fighting gravity
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  Speaker 2: just holding its ground.
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  Speaker 3: Right. It is completely stationary. But Path B.
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  Speaker 2: Here's where it gets really interesting.
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  Speaker 3: It does. For Path B, they use a quick magnetic
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  Speaker 3: pulse to push it up. Then they just switch the
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  Speaker 3: magnetism off.
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  Speaker 2: They snap the elevator cables.
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  Speaker 3: They snap the cables. Path B falls freely under gravity.
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  Speaker 3: It traces a purely ballistic trajectory, just like throwing a
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  Speaker 3: baseball into the air and letting it drop.
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  Speaker 2: So within a single quantum wave, a single atom, you
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  Speaker 2: have half of it fighting gravity to stay perfectly still,
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  Speaker 2: and the other half in a state of literal free fall,
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  Speaker 2: experiencing that localized weightlessness that Einstein talked about.
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  Speaker 3: Yes. Two entirely different gravitational realities happening simultaneously to one object.
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  Speaker 2: That is mind-bending. But, okay, they let it fall. Gravity
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  Speaker 2: does its work. But eventually they have to get the data.
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  Speaker 2: And we already said you can't just take a picture
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  Speaker 2: of it.
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  Speaker 3: No. If you try to measure it mid-drop, the atom
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  Speaker 3: picks a path, the wave collapses, and the experiment is
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  Speaker 3: totally ruined.
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  Speaker 2: So how do you measure it?
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  Speaker 3: To extract the data, you have to use another magnetic
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  Speaker 3: pulse to bring path A and path B back together.
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  Speaker 3: You reunite them.
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  Speaker 2: And because they are waves, when they reunite, they don't
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  Speaker 2: just clink together like marbles. They interfere.
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  Speaker 3: Exactly. They overlap. Where the peaks of the waves align,
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  Speaker 3: they amplify. Where a peak meets a trough, they cancel.
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  Speaker 3: It creates a very specific interference pattern.
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  Speaker 2: And reading that pattern lets them measure the quantum phase, right?
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  Speaker 2: The difference between the falling half and the stationary half. Right.
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  Speaker 3: The quantum phase accumulated on each path. Okay.
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  Speaker 2: Let's pause there for a second. Quantum phase is one
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  Speaker 2: of those terms that sounds super dense. Let's make sure
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  Speaker 2: we really grasp what's physically happening. What is the phase?
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  Speaker 3: Think of it like two identical perfect metronomes.
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  Speaker 2: Okay, ticking back and forth.
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  Speaker 3: Right, in perfect unison. A quantum wave has an inherent oscillation,
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  Speaker 3: just like a metronome. The phase is basically a measure
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  Speaker 3: of exactly where that metronome is in its swing at
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  Speaker 3: any given microsecond.
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  Speaker 2: So if they are perfectly in sync, their phases match.
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  Speaker 3: Exactly. But here is where Einstein's gravity comes back in.
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  Speaker 3: According to general relativity, gravity is a curvature of spacetime.
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  Speaker 3: Your position in a gravitational field changes your potential energy,
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  Speaker 3: and that actually affects how time flows for you.
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  Speaker 2: Right. gravitational time dilation. The closer you are to the Earth,
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  Speaker 2: the slower time actually ticks.
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  Speaker 3: Yes, and in quantum mechanics, a change in potential energy-like
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  Speaker 3: falling deeper into a gravity well alters the frequency of
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  Speaker 3: the wave function. It changes how fast the quantum metronome ticks.
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  Speaker 2: Wow, okay. So path A is held high up, stationary...
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  Speaker 2: Path B is dropped, falling lower into gravity. So path
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  Speaker 2: B's metronome actually falls out of sync with path A's metronome.
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  Speaker 3: It does. They are ticking at slightly different rates because
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  Speaker 3: they are experiencing gravity differently.
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  Speaker 2: So when you bring them back together, the waves are misaligned.
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  Speaker 2: One is a fraction of a beat behind. And that
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  Speaker 2: misalignment shifts the final interference pattern.
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  Speaker 3: That is exactly what they measured. The interference pattern is
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  Speaker 3: a physical, readable record of the time and energy differences
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  Speaker 3: between the two halves.
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  Speaker 2: So what does this all mean? When they looked at
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  Speaker 2: the pattern and ran the numbers, did Einstein win or
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  Speaker 2: did quantum mechanics win?
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  Speaker 3: Well, the measured phase perfectly matched what was predicted when
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  Speaker 3: applying Einstein's equivalence principle to a quantum wave.
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  Speaker 2: So Einstein's rules held up.
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  Speaker 3: Flawlessly. Even in the strange, jittery quantum realm, the probability
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  Speaker 3: wave reacted to gravity exactly the same way a falling
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  Speaker 3: set of keys would. Gravity treats them identically. As the
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  Speaker 3: lead author, Professor Ron Fulman, pointed out, This doesn't totally
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  Speaker 3: unify the two theories.
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  Speaker 1: Right. Yeah.
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  Speaker 2: Let me jump in there just so we're clear. This
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  Speaker 2: doesn't prove that gravity itself is quantum rounds.
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  Speaker 3: Absolutely correct. It proves consistency in this specific regime, not unification.
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  Speaker 3: We didn't just discover the graviton or prove that spacetime
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  Speaker 3: is pixelated.
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  Speaker 2: We just proved they can coexist in this one scenario.
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  Speaker 3: Right. Professor Vlachovadril at Oxford, who was part of the team,
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  Speaker 3: gave some great context here. He noted that we have
• 11:44
  Speaker 3: absolutely no theory saying quantum physics should fail when gravity
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  Speaker 3: is introduced. So this pushes quantum mechanics into its most
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  Speaker 3: intriguing frontier yet, and it survives.
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  Speaker 2: It's like mapping the edge of the known universe. But
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  Speaker 2: it immediately makes me wonder, if the rules hold up
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  Speaker 2: for a single, tiny, rubidium atom, how far can we
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  Speaker 2: push this before reality actually breaks?
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  Speaker 3: And if we connect this to the bigger picture, that
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  Speaker 3: is exactly what they want to find out next. Because
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  Speaker 3: there is a very famous idea... by Nobel Laureate Professor
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  Speaker 3: Sir Roger Penrose, who, by the way, was actually involved
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  Speaker 3: with this team.
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  Speaker 2: Oh, really? I didn't realize he was on this specific project.
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  Speaker 3: He was, yeah. And Penrose has argued for a long
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  Speaker 3: time that quantum mechanics could eventually break down, specifically if
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  Speaker 3: you use massive enough objects and hold them in superposition.
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  Speaker 2: For long enough. Because right now we never see large
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  Speaker 2: objects in superposition, right? I can't be in my car
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  Speaker 2: and my house at the same time.
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  Speaker 3: Right. The usual excuse is environmental noise heat, air molecules
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  Speaker 3: bumping into you and collapsing the wave. But Penrose thinks
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  Speaker 3: it's deeper. He thinks gravity itself is what breaks the superposition.
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  Speaker 2: How does that work? Why would gravity care if I'm
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  Speaker 2: in two places at once?
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  Speaker 3: Well, remember that mass curves spacetime. If you put a
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  Speaker 3: massive object into a superposition of two locations.
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  Speaker 2: Then its gravitational field is also in two locations.
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  Speaker 3: Exactly. You are asking the fabric of spacetime to curve
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  Speaker 3: in two entirely different ways simultaneously. Penrose calls this a
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  Speaker 3: blister in spacetime.
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  Speaker 2: A blister.
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  Speaker 3: Yeah, fundamental instability. And his math suggests the universe simply
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  Speaker 3: won't tolerate it. The more mass the object has, the
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  Speaker 3: bigger the blister, and the faster gravity steps in and
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  Speaker 3: forces the wave to collapse into a single solid reality.
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  Speaker 2: That is wild. But obviously, a single rubidium atom doesn't
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  Speaker 2: have enough mass to test that. Its gravity is basically zero.
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  Speaker 3: The blister is negligible. This current experiment didn't reach the
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  Speaker 3: mass or time scales to test Penrose's theory, but, and
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  Speaker 3: this is the exciting part, it lays the groundwork. The
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  Speaker 3: exact atom chip architecture they just invented. The Ben-Gurion team
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  Speaker 3: is already working on the next step.
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  Speaker 2: Upgrading the mass.
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  Speaker 3: Exactly. They want to use much heavier objects, specifically nanodiamonds.
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  Speaker 2: Nanodiamonds. Which are tiny to us, but compared to a
• 14:02
  Speaker 2: single atom.
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  Speaker 3: They are absolute behemoths. Millions of atoms bound together.
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  Speaker 2: So if they can cool a diamond, isolate it, and
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  Speaker 2: put the whole diamond into a superposition.
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  Speaker 3: They can then drop it.
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  Speaker 2: Oh, man. We are literally watching the boundaries of reality
• 14:15
  Speaker 2: being mapped out in real time. If they drop a diamond,
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  Speaker 2: and Penrose is right, the interference pattern would just vanish.
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  Speaker 2: The diamond would collapse on its own. Just because it's
• 14:25
  Speaker 2: too heavy for space-time to handle the paradox.
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  Speaker 3: It would fundamentally change our understanding of the universe. It
• 14:31
  Speaker 3: would show us the exact mechanism that creates the classical world.
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  Speaker 2: It's incredible. So just to recap this whole journey. They
• 14:37
  Speaker 2: managed to take an atom, split its reality into two
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  Speaker 2: distinct paths, hold one completely still while dropping the other
• 14:44
  Speaker 2: in free fall, and prove that Einstein's century-old theories still
• 14:48
  Speaker 2: govern the quantum realm. It's just a masterclass in experimental physics.
• 14:52
  Speaker 3: It really is. And this raises an important question, something
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  Speaker 3: really profound to think about. If researchers do eventually test
• 14:58
  Speaker 3: heavier things like those nanodiamonds and quantum mechanics does break
• 15:02
  Speaker 3: down exactly as Penrose suspects, could gravity actually be the
• 15:06
  Speaker 3: very mechanism that forces the fuzzy multiple choice quantum world
• 15:10
  Speaker 3: to collapse into the single solid classical reality you and
• 15:13
  Speaker 3: I experience every single day?
• 15:15
  Speaker 2: Is gravity the reason reality stays in one piece? That
• 15:19
  Speaker 2: is a wild thought to leave on. Thank you so
• 15:21
  Speaker 2: much for joining us on this exploration today. It's a
• 15:24
  Speaker 2: lot to wrap your head around, but I hope you
• 15:25
  Speaker 2: are leaving with a whole new perspective on how the
• 15:27
  Speaker 2: universe holds itself together. Keep questioning your everyday reality, and
• 15:31
  Speaker 2: we will be back to explore the next frontier soon.