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Light Just Broke Our Intuition About Newton’s Third Law · 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,
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  Speaker 1: and understanding begins at the smallest scales.
• 0:28
  Speaker 2: So, picture standing on the bank of this, uh, perfectly
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  Speaker 2: peaceful really wide.
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  Speaker 3: River okay picturing it you're just watching.
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  Speaker 2: The water flow by and a single autumn leaf falls
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  Speaker 2: from a branch right it lands softly on the surface
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  Speaker 2: and instantly starts drifting downstream with the current.
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  Speaker 3: Yeah naturally i mean that's just how fluids.
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  Speaker 2: Work exactly it makes intuitive sense objects in a moving
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  Speaker 2: fluid naturally drift along with the flow If you want
• 0:54
  Speaker 2: to move against that stream, well, you have to exert energy.
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  Speaker 3: Right. You need a motor or something.
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  Speaker 2: Yeah. You need a motorized boat burning fuel or, you know,
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  Speaker 2: a paddle hitting the water, pushing backwards so the water
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  Speaker 2: pushes you forward.
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  Speaker 3: You always have to pay that toll to physics.
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  Speaker 1: Yeah.
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  Speaker 3: Action and reaction.
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  Speaker 2: Right. But what if I told you that rule doesn't
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  Speaker 2: just apply to water? It applies to light waves, photons too.
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  Speaker 2: But here's the twist. scientists recently pulled off a laboratory
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  Speaker 2: breakthrough that completely shatters that intuition.
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  Speaker 3: It's honestly mind-bending.
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  Speaker 2: It really is. So our mission today is to explore
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  Speaker 2: this wild discovery where a bill of light was actually observed,
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  Speaker 2: like swimming upstream through a quantum fluid.
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  Speaker 3: And doing it without a paddle.
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  Speaker 1: Exactly.
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  Speaker 2: And the craziest part, it does this by completely violating
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  Speaker 2: Newton's third law of motion.
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  Speaker 3: Which is huge. I mean, it forces a reevaluation of
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  Speaker 3: fundamental mechanics... that we just universally accepted for centuries.
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  Speaker 2: Yeah, it rewrites the rule book. Okay, let's unpack this.
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  Speaker 2: To really grasp how light breaks the rules, I think
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  Speaker 2: we first need to understand the rules themselves, right?
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  Speaker 3: Right, right. So let's talk about Newton's third law action-reaction symmetry. Basically,
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  Speaker 3: for every action, there is an equal and opposite reaction.
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  Speaker 3: The classic rule. Exactly. Even when matter isn't in equilibrium,
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  Speaker 3: like taking your motorized boat, fighting the current, it's actively...
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  Speaker 3: creating a force to oppose motion, but it is still
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  Speaker 3: fundamentally obeying Newton's third law.
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  Speaker 2: Because it's pushing the water back, and the water pushes
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  Speaker 2: the boat forward.
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  Speaker 3: Yes. The symmetry is perfectly intact. The toll is paid.
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  Speaker 2: Wait, so quick question on that. Previous experiments have actually
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  Speaker 2: shown upstream motion in quantum systems before, right? But they
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  Speaker 2: didn't break Newton's law to do it.
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  Speaker 3: Yeah, that's correct. They absolutely didn't break the law. They
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  Speaker 3: achieved that upstream motion through a very legally compliant mechanism
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  Speaker 3: called called vortex shedding.
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  Speaker 2: Vortex shedding.
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  Speaker 3: Okay, how does that work? It's a really beautiful fluid
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  Speaker 3: dynamic concept. So to visualize it, imagine a boulder sitting
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  Speaker 3: in the middle of a really fast flowing river. Got it.
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  Speaker 3: As the water rushes past the boulder, it doesn't just
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  Speaker 3: glide perfectly around it in smooth, perfect lines. The flow separates.
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  Speaker 2: Oh, like it gets choppy.
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  Speaker 3: Exactly. It swirls and creates these rotating whirlpools or vortices
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  Speaker 3: in the wake right behind the rock. Right.
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  Speaker 2: Yeah, you can totally see those little eddies spinning off
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  Speaker 2: behind bridge pillars or rocks in a creek. It's just
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  Speaker 2: turbulent flow.
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  Speaker 3: And that turbulence actually carries physical momentum. So in certain
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  Speaker 3: quantum systems like superfluids, say liquid helium, chilled down to
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  Speaker 3: near absolute zero, a very similar effect occurs.
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  Speaker 2: Okay, so the quantum fluid gets turbulent too.
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  Speaker 3: Yeah. If an object moves through that superfluid fast enough,
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  Speaker 3: it literally sheds quantum vortices into the wake behind it.
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  Speaker 3: And the physical act of throwing off that vortex downstream.
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  Speaker 2: Wait, it imparts a kick backward to the object itself?
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  Speaker 3: You got it.
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  Speaker 2: Yeah.
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  Speaker 3: It generates a net upstream recoil momentum.
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  Speaker 2: Oh, wow. So the object kicks a vortex downstream, and
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  Speaker 2: in return, the vortex kicks the object upstream.
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  Speaker 3: Exactly. Momentum is entirely conserved.
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  Speaker 2: The toll is paid.
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  Speaker 3: What's fascinating here is that this new breakthrough doesn't do
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  Speaker 3: that at all. The upstream motion they achieved happens without
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  Speaker 3: shedding any vortices. None. None. There's no wake. There's no
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  Speaker 3: equal and opposite kick. They completely threw action-reaction symmetry out
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  Speaker 3: the window.
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  Speaker 2: Okay, so if there's no vortex shedding, we really have
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  Speaker 2: to figure out how these scientists did it. Like, where
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  Speaker 2: is the physical momentum actually coming from?
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  Speaker 3: Right, and that brings us to the core secret of
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  Speaker 3: the whole thing, a concept physicists call non-reciprocal interactions.
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  Speaker 2: Non-reciprocal interactions. Okay, let me try to make this relatable.
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  Speaker 2: Imagine if I give you a high-five. Ready.
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  Speaker 3: High-five.
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  Speaker 2: But when our hands meet, My hand feels a totally
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  Speaker 2: different force than your hand does. Like, I feel a
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  Speaker 2: light tap, but you feel a massive shove. That non-reciprocal, right?
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  Speaker 3: That is a perfect way to visualize the mathematical imbalance, yeah.
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  Speaker 2: But physically, in the real world, how is that even possible?
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  Speaker 2: I mean, if two objects collide, the force has to
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  Speaker 2: be mutual, doesn't it?
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  Speaker 3: Well, in a vacuum, yes. A non-reciprocal high-five between two
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  Speaker 3: isolated objects is physically impossible.
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  Speaker 2: Right, because Newton says so.
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  Speaker 3: Exactly. But the trick here is that we are not
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  Speaker 3: operating in a vacuum. A non-reciprocal interaction can actually happen
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  Speaker 3: if we introduce a mediating substance.
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  Speaker 2: A mediator.
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  Speaker 3: Yeah, an environment that selectively absorbs or alters the symmetry
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  Speaker 3: of the interaction.
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  Speaker 2: Oh, okay. So it would be like us high-fiving through
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  Speaker 2: a layer of, I don't know, smart gel.
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  Speaker 3: Smart gel, I like that.
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  Speaker 2: Yeah, like the gel acts as a mediator. It hardens
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  Speaker 2: instantly against the pressure of my hand, protecting me, but
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  Speaker 2: it stays perfectly soft on your side and transfers all
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  Speaker 2: the force into your hand.
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  Speaker 3: Yes, exactly. That is a much more accurate physical model.
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  Speaker 3: The environment itself is what dictates how the interaction gets distributed.
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  Speaker 3: That's wild. And we actually see a macroscopic version of
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  Speaker 3: this in biology, in what we call active matter.
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  Speaker 2: Active matter, like what?
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  Speaker 3: Like flocks of birds, schools of fish, or even swarms
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  Speaker 3: of bacteria. They move in highly synchronized, non-reciprocal ways because
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  Speaker 3: every individual bird or bacterium has its own internal energy source.
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  Speaker 2: Oh, I see. They aren't just bumping into each other
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  Speaker 2: like billiard balls.
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  Speaker 3: Exactly. They're constantly processing information from their environment and adjusting
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  Speaker 3: their thrust based on that. Right.
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  Speaker 2: But light doesn't have an internal battery. A photon doesn't
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  Speaker 2: have a tiny little motor to process its environment and
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  Speaker 2: adjust its thrust like a bird does.
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  Speaker 3: Which is exactly what makes engineering active matter out of
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  Speaker 3: light such an incredible challenge.
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  Speaker 2: I can imagine.
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  Speaker 3: Under normal circumstances, two beams of light don't even acknowledge
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  Speaker 3: each other. Like if you cross two laser pointers in
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  Speaker 3: a dark room.
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  Speaker 2: The beams just pass perfectly through one another like ghosts.
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  Speaker 3: Right. Photons just do not interact with other photons in
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  Speaker 3: a vacuum. So to force them to interact and specifically
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  Speaker 3: to force them to interact non-reciprocally.
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  Speaker 2: They had to build that smart gel environment. OK, here's
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  Speaker 2: where it gets really interesting. Let's break down the actual
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  Speaker 2: laboratory setup because the way they built this environment is
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  Speaker 2: just fascinating. They took theoretical math and real world lasers.
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  Speaker 2: And they sent those lasers through this thing called a
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  Speaker 2: nonlinear crystal.
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  Speaker 3: Yeah, the nonlinear crystal is the absolute secret ingredient here.
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  Speaker 2: How does it work?
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  Speaker 3: Well, in a normal linear optical medium, light just passes
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  Speaker 3: through predictably. But in a nonlinear crystal, the optical properties
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  Speaker 3: of the crystal itself actually change depending on the intensity
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  Speaker 3: of the light passing through it.
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  Speaker 2: Wait, the light changes the crystal?
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  Speaker 3: Yeah. The presence of one light beam actually alters the
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  Speaker 3: refractive index of the crystal. And that change then bends
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  Speaker 3: or affects the second beam. The crystal literally forces the
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  Speaker 3: photons to talk to each other.
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  Speaker 2: That is so cool. So setting the scene here, the
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  Speaker 2: physicists took two perfectly timed laser beams and fired them
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  Speaker 2: into this crystal. One was this broad, wide laser beam
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  Speaker 2: that acted as the fluid. Right.
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  Speaker 3: That's our optical river.
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  Speaker 2: Exactly. Then they took a second, much narrower laser beam
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  Speaker 2: to act as the swimmer. And by physically silting the
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  Speaker 2: angle of that broad fluid beam, they could dictate the
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  Speaker 2: direction and the speed of the optical river flowing through
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  Speaker 2: the crystal.
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  Speaker 3: It's brilliant. They essentially constructed a microscopic, highly programmable water
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  Speaker 3: ride built entirely out of light.
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  Speaker 2: All governed by the physics of that nonlinear crystal.
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  Speaker 3: Yes.
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  Speaker 2: So when they shoot the narrow swimmer beam into the
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  Speaker 2: broad fluid beam in a normal reciprocal setup, the swimmer
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  Speaker 2: just drifts downstream. It gets carried away by the flow,
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  Speaker 2: just like we'd expect.
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  Speaker 3: It's like the leaf in the river. Right. Right.
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  Speaker 2: But when they tune the lasers to trigger those non-reciprocal interactions,
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  Speaker 2: the magic moment happens. The narrow swimmer beam starts moving
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  Speaker 2: in the exact opposite direction of the broader fluid's flow.
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  Speaker 3: It's honestly stunning. It swims upstream against the current.
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  Speaker 2: No paddle, no motor, and no vortex shedding, just moving backwards.
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  Speaker 3: And, you know, the visual confirmation of that theory is striking, sure.
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  Speaker 2: Yeah.
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  Speaker 3: But the actual mechanics of why it moved backwards, what
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  Speaker 3: we call the inverted recoil, are even more compelling.
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  Speaker 2: Inverted recoil. Tell me about that. Because the swimmer beam
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  Speaker 2: doesn't just, like, statically exist inside the fluid beam, right?
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  Speaker 3: Not at all. As it passes through the nonlinear crystal,
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  Speaker 3: it continuously reshapes the optical fluid around it.
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  Speaker 2: So it isn't just cutting through the broader beam like
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  Speaker 2: a knife?
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  Speaker 3: No. Because of the nonlinear crystal acting as our smart
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  Speaker 3: gel mediator, the swimmer beam is actually altering the density
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  Speaker 3: of the fluid beam. Okay.
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  Speaker 2: Altering it how?
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  Speaker 3: Asymmetrically. Yeah. Because the interactions are tuned to be non-reciprocal,
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  Speaker 3: the swimmer beam causes the intensity of the surrounding fluid
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  Speaker 3: beam to become heavily skewed.
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  Speaker 2: Skewed, like uneven.
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  Speaker 3: Exactly. The fluid literally becomes denser or optically more intense
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  Speaker 3: on one specific side of the swimmer compared to the
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  Speaker 3: other side. Oh, wow.
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  Speaker 2: Okay, let me see if I can trace the causality
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  Speaker 2: here for a second.
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  Speaker 3: Go for it.
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  Speaker 2: So this rumor beam enters the fluid through the mediation
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  Speaker 2: of the crystal. It basically tricks the fluid into piling
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  Speaker 2: up unevenly around it. That uneven pileup creates a gradient
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  Speaker 2: in the light intensity. And because light actually exerts radiation pressure,
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  Speaker 2: that dense pile of fluid physically squeezes the swimmer forward,
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  Speaker 2: pushing it against the current.
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  Speaker 3: You've just described the inverted recoil mechanism perfectly. The swimmer
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  Speaker 3: manipulates the river's local environment to squeeze itself upstream.
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  Speaker 2: That is just... It's so counterintuitive.
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  Speaker 3: It really is. Because the forces between the swimmer and
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  Speaker 3: the fluid are fundamentally unbalanced by the crystal, that action-reaction
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  Speaker 3: symmetry is cleanly broken.
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  Speaker 2: So the swimmer generates its own directional drive from the
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  Speaker 2: inside out.
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  Speaker 3: Purely driven by the asymmetrical way it influences its immediate surroundings.
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  Speaker 2: It essentially builds its own momentum out of the uneven
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  Speaker 2: pressure it creates. But, you know, looking at the findings
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  Speaker 2: of the experiment... There's a nuance here to the fluid
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  Speaker 2: dynamics that I found really surprising.
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  Speaker 3: Oh, the fluid speed.
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  Speaker 2: Yeah. You would naturally assume that to get the absolute
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  Speaker 2: maximum upstream speed out of this swimmer, you'd want to
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  Speaker 2: crank the fluid flow up to the highest possible setting, right?
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  Speaker 3: Yeah, that's the logical assumption. Yeah. Turn the river into
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  Speaker 3: raging rapids, get the mass amount of light interaction, generate
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  Speaker 3: the strongest squeeze.
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  Speaker 2: Exactly. But the results show the exact opposite.
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  Speaker 3: Yeah, the upstream motion was actually strongest at intermediate fluid
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  Speaker 3: speeds and densities. It's very much... A Goldilocks effect.
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  Speaker 2: Not too fast, not too slow.
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  Speaker 3: Exactly. It operates within this really strict parameter zone. If
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  Speaker 3: the optical river is flowing too slowly, the upstream effect
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  Speaker 3: is basically negligible.
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  Speaker 2: Right.
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  Speaker 3: But if it's flowing too fast, the effect completely collapses
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  Speaker 3: as well. The swimming behavior only really peaks when the
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  Speaker 3: conditions are perfectly balanced.
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  Speaker 2: Well, if we think about the mechanism we just outlined,
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  Speaker 2: that limitation actually makes sense. I mean, the swimmer relies
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  Speaker 2: on carefully reshaping the fluid around it To create that
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  Speaker 2: asymmetric pileup of light. Right.
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  Speaker 3: It needs to warp its environment.
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  Speaker 2: So if the fluid is moving too slowly, there just
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  Speaker 2: isn't enough flow dynamic to generate a meaningful pressure gradient.
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  Speaker 2: It would be like trying to surf a ripple in
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  Speaker 2: a bathtub.
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  Speaker 3: That's a great analogy. And on the other extreme, if
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  Speaker 3: the fluid is moving at its absolute maximum velocity, it's
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  Speaker 3: just rushing past the swimmer so violently that that the
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  Speaker 3: swimmer doesn't have the time to properly reshape it.
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  Speaker 2: Oh, so the pileup just gets washed away?
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  Speaker 3: Exactly. The asymmetric distribution of the fluid just gets totally
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  Speaker 3: washed out by the sheer speed of the flow before
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  Speaker 3: it can exert any of that inverted recoil. So the
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  Speaker 3: ideal zone is that perfect intermediate speed where the swimmer
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  Speaker 3: has just enough time to warp the fluid and the
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  Speaker 3: fluid has enough momentum to squeeze the swimmer forward.
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  Speaker 2: Wow. It requires an incredibly delicate calibration between the object
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  Speaker 2: and its environment. But we should probably specify a critical
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  Speaker 2: distinction here regarding the physical nature of this whole experiment.
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  Speaker 3: Oh, regarding the fluid itself.
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  Speaker 2: Yeah. The setup we are discussing is an optical analog
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  Speaker 2: of a quantum fluid. They used light interacting within a crystal.
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  Speaker 2: They didn't actually perform this using a material quantum fluid
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  Speaker 2: like liquid helium or, you know, a Bose-Einstein condensate of
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  Speaker 2: ultra-cold atoms.
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  Speaker 1: Right.
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  Speaker 3: And if we connect this to the bigger picture, that
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  Speaker 3: distinction is important. But it doesn't diminish the finding.
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  Speaker 2: How so?
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  Speaker 3: While it is an optical analog, the mathematical frameworks governing
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  Speaker 3: how the light behaves in that nonlinear crystal are directly
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  Speaker 3: analogous to the exact equations that govern actual material quantum fluids.
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  Speaker 2: Oh, so the math is the same either way.
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  Speaker 3: Exactly. In the field of condensed matter physics, optical analogs
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  Speaker 3: serve as these highly robust, controllable test beds.
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  Speaker 2: Because working with actual quantum fluids is a nightmare, right?
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  Speaker 3: Oh, totally. They allow physicists to isolate and measure behaviors
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  Speaker 3: that would be nearly impossible to observe in a vat
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  Speaker 3: of liquid helium. I mean, that requires extreme cryogenic cooling
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  Speaker 3: and is just notoriously difficult to probe without destroying the
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  Speaker 3: quantum state entirely. Right.
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  Speaker 2: So by using light at room temperature inside a crystal,
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  Speaker 2: they can clearly isolate that non-reciprocal variable.
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  Speaker 3: Exactly. And map out the exact mechanics of the inverted recoil.
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  Speaker 3: The math holds up beautifully regardless of the medium.
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  Speaker 2: So what does this all mean? Why should you, the listener,
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  Speaker 2: care about some lasers and a crystal?
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  Speaker 3: Because the transition from this theoretical math to a physical
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  Speaker 3: optical demonstration is the necessary bridge to the future of
• 14:14
  Speaker 3: material science.
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  Speaker 2: Okay, color me intrigued.
• 14:17
  Speaker 3: Understanding non-reciprocal interactions in the quantum regime provides the key
• 14:21
  Speaker 3: insights we actually need to design entirely novel devices. We're
• 14:25
  Speaker 3: basically entering an era of engineering what we call active functionalities.
• 14:30
  Speaker 2: Active functionalities? That refers to systems that can process information
• 14:34
  Speaker 2: or like drive themselves in ways that current technology just
• 14:38
  Speaker 2: can't achieve, right?
• 14:39
  Speaker 3: Right. If we take this mechanism, this ability to convert
• 14:42
  Speaker 3: internal asymmetrical interactions into net directional momentum and apply it
• 14:46
  Speaker 3: to other platforms, the implications are massive.
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  Speaker 2: Give me an example. Where would we see this first?
• 14:51
  Speaker 3: The most immediate applications would definitely be seen in optical
• 14:54
  Speaker 3: technologies and quantum computing. Oh, okay. A major hurdle in
• 14:57
  Speaker 3: quantum computing right now is signal routing and energy loss.
• 15:01
  Speaker 3: Components often suffer from backscattering.
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  Speaker 2: Backscattering, you mean where information or energy just reflects backward
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  Speaker 2: and disrupts the whole system?
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  Speaker 3: Exactly. It's a huge problem. But by engineering non-reciprocal pathways,
• 15:13
  Speaker 3: you could design perfectly directional light sensors or signal routers
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  Speaker 3: where the information can only flow forward.
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  Speaker 1: Oh, wow.
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  Speaker 2: Completely immune to backward reflection because the environment itself physically
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  Speaker 2: prevents the reciprocal action.
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  Speaker 3: Yes. It fundamentally changes the mechanical possibilities at the microscopic scale.
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  Speaker 2: That is incredible. I mean, we started this whole conversation
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  Speaker 2: by looking at a leaf floating down a river, representing
• 15:38
  Speaker 2: that intuitive rule that everything in a fluid drifts unless
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  Speaker 2: it exerts energy to fight back.
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  Speaker 3: The standard interpretation of physics based on equilibrium and symmetry, yeah.
• 15:46
  Speaker 2: But then we saw how physicists took a narrow beam
• 15:48
  Speaker 2: of light placed it in a flowing optical fluid, and
• 15:52
  Speaker 2: made it swim against the current.
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  Speaker 3: Without a motor, without shedding vortices.
• 15:56
  Speaker 2: Right. They engineered an environment inside that nonlinear crystal that
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  Speaker 2: allowed the light to break action-reaction symmetry. The light literally
• 16:05
  Speaker 2: reshaped its own environment, creating that asymmetrical density gradient that
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  Speaker 2: squeezed it forward in an inverted recoil.
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  Speaker 3: They leveraged the environment to act as the engine itself.
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  Speaker 3: And the implications of that extend far, far beyond lasers
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  Speaker 3: in a crystal.
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  Speaker 2: Yeah, because we've proven the math using light. The next
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  Speaker 2: phase is applying this non-reciprocal loophole to the physical materials
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  Speaker 2: of the future.
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  Speaker 3: And this raises an important question for everyone listening. Consider
• 16:31
  Speaker 3: the long-term potential for fields like medicine or space exploration.
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  Speaker 2: Oh man, self-propelling materials.
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  Speaker 3: Exactly. If scientists can design microscopic quantum materials that inherently
• 16:43
  Speaker 3: exploit these non-reciprocal interactions... We could develop self-propelling structures that
• 16:48
  Speaker 3: navigate through complex, viscous environments without any external fuel.
• 16:53
  Speaker 2: Like targeting a specific cell through the human bloodstream.
• 16:56
  Speaker 3: Yes, or navigating a probe through the extreme radiation streams
• 17:00
  Speaker 3: of deep space. These materials wouldn't require a conventional battery.
• 17:04
  Speaker 3: They would just continuously manipulate their local environment, tricking the
• 17:07
  Speaker 3: surrounding fluid into pushing them exactly where they need to go.
• 17:10
  Speaker 2: A completely self-contained method of propulsion built directly into the
• 17:14
  Speaker 2: material itself. It is an incredible concept to consider. You know,
• 17:18
  Speaker 2: we really no longer have to fight the current if
• 17:20
  Speaker 2: we can simply rewrite the rules of the river.
• 17:22
  Speaker 3: It's only a frontier.
• 17:23
  Speaker 2: It really is. So keep observing the mechanisms of the
• 17:26
  Speaker 2: world around you. Keep questioning the fundamental rules. And the
• 17:30
  Speaker 2: next time you see an object drifting downstream, maybe consider
• 17:34
  Speaker 2: the physics required to make it swim the other way.
• 17:36
  Speaker 2: Until next time.