Speaker 1: Welcome to the quart Side Quantum Physics Podcast, an exploration
Speaker 1: of the fundamental structure of reality, where quantum laws govern matter, energy,
Speaker 1: and information. Here, uncertainty is a feature, not a flaw,
Speaker 1: and understanding begins at the smallest scales.
Speaker 2: So, uh, picture this. It's around May first, twenty twenty
Speaker 2: six and a team of physicists up in Toronto are
Speaker 2: looking at their lab instruments and they realize something that
Speaker 2: sounds well, literally impossible.
Speaker 3: Yeah it really does, right.
Speaker 2: They realized a particle had basically exited their experiment before
Speaker 2: it had fully entered it.
Speaker 3: Which I mean it completely shatters the most comforting constant
Speaker 3: in our macroscopic world, the ticking claw. Exactly. We all
Speaker 3: just expect the arrow of time to only point in
Speaker 3: one direction, you know, one second follows the next, cause
Speaker 3: always comes before effect. But down on the quantum realm,
Speaker 3: the universe operates under a totally different set of rules,
Speaker 3: and frankly, it doesn't owe us intuitive comfort.
Speaker 2: It definitely doesn't. And today we are taking you down
Speaker 2: into that microscopic landscape where the relentless forward march of
Speaker 2: the clock simply stops, or well, in this specific case,
Speaker 2: it turns completely around.
Speaker 3: Yeah, we're talking about negative.
Speaker 2: Time, right. We're going to figure out exactly what it
Speaker 2: means for a particle to spend a negative amount of
Speaker 2: time interacting with an atom. Like. We'll break down how
Speaker 2: scientists actually measured this without breaking the laws of physics,
Speaker 2: and why this discovery reshapes our entire fundamental understanding of causality.
Speaker 3: It's a huge paradigm shift.
Speaker 2: It is because while you experience time as this unstoppable
Speaker 2: one way street today, you're going to learn how time
Speaker 2: can effectively wear a negative sign.
Speaker 3: And to really grasp the mechanics of this, I mean
Speaker 3: we have to basically throw out all our every day
Speaker 3: assumptions about how objects move through.
Speaker 2: Space, just toss them right out the window.
Speaker 3: Yeah, pretty much. We have to look at the physical measurements,
Speaker 3: like the hard data from the laboratory and just trust
Speaker 3: them even when they scream against all our common sense.
Speaker 2: Okay, let's unpack this because understanding the architecture of the
Speaker 2: actual experiment is it's really the only way to understand
Speaker 2: this bizarre result.
Speaker 3: Definitely, you need the physical context, right.
Speaker 2: So, at the University of Toronto, researchers took a cloud
Speaker 2: of ultra cold atoms specifically rubidium atoms.
Speaker 3: Yeah, rubidium is great for this kind of optical work.
Speaker 2: And they cooled them down to just a fraction of
Speaker 2: a degree above absolute zero, just incredibly cold. And the
Speaker 2: reason they do that is so that thermal noise, you know,
Speaker 2: random heat vibrations, wouldn't wash out the really delicate quantum effects.
Speaker 3: You need it as quiet as possible in there, exactly.
Speaker 2: So then they fired this very weak laser pulse of
Speaker 2: photons straight into this freezing cloud of atoms, right, And.
Speaker 3: What's fascinating here is well, classically any object entering a
Speaker 3: medium has to spend a positive duration of time.
Speaker 2: Traversing it, like walking through a door.
Speaker 3: Yeah, exactly. A particle of light enters the cloud, it
Speaker 3: interacts with the atoms, it bounces around or gets delayed,
Speaker 3: and then you know, it exits the other side.
Speaker 2: And there are specific terms for this timing, right there are.
Speaker 3: So the time it takes for that whole pulse to
Speaker 3: travel through the material we call that the group deway
Speaker 3: and the actual physical duration the photon spends inside the
Speaker 3: atomic cloud, like physically interacting with the atoms, that is
Speaker 3: the dwell time.
Speaker 2: Okay, so dwell time, and in a normal everyday macroscopic world,
Speaker 2: that dwell time just has to be a positive number.
Speaker 2: It has to be like if you walk into a
Speaker 2: house to the front door and you leave through the
Speaker 2: back door, it takes a measurable positive amount of time.
Speaker 2: Even if you sprint like as fast as humanly possible,
Speaker 2: the duration is still greater than zero.
Speaker 3: Right, you can't leave before you arrive exactly.
Speaker 2: But in this experiment, the calculations for the photon's dwell
Speaker 2: time yielded and negative value, which.
Speaker 3: Is just it's so wild. The photon arrived at the
Speaker 3: detector earlier than it would have if it had just
Speaker 3: traveled through empty space.
Speaker 2: It took a shortcut.
Speaker 3: Yeah, it took a measurable shortcut through time.
Speaker 2: I know. A lot of the popular coverage tried to
Speaker 2: explain this using like a tunnel analogy.
Speaker 3: Oh yeah, I saw those.
Speaker 2: They said, imagine standing outside a long tunnel, right, and
Speaker 2: you're watching a car drive through it, and somehow the
Speaker 2: car exits the far end of the tunnel before the
Speaker 2: back bumper has even entered the front of the tunnel.
Speaker 3: Which is a fun image, but it's highly misleading.
Speaker 2: Yeah, it just describes the visual outcome. It doesn't explain
Speaker 2: the physics at all. It just makes it sound like magic.
Speaker 3: Yeah, the car analogy falls apart because a photon isn't
Speaker 3: a solid object like a car, right, It's a wave packet.
Speaker 2: And that distinction is basically the entire key to this phenomenon,
Speaker 2: isn't it.
Speaker 3: It is the whole ballgame. You have to think of
Speaker 3: the photon pulse as a bell curve of probability. It
Speaker 3: has this leading edge, a big peak in the middle,
Speaker 3: and then a trailing it.
Speaker 2: Okay, so it's smeared out exactly.
Speaker 3: So when this wave packet enters the resonant atomic cloud,
Speaker 3: the interaction isn't just uniform across the whole thing. The
Speaker 3: atoms and the cloud react to the light based on
Speaker 3: its specific frequency.
Speaker 2: So as the wave packet pushes into the cloud, the
Speaker 2: atoms don't just act like a solid brick wall.
Speaker 3: No, not at all. They actively reshape the wave reshape
Speaker 3: it how so well. The atoms absorb and re emit
Speaker 3: the light. But because of the specific highly tuned frequency
Speaker 3: of the laser they used, the trailing edge of the
Speaker 3: wave packet, the back of the bell curve is highly
Speaker 3: likely to be absorbed by the atoms. Yeah, while the
Speaker 3: leading edge passes through much more easily.
Speaker 2: Okay, wait, I think I have a better analogy than
Speaker 2: the car and the tunnel.
Speaker 3: Let's hear it.
Speaker 2: It's like sending a long freight train through a station. Okay,
Speaker 2: if the station master uncouples the entire back half of
Speaker 2: the train and just holds it at the platform, the
Speaker 2: remaining front half just keeps rolling down the track. So
Speaker 2: if you were to somehow calculate the mathematical center of
Speaker 2: the train that left the station, that center has suddenly
Speaker 2: shifted way forward compared to the original full train.
Speaker 3: That is a much better analogy. Yeah, the peak of
Speaker 3: the wave packet shifts forward because the back half was
Speaker 3: basically eaten by the atomic cloud.
Speaker 2: So that shifting peak gives the illusion that the entire
Speaker 2: wave traveled faster.
Speaker 3: Than light, exactly arriving early. The speed of that peak
Speaker 3: is what we call the group velocity, and group velocity
Speaker 3: can mathematically speaking exceed speed of light in a.
Speaker 2: Vacuum, which always freaks people out.
Speaker 3: Oh totally. But the crucial detail here is that the
Speaker 3: true leading edge of the wave, like the very first
Speaker 3: faint ripple of the signal, that never exceeds the speed
Speaker 3: of light, which.
Speaker 2: Means no actual information traveled faster than light.
Speaker 3: Right, Einstein's theory of relativity is totally safe.
Speaker 2: Few good news for Einstein.
Speaker 3: Yeah, he can rest easy. We can't use this to
Speaker 3: build a time machine or you know, send a lottery
Speaker 3: ticket to our past selves or anything. Because the absolute
Speaker 3: frontier of the signal still obeys that cosmic speed limit.
Speaker 2: Causality remains intact.
Speaker 3: It does. But that realization actually led to this massive
Speaker 3: debate in the physics community.
Speaker 2: Oh really, why, well think about it.
Speaker 3: If the early arrival is just a reshaping of the
Speaker 3: wave packet, like just an optical illusion of absorption, then
Speaker 3: the negative time is just a math trick.
Speaker 2: Oh I see, Like it's just a statistical quirk.
Speaker 3: Right. A lot of physicists argued it shouldn't have any
Speaker 3: real physical significance to the atoms themselves.
Speaker 2: But this is where the twenty twenty sixth experiment goes
Speaker 2: from a mathematical curiosity to a straight up mind bending
Speaker 2: physical reality. Because they didn't just measure the photon leaving
Speaker 2: the cloud.
Speaker 3: No, they didn't. They measured the atoms, which is incredibly
Speaker 3: hard to do.
Speaker 2: Yeah, it brings up this massive procedural wall because I mean,
Speaker 2: anyone who knows basic quantum mechanics knows about the observer.
Speaker 3: Effect, the good old observer effect.
Speaker 2: Right, you cannot look at a delicate quantum system without
Speaker 2: destroying it. Measuring a system collapse is its wave function.
Speaker 3: That's the textbook rule.
Speaker 2: Yeah, So how on earth do you ask a rubidium
Speaker 2: atom how long a photon hung out with it without
Speaker 2: ruining the entire experiment?
Speaker 3: And historically you know that exact barrier kept this entirely
Speaker 3: in the realm of theory because if you hit the
Speaker 3: atom with another laser to measure its state, you literally
Speaker 3: destroy the very interaction you're trying to observe.
Speaker 2: It's a catch twenty two totally.
Speaker 3: But the researchers overcame this using one of the most
Speaker 3: elegant techniques in modern physics. It's known as a weak measurement.
Speaker 2: Okay, weak measurement yea, which I have to say sounds
Speaker 2: like a total oxymoron.
Speaker 3: It does, doesn't it.
Speaker 2: Yeah, like either you measure something or you don't.
Speaker 3: Well, a standard, what we call a strong measurement is definitive.
Speaker 3: It collapses the state completely, But a weak measurement involves
Speaker 3: coupling the system. You want to measure to a sort
Speaker 3: of pointer system, but keeping that interaction incredibly almost absurdly.
Speaker 2: Subtle, subtle, how like I'm trying to think of it.
Speaker 2: Examples like trying to figure out the weight of a
Speaker 2: single snowflake. Oh, I like where this is going, right,
Speaker 2: Because you can't put one snowflake on a traditional mechanical
Speaker 2: scale without it melting or getting crushed. But if you
Speaker 2: let a million individual snowflakes fall onto an incredibly sensitive
Speaker 2: massive scale, you could like deduce the average weight of
Speaker 2: a single snowflake without destroying any specific one exactly.
Speaker 3: Wait, let me rephrase that a bit. But your snowflake
Speaker 3: analogy perfectly captures the statistical nature of the technique.
Speaker 2: Okay.
Speaker 3: Instead of a harsh spotlight that forces one single atom
Speaker 3: to declare its state, they let the passing photons slightly
Speaker 3: rotate the polarization of the light.
Speaker 2: Okay, so a tiny shift, right, It's.
Speaker 3: An almost imperceptible shift. But by doing this repeatedly over
Speaker 3: a massive on some we're talking millions of photons passing
Speaker 3: through millions of atoms, they extracted a statistical trend. They
Speaker 3: found the mean atomic excitation time, and they did it
Speaker 3: without destroying the quantum coherence of any individual interaction.
Speaker 2: So they gently monitor the rubidium atoms while the photon
Speaker 2: passed through, taking this huge ensemble average, and the atoms
Speaker 2: revealed their actual physical experience.
Speaker 3: Of the event, and the hard data is just stunning.
Speaker 3: Depending on how far the light's frequency was from the
Speaker 3: atom's natural resonance, the mean atomic excitation time was a
Speaker 3: highly specific negative value negative.
Speaker 2: Yeah.
Speaker 3: Specifically, it was negative point eight two times the amount
Speaker 3: of time light normally takes to cross that empty.
Speaker 2: Space negative point eight two.
Speaker 3: Plus or mine is a tiny murlder bearer.
Speaker 2: Yeah, that is just I mean, the atoms themselves confirmed it.
Speaker 2: The weak measurements showed the atoms physically experienced a negative
Speaker 2: interaction time. It did. The atom recorded holding onto the
Speaker 2: energy for less than zero seconds.
Speaker 3: Which means the physical system agrees perfectly with the reshaped
Speaker 3: wave packet.
Speaker 2: That is wild, and this dual confirmation is really what
Speaker 2: makes this a watershed moment.
Speaker 3: Oh absolutely. I mean, you have the outgoing photons timing
Speaker 3: arriving early on one side of the experiment, and then
Speaker 3: you have the internal state of the atomic ense recording
Speaker 3: a negative duration on the other, and they match both
Speaker 3: point to the exact same negative value. It builds beautifully
Speaker 3: on this twenty twenty four work by Danielle Angulo a
Speaker 3: from Steinberg and their colleagues. Back then, they show that
Speaker 3: photons made atoms seem to linger in an excited state
Speaker 3: for a negative.
Speaker 2: Duration, Right I remember reading about that.
Speaker 3: Yeah, so this new data just cements it. The atoms
Speaker 3: effectively pre excite. The phenomenon is deeply, deeply tied to
Speaker 3: the non local nature of quantum wave functions.
Speaker 2: But wait, if the atom actually physically experiences the negative time,
Speaker 2: we have to talk about how it knows to do that.
Speaker 2: What do you mean, Well, if the trailing edge of
Speaker 2: the wave is getting absorbed and the peak is shifting forward,
Speaker 2: why does the atom register and negative time only when
Speaker 2: the photon successfully passes all the way through?
Speaker 3: Ah? Okay. That brings us to a concept called quantum
Speaker 3: post selection, which honestly is vital to understanding why causality
Speaker 3: isn't broken here.
Speaker 2: Okay, post selection.
Speaker 3: The negative values only emerge mathematically and physically when we
Speaker 3: filter the results.
Speaker 2: So it's like, okay, it's like we ran a massive
Speaker 2: survey of a million people walking through a giant building,
Speaker 2: but at the end of the day, we throughout all
Speaker 2: the survey data except for the people who walked out
Speaker 2: of one specific exit door. Yes.
Speaker 3: Perfect. The negative time only appears because we are aggressively
Speaker 3: filtering for a very specific outcome.
Speaker 2: The photons that made it completely through.
Speaker 3: Exactly the ones that made it through without being fully scattered.
Speaker 3: That filtering process is the actual engine of the phenomenon.
Speaker 3: Interesting in quantum mechanics, a particle explores multiple probability paths simultaneously.
Speaker 3: So when we pre select the initial state you know,
Speaker 3: firing the laser, and then we post select the final
Speaker 3: state only looking at the photons that transmitted through, we
Speaker 3: are looking at a highly specific subgroup of quantum histories.
Speaker 2: We are essentially isolating the weirdest possible path.
Speaker 3: Exactly, we are isolating a path where virtual processes dominate
Speaker 3: the interaction. In this post selected group. The quantum interference
Speaker 3: creates a situation where the probability amplitudes mathematically demand a
Speaker 3: negative interaction.
Speaker 2: Time demand it. Wow.
Speaker 3: Yeah, And if we connect this to the bigger picture,
Speaker 3: Leading quantum theorist Howard Wiseman helped confirm this. He showed
Speaker 3: that these post selected negative values aren't just artifacts. They
Speaker 3: dictate the actual real physical kick the atoms receive.
Speaker 2: I just keep thinking about the reaction to this. When
Speaker 2: the news broke place like Reddit, you know, our science
Speaker 2: they just exploded.
Speaker 3: Oh it was everywhere.
Speaker 2: People were calling it a bomb on the standard view
Speaker 2: of temporal flow. But I imagine this is also an
Speaker 2: absolute nightmare for quantum computing. How so, Well, if you're
Speaker 2: trying to build a perfectly timed quantum logic gait and
Speaker 2: one of your photons takes a statistical shortcut and arrives
Speaker 2: before it left, I mean that throws the whole synchronization
Speaker 2: completely out of whackry Yeah, oh yeah.
Speaker 3: Managing that kind of temporal jitter is one of the
Speaker 3: hardest parts of scaling quantum processors. But at the same time,
Speaker 3: it's also a massive opportunity, really an opportunity, Yeah, because
Speaker 3: if we can harness and manipulate these negative dwell times,
Speaker 3: it gives us unprecedented control. Understanding these ultrafast, counterintuitive interactions
Speaker 3: allows us to design photonic quantum computers that manage entanglement
Speaker 3: with vastly greater precision.
Speaker 2: Oh I see, so what does this all mean for
Speaker 2: the tech? We could like intentionally use the negative delay
Speaker 2: to correct synchronization errors.
Speaker 3: Theoretically, yes, if one entangled particle is lagging behind, you
Speaker 3: could pass it through a specifically tuned resonant atomic cloud
Speaker 3: to give it a negative time kick.
Speaker 2: Just snap it right back into perfect sync with its
Speaker 2: partners exactly. Now.
Speaker 3: That level of engineering is at the absolute limit of
Speaker 3: physics right now, but it could also revolutionize things like
Speaker 3: slow light technologies and quantum memories member. Yeah, systems where
Speaker 3: we need to store light in an atomic state and
Speaker 3: retrieve it on demand. Factoring in these negative time scales
Speaker 3: allows for theoretically perfect fidelity in those memory transfers.
Speaker 2: Wow. But beyond the immediate technology, this finding fundamentally broadens
Speaker 2: how we even think about the universe, oh completely, because
Speaker 2: it joins this very exclusive club of bizarre negative concepts
Speaker 2: and physics that describe completely real phenomena. We usually think
Speaker 2: of physical properties as having a hard floor at zero, right,
Speaker 2: zero is usually the bottom, But the universe just doesn't
Speaker 2: always agree with us.
Speaker 3: No, it doesn't. And actually temperature is a perfect example
Speaker 3: of this.
Speaker 2: Wait really, because I've always been taught that absolute zero
Speaker 2: is the hard limit most people are It's the point
Speaker 2: where all atomic motion completely stops. You physically can't have
Speaker 2: less motion than no motion.
Speaker 3: Classically, you're right. Temperature measures the average kinetic energy of
Speaker 3: moving particles. But in deep thermodynamics, temperature is actually defined
Speaker 3: by how energy relates.
Speaker 2: To entropy, entropy being disorder. Right.
Speaker 3: Usually, adding energy to a system increases its disorder. If
Speaker 3: you heat up in ice cube, the rigid st ructured
Speaker 3: water molecules break apart and start moving chaotically as liquid water.
Speaker 2: More energy equals more chaos positive temperature exactly.
Speaker 3: But in certain specialized quantum spin systems, like say atoms
Speaker 3: locked in a magnetic lattice, there is an upper limit
Speaker 3: to how much energy the system can physically hold.
Speaker 2: Okay, a ceiling right.
Speaker 3: If you keep pumping energy into this bounded system, the
Speaker 3: atoms eventually have no choice but to align their spins
Speaker 3: perfectly just to absorb that maximum energy.
Speaker 2: Oh wait, so as it reaches maximum energy, the system
Speaker 2: actually becomes highly ordered again.
Speaker 3: Yes, the disorder goes down. Adding energy decreases entropy. That's
Speaker 3: wild and mathematically, the only way the thermodynamic equations can
Speaker 3: handle a state where adding energy lowers entropy is for
Speaker 3: the temperature value to actually flip to a negative number.
Speaker 2: Okay, so wait, that means a system with a negative
Speaker 2: temperature is actually mathematically speaking hotter than a system at
Speaker 2: an infinite positive temperature.
Speaker 3: It is, if you put them right next to each other,
Speaker 3: heat would flow from and the negative temperature system into
Speaker 3: the positive one.
Speaker 2: My brain just melted a little bit.
Speaker 3: It's totally counterintuitive. The math creates a negative, but the
Speaker 3: physical reality is just a different extreme state of energy allocation.
Speaker 3: We see similar mathematical leaps with effective negative mass in
Speaker 3: certain superfluids, where pushing an object makes it accelerate backward
Speaker 3: towards you.
Speaker 2: Which brings us to honestly the biggest philosophical ramification of
Speaker 2: all this.
Speaker 3: Let's hear it.
Speaker 2: If time can be negative in these quantum measurements, if
Speaker 2: the rubidium atoms and the photons are literally trading negative durations,
Speaker 2: what does that actually say about our everyday experience of time.
Speaker 3: It's a deep question.
Speaker 2: We treat the ticking clock as a fundamental, unbreakable property
Speaker 2: of the universe, but this experiment makes time look incredibly fluid.
Speaker 3: It does, and it heavily reinforces the theory that time
Speaker 3: is just emergent, emergent like wetness.
Speaker 2: What do you mean, well, a single molecule of H
Speaker 2: two O isn't wet, oh right, right, Wetness is an
Speaker 2: emergent property of billions of water molecules all interacting together.
Speaker 3: That's a great way to look at it, and time
Speaker 3: might operate the exact same way. Really, Yeah, the relentless
Speaker 3: forward earrow of time, the strict flow of cause and effect,
Speaker 3: it's likely a macroscopic illusion. It's created by the overwhelming
Speaker 3: statistics of entropy. At the human scale, there are so
Speaker 3: many particles interacting that the statistical probability of time flowing
Speaker 3: backward is essentially functionally zero.
Speaker 2: But when you zoom all the way down to a
Speaker 2: single photon interacting with an ultra.
Speaker 3: Coold atom, the rigid rules just wash away. At the
Speaker 3: fundamental quantum level, time isn't a rigid track at all.
Speaker 3: It's a localized statistical measurement, and sometimes those statistics yield
Speaker 3: a negative number. Negative time joins things like superposition and
Speaker 3: quantum entanglement as just another profound reality that proves our
Speaker 3: macroscopic intuition is just a tiny, tiny, simplified slice of
Speaker 3: the whole picture.
Speaker 2: So we started this with the assumption that time is
Speaker 2: an unstoppable one way street. But down at the microscype
Speaker 2: topic scale, looking at a cloud of ultra cold rubidium atoms,
Speaker 2: that streak completely dissolves.
Speaker 3: It really does.
Speaker 2: Using the delicate art of weak measurements, scientists allowed us
Speaker 2: to look closely at a wave packet traversing a medium.
Speaker 2: The leading edge pushes through, the trailing edge gets absorbed,
Speaker 2: and the peak leaps forward, seemingly outrunning the speed of
Speaker 2: light itself.
Speaker 3: It's an incredible feat of observation, and.
Speaker 2: By actually measuring the atoms themselves, we saw the physical
Speaker 2: reality of a negative dwell time. The atom literally records
Speaker 2: holding the photon for less than zero seconds.
Speaker 3: And it does all of this without breaking causality, and
Speaker 3: it doesn't violate relativity.
Speaker 2: Information is still bound by the speed of light exactly.
Speaker 3: But it forces us to recognize that our physical models
Speaker 3: are vastly more complex than a simple ticking clock. Physics
Speaker 3: just marches on revealing these hidden layers of strangeness that
Speaker 3: were always there, just waiting for the right probe to
Speaker 3: coax them out of the equations.
Speaker 2: The lab has spoken. By asking the atoms directly, scientists
Speaker 2: prove that time, at least in quantum interactions can wear
Speaker 2: a negative.
Speaker 3: Sign, it's an absolute triumph, which.
Speaker 2: Leaves us with a massive lingering question. If time itself
Speaker 2: can effectively reverse in these quantum interactions, what other impossible
Speaker 2: signs are lurking in our physics equations right now?
Speaker 3: Oh, there are probably plenty.
Speaker 2: Just dismissed as math tricks, waiting to be physically measured.
Speaker 2: Could these findings eventually be the missing link to understanding
Speaker 2: how space time itself emerges from the quantum foam.
Speaker 3: That is the multi billion dollar question.
Speaker 2: The universe is infinitely fascinating. The next time you look
Speaker 2: at a ticking clock, or just assume a rigid chain
Speaker 2: of cause and effect, Remember the rubidium atoms.
Speaker 3: Remember that things aren't always what they seem exactly.
Speaker 2: Remember that, deep down in the fundamental fabric of reality,
Speaker 2: that clock might just be ticking backward. Keep questioning everything,