analogue gravity, entanglement, non-locality, physics, Popular science, quantum mechanics, science
I have the good fortune of attending this year’s Nordita workshop for science writers, which is focused on quantum mechanics. It’s been a fascinating and educational experience, so I thought I should make an effort to write a post about the workshop each evening. Since I’m writing about an unfamiliar subject following a day full of lectures, these posts won’t be nearly as polished as the other stuff on Inspiring Science; I’m tired and I may have misunderstood things, so there will probably be some mistakes and clumsy explanations. On the other hand (and with that caveat), I think it’s better to share whatever understanding I’ve gained today and hope that someone will correct me where I go wrong. I’m not going to make any kind of attempt to be thorough, but simply write a bit about a couple of the things I found most interesting each day
The day started with a two different aspects at the heart of quantum weirdness, entanglement and superposition. Entanglement is when two or more particles are related in some way so when you measure the quantum state of one of them, you immediately know something about the quantum states of the others, even if they’re far enough away that you have no access to them. So if particle A and particle B are entangled and you measure the spin of particle A, you immediately know the spin of particle B, even if it’s light years away. Superposition is, in some ways, a closely related concept; it’s the idea that the particle(s) actually exists in all the possible different states until you measure it. It doesn’t really need a human to make the measurement, though; any kind of interaction with the environment — the particle giving off heat or another particle bouncing off it — will do the trick. Any interaction which carries information about the system is enough to force the superposition into one state or the other, so superposition doesn’t tend to last for very long.
To me, the interesting part of this is trying to think about what’s “really” going on. Entanglement and superposition together seem to imply what Einstein called “spooky action at a distance”: entangled particles A and B exist in a superposition of many possible states, and when you fix the state of A by measuring it, this causes the state of B to simultaneously become fixed. This raises the strange possibility of non-local causality: the action of measuring A causes B, which is not in the same location, to take a particular value. If you don’t like that idea, you can say that the state of the particles is undefined until you measure it. For example, the spin of the particle isn’t both up and down before measurement; it’s undefined, and the particle isn’t “real” until you measure it. In this interpretation, you don’t have to deal with non-local causality, since measuring the system (of entangled particles) just makes it “real” instead of undefined. Quantum mechanics won’t let you have it both ways; if particles are “real”, there has to be non-local causality, but if you think things aren’t “real” until they interact with something, you can stick to strictly local causality. From what I understood, quantum physicists don’t entirely agree on which interpretation is better.
Analogues for Gravity
Beyond the event horizon of a black hole, the gravitation pull is so strong that nothing, not even light, can get out. The idea behind analogue gravity is to create an analogous situation in a fluid. Imagine a stretch of water that’s gradually flowing faster and faster. Sound waves in the water will propagate in every direction as long as the water flows more slowly than the speed of sound (in water). But once the flow speed becomes supersonic — that is, faster than the waves’ speed — the waves can only move in one direction. The subsonic/supersonic transition is an acoustic horizon; sound waves beyond the horizon can’t travel out, but are dragged onward by the water’s flow.
If the transition to supersonic flows is done in a way which avoids shockwaves, the mathematical description of the system can be rewritten in a form analgous to the equations for waves propagating in a curved spacetime — that is, near the gravitational well of a black hole. In other words, relatively simple experiments involving flowing water can be used as analogues for some of the things that happen around black holes! Of course, the physical basis of the two systems is completely different, so you can’t actually map the findings from the water experiments directly onto black holes, but the fact that certain things emerge in these (much simpler) analogue systems raises the quesion of whether similar aspects of black hole physics are also somehow emergent and what the underlying dynamics would be. It’s even possible that gravity itself is somehow emergent, just as the gravity-like behaviour in the analogue systems emerges from the dynamics of the flowing water.
The last session was on quantum computing, and though I’d really like to write about it I’ve run out of energy. Maybe I’ll include something about it in the next post, or maybe I’ll save it and do a whole post on quantum computing one day.
Thanks for sharing! I’m happy you’re finding the time and energy to write about the day’s lecture. Maybe you could write more about entanglement in the future? 🙂
Thanks! I’m really enjoying the workshop and I’m quite happy to try to share what I’m learning. I’d definitely like to revisit some of these subjects and try to explain them better in later posts, so yes, I’m planning to write more about entanglement sometime! 🙂
Thanks for sharing – it sounds like an awesome workshop! I really enjoyed reading about analogue gravity. It’s a very neat idea and you explained it very well!
In terms of quantum weirdness, what does it mean that a particle isn’t “real” until you measure it? Doesn’t it exist, does it spring from nothing if you measure it? Or does it exist in some “unreal” sense?
That’s a good question, and unfortunately it’s also a bit challenging to answer. The key point is that each of the entangled particles is in a superposition until you measure it. For example, until you measure an entangled photon, it could have horizontal (H) or vertical (V) polarization. (Don’t worry about what H or V polarization means for now.) The issue of the particle not being “real” is a matter of how you interpret the superposition and the effect of measurement (whether it’s a person making a measurement or some other particle bouncing off it).
There’s one approach that says that the particle in superposition is both H and V polarized; when you measure it, the superposition collapses to one or the other, and that will also determine the polarity of the entangled particle. From this perspective, the particle has a defined stated (both H and V) before you measure it, so there’s no issue of whether it’s “real” or not. But then you have to accept that measuring this particle has an immediate causal impact on the particle entangled with it, despite the distance separating them. This is the “spooky action at a distance” that people don’t like. One reason to dislike it is that it may not always be clear which particle was measured first — for example, someone moving past quickly in a spaceship may see the two measurements as simultaneous, while we think one of them happened first.
The other way to think about the entangled particle is to say that its state is undefined before it gets measured. So it’s not that the particle is both H and V polarized, but that its polarity is undefined. (Bear in mind that that’s also different from having no polarity.) It’s tough to imagine what it means for a particle to have an undefined polarity (or spin, or whatever other state), but that’s what’s meant by “not real” in this context. I’ve been struggling to come up with a good image or explanation for this, but I’m not sure I have a good enough feeling of how it works myself. The best I can do is pass on something Rainer Kaltenbaek said when I asked him about it today, which is that all of the information about the polarities of the entangled particles is in the correlation between them (ie, the entanglement), so there isn’t really any information about the polarity of the particles themselves. I’m not sure if that will help others, but it helped me a bit.
It’s worth reading what Joy Christian said about all this, see Quantum untanglement: Is spookiness under threat?. Unfortunately Joy has been castigated for this. He challenged quantum weirdness, and was subjected to opprobrium for doing so.
Hmmn. That flowing-water analogy for gravity is misleading. Space isn’t flowing in a gravitational field. Instead the coordinate speed of light varies with gravitational potential. A better analogy would feature a variable speed of sound, see sonar, which reduces to zero at the black hole event horizon and at the beginning of the universe. A good question to tease this out is “Why doesn’t the light get out of a black hole?” You stand on a gedanken planet shining a laser straight up. The light doesn’t slow down or fall back or curve round. Now make the planet more massive, and the light still doesn’t slow down or fall back or curve round. There is no point where this starts to occur, even when you take it all the way to a black hole. So why doesn’t the light get out? Some will refer to the waterfall analogy, but it’s misleading. A gravitational field alters the motion of light and matter through space, but it doesn’t suck space in. We do not live in a Chicken Little world. The sky is not falling in! This Baez article is worth a read: