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.