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Apr. 25th, 2017 @ 04:53 pm QM: Why spooky action at a distance would be mysterious if it happened
Last post, I decided that what's "really there" for fundamental particles is typically a quantum thing, specifically, a probability wave of possible values a particle can have which appears to collapse into one particular place only when its interacted with.

However, this "collapse" sounds very suspicious. If two different particles emitted from the same particle decay (or something?) are known to have opposite spins, but not what those are, do you get all the usual wavelike behaviour, can each self-interfere, etc? Yes, of course. And yet, when you finally measure them, lo, the spins are still conveniently opposite.

Something that looks like collapsing to a single answer seems to happen, because when we measure them, we always do get a single answer. But that's not an event. If you measure one, does a spooky force reach out across the room to force the other to collapse at the same time? Does it collapse the value you measure, but still allow other properties of the particle to continue being multiple? That looks awfully like what happens, but it should seem wrong to start with, even before you ask "if you measure one particle, does the other know to wait until you interact with it, but store the answer you're going to find until then" and "if you measure them both a long way apart, does the collapse rush faster than the speed of light (aka backwards in time) to make sure both answers agree with each other?"

Any theory involving particles "knowing" or "waiting" or "choosing" depending on how you measure them sounds very unlike physics.

And yet, the particles go on behaving like probability waves until you measure them, and if they came from a shared source, then when you measure them, they DO agree. Just as if this spooky shit was happening. What might be going on?

Hypothesis 1

Whenever one particle collapses, a spooky force travels faster than the speed of light to the other particle, and then hangs around telling it what value it will have when it's finally measured.

This *works*, but hopefully you can see why it doesn't seem correct.

Hypothesis 2

Just like hypothesis 1, but we try to avoid thinking about it. This is not really satisfying, but it works and is a pragmatic default for many physicists. (Sort of Copenhagen interpretation?)

Hypothesis 3

Even while a particle is still smeared out across a probability of many potential positions/values, it has a hidden property which tells it how it's *going* to collapse when something interacts with it. Like, not necessarily "hidden", but basically some sort of determinism.

This is roughly Hidden variables interpretation (right?)

This would be fairly satisfactory except that it turns out it's impossible.

This is not very mysterious or controversial, but involves more simple probability than I can manage to wade through. Look up the EPR paradox or the Bell inequality. The idea is, you choose something like polarisation angle that could be measured at many different angles. You randomly choose to measure at different angles for two particles known to have opposite polarisation. There are various correlations between the probabilities when you measure the two particles at an angle to each other (the detectors neither parallel nor orthogonal). You can prove that no possible hidden value would make all those correlations true at once, but QM does and that's what's actually observed.

I can't really prove this to myself, let alone anyone else, but AFAIK no reputable physicists doubt that it's correct, only maybe what it means, so I'm willing to accept it as true.

There are still edge cases, like, people argue whether the experiments have ABSOLUTELY DEFINITELY proved this spooky collapse effect would have to go faster than the speed of light, rather than going at a possible speed (but depending what exact moment sets it off, etc). But I don't find any of that very persuasive. A spooky collapse effect which is triggered by measuring a particle and goes at the speed of light or below, while not ABSOLUTELY DEFINITELY ruled out, doesn't sound at all likely. I don't think anyone seriously expects that if they make the distance apart in those measurements a bit bigger, they'll suddenly get difference results: that's not how you expect physics to happen.

Hypothesis 4

Those weird quantum probability waves don't only exist for tiny particles, they happen just the same for everything including macroscopic objects, humans, etc, but you can't observe the effects except for tiny things (because to see interference you need something isolated from other particles, and you need to be able to detect its wavelength, which is way too small for anything bigger than a molecule).

I'm still working on understanding *why*, if that's true, it produces the effects we see. But most physicists, even ones who don't like this line of reasoning, seem to agree that it *would*.

This makes everything above non-mysterious. How does the collapse effect move around? It doesn't. Every "collapse" is just another probability thing of a scientist (and all the other macroscopic stuff) interacting with a particle and becoming two never-interacting possible scientists, one observing A, one observing B. We know both happen. We know, when we measure things light-hours apart and then compare notes, that we will be comparing notes with the version of the other scientist who observed the opposite polarisation to what we saw, while our shadow twin will be comparing notes with the other scientist's shadow twin.

The multiple non-interacting versions of the macroscopic world are called "many worlds" or "parallel universes" which admittedly makes them sound very implausible.

It seems like, this leaves some things to ponder, but resolves a very large part of the things people find mysterious. And yet, many physicists really don't like it. I need to read the bits of Scott Aaron's book about different interpretations[1], because I trust him to know more about this than me and he doesn't seem convinced.

Footnote [1]

The hypotheses above are called interpretations. I don't know if my ones exactly map onto the real ones. The name is because they all predict the same results, and yet seem quite different.

You can argue, "they're the same", but I don't quite agree. See for instance space outside our light cone -- we have no way of observing it, so the hypotheses "it's got physics just like ours but with different stuff there" and "it's all purple unicorns" are both possible, and yet, the first one seems a lot more like actual reality.

In both cases, it sort of doesn't matter, but you can imagine (a) which answer is most plausible, most useful, easiest to work with, or least ridiculous (b) if we're wrong and there IS some difference, which one would actually be found to be the one that exists.

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