For a few years now we’ve been hearing a lot about quantum computers and quantum entanglement and “spooky action at a distance” wherein lay members of the public, like myself are told that a particle that is here can be put in an entangled state with a particle somewhere else and both of them will reflect what is going on with the other. Distances mentioned have been meters. So, I have two questions:
How fast do the changed states travel? Has it been measured or is it just theory?
How far apart can this work in theory? Yes, we know in practice that it can be meters, but could it be miles? Light years? Across the Universe?
Again, I’m no physicist (although I did do a degree in physics), but as I understand it, there’s no limitation in theory on the distance between the photons - it could well be across the Universe.
The important thing to understand, as with all quantum entanglement experiments, is that relativity is not violated. There’s no FTL information transfer from one photon to the other. Beyond that, I’ll let the experts speak.
Entangled particles are entangled in the sense that measuring a parameter of one immediately defines the parameter of the other. This is independent of the distance between them, and the moment the parameter of one particle is measured, the parameter of its entangled partner becomes defined.
Say, for instance, for a given entangled pair, either one (but not both) could have a parameter of spin “up.” Measuring one particle sets it spin as “up” (it does not have this parameter set ((i.e. the spin is indeterminate)) until it is measured) and instantaneously its entangled partner, regardless of how distant it is, assumes “down” spin.
Because the particles do not assume up or down until measured, you get the notion of the spooky action at a distance.
What the whole thing tells me is that we do not know what particles are and more specifically we have no idea what space is. But that’s a different thread.
ETA: the distance, in theory, could be the breadth of the universe (a whole different thread, again).
Unless I am mistaken, we haven’t come up with the Big TOE yet to unite Quantum Mechanics and Relativity, so it seems to me a teeny hasty to say “relativity” is not violated…more like the one can’t speak the other’s language, or that the tiny world and the macro world are described with such disparate mathematical models that concepts in one aren’t easily translated to the other–another thread again.
I am working on my own big Theory of Everything, but haven’t finished it yet.
Is it correct in that you can’t make a particle spin up or down it can be either or and you can’t know until it is measured? So could entangled particles be viewed a pair of shoes one left one right and if you seperated them without looking you don’t which one is the left or right until you look and when you do see a left shoe you know the other one is right no matter how far apart they are. Is this right? If so no actual communication is passed between them so FTL is not happening.
“Here man,” he said, “Take this box. Our super-secret galactic space club will meet near Andromeda if it is spin up, or the Crab Nebula if it is spin down. We’ve got a more advanced version with 32 entangled pairs capable of selecting over four million unique locations this way, but you’re new here.”
I’ve never understood how the following two sentences are compatible:
Measuring the state of one particle in an entangled pair instantaneously brings the other particle in the pair to have a certain state.
No faster-than-light transfer of information occurs.
If my actions here instantaneously bring about a change there, then how is it that what has happened there doesn’t contain information about what has happened here, and how is it that the information didn’t get from here to there faster than the speed of light?
Every time I go poking around looking for information about entanglement, I fail to achieve clarity. It may be that this is the kind of thing that is really beyond a layman’s explanation or something.
I used to think I understood it, and thought it was just an epistemological point–that since the two particles have interacted in a certain way in the past, we know the state of one once we’ve measured the other. But apparently this isn’t the whole point. If I understand correctly, the present act of measurement is supposed to make it the case that the other particle has the state it has. That’s what I don’t get, or rather, I’m fine with that, but then I fail to see how it doesn’t constitute FTL information transfer.
What I don’t understand is: Why do we say information is not transferred? If you can influence the ‘up’ or ‘down’ state here, and it instantly affects the entangled partner somewhere ‘over there’, don’t you now basically have FTL digital communication? What am I missing?
That’s what I used to think, but popularizations I’ve read seem to be saying that somehow, so to speak, my seeing that it’s a left shoe here makes the other shoe a right shoe. Before the measurement, that wasn’t necessarily the case, so they say. If I’m understanding correctly.
That’s why I’ve never understood how there could be no FTL information transfer. It seems to me you could have two people, each with a shoebox with one shoe in it, and each agreeing not to measure the shoe until a particular time in the future. One stays on Earth, the other goes to Betelguise. Then at the agreed upon time, the one on Earth looks at his shoe. Then a split second later, the other guy looks at his shoe. It seems like the guy on Betelguise now knows, and can act on, information concerning the first guy’s shoe, the instant that shoe has acquired its state. (Because its supposed to be there’s no determinate state before the measurement, see?) That’s faster than light information transfer, isn’t it?
I don’t see a way to send signals this way, and maybe that’s what is meant by saying no information is transferred. But there are ways to coordinate activities in a way contingent on the measured state of the shoe remaining on Earth, and it seems like that requires faster than light information transfer.
I don’t get it. I don’t get it at all.
ETA: Hm, maybe the technical sense of “information” that people use in this explanations implies that measurement does not change informational states? A quantum superposition has the same information as a measured particle? Maybe?
That’s part of my question, I guess. It seems to me if you can measure the state, you’re not far off from setting the state. And if you can set here & measure there, you have communication. But, people in “the know” say it doesn’t work that way - and I don’t understand why.
Please clarify this comment. Who said ‘signals are sent,’ and in what way did that betray a fundamental failure to grasp the concepts? Answering these questions may help clear up some of the confusion some of us are expressing in this thread.
The shoes are always opposite, though. Imagine that the guy on Betelgeuse accidentally opened his box first. What difference would it make? He would still find a left shoe in there, and the guy on Earth would still find a righty. In other words, nobody can tell whether they measured it first or the other guy did. Because they’re always opposites no matter what, and when you measure it you collapse the wavefunction and get a particular answer no matter what. In other words, it doesn’t matter whether the other guy opened his box first or not, in both cases it is impossible to tell.
It’s not like the guy on Betelgeuse can watch the transition and say “The guy on Earth JUST NOW opened his box”. The wavefunction collapses when you measure it either way.
If I take a shot at this, please consider it the blind leading the blind. But here’s how I look at it.
Quantum theory, which is internally consistent and has predictive ability (two pretty good litmus tests for being “right”) says that after an act of measurement causes the waveform describing the system to collapse, the result is random. In other words, say some, the shoe is neither right nor left until they have been measured. The spin is neither up nor down until after the waveform collapses; once the two particles (now at points A and B) are disentangled by the act of measuring one of them at point A, the particle at point A becomes (randomly!) up or down and the particle at point B is constrained to the opposite paramater, instantaneously.
Was “information” transferred? Well, most of the arguments around that are more linguistic in nature. Certainly no person at point B could have any information about what happened at point A, so in the ordinary sense no useful information is transferred. But at a deeper level there have been a couple main complaints about the whole conceptual theory…
The first, of course, is Einstein’s (along with Podalsky and Rosen) who argued that the assignment of a given parameter could not be truly random. There must be some underlying local (unknown) variable which actually affected the assignment of the parameter. “God does not play dice with the Universe” is a shorthand way to describe this reaction.
The second, in my highly undereducated opinion, is deeper still: we are fairly clueless about what particles are, and utterly clueless about what the matrix (space) in which they exist, is. Therefore even though a mathematical model for how things appear may be descriptively and predictively accurate, it doesn’t mean it’s complete. Non-local behaviour that seems superficially to violate Relativity may become a lot less spooky when we understand what it means to be “local” in the first place. Consider that if space is not Euclidean on a large scale, the intuitive notion of a 3-D separation (and the consequence of throwing Time in there to get a “point” in Spacetime) falls apart completely. We talk loosely about space expanding as if it were expanding into something, but obviously it’s not; those are linguistic translations of mathematical models that may well describe what happens but don’t really adequately describe what is.
It’s obvious that particles are neither waves nor physical bits nor both. And until we figure what they really are, and what space really is, I think it’s fine to say quantum mechanics makes no sense in the big world (puts you in the company of Einstein) and that Relativity can’t describe the teeny world (puts you in the company of Bohr, Heisenberg, Planck and a bunch of others).
I don’t believe that’s true. If I am understanding the popularizations correctly, until the guy on Earth looks at his “shoe” (and remember, we’re using shoes as standins for subatomic particles here), there is no fact of the matter as to what kind of shoe is in the box near Betelguise. If the guy at Betelguise opened his first on accident, then the “shoe” might very well turn out to be right shoe instead of a left shoe.
Right, but the idea I was trying to illustrate was that they agree to open their boxes at particular times, without trying to find out whether the other guy opened his box at the right time or not. Of course the whole setup relies on each guy doing his part correctly, but that’s no different than almost any other coordinated activity between humans that you can think of, so I’m not sure what special significance you think that consideration is supposed to have in this situation.
Now, something else I have never understood is what “instantaneous” is supposed to mean in these descriptions of entanglement, since simultanaity is relative to a frame of reference. I’m not sure what is supposed to be meant by saying the changes of state happen “at the same time.”
But apparently, Bell’s theorem has been demonstrated empirically to be true, and precludes any kind of local “hidden variable” theory like this.
I have never been able to understand how this was demonstrated–again, possibly this is simply beyond a layman’s ability to understand?–but it is a result universally accepted amongst physicists. I just wish I could make sense of it.
You and I wish to determine one of n locations to meet, agreed upon in advance. n is large to escape the galactic police being able to just lie in wait at every location for us. We need log[sub]2[/sub]n bits to communicate this. So we create log[sub]2[/sub]n entangled pairs and put them in our boxes, which are synchronized locally, and my box is set to measure some binary property first. We each fly away at similar speeds (to avoid clocks slowing down relative to each other) and at the predetermined time, my box measures, collapses the waveform, and your box reads out shortly afterwards. Now we both know where to meet, according to the table mapping binary numbers to locations. ETA: of course your box reads out the negation of the states, relative to mine.
No, the information traveled between you and I when we set up the experiment. We each know that our pairs are opposites when we start, and nothing else changes when we open our boxes. It’s like blindly cutting a coin in half before we leave: If you have the head side, then you automatically know I have the tail side, even if we wait until we’re a billion miles apart to check which half we have.