I am not an expert by far…or even a journeyman…
but I’ve looked at spooky action at a distance this way:
Using the ‘many-worlds’ explanation of quantum mechanics when 2 objects are entangled and one is observed, like I open the shoe box and see the left shoe, then the universe splits into 2 and I went the left shoe ‘path’. The other box is automatically the right shoe in this universe…no splitting is needed…hence ‘entangled’.
No idea if that is a good way of visualizing it though…
There is nothing wrong with what you’re saying here. On the other hand, there’s also nothing useful with it. If it helps to keep your head from exploding, great (and in fact, I know a few physicists who do like pretty much this explanation, for this reason). But it won’t really tell you anything new about the results of any experiment.
How do two particles become entangled?
How do you seperate two entangled particles without effecting their entanglement.
How do they store and transport those seperated particles during the experiments?
Once one is determined to be a left shoe (and hence, the other a right) is that the end of it? Can they be measured again in 10 minutes and then the left is a right and visa versa?
Can the particles themselves actually be manipulated (thereby effecting their entangled pair)? Or are we just measuring one and confirming our assumption about the second after measuring the first. Can we actually make the one particle behave as we want by manipulating the particle we have? If not. What’s so special about this?
The easiest way to get two entangled particles is to create them in the same process. For example, if I have a pion that decays into an electron and a positron, the electron and positron will (at least initially) be entangled. They’ll remain entangled until something is done to them which affects the state of their spin (practically speaking, for charged particles, this usually involves putting them in a magnetic field).
Once you measure the state of a particle, it is no longer entangled, and will remain in whatever state you measured it to be in until you do something else to affect the state. If you perform the same measurement twice in a row, you’ll get the same result.
When you measure the state of one member of an entangled pair it INSTANTLY influences the state of its partner. The effect is not limited by the speed of light.
Indistinguishable’s answer is very good. It might help to understand this if you asked yourself the following question:
If I want to tell someone that a system is in state 1 how would I do this via correlated pairs? You’ll find you can’t to do it.
And the answer to my two questions in English is? I ask you because you are the one who seems most likely to have the knowledge to answer it. Or does you being in the know require that I remain uncertain?
Both theory and experiment so far say “instantly”. Definitely faster than the speed of light.
The record so far is 144 kilometers, I believe. But there’s no theoretical reason to think there’s an upper bound.
Thank you. So if I ever write my sci-fi stories I can posit a communication device that will work instantly anywhere in the universe. And on a more practical level, boomer subs can be in constant communication no matter how deep and have unbreakable code using a one time pad. (One time pad only necessary if you assume someone else has secretly entangled your particles.)
You can’t use quantum entanglement as an information channel (i.e., you can’t send messages over it; you can only read what it produces of its own accord, so to speak).
Huh?
Instantly in what frame of reference? (Different ones will count different events as being simultaneous.)
-FrL-
So what? No one has said anything about simultaneity. The effect must be instantaneous not necessarily simultaneous.
Heh. That just adds to the weirdness! Consider two entangled particles A and B. In some reference frames A may be measured first. In other reference frames B may be measured first. So if a message is being sent, is it travelling from A to B, or B to A?
That’s correct and normally this would mean that causality gets all screwy. But there’s no conceivable measurement that an observer can do that will distinguish the two particles.
“Well,” the appropriate response will come, “what does instantaneous mean if not ‘The effect is simultaneous with the cause’?”.
As you would likely agree, such a question no more has or needs an answer than a question like “Is X standing still and Y moving, or is Y standing still and X moving?”.
All the more reason to adopt the perspective that wavefunction collapse is not a physical phenomenon, as such; at least, it is not the sort of thing which has a genuine spatiotemporal location. There is no substantive question as to whether the wavefunction collapse occurred at A or instead occurred at B. And being not a spatiotemporal event, wavefunction collapse has no legitimate spatiotemporal location in relation to other events; in particular, there is no well-formed question as to whether it is part of a causal chain surpassing the speed of light. Trying to speak that way should be regarded as a category error.
Why does it need to be instantaneous? The only way to compare the two measurements is to bring the results together, which can only happen at the speed of light.
You asked three questions, I think:
*"1. How fast do the changed states travel? Has it been measured or is it just theory?
In fairness to Chronos, I think he’s giving you the specifics around the answer to your second question, which was “Has it been measured or is it just theory?”
The answer to that question is, “Actual measurements have been made from which we infer an assortment of things, among which is that the assignment of spin (the paramter those variously-angled detectors are measuring) does not occur until a measurement takes place.”
This is a critical inference, because there is a huge difference between simply deciding which one of an entangled pair has a previously-assigned spin (this lets you know instantly what the other far-away one must be), and assigning the spin at the moment of measurement (this creates the spooky action at a distance which is both instantaneous and simultaneous or whatever other linguistic term you want to apply to it).
So the short answer is, “Yes, there are supporting experimental measurements for the concept.” The longer answer, elaborated by Chronos, is what those measurements involve and why the conclusion is drawn that the spins of a particular particle pair are truly randomly assigned at the time of measurement. Because this assignment is random, the argument goes, it cannot be that we are simply establishing which spin a particle already had when it separated from its entangled partner. The inference drawn from what happens at detection is that the particle has neither up nor down spin until it’s measured, and when it’s it randomly becomes either up or down, immediately constraining its partner to the opposite spin.
There’s no weasel room here around time frames, instanteous v simultaneous and so on, because there is no time involved: it’s zero. That’s what makes it spooky in the first place. It’s not some propagation of information across the distance between point A and point B that is somehow “faster” than the speed of light. Speed needs time in the equation to exist. The time involved is not small. It’s zero.
ETA: Should read, “…and when it’s measured it randomly becomes either up or down…blah blah blah”
Missed the edit window for the key point I was babbling on about.
Exactly. (Though I might not have put the question in terms of causes and effects.)
I think I see what you mean here. Do you mean to say that a statement like “A waveform collapses” should be treated more like “2+2 = 4” than like “ball A hit ball B”? As in, the usefulness of “2+2 = 4” does not involve the sentence’s being true at a location in space and time?
Even if waveform collapse doesn’t happen at a location, it seems like the scientists (at least in their popularizations) make a pretty clear point about the thing that’s happening here happening “instantaneously” or “simultaneously,” and I’ve always wondered what they mean by this. Is it merely loose talk?
-FrL-
Agreed. And as you are well aware, the fun for theoretical physicists is coming up with wacko ideas for which no experimental and/or predictive models can be designed.
Any such ruminations are not “useful” exactly, but when general relativity and quantum mechanics are at such odds, it’s obvious that there is something else going on which will more beautifully explain what really happens. And recognizing that does keep my head from exploding over particle entanglement.