While reading an article on entanglement of particles I was struck by this bit: “The theory of quantum mechanics predicts that two or more particles can become “entangled” so that even after they are separated in space, when an action is performed on one particle, the other particle responds immediately. Scientists still don’t know how the particles send these instantaneous messages to each other, but somehow, once they are entwined, they retain a fundamental connection.”
If the particles were separated by more than a fraction of a meter, so that scientists could measure realtivistic effects, would they find that “instantaneous” or “immediate” really means “separated in time by the speed of light”, or would the entanglement propagate faster than the speed of light? Does anyone know if this has been explored?
Which leads me to a related question:
How good are our instruments nowadays? How far away from each other do objects have to be for scientists to be able to measure relativistic effects?
“Immediately” is a relative term, especially at a distance. “Immediately” could be a reporter’s incorrect interpretation.
If the entanglement travels faster than the speed of light, then how much faster? According to the Theory of Relativity, nothing can travel faster than the speed of light, including information. If the entanglement persists at a distance, you could theoretically construct a communication device that would be able to predict events in the future.
One is that the connection exists and can be instantaneously detected. Instantaneous is instantaneous. There are thousands of experiments on this (and hundreds of previous threads). There is no doubt that entanglement exists in just the way it is described. How can you ask if it has been explored when your own link cites an important experiment from Nature?
The other point is that - as far as known - no information is transferred and no communication between the entangled particles has ever been discovered. Therefore no violation of relativity exists, no matter what Einstein believed. Nor you can use it to send information faster than light or look into the future or anything else.
Again, all these points have been covered in zillions of threads. You should search those and presumably some of the respondents will post again here. But entanglement is interesting precisely because all the things you say about it are wrong. I’ll let the experts give the technical details.
“The speed at which entanglement effects travel” is not a well-defined concept. It’s not even always well-defined which particle collapsed the wavefunction of the other: From one reference frame, it might be A that collapsed the wavefunction of B, while from another reference frame, B collapsed A. Trying to bring in concepts like “speed” and “instantaneous” just muddies the waters even further (and it’s not like they weren’t muddy enough to begin with).
Create two entangled particles and send them off in opposite directions toward two scientists, Alice and Bob. When Alice measures the spin of her particle, it’s pointing up, and when Bob measures the spin of his particle it’s pointing down – they are always mirror images of each other.
How would you use this set up to send a message? Alice can’t set the spin of her particle. It’s going to be randomly up or down. And the same is true of Bob. They can only be sure that the behavior of the two particles was linked by comparing notes after they’ve made their measurements.
This might seem pretty unimpressive. Clearly when the entangled pair was originally created, one was set to be spin up and the other to be spin down. But … it’s possible to set up the experiment in such a way that you can rule out the spins being set in advance. (It’s complicated to explain … take my word for it.) Alice’s particle randomly picks its spin at the moment it’s measured. As does Bob’s. They just always happen to pick the opposite of each other.
If the particles are exchanging information “instantly” to achieve the observed effects of entanglement, experiments indicate the speed of transfer has to be at least 10,000c. Yes, that’s 10 thousand times the speed of light.
The measurements will always correlate no matter whether their seperation is timelike or spacelike. So yes for some reference frames it can appear to be instantaneous.
Sorry, but that’s not entirely true. There is, in fact, an instantaneous connection between entangled particles, and it can be used to convey information “instantly” between two nonlocal points (we’ll call them A and B) in space-time. The problem for a practical users, however, is that you need information about the states of both A and B in order to interpret any information that is relayed; otherwise, all you have is an uninterpretable string of literally random data. In order to convey information about those states, you have to send it by radio, laser beam, e-mail, facsimile, FedEx, garden gnome, et cetera; in other words, some conventional method that is restricted by relativity. Which means we have a physical principle that violates the explicit laws of special relativity, but is compatible in spirit. You’d think this would make people more happy, but instead they’re frustrated at why there should be such an annoying and inexplicable discrepancy in a theory that is otherwise so elegant and beautiful (relativity), especially by a theory that is comparatively ugly and incomplete (quantum mechanics). Attempts to rectify this disconnect, in the form of quantum field theories, and specifically quantum electrodynamics, have been successful in rendering predictions that are validated by experiment to extremely high degrees of precision, but which use a seemingly informal, ad hoc way of getting to a solution (renormalization, the topic of which deserves its own separate discussion).
Two of my favorite threads in this regard are [THREAD=437950]What is Quantum Entanglement?[/THREAD] and [THREAD=417038]Who/What counts as an “observer” in Quantum Mechanics?[/THREAD], which together pretty succinctly answer the questions of the o.p.
You can observe the effects of special relativity on everyday scales. A rare earth magnet dropped through a copper tube (a kit for which can be procured on line from a number of sources if you don’t want to scrounge the components themselves) will fall slowly through the tube due to Lenz’s law. An explanation for how special relativity provides the coupling between electrical and magnetic effects can be found here. Relativistic effects (special relativity) are regularly seen as part of the operation of any high energy particle accelerator. General relativity is a bit tougher, as gravitational fields that we encounter in everyday life have very shallow gradients, but we observe them as part of having to provide correction factors for synchronization of GPS satellites and other timing mechanisms that fly in space.
Well the funny thing is I’ve seen people really screw up explaining this. I can’t remember if it was the sci channel or what but they way they described it you had a pair of entangled particles, one spin up one spin down. Separate them and then switch the spin on one and the other one’s spin changes simultaneously. (Yes, I know this is wrong but if that was the way it was first explained to you it’s very confusing.)
Actually come to think of it I saw another version where the explaination was actually correct yet very confusing if you didn’t know what he was talking about. (I want to say the guy explaining it was Kip Thorne but I’m not sure. All I remember was it was a physicist I had actually heard of.) Basically what he said was that you had a pair of entangled particles. You separate them and then do something to set a property on one and the opposite will show up on the second one. He left out the fact when he said “Set the property on one” he didn’t mean pick a value and set it to that value. He basically meant look at it and that will force the particle to go into one of 2 states and not this indeterminant state. (Since I knew about this before hand I knew what he was talking about and didn’t get confused.)
Not sure many would agree with that. What is communicating with what? What method are they using?
I think the thing to say is that quantum entanglement on first glance appears to be a violation of relativity, but on a deeper inspection issues are really complex, there’s a lot of nuances which aren’t obvious. There doesn’t seem to be any explicit violation of special relativity, it’s very much down to interpretation.
Even then that is not quite what entanglement is as it could equally well describe a classical system where the values are simply unknown or a quantum system in a seperable state where the values are not determined.
What quantum entanglement really is, in simple terms, is when you can’t describe the state of a quantum system of particles in terms of the states of the particles, instead you have to describe the system as a whole.
In practical terms this means that you get correlations that are different from the correlations in a classical system or a quantum system that is in a seperable state.
I missed something here. What exactly are you objecting to?
I said no information and no communication.
Certainly the second particle always shows the opposite value, but if that is a transfer of information, what is being transferred? Isn’t that exactly the thing whose communication we can’t explain?
And if there is communication, then how does the communication work?
You say there is a connection, but that begs the question. Isn’t what makes entanglement interesting the very question of what the connection is and how it is “known”?
The other possible area of disagreement is that I said it doesn’t violate relativity, but your answer is that the inability of our ever knowing in less than than light speed what the particle “knows” keeps it from being a violation of relativity, which in practical terms is not a disagreement.
I know it took me a long time to understand entanglement because all of the explanations I heard just talked about effects that could be classically explained. “Well, so what?,” I would say, “I could do the same thing with a couple of envelopes and the Post Office.” “No,” they would say, “It’s weird and quantum mechanical!” “But what you just described isn’t weird at all!”
The key is that, while the parallel and orthogonal cases work just like you’d expect from a hidden-variable theory (the sort of thing you could get classically), they don’t work that way for angles in between.
There’s a very simple non-paradoxical explanation for the EPR Paradox.
In the standard experiment, two entangled photons are created on Monday and interact with filters on Tuesday and Wednesday respectively. Wednesday’s photon seems to “know about” the Tuesday interaction even though it’s outside the Wednesday filter “causality cone.”
But the experiment can be viewed another way. A photon from some irrelevant event in a very distant future galaxy travels backwards in time, interacts with the Tuesday filter, and continues its reverse-time travel to Earth. There it happens to end up in the physicist’s laboratory and interacts to become Wednesday’s photon. In this scenario, of course Wednesday’s photon has Tuesday’s “hidden variables” available – in a sense the two photons can be considered the same photon! Causality is operating future-to-past!
I’m sure this seems problematic. Why does the photon from Tuesday’s filter happen to arrive at just the right time and place in the physicist’s laboratory? Answer: Causality isn’t strictly future-to-past; in fact we know from common-sense observation that causality near Earth appears macroscopically to be well over 99.9999% of the past-to-future type. But causality’s time arrow may rely on statistical phenomena inapplicable at the particle level.
Retrograde causality certainly defies common-sense. But other explanations of the EPR Paradox seem to defy common-sense even more! Physicists are well aware of the retrograde-causality explanation for EPR Paradox; why don’t they speak of it more? (Last I looked it wasn’t even mentioned on a relevant Wikipedia page.)
Disclaimer: IANAP. I’ve read many “Modern Physics for the Intelligent Layman” books; they leave me confused; next bookstore trip I may look for “Modern Physics for the Stupid Layman.” :smack:
I don’t think my retrograde causality discussion is complete nonsense. But let me propose something which probably is nonsense! For me, one of the most mysterious things about quantum physics is the complex-valued “probabilities.” “Complex-valued” can be read as “having two real degrees of freedom.” I don’t have even a hint of the requisite math details, but is it possible that these two degrees of freedom arise because of the two causality directions (past-to-future and future-to-past)?
Forgive my gross ignorance on quantum mechanics as a whole, and in particular if this question is completely missing the point of something already explained regarding entanglement, but…
Assuming you have one entangled particle and your friend in the next room has the other, is there any way to determine, upon measuring yours, whether your friend’s has already been measured (regardless of the value recorded), or if instead you’re the “first” to nudge it and make the pair determine their values?
Nope, unless of course you ask your friend what time he made the measurement.
By the projection postulate the state of the system immediately after measurement will be the eigenstate corresponding to the result of the measurment. Practcially these means repeated measurements will yield the same result (barring the wavefunction being allowed to evolve into a state which is again in a superpostion of possible results).
In ‘Schrodinger theory’ (i.e. non-relatvistic quantum mechanics) you can always in principle (barring classical uncertainity in measurement) say who made the measurement that collapsed the wavefunction as there is never ambiguity with respect to time. The time parameter is the same parameter for all observers and is also not a dynamical observable subject to quantum uncertainty. In other words you just have to ask your friend at what time he made the measurement to determine who actually collapsed the wave
When you introduce special relatvity to the situation ambiguity arises as to who made the measurement as the time parameters of different observers are not necessarily equiavlent. However because Schrodinger theory is non-relativistic and the description of entanglement is inherently part of Schrodinger theory*, perhaps it’s not so suprising that such ambiguities occur.
*I say this because in entanglement the inseperable quantum subsystems explicitly describe particles, and describing quantum systems this way leads to problems in relativistic quantum physics. That’s not say though that the predictions of relativstic quantum phsyics differ in this situation.
While my tendency is to agree that there’s no means to communicate desired information faster than light, I don’t know why, and your explanation is making me ask this: but why? Specifically, what information needs to be conveyed via normal means that can’t be interpreted in the entangled pairs? For example, if we’re only looking at state changes, there’s no random data. State changes are the basis for electronic communications today. Who cares if what you’re measuring is spin up or spin down? Bob either observes that the state changes at his sampling frequency (bit set high) or that the state doesn’t change at his sampling frequency (bit set low). Alice presumably doesn’t have to measure her half of the pair; she’s changing the state by whatever technology will exist to set the state at the desired transmission frequency.
My assumptions are these (and probably wrong, causing my uncertainty): (a) The entangled particles always read the same for every subsequent measurement, i.e., once measured the up spin half of the pair will always measure up. (b) That Alice has some way to reverse the spin polarity, whether or not she has to read the initial polarity.
For me, one of the most mysterious things about quantum physics is the complex-valued “probabilities.” “Complex-valued” can be read as “having two real degrees of freedom.” I don’t have even a hint of the requisite math details, but is it possible that these two degrees of freedom arise because of the two causality directions (past-to-future and future-to-past)?