Why Couldn't Einstein Accept Non-Locality?

[peter0302]

If General Relativity is background-independent, and, as Einstein himself said, general covariance "takes away from space and time the last remnant of physical objectivity," why do you think it was so hard for him to accept the possibility that quantum mechanics also could be non-local? Are these two ideas so drastically different that one cannot support the other?

 

[DeepGround]

Non-Locality requires a faster then light signal. I don't think any of his equations include something like this, in fact they require that all signals max out at the speed of light.

He understood that Relativity was not the final solution to physics and that we just do not understand how to explain quantum mechanics, only how it works. He spent the rest of his life trying to find a universal theory that could explain these type of questions.

I think we are still searching..

 

[colorSpace]

Einstein also had strong opinions about philosophy. I'd think he enjoyed defeating common sense in favor of a scientific view, where scientists are the masters of the universe, beyond criticism from laymen. However he probably didn't like that non-locality seemed to have too much of a "supernatural" touch. When it came to quantum physics, he seemed to be strongly driven by his likes and especially his dislikes, even though he very much contributed to the whole thing. A paradox right there.

 

[peter0302]

Of course - my only point is that his own work proved that concepts of space and time are mutable and are defined by the matter that "occupies" them. If spacetime can be defined by the presence of matter, and shrunk/bent/what-have-you accordingly, why is it so hard to accept the possibility that quanta (esp. entangled quanta) appear to interact in a non-local fashion?

[Edit]
In other words, perhaps the "distance" between entangled quanta, or any quanta for that matter, is defined to be zero, and that it is only their interaction with other quanta that define spacetime and lead to concepts such as "spacelike" and "timelike" separation.

After all, for a photon traveling at "c", all distances are equal to zero. Thus, in their reference frame at least (the existence of which I realize is subject to debate), the entire universe is "local." So a hypothetical communication between them would not have to travel _any_ distance and, thus, would require no energy.

Has the apparent non-locality of entanglement effects been experimentally proven for massive particles or have non-locality tests been limited to photons?

[Edit2]
Has there been any attempt or success in framing quantum theory in a background-independent manner without spacetime?

 

[f95toli]

Another question one can ask is if Einstein would have accepted all the consequences of GR if someone else had suggested the theory?
We obviously can't know the answer to that question. But it is worth noting that the "old Einstein" and the "young Einstein" were quite different people. Einsten, like all of us, changed as he got older and was -at least at times- just as arrogant as the young Einstein accused some of the older famour scientists of his day to be. Einstein was at heart a 19th century physicist and QM was a 20th (and perhaps a 21th) century theory.

"Einstein Versus the Physical Review" in Physics Today sept 2005 (available online) is a good article.

That said, Einstein's criticism probably helped more than it hindered the development of QM in the early days. So perhaps we should be glad that he never accepted it?

 

[birulami]

Non-Locality requires a faster then light signal.

This is always something that confuses me as a layman. On the one side the talk about locality. On the other side the wave function and even fields penetrating the whole universe.

The wave function is modeled as a mapping from all of $R^3$, i.e. it is an object that "is" or "exists" everywhere. Why the assumption that signals must travel. There is this one mathematical object and something happens to it, e.g. it is subject to the application of an operator. No mathematician would ever question that this of course happens to the whole object, i.e. $\forall x\in R^3$, at the same time --- if you want to talk about time at all: it just happens.

If you observe near a certain position $x$ and your colleague tells you what happened at position $y$, you get correlations. No wonder, it was one of the same object that went bust (collapsed) in an instant.

So this is of course non-local, but there is no need to send signal speedier than light.

Ok, that's my view until I learn more.

Harald.

 

[Tom Delacour]

Not a conjecture; not even a guess; an idea:
Photons are their own anti-particles and ( not forgetting their lack of experiencing time, but from observer's perspective, ) a photon traveling back in time is just an antiphoton.
Imagine two spin-entangled photons A and B emitted in opposite directions from source S towards two widely separated detectors at time T.
Spin A is measured at time T+1. The interaction of A with the detector emits it's anti-self back to S at T which correlates A and B's spins, so at T+1, A's spin is detected as was 'originally' ( language problem here ) and also at T+1 or later, B's spin will be found to be correlated to A's.
Thus both A and B's spins are set at T and/or T+1, it makes no difference- spacetime as a whole is immutable. This makes sense in context of special relativity since future of observer C may be past of observer D, so all of spacetime is fixed and past/future makes sense to only massive objects.

 

[turbo]

Einstein rejected "spooky action-at-a-distance" and the Machian notion that inertia arises from matter's instantaneous interaction with all the other matter in the universe. In the process, he derived the idea that gravitational and inertial forces arise from matter's interaction with the space in which it is embedded. These concepts require that "empty" space is real, with real properties.

Fotini Markopoulou theorized some years ago that high-energy gamma rays from GRBs may be slowed WRT to less-energetic EM because they would interact more frequently with space and lose proportionately more energy than less-energetic EM. She hung her hopes on GLAST, but she may have been trumped by MAGIC. If other observations of GRBs can show that high-energy gamma rays trail the lower-energy EM AND that the delay is proportional to the redshift of the source, we've got some exciting new science to pursue.

 

[jVincent]

Personally I believe he just like many after him never saw the clear need to jump to nonlocality to explain what could (and can still!) be explained by more moderate means.
If i showed you a box and said it contained a million dollars, but that they would disapear if you opened it, would you believe me? Even if I gained the trust of the intire scientific community, would you still not have your doubts?
Sometimes things are impossible to both prove and disprove, and I personaly is of the oppinion that nonlocality is still one of those things. At least all the proff I have seen so far aren't exactly obvius in what they state, and aren't written for the layman.
On top of that, every time I find good experimental data, It comes with the advice to sort out a quarter of the results, otherwise it wound show the "clear" picture that it's supposed to.

I know that this is an oppinion opposed to current scientific development, but I'll be deligted the day I see the proff I need to change my mind. I just haven't seen it yet.

 

[Tanja]

I just wanted to add something regarding the understanding of non-locality and entanglement from the view of a non-scientist.

While learning for my exams I thought it might be a good idea to explain things to a friend, who doesn't know anything about physics. She just said: "I don't understand your problem. It's all very obvious. If you have e.g. two electrons, each with 2 possible spin alignments, and you know that two entangled electrons can't have the same spin because of energy conservation or any other conservation law, so, if you measure one, of course you will know the other. You only have two possibilities!"

I didn't know what to answer.

My knowledge of general relativity is very limited. Mathematically speaking non-locality is the fact that it's not possible to separate or decompose states into tensor products.
How is the non-locality (if there is any) in general relativity described?

Ok, the actions of these two theories are instantaneous. The covariance in GR and the correlation in QM might be a reason to find the same mathematical background.

Anyway, I don't understand Einstein, too. How can you support just one of this "spooky" theories?

 

[JesseM]

While learning for my exams I thought it might be a good idea to explain things to a friend, who doesn't know anything about physics. She just said: "I don't understand your problem. It's all very obvious. If you have e.g. two electrons, each with 2 possible spin alignments, and you know that two entangled electrons can't have the same spin because of energy conservation or any other conservation law, so, if you measure one, of course you will know the other. You only have two possibilities!"

This would be a type of "local hidden variables" explanation, but such explanations are ruled out because of the statistics the scientists see when they don't measure the spin on the same axis (the spins are only guaranteed to be opposite when they choose the same axis to measure)--any local hidden variables theory should satisfy the various "Bell inequalities" as proved by Bell's Theorem, but QM violates these inequalities, therefore QM results cannot be explained by a local hidden variables theory. Here's an analogy I came up with on another thread:

Suppose we have a machine that generates pairs of scratch lotto cards, each of which has three boxes that, when scratched, can reveal either a cherry or a lemon. We give one card to Alice and one to Bob, and each scratches only one of the three boxes. When we repeat this many times, we find that whenever they both pick the same box to scratch, they always get opposite results--if Bob scratches box A and finds a cherry, and Alice scratches box A on her card, she's guaranteed to find a lemon.

Classically, we might explain this by supposing that there is definitely either a cherry or a lemon in each box, even though we don't reveal it until we scratch it, and that the machine prints pairs of cards in such a way that the "hidden" fruit in a given box of one card is always the opposite of the hidden fruit in the same box of the other card. If we represent cherries as + and lemons as -, so that a B+ card would represent one where box B's hidden fruit is a cherry, then the classical assumption is that each card's +'s and -'s are the opposite of the other--if the first card was created with hidden fruits A+,B+,C-, then the other card must have been created with the hidden fruits A-,B-,C+.

The problem is that if this were true, it would force you to the conclusion that on those trials where Alice and Bob picked different boxes to scratch, they should find opposite fruits on at least 1/3 of the trials. For example, if we imagine Bob's card has the hidden fruits A+,B-,C+ and Alice's card has the hidden fruits A-,B+,C-, then we can look at each possible way that Alice and Bob can randomly choose different boxes to scratch, and what the results would be:

Bob picks A, Alice picks B: same result (Bob gets a cherry, Alice gets a cherry)

Bob picks A, Alice picks C: opposite results (Bob gets a cherry, Alice gets a lemon)

Bob picks B, Alice picks A: same result (Bob gets a lemon, Alice gets a lemon)

Bob picks B, Alice picks C: same result (Bob gets a lemon, Alice gets a lemon)

Bob picks C, Alice picks A: opposite results (Bob gets a cherry, Alice gets a lemon)

Bob picks C, Alice picks picks B: same result (Bob gets a cherry, Alice gets a cherry)

In this case, you can see that in 1/3 of trials where they pick different boxes, they should get opposite results. You'd get the same answer if you assumed any other preexisting state where there are two fruits of one type and one of the other, like A+,B+,C-/A-,B-,C+ or A+,B-,C-/A-,B+,C+. On the other hand, if you assume a state where each card has the same fruit behind all three boxes, like A+,B+,C+/A-,B-,C-, then of course even if Alice and Bob pick different boxes to scratch they're guaranteed to get opposite fruits with probability 1. So if you imagine that when multiple pairs of cards are generated by the machine, some fraction of pairs are created in inhomogoneous preexisting states like A+,B-,C-/A-,B+,C+ while other pairs are created in homogoneous preexisting states like A+,B+,C+/A-,B-,C-, then the probability of getting opposite fruits when you scratch different boxes should be somewhere between 1/3 and 1. 1/3 is the lower bound, though--even if 100% of all the pairs were created in inhomogoneous preexisting states, it wouldn't make sense for you to get opposite answers in less than 1/3 of trials where you scratch different boxes, provided you assume that each card has such a preexisting state with "hidden fruits" in each box.

But now suppose Alice and Bob look at all the trials where they picked different boxes, and found that they only got opposite fruits 1/4 of the time! That would be the violation of Bell's inequality, and something equivalent actually can happen when you measure the spin of entangled photons along one of three different possible axes. So in this example, it seems we can't resolve the mystery by just assuming the machine creates two cards with definite "hidden fruits" behind each box, such that the two cards always have opposite fruits in a given box.

My knowledge of general relativity is very limited. Mathematically speaking non-locality is the fact that it's not possible to separate or decompose states into tensor products.
How is the non-locality (if there is any) in general relativity described?

I don't know that much about the mathematical details of GR either, but I know it is a local theory in the sense of having a "light cone structure" where the values of physical variables at one point in spacetime depend on events in the past light cone of that point, but they do not depend on events with a spacelike separation from that point. So, I don't think it's accurate to say there is any "non-locality" in GR.

 

[peter0302]

How can you support just one of this "spooky" theories?

Because just one of them was *his* :)

 

[turbo]

Anyway, I don't understand Einstein, too. How can you support just one of this "spooky" theories?

I could be entirely wrong, but his rejection of spooky action-at-a-distance (1920 and later) to explain gravitational attraction and inertial effects in GR may have played into his discomfort with quantum theory, too. He was a firm believer in causality. At the time that he wrote "On the Ether" (1924) he had concerns not simply because quantum theory and GR seemed incompatible; he had deeper concerns that quantum theory might "bring the entire edifice down", referring to the possibility that GR might be supplanted by another more complete theory of which quantum theory would be a precursor or a sub-set.

 

[JesseM]

How can you support just one of this "spooky" theories?

Because just one of them was *his* :)

"Spooky" in what sense? Do you disagree with this comment from my last post?

I don't know that much about the mathematical details of GR either, but I know it is a local theory in the sense of having a "light cone structure" where the values of physical variables at one point in spacetime depend on events in the past light cone of that point, but they do not depend on events with a spacelike separation from that point. So, I don't think it's accurate to say there is any "non-locality" in GR.

 

[peter0302]

Well, I was being somewhat facetious, although Mach's Principle was, at least at first, an inspiration for Einstein, and Mach's Principle is just about as spooky as entanglement. The real point is that Einstein was a little selective in what he thought was spooky and what he thought was perfectly normal...

I was thinking about this this morning, actually, and a thought occurred to me that I'm sure has been considered by others too. Similarly to the way that the result of a distant observation influences what is observed locally with entangled particles, it is also true that the kinetic energy of a local object with respect to a distant object can be "instantaneously" changed by changing the speed of that distant object. Is this kinetic energy a physical reality and, if so, isn't this influence just as "spooky" as entanglement?

 

[JesseM]

was thinking about this this morning, actually, and a thought occurred to me that I'm sure has been considered by others too. Similarly to the way that the result of a distant observation influences what is observed locally with entangled particles, it is also true that the kinetic energy of a local object with respect to a distant object can be "instantaneously" changed by changing the speed of that distant object. Is this kinetic energy a physical reality and, if so, isn't this influence just as "spooky" as entanglement?

No, kinetic energy is frame-dependent, this is no more significant than the fact that if I have a coordinate system with my position defined to be the origin, then any change in my position relative to a distant object will "instantaneously" changes that object's x-coordinate.

 

[peter0302]

So then kinetic energy really has no physical meaning until the two objects meet. The "connection" seen in entanglement likewise has no physical meaning until the two measurements are brought together and correlated. In both cases, interaction is required before any physical manifestaiton emerges. Before that, it's just math. The math - in both cases - is non-local, while the manifestation of the result - in both cases - is local.

 

[JesseM]

So then kinetic energy really has no physical meaning until the two objects meet.

Huh? Kinetic energy is no more frame-independent when they meet than at any other time. Different frames can disagree on the kinetic energy of two colliding objects and still make the same predictions about all physical (frame-independent) aspects of the collision.

The "connection" seen in entanglement likewise has no physical meaning until the two measurements are brought together and correlated.

The outcome of each measurement is a real physical fact, not a frame-dependent one. I'm not sure what you mean when you say the correlations between two physical facts have no "physical meaning", the correlation is obviously not frame-dependent either.

In both cases, interaction is required before any physical manifestaiton emerges. Before that, it's just math. The math - in both cases - is non-local, while the manifestation of the result - in both cases - is local.

This sounds more like poetry than physics, it doesn't seem like you have any clear definitions of "physical meaning" or "physical manifestation" or "non-local" in mind (what does it mean to say 'math' is non-local, for example?)

 

[peter0302]

What I mean is that there are no physical manifestations of kinetic energy until there is a collision. You can observe an object whizzing by you and calculate its speed relative to a known distant object and know what will happen when they collide far in the future. That's what I mean by the math is non-local. You can calculate the result of what will happen light-years or years from here and now. But regardless, there is no physical aspect of the collision until they have _locally_ come together.

[edit]
Same thing with entanglement observations. You can predict on paper what the outcome will be but you cannot see any connection between the results until you've brought the results together. So, again, the entanglement doesn't really manifest itself until the information comes together locally. Until then, the observations are entirely observer dependent in that both have exactly a 50% chance of seeing a photon pass through a polarizer, and therefore both think that their observations were unhindered by the other. It's only when the two observers meet that they realize there was a correlation between their observations.

In the one case, we're dealing with colliding bodies, and in the latter case, we're dealing with information, but the idea is the same. And I say again that I don't think one is spookier than the other.

 

[JesseM]

What I mean is that there are no physical manifestations of kinetic energy until there is a collision.

What is a "physical manifestation"? Would you say there are any physical manifestations of other coordinate-dependent quantities like an object's velocity or its x-coordinate?

You can observe an object whizzing by you and calculate its speed relative to a known distant object and know what will happen when they collide far in the future. That's what I mean by the math is non-local.

I don't get it. Being able to predict the future from present observations is "non-local"? So any deterministic universe would be "non-local" in your sense? You can define words any way you please I suppose, but this sort of definition (and you still haven't given me a clear definition, just an example with some confusing commentary) doesn't seem to have any relation to the standard definition of locality, which just means that physical outcomes at a given point in spacetime can depend only on things in their past light cone. Do you agree that according to this definition, general relativity is a local theory, while no local hidden-variables theory could reproduce the statistics seen in entangled particles?

You can calculate the result of what will happen light-years or years from here and now. But regardless, there is no physical aspect of the collision until they have _locally_ come together.

What you seem to be saying here is that there is no "physical aspect of the collision" until they actually collide, but you haven't provided any definition of what you mean by terms like "physical aspect" or "physical meaning" (when I talk about facts or quantities being 'physical', I just mean they are coordinate-independent, but this has nothing to do with time, a fact is still coordinate-independent regardless of whether it's already happened or hasn't happened yet from my point of view). Then you go from this statement to the notion that there is something "nonlocal" about kinetic energy, but I don't follow your reasoning, and you haven't defined what you mean by nonlocal either. Again, without providing any clear definition of what you mean by the various terms you use, this just sounds like intuitive poetry rather than meaningful statements about physics.

Same thing with entanglement observations. You can predict on paper what the outcome will be

You can predict what statistics you will see after many measurements, but you can't predict the outcome of any particular pair of measurements.

but you cannot see any connection between the results until you've brought the results together. So, again, the entanglement doesn't really manifest itself until the information comes together locally.

What does "manifest itself" mean? If you just mean that you aren't aware of certain the outcomes of the measurements until the information from each one "comes together locally" in your brain, I suppose this is true, but this is true of any possible physical fact whatsoever. And why does this show that there is something nonlocal about entanglement?

In the one case, we're dealing with colliding bodies, and in the latter case, we're dealing with information, but the idea is the same. And I say again that I don't think one is spookier than the other.

In the classical case the outcome is dependent only on things in the past light cone of the event, in the case of entanglement no local theory where this is true can explain the correlations you get. That's why entanglement is seen as spooky. You can invent your own nonstandard definitions of terms if you want, but you can't deny that the above is true, regardless of what terms you use for it.