Exploring the Hypothesis: Is Everything Entangled in Physics?

[atyy]

ZapperZ: If you're still reading, this is, I think, the point where I have lost your train of thought. We have 'compelling evidence' of the coherent two-particle state, and of the three-particle state... the part I don't get is why you're so staunchly opposed to the "and so on" part.

On experimental procedures for entanglement verification
S.J. van Enk, N. Lutkenhaus, H.J. Kimble
http://arxiv.org/abs/quant-ph/0611219

I haven't understood the details of this paper, but anyway there are Alice and Bob as usual, plus:

Quinten who "believes in quantum mechanics but does not trust Alice and Bob";

Victor who "lets Alice and Bob teleport a state that he hands over", then checks that the output state is "close to his original input state to warrant the conclusion Alice and Bob must have made use of entanglement"; and

Rhiannon, who "just like Quinten, mistrusts Alice and Bob and performs her own measurements on states handed over to her by Alice and Bob. But unlike Quinten, she does not believe in quantum mechanics and tries to construct a local hidden-variable model that describes her measurement results."

So my question is for the universe, God is presumably Alice and Bob, and Quinten is ZapperZ? In which case, could it be possible that the universe is not an entangled state to start with, but if it is, could ZapperZ perform any ideal experiment to verify it?

 

[ThomasT]

IMO that's inaccurate. Interactions almost always result in entanglement; the problem is that the entanglement spirals out of control. Only an interaction involving the entire entangled system can detect it, but it's still there. Only in the collapse model of quantum ontology does the entanglement get destroyed (due, of course, to collapses) -- in the decoherence model the entanglement simply spreads exponentially, quickly becoming too vast to be able to observe.

Ok, but the decoherence model is still subject to thermodynamic laws and c. A particular entanglement is either lost via dispersion and dissipation or is, via substantial interaction, altered beyond possible detection, or destroyed completely. And the exponential spreading is limited by c. Some events are completely isolated from others, whether the universe is finite or not, given that our universe is expanding at c.

So, in what sense could it be said that the evolution of the universe is an entanglement? In what way might all the constituents of the universe be entangled with each other? How can we physically relate everything in the universe with everything else?

But, as he did state, it results in a coherent two-particle quantum state. And another interaction would result in a coherent three-particle state, and so on.

Good point, however none of the resulting ("and so on") states is entangled with the behavior of anything from which they are, not just for all practical purposes but in principle, isolated -- although there is still the possibility of talking about universal entanglement wrt the motion of the universe as a whole. Whether this can be done in any physically meaningful way is a matter of speculation.

 

[schroder]

This is indeed very important. Ultimately, one needs always a "theory of observation" (read, of subjective observation).

Not to belabor the (possibly) epistemological/philosophical argument about whether our perception of the Universe in 3-D is purely subjective and/or a product of how our brains are wired, but I think it is important to note that there is other compelling (mathematical) evidence for a Universe that has exactly three dimensions. Not the least of this is the inverse square law. This law applies not just to gravitation, but to all electromagnetic phenomena, including electric and magnetic fields. What other conclusion can we draw, other than that “space” or at least all objects in space, which adhere to the inverse square law, have extension in only three directions? Or is it possible for a body to have a field or radiate anything in a dimension in which it does not have a spatial extension?

 

[vanesch]

Not to belabor the (possibly) epistemological/philosophical argument about whether our perception of the Universe in 3-D is purely subjective and/or a product of how our brains are wired, but I think it is important to note that there is other compelling (mathematical) evidence for a Universe that has exactly three dimensions. Not the least of this is the inverse square law. This law applies not just to gravitation, but to all electromagnetic phenomena, including electric and magnetic fields.

Yes, you are right. There is clearly also some 3-D structure, simply because 3-D Euclidean distance is a very meaningful concept in many physical regularities (which w've written down as laws of nature). Now, whether that is some extra structure on top of a N-dim phase space, or whether that's all there is to it, is a matter of taste. The price you pay by considering *just* 3D space is that you loose the beauty of the symplictic structure (and the canonical transformations and so on) of phase space. The price you pay when only considering phase space is that you don't have a natural structure for Euclidean distance.

But you point out something very important: it are *formal* and *theoretical* reasons (next to philosophical ones) which may lead us to prefer this view over that one. Your argument was a formal one: if there's just phase space, how come we can express part of the dynamics in that phase space based upon an Euclidean distance in 3 D ? And that's indeed a good argument for 3D.

As I said before, this was just an analogy and maybe I went much too far in it: the original discussion was "is everything entangled" or is "nothing (or almost nothing) entangled".

And again, here we have a strong formal argument: if *everything* is entangled, we EXPECT observationally to observe NO quantum interference effects in all correlation functions that are of a lower order than including all degrees of freedom (of the universe). For all practical purposes, that means that "everything entangled" comes down to "no observable quantum interference". So that's perfectly in agreement with observation.
Of course, "no entanglement" also means "no observable quantum interference in correlation functions" - although they would allow so for quantum interference effects on single systems.

"Decoherence" can mean: irreversible entanglement with the environment, leading to "everything is entangled" and hence "quantum interference effects disappear" ;
or it could be just another word for "projection" or "quantum-classical transitition".

So both "everything entangled" and "nothing entangled" are views which are in agreement with the observation that we don't see quantum interference effects.

However, we have - as ZapperZ points out, with great difficulty - done experiments where the "decoherence is partial", or in other words, where a limited number of systems (2, or 3 or so) interact and "entangle". We then see, as expected, that individual systems don't show any quantum interference effects anymore, but that we DO have quantum interference effects in the correlation functions that correspond to the highest number of systems entangled (for 2, that is a 2-point function, for 3 it is a 3-point function).

 

[ueit]

No, not at all. This is not about brains as you (think you) know them, because a brain is then also totally different (it is a projection of a point in N-dimensional space on an M-dimensional hyperplane)

Sure, I've understood this.

there is a 1-1 relationship between this "single-point" brain in the M-dimensional hyperplane, and the "many points" brain in 3 D. Imagine for a moment that a true brain is a single point in the M-dimensional hyperplane. In order to function well, that is, to have evolutionary advantageous dynamics, it is such that patterns are recognized. It turns out that these patterns are most conveniently organized as "many points in 3D", because this then shows up most easily relationships that occur in interactions (the concept of 3D-distance and so on). So even a "single-point-in-M-dimensional-hyperplane" brain would probably give us sensations that correspond to a 3D space "out there".

I'm not saying that this is not possible in principle. However, all this talk cannot be considered a proof that 3d reality emerges from your proposed reality. May be it does, may be not. Therefore you still need to postulate all this in order to make your theory work.

Of course, I agree that the very fact that this 3D structure (including that 3D Euclidean distance thing) is so very well organizing (so well that our brains are wired up to interpret our sensory nerve pulses that way) is an argument in favor of saying that this might then be the best structure that corresponds to our sensations. But as I said, that's a matter of taste. If you are marveled by the simplectic structure of Hamiltonian phase space, then you might give this more importance over this 3D Euclidean distance thing.

Again, I disagree it's a matter of taste. A theory that takes our 3d-world as reality can ignore the observer completely. A theory that proposes a different reality must explain the observation act because this is the place where the 3D illusion (containing all the required experimental data) appears. So, such a theory is necessarily more complex, and this complexity should be balanced by an increase in explanatory power.

I don't think it would lead to inconsistencies, given that Hamiltonian dynamics is mathematically equivalent to Newtonian dynamics. Any inconsistency in the Hamiltonian dynamics picture would then translate in an equivalent inconsistency in Newtonian dynamics.

This is true if you take a Newtonian system that you know it's consistent and translate it into the Hamiltonian view. But this begs the question of what is the most fundamental description. If the Hamiltonian approach is the one, then you must work from the assumption of a brain being "a projection of a point in N-dimensional space on an M-dimensional hyperplane". You must show how you can recognize this point to be a brain, how it interacts with other objects and so on. If you choose a "wrong" point then this could lead to inconsistencies. In other words, forget completely about the 3d world, use only the new proposed reality and show in the end that 3d-world is emergent.

 

[ueit]

Ontological biases aside, you didn't really answer my question, you just restated it in an equally vague way, with equal problems. What does it mean to observe reality "as a state vector in a Hilbert space"? What about "as a point particle in a 3d space"? And how are they observationally different?

I see the world around me as a 3d space populated by objects that can be reduced at a collection of points. Or, to put it differently, I can imagine a computer simulation of our world based on points moving in a 3d space that is indistinguishable from reality.

On the other hand I have no idea what would mean an observation in your proposed reality (a universal wavefunction). I cannot see directly wavefunctions. You claim that I am a part of that reality, and when I make an observation (whatever that might mean in your description) I should "see" points moving in 3D space. Do you have a proof for that? Why exactly shouldn't I see 6D blobs waving in a 13D space for example?

 

[ManyNames]

Given the hypothesis that everything that exists came from an original singularity, would it not follow, in physics, that everything is "entangled"?

I don't know, maybe I'm way-off here, but somehow it seems plausible.

Maybe, if it did not arise from a singularity, because ordinary quantum rules do not hold from a singularity. If everything came from a single source which was a non-singularitarian solution, then perhaps we have matter and indeed, all matter and energy entangled in such way, we can two, four and eight particles at a time that are entangled.

So if we ever took up the time to observe a single electron, out of all the matter in the universe, which actually takes up less than 1% of all the spacetime in the universe, then maybe another electron located billions of light years away will have a spin-eignestate created for it. So yes, the theory is possible, as i have concluded.

 

[ThomasT]

And again, here we have a strong formal argument: if *everything* is entangled, we EXPECT observationally to observe NO quantum interference effects in all correlation functions that are of a lower order than including all degrees of freedom (of the universe). For all practical purposes, that means that "everything entangled" comes down to "no observable quantum interference". So that's perfectly in agreement with observation.
Of course, "no entanglement" also means "no observable quantum interference in correlation functions" - although they would allow so for quantum interference effects on single systems.

"Decoherence" can mean: irreversible entanglement with the environment, leading to "everything is entangled" and hence "quantum interference effects disappear" ;
or it could be just another word for "projection" or "quantum-classical transitition".

So both "everything entangled" and "nothing entangled" are views which are in agreement with the observation that we don't see quantum interference effects.

However, we have - as ZapperZ points out, with great difficulty - done experiments where the "decoherence is partial", or in other words, where a limited number of systems (2, or 3 or so) interact and "entangle". We then see, as expected, that individual systems don't show any quantum interference effects anymore, but that we DO have quantum interference effects in the correlation functions that correspond to the highest number of systems entangled (for 2, that is a 2-point function, for 3 it is a 3-point function).

You seem to be saying that we both do and don't observe quantum interference effects. So, which is it? Or am I just having a bad day?

My first thought about this is that we do see quantum interference effects, even up to macroscopic scales.

Anyway, what does seeing quantum interference effects (or not) have to do with whether it's possible that everything in the universe is entangled?

We know that some things are entangled.

But the only sense in which all things can possibly be entangled is wrt some holistic motion of the universe, eg. its rotation or its expansion.

Does that mean that it's plausible or possible that everything in the universe is entangled. Depending on what's meant by the terms, 'plausible' and 'possible', I guess it might, so I'll have to retract my original answer to pallidin.

Is there any physically meaningful way that such a 'possibility' could be formalized using qm?

I once asked a question about a 'universal clock' at sci.physics.research and got some very interesting replies. Maybe some of the creative qm experts here at PF can come up with something regarding everything in the universe being entangled wrt some property of the universe as a whole.

Could the degree of universal entanglement be related to the thermodynamic state of the universe?

Would it make any sense to say that everything's entangled wrt time asymmetry?