# Entanglement - the Latest Insights?

1. Jan 13, 2010

### wavering

Dr Chinese has an admirable exposition of Bell's Inequality, here:
http://www.drchinese.com/Bells_Theorem.htm

From this it appears (and please correct me if I am wrong) that a pair of entangled particles each have opposite spin BUT this spin is NOT a hidden value waiting to be revealed by measurement but is intrinsically undecided until the point of measurement. Indeed, I suspect that the question "what is the spin and position and momentum of a particle" is a meaningless question. Rather, perhaps, like asking whether a coin reads "heads" or "tails" when it is rotating in the air when you are deciding who will bat first in cricket (sorry, I'm English). It will not read either and the question is meaningless until you catch it which forces it to decide. No analogy is accurate but hopefully this may help somebody (it helps me)

So, you have two entangled particles which you have pushed hundreds of miles apart, and you force one of them to reveal its spin. Instantaneously (at least 10,000 times faster than the speed of light anyway according to Wikipedia) the other particle will be found to have the opposite spin if you care to check it. Presumably the experiment is done by using two ultra accurate clocks - you force one particle then the other one a minute fraction of time later.

You cannot use this to send information but clearly it is an amazing event (at least, it is to me) and seems to strike at the very basis of our (at least, my) understanding of the universe. In case you don't think this is amazing it appears that somehow a particle has told its colleague hundreds of miles away "I have just decided to read 'heads' so you will have to read 'tails'. I could equally well have decided to read 'tails' in which case you would have had to read 'heads' ". Although we cannot use this fact to send information, it seems to me that information has been sent from one particle to the other but how? Maybe through another dimension of some kind in which they are still touching?

Accordingly, I would expect an "explanation" to have profound implications and to be part of a TOE or GUT. Or maybe I am missing something?

Anyway, to my question, Dr Chinese states "Research continues on understanding of the true nature of particle entanglement".

Could Dr Chinese or any other member(s) explain the current state of thinking in terms that non specialists can understand?

2. Jan 13, 2010

### f95toli

No, it doesn't mean that. There are obviously many ways you can think about this, but there is no physical (and definitely no experimental) reason to believe that the experiment implies that the two particles are somehow in contact with each other (even via some channel that is inaccessible to us). One can for example interpret the experiment as meaning that it is only meaningful to talk about individual particles AFTER the detectors have reacted, i.e. before that we have some sort of state where the the whole notion of "single particle" is meaningless. This is a simplified non-realist interpretation. One could of course also use a non-local interpretation or even a non-realist non-local interpretation.
Personally I don't like interpretations....

3. Jan 13, 2010

### DrChinese

Thanks for the shout out and I will be glad to take a shot at the question...

There are a lot of new papers coming out on entanglement every month (I saw nearly 1000 in 2009), here is a link to those:

http://arxiv.org/find/quant-ph/1/abs:+OR+entanglement+bell/0/1/0/2009/0/1?per_page=100

To summarize:

a. There are a lot of new and "exotic" forms of entanglement being addressed. Some I think are fascintating include: hyperentanglement (multiple degrees of freedom), closing loopholes (which pushes limits of experimental design), entanglement of more than 2 particles, entanglement of systems other than photons.

b. There is both theoretical and experimental research into the rejection of realism by non-Bell avenues. Those include the older theorems by GHZ, Leggett, Hardy which support the Bell result independently.

c. Although there are many individuals doing great work, in the past year I have seen some good work in particular from groups including Adan Cabello (17), Jian-Wei Pan (10), Marco Genovese (7), Anton Zeilinger (6). Of course quality matters over quantity, but these are some of the top researchers to look out for.

d. A lot of work has gone into studying the interpretations of quantum mechanics. This includes the possibility of non-locality, usually going under the name of Bohmian Mechanics (BM), de Broglie-Bohm (dBB) or Pilot Wave Theory. We have a number of members on this board who are very knowledgeable about this (search PhysicsForums for Demystifier and you will learn a lot about this). Also, there are new papers on time-symmetric interpretations which are fascinating, and I would single out Relational Blockworld (RBW).

So there! If that doesn't give you enough to peek around at, I don't know what will. Most of the papers are not oriented to non-technical readers so they may be hard to follow, especially as to what the real point of the paper is. But if you find an area you would like to discuss more specifically, let us know and I am sure someone can help.

4. Jan 13, 2010

### wavering

Dr Chinese
Thank you for your detailed response. I think that I lack the intellect to follow all the ins and outs and I suspect that trying to explain things in terms of "classical reality" is a mistake. Nevertheless, if it is possible to summarise the current state of thinking in layman's terms it would be most helpful.

If there are no "hidden variables" ie variables which mean that it is inevitable that one particle will declare itself "heads" and the other as "tails", then surely this implies that the particles are somehow "communicating" - not in the sense of getting on the phone but in the sense that if one of them arbitrarily becomes "heads" the other is influenced subsequently (or maybe previously) to declare itself "tails"?

Or is it that although there are no hidden variables the nature of entanglement is such that the decision has already been made when they entangle as to which one will be heads? If so, then this sounds like a "hidden variable"

Or does the observation process somehow transcend time in the normal meaning of the word? Ie the decision is retrospective somehow

I am sorry if this is irritating (as so much layman's comment on QM is) but I am trying to understand precisely what it is that I don't understand - if you understand me!

5. Jan 13, 2010

### f95toli

No, as I wrote above it doesn't necessarily imply this. Remember that you can't -regardless of interpretation- really think about "classical" particles with well defined properties that are also well-localized in space (at least not at the same time) here.
The anti-realist approach I described above, where the particles do not really "exist" in the sense that they have individual properties before they are measured, is one option consistent with experiments but there are others.
Note that I am not advocating this particular interpretation, I just think it is important to point out that none of the "obvious" interpretations are consistent with experimental data. Moreover, there several possible interpretations of these experiments and no one can say for sure if one of them is "correct"; which interpretation people decides to use has more to do with philosophy and personal taste than science.

The main conclusions we have to draw from these experiment -regardless of interpretation- is that nature is very, very weird and that reality is not what it seems.

6. Jan 13, 2010

### wavering

f95toli
Many thanks for your two replies. I am genuinely seeking enlightenment here - I have no agenda to pedal. You wrote:
Well, I have no problem with particle/wave duality or with 4D space-time continuum but I am struggling to understand what you mean here. Please don't take that as a criticism - merely me being dense, I suspect.

Certainly QM is weird but that is hardly surprising - I would have expected macro physics to be an emergent property that differs from the underlying micro physics (in the same way that you can treat a thousand cars as a stream of dots but a single car is a totally different phenomenon and if you had only ever seen streams of thousands of them one car on its own would be just incredible and need a whole different bag of explanation)

So, is the problem that the mathematics is fairly clear but the physical mechanisms are not? For instance, you could write a whole set of equations that would describe how a transformer works giving the relationship between the input voltage and current and the outputs and it would be extremely accurate in prediction BUT unless you took it apart you would not have the faintest idea how it actually worked?

If this is the case, then is it that nobody really understands QM and that even the brightest people in the field are really just hiding behind the mathematics? Or is it just counter-intuitive and most of us are too thick and hide bound to be able to grasp it?

7. Jan 13, 2010

### f95toli

That is one way of looking at it, yes. Although be careful with the word "mechanism" here, it could be that some of these "weird" things are really just the way are, i.e. the real "mechanism" might be whatever (decoherence etc) causes our us to see particles with well defined properties etc in our everyday world.

I don't think that is an accurate description. We understand the physics quite well (although there are certainly many open issues), it is just that we don't know what to make of that knowledge.
To some extent yes (but I would object to the word "hiding", what is the alternative?), there is a reason why we talk about "quantum weirdness". It is just that the quantum world (which also happens to be the real world) is so strange. It is a bit like trying to understand the mathematical concept of infinity; no matter how much you study defintions etc you will never be able to imagine an infinite number of apples...

Last edited: Jan 13, 2010
8. Jan 13, 2010

### DrChinese

There are hidden variable interpretations that are viable, and indeed they feature "communication" which transcends the normal restrictions of space-time (c=speed of light). There are 2 such types: one is the Bohmian group, and the other is the time symmetric group (both of which I mentioned in my earlier post). Please keep in mind that there is no specific evidence for these views over other viable interpretations such as Many Worlds or the Copenhagen interpretations.

As f95toli says, there are problems with just about ANY physical way of thinking of the situation. Many a physicist has considered the stange qualities of QM without being able to arrive at a completely successful mental picture. There are always one or two little bumps that are contrived to make the model work, and so you end up with an imperfect or ad hoc model.

One picture that works well for me is this (also imperfect): imagine that there many paths for a particle to take from A to B (at which point a final measurement/observation/decoherence occurs), and each has a probability. These paths - probability waves - are allowed to interfere constructively and destructively to determine possible outcomes. An outcome is randomly chosen and collapse occurs. Now, this is a picture that is accurate but fraught with bumps, such as: how can a probability wave act as if it is real? Who/what picks the random outcome? Wouldn't the collapse itself be a non-local manifestation? And lastly, how do you explain partial collapse (which is seen in cases of hyperentanglement)? My point is not that my mental model is good, it is that no model really does a convincing job in all respects.

(The only exception is Demystifier's, just ask him. )

Don't let the bumps stop you from asking. And do your best to learn a bit about entanglement, quantum erasers, the double slit, etc. so you have some good examples to think about. And while you are at it, remember all of the incredible science that has resulted from quantum physics... much of it which is extremely complex but drives electronics, chemistry and other areas.

9. Jan 13, 2010

### ZapperZ

Staff Emeritus
It might be worth mentioning that the current issue of Rev. Mod. Phys. has a rather comprehensive review of not only the current progress in this topic, but also a very thorough background on EPR, entanglement, and Bell theorem and concepts.

M.D. Reid et al. Rev. Mod. Phys. v.81, p.1727 (2009).

Zz.

10. Jan 13, 2010

### DrChinese

You can find it here as well:

http://arxiv.org/abs/0806.0270

11. Jan 18, 2010

### pallidin

12. Jan 19, 2010

### wavering

Here is a thought (doubtless not original - with 7,000,000,000 people in the World it is very difficult to have an original thought). If the entangled particles are a few hundred miles apart then given that there is no such thing as absolute time, if they are both forced to confess at very close "times", then it is not possible to even theoretically say which one confessed first - not because of accuracy but because different observers see different time ie Alice says that Particle A was first and Bob says that particle B was first and they are both right.

If I am right, how does this effect things? I suspect the answer is that time does not come into it and is a big red herring ...

13. Jan 19, 2010

### eaglelake

A quantum experiment is nothing like its classical counterpart. Quantum events force us to reject the classical description of nature and to accept the fact that quantum events violate some of the most fundamental principles of classical physics. There is no classical explanation of “how the quantum experiment works”, but let me try to give a “quantum explanation”.

There are two essential features of a quantum experiment, both due to Bohr, that render it non-classical.

The first states that the experiment must have a measurement result; an experiment is not an experiment without an experimental result. ( Actually, Wheeler said,” "No elementary phenomenon is a phenomenon until it is a registered phenomenon.) Once the result is made , the experiment is over and done. The result is irreversible and it gives closure to the experiment. Prior to measurement, i.e. without closure, the experiment is undefined; the experiment “exists” only at the instant of closure.

The second is called the Non-Separability Principle. Briefly, it says that all parts of the experimental apparatus, including the measuring device and the measurement result, make up a single entity; all the parts are inseparable and they are not independent of one another. The experiment cannot be broken into its separate parts. Further, the experiment is not a sequence of physical events, as in a classical process.

Here, the two entangled particles are part of the whole experiment. They are not two independent entities. Assume we measure the usual spin component for entangled spin ½ particles and get the value +1 in detector A. Then we also get the result -1 in detector B, whether or not we bother to make the second measurement. This is because the two values +1 and -1 make up a SINGLE EXPERIMENTAL RESULT. Because of the entanglement, +1 in detector A is always accompanied by -1 in detector B. We always obtain both values together. It doesn’t matter whether we measure particle A alone, or particle B alone, or both particles independently. If we decide to measure both particles, which is unnecessary, it doesn’t matter when we make the measurements, or how far we think the particles are apart. The time and distance are irrelevant. The point is this: the entangled particles cannot be separated from each other, or from the rest of the apparatus. The experiment, as well as the theory, does not describe the behavior of two independent particles, and we should not be discussing them as such!

All we can say is that that in this particular experiment, the measurement result consists of two numbers, +1 and -1. Any discussion of two particles moving away from each other in 3-space is an attempt to force a classical description on this quantum event. The description you give is classical. This was tried in the EPR experiment and the calculated results were erroneous.