Realists? Tsk...

[Macoy]

http://physicsweb.org/articles/news/11/4/14

[Schneibster]

Interesting, a good link. I've been saying for quite a while that I thought that the combination of Aspect and the DCQE came very close to ruling out local realism; this adds possible evidence. I'll be interested to see how this plays out.

[Piggy]

As a person with practically no formal science education, a self-taught amateur, I understand some of what's being said in the article, but not enough to fully comprehend it.

It would be very helpful if someone could offer a plain-English synopsis for the layman.

I'm highly interested in this, and would very much appreciate a leg-up.

[Macoy]

Originally Posted by Piggy

As a person with practically no formal science education, a self-taught amateur, I understand some of what's being said in the article, but not enough to fully comprehend it.

It would be very helpful if someone could offer a plain-English synopsis for the layman.

I'm highly interested in this, and would very much appreciate a leg-up.

To be honest, it dosen''t seem that anyone's got anything to offer on this one - just like reality.

[UserGoogol]

How is the Many Worlds Interpretation effected by this? The way they seem to define "realism" in this seems to imply that the Many Worlds Interpretation is not realist in the sense they're using it, (because Many Worlds doesn't have hidden variables) but quantum physics is confusing, so I'm not entirely sure what "realism" really means.

[Schneibster]

Originally Posted by Piggy

As a person with practically no formal science education, a self-taught amateur, I understand some of what's being said in the article, but not enough to fully comprehend it.

It would be very helpful if someone could offer a plain-English synopsis for the layman.

I'm highly interested in this, and would very much appreciate a leg-up.

I didn't see this earlier or I'd have accommodated you. I can only plead I been busy.

The particular type of realism they're talking about is something called "local realism." This concept is the idea that despite the fact that some parameter of a particle cannot be measured, because it is conjugate under uncertainty to another parameter that has been measured, it still has a value.

First, you have to understand Heisenberg Uncertainty. What that means is that there are certain parameters of particles that can only be measured to a particular combined accuracy; the more accurately you measure one of them, the less accurately you can measure the other. The product of the uncertainty is approximately the value of Planck's constant, which is the constant of proportionality that determines the energy of a quantum for a given frequency.

Physicists don't know why things are this way; they just know that they are, because every attempt they've made to create a consistent theory that describes subatomic events as we actually see them, that doesn't incorporate uncertainty, just doesn't work. It's a required feature of any theory of subatomic physics that accurately describes the things we see.

Two such parameters are position and momentum. That is, the more accurately you can measure the precise position of a particle at a particular time, the less accurately you can measure the precise product of its mass and its velocity. Another pair is energy and time, and another is the value of the spin (technically, "spin angular momentum," which is a property of subatomic particles that behaves like angular momentum does, but is quantized, that is, can only occur in units of fixed size) about more than one axis of rotation. So, for example, the more accurately you determine the energy of a particle, the less you know about precisely when it had that much energy, and vice versa. Such pairs of variables are called "conjugate under uncertainty."

Now, spin is special, because it is quantized; a particle's precise value of spin is a small whole number multiple of 1/2. 0, 1/2, 1, 3/2, 2, and so forth. And spin is a characteristic of a type of particle; all electrons, for example, have spin 1/2. There are no spin 3/2 electrons, or any electrons with any spin but 1/2. So what do I mean when I say that the spin about two axes is conjugate under uncertainty? I mean its direction. The spin can be positive or negative; that is, 1/2 or -1/2. And what I mean is, if you know the direction about one axis, you cannot know the direction about any other. This situation comes about because, unlike position, or momentum, or time, or energy, spin is discrete. These other parameters can take on any value over a range, but spin can have only one of two values. Therefore, if you know the spin about one axis, you know it completely; there is no range. It is discrete. And that means that knowledge of its value means you cannot ever find out what the spin is about another axis at that same time.

So what local realism says in this case is, "even though you can't ever know what the spin about another axis was at that particular moment, because you were measuring it about a particular axis at that moment, it had a value."

And what these guys are saying is, "we've performed an experiment that shows that in fact, the spin about any other axis cannot have had a value at that time;" they're denying local realism. In other words, the answer to Einstein's famous question about whether the Moon is up there in the sky if no one is looking at it is, "no, it's not." (Actually, that's not true- decoherence says that you can't have this kind of uncertainty about a macroscopic object like the Moon, because it's made up of so many particles that, hmm, this is inaccurate but will give you the flavor, their uncertainties all cancel each other out. Something very like that, anyway.)

This sounds pretty wild to someone used to dealing with everyday macroscopic objects that don't disappear from over here and appear over there, but "over here" and "over there" are, from our point of view, pretty fuzzy in the first place for subatomic particles. That's the physical meaning of uncertainty. Just as relativity gives meaning to "fast" and "slow" by asserting that non-trivial things happen to something "fast" that make it behave according to rules that appear radically different from the rules that apply to things that are "slow," quantum mechanics gives meaning to "big" and "small" by asserting that non-trivial things happen to something "small" that make it behave according to rules that appear radically different from the rules that apply to things that are "big." But both relativity and quantum mechanics say that the behaviors we observe should come out the way we see them; it's just that we're used to things that are "big" and "slow," and things that are "fast" and "small" behave very differently from what we're used to.

So, the reason they have to do this experiment? It's because of something called the "EPR gedankenexperiment," a thought experiment dreamed up by Einstein and a couple of his associates named Podolsky and Rosen (E, P, and R). What they said was, suppose you take a pair of particles that have values that are dependent upon one another. For example, they both came from the same event, and you know what happened in that event, so that they have to have parameters that are in a fixed relationship to each other. This is generally due to a conservation law; for example, since momentum is conserved, if a particle disintegrates into two particles, their momenta have to add up to the original momentum of the original particle before it disintegrated. If it was zero, then they have to be equal and opposite so they add up to zero. Such particles, whose combined values have to add up to some known value, are called "entangled."

So let's suppose the entangled values are position and momentum. We know that since their momenta have to be opposite, so (in the frame of reference in which their common origin is (0, 0, 0)) do their positions, and for momentum to be conserved this has to be true at all times. OK, so let's suppose I measure the momentum of one particle, and at the same time the position of the other. Haven't I violated the Uncertainty Principle? Because knowing one momentum, the other has to be equal and opposite; and knowing one position, so again the other has to be equal and opposite, and doesn't that mean I know the position and momentum of both particles simultaneously?

There are only three ways out of this box, and one of them doesn't agree with the evidence.

The first is that perhaps uncertainty is wrong. The problem with that, and it's a problem that EPR all acknowledged, is that we can't make a consistent theory of subatomic physics that doesn't have uncertainty. What they said is, well, perhaps using current theories- but this just means that the theories of quantum mechanics can't be complete. There must be more to it than we can find out. Bohr and others at the time rejected this explanation, because they had more faith in their math than Einstein did. An interesting fact about Einstein is that while he was very good with the particular type of math he did, he wasn't all that good at the type that nuclear physicists at that time were using, and he wasn't very comfortable with it. As a result, he didn't see how completely paradoxical it would be to say that math was wrong.

The second is that perhaps locality is violated. Locality is the principle that local causes have local effects, and vice versa. In other words, there is no action at a distance. Einstein of course had a fit about this; after all, his primary achievement had been to create a theory that did what Newton could not: made gravity local instead of action-at-a-distance. This is because General Relativity makes gravity a matter of spacetime distortion, a curvature of spacetime, that surrounds matter; and the matter is touching the spacetime, and each portion of spacetime is touching the next, and they all warp together, with the amount of warping decreasing as the amount of spacetime increases, as you get farther from the matter that's making the distortion.

But there's a bit more to locality than that. Locality is also closely linked to one of the Postulates of Special Relativity: the postulate that the speed of light is finite and maximal. Now, a postulate in mathematics is an axiom; in other words, a basic "truth" that is accepted a priori. But because physics describes the real world, and we can measure the real world and check if the physics describes it, we have a method for testing postulates that does not exist in mathematics. And when the predictions made by a theory agree with the real world, most physicists accept that the postulates have been proven, along with the theory. So most physicists believed (and continue to believe) that locality violations are impossible, at least beyond the reach of uncertainty; it's impossible, of course, to measure the position and momentum of a particle at the same time, so that in turn means that the particle could violate the speed-of-light limit by just a little bit, and because we could never "catch it in the act," we could never tell.

But there's one more loophole: violation of local realism, that is, the assertion that conjugate parameters have values, whether we can measure them or not. This is also called "hidden variables," although it is not the only version of hidden variables, but only the simplest.

So the situation was, either quantum mechanics was incomplete, or locality failed, or local realism failed.

Now, when EPR came up with their experiment, they never dreamed that anyone would come up with a way to constrain the "hidden variables," that is, the values of position and momentum that were not measurable. But a genius physicist (and extremely talented mathematician) named John Bell came up with one in the early 1960s. What Bell realized was that although the spins on different axes were conjugate under uncertainty, the spin about the second axis would have a measurable, probabilistic effect if it had a real value. How he did this is very complicated, but he managed to prove to everyone's satisfaction that it was true. So from that, he was able to make predictions that differentiated between local realism and the straight predictions of quantum mechanics, which ignore local realism. This is the famous "Bell's Inequality," which if it was true would prove local realism, and if it were not would disprove it.

EPR had used their gedankenexperiment to show that unless locality or local realism were violated, then quantum mechanics had to be incomplete; then they went on to demonstrate why they thought that locality and local realism had to be true. Einstein's Moon comment is a disparagement of the idea that local realism could be violated; and whenever anyone proposed a locality violation, Einstein could mutely point to Special Relativity.

But by using Bell's Inequality, a series of experiments were proposed and carried out that showed that either local realism or locality had to be violated. Either that, or quantum mechanics had to be wrong- and wrong in ways we could measure, and we had, and it wasn't. The final experiment of this series was done by Alain Aspect in the 1980s, and is called the Aspect Experiment. Most physicists believe that this is the ultimate proof that Einstein was wrong, and the real physics of our universe is either non-local or not locally real.

So this is where all the business about "locality violations" comes from. IMO, it's woo, and I share that opinion with most physicists- but, you see, even though we accept that explicit measurable locality violations are impossible, we couldn't really be sure. Because although we could rule out incompleteness in QM, we couldn't distinguish between violations of locality, or violations of local realism.

What this experiment claims to have done is to rule out locality violations in favor of local realism violations. Thus the title of the thread. I have not evaluated it myself, and it will probably be a while before I get around to it- and I'd like to see, as well, what other physicists than the experimenters have to say on the subject. I have my doubts. OTOH, I believe (though I cannot yet prove it) that the combination of Aspect and another interesting experiment called the DCQE- Delayed Choice Quantum Eraser- rule out locality violations, particularly when SR is considered along with them. So we'll see what happens.

Feel free to ask questions, but first, pull up the threads I've done on here about Aspect and the DCQE. I hope that you will find them informative on these subjects. Feel free as well to do a little necromancy and ask any questions you feel are not well answered in those threads- I'll do my best to clear them up for you.

[Macoy]

Schneibster, i shall have to read your post again, without a hangover.

[Paul C. Anagnostopoulos]

Originally Posted by Schneibster

So this is where all the business about "locality violations" comes from. IMO, it's woo, and I share that opinion with most physicists
...
What this experiment claims to have done is to rule out locality violations in favor of local realism violations. ... I have my doubts.

I'm confused, although it's quite possible that I misunderstood what you said. You appear to have labelled locality violations woo, while at the same time having doubts about realism violations. How can you hold both these opinions when we agree that one or the other is required?

Also, don't the tests of Bell's inequality already tell us that local realism is violated?

~~ Paul

[Piggy]

Originally Posted by Schneibster

The particular type of realism they're talking about is something called "local realism." This concept is the idea that despite the fact that some parameter of a particle cannot be measured, because it is conjugate under uncertainty to another parameter that has been measured, it still has a value.

And what these guys are saying is, "we've performed an experiment that shows that in fact, the spin about any other axis cannot have had a value at that time;" they're denying local realism.

Thanks. Those statements alone clear up quite a bit for me (I am familiar w/ Heisenberg).

I don't have time at the mo to read the rest of it regarding the details of this experiment, but I'll get to it later today. Thanks much!

[Schneibster]

Originally Posted by Paul C. Anagnostopoulos

I'm confused, although it's quite possible that I misunderstood what you said. You appear to have labelled locality violations woo, while at the same time having doubts about realism violations. How can you hold both these opinions when we agree that one or the other is required?

I was writing too quickly. Locality violation is used to justify woo. It also could violate the speed-of-light limit, though not explicitly. But it's not woo itself.

Originally Posted by Paul C. Anagnostopoulos

Also, don't the tests of Bell's inequality already tell us that local realism is violated?

~~ Paul

No, they tell us that either local realism or locality is violated.

[Paul C. Anagnostopoulos]

Originally Posted by Schneibster

No, they tell us that either local realism or locality is violated.

I think I remember that, but then why did you say:

Quote:

So from that, he was able to make predictions that differentiated between local realism and the straight predictions of quantum mechanics, which ignore local realism. This is the famous "Bell's Inequality," which if it was true would prove local realism, and if it were not would disprove it.

and why does Wiki say:

Quote:

Per Bell's theorem, either quantum mechanics or local realism is wrong. Experiments were needed to determine which is correct, but it took many years and many improvements in technology to perform them.

~~ Paul

[Schneibster]

Because the mainstream view (and Bell's view) was and is that locality violations are prohibited by the postulates of Special Relativity, specifically the postulate that the speed of light is finite and maximal.

[Paul C. Anagnostopoulos]

Ah, so locality is more or less built into mainstream QM. If something's gonna be violated, it has to be realism.

Say Schneibster, can you give us a simplified explanation of why entanglement does not provide a way to transmit information instantaneously?

~~ Paul

[Schneibster]

In the view where local reality is violated, there is no transmission of information. This is consistent with the results when different axes are measured; the spins are the quantum distribution, not the expected distribution if the spin on the axis coordinated with that measured on the remote particle had actual existence on the local particle. When the spins are measured on the same axis, they are coordinated because they must be by conservation of momentum, otherwise an explicit violation of momentum conservation could be observed.

In the locality violation view, the value of the spin on the local coordinated axis is determined when the remote measurement is made, and takes on the value that it must to avoid explicit violation of momentum conservation; but because it is only the collapse and the value of the spin on the remote particle, no information can be transmitted, because if you alter it to impose a signal, the entanglement is broken, and if you don't, there's nothing to transmit.

[Piggy]

Originally Posted by Schneibster

because it is only the collapse and the value of the spin on the remote particle, no information can be transmitted, because if you alter it to impose a signal, the entanglement is broken, and if you don't, there's nothing to transmit.

Ooh. Thanks. That's the clearest, most concise comment I've read on that point, and it answers questions I've had for years. Very much appreciated!

[Piggy]

Originally Posted by Schneibster

Feel free to ask questions, but first, pull up the threads I've done on here about Aspect and the DCQE. I hope that you will find them informative on these subjects. Feel free as well to do a little necromancy and ask any questions you feel are not well answered in those threads- I'll do my best to clear them up for you.

Thank you SO MUCH for taking the time to write that!

I was already familiar w/ many of the concepts -- e.g. spin, the EPR gedankenexperiment, some basic principles of relativity and QM -- so I'm not freaked out by the weird world of the very small and very fast, but was not familiar w/ other key notions, such as Bell's inequality, which I've seen referenced but never followed up on (b/c the arguments in which it was embedded were over my head).

I'll do some more rooting around, and probably will have questions.

My writing partners are going to be pretty ticked at you, I think, b/c they're wanting me to concentrate on a project right now, and this is bound to siphon off a lot of my attention!

If you can recommend any specific threads that are relevant, or any online articles that I'm likely to understand, please post.

But seriously, I can't thank you enough for taking the time to pen that explanation.

Nobody I know personally (friends, family, business associates) understands why I'm so "obsessed" with astronomy, cosmology, and particle physics. But what I don't understand is, how can they be content with their lives and not be curious about the very fabric of reality?! Yeah, I know, I'm just a milltown boy who writes advertisements, but still, if there's anything more meaningful or important to comprehend than this... I simply have no idea what it might be.

[Schneibster]

You and me both, Pigster, that's why I went to the trouble to find out so much about it. In another universe, I'm a particle physicist- but don't live in nearly as nice a house.

[Jimbo07]

Originally Posted by Piggy

But what I don't understand is, how can they be content with their lives and not be curious about the very fabric of reality?! Yeah, I know, I'm just a milltown boy who writes advertisements, but still, if there's anything more meaningful or important to comprehend than this... I simply have no idea what it might be.

(with apologies to, and not directed at your family and friends, because I don't even know them)

Because they already know that the fabric of reality can be explained by gods, hobgoblins and water memory?

[Schneibster]

Search on Aspect and DCQE or Delayed Choice, poster Schneibster. You will get hits, and if you can wade through all the BS (a flamer woo made a mess in one of the threads), you'll find some stuff I hope you will find informative. I may or may not get to you before next weekend; things are moving quickly at Schneibster manor just at the moment. Be patient and I'll do my best to answer your questions.

[Piggy]

Originally Posted by Jimbo07

Because they already know that the fabric of reality can be explained by gods, hobgoblins and water memory?

That's the case for some. The Bible-believers are especially insulated. (A cow-orker opined just last week that Einstein and Hawking wasted their lives because they did not spend them investigating the real Truth. My step-father thinks Strobel's "Case for Christ" is a slam dunk and that global warming is simply "impossible".)

For others, they're just not interested, the way I'm not interested in, say, sports statistics, or "news" (read "gossip") about celebrities. For them, it's just stuff "out there" that doesn't matter to their daily lives.

For me, it tells me what my daily life is.

</hijack>

[Piggy]

Originally Posted by Schneibster

I may or may not get to you before next weekend; things are moving quickly at Schneibster manor just at the moment. Be patient and I'll do my best to answer your questions.

No problem. I'm going on vacation in a couple of weeks, and because of my CDO I'll be freaking out and running around frantically til then. So it may be late May before I really settle in to the trough anyway.

[Paul C. Anagnostopoulos]

Originally Posted by Schneibster

In the locality violation view, the value of the spin on the local coordinated axis is determined when the remote measurement is made, and takes on the value that it must to avoid explicit violation of momentum conservation; but because it is only the collapse and the value of the spin on the remote particle, no information can be transmitted, because if you alter it to impose a signal, the entanglement is broken, and if you don't, there's nothing to transmit.

Is the entanglement always broken? I thought there were ways in which we can entangle multiple particles, which would involve retaining existing entanglements while adding additional ones. Ah, but it would also involve not measuring the particles during the process, wouldn't it?

~~ Paul

[Paul C. Anagnostopoulos]

Originally Posted by Schneibster

In the view where local reality is violated, there is no transmission of information. This is consistent with the results when different axes are measured; the spins are the quantum distribution, not the expected distribution if the spin on the axis coordinated with that measured on the remote particle had actual existence on the local particle. When the spins are measured on the same axis, they are coordinated because they must be by conservation of momentum, otherwise an explicit violation of momentum conservation could be observed.

Can you expand on this? How does the experiment work, exactly? I'm not sure how they can measure the spin on any axes without encountering conservation of momentum.

~~ Paul

[Schneibster]

Originally Posted by Paul C. Anagnostopoulos

Is the entanglement always broken? I thought there were ways in which we can entangle multiple particles, which would involve retaining existing entanglements while adding additional ones. Ah, but it would also involve not measuring the particles during the process, wouldn't it?

~~ Paul

Actually, it is. Here's an interesting experiment that proves it.

Take a polarizer, run a light beam through it, and then orient another identical polarizer at 90 degrees to it, and run the light beam through that afterward. No light comes out. That's because the first polarizer only passed light polarized along its polarization axis, and none of that light is passed by the second because all of the light is polarized in opposition to its axis of polarization.

Now put another polarizer in the middle between them, at 45 degrees to both.

Light comes through the second one now.

How dey do dat?

Obviously, the plane of polarization was changed by the insertion of the second polarizer. But you have to understand that what polarizers measure is the spin of particles in the direction of the polarizer's axis of polarization. So what that means is that all of the particles that pass the polarizer have spin, for example, UP, in the plane of polarization. It's actually more complicated than that, but you get the idea. So when they hit the second polarizer, which only passes particles that have spin UP in its plane of polarization, none of them do, so none of them pass. But insert the third polarizer, which measures the spin in its plane of polarization, and viola, the spins in the plane of polarization of the second polarizer suddenly have a random distribution. So by measuring the spin in an axis midway between the two, suddenly some of them (about half) are changed by the measurement.

This is a very important experiment, and if you doubt the outcome, you should do it for yourself. Polarizing film is quite inexpensive; you can probably do this experiment for less than five dollars. And it proves conclusively that an interaction will break the entanglement, because it doesn't just alter the spin in one axis.

[Schneibster]

Originally Posted by Paul C. Anagnostopoulos

Can you expand on this? How does the experiment work, exactly? I'm not sure how they can measure the spin on any axes without encountering conservation of momentum.

~~ Paul

That's what Bell's Theorem says. He says, if the spins on two axes are coordinated, then it proves that the spin on the unmeasurable axis has a value; but if they aren't, then it doesn't. In other words, we can show this implicit violation of conservation of momentum, but we can't show the explicit one that would result if they weren't coordinated on the same axis. OTOH, if the spins are coordinated, then local reality is preserved. The lack of coordination proves the uncertainty principle isn't just you can't see it, but it doesn't have a value. You've seen to the heart of the matter, you just haven't understood what you've seen in context.

[Piggy]

Schneibster, I just want to tell you a little story from when I was a kid. It's about an experience that first opened me up to wonder, and really sparked my interest in the often paradoxical world of science, and the fact that things aren't really as they seem once we stop to think about them.

We were driving home one evening. I was maybe 6 or 7 years old, in the back seat, staring out the window at the landscape of rural Georgia, the familiar world of my childhood, watching the rows of crops pass me by. I already knew why the moon "stood still" above the "moving" trees and fields -- my father had explained that to me.

So I was thinking about motion, in my small boyish way. I was killing time, uninterested in the grown-up talk, playing silent games. I started trying to look at each row as it was directly in front of me, staring straight ahead without focusing on any landmark.

And I thought, "How long is each row right in front of me?" And almost as soon as the question occurred to me, I realized my dilemma. If it spent any time in front of me at all, then we would not be moving. We would be standing still at each and every place, every point, for some amount of time. And that couldn't be. Obviously, we weren't jerking along like that.

But if each row were not in front of me for any length of time, how could it ever be in that position (or any other) at all?

It was a puzzle. And no grown-up seemed the least interested in it, even though none of them seemed to have an answer.

So when, many years later (when Carl Sagan aired "Cosmos"), I encountered Einstein's ideas, my heart raced. I thought, "Yes, here is a man who understands!" And I saw that my dilemma with the beanrows had echoes and more profound and perplexing variants in the realms of the very small, and the very vast, and the very quick.

Here were grown-ups -- Einstein, Sagan, and others -- who saw that the elephant was not a hat. I loved them immediately. I still do.

[l0rca]

Does anyone else find the absence of local realism a thrilling idea, and almost common-sense one?

[Macoy]

To be honest, it dosen''t seem that anyone's got anything to offer on this one - just like reality.