< Earlier Kibitzing · PAGE 27 OF 27 ·
|Nov-26-14|| ||al wazir: <zanzibar: This statement implies a causal connection which is physically impossible.> It's been verified experimentally. Sorry about that.|
I think you're being misled by your envelope analogy. Even if you don't know what's in the envelope, whatever is there doesn't change. It is what it is. Not so with electrons or photons. Each electron is in a *superposition* of two states, spin up and spin down. It is *not* definitely one or the other.
Imagine that in two labs a coin is tossed. Suppose that the coins are correlated in the same fashion as electrons. Each coin can land heads or tails with equal probability; but whichever way it falls, *the coin in the other lab has to fall the other way* -- even if the labs are a light year apart. Why do they do that? Because (by my assumption) they obey the laws of quantum mechanics.
|Nov-26-14|| ||zanzibar: <al wazir> OK, but the state isn't|
|1>|2> implying two independent particles (somehow correlated).
The state is |1,2>.
The mystery is quantum measurement, having two particles (even if separated by a space-like interval) isn't different from having one particle.
You make a measure and have determined the state. Be it one particle or two.
What's the difference?
|Nov-26-14|| ||Shams: <al wazir> Weren't you the one who said, a year or so ago, that quantum entanglement only seems to imply action at a distance but doesn't really?|
|Nov-26-14|| ||al wazir: <Shams: Weren't you the one who said, a year or so ago, that quantum entanglement only seems to imply action at a distance but doesn't really?> Probably. (I could answer with some flip line about how I'm in a superposition of two states, "yes" and "no," but I won't.)|
Correct me if I'm wrong: I haven't used the expression "action at a distance." <Zanzibar> did.
|Nov-26-14|| ||nok: <quantum entanglement only seems to imply action at a distance but doesn't really?> Indeed. When one party updates its information, it doesn't mean the description of the other party is wrong.|
|Nov-26-14|| ||Shams: <al wazir> No I think you're right. I'm just trying to follow along, please ignore me.|
|Nov-26-14|| ||chancho: Albert Einstein, a real Player:
|Nov-26-14|| ||WannaBe: Chicks dig the long (differential) equations.
(Apologies to the MLB ad)
|Nov-26-14|| ||zanzibar: <al> but you did the word instantaneous(ly), which amounts to the same thing. |
And the statement <It's been verified experimentally.> also needs care.
<QM> has been verified experimentally - not the instantaneous transmission of information which I think you're suggesting.
The current standard model of physics has nothing going faster than the speed of light.
(The advanced Green's functions get throw out by hand)
* * * * *
OK, I admit the envelop analogy is flawed, since it suggests hidden variables.
I go back to the teeter-totter analogy.
If you have conversation of angular momentum and a spin-zero decay of positronium, you will always find one end down when the other is up. (|1R,2L> or |1L,2R>).
Again, suppose I have a single particle system with L,S degrees of freedom. Suppose L can be a or b, while S can also be a or b.
Start with a QM superposition:
1/sqrt(2)( |L(a),S(a)> + |L(b),S(b)> )
Now if I make a measure on L and get |L(a)> I know I get |S(a)> too. Even if I wait to measure S. That's because it's a quantum state, but it's not so mysterious.
What's the difference of a one-particle, two degrees of freedom quantum state and a two-particle, one degree of freedom quantum state.
You make a measurement, you find the state, and you know something about the part of the state you didn't measure.
Because the correlation is built into the state.
QM states are mysterious, not the two particles flying off to infinity.
|Nov-26-14|| ||zanzibar: Sorry, but to be explicit:
<<al> but you did the word instantaneous(ly), which amounts to the same thing.>
The <same thing> being action-at-a-distance (whether it's force or information transfer).
|Nov-26-14|| ||al wazir: <zanzibar: And the statement <It's been verified experimentally.> also needs care. <QM> has been verified experimentally - not the instantaneous transmission of information which I think you're suggesting.> No, *information* is not transmitted instantaneously. |
Here, from the wiki article on"quantum entanglement," is a summary of the current state of affairs: <Schrödinger was dissatisfied with the concept of entanglement, because it seemed to violate the speed limit on the transmission of information implicit in the theory of relativity. Einstein later famously derided entanglement as "spukhafte Fernwirkung" or "spooky action at a distance." The EPR paper generated significant interest among physicists and inspired much discussion about the foundations of quantum mechanics (perhaps most famously Bohm's interpretation of quantum mechanics), but produced relatively little other published work. So, despite the interest, the flaw in EPR's argument was not discovered until 1964, when John Stewart Bell proved that one of their key assumptions, the principle of locality, was not consistent with the hidden variables interpretation of quantum theory that EPR purported to establish. Specifically, he demonstrated an upper limit, seen in Bell's inequality, regarding the strength of correlations that can be produced in any theory obeying local realism, and he showed that quantum theory predicts violations of this limit for certain entangled systems. His inequality is experimentally testable, and there have been numerous relevant experiments, starting with the pioneering work of Freedman and Clauser in 1972 and Aspect's experiments in 1982. They have all shown agreement with quantum mechanics rather than the principle of local realism. However, the issue is not finally settled, as each of these experimental tests has left open at least one loophole by which it is possible to question the validity of the results.> http://en.wikipedia.org/wiki/Quantu...
Here's the experimental test (cited above) that confirmed "spookiness": http://journals.aps.org/prl/abstrac...
|Nov-26-14|| ||al wazir: From the same article (I should have copied this instead of what I just posted): <Experiments have been performed involving measuring the polarization or spin of entangled particles in different directions, which—by producing violations of Bell's inequality—demonstrate statistically that the local realist view cannot be correct. This has been shown to occur even when the measurements are performed more quickly than light could travel between the sites of measurement: there is no lightspeed or slower influence that can pass between the entangled particles. Recent experiments have measured entangled particles within less than one part in 10,000 of the light travel time between them. According to the formalism of quantum theory, the effect of measurement happens instantly. It is not possible, however, to use this effect to transmit classical information at faster-than-light speeds (see Faster-than-light → Quantum mechanics).>|
|Nov-26-14|| ||zanzibar: <al wazir> ok then, the paper you cite is the Aspect paper.|
The wiki use of <local realism> is what I would call <hidden variables> I think.
Mermin's work from 1985 was basically inspired by the recent Aspect et al experiment:
<Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell's Inequalities
Phys. Rev. Lett. 49, 91 – Published 12 July 1982>
It is expected to violate Bell's Inequalities if <QM> is correct.
I believe there are other even larger measured violations subsequent to the 1982 result. I have to look it up...
According to this 2013 book:
<The Quantum Divide (2013)>
The largest observed violation (at 1200 std) is from a 2005 expt by Altepeter, Jeffery, Kwiat at U. Illinois.
The abstract is here:
<Here we present experimental realizations of two new entanglement detection methods: a three-measurement Bell inequality inequivalent to the Clauser-Horne-Shimony-Holt inequality and a nonlinear Bell-type inequality based on the negativity measure. In addition, we provide an experimental and theoretical comparison between these new methods and several techniques already in use: the traditional Clauser-Horne-Shimony-Holt inequality, the entanglement witness, and complete state tomography.>
The three measurement idea probably inspired Mermin's more involved gerdankin used in his Berkeley talk.
|Nov-26-14|| ||zanzibar: <al> I assume <According to the formalism of quantum theory, the effect of measurement happens instantly.> was copied from a wiki article and not a PRL or other physics paper?|
|Nov-27-14|| ||al wazir: <zanzibar: I assume <According to the formalism of quantum theory, the effect of measurement happens instantly.> was copied from a wiki article and not a PRL or other physics paper?> Yes. I apologize for leaving that ambiguous.|
|Nov-27-14|| ||Joshka: <ernieb> That's absurd. We observe what is there cause we CAN SEE IT!! To nail your statement in a very clear example: You are saying the moon is only there cause we can observe?? It's not really there??LOL|
|Dec-04-14|| ||Sneaky: <my recommendation steadfastly remains> I agree, that was a good video.|
I *still* haven't quite gotten my head around this Bell experiment, but I get the feel for it's nature. What it comes down to is that if you suggest that the particles have definite spins (or "colors" in Mermin's metaphor) then you run into contradiction with empirical evidence.
Here are my takeaways so far:
1. The reason why the "hidden variable" explanation (e.g. my envelope metaphor, or Einstein's left/right glove metaphor) doesn't work is *very* subtle. It can't be explained concisely. You have to get into the guts of a rather complicated experiment and even then it's not obvious until you really reflect on what's going on.
2. Einstein was no dope for suggesting the hidden variable explanation; there was every reason to believe that. It's easy to see why hidden variables cannot explain the classic double-slit experiment, but it took some real ingenuity to show that it can't explain entanglement.
3. 99% of the people who discuss entanglement have only a vague idea of what they are talking about, if even that.
|Dec-04-14|| ||ughaibu: Hey Sneaky! Here you go: http://arxiv.org/pdf/0807.3286.pdf|
|Dec-04-14|| ||Sneaky: Um, by "Bell experiment" I suppose I meant "Aspect experiment" — Mermin wasn't entirely clear on that.|
I am also curious as to what he means by "stuff left behind." Surely electrons don't leave skid marks?
And am I to understand that the experiment requires a three-way entanglement?
|Dec-04-14|| ||Sneaky: Thanks ughaibu.|
|Dec-05-14|| ||WannaBe: Princeton have opened up the Einstein (eLibrary) papers!!! And it's Freeeeeeeee! Wheeeeee.|
|Dec-05-14|| ||Jim Bartle: Sure it's not "relatively" free?|
|Dec-05-14|| ||Shams: <JB> You're going to the timeout room for that one.|
|Dec-05-14|| ||Jim Bartle: Theoretically.|
|Dec-12-14|| ||Sneaky: OK, here's my updated envelope-analogy and the reason why it fails.|
To restate the idea: suppose we have 6 playing cards: a red ace and a black ace, a red king and a black king, a red queen and a black queen. We have two envelopes with special three-way pouches, labelled "A Q K", so that each three-way envelope contains an ace, a king, and a queen. Randomly and without looking, I put an ace into the A-pouch on both envelopes, the queens into the Q-pouches, and a king into the K-pouches.
Furthermore, we have to accept the proposition that it's impossible to open one of the pouches without destroying the other two cards. It's booby trapped with incendiaries or something.
Now I mail one of the envelopes to my friend 5000 miles away, and at agree to peek at our ace-pouches at exactly the same time. Obviously, if he sees a red ace I will see a black one, and vice versa. Nothing mysterious is going on there. No reason to invoke "spooky action at a distance."
But here's the big wrinkle:
I look in any of three pouches at random, I have a 50/50 chance of getting a red card, right?
What if my confederate opened his A-pouch before I even received my package, and saw a black ace? Surely then it would be logical to assume that the odds of my finding a red card are more than 50/50, right? He knows that one of my choices must be red, and the other two could be anything, so it must be more than 50%. (If my mental math is right, the odds of getting a red card should be 2/3.)
But that's where my metaphor breaks down with reality. If these envelopes truly behaved in a quantum mechanical way, my chances of getting a red card would *still* be 50/50 even when my friend has discovered I am holding a red ace. This occurs because the odds of my finding a red card in the non-ace slots mysteriously goes down, so that the overall odds are still 50/50.
Could this be because there is some principle that makes a red ace imply less likeliness of a red queen or king? Perhaps the deck isn't very well shuffled, or like-colored cards repel one another, or some weird principle like that is in play?
But that doesn't make any sense, because it was entirely arbitrary which pouch my friend peeked into. He might have said, at the very last moment, "Forget this plan to look at our ace pouches. I'm going to look at my queen pouch on a whim." In that case the color of the queen would be known and the probability of the colors in the other two pouches would shift.
So at this point I have to tear my envelopes and cards to shreds as they are entirely insufficient to model what's going on here. Wow. It's like nature is trolling us.
< Earlier Kibitzing · PAGE 27 OF 27 ·