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Bounding the speed of ‘spooky action at a distance’

arxiv.org

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Re: Bounding the speed of ‘spooky action at a distance’

#31

Post by tedsanders »

Filligree wrote: post #25
tedsanders wrote: post #23

It's like you and a friend have a deck of cards. You each take a card from the deck, and then travel a million miles away. Then, at the crack of noon (synchronized to GST, of course), you both look at your cards. Your card is the 3 of Clubs. You travel back to meet your friend, and discover that his card (which he measured at noon, just like you) was the 7 of Diamonds. You repeat this experiment a gazillion times and…

What you're describing is a hidden-variable theory, which has been ruled out by Bell's Theorem. Sorry.

No need to be sorry. :)

The point of my post is to convey the sense that this article headline has when it says that "spooky action" has a speed.

Then, in the last paragraph, I ask whether the example I gave is actually analogous to quantum mechanics. I think it is, particularly if you see things through the many-worlds interpretation.

In the many-worlds interpretation, you and your card exist in a superposition of 52 states, 1 for each possible card. When you look at your card, your wavefunction separates into 52 different experiences and that effect propagates out into the universe at a speed less than the speed of light. Nonetheless, when you compare your results with your friend, you find that you never got the same card, even though you looked at the same time.

Nitpick: Bell's Theorem doesn't rule out hidden variable theories. Only local hidden variable theories.

Re: Bounding the speed of ‘spooky action at a distance’

#33

Post by lisper »

scythe wrote: post #15

So what they actually measured is something that appears to go faster than light in the reference frame of measurement. This does not mean they measured the actual speed of the interaction. Instead, the point of this experiment is to lose a loophole in Bell test experiments: simply put, Bell experiments had previously proven that this seemingly-"non local" (bear with me) state exists and preserves dntanglement over l…

You might enjoy these then:

http://blog.rongarret.info/2011/09/faster-than-light-probabl...

http://www.flownet.com/ron/QM.pdf

Re: Bounding the speed of ‘spooky action at a distance’

#34

Post by eoinmurray92 »

Everyone here is getting confused by the fact that you cannot control the polarity/polarization of the qubits before/as you send them.

This means that one cannot send information faster than light.

What is useful however is the fact that when you send entangled qubits very far away, and measure them is the fact that the results of the measurements are always correlated/anti-correlated.

As an example.

Alice has a pair of entangled qubits, they are in a superposition of 1 and 0 (lets say they are an correlated pair, such that when measured both quibits will always agree).

She sends one to Bob.

Bob and Alice now hold one qubit each out of the entangled pair. They both measure their qubits. The measurement is probabilistic, you have a 50/50 chance of getting a 1 or a 0, but Bobs quibits and Alices quibit will agree everytime.

Since you can't control the outcome of the experiment you cannot decide which bit to send and hence cannot send information.

The useful result is that Alice and Bob have a string of bits which only the two of them know the arrangement of. They can use the string of bits as a one-time pad for encryption.

I have simplified this immensely but I think the point gets across.

Source : I currently undertake research into generating qubits for quantum computation and communication purposes.

Re: Bounding the speed of ‘spooky action at a distance’

#35

Post by GuiA »

didgeoridoo wrote: post #28
tedsanders wrote: post #10

Suppose you and a distant friend both agree to wave a flag at 1200 GST. To an alien observing Earth, it looks as if the "human flag-raising interaction" is an instant effect that "travels" faster than the speed of light, since the flag raising happens at the same time in two places. But really, nothing is travelling. You and your friend just pre-agreed to do something at the same time at different places. If energy o…

Almost, but spooky action is quite a bit spookier. Suppose you and your friend both agree to MAYBE wave a flag every day at 1200 GST, then communicate no further about it. The alien would always observe that either both flags are being waved, or both are not.

(My physics education is limited to some undergrad level classes, so sorry if my interpretation is erroneous)

Could it be that spooky action has underlying causes that we can't tell yet? For example, reusing the flag metaphor- you and your friend decide to MAYBE wave a flag every day at 1200 GST, and then in practice it happens that every sunny day you both wave it (because hey it's sunny, everybody likes to go outside) and on rainy days no one does (because it's warm and dry inside in your house).

An alien with no knowledge of meteorology and our reaction to it observing this would always observe that either both flags are waved, or both are not, in a seeming random matter, where in fact it is not random at all.

Re: Bounding the speed of ‘spooky action at a distance’

#36

Post by GhotiFish »

didgeoridoo wrote: post #28
tedsanders wrote: post #10

Suppose you and a distant friend both agree to wave a flag at 1200 GST. To an alien observing Earth, it looks as if the "human flag-raising interaction" is an instant effect that "travels" faster than the speed of light, since the flag raising happens at the same time in two places. But really, nothing is travelling. You and your friend just pre-agreed to do something at the same time at different places. If energy o…

Almost, but spooky action is quite a bit spookier. Suppose you and your friend both agree to MAYBE wave a flag every day at 1200 GST, then communicate no further about it. The alien would always observe that either both flags are being waved, or both are not.

It's far less spooky when you consider both you and your friend are basically the same person, and the criteria you use for considering whether or not to raise the flag are the same.

That's my understanding of entanglement, but I get the feeling it's a wrong view.

Re: Bounding the speed of ‘spooky action at a distance’

#37

Post by lisper »

didgeoridoo wrote: post #28
tedsanders wrote: post #10

Suppose you and a distant friend both agree to wave a flag at 1200 GST. To an alien observing Earth, it looks as if the "human flag-raising interaction" is an instant effect that "travels" faster than the speed of light, since the flag raising happens at the same time in two places. But really, nothing is travelling. You and your friend just pre-agreed to do something at the same time at different places. If energy o…

Almost, but spooky action is quite a bit spookier. Suppose you and your friend both agree to MAYBE wave a flag every day at 1200 GST, then communicate no further about it. The alien would always observe that either both flags are being waved, or both are not.

It's even worse than that.

You each have four flags. The first flag is black on one side, red on the other. The second is black on one side, green on the other. The third is white on one side, red on the other, and the fourth is white on one side, green on the other. (In other words, one side is black or white, the other side is red or green.)

The flags have the peculiar property that as soon as they are exposed to enough light to see what color they are, they burst into flames. So you can look at one side of a flag, or the other, but you can never see both sides of the same flag.

You and your (entangled) friend each choose a flag. The alien observer observes the following:

1. Your choice of flags, considered independently of your friend's choice, is indistinguishable from random. Likewise, your friend's choice of flag, considered independently of your choice, is indistinguishable from random.

2. Nonetheless, if the alien looks at the black-white side of both flags, they are always the same. Likewise, if he looks at the red-green side, they are always the same.

Re: Bounding the speed of ‘spooky action at a distance’

#38

Post by Cushman »

tedsanders wrote: post #8
ChrisClark wrote: post #5

Earlier quoted context omitted.

Then he didn't understand what you were asking. It can easily be faster than the speed of light because no information can be transmitted through this phenomenon.

If no information or energy is transmitted, then what exactly is traveling faster than the speed of light? Nothing. It's just as if you and a distant friend pre-agree to lift a flag at 1200 GST. The "flag-raising interaction" is instant, "travelling" faster than the speed of light, but really nothing is travelling faster than the speed of light. You and your friend just pre-agreed to do something at the same time in…

Things can travel faster than light. Say you sweep a spotlight across the moon. You could easily observe a spot of light traveling from one side of the moon to the other much faster than the speed of light. The spot of light is a "thing" the way we describe it; look at it, it's moving. But it can't be used to send information from one side of the moon to the other, so it's not a "thing" the way the universe describes it.

Re: Bounding the speed of ‘spooky action at a distance’

#39

Post by scythe »

lisper wrote: post #33
scythe wrote: post #15

So what they actually measured is something that appears to go faster than light in the reference frame of measurement. This does not mean they measured the actual speed of the interaction. Instead, the point of this experiment is to lose a loophole in Bell test experiments: simply put, Bell experiments had previously proven that this seemingly-"non local" (bear with me) state exists and preserves dntanglement over l…

You might enjoy these then: http://blog.rongarret.info/2011/09/faster-than-light-probabl... http://www.flownet.com/ron/QM.pdf

My favorite interpretation so far:

http://en.wikipedia.org/wiki/Relational_quantum_mechanics

It's sort of like many worlds with a whole lot of unnecessary baggage stripped away.

Re: Bounding the speed of ‘spooky action at a distance’

#40

Post by Yossarian_Lives »

tedsanders wrote: post #10

Suppose you and a distant friend both agree to wave a flag at 1200 GST. To an alien observing Earth, it looks as if the "human flag-raising interaction" is an instant effect that "travels" faster than the speed of light, since the flag raising happens at the same time in two places. But really, nothing is travelling. You and your friend just pre-agreed to do something at the same time at different places. If energy o…

I'm inclined to agree with your premise, but your analogy implies that it's as a result of our pre-synchronisation that there's no FTL (faster than light) communication taking place. Entanglement is weirder than that though as Bell inequalities attest - it's as if I raised my flag at 11:59 and you did too despite the prior agreement.

I'm not sure if current physics has an explanation, but the last time I read around this, it seemed that the relativist position was that no FTL communication was occurring between particles because a transfer of information would require a slower than light communication between us so that we both knew what to measure. Or to pose it a different way - even if the electrons seem to communicate faster than light, technically we can't tell that it happened until later.

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