Bounding the speed of ‘spooky action at a distance’

http://arxiv.org/abs/1303.0614

by espinchi • 13 years ago

47 52 13 years ago

arxiv.org

loceng• 13 years ago

This would be helpful for intergalactic domination. Kidding.

digz• 13 years ago

I sent this to my friend who has a phd in physics and is a post-doc at Cambridge and he said it's bogus.

ChrisClark• 13 years ago

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.

dasil003• 13 years ago

Whoah you're jumping to a strange conclusion here. How do you know he didn't actually look at the paper and respond based on the contents rather than first thing that happened to pop into your mind?

n3rdy2• 13 years ago

> 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 its observable then how could information not be transmitted?

I'm the furthest thing from a physicist but if quantum entanglement causes an atoms polarity to change because millions of miles away another atom it is entangled with did the same, that would seem like an excellent way to encode information into bits, supposing you can change the atoms polarity at a whim.

tedsanders11• 13 years ago

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 different locations.

mratzloff• 13 years ago

My understanding is there are scientists working on quantum entanglement communication. If so, I'm not sure why you assert no information transfer. (Not a physicist.)

tedsanders11• 13 years ago

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 or information is not transmitted, then what is? I'd argue nothing.

(-A physicist)

_red• 13 years ago

But it takes energy to arrange, synchronize, and release that energy (ie. that synchronization didn't come without an energy cost).

shardling• 13 years ago

It's still worthwhile to check that it really does work like that.

But somehow, no one ever titles these articles "Physics continues to work exactly as expected." :)

tedsanders11• 13 years ago

I wholly agree. :)

bpicolo• 13 years ago

Except in the case of the top article,as far as I understand, information could in theory be transmitted thanks to time. A spin change can represent a 1 bit, and the lack of a spin change at a time can represent a 0 bit.

tedsanders11• 13 years ago

No, entanglement cannot transmit information. It does not (necessarily) violate speed-of-light causality.

Like eoinmurray92 said, entanglement can be useful for cryptography.

It can also be useful for quantum computing, where you need to find clever algorithms where the wrong answers "cancel out."

eoinmurray925• 13 years ago

It can be used to share a cipher (one-time-pad) because you and I would know the arrangement of bits that the other person has, and no-one else would/could know. However the measurements on the qubit's are probabilistic, so we cannot control the arrangement of bit that we have. We will only know if the are correlated/anti-correlated.

Filligree2• 13 years ago

What you're suggesting sounds like a hidden variable theory, which have been thoroughly disproven (Bell's Theorem).

Quantum mechanics cannot work through merely pre-arranging actions.

tedsanders11• 13 years ago

To nitpick: Only LOCAL hidden variable theories are disproven by Bell's theorem.

However, I agree with your point that my example is not compatible with quantum mechanics.

I only meant to give the sense that nothing is travelling, even though you can semantically say something is.

scythe3• 13 years ago

See my comment on reddit here for why it's not quite that simple:

http://www.reddit.com/r/askscience/comments/19w1ag/why_is_qu...

(Yes, the probabilities are fudged)

didgeoridoo• 13 years ago

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.

GuiA• 13 years ago

(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.

GhotiFish• 13 years ago

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.

lisper2• 13 years ago

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.

Yossarian_Lives• 13 years ago

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.

scythe3• 13 years ago

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 long distances. The loophole was that it previously wasn't possible to time the ecperiment well enough to make sure that the whole thing didn't occur in some unexpected, subluminal way. Now we know it couldn't have occurred by some sort of classical interaction between the entangled qubits.

So how fast is the interaction, really? Well, it turns out that if something moves even 0.0001% faster than light, we can always find a reference frame in which it moves a million times faster than light, or infinitely fast, or even in which it appears to move at some huge negative velocity: back in time. See "tachyonic antitelephone". In other words, once something is faster than light, it is, essentially, instantaneous. This is in accordance with standard predictions of quantum mechanics.

Source: I'm a physics grad student.

lisper2• 13 years ago

Bounding the speed of ‘spooky action at a distance’

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