Live data from Hacker News

Bounding the speed of ‘spooky action at a distance’

arxiv.org

11–20 of 55 posts

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

#11
post #4

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.

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.

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.)

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

#12

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…

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

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

#13

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…

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." :)

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

#14

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…

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.

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

#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 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.

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

#16
post #7

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.

> 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 wou…

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 are amazed!! No matter how many times you run it, you and your friend never get the same card!!!

It's as if the cards are sending spooky signals to each other.

And after you double check that you and your friend are both looking only at 1200 GST, you realize that this spooky signal travels faster than the speed of light (since the it takes no time to "travel" a million miles).

It's like magic, eh?

Not really. The fact there is one of each card means it's impossible for you and your friend to have the same card. No magical spooky signals are being sent.

(I think the quantum mechanical case is analogous to this contrived example. The difference in real quantum mechanics it that you really don't know what card you have until you look at it. However, that doesn't mean that your card is sending a faster-than-light spooky signal to tell the other card what to be. And you certainly can't use it for signalling or anything useful.)

((Reposted to main thread with some slight edits, in case people find this comment useful or interesting.))

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

#18

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…

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.

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

#19

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…

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)

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

#20
post #14

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…

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.

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."

Post reply on HN