Quantum entanglement shows that reality can't be local • Hacker News Board

Re: Quantum entanglement shows that reality can't be local

#31

Post by Osmium » Tue, Oct 30, 2012, 7:12 PM UTC

spuz wrote: post #22

Osmium wrote: post #9

Earlier quoted context omitted.

"However, both particles have the property from their inception, and so no faster-than-light or non-local interpretation should be needed." This is incorrect, both particles have a superposition of k and ~k (to use your terminology). It is not the case that when the wavefunction collapses, we just suddenly find out the "internal state" of the particle. This "internal state" simply does not exist until the wavefunctio…

> both particles have a superposition of k and ~k (to use your terminology). ... This "internal state" simply does not exist until the wavefunction collapse has occurred. What would the implications be if particles did have an internal state?

It would be a violation of Bell's theorem as we currently understand it. In other words, it can't happen, unless our understanding is wrong. (For more info, the wiki on Bell's theorem has been linked to elsewhere in this thread.)

Re: Quantum entanglement shows that reality can't be local

#32

Post by aroberge » Tue, Oct 30, 2012, 7:12 PM UTC

ecliptic wrote: post #20

If you were able to construct a rigid beam 25 million miles long wouldn't you be able to transmit data (push a button) on the other end faster than the speed of light?

It is impossible to construct a perfectly rigid beam, in the sense that you are thinking of. A "rigid beam" would be made up of matter and, as you push on one end, you would trigger a wave that would propagate from neighbour to neighbour until it reaches the other end.

Re: Quantum entanglement shows that reality can't be local

#33

Post by fixermark » Tue, Oct 30, 2012, 7:14 PM UTC

fixermark wrote: post #4

One thing I've never understood about interpretation of quantum entanglement experiments: Based on what I've learned of these experiments, it seems to me that the least mind-bending interpretation is that the entanglement event results in two particles that have some complementary property k (i.e. one particle has k and the other has ~k) and which one of the two, k or ~k, is had by one of the particles is unknowable…

(Replying to my own comment as basically every response raised Bell's Theorem)

This is what I like about Hacker News; when I ask a deeply technical question in a field removed from my own, there are plenty of people who can point me in an interesting direction.

Bell's Theorem was the aspect of observational measurement that I was missing, and it's absolutely fascinating. Even moreso in that it was envisioned mathematically before we had the technology to prod at its claims. Thank you everyone for showing it to me.

Re: Quantum entanglement shows that reality can't be local

#34

Post by Liron » Tue, Oct 30, 2012, 7:17 PM UTC

I hate this article, because locality is an important property that we think reality does have.

It's like saying "Gravity violates conservation of energy!!!" No it doesn't; quit playing fast and loose with well-understood principles.

Re: Quantum entanglement shows that reality can't be local

#35

Post by ecliptic » Tue, Oct 30, 2012, 7:17 PM UTC

MBlume wrote: post #24

ecliptic wrote: post #20

If you were able to construct a rigid beam 25 million miles long wouldn't you be able to transmit data (push a button) on the other end faster than the speed of light?

Nope. Pushing on the end of the beam sends a pressure wave through the beam. The button gets pushed when the wave reaches the other end. Wave travels significantly slower than light.

If you push on the beam and the other end does not move right away doesn't that imply that the beam has compressed and is not rigid?

Re: Quantum entanglement shows that reality can't be local

#36

Post by Osmium » Tue, Oct 30, 2012, 7:17 PM UTC

hasenj wrote: post #29

fixermark wrote: post #4

One thing I've never understood about interpretation of quantum entanglement experiments: Based on what I've learned of these experiments, it seems to me that the least mind-bending interpretation is that the entanglement event results in two particles that have some complementary property k (i.e. one particle has k and the other has ~k) and which one of the two, k or ~k, is had by one of the particles is unknowable…

I agree with you. And further more, wouldn't entanglement itself be some sort of evidence that the particle does have an internal state even before being observed? For instance, I could phrase things like this: - Let particles A and B get into an entangled state - Measure the state of particle A, see that it has property k. - Predict that B has property ~k even though it's not being measured yet. - Measure B. If B ha…

"If B has property ~k, then we've proven that B had an internal state before we performed the measurement."

This part is the flaw in the logic. That doesn't prove that B has an internal state. All it proves is that, in some sense, B's state is related to A (assuming you repeat the experiment until you reach statistical significance).

Entanglement is nothing more than saying one particle is connected, in some sense, to another. Like a see-saw: if one side goes up, the other must go down. It's only "confusing" in the sense that the particles are physically separated, and the link between them is not mediated by a force as we understand it. Instead, they're linked by their very nature. It's almost as if you have just one particle, that's somehow been split into two and spatially separated. That's the conceptually difficult part.

Re: Quantum entanglement shows that reality can't be local

#37

Post by morpher » Tue, Oct 30, 2012, 7:18 PM UTC

fixermark wrote: post #4

One thing I've never understood about interpretation of quantum entanglement experiments: Based on what I've learned of these experiments, it seems to me that the least mind-bending interpretation is that the entanglement event results in two particles that have some complementary property k (i.e. one particle has k and the other has ~k) and which one of the two, k or ~k, is had by one of the particles is unknowable…

This article[1] by David Mermin does a good job explaining why the second particle couldn't have always been a ~k particle.

[1] http://www.phy.duke.edu/undergraduate/physics-articles/mermi...

Re: Quantum entanglement shows that reality can't be local

#38

Post by Osmium » Tue, Oct 30, 2012, 7:19 PM UTC

MBlume wrote: post #27

nessus42 wrote: post #23

This article completely ignored the Many Worlds Interpretation, which preserves all of realism, locality, and Relativity.

Yeah. It shouldn't be that surprising that when you arbitrarily assume that part of the wavefunction magically disappears sometimes, you get weird results like non-locality.

Could you expand on this a little more? I'm not quite sure I follow.

Re: Quantum entanglement shows that reality can't be local

#39

Post by danielweber » Tue, Oct 30, 2012, 7:19 PM UTC

ChuckMcM wrote: post #3

This is for me the most interesting part of the whole teleportation / entanglement research. When these papers started coming out we debated the notion of 'faster than light' communication. The counter argument was that you had to move the particles apart and that was constrained by the speed of light. Then the question of "when" the state was resolved was pondered. There were two thought experiments proposed at this…

and have the alien equivalent of the FCC come out to the planet to arrest the scientists for transmitting without a license

Or the channel is already filled completely with spam.

Re: Quantum entanglement shows that reality can't be local

#40

Post by jmaygarden » Tue, Oct 30, 2012, 7:21 PM UTC

ffk wrote: post #8

This reminds me of the Ansible, a communication device allowing instant or near instant faster-than-light communication. It is seen in many science fiction works and often justified as quantum entanglement in practice. https://en.wikipedia.org/wiki/Ansible

I didn't realize that Ursula Le Guin coined that term. My first recollection of it was from Orson Scott Card and later, Dan Simmons. In any event, entanglement certainly doesn't look like it will lead to an ansible yet.