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

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

#61

Post by jessriedel » Tue, Oct 30, 2012, 7:54 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…

Physicist here. NegativeK is correct. Bell's theorem is the key to understanding why your explanation doesn't work. Bell's theorem, and the associated experimental demonstration, has been called the most profound discover of science. It's well worth a few hours of your time to try to understand it. You don't need to know any quantum mechanics or advanced math to do so.

Incidentally, I'll mention that your argument is essentially what Einstein et al were arguing for in their famous EPR paper. The fact that it's even possible to rule out local hidden variable theories is highly surprising, and is what makes Bell's discovery (30 years after EPR framed the problem) so monumental.

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

#62

Post by elteto » Tue, Oct 30, 2012, 7:55 PM UTC

IvoDankolov wrote: post #52

cs702 wrote: post #44

I've always found classical Newtonian physics, the Special Theory of Relativity, and (to a lesser extent) the General Theory of Relativity to be understandable at an intuitive level, but quantum phenomena just baffles me, even when I think I "understand" it. PS. IvoDankolov: I find it difficult or impossible to think intuitively about quantum phenomena.

In what terms do you think you "understand" it? What do you make of this problem with distant entangled particles? The double slit experiment and interference in general? The Heisenberg uncertainty principle? (Or as I'd like to call it, Heisenberg's horribly mislabeled-in-order-to-confuse-students principle) A shot in the dark - many of the problems with coming to terms with quantum mechanics arise from trying to imp…

Exactly, we humans try to learn new things by relating to what we already know. This leads to trouble when we encounter new subjects that have no connection to previous experiences, because we have a hard time relating to it. However, the problem with people finding QM esoteric and weird is a bit more nuanced than that.

In most of the other areas of knowledge we can make progress because they do resemble reality in ways that we are familiar with. For example rotation in classical mechanics or special relativity can be somewhat confusing to a beginner, but if we think a little bit deeper than what we are used to then we can see that the results make sense and match what we experience. And also this experience is consistent at different scales. Now you move to QM and the first thing that hits you is that reality "breaks" after some point in the size scale, beyond that everything is different, with uncertainties, probabilities and a bunch of other odd properties. Learning QM is an exercise in mind-stretching, even for the most capable of us. For me the problem is not that it is different, but why is different. Why do we have such a gap between the large scale and the small scale? That is what baffles me.

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

#63

Post by derleth » Tue, Oct 30, 2012, 7:56 PM UTC

morpher wrote: post #37

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...

PDF

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

#64

Post by hasenj » Tue, Oct 30, 2012, 8:01 PM UTC

Osmium wrote: post #36

hasenj wrote: post #29

Earlier quoted context omitted.

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, t…

Why not choose the mundane and least astonishing explanation?

If an object A is moving (with a known momentum) and hits a stationary object B, we can measure the momentum of either A or B and instantly know the momentum of the other object without having to measure it. The two objects are somehow "connected"; their state (momentum) is "entangled".

Why is quantum entanglement different?

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

#65

Post by zellyn » Tue, Oct 30, 2012, 8:14 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…

Thank you for asking this so well. I have been keeping the page hanging around in a tab, waiting until the answers showed up!

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

#66

Post by jere » Tue, Oct 30, 2012, 8:21 PM UTC

Can someone provide or point to a layman's definition of "realism" in quantum mechanics?

Recently on a call-in podcast I listen to, a caller has claimed that the following article shows that a) realism has been disproven and b) that proves we are all in a computer simulation: http://physicsworld.com/cws/article/news/2007/apr/20/quantum...

>Now physicists from Austria claim to have performed an experiment that rules out a broad class of hidden-variables theories that focus on realism -- giving the uneasy consequence that reality does not exist when we are not observing it

Sure. Anyway, I'm certain this guy is equivocating, but I don't even know where to read up on the relevant topics. And I read articles like these and they keep using the word realism without defining it.

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

#67

Post by Ingaz » Tue, Oct 30, 2012, 8:23 PM UTC

nessus42 wrote: post #23

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

Correct if I'm wrong, but I think that Many Worlds Interpetation does not explain a thing. It's like religion - one can think it's possible, other - impossible. But it can't be proved or disproved

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

#68

Post by powertower » Tue, Oct 30, 2012, 8:23 PM UTC

I'm starting to believe that we confuse the mathematics of quantum mechanics with being the actual underlining physical phenomena, to the point where we start to believe that "wave function collapse" are real, rather than just a way of looking at something via probabilities; or being the models, or abstractions, or the artifacts of the maths involved.

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

#69

Post by zellyn » Tue, Oct 30, 2012, 8:24 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…

Nobody seems to like my theory that the programmers of the simulation we're all living in were deferring procedurally generating that level of detail until it was necessary because we'd actually examined it closely.

They're probably still cursing our Hubble telescope, not to mention computers with their decreasing semiconductor scales.

This comment probably just triggered their simulation-awareness detector. :-)

Actually, I think there's a silly but fascinating SciFi short story waiting to be written about assembling a group of people to try to privilege-escalate out of the simulation without triggering the simulation-awareness detectors. "I need you to think about this, but not all at the same time, or in the same place. Oh, and try to think about it only in vague terms."

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

#70

Post by Symmetry » Tue, Oct 30, 2012, 8:27 PM UTC

Osmium wrote: post #38

MBlume wrote: post #27

Earlier quoted context omitted.

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.

Lets say you have some experiment which gives you a 1 or a 0, and you have two entangled particles which will give the same result depsite the result being random and the measurements being taken outside each other's lightcones. In Collapse QM there was some instantanious communication making that possible.

In Many Worlds you'd find that for each measurement the wavefunction split into two branches, one corresponding to 1 and one to 0, in each location. From the sites of both measurements the decoherence spreads outwards as particle interactions happen, and when they finally touch the two 1 segments and the two 0 segments merge, and there is one area split along 1/0 in the wave function instead of two.

That's all horribly simplified, but should get the idea across.