Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

How Bell’s Theorem proved ‘spooky action at a distance’ is real

quantamagazine.org

Post reply on HN
Print view View original

101–110 of 356 posts

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#101

Post by ThePhysicist »

There's no spooky action at a distance. Let's imagine we have an entangled qubit system that consists of a superposition of the states (0,1) and (1,0), i.e. either part A is in state 0 and part B in state 1 or vice versa. When we perform a measurement on the first part of the system and obtain 1, it simply means that we have "branched" into the (1,0) state of the system. This branching is usually irreversible because of the decoherence caused by the measurement (which itself is just an ordinary quantum process). There is no information exchange or any type of exchange between the two parts of the system going on, we simply branch into a part of the probability space defined for the system. The question whether the other branches still exist then leads to either the "classical" interpretation of quantum mechanics or the "many worlds" interpretation. The latter seems to be favored today as we know that there's nothing special about the measurement process that causes the collapse of a wave function (it's a quantum process in itself), but in the end there's not really a way to test this so it's really more of a philosophical question.

Articles about "spooky action at a distance" should really mention this, as we have a much better understanding of the measurement process in quantum mechanics today than Einstein et. al. had when they wrote their paper.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#102

Post by guyomes »

nsxwolf wrote: post #27

I'll never understand entanglement. Every explanation makes me wonder why it can't be used to instantaneously send a message. I never fully understand the explanations why it can't be used to do so. I don't understand how you can be sure about the state of the other particle, what if someone already measured it and then did something to it?

I find that this article [0] from Conway and Kochen is helpful. The authors do not really explain the paradoxes of quantum mechanics. Instead they reduce them to minimal fundamental axioms that have been tested and observed experimentally, even though they are arguably highly counter-intuitive (notably SPIN and TWIN). Based on those axioms, the authors show that you cannot send a message through entanglement. More precisely, they show that a particle has a free will, in the sense that the result of a measurement on it "is not a function of properties of that part of the universe that is earlier than this response".

[0]: https://www.ams.org/notices/200902/rtx090200226p.pdf

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#103

Post by mmaroti »

abetusk wrote: post #53
nsxwolf wrote: post #27

I'll never understand entanglement. Every explanation makes me wonder why it can't be used to instantaneously send a message. I never fully understand the explanations why it can't be used to do so. I don't understand how you can be sure about the state of the other particle, what if someone already measured it and then did something to it?

Let's say particles have a 'direction angle' that we can measure with a detector that only gives 'up' or 'down' relative to a direction angle measurement. We can change this direction angle measurement with a knob to set what the measured 'up' and 'down' answers are relative to the detector's direction angle. Further let's say particles can be quantum entangled so that when when two detectors are placed very far apar…

I think you need more assumptions. This satisfies your requirements:

X(0) = 0000, Y(a) = 0001, X(a) = 0011, Y(2a)= 0111.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#104

Post by gpsx »

gpsx wrote: post #38
willyg123 wrote: post #4

What is the prevailing theory to explain quantum entanglement? Must there be another dimension we cannot access or measure that is not subject to the laws of relativity? (I understand the laws of relativity break down at the quantum level but please ELI5)

I think people get confused when they think that each object has a wave function. This is not correct. The universe has one wave function. The wave function consists of a bunch of possible states along with the coefficient for each state. You can think of each state as being a distinct snapshot of what the universe might look like - including for example the position and spin of each particle. In the example of two e…

I want to add to my above comment. Non-entanglement is a special mathematical case, but it happens quite often. If the two particles never interact in any way, then the special condition will be true and they will not be entangled. There is another case where the particles _appear_ not to be entangled. This is when the wave function is so jumbled that even though the particles are entangled you can't detect it. This is called a decoherence. This also happens quite often and is why macroscopic quantities don't exhibit entanglement and hence quantum behavior.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#105

Post by virgil_disgr4ce »

ThePhysicist wrote: post #101

There's no spooky action at a distance. Let's imagine we have an entangled qubit system that consists of a superposition of the states (0,1) and (1,0), i.e. either part A is in state 0 and part B in state 1 or vice versa. When we perform a measurement on the first part of the system and obtain 1, it simply means that we have "branched" into the (1,0) state of the system. This branching is usually irreversible because…

If there's one single phrase I wish I could erase from history it's "Spooky action at a distance." Ugh. It bugs me a lot that Quanta made these misleading statements that just continue the confusion over what should be a more widely-understood core feature of the universe we live in.

Tangentially, I wish "interaction" would come to replace "measurement," especially in the context of decoherence. The universe is branching *constantly* everywhere as various quantum systems interact.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#106

Post by AnimalMuppet »

morpheos137 wrote: post #75
TheOtherHobbes wrote: post #69

Earlier quoted context omitted.

The only difference is that in QM the marbles don't really exist until you look at them. Somehow they still manage to align themselves so if one person sees red the other sees blue. Although it's even more accurate to say that if one person sees [colour] the other person sees [opposite colour]. The colours are random, but the relationship between them is fixed. Very crudely (and rather misleadingly but never mind) th…

>we have no idea where it is. It is probably encoded in the cosmic horizon a la Green's Theorem.

You're giving "probably" quite a workout there.

For it to be encoded at the cosmic horizon, it has to communicate with the cosmic horizon. It's hard to see it doing so, within the time frame of the experiments, without superluminal communication.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#107

Post by virgil_disgr4ce »

Kranar wrote: post #66
tylerhou wrote: post #56

Earlier quoted context omitted.

> Rather, both balls are in a superposition of being both red and blue simultaneously, and it is not possible in principle to assign a color to either one of them until the moment a measurement is made. I don't disagree, and (clearly) I make a measurement when I show you the color of a ball. Before I show you a ball, I would also say that the colors of the balls are in a superposition. > major revolution in physics i…

Fair enough we'll simply disagree on that. Entanglement is not a property about wave functions and really has nothing to do with waves. It's a logical consequence of the uncertainty principle and was ironically deduced by Einstein, Rosen, and Podolsky (EPR Paradox) as a way to argue that quantum mechanics is an incomplete description of physical reality. Being that it's strictly a consequence of the uncertainty princ…

This comment is the most helpful thing I've ever read about entanglement. Thank you!

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#108

Post by joe_the_user »

ThePhysicist wrote: post #101

There's no spooky action at a distance. Let's imagine we have an entangled qubit system that consists of a superposition of the states (0,1) and (1,0), i.e. either part A is in state 0 and part B in state 1 or vice versa. When we perform a measurement on the first part of the system and obtain 1, it simply means that we have "branched" into the (1,0) state of the system. This branching is usually irreversible because…

What you're describing could be done with classical physics. Have one penny and two lockets. Place the penny blindly in one of the two lockets. Take one locket across the world. Opening it instantly lets you whether the other locket has contains the penny.

And the point of this description is this is not what's weird about quantum entanglement.

What's weird about quantum entanglement is you have two different measurement types that are non-orthogonal and they combine according to quantum logic rather than classical logic [1]. Having a particle in a state of this sort can't be explained by any analogy to discreet events occurring beforehand.

[1] https://en.wikipedia.org/wiki/Quantum_logic#Quantum_logic_as...

Edit: Whether this is "spooky action at a distance" is in the eye of the beholder. One thing is isn't able to be is fully reducible to actions happening on something like a "classical time line" but another thing is isn't to able to do is transmit information.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#109

Post by [deleted] »

lisper wrote: post #13
ericb wrote: post #3

If we were in a simulation, would the speed of light be the processing speed of the universe as each area re-renders, and spooky action at a distance be two variables pointed to the same memory location, populated with a lazy-loaded value, with copy-on-write semantics? edit: seems like it is lazy loaded, so revised my summary.

That's not a bad analogy, but you have to be very careful here because no classical analogy can be a perfect fit for entanglement. The wave function is deeply and fundamentally different than our classical reality, and there is no way to reproduce its behavior classically. Among the fundamental differences is the fact that classical information can be copied but quantum states cannot be cloned. This is IMHO the singl…

[deleted]

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#110

Post by subroutine »

choeger wrote: post #76

Does the article do justice to the hidden variables hypothesis? In case of the hidden variables, the spin is a (3-dimensional?) value that is identified by the measurement result. In case of quantum theory we have have a probability distribution. How is that probability distribution different from a hidden variables, except that it's not a straight number but a function instead? Speaking as a programmer, is the diffe…

I believe the QM interpretation is that probability distributions are to be taken literally - a flipped coin under a napkin is both heads and tails with P=.5

Hidden variables on the other hand acknowledges the probability, but contends nevertheless that the coin is actually in a specific but unknown state.

Post reply on HN