Quantum entanglement...ftl light and no communication

[jarablue]

Let me preface what I'm about to say that this has nothing to do with communication or the potential for communication faster than light. I am just talking about the actual change in the particle at a distance. Is that why it's called spooky? Because apparently it betrays the light speed limit? I mean I understand in a Layman's way that communication is not possible and I'm not worried about that I'm talking about the acknowledgment of the other particle breaking the speed of light. Is it some sort of Wormhole that's going on that we don't know about?

 

[Megalodon]

Let me preface what I'm about to say that this has nothing to do with communication or the potential for communication faster than light. I am just talking about the actual change in the particle at a distance. Is that why it's called spooky? Because apparently it betrays the light speed limit? I mean I understand in a Layman's way that communication is not possible and I'm not worried about that I'm talking about the acknowledgment of the other particle breaking the speed of light. Is it some sort of Wormhole that's going on that we don't know about?

The underlying mechanism is not known and there are multiple competing explanations, none of which appear to be possible to distinguish from any of the others by any feasible (or even infeasible) experiment.

Broadly speaking, you either have to give up on the idea of locality, or, you have to suppose there's hidden variables.

I think there's a lot of interest in the idea that locality is emergent rather than fundamental as that helps out in other areas like black holes, but as far as I know there's no falsifiable predictions arising from that idea.

 

[UserJoe]

Let me preface what I'm about to say that this has nothing to do with communication or the potential for communication faster than light. I am just talking about the actual change in the particle at a distance. Is that why it's called spooky? Because apparently it betrays the light speed limit? I mean I understand in a Layman's way that communication is not possible and I'm not worried about that I'm talking about the acknowledgment of the other particle breaking the speed of light. Is it some sort of Wormhole that's going on that we don't know about?

Entanglement is not some magical faster than light communication. It just means that the properties of the particles are correlated with each other because they were entangled when they were close together. For example, you can prepare an entangled state of 2 electrons that has zero spin. You don't know the spin of either electron. It can be either up or down. If you measure the spin of one and it is up you know the spin of the other has to be down when you measure it. It doesn't matter how far away from each other they are. There is no communication between them. People mistake the correlation between them as spooky action between them.

 

[ajk48n]

From what I understand, if there are no hidden variables, then when the measurement of the first spin occurs, is the exact moment when the value of both entangled electrons is determined.

So while there may not be communication between them, something spooky is happening. "Spooky" here is just my placeholder for "uh, we don't know how this actually works"

 

[Defenestrar]

I have two spinning flywheels in contact with one another (entangled) and they happen to have a line painted straight across both of them. When I separate them the position of the lines will still be in sync.

 

[ajk48n]

Isn't that the assumption of a hidden variable though? Or maybe better put, the assumption that the spin direction is decided before the act of measurement and before the electrons are separated?

 

[redleader]

I am just talking about the actual change in the particle at a distance. Is that why it's called spooky? Because apparently it betrays the light speed limit?

It's spooky in the sense that something far away can be determined by something near, which is weird. There's no problem with the speed of light though since you can't send information using entanglement, so nothing is actually moving or transmitting faster than c.

 

[rain shadow]

Bell's Inequality (which has been experimentally verified) is the weird part. I can't even begin to explain it, but it shows that there can be no hidden variables, because the statistical outcome of measuring many pairs of entangled particles indicates that the local choice of the method of measurement (usually characterized as varying the polarization angle) changes the outcome of the remote measurement in an (FTL) way that cannot be explained by hidden variables.

This has given rise to a few interpretations, some of which seem intuitively compelling, although none really make testable predictions. It just appears that this is just how quantum entanglement works and we don't currently have any analogy or deeper explanation that helps us comprehend what's actually going on.

 

[rain shadow]

Speculation.

For several years I've been kind of mentally prodding at the idea that quantum entanglement (or the information about it) itself is traveling at the speed of light in a way that enables relativistic length contraction a la the barn door paradox. This velocity is separate from the movement of the entangled particles or the observer. It would not necessarily explain anything, but it would connect relativity with QM in (what I think is) a surprising way.

The idea isn't that crazy when the entangled particles are photons. To them there is no distance or time in the universe, so whatever entanglement they have could more easily be seen as local, as everything is local to photons. Extending the idea to particles with mass is quite difficult, and would mean detaching the particles' velocities from the quantum information velocity.

Can't find anyone else working along these lines, very likely the whole thing just falls as just being another non-testable explanation.

 

[SnoopCatt]

Question: is this a useful analogy for quantum entanglement?

Imagine that we take a coin and using a very fine saw, we slice it into two thinner disks. One half-coin will have a head on one side, and the one will have a tail on one side. These half-coins are now "entangled". If one half-coin is chosen at random and moved to a distant location, we won't know until we look at it whether it is the head or the tail. And the same with the one that is kept local - it could be either. But as soon as we observe one, the other can only be opposite.

 

[redleader]

Question: is this a useful analogy for quantum entanglement?

Imagine that we take a coin and using a very fine saw, we slice it into two thinner disks. One half-coin will have a head on one side, and the one will have a tail on one side. These half-coins are now "entangled". If one half-coin is chosen at random and moved to a distant location, we won't know until we look at it whether it is the head or the tail. And the same with the one that is kept local - it could be either. But as soon as we observe one, the other can only be opposite.

Thats a good analogy for why you can't send information between entangled particles, but that's actually more like the hidden variable theory in that the answer was predetermined when you put it into the envelope.

 

[Dmytry]

I wonder if its any easier under many-worlds interpretation... each side observes all of the outcomes, but only the compatible outcomes can be discussed (observers of incompatible outcomes don't get to communicate with one another).

 

[Megalodon]

I wonder if its any easier under many-worlds interpretation... each side observes all of the outcomes, but only the compatible outcomes can be discussed (observers of incompatible outcomes don't get to communicate with one another).

It works equally well if you get rid of locality. Assume the particles are connected by wormhole which collapses (or whatever non-local idea you prefer) when they're measured. You still get compatible outcomes.

 

[Shavano]

Let me preface what I'm about to say that this has nothing to do with communication or the potential for communication faster than light. I am just talking about the actual change in the particle at a distance. Is that why it's called spooky? Because apparently it betrays the light speed limit? I mean I understand in a Layman's way that communication is not possible and I'm not worried about that I'm talking about the acknowledgment of the other particle breaking the speed of light. Is it some sort of Wormhole that's going on that we don't know about?

I think it's like a card trick. You take a polaroid of the king of diamonds and the king of clubs. You go into a dark room and totally by feel you put them into two different envelopes and seal them. Then you paste sticky labels on them with an addresses of two colleagues, one in Sydney and one in Chicago. You drop them both in the mail in Rome. Via your podcast, you show both colleagues the Polaroid. The one in Sydney arrives first and your colleague in Sydney opens it, discovers the king of clubs, thus teleporting the king of diamonds to Chicago.

Was the king of diamonds in the envelope in Chicago all along, or did you mail a quantum superposition of two cards. Isn't the math the same either way?

 

[Shavano]

From what I understand, if there are no hidden variables, then when the measurement of the first spin occurs, is the exact moment when the value of both entangled electrons is determined.

Or discovered, but perhaps I make a distinction where there is no difference.

So while there may not be communication between them, something spooky is happening. "Spooky" here is just my placeholder for "uh, we don't know how this actually works"

Or something dreadfully ordinary.

 

[ajk48n]

Or discovered, but perhaps I make a distinction where there is no difference

I could well be off base here, but I think it's specifically "determined", not "discovered"

Discovered would imply that there was a value that already existed which we are now finding out.

But from I understand, when the measurement is made, that's the instant an actual value exists for the first time (which is what I meant by determined

English is really not equipped to describe quantum mechanics precisely

 

[Defenestrar]

English is really not equipped to describe quantum mechanics precisely

What language is?

 

[dmsilev]

What language is?

What, you don't speak N-dimensional matrix algebra?

 

[Dmytry]

It works equally well if you get rid of locality. Assume the particles are connected by wormhole which collapses (or whatever non-local idea you prefer) when they're measured. You still get compatible outcomes.

Yeah, that works too but then you're left with the weirdness of having non-locality that is local (prohibits actual communication).

 

[Megalodon]

Yeah, that works too but then you're left with the weirdness of having non-locality that is local (prohibits actual communication).

I think that's a smaller issue, on the whole. And if you introduce the idea of locality breaking down it's potentially a useful approach to other problems, like various issues introduced by black holes. If locality can break down you might not need a singularity, and it may be helpful for the information paradox.

 

[Shavano]

I could well be off base here, but I think it's specifically "determined", not "discovered"

Discovered would imply that there was a value that already existed which we are now finding out.

But from I understand, when the measurement is made, that's the instant an actual value exists for the first time (which is what I meant by determined

English is really not equipped to describe quantum mechanics precisely

That requires either information to travel faster than the speed of light or causality to work backwards.

 

[ajk48n]

Hopefully we're not talking past each other here, but isn't what I said the correct description of what happens, as far as the current understanding goes?

The Bell inequality seems to imply that there is not an existing value until the moment of observation.

 

[rain shadow]

One thing I'm curious about is what happens to quantum information when the particles are e.g. photons or neutrinos that happen to travel so far across the universe that not only are they billions of [light] years outside of each other's light cones, but the space between them is expanding apart faster than light, so no measurement could ever be done and have the results checked.

This puts a stick into the many worlds hypothesis as it will never be possible to figure out which of the many worlds would be allowed to exist...there is no way to (ever) determine which one has measurement results consistent with preservation of QM information.

 

[ajk48n]

that happen to travel so far across the universe that not only are they billions of years outside of each other's light cones

I'm not sure if that could physically happen, because one of the particles would have to travel to the other location, and would travel there at less than the speed of light.

but the space between them is expanding apart faster than light

That's an interesting question, and not an idea I've come across before. If I had to make a completely uneducated guess, the answer would be "we dunno"

 

[Shavano]

Hopefully we're not talking past each other here, but isn't what I said the correct description of what happens, as far as the current understanding goes?

The Bell inequality seems to imply that there is not an existing value until the moment of observation.

Maybe. I honestly don't understand that part. I don't see any way it can be distinguished which of many worlds (assumes there are outcomes you can't examine), the Bohm interpretation (nonlocal hidden variables you can't examine), or something else you can't examine is "correct." So to avoid stating positively what we can't observe, the Copenhagen interpretation (it just works like the math says and we probably shouldn't bother ourselves about stuff we can't examine until we figure out a way to do that) seems preferable.

One thing I'm curious about is what happens to quantum information when the particles are e.g. photons or neutrinos that happen to travel so far across the universe that not only are they billions of [light] years outside of each other's light cones, but the space between them is expanding apart faster than light, so no measurement could ever be done and have the results checked.

This puts a stick into the many worlds hypothesis as it will never be possible to figure out which of the many worlds would be allowed to exist...there is no way to (ever) determine which one has measurement results consistent with preservation of QM information.

I don't see what's interesting about that. There's always been things we couldn't know.

 

[rain shadow]

I don't see what's interesting about that. There's always been things we couldn't know.

Not just that we couldn't know, the universe itself could not know, leading to the possibility that e.g. spin would not be conserved.

I just broke physics. Sorry!

 

[ajk48n]

So to avoid stating positively what we can't observe, the Copenhagen interpretation (it just works like the math says and we probably shouldn't bother ourselves about stuff we can't examine until we figure out a way to do that) seems preferable

Yeah that's probably best. All good points in your full reply as well

 

[Shavano]

Technically, that's how everything is, right? I mean we only know spin is conserved in the kinds of experiments physicists have devised to measure spin. Tomorrow they might (unlikely but possible) discover some new interaction that they didn't think was possible because it breaks spin conservation. Or lepton number or momentum or whatever. We build confidence that these principles are "right" because they keep holding up in experiment after experiment and so far experiments designed to falsify them keep failing.

And in a way we don't know that the properties we observe to be conserved are fundamental properties. Sometimes physicists have found out they're not. They used to think mass was conserved, then found out that's not exactly true, but mass-energy seems to be conserved, and it wasn't grossly wrong in the context in which it was thought to be true. People thought atoms were indestructible tiny spheres until they demonstrated they weren't and were composed of tinier, simpler particles but they thought that was all there were. And then they found out protons and neutrons were composed of still simpler but more confusing particles, and then the existence of a whole truckload of particles with weirder properties.

There's no guarantee we're at the bottom with the standard model, but it's held up pretty well. Maybe at some point they figure everything's much better explained if spin isn't really a thing, just a peculiar manifestation of some new property they don't have a name for yet.

And that would break physics, but it's EXCITING when physicists break physics. It means they get to learn new things!

 

[UserJoe]

It works equally well if you get rid of locality. Assume the particles are connected by wormhole which collapses (or whatever non-local idea you prefer) when they're measured. You still get compatible outcomes.

Pilot wave theory is a determinist, hidden variable theory championed by Bohm and others. It is a non-local theory that gives the same results as quantum mechanics as was recognized by Bell in his EPR paradox paper. Quantum mechanics cannot answer the questions people are asking in the thread. It only calculates the probability of at the measurements will show and does not give any information beyond that. You can't ask questions that it doesn't answer. Here's a video of a well-known lecture about the strangeness of quantum mechanics and Einstein's problems with it.

 

[Megalodon]

I'm not sure if that could physically happen, because one of the particles would have to travel to the other location, and would travel there at less than the speed of light.

No it absolutely can happen, because the universe's expansion is accelerating. We can see galaxies in the sky today whose light from when they are the same age as the Milky Way is now will never reach us (and light we emit now will never reach them). All you need for rain shadow's scenario to happen is for entangled photons to be emitted in different directions and diverge over billions of years. The accelerating expansion of the universe will take care of the rest.

 

[UserJoe]

No it absolutely can happen, because the universe's expansion is accelerating. We can see galaxies in the sky today whose light from when they are the same age as the Milky Way is now will never reach us (and light we emit now will never reach them). All you need for rain shadow's scenario to happen is for entangled photons to be emitted in different directions and diverge over billions of years. The accelerating expansion of the universe will take care of the rest.

Some of the different cosmological horizons are discussed in the linked wiki article.

 

[Shavano]

No it absolutely can happen, because the universe's expansion is accelerating. We can see galaxies in the sky today whose light from when they are the same age as the Milky Way is now will never reach us (and light we emit now will never reach them). All you need for rain shadow's scenario to happen is for entangled photons to be emitted in different directions and diverge over billions of years. The accelerating expansion of the universe will take care of the rest.

What are the error bars on the acceleration of the universe's expansion?

 

[Megalodon]

What are the error bars on the acceleration of the universe's expansion?

I don't think that matters for the purpose of this thread because my assertions are true for large enough distances regardless.

 

[MilleniX]

What are the error bars on the acceleration of the universe's expansion?

Off the top of my head, they're about 5-10% of the magnitude of the acceleration. They are very far from encompassing 0 as a possible value.

 

[dmsilev]

Off the top of my head, they're about 5-10% of the magnitude of the acceleration. They are very far from encompassing 0 as a possible value.

The progress in the field over the past couple of decades has been amazing. When I was in grad school, the standing joke was that cosmologists put their error bars in the exponent of whatever it was they were measuring/calculating. Now, the phrase "precision cosmology" is an actual thing.

 

[MilleniX]

When I was in grad school, the standing joke was that cosmologists put their error bars in the exponent of whatever it was they were measuring/calculating.

I don't know how long ago that was for you, but that joke was definitely still told as of a decade ago. At least some of the time, it came from astrophysicists who were proud that they could do much better, though.

 

[Shavano]

No it absolutely can happen, because the universe's expansion is accelerating. We can see galaxies in the sky today whose light from when they are the same age as the Milky Way is now will never reach us (and light we emit now will never reach them). All you need for rain shadow's scenario to happen is for entangled photons to be emitted in different directions and diverge over billions of years. The accelerating expansion of the universe will take care of the rest.

I don't plan to live long enough to see rain shadow's experiment play out.

 

[UserJoe]

I don't know how long ago that was for you, but that joke was definitely still told as of a decade ago. At least some of the time, it came from astrophysicists who were proud that they could do much better, though.

There's a famous quote from Lev Landau that "Cosmologists are often in error but never in doubt."