Why does quantum entanglement not allow ftl communication

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Quantum entanglement does not allow for faster-than-light (FTL) communication because the information from entangled particles can only be extracted through correlations that require classical communication, which is limited by the speed of light. While entangled particles exhibit correlated behaviors, measuring one does not transmit usable information to the other without prior agreement on measurement methods, thus maintaining causality. Theoretical frameworks like "no signaling theorems" reinforce that entanglement cannot be used for FTL communication, as any information appears random without classical communication. Some researchers, like John Cramer, are exploring potential loopholes through nonlinear modifications to quantum mechanics, but these remain speculative. Overall, current understanding and experiments suggest that FTL communication via quantum entanglement is not feasible.
  • #91
DrChinese said:
But add Bell's Theorem, and you now know that either there are FTL (nonlocal) influences OR that particles do not have well determined properties outside of the context of a measurement (contextuality or nonrealistic).

Is that the same thing as non-deterministic vs. deterministic, or quantum vs. classical physics?


Could be either (or both). So it depends on which interpretation of QM you choose to adhere to. Many Worlds or Bohmian Mechanics are examples interpreting differently. Both follow Bell. You merely have to decide *which* classically intuitive idea you wish to abandon. There is nothing inconclusive about needing to drop one or the other.

I didn't know Bohmian Mechanics was considered as equally valid theory. I know of Bohmian Mechanics interpretation of double-slit experiment, but I've never seen anything like that for these types of experiments about quantum entanglement. Is Bohmian Mechanics deterministic, or maybe probabilistic, theory, and is there actually any difference between these two types of "determinism"?
 
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  • #92
Nor am I a professional physicist, but if you will look at my recent post, I'm sure you will be interested. It deals with the practical application of quantum entanglement in communication
 
  • #93
What is the main reason to think that there has to be state transfer? In other words What is the main reason to think that relationship of entangled particles is not simply hidden until the measurement?
 
  • #94
PhysicalArch said:
In other words What is the main reason to think that relationship of entangled particles is not simply hidden until the measurement?

That's a valid view.

You will find a lot of hidden assumptions of that sort in discussions of EPR type stuff.

That's why it was important Bell came up with his theorem, because it puts it on a sound basis devoid of semantics.

Thanks
Bill
 
  • #95
I got huge problem with QM because i know little math.
Bit more complicated algebraic equation and I don't even try to read it.

QM seems to go deep in laws that differ from ones that our minds evolved for.
Example - if we learn Newtonian physics, we learn maybe one new concept at a time. Few individual concepts don't make good insight but because it explains phenomena of everyday movement we can map it on previous knowledge, fill gaps with intuition. That is nice and natural way to learn.

Then history takes a turn and we have to learn laws that need lot more work to fit in our intuitive world view.

R.Feynman explained this very nicely in one of his interviews.
Maybe only reason QM is hard is that it is too new and too different mind model from anything before.

How did pioneers got so far so quick, when huge masses of people do way worse on already obtained and explained laws of quantum physics? Even Einstein had much more problem with it than bit more involved researchers.

That is good question to answer not only for benefit of QM research but for understanding of ourselves. What does it mean to learn something most unrelated to your basic knowledge.

For me answer to that comes almost easy. Critical thinking has been slowly developed for millennia, marginalized as useless overthinking or simply philosophy.
To truly know something at its full depth you have to be able to investigate in steps like these:
  • What what is it that makes something different from similar things.
  • Is that sufficient to make it essentially different or does it only change the name
  • How is it, in a way, essentially the same as other different things with different names and functions
It can be hard time consuming work to understand everything like that. We probably do that only with most important things in our lives (hopefully). We can get thru everyday life with recognition of general situations we are in and what premade optimized thought should be used. Deeper investigation of things naturally is left for very stressful or curiously playful situations.

For QM pioneers there were two factors that made it exceptionally easy to learn:
  1. Deeper knowledge. As it was top cutting edge science, They had to find and check everything new by themselves. It was impossible for them to do easy thing and use pre-made concepts.
  2. As it was their life work with real possibility of gaining global recognition, they were strongly motivated to the strongest research possible.
After few loudest discoveries everyone's motivation diminished. Not only because there was less to discover but because fame got harder to attain. Disagreement between interpretations indicates that depth of understanding differs widely even up to this day.

It's interesting because when it becomes clear that even entitled persons cannot understand each others interpretations, maybe it can turn out to be transformative to culture of teaching and learning, you know, the one that is in a very sad state. Maybe education can move away from role of social conditioning and take a good hard look of what learning really is about. Or maybe we need more important and harder discoveries for that to happen.

What a long rant :D tnx if you read it

As for my previous question, what makes one believe that there is state transfer instead of hidden variable, I was able to imagine right after I defined it as a clear question. Just after I clicked submit button. And I must say ability to imagine in somewhat familiar form makes it so much easier to understand.

I imagine that 2 entangled particles are like 2 solid spheres. They are opaque and look the same. Whenever you cut one in half you see the cutting angle against inner structure and imprint of the cutting tool let's say knife. At the same time knife gets imprinted from inner structure of this first sphere
You can read information about spheres insides but somewhat masked with information about knife. Same with knife, you can read information that you didn't know but it is limited by the process. You can never get much better deal of information because all knifes and spheres come in similar sizes.. and too big knife just doesn't cut it

You still have other solid whole mystery sphere left and whenever and wherever and with whatever you cut it. It will cut in same angle and it will give that same information only this time what's missing is the part that you already have from first ball. You may use different knife but even that by its specific imprint won't take missing information away.
From two pieces of matter you get information that describes whole.

Imagine like this and you can draw intuitive conclusions.
Example:
Things on quantum scale has unseen properties. Does it mean Q scale is somewhat different? Looks like physical law acts increasingly different on another scale. That is simple statement that goes against intuition. Why does it go against intuition? Probably because evolution perfected our mind like that. There is even stronger intuition that laws should be consistent against distances. We evolved like that because on our scale things happen consistently. Do we know this intuition is right about scaled or far away physics? I don't think so. Evidence is against. Warped space, stretched time. Universal speed limit, density limit. Ofc there is reason why our scale is consistent its not only illusion of mind we are biased by it because its there. If laws are scale relevant there might be change in laws on larger scales, that goes well with fact that we have limits that prevent us from moving out in structures of large scale or moving down in structures beyond small. The fact that we are fundamentally limited could be mistaken for notion that there is nothing there beyond our reach. Not evidence based. Intuition based. Imagine that evolution would have made us with strong intuition that not every place is acting the same, not every size acts as in the same place. If it was so, how would it change our way of doing research, looking for evidence, discarding data that feels useless. We would have totally different evidence based models. We would imagine unreachable beyond in different way.

These may be superficial and irrelevant conclusions for current research, yet still, for personal understanding that is so much better than learning from abstract descriptions about things that you don't know in detail. Without intuition to fill missing spaces, i cannot think of a way to build whole coherent mental framework.

That leads to questions about intuition.
  • How does intuition work
  • What does it work with
  • Why does it work like that
  • What is it similar to and what are critical points that make it work better
  • How did it came to be
  • Can we engineer it? Maybe quicker, better and more optimal for our needs, keep it up to date
  • What would it take
  • If we can do it what could be the consequences
  • How is it going to be used
  • is it worth the risk

There can be more or better questions of course :)
 
  • #96
PhysicalArch said:
How did pioneers got so far so quick, when huge masses of people do way worse on already obtained and explained laws of quantum physics? Even Einstein had much more problem with it than bit more involved researchers.

Generally speaking the math is smarter than we are.

Once Dirac came up with his transformation theory in 1927 QM was basically complete.

Figuring out what it meant took a lot longer and is still going on.

Thanks
Bill
 
  • #97
bhobba said:
Generally speaking the math is smarter than we are.

Once Dirac came up with his transformation theory in 1927 QM was basically complete.

Figuring out what it meant took a lot longer and is still going on.

Thanks
Bill

The Dirac Equation seems really an example of the math being smarter than we are. He only had one goal: to make a first-order differential equation that was the relativistic generalization of Schrodinger's equation. But the math forced him to a theory that had particle spin, antiparticles, particle creation, etc.
 
  • #98
stevendaryl said:
The Dirac Equation seems really an example of the math being smarter than we are. He only had one goal: to make a first-order differential equation that was the relativistic generalization of Schrodinger's equation. But the math forced him to a theory that had particle spin, antiparticles, particle creation, etc.

It is.

And its notable other physicists thought Dirac too smart by half. But all he thought he was doing was mucking around with equations.

Some people attracted to physics say its just math, it can't be reality, yada, yada yada, regular posters here know the drill. Physicists did not go down that route without reason - and Dirac's success certainly played a part in it.

Thanks
Bill
 
  • #99
Let's do a thought experiment.

A long time ago in a galaxy far far away, somebody used his entangled particle generator to create a gazillion pairs of entangled particles.
The first of each pair was shipped to Alice Alien, who lives several thousand light years away from us. My neighbour Bob just received the other ones.

Alice measures the particle spin of each partice. All in the same direction.
Bob measures the spin of each particle in a random direction and plots the results.

q: what will the graph look like?

[PLAIN]https://upload.wikimedia.org/wikipedia/commons/7/77/StraightLines.svg[URL='https://upload.wikimedia.org/wikipedia/commons/7/77/StraightLines.svg'][/URL]

https://upload.wikimedia.org/wikipedia/commons/7/77/StraightLines.svg

I think it'll look like the dotted curve in the graph above.
q: Does Bob learn the direction Alice is measuring at?
q: Does this violate the no-cummunication theorem?
 
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  • #100
realbart said:
Let's do a thought experiment.

A long time ago in a galaxy far far away, somebody used his entangled particle generator to create a gazillion pairs of entangled particles.
The first of each pair was shipped to Alice Alien, who lives several thousand light years away from us. My neighbour Bob just received the other ones.

Alice measures the particle spin of each partice. All in the same direction.
Bob measures the spin of each particle in a random direction and plots the results.

q: what will the graph look like?
The graph you have enclosed is a plot of the correlations between Alice's and Bob's measurement results. Bob (without knowing Alice's results) can not make this plot by himself.
 
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  • #101
Which makes the answer to the first question "only if Alice tells him her outcomes", and that makes the answer to the second "no."
 
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  • #102
realbart said:
Let's do a thought experiment.

...

q: Does Bob learn the direction Alice is measuring at?
q: Does this violate the no-cummunication theorem?

Welcome to PhysicsForums, realbart!

Bob does NOT learn anything from Alice. The "no-signaling" theorem remains intact.

As Heinera says, Alice must first send her results by traditional means (signals propagating at c) to Bob in order for Bob to prepare your graph.

What Bob sees is the same 50-50 statistical result at any angle setting he chooses. Ie no different than flipping coins.
 
  • #103
Heinera said:
The graph you have enclosed is a plot of the correlations between Alice's and Bob's measurement results. Bob (without knowing Alice's results) can not make this plot by himself.

You're absolutely right, Heinera. I expected to see the graph as function of the angle between the measurement and the Z-axis.

So let's say Alice will measure every particle spin along the Z-axis.
What will Bobs measurements be along the horizontal plane? (perpendicular to the Z-axis)
 
  • #104
realbart said:
You're absolutely right, Heinera. I expected to see the graph as function of the angle between the measurement and the Z-axis.

So let's say Alice will measure every particle spin along the Z-axis.
What will Bobs measurements be along the horizontal plane? (perpendicular to the Z-axis)
Bob's measurements will be completely random to him, no matter what axis he chooses. They will be "up" or "down" with 50/50 % probability.
 
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