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Questions about entanglement from a non-physicist

  1. Feb 21, 2013 #1
    I posed these questions via email to a few actual physicists, with the only responses received coming from those that admit to not fully understanding the subject matter. And maybe it's too simplistic for the others. I've read countless articles but haven't come across satisfactory answers, at least not easy enough for me to wrap my head around. I'm hoping some of you can help me make sense of things...


    1. How do we know that engtangled particles aren't just in sync? Is there no chance that they're both set on the same course, until one of them is knocked off of it? I'm assuming this has been experimentally verified but no article is crystal clear on how.


    2. So if we know they definitely are entangled beyond that first event, then it seems logial to think that an advanced society could harness this to send data. I keep reading that the information that can be gathered from quantum teleportation comes only after one particle is compared with the other, so we're limited by the speed of light. The why, that's confusing, and not sure it's science that I will be able to understand soon. What I'm wondering though is, if there's SOMETHING being transferred, even if it's just seemingly random quantum events, that does mean there's a channel outside of regular space, does it not?

    People believe we can potentially travel faster than the speed of light by warping space. So there are theoreticaly shortcuts. Why doesn't entanglement represent another that we could use? Couldn't we one day discover a method of exploiting this new dimension?

    It just sounds to me like this no-information rule is a self-imposed limitation without considering possible ways we could eventually learn to transfer quantum data into more useful purposes. Sorry if the answer is obvious to you guys but I've been struggling to figure this out for the past year. I currently see it as a channel, that connects the smallest parts of matter, that we may never access but I don't get why it's deemed theoretically impossible.
     
  2. jcsd
  3. Feb 22, 2013 #2
    1. What do you mean by "in sync" and "on the same course"? The EPR-type experiments confirmed that Bell's inequalities are violated. On many (most) interpretations of qm there is entanglement.

    2. We can't exploit entanglement to send faster than light signals because quantum particles do not have definite properties before measuring them. If you want to find out their properties (so that you could send the signal) then the act of measurement destroys the entanglement.

    Don't think of entanglement as a channel. If quantum physics teaches us anything, is that classical representations, pictures, images, concepts are inadequate.
     
  4. Feb 22, 2013 #3
    1. I mean... both particles are set on the same course, the same "random" sequence from there on out. Until one particle is interferred with after which time it wouldn't sync up with the other particle when measured. Which would mean they aren't actually entangled. I believe they are actually entangled however I don't understand how they ruled this out experimentally.

    2. I think your definition helps. At least in part. The actual non-classical definition is a total mind-destroyer though, all possibilities existing at once.
     
  5. Feb 22, 2013 #4
  6. Feb 22, 2013 #5

    DrChinese

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    1. Some people believe there is action at a distance, and the normal limit of c for the propagation of effects does not apply to entanglement. But this is not universally agreed.

    2. Most scientists do not believe this. A lot of Star Trek viewers do. I don't, although I watch a lot of ST. :smile:

    3. You are free to reject whatever you like. This "self imposed" limitation is a part of the most accurate and useful theory in science. Thousands of different experiments have been run to test it. Obviously it has passed all to date. So it has been well considered. Obviously, a better idea may come along tomorrow. But someone would have to think it up first.

    Personally, I think FTL signalling is not possible in our universe.
     
  7. Feb 22, 2013 #6
    Can it not be considered that the two photons are "entangled" because they originated from the same event?

    Oh weird, just read about local hidden variables. I guess that isn't why they are "entangled", but don't understand the reasoning how local hidden variables is proved impossible.
     
  8. Feb 22, 2013 #7

    DrChinese

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    Bell's Theorem explains this. Google that or check out the link from my signature line.

    Basically, "local hidden variables" (LHV) is a reasonable hypothesis for measurements of the same angle settings for Alice and Bob. But there are other settings at which the results are inconsistent with LHV.
     
  9. Feb 22, 2013 #8
    Ah, that might clear up some of my confusion. "Local hidden variables" never made total sense, I think because of the usage of "entanglement". The way I'm familiar with the word it would imply that there's a continued connection that affects each particle, as opposed to a synchronicity that's initially set in motion, like two clocks.

    If it's just a case of language then I can now read and understand these Bell experiments, and others, more clearly.
     
  10. Feb 22, 2013 #9

    DrChinese

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    I would say it is much more like a "continued connection" than "two clocks". If they were like clocks, they would be deterministic and they would have a classical relationship. That description leads to contradictions and incorrect predictions.

    Although most experimental entanglement is from things that start out as entangled pairs, such is not a requirement. It is possible to entangle particles that have never interacted, for example. This can be done by a process called entanglement swapping. Here is a recent experiment using this:

    http://arxiv.org/abs/1209.4191

    It should be clear that the entangled relationship shown in the paper defies a classical description. Keep in mind that you can also create pairs that actually are fully "synchronized" (example: phase locked lasers) and they will not be entangled.

    The point of Bell's Theorem is to explain how a contradiction arises.
     
  11. Feb 22, 2013 #10
    Thanks for all the info. Bell's Theorem will hopefully make more and more sense as I wrap my head around some of the basics. These Eisenberg experiments are fascinating.
     
  12. Feb 22, 2013 #11
    Actually this is still clear as mud. I now understand a bit better what scientists are finding, but... if this entanglement isn't actually FTL communication via their own channels, then it is, as I understand, that every possibility is happening at once? And we're tapping into this somehow?

    I'm very sorry to waste the time of physicists on such basics but I've been struggling with these questions on and off for the past year, and am willing to collect some intellectual charity. Maybe someone has an analogy that gets through.

    My purpose, besides general knowledge, is to consider possible forms of travel and communication for a story I'm writing that takes partially in the next century, and partially in a millenium. I also don't believe FTL travel is likely to happen in this universe of ours. However what I thought might be possible is to entangle two points locally, deliver one to another section of the galaxy and then have a form of teleportation. Now though it's seeming less and less plausible. I don't want to think small, but I dislike fiction based on sketchy science.
     
  13. Feb 22, 2013 #12

    DrChinese

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    You aren't wasting anyone's time, everyone who responds does so because they enjoy it. :smile:

    I think good sci fi is based on some science, but the whole point is to stretch a bit. Conceptually, entanglement involves points separated in space or time or both. You could always write it as if entanglement DID allow signalling.
     
  14. Feb 22, 2013 #13
    I can accept sci-fi that explores ideas that are not theoretically disproven. And I do realize that any actual attempt to foretell the future beyond the next few decades takes some luck. But if something looks like it's not allowed by a pretty good understanding of physics then I have little interest in pursuing that line of thinking.

    For instance, most sci-fi writes in FTL travel because it allows them to explore the far reaches. Meet with alien life. Which is fun, but it's been done and I would feel like I was cheating if I used it as a plot point without any explanation. When you give explanation though you run the risk of being falsified at any time. The point for me is to create a future that remains plausible for as long as possible. While also telling an intruiging story.

    In my own view, the fact that we haven't met up with any other intelligent civilization is a strong argument for the universal speed limit being steadfast. Without it there doesn't seem to have been enough time to colonize much more than one's own galaxy. And even that's a stretch.

    Being able to send information from point to point though instantaneously, after one has travelled the great distances to established an entangled port, that's a desirable narrative. But the only stuff I've come across by scientists that believe such a thing is theoretically possible, well there's little written on it and what I did find is years old. Plus I'm not a physicist so much of it is over my head. It just doesn't look possible.

    That begs the questions, what IS possible. It's frustrating but also exciting that it's such a mystery.
     
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