Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Entanglement - Does FTL via entanglement violate causality or relativity?

  1. Jun 15, 2011 #1
    Does FTL transmission of information (via 1 entanglement or 2 any means) violate causality?

    FTL = faster than light


    1. I argue that FTL (of information) does NOT violate causality or relativity.

    2. Also FTL (of massless information) is possible via quantum entanglement. All you need is a DCQE setup (without EM radiation) or any apparatus that can exploit entanglement for information transfer purposes.


    For 1 above:

    relativity says mass (rest mass) cannot travel FTL, however in information transfer via quantum entanglement no mass is being transferred only information is.

    quantum entanglement provides a way to transmit information without mass (having to carry it)

    also, according to my hypothesis, quantum entanglement happens in channel/dimension other than space-time or is not effected by space time.

    There is NO mass involved, so no problems with relativity.

    Nor with causality. The future is not/cannot effecting the past.

    For 2 above:

    All you need is a DCQE setup (without "noise" EM radiation) or any apparatus that can exploit entanglement for information transfer purposes (in which "noise" EM radiation is kept close to zero).
     
    Last edited: Jun 15, 2011
  2. jcsd
  3. Jun 15, 2011 #2

    Ken G

    User Avatar
    Gold Member

    Entanglement never transmits information FTL, this is a well-known feature of entanglement that appears over and over in entanglement experiments. Indeed, any physicist who is unaware of either the entanglement, or the outcomes of experiments on the entangled partner, can do normal quantum mechanics on the particles they get, and never have the slightest inkling that anything weird is happening. That's why no information is ever transmitted by entanglement at all, let alone FTL information. Entanglement experiments invariably involve coincidence counting, which requires that information is being transmitted in the usual sub-FTL ways to do the experiment at all.
     
  4. Jun 15, 2011 #3
    Ken G,

    If we are able to somehow remove EM radiation "noise" then we would not need the coincidence counter.....

    we could then manipulate the path of the signal photon (eraser or no eraser) and read off the result from the idler photon.
     
  5. Jun 15, 2011 #4

    SpectraCat

    User Avatar
    Science Advisor

    That is nonsense ... Ken G is right. It has NOTHING to do with noise. Information is only contained when you compare the measurement streams between the two ends of the communication channel, whether those are coincidence counters (which by the way are not only there to remove noise), or something else.

    Assume you sit at one end of the stream .. you are making measurements on a quantum system. Explain *in detail* how you will tell if a given measurement you take reflects a value that has been pre-determined by the action of someone else at the other end of the channel, or if *your* measurement collapsed the wavefunction?

    Or better yet, read one of the 18,576,432,719 (approximate value) threads on this topic posted to this very forum which explain in detail why what you suggest is impossible.
     
  6. Jun 15, 2011 #5

    Ken G

    User Avatar
    Gold Member

    No, the coincidence counter is of much more fundamental importance, it has nothing to do with noise unless you give unsubstantiated importance to noise, like calling it the basis of quantum uncertainty or some such belief.
     
  7. Jun 15, 2011 #6
    No quantum cell phones then? Rats.
     
  8. Jun 15, 2011 #7
    In order to understand what you are saying (and I might agree with you, once I understand this fully) I will play devil's advocate (by taking the view that FLT is possible by entanglement):

    In a DCQE setup (with zero "non-entangled" noise, i.e. a highly hypothetical experiment controlled in such a way that only entangled pairs strike the detectors, i.e. all EM radiation (other than from the entangled pairs that are sent one-by-one) is removed):

    we can place (or not place) quarter wave plates before the double slit in the path of the signal photon, the idler (placed say 1 light minute away) behavior will tell us whether there was a QWP in front of the signal's path or not.

    Now the question is:

    can we know about idler behavior with just having QWP's in front of signal path, or do we need additional instrumentation (such as the coincidence detector and/or polarizer)?

    if yes then what's the reason for the need for additional instrumentation?
     
    Last edited: Jun 15, 2011
  9. Jun 15, 2011 #8

    Ken G

    User Avatar
    Gold Member

    That's fine, removing the noise means we are doing a gedankenexperiment, but that's pretty much how we understand entanglement anyway.
    You can never tell by looking at one entangled partner anything that did or did not happen to the other entangled partner. This is the point. Instead, you have to be told what happened to the partner, and only then does the behavior of the one you observe seem affected in some mysterious way by that partner. Only once you are told what happened to the partner, this is key-- to see anything in entanglement experiments, you have to slice the data you have based on knowledge of the data from the partner. If you don't do that, you never see anything that gives you any hint about the partner, as required by causality.
     
  10. Jun 15, 2011 #9
    Good post Ken.


    Well articulated in layman language...:)

    I agree but to really validate it, I will double check the above via creating various tweaks/scenarios in the DCQE experiement. let me think a few and I will post here...on this thread....

    Experiment 1:

    In DCQE, if we place
     
    Last edited: Jun 15, 2011
  11. Jun 18, 2012 #10
    I don't think FTL can happen or QE can be used to transmit information without comparing (Alice and Bob), that said, I am trying to figure out the wrong assumptions (and misunderstandings) in the below experiment.

    The below experiment is based on the idea that to do which-way or no-which-way can be pre-determined by Alice (or Bob). In short - Alice does no-which-way (or which-way) which shows up as interference (or no-interference) pattern on Bob's screen.

    Alice and Bob have a pair of say 1 billion entangled photons. Assume (big assumption) - noiseless environment at both ends.

    Assume Alice is doing single particle double slit experiment. In bunches of say 10,000 at a time, so that the pattern is unmistakable.

    And Bob is simply watching his same bunch of 10,000 photons on a detector to see if it's interference pattern or no-interference-pattern.

    When Alice wants to send yes/1 she does NO-which-way on a bunch of 10,000 photons
    When Alice wants to send no/0 she does which-way on the next bunch of 10,000 photons

    if Alice decides to do no-which-way won't there be an interference pattern at Bob's end?
    Is a co-incidence counter still needed? why? What would it be filtering if there is no noise?

    if Alice decides to do which-way won't there be an absence of interference pattern at Bob's end?
    Is a co-incidence counter still needed to filter photons? why?
     
    Last edited: Jun 18, 2012
  12. Jun 18, 2012 #11

    Ken G

    User Avatar
    Gold Member

    No. There will never be an interference pattern in the raw data at Bob's end, it makes no difference what Alice does or does not do. That's why no signal can be sent that way.
    It is needed if you ever want to see an interference pattern in Bob's data. You must sort Bob's signal by the results of Alice's, and it is only in the sorted data that you can see interference-- if Alice's outcomes were no-which-way. The coincidence counter filters Bob's data based on Alice's results, noise plays no important role.
     
  13. Jun 18, 2012 #12
    thanks Ken G

    what else, besides the correlated results of Alice's, does the raw data at Bob's end consist of?
     
  14. Jun 18, 2012 #13

    Ken G

    User Avatar
    Gold Member

    Look at the socks analogy, which does not contain the subtleties of entanglement but serves for this point. Let's say you have 10 individual socks, each of a different color. 5 are left socks, 5 are right socks, and it all looks completely random-- you see no connection at all between color and handedness, you only see that you have no pairs. But then the phone rings, and Alice tells you that she has 5 right socks and 5 left socks also-- and when she reads off the colors of her socks, you now do see a clear pattern, when you sort your socks into two piles based on the colors of Alice's left and right results. If Alice never looked at her socks, she couldn't call you, and you would never see any pattern in your data. If she looked, but didn't call you, you still see no pattern. Only when she communicates the correlations, via standard slower-than-light communication, can you ever see any kind of pattern in your data. The phone is the only way you will ever know anything that Alice is doing, yet when you sort your socks based on the colors she lists, you see a connection between color and handedness, a connection you could never see on your own.
     
    Last edited: Jun 18, 2012
  15. Aug 13, 2012 #14
    Imagine if we lived on a two dimensional plane, and we had NO understanding whatsoever of a height dimension.
    With that in mind:
    What if a 3 Dimensional being put a ring (vertically) through our two dimensional plane? We would have NO way of knowing that it was actually a ring. Instead, we would see two separate "dots or lines" from each side if the ring that appear on our plane. To us as 2 dimensional beings, we would see two separate dots that would seem to have no connection BUT if we were to "move" one of the dots, the other one would move with it instantaneously!
    When is reality we may be moving the ring sideways, it would appear to us as two dimensional beings that the second dot (other side of the ring) was moving instantaneously even though on another dimension they are part of the same thing. We may describe what was happening as "information" moving faster than the speed of light.
    With this idea no Relativity laws are broken since there inst any "information" traveling to begin with. Maybe two entangled atoms are part of the same thing (like the dots and ring) just in higher dimension then ours. So when you do something to one, the other will react Instantaneously. This means light can still be the ultimate speed limit.
    Obviously this is theoretical, but its a different way of looking at a very confusing phenomena.
     
  16. Aug 13, 2012 #15

    phinds

    User Avatar
    Gold Member
    2016 Award

    Thank you for that Ken. That's the most straightforward way I've ever seen of explaining the "no FTL info via entanglement" to a layman.
     
  17. Aug 13, 2012 #16

    Demystifier

    User Avatar
    Science Advisor

    The socks analogy was first introduced by Bell, to explain why that analogy is NOT good to explain what is going on in the quantum case.
     
  18. Aug 13, 2012 #17

    martinbn

    User Avatar
    Science Advisor

    The point of Ken G example with the socks was to explain what the role of the coincidence counter is, not what is going on in the quantum world. He explained that earlier.
     
  19. Aug 13, 2012 #18
    This is the only part that I don't understand. Why would there not be an interference pattern?

    what am I missing here?

    don't we have only those photons which are no-which-way (for both Alice and Bob)? (because we removed the noise) and hence they should form an interference pattern?

    are we not sending only the no-which-way photons (for Alice and Bob)?

    the must be something wrong in my understanding of the experiment
    _________________________________________________________________

    i think I got the answer (please let me know if it's correct) after reading through one of the earlier posts in this forum:

    I was not aware of any of the below:

    1. Entangled photons, for some reason, never produce an interference pattern, to begin with.....so that resolves my query because if there is no pattern then you cannot send information FTL.

    To get the interference pattern you would have to compare (alice and bob) which can only be done at, or below, speed of light

    2. The entanglement is broken as soon as Alice's photon goes through single particle interference. Thus Bob no longer has the entangled photon.

    3. single- and two-photon interference are complementary. You cannot have both at the same time.


    Note:The post I was referring to was -- https://www.physicsforums.com/showthread.php?t=511509. thanks DrChinese and Cthuga
     
    Last edited: Aug 13, 2012
  20. Aug 13, 2012 #19
    I think what Ken is saying that there is no interference in the data at Bob's end.

    By correlating coincidence counts between Alice and Bob, interference patterns emerge. (when no which-way info is present)
     
  21. Aug 13, 2012 #20
    But doesn't entanglement imply some kind of superluminal transmission of information to that particular quantum system, even though the information is not acccessible to any other physical system? I believe this is Maudlin'a argument? I always understood it as instataneous "hidden signals" between such systems but as long as these "signals" are hidden/private, no contradiction with relativity occurs.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook