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Quantum Experiment Shows Effect Before Cause 465

Posted by Soulskill
from the enjoy-the-headache dept.
steveb3210 writes "Physicists have demonstrated that making a decision about whether or not to entangle two photons can be made after you've already measured the states of the photons." Here's the article's description of the experiment: 'Two independent sources (labeled I and II) produce pairs of photons such that their polarization states are entangled. One photon from I goes to Alice, while one photon from II is sent to Bob. The second photon from each source goes to Victor. Alice and Bob independently perform polarization measurements; no communication passes between them during the experiment—they set the orientation of their polarization filters without knowing what the other is doing. At some time after Alice and Bob perform their measurements, Victor makes a choice (the "delayed choice" in the name). He either allows his two photons from I and II to travel on without doing anything, or he combines them so that their polarization states are entangled. A final measurement determines the polarization state of those two photons. ... Ma et al. found to a high degree of confidence that when Victor selected entanglement, Alice and Bob found correlated photon polarizations. This didn't happen when Victor left the photons alone.'
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Quantum Experiment Shows Effect Before Cause

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  • by Tanman (90298) on Tuesday April 24, 2012 @04:38PM (#39787321)

    Nevermind -- why bother telling you if you already know :-(

  • Sigh... (Score:5, Funny)

    by InvisibleClergy (1430277) on Tuesday April 24, 2012 @04:41PM (#39787373)

    *Looks at physics degree.*

    *Tosses it in the trash.*

  • Paradoxical (Score:5, Funny)

    by myrdos2 (989497) on Tuesday April 24, 2012 @04:41PM (#39787375)
    Victor should decide not to entangle the photons whenever Alice and Bob's polarizations are correlated. That'll rip physics a new one...
    • shhh! That's the halt instruction for our universe.

      • by myrdos2 (989497)

        Hmmm... if we were to keep Alice and Bob one light-millisecond from Victor, could we send messages back in time? Imagine we're continually repeating the process of emitting photons as in the summary.

        1) An earthquake happens. Victor entangles the photons.
        2) Alice and Bob detect correlated polarizations, and instruct Victor to entangle his photons.
        3) This process continues, bringing us one millisecond backwards in time at each step. (Minus the time to perform the measurements and inform Victor)
        4) Lab sta

        • by hvm2hvm (1208954)
          Yeah, I really like that. Kind of like the signals sent in medieval times (and in LOTR for example) by lighting up torches(towers) at predefined locations in a domino effect.

          Still, creating a paradox would be cooler - i.e. telling victor to entangle the photons when they get different results or vice versa. Maybe that makes time go backwards or something like that just to avoid the paradox
        • by dward90 (1813520)

          I'm not sure how serious you are, but I'll point out the problem at the risk of killing the joke. The issue is in step 2. Photons travel at the speed of light (by definition). Because we cannot send information faster than the speed of light, the photons arrive at Victor strictly before any message from Alice and Bob.

          • by myrdos2 (989497)
            I suppose using any of these techniques to slow light would destroy the entanglement? http://en.wikipedia.org/wiki/Slow_light [wikipedia.org]
          • by skids (119237)

            Photons can also bend their paths, so just because they travel from Alice/Bob to Victor doesn't mean Victor is that many light-meters away. The photon could be traveling in a circle. But then, I don't know what that does to polarization :-)

          • by Baloroth (2370816)

            Not true: we can make photons travel much much slower than c (the speed of light in a vacuum), while transmitting the information up to c, which means we could certainly communicate information faster than the photons travel to Victor. In fact, most fiber optic cables IIRC transmit light at ~3/4 the speed of light in a vacuum.

            Of course, it still almost certainly wouldn't work. I actually wouldn't be surprised if anyone setting up such a system noticed that Victor entangling the photons didn't correlate Ali

          • Re:Paradoxical (Score:5, Interesting)

            by Ruie (30480) on Tuesday April 24, 2012 @05:20PM (#39787917) Homepage

            I'm not sure how serious you are, but I'll point out the problem at the risk of killing the joke. The issue is in step 2. Photons travel at the speed of light (by definition). Because we cannot send information faster than the speed of light, the photons arrive at Victor strictly before any message from Alice and Bob.

            Just use a fiber optic cable to make them wait longer. Or bounce between mirrors in a zigzag - this way light trajectory can be long, but the spatial distance can be short.

            • by kipsate (314423)

              Just use a fiber optic cable to make them wait longer. Or bounce between mirrors in a zigzag - this way light trajectory can be long, but the spatial distance can be short.

              This would cause the photon to interact and hence the waveform to collapse.

    • by TexVex (669445)
      Entanglement implies correlation, but correlation does not imply entanglement.
      • by hvm2hvm (1208954)
        Same thing... Entanglement implies correlation -> if victor decides not to entangle the photons (for whatever reasons) and alice and bob get uncorrelated measurements it's still a paradox, isn't it?
    • Victor should decide not to entangle the photons whenever Alice and Bob's polarizations are correlated. That'll rip physics a new one...

      That would require observing the Alice / Bob results first, thus "changing" the Victor photons before he can do anything.

      • by hvm2hvm (1208954)
        So once victor knows from alice and bob whether the measurements are correlated, the photons are already entangled/not entangled? That seems like a method for passing information into the future...
  • causality (Score:5, Interesting)

    by Black Parrot (19622) on Tuesday April 24, 2012 @04:42PM (#39787385)

    AIUI, the notion that information can't be transferred faster than the speed of light is based on the fact that it would violate causality. I have wondered whether causality is an assumption rather than an actual property of the universe.

    If it is (I'm not qualified to interpret this experiment), we'll have a lot of new physics coming down the pike over the next few decades.

    • by RyuuzakiTetsuya (195424) <taiki&cox,net> on Tuesday April 24, 2012 @04:46PM (#39787461)

      Given that now cause/effect are now uncertain...

      are you sure about that? :)

    • by skids (119237)

      the notion that information can't be transferred faster than the speed of light is based on the fact that it would violate causality

      Well, it would pose problems to the way modern physics interprets the concept of spacetime (and not without ample experimental evidence.). If you asked someone with a more newtonian view of the universe, something traveling over the speed of light isn't going back in time, and so there's no causality violation.

      Anyway this experiment would seem to leave open the question as to whether the Victor measurement could be performed in time to inform Alice and Bob, unless that can be ruled out by other factors.

    • by Chris Burke (6130)

      I have wondered whether causality is an assumption rather than an actual property of the universe.

      Causality is absolutely an assumption, one that physicists have understood that they are making -- among others -- for a long time. It might be an invalid assumption that only appears to be correct most of the time.

      And what a fucking weird world would that be? Could we even reason about such a universe? It might be impossible.

      I know I'm not going to let go of this assumption until there is some very, very convincing evidence.

  • by dc5464 (869897)
    I don't even know where to begin with this one.
    • Re: (Score:2, Funny)

      by Anonymous Coward

      don't worry, you've already decided......

  • one month until a New-age quack publishes a book on how to harness this phenomenon for better health, improved intimacy, and financial success!
  • by Grayhand (2610049) on Tuesday April 24, 2012 @04:46PM (#39787459)
    Victor is Bipolarized making him erratic and unpredictable. Might want to try adding lithium atoms into the mix and see if the results stabilize.
  • I was already confused before reading the article, that proves effect before cause.

  • More studies have to be done to see how the correlation relates to the time interval between observation and choice, the current setup was just a few nanoseconds' delay.

    • by plalonde2 (527372)
      10:1 says that once the (alice->victor | bob->victor) delay is longer than the speed of light delay from alice->bob the effect vanishes. The result seems consistent with causation being an effect at slower scales than the speed of light, which comes back to the basics of modern physics: Everything is goofy when you get near C.
      • by jd (1658) <imipak @ y a h o o .com> on Tuesday April 24, 2012 @06:59PM (#39788893) Homepage Journal

        Ok, let's rephrase the experiment. You have four photons - A, B, C, D. A starts off entangled with B, C starts off entangled with D.

        What the experiment appears to show is that if B is then entangled with C, then A is effectively entangled with D. In other words, entanglement is transitive. What it does NOT show is a violation of causality, unless I'm seriously misunderstanding the results.

        (There may be other alternative explanations, but I'm satisfied that the results can be explained without resorting to violations of causation.)

        However, I am going to throw in another thought -- IF it is established that causation is indeed violated, the Many Worlds theory of quantum mechanics must be false. (The Many Worlds theory says that the universe splits at the event, and that the measurement simply tells you which universe you're in - until then, there's a given probability you're in any of the possible universes. However, the event hasn't taken place at the time of the measurement here, so all probability waves must coexist, so you should observe every possible state. This isn't what's observed. Ergo, one or both of Many Worlds and Violation of Causality must be wrong.)

  • by agent_vee (1801664) on Tuesday April 24, 2012 @04:49PM (#39787509)
    1. PROFIT!!!
    2. ???
    3. Collide some photons!
  • by Anonymous Coward on Tuesday April 24, 2012 @04:53PM (#39787573)

    I really which quantum people would stop acting like they know what they are talking about.

    This is just a really shitty description/way of looking at a series of events and is more or less wrong in the same way that saying your traveling back in time by looking at old stars in the sky from far off distances.

    The only thing out of order here is the observers note taking and logic. Due various other quantum flux it may appear to happen in a certain order even though it didn't and its just a matter of appearance due to propagation effects.

    Its a bad observation and bad description of that observation, not a causality violation.

    • by marcosdumay (620877) <marcosdumay AT gmail DOT com> on Tuesday April 24, 2012 @05:48PM (#39788179) Homepage Journal

      Due various other quantum flux it may appear to happen in a certain order even though it didn't and its just a matter of appearance due to propagation effects.

      That phrase is also missing some better note taking and logic. Does it have any meaning? It's not a bad observation at all, altought I'd agree that is a bad description.

      To summarize the article, scientists confirm (again) that Quantum Mechanics works as designed. Despite all the naysayers (ok, there aren't many anymore), and the despair of people trying to create any deep understand over what is a purely pragmatic model, the Universe works exactly the way QM says it will. On a related notice, causation is preserved, unless you want any deep understanding of it.

  • Just a glitch in the matrix....
  • by Chris Burke (6130) on Tuesday April 24, 2012 @04:59PM (#39787643) Homepage

    Or at least try...

    So a key part of the experiment was that the pair of photons sent to "Victor" went through a 104 meter cable to ensure that whatever Victor did, Alice and Bob measured their polarizations first.

    Presumably, one could extend this cable to increase the amount of time between Alice and Bob's measurement and Victor's decision to entangle or not.

    Presumably long enough for Alice and Bob to send the result of their measurement to Victor.

    And then instead of an RNG, Victor chooses to entangle based on whatever would contradict Alice and Bob's measurement.

    Come on, we have to try...

    P.S. the paper says they aren't violating causality, and it only looks like they are if you're looking at it wrong.

    • by mark-t (151149)
      I might suggest that although their measurements are made before Victor's decision, I expect that the results of their measurements could not reliably be communicated to Victor prior to his decision.
      • by dgatwood (11270)

        I would suggest that their measurements were not made before Victor's decision, but merely before Victor's realization of that decision. Victor believes that his decision was random, but in fact it was actually biased strongly (and possibly determined entirely) by the overall state of the universe as a whole.

        Newtonian physics would suggest that if you could simultaneously know the position, velocity, spin, etc. of every particle in the universe, you could know the future. If one particle is entangled wit

  • by narcc (412956) on Tuesday April 24, 2012 @04:59PM (#39787647) Journal

    The experiment in the article is ... awesome. Though if history is any indication, hoards of raving Slashdoters will try their damnedest to force this into a classical mechanistic world-view.

    So here's a fun experiment you can do at home! (Craftsmanship is important for good results.)

    1) Start by setting up up a classic double-slit experiment. A laser pointer and some household junk is all you need.

    * Observe the interference pattern.

    2) Stop denying that you went to see "Avatar" 36 times and grab a couple pairs of 3D movie glasses.
    2a) Alternately, you can just buy a polarizing filter sheet. (this is the better way)

    3) Being careful to note orientation of the filter, place the filters in front of the slits with one oriented 90 degree to the other. (This is only tricky because the distance between the two slits is so small.)

    * What happened to the interference pattern? You "tagged" the individual photons so that you could, in principle, know which slit they passed through, so instead of going through both, they went through just one.

    4) Place a third sheet of polaroid between the slits and the detector screen, oriented half-way between the two other filters (if one sheet is vertically oriented and the other horizontally, this sheet will be oriented at 45 degrees)

    * The interference pattern is back? WTF? You took the tag away, so that you couldn't know which slit a photon passed through. You "erased" the which-path information so each photon went through both slits, instead of just one of them.

    Do the experiment. Accept that the physical world is weird as shit. Shut-up and calculate.

    • by geekoid (135745)

      Some men just want to watch the world learn.

    • by daaxix (218354) on Tuesday April 24, 2012 @07:00PM (#39788901)

      I am an OSGS (Optical Sciences Graduate Student) and you don't need Quantum Mechanics to explain the experiment above, all you need is classical wave optics.

      Linear polarization is electric field in a specified direction, lets say you have the electric field oscillating in the x direction and in the y direction for the first slit and the second slit respectively. Those directions are orthogonal to one another, so cannot interfere (the inner product is zero). But, if you have some component from both slits in some direction (for your example you will be getting out sqrt[2]/2 of the x component in the 45 degree direction and sqrt[2]/2 of the y component in the 45 degree direction when you insert the 45 degree polarizer, which is basically equivalent to the no polarizer case except you have reduced the amplitude). Then you have slit interference in the classical sense as illustrated here : http://astro1.panet.utoledo.edu/~lsa/_color/14_interference.htm, you will have to scroll down to see the two slit interference. Note that we see a sinusoidal pattern because our eyes view the time averaged irradiance (intensity) of the wave pattern, the the wave pattern itself.

      What is different about the quantum case is that you can send, say electrons, through the slits *indivdually*, one at a time and they somehow interfere, that is what is intuitively strange.

  • by wierd_w (1375923) on Tuesday April 24, 2012 @05:00PM (#39787661)

    What this article is saying, is that victor's decision to entangle his photons has a direct effect on the results that alice and bob get from their double blind measurements.

    So, either there is retrograde communication on time's axis, or....

    The decision that victor makes is predetermined, by the act of measurement undergone by alice and bob. (Meaning victor doesn't really have as much free will as he thinks he does.)

    Proposed followup experiment:

    Alice and bob examine their photons, tell each other, but not victor. Victor decides to entangle or not entangle. Examine new correlation.

    This will test "does a correlation between alice and bob indicate that victor will entangle?".

    If it does, you have a reasonably strong test case for many worlds.

  • by Spy Handler (822350) on Tuesday April 24, 2012 @05:01PM (#39787665) Homepage Journal

    Subatomic laws
    Scientific pause
    Synchronicity

  • by rritterson (588983) on Tuesday April 24, 2012 @05:04PM (#39787715)

    One explanation of the results, should they hold up is that Alice, Bob, and Victor's actions were predetermined before the photons were generated and thus had to correlate.

    You could say that the actors then had no free will, or you could imagine a scenario where somehow the actions of all three were entangled via an earlier free will choice.

    • by tompaulco (629533)
      You could say that the actors then had no free will, or you could imagine a scenario where somehow the actions of all three were entangled via an earlier free will choice.
      Or something non-observable already contains the information about the entanglement and the acts of Victor, Alice and Bob are all determined by the non-observable information holder.
  • ... what happened to the cat?

  • Alice get Ia and Bob gets IIa and Victor get Ib and IIb. Alice and Bob do their measurements. Alice and Bob then can compare their measurements and find that Victor was fiddling with the equipment on his end or not. What constitutions "no communication passes between them during the experiment"? Does Victor have to do his measurements to determine that the experiment is over, or Can I shoot Victor as the only one who knows whether he diddled the equipment or not and his message would still exist in the han
  • by cvtan (752695) on Tuesday April 24, 2012 @09:22PM (#39790239)
    This is even better than the news item about how the Psychic Network went out of business due to unforeseen financial difficulties.
  • by Suiggy (1544213) on Tuesday April 24, 2012 @09:33PM (#39790343)

    The researchers are assuming the actions of Victor to select a specific polarization and entanglement are somehow independent of the entire quantum configuration space. In other words, they're assuming free will, and the existence of external magical souls that are somehow independent of reality.

    If you assume determinism, Victor's actions should be consistent with the configuration space, and so when measurements are made by Bob and Alice that are correlated, it increases the probability that Victor will choose to entangle.

  • by quax (19371) on Wednesday April 25, 2012 @01:14AM (#39791539)
    The authors of the paper [arxiv.org] are actually quite clear on it:

    If one views the quantum state as a real physical object, one could get the seemingly paradoxical situation that future actions appear as having an influence on past and already irrevocably recorded events.
    However, there is never a paradox if the quantum state is viewed as to be no more than a âoecatalogue of our knowledgeâ. Then the state is a probability list for all possible measurement outcomes, the relative temporal order of the three observerâ(TM)s events is irrelevant and no physical interactions whatsoever between these events, especially into the past, are necessary to explain the delayed-choice entanglement swapping.

    The wave-function is nothing but a correlation machinery that organized nature's limited resources to properly fall into place (without upsetting causality as the correlations can only be sorted in hindsight).

    This demystified view of QM is still very much overshadowed by the Quantum Hippie version [wavewatching.net] that makes for better headlines. I.e. non of the pop science sites clearly report this tidbit of the authors wisdom. Causality violation draws more web traffic.

  • by aaronb1138 (2035478) on Wednesday April 25, 2012 @03:30AM (#39792123)

    Sounds very similar to the delayed choice quantum eraser experiments performed by Wheeler et al. The main difference sounds like the use of polarized filters instead of the double slit diffraction.

    http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser [wikipedia.org]

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