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Physicist Trying To Send a Signal Back In Time 685

Posted by samzenpus
from the don't-be-so-gullible-McFly dept.
phil reed writes "University of Washington physicist John Cramer is attempting to send a signal back through time." From the article: "We're going to shoot an ultraviolet laser into a (special type of) crystal, and out will come two. lower-energy photons that are entangled," Cramer said. For the first phase of the experiment, to be started early next year, they will look for evidence of signaling between the entangled photons. Finding that would, by itself, represent a stunning achievement. Ultimately, the UW scientists hope to test for retrocausality — evidence of a signal sent between photons backward in time. The test will involve sending one of the photons down 10 miles of fiber optic cable, delaying it by 50 microseconds, then testing a quantum-mechanical aspect of the delayed photon. Due to quantum entanglement, the non-delayed photon would need to reflect the measurement made 50 microseconds later on the delayed photon. In order for this to happen, some kind of signal would need to be sent 50 microseconds back in time from the delayed photon to the non-delayed photon. (Confusing? Quantum physics is like that.)
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Physicist Trying To Send a Signal Back In Time

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  • by AEton (654737) on Thursday November 16, 2006 @06:31AM (#16866882)
    @BEGIN MESSAGE
    @author 321260
    @target_time 0519
    @subject I heard about this
    @content Yesterday
    @EOM
  • by Anonymous Coward on Thursday November 16, 2006 @06:38AM (#16866916)
    Okay, we all know spooky action at a distance involves guaranteeing that one measurement can determine the outcome of another billions of miles away; but this cannot actually be used for communication because there is no way of fixing the state of one, just measuring it, forcing an eigenstate. But the article suggests that we are *setting* "wave or particle-ness" (what? What quantum state is that referring to?) on the photon travelling the extra distance and transmitting that information back in time to the photon that first hits the detector. eh?

    Please type the word in this image: predict
  • That's not a signal. (Score:5, Informative)

    by i_should_be_working (720372) on Thursday November 16, 2006 @06:47AM (#16866968)
    It carries no useful information, and it's not going 'backwards in time'. It's just two entangled particles outside of each other's light cone. Once one particle is found to be in a certain state, the state of the other particle will be instantly known, but no information is traveling back in time or faster than the speed of light.
     
    It would be cool to see it actually happen, since previous entanglement experiments have never put the particles outside of each other's light cone, but the effect is something that physicists have understood (as much as anything in quantum physics is) for decades. In the article one of them say they don't really expect it to work, but I'd guess this is for technical reasons. No one expects that it won't work for theoretical reasons.
  • The future called. (Score:5, Informative)

    by Anonymous Coward on Thursday November 16, 2006 @07:00AM (#16867078)
    It wants its news back.
    Quoting from:
    http://en.wikipedia.org/wiki/Quantum_entanglement [wikipedia.org]

    Although two entangled systems appear to interact across large spatial separations, no useful information can be transmitted in this way, so causality cannot be violated through entanglement.

    The slashdot editor's brains seem to be traveling back in time though.
  • by OeLeWaPpErKe (412765) on Thursday November 16, 2006 @07:03AM (#16867092) Homepage
    They're sending a RANDOM signal back in time.
  • by DMiax (915735) on Thursday November 16, 2006 @07:12AM (#16867168)

    I actually graduated in quantum information, this is no news and it is wrong.

    I explain my opinion:

    - Entanglement has been observed, pairs of fotons and spin of electrons can be correlated in a manner impossible to describe in classical physics.

    - The experiment described does not even measure entanglement, as you could achieve the same result classically:
    Say I have a black ball and a white ball, I put one at random in a closed box, the other one in another box. Say the boxes are put 1000 miles away from each other, from the content of one of the boxes I can predict which ball is in the other one, as I can check later.

    The point is that they are not choosing in which state (of polarization) the light will be in the moment they measure the first time. So they aren't going to send any message ever this way. To do it they would require a classical channel wich works as we expect...

    For the proof of entanglement one must implement physically the Bell's system [wikipedia.org] or the Greenberger-Horne-Zeilinger one (I have no link), and SURPRISE! it has already been done.

  • Seems familar (Score:1, Informative)

    by Anonymous Coward on Thursday November 16, 2006 @07:22AM (#16867226)
    This is what happens in Timescape, by Gregory Benford! I wonder if this guy's read it? Certainly looks like he might suffer the same fate as the protagonists of the book - being labelled a crackpot!
  • by Per Abrahamsen (1397) on Thursday November 16, 2006 @07:28AM (#16867268) Homepage
    If they measure the non-delayed particle *before* the 50 ms have passed, the quantum state of the delayed particle will already be fixed at the time they get around to measure it.

    On the other hand, if they wait 50 ms before measuring the non-delayed particle, they aren't really sending much of a signal back in time.

    It isn't much use to send message back in time, if you aren't allowed to read them before the present time.
  • Re:Seems familar (Score:2, Informative)

    by mikesum (840054) on Thursday November 16, 2006 @08:06AM (#16867476)
    Damn, you beat me to it. In the book (from 1979 or 80) the future of 1998 is dying, and we have to send a message back in time to warn our past selves. This involves firing a tachyon beam into space where past Earth was. It will interfere with a physics experiment in 1962. One of the problem with transmitting a message back in time to 1962 is that the scientist doesn't know that he has a reciever. He wonders about the strange interference plaguing his data. Of course there is a big build up, but the ending sucks. It sucks hard.
  • by Anonymous Coward on Thursday November 16, 2006 @09:36AM (#16868362)
    To figure this out, you need to understand the relationship between the models of science and the real world. Clearly there is a direct and very thorough correspondance between the two, because after all our goal in science is to make the correlation between predictions and observations as strong as possible.

    But there's a huge gulf between saying that and then making a leap into assuming that our models directly reflect the structure of reality. They don't. We don't know anything about the actual structure of reality at all, and have no means of ever knowing anything. It's all entirely indirect, and in a funny sort of way, purely circumstantial.

    That's no hardship at all in science, in fact it's an opportunity --- it means that we can refine and redefine our models for all eternity, and hence achieve ever greater things. Arriving at a final Truth is simply not possible, not even in infinite time, because models are just models, and never the real thing. A rather cute observation that hammers home this point is that reality doesn't even use mathematics --- it's purely our own invention, the clockwork for our scientific models.

    Given the above, the explanation for the quandry created by TFA can be explained pretty simply (although not particularly accurately, as Godel applies here): by Bell, the communication *must* be regarded as occurring across time, because if we don't think of it that way then our axiomatic framework in relativity breaks down, and we don't want that. So really, it's just a fudge to save the model, as it's a damn good (ie. useful) model.

    But that doesn't say much about what's really happening. :-)

    If you think that's wierd, you're wrong. Science is full of submodels of limited applicability which can't be reconciled. That doesn't stop them from being the foundation of progress.
  • by radtea (464814) on Thursday November 16, 2006 @10:37AM (#16869094)
    Actually the experiment is designed properly. The thing is, they are already going to misinterpret the results. Quantum entaglement means that at the moment of setting wavefunction of one of the particles, the wavefunction of second particle is immediately changed to "second" possible state

    I believe they are hypothesizing actual signalling to occur as follows. Call the two detectors Ap (for prompt arm) and Ad (for delayed arm), and the two photons Pp and Pd for the same reasons.

    Ap and Ad are not the same. Ap has some capacity to respond to the photon in two different ways. I don't know what they're planning, but conceptually some kind of double-slit apparatus followed by a two-layer detector that has one layer capable of determining which slit the particle passed through, followed by another layer that is sensitive only to photons in interference maxima that have classically very low probabilities. So if you detect the photon in layer 1 it is behaving as a particle, in the layer 2 it is behaving as a wave.

    On the other end, at Ad, rather than giving photon Pd a "choice" of what to do, you have two different detector systems: one that is an interferometer, one that is a localized particle detector. One or the other gets switched into the beam "after" the photon has been detected at Ap. With correct placement of the detectors it should be possible to give the term "after" an absolute meaning.

    The claim is that the results of the measurement of Pp by Ap will necessarily reflect the choice made by the experimenter at Ad. So if Pp is detected "as a particle" it will be "because" the experimenter has chosen to detect Pd "as a particle" some time "later", and similarly if Pp is detected "as a wave". The heavy use of scare quotes is due to my respect for relativity and disbelief in strong quantum ontologies.

    I hope I have made this seem plausible, although it is all wrong.

    The perfect linearity of quantum reality ensures that when one gets down to the detailed computations there is an exact balance between terms that wipes out any possibility of transmission of information by this means. This experiment is testing this aspect of reality, and if no one has been able to explain to them "exactly" why it won't work it is because no one has bothered to do the detailed analysis of their apparatus that would be required. When detector efficiencies are folded into the mix the analysis can become quite complex, and you really need to do that if you want to test causality in this manner. If you want to simply demonstrate that the conventional interpretation of QM predicts no knowable information will be transmitted the analysis is much easier.

    So this is a pretty ordinary test of the linearity of quantum reality, and as they say, it is virtually certain that no transmission of information will occur. Unfortunately, given the truly terrible standard of communication demonstrated by this article it is likely that that fact will never be clearly understood by the public.
  • Douglas Adams (Score:4, Informative)

    by Headcase88 (828620) on Thursday November 16, 2006 @11:08AM (#16869492) Journal
    The major problem is quite simply one of grammar, and the main work to consult in this matter is Dr. Dan Streetmentioner's Time Traveller's Handbook of 1001 Tense Formations. It will tell you for instance how to describe something that was about to happen to you in the past before you avoided it by time-jumping forward two days in order to avoid it. The event will be described differently according to whether you are talking about it from the standpoint of your own natural time, from a time in the further future, or a time in the further past and is further complicated by the possibility of conducting conversations whilst you are actually travelling from one time to another with the intention of becoming your own father or mother...

    To resume: The Restaurant at the End of the Universe is one of the most extraordinary ventures in the entire history of catering. It is built on the fragmented remains of an eventually ruined planet which is (wioll haven be) enclosed in a vast time bubble and projected forward in time to the precise moment of the End of the Universe. This is, many would say, impossible.

    In it, guests take (willan on-take) their places at table and eat (willan oneat) sumptuous meals whilst watching (willing watchen) the whole of creation explode around them. This is, many would say, equally impossible.
    You can arrive (mayan arivan on-when) for any sitting you like without prior (late fore-when) reservation because you can book retrospectively, as it were when you return to your own time. (you can have on-book haventa forewhen presooning returningwenta retrohome.) This is, many would now insist, absolutely impossible.

    At the Restaurant you can meet and dine with (mayan meetan con with dinan on when) a fascinating cross-section of the entire population of space and time. This, it can be explained patiently, is also impossible.

    You can visit it as many times as you like (mayan on-visit re-onvisiting... and so on-for further tense-corrections consult Dr. Streetmentioner's book) and be sure of never meeting yourself, because of the embarrassment this usually causes. This, even if the rest were true, which it isn't, is patently impossible, say the doubters.
  • Re: The Future (Score:4, Informative)

    by iamghetto (450099) on Thursday November 16, 2006 @12:05PM (#16870344) Homepage
    Not to get to technical about hypothetical time travel, but Professor of Physics at the Univ. of Conn., Dr. Ron Mallett is one the leading physicists actually dealing with the plausibility of time travel. According to his work (and coincidentally John Titor as well) we can't actually go back or forward into our same time lines. We can go back to 1800 AD, but it we'd do so by side-stepping to a parallel 1800 AD, not ours own. We cannot traverse time in a reverse fashion, but rather step outside of it then step back in.

    There would be discrepancies in the timelines for that reason. John Titor, for example, couldn't know exactly what would happen in our timeline but can only relate his own timeline which is basically an approximation of ours.
  • by Xerxes314 (585536) <clebsch_gordan@yahoo.com> on Thursday November 16, 2006 @12:35PM (#16870796)

    IAAP, and this point of view (that in standard quantum mechanics no information is transmitted superluminally) is entirely correct. This will just be one more shoddy example of science reporting in a very very long line. The only question here is who got their basic facts wrong?

    The physicist in question really ought to know better. Did he lie to the reporters in order to get press for his experiment?

    The newspaper ought to have done some basic fact-checking; reading Wikipedia would be enough to figure things out in this case. Did they lie to the public to make the story more interesting?

    So let's do some digging. The physicist in question is a proponent of the "transactional interpretation" of quantum mechanics (not coincidentally invented by this same guy). In this interpretation, particles may send signals back in time that "handshake" with other particles in the past; however, they do so in such a way that ordinary causality is always correct. See, for example, Cramer's paper at http://www.npl.washington.edu/npl/int_rep/qm_nl.ht ml [washington.edu] where he says:

    Can quantum nonlocality be used for faster-than-light or backward-in-time communication? Perhaps, for example, a message could be telegraphed from one measurement site of the EPR experiment to the other through a judicious choice of which measurement was performed. The simple answer to this question is "No!"

    So that seems to answer that question. However, he goes on to muddy the water by suggesting that quantum mechanics as verified by every experiment to date is actually very slightly wrong, that quantum theory is actually slightly nonlinear. In that case, the delicate conservation of our usual notion of causality will break down and superluminal signals become possible again. Virtually nobody believes this is the case, but I suppose that shouldn't stop us from checking just to be sure. After all, sometimes what nobody believes still turns out to be true.

    The blame here (as so very often) must fall on the reporters. Let's examine some of their shoddy work:

    The problem with quantum theory, put simply, is that it's really weird.

    That's not a problem with quantum theory; it's a problem with what you think is weird.

    One of the paradoxes of interest to Cramer is known as "entanglement." It's also known as the Einstein-Podolsky-Rosen paradox, named for the three scientists who described its apparent absurdity as an argument against quantum theory.

    Like the twin paradox, this is not a paradox at all. Quantum mechanics predicts something. EPR say, "Hey, that sounds weird and wrong." Experiment verifies quantum mechanics. Once again, the problem is with what is perceived as being "normal", not with quantum theory.

    If one of the entangled photon's trajectory tilts up, the other one, no matter how distant, will tilt down to compensate.

    This one is the core conceptual problem with the whole article. It should read:

    If one of the entangled photon's trajectories is measured to be up, the other one, no matter how distant, if measured will be measured to be down.

    That doesn't sound very weird at all, which is why reporters persist in getting it wrong. People like to think quantum mechanics is weirder than it is; it adds some kind of mystical aura to the whole thing. But the universe is plenty weird and interesting even when you get all your facts right. I hope eventually the popular writing on quantum theory will reflect that.

  • by Anonymous Coward on Thursday November 16, 2006 @01:40PM (#16871948)
    Original (and I have an old 'Penguin' book to prove it) was:

    There was an old lady called Bright
    who travelled faster than light.
    She went out one day
    In a relative way
    and came back the previous night.

    Scansion needs a biblical pronunciation of 'travelled', which perhaps the UK spelling helps with (checked in OED, before you lot jump on it). For connoisseurs, the other one in the same chapter was:

    There was a young fellow named Fisk
    Whose fenced exceedingly brisk.
    So fast was his action
    The Fitzgerald contraction
    reduced his rapier to a disk.

    (with all due respects to that 1887 comedy duo 'Michelson and Morley', of course)
  • Re:Grammar Nazi (Score:2, Informative)

    by beckerist (985855) on Thursday November 16, 2006 @02:07PM (#16872366) Homepage
    aaaaaand RIGHT THERE is where it turns into overkill.

    I must admit though, there are some impressive grammatical skillz (including this parent and above) going on! One question though. If "screwed" turns to "scrod" and inherently "scrodded," what was that young Atlantic Haddock I had for lunch today? Scurred? Now this is just getting all Chingy!
  • by Anonymous Coward on Thursday November 16, 2006 @02:31PM (#16872736)
    yes the idea is that the delayed photon is changed.
    What they try to do is they choose with their moving detector how they want the delayed photon to be detected.
    The question they try to answer: if we choose for example delayed photon to be detected as wave and we know that 50mksec earlier the other photon was detected as particle - will we be able to detect the delayed wave-photon ?
  • Re: The Future (Score:5, Informative)

    by iabervon (1971) on Thursday November 16, 2006 @03:07PM (#16873402) Homepage Journal
    There are a bunch of things that make this less useful for doing weird things:

    You can't send a message back in time. You can only receive a message from the future. That is, you can only send a message back in time to a point where you had arranged to get it. It's like an box that you take stuff out of before you put it in; things go back in time to the point where you took stuff out, not to any other time. So there's no issue with the fact that we're not getting messages from the future; the time before the time machine is invented is inaccessible.

    You can't tell what it says in the past. This is where quantum is weird. Basically, what happens is that person A receives the message, which is a series of dots to put in a picture. It looks like random static. Then person B sends the message, which consists of choosing, for each dot, "bell" or "bars". Then they talk to each other, and they find that if you look at only the "bell" dots, the picture is a bell, and if you look at the "bars" dots, it's a set of bars. Since all of the data is collected by A before B chooses, they have to come to the conclusion that something really weird is going on, and the choice later clearly affects the data that was already written down. But they can only come to this conclusion after the experiment is over; before the message is sent, the received message can't be interpreted, although all of the observations can be taken.

    This of it like this magic trick: the audience gets a deck of cards with a variety of backs which they examine in detail. A volunteer on stage shuffled a second deck of cards, writes down a few numbers between 1 and 52, and draws the cards with the given numbers (i.e., for 10, draws the 10th card in the shuffled deck). When the volunteer announces the set of names, they all turn out to have the same backs in the audience's deck. The volunteer chose freely, the deck was really random, and the audience saw the fronts and backs of all of the cards in their deck before the choice was made. If the trick is repeated with fresh decks, it always works. We have to conclude that the volunteer is affecting the construction of the deck in the past, but we're only impressed after it's all over, and we have no idea what the volunteer is going to choose in advance. Even if we agree on a set of numbers to pick if the stock market goes up and a different set to pick if it goes down, we can't tell by looking at the audience's deck which it will be, but the trick still works.
  • by tylersoze (789256) on Thursday November 16, 2006 @06:35PM (#16877130)
    I can understand why you would think the guy was a crackpot if he was on Art Bell. Had I not been familiar with his work and theories beforehand I'd probably have the same knee jerk impression. Not having heard the interivew I can't comment on if he sounded "crazy" or not :) but I've been very interested in his Transactional Interpretation of Quantum for quite some time having come across it while I was researching Wheeler-Feynman absorber theory in grad school. I even came across it recently as a well known physicist's (Lee Smolin I think, the loop quantum gravity guy, although I guess the Super String guys would consider him a crackpot :) favored interpretation in a mainstream physics book I just read.

    http://www.npl.washington.edu/ti/ [washington.edu]
    http://www.npl.washington.edu/npl/int_rep/dtime/no de2.html [washington.edu]
    http://en.wikipedia.org/wiki/John_Cramer [wikipedia.org]
    http://en.wikipedia.org/wiki/Transactional_interpr etation [wikipedia.org]

    I'd suggest you guys also look into the controversy over the various interpretations of QM among physicsists nowadays. Hell how anyone ever thought Copenhagen made any sense is beyond me. :)

    http://en.wikipedia.org/wiki/Interpretation_of_qua ntum_mechanics [wikipedia.org]
  • Re:Grammar Nazi (Score:3, Informative)

    by LouisZepher (643097) on Thursday November 16, 2006 @08:39PM (#16878574)
    This is /., everything is an obscure reference here, but don't panic, just relax and pour yourself a nice refreshing ouisghian zodah.

To be a kind of moral Unix, he touched the hem of Nature's shift. -- Shelley

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