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Cramer's time travel experiment

  1. Apr 18, 2007 #1
    There was some publicity recently about John Cramer not being able to get funding for an experiment that he claims could prove "retrocausation" or backward time travel. I'm confused as to what the experiment is supposed to show. Cramer seems to be saying the outcome will depend on whether or not his transactional interpretation of QM is true, but that can't be the case, since all interpretations are supposed to make the same empirical predictions. Elsewhere, he seems to suggest the "sending information into the past" part works only if the world deviates from pure QM with small nonlinear effects. Does anyone understand what's going on?

    I certainly don't think QM lets you send information into the past, but the experiment still bothers me. Let's say that Cramer is right, and this sort of experiment really does let you set up a causal loop. If this is something Nature hasn't safely tried out yet (and I don't know if that's true -- anyone?), couldn't there be all sorts of risks we don't know about? For one thing, it seems to me that once you have a time machine, it's consistent with the laws of physics for absolutely anything to come out, so long as it goes back in later. People have made some predictions of risks (like black holes and strangelets eating the Earth) associated with particle accelerators, but at least we have strong evidence that those risks aren't real, since they would already have happened naturally due to cosmic rays. Do we have some similar argument why we know successful time travel experiments wouldn't blow up the world?

    If the answer is, "Oh, don't worry, there's only a 1 in 1000 probability that this experiment will destroy the universe", I don't find that very reassuring. Is doing the experiment worth one thousandth of a universe?

    On the other hand, if you can destroy the world with a 20,000 dollar experiment, I guess that in the long run we're screwed anyway.
    Last edited: Apr 18, 2007
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  3. Apr 20, 2007 #2
    I have not heard of this one but ...wait a minute. So what is he proposing to actually do in the experiment? He might try to prove something fancy, but what is the apparatus? What is really expected to be observed? What is he doing that doesn't happen every day on our little planet, Earth, and why are we all of a sudden at risk of being sucked into the void over a $20,000 experiment?

    Hey, I can do an experiment (say $19999 worth, with spitballs and cotton and see if time stops.
    But don't lose sleep over this one....
  4. Apr 20, 2007 #3


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    I have looked at this, and it DOES happen every day on our planet. So not to worry... :tongue:

    Cramer's is a essentially a Bell test setup attempting to measure entangled photon interference. There is no signalling to the past any more than there is instantaneous signalling in a Bell test. Only when the results at both sides are compared does a pattern emerge.

    In no ways does this experiment invalidate anything about oQM in favor of Cramer's Transaction Interpretation (TI). TI definitely has its own baggage, somewhat akin to the baggage of Many Worlds or Bohmian Mechanics. In this case, there are "offer waves" and "acceptance" waves.

    Actually, there are some things to like about TI. In many ways it does help to make sense of what seems to occur. The thing I like most about it is that it postulates physical effects going from the future back to the past. This is sometimes referred to as retrocausality. However, this effect is limited in such a way as to prevent the kind of paradoxical loops that are the stuff of sci-fi books.
  5. Apr 20, 2007 #4


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    This description sounds a bit like the delayed choice quantum eraser, is Cramer's setup similar to that?
  6. Apr 20, 2007 #5


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    It does look very similar to my eyes as well. The following link provides a much more detail description of the basic experiment. The ones to look at especially are Figure 3 (which is the Dopfer experiment) and Figure 2, which is the double slit with entangled photons. The author is Anton Zeilinger, so you know the analysis will be accurate.

    Experiment and Quantum Physics (1999)

    You can see more easily that nothing about the HUP is violated in this experiment. The same is true when you insert a optical fiber delay loop on one side. The only time any interference can be seen is in a subset of detections containing no "which way" information. This part is like the quantum eraser, as you point out.

    Is it retrocausality, as Cramer says? That is a matter of interpretation, in my opinion. I personally think that oQM must exhibit something like retrocausality in order to explain the mechanics of entanglement - but I would emphasize that is just one viewpoint.
  7. Apr 24, 2007 #6


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    The question is..Can QM have NOn-linear effects ??...by imposition and correspondece principle with Classical mechanics Schröedinguer equation is purely linear ,anyway why can you (according to cramer) send info to the past but not to the future ??..
  8. Jun 12, 2007 #7
    I suppose you've got to give him credit, so many crackpot ideas get stuck at the "it's a conspiracy denying me any funding" stage. I just can't convince myself, if there were any shred of question in this, that Zeilinger wouldn't have done it already.
  9. Jun 13, 2007 #8
    I don't care what the experiment is or even if it has the slightest amount value, he has raised a public interest in physics and that is the greatest result of all.

    CraigD, AMInstP
  10. Jun 15, 2007 #9
    Now that it's going to happen, I have to ask.
  11. Jun 15, 2007 #10


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    Ontoplankton, you could always just email Cramer and ask him your questions. He is a pretty nice guy and I'm sure that if you kept your question brief and coherent (i.e., non-crackpot-esque) he would be happy to answer.
  12. Jun 17, 2007 #11
    Here's a new(?) explanation of the experiment: http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf

    OGP: I don't know, I might, but I'm reluctant to say something amounting to "prove your experiment for sure won't blow up the world" without a good idea what I'm talking about. I'd still be obliged if someone here could confirm/explain that the experiment is not somehow going into unexplored physics territory. (Thanks, everyone, for the answers so far.)
    Last edited: Jun 17, 2007
  13. Jun 17, 2007 #12
    Look at fig. 1 in the pdf you linked. The top right part of the apparatus is designed to produce (then separate) pairs of entangled red photons, which are collinear and "momentum" entangled, which is presumed to mean that if a photon goes through slit A at S1, the entangled partner photon must go through the corresponding slit A' at S2, and so forth.

    Cramer's idea is that by choosing (at S2) to either measure "which slit the photons go through" or to not make that measurement, he thinks he can affect whether the camera (at S1) registers an interference pattern (as would occur if there was no way in principle to determine which slit the partner photon went through) or no pattern (as occurs if the "which-path" information is measured).

    I'm not the first to predict this will fail in the following way: regardless of what happens at S2, there will no net interference pattern at S1 (because it would be possible in principle to determine the which-path information at S2). Moreover, the optical combiner C is measuring and then classically discarding available information about the superposition of "which-path" states that the VLP/"idler", information which would be necessary to reconstruct the interference pattern from classical correlations with the HLP/"signal" photon.

    You don't need to trust my word on this, compare with the experiment published with the title Delayed "Choice" Quantum Eraser. If you look at the raw data from that experiment, you'll conclude Cramer's camera will measure "1" all of the time, never "0".

    Now, if this wasn't the case, then sure, the experiment could be a way to transmit information (from S2 to S1) "instantly" (or even backwards in time, violating causality, the very paradox of which might be hypothesised to destroy reality). But quantum physics just doesn't seem to work that way (and if it did, you'd expect those world-destroying causality-violations would already be constantly occurring, due to quantum entanglements amongst the air particles around us, without ill effect). And of course, Kim, et al, have already done basically the exact same experiment, which I'm surprised Cramer hasn't even referenced (I think I'll try emailing him.. it'd be a shame if he just hadn't seen the other work, surely it would be worth explaining the difference if he thought there was one, after all the time-symmetric interpretations of QM seem otherwise promising).
    Last edited: Jun 17, 2007
  14. Jun 18, 2007 #13


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    Hopefully his experiment *is* going into unexplored territory to some small extent... otherwise it would not be a cool experiment.

    P.S. I can promise you that Cramer's experiment is certainly not going to blow up the world. If you are worried about the world blowing up then I encourage you to act pragmatically and work for nuclear non-proliferation. :)
  15. Jun 18, 2007 #14


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    ma teory

    So basicaly they are two teories for teletransportation:the one that copies u and then kills u, and the other one that transformes u into a puzzle and then remakes u. But i have another theory that i came in here for u all to say if its right or wrong.So i first thought in framerates in games, if u have low framerates and u walk frontways u dissapear and appear somewhere infront instantly.Then i thought if real life movement was done by small teleportations that human eye sore them like continuous movement and if so, would it be posible of making those small teleportations bigger, in the game example, have slower framerates.The only bad thing about this is inertia, u need to teletransport that before u teletransport the object, basicaly we need to invert inertia. posibly it sounded like nonsence to u but... im only 15 o:).I think posibly is out off topic but i think u need teleportation for time travel.which is posible because if u travel in time to kill ur dad u just cant, because u werent destined to do it.and viseversa.:approve:
  16. May 27, 2009 #15
    Did John Cramer make the experiment at last? Anyone knows what happened with this issue?
    Last edited: May 27, 2009
  17. Feb 16, 2011 #16
    No offense, but I'm just wondering how much research and thought is behind what you wrote. Cramer knows a good bit of physics, but he's a crackpot, of sorts, with an agenda. Take it with a grain of salt, and, really, don't worry.
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