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Discover Magazine Article Backward Causality

  1. Sep 15, 2010 #1


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    I'm new so I hope I post at the right place ( I think I posted wrongly once already).
    I was just reading about this article on Discover Magazine which suggests that experiments show that events in the future may affect results in the past.

    Has anybody read it – does it suggest retrocausality?

    Some excerpts are here.
    Just last year, physicist John Howell and his team from the University of Rochester reported success. In the Rochester setup, laser light was measured and then shunted through a beam splitter. Part of the beam passed right through the mechanism, and part bounced off a mirror that moved ever so slightly, due to a motor to which it was attached. The team used weak measurements to detect the deflection of the reflected laser light and thus to determine how much the motorized mirror had moved.

    That is the straightforward part. Searching for backward causality required looking at the impact of the final measurement and adding the time twist. In the Rochester experiment, after the laser beams left the mirrors, they passed through one of two gates, where they could be measured again—or not. If the experimenters chose not to carry out that final measurement, then the deflected angles measured in the intermediate phase were boringly tiny. But if they performed the final, postselection step, the results were dramatically different. When the physicists chose to record the laser light emerging from one of the gates, then the light traversing that route, alone, ended up with deflection angles amplified by a factor of more than 100 in the intermediate measurement step. Somehow the later decision appeared to affect the outcome of the weak, intermediate measurements, even though they were made at an earlier time.
  2. jcsd
  3. Sep 15, 2010 #2


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    I don't take seriously any weird experimental result based on weak measurements. I have explained my reasons in my blog.
  4. Sep 15, 2010 #3


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    Thanks. Again, forgive me if I'm ignorant but isn't Discover magazine a reputable publication that wouldn't publish the results if they thought it was trash?

    So sorry to disturb.
  5. Sep 15, 2010 #4


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    I'm not saying it's trash. Indeed, some respectable physicists think that weak measurements are great. Yet, some other respectable physicists disagree. In other words, the meaning of weak measurements is controversial. So we can conclude that the value of this publication is controversial, but it does not mean that it is trash.
  6. Sep 15, 2010 #5


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    Discover magazine is just some publication format reporting on scientific findings published elsewhere. If you have a look at the original article in PRL (a viewpoint on it including a link to the free original article is linked in the Discovery magazine article), you will already find the following comment in the viewpoint:
    "The explanation can be fully given by standard quantum mechanics, involving regular past-to-future-only flow of time. But the explanation is cumbersome and involves very intricate interference effects in the measuring device. Assuming that time flows in two directions tremendously simplifies the problem. As far as I can tell, Aharonov, Albert, and Vaidman hold the view that one should indeed accept this strange flow of time. I fully agree. Not everybody agrees though, and this is one of the most profound controversies in quantum mechanics."

    In my opinion simplifying a problem might be a reason to consider this idea of time flow in both directions as a convenient mathematical tool, but as standard qm is completely sufficient to describe the results, the experimental results do not suggest that retrocausality is unambiguously demonstrated. Just having cumbersome calculations does not affect the validity of standard qm.
    Last edited: Sep 15, 2010
  7. Sep 15, 2010 #6


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    Yes, it suggest retrocausality. But that is an interpretation, and is not really required as an explanation. There are other experiments, for example, which suggest non-locality. And others which suggest realism is not respected.

    There are some amazing retrocausal experiments. My favorite, from an extremely well-respected research group, is:


    "Such a delayed-choice experiment was performed by including two 10 m optical fiber
    delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed
    by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."

    All of this is also consistent with Cthugha's comments.
  8. Sep 15, 2010 #7


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    Thanks everybody,

    But one more question, forgive me if I'm naive.

    In my opinion simplifying a problem might be a reason to consider this idea of time flow in both directions as a convenient mathematical tool, but as standard qm is completely sufficient to describe the results, the experimental results do not suggest that retrocausality is unambiguously demonstrated. Just having cumbersome calculations does not affect the validity of standard qm.[/QUOTE]

    Does the fact that one solution is more convenient than one cumbersome one mean it is more likely?
  9. Sep 15, 2010 #8


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    No, not more likely. :smile:
  10. Sep 16, 2010 #9
    Given a certain view of reality, retrocausality is simply nonsensical. One of the problems that modern physics faces is that it has no fundamental 'dynamical' laws. It's entirely possible that when such laws (or law) are formulated, then concepts like backward time travel and backward causation will be obviated.

    The preeminent conceptual problem is that we can't be living in an evolving universe and also be able to 'travel backward' in 'time'. In an evolving universe, 'retrocausality' is prima facie nonsensical.

    The convenience of certain formulations has to do with ... well, convenience, and not necessarily anything to do with reality.

    For me, the logical extension of conversations about stuff like retrocausality is that there's really no point in talking about anything.

    Nature is indeterminate, anything can happen at any time, and there are no fundamental dynamical laws to discover.
  11. Sep 16, 2010 #10


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    So, for some experiments some interpretations look more natural, while for other experiments some other interpretations look more natural. Thus, in a choice of a favored interpretation, one should not ask which interpretation looks the most natural for some specific experiment. Instead, one should ask which interpretation looks the most natural for ALL experiments. But the only way to think of all experiments at once seems to be not to think about experiments at all. Instead, one should only think about the general structure of the quantum THEORY, because all these experiments have a common feature that can be explained/predicted by the same theory. That's why I think that theorists have much better chances than experimentalists to construct a good interpretation of quantum mechanics, and that, for interpretational issues, various cool experiments act only as a red herring.

    Nevertheles, I do not think that experiments should not be done. Indeed, if one day some experiment turn out to disagree with the quantum theory, THEN the experiment will become more important than the theory, even for the interpretational issues.
  12. Dec 30, 2010 #11
    Hi all,

    In my reading of the Discover article when I stumbled upon it a couple of months back, I was struck by the statement that it did not matter whether the final measurement took place, the observed impact at the weak measurement was still recorded.

    Time for some armchair physics:
    The implication I'm tempted to draw from this is that it undermines a purely retro-causal theory in favour of the "intent" of the observer to take the measurement making the collapse of the superposition [due to a final measurement] more probable, thus the observed indication of a collapse at the earlier weak measurement stage more probably too.

    So the weak measurement would be more likely to indicate a collapse to a single state even where the final measurement is not taken, but was likely.

    Any effect the intended final measurement appears to have on the earlier weak measurement would sit within a frame of time after the causal intention. To my mind, this preserves causality - not sure if that's borne out mathematically.

    Where I'm coming from in this is that this experiment put me in mind of a rough thought experiment I came up with (while still studying) relating to one of the interferometer setups that involves beam splitting and recombining wherein one introduces an additional element that increases probability of one the channels being blocked (*), and measuring the impact this has on interference patterns in the result set where the channel was not blocked....

    (*) which at the time I'd considered might be as simple as the mere intention of the observer to block the channel.

    Assuming the above doesn't read as complete twaddle, what do you think of my interpretation?
    Could the intention to [take a measurement that would cause a] collapse the superposition be a mechanism that preserves causality within the set time-frame? (between the intention and action/inaction of the measurement)

    In anticipation.
    Layman Bill

    Having not studied Physics academically since an abandoned A-Level course, I'm a relative layman in this arena, so please be gentle.
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