Time-reversed causality and reality of space and time

[premjan]

Is it possible for something later in time to change something earlier in time? According to the delayed choice quantum eraser experiment (http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm), it is possible. In this experiment, basically there are two entangled photons that are detected, one earlier in time and one later. The later photon may be detected in such a way as to either reveal its source (slit A or slit B in the traditional Young's double slit experiment) or in a way that hides the slit choice of the photon. Note that as the two photons are entangled, they both originate from the same slit. If the second photon is measured in a way that reveals the slit choice of the photon(s), the first photon displays no interference pattern. If the second photon is measured in a way that does not reveal the slit choice of the photons, the first photon displays an interference pattern. It is also possible for this effect to occur instantaneously across large distances of space. It appears to effectively negate the reality of temporal order and of spatial separation at least for quantum effects (well, this was already known to Einstein as a spooky effect). One thing about this experiment as described that is a bit puzzling is why they insist on a step motor for measuring the interference pattern of the first photon at a particular position - couldn't they just use a rectangular CCD?

This has been discussed in detail in another thread (http://www.iidb.org/vbb/showthread.php?t=105684).

[EricK]

If you accept that the universe is deterministic and that the equations which describe its evolution are symmetric w.r.t. time then the simplest conclusion is that in whatever way the past can be said to affect the future, so the future affects the past in exactly the same way.

[premjan]

The only problem with the present (or future) affecting the past is that the past has already had diverging effects visible in the present. Hence affecting the past would amount to erasing and instantly reinscribing a lot of information in the present. Whereas I am not sure that is what is happening in the delayed choice quantum eraser. Perhaps information is not actually flowing from present to past. Rather certain facts about the past remain uncommitted or uncrystallized until the point at which we actually measure them. Though Schneibster in that earlier thread seemed quite certain that the present was actually affecting the past directly which ought to cause the grandfather paradox.

Also, time is probably just the perceived track of information or entropy. Our conscious awareness may exist outside of time in a sense.

[EricK]

If you have the preconceived notion that the past should be treated differently to the future then you will never accept theories which imply that they are qualitatively the same.

But suppose those theories are actually right? Why is saying "the present is like this because of the choices we made in the past" any more revealing than saying "we are making these choices now because of the state of the universe in the future"?

[premjan]

I don't have a problem with thinking of time as symmetric in past and future. It is mostly a problem of potential inconsistency (the grandfather paradox). At the macroscopic level the present influences the future and the future doesn't influence back. Well, "influence" may be a loaded term here as we don't know that the future is not determined. However the past is determined, once we have formed memories of it. According to QM anyway the future (or past!) is not determined until we measure it. But the past is determined because we remember many details of it. Also, if the present could influence the past, we could potentially take actions that eliminate our existence which causes a causal inconsistency.

[EricK]

But if the present causes the past in the same way the present causes the future then we simply couldn't take those actions which would lead to inconsitencies, because those are not the actions which would be caused.

This just follows from a deterministic universe with time symmetric equations.

[premjan]

Maybe we couldn't take actions which cause inconsistencies. That's possible, but I am certainly able to conceive of such actions, so the question is still moot. The universe must in some way prevent causal inconsistency. And apparently it does - in the delayed choice quantum eraser experiment, as the so-called retroactive change in the signal photon is actually possible only when the signal photon has not already been measured. Rather the correlation between the signal photon and the measured / erased state of the idler photon is measured.

[Caine]

Technically this effect is not really a backward causality if you use certain interpretations of QM - mainly the transactional interpretation I believe. Of course those other interpretations use explanations just as strange and counterintuitive, but they do not necessarily require a deterministic universe.

[uberhobo]

Is it possible for something later in time to change something earlier in time? According to the delayed choice quantum eraser experiment (http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm), it is possible. In this experiment, basically there are two entangled photons that are detected, one earlier in time and one later. The later photon may be detected in such a way as to either reveal its source (slit A or slit B in the traditional Young's double slit experiment) or in a way that hides the slit choice of the photon. Note that as the two photons are entangled, they both originate from the same slit. If the second photon is measured in a way that reveals the slit choice of the photon(s), the first photon displays no interference pattern. If the second photon is measured in a way that does not reveal the slit choice of the photons, the first photon displays an interference pattern. It is also possible for this effect to occur instantaneously across large distances of space. It appears to effectively negate the reality of temporal order and of spatial separation at least for quantum effects (well, this was already known to Einstein as a spooky effect). One thing about this experiment as described that is a bit puzzling is why they insist on a step motor for measuring the interference pattern of the first photon at a particular position - couldn't they just use a rectangular CCD?

It looks like the original experiment was designed in 1982, back when a CCD may have been prohibitively expensive. I'm sure a modern experiment would use a CCD.

[Caine]

Technically this effect is not really a backward causality if you use certain interpretations of QM - mainly the transactional interpretation I believe.

Whoops I mean the transactional interpretation is the main one which does agree with backward causality. Copenhagen and Bohm consider it completely differently.

[Jesse]

Repeating my comment on the "why is qm so hard to accept?" thread:

There's no need to imagine anything is changed "retroactively" in the DCQE experiment. You certainly can't use it to send a message back in time, since no matter whether the which-path information in the "idler" photons is erased or not, the pattern of all the "signal" photons on the screen never shows an interference pattern. Here's a little explanation I posted on another thread which may help you follow what's going on in this experiment: Even in the case of the normal delayed choice quantum eraser setup where the which-path information is erased, the total pattern of photons on the screen does not show any interference, it's only when you look at the subset of signal photons matched with idler photons that ended up in a particular detector that you see an interference pattern. For reference, look at the diagram of the setup in fig. 1 of this paper:

http://xxx.lanl.gov/PS_cache/quant-p...03/9903047.pdf

In this figure, pairs of entangled photons are emitted by one of two atoms at different positions, A and B. The signal photons move to the right on the diagram, and are detected at D0--you can think of the two atoms as corresponding to the two slits in the double-slit experiment, while D0 corresponds to the screen. Meanwhile, the idler photons move to the left on the diagram. If the idler is detected at D3, then you know that it came from atom A, and thus that the signal photon came from there also; so when you look at the subset of trials where the idler was detected at D3, you will not see any interference in the distribution of positions where the signal photon was detected at D0, just as you see no interference on the screen in the double-slit experiment when you measure which slit the particle went through. Likewise, if the idler is detected at D4, then you know both it and the signal photon came from atom B, and you won't see any interference in the signal photon's distribution. But if the idler is detected at either D1 or D2, then this is equally consistent with a path where it came from atom A and was reflected by the beam-splitter BSA or a path where it came from atom B and was reflected from beam-splitter BSB, thus you have no information about which atom the signal photon came from and will get interference in the signal photon's distribution, just like in the double-slit experiment when you don't measure which slit the particle came through. Note that if you removed the beam-splitters BSA and BSB you could guarantee that the idler would be detected at D3 or D4 and thus that the path of the signal photon would be known; likewise, if you replaced the beam-splitters BSA and BSB with mirrors, then you could guarantee that the idler would be detected at D1 or D2 and thus that the path of the signal photon would be unknown. By making the distances large enough you could even choose whether to make sure the idlers go to D3&D4 or to go to D1&D2 after you have already observed the position that the signal photon was detected, so in this sense you have the choice whether or not to retroactively "erase" your opportunity to know which atom the signal photon came from, after the signal photon's position has already been detected.

This confused me for a while since it seemed like this would imply your later choice determines whether or not you observe interference in the signal photons earlier, until I got into a discussion about it online and someone showed me the "trick". In the same paper, look at the graphs in Fig. 3 and Fig. 4, Fig. 3 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D1, and Fig. 4 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D2 (the two cases where the idler's 'which-path' information is lost). They do both show interference, but if you line the graphs up you see that the peaks of one interference pattern line up with the troughs of the other--so the "trick" here is that if you add the two patterns together, you get a non-interference pattern just like if the idlers had ended up at D3 or D4. This means that even if you did replace the beam-splitters BSA and BSB with mirrors, guaranteeing that the idlers would always be detected at D1 or D2 and that their which-path information would always be erased, you still wouldn't see any interference in the total pattern of the signal photons; only after the idlers have been detected at D1 or D2, and you look at the subset of signal photons whose corresponding idlers were detected at one or the other, do you see any kind of interference. Also, I added a bit to the wikipedia page on the DCQE (http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser) to make it clear that the results of the experiment can be explained without any backwards-in-time-influences by most interpretations of QM: It might initially seem that the "choice" to observe or erase the which-path information of the idler can change the position where the signal photon is recorded on the detector, even after it should have already been recorded. However, as noted above, the total pattern of signal photons never shows interference, and it is only when one looks at a subset of signal photons whose idlers were seen at a particular detector that an interference pattern can be recovered. So, the experiment would certainly not allow one to send a message back in time, and whether the experiment requires any sort of backwards causality to understand it would depend on one's interpretation of quantum mechanics (http://en.wikipedia.org/wiki/Interpretation_of_quantum_mechanics). The transactional interpretation (http://en.wikipedia.org/wiki/Transactional_interpretation) would interpret the results in terms of genuine backwards causality, but other interpretations such as the Copenhagen interpretation ( http://en.wikipedia.org/wiki/Copenhagen_interpretation), the Bohm interpretation (http://en.wikipedia.org/wiki/Bohm_interpretation) and the many-worlds interpretation (http://en.wikipedia.org/wiki/Many-worlds_interpretation) would predict the same experimental results without the need for backwards causality. For example, according to the Copenhagen interpretation the initial measurement of the position of the signal photon (whose probability distribution would not show interference if it were measured first) would discontinuously alter the wave function (http://en.wikipedia.org/wiki/Wave_function) of the combined signal/idler system, affecting the probabilities that the idler would be detected at different locations. If the signal photon was detected by detector D0 at a position near a peak of the D0/D1 joint detection graph (fig. 3) and a trough of the D0/D2 joint detection graph (fig. 4), this would increase the probability that the idler would be detected at detector D1 and decrease the probability that the idler would be detected at D2; likewise, if the signal photon was detected at a position near a peak of the D0/D2 joint detection graph and a trough of the D0/D1 joint detection graph, this would increase the probability that the idler would be detected at D2 and decrease the probability it would be detected at D1. This would ensure that both correlation graphs showed the correct interference pattern, with the interference patterns now explained in terms of the initial measurement of the signal photon affecting the probabilities of the later measurement of the idler rather than the other way around.

[Caine]

Also, I added a bit to the wikipedia page on the DCQE (http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser) to make it clear that the results of the experiment can be explained without any backwards-in-time-influences by most interpretations of QM:

Should you really list figures that are in a completely separate article? The explanation is pretty confusing for the Copenhagen version (although I must admit it is much better than the Bohm approach).

[Jesse]

Should you really list figures that are in a completely separate article? Well, it would be pretty hard to explain the setup and results without using diagrams, and since the paper is available free for anyone to look at, I thought it was OK to refer to the paper's diagrams. If other wikipedians object, it would also be pretty easy to just upload copies of those diagrams to the wikipedia article, although we'd need to get permission from the paper's authors first. The explanation is pretty confusing for the Copenhagen version (although I must admit it is much better than the Bohm approach). It is somewhat confusing, although I think if you've already understood the explanation of the different parts of the DCQE setup in the previous section, and you follow along looking at the diagrams as you read, it's not too bad. If you have any suggestions for how to make it clearer they'd be welcome.