Science a GoGo's Home Page Forums General Discussion General Science Discussion Forum Have I caused someone to kill his grandfather ?

It could be quite rational in the case of a photon which travels at "c" and would therefore be averywhere at once, but, as redewenur points out, is unlikely to be the case if an electron is used in the experiment. Can this experiment be done with electrons?

Yes, I've read that such experiments can be done with electrons, protons, and even neutrons, but are most easily performed using photons.

Hang on, who made the choice about erasing the path? Not the human operator, they just looked at what the detectors told them. I think this is not too amazing. Suppose the initial photon knows what it's going to do, and does it. That sounds like it would be consistent with your explanation. Sometimes it decides to have its path erased, and shows that on the signal photon detector, then later on shows its decision again on the idler detectors.

Or are the beam splitters controlled by the operator? In that case I think it can't work as described because it would lead to faster-than-light or backward-in-time communication. I suppose this is what you're all talking about :P You could have the splitters and detectors in one location, and the signal photon detector a long way away. Keep putting photons in. When the idler-operator wants to send a message he sets the splitters to 'erase path'. Then either instantly or before(!), the long-way-away signal photon man will see an interference pattern.

Haha, I just read this now. And it reminds me of exactly the same thinking I did when I saw Back to the Future. I came to the same conclusions as you. And it still pisses me off that people were fading out of photos in the 1-monoverse movie, which implied a whole lot of inconsistencies.

Well, the upshot of this experiment is that you can determine whether an entangled photon behaves like a wave or a particle after it has been detected by making a conscious decision in the present whether to preserve information about which slit it went through or erase such information. The first photon will have reached its detector before its 'twin' gets detected due to the shorter path it is forced to travel. So, an event that happened in the past can be determined by what is done in the present; this is the paradox. It is though the actual possibility to gain information about which slit the photon went through is sufficient to make the first photon behave like a particle, even if the information isn't looked at by the experimenter. How does the first photon know?

The beam splitters and all the other apparatus are set up beforehand to enable knowledge of which slit the photon went through to be available, but this information is isolated from the first photon that 'thinks' it has no knowledge of which slit it went through, therefore, shows interference patterns indicative of this, and behaves like a wave. But, if the experimenter chooses to preserve such knowledge AT A LATER TIME this will be reflected by the photon's behaviour to act like a particle; if the experimenter chooses to erase the information, again, AT A LATER TIME, this will be reflected by the first photon, but it behaves like a wave, which happens when information is unavailable.


The first photon has no way of knowing which slit it went through, therefore, it will exhibit an interference pattern due to no information. However, the delayed decision whether to keep or destroy information gathered by various other detectors about its entangled 'twin' affects whether the first photon behaves like a wave or a particle. In a sense, it is the conscious decision by the experimenter after the event that seems to directly affect what the first photon does.

I think this is the question that David Deutsch asks, and his answer seems to be that they are purely mathematical, hence his multiverse interpretation. Is there any way the waves could be considered "real"?

Abacus9900. Your description of the 'delayed choice quantum eraser experiment' is the best I have seen. If you are not already an author of PS books, perhaps you should consider that. "Hitch-hikers" like me need clear explanations.



Lost here! In the case of a single photon, does the device generate a second photon which is then entangled with the first? If this is the case, how does the second photon, which did not exist when the first photon passed through the slit(s) "know" which slit(s) the first photon came through?

I have more questions, but I think I must have this sort of information in "bite sized" chunks.

Could it not be the case that any apparent backward-in-time communication is observable only in the F of R of the observer?

Thank you Bill S., but it took me a while to really get my head around it and I'm still not sure I have fully grasped it! I only wished I had the talent to write books!







Well, I do not claim to be an expert by any stretch of the imagination, but I think the basic explanation is that when a photon is 'split' into two entangled photons they still act as one system (whatever that means). I think they share the energy of the original photon. I don't think anyone really knows how two entangled photons are able to be aware of what happens to each one. What puzzles me is how can something as simple as a photon 'know' what a complicated piece of apparatus as used in the experiment we have been discussing is measuring? On top of that it seems to be aware of what the experimenter is doing at a later time!


It kind of makes you wonder whether our simplistic understanding of our four dimensional spacetime universe needs radical revision in order to account for all these strange events. David Deutsch could be right!

This is probably ultra-naive, but I thought a photon was a quantum of light, that is why I wondered if the device had to create a second photon and somehow entangle it with the first.

Yes, DD could well be right, he certainly makes a good case. You will be aware of my discussions with Kallog about infinity. In the back of my mind is the thought that something about the nature of infinity should throw some light (no pun) on quantum strangeness. Getting the ideas to run smoothly is the problem. I took my wife's advice and wrote them down. Things seemed to go well for a while, but I keep finding holes in the "bag" I put them in. Now Kallog keeps making more holes, but that has to be a good thing.

Yes, a photon is a quantum of light Bill S. but there is a way of splitting up one photon into 'twins.' These twins will be 'entangled' which, as I mentioned earlier, means they act as one system, although exactly how is beyond me and probably even beyond the experts. I do know it is a very delicate operation to achieve. Even entangled photons many lights years separated will have a 'spooky' connection.



Well, this is what science is all about, Bill S. - somebody has a bright idea then other people try to find holes in it. This is a healthy situation since many 'good' ideas will not be correct. Imagination is beneficial I think because without it where would be be? The role of science is to rigorously examine ideas of course.

Does this mean it is possible to have half a quon (sensu, Nick Hrebert), or does another photon materialise from somewhere, or is it just one of those things that seems to happen G O K how?

Bill S., you have exhausted my limited knowledge of quantum mechanics. Sorry.

I think there's no question about it. There are some very big surprises in store as we learn more about the universe...

"But we should be open to the prospect that some aspects of reality...might elude us simply because they’re beyond human brains, just as Einstein’s ideas would baffle a chimpanzee...Even so, that need not mean that the fundamental questions were for ever unanswerable. That’s because we humans need not be the culmination of the evolutionary tree...there is time enough for dramatic posthuman evolution...And for those species that come after us, even the most baffling problems that we can pose may be as straightforward as simple arithmetic is to us"

- Professor Martin Rees, President of the Royal Society and Astronomer Royal

http://www.timesonline.co.uk/tol/comment/columnists/guest_contributors/article7140578.ece

Maybe. But from my understanding there would be backward in time actually seen by the observer. FoR or not, that's either impossible, or this simple experiment will shatter the world as we know it.

I think it must be impossible because the world isn't shattered yet.

These two statement appear to contradict each other. Can the experimenter control the beam splitter or not? You said it had a 50% chance, which sounded like it was random.

If the experimenter can quickly change the beam splitter to "preserve information" after the photons are in flight, and even after the signal photon has hit it detector, then you have FTL or backwards-in-time communication. I'm certain this is impossible by current physics knowledge.

Otherwise it sounds like the other entangled particle experiments which seemed like FTL but in fact weren't because no information could be transmitted FTL.

I need to clarify what you are saying here. Are you saying that if anyone observed past directed TT, even if only in his/her F o R, that would "shatter the world as we know it"? I find it difficult to believe that you are talking about a physical effect; perhaps you mean that it would throw the scientific world into chaos. That would depend, not so much on the observation being made, as on the scientific world accepting it as real. Think of the situation we are in; abacus9900 is presenting what looks like evidence of retro-causality, but you find it difficult to accept.

I was thinking along these lines only the other day.


Perhaps QM has confronted Homo-Sapiens with the question of whether humans are evolved enough to really understand it. After all, we have evolved to deal with large 'real-worlds' objects and therefore have developed a brain adapted to this end. The world of quantum objects is a very new and bewildering one for us to try to understand because it seems so contrary to commonsense notions of reality. We may have to adapt considerably more than we have up till now to get a real handle on such phenomena. Why should we assume we have achieved the pinnacle of evolutionary development?

No, the beam splitters are arranged beforehand with four detectors, two of which give information about the paths of the 'twin' photons and which slit the original photons (before being 'split') went through and two of which do not provide definite path information (i.e. could have gone through one or the other slit). So, the configuration of the apparatus in the experiment insures that both information and no information are available to the experimenter at the same time. The experimenter then has the choice to either preserve information or to erase it. Again, though, this can be done AFTER the first photon has reached its own detector and been registered in a special device as a wave (since no information available at that point). Remember, the first photon will always reach its detector before its 'twin' reaches its detector because it has a slightly shorter path to travel and so will be checked first, yet this result is able to be changed by the experimenter's choice AFTER it has happened.

I mean it both as the world of science and kind of the physical world. Retro-causailty!!! How can that not be hard to accept? It creates endless paradoxes.

What if you see what your decision will be before you make it? Then you can change your mind. But somehow you're bound to do it the way you know you must. It's too impossible.

Oh actually, hmm I suppose you can't see the results before you make your decision because of light travel time from there. But it's still communication backwards in time, which would have paradoxes too.

If something causes a paradox then it cannot happen.