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Consider the following scenario, (the “grandfather paradox”). I decide that I will try to build a time machine which will allow me to travel back in time in order to kill my grandfather, and thus prevent my birth. In a single universe this creates a serious paradox, but the multiverse appears to offer us a way out of this by creating a new universe in which I was never born. However, we should look more closely at this situation. My decision to try to build a time machine creates a range of paths through different universes, all of which become “real”. The range must include paths in which I do, and do not actually make this decision.
The paths in which I do make the decision must include paths in which I do, and do not succeed in building the time machine.
The paths in which I build the machine must include paths in which the machine is, and is not successful.
The paths in which the machine is successful must include paths in which I do, and do not travel back to the appropriate point in time.
The paths in which I travel back to the appropriate point in time must include paths in which I do, and do not kill my grandfather.
All these paths are created by my initial choice. They must all become real, whatever my first decision happens to be. Even if I decide not to try to build the time machine, this decision will mean that another version of me, in another universe will decide to build the machine; so all the ramifying possibilities mentioned above, and perhaps lots more, will become “real”. So, if time travel to the past is, or ever will be, possible, and if the multiverse theory is right, and if I even think about building a time machine, then all these possibilities become “realities”, whatever decision I make. The point I seem to have reached here is that I have to accept that whatever choice I make in this, or any other matter, makes absolutely no difference in the context of the multiverse, because any choice I make will be countered by the choices made by other versions of me in other universes.

Does it make any difference if we assume that the universes are not created by the making of choices, but already exist? The answer must be “no”, because even if the universes have always been there, the paths through them, taken by the various versions of me, will be governed by the decisions made by each of those versions of me as we (I?) make our ways through our different worlds. Throughout the multiverse, all possible paths will always be taken, or so we are told. Of course, any decision I make will be of importance to the version of me that I recognise in the universe I experience as real, but the multiverse theory says that the other versions of me are just as real, and just as really “me” as the one I experience. It seems that, in the multiverse, I have no monopoly on “me-ness”. It also seems that if I want to travel back in time, all I have to do is decide not to build a time machine and use it to go back and kill my grandfather, and this will ensure that I will do just that in another universe. Who needs to build time machines?
What have I just done?
Bill: "What have I just done?"

I think you've said that, whether or not certain quantum phenomena can be explained by a multiverse hypothesis makes no difference to our daily lives.
My intention was to instigate a discussion about multiple universes, rather than how we might direct our daily lives. I half expected Kallog to pour scorn on the idea of alternative universes and multiple Kallogs!, but I guess he (she?) doesn't like to be predictable.
Ah, excuse me then. Actually, on second thoughts it seemed you might be implying that belief in such a multiverse could provide a rationale for unethical behaviour; or at least, a reason for modifying behaviour in some way. Apparently not.

Personally, I'm not scornful of the various multiverse ideas (there are at least five, as far as I know), but...

While some reputable scientists, including David Deutsch, take the existence of this kind of multiverse to have been demonstrated experimentally by quantum computation - albeit thus far limited to a few qubits of processing power - it seems that others, despite the logic of Deutsch's reasoning, deny that it necessarily demonstrates any such thing, and are disinterested in what they regard as metaphysics.

For all we know, there may be several types of multiverse coexisting. Infinities within infinities within infinities...On the other hand, maybe not.
David Deutsch does seem to have a lot of faith in the ability of quantum computing to support his multiverse ideas, but I suppose he knows a lot more about QC than most people. Personally. I was more impressed with his interpretation of the double-slit experiment, but even there, I am more inclined towards the de broglie - Bohm pilot wave theory, than to the idea that interference comes from a multitude of other universes.
Originally Posted By: Bill S.
...but even there, I am more inclined towards the de broglie - Bohm pilot wave theory, than to the idea that interference comes from a multitude of other universes.

Why is that your inclination, Bill?

The reason I ask is summed up in a Physics Forums post by Frame Dragger:

"I am a little disturbed by the number of arguments that center on liking one theory over another for reasons that seem to have less to do with the science and more to do with what is most palatable. I'm not sure why a work in progress (QM) needs an interpretation that makes a series of assumptions for the sake of remaining at least semi-classical."

http://www.physicsforums.com/showpost.ph...mp;postcount=13
Originally Posted By: redewenur
Why is that your inclination, Bill?


There is more than one reason for this, but perhaps the simplest is that Ockham's Razor might lead us to prefer one universe over a multiplicity of universes, unless we have good reason to opt for the latter. (Don't ask me why I prefer Ockham to Occam, in spite of the spell checker)
Probably because the man's name was William of Ockham.

BTW, you may well have seen this cartoon. It's my favourite 'lecture' on the double-slit experiment:

http://www.youtube.com/watch?v=wEzRdZGYNvA
Thanks. Good "lecture".

Have you come across David Deutsch's idea that when photons go through the double slit set-up singly, it is virtual photons from other universes that do the interfering? I would love to hear someone's opinion on that.
Yes, I have. D.D. is an entertaining guy, and he's not shy of publicly contradicting his famous peers. I could listen to him all day.

A single particle taking all possible routes simultaneously vs. many particles from many universes, each taking a single different route - and either way, the act of observing causing the elimination of all but a single route. Deutsch is a brave man, as was Hugh Everett** half a century ago, but I suspect he's got all the evidence he's going to get; and that's not enough for many.

My opinion? The options are equally incongruous with reality as we otherwise know it, but both are in accord with experimental data.

** Back in the distant days of 12" monochrome TV, I watched a programme(olde worlde English) about the many worlds theory. I reckon that must have been one of the first mass media explanations of Everett's hypothesis.
Some serious cosmologists take the idea of the 'many universes' model seriously nowadays. I think, to enable science to work towards a grand unification of all the known forces, it is necessary to postulate the 'multiverse' since there appears to be little choice when attempting to producing a viable theory.
The only rational approach to this is to hypothesise that the photon is able to be in several places at once. Of course, the problem is a photon cannot really be considered an 'object' in the way that ordinary everyday objects are defined.
Originally Posted By: abacus9900
The only rational approach to this is to hypothesise that the photon is able to be in several places at once.


I would be inclined to go further and say that because the photon travelling at "c", it is everywhere within the set-up at the same time, in its own F of R.

With regard to the double slit experiment, we should remember that the major problem involves sending single photons through the setup, and finding that, provided there is no observation made of the passage of each photon, an interference pattern results which, with time, builds up into a pattern that is indistinguishable from that produced by continuous waves of light passing through the same slits. There can be no doubt that something interferes with the single photons as they pass through the slits, but what could it be? The idea that this something could be a “probability wave” leaves the more down-to-earth among us wondering what a probability wave might be, and if, in fact, it is anything more than a semantic convenience. However, its established place in scientific thought demands that it be considered.

Various candidates have been suggested as agents of interference, including:
1. Probability waves.
2. Pilot waves.
3. Photons from other universes.

If any of these is correct, the question still remains as to how observation disrupts the mechanism. How does observation collapse the probability waves, disrupt the pilot waves or exclude the interference from a vast number of other universes?

The simplest answer would seem to be that the time lapse involved in the experiment occurs only in the F of R of the observer. In its own F of R the photon is at every point in the set-up at the same time.

In the photon’s F of R:
1. There are no probability waves, the outcome is always a certainty.
2. The act of observing disturbs the photon and its pilot wave, but because this is part of the infinite existence of the photon, its appearance as part of a sequence of events occurs only in the observer’s F of R.
3. Photons from other universes will also be travelling at “c”, so they too will be at every point in the set-up at the same time, so the above reasoning will apply to them as well.
Are any of these waves real or purely mathematical though?
Quote:


The simplest answer would seem to be that the time lapse involved in the experiment occurs only in the F of R of the observer. In its own F of R the photon is at every point in the set-up at the same time.




If that is in fact the case, would a reasonable hypothesis be that it is the specific experimental set-up plus the experimenter(s) own awareness, acting in given time-frame, that 'experiences' the concept of a 'photon?' We have to be aware that in something like the double-slit experiment we are forcing nature to yield outcomes we can conceptually deal with, i.e. 'particles' and 'waves.' In other words, this may not really be anything to do with the way nature is but the way 'we' are! If I am right here then the implication is that science is fundamentally subjective inasmuch as it must always have to be 'interpreted' in terms of the 'language' of out native conceptual attributes.
Let's remember that the electron, which does have mass, behaves just like the photon in the double-slit experiment.

http://www.upscale.utoronto.ca/PVB/Harrison/DoubleSlit/DoubleSlit.html
I don't know whether you are familiar with the 'delayed choice quantum eraser experiment', but what happens, basically, is that a photon, say, is sent through a double slit apparatus.


Now, the photon either goes through one or both slits, ok?


So far, this is just like the original double-slit experiment.


Placed right behind the slits, however, is placed a device that converts the photon(s) into two identical 'entangled' photons.


One of the twin photons acts as what they term the 'signal' photon, which is directed to a detector. This detector can be examined later on to see whether an 'interference' pattern has occurred.


The other entangled photon, called the 'idler' photon is directed by a prism that sends it in two separate directions, depending on which slit it came from.


Now, beyond this are placed beam splitters that each have a 50% chance of allowing the idler photon(s) to pass or to be reflected.


The beam splitters are arranged in such a way that allow the idler photon(s) to be detected by several detectors.



If the idler photon is detected at a particular detector then it could only have come from slit B.


On the other hand, if the idler photon is detected by another detector it must have come from slit A.


If the idler photon is detected at other detectors then it might have come from either slit.


All this comes down to the fact that some detectors reveal information, others reveal no information about the path of the photon(s) to the signal photon(s) with which it is entangled.


If the idler is detected at some detectors then the 'which-path' information is erased, so it is impossible to know whether it came from slit A or B.


When a human experimenter looks at the signal photon(s) whose entangled idlers were detected as 'erased which-path' they find an interference pattern (because lack of knowledge).


However, when they look at the signal photon(s) whose entangled idlers *were* detected by the detectors that yielded information about which slit they came from, they found no interference patterns.


This result is similar to that of the double slit experiment, since interference is observed when it is not known which slit the photon went through, while no interference is observed when the path is known.


The astonishing thing with this experiment though, is that the choice of whether to keep or erase the 'which-path' information of the idler photon(s) need not be made until *AFTER* the status of the signal photon has already been measured by its detector, which would have occurred at an earlier point in time than the detection of the idler photon! So, making a conscious decision to either preserve or erase 'which-path' information about the idler photon(s) seems to magically determine whether there will be an interference pattern or no interference pattern shown by the signal photon, which had already been measured!!


Is this rational?
Originally Posted By: redewenur
Let's remember that the electron, which does have mass, behaves just like the photon in the double-slit experiment.


If the experiment is conducted in a vacuum, how fast does the electron travel?
Bill

An electron can be accelerated close to c, but no amount of energy will enable it to reach c (speed of light in a vacuum).

But as a matter of interest:

http://en.wikipedia.org/wiki/Cherenkov_radiation

"Cherenkov radiation results when a charged particle, most commonly an electron, travels through a dielectric (electrically polarizable) medium with a speed greater than that at which light would otherwise propagate in the same medium."
Originally Posted By: abacus9900
So, making a conscious decision to either preserve or erase 'which-path' information about the idler photon(s) seems to magically determine whether there will be an interference pattern or no interference pattern shown by the signal photon, which had already been measured!!


Is this rational?

It's insane! Psychic photons smile

Wheeler got the ball rolling back in 1978 with his delayed choice thought experiment, and in more recent years there have been several experiments similar to the one you describe. Was that the one by Leonard Mandel's team at theUniversity of Rochester?

http://www.fortunecity.com/emachines/e11/86/qphil.html

- haven't had time to read it yet.
Originally Posted By: abacus9900
So, making a conscious decision to either preserve or erase 'which-path' information about the idler photon(s) seems to magically determine whether there will be an interference pattern or no interference pattern shown by the signal photon, which had already been measured!! Is this rational?


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.
Originally Posted By: abacus9900
The astonishing thing with this experiment though, is that the choice of whether to keep or erase the 'which-path' information of the idler photon(s) need not be made until *AFTER* the status of the signal photon has already been measured by its detector, which would have occurred at an earlier point in time than the detection of the idler photon! So, making a conscious


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.

Originally Posted By: Bill S.
Consider the following scenario, (the “grandfather paradox”). I decide that I will try to build a time machine which will allow me to travel back in time


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.
Originally Posted By: Bill S.
Originally Posted By: abacus9900
So, making a conscious decision to either preserve or erase 'which-path' information about the idler photon(s) seems to magically determine whether there will be an interference pattern or no interference pattern shown by the signal photon, which had already been measured!! Is this rational?


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?




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?
Quote:


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.



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.
Originally Posted By: abacus9900
Are any of these waves real or purely mathematical though?


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.

Quote:
..behind the slits, however, is placed a device that converts the photon(s) into two identical 'entangled' photons.


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.
Originally Posted By: K
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..


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

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.



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!



Quote:
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?





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!
Originally Posted By: abacus9900
when a photon is 'split' into two entangled photons


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.
Quote:

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.
Originally Posted By: abacus9900
but there is a way of splitting up one photon into 'twins.'


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?
Originally Posted By: Bill S.
Originally Posted By: abacus9900
but there is a way of splitting up one photon into 'twins.'


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.
Originally Posted By: abacus9900
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.

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
Originally Posted By: Bill S.

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


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.
Originally Posted By: abacus9900

The beam splitters and all the other apparatus are set up beforehand to enable knowledge of which slit the photon

a wave. But, if the experimenter chooses to preserve such knowledge AT A LATER TIME this will be reflected by the


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.



Originally Posted By: K
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 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.
Originally Posted By: redewenur
Originally Posted By: abacus9900
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.

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



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?
Quote:

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.






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.


Originally Posted By: Bill S.
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 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.
OK, a look at Wikipedia says this experiment can't cause FTL communication, and certainly not backward-in-time communication.

It seems you somehow can't actually read the results from the signal detector until after the idler photon has reached its detector. Can't quite work out why, but maybe its because you need multiple photons to form an inteferance pattern.
Quote:

It seems you somehow can't actually read the results from the signal detector until after the idler photon has reached its detector. Can't quite work out why, but maybe its because you need multiple photons to form an inteferance pattern.



Well, I may have this wrong as it is very confusing, but the photon that reaches its detector first (and this is designed to always happen) will always show an interference pattern since it has not, at that point, any information as to which slit it went through.

After the idler photons have been detected we seem to have the situation where there is both information about which slit the original photons went through and also no information about this (according to which set of detectors is checked).

My understanding is (and I may be wrong about this) that the experimenter now has the choice of whether to keep the information about 'which-path' (the true path of the original photon, thus telling us which slit it went through) or erasing it. If the true-path info is kept then the signal photon will automatically change its orientation to a clump pattern, representing a particle like aspect. It will do that because now, information has been made available indicating which slit the original photon went through. On the other hand, destroying this information will simply mean the signal photon displays the interference pattern and behave like a wave since no information is available any longer.

Wiki is very conservative and tends not to express conclusions that seem unscientific, however, nobody has really explained this phenomenon to date, although many have tried.


Confused? You will be!!
Originally Posted By: abacus9900

the signal photon will automatically change its orientation to a clump pattern, representing a particle like aspect. It will do that because now, information has been made available indicating which slit the original photon went


But it can't change because it's only one photon that only hits the detector once, then it's gone.

Regardless of Wikipedia, as long as this experiment has actually been performed, then it's not communicating FTL because that's still accepted as impossible.

Sure you might make some interpretation of what's happening and it somehow appears to be FTL. But at the end of the day, there's no way you can rig the machine to actually use as a telegraph to transmit data FTL.
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But it can't change because it's only one photon that only hits the detector once, then it's gone.



That is the paradox. How can a photon that has already been detected as being a wave change to a particle later on depending on the whim of a human being?


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Regardless of Wikipedia, as long as this experiment has actually been performed, then it's not communicating FTL because that's still accepted as impossible.



Well, the same principle operates on two entangled particles who could be at opposite ends of the universe. Although no information as such can pass between them one of the 'twins' is immediately aware of any measurement made on the other. The one that has not had some property measured on it directly will instantaneously take up a complimentary position of the property measured on the other one. It's a kind of symmetry. Somehow there is 'spooky action at a distance' taking place outside the normal relativistic constraints of space time, i.e. greater than luminal speed.


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Sure you might make some interpretation of what's happening and it somehow appears to be FTL. But at the end of the day, there's no way you can rig the machine to actually use as a telegraph to transmit data FTL.




No, but then these effects might be taking place outside of our normal concept of space and time, I really don't know. Certainly, this is the basic principle underpinning quantum computing, i.e. many parallel computations taking place over superimposed particles. David Deutsch sites this idea as supporting the idea of 'many worlds.'
Originally Posted By: K
Retro-causailty!!! How can that not be hard to accept?


Of course it is hard to accept. At least it is for me, but the more P Sci. books I read, the more I wonder if that is just my poorly informed personal prejudice. I can usually find arguments to support my view, but that could be because I "don't have the math"
Originally Posted By: abacus9900
No, but then these effects might be taking place outside of our normal concept of space and time,


Kallog will not like this, because it points towards a physical infinity, but if we are looking at photons which travel at "c", then they can be at every part of the experimental set-up at the same time, in their own F of R; so there is no question of FTL communication or retro-causality; only the appearance of such in the F of R of the observer.
Originally Posted By: abacus9900

Although no information as such can pass between them one of the 'twins' is immediately aware of any measurement made on the other. The one that has not had some property measured on it directly will instantaneously take up


Yes, and I suppose it's amazing if you understand it well enough. But the overall effect is very non-amazing. If you didn't know how it worked, you'd just say "oh well when the entangled pair was together they agreed on what result to produce if ever either of them was observed."
Originally Posted By: K
Yes, and I suppose it's amazing if you understand it well enough. But the overall effect is very non-amazing. If you didn't know how it worked, you'd just say "oh well when the entangled pair was together they agreed on what result to produce if ever either of them was observed."


If you regard QM as a "window" into the infinite; entanglement becomes understandable. It is no stranger than talking about needing to involve other dimensions in order to make sense of observations; especially if we then have to accept that these dimensions are rolled up "infinitely small".

If the quons, in their F of R, exist in infinity, there is never any question of physical separation between them, either in space or time. Instantaneous communication is always possible, because the separation is only in the F of R of the observer.
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