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still_unsure
Registered Member
Date Joined Jun 2001
Total Posts : 54
| Posted 12/3/2001 3:12 PM (GMT -6) |   | We discussed Thomas Young's double slit experiment some time ago on another thread (i can't remember which) and a thought occured to me while I was pondering the "Relativity theory and sequencelessness" thread.
John Wheeler posed that it was possible to "look back" from the screen at the time the particle arrived and determine which slit the particle had travelled through. This observation would, of course, cause the quantum wave to collapse to a particle nature. The decision about whether or not to look back would thereby determine if the wave nature of the particle would collapse and the interference pattern disappear. Interestingly, this observation takes place AFTER the particle has travelled through either/both slits. In effect, backward causation. He calls this "the delayed-choice experiment". This effect has been experimentally confirmed.
Wheeler has further extrapolated this thought experiment to the extreme. He cites the possibility that the gravitational field of a a black hole or a galaxy, acting as a lens, could take the place of the slotted screen and allow the delayed-choice experiment to be performed over vast distances and time-scales (millions or even billions of years!) I don't know how to reconcile this evidence with the open view. It seems to point to either a "block universe" interprtation of space-time or, if you consider God the "sustainer" of our reality, a timeless deity (one who experiences sequence simultaneously). How can you possibly square the open view's idea of a formless future with my ability to affect the quantum state of a photon billions of years ago?
A little help!
Rob
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GregBoyd
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Date Joined Jul 2001
Total Posts : 1818
| Posted 12/3/2001 6:49 PM (GMT -6) | | Great question stillunsure. I have pondered over the EPR experiment for years. I am not as brushed up on this stuff as I was several years ago, but here's how I process it.
* Even if we had to admit supraluminal information (the affected electron "communicates" to the other paired electron how it is affected at a speed faster than light) we would only need to do so at a quantum level. This need not affect our assessment of the irreversibility of time on a phenomenological level, any more than Einstein's theory destroys Newton's laws on a phenomenological level.
* It may be that the best solution to the problem posed by the EPR experiment is to suppose there is an "order" (something like Bohm's "implicate order") which transcends the original division of the particles. On this level the two pair particles are never actually split. They are part of a whole. Hence, affecting one "part" of the whole simultaneously affects the whole. We may perhaps conceive of the original pair as an IDEA. When you affect one particle, you affect the entire IDEA. (I just read a book by Gerald Schroeder [The Hidden Face of God] in which he argues that the ultimate reality "behind" quantum physics, and all of life, is INFORMATION -- pointing to the WISDOM of a Creator). This avoids supposing that information is conveyed at speeds faster than light.
* If one supposes that causation can go backwards, innumerable contradictions ensue. Hence the vast vast majority of people who think on the issue agree that it is an unintelligible notion. (Just watch Back to the Future, or The Terminator, or Twelve Monkeys, to see the nonsense it creates).
What do you think?
Doc
"Three is the magic number, oh yes it is, it's the Magic Number" - De La Soul | | Back to Top | | |
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still_unsure
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Date Joined Jun 2001
Total Posts : 54
| Posted 12/4/2001 10:17 AM (GMT -6) | | Hi Doc:
The EPR experiment has intrigued me for some time as well. The apparent instantaneous signalling between particles is baffling. However, the seeming backward-in-time causation of Wheeler's "delayed-choice" experiment or Marlan Scully's "Quantum eraser" is just plain mind bending.
I was thinking about it last night and it occured to me that perhaps what is being observered in the Wheeler example is simply a demonstration of special relativity. The photons in his example must travel at light speed and therefore do not experience sequence(time is shrunk to non-existence). Their emission, quantum state(reaction to the observer), and arrival at the screen all happen simultaneously in their time frame. Pehaps what we percieve as backward-causation is simply the simultaneity of their timeframe being translated into ours by our observation. Does that make sense??
I don't know if the experiment would produce the same result with subluminal particles...my hope is that it wouldn't but my gut instinct is that it would.
Regarding the EPR experiment, I like your second explanation...that the particles are still linked in some manner. I think it must be closest to the truth, although this idea leads some to the "many worlds" interpretation of reality, which I find a bit absurd. I think it is interesting that we theists alone have a way around this conclusion.
One final point, I understand your philosophical concern about backward causation but I don't think it applies in this case. Einstein's theory doesn't necessarily prohibit backward causation...only supraluminal signalling or information exchange. In these experiments, although we seem to be able to alter past quantum states, it is still impossible to signal the past because any attempt to decode information sent via this method would be defeated due to quantum fuzziness.
In the end, it seems I'm left with 2 possibilities, either quantum particles somehow "know" my future decision about whether or not to observe their location, or, God, experiencing "sequence" in simultenaety collapses the quantum state of the particle simultaneously with my observation, giving the "appearance" of backward causation.
God bless
Rob
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still_unsure
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Date Joined Jun 2001
Total Posts : 54
| Posted 12/5/2001 3:17 PM (GMT -6) | | Hi Doc:
I've been puzzing over this topic and I'm not entirely certain it is in fact backward causation. I've done a little reading on it and it seems the MIND of the observer must be considered an integral part of the quantum system. If this is the case, I think it may be a loophole as the reality we look for is the reality we will find. Any thoughts? How about you Magneton?
Rob
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Magneton
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Date Joined Aug 2001
Total Posts : 132
| Posted 12/6/2001 9:49 AM (GMT -6) | | Hi, Still_unsure,
Good insight that the role of mind is a consideration. It reminds me of the story of three baseball umpires:
One says, "I call them as I see them." This is the instrumentalist, who judges theories and observations by their usefulness, rather than by correspondence to reality (which is inaccessible due to experimental or perceptual limitations).
Another says, "I call them as they are." This is the realist, who views our observations as a representation of physical reality (or a partial representation, in the case of the critical realist).
The other says, "They ain't nothing till I call them." This is the idealist, who says reality is mental: the wavefunction only collapses in the mind of an observer.
I believe Wheeler is in this third camp, and talks about an "observer-created universe." I'm pretty suspicious of this sort of thing. It always seems to lead to grand explanations of consciousness in the universe (Bohm, Tipler).
I haven't studied the two-slot delayed-choice experiment in depth, but I have a gut feeling there is a fallacy involved. It is an experiment to determine whether a photon behaved as a particle (went through a particular slot) or a wave (exhibits interference). We can apparently influence after the fact (i.e. after it has passed the slots but before it arrives at the detector) which of these it had done earlier. The paradox seems to come from the inadequacy of our perceptual description of a quantum object as either a particle or a wave.
There is an Indian physicist, R. Srikanth, who has been studying quantum nonlocality, EPR, delayed choice, etc. Here is the abstract of a paper (s)he wrote, from the viewpoint of quantum field theory. If I understand him/her right, the particle/wave "choice" is not made until it hits the detector, and thus is not really delayed.quote:
Wheeler's delayed choice experiment, a well known manifestation of the complementarity principle, has proved somewhat difficult to physically interpret. We show that, restated in quantum field theoretic language, the experiment submits to a simple explanation: that wave- or particle-nature is imposed not at the slit plane but at the detector system. The interpretational difficulty conventionally encountered is due to the assumption of enforcement of complementarity at the former.
http://www.iiap.ernet.in/~website/personnel/srik/ (See frame on foundations of QM)
Hope this is helpful.
Blessings,
Magneton
"Man tries to make for himself in the fashion that suits him best a simplified and intelligible picture of the world; he then tries to some extent to substitute this cosmos of his for the world of experience, and thus to overcome it. This is what the painter, the poet, the speculative philosopher, and the natural scientists do, each in his own fashion. Each makes this cosmos and its construction the pivot of his emotional life, in order to find in this way peace and security which he can not find in the narrow whirlpool of personal experience." - Albert Einstein
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GregBoyd
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Date Joined Jul 2001
Total Posts : 1818
| Posted 12/10/2001 4:44 PM (GMT -6) | | I'd have to get caught up on quantum mechanics the last five years (which is a long time in this field) and brush up on the details I once knew to comment any further. I rest at night with the convinction that whatever implication this stuff might have on a quantum level, it does not have any necessary implications for reality at a phenomenological level -- and the phenomenological level is just as real -- has just as much a claim to ontology -- as the quantum field. On the phenomenological level, I have every good reason (biblical, philosophical, experiential and empirical) to conclude that time is irreversible, sequence is real, and the future is in part composed of possibilities.
Doc
"Three is the magic number, oh yes it is, it's the Magic Number" - De La Soul | | Back to Top | | |
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still_unsure
Registered Member
Date Joined Jun 2001
Total Posts : 54
| Posted 12/11/2001 3:00 PM (GMT -6) | | Hi Magneton and Doc:
Thanks for the reply Magneton, I'll check out the link further. Doc, thanks for the input, I think I need to curb my tendency to run around half-cocked with information I don't fully understand. I am also beginning to think I would be better off giving up on trying to understand physics and theology and taking up something simpler...maybe kite-flying or checkers.
I just purchased "Satan and the problem of evil" as well as Pinnock's new book. Just thought I'd let you know I'm tackling your's first! But wow!, 450+ pages...I was pleasantly suprised at the size. Looks like it was worth the wait!
Rob
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Magneton
Registered Member
Date Joined Aug 2001
Total Posts : 132
| Posted 12/12/2001 7:23 AM (GMT -6) | | Still_unsure,
Don't give up on trying to understand physics or theology. None of us do. You're raising very good questions. The following quote is from a Nobel-prize-winning physicist.
Magneton
"I think that I can safely say that nobody understands quantum mechanics."
Richard Feynman
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