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Cosmic string
New Member
USA
37 Posts |
 Posted - 07/28/2002 : 22:59:47
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John A. Wheeler has proposed a thought experiment which, to me at least, sounds highly implauseable (I might be wrong). It is as follows:
When the Young light interference experiment is performed, interference bands form on the screen behind the double slit, indicating a wave which passed though both slits interfering with itself. When a photon detector is used, however, photons pass through only one of the slits each, and hence the interference bands don't form (is this correct?). This implies, according to the thought experiment, that the observations we make now affect the present, because the choice to use a photon detector or not influences whether the light was emitted with more wavelike properties or more particlelike properties. So far, so good; no violations of causality yet. But now imagine the same experiment on a cosmic scale. The emitter is a quasar 12x10^9 ly away, and the double slit is a gravitational lens between us and the quasar. If we bounce the light from each lensed image off mirrors such that the two images interfere with eachother, we should see interference bands, indicating a predominantly wavelike nature. But what if we used a photon detector? The Young experiment suggests that we would not then see the interference bands. This poses a dilemma: either our observations now influenced how the light was emitted 12x10^9 years ago (violating causality), or the light changes from a wavelike particle (or particlelike wave) to one or the other when we make the observations.
Wheeler has proposed the first possibility with this thought experiment, but he also suggests that any interaction can produce this effect. Andrei Linde, on the other hand, thinks that a sentient observer is necessary for this to happen. Personally, I think Linde is off his rocker. I also think that, while Wheeler makes some valid points, Occam's razor points to the light changing its form now rather than in the past (violating causality). But then again, I know next to nothing about physics compared with him.
Thought experiments have, in the past, been dubiously unreliable at times (anyone remember the Einstein-Podolsky-Rosen thought experiment?). Therefore, in addition to discussing the implications of the Wheeler experiment, I think it would be useful to analyze the chain argument used to see if each conclusion must follow from its basis.
“The truths of religion are never so well understood as by those who have lost the power of reasoning.” --Voltaire
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Lars_H
SFN Regular
Germany
630 Posts |
 Posted - 07/29/2002 : 04:02:50 [Permalink]
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I am not an expert on this, but the way I understand it the photons normally always go both ways. Only when we look closer at a photon it ceases to have gone one way or the other with a 50-50 chance and becomes and actual particel that has taken one path.
Causality is not really being violated, because if the photon in question had interacted with something during 12x10^9 y (or however long it took) it would already have lost its wave form before reaching us.
It doesn't matter 'when' the photon changed because we can't tell. Any attempt to tell would cause the collapse of the wavefunction itself.
Just like opening the box with Schroedingers famous feline will not violate causality by going back in time and killing the poor animal in the past. The Cat is already half dead and half alive the poisioning has already happened with a certain probabilty and all the opening does, is collapsing the propabilities into one actuallity.
No time travel here.
About the sentient observer bit, I honestly don't know enough about it to make much comment. It would be kind of like listening to the sounds collapsing trees make when nobody is listening to find out how stuff behaves when no sentient observer is present I guess.
I gerally am a bit septic about any theory that attributes the human consciousness more importance and powers then absolutely neseccary, but I think quantum physics is an exception as it literally is the dreams that stuff are made of 
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Cosmic string
New Member
USA
37 Posts |
Posted - 08/10/2002 : 03:04:20 [Permalink]
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quote:
About the sentient observer bit, I honestly don't know enough about it to make much comment. It would be kind of like listening to the sounds collapsing trees make when nobody is listening to find out how stuff behaves when no sentient observer is present I guess.
I gerally am a bit septic about any theory that attributes the human consciousness more importance and powers then absolutely neseccary, but I think quantum physics is an exception as it literally is the dreams that stuff are made of
I misunderstood Linde's sentient observer requirement. It explicitly does not require humans. The claim is not that intelligent beings are required for a universe to exist; it is merely that without observers, the claim that there is a universe can't be tested. It therefore can't be falsified and hence is not a scientific claim but is instead mysticism. The only problem is: where do you draw the line between sentient beings and inanimite stuff? Since conciousness is, on the deepest level, trillions of subatomic particles interacting in complex ways entirely determined by the laws of physics, are particles sentient? Is this question even falsifiable?
“The truths of religion are never so well understood as by those who have lost the power of reasoning.” --Voltaire |
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Chippewa
SFN Regular
USA
1496 Posts |
Posted - 08/10/2002 : 11:17:58 [Permalink]
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Also...if you had both types of detectors operating at the same time, and they showed different results, wouldn't that indicate that the different data is within the design of the detectors, and that we're dealing with the old idea of "wavicles"? -- i.e. Measuring two aspects of the same light?
As for Linde's concepts, I'm not sure either. I agree that he sounds more on the side of the mystical, but I'm not qualified to better address his ideas as I haven't read enough of him. If there are no observers then the universe doesn't exist? (Maybe for them.) "If a tree falls in the forest and there's no one there to hear it" - - - it makes a sound that no one heard when it fell.
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"Speaking without thinking like shooting without aiming." - Charlie Chan |
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Cosmic string
New Member
USA
37 Posts |
Posted - 08/10/2002 : 16:08:50 [Permalink]
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quote:
Also...if you had both types of detectors operating at the same time, and they showed different results, wouldn't that indicate that the different data is within the design of the detectors, and that we're dealing with the old idea of "wavicles"? -- i.e. Measuring two aspects of the same light?
Perhaps such experiments with electrons would be easier to consider this for. When no particle detector is used, interference bands are measured. This experiment could be thought of as an electron-wave detector because it detects the wave properties of the electrons. Now add a particle detector, which consists of a bright light behind the slits and a device to detect the flash of an electron colliding with a photon. Now each electron is measured to go through one slit or the other. As we lower the energy (frequency) of the light used to detect the electrons, the flashes become more 'smeared out' by uncertainty. Eventually they become spread out so wide that it is impossible to tell which slit each electron went through. Now interference bands reappear (because there is so much uncertainty that the electrons go through both slits), even though we are still using a particle detector. Essentially, the amount of uncertainty determines whether the electrons are observed to behave as particles or waves. Once an observation is made of a subatomic particle, it is observed to behave as either a particle or a wave in this experiment (just like Schrodinger's cat is either alive or dead when you open the box, not both) as I understand it. And because the experiment itself is a wave detector, when a photon detector is also used, the experiment demonstrates what happens when both detectors are used simultaneously. Anyone able to better articulate this?
quote:
If there are no observers then the universe doesn't exist?
No, just that the statement that the universe exists can't be tested (even in principle). Since it can't be tested, it can't be falsified and hence is like claiming that an invisible, flying, incorporeal dragon is in your garage. Even if that universe exists, there are still no observers to test that hypothesis. Since it can't be tested/falsified, it is not a scientific claim/question and instead is more mysticism than science. This says nothing of whether that universe really exists or not.
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"If a tree falls in the forest and there's no one there to hear it" - - - it makes a sound that no one heard when it fell.
The tree in the forest is not the same. In principle, the effects of the tree's sound while falling could be measured later, making the claim falsifiable (in principle) and hence a scientific claim. Of course, the effect is so small and so swamped out by other factors that this could probably never be actually tested. But the existence of an uninhabited universe can't - even in principle - be tested.
“The truths of religion are never so well understood as by those who have lost the power of reasoning.” --Voltaire |
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Chippewa
SFN Regular
USA
1496 Posts |
Posted - 08/25/2002 : 23:20:46 [Permalink]
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quote:
"Anyone able to better articulate this?"
I think you've done a splendid job on all three answers. Thank you.
And we haven't even brought up what is perhaps a third alternative with regard to the seeming paradoxical results of a cosmic version of these light experiments; possible interference from a parallel universe or a "multiverse." (Which I have to refresh my memory about before writing further.)
"Speaking without thinking like shooting without aiming." - Charlie Chan |
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