Physics of Light Question

[Tuckerfan]

Okay, so I’m reading the latest issue of Discover magazine and there’s an article on the current work that physicist John Wheeler is doing and that Wheeler believes that human consciousness shapes not only the present but the past as well. As proof of this, Wheeler describes an experiment involving light, and I’m trying to reconcile what he’s describing with Einstein’s theory of relativity and coming up empty. I’m hoping that someone can explain it to me better than the article does.

The experiment goes something like this (I’m paraphrasing to avoid copyright issues, so if I get things a little scrambled, forgive me.): Light can be observed to act as either a particle or a wave, the classical demonstration of this is that detectors are set up beside two parallel slits that light is being shown through. With the detectors placed there, the photons go through either one slit or the other. If the detectors are removed, and a piece of photographic film is placed so that the light strikes the film after passing through the slits, it creates an image on the film of alternating patterns of light and dark stripes, indicating that the light is acting as a wave. So far, so good, right?

Here’s the part that sends my brain screaming into the night and reduces me to a gibbering idiot: If you replace the light source with a quasar and the slits with a pair of galaxies, you’d get the same result, which means

The measurements made now, says Wheeler, determine the photon’s past.

Huh? Doesn’t that involve something operating a level faster than light? What’s going on here? And does this mean that Einsteins cemetary neighbors refer to him as “Whirly-gig Al?”

[Revtim]

That “something” is what Einstein called “spooky action at a distance”, and it’s FTL nature is the reason why he never fully accepted quantum physics.

I’m not knowledgable enough to explain the “something”, but probably others here are. You could also try searching on “Spooky Action Einstein” and you’ll likely come up lots of info, hopefully some useful.

[Nametag]

*Originally posted by Revtim *
**That “something” is what Einstein called “spooky action at a distance”, and it’s FTL nature is the reason why he never fully accepted quantum physics. **

I’d just love to hear how old Al pronounced “spooky.”

[bbeaty]

*Originally posted by Tuckerfan *
Here’s the part that sends my brain screaming into the night and reduces me to a gibbering idiot: If you replace the light source with a quasar and the slits with a pair of galaxies, you’d get the same result, which means Huh? Doesn’t that involve something operating a level faster than light? What’s going on here? And does this mean that Einsteins cemetary neighbors refer to him as “Whirly-gig Al?”

That’s what everyone has been arguing about since the 1970s. It’s Quantum Weirdness. Not the Shrodinger’s Cat weirdness, but the other kind.

Also search for “Bell Inequality” and “EPR paradox” for info about it. Also “collapse of the wave-function.” The weirdness was first proved experimentally by A. Aspect, so searches on “Aspect Experiment” will also turn up some articles.

Good beginner’s books on this: TAKING THE QUANTUM LEAP, and IN SEARCH OF SCHRODINGER’S CAT

Here’s the paradox stripped to it’s roots. If a single tiny light source emits light, the light expands in a sphere-shaped wave. You can prove this experimentally (and also it’s part of antenna physics.) However, if that light strikes an object, it acts as particles, AND THE SPHERE-WAVE VANISHES. The sphere-wave can be thousands of light-years across, but when the light behaves as a particle (hitting a silver molecule in some camera film, for example) the wave is gone and the particle remains… even if the wave is enormous. Somehow the entire wave must be instantly “informed” so it can properly vanish. If it didn’t, then pieces of photons might be found in many distant places. But we know that this doesn’t happen: only single photons are ever detected.

In the above version, no doubled path is needed, and no lensing galaxies are needed. All you have to do is expose some film to very weak starlight. The film will record the individual photons which strike it. Every time a photon strikes it, that photon has to inform the entire gigantic lightwave-sphere that the photon is out of the running. Since the distant atom emits single photons, we can’t have one photon be sent out, and more than one be received!

All this weird stuff is a consequence of light behaving as waves and/or particles. If it was just waves or just particles there’d be no problems.

PS, people have been trying to figure out a Faster Than Light (FTL) communication system ever since this stuff was discovered. No luck. It apparantly cannot be harnessed for communication. The atoms and photons seem to talk together instantly, but we can’t listen in or use that FTL communications channel.

I remember a good Dilbert about this. Dilberts garbage man finds Dilbert’s plans for a FTL entanglement communication system in the trash. The garbage man looks at it, then points out the mistakes and proves that it can’t work. Then he makes some changes so it WILL work.

When I grow up, I don’t want to be Einstein. And I don’t want to be a powerful wizard. Me, I want to be Dilbert’s garbage man.

[Chronos]

The measurements made now, says Wheeler, determine the photon’s past.

The key here is what you mean by “determine”. The measurements do find something out about the photon’s past, but they do not cause something in the photon’s past: They only cause things at the time of measurement and later. It should be noted, by the way, that none of this has any connection to human consciousness. It’s quite possible for something to act as a quantum-mechanical measurement without anyone knowing of the results. Since Wheeler is a respected physicist who generally knows what he’s talking about, I’m going to guess that he’s been quoted out of context here.

[Tuckerfan]

Chronos, you might be right, but my take on the article is that Wheeler means that the reading affects what happened at the point of the photons origination. The title of the article is Does the Universe Exist if We’re Not Looking? To reenforce the point that its acting upon the past the article goes on to state

Wheeler conjectures we are part of a universe that is a work in progress; we are tiny patches of the universe looking at itself-and building itself. It’s not only the future that is still undetermined but the past as well. And by peering back into time, even all the way back to the Big Bang, our present observations select one out of many possible quantum histories for the universe.

and

Wheeler suspects that most of the universe consists of huge clouds of uncertainty that have not yet either interacted either with a conscious observer or even with some lump of inanimate matter. He sees the universe as a vast arena containing realms where the past is not yet fixed.

Perhaps we need to put in a call for the Bad Astronomer.

[Pleonast]

Excellent summary of the essential problem, bbeaty. Most of this spooky-at-a-distance weirdness is an artifact of a particular interpretation of quantum mechanics. It’s because we insist on thinking of a photon as either a particle or a wave. And so when its behavior switches modes, there’s this spooky action.

Remember, a photon is not a particle and it is not a wave, it’s a quantum object with some properties of both. We can approximate it’s behavior as a wave or particle depending on its environment, but then we shouldn’t complain when our approximation fails.

[scm1001]

The other controversial question is whether conciousness has anything to with it at all. A bit like the “does a tree falling in the forest make a noise if no-one is there to hear it” sort of thing.

The point is somewhere the quantum uncertainty has to collapse. In several peoples perspective, this is intrinsically bound up with the observer, as the mathematics do not seem to describe a place where it collapses - ergo conciousness is crucial. Other people (me among them) see any large object as a potential observer, and the collapse is simply one of scale. Basically it is a mystical versus realist approach to the universe (I am not using mystical in a perjurative sense, hell quantum stuff is so weird that there is no reason that the universe isn’t, but realist in the sense there is an independent reality out there whether or not I am looking at it)

The mid range region where quantum uncertainty starts to collapse is certain to be one of the most fascinating in the next few years. Several research groups are constructing experiments to try and observe this collapse in action on quite large (micron sized) objects

[Revtim]

*Originally posted by Chronos *
**The key here is what you mean by “determine”. The measurements do find something out about the photon’s past, but they do not cause something in the photon’s past: They only cause things at the time of measurement and later. **

That’s open to interpretation. Most of the stuff I’ve read seems to indicate that this type of experiment does in fact cause a change in the past from quantum uncertainty to certainty. That’s one of the freaky counter-intuitive things about QM that the literature always makes a strong point on.

[Ring]

In between measurements quantum particles are superpositions of states. Measurement devices are composed of quantum particles, and the original quantum particles must become superpositions of the measurement device’s states. The retina of the eye is composed of quantum particles and so are the optic nerve and the brain.

Since all matter is composed of quantum particles, what can possibly cause this chain of linear superpositions to collapse. It can only collapse when it encounters a system that is not subject to the laws of QM.

Wheeler posits that our own evolution involved a series of quantum events, which only our own present consciousness conjured into existence

The implication here is that we continued to evolve as a quantum system until we became conscious, and at that point we teleologicaly collapsed our own wavefunction. The ultimate question here is what would the universe be like if there were no conscious beings? And the answer is…….?

What difference could it make, who would even there be to ask the question?