Always have fun with this - What collapses a wave?

twistor59 wrote:You mean like this:

slits.jpg

Yep

twistor59 wrote:

What we would expect from that are tracks that must be traceable back through a specific slit

Is that really true though ? You wouldn't be able to do that with 100% certainty. For example the bottom cloud chamber track could have come from an electron that came through the lower slit, line of sight to the source, or one that scattered off the top surface of the upper slit. To know which slit an electron came through, wouldn't I need a detector at the slit itself ?

Hmmm. There will definitely be edge effects, yes. The slits have an edge, and so does the boundary of each 'cloud-chamber' hole. There will also be tracks that bounce from detector surface back to slit-screen, possibly multiple times, before entering one of the 'cloud-chamber' holes. And it may be impossible to get an electron into a directional detection device without it being refracted - this may destroy the information that I kind of assumed a directional detector would give us.

Suppose we had a knob that changed the percentage of the detector surface given to 'directional' and 'non-directional' detection modes. Hard left, we allow every electron to enter the cloud chamber, and can record its precise trajectory, at least from the moment it passes the plane of the detector. Hard right, and we effectively have the conventional detector that quantum-classically gives an interference pattern.

The question is, what would we expect at hard left? By plotting the points at which electrons enter the chamber, we will either get an interference pattern or two clear spots, with some stray track origins due to additional possible paths. If refraction destroys the directional signal, we just have regular quantum weirdness - paths keep the "which-slit" information fully uncertain, and the interference pattern is simply a regular Young's-slit head-scratcher, projected upon a slightly different kind of screen.

We could test the device with a single slit. We would expect a single, slightly diffuse spot at the points of entry. Would our tracks lead away from that spot in a ballistic cone, or would the refraction problem destroy even that information?

Or, maybe to simplify it a bit more - instead of a photographic screen as a detector, we just have a cloud chamber or spark chamber placed at some distance from the slits (to allow for spreading and interference). This "chamber" is an idealised version where we don't have to get through any, say, glass to enter the device.

At the left hand side of this chamber you might expect to see sources of tracks, with most tracks emanating from points corresponding to where there would be a sharp line in the traditional setup. The difference is that now for each scintillation, we get a whole track instead of a dot.

I need to think a bit about what extra information you could glean from that...

Templeton wrote:Always have fun with this - What collapses a wave?

I thought Wavefunctions collapsed, not waves per se.

With the above experimental setups, aren't we just trying to do the Wheeler's delayed choice experiment (for electrons rather than photons) in a needlessly complicated way ? I believe the delayed choice experiment has already been done using electrons rather than photons, and the results have been identical.

Tangentially, this guy seems to think that photons don't actually exist:

[youtube]http://www.youtube.com/watch?v=Ym8_D7_kBD0[/youtube]

What do you think?

Can't listen to sound (I'm at work at the moment, so will have a look later).

I think the cloud chamber workaround will run into the usual problems with HUP and the either the interference will vanish or it will not be possible to determine which slit.

GenesForLife wrote:
I thought Wavefunctions collapsed, not waves per se.

Very well, What collapses a wave "function"?


Energy moves in waves (not all) therefore what causes the wave function to collapse and become a particle?

Templeton wrote:

GenesForLife wrote:
I thought Wavefunctions collapsed, not waves per se.

Very well, What collapses a wave "function"?


Energy moves in waves (not all) therefore what causes the wave function to collapse and become a particle?

But does it collapse? The many-worlds interpretation says that it does not collapse - it just appears to for each observer after splitting (and the modern idea of splitting is that it would be due to decoherence).

Everett, H., 1957. Relative State Formulation of Quantum Mechanics. Reviews of Modern Physics, 29, pp.454-462.

Price, M. C., 1995. The Many-Worlds FAQ. [Online] The Anthropic Principle. Available at: http://www.anthropic-principle.com/prep ... orlds.html [Accessed 12 December 2010]. (Also available at: http://www.hedweb.com/everett/everett.htm [Accessed 12 December 2010] and http://kuoi.com/~kamikaze/doc/many-worlds-faq.html [Accessed 12 December 2010].)

Tegmark, M., 1997. The Interpretation of Quantum Mechanics: Many Worlds or Many Words?. [Online] arXiv:quant-ph/9709032. Available at: http://arxiv.org/abs/quant-ph/9709032 [Accessed 8 December 2010]. (Also available at: http://xxx.lanl.gov/PS_cache/quant-ph/p ... 9032v1.pdf [Accessed 12 February 2011].)

(Max Tegmark was one of the scientists in the TV programme being discussed.)

What's the general feeling in the scientific community on the Many Worlds theory?

I saw a documentary about Everett a while back saying that it's gaining more and more respect with every passing year.

I can sympathise with Einstein... there's something deeply anti-intuitive about the Copenhagen Interpretation imho.

Just returning briefly to your previous point:

Allan Miller wrote: We 'know' a particle has arrived at the detector because we see a flash - but does that really mean that a particle has traversed the apparatus? If the detector consisted of atoms at various states of closeness to firing, and you bathed it in a uniform wave which continually pushed each atom closer to firing, then the next 'flash' would be detected from the atom nearest to its firing energy when we started looking. When it fires, it drops back to the ground state. The detector is simply a randomised collection of atoms at various intermediate energy states, and the interference pattern builds up because the wave front is non-uniform - the interference is real.

There is an analysis in "Optical coherence and quantum optics By Leonard Mandel, Emil Wolf" - if you do a google books search for that you can get it (the URL from google books itself is mega-long). Section 9.1 onwards presents a semiclassical analysis, where you have quantum atoms in the detector, but a fully classical EM radiation field - it gives the Poisson distributed detector clicks that we see.

MacIver wrote:What's the general feeling in the scientific community on the Many Worlds theory?

I saw a documentary about Everett a while back saying that it's gaining more and more respect with every passing year.

I can sympathise with Einstein... there's something deeply anti-intuitive about the Copenhagen Interpretation imho.

I'm not sure there is a general feeling. There are many different opinions on it, but I think the important question is whether it has observable consequences or not. I suspect not, but I wouldn't be surprised if a google search turns up some particular case where MWI is consistent with theory A but not theory B.

twistor59 wrote:Or, maybe to simplify it a bit more - instead of a photographic screen as a detector, we just have a cloud chamber or spark chamber placed at some distance from the slits (to allow for spreading and interference). This "chamber" is an idealised version where we don't have to get through any, say, glass to enter the device.

At the left hand side of this chamber you might expect to see sources of tracks, with most tracks emanating from points corresponding to where there would be a sharp line in the traditional setup. The difference is that now for each scintillation, we get a whole track instead of a dot.

I need to think a bit about what extra information you could glean from that...

Yes, that’s the kind of thing I had in mind – the traditional detector is a plane through an assumed 3D set of paths, so it might be possible to have all the path information and extrapolate back. But I think I omitted to consider scale. Cloud chamber tracks are enormous compared to the size of an electron, and compared to the wavelength (and hence slit size and spacing). I don’t think they would ‘point’ to one slit or the other with sufficient accuracy. Damn you, Quantum World, you win again!

twistor59 wrote:Just returning briefly to your previous point:

Allan Miller wrote: We 'know' a particle has arrived at the detector because we see a flash - but does that really mean that a particle has traversed the apparatus? If the detector consisted of atoms at various states of closeness to firing, and you bathed it in a uniform wave which continually pushed each atom closer to firing, then the next 'flash' would be detected from the atom nearest to its firing energy when we started looking. When it fires, it drops back to the ground state. The detector is simply a randomised collection of atoms at various intermediate energy states, and the interference pattern builds up because the wave front is non-uniform - the interference is real.

There is an analysis in "Optical coherence and quantum optics By Leonard Mandel, Emil Wolf" - if you do a google books search for that you can get it (the URL from google books itself is mega-long). Section 9.1 onwards presents a semiclassical analysis, where you have quantum atoms in the detector, but a fully classical EM radiation field - it gives the Poisson distributed detector clicks that we see.

Yes, that’s much like Lamb’s analysis in the 1968 paper (and my more hand-wavy version!). It’s easier to say ‘photons don’t exist’ than “electrons don’t exist”, though.

Another interesting program recently was the secret life of waves.The point was made there that nothing actually travels other than energy when a wave propagates. It is of course possible to surf along a wave – to pick up some of the energy in a material object and gain propulsion in the same direction – but we don’t see electrons as surfing their wave, but in some way actually being the wave. Nonetheless, it would be possible to simulate their quantum behaviour on a still pond, too. A ripple machine and two slits would give an interference pattern. But if, instead of looking at it, we tried to detect it energetically, with detectors that fired only when they had received a certain amount of energy (and we didn’t know how close any given detector in the array was), they would suggest the arrival of “ripplons”. Stick one of these detectors after each slit, and they detect only those “ripplons” that don’t contribute to the interference pattern - and so, naturally enough, there is no interference.

This is where I think a path detector may be interesting. If nothing material actually passes, the electron paths manifest in the 3D detector might bear little or no relation to the direction of the incoming radiation.

MacIver wrote:What's the general feeling in the scientific community on the Many Worlds theory?

It seems surprisingly popular for such a hare-brained notion! It’s just too extravagant and ad hoc IMO. No explanation is offered as to how these two worlds interfere with each other before splitting for good. And it’s not just one extra world per slit. The photon ends up at a specific point in the detector (if it’s really a particle), and there must be a world for each separate point it could have arrived at. Which we can generalise to every potential target for every photon emanating from every source everywhere. A lot of universes.