Quantum Rebellion?

[II Gyan II]

The New Scientist's July 24th issue has its feature article titled 'Quantum Rebellion' with the intriguing byline 'Nothing exists till it is measured'. Since the archive is subscribers-only, I couldn't read the article or the editorial. Till now (http://www.kathryncramer.com/wblog/archives/000674.html).

The article concerns a double-slit experiment, conducted by a researcher at Harvard, Shariar Afshar (http://users.rowan.edu/~afshar/). Before reading further, familiarize yourself with the Principle of Complementarity (http://www.britannica.com/nobel/micro/138_65.html), if you haven't already.

Basically, the experiment shows light displaying wave and particle aspects at the same time, in violation of complementarity. Supposedly, and I'm not too qualified to render this judgement myself, this falsifies (http://www.lns.cornell.edu/spr/2004-05/msg0060855.html) the Copenhagen interpretation and the Many-Worlds interpretation as well.

If the experiment is valid and verified, what conclusions can conservatively be drawn?

[SentientMeat]

I'm not sure this is quite so revolutionary as the title suggests (but then again, that's what New Scientist does every week to sell copies, and good luck to them!). As the discussion below the article admits: However, reading various responses online it seems likely that the presence of the wires themselves introduces some new uncertainty about which slit the photon went through, because the wires can scatter photons so that a photon going through the left slit can end up in the right detector and vice versa. One poster on sci.physics.research explains this with a nice thought-experiment about enlarging the size of the wires until they are almost touching, forming, in effect, a new series of "slits"My own view of how to interpret QM is that M Theory (http://en.wikipedia.org/wiki/M_theory) will hopefully provide a rather more rigorous formulation of Many Worlds (http://en.wikipedia.org/wiki/Many-worlds_interpretation) which does not quite appeal to the infinite for each electron or photon, while avoiding the problems inherent in the simple hidden variable (http://en.wikipedia.org/wiki/Hidden_variable) interpretation.

[II Gyan II]

As the discussion below the article admits:

Elsewhere, someone rebutted that the sci.physics.research poster misconstrued the experiment:


The experiment involves three different processes.
1) Create a standard double slit, but add a lens so that two photodetectors each detect light from a single slit. Observe the EM flux everywhere, and find that all light is indeed focused on those two detectors.

2) Pick just one location where the double-slit setup would create wave-like negative interference, and place one metal wire at that location. Then cover up one slit, and measure EM flux everywhere. You'll find that some photons collide with that wire and get scattered, and that less light will hit the photodetector.

3) Having established that light particles get scattered by that single wire, uncover the first slit. Now, measure the flux everywhere and find that no light gets scattered. That is, all light hits those two photodectors.

[SentientMeat]

Then I'm afraid I must admit I'm in over my head here! I'll leave the discussion to those more capable than I.

[II Gyan II]

No one?

An update (http://www.kathryncramer.com/wblog/archives/000687.html):


Dear Friends,

I have a wonderful news! I just received an e-mail from my team at Rowan University: The single-photon experiment confirms my earlier findings! Bohr and Copenhagen are history!!!

Thanks for your help and support.

Best regards.
Shahriar S. Afshar

[BrainGlutton]

How is this any different from the Schrodinger's Cat paradox?

[E-Sabbath]

Wait. Wavicle action at the same time? This... this _does_ change things, and seriously, insofar that... okay. Insofar that it is not "This is true, but this is true, and both are true, as long as you don't measure it. If you measure it like a particle, it is a particle, if you measure it like a wave, it is a wave. But it will never be both at the same time, that is, it is indeterminately both until measured.

If it is measured, and measured as both a wave and particle at the same time, a lot of fairly basic quantum mechanics theories are wrong, I think, because it's no longer.. uhm, deterministic? It is not one or the other but both at once, a different sort of particle.

[JasonFin]

I think that article does physics a disservice by describing Bohr's "principle of complimentarily" (that something can be a wave or a particle but not both at the same time) as the orthodox view. It simply isn't precise enough to be useful. This principle can be interpreted as a summary or a simplification of the actual laws particles obey, but I doubt many practicing physicists would accept it as a perfectly true statement about the way they think the world works.

In standard quantum theory, a particle is considered to be a wave packet: a combination of an infinite number of waves of continuously varying frequency. It's possible for a packet to be relatively confined to a single spot in space ("particle-like"), or for it to travel coherently, with little dispersion in momentum ("wave-like"). There are an infinite number of possible states that match neither of these descriptions, or both, to varying degrees; but for a wave packet to have an arbitrarily precise position it must have an arbitrarily imprecise momentum, and vice versa. This behavior is perfectly described by the ordinary mathematics of wave propagation (leaving complications accompanying "measurement" out of it).

A quick search for "uncertainty principle wave packet" gave me this page (http://www.phys.virginia.edu/classes/252/Wave_Packets/Wave_Packets.html) which appears to describe the orthodox view reasonably well.

In short, the result of this experiment is exactly what one would expect based on the rules of quantum mechanics. All it does is demonstrate in an obvious way that Bohr's principle cannot be exactly true, but that shouldn't surprise anyone with detailed knowledge of the theory.

[II Gyan II]

This is over my head, as well. So, I'll post another clarification on the supposed significance of this experiment.


Anyway, what's interesting about the experiment isn't that light is being observed acting like a wave in transit and as a particle when it hits the detector - that happens all the time, and isn't surprising at all. What's surprising is that, if you can "follow" the light for the whole time - that is to say, you know which slit each photon through and where they're going to end up - they should act like particles the whole time, and there should therefore be no interference pattern. To quote a standard textbook (Introductory Quantum Mechanics, by Richard Liboff):

"We conclude that if it is possible to observe which slit [photons] go through, their interference pattern is destroyed. In observing the position of the [photons], their wave quality (e.g., interference-producing mechanism) diminishes."

(Liboff used electrons instead of photons as an example, but the principle is the same.)

This is determined by the Heisenberg uncertainty principle, which is very closely linked to Bohr's idea of complementarity. This concept is poorly explained by the article - what it actually states, basically, is that if the photon exists within a well-defined locality of space, momentum will be ill-defined, and it will act like a particle. If it doesn't exist within a well-defined locality, momentum can be defined more precisely and it acts more like a wave (although it will still always register on a detector in a particle-like way.) So, if you know which slit it goes through (locality defined), it should act like a particle - no interference pattern.

What Afshar has done is set up a double-slit experiment where, because a lens directs the photon streams to different detectors, he knows which photon came from where. So they should be behaving like particles the whole time. Then he plunks down a set of wires right where there would be interference nulls if the photons were being wave-like. If the photons are behaving as particles, like they should, photons should hit the wires, scatter, and the image at each lens should get all fuzzy, as it does with a non-interfering single slit. Instead, nothing much happens, which is what you'd expect to happen if the wires were in the null points of a wave interference pattern. This is a very interesting result.

Now, there's an obvious problem with this, which was brought up by the article you link to. Putting a set of wires in like that is essentially creating a whole new set of slits after the first pair, which could be effectively destroying the "we know where each photon comes from" phenomenon. One way to test this might be with a single-photon experiment, which is probably what's being referred to in the link in Gyan's "update" post; if you fire single electrons at each slit, and every time they show up at the detector they're supposed to even if the wires are there, that's supporting evidence that the wires aren't making things screwy (single photons should behave exactly as photon streams do, including with regard to interference patterns.)

So, it looks like this is a pretty interesting experiment, since it seems to contradict predictions made by Bohr, Heisenberg, and others which have stood up to a lot of tests and experimentation. I can understand why he might be excited about it. However, it'll require a lot more investigation and experimentation to determine exactly what's going on, and it hardly invalidates Einstein, Bohr, or quantum mechanics in general - but it might be a step towards correcting, amplifying, or even contradicting some of their work in areas which are still poorly understood.