Delayed-choice and double slits

Send single photons through a double slit and they will somehow interfere with themselves to produce an interference pattern, as if they were waves. That’s quantum mechanics for ya. ‘Cept it don’t work if the photons are being watched, in which case each photon appears to pass through one slit or the other, as if it were a particle.

Somehow the photon knows it is being watched, which sends physicists and philosophers a-shiverin and and a-tremblin back to their textbooks.

In 1984, the American physicist John Wheeler dreamt up a way of tricking the photons. He suggested switching the method of observation after the photon had passed through the slit.

That means sending a photon through the slits and only then making the decision to record its arrival observed or unobserved. How would the photon “know” what you’ve decided?

Now Alain “Hidden” Aspect at the Laboratoire Charles Fabry de l’Institut d’Optique in France and a few pals have performed the trick for the first time. They say that photons are not so easily tricked: switching the method of observation actually changes the outcome of the experiment, which means the photon somehow knows what you’ve done.

So how to explain this. Either some unknown laws of physics are telling the photons that a switch has taken place during the experiment (and do so at several times the speed of light) or the wave-particle nature of quantum mechanics is correct.

Take yer pick.

Ref: arxiv.org/abs/0710.2597: Wheeler’s Delayed-choice Thought Experiment: Experimental Realization and Theoretical Analysis

3 Responses to “Delayed-choice and double slits”

  1. [...] You might have heard about the famous double slit experiment, in which particles (light or electrons) fired at a wall with two vertical slits in it, will behave like waves when they pass through and make an interference pattern on the screen. This is even true when a single particle is used: the weird thing is that the single particle will interfere with itself (oo-er, missus) and produce a wave-like pattern. Weirder yet is that this doesn’t work when you watch it happen. Scientists have been trying to sneak a peek at what’s happening without much success. They haven’t managed to do it yet – read about the latest attempt here. [...]

  2. enots says:

    It is fundamental to all logical thinking that something is either A or not A.

    That QM is a behavioral model and requires a wave-particle duality is an indication that it is incomplete.

    The underlying fundament of this behavior must be a wave-particle unity model.

  3. Zephir says:

    By Aether Wave Theory the vacuum is formed by foam and the particles are moving like wave packet (dense condensed blobs of foam) through it. Because the Aether foam is chaotic, the path of wave packets, which are following the most dense actual path by the principle of least action, is somewhat random as well. Therefore the free particle falls into target in randomly distributed points and nothing very strange is about it.

    But the particle foam is behaving like dense system of water surfaces as well. During motion of every particle near density gradient the so called the deBroglie wave is formed. This wave is always perpendicular to the particle motion direction and it looks like the artifact, analogue to wave, which is formed above the fish, swimming fast beneath the water surface.

    http://superstruny.aspweb.cz/images/fyzika/quantum/fish_flowing.gif

    The whole trick is, the deBroglie wave is spreading by the speed of light around particle in motion and it’s much large, then the whole particle, so it advances the particle motion and it can interact/interfere with obstacles, like the double slit in typical flabelliform patterns.

    http://superstruny.aspweb.cz/images/fyzika/quantum/normaldifr.gif

    Because the vacuum is behaving like foam, it means, it gets more dense temporarily during shaking, like common soap foam. So that the deBroglie wave makes the vacuum more dense at the places of interference patterns amplitude and the particle wave packet (which follows the most dense places of vacuum, as above stated) tend to follow the paths of flabelliform patterns. The resulting situation would appear like this picture:

    http://superstruny.aspweb.cz/images/fyzika/quantum/quantum_split.jpg

    It means, the path of particle passing through double slit wouldn’t be random anymore and it will approach the target in the flabelliform patterns, which are the consequence of the semi-classical behavior of Aether foam, in fact.