The "double slit" or two slit experiment is a classic demonstration of how physics can act in ways counter to common expectations.

The basic set up of the experiment is simple, a laser is placed so that it shines onto an opaque screen with two close, narrow, parallel slits cut into it. A second, matte screen is put behind the first so that the light cast by the laser through the slits is projected onto it.

In many ways light acts like a wave. As the coherent light from the laser passes through the narrow slits it is diffracted into two spreading frustums (like a pair of trapezoids with their narrow ends at the slits). The second screen is set up to be far enough from the first that the two projections cross before reaching it. The two waves interfere constructively and destructively and a ripple-like pattern is cast onto the second screen.

At first glance this is a nice demonstration of the wave nature of light. The interesting part of this experiment occurs when you consider the wave/particle duality of light. Light can also be considered as a stream of quantized particles. That is, you can look at light (yeah, I know) as being made up of indivisible equal packets called photons.

This theory of light doesn't seem to conflict with the experimental results; The stream of photons produced by the laser is blocked by the screen, except at the slits where it spreads out by ricocheting off of their edges, and the two spreading streams of photons emerging from the slits interfere with each other as they collide, producing the same results. However if an electron gun is used to fire photons at the screen one at a time, and a light sensitive surface such as a photographic plate is placed in front of the second screen to record where each photon hit, an interference pattern develops, identical to the one produced when the photons are sent out in a stream.

This represents an enigma. If only one photon is being sent out at a time then there are never two streams of photons to interfere with each other. Some physicists believe that photons in parallel universes are interfering with the serially released photons. These are of course being fired by alternate experimenters. Others believe that this represents a failure in quantum theory.

In the beginning of the 20th century, Louis de Broglie suggested that, analogous to light, matter too had both particle and wave properties. Several experiments where crystals were bombarded by electrons showed diffraction patterns, supporting this theory. However, one of the most interesting experiments was done by C. Jönsson in 1961.

Jönsson repeated Young's double slit experiment, but instead of using light he used a beam of electrons. The same interference pattern was observed, and the wavelength of the electrons was found to agree with the de Broglie wavelength.

One would think that a beam of mutually interferencing electrons would give rise to the observed pattern, but that a single electron would not be able to do so. Later, the Jönsson experiment was repeated with an electron source that emitted only a few electrons per hour. Using photographic plates and extremely long exposure times, the interference effect was still found to occur. Even today this effect boggles the mind of physicists.

This experiment has two important outcomes, both of which are highly confusing, and demonstrate that the universe does not work in ways that we are equipped to understand.

To set up this experiment, you get a light source, a photographic plate, and a barrier with two slits in it. You set up the barrier so that the only way that any light can reach the photographic plate if it goes through the slits.

You then shine the light towards the barrier, and thus though the slits. Instead of getting neat little slits of light on the photographic plate, you get an interference pattern. This shows that the photons of light are waves, and that the waves are interfering with each other. Some waves strengthen (amplify) each other, giving you bright bars, and some waves weaken each other, giving dark bars. This is elementary physics; nothing mysterious about it at all.

The interesting bit is, you can see the pinpoints where each photon hit the photographic plate. The interference patterns are made up of hundreds of individual photons impacting the screen. When they hit the screen, they don't act as waves, they act as particles. Photons travel as if they are waves and arrive as if they are particles.

It gets more confusing. Redo the experiment, but this time you sent the light out one photon at a time. You will still get an interference pattern. There is only one photon, but it still acts as if other photons are interfering with its wave.

One hypothesis (I believe it is the strongest), is that the photon is interfering with itself. Quantum mechanics is consistent with the claim that particles can exist in multiple 'life courses' until they are observed, but collapse into one course when observed. As you do not observe which slit the particle goes through, it literally goes through both. The photon is interfering with its own other possible existences. Yes, it's weird. But it appears to be true, or as close to true as we can currently conceive, and it seems to be representative of how all small particles work. For a more technical description, see Young's Slits

This experiment has been redone many many times, including with a number of interesting variations. For example, it has been repeated using carbon buckyball molecules, and they were found to work the same way as photons; they travel as waves, show interference patterns, and arrive as particles (but in the locations that would be expected from an interference pattern). But because buckyballs are much larger than photons, they are also much easier to measure. In one experiment, detectors using photons to detect the path of the buckyballs were placed at each slit; although nothing else was changed, the buckyballs did not travel through both slits (as they do when not measured); instead each buckyball traveled through only one slit, and no longer arrived at the screen in an interference pattern, instead arriving in two patches behind the slits (just as traditional Newtonian physics would have expected). Observing them at the slit 'made them choose' which slit they went through.

This experiment was carried further: this time, the location of the buckyballs were not measured at the slits, but just before hitting the plate. This experiment also found that observing the position of the particles resulted in them hitting the plate without an interference pattern, as if they had traveled as particles along their entire path. This suggests that because they would be observed later, the buckyballs 'chose' to not travel as waves. This highly counter-intuitive experiment is known as Wheeler's delayed choice experiment.

You can find even more weirdness using quantum erasers and delayed choice quantum erasers.

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