The Most Fantastic Scientific Experiment Ever

I know I have mentioned this experiment before, but I could look at it once a month (or a day, or an hour) and still be newly amazed every time.

Let’s start with the basic Double Slit experiment, first performed by Young all the way back circa 1801. He passed a light source through a single slit and then through two narrow slits (the diagram below exaggerates the distance between the two slits). Because the light waves interfere with each other, multiple bands of light appear on the backing. At first one might expect just two bands of light from the light passing through the two slits, but the wave nature of light maximizes and reinforces the light in some areas and cancels with light waves in other areas. This is called interference.

Now is where it gets interesting. Richard Feynman brought up the idea of the photons, or electrons, going through the two slits being like firing a machine gun through the two slits. Obviously, although being fired very rapidly, only one bullet comes out of the machine gun at time. So, if they come out one at a time they would go through the slits one at a time. If so, how could they interfere with each other. Based on this kind of thought a modified experiment was designed. It was the same Double Slit setup, but this time the photons (or electrons) would be made to come out one at a time.

Here is a interactive graphic showing this concept: Light Canon.

Much to the experimenters surprise, when firing one at a time they continued to form bands, indicating an interference pattern. Odd indeed. In fact it is so odd, only two theories eventually evolved to explain this result, and both are equally mind-blowing and beyond all that was considered either normal, or scientific, up to that point. They are:

1. The one particle is going through both slits at the same time. They are superposed in more than one physical location, simultaneously. Of course in classical physics, and by all we hold dear of our everyday intuition of how the universe is, something can’t be in two places at once. Yet it is.
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3. The particle is going through one slit in one universe and while another particle goes through the other slit in another universe. This is the multiverse theory. This theory proposes that there are alternative equally valid universes for each possibility that exists. Again, mind-blowing.

No matter which explanation is used, the math on how to figure it out is about the same. Rather than particles being in a definite place at a definite time, apparently small particles like photons and electrons only have a probability of being in a particular place when observed, but are actually in all possible places. This comes out being a probability function that looks like a wave. The highest point on the wave is the most probable location when observed. But the idea of observations leads the next amazing step of this experiment.

Eventually, researchers came up with the idea of putting a detecting device next to one of the slits to find out which slit the one particle was actually going through. So, for example, they might put a device that detects an electron passing by the right slit, and makes a click noise if it does. No detector is on the left side. So if one electron is shot out and a click is heard then they would know if it went through the right slit. If no click was heard they would know it went through the left side. So what do you think the pattern will be on the wall behind the two slits? Well, it turns out that if this observation is made, of which slit the electron goes through, then the interference pattern disappears. Now there are just two bands directly behind each of the slits. Truly, amazing. The fact of observing the electrons has changed the physical behavior. You might think that the detection device itself altered the behavior, but it can be shown that just deducting that it went through the left slit, because no click was heard, will still alter the physical behavior. So it is just the obtaining the information, with no physical interference, that changes what happens physically.

This phenomena is so non-intuitive, so contrary to traditional physics, that many scientists just got confused and blew it off. Many it appears ignored the results, and pushed them out of consciousness. Yet, these results are easily reproducible, and have in fact been duplicated innumerable times. Quantum scientists are able to work with the data and in fact use the results for many technological practical purposes. But to me this is easily the most amazing experiment ever.

See this animated video that covers much of what was discussed here: Quantum Physics Double Slit Experiment - What The Bleep Movie