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Young's double slit experiment

Probably one of the most outstanding contributions to physics is Young’s infamous double slit experiment.

The key phrase that you will hear again and again with regards to the experiment is interference pattern. The term simply means the interaction between particles. An easy way to think about this is when drops of rain fall into a pool of water. When the ripples move out from the point of impact the waves merge and interact with each other as they expand outwards. This is what is known as an interference pattern.

The double slit experiment is basically involves a detector sitting behind a screen which contains 2 slits in it. The experiment works by firing particles or matter at the screen and and registering what is coming through by looking at the detector. Sometimes to screen will have a slit closed, so sometimes there will be 1 slit and sometimes there will be 2 slits open. So goo so far.

Light

Imagine there is one of these double slits screens and behind it a wall. When performing this experiment by shining light on onto the screen with one slit open you will see a pattern like this.

Now with both slits open.

What you are seeing in the above is an interference pattern. Where light is coming through both of the slits, the waves of light are interacting with each other like the drops of water on a pond.

Marbles

Now if we preform this experiment with marbles instead of light and now we are to fire marbles at the screen.

With one slit open we will see a pattern like this.

With both open something like this.

You might be wandering what all of the fuss is about. Everything is behaving as would be expected. So far would it be safe to say we know and have established the difference between what happens when we do this experiment with waves and particles.....?

This is where it becomes interesting.

Electrons

If we now do this experiment with electrons which are to tiny bits of matter. And we send them through one at time and only send the next one through when a hit has been recorded on the screen. We would expect to see something similar to what we observed with the marble experiment. But is that what we get :) .

With one slit open.

And now (play close attention) with both!

This experiment has been carried out countless times in many different places and it always gives the same result. What is happening? The implications are that one single particle, a small piece of matter is behaving like an ocean of waves. What people did first was to try and find out which slit the electron was going through. So they put a detector near each slit. They recorded which slit each electron went through, but when they looked at the results of the experiment the interference pattern had gone! Interestingly enough when the apparatus that detect which slit the electron goes through is turned off the interference pattern comes back.

This experiment has been done in many situations. For instance turning on the detector on and off while the electron is in mid flight. But the resulting fact seems to be that when we try to pin point the location of the particle it behaves normally, and when we are not looking it behaves like a wave.