• Question: What is your favourite experiment with particles?

    Asked by stefin456 to Tim, Kristian on 21 Nov 2013.
    • Photo: Kristian Harder

      Kristian Harder answered on 21 Nov 2013:

      Ha, you are probably expecting me to vote for the CMS experiment at the LHC, or any massive particle physics experiment that I’ve been involved in, right?
      Wrong!
      For me, the winner is….

      THE DOUBLE SLIT EXPERIMENT!

      Specifically, the version of it that sends single electrons through the double slit at a time.
      The reason is that this experiment is the single most impressive demonstration of how wrong our intuitive ideas really are about what matter is. You might do this experiment with a laser in school. Shining a laser through two slits, you get a stripey pattern on a screen behind the slits, because light behaves like a wave, and waves can add up or cancel out, depending on whether peak of one wave hits peak of the other wave, or peak hits trough. We call this “interference”, and only waves can do that. So far so good.
      Now, do this experiment with electrons. Electrons are said to be particles. So they can’t really cancel out, can they? Well, they do! Even with electron beams instead of laser light, you get interference patterns. So that’s lesson one in quantum physics: particles sometimes also behave like waves. Ok. Kind of acceptable for most people.
      Finally, do this experiment with electrons again, but slow it down so much that only one electron gets sent at a time. Now it has nothing to interfere with, it must go either through one or through the other slit, and there can’t be an interference pattern, right?
      WRONG! If you do this over and over again and mark where the electrons ended up hitting the screen, you*still* get an interference pattern! The only explanation for that is that each electron basically goes through both slits at the same time, and does some wave-like interference *with itselfMATOMO_URL
      If that isn’t odd, then I don’t know what. 🙂 But *that* is what matter is like.

    • Photo: Tim Hollowood

      Tim Hollowood answered on 21 Nov 2013:

      My favourite experiment is only a thought experiment devised by Schrodinger to show what a strange theory quantum theory is. Here’s a version of it:
      you shine a very weak light so that it gives out a single photon (particle of light) at a half-mirror. The half mirror lets through the photon with a probability 1/2 or reflects it with probability 1/2. If the photon goes through then a detector sends a signal to electrocute a poor cat.
      Now according to quantum theory, which is our best theory of the universe, the cat is in a state which is a strange superposition of dead and alive. This is obviously a problem because whenever we do the experiment (obviously without the cat!) then we always gets definite answer. So there’s something not quite right with our quantum theory but no one knows quite what to do about it. So it’s very thought provoking.

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