Quantum Mechanics: Double Slit Experiment

This stuff is definitely sci-fi weird, and probably not appropriate for younger grades (although we did have a seven year old reiterate in his own words this exact phenomenon to a physics professor, so hey… anything possible! Which is why we’ve included it here.)

This experiment is also known as Young’s Experiment, and it demonstrates how the photon (little packet of light) is both a particle and a wave, and you really can’t separate the two properties from each other. If the idea of a ‘photon’ is new to you, don’t worry – we’ll be covering light in an upcoming unit soon. Just think of it as tiny little packets or particles of light. I know the movie is a little goofy, but the physics is dead-on. Everything that “Captain Quantum” describes is really what occurred during the experiment. Here’s what happened:

So basically, any modification of the experiment setup actually determines which slit the electrons go through. This experiment was originally done with light, not electrons. and the interference pattern was completely destroyed (as shown in the end of the video) by an ‘observer’. This shows you that light can either be a wave or a particle, but not both at the same time, and it has the ability to flip between one and the other very quickly.  (The image at the left is a photograph of an interference pattern – the same thing you’d see on the wall if you tried this experiment.)

So, both light and electrons have wave-particle characteristics. Now, take your brain this last final step… it’s easier to see how this could be true for light, you can imagine as a massless photon.

But an electron has mass. Which means that matter can act as a wave.  Twilight zone, anyone?

Read more about this in our Advanced Physics Section.


20 Responses to “Quantum Mechanics: Double Slit Experiment”
  1. bac051201 says:

    what would happen if there were 3 or 4 slits what if there were an immense amount how would it change

  2. Aurora says:

    All very good questions. 🙂 What do you think?

  3. Michelle Stevens says:

    I’m confused. When you said that it is impossible to watch light without detecting it (and if detecting it interferes with the experiment) then is the light we see every day really light? And also, how do the photons know if they have been detected?

  4. Amber Simms says:

    AGH! My brain hurts! Very interesting though!

    -Jacob Simms

  5. Aurora says:

    It’s a great video, isn’t it? The rest of the video isn’t quite like this part, and some may not be suitable for kids. It’s from “What the Bleep Do We Know”.

  6. Carol Brown says:

    May we watch the entire video somewhere? It is great! Thank you.

  7. Aurora says:

    Wow – that’s a really hard question to answer without going all geeky with graduate level terminology, so I would have to explain that measuring the position destroys knowledge of speed, so you have no idea where the next position will be. It’s just the way the universe is. You can’t know both… it’s either one or the other.

    I would also suggest asking your questions on the Physics Forum so you can get more folks helping you out as you learn and grow your ideas!

  8. says:

    Hi Aurora, I’ve got a thought experiment that seemingly defies the uncertainty principle: Imagine you’ve got a square one inch box full of helium atoms; you have a machine that will measure the position of a particular atom A to a degree of accuracy we will call C and this machine takes a measurement of A’s position every quadrillionth of a second. Now as C increases wouldn’t our knowledge of A’s velocity also increase? Because measuring position every unit in time is the very definition of measuring speed.

  9. Aurora says:

    Haha… there’s a science to words, isn’t there? I am not sure what kind of phrase you’re looking for though… is it for the double slit experiment?

  10. Jodi Schreiber says:

    Can you give me a phrase to use in everyday context? I try to say things like “hey, would you please break that circuit” instead of “turn off the light”. If I were more sciency, I’d write a children’s book with phrases like that. Maybe YOU should, Aurora! Can you think of a phrase to plug this concept into everyday context?

  11. Aurora says:

    Yes, learning about Chemistry opens us up to a whole world of questions! But you should know that it is actually the number of protons that defines an element, and not the number of electrons. Thus Oxygen is number 8 on the table because it has 8 protons. If an atom loses or shares an electron (or two) the atom is called an “ion,” but it is still the same number on the periodic table. If you want to know more about specific elements on the table and their properties, check out this interactive period table here.

  12. Lorelei Grecian says:

    That is unbeleiveable, its like the electrons have eyes or something… but I don’t get some of that stuff. 🙁 There is to much to wrap my mind around. Like also in this unit how things from the periodic table can change into other things because on electron changes into something. ????

  13. Aurora says:

    You’re right – it is hard to imagine. This video is part of a much longer video that I didn’t create. Here’s the part that they left out:

    in order to see small particles (like photons), you need to detect them. But the trouble is, if you detect them, then you are interfering with the experiment. Photons are one of the very few things you can’t detect without altering the experiment, because you are trying to detect light itself. You can’t “watch” light without detecting it… does that make sense?

    So yes, the observer location will alter the experiment. Light usually act a like a wave if you leave it alone, but when you start trying to detect it or make it act like a particle (as in this experiment), then it acts like a particle.

    Recently, we’ve had studies done where electrons exhibit this same phenomenon – it’s a wave sometimes, and other times it is a particle. Which means that matter can be a wave or a particle. Does that twist your brain into a pretzel now?

  14. Sarah Stone says:

    This is hard to wrap one’s brain around! If the presence of the “observer” was what changed the pattern, then why wasn’t it just always a double slit pattern since both experiments were observed? Does the location of the “observer” affect the pattern? Do the electrons have to somehow be able to “see” the observer?

  15. Aurora says:

    This experiment has to do with the photon, which is a packet of light. The electron is the particle that zips around in a cloud around the nucleus. When the electrons relax into a lower state, they can emit a photon during the process.

  16. Sonya Fiebig says:

    How does this relate to how electrons move around the nucleus?

    Laura Fiebig

  17. Debra Thomson says:


  18. sevy keble says:

    I know its real! Isn’t it amazing?

  19. Aurora says:

    Kind of Well, I warned you it was goofy… but the strange part is how the stuff he talks about is REAL… we did the same thing in the lab in grad school.

  20. sevy keble says:

    Really funny video, and kind of mysterious!
    sevy keble 🙂

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