I'm fascinated by the fundamental questions raised by the Double Slit Experiment at the quantum level. I found this "Dr Quantum" video clip which seems like a great explanation. But is it scientifically accurate?
Answer
A bad thing about the video is how they explained the part where you try to observe which slit the electron goes through. They made it sound more mysterious than it really is.
What we have to ask ourselves is: what does it mean to observe an electron? What does it mean to observe anything? If we want to look at something, we need light. We see things because light is reflected off objects and our eyes collect this light which is then interpreted by our brains.
If we want to see which slit the electron goes through, we shine light upon it, but this fundamentally alters the experiment. Small particles are very sensitive to perturbations and shining light on an electron is a big perturbation. Now, and this is technical, the Heisenberg uncertainty relation tells you how much the electron's path will be perturbed by the light. The path is more perturbed as the energy of the photon is greater, but to determine the position of the electron accurately, you need high energy according to Heisenberg. High energy means perturbing the electron a lot and as a consequence destroy the interference pattern.
So, you might want to give up on accuracy to avoid perturbing the path of the electron too much, but if you do that, the Heisenberg relation will show you that you have to diminish the energy of the photon so much that you will not be able to locate the electron anymore. The interference pattern on the other hand will reappear.
More details can be found in the Feynman Lectures, Volume 3, Chapter 1.
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