Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Double Slit Experiment Explained
Name: Boii2
Status: other
Age: 18
Location: N/A
Country: N/A
Date: 9/30/2003


Question:
Hi, I have a question to ask regarding the double slit experiment. In the double slit experiment, when we allow just one electron (or in that case, even a photon) to pass through the slits (both open), we consider the probability functions of the electron and say that the this probability function splits up into 2 and after passing through the slits reinforce. That is to say that the electrons pass through both the slits simultaneously (provided that no one is observing). I find this pretty hard to comprehend especially when I think about energy conservation. How is that one electron now appears as two violating the Einstein equation E=m*c*c (I have never found any mention of this anywhere else, so I understand that my question is inherently absurd but please clear my doubts).


Replies:
Boii2,

You have hit on one of the most confusing things in quantum mechanics. When you are measuring properties of an electron, the electron behaves as a particle. It has standard electron mass and other standard properties. This only holds at the instant you do the measurement. The more time a particle spends without being measured, the more random the electron becomes. If you measure which hole the electron passes through, then it will only pass through one of the holes. If you do not measure which hole, then it will pass through both holes as a wave. This fact is known by measuring the angle of the electron's motion after passing through. I do not think any person truly understands why this happens. Richard Feynman, perhaps the greatest quantum physicists, once said that anyone who claims to understand quantum physics must be telling a lie. Accept the fact that nothing tiny is completely a particle or completely a wave. Every individual particle or wave has both properties. A photon of light tends more toward wave properties. An electron tends more toward particle properties. Still, both are somewhere between particle and wave.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College


It *is* pretty hard to comprehend, but you do not want to be thinking about one electron appearing as two electrons. The fact is that you do not know where the single electron is with enough accuracy to say which slit it goes through. (If you do a measurement to determine which slit the electron goes through, the measurement will destroy the interference pattern.) Note that physicists did not make this up, and they do not make it happen. All they did was to notice that this is the way electrons behave, and then they built a mathematical formalism consistent with this behavior, and other known behaviors, so that they could guess what an electron might do in different circumstances. The formalism describes what electrons do with fantastic precision, but -- as you have noticed -- it does not make much intuitive sense, and it is pretty hard to put into words.

Tim Mooney


If you have difficulty understanding the "double slit" experiment, you join a large audience of some of the best minds in physics. What makes the experiment so frustrating and infuriating is that it is so simple, in principle. It applies to electrons as well as photons and presumably to other "particles" as well. To the best of my knowledge, there is no interpretation that is generally accepted by scientists who know a lot more quantum mechanics than I. All sorts of very "strained" explanations have been offered. Here is a recent web page that will let you observe the fray, but I do not think it can be called an "explanation".

http://www.sciscoop.com/story/2003/7/21/1715/66159

Vince Calder


It is, of course, impossible to really comprehend as quantum mechanics is only significant for objects much smaller that we have experience with. It is, after all, quite mysterious that when you release a ball, it falls without anything visible doing anything to it. However, we have observed that so many times and in so many ways that it seems reasonable and even obvious to us.

It might help if remember (as you correctly and with considerable insight point out) that an electron passes through both slits PROVIDED THAT NO ONE IS OBSERVING. Actually, the wave function of the electron always spreads out throughout the universe, although with very little amplitude where it is very unlikely to be. Then when one observes it, the wave function instantaneously collapses to the point where the electron is observed. The probability of it appearing at any point is proportional to the square of the wave function at that point which will, of course, depend on whether the electron wave function "went through" two slits. This is the "Copenhagen" interpretation of quantum mechanics and how that really works is not terribly well understood and, in fact, there is disagreement about it.

The crucial point is that the electron never "appears as two". When observed, it is just one.

Best, Dick Plano...



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory