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Name: Steve
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Hi I am currently reading "The Elegant Universe" by Brian Greene. (Which is a fantastic book.) He refers to Feynman's formulation about the double slit experiment, which says that there is a probability that an electron can travel infinite paths to get through both slits. This means that information can travel faster than the speed of light, which Einstein says is not possible. Am I missing something or is this another inconsistency between Relativity and Quantum Mechanics?

Hi Steve

I suggest picking up a copy of QED, The Strange Theory of Light and Matter, by Feynman. It is a short text of about 150 pages and written in terms a non-expert can easily grasp. Chapter 3 presents an eloquent explanation of the double slit experiment, but please read the first two chapters to get a foundation. The discussion of light reflected by a mirror (in which ray diagrams show the angle of incidence equals angle of reflection) is explained within the context of an infinite number of possible paths. You will find that there is no conflict between relativity and QM. Personal note: I like Brian Greene's stuff very much, but I find his discussions to be sometimes disjointed and assume a level of understanding inconsistent with the presumed target audience.

Hope this helps.

Bob Froehlich


Congratulations, you answered your own question :)

This is another inconsistency between Relativity and Quantum Mechanics.

Perhaps you can be part of the effort to solve the problem.

Sincere regards,

Mike Stewart


Feynman's path integral techniques do not mean that an electron travels through one slit and then goes back to travel through the other. The electron, like all particles, also has wave properties. If there is no way to tell which slit the electron passes through, then the electron interferes with itself like when light waves and sound waves pass through double slits. The electron passes through both slits together, as a wave rather than a particle. If you do have a way to tell which slit the electron passes through, then the electron can go through only one of the slits, and interference doesn't happen. If something is measured in quantum physics, then it has only the quantity measured. If it is not measured, than it has all possible values. Measurement forces one of the possible values to be "chosen". For relativity, a particle is always a particle, and a wave is always a wave. For quantum physics, each item has properties of both.

As for the term "infinite paths", this means an infinite number of paths, not paths if infinite length. Each electron final position has a certain probability of happening. Only one final position happens for an electron, but you cannot predict what it will be. After sending many electrons through, the most likely final positions will have received most of the electrons.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

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