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A QED question

I was watching a lecture about QED given by Richard Feynman. Part of the lecture was about the reflection of light. This is what I think he said, paraphrased: When shining a light at a reflective surface the probabability that the light will take the shortest path or the longest path to a detector, placed anywhere, is equal. However, the AMPLITUDE of the probability is not equal, but varies. The light particles with the highest amplitude are the ones we see when the incident angle and the exit angle are the same, the shortest path. So I think Feynman said the probablity can be the same but the amplitude of the probability can be different. Is that correct? If so, can anyone point me to a web site that explains this in a way a layman can sort of understand? Thanks, Eric

Reply to
etpm
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Are you sure he didn't mean that the photon was the same everywhere, but that the amplitude of the probability was different from place to place?

AFAIK, "probability" and "probability amplitude" are the same thing -- either quantum physicists use these terms differently than I'm used to (possible), or you misheard he said (presumably possible, although I don't know how many times you listened), or he _meant_ what I'm saying above (or something similar) but he didn't state it well.

Reply to
Tim Wescott

Feynman was stating Fermat's principle, but in a quantum mechanics framework.

What Fermat actually said was that the actual path is an extremum, which can be the longest or the shortest. In practice, it's always the shortest. An extremum is a point where the slope of path length versus choice of nearby path is zero.

If you want a bigger explanation, try posting the question on sci.optics.

Joe Gwinn

Reply to
Joseph Gwinn

He refers to COMPLEX (number) propabilities..

formatting link

No. :)

Kristian Ukkonen.

Reply to
Kristian Ukkonen

What I'm asking about Joe is the statement: The probabilities are equal but the amplitude of the probabilities are not. Is this correct? I understand that the light can be reflected along longer paths, that diffraction grating can be used to show this. It's this amplitude thing that I'm not sure about. Eric

Reply to
etpm

Thanks Kristian, the link you provided makes sense. I think I get it now. So even though you said there is no web site you pointed one out to me. I understand the joke even though I don't really understand QED. I'll keep plugging away at it though, it's so interesting and entertaining. Eric

Reply to
etpm

I have the book. I advise getting one and reading a section - and you can look it over and over to gain an insight when needed.

Mart> I was watching a lecture about QED given by Richard Feynman. Part of

Reply to
Martin Eastburn

Because the probabilities are complex numbers, they can be equal in magnitude and yet point in different directions. The animated drawing near Probability Amplitudes in

gives the general idea.

Joe Gwinn

Reply to
Joseph Gwinn

That's what I'm gonna do. I have other books by and about Richard Feynman. Entertaining reading. So now I need to get all of his lectures in book form that are available. I did find a web site devoted to these works. Fo someone without a strong math background the stuff he talks about is a little hard to grasp sometimes. But it sure is fun! Eric

Reply to
etpm

It is an interesting read - read the notes in the front of the book - whence it came - and how and why - Friend, wife and himself!.....

Mart> >

Reply to
Martin Eastburn

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