Wednesday, December 13th, 2017

Two physicists


Feynman You may remember my mention of a prolific writer-theoretical physicist named Stephen Hawking. He is the author of a book (among many others) called A Briefer History of Time. It is not easy to figure out what he is talking about, even though he writes for a non-technical audience.

Let me introduce you now to another theoretical physicist (I think that’s what we should call him) named Richard P. Feynman. He also has written many books, some being popular explanations of what he does as a physicist and some more rigorous. Right now I am reading his QED: The Strange Theory of Light and Matter. It falls in the middle ground between popular treatment and rigor. Here is a sample of his fresh style of writing and lecturing. He is talking about the reflection of light off glass, more specifically about the partial reflection of light off both the front and back surfaces of a sheet of glass.

The situation today is, we haven’t got a good model to explain partial reflection by two surfaces; we just calculate the probability that a particular photomultiplier will be hit by a photon reflected from a sheet of glass. … I am going to show you “how we count the beans”–what the physicists do to get the right answer. I am not going to explain how the photons actually “decide” whether to bounce back or go through; that is not known. (Probably the question has no meaning.) I will only show you how to calculate the correct probability that light will be reflected from glass of a given thickness, because that is the only thing physicists know how to do! What we do to get the answer to this problem is analogous to the things we have to do to get the answer to every other problem explained by quantum electrodynamics.

You will have to brace yourself for this–not because it is difficult to understand, but because it is absolutely ridiculous: All we do is draw little arrows on a piece of paper–that’s all!

He goes on to draw a lot of little arrows.

I have just shown you how this strange feature of partial reflection can be accurately calculated by drawing some damned little arrows on a piece of paper. The technical word for these arrows is “probability amplitudes,” and I feel more dignified when I say we are “computing the probability amplitude for an event.” I prefer, though, to be more honest, and say that we are trying to find the arrow whose square represents the probability of something happening.

This breezy style of writing makes me think I understand what he is saying. It’s somehow strangely satisfying, not that I’ve ever lost much sleep over the question of light reflecting off of glass.

Dave, always willing to be more satisfied with God’s world.

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