QED by Richard P. Feynman

Author:Richard P. Feynman
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2013-06-14T16:00:00+00:00

1 The areas of the mirror whose arrows point generally to the left also make a strong reflection (when the areas whose arrows point the other way are erased). It’s when both left-biased and right-biased areas reflect together that they cancel out. This is analogous to the case of partial reflection by two surfaces: while either surface will reflect on its own, if the thickness is such that the two surfaces contribute arrows pointing in opposite directions, reflection is cancelled out.

2 I can’t resist telling you about a grating that Nature has made: salt crystals are sodium and chlorine atoms packed in a regular pattern. Their alternating pattern, like our grooved surface, acts like a grating when light of the right color (X-rays, in this case) shines on it. By finding the specific locations where a detector picks up a lot of this special reflection (called diffraction), one can determine exactly how far apart the grooves are, and thus how far apart the atoms are (see Fig. 28). It is a beautiful way of determining the structure of all kinds of crystals as well as confirming that X-rays are the same thing as light. Such experiments were first done in 1914. It was very exciting to see, in detail, for the first time how the atoms are packed together in different substances.

3 This is an example of the “uncertainty principle”: there is a kind of “complementarity” between knowledge of where the light goes between the blocks and where it goes afterwards—precise knowledge of both is impossible. I would like to put the uncertainty principle in its historical place: When the revolutionary ideas of quantum physics were first coming out, people still tried to understand them in terms of old-fashioned ideas (such as, light goes in straight lines). But at a certain point the old-fashioned ideas would begin to fail, so a warning was developed that said, in effect, “Your old-fashioned ideas are no damn good when …” If you get rid of all the old-fashioned ideas and instead use the ideas that I’m explaining in these lectures—adding arrows for all the ways an event can happen—there is no need for an uncertainty principle!

4 Mathematicians have tried to find all the objects one could possibly find that obey the rules of algebra (A + B = B + A, A * B = B*A, and so on). The rules were originally made for positive integers, used for counting things like apples or people. Numbers were improved with the invention of zero, fractions, irrational numbers—numbers that cannot be expressed as a ratio of two integers—and negative numbers, and continued to obey the original rules of algebra. Some of the numbers that mathematicians invented posed difficulties for people at first—the idea of half a person was difficult to imagine—but today, there’s no difficulty at all: nobody has any moral qualms or discomforting gory feelings when they hear that there is an average of 3.2 people per square mile in some regions. They don’t try to imagine the 0.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.