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by Thomas Hinks on June 21st, 2005

Thomas Hinks

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How can the light from a galaxy possibly tell us whether it is approaching or receding? The speed of receipt is not supposed to be affected by the speed of the sender. If it was, the effect would be a change of frequency. But it is!

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  • by Aminor on June 21st, 2005

    Aminor

    The speed of receipt is indeed affected and there is indeed a change of frequency, resulting in a "red shift," which gives us information about the speed at which something is receding from us. It's frequently described as a Doppler effect, similar to what happens when a train whistle passes us. That's OK as far as it goes, but be careful with the analogy -- Doppler effects on sound on earth are mediated by the atmosphere, making significant differences from what happens with things radiating in space.

    You don't say so, but it sounds like your question springs from a conflict between what you know to be observed effects -- the red shift -- and what you may have heard about the speed of light being invariant, as is required by the Special Theory of Relativity (and modified by the General Theory). (Actually the spped of light being invariant was required by math that came into existence before Einstein; what he did was resolve the conflicts between that and the rest of what we knew about the universe by realizing that time and space must not be invariant.) If you're confused by that stuff, welcome to the club.

    The good news is that there are a lot of good books out there aimed at sorting out relativity for a general, non-mathematical audience. Einstein himself wrote one in 1916, and he's a wonderfully clear, understandable writer (what little math couldn't be left out you can skip if need be -- the book is about the ideas). And there are a lot of others, e.g.,

    Epstein, Lewis C., "Relativity Visualized" (Insight Press, 1988). "Pictures to aid the mind's eye in understanding difficult concepts." Epstein is as engaging a writer about physics as you're going to find. He is dedicated to creating pictures that will communicate underlying ideas (and is disdainful of people who think they understand physics because they understand the math, without ever having really thought about the ideas). He is also the author of "Thinking Physics" (Insight Press 2002), which is a wonderful compendium of one- and two-page problems about everyday phenomena designed to lead the reader inexorably to, well, thinking physics. (I've recommended it elsewhere on this site.)

    Schwinger, Julian, "Einstein's Legacy" (Freeman, 1987). Schwinger is a Nobel laureate in physics, and I believe this is one in a series of Scientific American books about the sciences. Still directed at a general audience, but maybe not quite as general -- a little more knowledge assumed, a little more detailed information laid out. What I've seen of this series are all very useful.

    Comments

    • Good answer on stuff that can make your head hurt.

      jalex137

      by jalex137 on September 28th, 2005

    • Am I right to correct you on the following; "red shift" as you described it would reveal that the stellar body in question is receding, however if it were approaching there would be a blue shift? Is this correct terminology?
      .
      It was a great answer, but he did ask how to distinguish between a receding or an approaching body, and I was under the impression that "red shift" was the result of the lower frequency resulting from the stretching of the waveform as the source moves away only.

      TheicidalManiac

      by TheicidalManiac on October 12th, 2009

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