Posted by: T. Boyd | October 27, 2009

Relativity for Relatives, Part II

Let all the earth fear the LORD: let all the inhabitants of the world stand in awe of him (Psalm 33:8)

I discussed last time that the speed of light is constant as measured by the observer no matter how fast he himself is moving.  For example, if a space ship were moving at a speed of 90,000 miles/sec, toward a star, a beam of light from that star would still measure as having a speed of 186,000 miles/sec, the same as it would measure when the space ship is at rest compared to the star.  Classical physics, and common sense, would say that it should measure 186,000 + 90,000 miles/sec, but common sense fails here.

This result, when predicted in 1900 or so by Einstein and Lorentz, did not seem credible, but it has been shown many times to be true with experiments.  Even more incredible are the consequences of this:   moving clocks slow down (as measured by an observer “at rest”), and moving objects get shorter along the axis of the motion, again as measured by a person at rest.  These are called “time dilation” and “space contraction“, respectively.

Why is it called relativity?  Let’s imagine 2 identical rocket ships coasting by each other in empty space at a high velocity.  Each could say that the other ship was moving and he himself was at rest.  There is no detectable way for them to say who is moving.  It is all relative.

And the time dilation, etc. is reciprocal.  Each space traveler would observe that the other ship’s clock was losing time compared to his own clock and that the other ship was shorter than his own.  How can this be?

Welcome to the weird world of relativity, the effects that normally can only be detected by very precise instruments,  like atomic clocks, or in the case of objects moving near the speed of light.  For example, one early observation was that certain cosmic ray particles (called mesons) created in the earth’s upper atmosphere, with well known, very short life spans, would not be seen as plentifully as they are at the  earth’s surface unless their life-spans were extended about 10 times by moving at about 99.5% of light speed.

There is an easy-to-read and entertaining fantasy book written by George Gamow in 1946, called Mr. Tompkins in Wonderland.  (An updated version, called Mr. Tompkins in Paperback, is available at bookstores – I just found some used ones on-line for $3).  Mr. Tompkins attends a lecture on relativity and then dreams he is in a wonder land where the speed of light is 10 miles/hour.  So these effects upon time and length are obvious just by watching ordinary motion.  For an example, a bicycle goes by and looks much shorter in length than it should.  And as the cyclist speeds up, the length of the bike shrinks even more.

Mr. Tompkins, then gets on a bicycle to catch the other rider.  When he gets to moving, he notices that the buildings on the street start getting skinnier and skinnier the faster he moves.  And then he realizes why it is called “relativity” – each observer sees things from his relative frame of reference,  He notices that the distance to the other biker shrinks, and that the other biker himself looks normal when they are traveling at the same speed.  The story is wonderful fiction, yet reflects a reality that we don’t ordinarily see because the speed of light is so great in our universe.

The discovery of the law of relativity has uncovered another mysterious wonder of creation that continues to amaze God’s sons and daughters and causes them to honor and glorify Him.


  1. From Sharon T (via e-mail):

    Awesome, Boyd. Veerrrrrry interesting!!

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