Mathematics 5
Winter Term 2000
The World According to Mathematics
Dwight Lahr and Josh Laison
Class Discussion: Week #9
Today we are going to conduct a thought experiment involving objects moving at speeds
close to the speed of light. We will begin with an excerpt from Mathematics and
Knowledge: Models of Reality, Section 9.10, a dialogue between Yma and her mother:
Mass is a measure of the substance, the stuff, of the rock.
Hmm. That's pretty fast. I have heard that nothing can go faster than the
speed of light. That's true, isn't it?
Yes, it certainly is. One of Einstein's assumptions in his theory of special
relativity is that the speed of light is always the same, no matter the velocity
of the source or the recipient.
Can you elaborate on that a bit?
Certainly. Suppose a train is moving at 80 miles per hour, and someone on
the train throws a ball at a speed of 10 miles an hour to a friend several rows
ahead. If you are standing on the platform watching the train go by, then
what do you think you see as the speed of the ball?
Well, I guess I would see it as the sum of the two speeds: 80 + 10 = 90
Yes, in other words, from your perspective on the platform, you have to add
the two speeds to get the speed of the ball.
Now, suppose the train is going at, say, three-fourths the speed of light and
someone shines a flashlight toward the front of the train. What is the speed
of the light that you observe from the platform?
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