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light in space
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Question:
1. According to the books, the speed of light in a vacuum
is 300,000 km per second. If you send out a sudden pulse
of light in space, does it have to accelerate to that speed?
2. If you could make a _very_ long tube in space, with
a mirror at both ends (perfect mirrors, and perhaps long
enough to reach from earth to Venus), could you open one end,
shine a bright light in for a few seconds, then slide the
mirror back in place, trapping the beam of light in there?
Would the light beam keep bouncing back and forth?
Replies:
1. No, light starts out at the speed of light - it does not
have to accelerate. What does happen is that the amplitude
(of the electric and magnetic fields) gradually increases
so that at the start of the pulse the amplitude is small,
it then rises to a peak, and then falls back down to
zero.
2. Yes, you sure could do that. In fact, that is essentially
the way some experiments on fiber optics work (and somewhat
related to the way lasers work). Apparently a recent
experiment by some Japanese researchers has sent light
pulses round and round a fiber optic cable for some
180 million miles - that is getting into astronomical
distances right here on earth!
Arthur Smith
But why does not light "have to accelerate"? In truth it does,
but the acceleration to light speed is instantaneous. That is because light
is made up of massless photons. The force that creates the photons
gives them infinite acceleration, so they reach the speed of light
instantaneously.
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Update: June 2012
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