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A friend and I had an argument we cant resolve. Imagine a rocket traveling through space. Inside the rocket there is a electron beam pointing at the wall, assuming the beam is in a vacuum, how much would the beam be deflected by velocity and acceleration?
- chris (age 18)
The beam will not be deflected at all by the rocket's velocity. In
other words, whatever the velocity of the rocket, the beam will head
for the same point on the wall. In fact, this insensitivity to velocity
is universal, for all physical effects. Galileo was the first to
suggest something like that. It's now known as the principle of
relativity. Of course, what it really means is that there's no physical
meaning to saying what the rocket's velocity is, since there's no way
to test it. All that's meaningful is to say what the velocity is
relative to something else, such as a nearby planet.
Acceleration is quite another matter. If the rocket is
accelerating, the wall will change its motion while the beam is heading
toward the wall, changing the spot where the beam hits. So in that
sense the acceleration is absolutely there. Einstein noticed, however,
that the effect of the rocket accelerating up looked just like that of
the rocket not accelerating but rather the beam being pulled down by a
gravitational field. So what really produces the the observable
physical effects is not acceleration in the normal sense but rather the
difference between the acceleration and any acceleration due to a
gravitational field. In other words, the acceleration due to forces
such as those produced by firing the rocket has physical effects which
can be measured inside the rocket, without reference to the motion of
any nearby objects.
(republished on 07/11/06)
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