The laws of physics are the same as long
traveling at a constant speed.
Suppose you’re in a vehicle that’s closed up—the
doors are shut, the windows are blacked out, and you have no way
to see the outside world. How can you tell whether you’re
sitting still or hurtling along in a straight line at a steady
100 kilometers an hour?
According to physicists, there is no way to tell.
In a closed vehicle, there is no experiment you can do to figure
out whether you’re standing still or moving at a constant
velocity (that is, in a straight line and at a constant speed).
Pour water into a glass, bounce a ball, swing a pendulum—do
any experiment you can imagine. The laws of physics that dictate
how liquids pour, balls bounce, and pendulums swing are the same
at all constant velocities, whether you’re moving at 100
kilometers or 0 kilometers per hour.
If the vehicle speeds up or slows down, then you can detect the
change. But as long as it’s moving at a steady velocity,
there’s no way you can tell.
Physicists call the place from which motion is measured a “frame
of reference.” As a physicist might put it, “all laws
of physics are the same in all uniformly moving frames of reference.” This
assumption is fundamental to Einstein’s special theory of
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