The laws of physics are the same as long as you’re 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 relativity.

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