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No matter who you are, where you are, or how quickly you’re moving, the laws of physics will appear exactly the same to you as they will to any other observer in the Universe. This concept — that the laws of physics don’t change as you move from one location to another or one moment to the next — is known as the principle of relativity, and it goes all the way back not to Einstein, but even farther: to at least the time of Galileo. If you exert a force on an object, it will accelerate (i.e., change its momentum), and the amount of its acceleration is directly related to the force on the object divided by its mass. In terms of an equation, this is Newton’s famous F = ma: force equals mass times acceleration.

But when we discovered particles that moved close to the speed of light, suddenly a contradiction emerged. If you exert too large of a force on a small mass, and forces cause acceleration, then it should be possible to accelerate a massive object to reach or even exceed the speed of light! This isn’t possible, of course, and it was Einstein’s relativity that gave us a way out. It was commonly explained by what we call “relativistic mass,” or the notion that as you got closer to the speed of light, the mass of an object increased, so the same force would cause a smaller acceleration, preventing you from ever reaching the speed of light. But is this “relativistic mass” interpretation correct? Only kind of. Here’s the science of why.

The first thing it’s vital to understand is that the principle of relativity, no matter how quickly you’re moving or where you’re located, is still always true: the laws of physics really are the same for everyone, regardless of where you’re located or when you’re making that measurement. The thing that Einstein knew (that both Newton and Galileo had no way of knowing) was this: the speed of light in a vacuum must be exactly the same for everyone. This is a tremendous realization that runs counter to our intuition about the world.

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Credit: Jahobr/Nevadawest of Wikimedia Commons/  This moving, zipping star field appears to depict an ultra-relativistic motion through space, extremely close to the speed of light. Under the laws of relativity, you neither reach nor exceed the speed of light if you’re made of matter. You might be able to approach it if you had a large-enough amount of an efficient-enough fuel, but you still need to obey the rules of relativity

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