Tachyons are hypothetical particles that always travel faster than the speed of light. Einstein showed that such particles would allow for communication back in time, which opens up all sorts of problems with a fundamental rule of the universe. While physicists haven't proved that tachyons can't exist, there's good reason to believe they don't.
The barrier that nothing with mass can travel at the speed of light isn't just an expression of the limitation of engineering or a representation of a failure of imagination. It's baked into the very laws of the universe, as expressed by Einstein's theory of special relativity.
Let's say you want to start traveling faster than the speed of light. You start from rest and give yourself a little nudge. Because you have mass, your nudge has to overcome a bit of inertia to get you going, but you eventually get going. You light up a rocket, for example, and you blast off.
But once you're off the launchpad, you don't stop. You have some superadvanced engine that allows you to keep pushing, causing you to continue accelerating. At speeds much lower than the speed of light, everything makes sense: For every second you fire your engines, you get the same amount of acceleration and the same boost in your velocity.
But as you approach the speed of light, something funny starts to happen. The same amount of energy put into your engines starts giving you less and less acceleration, so you get less velocity bang for your buck. Despite working your engines to the extreme, you find yourself inching closer to the speed of light but never reaching it. At some point, you realize that to achieve light speed, you need to put an infinite amount of energy into your engines — which you don't have.
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