system
Given the following post, respond as if you're commenting on Reddit:
human
ELIF: How a tachyon can travel faster than light.

In fact, explain like I'm four.
gpt
They can't. Tachyons are pure sci-fi nonsense.

If you start with the theory of relativity, there are some basic equations that describe how things move around. What those equations are aren't important right now, but suffice to say that they exist. What they say, in a nutshell, is that nothing can ever move faster than a speed called *c*, which is the speed at which light propagates, so we call *c* the speed of light.

But somebody noticed, years and years ago, that if you turn one of the numbers in one of those equations from positive to negative, those equations appear to imply that there could exist particles that can never move *slower* than the speed of light! These hypothetical particles got dubbed "tachyons," after the Greek word meaning "speedy."

Sci-fi writers, unsurprisingly, latched on to this like drowning men clinging to a log. It's jargonny and fantastical, but it's at least *vaguely related to* real science, so it became a very popular idea.

The problem, though, is that the people who've read those sci-fi books latched on to the idea of tachyons and thought it was a real thing. "They're just particles with negative mass," they think to themselves, and then they go on about their lives unaware that they've actually been grossly misinformed.

To understand why this is the case, we have to talk about what the word "mass" means. We all went to school so we all think we know: mass is related to how heavy a thing is. But the essential truth is a lot more subtle than that. Mass is what's called a "restoring force." It exists in the equations that describe how things move as a term that tends to "push" a particle from an excited state back into its ground state. That's why, for example, inertia exists. Things which are just sitting there not doing anything don't spontaneously fly off into space. That's because those things have mass, which means they must be acted on in order to change their motion.

If you replace a positive mass with a negative mass, yes, the equations of relativity tell you that the particle will move faster than the speed of light … but the questions of quantum field theory tell you something different. They tell you that that restoring force we talked about that comes from a particle having positive mass, when you change that positive to a negative, suddenly becomes an *exciting force.* Instead of particles being in their ground state all the time (their state of lowest energy), getting exited into a higher state by some process, then *dropping back* to their ground state again, particles with negative mass would have a ground state that's their *highest* energy state! Such particles would *spontaneously* drop into excited states, because those excited states are actually *lower* energy states, due to the particles having negative mass.

Which means *if it were possible* for negative-mass particles to exist, yes, they would propagate faster than light, and they would be rightly called tachyons … but such particles would *also* spontaneously appear in space out of nothing (because it's energetically possible for them to, since their excited states are lower-energy than their ground state). This would be happening *all the time,* in an exploding cascade of more and more particles filling the universe. Since negative-energy particles would *anti-*gravitate (a particle's gravity is related to how massive it is), this would have the effect of pushing every *real* particle in the universe away from every other particle, tearing apart things like planets and stars and galaxies and atoms until nothing existed but a uniform gas of protons and electrons, and that would be happening *everywhere, all the time.*

So the fact that the universe *exists* is conclusive proof that there can be no such thing in the universe as a tachyon. It's just sci-fi nonsense.