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What exactly are tachyons? If they can travel faster than light, how much faster can they go?
Answer 1:

You're learning about tachyons in 10th grade? That is quite ambitious. Tachyons are studied in an area called particle physics, and I must say this is a bit out of my league, but I'll give you some general thoughts. Tachyons are hypothetical particles resulting from what physicists call a thought experiment. Back in the 1960s, some physicists wondered what would happen if matter could travel faster than the speed of light, something that is supposed to be impossible according to the Theory of Relativity. So these particles may or may not exist because they have not been proven or disproven by real experiment as of yet. What people have done is apply existing formulas to the unique properties of tachyons (like imaginary mass!). What comes out is a particles that go faster when they lose energy with a MINIMUM velocity of the speed of light and a maximum velocity of infinity! Hope that helps Ben, theoretical physics is a weird place and is not too far off from philosophy.

Answer 2:

Let's start by saying that no one has ever seen a tachyon -- they don't exist in a universe that makes sense compared to itself.Since this is a question from the 10th grade, I'll go into a bit of detail.

In Einstein's theory of relativity, the "mass" of an object increases as it goes faster, becoming infinite at the speed of light, so it takes an infinite amount of energy (remember E=mc^2 means energy and mass are the same) to reach the speed of light. This is why special relativity says we cannot go faster than the speed of light. So what we talk about in physics is the mass of the object when it is sitting still, the "rest mass." If an object has a positive rest mass, it goes slower than the speed of light; if it is like light with a zero rest mass, it moves at light speed. What we call a tachyon is a particle (a fundamental particle, like an electron) that has an _imaginary_ rest mass. Naively, putting this into the equations of relativity (1) doesn't make a lot of sense and (2) seems like it would allow a particle faster than light.

I say naively because this is not really the full picture. To understand, we need to know what a fundamental particle is. Imagine that the universe is a ball sitting in a valley between two hills. If the ball gets bumped a little up one hill, it rolls back down and wobbles back and forth for a while. What we call a fundamental particle, like an electron, is really that wobble. The mass of the particle is given by how steep the hills are right near the valley. If we have a tachyon, this is really like the universe ball is at the _top_ of a hill between two valleys -- the mass is still given by how steep the hill is, but now the hill is going down. Then the "tachyon wobble" is when the ball moves away from the top of the hill. But then the ball won't go back to the top of the hill; it will go to a valley, somewhere else! So a tachyon isn't a particle in the usual sense because it's not a small wobble. A tachyon is really an instability in the universe, just like a ball at the top of a hill isn't stable. This is why I say a universe with a tachyon doesn't "make sense."

In the standard model of particle physics, this subject actually is very important. There is a particle called the Higgs particle that, in the early universe, was kept at the top of a hill by the high temperatures that were present in the Big Bang. As the temperatures dropped,eventually the Higgs particle could move around and slid down the hill.
So as soon as it could tell it was a tachyon, it rolled away from the top of the hill and stopped being a tachyon. When that happened, the whole universe changed, leaving things the way they are today. This Higgs particle is the only particle in the standard model that hasn't been seen in experiments yet, but scientists believe it will be discovered within about 10 years at some new experiments.
Hopefully, you can find some information about this in popular magazines, so let me encourage you to try! In fact, there might be several different Higgs particles -- that
would be a very exciting discovery, for lots of reasons! But describing those is for another question...

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