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=mc2 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...
Update 2017. Note from ScienceLine
Moderator:
Please read the news about Higgs Boson from
July 2012 on the link below.
Higgs Boson particle
"Scientists from two experiments at the Large
Hadron Collider in Europe confirmed the existence
of a new heavy particle, likely to be the
long-sought Higgs boson, thanks to troves of
particle-collision data that yielded
discovery-level certainty upon analysis. The
results, announced at a major particle physics
conference in Melbourne, Australia, mark the
culmination of a search for a heavy particle
believed to give mass to elementary particles such
as electrons and quarks."
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