When time runs backwards

Special relativity tells us that nothing can be accelerated up to a
speed greater than that of light, but it does not rule out things
travelling faster than the speed of light as long as they always
remain on the other side of the light speed barrier. You see the
speed of light is a two-way barrier; nothing moving slower than
light can ever go faster than light and nothing already faster than
light can slow down to a speed below that of light. Physicists even
have a name for hypothetical ‘superluminal’ particles that travel
faster than light. They are called tachyons and, if they exist, would
have some strange properties. For instance, since time slows down
the closer a particle gets to the speed of light, until at light speed
time stands still, we can take this a step further and see that, for
tachyons, time would be running backwards. To us, tachyons
would be travelling backwards in time! Tachyons would not be
like normal particles that slow down as they lose energy. Instead,
they speed up, and when a tachyon has lost all its energy it will be
travelling at infinite speed!
Even though special relativity predicts that such particles
could exist, no evidence whatsoever has been found for them and most physicists do not believe they exist. Anyway, they are too
weird to contemplate, even by the standards of modern physics.
There is one sure example of particles travelling faster than
light though. The one thing I have not mentioned yet is that this
maximum speed limit I have been discussing refers specifically
to light travelling through empty space. This is called the speed
of light in the vacuum. When light travels through a transparent
material such as glass or water, it moves more slowly. This is what
gives rise to refraction (the reason a spoon looks bent when placed
in a glass of water and why a swimming pool looks shallower than
it really is). Because of this, it is possible for a particle to be moving
through such a medium at a speed that is greater than the speed of
light through that medium. When electrons move through water
faster than the speed of light in water, they emit a pretty blue light
known as Cˇ erenkov radiation. This is the light equivalent of a
sonic boom when a jet breaks through the sound barrier.
Finally, there are a number of examples where it looks as
though the speed of light barrier is being broken, but which on
closer examination show this not to be the case. The most famous
of these is known as the searchlight paradox. In Chapter 2 we
met the rapidly spinning neutron stars, known as pulsars. Some
of these can spin at over one hundred times per second. As they
spin, they emit an intense beam of radio waves which sweeps past
the Earth like a searchlight with the same frequency as the rotating
pulsar. But since pulsars are so far away (usually thousands of
lightyears) the spotlight that this beam shines onto the Earth must
be sweeping past in such a gigantic circle that it would have to
travel at trillions of times the speed of light! Oncloser examination,
however, we can see that this is not so. The radio waves coming
from the pulsar are in the form of photons (since they are just
electromagnetic radiation) and it is these photons which are the
things that are being emitted from the pulsar and they always
travel at the speed of light. The confusion arises because we think
there is something moving round in a circle, but all the photons
are travelling radially outwards. No physical object is actually
moving in this large circle at all. Think of the photons as being like
water sprayed out of a garden sprinkler.