| MadSci Network: Physics |
Hi Jason,
Good question! I could answer this question with a gentle introduction to
Relativity (Groans from the back of the class) or I can give you a short
answer. I think I'll do both and, if you're feeling brave, you can read both!
The mass of an object does increase as it gets faster, and theoretically
infinite at the speed of light. What this really means is that only massless
things like photons (particles of light) actually travel at the speed of
light. Massive bodies like electrons or rockets can only ALMOST get there.
This is just because the force required to accelerate them closer and closer
to light speed increases as they get heavier. So you would need an infinite
force to actually reach it.
Why do such effects occur? It's basically because times and distances as
we understand them day-to-day behave very strangely if you are moving fast.
Imagine someone leaning out of a train window and gently tossing a baseball
in the opposite direction, back along the track. He sees the ball move away
from him slowly. Someone standing by the railway line will see the ball moving
in the same direction as the train, just slightly slower. This dependance of
speeds on the motion of whoever is observing them is normal, everyday
'classical' physics. It assumes, sensibly, that time ticks along at the same
rate however fast you move, and that space stays the same size and shape.
However, the speed of light is actually constant, unvarying, fixed. This has
been checked many many times by experiments, and there are also good (but
subtle) theoretical reasons for this being true. So, imagine the person on the
train now shines a flashlight back the way he or she came. According to our
common-sense example with the baseball, the speed of the light should look
slower to the person on the ground than it looks to the person in the train.
In fact, they both see the same speed. Distances and times ACTUALLY CHANGE.
The train is 100m long when it's at rest, and the person on the train measures
it to be the same length when it's moving at, say, half the speed of light.
To the person by the track, it looks as though it's 87m long. Worse still,
it's the same height, so it looks like a weird fat train, and all the people
on it have thinner bodies. What's more, the clocks on the train
are all running more slowly than the clock on the station platform.
The reason
it took so long for people to measure these effects was that the speed of
light is so high: seven hundred million miles per hour.
Relativity shows us that these
strange effects only become noticable at speeds of more than about a hundred
million mph. For example, the fastest car available, which can drive at
maybe 200mph on a straight racetrack, appears longer to the driver than to
you, standing by the track, but the difference is much less than the width
of an atom. Likewise, the driver's watch runs slower, but she'd have to drive
for six hundred thousand years for her watch to lose just one second on yours.
If you wanted to make something go faster and faster, you'd end up slowing
ITs version of time more and more relative to yours. In theory, you can make it
go so fast that time stops dead, but that would need infinite force. There
is no number bigger than infinity, so you can't make time reverse by pushing
harder than infinity!
I hope this helps a little, but if you want to know more
there are plenty of good books on Relativity which are written for the
non-scientist. Many of them will give you a good understanding of the theory,
and maybe you'll even be tempted to look at the equations (they're not much
more complicated than our 'GCSE' or 'O' level exams here in Britain: just some
algebra and a little bit of really simple calculus. And after all, there's no
better way of learning maths than using it to tackle something interesting!)