134 The Poetry of Physics and The Physics of Poetry
physics, however. Science fiction writers, since the advent of the Theory
of Relativity, have exploited the relativistic time machine, described
above, to project their characters into the future. In order to project
backwards in time, these writers usually have their character travel faster
than the speed of light, a violation of the Special Theory of Relativity.
But how does traveling faster than the speed of light project one
backwards in time? It really cannot. If one could travel faster than the
speed of light, one could overtake light signals that were emitted from
Earth a long time ago in the past. By intercepting these signals, one could
observe events that occurred hundreds of years ago such as the building
of the pyramids or the extinction of the dinosaurs. While this is not
equivalent to actually living in the past, it represents traveling into the
past in a certain sense. One can see how a science fiction writer might be
able to imagine his character traveling backwards in time. Unfortunately,
it is not possible for us to travel faster than c, so there is no hope
for seeing back into the past. We can, however, anticipate traveling
into the future but this will require tremendous advances in space
travel technology. At the moment, we can only send π mesons into
the future.
Our claim that man cannot travel faster than the speed of light is
based on experimental facts. Accelerators have been built, which
accelerate electrons and protons through electric potentials. The velocity
of the charged particles increases as a result of their absorbing electrical
energy. It is found that, as the velocity of the charged particle approaches
c, a large amount of energy is required to increase the particles’ velocity
by even a small amount. One finds, in fact, that no matter how much
energy is transferred to the particle, its velocity never reaches c.
Apparently, the velocity of light is the ultimate velocity of the universe.
Since all matter is composed of particles, such as electrons and protons,
these experimental results show that the velocity of light may never be
exceeded.
In order to understand why particles cannot go faster than c, it is
necessary to examine the behaviour of the mass of a particle as a
function of its velocity. In fact, the mass of the particle becomes infinite
as the velocity of the particle approaches c. Since the mass of a particle
depends on its velocity, it is useful to define its rest mass, which is the
value of its mass when it is at rest, and which we denote by mo. The mass
of a moving particle, with respect to a given frame of reference, is related
to its rest mass, mo and its velocity v, with respect to that frame by the