The Special Theory of Relativity 133
laboratory lifetime, because of uniform motion, is also observed for
the short-lived π meson, an unstable elementary particle that decays at
rest in 10-8 sec. When the π meson is accelerated in a circular accelerator,
it is found that its lifetime in the lab frame increases in the same manner
as its lifetime in the lab increase when it is in uniform motion. The π
meson, which makes a round trip around the accelerator, comes back
younger than a π meson, which remains at rest in the laboratory. In other
words, the π meson twin, which actually makes the journey, comes back
younger.
Now that we have experimental proof that a space traveler will
come back younger than his twin, we know how to build a time machine.
H.G. Wells, in his famous short novel, The Time Machine, describes a
device that allows its passenger to travel into the future. When this story
was first written, the idea of a time machine was pure fantasy. This idea
is no longer a fantasy. The machine, which accelerated the π meson, is a
time machine, which projects the π meson into the future. If the π meson
had not been accelerated, it would have decayed within 10-8 sec, like all
the other π mesons, which were created at the same time as it was.
Instead, as a result of its motion, it found itself still in existence long
after the time it would have naturally ceased existing if it were at rest. It
was existing in the future, i.e. the time after it was suppose to have
decayed.
If it were possible to build a spacecraft that allowed a man to
travel at velocities near the velocity of light, then one could travel
into the future by making a round trip journey. The closer to c at
which one travels; the farther into the future one would be projected.
For example, if one wished to devote one year of his life to traveling into
the future and one’s space ship would travel at the velocity v, then he
would find himself projected 1/ 1 −v^2 /c^2 years into the future. If he
traveled at v = 0.99995c he would be projected 100 years into the future
whereas, and if he traveled at v = 0.999999995c, he would be projected
10,000 years into the future. If one was unhappy about the point in time
one arrived at, one could always get back into the space ship and travel a
bit further.
The only problem with our time machine, however, is that once one
traveled into the future, it would not be possible to travel back to the time
one started from. H.G. Wells’ time machine did not suffer from this
problem. His device was purely a product of his imagination. Our device,
while not practical at the moment, does not violate any of the laws of