Redefining Reality - The Intellectual Implications of Modern Science

H. A. Lorentz had found, when you are moving, the lengths you
measure are squished in the direction of motion. If you walk with a
pen sticking straight out in front of you, the pen would get shorter.
The closer you go to the speed of light, the smaller the pen gets.
In this way, length moves from a real property of the world to a
property of our perspective.
Einstein’s theory of relativity transforms distance from the invariant
fact Newton thought it was, that is, something true of the world
itself, into a covariant fact—something just true of how one person
sees the world. Time works in the same way. Durations stretch, that
is, time slows down when you move, and the faster you go, the
slower your watch goes when observed by someone at rest.
Putting these two together, Einstein showed that our commonsense
picture of adding velocities is wrong. According to Newton, if
you’re on a people mover in an airport traveling at 2 miles an hour
and you want to calculate how fast a traveler passing you at 5 miles
Einstein calculates that the speed will be slightly less.
For the speeds of daily life, this decrease isn’t noticeable, but
the effect makes it so that if something is moving at the speed
of light, adding any other velocity to it keeps it moving at the
speed of light.
Thus, the speed of light is not only constant but a limiting
speed. There is no way to get something to move faster.
Einstein realized that this led to a startling conclusion.
If we think about the interactions between particles, there are
certain conservation laws that must be observed: conservation of
momentum and conservation of energy. You cannot get energy
or momentum from nowhere, and what goes in must come out.
Momentum is mass multiplied by velocity (), and the kinetic
energy of a body is 1 /2 2 . But because the theory of relativity
makes velocity dependent on the frame of reference, these
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