The Special Theory of Relativity 123
frame of reference at rest, Einstein showed that any frame of reference
not accelerating may be considered at rest and the motion of all the other
objects in the Universe may be taken with respect to it.
We have all experienced the relativity of motion while riding in a
train or a subway car. Remember the sensation when your car was at rest
in the station and you looked out the window to see another car besides
yours moving out of the station. For a moment it feels as though your car
is in motion. In fact, unless you can observe the ground or some other
fixed structure in the station it is impossible by just looking at the other
car to determine whether your car or the other car is in motion. This
example illustrates the relativity of motion. Before Einstein, physicists
believed they could always tell which car was really in motion. Einstein
has shown this is pure folly. You can determine which car is moving
with respect to the Earth but there is no way of determining whether the
Earth and the stationary car are really at rest and the moving car is in
absolute motion or vice-versa.
In the Einsteinian universe, the whole notion of space changes; it is
no longer defined in the absolute sense in which it is in the Newtonian
universe. For Einstein, space does not exist per se. Space is a relationship
between physical objects. If there were no objects in the universe, there
would be no space in Einstein’s universe, whereas in Newton’s universe,
there still would be space. Space has an a priori existence for Newton.
It is the container, which holds the universe. If one considers a finite
universe from the Newtonian point of view one can discuss the space
outside the universe whereas in the Einsteinian worldview, one cannot.
Space for Einstein is a relationship, whereas space for Newton is
something, which has existence and reality.
Let us illustrate their two contrasting viewpoints by considering
the space between my two outstretched hands. What happens to that
space when I put my hands down at my side? From the Newtonian point
of view, the space in front of me is still there, just sitting there ready to
be filled, if necessary. From the Einsteinian point of view, once my
hands have disappeared, we can no longer talk about the space between
them or the space that was between them. The reasoning is similar to the
one employed in the riddle: Where does my lap go when I stand up?
My lap is the relationship of my legs when I am seated and no longer
exists when I stand. In a similar fashion, according to Einstein’s way
of thinking, space is also a relationship of physical objects, which
disappears when those objects disappear.