These questions can all be considered and explained using the theory of relativity proposed by Albert Einstein in 1905. This theory, as its name suggests, deals with measurements made in systems that are in motion relative to each other. Such a system is known as a frame of reference. For example, consider a train moving along a railway track.
Measurements made in the train are relative to
the train and the train is therefore one frame of reference, while measurements made at
the side of the track are relative to the ground, the other frame of reference. For example
if you swing a ball round your head in the train on a piece of string you see it moving in a
circle while someone watching from the side of the track will see it move in a much more
complicated orbit.
A plane flying is yet another frame of reference, as is a car
moving along a motorway or a ship sailing at sea. You can appreciate that there are many
possible frames of reference all equally important to the people within them who think
that their frame of reference is the "centre of it all".
While you read this you are
probably sitting still - but 'still' relative to what? Even if you are in a building, the building is fixed
to the Earth that is itself rotating and also travelling round the Sun. The Sun is moving within our
galaxy and the galaxy is moving relative to others in the universe.
However Einstein's
theory of relativity is most important when we are dealing with objects moving at high speeds,
and by high speed we mean a substantial fraction of the speed of
light.
If we assume these, then we
must abandon some of our other more traditional ideas, such as the constancy of mass, length
and time. This means that if one object is moving relative to a frame of reference, then the mass
and length of the body measured from the frame of reference will be different from those
measured with instruments travelling with the body. Even more unusual, time measured by a
clock travelling with the body will differ from that measured by a clock at rest in the frame of
reference!
Length appears to get smaller, mass increases and time appears to pass
more slowly in a moving frame of reference when viewed from a stationary frame. Any such
differences are very small, however, unless the relative velocities are very large, that is,
approaching that of light.
Imagine for a moment that we live in a world where the velocity of light is small, say 20 ms-1 (nearly 45 m.p.h.). Then the predictions of the theory of special relativity would become much more obvious (see Figure 1).
If we stood at a street corner in this strange world
and watched traffic passing by, then all the cars would appear shortened and even people would
appear a little thinner than they were when standing still. If you put a friend in a shopping trolley
and tried to push a trolley along, not only would it seem to get thinner (as observed by a person
watching you go by) but you would also find that as you went faster and faster the trolley would
seem heavier and heavier and so become more difficult to accelerate.
Imagine that you
had gone to the station in the morning to say goodbye to your friends who were going by train to
the nearest town (at no more than 45 m.p.h.) and agreed to meet them there in the evening.
They would seem to have aged little, but to them you would have looked a lot older - time for a
moving frame of reference runs more slowly than for one at rest!
