48 Relativity
When an object moves, its motion is measured relative to other objects. If we drive
along a major road at 70 miles per hour (mph) and another car is driving outside us at
70 mph in the same direction, our speed relative to this car is zero. Yet we are both
travelling at 70 mph relative to the ground. And our speed is 140 mph relative to a car
driving at 70 mph on the opposite carriageway. The theory of relativity changed this
way of thinking.
The theory of relativity was set out by the Dutch physicist Hendrik Lorentz in
the late 19th century, but the definitive advance was made by Albert Einstein in
- Einstein’s famous paper on special relativity revolutionized the study of
how objects move, reducing Newton’s classical theory, a magnificent
achievement in its time, to a special case.
Back to Galileo
To describe the theory of relativity we take a tip from the master himself:
Einstein loved to talk about railway trains and thought experiments. In our
example, Jim Diamond is on board a train travelling at 60 mph. From his seat at
the back of the train he walks towards the cafeteria car at 2 mph. His speed is 62
mph relative to the ground. On returning to his seat Jim’s speed relative to the
ground will be 58 mph because he is walking in the opposite direction to the
train’s travel. This is what Newton’s theory tells us. Speed is a relative concept
and Jim’s direction of motion determines whether you add or subtract.
Because all motion is relative, we talk about a ‘frame of reference’ as the
viewpoint from which a particular motion is measured. In the one-dimensional
motion of the train moving along a straight track we can think of a fixed frame of
reference positioned at a railway station and a distance x and a time t in terms of
this reference frame. The zero position is determined by a point marked on the
platform and the time read from the station clock. The distance/time coordinates
relative to this reference frame at the station are (x, t).
There is also a reference frame on board the train. If we measure distance
from the end of the train and time by Jim’s wristwatch there would be another