French mathematician, Henri Poincare. Einstein’s arguments were closer to physics than those of Poincare,
who regarded this problem as mathematical. Einstein is usually given the credit for the new theory, but
Poincare is remembered by having his name attached to an important part of it.
The fundamental postulate of the theory of relativity, as it was called, was that the laws of science should be
the same for all freely moving observers, no matter what their speed. This was true for Newton’s laws of
motion, but now the idea was extended to include Maxwell’s theory and the speed of light: all observers should
measure the same speed of light, no matter how fast they are moving. This simple idea has some remarkable
consequences. Perhaps the best known are the equivalence of mass and energy, summed up in Einstein’s
famous equation E=mc2 (where E is energy, m is mass, and c is the speed of light), and the law that nothing
may travel faster than the speed of light. Because of the equivalence of energy and mass, the energy which an
object has due to its motion will add to its mass. In other words, it will make it harder to increase its speed. This
effect is only really significant for objects moving at speeds close to the speed of light. For example, at 10
percent of the speed of light an object’s mass is only 0.5 percent more than normal, while at 90 percent of the
speed of light it would be more than twice its normal mass. As an object approaches the speed of light, its mass
rises ever more quickly, so it takes more and more energy to speed it up further. It can in fact never reach the
speed of light, because by then its mass would have become infinite, and by the equivalence of mass and
energy, it would have taken an infinite amount of energy to get it there. For this reason, any normal object is
forever confined by relativity to move at speeds slower than the speed of light. Only light, or other waves that
have no intrinsic mass, can move at the speed of light.
An equally remarkable consequence of relativity is the way it has revolutionized our ideas of space and time. In
Newton’s theory, if a pulse of light is sent from one place to another, different observers would agree on the
time that the journey took (since time is absolute), but will not always agree on how far the light traveled (since
space is not absolute). Since the speed of the light is just the distance it has traveled divided by the time it has
taken, different observers would measure different speeds for the light. In relativity, on the other hand, all
observers must agree on how fast light travels. They still, however, do not agree on the distance the light has
traveled, so they must therefore now also disagree over the time it has taken. (The time taken is the distance
the light has traveled – which the observers do not agree on – divided by the light’s speed – which they do
agree on.) In other words, the theory of relativity put an end to the idea of absolute time! It appeared that each
observer must have his own measure of time, as recorded by a clock carried with him, and that identical clocks
carried by different observers would not necessarily agree.
Each observer could use radar to say where and when an event took place by sending out a pulse of light or
radio waves. Part of the pulse is reflected back at the event and the observer measures the time at which he
receives the echo. The time of the event is then said to be the time halfway between when the pulse was sent
and the time when the reflection was received back: the distance of the event is half the time taken for this
round trip, multiplied by the speed of light. (An event, in this sense, is something that takes place at a single
point in space, at a specified point in time.) This idea is shown here, which is an example of a space-time
diagram...
Figure 2:1
Using this procedure, observers who are moving relative to each other will assign different times and positions
to the same event. No particular observer’s measurements are any more correct than any other observer’s, but
all the measurements are related. Any observer can work out precisely what time and position any other
observer will assign to an event, provided he knows the other observer’s relative velocity.
Nowadays we use just this method to measure distances precisely, because we can measure time more
accurately than length. In effect, the meter is defined to be the distance traveled by light in
0.000000003335640952 second, as measured by a cesium clock. (The reason for that particular number is that
it corresponds to the historical definition of the meter – in terms of two marks on a particular platinum bar kept
in Paris.) Equally, we can use a more convenient, new unit of length called a light-second. This is simply
defined as the distance that light travels in one second. In the theory of relativity, we now define distance in
terms of time and the speed of light, so it follows automatically that every observer will measure light to have
the same speed (by definition, 1 meter per 0.000000003335640952 second). There is no need to introduce the
idea of an ether, whose presence anyway cannot be detected, as the Michelson-Morley experiment showed.
The theory of relativity does, however, force us to change fundamentally our ideas of space and time. We must
accept that time is not completely separate from and independent of space, but is combined with it to form an
object called space-time.
It is a matter of common experience that one can describe the position of a point in space by three numbers, or
coordinates. For instance, one can say that a point in a room is seven feet from one wall, three feet from
another, and five feet above the floor. Or one could specify that a point was at a certain latitude and longitude
and a certain height above sea level. One is free to use any three suitable coordinates, although they have only
a limited range of validity. One would not specify the position of the moon in terms of miles north and miles
west of Piccadilly Circus and feet above sea level. Instead, one might describe it in terms of distance from the
sun, distance from the plane of the orbits of the planets, and the angle between the line joining the moon to the
sun and the line joining the sun to a nearby star such as Alpha Centauri. Even these coordinates would not be
of much use in describing the position of the sun in our galaxy or the position of our galaxy in the local group of
galaxies. In fact, one may describe the whole universe in terms of a collection of overlapping patches. In each
patch, one can use a different set of three coordinates to specify the position of a point.
An event is something that happens at a particular point in space and at a particular time. So one can specify it
by four numbers or coordinates. Again, the choice of coordinates is arbitrary; one can use any three
well-defined spatial coordinates and any measure of time. In relativity, there is no real distinction between the
space and time coordinates, just as there is no real difference between any two space coordinates. One could
choose a new set of coordinates in which, say, the first space coordinate was a combination of the old first and
second space coordinates. For instance, instead of measuring the position of a point on the earth in miles north
of Piccadilly and miles west of Piccadilly, one could use miles northeast of Piccadilly, and miles north-west of
Piccadilly. Similarly, in relativity, one could use a new time coordinate that was the old time (in seconds) plus
the distance (in light-seconds) north of Piccadilly.
It is often helpful to think of the four coordinates of an event as specifying its position in a four-dimensional
space called space-time. It is impossible to imagine a four-dimensional space. I personally find it hard enough
to visualize three-dimensional space! However, it is easy to draw diagrams of two-dimensional spaces, such as
the surface of the earth. (The surface of the earth is two-dimensional because the position of a point can be
specified by two coordinates, latitude and longitude.) I shall generally use diagrams in which time increases
upward and one of the spatial dimensions is shown horizontally. The other two spatial dimensions are ignored
or, sometimes, one of them is indicated by perspective. (These are called space-time diagrams, like Figure
2:1.)
Figure 2:2
For example, in Figure 2:2 time is measured upward in years and the distance along the line from the sun to
Alpha Centauri is measured horizontally in miles. The paths of the sun and of Alpha Centauri through
space-time are shown as the vertical lines on the left and right of the diagram. A ray of light from the sun
follows the diagonal line, and takes four years to get from the sun to Alpha Centauri.
As we have seen, Maxwell’s equations predicted that the speed of light should be the same whatever the
speed of the source, and this has been confirmed by accurate measurements. It follows from this that if a pulse
of light is emitted at a particular time at a particular point in space, then as time goes on it will spread out as a
sphere of light whose size and position are independent of the speed of the source. After one millionth of a
second the light will have spread out to form a sphere with a radius of 300 meters; after two millionths of a
second, the radius will be 600 meters; and so on. It will be like the ripples that spread out on the surface of a
pond when a stone is thrown in. The ripples spread out as a circle that gets bigger as time goes on. If one
stacks snapshots of the ripples at different times one above the other, the expanding circle of ripples will mark
out a cone whose tip is at the place and time at which the stone hit the water Figure 2:3.
Figure 2:3
Similarly, the light spreading out from an event forms a (three-dimensional) cone in (the four-dimensional)
space-time. This cone is called the future light cone of the event. In the same way we can draw another cone,
called the past light cone, which is the set of events from which a pulse of light is able to reach the given event
Figure 2:4.
Figure 2:4
Given an event P, one can divide the other events in the universe into three classes. Those events that can be
reached from the event P by a particle or wave traveling at or below the speed of light are said to be in the
future of P. They will lie within or on the expanding sphere of light emitted from the event P. Thus they will lie
within or on the future light cone of P in the space-time diagram. Only events in the future of P can be affected
by what happens at P because nothing can travel faster than light.
Similarly, the past of P can be defined as the set of all events from which it is possible to reach the event P
traveling at or below the speed of light. It is thus the set of events that can affect what happens at P. The
events that do not lie in the future or past of P are said to lie in the elsewhere of P Figure 2:5.
Figure 2:5
What happens at such events can neither affect nor be affected by what happens at P. For example, if the sun
were to cease to shine at this very moment, it would not affect things on earth at the present time because they
would be in the elsewhere of the event when the sun went out Figure 2:6.
Figure 2:6
We would know about it only after eight minutes, the time it takes light to reach us from the sun. Only then
would events on earth lie in the future light cone of the event at which the sun went out. Similarly, we do not
know what is happening at the moment farther away in the universe: the light that we see from distant galaxies
left them millions of years ago, and in the case of the most distant object that we have seen, the light left some
eight thousand million years ago. Thus, when we look at the universe, we are seeing it as it was in the past.
If one neglects gravitational effects, as Einstein and Poincare did in 1905, one has what is called the special
theory of relativity. For every event in space-time we may construct a light cone (the set of all possible paths of
light in space-time emitted at that event), and since the speed of light is the same at every event and in every
direction, all the light cones will be identical and will all point in the same direction. The theory also tells us that
nothing can travel faster than light. This means that the path of any object through space and time must be
represented by a line that lies within the light cone at each event on it (Fig. 2.7). The special theory of relativity
was very successful in explaining that the speed of light appears the same to all observers (as shown by the
Michelson-Morley experiment) and in describing what happens when things move at speeds close to the speed
of light. However, it was inconsistent with the Newtonian theory of gravity, which said that objects attracted
each other with a force that depended on the distance between them. This meant that if one moved one of the
objects, the force on the other one would change instantaneously. Or in other gravitational effects should travel
with infinite velocity, instead of at or below the speed of light, as the special theory of relativity required.
Einstein made a number of unsuccessful attempts between 1908 and 1914 to find a theory of gravity that was
consistent with special relativity. Finally, in 1915, he proposed what we now call the general theory of relativity.
Einstein made the revolutionary suggestion that gravity is not a force like other forces, but is a consequence of
the fact that space-time is not flat, as had been previously assumed: it is curved, or “warped,” by the distribution
of mass and energy in it. Bodies like the earth are not made to move on curved orbits by a force called gravity;
instead, they follow the nearest thing to a straight path in a curved space, which is called a geodesic. A
geodesic is the shortest (or longest) path between two nearby points. For example, the surface of the earth is a
two-dimensional curved space. A geodesic on the earth is called a great circle, and is the shortest route
between two points (Fig. 2.8). As the geodesic is the shortest path between any two airports, this is the route
an airline navigator will tell the pilot to fly along. In general relativity, bodies always follow straight lines in
four-dimensional space-time, but they nevertheless appear to us to move along curved paths in our
