By Joseph Choonara
Downloading PDF. Please wait... Issue 1945

How Einstein explored the light fantastic

This article is over 17 years, 2 months old
Our series on science continues with a look at the theory of relativity
Issue 1945
Einstein lecturing
Einstein lecturing

I showed last week how Isaac Newton developed laws of motion that allowed a materialist picture of the universe to emerge. This materialism was of a mechanical kind — with matter interacting like parts in a giant clockwork mechanism set into motion by god.

This view of nature began to break down from the late 18th century. A number of important discoveries — from Charles Darwin’s theory of evolution to new theories about the origin of the solar system — showed that nature had a history and had evolved.

And, as the industrial revolution got into full swing, new technologies, especially the steam engine, required a new understanding of physics.

Scientists saw how heat energy could be transformed into motion in the piston of a steam engine, giving a more fluid picture of nature than Newton’s vision.

They began to develop the idea that gases, and all matter, could be collections of tiny particles — and that the temperature of matter reflects the motion of these particles.


Newtonian mechanics was not enough in order to be able to understand the behaviour of the billions of particles making up a gas. A new science, thermodynamics, which studied flows of heat and energy was born.

A further development would have a direct impact on Albert Einstein. Newton’s mechanics implied a kind of relativity in nature.

For example, if you stood on top of a train moving at 50 miles per hour (mph) and threw a ball forwards so it moved away from you at ten mph, you would expect a friend stood by the side of the track to see the ball move away from them at 60 mph.

There is no absolute value for the speed of the ball, it depends which “frame of reference” you are in — stood on the train, or by the track. Both observers can work out what the other will see, as long as they know how fast the train is travelling.

This relativity seemed to work for everything — except for light. In the 19th century, the physicist James Clerk Maxwell showed how light was the product of an interacting electric and magnetic field.

The equations he produced predicted a value for the speed of light. The problem was that the speed of light turned out to be independent of the speed of the object producing it.

If you stood on the train and pointed a torch forward, both you and your friend stood by the track would measure the same speed for the light emitted from the torch.

Special relativity

This perplexed Einstein. How could two of the best established theories of physics — Maxwell’s equations and Newton’s laws — contradict each other? His solution, published in 1905, was the special theory of relativity.

The theory was based on two principles. Firstly, he said that the laws of physics must work the same way in every frame of reference. Secondly, he said that observers in different frames of reference should always measure the same value for the speed of light.

The only way both of these principles can be true is if we ditch many of our notions of time and space. Space and time become entangled so that observers, moving relative to each other, no longer agree whether any two events happen simultaneously or not.

If the train in our example began moving close to the speed of light, the observer by the side of the track would see the train contract in length.

A clock placed on the train would appear to tick more slowly. A weight placed upon it would seem to get heavier. However, to a person travelling on the train, everything on the train would appear perfectly normal.

The weird effects described by Einstein are only noticeable at speeds close to the speed of light (299,792,458 metres per second) relative to the observer, which is why the Newtonian physics we learn at school works for most purposes.

Two other things followed from Einstein’s theory. Firstly, it led to his famous equation E=mc². This equation showed that a small mass could, in theory, be converted into a huge energy — which is what happens when a nuclear bomb goes off.

Secondly, the general theory of relativity, developed later, deals with bodies that are not just moving with constant speed, but accelerating relative to each other. It shows how mass and energy distort the fabric of space and time to give rise to gravitational forces.

Next week I will draw some conclusions about how revolutions in science take place.


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