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Einstein sets limit on light speed

Einstein's theory on relativity changed the way we see the universe. Nasa

In 1905 Albert Einstein published a series of papers that turned contemporary physics on its head and changed our perception of the universe forever.

While E=mc² has become synonymous with Einstein’s name, it was just part of his wider research into the relativity of space and time.

Special relativity

At the beginning of the 20th century, scientists knew that light was an electromagnetic wave, which travelled at 300,000 kms a second.

Because light was a wave, they assumed it needed a medium to travel through, just as sound waves move through the air and water waves travel across oceans and lakes.

The medium in space was called the ether, but it was posing a dilemma because it could not be detected.

Experiments had failed to find it, and motion through the ether did not affect the speed of light, which was always the same irrespective of whether the observer was travelling towards or away from the light source.

In one of his 1905 papers, Einstein proposed that the ether did not exist. This seemingly innocuous suggestion was in fact revolutionary.

The speed of any moving object is perceived differently by different observers. To someone standing on a railway platform, for example, the speed of a ball thrown on a passing train would be faster than that a ball thrown on the platform itself. The reason is because the speed of the train is added to that of the ball.

Einstein reasoned that the ether was not there because the speed of light is always constant, no matter who measures it. Also, this meant that something very odd was happening to space and time, which traditionally determine speed.

Until 1905, time was seen as absolute, ticking away at the same rate for everyone in the Universe. Thanks to Einstein we now know that people measure time differently depending on how they move.

Slowing down

Time running in a spacecraft moving with a very high speed relative to ourselves would appear to us to be running very slowly, but to the person in the spacecraft it would be running at the normal speed.

Furthermore, our time would appear to be running very slowly to the person in the spacecraft.

The idea that time passes at different rates has now been proved. Scientists synchronized two atomic clocks and flew one around the Earth on a plane. When the clock returned to Earth, it was a fraction of a second behind the one that stayed on the ground.

Space is also affected. If you stood on a platform and watched a train pass by at 60 per cent of the speed of light, the train would appear to be shorter than it is.

These ideas are difficult to grasp because they’re not part of our everyday experience. But as speeds approach the speed of light, the effects become more and more obvious.

Special relativity is a combination of two simple ideas: all motion is relative, and the speed of light is constant.

Both were well known – from classical physics and the work of James Clerk Maxwell. But it was Einstein’s genius to question assumptions that everyone simply accepted.

Einstein’s main paper on the Special Theory of Relativity was titled On the Electrodynamics of Moving Bodies (June 1905).


Three months after the main paper on relativity was published, Einstein reported a result that he’d overlooked – mass and energy are related and they can be changed from one to the other.

It was the forerunner of the world’s most famous equation, E=mc², in which the energy content of a body is equal to its mass multiplied by the speed of light squared.

The equation shows that very small amounts of mass can be converted into a very large amount of energy and vice versa.

Eventually the equation explained how radiation worked – how a lump of uranium could throw out constant streams of high-level energy. It also explained how stars could burn for billions of years.

But in 1905 Einstein merely said that when an object sends out energy, it loses a small amount of mass in the process. Only later did he grasp the full implications.

It was the observation that nothing can exceed the speed of light which led Einstein to relate energy and mass. As an object travels faster, its mass increases.

As the mass of an object increases, it takes more and more energy to increase its speed any further. Eventually as the objects get close to the speed of light, it becomes so massive that no amount of energy will make it go any faster.

The title of the paper: Does the inertia of a body depend on its energy content? (September 1905)

swissinfo, Vincent Landon

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