Light speed

Michelson Interferometer

During the 1880's a series of experiments were performed that showed that light actually travels at a constant speed and it was this knowledge that later led Einstein to write his famous paper on Special Relativity. So how did we get from measuring the speed of light to Special Relativity?

This is a secong blog on Light, a previous post showed how Ole Rømer's had used the orbit of Io around Jupiter to work out the speed of light. While the results were out by a considerable way, they did show that the speed of light was finite and very quick. Improvements in its measurements showed that it was around 300,000 km per second! An incredible speed.


It was known that light could travel through a vacuum, but it was thought that the vacuum must contain some medium to propogate light. Much in the same way that sounds waves need air. As we know light travels really fast so coming up with experiments to try and detect the aether was going to take some smart thinking.

It was thougth that the earth must being moving through the aether during its orbit around the sun. So by measuring the speed of light in different directions at various different times, it should have been possible to measure the motion of the earth relative to the aether and this would have shown up in experiment as small differences in the speed of light. Now it was know that the speed of the earth around the sun is about 30 km/s, so the detectable effect would be this value divided by the speed of light,

30/300000 = 1/10000

about 1% of 1%. The idea is that if you are travelling in the same direction as the aether the apparent speed of light would be given by

apparent speed of light = speed of light - your speed

much in the same way that when you are travelling on the motor way, all the traffic on your side of the road seems to be almost stationary relative to you, where as on the other side of the road the traffic is belting passed.

White light interference pattern

Michelson came up with a piece of apparatus know called an interferometer (see the picture above) , sent a single source of white light through a half silvered mirror. The mirror split the beam at right angles sending half the light along path 1 and the second on path 2. The light is reflected back and meets up on the far side of the mirror and travels into an eye piece. It shows up in the eye piece as an interference pattern, similar to that shown here.

If the Earth where travelling through the aether then the small change in the speed of light would cause the pattern to drift to the left or right. The first experiments did not show the expected effect. Six years later in 1887 Michelson and Morley repeated the experiment with better equipment which would be more than capable of measuring the small shift that was expected. The result? Probably the most famous "failed" experiment in physics. They did not detect any difference in the speed of light that could not be attributed to experimental error. So what happened next? Well not a lot really, additional experiments were formed but they all came back the same. Light appeared to be a constant. It was almost 30 years later that one man realised that they had actually made one of the most important discoveries in modern physics. The discovery? The speed of light is a constant!

How can light be a constant speed? And why can't things move faster than it? This doesn't make sense. If you are travelling at 3/4 the speed of light and you see someone coming towards you at 3/4 speed of light then surely relative to each other you are travelling at 3/4+3/4 = 6/4 which is much faster than light? The answer is no, your relative speed must be less than the speed of light.  It is still thought that the speed of light is the universal maximum speed limit. Also, any object with mass cannot reach the speed of light. The only things that travel at the speed of light is light, and potentially, gravity waves (though I don't think these exist in the form we think).

(Speed of light being maximum speed in universe - at the time of writing some results have come out of CERN that cast doubt on this. If these results are true then they will change pretty much everything!)

So Einstein ponders how things look to someone moving and someone standing still and realises that the results should be the same. So he began to think about a wire  moving in a magnet field.  If you are sitting on the magnet when it is stationary there is the magnetic field and no electric field. In the conductor which is moving there is something called an electromotive force due to the magnetic field. This causes a current to flow in the wire.

A wire passing through a magnetic field

But if you are travelling on the magnet and the conductor is stationary there will be an electric field near the magnet due to the moving magnetic field.

Hold on a minute!

On the moving magnet we have an electric field, but when the magnet is stationary no electric field near the magnet? Yep.

Einstein realised that although they appeared different they must actually be the same.

Doesn't quiet make sense? Know what you mean!

Think about that situation described above for a minute. The two appear to be quiet different and yet Einstein realised that they must actually be the same and what is different is the way we are looking at things. 


This is not all however, he then goes one step further and says that light has a constant speed in a vacuum which does not matter on the motion of the emitting body!

You may be thinking, ok, he's had a couple of fairly wild ideas, and ....

From these two ideas he goes on to show how the unit of time is not fixed but is dependent on the velocity of the observers. How space contracts , how mass alters depending on how fast we are travelling, how space and time are not actually independent but are actually part of something different that is now called space-time, and on it goes...

Amazing! It would have had impressed even the great Sherlock Holmes.

With Einstein's paper came the realisation that time is not a seperate entity to space. This is a mind bending idea because there is nothing in every day life to suggest that time and space cane be related in any way and yet they are. It amazes me that anyone was able to realise this, it just seems some unlikely.

So what does this tells us about the nature of times arrow? To be honest I have some ideas, but they are for another day.

Think I may have to update my post on the positron though. After all, a positron may be no more than an electron travelling backwards in spacetime.