Gravitational Waves

100 Years ago



Albert Einstein published his General Theory of Relativity about which we have all heard . I am going to go through a quick history of it’s achievements  culminating in the detection of gravitational waves earlier this year.Gravitational Waves are really exciting for all sorts of reasons but don’t expect them to be useful in everyday life (whenever someone says this someone else discovers something useful so who knows ?).

Isaac Newton

He discovered that the same force which makes the apple fall from the tree governs the orbits of the planets round the sun and indeed the motion of everything as it was understood at the time. That law was his famous inverse square law which I won’t reproduce here .Occasionally things were not quite right with planetary orbits but when the astronomers took a closer look they usually found a companion or a moon and that fixed things up for the theory. Except for Mercury


General Relativity

General relativity is a bit challenging mathematically (at least I find it so) .All I can say about it here is that the basic idea is that a mass distorts space and time . A few obvious thing about that:it must be a very small effect because we don’t perceive these distortions (a bit philosophical ) and to see anything at all you need be looking a very big things like a planet.You are probably ahead of me here but Einstein was able to correctly calculate the the motion of Mercury and this was the first big triumph for the theory. ( I don’t think that was hugely exciting but it was brilliant). The next thing which followed shortly after was exciting. A prediction of the theory  is that the path of a ray of light will be altered by the presence of a mass .Again it must be a very small effect otherwise we would see some strange effects like light being bent round corners .In about 1919 there was  a complete Solar eclipse the moon was directly between us and the sun,the moon is sufficiently massive to produce an effect and sure enough the star map ( which could be measured when were in darkness for about 6minutes) was distorted.

Gravitational Waves

I am not sure ,maybe someone else knows ,but I seem to remember that Einstein himself didn’t really believe in gravitational waves but they were predicted by his theory.


To see anything at all you need extremely big masses  (bigger than planets) and be able to see very small changes in distance. In 2016 the LIGO (Laser Interferometry Gravitational Observatory)team detected  2 black holes colliding by using a laser interferometer.A laser interferometer is an instrument which picks up very small changes in the distance between 2 mirrors.

Looking Back in Time

Light travels at about 118000 miles a second. The distance light travel in a year is called a light year so a light year is distance.  If we lived on a planet 1 light year away and we could pick up BBC 1 we would be watching one year old episodes of East Enders. So if we can see light from objects billions of light years away we are seeing them as they were billions of years ago.

Why this is Exciting

The universe is about 13.8 billion years old (and the big bang happened on a Tuesday).It turns out the universe was not transparent for the first 400million years so we can see back 13.4 billion years with optical and radio astronomy. So we can’t use them to look at the big bang.


The theory tells us we can see much closer to the big bang with gravitational waves .

ps The event detected is now thought to be 2 black holes colliding inside a star 
pps Not sure it was on a Tuesday