In an interview on The Atom

Your first book is Subtle is the Lord: The Science and the Life of Albert Einstein by Abraham Pais

This is a book I read in my final year as an undergraduate. It’s a biography of Einstein but there is a lot of technical science and mathematics in it too. It’s written by a physicist so at the time I didn’t follow all of the maths. This is a huge book so I surprised myself by wading through it, but it is this book that really built up my passion for physics and made me sure that this was what I wanted to do with my life. Biographies are wonderful for making things come to life. You learn about Schroedinger’s equation, a differential equation in quantum mechanics, a mechanistical recipe, but if you read a biography you learn that Erwin Schroedinger was this incredible womaniser, a fascinating, charming man who, while he was arguing with Niels Bohr about quantum mechanics, was having dodgy affairs with teenage girls and his wife knew about it. So, this book was the first time I had a really good look behind the iconic Einstein, the Einstein as an old man sticking his tongue out, holding his trousers up with a piece of cord. That’s the Einstein everyone knows, but the really great Einstein was the guy in his 20s who did all this great work, a guy with all this time on his hands to think deep thoughts. He was almost an amateur at the time in comparison with the big names based at the top universities, an amateur figuring out what no one else had done before.

You’re talking about the 1905 stuff he won the Nobel Prize for? The theory of relativity?

That’s not what he won the prize for. There were a few months in 1905 when he published four earth-shattering papers. One of them was on Brownian motion. When you put specks of particles of pollen seed in water you can see them jiggling about. What, he wondered, is the life force inside water that makes them do that? He proved that it was the thermal vibrations of water molecules. This was the first mathematical proof that atoms exist. If that was all he’d ever done in his career he could have retired famous.

And that’s what he won the prize for?

No. He won the prize for the photoelectric effect. If you shine ultra-violet light at a metal surface the light can knock out electrons from the surface of the metal. So the light has an oomph to it. At the time people thought light was a wave and if they sent brighter light then electrons would be knocked out with greater energy. But it didn’t happen. Einstein understood that light isn’t exactly a wave. It’s made of particles, what we call photons. The only way you can knock off electrons more energetically is to change the wavelength. He won it for this because to win the Nobel Prize your theory has to be backed up by experimental evidence. Relativity was, at the time, just a theory.

What was the theory?

Well, there are two. When he came up with the special theory of relativity in 1905 people had almost got there before him. Again, this was to do with the nature of light. Sound waves need air, water waves need water. What is the medium you need to carry light waves? What is the oscillating thing that light travels through? It must be invisible to us and must pervade all space or the light from the sun and stars would not reach us. At the time scientists called it the ‘ether’. But when they did experiments it seemed not to exist. No one could understand how, but Einstein proved it doesn’t exist – light travels through empty space and doesn’t need a medium. I should say that I teach this to undergraduates and this one concept takes me all term to explain so it’s hard to do it in a few sentences.

Give it a go.

Light will travel at the same speed according to all observers no matter how fast they are moving in relation to each other. So, if you stay still but I head off chasing a beam of light in a space rocket that travels at three quarters of the speed of light then you would expect that when I looked out of the window it would appear to be going more slowly than it would to you on the ground. But, actually, we both see it moving away from us at the same speed.

How?

Because in my space rocket my time is running more slowly than yours. This is where all the stuff about time as the fourth dimension comes from, the space-time continuum. And the fourth paper, the E=mc² one, was an afterthought. When I ask people what they know about the theories of relativity this equation is what they come up with, but it was a consequence of the more important first paper about the way time slows down and space stretches.

Space stretches?

Yes, the easiest way to explain why this happens is think of speed is distance over time. So, if speed (of light) is constant and time changes then distance has to change too. If you are travelling very fast, close to light speed, then your clocks will appear to outside observers to be running slower and you will look squashed up and flatter. This is not an optical illusion.

Things actually become squashed?

Well, it seems like an optical illusion because you don’t feel any different. You don’t feel yourself becoming squashed up but the point is that everyone’s point of view is equally valid. The popular way of describing this would be to say: ‘Everything’s relative.’

But this is philosophy. If I see you as squashed up and short it doesn’t mean you are.

No. Einstein says there is no absolute constant for length or time. If something that is 1m long is moving very fast it looks shorter. There is no frame of reference because nobody can say that they are truly stationary. It is democratic – you can’t say my clock is fast and yours is slow.

But we do though.

Yes, because we don’t move anywhere near the speed of light so these effects are tiny enough for us to ignore in ordinary life. But in things like GPS systems, satellites, these effects have to be taken into account – a satellite is moving round the earth so they only work if we do take into account the fact that the clocks on the satellites slow down. This all follows from the weird nature of light. And that’s just the special theory of relativity. There’s also the general theory of relativity.

Go on.

We all know about Newton’s law of gravity. The apple falls on his head because of this invisible force of attraction. It turns out that this is actually a very crude explanation for what happens. What Einstein said was that actually anything with mass, which therefore has a gravitational pull, actually curves space around it. When the earth orbits around the sun it is just following the curved path in space-time. Einstein’s general theory even explained black holes and how the universe began with the Big Bang.

How did he explain black holes?

When a massive star runs out of nuclear fuel, has no more hydrogen to change into helium (this is thermonuclear fusion), it stops shining and there is nothing to stop it collapsing under its own weight. It has been inflated by heat and energy but then it collapses. If a star is massive enough it collapses under their own weight so violently, getting denser and denser and curving space around it until it literally punches a hole in the universe.

If the universe is curved can it be infinite?

Yes, and it could be curved but finite. We don’t know. It could be infinite but it is impossible to imagine because we can’t visualise higher dimensions. So we can simplify it a little: a sheet of paper could be infinite but you could still punch a hole in it. You could even imagine this hole leading to another sheet underneath it, which would be a parallel universe. Einstein suggested that there could be a parallel universe at the other end of a black hole. Black holes could be like tunnels leading back out of another black hole via what is called a wormhole. Like Alice Through The Looking Glass, they become gateways to somewhere else. It’s mathematically possible but can’t be proved yet. Einstein’s theory of relativity works because it’s been tested many times, so if it predicts this other more exotic stuff that we can’t test we still have to take it seriously.