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The World According to Physics by Jim Al-Khalili

OUT MARCH 10th 2020

The World According to Physics
by Jim Al-Khalili


Professor of theoretical physics Jim Al-Khalili explains why Einstein didn't win the Nobel Prize for relativity and how biography is a great way of bringing science to life. He talks us through the books that inspired his own passion for physics.

The World According to Physics by Jim Al-Khalili

OUT MARCH 10th 2020

The World According to Physics
by Jim Al-Khalili

Buy all books

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 twenties 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.


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 to think 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 its 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.

Book two. Quantum Mechanics by Albert Messiah.

This is a graduate level physics book, not an undergraduate book. I read it when I started my PhD in theoretical physics. It was originally written in French and it’s where I really learned about quantum mechanics. It goes further than all that undergraduate crap about atoms being in two places at once and it throws a lot of the laws of physics out of the window and says…

Wait. An atom can be in two places at once?

Yes. You cannot say for sure where an atom is until you look at it. Looking at it makes it decide. Only when you look at it is it in one place.

How do you know? Do you pretend you’re not looking?

Well, yes. I mean, you don’t look. You send an atom through something with narrow slits in the top and bottom and you hear see a blip when it hits the other side. You send loads of atoms and so would expect to see loads of hits at the top and the bottom adjacent to the two slits. But you don’t. What you get is an interference pattern like you would with waves washing through both slits at the same time. But you get his effect even when you send only one atom at a time. But if you spy on it then it only goes through either the top or the bottom and not both at once. It’s as though it knows you’re looking.


If I knew that then the King of Sweden would be calling me up with the Nobel. It’s counterintuitive and weird but we’ve learned to accept it. Quantum mechanics is hugely accurate – most of modern scientific development is based on it and on what it tells us about the subatomic world, but at its heart it says that an atom can be in two places at once. We’ve learned to live with that.

If atoms are so unreliable why isn’t everything fluid?

You mean why doesn’t your hand go through a table? Because electromagnetic forces hold the atoms together and give rigidity. Atoms themselves are mostly empty space – there’s a nucleus and then these electrons buzzing around. The nucleus has a positive charge and the electrons have a negative charge, so the negative charges repel each other and it is this electromagnetic force between the atoms in my hand and the atoms of the table that give rigidity and solidity.

Your third book is Surely You’re Joking, Mr Feynman!: Adventures of a Curious Character by Ralph Leighton, Richard P Feynman and Edward Hutchings.

Feynman is the most colourful character in physics and he just showed how much fun and what an incredible adventure science was and is. He just makes you think: Wow! How can the world be like that?! Wait until I tell everyone! I mean, you think the paranormal is amazing – let me tell you about quantum mechanics!

“If you think the paranormal is amazing, he says, wait till you hear about quantum mechanics.”

Feynman epitomises this attitude, the absolute joy of seeking answers. He was a genius but he was also an extrovert and loved playing pranks. He was a very young scientist in Los Alamos, on the Manhattan Project [the WWII Allied collaboration to build the first atom bomb] and he recounts how he found a hole in the fence. There was all this security but he didn’t tell anyone he’d found a hole; he just came in through security in the morning and then crept out the hole and came in again, trying to see how many times he could do it before security noticed he hadn’t actually left. He just had this wonderful sense of fun and this book relates episodes from his life.

What’s the best one?

Well, there’s a bit where he’s making spaghetti with a young researcher and he notices that whenever you break a stick of spaghetti it never breaks in two. It breaks into three pieces. The middle bit pings off. We would just say: ‘That’s weird.’ But Feynman studied it mathematically, looking at the properties of spaghetti and its bendiness.

He was one of the committee looking at the space shuttle Challenger disaster and he realised that it was one of the rubber O rings in the fuel tanks that had got cold and brittle and had snapped. At the press conference he took a tiny rubber O and dropped it into his glass of iced water to demonstrate. He brought science alive to people.

Your next book is The Born-Einstein Letters,1916-1955: Friendship, Politics and Physics in Uncertain Times by Albert Einstein and Max Born.

Max Born is one of the unsung heroes of the quantum revolution and he was at least as influential as Bohr, Schroedinger, Heisenberg. The Born-Einstein Letters is a collection of the correspondence of these two men over decades and decades. You know – ‘Dear Albert…’ ‘Dear Max…’ The letters map out the whole of the theory of 20th century physics but include all the conflict and personal life, the head-scratching. There’s no commentary at all, just the letters.

Lastly, The Emperor’s New Mind by Roger Penrose.

Penrose is rather less well-known than his young sidekick Stephen Hawking, but between them they developed the theory of black holes. Einstein had provided the mathematical framework with his general theory of relativity but they studied them theoretically to understand their properties.


Well, spotting and identifying a black hole is astronomy. This has nothing to do with that – it is theoretical work. Anyway, in this book Penrose brings together different areas of modern science like quantum mechanics and thermodynamics, which says that if things are left to their own devices they decay and unwind and disorder increases.

How do you mean?

If you take an ordered pack of cards and shuffle it they will get mixed up. That doesn’t work the other way round. If you take a disordered pack of cards and shuffle it they won’t come back up in the right order. Well, they might but the odds are so tiny that we can ignore them. Things do not spontaneously order themselves. So, the question is, unless you have a divine creator, how are we, such organised and developed structures, here?

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His ultimate idea is that he wants to explain consciousness using logic, philosophy, quantum mechanics. This is very controversial and has spawned all kinds of inter-disciplinary conferences with theologians, philosophers, physicists, all exploring the ideas that come from this book. He does come up with a way to get complexity out of randomness without God. There are proteins in the brain, tubulin, which can have two shapes at the same time, a quantum superposition, but when consciousness clicks in they choose one particular shape. But this idea of how we get complex structures from random chance and simple rules is fascinating.

Do you believe in God.

I don’t.

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Jim Al-Khalili

Jim Al-Khalili OBE FRS is a scientist, author and broadcaster. He holds a Distinguished Chair at the University of Surrey where he is a Professor of Theoretical Physics and Public Engagement in Science. He is currently President of the British Science Association