Trinity’s Professor of Theoretical Physics David Tong has finally published his lecture notes!

After more than five million downloads of his free lectures and 20 years of teaching, Professor David Tong has published four textbooks with Cambridge University Press: Classical Mechanics, Electromagnetism, Quantum Mechanics, and Fluid Mechanics.

And there is more to come: the series will cover theoretical physics from Newton and Galileo to modern understandings of the fundamental laws.

‘In telling the full story, I get to highlight various threads that might get lost and reveal connections between different areas that might otherwise seem unrelated,’ said Professor Tong.

‘You may wonder why you should buy the books when the lecture notes are freely available,’ he writes on his website, David Tong: Cambridge Lecture Notes on Theoretical Physics. Among the reasons: the books have more content, better explanations and are great value. ‘I convinced the publisher to sell the paperbacks at half the price of most other textbooks.’

The cover of each book features a rare, sometimes unique, manuscript or book from the Wren Library by leading physicists who spent time at Trinity.

Classical Mechanics features Ernest Rutherford’s personal copy of Newton’s Principia, which he annotated to check Newton’s maths. The cover of Electromagnetism is a watercolour painting by James Clerk Maxwell. Slides of the wave-like behaviour of an electron – from a Nobel Prize-winning experiment by GP Thomson, son of JJ Thomson – is the cover of Quantum Mechanics.

Professor Tong (pictured above) explains his aim in writing the textbooks, why his lecture notes are freely downloadable, and how he got into physics in the first place.

What do you hope readers will gain from your new books?

I hope they learn physics obviously. But also, I hope, they learn physics the right way, understanding how everything follows from a few basic principles. Some of those principles evolve, and are refined, as we move through the journey but a few are constant companions – ideas of symmetry, and energy, a curious idea called the gauge principle. These are all topics that flourish as physics develops, but I’ve tried to make sure that they are, at least to some extent, with us from the beginning.

Also, there’s a balance when teaching theoretical physics. The subject has a reputation for being hard, and that reputation is well-deserved. There’s no doubt that theoretical physics contains the most beautiful set of ideas in the world, but it’s hard to remember that when you’ve spent the last three hours figuring out how on earth you get from one equation to the next. So, the challenge when teaching physics is to remind the student of the bigger picture, of the grand themes, while explaining where that minus sign went and why the factor of 2 is important.

Why are your lecture notes freely available?

I’ve been writing lecture notes on theoretical physics for more than 20 years. Most people teach a course and stick with it, but I love teaching new topics as it’s by far the best way to learn. I’ve averaged a new course every year. There are now more than 20 lecture notes, totalling more than 4000 pages, covering everything from string theory to mathematical biology. All of them are freely available to download on my webpage and form the backbone of the books. It’s been very gratifying to see these notes become one of the default resources for students and researchers around the world, with over 7 million page views and 3.5 million individual users.

How did you get into physics?

I was probably a little later to the party than most. I was good at maths and physics at my comprehensive school in Crawley, but I thought that meant I should be an accountant or do something with computers. I had no idea that there existed a job where you could think about black holes and particle physics for a living.

My eyes were opened when I bought a copy of Stephen Hawking’s Brief History of Time with a WH Smith book voucher for my 17^th birthday. That was the first time I learned about quantum mechanics and black holes and many other wonderful things. It was also when I decided to do physics at university. Many years later I told Stephen this story and got one of his famous smiles in reply.

What’s your favourite theory or principle of physics and why?

They’re all gorgeous! There’s something special about the fundamental laws – the Dirac equation describing matter, or the Einstein equations of General Relativity that describe gravity. One of the magical things about physics is that as you look deeper, the laws of physics become simpler. Not easier! But definitely simpler – there is an inevitability to the laws of physics and you realise that they really couldn’t be any other way. That’s very special.

But sometimes messy is good too. There is a famous and wonderful equation that describes how fluids flow, called the Navier-Stokes equation. The equation itself is simple, but it contains multitudes, with astonishing complexity emerging from its solutions. It is used to explain how planes fly, and to model the climate, but it also contains many of the most important unanswered questions in theoretical physics, like how turbulence arises. One of the books is entirely devoted to that story.

You have studied and taught physics around the world. How did you end up at Cambridge?

I wanted to go to Oxford, but they didn’t feel the same way about me. Instead I went to Nottingham where I got a great education – not quite as pressured or as advanced as in Cambridge, but probably what I needed at the time.

I did a Master’s at King’s College London and a PhD at the University of Wales in Swansea. Then I had a bit of an adventure. I applied for 120 postdocs – essentially every institution in the world that does my kind of physics – and got one offer, from the Tata Institute of Fundamental Research in Mumbai. I happened to be reading Vikram Seth’s A Suitable Boy at the time and India seemed so enticing (and, for a boy from Crawley, a little scary) and that sealed the deal. Going to India was one of the best decisions I’ve ever made, resulting in a lifelong attachment to the country and many friends.

Back in London for a second postdoc, I realised I had itchy feet. So I gave up my apartment and spent a year at Columbia University, three years at MIT, and a few months at Stanford. When I came to Cambridge as a lecturer I assumed that I’d just keep moving. But it turns out it’s quite nice here.

What’s your advice to anyone starting a physics degree?

That’s simple: enjoy it. Easier said than done when trying to solve an examples sheet at 2am, but ultimately you have to remember what brought you to physics in the first place. And the highlights, when they come around – quantum mechanics and general relativity – are genuinely breathtaking.