A Brief History of Time

"It was fascinating because when it came out, I was working at Yorkshire Television and I saw a review in The Sunday Times. I went in the next morning to the library which was underneath our office and I said to the ladies there I would really like to see a copy of this. And Sheila, who was the boss, said, ‘I’ll have it on your desk in the morning’, which was actually quite unnecessary because I had just said I wanted to see a copy at some stage. But she then rang up her favourite suppliers and they had sold out. And she rang bookshop after bookshop but they had all sold out. Well, I actually did read it. I got stuck in Chapter Six and I then read it again recently and it is a lovely book. It is very hard because he is trying to describe very complicated things, but he has actually done a very good job. I have started on his latest book, The Grand Design , which I think is rather easier. But this book is important not just because he is stuck in a wheelchair and is a brilliant cosmologist but also because it is a really difficult subject aimed at the general reader. This sort of cosmology, looking at whether or not black holes emit radiation, is a very esoteric sort of question. It is not like Stonehenge where people ask things that we can all understand, like, if you look through this gap can you see Capella. Whether or not black holes emit radiation is a much more difficult question even to think about. So it was quite brave of him to try and I think he did a grand job. And, of course, it was enormously hyped because of his awful condition and that didn’t do that book any harm at all. And I think for anyone like me who is interested in science even if we can’t understand it all, it gives us some insight into what these chaps are thinking about. If someone asks me what cosmology is I haven’t a clue, but if I read this at least I have more of a clue. I may not get it all but I get some gist of what is going on in the minds of these physicists."

"Most of these are somewhat older books that contributed to my own education. A Brief History of Time , for example, is a book that I read when I was in high school, or maybe even before that, and it was one of the books that got me really excited about cosmology. It has held up pretty well – we have discovered a lot about how the universe works since then, but it still gives you a very good overview of what we know. I can’t tell you how many people I’ve spoken with – my generation, or even a little younger – who basically got into astrophysics and cosmology because of that book. So that is a very central one."

"This is another classic, it had to be on the list. This is a book that came out after Weinberg’s, but before the others on the list. It was one of the few books written about this stuff at the time. As with Weinberg, it’s interesting to read a book by one of the 20th century’s greatest scientists. Everybody knows Hawking’s greatest contributions: understanding that black holes radiate light and other particles, that they contain entropy and all these things that no one imagined before him. Hawking and Roger Penrose also worked out the Big Bang singularity, the very moment of creation. To hear him describe some of these things with his own word choices, his own phrasing—not to mention his own personal biography and his disability—there’s no other book like it. He’s very witty and has a profound sense of sarcasm too. Given all the struggles he had in his life, you might expect him to have a somewhat darker outlook on things, but not at all. He’s just witty and clever and always making a joke. Certainly the stuff about black holes hasn’t changed, though we’ve discovered more things about black holes. No, but he was writing about them before we knew they existed. The idea that black holes could exist as a theoretical construct goes back to Karl Schwarzschild in the late 1910s. But, in the 1960s, physicists started to take them seriously as something that might actually be out there and probably was out there. The Cygnus X-1 black hole was finally discovered in 1971. At the time Hawking made a famous bet about it and I forget if he won or lost that bet, but it was still a controversial question at the time. Now we know of a large numbers of black holes. Every galaxy has a supermassive black hole in the center. We’ve observed the gravitational waves of merging black holes. Black holes are no longer a questionable subject. But a lot of the stuff we think about black holes and how they work is exactly what Hawking wrote down back when he was a young scientist. Yes, he wrote a book called Relativity that he intended to be a popular discussion of it, but if someone were asking my advice about a book to get a feel for the theory of relativity at a non-scientific, non-expert level, I wouldn’t pick that one. It’s incredibly important historically to hear it from Einstein’s own pen and there is something special about that, but there are probably more accessible ways to learn about relativity. There are tons of things in physics I don’t understand. Tons of things. Life is finite. You can learn a finite number of things in a finite length of time and I’m okay with that. I get to learn a lot of cool things and how disappointing would it be to find out you just learned the last thing! Now you know everything. There’d be nothing exciting to do ever again. That sounds terrible. I don’t want to live in that world. Let’s just pretend he did it that way. So what Isaac Newton worked out in the Principia , his masterpiece (he had many masterpieces, but if you had to pick one, that would be it), is that the force with which, for example, the Earth pulls on an object is proportional to the mass of that object. His most famous equation is force = mass x acceleration. So in the force, there’s a mass and in the m x a, there’s a mass. So those divide out. You can remove them from both sides of the equation and now you have, on one side, just a number and that equals ‘a,’ the acceleration. So as far as gravity is concerned, that hammer and that feather are going to be accelerated towards the Earth in exactly the same way. Now, like you said, there’s air resistance so that complicates things. But let’s say you did it in a vacuum where you’ve emptied all the air out, or on the Moon where there’s no air resistance. When I drop that hammer and that feather, they’ll just fall in unison together. They’ll be accelerated in exactly the same way. Another way to think about it is connected to something called ‘the equivalence principle.’ This was something Einstein thought about when he was constructing his theory of relativity. Physicists use the word mass to mean two different things. On the one hand, there’s the quantity which says, ‘the more mass something has, the more it feels gravity and the more it pulls on things gravitationally.’ So big, heavy objects attract things through the force of gravity more than light objects. We call that gravitational mass. The second kind of mass is inertial mass. Something with a lot of inertial mass takes a lot of force to accelerate or decelerate. “In 1905, Einstein introduced relativity and the first ideas of quantum physics. These things didn’t just build upon Newtonian physics, they tore Newtonian physics to the ground” What Einstein was concerned about is why it is that all the things with a lot of gravitational mass also have a lot of inertial mass. In fact, why does something always have exactly the same amount of gravitational mass and inertial mass? Why aren’t there giant objects that are really hard to accelerate but that don’t pull on things gravitationally? Or why aren’t there things that are small and easy to accelerate that pull a lot gravitationally? But there aren’t any. And Einstein worked out this is because all mass is is energy. And if you understand, as Einstein showed us, that all gravity is, is the geometry or curvature of space and time, then these two things have to be the same. Now I don’t expect that to make sense the first time you hear it. It’s a lot to unpack. But that’s the profound idea that was built at the foundations of Einstein’s theory and, really, is the same thing that Galileo was messing with. He said a heavy thing and a light thing are going to be accelerated by gravity in the same way and for the same reason a really heavy thing is going to take a lot of force to accelerate, a light thing is going to be very easy to accelerate. I think everything worth understanding takes some going back and revisiting. I read a lot of things in fields that are not my own and all the best stuff I missed the first time around. It takes a while to get the best ideas."