I had a lovely time on Wednesday evening giving a talk based on How to Build a Time Machine at Pewsey Library. I don't know what it is about Pewsey, but this is the second time I've spoken there, and again we had some brilliant questions, which tend to range over all of physics.
A couple were on gravity, which is rather nice as it's the subject of my next St Martin's Press book, due out later this year. And one was particularly timely. Someone asked, given that both electricity and magnetism have positive and negative aspects, was there anything that repelled gravitationally, rather than attracting.
It's timely because an experiment is underway to try to determine whether antimatter is gravitationally attracted by matter or repelled by it. I had always assumed antimatter was just like ordinary matter, behaving exactly the same way in everything except its electrical charge. So I was quite surprised when reading a book by George Gamow on gravity that he suggested it might be repelled gravitationally by ordinary matter. When a scientist of Dr Gamow's stature suggests something, you take it seriously.
You might think this is trivial to test, but it's not. Firstly we've only got tiny amounts of antimatter - and it doesn't usually stay around long before annihilating with normal matter. And also gravity is a very weak force. It might not seem it if you try to jump off the Earth, but just think about it. When I hold a fridge magnet near the fridge and let go, it has the whole Earth pulling it downwards and just a tiny magnet pulling it towards the fridge. The magnet wins. Gravity is vastly weaker than electromagnetism, making it very difficult to detect and distinguish gravitational effects in tiny particles of antimatter.
It will be fascinating to find out which way the antimatter goes. Apart from anything else, it has an implication for the principle of equivalence. This was what inspired Einstein towards general relativity, his theory of gravitation. The idea of equivalence is that if you were in an enclosed spaceship with no connection with the outside world, at any point in the spaceship you couldn't tell if you were feeling a gravitational pull or being accelerated by the ship's motors. The effect would be identical. They are equivalent. But if you had a piece of antimatter, and it is indeed repelled by ordinary matter, you would be able to distinguish. It doesn't really matter for general relativity, but it would mean a proviso had to be inserted into equivalence.
Let's wait and see. I rather hope the antimatter is repelled by matter. After all, it would make the universe even more exotic.