Question: What is your favourite subject?

Stewart Martin-Haugh answered on 8 Nov 2018:

Physics, unsurprisingly. I can be more specific: my favourite bit of physics is general relativity.

It starts off with something that seems obvious – all objects have mass (weight) and they have the same mass under gravity as under acceleration. If I push you, the amount of force needed depends on your mass. When you stand up, you feel gravity dependent on your mass in exactly the same way. There’s no reason why this should be true! You could have a pushing mass and a gravity mass.

Einstein realised this was weird and managed to find a new theory of gravity out of it. This gave more accurate answers for motion of planets etc, and we haven’t found a better theory.

It shows that there can often be something obvious you’re missing, and it can be really important.