The massive problem of trying to fully explain what mass actually is

LAST month, I wrote about the challenge of explaining space-time. To give people a feeling for what I mean when I say that space-time is curved, sometimes I talk about a rubber sheet with a ball sitting on it. I point out that the ball causes the sheet to curve and the curvature of the sheet literally shapes where the ball will roll. I say we can think of this as an analogy for our local star – the sun – curving space-time around it. How is the sun able to do this? Because it has mass.

Increasingly, I think it …