![The massive problem of trying to fully explain what mass actually is 1 A simulation of gravity showing curved space-time. The ball represents the sun and is resting on a sheet of plastic that stretches under its weight. The curved sheet of plastic demonstrates the way a gravity curves space.](https://i0.wp.com/images.newscientist.com/wp-content/uploads/2023/06/19154119/SEI_160372796.jpg?resize=696%2C464&ssl=1)
TED KINSMAN/SCIENCE PHOTO LIBRARY
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 …