Lots of tributes to Stephen Hawking out there after his recent death, last week. He is well known for his work on black holes for example. In the link at https://phys.org/print440413851.html ... we are told black holes were the remnants of supermassive stars that had fallen in on themselves, contracting to such a small size that gravity became so strong even light cannot escape. Hawking filled in some of the holes in the theory. Why do black holes exist? Answer. Because gravity exists and the speed of light is not infinite.

Stephen Hawking is also featured at https://phys.org/print440412776.html ... and his developing 'M-theory' - how to fit all the fundamental interactions of the Standard Model into a coherent mathematical system (including electro magnetism). We are told that despite an accurate description of large scale phenomena such as orbits and galaxy dynamics general relativity breaks down at very short distances. According to the Standard Model all forces are mediated by specific particles. For gravity this involves a particle known as the graviton. However, when trying to calculate how these gravitons interact, nonsensical infinities appear. A consistent theory of gravity should be valid at any scale and should take into account the quantum nature of fundamental particles, we are told. This could accommodate gravity in a unified framework with the other fundamental interactions - a theory of everything if you like. This is what M-theory is all about.

Back in the 1970s theoretical scientists realised that rather than describing the universe in point like particles you could instead describe it in terms of tiny oscillating strings (tubes of energy) and this turned out to appear to solve many problems. For example, a particular oscillation of the string could be interpreted as a graviton. Hence, string theory, it is said, can describe its interactions mathematically without getting strange infinities. Thus, gravity was finally included in a unified framework.

Theoretical physicists have since directed a lot of time to string theory - but it has had its ups and downs. However, we had 'supersymmetry' added on to string theory and we also have the idea of ten space time dimensions. With M-theory we have the possibility of eleven dimensions - but all this is of course to be regarded as 'possibly so' as M-theory is not reproducable as an experiment.