The picture is of Emmy Noether, a theoretical physicist/mathematician from the first half of the twentieth century. Despite coming up with one of the deepest theorems in physics, she is not well known, in particular, her contemporary Marie Curie is much better known.

A large majority of the physicists who have revolutionised our understanding of the natural world have been men. This leaves a shortage of role models for young female physicists; a shortage that often seems to be filled by mentioning Marie Curie over and over again. I am not sure why Noether is not mentioned more, not least to avoid giving the impression that early 20th century physics was done by 100 men plus Marie Curie.

Anyway, back to Noether’s theorem. It is very elegant. It basically says that conserved quantities come from symmetries. For example, momentum is conserved because of translational symmetry; translational symmetry is just a fancy way of saying that space is the same everywhere. The properties of space where I am typing this are the same as where you are reading it, and the same in the Andromeda galaxy, etc. This then forces momentum to be conserved. Simple as that.

Conserved quantities like momentum are tremendously important in physics (as are symmetries). And because they cannot be created or destroyed, the only thing they can do is move around, and so physicists spend a lot of time working out how conserved quantities, such as momentum, move around.

The motion of momentum is crucial to understand everything from clusters of galaxies to cells. I have been thinking about how momentum moves around inside the cells of our body, in particular how this affects how proteins move inside our cells. Inside our cells, proteins are pulling on other proteins, i.e., exerting forces on each other. Of course, as Newton’s 2nd Law tells us, when a protein exerts a force on another protein it accelerates, and so acquires momentum.

The protein which gains the momentum will then move along the direction of the momentum, until it loses this momentum. As momentum is conserved, the momentum of a protein cannot just disappear, it just diffuses from the protein that was accelerated into the surrounding liquid inside the cell. The insides of our cells are basically liquid, but maybe 10 or 100 times more viscous than water, i.e., about as thick as runny custard.

Now, although people often don’t think of it this way, how thick, or how viscous, a liquid is, is a direct measure of how fast momentum diffuses in it. If you are, say, making custard, and are checking how thick it is, you are effectively assessing how fast momentum moves inside it.

So, the fact that the liquid of proteins, DNA, etc., inside cells is viscous, means that momentum positively zips around inside them – a lot faster than it does in water. Almost as soon as one protein has pulled on another to accelerate it, the momentum is gone. This means that the motion of proteins that are being pulled and pushed by other proteins must be quite jerky. This may have consequences for how they function inside our bodies, but at the moment I can’t quite think of what these consequences could be.