In simple terms, it refers to theories that modify Einstein’s general theory of relativity (GTR) in order to explain certain phenomena that GTR can’t quite handle on its own.

One such phenomenon is the gravitational time delay a fancy way of saying that light takes longer to travel through space-time when it passes near massive objects like black holes or neutron stars. This effect was first predicted by Einstein in 1916, but it wasn’t until 1960 that we actually observed it for the first time!

So how does this work? Well, let’s say you have a beam of light traveling through space-time towards Earth from a distant star. If that beam passes near a massive object like a black hole or neutron star, it will experience a gravitational redshift essentially, its frequency (and therefore wavelength) will decrease due to the curvature of spacetime caused by the mass of the object.

This might sound confusing at first, but think about it this way: imagine you’re standing on top of a hill and shouting down to someone at the bottom. Your voice will reach them more quickly than if they were standing at the base of the hill this is because the curvature of the Earth causes your voice to travel in a straight line towards their ears, rather than having to follow the contours of the land.

Now imagine that instead of shouting, you’re sending a beam of light through space-time. The same principles apply: if the path it takes is shorter (due to passing near a massive object), then it will reach its destination more quickly. However, because light travels at such incredible speeds, this effect is incredibly small in fact, for most practical purposes, we can ignore it altogether!

But what about when we’re dealing with objects like black holes or neutron stars? Well, that’s where modified gravity comes into play. In these theories, the curvature of spacetime caused by massive objects is different than what Einstein predicted in GTR this means that light will experience a greater gravitational redshift (and therefore take longer to travel) when passing near them.

Gravitational time delay in modified gravity: a concept so mind-bending, it’ll make your head spin faster than a spinning top on juice!