In physics there are many values that are called “constants.” These include the speed of light in a vacuum, the gravitational constant, the fine-structure constant, and many others. These values have often been measured quite precisely; many are well-known. And yet…
We really need to question whether they are actually constant. Certainly we haven’t noticed changes in these values. They seem perfectly solid. But we’ve only been applying science to the Universe for the past few hundred years. We haven’t been measuring things long enough to really know whether their values change. It may be that over the course of millions or billions of years, the values of some “constants” actually do change.
Aside from whether values remain constant over time, we also don’t know whether they remain constant over distance. A case in point is the gravitational constant, often represented by the letter ‘G’. It represents how strongly matter attracts matter. We have measured it to have a certain value. There is no reason to believe that it changes; but there is also no evidence to show that it does not change.
There are two mysterious quantities in the Universe, Dark Energy and Dark Matter. Dark Matter is used to explain why galaxies don’t tear themselves apart as they rotate. According to how much visible matter there is, galaxies should not be able to rotate as fast as they do. One hypothesis is that there is Dark Matter – a form of matter we cannot see or interact with, but which exerts enough gravitational attraction to keep the galaxies from falling apart.
Dark Energy is an attempt to explain why the expansion of the Universe is not slowing down, but is actually speeding up over time. It is as though there is some sort of repulsive force driving the galaxies away from each other.
There is an idea called Modified Newtonian Dynamics, or MOND. It suggests that at the scale of galaxies, gravity is actually stronger than predicted. And at the scale of the Universe, gravity becomes repulsive, more or less “antigravity.” There are reasons why MOND is not widely accepted, but the point is that we assume gravity works the same at all scales, small to large. Maybe this assumption is incorrect.
There are dozens of physical quantities that are considered constants; but that is simply an assumption we make. We have no reason to think we’re wrong, but as I said, we can’t exclude the possibility. If we are wrong, then we will need to make some changes in our physical theories..