[Physics] What unit of measurement is used to indicate the warping of space

general-relativitygravity

It is my understanding that planetary bodies bend/warp space and that warping results in gravitational lensing (among other things).

My question is what unit of measurement is used to describe the degree to which space is warped? Is there a maximum limit that space can be bent?

Best Answer

While "warping of space" is a perfectly good way to think about it, to be precise we talk about the "curvature of spacetime". General relativity is the standard modern theory of gravity, and it describes this curvature with something called the Riemann tensor (although there are numerous other good ways to talk about curvature). A tensor is — to put it very roughly — a collection of numbers, and you can't boil it down to just one number. But each of the numbers in this Riemann tensor has units of $1/\mathrm{distance}^2$. Basically, the Riemann tensor measures the rate of change of the rate of change (yes, both) of space's shape as you move along different directions — so each of those rates of change brings in one factor of $1/\mathrm{distance}$, which is how you get a total of $1/\mathrm{distance}^2$.

There's no known maximum limit to how big the curvature can be. In our current theory, at least, there are places where it can become infinite. These are called gravitational singularities. For example, we believe that singularities are found inside of black holes. Of course, we also believe that the theory of general relativity is incomplete, and needs to be combined with quantum theory somehow. When this is done, most physicists expect that singularities will disappear and be replaced by some crazy quantum phenomenon. As just a vague ballpark guess, most physicists would expect that craziness to kick in once the curvature gets anywhere close to $1/\ell_{\mathrm{P}}^2$, where $\ell_{\mathrm{P}}$ is the Planck length — which might be something like a limit to the curvature.

In certain special cases, you can measure warping in different ways. Relatively recently, one important measurement of warping has been the detection of gravitational waves. Because they are very weak, we don't have to deal with a lot of the headaches that you normally run into in general relativity, which means that we can get away with measuring the warpage with something called the strain. The strain is the ratio between how much the gravitational wave changes the length of an object it passes and the normal length of that object. The units of strain, therefore, are just $\mathrm{distance}/\mathrm{distance}$ — which is to say that it's dimensionless.