Energy spacetime warping

Question

1. If energy warps spacetime, then does light warp spacetime?

2. And if special relativity says that things near the speed of light increase in relativistic mass, then does light have a relativistic mass?

3. Also, if energy warps spacetime then is it possible to have a black hole form from pure energy, instead of resulting from a massive object?

Answer 1

Spacetime curvature is caused by the density and flow of energy and momentum. Light has all of these things, and so it causes a slight amount of spacetime curvature.

A photon of light with frequency $f$ has energy $E=hf$ where $h$ is Planck’s constant, so it can be defined to have a relativistic mass $E/c^2=hf/c^2$. But the whole concept of relativistic mass has become obsolete because it is confusing and accomplishes nothing. (Whenever you think “relativistic mass”, just think “energy” instead. They are proportional, and physicists often use units in which the proportionality constant is 1, so having these two concepts is pointless.) You should think of “mass” as the Lorentz-invariant mass, which for a photon is zero.

In principle, a sufficiently dense cloud of self-gravitating photons could collapse to form a black hole. There is no reason to believe that such photon clouds have ever existed in the universe. The black holes they would create would be the same as the black holes created by matter.