Could gravitational waves and electromagnetic waves/light be the same thing?

Question

They travel at the same speed, why? Maybe they are the same thing, but seen from different perspectives.

Answer 1

Could gravitational waves and electromagnetic waves/light be the same thing?

I think most physicists would say they aren't, in that gravitational waves are thought to be quadrupole waves, whilst electromagnetic waves are thought to be dipole waves. However it's worth noting that the photon can be considered to be a singleton electromagnetic wave, and that it has an non-zero "active gravitational mass". As per Einstein's E=mc² paper, the photon conveys inertia between the emitting and absorbing bodies. It conveys energy, and it conveys a gravitational effect. And one thing that really struck me was Einstein talking about field theory in 1929. He was talking about gravitational fields and electromagnetic fields, and said this:

"The two types of field are causally linked in this theory, but still not fused to an identity. It can, however, scarcely be imagined that empty space has conditions or states of two essentially different kinds..."

So I think there may be a sense where the photon isn't just an electromagnetic wave, but is a gravitational wave too, of sorts. Not the only sort, but enough to throw a spanner in the works for things like quantum gravity and LIGO.