1
$\begingroup$

Mass is said to create curvatures in space-time thereby creating gravity, yet technically the smallest movements, even on Earth, create gravitational waves. Are there different "types" of disturbances of space-time? How?

$\endgroup$

1 Answer 1

2
$\begingroup$

Spacetime curvature is actually caused not by mass but by the density and flow of energy and momentum. These quantities are encapsulated in something called the “energy-momentum-stress tensor”. Even massless particles like photons can cause spacetime to curve because they have energy and momentum even though they don’t have mass. But this is a tiny theoretical effect, and of course realistically it is the energy of massive particles that causes gravity.

So I’ll answer your other question by talking in terms of mass. If a distribution of mass is sitting still, it creates a static gravitational field that decreases at large distances like $1/r^2$. But if it is moving in a certain way, it radiates a gravitational wave whose field decreases like $1/r$. In order to radiate, the quadrupole moment of the mass distribution must have a nonzero third time derivative. (Or the octupole moment can have a nonzero fourth derivative, etc.) This means, for example, that a spinning sphere does not radiate, but a dumbbell spinning end over end does.

$\endgroup$
1
  • $\begingroup$ Even massless particles like photons can cause spacetime to curve because they have energy and momentum even though they don’t have mass. But this is a tiny theoretical effect, and of course realistically it is the energy of massive particles that causes gravity. The effect is neither tiny nor theoretical. A significant fraction of the mass of ordinary matter comes from the energy in the internal electromagnetic fields of the nuclei. And the entire universe was once dominated gravitationally by radiation. $\endgroup$
    – user4552
    Jan 1, 2019 at 1:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.