# A Why binary systems for gravitational waves?

1. Feb 20, 2017

### binbagsss

So a spherically symmetric object, by Birkhoff's theorem, does not emit gravitational waves.

Is this why we look to binaries, so that there is some rotation of the objects with respect to each other breaking the spherical symmetry? Or mainly because the gravitational radiation is much greater as needed, or is it both reasons?

What single sources, if the sensitivity of detectors was sufficient enough, would emit gravitational waves . Like a single rotating black hole? (or is this spherically symmetric, if it rotates in a certain way)?

2. Feb 20, 2017

Staff Emeritus
You need a changing quadrupole moment. A spinning BH does not have a changing quadrupole moment.

3. Feb 21, 2017

### binbagsss

is this related to the way the mass is distributed?
if the mass is spherically symmetrically distributed about the rotation axis, still spherically symmetric and no quadrople moment?
Can the mass be distributed in such a way relative to the rotation axis that there can be a non-zero quadrople moment?

4. Feb 22, 2017

### Ben Niehoff

Yes. The simplest such non-uniform distribution is when the mass takes the form of a binary system.