# Why does the h tensor represent gravity waves?

• the_pulp
In summary, the conversation discusses the difference between scalar and tensor perturbations and why tensor perturbations are considered to be gravity waves. The speaker is seeking clarification on why tensor perturbations are labeled as gravity waves while scalar perturbations are not. The expert provides an explanation that gravity waves are defined as metric perturbations and require an oscillating quadrupole moment as a source. The speaker confirms their understanding and acknowledges that they will review and study the concept further.

#### the_pulp

What makes it more "gravity-wavy" than the fi or psi scalar of the vector perturbations? (Im talking about metric perturbations)

Thanks!

What is the difference between a scalar and a tensor?

the_pulp said:
What makes it more "gravity-wavy" than the fi or psi scalar of the vector perturbations? (Im talking about metric perturbations)

Thanks!

Can you first figure out why you need to perturb the metric tensor? Then, if you write $g_{\mu\nu} = \eta_{\mu\nu} + \mbox{first order perturbation}$, how do you balance the indices?

dextercioby said:
Can you first figure out why you need to perturb the metric tensor? Then, if you write $g_{\mu\nu} = \eta_{\mu\nu} + \mbox{first order perturbation}$, how do you balance the indices?

Sorry can you be more explicit? Thanks!

the_pulp said:
What makes it more "gravity-wavy" than the fi or psi scalar of the vector perturbations? (Im talking about metric perturbations)

Um, the fact that "gravity waves" are metric perturbations, by definition?

I'm confused about what you're asking here. "Scalar", "vector", "tensor", "gravity wave", etc. are all just labels. If you're asking about why we choose certain labels for certain concepts, I don't see how we're going to answer that here; you'd have to ask the people who came up with the labels. If you're asking about some actual physics, what physics?

Ok, I'll be more clear myself. I am reading mukhanov "physical foundations of cosmology". There he says that metric perturbations can be decomposed in scalar, vector and tensor perturbations. Then he says that the tensor perturbation, represented there with letter "h" are Gravity Waves. Why does he say that for the h fluctuation but not for the scalar fluctuation? (Represented there by greek letters fi or psi)

Hope that helps to make you understand my doubt.

Because, as PeterDonis said, gravity waves are metric perturbations, by definition. To produce gravitational waves, you need an oscillating quadrupole moment as a source, in contrast to electromagnetic radiation, which requires an oscillating dipole moment.

Ok, so, what I have wrong is my comprenhension of Gravity Waves. What I sort of understood from Leonard Susskind videos was that a short perturbation (of any kind, dipole, quadrupole, etc.) in a far place produces gravity waves but I will see that again (and I will go to my GR books, I always overlooked the "Gravity Waves chapter").

Thanks