"The existence of such [gravity] waves predicted by Einstein’s theory of general relativity was beautifully confirmed by the observations of the binary pulsar PSR 1913+16 by Russel Hulse and Joseph Taylor (who were awarded the Nobel Prize in 1993)"
Really? Science articles (SciAm etc...) had led me to believe that gravity waves had yet to be detected.
Gravitational waves have not been directly detected by laboratory instruments here on Earth. The LIGO and VIRGO collaborations are looking for gravitational waves by using large laser interferometers, but haven't detected any yet.
The article is referring to the first indirect detection of gravitational waves. Binary stars emit gravitational waves which carry energy away from the system. The loss of energy causes the two stars to spiral into a closer orbit until eventually they merge. Using general relativity, you can predict how the semi-major axis will change as a function of time [1]. Hulse & Taylor measured the change in semi-major axis in a binary pulsar and found that it matched the predictions of general relativity.
In this case two neutron stars were orbiting each other. Relativity predicts they orbit would be unstable due to energy loss caused by gravity waves. Observation confirmed decaying orbit, we have not actually observed the gravity waves them selfs.
I think I read somewhere that gravity waves travel at the speed of light. Does anyone know if they can be slowed down like light through glass/other materials?
Yes, there is an index of refraction for gravitational waves passing through matter. As the gravitational wave passes through, particles in the material oscillate, producing their own gravitational waves. The combined gravitational wave from the original gravitational wave plus the secondary gravitational waves is broader. The increased wavelength implies a decreased speed. An idealized equation for this index of refraction can be found in Eq. 18 here:
The paper calculates the index of refraction for the Earth and finds that n - 1 ~ 10^-17, so it's a very small effect indeed. For something much more dense and stiff, like a neutron star, it would be higher by many orders of magnitude. But without knowing the equation of state of a neutron star it's impossible to calculate n.
"For instance, the mountains on the surface of the Crab pulsar could be as high as a few meters, while in PSR 1957+20 they cannot exceed a few microns!"
What's the reason for the difference? What causes one pulsar to be able to have mountains a million times as high as another?
It's mostly due to the total mass of the star, since a more massive star has greater gravitational force at its surface, overwhelming the structural forces in the particles.
That doesn't sound right. Neutron stars vary by no more than a factor of two in mass; that's not enough to account for six orders of magnitude variation in height of highest mountains.
>The size of the highest mountain of order ∼ εR can be roughly estimated from the inequality (292).
>P, P˙ and I are, respectively, the pulsar’s period, period derivative and moment of inertia [in inequality (292)].
Those terms are all just multiplied together in the inequality, so changes in any of them could affect epsilon (the "dimensionless parameter characterizing deformations of the star")
So a neutron star with a mountain would both emit gravitational waves and slow down due to that emission? Does that imply that pulsars that do not slow down are all perfectly spherical? (or at least perfectly symmetrical across their rotational axis?)
http://en.wikipedia.org/wiki/Dragon's_Egg