
Mountains on neutron stars - qwerta
http://relativity.livingreviews.org/open?pubNo=lrr-2008-10&amp;page=articlesu46.html
======
thearn4
`Dragon's Egg` is a great sci-fi story that deals with mountains and life on
the surface of a neutron star.

[http://en.wikipedia.org/wiki/Dragon's_Egg](http://en.wikipedia.org/wiki/Dragon's_Egg)

------
AceJohnny2
"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.

~~~
antognini
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.

[1]
[http://adsabs.harvard.edu/abs/1964PhRv..136.1224P](http://adsabs.harvard.edu/abs/1964PhRv..136.1224P)

------
concernedctzn
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?

~~~
antognini
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:

[http://link.springer.com/article/10.1007%2FBF00891464#page-1](http://link.springer.com/article/10.1007%2FBF00891464#page-1)

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.

------
ianstallings
It's interesting that a mountain only a few meters tall might be the hardest
mountain in the universe to climb.

------
rwallace
"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?

~~~
fragsworth
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.

~~~
rwallace
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.

------
ChuckMcM
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?)

~~~
qwerta
Pulsar will always slow down, thanks to magnetosphere interacting with
interstellar environments.

