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Are there any fields of astrophysics with no general relativity?

  1. Apr 22, 2012 #1
    Is there any research in astrophysics today that does not involve general relativity?
  2. jcsd
  3. Apr 22, 2012 #2


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    Well, not really, but, there are any number of situations where velocity is not an issue or is so low that relativity has no meaningful effect - like exobiology.
  4. Apr 22, 2012 #3


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    Physics of stars - a lot is known already. Galaxy formation.
  5. Apr 22, 2012 #4
    Can such large masses really be modeled with classical mechanics?

    What I meant to say was, is there any astrophysics done that doesn't require extensive knowledge of general relativity? Technically speaking, there's general relativity everywhere.
  6. Apr 22, 2012 #5


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    Absolutely. As far as I know, MOST astronomers are not experts at general relativity. Anyone doing planetary astronomy of course does not need it. Stellar physics does not need it. The study if the interstellar, intergalactic medium does not need it. The dynamics of individual galaxies does not need it. Really, only two (main) groups of people use GR on a day to day basis. These would be:
    1) People modeling compact objects such as neutron stars, black holes, or white dwarves.
    2) Cosmologists.

    This might just be failure of imagination on my part, but I've talked to a fair few astronomers and almost unless they're explicitly in one of these two categories they mostly moan about the index manipulation twenty years ago in graduate school.

    (I just remembered there's a lot of lensing work going on. But in this, GR is treated rather as a black box. Sure, it's necessary to know qualitatively what's going on, but to do the actual work no deep knowledge of GR is required.)
  7. Apr 23, 2012 #6

    I'm curious about astrophysics and want to read a technical book on it for fun, which is why I asked about how much relativity is in astronomy. It would do me no good to buy a book that I can't understand.

    My B.S. is in Chemistry, so I'm very ignorant about astronomy. I heard that thermodynamics and statistical physics are very important in theoretical astronomy. Is that true? If so, that's very interesting, because those are the same things that are important in materials.
  8. Apr 24, 2012 #7


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    Yes, because GR is only relevant where relativistic affects are significant.
  9. Apr 24, 2012 #8


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    Of course, in certain situations these things are very important in astronomy. Particularly in solar system physics, chemistry and thermodynamics are important in determining the various phase of matter in a protoplanetary nebula, or formation of planets. Statistical physics and thermo are generally useful also when studying gas clouds or stars.

    Astrophysics is such an incredibly broad thing, that literally any branch of physics is used by someone who considers themselves an astrophysicist. I think the best way to approach the field in a serious manner is to identify an object or situation that's of interest to you. Are you interested in how stars form? How they work? How planets form? How galaxies form? About gas between stars? About the universe as a whole? The list goes on for a long time. Of course the answer might be all of the above, but try to latch onto one particular topic to begin with and study in at least some depth.
  10. Apr 24, 2012 #9
    thank you for the response. i think i could usefully read some books on this subject.

    hopefully, it will be a good experience over the long and boring summer. can't party every day =)
  11. Apr 27, 2012 #10
    Well, a rotating star that collapses has to take into account relativistic effects.

    Jets emitted from stars going supernova can reach speeds where it matters.

    Supernova effects themselves are obviously full of relativistic interactions.

    Neutron stars, black holes, hypervelocity partner ejection, all in all I don't see a reason to specifically avoid learning about GR while being interested in any branch of physics.

    Isn't that kinda like trying to avoid learning how to multiply while being interested in math?

    If you're scared by things like the Einstein Field Equations, that just means you're not an eldritch horror from beyond the stars, you don't have to know those by heart.
  12. Apr 28, 2012 #11
    In effect, most astrophysical phenomena are well within the Newtonian limit. It is easy to estimate the size of the departure from that limit:

    Kinetic: (v/c)2
    Potential: (G*M)/(r*c2)

    In gravity-dominated systems, the virial theorem indicates that these quantities will be rather close, so one usually needs to calculate only one of them.

    I suggest that you people try calculating some departure sizes.

    In most cases, the departure is very small, and in most of these cases, it is too small to measure. Exceptions:
    • Orbits of planets and spacecraft in the Solar System. Weak.
    • Visible light and radio waves passing near the Sun. Weak.
    • Close orbits, like those of some binary pulsars. Weak.
    • Neutron stars. Strong.
    • Black holes. Strong.
    • The inner parts of neutron-star and black-hole accretion disks. Strong.
    • Gravitational lensing. Weak.
    • Cosmology for z >~ 1. Strong.

    In the weak case, one needs high precision to detect GR effects. High precision like high angular resolution or high-precision timing. One can express several of these effects in "post-Newtonian" form, with the Newtonian equations of motion with correction terms added. For propagation of light, one can treat space-time as having a variable refractive index, with the variation being another post-Newtonian effect.

    Most of the tests of GR have been of its weak-case post-Newtonian effects.

    In the strong case, one needs to use the full apparatus of GR, or whatever similar theory one may wish to evaluate. However, it's VERY hard to resolve non-cosmological strong-case objects, and that can make it difficult to test hypotheses about them. Try estimating the angular sizes of the Crab Nebula pulsar, the Cygnus X-1 black hole, our galaxy's central black hole, and M87's central black hole.
  13. May 1, 2012 #12
    Most of it doesn't. Some of it just involves doing some first-order approximation.

    Supernova calculations typically are run with general relativity turned off. You find that it doesn't make a difference, and once you find it doesn't make a difference, then you just eat computer CPU for no good reason.
  14. May 1, 2012 #13
    No it doesn't. (I wrote a dissertation on this).

    The interesting stuff in collapsing stars happens at > 0.5 solar masses where GR turns out to be not relevant. You can calculate the GR correction factor and what people have found is that it turns out that it's near zero for any of the interesting parts.

    Special relativity yes. GR no.

    They actually aren't.

    You do have to know a decent amount of GR to show that it doesn't matter. It turns out that in none of these situations does GR matter to the calculation. Even if the interior of the supernova turns into a black hole, it turns out that it doesn't change the calculation. When you calculate that a black hole looks like, it looks like a "frozen star", which is how it behaves if you go Newtonian.

    Actually you do. They aren't that difficult.
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