1
$\begingroup$

The earth rotates, and its axis of rotation precesses due to the gravitational pull of the sun and moon and other planets upon the mass of the earth.

If an earth-sized, rotating black hole was in orbit around the sun, would its axis of rotation precess?

EDIT: I forgot that an Earth mass black hole would have a supertiny horizon. And making the horizon Earth radius means that it's heavier than the sun. So let's just scale it all up.

Consider a "solar system" in black hole equivalent: a "sun" black hole with a horizon the size of our sun, an "Earth" black hole with a horizon the size of our Earth and the same angular momentum as our Earth, and a "moon" black hole, orbiting the "Earth", with a horizon the size of our moon which is orbiting "Earth". Does the "Earth" precess? Does it experience any tidal forces?

Improve this question
$\endgroup$
2
  • $\begingroup$ Earth mass black hole or earth size? Earth size would be over 2,000 solar masses, so the sun would orbit it - pretty fast too. $\endgroup$
    – userLTK
    Feb 21, 2015 at 9:28
  • $\begingroup$ A Schwarzschild black hole would not precess because it is (a) spherically symmetric and (b) not spinning. How a Kerr black hole would behave I'm not sure - that seems to me a very interesting question. $\endgroup$
    – John Rennie
    Feb 21, 2015 at 10:09

2 Answers 2

Reset to default
2
$\begingroup$

Since there are no extra rules for black holes they should follow the same laws. In general relativity this effects are called the de Sitter- and the Lense Thirring-effect which have been verified with big trumpets call by the famous Gravity Probe B.

Improve this answer
$\endgroup$
2
  • 1
    $\begingroup$ I don't think normal Earth precession happens because of GR effects. I think it's much more prosaic -- the tides pull on the distributed mass of the Earth. A black hole has all its mass at the singularity, so no torque. $\endgroup$
    – Ross Presser
    Feb 21, 2015 at 17:21
  • $\begingroup$ I'm accepting this answer. The GR effects you mention are undoubtedly responsible for black hole precession. This video shows an impressive simulation of this. It still seems to me now that the non-GR precession experienced by the Earth, which is much greater in magnitude than the GR precession effects, would be absent in a black hole situation. $\endgroup$
    – Ross Presser
    Sep 6, 2018 at 19:39
1
$\begingroup$

Assuming earth mass, precession would be reduced practically to zero because tidal effects would be practically zero. Precession, at least according to this site, has to do with tidal effects and a planet being malleable. A black hole - 2/3rds of an inch in diameter would experience essentially zero tidal effects and it would be far less prone to bulging (if it bulges at all) - so in practical terms, it would not have any recognizable procession.

As to whether a large black hole with a significant radius, orbiting another black hole might have a precession - sorry, but I have no idea. Interesting question.

http://astro.wsu.edu/worthey/astro/html/lec-precession.html

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.