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Comment by michaelcampbell | Hacker News Reader

It will add mass to the black hole though, right? So that will affect orbits, no? Or is that just so infinitesimally small that it can reasonably be ignored?

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The mass is already roughly there. One thing to bear in mind - if you replaced the Sun with a black hole of equal mass, planets wouldn't change orbits. It's the same gravity and centre of mass, just a different density/volume.

This is true for Newtonian gravity, but only approximately true for general relativity (or so I understand it).

This includes the effects of general relativity. As long as the mass is the same, the orbits of planets will remain the same.

According to wikipedia[1] the converse of the Shell Theorem is (nearly) true:

Suppose there is a force F between masses M and m, separated by a distance r of the form F = Mmf(r) such that any spherically symmetric body affects external bodies as if its mass were concentrated at its centre. Then what form can the function f take?

The form of f allows Newtonian gravity but not Einsteinian.

[1] http://en.wikipedia.org/wiki/Shell_theorem#Converses_and_gen...

Interesting. However, I would have thought that planets are far out enough that classical is accurate enough. This sort of thing is why I specified planets as opposed to orbits in general.

The key point there is "spherically symmetric". The Sun isn't. It bulges around its equator thanks to rotation, as does everything. That does have effects on planetary motion; an object in an inclined orbit spends a bit more time a bit farther away from a bit of the Sun's mass. Replace the Sun with a black hole of equal mass and you don't have that oblateness. The effect on planetary orbits would be very small, so macroscopically the Solar System would still be the same, just with very slight differences in orbital speeds and periods.

All that mass is already orbiting (on a galactic scale) right on top of the mass. It'll affect our solar system's orbits in the same way that the sun moving a few microns affects them, I'd imagine.

The black hole is estimated around 4 million solar masses, the gas cloud around 3 earth masses.

So by "spring to life" the writer meant "we may, if we're lucky, finally see a glint of something". It's very exciting, yet the writer put it in terms that will scare the living hell of any uneducated person like me. Glad there's always HN.

I was thinking of the exact same thing.

It's ironic when a piece of writing tries to present you with factual information begins with a deceiving title, all in the name of attention grabbing.

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