Do black holes have any translational motion?

[Antonio Lao]

Can black holes translate ?

Black holes are theorized to have spin or rotational component of motion. But do black holes have a translational component of motion? Relatively speaking, black holes are more at rest than anything I know in the visible universe.

 

[Antonio Lao]

My theory is that if a star stop moving, losing its translational motion, it is more likely to become a nova or supernova and then becoming a black hole.

 

[zefram_c]

Unlike spin, net translational motion is not an internal property of a system being considered. Keep in mind that each galaxy probably contains a supermassive black hole at its center, and galaxies can be in any number of states of relative motion.

 

[Antonio Lao]

and galaxies can be in any number of states of relative motion.

Is this the reason why a galaxy can never explode but only its individual stellar component? What about quasars? Are they not exploding galaxies?

 

[zefram_c]

galaxies can be in any number of states of relative motion

I meant this to say that galaxies (and the black holes at their centers) are in motion relative to one another, i.e. if we would claim that the BH in our own galaxy is stationary, then BH's in the centers of other galaxies would naturally appear to us as moving. So yes, BH's can have translational motion, but it depends on the frame of reference you consider. As far as its angular momentum is concerned, however, everyone in inertial frames will agree on it.

Is this the reason why a galaxy can never explode but only its individual stellar component?

Galaxies are generally stable against (further) gravitational collapse; the matter that was close to the center collapsed in the BH and the stars in the rest of the galaxy orbit the center of the galaxy. The vast majority of these orbits are non-relativistic and Newtonian mechanics is sufficient to show that the orbits are stable.

Stars on the other hand are NOT inherently stable against the collapse. That's why stars form from collapsing hydrogen clouds in the first place. Here's a short list of why various stars are stable (ie what what mechanism acts against gravity)
Main sequence (like our sun): pressure of the photons emitted in nuclear reactions. In fact it takes 100 million years for a photon created at the core to escape through the surface!
Red giants / Supergiants: Other nuclear reactions from fusing elements heavier than hydrogen.
White / black dwarves: electron degeneracy pressure
Neutron stars: neutron degeneracy pressure

What about quasars? Are they not exploding galaxies?

I will have to refer you to someone more knowledgeable regarding quasars; my knowledge of the way they generate their energy is limited.

 

[Antonio Lao]

zefram c,

Thanks for all these information on cosmic activities.

 

[jamie]

Black holes are theorized to have spin or rotational component of motion. But do black holes have a translational component of motion? Relatively speaking, black holes are more at rest than anything I know in the visible universe.

Not all black wholes have spin.
those that do are generally expected to bulge around the centre.
We call a black hole a singularity because no laws of physics can explain what is going on inside and because we cannot directly observe them we cannot see what is going on.

 

[force5]

Not all black wholes have spin.
those that do are generally expected to bulge around the centre.
We call a black hole a singularity because no laws of physics can explain what is going on inside and because we cannot directly observe them we cannot see what is going on.

Hi Jamie;

How do we know that all Blackholes do not spin? I just assumed they did. Can you please explain?

Thanks for any explanation.

 

[zefram_c]

Generally a star has some internal angular momentum, like spinning around its own axis. Even when the star collapses, this has to be conserved. After the collapse is complete, the core - whether it's now a white dwarf, neutron star or BH - usually retains at least some angular momentum. So in practice, it's virtually imposssible to find a BH with no spin whatsoever. But one can certainly write down a BH solution that does not contain any angular momentum. To make it easier for you to look this up, they're called as follows:
L=0, Q=0 : Schwarzschild black hole / spacetime geometry
L>0, Q=0 : Kerr black hole / spacetime geometry
L=0, |Q|>0 : Reissner-Nordström ...
L>0, |Q|>0 : Kerr-Newman ...
The "black holes have no hair" theorem states that a black hole is completely characterized by its values of M, L, and Q so this should be it. I don't know if that theorem is proved or not.