[thedoc (OP)]

Steven Wasmer  asked the Naked Scientists:
   
Good morning Scientists,

If it's true that an objects mass increases with it's velocity, and that black holes have infinite mass, can black holes have any velocity of their own? If not could they therefore be used as reference datumns to measure the universes expansion rate?

Thanks for your continued phenomenal work!

Steve
Philadelphia, PA

What do you think?

[Kryptid]

Black holes don't have infinite mass. According to Relativity, they have infinite density (which is a different thing). Once we get a good quantum theory of gravity, I believe we will discovery that nothing about a black hole is truly infinite any way.

[Emc2]

I do not believe Black Holes have infinite mass or density.

  I believe matter breaks down to it's elementary particles inside of a black hole.
 
 so any calculation for inside of a black hole, will show "infinity", because it might as well be..

  impossible to now the mass of "all" elementary particles in the universe that "could" fit into a black hole.....

    trying to multiply anything by "unknown" = infinity

   total mass of all elementary particles in the universe x "anything" = unknown, hence - might as well be infinity....

[imatfaal]

Further to Supercryptid's clarification about the mass - nothing has velocity per se, it has velocity relative to something else, with reference to a rest frame.   There is no absolute frame of rest - so there is no way to judge if something is moving without that being in reference to another object. 

Therefore some black holes will have immense speeds when viewed from the rest frame of other objects - but have zero velocity from the perspective of an object in the same frame as the blackhole itself.  so no - they cannot be used as some form of benchmark in this manner, as useful as that would be

[yor_on]

A objects mass is the so called 'rest mass' defined as being the same no matter what frame of reference you use when/for measuring the same.  Black Holes has different 'gravity' attracting other particles etc, depending on their size/mass/density as measured by us outside the event horizons. The main stream definition of a 'inside' though is that we don't know what happens there, although we have all sorts of ideas of how to describe it. Assuming that all centers, no matter the Black holes size/mass/density, are the same we get a description of where the physics we use outside the event horizon breaks down. If 'energy' can exist on its own then that must be what exist there and if 'energy' also can produce a gravity then that is what it does. And although that gravity will differ to us watching it outside the Event horizons, the question then becomes if all centers have a same expression?