Relativity & Quasars: Spatial Contraction & Density

[b2386]

Hi all,

Just a quick question.

Suppose I am traveling at relativistic speeds. An object moving with respect to my frame has undergone spatial contraction. In my frame, does the object's density increase?

 

[pervect]

Yes, the density does increase.

Energy density, the sort used in the stress-energy tensor, increases by a factor of rougly gamma^2, gamma = 1/sqrt(1-(v/c)^2). This happens by a combination of length contraction and the fact that relativisticly moving objects have kinetic energies on the order of their rest mass.

 

[b2386]

OK, let's change the situation slightly.

Suppose I am traveling at relativistic speeds. A quasar moving with respect to my frame has undergone spatial contraction. In my frame, does the quasar become a black hole if it is traveling at sufficient speed?

 

[jtbell]

(link) http://www.physics.adelaide.edu.au/~dkoks/Faq/Relativity/BlackHoles/black_fast.html