# Relativity and Quasars

1. Sep 25, 2006

### 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?

2. Sep 25, 2006

### pervect

Staff Emeritus
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.

3. Sep 26, 2006

### 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?

4. Sep 26, 2006

### Staff: Mentor

I Covariant Form of $E_{1}E_{2}|\vec{v}|$ Today at 3:38 PM