Black holes are actually quite easy to construct in special relativity. We know the famous E=mc^2 derived from special relativity, which gives energy as a function of mass. The gravitational potential energy of a point mass of mass m in a gravitational field made by another mass of mass M at a distance of r is -GMm/r. (G being the gravitational constant.) The total energy stored in mass m is mc^2, so it cannot possibly escape the gravitational field of mass M if GMm/r > mc^2 by conservation of energy. Solving for this, we get that r < GM/c^2 is sufficient to "trap" the mass m in M's gravitational field.

Although we assumed M and m were point masses, the equations hold so long as M is a sphere of uniform density of radius less than r. So if we take a mass M, it must be smaller than GM/c^2 for something to be sufficiently close to it to be trapped within its gravitational field. This gives the event horizon of the black hole. The actual results have general relativity considerations to be taken into account, but IIRC this simple calculation is accurate to about a factor of 2. It should be noted that

*any* mass can make a black hole so long as it's compressed small enough ( by this simple model - at some point quantum mechanics plays a role and I know nothing about that).

@ ___: The correct equations for special relativity have already been given. Yours do not take into account any special relativity: time dilation or space contraction (depending on reference frame).