Thorium is thought of as radioactive and "dangerous" because its half life is 10 billion years. However, most rhenium is radioactive with a half life of 40 billion years even though rhenium does have a stable isotope which comprises a minority of natural rhenium atoms. Why don't we hear more ab...

Even if they move toward each other at relativistic velocities and not head-on? I looked at questions Black hole collision and the event horizon and Dynamics of Event Horizons between head-on colliding non-rotating black holes, but did not find the answer.