Hi all - this (seeming) paradox has been brought up on a few other forums with less than illuminating results. I'm curious to see what the solution is. Okay, so, you're watching a massive object approach the speed of light. From your reference frame, you are measuring the increase in apparent mass due to relativistic effects. This thing goes faster and faster and gains more and more energy, and so appears to get more and more massive. At some point, this thing will appear to be so massive that it should collapse under its own gravitation to form a black hole. However, from *its* reference frame, its apparent mass is constant - just its rest mass - and so clearly no black hole forms. That is, from one reference frame this thing collapses, while from the other reference frame it does not: paradox. How is this reconciled? Where am I going wrong?