system
Given the following post, respond as if you're commenting on Reddit:
human
Does a mass particle, traveling close enough to the speed of light, appear to be a black hole?

Is it correct to think that a particle with mass, as it travels close to the speed of light, that it's mass will increase and it will contract to the observer? Thus it's density will increase. Would you expect it to behave like black hole?
gpt
No, not exactly. The "relativistic mass" thing is just a (partially) useful interpretation of the physics. The fact is that when an object increases its energy, its momentum continues to grow in a way that is no longer defined by mv (mv is in fact just an approximation of momentum for sufficiently slow moving objects). This can be *thought of* as an increase in the mass part of the equation, but the particle doesn't truly gain mass. 

Furthermore if you're moving alongside that massive object, it appears to be at rest and only have its "rest mass;" so it clearly doesn't look like a black hole to that observer. 

tl;dr: black holes only refer to the rest mass of an object.