Object appearing

Question

I would like to know if it was possible to make an object appear or disappear in a room what would happen with the addition of subtraction of this mass? If it was the size of a human would it cause a shockwave/explosion if it appeared and cause a black hole if it disappeared?

Answer 1

Mass suddenly existing and ceasing to exist in a certain place would be scientifically impossible, but I'll give some answer here, even though it will be absurd. 

 

If we assume the object appeared instantaneously, that means that the mass of the air that originally occupied the space also has to be displaced instantaneously. In other words, the air has to move out of the way at infinite speed. If it has infinite speed, it will never slow down to a finite speed, and will instantaneously rip through the entire universe. This is obviously impossible, so we could also assume that the object appears quickly, but not instantaneously. The fastest it could happen is at the speed of light, so then the air would move outward at some speed that decreases over time. Depending on the initial speed of the displaced air, the result could range from a violent explosion to a barely noticeable breeze.

 

Now for disappearing. If your object disappears, instantaneously or not, the air will just rapidly diffuse to occupy the resulting vacuum. If the air in the room was anywhere near normal atmospheric pressure, you probably wouldn't notice anything at all besides a very slight drop in pressure. It probably wouldn't even make a loud noise. No explosions. No black holes.

 

Supposing you did find a way to break physics and transport objects around a la Star Trek, you would greatly reduce the risk of universe-ending incidents if you simultaneously switch the object you are transporting with the air it will be displacing. Then you don't have to worry about the displaced air because it will be in the location that your object used to be in.