What happens to a substance if its speed is more than the speed of light?

What happens with a substance if its speed is more than the speed of light? I think that it will disappear or replace somewhere but it wouldn't be destroyed!!!

We do not know what happens to a substance if it moves faster than the speed of light for the very simple reason that it can never move faster than the speed of light. The speed of light poses a fundamental limit to the speed that an object can take, relative to objects nearby it. In fact, no object with any finite rest mass can move at the speed of light. That is why all the particles that move at the speed of light (e.g. photons) have zero rest mass. As a particle with mass approaches the speed of light, its energy increases and becomes infinite at the speed of light, which is the reason why it can never be accelerated to reach that speed. This has actually been verified by experiments, and it has been shown that nothing moves faster than the speed of light.

However, the above discussion only applies to objects on small scales in the universe -- for example, if you take a baseball or a planet or a star or a galaxy and try to accelerate these objects to the speed of light relative to objects nearby them, it is impossible to do. However, there is nothing which prevents objects that are separated by huge distances from moving relative to each other faster than the speed of light. Over these large distances, the effects of the universe's expansion become important, and the above discussion no longer applies.

Technically speaking, the speed of light limit only applies when you are in an "inertial frame" -- that is, sitting where you are, without any forces acting on you, and measuring the speed of an object that moves past a ruler and clock that you are holding in your hand. Across the large distances in the universe, however, we have a very different set of circumstances. No one is in an inertial frame, because everyone is being accelerated with respect to everyone else, due to the universe's gravitational field and the fact that the universe is expanding. In effect, the universe's expansion isn't really due to galaxies moving "through space" away from each other, but rather due to the stretching of space itself, which isn't governed by the same limits that we are.

Thus, although it's impossible to move through space (locally) faster than the speed of light, and it's impossible for anyone within the universe to send off a piece of "information" faster than the speed of light, it is still possible for the distances between faraway galaxies to increase faster than the speed of light, due to the rate at which the space between them is stretching. This faster than light "travel" doesn't have any effect on the material that makes up the galaxies (for example, their energy does not become infinite in any meaningful sense), since they aren't really moving with respect to each other in any way that they can measure directly.

March 2002, Jagadheep D. Pandian (more by Jagadheep D. Pandian) (Like this Answer), Dave Rothstein (more by Dave Rothstein) (Like this Answer)

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