## “The Speed of Spacetime”, not the “Speed of Light”

Posted on June 25th, 2009 — permalink

That thing that we call the “Speed of Light” — which is, numerically, 3×108 meters per second, or, more elegantly, one light-year per year — is in fact not a property of light itself. Rather, it is a property of spacetime.

Relativity tells us that there is a fundamental speed scale to the Universe. That speed is the “speed of light”. Why it has this value, we don’t know; as far as relativity is concerned, it’s an initial condition, something that just is a property of our particular Universe. This speed scale can also be seen as the natural conversion between space units and time units — the natural scale of our Universe is such that there are 3×108 meters in one second. This speed scale is necessary to talk about “spacetime” instead of space and time as separate things. And, according to the theory of Relativity, they are not separate things. Depending on how one person is moving relative to another person, what one person considers to be pure time will be a mixture of space and time to the other person. How mixed it is depends on their relative velocity.

Relativity has an ultimate speed limit, a speed that nobody with non-zero mass can attain, but can only approach. Nobody is moving faster than the speed of light relative to anybody else. As they accelerate more and more, as they gain more and more momentum and energy, they get closer and closer to the speed of light, but they never quite get there. Meanwhile, particles with zero mass always move at exactly the speed of light (in the absence of a medium with which they interact). Photons happen to be particles of zero mass, but they aren’t the only ones. Gravitons also move at the speed of light– although we have yet to individually detect Gravitons. We have, however, detected (indirectly) gravitational waves, and those (just like light waves) move at the speed of light. That is, the speed of gravity is the same as the speed of light. And, this is no concincidence. In fact, both of them are moving at something more fundamental, the basic speed scale of spacetime.

For this reason, calling this speed limit “the speed of spacetime” would be more accurate than calling it the speed of light– although I grant you that that might be more confusing, and for general use “speed of light” is probably a better term.