# Q & A: Is relativity based on light?

Q:
Why is the theory of relativity based on light? here I want to add an alternate example to the twin clocks example of relativity:- Suppose u cannot see or detect light(suppose u r blind),and u r on one side of a planet. From the other side a relatively large meteorite crashes onto to the surface. At the same time on the other side of the planet u r monitoring the gravitational field of the planet as well as its mass which shows up on a Braille interface using a very fast machine(hypothetical). technically u as an observer can see/feel the objects position through space. Imagine instead of earth a very big planet(hypothetically) with a very huge gravitational field. Now in this situation could u get the information about the meteorite faster than light? if so what would that mean for relativity? if u do get information faster than light, then the observer here will not have been seeing/feeling the large meteorite in the past or future like a tachyon. He will have seen/felt the meteorite faster than light. even if the meteorite is travelling as fast as light(ignoring the consequence of the impact) the observer will perceive it before the object reaches the planet and not at the same time. Please correct me if I m wrong.
- Soham (age 24)
India
A:

Relativity is not based on light. The existence of something that travels at the speed limit, e.g., light just lets us tell some stories about how things work that let us derive some of the equations of relativity with very simple math. Otherwise the path to derive the same equations would be slightly more complicated.

We've discussed this before: .

Gravitational signals also travel at the same speed limit. We don't have a direct measure of that, but as you suspect the whole structure of relativity would fall apart if it weren't true. The slowing of binary pulsars by gravitational radiation confirms this conclusion quantitatively, although indirectly.

Mike W.

(published on 03/13/2015)