I've been wondering about relativity for a while now.

• We seem to measure time by using the speed of light.
• Light can be slowed down or stopped

We assume light doesn't change in a vacuum - yet, how can we know it isn't changing relative to everything right now? If we can affect light, it seems we should expect other known/unknown forces to have the same power.

Basically, is our whole view of time measurement based on light which could theoretically be affected by unknown forces right now, in the past, or in the future?

So my question is, how can we know for sure time is certain? Might it have been relative in the past/present/future?