Jdeforrest wrote:
You’re mixing up two different things here. You can have a vacuum in a strong gravity field, or a non-vacuum with hardly any gravity. A non-vacuum means that it’s travelling through some substance, such as air, water, or teflon (these are things for which I happen to know the approximate speed). I’ve heard that the reason light slows down going through things is that it still travels from sub-atomic particle to sub-atomic particle at c, but it gets delayed some when it interacts with the particles, being absorbed and re-emitted.