Light travels "as fast as it can." If light travelled slowly, then, by the momentum equation of special relativity, we'd still ramp our energy limits long before we saw this sort of stuff. In fact, if things took so much more energy to move about, macro-scale creatures such as ourselves would probably not have developed.
I think the author meant "what if light travelled at only a fraction of the speed it physically could?" which is a much more ridiculous question, but with even more interesting implications, especially if you don't also scale down the speed of gravitational transmission et all to match.
I remember looking into the question of gravitation transmission before. As far as I could amatueurly determine, gravitational transmission is an unanswered question. This is to say, if the sun "dissapeared" would the Earth feel it straight away (because spacetime had been altered) or would it take as long as the time for light to reach us because gravitrons (if they exist) could only travel as fast as the speed of light?
I'm aware of the whole limit on information transmission so I assume the latter, but I thought it was still unanswered.
I think the real problem with that question is deciding exactly what you mean by "disappear." If you blow the sun up, it still has all its mass, it's just spreading apart, and the effect this has on Earth can be graphed continuously. Likewise if you just move it somewhere else really fast (because you can't move it faster-than-light.) If you convert it completely into energy, by the mass-energy equation, it still has all its mass.
Thinking about gravity becomes a lot simpler when you realize that every one of those particles of the Sun is quantum-entangled with all the particles of the Earth. If you want to "remove" the Sun, you have to decide what that means for the decoherence of the configuration subspace consisting of (what was previously) Sun + Earth.
Configurations only evolve toward more definite subsections of themselves, not completely "new," remote amplitudes. Also, as far as we can tell, decoherence is computable from the completely local neighborhood of the amplitude-mass in question. Thus, gravitational transmission has to take some amount of time, in the same way that one domino can't knock down another across the room instead of going through the dominos in-between.
The interesting thing is that some of the particles of the Earth (analogously, the intermediate dominos) would be immediately affected—but it would take Au/C before enough of them were affected that that decoherence would become detectable.
I've never been given that view before. If I understand you correctly, gravity does have an effect faster than the speed of light but there is no detectable effect faster than the speed of light?
I think the author meant "what if light travelled at only a fraction of the speed it physically could?" which is a much more ridiculous question, but with even more interesting implications, especially if you don't also scale down the speed of gravitational transmission et all to match.