I'd like to understand something. Namely the interaction between gravity and light. Here's what I know: (or what I think I know -- not a physics wizard. And I hate it when people use the word 'wizard' in such manner. But that's irrelevant.) 1. Gravity bends the path of a photon because the space around it is curved. 2. Time slows down inside a gravity field. 3. The reach of gravity is infinite and it propagates at the speed of light. 4. Photons spend some of its energy when travelling through a gravitational field. (I'm not so sure about this one, actually. I think it's called a gravitational redshift.) Here's what I don't know: (I don't know many things, but still.) 1. If gravity has effects on time, can time exist without gravity? If there were no particles with mass in our universe, would time still exist? Are mass and gravity needed for space to exist? Or in a massless universe, would time pass... quicker? (As much sense as that makes...) 2. Does time pass at a different speed when observed by a person in intergalactic space and a person on Earth? Suppose synced clocks were given to two people. One sits on Earth the whole year and the other pops into intergalactic space where there are no bodies of great mass for lightyears and then he would pop back after a year. Would the clocks show any difference? 3. Could a gravitational redshift explain the observation that everything is moving away from us in space? (Redshifts.) 4. What happens to a photon when it... (this will sound stupid, hold on to your chair) runs out of energy when flying through a gravity field? 5. Can the expansion of the universe be tied to the expansion driven by gravity? What I mean is could gravity be creating space at the boundaries of our universe? Related to question 1, I suppose. And now for two unrelated ones... 6. Could the ever-elusive tachyon actually be a particle that acts like the Alcubierre drive, but does it naturally? How would normal matter react when driven through by a spaceship with the Alcubierre drive? Yes, well, some questions are probably off-topic and some are probably stupid... Anyway, I'd appreciate any and all replies to these questions. They've been bothering me for quite some time now.