What is Time and Space

I'm sure there are better answers, but my short answer is that of course the universe can have no one size or age from all perspectives. That's what relativity tells us. The amount of time that has passed between events, and the amount of space that exists between two points, is relative to the observer's reference frame. There is no one right answer.But there may be a more meaningful question than the one you raise about how old the universe would appear if you were traveling on a photon. This oft-mentioned Einstein thought experiment has never proved helpful to me because it is never described, at least not in anything I've read, what Einstein thought he would see while traveling on the photon. It might be better to ask how old a universe an observer in a galaxy 10 billion light years away would see. Between ourselves and this observer space is expanding at a collosal rate, yet because of the high degree of uniformity in the structure of the known universe (that we assume for the moment applies to the entire universe) we know that this distant observer would also see a universe 13.7 billion years old.Another interesting question to ask is what an observer in a heavy gravtitational field would see. Let's say life somehow evolves on the surface of a neutron star, which has a very strong gravitational field, and let's assume this neutron star is relatively close to us within our own galaxy so that we know he sees pretty much the same thing we see when he peers up at the sky. How old a universe would this observer see?I don't think I can answer this question. I know that when we turn our telescopes on this observer we would see time passing by more slowly for him, because he's in a strong gravitational field. But I'm not sure what he would see when he looks at us or the rest of the universe.There's a couple people here who are very good at this type of question, Eta and Sylas. Hopefully they'll chime in, because I know more people than just us are interested in answers to questions like this.--PercyThis message has been edited by Percy, 07-29-2005 09:10 AM

I don't own Greene's book so I can't look it up, but by "notion of time" are you sure he means the same thing as "perception of time"? The excerpt isn't long enough to tell, but I would have guessed that he means that time has the same nature throughout the universe, and that we can figure out what the perception of time would be from another reference frame that is moving or accelerating relative to our own.--Percy

Okay, now I'm really confused. You'll have to explain the theory of invariantivity to me. --Percy

Right. In other words, the expansion of space does not change the relationship of clocks that are not moving with respect to one another.In even more detail, consider two clocks, both stationary with respect to you, but one is in front of you and the other is 10 billion light years away. The distant clock will appear to be receding from you at great velocity because of the expansion of space, but it is really stationary with respect to you. Both clocks will keep perfect time, meaning each will click off seconds at precisely the same rate. Earth's gravity isn't strong enough to bend light around its circumference. For that you need to be at the event horizon of a black hole.--Percy