Vocino Labs

GDR writes:

I find time as defined by relativity fascinating and confusing.

For example we say that a particular star is 10 billion light years away. We are saying that the light from this star took 10 billion years to reach us travelling at the speed of light. If however we had a seat on the photon travelling here no time would have passed at all. If you have zero time, then no matter what the velocity is, you have zero distance. In other words from our perspective the universe is huge, but from the perspective of a photon we are back to a singularity with the Earth and the star being co-located. As I see it, as photons are always travelling at the speed of light, it is still existing at the exact same instant as when it came into being. Which view represents reality?

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It just seems to me with my extremely limited understanding; relativity makes it impossible to say that the universe is a particular size or age because we can only measure things from our perspective on space and time.

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 colossal 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 gravitational 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.