The light cone of our galaxy is a lot different to the light-cone of a galaxy 5 bly away. Our Hubble volumes are much different. Everyone is born in/at a different space/time.

Can I see stars in my light cone that you can never see?

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    $\begingroup$ You mean light from stars that wont reach us until after we're dead? Yeah. $\endgroup$ – CoilKid Mar 14 '15 at 22:37
  • $\begingroup$ You might find the answers here relevant--the only way someone can see events that you'll never see (imagining you're immortal) is if their worldlines exit the past light cone you will have at t=infinity, the one labeled "event horizon" in Christoph's answer (and also Fig. 1 in the Lineweaver & Davis paper I link to in my answer). Once their worldline exits that cone, their own past light cone can include events that will never be in your past light cone. $\endgroup$ – Hypnosifl Mar 15 '15 at 0:10

A light cone extends behind (and in front of) a particular point in space-time. So if you want two people to have different light cones, they need to be separated in space or in time. Unfortunately, humans are never going to be very separated in space--any event that enters your light cone will enter mine a few milliseconds later, even if I'm on the other side of the world. The facts of stellar evolution mean it is unlikely an entire star would be "born" in this time frame, so I think the two of us will pretty much always agree on whether a particular star is there or not.

  • $\begingroup$ Oh, didn't think of that. I guess I was thinking of something occluded being unblocked. $\endgroup$ – CoilKid Mar 14 '15 at 22:42
  • $\begingroup$ Z. I was thinking of a star forming during rapid expansion. Would that help my case and increase the time from milliseconds? $\endgroup$ – Jitter Jul 19 '15 at 12:02

"Everyone is born in/at a different space/time"

that's irrelevant - what's relevant is your path through space time.

So it could be that you are in North America during 2011 and I am in Europe during 2011.

"Can I see stars in my light cone that you can never see?"

Sure, it's easy to make a trivial example. There's a supernova that lasts for only one hour, sometime in 2011. For some reason, the moon (say) blocks the view of that for the whole hour from North America, but, it is visible for the whole hour from Europe.

I would see it and you would not. It's that simple.

Again, "Everyone is born ..." is totally unrelated. It depends on your path through spacetime between birth and death.


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