Imagine 3 objects: A, B, and C, positioned along a straight line in space:


A and C move at the speed of light towards B.
A and C are at an equal distance from B.
A and C are inside B's observable universe, close enough to eventually reach B.
A and C are outside each other's observable universe.

From B's point of view both A and C should be able to reach B at the same time, but from A's point of view C is outside of the observable universe so it can never be reached.

How is it possible that both A and C can reach the same point at the same time even though they are too far to ever reach each other?

  • 2
    $\begingroup$ You, at this exact moment, are outside. my observable universe (more precisely, outside my past light cone). Does it seem odd to you that we might nevertheless shake hands someday? $\endgroup$
    – WillO
    Jan 26, 2019 at 19:59
  • $\begingroup$ The light we see now in telescopes - the specific photons that reach our telescopes right now - have been outside our past light cone until this very moment. If I'm outside your past light cone I still might be in your observable universe or even right next to you, approaching you at the speed of light and you'll see me in a moment when your past light cone will include my position in space-time. What matters is that our future light cones intersect. (in my thought experiment they intersect at point B in space at an unspecified time) $\endgroup$
    – potato
    Jan 26, 2019 at 20:37

1 Answer 1


You make an assumption that is not justified in this situation :

from A's point of view C is outside of the observable universe so it can never be reached

The fact that A is outside C's observable universe just means that A can't know anything about C. But the light from object C (or in this case the object C itself) will eventually reach A. The general lesson here is that the observable universe is not unchanging, it rather is a bubble that grows at the speed of light.

  • $\begingroup$ Right, the light just didn't have enough time to reach us from there yet. thanks for your answer. I mixed up 2 different concepts then. What I was trying to test in my thought experiment is the ability of things to reach each other when there is too much space between them so that the expansion of space creates extra space between them faster than they can cross it. I guess that limit would be far beyond the edge of the observable universe. $\endgroup$
    – potato
    Jan 27, 2019 at 0:34

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