# Observable universe thought experiment

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

A....................................B...................................C

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?

• 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? Jan 26 '19 at 19:59
• 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) Jan 26 '19 at 20:37