- #1
Warp
- 128
- 13
Here's a puzzle that I have been wondered about. I know the answer, but I do not understand the reasoning for it. Could someone explain it to me in a way that a layman like could understand in an intuitive manner? (The math involved in GR equations is too complex for me to grasp.)
General Relativity does not forbid (but on the contrary predicts) the distance between two points in space increasing faster than c. This can be caused, for example, by the metric expansion of the Universe (and is, in fact, as far as we know, happening as we speak). As the Universe expands, the parts of it that are farther away from us than a certain distance are receding from us faster than c. This means that the observable universe (ie. the part of the universe that we can observe) is smaller than the entire universe. We cannot observe anything from beyond this distance. (Even though the distance between two points can increase faster than c, this still does not allow traveling at superluminal speeds. Nothing can move from one point to another faster than c.)
Now, the observable universes of, for example, two planets can overlap, even if the planets are so far apart from each other that they are receding from each other faster than c. In other words, if we have a planet A and a planet B that are so far apart that they recede from each other just slightly faster than c, A will be outside of the observable universe of B and vice-versa, and no communication whatsoever is possible between them (according to GR). However, the observable universes of A and B overlap, if their distance is not too great. This means that both A and B can, for example, observe the same star (which is in the overlapping part).
Now here's the puzzle: If both A and B can observe the same star, that means that the star can likewise observe both A and B (because they are both inside the star's observable universe). But this means that someone located at the star could, for example, take a photo of A and send it to B, all this at subluminal speeds. But this breaks the premise that no information can be sent from A to B because they are receding from each other faster than c.
This seems like a contradiction. What is the correct answer to this, and why?
General Relativity does not forbid (but on the contrary predicts) the distance between two points in space increasing faster than c. This can be caused, for example, by the metric expansion of the Universe (and is, in fact, as far as we know, happening as we speak). As the Universe expands, the parts of it that are farther away from us than a certain distance are receding from us faster than c. This means that the observable universe (ie. the part of the universe that we can observe) is smaller than the entire universe. We cannot observe anything from beyond this distance. (Even though the distance between two points can increase faster than c, this still does not allow traveling at superluminal speeds. Nothing can move from one point to another faster than c.)
Now, the observable universes of, for example, two planets can overlap, even if the planets are so far apart from each other that they are receding from each other faster than c. In other words, if we have a planet A and a planet B that are so far apart that they recede from each other just slightly faster than c, A will be outside of the observable universe of B and vice-versa, and no communication whatsoever is possible between them (according to GR). However, the observable universes of A and B overlap, if their distance is not too great. This means that both A and B can, for example, observe the same star (which is in the overlapping part).
Now here's the puzzle: If both A and B can observe the same star, that means that the star can likewise observe both A and B (because they are both inside the star's observable universe). But this means that someone located at the star could, for example, take a photo of A and send it to B, all this at subluminal speeds. But this breaks the premise that no information can be sent from A to B because they are receding from each other faster than c.
This seems like a contradiction. What is the correct answer to this, and why?