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> Every second, 20,000 stars become newly unobservable from Earth. Inquiring minds want to know. 1) If we had an instrument that measures the gravitational fi

> Every second, 20,000 stars become newly unobservable from Earth. Inquiring minds want to know. 1) If we had an instrument that measures the gravitational field due to one of these stars, would it drop discontinuously from a small positive value to zero, implying something like a gravitational wave shaped like a step function, or is there some reason for it to smear out? 2) If there are observable stars far enough away from us that these disappeared stars are still observable at their positions, should we detect the observable stars being affected by gravitational forces appearing to come from nowhere? 3) If the remaining observable universe is always ascending from a gravitational well, does relativistic time dilation imply that the rate of expansion we perceive is always faster than the actual rate but gradually slowing down to it? edit: time dilation was the wrong way around

corey_moncure 5y ago
I think if the information can't travel to you by the shortest, most direct path through vacuum without being sublimed away by spacetime expansion, then there's no other path for that information to get to you. The observed positions of faraway stars are also delayed by a proportional amount of time, so I think you can see the continued past effects of the unobservable star on an observable star, but nothing from after it became unobservable to you directly. I think that's the case in an expanding universe, but it might be the other way around in a contracting universe!
nh23423fefe 5y ago
1. I don't think so. I think this is because the "moments" are different. A star becomes unobservable once it passes from the region within the particle horizon to the region outside the particle horizon. But passing into this region can't affect all the information on the way to us. That's because the region doesn't exist until we trace all light path backward. So start right now and trace out 14B years of light paths. You will find that star is within the region. However some point in the future you notice that the star passed beyond the horizon. The star still exists it still emits light, but that light will never reach us. We don't see the star "go away" as it passes beyond the horizon. Instead one day the future we will receive the last piece of information about the star redshifted to infinity. I think this means we observed a decay to some constant and that constant should be zero. This argument feels very similar to black hole arguments, so rings true to me. 2. I think this is trying to state something paradoxical. But it's not really a paradox. Everything we observe is within our particle horizon. Everything has it's own particle horizon. These regions don't overlap. I think if I restate what you said without the objects the paradox dissolves. > If there are points (P) in our particle horizon far enough away from us that these newly unobservable (U) points are still within the respective particle horizon of the points (P) in our horizon, should we detect the points (P) our particle horizon as being affected by points in their own horizons? Of course, why should what we can observe in future affect what had happened in the past at some distant point.

corey_moncure 5y ago

I think if the information can't travel to you by the shortest, most direct path through vacuum without being sublimed away by spacetime expansion, then there's no other path for that information to get to you. The observed positions of faraway stars are also delayed by a proportional amount of time, so I think you can see the continued past effects of the unobservable star on an observable star, but nothing from after it became unobservable to you directly. I think that's the case in an expanding universe, but it might be the other way around in a contracting universe!

nh23423fefe 5y ago

1. I don't think so. I think this is because the "moments" are different. A star becomes unobservable once it passes from the region within the particle horizon to the region outside the particle horizon. But passing into this region can't affect all the information on the way to us. That's because the region doesn't exist until we trace all light path backward. So start right now and trace out 14B years of light paths. You will find that star is within the region. However some point in the future you notice that the star passed beyond the horizon. The star still exists it still emits light, but that light will never reach us. We don't see the star "go away" as it passes beyond the horizon. Instead one day the future we will receive the last piece of information about the star redshifted to infinity. I think this means we observed a decay to some constant and that constant should be zero. This argument feels very similar to black hole arguments, so rings true to me. 2. I think this is trying to state something paradoxical. But it's not really a paradox. Everything we observe is within our particle horizon. Everything has it's own particle horizon. These regions don't overlap. I think if I restate what you said without the objects the paradox dissolves. > If there are points (P) in our particle horizon far enough away from us that these newly unobservable (U) points are still within the respective particle horizon of the points (P) in our horizon, should we detect the points (P) our particle horizon as being affected by points in their own horizons? Of course, why should what we can observe in future affect what had happened in the past at some distant point.