3 ms·

The description in the article is seriously misleading. Your misgivings about what it appears to mean are entirely correct: it doesn't mean what it appears to m

by pdonis 3y ago

The description in the article is seriously misleading. Your misgivings about what it appears to mean are entirely correct: it doesn't mean what it appears to mean. The effect in question is only an appearance; it does not mean that clocks 1.5 billion years after the Big Bang "actually" ran 5 times slower than clocks today (as you point out, it is impossible to even make such a comparison). It just means that, because of redshift effects, when we look at light from events that happened 1.5 billion years after the Big Bang, we see those events appear to happen 5 times slower than similar events today. But we would see the same thing if we watched events happening on a spaceship that was flying away from us with the same redshift.

raattgift 3y ago
Reciprocity in Minkowski space is relevant. In the special relativistic setup in your final sentence, the spaceship occupants could in principle watch our clocks and observe that ours are ticking slowly, with the same redshift. Observers at a high cosmological redshift cannot watch our clocks at all. They can only look at the even more distant past of what will become our clocks in billions of years, and there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors. (Indeed the gravitational redshift from the metric sourced by Earth (assuming "us" are Earthbound) breaks the reciprocity -- if only slightly -- that would be observed in Minkowski space.)
- pdonis 3y ago
  > Observers at a high cosmological redshift cannot watch our clocks at all. This is true, but it doesn't change the fact that the "time dilation" described in the article is like the time dilation in special relativity, and is not like gravitational time dilation, with respect to what the article is talking about. > there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors. Indeed, there is every reason not to think this. But that still doesn't change the fact that the "time dilation" due to cosmological redshift is like the time dilation in SR, not gravitational time dilation, with respect to what the article is talking about.
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by pdonis 3y ago

raattgift 3y ago

Reciprocity in Minkowski space is relevant. In the special relativistic setup in your final sentence, the spaceship occupants could in principle watch our clocks and observe that ours are ticking slowly, with the same redshift. Observers at a high cosmological redshift cannot watch our clocks at all. They can only look at the even more distant past of what will become our clocks in billions of years, and there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors. (Indeed the gravitational redshift from the metric sourced by Earth (assuming "us" are Earthbound) breaks the reciprocity -- if only slightly -- that would be observed in Minkowski space.)

pdonis 3y ago
> Observers at a high cosmological redshift cannot watch our clocks at all. This is true, but it doesn't change the fact that the "time dilation" described in the article is like the time dilation in special relativity, and is not like gravitational time dilation, with respect to what the article is talking about. > there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors. Indeed, there is every reason not to think this. But that still doesn't change the fact that the "time dilation" due to cosmological redshift is like the time dilation in SR, not gravitational time dilation, with respect to what the article is talking about.

deleted 3y ago

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