APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

[don_b]

Chris is defining 'now' to be the photons that are arriving 'now'. In this image, we see photons that left the comet 6 minutes ago, and photons that left the galaxy 25 million years ago. Both are arriving 'now'. They are in the same image, the same context. There is no mention of what may have happened to the galaxy in the 25 million years since the photons left. We will have to wait and watch to find out.

We can make this REALLY complicated by including relativistic effects of time. Time is not constant, which is mind blowing. As an extreme example, in the context of those photons from the galaxy, they left that galaxy and arrived at earth at the same time. Since they travel at the speed of light, time did not pass for them. True, bizzare, but not helpful. Go back to the first paragraph. Stick to an earth based context and 'now' is what the universe looked like to the camera that took the beautiful APOD.

[johnnydeep]

I don't know. It seems to me that what Chris, alter-ego, and Ann seem to be arguing above comes uncomfortable close to saying that the passage of time is (mostly) irrelevant when it comes to understanding astronomical images.

Speaking for myself (and I guess the others, too) I'm not remotely saying that. What I'm saying is that in most contexts, "now" is the moment we observe something. It is pointless to ask "what is that object like 'now' because it took time for the light to reach us?" One of the most fundamental ideas of relativity is that "now" is different in different places, because of the nature of spacetime.

That does not mean that time isn't important! We look at pretty much everything in terms of how it has evolved and changed with time. And we may well use that information to try and understand what it will look like in the future. But we don't treat our observations as somehow false or incomplete because our frame of reference is different from that of the object in some different part of spacetime.

Ok, got it! And I can agree with all of that. Finally!

[alter-ego]

I don't know. It seems to me that what Chris, alter-ego, and Ann seem to be arguing above comes uncomfortable close to saying that the passage of time is (mostly) irrelevant when it comes to understanding astronomical images.

Speaking for myself (and I guess the others, too) I'm not remotely saying that. What I'm saying is that in most contexts, "now" is the moment we observe something. It is pointless to ask "what is that object like 'now' because it took time for the light to reach us?" One of the most fundamental ideas of relativity is that "now" is different in different places, because of the nature of spacetime.

That does not mean that time isn't important! We look at pretty much everything in terms of how it has evolved and changed with time. And we may well use that information to try and understand what it will look like in the future. But we don't treat our observations as somehow false or incomplete because our frame of reference is different from that of the object in some different part of spacetime.

I agree, my stance exactly.