Astronomers know all about light travel time. It is called “aberration”, aka light-time correction, and has been taken into account for centuries.
I don’t know what it means to say that they are the same reference frame, or nearly the same, when they are separated by so much space 
In this shared coordinate space, where on the Time dimension am I marking the events “Betelgeuse goes supernova” or “Someone posts the #61 post into the thread 'Betelgeuse losing brightness
'”?
My important take-away is that if the sun goes supernova, then when all those photons finally reach us, it will be fast and painless. That’s all I need to know. :eek:
There is no mechanism for (the nearby) sun to explode like that, unless I’m missing something.
The sun is not going to supernova. Nor is Sirius or any other nearby star. Betelgeuse is probably the closest star that is going to supernova. It’s certainly the closest red supergiant, although it’s not required for a star to be one in order to supernova.
Rigel, a blue supergiant, is also nearby and may go supernova in the not-too-distant future, but it will first transform into a red supergiant. (Was the red supergiant Betelgeuse once a blue supergiant?)
Betelgeuse, BTW, is the answer to the trivia question: Which star has the 3rd largest apparent diameter as viewed from Earth? The #2 slot is held by R Doradus, a variable red giant 200 ly distant. Which star holds the #1 record? Our own Sun.
Being separated in space is completely irrelevant to reference frames. If two objects are at rest relative to each other, they are in the same reference frame. Both will agree on time intervals between events located at either, or at anywhere else in the same reference frame.
Yes, they will agree on time intervals for events that happen in one place, but not on time sequence for events that are located at different points and they will not agree on what they respectively call now. And that, I think, is the point we are talking past each other.
They will agree on time sequences of all pairs of events everywhere. And they won’t agree on “now”, which is a single event in both time and space, but they will agree on the question of whether two events occurred at the same time.
An astronomer on Earth, working in Earth’s reference frame, would say that Betelgeuse’s explosion happened 642 years before it was observed on Earth. An astronomer near Betelgeuse, assuming he survived, and working in Betelgeuse’s reference frame, would also say that Betelgeuse’s explosion happened 642 years before it was observed on Earth. If there were a mirror halfway between Betelgeuse and Earth, such that the astronomer at Betelgeuse could see the reflection of the explosion, then both astronomers would agree that the astronomer at Betelgeuse saw the reflection at the same time that the astronomer at Earth saw the original explosion.
There is no such thing as “two events that happen in one place”.
I should have written “two events that take place in the same place but at different times”, and I hope that is possible. Of course, if you argue that no place remains the same because pantha rhei and so on and therefore no event can take place there anymore because there no longer exists, well, yes. Thanks for helping me improve the precision of my written expression.
I think you understood me but I’m not sure because your initial clarification still doesn’t work. How can you determine whether two events are in the “same place”? It doesn’t mean anything without an absolute frame of reference. Two events can seem to be in the same place relative to an inertial observer, but a different inertial observer will not necessarily agree. This is just a consequence of Galilean relativity, not even Einsteinian.
So you first specify a frame of reference, and then say that in that frame of reference, they’re in the same place. No problem.
I’m confused by this whole discussion. Differences in frames of reference arise from relative velocities, not from physical distance. As mentioned, if two bodies are at rest wrt each other they are in the same frame of reference. The fact that one observer may not become aware of an event for while because of significant physical separation, is irrelevant, we can calculate time of the event based on speed of light/information. Two observers who are motionless wrt each other will agree on any simultaneous events, even if they have to wait to see both events. i.e. “that explosion happened 600 years ago in our mutual frame of refence, I did not know that until I just saw it now, 600 years after you did.”
There will be a slight difference in perceived “simultaneity” of distance separated events between us and the Betelguesians, because of that gamma factor, but very slight - essentially, we are in the same frame of reference as Betelgeuse.
So, obviously I misstated matters when I said there’s “a” natural frame of reference for the universe. There’s a natural comoving coordinate system, and a natural comoving local frame at each point, but (as Chronos noted) it’s a different frame at each point, albeit the difference is only significant at intergalactic distances.
That implies a natural way to measure the age of the universe - the elapsed time on a clock that has always been at rest in the local comoving frame. And that would be the natural way to talk about the current age of the universe (for us, here, now), and the natural way to describe the time that light was emitted from a distant galaxy that we see - the age of the universe for that galaxy at that time.
I don’t think there’s any contradiction between this and how we think about spacetime under SR.
That’s certainly one valid way to discuss cosmological measurements. But you have to be careful, on cosmological scales, to specify exactly what you mean, because a lot of things that we think of as having well-defined meanings cease to do so when used on cosmological scales. There are a half-dozen different ways to define “distance”, for instance, and on small scales, they all agree, but on large scales, they’re all different.
That’s what redshift measures in extragalactic observations, right? It corresponds (under a GR model) to the age of the universe when the light was emitted. Astronomers don’t tend to talk about the distance of observations at that scale, because it’s not so clear what that means.
Well, that, and even if you’re clear on what notion of distance you mean, you still need to calculate it, and that depends on precisely how the Universe has been expanding since then, which has some uncertainties. But the redshift is clearly and directly measured, to a high degree of both precision and confidence.
You may have to settle for that neutrino pulse.
Just how dangerous to us would Betelgeuse going supernova be? Would it cause health or climate effects dangerous to us or infrastructure we depend on? Would the delivered thermal energy contribute to global warming? Would the radiation cause massive deaths from radiation poisoning, a more moderate effect in increased cancer rates or birth defects, problems with livestock, mutated supergerms, a zombie apocalypse? Would it crash the Internet or screw up my TV reception? And how long a delay would there be between when the visible light that lets us see the event arrives and the following storm of radiation arrives?