Hours, and 21 million years, after it happened.
Both space and time are relative, so if you're choosing Earth as your reference point, the supernova is happening right when you're seeing it.
Relative time is a real thing, certainly and messes with the idea of simultaneous.
But transit time is not that. We and the supernova are more or less in the same frame of reference (we are not accelerating relative to one another, and there isn't much relative velocity difference either).
And because of that it is correct to say it happened xxx years ago.
No, that is incorrect. We observe that the light from the supernova reached us now, yes. But we are well aware that it actually happened long ago.
Both us and the distant observer agree about the order and relative timing of the events.
It's only when we are not in the same frame of reference that we would disagree, and that is not the case here.
Also, http://newscenter.lbl.gov/feature-stories/2011/08/25/superno... is perhaps a more official story. They too don't mention that it's discovery was a few hours after it was possible to observe the explosion, not a few hours after the actual explosion.
can it look like a planet-wide intelligence to an outsider? :) . Interesting to compare to the origins of cybernetics :
http://en.wikipedia.org/wiki/Norbert_Wiener#During_and_after...
It seems to me that detect-track-target feedback-looped principle was one of key in the development of intelligence of animals and humans, and it seems to continue its work in developing the intelligence beyond that.
It may help you to look at it like this: a supernova is NOT the star exploding, it is the observation of the star exploding. In one sense, this is wrong, but in another it is really the only freaking thing a supernova can be. Therefore since it wasn't a supernova until humanity saw it being a supernova, it is currently happening.
Further, the article only says the supernova is brightening, and that the telescope saw the star early in its explosion a few hours ago. From the reference point of observability, no claim is made that something 21million light years away is being watched now, just the effects of it are being watched right now.
Relax and go watch the show.
Actually, that statement is nonsensical. Choosing the Earth as your point of reference, the supernova is happening exactly when we see it happening.
Putting a specific time span on how far in the past it is without further specification is dubiously meaningful, saying it is in the past is not nonsensical. Everything we see is always in the past, even if it's just on the other side of the room.
Most educated people understand the principle of relativity (which states that the laws of physics behave consistent in all frames of reference) and the fact that the speed of light is constant regardless of your frame of reference, but we tend to forget the resultant phenomenon called the relativity of simultaneity. Two observers in different frames of reference can't even agree on the order of events, much less the "exact time" they occurred.
Now, you can certainly propose the semantic axiom that "everything happened in the past due to the finiteness of the speed of light," but I find that axiom to be pretty useless in scientific discussion. You can suppose that millions of years of stuff has happened to that supernova after "now," but none of those events could have any causal effects on us.
Please keep in mind that we and the supernova are more or less in the same frame of reference. So all the ambiguity of simultaneity does not apply here.
Only in the case that neither A can observe B nor B can observe A, which we denote by saying that the events are in their relative present, the ordering becomes relative to the frame of refrence, but that case is irrelevant in the present discussion, as we can observe the nova and so have established the fact that it (or at least its beginning) is in our past. Note that its maximum, which is predicted to be observable around next friday, is in our present (we can not influence nor observe it yet), while our observation of it is in our future (we can still make plans for where to watch it), at least until next friday, when both of these events simultaneously move into our past.
No, that is not true. You do not say point of reference, you say frame of reference - and for a good reason. Huge areas of space can all be in the same frame of reference.