[sophacles]

These conversations are useless. Everyone knows that anything we see is strictly in the past and already has happened. Yet for practical purposes, a week ago there was no supernova there. Today a supernova is there.

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.

[baddox]

> Everyone knows that anything we see is strictly in the past and already has happened.

Actually, that statement is nonsensical. Choosing the Earth as your point of reference, the supernova is happening exactly when we see it happening.

[jerf]

I don't think it's nonsensical. There is a meaningful way in which it is in the past. Even if you turn yourself into photons, match positions with the supernova remnants, then return to physical form, the supernova is going to evolve though time for quite a ways beyond the current state we observe it in before your physical form has its rendezvous, even if the trip has zero duration for you.

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.

[baddox]

If you set a reference point, then I think it's perfectly sensical to say two events happen simultaneously or "now," but without a set reference point those notions are completely nonsensical.

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.

[ars]

> Two observers in different frames of reference can't even agree on the order of events

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.

[fhars]

I fear you don't actually undestand several core concept of special relativity, among these past, present, causality and simultaneity. Of these, only the last one is relative to the reference frame. Temporal ordering of events is absolute for events that are causally related, if event A is in the past of event B, which is defined as event B being able to observe event A or any of its consequences, then event A happens before event B in every frame of reference. Only the time difference between A and B depends on the frame of reference (for an observer at rest relative to the microwave background, there are 21 million years between the supernova and our observation of it, but a neutrino emitted by the nova passes though the earth only moments after it was created in its own reference frame, as it travels almost at the speed of light relatie to the nova and the earth). Likewise, we say that B is in the future of A if B can observe A or any of its consequences.

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.

[jerf]

fhars is correct. I'll say that you're getting closer than most people get to these concepts, but you seem to have mistaken partial ordering for no ordering. There is still a partial ordering of events, and we can now say that the supernova is in our past without ambiguity.

[ars]

> Choosing the Earth as your point of reference, the supernova is happening exactly when we see it happening.

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.

See: http://en.wikipedia.org/wiki/Einstein_synchronisation