Comments on: AAS #4: Supernova expands as we watch!

By: Peter

Peter — Sun, 11 Jan 2009 18:26:24 +0000

Why is anyone talking about relativity affecting the various discussions of “when” the SN event happened in our frame of reference? Cas A is close enough and in the same galactic arm as we are so the separation velocity between us is so low that relativistic effects are of piddling importance. Even the radial velocity of the expanding layers of the Sn are low enough that the doppler shifting in radaition frequency is very low.

This is not a relativistic issue at all. The only issue with discussing time here is the transit time of the radiation from there to here, otherwise simulanity is not affected – good old Newtonian calculation based on the distance and velocity c give pretty accurate numbers for the transit time.

The real issue is for what we are observing, do we talk about it t with t=0 defined as the actual time in our reference frame the SN occured – ie 10330 years ago and then convert every observation back to that origin or do we ignore the transit time and talk about t=0 as when the first radiation from the SN explosion would have first been observable here? Obviously if we want to easily talk about the evolution of the event – ie what it looked like one day after, or 1 year after the explosion then it is far easier to ignore transit time.

For SN’s this makes sense – the interesting events are fast compared to the transit time for most SN’s we observe, For the far reaches of the galaxy though, when we talk about highly redshifted objects obviously we are interested in how far back into time we are looking and time here is dicussed as if it stretched back to the big bang.

The problem comes for intermediate objects – that evolve on the same sort of time scales as the transit time- ie a planteary nebula which is 10000 light years away and has been expanding for 20,000 years – what we see appears to be 10,000 years of expansion. What do we say is the age of nebula, how do we answer when did it explode? It is 20000 years too late for the poor aliens whose planet orbitted the star, but what we see is a nebula 10000 years after the original explosion. The bald statement “it happened x years ago” is less than precise unless we have a solid convention in discussing this sort of event. For astronomers studying “local” events in our and nearby galaxies there is a solid convention which is to ignore the transit time and discuss the events only in terms of what we observe – we see a 10000 year old nebula so that is how old it is.

Again this is NOT a relativistic issue.

By: Visibly expanding supernova remnant : Stochastic Scribbles

Visibly expanding supernova remnant : Stochastic Scribbles — Sun, 11 Jan 2009 00:32:10 +0000

[...] I get a tingly feeling when I see cosmic objects change within our lifetimes. It’s no exception when I learn about a video from the Chandra X-ray observatory that shows how the Cassiopeia A supernova remnant expanded over a seven year period via Bad Astronomy. [...]

By: Tom Marking

Tom Marking — Fri, 09 Jan 2009 00:29:13 +0000

k9_kaos “I look forward to reading a more detailed explanation from you.”

Detailed explanation? It must be all in the book. (”Death from the Skies” is available from amazon.com for the LOW, LOW price of $16.25 … hint, hint)

*crickets*

By: Bjoern

Bjoern — Thu, 08 Jan 2009 13:55:16 +0000

@Phil:
Well, IMO, I *am* thinking relativistically. Have you looked at what I actually wrote in my comments? I took great care in specifying which frame of reference I talked about, and provided the actual spacetime coordinates of the relevant events – as did Tom Marking in his last comment.

Even in the theory of Special Relativity, *not* everything is relative – one can make some definite statements, and the statement that the SN explosion happened 10,330 years ago *in our frame of reference* is definitely right. (note that I am *not* talking about when that explosion became visible to us – I’m talking about when it actually happened)

By: k9_kaos

k9_kaos — Thu, 08 Jan 2009 07:52:32 +0000

“Literally, as far as we are concerned, that star really did blow up 330 years ago, not 10,330.”

Phil,
Does this mean that the star blew up 330 years ago in our reference frame?

I’m familiar with relativity (I’ve read Einstein’s little book) and I understand that there are some weird effects of relativity, for example, two events that are simultaneous in one reference frame may not be simultaneous in another (particularly if the two reference frames are moving relative to each other). Not to mention Lorenz contraction and time dialation.

However, I don’t remember reading about the effect you described. I’m sure there is a lot more I have to learn. I look forward to reading a more detailed explanation from you.

By: Tom Marking

Tom Marking — Wed, 07 Jan 2009 19:36:38 +0000

@Adrian Lopez “when do the people on Earth get to see the explosion, and how old is the explosion from my point of view on Cas A once the people on Earth get to see it?”

Again, from the point of view of the CasA-ans (to coin a phrase – they must be tough SOBs to have withstood a supernova explosion, but that’s another topic) the event you are speaking of (i.e., when do Earthlings get to see the explosion) is also INFERRED – they have no direct experience of it. But they can calculate it based on the distance and the speed of light. So again, 2 events = 2 points in Minkowski 4-space, NOT 1 event = 1 point in Minkowski 4-space.

I hope they are not having this same argument on the CasA blog.

By: Adrian Lopez

Adrian Lopez — Wed, 07 Jan 2009 19:07:45 +0000

“Tom Marking, you and everyone else disagreeing with me are not thinking relativistically.”

Thinking relativistically, sure… but from where?

Let’s think forward instead of backwards. You are at Cas A, and it has just exploded. Earth is 10,000 light years away. In that case, when do the people on Earth get to see the explosion, and how old is the explosion from my point of view on Cas A once the people on Earth get to see it?