h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I came across a reference to this on an astronomy page. But it lacked a definition. Can anyone give me a definition or description of this system and its usefulness/applications?

Interesting, I googled it and got this:

http://en.wikipedia.org/wiki/Light_cone_coordinates

No wonder you're asking here

I *think* it's volume light occupies within a certain time, and so it's useful when deciding when things happen relative to each other. IE, within the lightcone, events can be affected by the event at the centre, but outside it it's impossible for anything to be affected by the event.

Maybe.

methinks a physicist is required here.

sorry simpost there.

Yeah, that link's the precise mathematical equivalent of what I said.

I went upstairs and jumped up and down on the bed after I read the Wiki entry and it was still way over my head...

t.

thanks Orcus, a most helpful link that (what, me? sarcasm?)

pedro:"it's useful when deciding when things happen relative to each other"

that would fit in with the context, which was of a supernova 10,000 lightyears away which we can see now but was described as going off 330 yrs ago (not 10,330 yrs) which I sort of understand but not fully. I know it is all to do with relativity and light speed and so forth and that the 10,000 ly away = 10,000 yr ago only works if you assume a universal standard time which we know is not right because of relativity.

turvy:"I went upstairs and jumped up and down on the bed after I read the Wiki entry and it was still way over my head..."

http://en.wikipedia.org/w/index.php?title=Light_cone_coordinates&oldid=217507985

Oh I think that's remarkably clear, don't you?

I recall reading something about lightcones in The Science of Discworld III. Anything within a lightcone can have no effect on anything outside the lightcone. This is because nothing travels faster than light. Lightcone coordinates are probably something to do with mapping this mathematically, but that Wikipedia article is ... somewhat less than helpful.

TRiG.

hmmm, but none of this explains why a supernova 10,000 ly away that we can see only happened 330 yrs ago and not 10,330 yrs ago.
I suspect it has something to do with overlapping lightcones and relative time lines.

I wonder if light cone coordinates give points only inside the light cone - that is either in the past or the future - so not points that are too far away in space and not far enough in time to either affect or be affected by the here and now.

Do you understand what light cones are, Ictoan, or are you looking for an explanation of them as well?

I understand the concept of lightcones, yes. That isn't too complicated. I just don;t know what light cone coordinates are and why that means the 330yr ago instead of 10,330 yr ago.