How can we be sure

[sidkid]

that any of the stars we see in the night sky are actually still there? I know we can determine the natural life-span of a star, but how do we know for definite that any star is actually still there. Is it that we can only assume they are all still there? (I posed this question in a recent supernova thread but received no response).

[rojash]

"How can we be sure"
We can't. But the whole concept of "still there" is meaningless when you are dealing with relativistic distances.

[Postdog]

They are there in our timeline - I see it, therefore it exists.

[sidkid]

Rojash - can you explain further why 'still there' is meaningless...surely something is either there or not irrespective of the distance away it is....and why is the distance to a star 'relativistic'? Surely they are all 'hard' measurable distances (albeit rather large)

Postdog - thanks for answer although I would phrase it as I see it but does it still exist.....?

[Noraq]

We don't know....yet.

[jomifl]

Depends on whether you think that time travels at the speed of light or not. They could be there for us but not for others.

[wildwood]

Common sense would determine that if the almighty decided to instantly extinguish the sun we wouldn't notice until 8 or so minuter later. Maybe if he did so with the Andromeda galaxy we also wouldn't know about it for 2+ million years.

But you just never know with science....

[beso]

The stellar life cycle is one of the simplest of all scientific phenomena. The bigger they are the faster they burn. The life of any star can be calculated from its mass.

Rojash is alluding to the fact that the concept of "now" looses its meaning at relativistic distances.

[sidkid]

Beso - thanks for response - I mentioned in the question that I know that we can calculate the life-span of any star - but the point of the question is how can we be sure that any star is still there. I also understand what Rojash is alluding to. I just don't understand why he thinks they are relativistic distances. They are measurable. We should also take into account the probable existence of very advanced life-forms out there who may have the technology to 're-arrange' things. Take NGC 5457 the Pinwheel Galaxy for example, location of the recent supernova. Its light is only just reaching us after travelling for 21million years. Who can possibly tell what has happened to the galaxy (for definite) in all those intervening years?

[modeller]

sidkid // Its light is only just reaching us after travelling for 21million years. //
What do you mean it is only just reaching us ? Today ? This year ?
This millenium ?
It may have taken that time to reach us but it could have been reaching us for millions of years. Not that that answers your question but I just wondered at your statement of '' just ''. It also raises a further question are there galaxies out there whose 'light ' has not reached us yet ?
Statistically there should be countless galaxies far more advanced than ours and yet none have sent out signals that could indicate intelligent life. W

[sidkid]

Modeller - the light we are seeing from the Pinwheel galaxy started out on its long journey 21 million years ago. We know this because the galaxy is 21 million light years away from us. We therefore know for certain that the galaxy was in existence 21 million years ago...but what we don't know is what, if anything, has happened to it in the meantime?.....or do we? Regarding your last statement, how do you know no-one has sent out signals? A civilisation living in the Pinwheel galaxy could have started transmitting 20.99 million years ago. Unfortunately the transmission won't reach us for another 10,000 years.

[boxtops]

Good question, sidkid.

[jomifl]

Thats the way it is Sid, you'll just have to be patient..If the light hasn't got here yet then it hasn't happened.. some would say.

[sidkid]

They might..........but I wouldn't.