Ok I was wondering this the other day. I was staring up at the stars the other day I was on a lake down the road from my house. No unnatural light, and one could see ‘every star’ we all know those kinds of nights, atmospheric conditions are perfect and you can see very well.
What I was thinking, is if you are looking at a star and it is say 1000 light years away. Lets say that star explodes, in a supernova.
I thought it takes a certain amount of time for the light from that star to get to our eyes here on earth. essentially the light we see is a thousand years old.
First question, a star explodes and dissappears, 1000 light years from earth. Lets say it takes 900 years for the visible remnants to go away. What are we seeing then. A star? The residual light from a star? And if the light is residual, what exactly are we seeing? Ghostly affirmations from above, or scientifically explainable.
How can we see a star that is 10,000 light years away, when it could have been theoretically gone thousands of years ago.
From a purely physical standpoint, we’re not actually seeing anything; our retinas are registering the light either emitted from a source or reflected off an object. And, of course, since the speed of light is a finite velocity, it takes time for that light to travel from the source/object to our retinas.
The time is usually a trivial amount until you start dealing with astronomical distances. Then you start getting some serious lag. So, for instance, light needs approximately 2 million years to cross the gap between our humble little home and the Andromeda Galaxy - and what we’re seeing is the collective light of the stars there as they were two million years ago.
If you like, you can consider it a sort of photograph, like one of a long-dead ancestor you never met. You’re not actually looking at the thing itself, you’re looking at an image of it that was formed at some definite time in the past.
I cannot recall a better example of answering your own question. You are correct. We don’t see any change in an object until the light gets to us. If something happened since then, we won’t see it until the light reaches us. This is no more a ghost than a letter mailed by a person just before death comes from “beyond the grave.”
The light you see isn’t “residual” It’s light. It takes a fininte time to travel a distance. It takes about eight minutes before we see events that happen on our own sun, about a second and a quarter for the moon. The effect is the same if you’re holding a water nozzle in the air and shut it off. The flow stops but the water that came out travels in the same arc.
The same is true of looking at anything; when you look at that apple there on the table six feet away, you’re not seeing it as it is now, you’re seeing the light that was reflected off it a teeny tiny fraction of a second ago; the fact that light travels so very fast only makes it seem instantaneous.
Actually, a better analogy would be the light from a light bulb six feet away because, like the distant stars, it’s source of light, not just reflecting it like the apple.
Since time & space as we understand them are in fact relative to the speed of light, what does it mean to say that the star is ‘long-dead’? Is it really? How would you determine this? Now, after the light reaches you, you might calculate when it occurred where you were before, and say “the star burned out 1000 years ago (Earth time)”, but before then, there’s no way to communicate that information. Also, you actually need other objects in the universe beside that star to determine the star’s age.
So, maybe that star could be said to exist for 1000 years after it exploded. But suppose a new spaceship that can fly almost at the speed of light is developed. You board the ship and head off towards a promising star 1000 light-years away. In only one hour, you’ve traveled 500 light-years. Suddenly you notice that the star’s just gone nova. Now, for how long did that star exist? On earth, it happened 500 years ago, but for you, it only happened an hour ago (when you started out). And if you turn around and head back to earth, you’ll arrive shortly after the nova appears in the sky – about 1000 years after you left. All of earth’s astronomers (or the apes and/or robots and/or aliens who now rule the planet) will agree that it happened 1000 years ago, but for you, it was only two hours ago.
Of course you’re both right; the amount of time that’s passed just depends on the observer.
[The above numbers may be imprecise, but I don’t think I screwed up anything conceptually.]
Panamajack, I think it’s time for your medication You are on the right track but need to take relativisit time dilation into effect. Your intrepid space traveller’s perception of time will be vastly different from those waiting at home. If he’s going near the speed of light not only will making a U-turn be bit of a bitch, but he’ll think he’s barely been gone when he returns to the Y3K party being thrown by the damn dirty apes/aliens/killbots back on terra firma.
Phlosphr, a different way of looking at the situation is to think of the light from distant stars as something that allows us to see back in time.
The analogy that I find most useful is the U. S. Mail. Suppose I get a newsy letter from my sister 3000 miles away, telling me all the things she’s doing. Do I know what her life’s like now? No, I know what her life was like a few days ago, when she sent it. Since that time, she could have gotten a promotion at work, or won the lottery, or (God forbid) died, and I wouldn’t necessarily know yet. Likewise, when I look at a star, I see how it was some years ago, not how it is now. In the meantime, it might have died, and I wouldn’t necessarily know.
If you want to know when the star died, then you’ve got to specify whether you’re talking about relative measurements or absolute. If, for instance, astronomers on Earth see a supernova 3,000 lightyears away, then they’ll say that the star exploded 3,000 years ago, not that it exploded today. This is dependant on our reference frame, though: A person passing us in a spaceship at high speed might give a very different number for when the star exploded. If you want to say something about the time, then you can’t give numbers, you can just put it into one of four categories: An event happened in the past if light from it has had time to reach us. An event is in the future if light has time to reach from us to it. An event is “now” only if it occurs at this time and at this place. Any other event, which is farther away in space than it is in time, is what’s called “elsewhen”.
Back to the original question, although it’s possible that a star that you’re seeing is currently dead, it’s probable that all the individual stars you can see with your naked eye are still alive… Except maybe Betelgeuse. It’s a little over 400 lightyears away, and it’s estimated that what we’re seeing (from 400 years ago) is a star that has less than a thousand years left to live. In other words, there’s roughly a 40% chance that Betelgeuse has already died (in the frame of reference of the Earth).
You are on the right track but need to take relativisit time dilation into effect. Your intrepid space traveller’s perception of time will be vastly different from those waiting at home. If he’s going near the speed of light not only will making a U-turn be bit of a bitch, but he’ll think he’s barely been gone when he returns to the Y3K party being thrown by the damn dirty apes/aliens/killbots back on terra firma.