# How can we know, today, that there's something from 100 light-years from here?

In my understanding, to take a picture of something that is 100 light-years from here, our "camera" would have to travel 100 years at light speed, take the picture, send to us, and 100 years later we would receive it.

So, how do we have pictures of things that are 100 light-years from here?

-
Note that the distance is irrelevant, we can only measure the past. The present is an illusion, if you think of it. – Sklivvz Dec 6 '11 at 18:01
No, camera stays here. If you take a picture of something 30 m away, your camera doesn't travel 30 m to take the picture, and then send the image 30 m back to you. The camera registers light that has travelled 30 m from the object. (And in astronomy, the camera may be attached to a telescope.) – UncleBens Dec 6 '11 at 18:14
This is a Schrodinger's cat type problem. – Chad Dec 6 '11 at 20:14
@ChrisWhite: Minor comment to the edit(v2): I would suggest to use tags like e.g. special-relativity, time, and causality, rather than astronomy and astrophotography, since it is a conceptional question. – Qmechanic Feb 14 '13 at 13:51

Because it's not just a picture of something 5.87862537 x 10^14 miles away (the total distance in 100 light years), it's 100 years in the past. In other words, it's already happened, and it's simply taken the light (or other electro-magnetic radiation) that long to get here. If some aliens were pointing a camera at us from 100 light years away, they could observe all manner of interesting events.

For instance, the Earth is actually about 8 light-minutes from the sun - it takes 8 minutes for it to get 'here'. If the sun was to go out suddenly (a terrible tragedy, to be sure), we wouldn't find out... for 8 minutes (err, just eyeballin' it, no special detectors).

-
+1 Thanks for answering! But, if we only get the light that is comming to here, how do we know how many time this light take to arrive? – Tom Brito Dec 6 '11 at 18:31
We can tell how far away some stars are by paralax. Other ways of measuring the distance to stars are noting it's apparent luminosity, comparing its spectrum to known distances, and the like. We know that light has a fixed speed, so thus we know how long ago it left its source. – JasonR Dec 6 '11 at 19:43
If you look at a car in the distance, what you are seeing is light that has travelled from it to your eye. We can work out how far it must be by comparing how small it looks against other cars. There are similar techniques to compare a distant star to others that look similar. – Rory Alsop Dec 7 '11 at 12:02
Even with special detectors we wouldn't find out for 8 minutes surely? As no information can travel FTL, and the sun disappearing is certainly information. – Jonathan. Dec 13 '11 at 0:14
@RoryAlsop your example is a little confuse, because, unlike cars, the things on the universe have very different sizes. – Tom Brito Jan 2 '12 at 18:22

You can see an analog of light by using sound. When an aeroplane passes overhead and you look at where the noise is coming from you find that the aeroplane is not there, it is in a differen position to where your ears tell you it is.

-

Simply as I can put it is that when you take the picture of the object 100 light years away your camera is recieving light that was sent 100 years ago . But the object isn't necessarily in that spot anymore that's just were the object was 100 years ago when it sent the light.

-

You would receive the photo as long as the light of the photografer taking the photo reaches you (just few seconds of delay between the 2 ), so basically you would receive the same data again, so the photographer sending the photo is no more reliable than the subject's light itself. Receiving the photo makes no stronger guarantee of subject being still there after 100 years.

-