|MadSci Network: Astronomy|
Hi Koen, Your question touches some very deep parts of our understanding of the universe. Let's look at what we mean by "location", first, and use an example. Where am I located? As I'm currently typing this, I'm sat in a chair in an office in Glasgow. So we've got a time and a position. But by the time you read it, I could well be somewhere else. The information you've inferred about my location (chair, office, Glasgow, "now") is likely to be out-of-date due to the time it takes that information to reach you. What does this mean? Well, when that information finally reaches you, at the moment you call "now", you'll be receiving a snapshot of my location at a point in time that I would then be calling my "past". This is caused by a "finite speed of information" which also applies to visual information conveyed by the speed of light. Everything you see, from your computer monitor, to the planets, and on to the furthest stars, is a portrayal of objects in the past. While your monitor may not have moved much in the couple of nanoseconds it takes for its light to travel to your eyes, the planets and further stars may have moved great distances. Jupiter moves over 30000km in the 40-or-so minutes it takes its light to reach us. Arcturus (36.7 light years away) moves about 140 billion kilometres since that star's light set out. Now factor in your light bending. For nearby stars and galaxies this effect is really very small, but it can be measured when light passes near to the surface of the Sun, and it can be highly noticeable over greater distances when larger gravitational masses are involved. Here is a good example of light from a distant galaxy being bent by the gravity of a nearer one. What does this all mean with regards to location? In effect, we never know the present location of an object, as there is no universe-wide sense of "now" as a point of reference, no "infinite speed of information" to back up such a view. Astronomers are looking into time when they receive information about the universe, and while the rough location of objects is easy (Jupiter is orbiting the Sun, Arcturus is a near neighbour star in this galaxy) our ideas of "location" for distant objects are relative to our position in time and space as viewers. Can you see that these "locations" would necessarily be different if our position was different? I hope this helps answer your question - and makes you think more about the effects that relativity can produce!
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