What does it mean that we can only see the universe in the past?Billio38 - 6 Answers
Speed of light is 300000000 meters per second.
If the distance is more it takes more time for the light to travel.
It takes around eight minutes for light from sun to reach earth.
Similarly if distance is more it will take even longer for the light to travel.
In Universe the distances are immensely huge and since you need light to see what is going on in the dark, it takes a long time for the light to reach us, even years. So actually while an explosion occurs and we see it after a year (distance was large, it took the light of explosion one year to travel to earth) on earth something else may be happening there at present which we will be able to see after another year. :)
It is true that by the use of telescopes we are able to see the universe of the past, because anything is visible only when it reflects any kind of electromagnetic radiation from the electromagnetic spectrum. Since distances in the universe are so large, the light which got reflected years ago from any particular place in the universe reaches the earth's telescopes after a few million years (as the place is a few million light years away, and a light year is the distance light travels in one year).
Thus, the light reaching the telescopes now, had been reflected millions of years ago.
As everything is such a long way away, it takes light 1000's of years to reach us.
Lets say a star exploded 1000 light years away. The light from the explosion starts to travel. It takes 1000 years to reach us. By the time we SEE the explosion (the light that started travelling 1000 years ago), 1000 years have past. We are looking at something that actually happened 1000 years ago.
Yes, as mentioned by the above users, as the distance is very large, light takes a very long time to reach the earth. For instance, it takes 8 minutes for light from the sun to reach us. This means whatever happens on the sun, we can see it only after 8 minutes, I.e, we are viewing the past of the sun by 8 minutes....
I suppose I should have entered this question into the philosophy section, because my question is trying to get at how this fact is significant in understanding human knowledge and science, as well as how this fact might (or might not) effect our physical cosmogonies....
So when you see the explosion, it's already happened. It's just taken so long for its light to reach us....