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Author Topic: How do telescopes look back in time?  (Read 6889 times)

Mustafa

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How do telescopes look back in time?
« on: 25/05/2011 10:01:03 »
Mustafa asked the Naked Scientists:
   
HI,

I have a troubling physics question, that has lingered in my mind for a while, I can't seem to understand it.

All the matter that was once part of the singularity, flowed out in the universe to form the universe as we know today after the big bang. So the milky way, solar system, our Earth, all the matter in it, and around it was once present at that one point in time and space.

I often read and hear scientists mentioning, that when we look back at the universe through our powerful telescopes, we essentially are looking back in time, because light has taken that many years to reach here and hence we see them as they were that many years ago.

Now here is where I lose it a little, if light has taken that many years to get to this point, where we can use a telescope to actually form the image, how did we (as in matter) get here before light(also made of matter) did. this galaxy or solar system or earth for that matter did not form here spontaneously, this matter was part of some other piece of matter, a nebula lets say, which is part of our galaxy, which probably formed over billions of years colliding with other galaxies, and may be starting out from a massive black hole of some kind.

And if that is the case, how come I can be sure that many of the galaxies that we see through the telescopes are not just the older versions of the milky way, I understand that we would have to travel faster then light or close to it for that to happen, but like I said, if we are witnessing the past through telescopes we have already traveled faster then light at some point in time.

Can anyone clarify this for me, and save me from myself.

Mustafa

What do you think?
« Last Edit: 25/05/2011 10:01:03 by _system »


 

Offline Supercryptid

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How do telescopes look back in time?
« Reply #1 on: 25/05/2011 20:54:38 »
An interesting question. The answer has to do with the expansion of space. In the early Universe, space expanded at speeds much greater than that of light. Since space isn't matter nor energy, it isn't limited by Relativity in the same way as those are.

Here's a simplified way of looking at it.

Imagine that you have Alice and Bob in free space in space suits. They are fairly close together, say 1 meter apart. Suddenly, you introduce a massive expansion event in space, an expansion event which exceeds the speed of light. In an hour, Alice and Bob are now 1 lightyear apart from each other when the expansion stops (or at least slows down greatly). Neither of them actually travelled anywhere; new space was created between them which gives the illusion that they have moved away from each other. Waiting one year, Bob then uses a telescope to look at Alice. The light that has reflected off of her (or light generated by a flashlight or other device she happens to have) has now reached Bob. He can see her now, but it is an image of what she was doing 1 year ago.

As you can see, there was no requirement for him to outrun the light she emitted to get to where he was before the light did.

(By this way, the hypothetical enormous expansion event ignores the fact that Alice and Bob may have been exploded by the expanding space ripping their atoms apart. They are kept alive for the sake of the experiment.)
« Last Edit: 25/05/2011 20:57:13 by Supercryptid »
 

Offline Phractality

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How do telescopes look back in time?
« Reply #2 on: 25/05/2011 23:14:53 »
When you watch fireworks, you hear the sound several seconds after the explosion. You are hearing into the past the same way that a telescope sees into the past. If you are present where and when a bomb explodes, you would have to travel faster than sound to hear the bang at another time and place. If billions of bombs went off all around you, the atmosphere would expand, and it would take longer for the sound from nearby bombs to reach you; they would not be so near when the sound reaches you. The sound travels thru the air at the speed of sound, but the rate of change of distance between you and the sound is slower, and the rate of change of distance from you to the sound source is faster. 

We can't travel faster than light, so we can't see our own past in a telescope; we merely see other galaxies as they were millions or billions of years ago. In Euclidean space, gravity bends the path of light; if all the mass is contained in a finite space, the light may follow a closed loop back to where it started, but not back to when it started. So there are no images of our galaxy out there with us looking back at ourselves. In Minkowski space-time, the path of light is the definition of a straight line, so even though light may loop back to where (but not when) it started, it does so on a straight line.
 

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How do telescopes look back in time?
« Reply #2 on: 25/05/2011 23:14:53 »

 

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