Doppler effect

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alankennedy
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Doppler effect

Post by alankennedy » Wed Mar 03, 2010 11:21 pm

I asked on some [other] forum about the "theory" and the "fact"...
and wondered what we might look like from 13-15 billion light years from the other side of how far we deem we can "see"

when we have the image of [Einstein] mass presented as creating a dent in the continuum, we need to bear in mind that it is as if representing one drop of rain on a still lake - the "gravity/ripple effect"...
but the matter, in fact, is more like a single raindrop amongst all in a tempest tossed sea, and; "it's raining!"

it's probably a dark matter
brought to light

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Re: Doppler effect

Post by The Code » Thu Mar 04, 2010 12:50 am

Errr, Run that past me again.
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Chris Peterson
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Re: Doppler effect

Post by Chris Peterson » Thu Mar 04, 2010 7:01 am

alankennedy wrote:I asked on some [other] forum about the "theory" and the "fact"...
and wondered what we might look like from 13-15 billion light years from the other side of how far we deem we can "see"
The farthest we can possibly see is to the edge of the observable Universe, about 47 billion light years. Anything beyond that is not observable, and neither would be be observable to someone beyond that- whether by 1 km or 15 billion ly.
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wonderboy
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Re: Doppler effect

Post by wonderboy » Fri Mar 19, 2010 4:03 pm

Heres the thing. if we can see 47 billion years into the past (because the observable light we're seeing has taken 47 billion years to reach our retinas) then surely people that far away would look at us and see us as we were 47 billion years in the past. I've said before that if you had unlimited powers of magnification and clarification then technically you could see things happening on planets billions of light years away. Its mind boggling and I love it.
"I'm so fast that last night I turned off the light switch in my hotel room and was in bed before the room was dark" Muhammad Ali, faster than the speed of light?

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Chris Peterson
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Re: Doppler effect

Post by Chris Peterson » Fri Mar 19, 2010 4:33 pm

wonderboy wrote:Heres the thing. if we can see 47 billion years into the past (because the observable light we're seeing has taken 47 billion years to reach our retinas) then surely people that far away would look at us and see us as we were 47 billion years in the past.
The Universe is only 13.7 billion years old. That's as far back in time as anybody can see. It so happens that when we see a photon that was produced 13.7 billion years ago (actually, we don't see photons quite that far back, but lets ignore that fine point), the point where that photon was emitted is now 47 billion light years away (because space has been expanding over that 13.7 billion years). So the oldest thing we can see is 13.7 billion years old, and the most distant thing we can see is 47 billion light years away (but was much closer when it produced the light we see now).
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wonderboy
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Re: Doppler effect

Post by wonderboy » Fri Mar 19, 2010 4:39 pm

ahhhhhhhhhhhhhhh i get ya, but surely the same thing applies. I just got my timing out. this is where I get confused and I'm going to try and explain myself clearly. So before the big bang we dont know what there was. lets just go with the theory that it was a ball. So that ball explodes I go one way, and you chris, go the complete opposite direction. we were both created at the same time and since the speed of light is the fastest thing in the universe, then surely we have been in "visual" contact since the very begining, otherwise the speed of the explosion which thrust us apart was faster than the speed of light?


Do you understand that? I do, its just hard putting it across.
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rstevenson
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Re: Doppler effect

Post by rstevenson » Fri Mar 19, 2010 6:37 pm

For further boggling, consider this. Objects sufficiently far apart in the universe may actually be moving away from each other at greater than the speed of light. How so? In fact, neither object is actually moving that fast -- they may not have much of a local velocity at all. But the expansion of the universe has pushed them so far apart that they are now receding from each other at some apparent velocity >c. Read more here, in the last paragraph of this section in particular.

Rob

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Chris Peterson
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Re: Doppler effect

Post by Chris Peterson » Fri Mar 19, 2010 6:38 pm

wonderboy wrote:So before the big bang we dont know what there was. lets just go with the theory that it was a ball. So that ball explodes I go one way, and you chris, go the complete opposite direction. we were both created at the same time and since the speed of light is the fastest thing in the universe, then surely we have been in "visual" contact since the very begining, otherwise the speed of the explosion which thrust us apart was faster than the speed of light?
Not necessarily. We could have been causally connected early on and disconnected now. That's because you and I could be separating from each other faster than the speed of light. That doesn't break any rules, since no information is being exchanged between us. It is generally assumed that the Universe is much bigger than the 93 billion light year bubble of it that we can observe. The parts outside that bubble are beyond our ability to observe because they are receding at more than C.

Put a little differently, we see the rate of expansion of the Universe increase with distance from the observer. 47 billion light years (or 13.7 billion light years if you think simply in terms of light travel time) is the distance at which the expansion rate exceeds C, so anything beyond that is not observable. But most of the Universe is probably beyond that.
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Amir
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Re: Doppler effect

Post by Amir » Fri Mar 19, 2010 7:13 pm

Chris Peterson wrote: The farthest we can possibly see is to the edge of the observable Universe, about 47 billion light years. Anything beyond that is not observable, and neither would be be observable to someone beyond that- whether by 1 km or 15 billion ly.
Chris,
whenever you talk about observable universe i get confused, and i didn't get around to ask you about it:
how do we know the size of observable and whole universe when haven't it observed yet? and we don't know the size of the universe just after BB (or do we?)
is it speculated from simulations or complicated calculations, or could be explained in simple language?
Amir H Taheri

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Chris Peterson
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Re: Doppler effect

Post by Chris Peterson » Fri Mar 19, 2010 7:32 pm

Amir wrote:how do we know the size of observable and whole universe when haven't it observed yet?
The observable Universe is just that part of the Universe that we might reasonably expect to be accessible, since it hasn't sped up to the point that it is receding faster than light. All you need to do is consider Hubble's Law with a speed of C to get the distance to the edge of the observable Universe (it's actually slightly more complicated than that, but this idea should get you close). Nobody really has a clue about the size of the entire Universe, other than the fact that it appears to be at least as large as the observable Universe. Most people think it is much larger, perhaps even infinitely large. But the only way to eventually have a confident answer will be to further develop cosmological theory.
Chris

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