imjeffp
Senior Space Cadet
Reged: 09/30/03
Posts: 4436
Loc: Cedar Park, Texas
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OK, I've managed to wrap my brain around the idea that with a constant speed of light, then both time and distance become relative to the observer. A clock moving at a significant percentage of C will appear to be running slow compared to one at rest relative to the observer. A measuring stick moving will appear shorter than one at rest.
So--an expanding universe. The further away, the faster the expansion, right? A very distant galaxy could be moving at a fair clip relative to us. How do relativistic effects affect observations? There's something about it I can't quite figure.
Also--could a distant galaxy be receding at a relative velocity greater than C? Essentially, there would be an 'event horizon' beyond which we can make no observations--like an inside-out black hole.
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matt
Vendor (Scopemania)
Reged: 07/28/03
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Loc: Chaville, France
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"How do relativistic effects affect observations?"
Actually the galaxies are not receding from us - it's space itself which is expanding. This is why redshift is different from the Doppler effect. I know it's difficult to admit, but get over it. I did.
"Also--could a distant galaxy be receding at a relative velocity greater than C? Essentially, there would be an 'event horizon' beyond which we can make no observations"
Yup. We only see part of the universe, and we will even reach a day (which will be a dark day for galaxy observers such as me, but we still have some billions of years left) when no galaxies will be visible beyond the local group.
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Matt
CI700 mount with various scopes on top.
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imjeffp
Senior Space Cadet
Reged: 09/30/03
Posts: 4436
Loc: Cedar Park, Texas
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Quote:
Actually the galaxies are not receding from us - it's space itself which is expanding.
Huh? 
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matt
Vendor (Scopemania)
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yup.
It's hard to believe that space is expanding, not the galaxies fleeing away from each other, because we think space is vacuum, so vacuum can't dilate. But really smart people tell us it does, and most smart people who work on the subject agree on this now (or they just agree not to look like "dummies" in front of their colleagues. I imagine a meeting of AA (astrophysicists anonymous) with young researchers coming to tears admitting they don't understand how space can expand, and how gravitational lenses happen  ).
I often have in my imaginaion a vision of a map of the universe with galaxy clusters everywhere, and a scale on the X, Y and Z axes, and it's not the galaxies flying apart, it's the graduations on the axes getting smaller. It both thrills and baffles me.
About gravitational lenses, same story. It's not the mass of galaxy clusters bends light, the light still travels in a straight line but the "fabric" of space is bent by mass. Neat, huh?
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Matt
CI700 mount with various scopes on top.
Edited by matt (03/02/04 03:27 PM)
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rboe
Numbfinger
Reged: 03/16/02
Posts: 39767
Loc: Phx, AZ
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The clever people always use the expanding ballon as an example of an expanding space. So I will too.  If space is your ballon, puff your cheeks, and expand your universe. If you premarked your ballon with dots (galaxies - because I'm not into doubles) then measured them as you huff and puffed; well shoot, you don't need to measure them, of course they will get further apart as the ballon material stretches.
Of course the universe does not have Thor inflating it at a constant rate - Thor throws hammers. But inflate it does. But the universe is not a ballon with galaxies on its' surface, it's more of a funky volume with internal fabric bending with massive masses. But all these analogies break down fairly quickly so don't carry this one too far. It's just supposed the help you understand this expanding universe thing. The clever people said so. 
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Ron
NS11GPS
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Anonymous
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To make more things 'simple'; the smart people do not know yet if the big man keeps on huffin and puffin till the balloon explodes or if he get's tired and let the air out again... The funny thing about this explaination is; if you travel fast enough around the balloon, you'll end up were you started. i wonder if this goed for the universe too...  Heck if i know!
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rboe
Numbfinger
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Loc: Phx, AZ
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I do believe they have come to the conclusion the ol' ballon will just bigger and bigger until the fabric is so thin you can't see it anymore.
Which begs the questions; if you did start walking around the ballon; is it fair they keep making the ballon bigger? With it getting bigger is it even possible to make it around before it's too big or how fast will you have to go to do one lap? Once it becomes so big that the only one you can see is yourself - will you be truely alone then? I think these are quesions on the Graduate Entrance Exams for Astrophysics. 
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Ron
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imjeffp
Senior Space Cadet
Reged: 09/30/03
Posts: 4436
Loc: Cedar Park, Texas
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I like the raisin bread analogy myself: as the bread bakes, the raisins get further apart.
But it still doesn't get to what I'm wondering. Is (insert distant galaxy here) moving relative to us at high velocity? Yes? Then how does relativity affect our observations of it? It should appear smaller and "aging" slower, right?
As to the nature of the universe, I like to use this thought process. Imagine a 2-dimensional-being on a 3-dimensional sphere. The surface is finite, but endless. An expanding sphere would cause all other points to be receding. The shortest distance from A-B would be a curved line through another dimension. A 3-dimensional being could do the impossible--enter and leave a closed container, become visible and invisible, and change its shape.
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desertstars
Deja moo
Reged: 11/05/03
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Loc: Tucson, AZ
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And, of course, now you understand why some of us stuck with majors in biology!
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Tom W.
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rboe
Numbfinger
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The shortest distance between A & B would be a straight line on the surface of the sphere - in the same dimension as you 2D being. Not a different dimension.
Since a 3D being would only had height, the curved line would remain. Your straight line would appear curved. Suddenly biology sounds much better. I was much better at this when it was fresh. 
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Ron
NS11GPS
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Anonymous
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Quote:
But it still doesn't get to what I'm wondering. Is (insert distant galaxy here) moving relative to us at high velocity? Yes? Then how does relativity affect our observations of it? It should appear smaller and "aging" slower, right?
Ok, here's the catch. As far as i understand, latest info says that increased expansion of the universe started about 4 to 6 billion years ago.
Knowing this, when looking at fainter objects (thus looking further back in time), until that thresshold, object should have been moving slower and slower appart, because you're looking back in time. Beyond the 4 to 6 billion year treshold, things start moving up agian... Get it?
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desertstars
Deja moo
Reged: 11/05/03
Posts: 30057
Loc: Tucson, AZ
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Yeah. Sure. 
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Tom W.
SVP8 'She turned me into a 3-legged Newt' EQ
Ralph, the All-Purpose 102mm Refractor
Under the Desert Stars
Alcohol and calculus do not mix. Please don't drink and derive.
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Anonymous
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hmmmpff...this is hard to do in a nutshell...
every tried to read Stephen Hawkin's 'the Universe'?
it explains a lot
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desertstars
Deja moo
Reged: 11/05/03
Posts: 30057
Loc: Tucson, AZ
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Quote:
hmmmpff...this is hard to do in a nutshell...
every tried to read Stephen Hawkin's 'the Universe'?
it explains a lot
That's for sure!
All kidding aside, I think I have as much a handle on this as any non-physicist can have, and mostly thanks to the efforts of those scientists who have taken the time and trouble to find words that fit: Carl Sagan, Stephen Hawkin, Brian Greene, among many, many others.
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Tom W.
SVP8 'She turned me into a 3-legged Newt' EQ
Ralph, the All-Purpose 102mm Refractor
Under the Desert Stars
Alcohol and calculus do not mix. Please don't drink and derive.
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Anonymous
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Quote:
I like the raisin bread analogy myself: as the bread bakes, the raisins get further apart.
But it still doesn't get to what I'm wondering. Is (insert distant galaxy here) moving relative to us at high velocity? Yes? Then how does relativity affect our observations of it? It should appear smaller and "aging" slower, right?
the fact that the universe is expanding does change the way you observe things.
One thing that changes is the time frame in which you're seeing things. For example...imagine you were at the eyepiece constantly monitoring changes of a celestial object 5 billion light years away for one whole week. The changes that you can see happen in a whole week wouldn't have happened in a week at the actual object you're observing. Since the universe is expanding, light from that object will take longer to reach you than it would if the universe was static...so you'll see the changes happening slower than they actually did. So you're correct about that assumption, I believe.
I'm not a scientist myself, and also have only a very superficial understanding of the consequences of general relativity (from Hawking, sagan and the gang ).
However, I read about this exact problem when reading "The extravagant universe" by Robert Kirschner...it's a book about one of the 2 major teams involved in studying type 1a supernovas to find out if the expansion of the universe is accelerating or not (which they found it was). One of the problems they had to take into account when trying to figure out the peak curve and time a supernova took to dim was exactly due to the efects of general relativity making dimming "seem slower" when viewed from earth than it actually was at the supernova.
(I also like the raisin bread analogy, and also the ant on an expanding rubber band analogy too, since I find it makes it easier to understand that the "rubber band" is expanding, but the ant isn't).
Carlos.
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Anonymous
Unregistered
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Hawking is the only one who could ever explain it to me that I could understand.
Think about someone watching us from 5 billion LY's away. Our time seems normal to us, which is relative anyway) but to those watching us it would take forever for them to see a change.
Ron, you beat me to the balloon model.
I can really get wrapped up in these mind exercises when I'm in the mood. The twin's paradox is a real mind-bender. It's the only way I can think of to time travel without time traveling. Relatively speaking, of course. 
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summitlake
scholastic sledgehammer
Reged: 09/05/03
Posts: 878
Loc: Castro Valley CA
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Great fleas have lesser fleas upon their backs to bite 'em, And little fleas have lesser fleas, and so on ad infinitum, And the great fleas, themselves, in turn, have greater fleas to go on, While these again have greater still, and greater still, and so on. -- attributed to Augustus DeMorgan, 1872
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Alex
Meade LX90 8" SCT
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Victor Kennedy
Pooh-Bear
Reged: 05/22/03
Posts: 9850
Loc: Slovenia
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Another one, maybe even a little easier to understand than Hawking, is Understanding Relativity: A Simplified Approach to Einstein's Theories, by Leo Sartori.
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To err is human; to moo is bovine.
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imjeffp
Senior Space Cadet
Reged: 09/30/03
Posts: 4436
Loc: Cedar Park, Texas
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I've been re-reading The Physics of Star Trek along with A Brief History of Time.
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StarWars
Postmaster
Reged: 11/26/03
Posts: 11661
Loc: Cyber Space
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Quote:
I do believe they have come to the conclusion the ol' ballon will just bigger and bigger until the fabric is so thin you can't see it anymore.
Which begs the questions; if you did start walking around the ballon; is it fair they keep making the ballon bigger? With it getting bigger is it even possible to make it around before it's too big or how fast will you have to go to do one lap? Once it becomes so big that the only one you can see is yourself - will you be truely alone then? I think these are quesions on the Graduate Entrance Exams for Astrophysics.
The universe will continue to expand like the balloon and then collapse. The collapsing space will direct all matter (Galaxies) back to an area the size of a golf ball and then explode once again (Big Bang)...This is called the Closed Universe!!
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