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Author Topic:   Did the expansion rate of the universe exceed lightspeed?
cavediver
Member (Idle past 3665 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 60 of 86 (460258)
03-13-2008 6:48 PM
Reply to: Message 57 by LouieP
03-13-2008 12:14 PM


Re: Speed of Light
i assume that there is then a relationship between time and space then.
so if you were going at .999 the speed of light would your time be .001 of normal?
Yes, there most certainly is a relationship between time and space - this is the heart of Special Relativity. However, the relationship is based upon hyperbolic geometry, so at .999c, your time would be the square root of (1 - .999^2) as a fraction of normal, so about .045
At .5c, your 4-velocity would be projected .86 into time and .86 into space. But an observer would simply see you travelling at .5c - if he looked closely, he would see your time having slowed, by watching your watch tick, or your heart beat. He would also see your length contracted in the direction of travel. Both of these are the result of the above space and time projections.
There is no way to travel such that you age the same as the person who stays behind. The longest path through space-time is that that experiences no acceleration. Any acceleration will always shorten the path, and hence reduce your age between two events.
in the future there could be "time jumpers" that do what they can to learn as much as possible while the world ages behind them. crazy!
Yes, it is very tempting to jump through time to see what happens - I'd certainly be tempted, though given the current state of our space-faring technology, I don't hold much hope for a near-c spacecraft in my lifetime! Alternatively, you can get the same effect by hanging around a black hole. Sadly, to 'hang' around a black hole requires propulsion systems that could easily take you to near-c.

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cavediver
Member (Idle past 3665 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 66 of 86 (460340)
03-14-2008 9:13 AM
Reply to: Message 62 by LucyTheApe
03-13-2008 11:50 PM


Re: acceleration
Cavediver, If I pick up a stone and take it to the end of the universe, and drop it, and there's nothing except the earth, how fast is it going when it hits the ground?
If there were only the earth in the Universe, your Universe would be infinite in extent, and described by the Schwarzschild geometry (with a slight Kerr-Newman perturbation). In this case, lyx2no is correct - the stone would be travelling at around 11.2 km/s when it hit, assuming it was released from some sufficently large distance away, and ignoring air resistance. It is independent of exactly how far away it is dropped, and as space is infinte in this case, you can go as far away as you like.
In an expanding Universe, a stone sufficiently far away will never make it to the Earth as the expansion will work to increase the distance between the stone and the Earth.

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cavediver
Member (Idle past 3665 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 74 of 86 (460363)
03-14-2008 1:05 PM
Reply to: Message 68 by ICANT
03-14-2008 10:57 AM


Re: acceleration
Could you explain to me how that two of those objects that are 2.5 billion light years apart at present could end up in the same place?
There are no objects 2.5 billion ly apart that will end up at the same point (neglecting re-collapse) - you are thinking of M31, the Andromeda galaxy, and The Milky Way. These are a mere 2.2 million ly apart - practically on top of each other from a large scale structure point-of-view. And a collison in 2.5 billion years means that they are moving at a small fraction of c with respect to each other.
Neighbouring galaxies (such as ours and M31) are largely unaffected by the expansion of the Universe, becasue local gravity dominates. The expansion visibily operates on the scale of clusters and super-clusters of galaxies.

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cavediver
Member (Idle past 3665 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 80 of 86 (460396)
03-14-2008 5:41 PM
Reply to: Message 76 by Percy
03-14-2008 3:50 PM


Re: Question About Expansion Rate and Clocks
Naturally they'd both have the same amount of time Doppler effects, but wouldn't the nearby galaxy experience a large relativistic time dilation relative to ourselves and the distant one not?
No, both would exhibit time dilation. You might differentiate them and call one relativistic time dialtion, and the other cosmological time dilation, but both are caused by identical phenomena - their time directions are rotated with respect to ours. In the former, the rotation is casued by the relativie velocity; the latter, by the actual curvature of space-time tipping over the distant light-cone with respect to ours.
Edited by cavediver, : No reason given.

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 Message 82 by Percy, posted 03-14-2008 8:36 PM cavediver has replied

  
cavediver
Member (Idle past 3665 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 83 of 86 (460450)
03-15-2008 6:59 AM
Reply to: Message 82 by Percy
03-14-2008 8:36 PM


Re: Question About Expansion Rate and Clocks
Sylas was addressing Tired Light, which assumes a static source,non-expanding space, and some mechanism to redden the light as it travels towards us. It cannot explain observed time dilation in observed objects, such as seen in th SN spectra that Sylas mentioned. However, it is very difficult to differentiate the relative contributions of cosmological and relativistic red-shift for a single object. That is where statistical analysis of large numbers of objects becomes valuable.
Time dilation is the same regardless of the direction. Doppler is highly direction-dependent. Blue-shifts remain blue-shifts, although lessened, after time-dilation correction. Transverse
objects experience red-shift solely from time-dilation. Red-shifts are accentuated by time-dilation. So Arp still has to explain why all his galaxy-associated high-velocity quasars are all heading away from us!!
Edited by cavediver, : No reason given.

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Replies to this message:
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cavediver
Member (Idle past 3665 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 85 of 86 (460506)
03-15-2008 7:30 PM
Reply to: Message 84 by Percy
03-15-2008 4:36 PM


Re: Question About Expansion Rate and Clocks
Information is sparse, but it appears that a survey of the local group reveals that all eleven companion galaxies have a redshift relative to their parent galaxies (is this true?), when it should be roughly 50%.
I've just perused the fourteen companions of M31 listed on Wiki - seven are higher z than M31, five on lower (more negative z), and there is no data for two. Hmmm...

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