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Old 10-10-2007, 11:06 AM   #16
Choobus
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RenaissanceMan wrote View Post

The question I would ask... is can they still interact at all? I'm going to assume not.
Your assumption is wrong.

If two galaxies are moving away from each other in opposite directions, both moving at 0.8c (say) then a third party in the middle will see the distance between them growing at a rate consistent with 1.6c. However, this is only true from the perspective of the third party. It is the speed of galaxy A relative to galaxy B that matters, and this can never exceed c. If you want to know the relative speed between the two you have to use the formula

VR = (VA+VB)/(1+(VAVB/c^2)). so in this case you get VR = 0.98c and light from one galaxy will reach the other.

That, my friend, is why Albert would bitchslap you like you owed him money.

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Old 10-11-2007, 05:05 AM   #17
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Try this
Thanks. This is a very interesting read.
I get a couple of its points despite having been away from formal physics for some time.

If the universe(s) are actually as simple (lacking in information) as the paper says, it confines any possible creator-god to such a tiny box as to be no more powerful or intelligent than the force needed to topple a pencil balanced on its point.

In a developing system, randomness adds to the information content by decoupling it from cause-effect. This supports the ability of random mutation, with subsequent selection, to increase the information defining a biological entity.

As I read it, the paper says that attributes we ascribe to our reality are really something else that we cannot, even in principle, correctly observe or discover. For example, there is something in the wider reality that is not light, but which appears to be light by every means we have of perceiving it and the apparent effects it seems to have on every object that we think is illuminated. Supposing that it is true that light doesn't really exist on a universal level, such operations as photosynthesis suggest that light does exist on a local reality level. So we can continue breathing oxygen with a clear conscience.

The notion that the universes in the multiverse are superpositions is interesting, but I do not see that it makes any other universe than our "own" any more accessible. The multiverse concept is a convenient place to stick unwanted or difficult notions like alternate values in the fine-tuning hypothesis.

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Old 10-11-2007, 08:27 AM   #18
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Choobus wrote View Post
Your assumption is wrong.

If two galaxies are moving away from each other in opposite directions, both moving at 0.8c (say) then a third party in the middle will see the distance between them growing at a rate consistent with 1.6c. However, this is only true from the perspective of the third party. It is the speed of galaxy A relative to galaxy B that matters, and this can never exceed c.
Is this still true if the recession is caused by the expansion of the universe? My understanding was that this is only true of galaxies travelling in space, not of recession caused by the expansion of space itself.

On the same topic, I guess, I did read somewhere that the total energy of the universe amounted to zero, because gravitational potential energy (which for some reason is negative) cancels out all the rest. But I can't remember where this came from. If you have any idea if this is the case and why I'd appreciate an explanation.

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Old 10-11-2007, 09:39 AM   #19
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Is this still true if the recession is caused by the expansion of the universe? My understanding was that this is only true of galaxies travelling in space, not of recession caused by the expansion of space itself.

On the same topic, I guess, I did read somewhere that the total energy of the universe amounted to zero, because gravitational potential energy (which for some reason is negative) cancels out all the rest. But I can't remember where this came from. If you have any idea if this is the case and why I'd appreciate an explanation.
I think that if relativity is violated be the expansion opf space you will get all sorts of causality problems. I think it is still correct, but I could be wrong.
The total energy of the universe may be negative because of all the dark energy, which is supposed to have some unusual properties. It's still a bit of a mystery actually, but in the last few years there have been some really amazing data from the Hubble and Chandra telescopes looking at type 1A supernoave (whiich are like "standard candles" and very useful for measuring the expansion of the universe). I think now it is pretty well established that the expansion of the universe is now accelerarting, and that there was a transition to this phase about 5 billion years ago (before which is was slowing down). This tells us something about dark energy, which has a "negative pressure". I don't know why the GPE would be negative, I would think it would be like a pendulum, converting kinetic energy to potential energy.

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Old 10-11-2007, 10:12 AM   #20
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GPE from here http://www.sparknotes.com/testprep/b...section3.rhtml but I don't know if it actually makes sense. Wikipedia says the same thing but when I put this to some mathematicians of my acquaintance they ridiculed the idea that we claim 0 GPE at infinity.

The recession of the universe thing I'm pretty sure is true. I saw it in a SciAm article (P5 of the PDF) so it must be correct

Actually before I saw that article I assumed the two worked out as equivalent.

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Old 10-11-2007, 10:17 AM   #21
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I prefer GFE.

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Old 10-11-2007, 10:34 AM   #22
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Old 10-11-2007, 10:46 AM   #23
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GPE from here http://www.sparknotes.com/testprep/b...section3.rhtml but I don't know if it actually makes sense. Wikipedia says the same thing but when I put this to some mathematicians of my acquaintance they ridiculed the idea that we claim 0 GPE at infinity.

The recession of the universe thing I'm pretty sure is true. I saw it in a SciAm article (P5 of the PDF) so it must be correct

Actually before I saw that article I assumed the two worked out as equivalent.
Looks like I was wrong about the receeding galaxies. Even if they cannot interact though they may still have some quantum entanglement since they were once part of the same soup.

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Old 10-11-2007, 11:14 AM   #24
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I love this place.
Yeah, there's been some kick-ass science and philosophy going on around here the last couple of days. I'm about to tape up my glasses and wear a pocket protector just for shits and giggles.
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Old 10-11-2007, 11:40 AM   #25
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Your assumption is wrong.

If two galaxies are moving away from each other in opposite directions, both moving at 0.8c (say) then a third party in the middle will see the distance between them growing at a rate consistent with 1.6c. However, this is only true from the perspective of the third party. It is the speed of galaxy A relative to galaxy B that matters, and this can never exceed c. If you want to know the relative speed between the two you have to use the formula

VR = (VA+VB)/(1+(VAVB/c^2)). so in this case you get VR = 0.98c and light from one galaxy will reach the other.

That, my friend, is why Albert would bitchslap you like you owed him money.
What a brain you have Choob! I envy your axons.

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Old 10-11-2007, 11:41 AM   #26
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What a brain you have Choob! I envy your axons.
yeah, but I was wrong! (at least, I was wrong if he was talking about galaxies moving through space rather than the expansion of space itself).

By the way, even though I was wrong about the recession it still doesn't mean that there would be no interaction between the galaxies because the Hubble constant is not a constant (see page 7 of pdf).

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Old 10-11-2007, 12:07 PM   #27
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yeah, but I was wrong! (at least, I was wrong if he was talking about galaxies moving through space rather than the expansion of space itself).

By the way, even though I was wrong about the recession it still doesn't mean that there would be no interaction between the galaxies because the Hubble constant is not a constant (see page 7 of pdf).
I am not that good at computing, that's the reason I admire anyone that can. I know those equations are delightful music to physicists I wish I wasn't tone deaf to them and could hear them..

Christians and other folks infected with delusional beliefs think and reason like schizophrenics or temporal lobe epileptics. Their morality is dictated by an invisible friend called Jesus.
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Old 10-11-2007, 05:49 PM   #28
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You can hear the music, Cal. But just a word of warning- when riffling the pages of a 300-page thermodynamics text, the swishing sound some people seem to think is making them more informed isn't truly it. Truth be told, there are subjects that would be harder for me than physics! But that is only because I don't find them interesting, not because they are difficult.
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Old 10-11-2007, 05:52 PM   #29
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I prefer GFE.
Government Furnished Equipment? You getting your women from the gov, now?
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Old 10-13-2007, 08:48 PM   #30
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Yeah, there's been some kick-ass science and philosophy going on around here the last couple of days. I'm about to tape up my glasses and wear a pocket protector just for shits and giggles.
You mean I'm not supposed to have been wearing a pocket protector all this time? I can't begin to contemplate cosmology without one. Damn, must have missed another memo!

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