Cosmological redshift: how much energy has gone missing?

[schwarzchildradius]

I don't recall seeing anything there about "t=0" and the beginning of the universe and "physics breaking down"

he says that right on the web page.

 

[marcus]

Originally posted by schwarzchildradius

quote:
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I don't recall seeing anything there about "t=0" and the beginning of the universe and "physics breaking down"
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he says that right on the web page.

Er...I will post the URL for the fifth time in a row.
There are ten paragraphs on the page and a picture which
gives the key to why energy conservation is not proven in GR.
It's a sphere showing parallel transport of tangent vectors.
There is nothing on the page about "t=0" and "physics breaking down".

What I believe I'm testing is what happens when you ask a question and request a link, and then I give you a link bearing on the question. The issue is trolldom, I believe.

Here is the link again:

http://math.ucr.edu/home/baez/einstein/node2.html

In which of the ten paragraphs does it talk about "t=0"
and "physics breaking down"?

We two are certainly burning up the wires with all this communication aren't we?

 

[schwarzchildradius]

Now a miracle occurs...
so, true it doesn't say the words "physics breaks down."

 

[marcus]

Originally posted by schwarzchildradius
Now a miracle occurs...
so, true it doesn't say the words "physics breaks down."

My link was to page 2 because of the bearing on your question

http://math.ucr.edu/home/baez/einstein/node2.html

Your link is to page 7

http://math.ucr.edu/home/baez/einstein/node7.html




On page 7 Baez says: [[...Now a miracle occurs. By all rights, this equation should only hold at t = 0 for a small ball of initially comoving particles in free fall. However, in the special situation we are considering, it holds at all times for arbitrarily large ball of galaxies, even though the galaxies are not comoving! I wish I knew a simple reason why this works...]]

He is not saying "the physics breaks down" but that the model continues (fortunately and unexpectedly) to hold for all time than only around a specific moment t = 0.

What he is doing is deriving a form of the first Friedmann equation from the simplified GR equation that he started with.
The whole essay is "GR made simple" and at some point one wants to get the Friedmann equation from the main Einstein one---he is doing this and saying "the physics works here remarkably well and I can't explain why it does in a simple way but just take it on faith that the equation works for all time under assumptions of homog and isotropy."

I don't think one can understand this out of context. Indeed you have misunderstood the passage with a complete 180 degree twist! Although it says Big Bang in the title on page 7 this does not mean that t = 0 refers to the very beginning of the universe and he is saying that the equation does NOT break down.

At issue for me is I need to learn what happens when you ask a question and I find something on web that bears on it and give you a link to a specific page (in this case with a specific picture).
Then is it likely to happen that you get to talking about something that is not on the page and misinterpreting other stuff on the wrong page and, as in this case, making up "physics breaks down" phrases that arent even there and arent part of the authors meaning even on the wrong page! If that happens it means discussion by means of shared web-pages is not workable in this case. For whatever reason---maybe you don't like me, or you are feeling ornery or whatever. It doesn't matter what the reason is, the point is I should not try to share web pages and discuss them with you cause it doesn't work. No hard feelings however

 

[schwarzchildradius]

as long as you have no problems with infinite energy existing in an infinitely small volume, then physics has no problem describing the conditions of t=0.

 

[marcus]

Originally posted by schwarzchildradius
as long as you have no problems with infinite energy existing in an infinitely small volume, then physics has no problem describing the conditions of t=0.

Are you still discussing John Baez's essay
"the Meaning of Einstein's Equation"?

http://math.ucr.edu/home/baez/einstein/einstein.html

It has 9 pages linked together and he calls it "a brief introduction to General Relativity." I still go back over certain pages from time to time because it is a masterful job of making an honest presentation of the mathematics short and understandable.

I suggested you look at page 2 , "Preliminaries".

I can't tell what you are discussing. It does not seem to be page 2.

On many of the pages (not page 2 but others) he uses "t=0"
to stand for some normal reference time, like the present, after the universe is well on its way and evolving normally. Think of it as noon GMT on March 1----some arbitrary time zero. In no case does he use "t=0" to refer to a singularity. Nothing in the essay concern infinite densities. So it has been four days now since I suggested the essay to you and it clearly has not helped you.
I'm not clear about why. But probably means I shouldn't recommend online physics links to you.

 

[schwarzchildradius]

You've got a weird attitude. That particular link has been discussed on PF last year. In any case, t=0 refers to the singularity at the beginning of time. If you have a solution/explanation for this, we would all love to hear of it!

 

[marcus]

Originally posted by schwarzchildradius
You've got a weird attitude. That particular link has been discussed on PF last year. In any case, t=0 refers to the singularity at the beginning of time. If you have a solution/explanation for this, we would all love to hear of it!

The explanation is simple, in Baez essay t=0 nowhere refers to the beginning of time. You totally failed to come to grips with the meaning of his essay, if you think that.
His essay is called "The Meaning of Einstein's Equation"
If there is anyone still at the board who took part in
a discussion "on PF last year" of that online essay, I invite
you to ask them if t=0 means the beginning of time
in that essay. If anyone thinks that, they totally failed to
grasp the equations and should go see a mentor like Tom
or damgo.

I am sorry but I don't feel I should spend any more time on this.
I am not sure I am getting a straight story from you, what with the "physics breaks down" business which was not in Baez essay and our failure to connect about the meaning of basics like the timescale. Adios. Bye.