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Author Topic:   what is the big bang and how do i understand it?
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 31 of 122 (234726)
08-19-2005 5:29 AM
Reply to: Message 28 by New Cat's Eye
08-18-2005 4:43 PM


Re: tall stack of earths
What's your opinion, if you had to pick one? Which one do you like more?
I've always found finite more aesthetic mathematically. It makes for much more interesting global topology. But I have no problem with infinite.
Does that mean we can't still explore the idea?
Not at all But it is a difficult field in which to make sensible conjectures without a thorough grounding in the subject (i.e. be actively working in the field) And by sensible, I mean consistent with the known mathematics. The analogies of relativity and cosmology have created more confusion than clarity, and they must not be used a starting points for extrapolation. As an example, the book "Has Hawking Erred?" was based upon the fact that us relativists had never explained what the thickness of the rubber represented in the balloon model of cosmology, nor had we explained what was inside or outside of the balloon...
Got any suggestions for a book I should read? I've got the math from getting a B.S. in engineering
That's all the maths you need for relativity. If you're happy (well, aquainted with will do) diff equations and you've a good grounding in vector calculus, then you could go straight to a GR textbook. My favourite for someone in your position is D'Inverno (An Introduction to General Relativity). It is an excellent mix of maths and phsyics, and is also quite readable. But be prepared to put some work in... otherwise, it's probably back to Greene's book. I've never read it, but many here seem to like it.
The other globe is the same universe, just earlier in time.
This sounds so reasonable and believable, that it is so hard to convey the fact that it just doesn't make sense You cannot progress through the singularity to "earlier in time". If you do bolt on an extra globe, you have no guarentee that its time is before our time, is after our time, or is totally seperate from out time. The "neck" radically changes the geometry (and topology) to ensure that progress backwards before t=0 is well defined.

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 32 of 122 (234727)
08-19-2005 5:45 AM
Reply to: Message 30 by New Cat's Eye
08-18-2005 8:29 PM


Re: tall stack of earths
How long was the universe at singualrity before it began to expand?
This question is totally reasonable, but it is just so ill-defined. I don't mean this anyway as a criticism. It is just a freaky application of conventional thinking (how long) to a highly non-conventional concept (singularity). I could say that it was no time at all... but that would be wrong, as time itself has no definition there. If we smooth off the singularity with the no-boundary proposal, then this region involves imaginary time. Now what does this mean? Passage of time is a reasonable concept well away from the singularity, but it is not globally defined.

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RAZD
Member (Idle past 1404 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 33 of 122 (234989)
08-20-2005 10:11 AM
Reply to: Message 26 by cavediver
08-15-2005 8:38 PM


Re: tall stack of earths
Hmmm, I owe RAZD a big reply, but I'm sure he won't mind if I fit this one in first
Message 23 when you have time ...
no hurry.
I'm just sitting here,
paiting watiently
...
...
like chopped liver ...

we are limited in our ability to understand
by our ability to understand
RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}

This message is a reply to:
 Message 26 by cavediver, posted 08-15-2005 8:38 PM cavediver has replied

Replies to this message:
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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 34 of 122 (234991)
08-20-2005 11:24 AM
Reply to: Message 33 by RAZD
08-20-2005 10:11 AM


Re: tall stack of earths
Yeah, yeah, alright... I'm trying to run a business here
I don't think of tweaking one or the other so much as of new ways of thinking about them that alleviate the problems -- a new way of thinking about time for instance.
Absolutely, and time is the biggie... to GR time is just a dimension with an odd signature in the metric. It's not something you travel along or through... there is no dynamics in GR (GR is what I take as God's eye view on reality) But we know that our place in time is much more exotic than this. We travel through time, whatever that means. By incorporating time as just a dimension, relativity has brought far more confusion to the nature of reality than it has clarity!
but the equations still only have space factors
No, this isn't true. In fact it was precisely the opposite that gave the hint that Maxwell's EM was not all it seemed... take EM:
div B = r
div E = 0
curl E = -dB/dt
curl B = dE/dt + J
where E, B and J are 3-vectors.
Now form the Faraday 4-tensor:
and just use
(where J^0 is r) and
You are hard pressed to distinguish time and space here.
and there is no relationship between gravity and distance in time (that I am aware of as a physics "spectator")
There most certainly is. The best demonstration I can think of is Wheeler showing the curvature of space-time around the earth (taken from Gravitation, MTW, page 33)
Throw a ball through the air, then fire a bullet, both from ground level such that both hit the ground 10m away. The ball has horizontal velocity 5m/s and takes 2s, rising 5m in height. The bullet has horizontal velocity 500m/s and takes 0.02s, rising 0.0005m in height. They both follow curved paths, but their radii of curvature are very different. Why should this be, given that the space-time curvature is the same? Becuase we're only looking at the spatial distance and not the temporal. The ball has travelled not 10m, but sqrt(10^2 + (2 x 3e8)^2) = 6e8m when time is factored in. The bullet has travelled sqrt(10^2 + (.02 x 3e8)^2) = 6e6m. The respective heights over the adjusted base-line distances now give identical radii of curvature... essentially the radius of curvature of space-time at the surface of the earth.
So gravity and distance in time is inextricably linked...
More to come
This message has been edited by cavediver, 08-20-2005 11:25 AM

This message is a reply to:
 Message 33 by RAZD, posted 08-20-2005 10:11 AM RAZD has replied

Replies to this message:
 Message 36 by RAZD, posted 08-20-2005 1:51 PM cavediver has replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 35 of 122 (234998)
08-20-2005 12:32 PM
Reply to: Message 33 by RAZD
08-20-2005 10:11 AM


Re: tall stack of earths
I'm having trouble resolving this apparent contradiction. It seems that string theory does not require dimensions, but when they are developed to explain observations that then they have to be there?
We formulate a string theory in N dimensions. We then try to make the mathematics correpsond to reality. We find that unless N is some specific value, we get unreal physics. By reality, I mean things like unitarity, or conservation of probability; the things that make science possible. So N is fixed right at the beginning. In bosonic theory, N is 26. In superstring theory, N is 10. We have known for 80 yrs or so that extra dimensions are good, as they enable us to unify the various forces.
But this still does not negate the fact that we do not have any corroborating evidence [of gavitational waves or gravitons]
Well, we certainly have indirect evidence of gravitational waves. The spin-up of a neutron-star pair has been measured with exceptional accuracy, and agrees with the GR predicted rate based on the system's energy loss due to grav wave emission. I'll have to dig out references.
And we have the rest of our confirmation of GR. Grav waves are just one prediction. Should we worry that we haven't directly detected them, when our instrumentation is only scratching at the sensitivity required? Whatever lies behind GR as some more fundemental theory, I will bet BIG money that it will not contradict GR's grav wave predictions. It will make modifications of course at QG limits...
This leaves the space between stellar systems for the {stage of actions} for the dark stuffs
It's usually the halo that is thought to be the residing place of the dark matter. I'm a long time out of this, but I would have thought that dynamics and observations of the globular clusters would tell us something. And the luminous matter is thought to be ~10% of the total mass of the Galaxy. The 4% figure comes from considering the entire universe.
And had humans on hand to make corrections as needed. More to the point are the {mars landers} of recent fame and misfortune, and they were programmed by GR (IIRC).
I'd have to see references to be convinced. Random noise from solar radiation and wind would surely wamp any GR corrction.
That Pioneer Anomaly stuff was great. Thanks for the references. But consider the level of effect they are looking at, and the level of precision of the calculations. They are using post-Newtonian approximation n-body work to determine if an 8e−8 cm/s2 acceleration is real! That is a measure of the accuracy of GR in the solar system... we can account for all but 8e−8 cm/s2 with the theory. I think we could go many orders of magnitude greater than this and still not lose a Mars probe due to bad physics!
How is the value of {G} determined? What should it be?
Easy, it's 1 What it is to be a theorist...
Seriously though, the value of G is one of the two free parameters in GR. GR cannot really be decribed as empirical just becasue we use observation to fix a couple of free parameters!
Note that one of the problems that I have with the ekpyrosis theory is that it explains the gravity anomaly by having a 'mirror' universe in the other sheet(s) and that gravity carries from one to the other. The problem here is that there is no reason for the other universe to end up with the same mass distribution, so there should be anomalies within anomalies.
I think this is a valid point, but I will have to go back to study "ekpyrosis" a little more before making a serious comment. At the same time, ekpyrosis is just a string based cosmological model, albeit novel, of which we have literally thousands!
Instead consider that each subatomic particle travels in time (similar to the stacks of earth concept). The existence of particles as lines in time...
This is precisely the view of SR, GR, QFT, etc...
The existence of particles as lines in time does not affect the observed behavior of particles at a point in time because it is "pulled" equally forward and back by its "time shadows" and the effect of "time shadows" gets absorbed into Newtonian {g} calibrations and Einsteinian {G} calibrations
But I'm not sure what you are saying here... can you expand a bit?

This message is a reply to:
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RAZD
Member (Idle past 1404 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 36 of 122 (235007)
08-20-2005 1:51 PM
Reply to: Message 34 by cavediver
08-20-2005 11:24 AM


Re: tall stack of earths
cavediver writes:
take EM:
div B = r
div E = 0
curl E = -dB/dt
curl B = dE/dt + J
etc
Guess I asked for that ...

--------Q
IKN

{{{digs out old math books, blows dust off covers, sneezes ... }}}
ermmm
I'll have to get back to you ...
(I'll get to your next one later)

we are limited in our ability to understand
by our ability to understand
RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}

This message is a reply to:
 Message 34 by cavediver, posted 08-20-2005 11:24 AM cavediver has replied

Replies to this message:
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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 37 of 122 (235034)
08-20-2005 4:56 PM
Reply to: Message 36 by RAZD
08-20-2005 1:51 PM


Re: tall stack of earths
Don't worry My point was just that space and time seem to have different roles when EM is looked at traditionally, but the simple dimensionality of time becomes apparent when using the language of tensors.
Did you look over the later points in the post? They weren't quite so gory...

This message is a reply to:
 Message 36 by RAZD, posted 08-20-2005 1:51 PM RAZD has replied

Replies to this message:
 Message 38 by RAZD, posted 08-21-2005 12:08 AM cavediver has replied

  
RAZD
Member (Idle past 1404 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 38 of 122 (235162)
08-21-2005 12:08 AM
Reply to: Message 37 by cavediver
08-20-2005 4:56 PM


time shadows, empirical g forces, etc.
Did you look over the later points in the post? They weren't quite so gory...
I just lost the post I was working on and only recovered 1/4 of it here, so I may be a little disjointed in this response. That was gory. Onward into the fray:
cavedigger, msg 34 writes:
Throw a ball through the air, then fire a bullet, ...The ball has travelled not 10m, but sqrt(10^2 + (2 x 3e8)^2) = 6e8m when time is factored in. The bullet has travelled sqrt(10^2 + (.02 x 3e8)^2) = 6e6m. The respective heights over the adjusted base-line distances now give identical radii of curvature...
Not sure how you get a 60,000,000 (6e8/10) fold difference in distance there other than assuming an orthogonal triangle made from distance and time and finding the hypotenuse ... doesn't this assume an equivalence between (1m) and (1s x 3e8)? I realize that 3e8 is the speed of light and this makes the units work out, but is this valid? or is there another constant involved? This seems to be the {math elegance} problem to me.
I assume that the trajectory heights are also "adjusted" to then calculate the radii, but then I have to wonder what radii are being measured...
cavediver, msg 35 writes:
In bosonic theory, N is 26. In superstring theory, N is 10. We have known for 80 yrs or so that extra dimensions are good, as they enable us to unify the various forces.
So N is fixed right at the beginning.
at 26 or 10? or whatever? This is one of the {elegant math} problems imho. and N is whatever it takes to make the predictions match the observations ...
The spin-up of a neutron-star pair has been measured with exceptional accuracy, and agrees with the GR predicted rate based on the system's energy loss due to grav wave emission. I'll have to dig out references.
What do the other theories predict? One of the things that is supposed to validate the theory of ekpyrosis is a lack of gravity waves: how does this handle this situation (tough question I understand, and unfair if you aren't familiar with ekpyrosis, but you can understand my reason for asking: I don't really need an answer unless it is easily obtained).
Grav waves are just one prediction.
But that is not all that is missing. It seems to me that not one thing predicted by (any of the) gravity theories has been found.
It's usually the halo that is thought to be the residing place of the dark matter.
And the luminous matter is thought to be ~10% of the total mass of the Galaxy.
So this halo exists just outside the solar system (in the area that the pioneer satellites are just entering) and yet it is composed of 89.986% of the solar system (the planets = 0.14% of the mass of the sun which is the 10% luminous matter here) and yet it doesn't interfere with any observations beyond the halo while being simultaneously dense, undetectable and universally distributed in a spherical shell ... is it made of salt?? (sorry, poor joke, but I think you can understand my problem here)
Random noise from solar radiation and wind would surely wamp any GR corrction.
Well I could be wrong on the use of GR for the Mars landers. Or they did it because they could, or because they lost a couple to {math\unit} errors and were gunshy, or because they were government contractors on an hourly payscale .
That Pioneer Anomaly stuff was great. Thanks for the references. But consider the level of effect they are looking at, and the level of precision of the calculations. They are using post-Newtonian approximation n-body work to determine if an 8e−8 cm/s2 acceleration is real!
Thanks, I thought so too. But also consider that this amount of acceleration is the same order of magnitude as is needed to correct the predicted behavior to match the observed behavior of large galactic systems. That to me is more than coincidence.
Seriously though, the value of G is one of the two free parameters in GR. GR cannot really be decribed as empirical just becasue we use observation to fix a couple of free parameters!
Ah, so close.
Let's start with Newton's general gravity equation. We observe that the force of attraction is proportional to the mass of object {A} and the mass of object {B} and inversely to the distance between them. We take extensive measurements at very fine precision and determine the value of G in the equation
F = GmM/d2
but
we cannot distinguish the effect of general relativity on this value, and further, any acceleration value of the amount given above is outside the level of accuracy here, and both get buried into the value of G even though they don't depend on the same generally observed relationships. The observations are the trump cards here and the formula is adjusted to match calculations to observations. That is the essence of an empirical formula, yes?
Now on to Einstein's formula:
Gab = {8(pi)G/c4}*Tab
from General relativity - Wikipedia:
The Einstein field equation reduces to Newton's law of gravity in the limiting cases of a weak gravitational field and slow speed relative to the speed of light. In fact, the constant, {8(pi)G/c4}, appearing in the EFE is determined by making these two approximations.
(bold red mine for emPHAsis)
And again, any effect of the above noted acceleration is buried into the value calculated for {G}, just as it was in the Newtonian value. You still have an empirical result.
The only difference is that when observation does not match calculation, instead of changing the formula, we now change the nature of the universe modeled in order to have it match the calculated formula driven result.
It will be interesting to see if {Gravity Probe B} will be able to detect {{dark stuffs}} effects while in earth's orbit eh?
but I will have to go back to study "ekpyrosis" a little more before making a serious comment. At the same time, ekpyrosis is just a string based cosmological model,
Understood. One of the things that is attractive about the extra dimensions is that it explains where the subatomic particles go as they dance between their observable avatars in our limited {world\observation}: they would exist as physical reality rather than as a probability cloud, and only appear to be changing from one into another into another ... there may even be particles that we would never see (gravitons?)
time shadows, absorbed into Newtonian {g} and Einsteinian {G}
But I'm not sure what you are saying here... can you expand a bit?
Hope the above rant on {empirical} helped with the {g} and {G} value absorption comment.
The existence of particles as lines in time...
This is precisely the view of SR, GR, QFT, etc...
Just a thought exercise: what is the gravitational attraction between two (time) lines as opposed to the attraction between (space) points (balls of matter)? Even if the lines are not parallel or even straight, the attraction will not be related to 1/d2 even in a gross approximation, eh? Two skewed non-intersecting curves arcing away from each other will still display an effect that is not congruent with that model even "in the limiting cases of a weak gravitational field and slow speed relative to the speed of light" -- these are the "time shadows" of the objects in {previous moment} and {next moment} affecting the behavior of the observed particles in the {current moment}.
If time does need to be multiplied by the speed of light to correlate to distance in space, then this effect would be small indeed, and probably only noticeable at a ... galactic scale?
And we're back to where we started (is the barman calling "time"?)
Enjoy.
This message has been edited by RAZD, 08*21*2005 12:09 AM

we are limited in our ability to understand
by our ability to understand
RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}

This message is a reply to:
 Message 37 by cavediver, posted 08-20-2005 4:56 PM cavediver has replied

Replies to this message:
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 Message 44 by cavediver, posted 08-21-2005 12:25 PM RAZD has replied

  
jsmall
Inactive Member


Message 39 of 122 (235179)
08-21-2005 1:27 AM


To get away from the math for a moment.
I love reading this thread, the math is just enough that I can handle it and the 'narrative' just about my level as well. One thing has always bugged me though. Many people say either there was no 'before' the big bang and/or there is no 'outside' of the universe (it isn't expanding 'into' anything). I much prefer to see (but hardly ever do) that we don't KNOW what it's expanding into, or there is currently no way for humans to interact with anything 'outside' the universe, so we don't know. Or that our current math and physics have no way to explain what was going on before the big bang, b/c there was no information, or the 'bang' randomized it.
If, by chance, our universe is a result of a white hole, then there most definitely was a before and the universe is probably expanding into something. Or other scenarios of where the matter might have 'come' from would allow for a before or a tipping point to begin expansion or something. It just doesn't seem like good science to flat-out say there is nothing out there. Perhaps the maths point out that there couldn't be, I don't know.
Or if we can imagine that there may be another universe somewhere (or somewhen or some-dimension), what if one day it interacts with our universe? They either overlap, collide, or begin to tug at each other or something. I understand we don't know, but I don't think we should say it therefore doesn't exist.
I'm open if anyone has good explanations of why my 'what ifs' are nonsense.

Replies to this message:
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SoulSlay
Member (Idle past 5610 days)
Posts: 44
From: billy's puddle, BC
Joined: 10-26-2004


Message 40 of 122 (235185)
08-21-2005 2:23 AM
Reply to: Message 24 by New Cat's Eye
08-15-2005 7:47 PM


Crunch or no crunch, that is the question
There is a thing called 'escape velocity'. Basically, it is the precise velocity required for an object(rocket) to escape another object's(planet) gravitational field.
The idea is, if the object(rocket) is travelling under its escape velocity, the ever-decreasing gravitational field felt from the other object(planet) will still be just enough to stop the moving object(rocket), and pull it into a return dive.
If the object(rocket) is travelling at a velocity higher than its escape velocity, it will be able to continue moving outward to infinty despite the pull from the other object(planet).
So, for the question of crunch or no crunch, I suppose it would depend on how much mass the collective universe has, and at what velocity all of its peices are flying out at.
This message has been edited by SoulSlay, 08-21-2005 02:24 AM

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RAZD
Member (Idle past 1404 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 41 of 122 (235206)
08-21-2005 7:37 AM
Reply to: Message 39 by jsmall
08-21-2005 1:27 AM


Re: To get away from the math for a moment.
I much prefer to see ... that our current math and physics have no way to explain what was going on before the big bang
That is essentially the essence. Note that it is entirely possible for a singularity to develop within our universe and that it expands internally in just the same way, unable to breach the wall of the singularity, and also not be noticeable in this universe. The incredible shrinking frame of reference.
Ekpyrosis posits the universe being caused by the collision of multi-dimensional sheets, with any number possible, and thus {only one} highly unlikely (especially as two sheets are involved), and also posits that they can repeat on top of previous expanded universes.
See Space.com Article on Ekpyrosis (click) for some {commentary\explanations}. This was written in 18 April 2001, so there might be more up-to-date info on the web. If you google use {ekpyrosis theory -CD} (there is a band with that name).
Or other scenarios of where the matter might have 'come' from
My understanding is that gravity enters the equations as negative {mass\energy} and that when you total all the {mass\energy\gravity} in the universe, you end up with a net zero (0) result -- that matter doesn't "come" from anywhere, it is just (temporarily) separated from the rest of the picture. If this is so then the {end entropic death of the universe} would be a {dissolving\fading\evaporating} of the universe away into the {nothing it started from}.
Enjoy.
{{Added by edit: welcome to the fray}}
This message has been edited by RAZD, 08*21*2005 07:37 AM

we are limited in our ability to understand
by our ability to understand
RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}

This message is a reply to:
 Message 39 by jsmall, posted 08-21-2005 1:27 AM jsmall has not replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 42 of 122 (235224)
08-21-2005 10:11 AM
Reply to: Message 38 by RAZD
08-21-2005 12:08 AM


Re: time shadows, empirical g forces, etc.
I just lost the post I was working on and only recovered 1/4 of it here
I sympathise... know only too well how that feels
But you've still managed a post of Holmesian proportion so i will have to tackle it a bit at a time...
Not sure how you get a 60,000,000 (6e8/10) fold difference in distance there other than assuming an orthogonal triangle made from distance and time and finding the hypotenuse
Precisely This is the core of relativity. Special Relativity is simply the statement that space and time form a four-dimensional geomtry with distance function (metric)
ds = -c.dt + dx + dy + dz
This IS just Pythagoras, with a strange minus sign in front of time.
I realize that 3e8 is the speed of light and this makes the units work out, but is this valid?
That is what "c" is... it's the conversion factor for converting our units of time into our units of space. We always set c=1, so the metric becomes
ds = -dt + dx + dy + dz
So, ok, I cheated a little... I kept a plus sign in my calculation, but it illustrates the point... and adding or subtracting 10^2 makes little difference to the dominating time component.
I assume that the trajectory heights are also "adjusted" to then calculate the radii
Not at all. All units were rendered into metres, so it's just a simple piece of geomtry.
but then I have to wonder what radii are being measured...
They are radii of curvature. Remember when you used to calculate radii of curvature of some function? It's simply the radius of a circle that will "fit" the curve at that point. It is not a radius of anything, just a measure of curvature. Here we are simply comparing the two radii, and finding them equal.
Now I've told you what SR is, I better tell you what GR is...
take the simple distance function of a 2d plane:
ds = dx + dy (Pythagoras)
I can change to polar coordinates and I get
ds = dr + r dq
Nothing has changed.
But what if I now choose a metric
ds = dr + sinr dq
This is now very different. This surface now has curvature, and in fact it is the distance on a sphere. Changing the coefficients of the differentials in the metric introduces curvature to our surface.
So a theory of curved space-time simply takes the metric of SR and sticks in functions of the coordinates in front of each term of the metric... e.g.
ds = -A(t,x,y,z)dt + B(t,x,y,z)dx + C(t,x,y,z)dy + D(t,x,y,z)dz (cross-terms are allowed as well, e.g. E(t,x,y,z)dt.dx )
Now GR tells us what A, B, C, and D are allowed to be
So for example,
ds = -(1-2M/r) dt + 1/(1-2M/r) dr + rdq + rsinqdf
is allowed under GR and is the Schwarzschild space-time (black hole).
As a general point to anyone reading this, notice how the space-time (existence) is given by the distance function. There is NO CONCEPT WHATSOEVER of anything "outside" the universe (including "before" and "after") The metric is all there is and it is totally contained within the reality it is describing. This is the amazing thing about GR. It is is completely self-contained and self-referential.
This message has been edited by cavediver, 08-21-2005 12:29 PM

This message is a reply to:
 Message 38 by RAZD, posted 08-21-2005 12:08 AM RAZD has not replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 43 of 122 (235232)
08-21-2005 11:17 AM
Reply to: Message 38 by RAZD
08-21-2005 12:08 AM


Re: time shadows, empirical g forces, etc.
at 26 or 10? or whatever? This is one of the {elegant math} problems imho. and N is whatever it takes to make the predictions match the observations ...
No, N is whatever it needs to be to make the theory actaully work in the first place. N=26 comes from the simplistic bosonic theory. The real string theories have N=10. There is no choice, it is not a free parameter. This is unlike GR, where N can be anything you want it to be. The maths works in all dimensions. This is a problem with GR... it doesn't tell us what the dimension of space-time should be.
What do the other theories predict? One of the things that is supposed to validate the theory of ekpyrosis is a lack of gravity waves: how does this handle this situation
Most valid alternative gravitational theories have observational problems in one place or another. The lack of gravity waves in Ekpyrosis is only their imprint in the Cosmic Microwave Background. It is not that gravitational waves in general do not exist in Ekpyrosis. And as mentioned, Ekpyrosis is not a theory of gravity. It is a possible scenario within string/M-theory. It just pushes back the big questions... it doesn't answer any other than our standard cosmological questions.
But that is not all that is missing. It seems to me that not one thing predicted by (any of the) gravity theories has been found.
Well, if we're talking GR I would say that it is one of the most successfully tested theories ever... I'll be lazy and refer you to Tests of general relativity - Wikipedia
So this halo exists just outside the solar system (in the area that the pioneer satellites are just entering)
No! The halo of the Galaxy, not the Solar System. Big difference. Apart from the Pioner Anomaly, there does not appear to be any deviation from GR in the Solar System. And given the size of the Pioneer Anomaly, I would take that as a measure of its success. The dark matter only reveals itself on the Galactic scale by the speed of rotation of the entire disc of the Galaxy.
But also consider that this amount of acceleration [seen in the Pioneer Anomaly] is the same order of magnitude as is needed to correct the predicted behavior to match the observed behavior of large galactic systems. That to me is more than coincidence.
Who said this? Given that the galactic rotation curves require 90% dark matter, this has to be complete nonsense. The difference in rotation curve is
A is prediction with luminous matter, B is the observed. You're not going to get that with the Pioneer Anomaly!

This message is a reply to:
 Message 38 by RAZD, posted 08-21-2005 12:08 AM RAZD has replied

Replies to this message:
 Message 48 by RAZD, posted 08-25-2005 10:40 PM cavediver has replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 44 of 122 (235245)
08-21-2005 12:25 PM
Reply to: Message 38 by RAZD
08-21-2005 12:08 AM


Re: time shadows, empirical g forces, etc.
Ok, skipping your middle section just for now...
One of the things that is attractive about the extra dimensions is that it explains where the subatomic particles go as they dance between their observable avatars in our limited {world\observation}
I understand your picture, but there is much deeper connection between the extra dimensions and "particles". For instance, in Kaluza-Klein theory, we have GR in five dimensions. We roll one dimension up into a small circle, to give us our usual 3+1 dimensions. You do the maths, and instead of just gravity in 4 dimensions, we have magically got electromagnetism as well! AND another "scalar" field of the type required by inflation and quintessence. So the photon in this picture is just a residue of the 5 dimensional graviton, as is this scalar particle. Now that's not bad from one extra dimension!
Just a thought exercise: what is the gravitational attraction between two (time) lines as opposed to the attraction between (space) points (balls of matter)? Even if the lines are not parallel or even straight, the attraction will not be related to 1/d2 even in a gross approximation, eh?
Ok, you're mixing ideas here... namely 3d Newtonian ideas with 4d GR ideas. There is no gravitational attraction, there is no gravitational force. There is only curvature of space-time. The time lines, if not actively accelerating, will follow the straightest path through space-time. These "straight" paths will be distorted by the presence of mass. I explained this recently in the "What is Space and Time" thread so I'll just add it in here
quote:
What track through space-time do we take? Well, just like everyday experience we follow "straight" lines. We call these geodesics. Think of the aircraft following a great circle path around the earth... it's not straight in the usual definition, but it is the straighest path over a curved globe. It is the path something will move along if pushed and then does not subsequently accelerate.
Mass/energy curves space-time, so the geodesics are curved away from naive ideas of straight. Discovery has just returned from following a straight line which happens to be so curved it closes up into a "circular" path all the way around the earth. The orthogonal straight line (geodesic) to the orbit is a path straight towards the earth. If you "fall" along these paths, you are following the curvature of space-time. You experience no acceleration, no force. Hence being weightless both in orbit and in free-fall (ignoring air-resistance). To deviate from a geodesic, you need to exert a force.
This is the crux... it led Einstein to GR. The 1g we experience on the earth is not gravity. It is the up-push from the earth forcing us off our preferred geodesic which is to "fall" to the centre of the earth.
If we increase this force, by use of Discovery for instance, we increase this 1g force and we move further away from the geodesic. In fact, with Discovery, we exert sufficient force to move us onto a new orbital geodesic.
If you jump out of plane, nothing accelerates. You just move naturally. It is, as you say, the surface of the earth accelerating towards you.

This message is a reply to:
 Message 38 by RAZD, posted 08-21-2005 12:08 AM RAZD has replied

Replies to this message:
 Message 59 by RAZD, posted 08-27-2005 9:05 PM cavediver has not replied

  
Caravaggio
Inactive Member


Message 45 of 122 (235257)
08-21-2005 1:42 PM
Reply to: Message 31 by cavediver
08-19-2005 5:29 AM


another chance?
I'm awful when it comes to actual mathematical computations, but because of a sci-fi story I've been writing I've been trying to understand the concepts of the ways that matter moves around. My questions have brought me to this forum and coincidentally, the story relates directly to this thread, so I had to register to beg for some clarification...
Proposal:
"The other globe is the same universe, just earlier in time."
Cavediver:
"This sounds so reasonable and believable, that it is so hard to convey the fact that it just doesn't make sense You cannot progress through the singularity to "earlier in time". If you do bolt on an extra globe, you have no guarentee that its time is before our time, is after our time, or is totally seperate from out time. The "neck" radically changes the geometry (and topology) to ensure that progress backwards before t=0 is well defined."
This is the part I'm not clear on. It's like your saying that since no one is in the woods to hear the tree fall, it doesn't make a sound. Now if, by some miracle, the big crunch does happen, then just because all living things will die during or before it doesn't mean that time won't go on does it? Doesn't time exist even if we're not here to observe it?
Appreciate any replies here.
This message has been edited by Caravaggio, 08-21-2005 01:44 PM

This message is a reply to:
 Message 31 by cavediver, posted 08-19-2005 5:29 AM cavediver has replied

Replies to this message:
 Message 46 by cavediver, posted 08-21-2005 6:14 PM Caravaggio has not replied

  
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