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Moonhead
2007-Nov-11, 06:16 PM
Time again for a couple of incidental uneducated questions that I'm wondering about!
__________________________________________________ ____________

[1] Imagine the entire universe was an almost-complete-vacuum with just two electrons in it, seperated by a vast distance of, say, a billion billion lightyears...

1a1. If they would be in an initial rest (suppose time was frozen until our thought experiment, or they were held at their place by some deity or whatever), would these two electrons be attracted by each other?

1a2. If so, would the speed at which they would be moving towards increasing, and if it is, would it increase until almost lightspeed?

1b. Is there a maximum distance at which gravity ends? Do scientists actually know this, or have ways to either observe or calculate (or both) the answer to this question?

__________________________________________________ ____________

[2] I read somewhere that the moon is drifting away slowly but steadily, and will eventually get loose from orbiting the earth...

2a1. Would the moon then nicely and peacefully fit in its own orbit around the sun?

2a2. If so, would it be more likely to get between earth and Venus, between earth and Mars, beyond Venus, or beyond Mars?

2b. Could it go on a collision course with the earth or another planet, or the sun?

2c. Can the question where the moon would go be theoretically nailed down, or is it a chaotic process like a weather system?

2d1. Would the earth's distance to the sun and/or orbit around the sun change if there would be no moon around it (without the effects of the actual losing of the moon as in 2c2. Read this Q as if there had never been a moon at all)?

2d2. Would the process of losing the moon give the earth some spin, or otherwise disrupt its orbit around and/or distance to the sun?

__________________________________________________ ____________

Thanks!

Neverfly
2007-Nov-11, 06:33 PM
Gravity is the weakest of all forces. The only reason it holds the influence that it does is because of the masses involved.

The universe would end long before your two electrons kissed. Incidently, they may even repel eachother.


Along similar lines, the solar system will see a great many changes far in the future as the sun exhausts its nuclear fuel. It is likely that the sun will expand into a red giant before the moon escapes Earth orbit.
Also, simulations predict that the Moon will never actually escape- coming to a halt at about the 70 day cycle.

Moonhead
2007-Nov-11, 06:54 PM
Gravity is the weakest of all forces. The only reason it holds the influence that it does is because of the masses involved.

The universe would end long before your two electrons kissed. Incidently, they may even repel eachother.


Along similar lines, the solar system will see a great many changes far in the future as the sun exhausts its nuclear fuel. It is likely that the sun will expand into a red giant before the moon escapes Earth orbit.
Also, simulations predict that the Moon will never actually escape- coming to a halt at about the 70 day cycle.

Thanks for the answers!! Clear on most issues.

Yet, gravity, although the weakest fo forces, doesn't have an actual "max reach" or so, does it?

frankuitaalst
2007-Nov-11, 06:58 PM
As far as we know now the attractive force is proportional with 1/r▓ , r beeing the distance between the objects . So this force exists in any circomstance. At "infinity" the attraction becomes zero . But what is infinity ?

Moonhead
2007-Nov-11, 07:07 PM
As far as we know now the attractive force is proportional with 1/r▓ , r beeing the distance between the objects . So this force exists in any circomstance. At "infinity" the attraction becomes zero . But what is infinity ?

Infinity only exist as a totality... There is no "at infinity".

So, gravity never actually ends and it also doesn't need a contacting "ping", right? (I mean, two objects do not need to probe each other before gravity "comes into action", right?)

antoniseb
2007-Nov-11, 07:15 PM
[1] Imagine the entire universe was an almost-complete-vacuum with just two electrons in it, seperated by a vast distance of, say, a billion billion lightyears...

If they would be in an initial rest (suppose time was frozen until our thought experiment, or they were held at their place by some deity or whatever), would these two electrons be attracted by each other?

OK, so we have this thing called universal expansion. Would we still have so much of it if the only thing in the universe was two electrons? Second, the charge of the electrons would overwhelm the gravitational attraction (IF the universal expansion allowed the two particles to influence each other).

crosscountry
2007-Nov-11, 07:18 PM
Time again for a couple of incidental uneducated questions that I'm wondering about!
__________________________________________________ ____________

[1] Imagine the entire universe was an almost-complete-vacuum with just two electrons in it, seperated by a vast distance of, say, a billion billion lightyears...

1a1. If they would be in an initial rest (suppose time was frozen until our thought experiment, or they were held at their place by some deity or whatever), would these two electrons be attracted by each other?

1a2. If so, would the speed at which they would be moving towards increasing, and if it is, would it increase until almost lightspeed?

1b. Is there a maximum distance at which gravity ends? Do scientists actually know this, or have ways to either observe or calculate (or both) the answer to this question?

1a1 - it would take a billion billion light years before they "saw" each other. They would be attracted gravitationally. Exactly at that time the electromagnetic force would also turn on causing a replusion. The NET force would be replusive.

1a2 if you had chosen two non-charged particles they would accelerate towards each other - and thus gain speed. Eventually they could gain enough speed to approach that of light, but probably they wouldn't. It depends on initial conditions, and the calculations are more than I want to do right now.

1b gravity never ends like you are asking - as far as we know. Most likely never.
_____

__________________________________________________ _______

[2] I read somewhere that the moon is drifting away slowly but steadily, and will eventually get loose from orbiting the earth...

2a1. Would the moon then nicely and peacefully fit in its own orbit around the sun?

2a2. If so, would it be more likely to get between earth and Venus, between earth and Mars, beyond Venus, or beyond Mars?

the moon will never get loose from orbiting the earth. the reason it moves away is that it takes energy from the rotation of the earth and our tides. Eventually the rotation of the earth will match one lunar month (that will change too), and the moon will STOP moving away.
2a1 N/A
2a2 - N/A



2b. Could it go on a collision course with the earth or another planet, or the sun?

2c. Can the question where the moon would go be theoretically nailed down, or is it a chaotic process like a weather system?

2d1. Would the earth's distance to the sun and/or orbit around the sun change if there would be no moon around it (without the effects of the actual losing of the moon as in 2c2. Read this Q as if there had never been a moon at all)?

2d2. Would the process of losing the moon give the earth some spin, or otherwise disrupt its orbit around and/or distance to the sun?



2d1 if the moon were never here the earth system would have had less total mass, thus we would be slightly farther out than we are now.

2d2 we are actually losing spin due to the friction of the tides. that energy is transfered to the moon distance. again, at a certain point the earth will rotate at the same rate as the moon. That rate will be at a slower rate than one lunar month is now.

Moonhead
2007-Nov-11, 09:10 PM
OK, so we have this thing called universal expansion. Would we still have so much of it if the only thing in the universe was two electrons?

Well, you tell me! :lol:


Second, the charge of the electrons would overwhelm the gravitational attraction (IF the universal expansion allowed the two particles to influence each other).

I should have used neutral particles as Crosscountry suggests.

Btw, would the electromagnetic force work on large distances in this example? (not taking universal expansion into account)?


1a1 - it would take a billion billion light years before they "saw" each other. They would be attracted gravitationally. Exactly at that time the electromagnetic force would also turn on causing a replusion. The NET force would be replusive.

1a2 if you had chosen two non-charged particles they would accelerate towards each other - and thus gain speed. Eventually they could gain enough speed to approach that of light, but probably they wouldn't. It depends on initial conditions, and the calculations are more than I want to do right now.

1b gravity never ends like you are asking - as far as we know. Most likely never.


That was indeed my core question; the example was just a way to visualize it. Indeed I'd rather chosen some neutral particles.


the moon will never get loose from orbiting the earth. the reason it moves away is that it takes energy from the rotation of the earth and our tides. Eventually the rotation of the earth will match one lunar month (that will change too), and the moon will STOP moving away.


Kinda comforting thought that the moon's gonna stick around until the very end!



2d1 if the moon were never here the earth system would have had less total mass, thus we would be slightly farther out than we are now.

2d2 we are actually losing spin due to the friction of the tides. that energy is transfered to the moon distance. again, at a certain point the earth will rotate at the same rate as the moon. That rate will be at a slower rate than one lunar month is now.[/quote]

antoniseb
2007-Nov-11, 09:27 PM
Well, you tell me!
Who knows? I suspect that it would be expanding even faster than it is now, but if we also specify no dark energy, then who knows?


I should have used neutral particles as Crosscountry suggests.
Probably the neutralino (suggested dark matter particle) would be best. With the distances you are talking about the nucleus of a neutral Hydrogen atom might decay before the experiment was over.


Btw, would the electromagnetic force work on large distances in this example? (not taking universal expansion into account)?

As far as we know, but it has never been tested over distances like a billion billion light years as you suggest in the OP.

Lord Jubjub
2007-Nov-11, 10:11 PM
What if the Earth were still spinning fast enough so that the Moon would eventually escape? What kind of orbit would the Moon have with respect to the rest of the solar system?

Kaptain K
2007-Nov-11, 10:37 PM
Electromagnetic force and gravity are both inverse square relationships, so both go on for ever - closer and closer to (but not reaching) zero.

neilzero
2007-Nov-12, 12:17 AM
Electrons repell each other, but the effect would be negligible at one kilometer, but not zero even at a billion, billion light years. Also gravity propagates at the speed of light, so it would take a billion, billion years before the electrons started to repell each other.
The Moon is presently tide locked to Earth and moving away about (one?) centimeter per century. This will decrease in a billion years or so to one millimeter per century. Much later the the Earth will tidelock to the Moon and stop moving away. The moon will then orbit millions of kilometers from Earth, so a modest impact could result in the moon going into a solar orbit quite close to the Earth's orbit. My guess is closer to Mars is as likely as closer to Venus. Perhaps most likely is an Earth crossing orbit which would eventually result in the collision of Earth and moon. This likely will be after our sun becomes a white dwarf. Possibly very advanced humans can prevent the Earth from slowing it's rotation, allowing the moon to move away from Earth to perhaps 20 million kilometers radius orbit. At about this time, the moon would be perturbed into a solar orbit by Venus or Mars.
A reasonable guess is: with only two electrons; the big bang and expanion of the Universe would not have occured, but we have no data, so only guesses are possible. Neil

crosscountry
2007-Nov-12, 12:19 AM
Electromagnetic force and gravity are both inverse square relationships, so both go on for ever - closer and closer to (but not reaching) zero.



I think that is the only question left unanswered.

Moonhead
2007-Nov-12, 07:25 AM
Also gravity propagates at the speed of light, so it would take a billion, billion years before the electrons started to repell each other.

I didn't know that! I thought I read somewhere it worked instantly... It makes sense it would take time like you suggest, but I thought it was one of the myseries about gravity: that it worked instantly...

Do 'we' (meaning mankind, or more specifically its scientists) really know?

astromark
2007-Nov-12, 09:53 AM
Gravity works at the speed of light. Not instantly.

Fraunkensteen
2007-Nov-12, 12:07 PM
I was under the impression that gravity IS the warping of the space-time fabric of the universe, and therefore works instantaneously regardless of the distances involved. The graviton is the communicating entity, which is limited by the speed of light, but the warping of space-time gives the appearance of operating at greater-than-light speeds.

Moonhead
2007-Nov-12, 01:07 PM
I was under the impression that gravity IS the warping of the space-time fabric of the universe, and therefore works instantaneously regardless of the distances involved. The graviton is the communicating entity, which is limited by the speed of light, but the warping of space-time gives the appearance of operating at greater-than-light speeds.

That does sound like it makes sense...

Yet, a wikipedia article (http://en.wikipedia.org/wiki/Speed_of_gravity) suggests that gravity does indeed propagate at the speed of light. Reading it, I get the impression that even scientists haven't got it nailed down for sure.

Would it (gravity propagating at the speed of light and not acting instantly) not offer an easy & obvious explanation for the accelerating part of the expansion of the universe? :surprised

Nicias
2007-Nov-12, 01:36 PM
1a2 if you had chosen two non-charged particles they would accelerate towards each other - and thus gain speed. Eventually they could gain enough speed to approach that of light, but probably they wouldn't. It depends on initial conditions, and the calculations are more than I want to do right now.


This is incorrect, if the particles have finite radius, then when they are just kissing, they will have been traveling at their relative escape velocity (run the system backwards.)

Moonhead
2007-Nov-12, 03:03 PM
Electromagnetic force and gravity are both inverse square relationships, so both go on for ever - closer and closer to (but not reaching) zero.

Yet, if indeed gravity propagates at the speed of light, there must be a "gravitational horizon" - at which gravity is actually zero...

Assuming the gravity of a given particle propagates at the speed of light, it is like a spherical horizon around that particle, that is growing at the speed of light, starting when the universe came into existence. Yet, because the universe is expanding, particles very distant from "our" particle will never get within its gravitational horizon...

Wouldn't that explain the acceleration of the expansion of the universe without needing dark energy to explain it? When I try to visualize it, it would seem to me that the expansion of gravity would act like a katalyzer on both expansion and contraction of the universe (so, if the universe had been contracting, it would have been acceleratingly contracting...)

Btw, intu´tively I feel more comfortable with gravity not propagating at any speed at all, but just being instantly present from the "beginning" to the "end" (but intu´tively I'd also vote for Hoyle's steady state universe, which is more or less proven to be wrong.) Nonetheless the "speed of gravity" is an intriguing idea! I guess I just have to get used to it... hence my questions.

Moonhead
2007-Nov-12, 03:10 PM
(deleted explicit comment I made becaue the edit function don't work properly)

Nereid
2007-Nov-12, 04:10 PM
Yet, if indeed gravity propagates at the speed of light, there must be a "gravitational horizon" - at which gravity is actually zero...

Assuming the gravity of a given particle propagates at the speed of light, it is like a spherical horizon around that particle, that is growing at the speed of light, starting when the universe came into existence. Yet, because the universe is expanding, particles very distant from "our" particle will never get within its gravitational horizon...All of these questions - and many, many more - are answered internally consistently and (as far as we can tell, to date) consistently with observation and experiment by applying General Relativity (GR) to the universe.

Perhaps the hardest concept to get your head around is that 'gravity', in GR, is just geometry (no particles or propagation required).

Would you be interested in reading up on GR, how it can be applied to the universe, and thus how these questions of yours can be answered?
Wouldn't that explain the acceleration of the expansion of the universe without needing dark energy to explain it? When I try to visualize it, it would seem to me that the expansion of gravity would act like a katalyzer on both expansion and contraction of the universe (so, if the universe had been contracting, it would have been acceleratingly contracting...)

Btw, intu´tively I feel more comfortable with gravity not propagating at any speed at all, but just being instantly present from the "beginning" to the "end" (but intu´tively I'd also vote for Hoyle's steady state universe, which is more or less proven to be wrong.) Nonetheless the "speed of gravity" is an intriguing idea! I guess I just have to get used to it... hence my questions.There are ways to present and explain GR-based cosmology without (much) math or equations; for example, here (http://www.astro.ucla.edu/~wright/cosmolog.htm).

However, a lot of this stuff is counter-intuitive, and trying to think it through for yourself, without reference to the details of the actual theory (GR) is likely to be frustrating and lead you to many false conclusions ...

Moonhead
2007-Nov-12, 05:24 PM
Perhaps the hardest concept to get your head around is that 'gravity', in GR, is just geometry (no particles or propagation required).

Hmmm... That was actually what I thought how it was according to present-day mainstream science - until people over here suggested gravity did indeed propagate, which I also read back in a wikipedia article... So, that inspired my latest thoughts on it, posted above.


Would you be interested in reading up on GR, how it can be applied to the universe, and thus how these questions of yours can be answered? There are ways to present and explain GR-based cosmology without (much) math or equations; for example, here (http://www.astro.ucla.edu/~wright/cosmolog.htm).

Yes, I would, hence my questions here. Thanks for the link! (Btw why do many good astronomy sites look as if they were made for Netscape 3.0? Probably because they are! :shifty: :lol: )


However, a lot of this stuff is counter-intuitive, and trying to think it through for yourself, without reference to the details of the actual theory (GR) is likely to be frustrating and lead you to many false conclusions ...

Well, I think you're right... So it's cool you send me a site that tries to get around heavy math...

If I do not get certain concepts, e.g. because of math, or maybe because English ain't my native language (although I don't think that will be a problem) I'll come to this forum and ask for clarification.

Moonhead
2007-Nov-12, 06:14 PM
There are ways to present and explain GR-based cosmology without (much) math or equations; for example, here (http://www.astro.ucla.edu/~wright/cosmolog.htm).

The tutorial on this site contains way too much equations, formula's and "slang expressions"; between those, the explanations are clear as long as the math can be ignored, which mostly doesn't seem to be the case...

Well, too bad for me, and also a bit sad for science, which apparently has crossed a line with regards to being able to inspire the non-initiated... (That, I think, is one of the reasons that present-day philiosophers keep themselves limited to decadent "human relationship" bull****, as the real questions of life are beyonf their understanding... Before the 20th century, cosmology was also the domain of philosophers. Maybe this is why many people tend to flee again into superstition (which imho includes religion))

The FAQ on the site seems inviting though.

crosscountry
2007-Nov-12, 06:20 PM
This is incorrect, if the particles have finite radius, then when they are just kissing, they will have been traveling at their relative escape velocity (run the system backwards.)



that is incorrect. If you run it backwards they they never will escape, thus they are moving slower than escape velocity - it is zero at the final distance, and that cannot be. remember, we are not working with infinities only large distances. And for point like particles, large distances are just like short distances.

Nereid
2007-Nov-12, 08:01 PM
The tutorial on this site contains way too much equations, formula's and "slang expressions"; between those, the explanations are clear as long as the math can be ignored, which mostly doesn't seem to be the case...

Well, too bad for me, and also a bit sad for science, which apparently has crossed a line with regards to being able to inspire the non-initiated... (That, I think, is one of the reasons that present-day philiosophers keep themselves limited to decadent "human relationship" bull****, as the real questions of life are beyonf their understanding... Before the 20th century, cosmology was also the domain of philosophers. Maybe this is why many people tend to flee again into superstition (which imho includes religion))

The FAQ on the site seems inviting though.Sorry to hear that :(

There are a number of quite popular books on the BBT and related topics; of necessity they all contain (or should contain!) material on GR that is free of math etc ... in fact, I remember reading a good one just the other day, now where is it!?!*>^%? I can't find it now; maybe later.

Anyway, back to your questions ...
Yet, if indeed gravity propagates at the speed of light, there must be a "gravitational horizon" - at which gravity is actually zero...

Assuming the gravity of a given particle propagates at the speed of light, it is like a spherical horizon around that particle, that is growing at the speed of light, starting when the universe came into existence. Yet, because the universe is expanding, particles very distant from "our" particle will never get within its gravitational horizon...This is, more or less, right.

Another way to look at it is to consider that 'light' and 'gravity' travel at the same speed, so two parts of the universe can be said to be 'causally connected' if light (and thus gravity) from one part can be seen in the other part.
Wouldn't that explain the acceleration of the expansion of the universe without needing dark energy to explain it? When I try to visualize it, it would seem to me that the expansion of gravity would act like a katalyzer on both expansion and contraction of the universe (so, if the universe had been contracting, it would have been acceleratingly contracting...)This is one of the parts where you may need to get some of the counter-intuitive parts under your belt ...

One - not particularly good - analogy is with escaping from Earth: if you get shot out of a canon (on an airless world, with no other objects forever) straight up, you'll come down again, eventually, unless you have a speed that's equal to or greater than the 'escape velocity'.

Similarly with the universe; if it's made of mass-energy in the form of baryons (and other matter) and photons, it will eventually stop expanding, and contract (if the mass-energy density is above the critical value), stop (at), or never stop (below the critical value).

In the case of dark energy (DE), note that the observations are that the universe seems to be expanding at a rate which is accelerating with respect to that of a universe with no DE! Also, consistent with DE, is the cosmic jerk (http://www.newscientist.com/article/dn4264.html) (this isn't a particularly good source, but the best I could find in a hurry).

Now there are a whole lot of t's to be crossed and i's to be dotted here .... maybe later ...

Moonhead
2007-Nov-12, 09:26 PM
Another way to look at it is to consider that 'light' and 'gravity' travel at the same speed, so two parts of the universe can be said to be 'causally connected' if light (and thus gravity) from one part can be seen in the other part.

Okay. I understand that... Even knew about it, but sofar it hadn't occured to me gravity was a factor in this, too. Isn't this called the particle horizon? Maybe not, but this phrase suddenly surfaces in my mind...


Similarly with the universe; if it's made of mass-energy in the form of baryons (and other matter) and photons, it will eventually stop expanding, and contract (if the mass-energy density is above the critical value), stop (at), or never stop (below the critical value).

In the case of dark energy (DE), note that the observations are that the universe seems to be expanding at a rate which is accelerating with respect to that of a universe with no DE! Also, consistent with DE, is the cosmic jerk (http://www.newscientist.com/article/dn4264.html) (this isn't a particularly good source, but the best I could find in a hurry).

I undertstand (I think).

So, without DE, the universe could be expanding on a steady course 'at best', but observations show it's accelerating. So, there must be something causing the acceleration, hence: dark energy.

The role of the 'causality horizon' in all of this, is something I cannot picture right now, but I believe you when you indicate it isn't related (at least not as a possible cause of the DE phenomenon)

I'll check that cosmic jerk site out too. Gotta a lot of reading to catch up on! But it's interesting stuff, so well worth.


Now there are a whole lot of t's to be crossed and i's to be dotted here .... maybe later ...

Thanks for the enlightenment! Really appreciated it & your patience. And that also goes to the other people here, pointing things out.

Fraunkensteen
2007-Nov-12, 10:49 PM
That does sound like it makes sense...

Yet, a wikipedia article (http://en.wikipedia.org/wiki/Speed_of_gravity) suggests that gravity does indeed propagate at the speed of light. Reading it, I get the impression that even scientists haven't got it nailed down for sure.

Would it (gravity propagating at the speed of light and not acting instantly) not offer an easy & obvious explanation for the accelerating part of the expansion of the universe? :surprised


According to mathematician & Cosmologist, Brian Green, in his 2004 book, "The Elegant Universe", who is also a leading proponent of Super-String Theory and M-Theory, the exponential expansion of the universe happened VERY early after the Big Bang and continued for a short time (cosmologically); maybe 100,000 years, and then slowed.

I don't think physicists have it nailed down yet either. The problem is the direct testability of the theoretical proposals. It used to be that tests were done, and then the theorists would pull the results together to make "laws." Now it is reversed. Theoretical cosmologists and particle physicists propose theory based upon the "beauty", "elegance", and "symmetry" of mathmatical equations, and then look for indirect evidence to support the mathmetical logic. Direct testing can not be done, perhaps forever, on distances of sub-planck length. But most physicists are enthusiastically in the String / M- camp, because many pieces of unification are coming together.

crosscountry
2007-Nov-12, 11:18 PM
According to mathematician & Cosmologist, Brian Green, in his 2004 book, "The Elegant Universe", who is also a leading proponent of Super-String Theory and M-Theory, the exponential expansion of the universe happened VERY early after the Big Bang and continued for a short time (cosmologically); maybe 100,000 years, and then slowed.

I don't think physicists have it nailed down yet either. The problem is the direct testability of the theoretical proposals. It used to be that tests were done, and then the theorists would pull the results together to make "laws." Now it is reversed. Theoretical cosmologists and particle physicists propose theory based upon the "beauty", "elegance", and "symmetry" of mathmatical equations, and then look for indirect evidence to support the mathmetical logic. Direct testing can not be done, perhaps forever, on distances of sub-planck length. But most physicists are enthusiastically in the String / M- camp, because many pieces of unification are coming together.


I have that book. He goes on to say that the universe is expanding currently - and not just that, it's accelerating to boot.

Nereid
2007-Nov-13, 08:57 PM
[snip]

There are a number of quite popular books on the BBT and related topics; of necessity they all contain (or should contain!) material on GR that is free of math etc ... in fact, I remember reading a good one just the other day, now where is it!?!*>^%? I can't find it now; maybe later.

[snip]"The State the Universe, a Primer in Modern Cosmology", by Pedro G Ferreira (http://www.orionbooks.co.uk/MP-40715/The-State-of-the-Universe.htm)

I found his treatment of (general) relativity unusually good - no math yet pertinent and as accurate as you could ask for (without math).

Moonhead
2007-Nov-15, 10:56 AM
"The State the Universe, a Primer in Modern Cosmology", by Pedro G Ferreira (http://www.orionbooks.co.uk/MP-40715/The-State-of-the-Universe.htm)

I found his treatment of (general) relativity unusually good - no math yet pertinent and as accurate as you could ask for (without math).

Hey, thanks for the tip!

I was in a booksotre yesterday, but I forgot to write the title down (Besides, I'm in the Netherlands and it's not like they have every book in English on astronomy) and now I bought Many Worlds in One by Alex Vilenkin, The Fabric of the Cosmos by Brian Greene, and already had bought Dark Cosmos by Dan Hooper.

These books might not exclusively focus on the thing dicusssed in this thread, but they do give an account how it all came to be.

I'll keep an eye open for Fereirra's book!

loglo
2007-Nov-17, 01:21 AM
There is also the thread here (http://www.bautforum.com/questions-answers/63564-speed-gravity.html) discussing the speed of gravity and how it can "hide" its propagation delay.