Gravity never zero

[AlexG]

Here's an idea.

Why don't you read the whole thing? Einstein: The Special and General Theory of Relativity.

Start at the beginning, and stop when you reach the end.

It's a much better source than reading random paragraphs on wiki.

 

[Robittybob1]

Here's an idea.

Why don't you read the whole thing? Einstein: The Special and General Theory of Relativity.

Start at the beginning, and stop when you reach the end.

It's a much better source than reading random paragraphs on wiki.

That is what I had decided to do, once I noticed the whole book is there to read.
Up to chapter 3. Thanks for the suggestion.

 

[OnlyMe]

"Einstein showed that this is true - hence the previously unaccounted-for anomaly in the orbit of Mercury."

How about E = mc[sup]2[/sup]?

I don't see any real or direct connection between $$E = mc^2$$ and an explanation of how Einstein's field equations addresses the advancement in the perihelion in Mercury's orbit.

You mean, for example you missed this:
http://en.wikipedia.org/wiki/Tests_of_general_relativity#Perihelion_precession_of_Mercury
or this:
http://farside.ph.utexas.edu/teaching/336k/Newton/node116.html

Neither of these references address that issue or gravitation resulting from energy.

http://www.physicsforums.com/showthread.php?t=304975
for example.

The only really relevant post in your above reference was,
http://www.physicsforums.com/showpost.php?p=2148475&postcount=10 and I don't think "atyy" was saying what you think he was saying. Pay special attention to the following extract,

atyy
In that language, it is the stress-energy-momentum of stuff which causes spacetime curvature which is gravity.​

While we have some theoretical models that suggest that energy contributes to a gravitational field, we really have no way of proving that in the abscence of mass/matter any gravitational field can exist.

 

[Dywyddyr]

I don't see any real or direct connection between $$E = mc^2$$ and an explanation of how Einstein's field equations addresses the advancement in the perihelion in Mercury's orbit.

Since mass and energy are interchangeable then sufficient energy acts as a mass.

Neither of these references address that issue or gravitation resulting from energy.

They were posted to answer:

I had some sleep and closed the Google search on Einstein and GR WRT Mercury

The only really relevant post in your above reference was,
http://www.physicsforums.com/showpost.php?p=2148475&postcount=10 and I don't think "atyy" was saying what you think he was saying. Pay special attention to the following extract,

atyy
In that language, it is the stress-energy-momentum of stuff which causes spacetime curvature which is gravity.​

Maybe not, but the post I quoted (post 2 by Forestman) does say what I've been saying.

While we have some theoretical models that suggest that energy contributes to a gravitational field, we really have no way of proving that in the abscence of mass/matter any gravitational field can exist.

So energy in "sufficient quantities" doesn't "act" like matter?

 

[OnlyMe]

Since mass and energy are interchangeable then sufficient energy acts as a mass.

If this were true as it appears you are stating it to be, there would be no need to distinguish m from E, except as a magnitude of E.

Maybe not, but the post I quoted (post 2 by Forestman) does say what I've been saying.

The point I was attempting to make was that atyy's post essentially corrects the earlier discussion. Forestman was apparently getting his information from a lay publication... Not always the best source of good information.

So energy in "sufficient quantities" doesn't "act" like matter?

What kind of energy? Take a proton and accellerate it to near c, does it make a greater gravitational contribution than a proton at rest? While energy content within a system containing matter "may" contribute to the overall gravitational field, that contribution is trivial compared to the relationship between matter/mass and gravity and may not be relevant at all were there no mass involved.

As far as gravity is concerned, $$E = mc^2$$ cannot be directly applied and produce any meaningful results. I don't think you can interpret this equation as meaning that energy and mass are completely interchangeable. It does lead to some evidence that mass may be created given enough energy and the right conditions and that mass can be destroyed, via fission, fusion and mater/antimatter reactions, releasing energy in the process.

None of these can be directly associated with energy at any level creating a gravitational field, in the abscence of mass/matter.

 

[Robittybob1]

If this were true as it appears you are stating it to be, there would be no need to distinguish m from E, except as a magnitude of E.



The point I was attempting to make was that atyy's post essentially corrects the earlier discussion. Forestman was apparently getting his information from a lay publication... Not always the best source of good information.



What kind of energy? Take a proton and accellerate it to near c, does it make a greater gravitational contribution than a proton at rest? While energy content within a system containing matter "may" contribute to the overall gravitational field, that contribution is trivial compared to the relationship between matter/mass and gravity and may not be relevant at all were there no mass involved.

As far as gravity is concerned, $$E = mc^2$$ cannot be directly applied and produce any meaningful results. I don't think you can interpret this equation as meaning that energy and mass are completely interchangeable. It does lead to some evidence that mass may be created given enough energy and the right conditions and that mass can be destroyed, via fission, fusion and mater/antimatter reactions, releasing energy in the process.

None of these can be directly associated with energy at any level creating a gravitational field, in the abscence of mass/matter.

So how do you envisage what happened in the moments after the BB, for this discussion was based on what I read that it was only after the "matter" cooled did particles with mass form. So if there was a time before particles was there gravity, so the very early expansion might not have been held back by gravity?

 

[arfa brane]

As far as gravity is concerned, $$E = mc^2$$ cannot be directly applied and produce any meaningful results.

That is demonstrably false: $$E = mc^2$$ is a relation that "falls out" when you relate 4-vectors in Minkowski space in a way that generalises Newton's laws of motion, particularly the 2nd law.

You should I think, read a bit more about Einstein's theories before making proclamations about them.

 

[OnlyMe]

As far as gravity is concerned, cannot be directly applied and produce any meaningful results.

That is demonstrably false: $$E = mc^2$$ is a relation that "falls out" when you relate 4-vectors in Minkowski space in a way that generalises Newton's laws of motion, particularly the 2nd law.

You should I think, read a bit more about Einstein's theories before making proclamations about them.

arfa brane, note the bold portion above. Also we are talking general relativity and curved spacetime, a post Newtonian view.

The comment was and is directed toward the use of this single equation, as a reference to support the notion that energy is responsible for the creation of a gravitational field, on its own.

It is easy to say that it does, another thing entirely to demonstrate.

I'll say it in another way. The equation, $$E = mc^2$$, does not demonstrate or prove that energy generates a gravitational field, in the abscence of mass/matter. And I still maintain, that though aspects of the implications apparent within the equation have become entrenched in most of today's physics, trying to define gravity, as a function of this equation, does not provide a meaningful explaination or description of experience....., as far as that experience involves gravity.

 

[OnlyMe]

So how do you envisage what happened in the moments after the BB, for this discussion was based on what I read that it was only after the "matter" cooled did particles with mass form. So if there was a time before particles was there gravity, so the very early expansion might not have been held back by gravity?

As far as the BB model is concerned everything about the universe we live in today, is an emergent phenomena. Be that space, spacetime, gravity matter and antimatter, etc... The BB is the starting point for everything we know......

There are some who would say that gravity existed before the BB but was overcome, by the initial expansion. A very crude example would be, in much the same way a supernova overcomes the gravitational forces involved. Others would suggest that gravity did not emerge until matter had condensed from the initial expansion stage.

Personally I am not partial to the BB model, though I really wouldn't hazard a guess as to how it all began.... I do believe that gravity is emergent and likely, though it pains me to say it, a phenomena emergent from QM, though as yet we have not been successful in developing a model of quantum gravity consistent with experience. GR remains our best explanation/description at present.

 

[Robittybob1]

As far as the BB model is concerned everything about the universe we live in today, is an emergent phenomena. Be that space, spacetime, gravity matter and antimatter, etc... The BB is the starting point for everything we know......

There are some who would say that gravity existed before the BB but was overcome, by the initial expansion. A very crude example would be, in much the same way a supernova overcomes the gravitational forces involved. Others would suggest that gravity did not emerge until matter had condensed from the initial expansion stage.

Personally I am not partial to the BB model, though I really wouldn't hazard a guess as to how it all began.... I do believe that gravity is emergent and likely, though it pains me to say it, a phenomena emergent from QM, though as yet we have not been successful in developing a model of quantum gravity consistent with experience. GR remains our best explanation/description at present.

Thanks for replying.

 

[AlexG]

arfa brane, note the bold portion above. Also we are talking general relativity and curved spacetime, a post Newtonian view.

The comment was and is directed toward the use of this single equation, as a reference to support the notion that energy is responsible for the creation of a gravitational field, on its own.

I wouldn't use that single equation as a reference, but I would cite chapters 15 and 23, taken together of Einstein: The Special and General Theory of Relativity

Read them, think about them, and then get back to us.

 

[OnlyMe]

I wouldn't use that single equation as a reference, but I would cite chapters 15 and 23, taken together of Einstein: The Special and General Theory of Relativity

Read them, think about them, and then get back to us.

Make some point, Alex. Explain your reference.

It has been years since I read the little book, so I went back opened it up to be certain of what you might be referring.

Chapter 15 is discussing kinetic energy and that assumes the presence of some mass/matter. There is nothing about gravity generated independently from energy on its own. Inertial mass of an object does not increase its graviitational mass. Nor does it have an effect on the object's gravitational field, so long as the motion is inertial.

Chapter 23 is not as straight forward and is best understood within the context of the previous chapter. Together they essentially summarize the principle of equivalence, in that they compare the effect of an accelerating environment and a gravitational field. Again there is no reference that suggests that energy in and of itself creates a gravitational field.

In the case of the principle of equivalence it is not the kinetic energy that generates the equivalence – directly. It is the observer's inertial resistance to constantantly changing motion. In chapter 23 this is accomplished via the centrifugal force of a spinning disc. This emphasizes the fact that the motion itself is only a contributory component. The inertial resistance and gravitationally equivalent force is always at 90 degrees to the instantaneous motion and acting away from the center of rotation, rather than 180 degrees to the direction of motion as would be the case, in the closed box example offered elsewhere.

To be sure, we can through application of the principle of equivalence, create an environment that is indistinguishable from a gravitational field standing on a planet. This is accomplished through the expenditure of energy of one sort or another. However, it is not the expended energy that creates a gravitational field. It is inertial resistance to a change in motion.

Again, what I was addressing initially was that energy abscent mass/matter cannot be confirmed as creating a gravitational field.

Neither of these chapters even begins to deal with $$E = mc^2$$, as a contributing source of gravity.

Mass has for sometime now been generally considered to be a specialized expression of energy, but no one has really confirmed any of the few theoretical models presented. And we have no real observational experience that excludes mass/matter, from any observable gravitational system.

 

[Robittybob1]

I wouldn't use that single equation as a reference, but I would cite chapters 15 and 23, taken together of Einstein: The Special and General Theory of Relativity

Read them, think about them, and then get back to us.

The surprise so far was in Chapter 23 where the circumference of a rotating disc is less than 2 * pi * R !!! Is this fact been proven?

 

[AlexG]

Again, what I was addressing initially was that energy abscent mass/matter cannot be confirmed as creating a gravitational field.

Neither of these chapters even begins to deal with , as a contributing source of gravity.

Sorry, I meant to reference chapter 29, not 23.

b.If there is any matter in the domain under consideration, only its inertial mass, and thus according to Section XV only its energy is of importance for its effect in exciting a field.
c.Gravitational field and matter together must satisfy the law of the conservation of energy (and of impulse).

But hey, read the whole thing.

 

[hansda]

Sorry, I meant to reference chapter 29, not 23.



But hey, read the whole thing.

I read chapter 29 .

Is it that , from the precession of the planet Mercury ; a conclusion is made that : ' sufficient energy causes gravity ' ?


This can be a case of the effect of " frame-dragging " also .


Frame-Dragging affects gravity ; that does not mean , sufficient energy converts to mass to affect gravity .

 

[hansda]

This is part of your post #90 .

It's quite simple: gravity didn't "form" as a force until after the BB occurred.

This is part of your post #106 .

And you're still ignoring the fact that with sufficient energy and no mass there will be gravity.

Your above two statements are contradicting .

 

[OnlyMe]

Sorry, I meant to reference chapter 29, not 23.

But hey, read the whole thing.

There ain't much meat to the above post. What is it, "that you believe", any of these references are saying? If you are asking for explanation make that clear. If you believe you see something here that has been missed or misunderstood, present your thoughts. It is hard for anyone to know what you are thinking or how you interpret something when you just post a reference and say read it!

Chapter 29 again says nothing that suggests that the gravitational field of any object is created directly by Energy.

What it does say is that Mercury is close enough to the Sun that to understand its orbital precession, the kinetic properties of the Sun's own motion become relevant. Within GR spacetime is dynamically interacting with gravitating masses.

Your above quote from Chapter 29...
(b) If there is any matter in the domain under consideration, only its inertial mass, and thus according to Section 15 only its energy is of importance for its effect in exciting a field.

(c) Gravitational field and matter together must satisfy the law of the conservation of energy (and of impulse). ​

Note the portion in red above. It does not say adding to the field...

Here what is being presented is that any kinetic motion of the gravitating mass, does have an affect on its gravitational field. (Space and/or spacetime interact dynamically.) In essence the kinetic motion of the Sun, in this case, is transferred to Mercury, resulting in the previously unexplained observed perihelion advancement.

In part Hansda, is correct, there are hints here of what we have come to understand as "frame dragging"... The Lense-Thirring effect (AKA frame-dragging) was first presented in 1918. Einstein's, "Relativity: The Special and General Theory" was first published in 1916. My copy is a 1924 revision. I do recall that there was some correspondence with Einstein prior to 1918, but I am unsure that this correspondence predates the 1916 publication and it is possible that the 1916 publication led to the correspondence and later publication of the Lense-Thirring effect.

Basically what chapter 29 is saying is that, the gravitational field of an object, is affected by the kinetic motion of that object. While this must be true for both linear and angular motions, it is only the angular momentum of the Sun in this case that is important.., and its affect on the Sun's gravitational field and Mercury's orbital precession.

This dynamic relationship later became known as the Lense-Thirring or frame-dragging effect.

And again there is nothing here that suggests that any kinetic aspect, of a gravitating mass, adds to the gravitational field. Only that the kinetics of a gravitating mass must also be dynamically expressed in its gravitational field.

 

[Dywyddyr]

This is part of your post #90 .
This is part of your post #106 .
Your above two statements are contradicting .

Nope, you're misconstruing.

 

[Robittybob1]

Nope, you're misconstruing.

Misconstruing! Not completely for I thought you had changed horses midstream too. I think you need to explain what you meant in each statement.

 

[Dywyddyr]

Misconstruing! Not completely for I thought you had changed horses midstream too. I think you need to explain what you meant in each statement.

Ho hum...
Until after the BB gravity as force didn't exist, therefore it couldn't apply.