Einstein's Field Equations and Poisson's Equation

[Dale]

The second d of gravitational field does not equal zero outside

Well, the vector Laplacian of the gravitational field (one possible meaning of "the second d") is in fact 0 outside a point mass. However, since there are three dimensions there are an infinite number of possible second derivatives of the gravitational field. I don't know which one you are specifically referring to as "the second d", but you are correct that many of the possible second derivatives (besides the Laplacian) may be non-zero in vacuum.

The lablacian of the potential equals zero outside and undefined at the origin

For a point mass, yes. What is the density of a point mass?

If poission equation relates Lablacian of Potential which is always zero why not just write it ; L(potential of GF)=zero ?

Why don't you answer this question yourself by doing the following exercise. Start with the known expression for the gravitational potential inside a solid uniform spherical mass (i.e. a solid ball, not a hollow shell) of unit density (http://en.wikipedia.org/wiki/Gravitational_potential#Spherical_symmetry), take the Laplacian of that expression and see what you get. And yes, I can do this exercise but then how will you learn.

the only thing it say is that; outside the source
the lablacian of the potential equals zero and it cannot be applied in the presence of matter ,(in both cases there is no method to characterize the differences in gravitational properties and relate it quantitatively to matterial properties

I don't know how you come up with any of this. This whole paragraph is simply incorrect. Please do the exercise above to see for yourself.

 

[lugita15]

The second d of gravitational field does not equal zero outside
The lablacian of the potential equals zero outside and undefined at the origin
If poission equation relates Lablacian of Potential which is always zero why not just write it ; L(potential of GF)=zero ?
also if poisson equation relates lablacian of the potential to the quantity 4*pi*G*dinsity (as you all claim) it can not be used as a model to find the constant "4PiG'' in EFEbecause Poission equation -as you understand it -can not relate the density of matter in a point to the properties of gravitational field in the absence of matter . the only thing it say is that; outside the source
the lablacian of the potential equals zero and it cannot be applied in the presence of matter ,(in both cases there is no method to characterize the differences in gravitational properties and relate it quantitatively to matterial properties While in EFE both in the presence and absense of matter in a point there exist a method to characterize gravitational field
(reply this question and leave my mistake in previous posts of using the word "lablacian of gravitational field'' instead of "second d of gravitational field dg/dr, g=dr/dt '')

I can't make sense of a lot of what you're saying, but I think you have one major misunderstanding about Poisson's equation which is leading to a lot of smaller confusions. It is true that for each point where there is no matter, Poisson's equation simply states that the Laplacian of the potential is zero. But the mere fact that the Laplacian is zero is not sufficient information to determine what the value of the potential is at that point, because there are an infinite number of possible functions whose Laplacian is zero. So you require some additional information, known as boundary conditions, to solve Laplace's equation (which is just Poisson's equation with the right hand side equal to zero). The boundary conditions are in the form of the distribution of mass throughout space; in other words you need to specify density as a function of position. Once you know the mass distribution, then you have sufficient information to solve the potential, because there is a theorem that says that there exists a unique solution to Laplace's equation for each set of boundary conditions.

The bottom line is that even though the Laplacian of the potential at a point of empty space is always zero, still the potential itself can and does depend on the magnitude and distribution of the surrounding matter.

 

[Dale]

The bottom line is that even though the Laplacian of the potential at a point of empty space is always zero, still the potential itself can and does depend on the magnitude and distribution of the surrounding matter.

I made this point back in post 15 (https://www.physicsforums.com/showpost.php?p=3086463&postcount=15) but hopefully he will pay attention now that more than one person is saying the same thing. Although, I am not placing any bets on it

 

[lugita15]

I made this point back in post 15 (https://www.physicsforums.com/showpost.php?p=3086463&postcount=15) but hopefully he will pay attention now that more than one person is saying the same thing. Although, I am not placing any bets on it

Well, I guess it bears repeating.

 

[Mueiz]

1\ I agree that the Lablacian outside the source =0
2\ I disagree that the left hand side of poisson equation is the Lablacian of the Potential
3\ I claim that the left hand side of poisson equation is dg/dr g is acceleration of gravitational field .r^2 = x^2+Y^2+Z^2 (also = C^2t^2)
and I can prove that dg/dr =4*pi*G*density
4\I want anyone who disagree just to prove that Ladlacian of Potential =4*pi*G*densitynoone so far did this and is not found in all links you posted to me
5\Noone reply my basic argument in my last post that ;Possion equation as you understand it can not be useful as a model to EFE
(These are the points of Discussion but not the zero value of L of potential and whether it means that the gravitational field is zero or my mistake of using the wrong phrase.)

 

[Dale]

2\ I disagree that the left hand side of poisson equation is the Lablacian of the Potential
3\ I claim that the left hand side of poisson equation is dg/dr

Are you blind?

http://en.wikipedia.org/wiki/Gauss'...vitational_potential_and_Poisson.27s_equation
gives Poisson's equation as \nabla^2 \phi = 4 \pi G \rho

http://en.wikipedia.org/wiki/Poisson's_equation
gives it as \nabla^2 \varphi = f

http://en.wikipedia.org/wiki/Electrostatics#Poisson.27s_equation
gives it as \nabla^2 \phi = -\frac{\rho}{\epsilon_0}

http://farside.ph.utexas.edu/teaching/em/lectures/node31.html
gives it as \nabla^2 \phi = -\frac{\rho}{\epsilon_0} and also gives its most general form as \nabla^2 u = v

All of these are direct links already posted in this thread and all of them confirm that the left hand side of Poisson's equation is the Laplacian of the potential whether you are dealing with a gravitational potential, an electrostatic potential, or some arbitrary potential. You have absolutely no excuse to be posting nonsense like this after so many references have already been provided.

Please retract your statements and then do the exercise I suggested and post your work.

 

[Mueiz]

Are you blind?

I am not blind but I see what is in front of me rather than what I wish to see as you do which is worse than being blind!
None of these Pages contain a proof that Lablacian(GM/r)=4*pi*G*density
If wikipeadia say A becomes B this is not a proof
Not all that Wikipedia says is correct ..see what the ( Holy) wikipeadia says about Faster-than-light spots of light.

 

[Dale]

I see what is in front of me

Apparently not given the number of links to Poisson's equations that you didn't see even after we put them in front of you.

None of these Pages contain a proof that Lablacian(GM/r)=4*pi*G*density

I was responding to your erroneous claims 2 and 3 regarding Poisson's equation, which is clearly defined in those links and in any textbook on the subject. We need to clear those up and you need to post the results of the exercise before going on to claim 4. The left hand side of Poisson's equation is clearly the Laplacian of the potential. If you cannot even understand that with so much evidence provided then any further discussion is pointless.

Are you ready to retract points 2 and 3 and post your work on the exercise?

 

[Mueiz]

This is the exersise:



Code:
In[1]:= << VectorAnalysis`

In[2]:= SetCoordinates[Spherical[r, \[Theta], \[Phi]]];

In[3]:= Laplacian[-((G M)/r)]

Out[3]= 0
(this is your work in Post #28)
Everyone -even the blind - can see that Laplacian[-((G M)/r)] =0 and not 4*pi*G*Density
Now can anyone prove that Laplacian[-((G M)/r)]=4*pi*G*Density?
If you do this (and you and everyone can not) I will stop the discusson and believe in your opinion.

 

[Dale]

Everyone -even the blind - can see that Laplacian[-((G M)/r)] =0

Excellent copying and pasting, but that wasn't the exercise. The exercise was to find the Laplacian of the potential inside a spherical mass, not outside. The potential inside a spherical mass is not -((G M)/r). See the link which I gave above in post 51:

http://en.wikipedia.org/wiki/Gravitational_potential#Spherical_symmetry

Mueiz, the gravitational potential inside a spherical mass is first-semester freshman physics material. If you don't know that, then you have no business trying to learn the EFE. I applaud your ambition, but you need to go back and learn basic Newtonian physics first. I mean, if you don't even know freshman physics nor differential equations then you are simply not equipped to understand the proof that you are asking for. We can post it (it is easy enough to find on the internet) but you won't be able to understand it.

Anyway, please complete the exercise and post your results and then we will proceed from there. Hopefully the exercise itself will be a valuable learning experience and will help prepare you for the proof.

 

[Mueiz]

Excellent copying and pasting, but that wasn't the exercise. The exercise was to find the Laplacian of the potential inside a spherical mass, not outside. The potential inside a spherical mass is not -((G M)/r). See the link which I gave above in post 51:

http://en.wikipedia.org/wiki/Gravitational_potential#Spherical_symmetry

Mueiz, the gravitational potential inside a spherical mass is first-semester freshman physics material. If you don't know that, then you have no business trying to learn the EFE. I applaud your ambition, but you need to go back and learn basic Newtonian physics first. I mean, if you don't even know freshman physics nor differential equations then you are simply not equipped to understand the proof that you are asking for. We can post it (it is easy enough to find on the internet) but you won't be able to understand it.

Anyway, please complete the exercise and post your results and then we will proceed from there. Hopefully the exercise itself will be a valuable learning experience and will help prepare you for the proof.

There is no proof in this link to the formula.. what is there is only that he is using the formula
to recover the density.
Those who do not know what the meaning of proof is, and think that to find an equation in the the Holy wikipeadia without proof is a proof need not only go back and learn basic physics but ask themselves whether they are able to learn physics or not.

 

[Mueiz]

Excellent copying and pasting, but that wasn't the exercise. The exercise was to find the Laplacian of the potential inside a spherical mass, not outside. .

I will leave you reply to yourself (in your post#25)

Please do the exercise I suggested above. Start with the known gravitational potential outside of a point mass and then calculate the Laplacian and see what you get..

!
This is enough and I prefer to stop here.
But truly I often find your posts very beneficial like your first post in this thread and many others, but sometimes you seem to have no aim except for rejecting my arguments.

 

[Born2bwire]

http://farside.ph.utexas.edu/teaching/em/lectures/node31.html

 

[Dale]

I will leave you reply to yourself (in your post#25)

Yes, I already solved the exercise from post 25 and posted the result a long time ago. I was referring to the new exercise I suggested in post 51 which you have not solved nor posted. Please do so, if you desire to learn about Poisson's equation and Newtonian gravity.

But truly I often find your posts very beneficial like your first post in this thread and many others, but sometimes you seem to have no aim except for rejecting my arguments.

Only when your arguments are in conflict with known science.

 

[Dale]

what is there is only that he is using the formula
to recover the density.

Feel free to look elsewhere for the formula for the potential inside a uniform sphere. It is well known. Just cite your source, and then post your work on the exercise.

 

[Mueiz]

DaleSpam is right ..I apologize for Criticize his position falsely ... but his involving the case of the point mass and outside application of Poission Equation in a question related to using Poission equation as a model was not suitable ..Poission Equation is the Lanlacian of the potential (which is also equal dg/dr at far points from the source)
What is written in wikipeadia is imcomplete .because there should be a proof that begin in
from lablacian of ((4/3)G*pi*r^3)/r) to end at 4*pi*G*density.
I apologize for the second time for any bad word against him and thank him for correcting me

 

[Dale]

DaleSpam is right ..I apologize for Criticize his position falsely

Apology accepted, and I will try to avoid the unnecessary side comments in the future.

If you have further questions don't hesitate, and I would still recommend the exercise calculating the Laplacian inside a uniform spherical mass.

 

[Mueiz]

Apology accepted, and I will try to avoid the unnecessary side comments in the future.

If you have further questions don't hesitate, and I would still recommend the exercise calculating the Laplacian inside a uniform spherical mass.

No nead for exersise I think it is better for me to open a new thread concerning zero gravitational field where i am save from complicated mathemaics and exeperimental evidences:
In fact there is much to be learned from the relation between Poissin's Equation and Einstien Field Equation even the statement of Einstein " Poisson Equation is used as a model to Field Equation '' nead futher discussion. it is a v good example how to get some information from the old theory to build new one.

 

[Dale]

In fact there is much to be learned from the relation between Poissin's Equation and Einstien Field Equation even the statement of Einstein " Poisson Equation is used as a model to Field Equation '' nead futher discussion. it is a v good example how to get some information from the old theory to build new one.

It is good not just for understanding gravity but for also understanding science in general. By the time Einstein got around to GR Newtonian gravity had been around for a long time and there was a lot of experimental evidence supporting it. One thing that GR absolutely had to do was to reduce to Newtonian gravity in the appropriate limit. The same was true with Quantum Mechanics, it had to reduce to classical mechanics in the appropriate limit, and the same will be true with any future breakthrough theory, it must reduce to today's experimentally confirmed physics in the right limit.

 

[atyy]

An interesting discussion about guessing general relativity from Newtonian gravity is found on p 24 of http://arxiv.org/abs/gr-qc/0506065

 

[Mueiz]

Yes this is an interesting discussion ... but I want briefly and clearly :
1/ How to use PE to get information about EFE
2/ Reply to the criticisms that PE is Newtonian theory so not correct ..then is it correct to use it as a model for the desaired field equation ?
This is what i want (logic) but not the history of failure and success of Einstien in his way to the Field Equation ,because it is not always true that the way theoreis is discovered is the best and clearest.
Henri poincare says: It is by logic we prove, but by intuition that we discover.