Why two mass attracts each other?

[1100f]

See the above quote from your wiki reference. The "unique attractive force due GR" causes additional precession and due to this the planet Mercury takes "additional time" for completion of its orbit. This "additional time" suggets the effect is repulsive. [note: behaviour of precession is not linear.]

Sorry, but a potential of the form $$U(r)\ \alpha \ -{1\over r^3}$$ is an attractive potential. The force that is generated by this sort of potential will be $${\bf F}\ \alpha\ -{{\bf r}\over r^5}$$.
It is the direction of the force that suggests that it is an attractive force, not the "additional time".
As you quote from wiki: "the unique ATTRACTIVE force ..."
The attractive force is attractive. This not a repulsive attractive force.

 

[eram]

Sorry, but a potential of the form $$U(r)\ \alpha \ -{1\over r^3}$$ is an attractive potential. The force that is generated by this sort of potential will be $${\bf F}\ \alpha\ -{{\bf r}\over r^5}$$.
It is the direction of the force that suggests that it is an attractive force, not the "additional time".
As you quote from wiki: "the unique ATTRACTIVE force ..."
The attractive force is attractive. This not a repulsive attractive force.

I think he was referring to the "repulsive "centrifugal" potential energy" in the wiki.

 

[hansda]

Sorry, but a potential of the form $$U(r)\ \alpha \ -{1\over r^3}$$ is an attractive potential. The force that is generated by this sort of potential will be $${\bf F}\ \alpha\ -{{\bf r}\over r^5}$$.
It is the direction of the force that suggests that it is an attractive force, not the "additional time".
As you quote from wiki: "the unique ATTRACTIVE force ..."
The attractive force is attractive. This not a repulsive attractive force.

"Additional Time" is mentioned here. (see Perihelion precession of Mercury)

 

[1100f]

I think he was referring to the "repulsive "centrifugal" potential energy" in the wiki.

No. In classical mechanics, the centrifugal force also exists. I remind you that if you solve the central force problem (only two-bodies interraction), when you write the equations of motion in spherical coordinates you get the following results:
1. From the equation in the angular coordinates you get that the motion is in a plane and you also get the Kepler law of area (which comes in fact from conservation of angular momentum).
2. The radial equation contains the repulsive centrifugal part. The radial equation is like a 1 dimensional problem with the potential given by an "effective potential" which is the original potential and the repulsive part given by the centrifugal part. If your potential is of the form $$-{1\over r}$$ or $$r^2$$ .i.e only r to the power -1 (gravitation or attractive Coulomb) or r to the power 2 (3D isotropic harmonic oscillator), and only in those two cases the solution are closed orbits. There is no precession and no "Additional time". If your potential contains a part which is $$r^n$$ with $$n\neq -1,2$$ then there will be a precession.

In the Schwarzchild problem, the Einstein equation gives you the metric from which you can solve the equation of motion, and the solution is as Markus said. However, in the weak field approximation, the equation of motion in the radial coordinate is like in the non relativistic equation with an addition term which is equivalent to an attractive force (see my earlier post). This term gives the "additional time" and the precession.

Due to this "perihelion precession", the planet Mercury takes "longer time" to complete its orbit. This suggets a repulsive effect.

Or maybe, due to an extra attractive force, there is a precession so that the orbit is complete after some rotaion of its major axis, so that the orbit is longer than without the precession so you get an additional time. So that in fact the additional time suggests an attractive effect

 

[IncogNegro]

I like to think it happens because negative values force them to group together.

 

[eram]

I like to think it happens because negative values force them to group together.

Do you realize that no one can understand what you're saying?

 

[Markus Hanke]

I think this is a very wrong statement, you made. Can there be any "precession" without any application of force? This 'precession' is accounted correctly by GR only.

You have answered your own question; the precession is account for perfectly by GR, which is a purely geometric theory. There are no forces involved.
It is actually fairly ridiculous that you are telling me I made a "very wrong statement" - tell me, have you ever actually done, or even seen, the Mercury perihelion precession calculation in GR ? If not, what do you base your above statement on, exactly ?

The "unique attractive force due GR" causes additional precession

No ! If you must rely on quotations, then at least quote it right - it says energy, not force.

 

[ash64449]

HOW?

Ask To Marcus. He knows them very well.. He is an expert in GR.(SR too!!!) I just know that it can explain.. That's all. I don't know that math.

Due to this "perihelion precession", the planet Mercury takes "longer time" to complete its orbit. This suggets a repulsive effect.

Well,I thought Your question was Why Gravity was MAINLY attractive. Right?

Does taking larger time always correspond to repulsive effect? Sorry. I don't know much about precession. But I thought your question was why gravity was 'mainly attractive'.

 

[IncogNegro]

Do you realize that no one can understand what you're saying?

Short answer. Yes. I work mostly on a subconscious level of intelligence.

 

[Markus Hanke]

HOW?

Due to the presence of a cosmological constant. This is described by the Friedmann-Lemaitre-Robertson-Walker metric, a cosmological solution to the field equations.
This does not, however, have any bearing on the attraction between gravitationally bound bodies, which is described by a different class of solutions to the EFEs.

 

[ash64449]

Due to the presence of a cosmological constant. This is described by the Friedmann-Lemaitre-Robertson-Walker metric, a cosmological solution to the field equations.
This does not, however, have any bearing on the attraction between gravitationally bound bodies, which is described by a different class of solutions to the EFEs.

Cosmological Constant?

I don't know much about GR. But i would like to ask a doubt. I have heard that Einstein has got through his field equations that universe had a natural tendency to expand. But he thought that the universe was static. So to make the universe appear static,I think he added the cosmological constant. And then Fredmann proved that universe was indeed expanding..

Anything wrong?

NOTE: I don't know much about GR Math.. So no need to paste any sort of equations.

 

[IncogNegro]

That explained the cosmological constant better than I've ever heard.

 

[ash64449]

That explained the cosmological constant better than I've ever heard.

Sorry,i didn't understand. Can you elaborate so that i can understand? It is a request.

 

[rr6]

<OUT<Perception >IN< Perception >OUT>

I guess it is like and experience of very long moment between two seconds. So was that due to a contraction of space, or acceleration of chemistry?
r6

Again, sometimes we have sensoral perceptions of extended moments-of-time and senses are related to our biological chemistry. If certain aspects of our nervous system chemistry interactions are accelerated, may be cause of extended time sensations or no time sensations.

At speed-of-radiation, time is said not to exist, or be perceived as existent and is said to be a constant relative to all observers and their various speeds.

That is indeed a strange phenomena. It appears to me that there are some phase changing occurrances. Gravitational spacetime/gravitons > EMRadiation/photons > Plasma/Gas/vapor( less associative > liguid/fluid( more associative ) > ice/solid( more asssociative ) > neutron star( more associative ) > black hole( maximum association ).


However, is there a phase-changing between gravity/gravitons > photons > and our electron related chemistry? Obviously there is because we do not see the connections between gravity and those electrons except via GRelativity mathematics.


So the metaphysically mathematical shape/pattern of the auto affects its molecular air-flow. So is it purely a metaphysical geometrical shape/pattern that affects teh physical/energy we call mass? I don't think so.

Bosnic force--ex;

1) two fermionic quarks( mass ) ergo bosonic energy/physical

2) EMradiation( photon/massless )---ergo bosonic energy/physical

Gravitational spacetime;

1) quasi-physical/energy,

2) metaphysical geometry ergo shape/pattern.

If mass did not attract we would not exist. The why is related one or more of the above i.e.there is force connection and not just geometrical shape/pattern.


r6

 

[Markus Hanke]

Cosmological Constant?

I don't know much about GR. But i would like to ask a doubt. I have heard that Einstein has got through his field equations that universe had a natural tendency to expand. But he thought that the universe was static. So to make the universe appear static,I think he added the cosmological constant. And then Fredmann proved that universe was indeed expanding..

Anything wrong?

NOTE: I don't know much about GR Math.. So no need to paste any sort of equations.

You are correct that it would expand even without cosmological constant; however, what we observe is that it expands at an accelerating rate, and that necessitates the presence of the constant.

 

[Markus Hanke]

@rr6 : Incomprehensible gibberish.

 

[hansda]

Due to this "perihelion precession", the planet Mercury takes "longer time" to complete its orbit. This suggets a repulsive effect.

Or maybe, due to an extra attractive force, there is a precession so that the orbit is complete after some rotaion of its major axis, so that the orbit is longer than without the precession so you get an additional time. So that in fact the additional time suggests an attractive effect

See here for "perihelion precession of planets". Consider this quote from the link:

The Solar System consists of eight major planets (Mercury to Neptune) moving around the Sun in slightly elliptical orbits which are approximately co-planar with one another. According to Chapter 5, if we neglect the relatively weak interplanetary gravitational interactions then the perihelia of the various planets (i.e., the points on their orbits at which they are closest to the Sun) remain fixed in space. However, once these interactions are taken into account, it turns out that the planetary perihelia all slowly precess around the Sun. We can calculate the approximate rate of perihelion precession of a given planet by treating the other planets as uniform concentric rings, centered on the Sun, of mass equal to the planetary mass, and radius equal to the mean orbital radius. This is equivalent to averaging the interplanetary gravitational interactions over the orbits of the other planets. It is reasonable to do this, since the precession period in question is very much longer than the orbital period of any planet in the Solar System. Thus, by treating the other planets as rings, we can calculate the mean gravitational perturbation due to these planets, and, thereby, determine the desired precession rate.

Here we can see that "perihelion precession of planets" is due "interplanetary gravitational interactions". These "interplanetary gravitational interactions" are "attractive forces" from planetary point of view. These "interplanetary gravitational interactions" can be considered as "repulsive" from "the Sun" point of view.

Now consider the "perihelion precession of Mercury". Here the planet Mercury is undergoing additional precession. This "additional precession" of planet Mercury is not due to any "interplanetary gravitational interactions" but due to the "interaction with the Sun". So, this "additional precession" of planet Mercury(being in the similar sense with other planetary precessions) also can be considered as "repulsive" from the "Sun point of view".

 

[hansda]

You have answered your own question; the precession is account for perfectly by GR, which is a purely geometric theory. There are no forces involved.
It is actually fairly ridiculous that you are telling me I made a "very wrong statement" - tell me, have you ever actually done, or even seen, the Mercury perihelion precession calculation in GR ? If not, what do you base your above statement on, exactly ?



No ! If you must rely on quotations, then at least quote it right - it says energy, not force.

I think you haven't yet got the point. See my earlier post #717. I explained there as to why "perihelion precession of Mercury" should be considered as "repulsive" from "the Sun" point of view.

 

[hansda]

Well,I thought Your question was Why Gravity was MAINLY attractive. Right?

Gravity is mostly attractive but it can be repulsive also.

Does taking larger time always correspond to repulsive effect? Sorry. I don't know much about precession. But I thought your question was why gravity was 'mainly attractive'.

See my post #717. I explained there, how gravity can be repulsive also.

 

[1100f]

Gravity is mostly attractive but it can be repulsive also.



See my post #717. I explained there, how gravity can be repulsive also.

You explained nothing. You said that the attraction of the other planets on mercury is repulsive from the sun pov so you conclude that the attractipn from the sun is repulsive.