# How does Gravity propagate ?

Discussion in 'Physics & Math' started by Dicart, Jun 9, 2022.

1. ### DicartRegistered Senior Member

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465
Hello.

I was wondering how we know the gravity field expand from its source.
Theoricaly as soon as "a mass" (it could be "pure energy "but let us do simple work) appear , a field spread and expand at light speed from this point, curving space (or making it possible to attrract an other mass, the alternativ newtonian model).

So fare so good...
But how do we know this ?
Ocham rasor ? Obvious thinking ? Observation ?

Hum... <But what say science ?
Nothing as far as i tried to figure out (but i am open minded and perhaps someone can give me some proof of it).

An alternativ possibility (I dont say it is the truth, but unlike there is a proof against this, this is a valid hypothesis) :
Mass is spreading some "mass particles", graviton or whatever.
These particles "sprays" and propagate at lights speed from the mass.
So. A mass m has a flux of m*k.
Some of theses particles propagates at speed of light and the majority remains at proximity of the mass.
The volume around the mass is filled (because of the flux) with these "mass particles" and are curving space.
Therefore, around the mass, we have a 1/d*d force.
But far from the mass, we have only a few "mass particles" that have colonised the space-time.
This is why, the question : What is the gravitational force at d distance ? is unrelevant without specifying ... when.,

The gravitational field depends on the filling of the space with these "mass particles".

The problem is very accurate if you consider that in a galaxy, a star is loosing mass and therfore the flux in wide space of the "mass particles" can be very different of the actual state of the star.

So the question.
Do we know (any observation ?) if the value of the gravitational force (space curvature) is realy spreading with the anticipated value ?

3. ### SsssssssRegistered Senior Member

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302
How are you going to make this source just appear?

5. ### DicartRegistered Senior Member

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465
Yes sure....the apppatition of the mass doesent appear in nature.
But this could be done artificialy (i hope you can give me more than one possibility to do so).
Does it change the problem ?
Let say it is a "tougth experiment" for simplification (i could have taken a real case but im am prety sure your will not understand,... first because we dont have the datas... all is already upon your eyes but you dident see anything... or did you ?).

7. ### originHeading towards oblivionValued Senior Member

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11,803
Because the gravitational field is stronger near the source, the field strength drops off by 1/r^2 as you move away from the source.

8. ### DicartRegistered Senior Member

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465
Sure...
And how do YOU know this ? (did you understand the problem of the when ?)

9. ### originHeading towards oblivionValued Senior Member

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11,803
Because have spent some time studying science and physics. All experiments, observations and space travel confirms this.
This appears to be a problem that only exists in your mind.

10. ### DicartRegistered Senior Member

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465
Space travel !!!!????
Are you joking young padawan ?

Perhaps, actualy it is a problem that you dont have understand in your mind ?

11. ### DaveC426913Valued Senior Member

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18,687
Space travel is no joke. Here's two hundred or so:

12. ### James RJust this guy, you know?Staff Member

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38,925
It doesn't. A field exists everywhere in space at all times.
Start with the basics of the gravitational model we're using. Then use clever maths and learning to deduce the consequences of one's mathematical premises. After that, it's a matter of checking predicted results against experiments or observations.

It's essentially the same process whenever we do science.
A quantum picture of gravity would say that changes in gravity are propagated by particle-like bosons called "gravitons", which are similar to what you describe.
You forgot to specify a surface and explain what "k" is.
So some propagate at the speed of light and others do not. What determines which ones propagate and which ones remain stationary?
Is there an infinite number of these "mass particles", then?
You forgot to explain how this follows from your postulates.
You just said the gravitational force is 1/d*d, didn't you?

When what?
Does it?
I agree that the problem is very accurate. It seems obvious that flux would be very different to the actual state of a star.
I know a few.
We can't directly detect the "speed of gravity", if that's what you're asking. We have various theories about it.

13. ### DicartRegistered Senior Member

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465
Yes it is, but knowing this is almost useless.

A field exists and have alway existed (almost since Univers appeared) but the value of the local intesity (the curvatur if you like) change.
This change could be measured and here we would have a usefull scientific observation.

What i meant here is : How do we know HOW the gravity field expand from its source (we can use newton's shell thorem to do the approximation the total mass of a star is a point)
Does it expand (the change) with the speed of light AND according to the 1/d*d diminishment rule ?
Or does it expand (the change) with the speed of light (for a front wave) but not accordingly to the 1/d*d diminismnet rule ? (per example 0.3/d*d at first and THEN as time passes it become 1/d*d)

k is some constant unknown (but you can easily know the unity)
it permit to use the mass to calculate the force (or the curvature)
The flux is as you like, it can be the intensity like if you calculate the energy spread by the sun.

Thats the good question...
Or pherphaps because graviton gravitate (yes this the difficult question) it is like in some liquid : Some molecules can exit the "liquid" (evaporation at high speed)
For some reasons, perhaps some gravitons are helped by the gravitational waves (we know that gravitational waves are traveling a speed of light) or like in a liquid there are more energetic molecules that others.

14. ### DicartRegistered Senior Member

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465
Not necessary.
The best to underestand the problem is to start with the propagation of the photons coming from a star (this science is very well known).
Why is then there a problem (this was my assumption) with the gravity field and not with the elecromagnetic field ?
I already answered this question : Because graviton can not act like a photon, or there would be a serious total energy problem.

1).With the elecromagnetic field :
Photons are emitted from the star at a steady rate, so we have some " emiting flux" corresponding at some energy, let say E=k*m and we can say what energy we can retrieve at every point of the space (that could be very large (light years)).
The total energy we can retrieve at some distance d (on the sphere of radius d... so we can discard the surface of the emiting star) is E/(Pi*d*d).
Therefore whe can sum the energy available fore every sphere with radius d (so if you have some mathematic knowledge you understand that there are an infinity of spheres with infinitesimal thickness)
With d from 0 to dmax. (dmax depends on the duration of the start of emiting for the star dmax=C*t)
The sum give you exactly the right amount of energy emited from the Star !
Yes, science power !

But...

2). With the gravitational field :
Gravitons are emitted from the star at a (unknown really but we can take it as a good hypothesis) steady rate, so we have some "emiting flux"... blablabla etc..
The sum SHOULD give you exacty the right ampount of energy emited from the Star !
Science power too.
But...

Whats wrong with 2 ?

Graviton do not act as the photon.
It is a quantum particle (it is a part of) but it is not a quantic particle (his behavior is not ruled by quantic mechanism). If you dont agree, just take it as an hypothesis.

So, concerning the 1). we usualy forget that the photon particle, to be accurate with the energy repartition in space, is also a wave.
This permit the photon to occupy a large amount of space, unlike a water molecule could do (when it it in some liquid).
This mean : The weave spread in space BUT if you act with one portion of the space where "the photon" is... INSTANTLY (or at speed unknown and very faster then light) the same photon
can not interact AGAIN with something else.
This forbid at somepoint the use 2 times of the energy of the wave at 2 distant positions.

Very difficult to understand ?
Yes, because light wave is not a unique photon but is composed with billions of photons intricated.
OK then.
Take it easy and try again... with 1 photon.
Same with 1 graviton.

The difference, if you want really to undestand, is : The graviton ACTUALLY bend space (therefore it needs energy)
BUT photon is only POTENTIALLY furnishing energy.

So yes, the graviton dosent expand like photons or it would be a violation of the energy EMMITED by the star (ligt or gravity) BECAUSE graviton ACTUALY bend space unlike the photon
who is POTENTIALY proposing energy for the one who can take it inside the space occupied by photon.