# Does Gravity bend Gravity?

## Main Question or Discussion Point

As the experiment proving Einstein's Relativity where the light waves from a star bend from the gravity of the Sun when viewed from a solar eclipse.

Does Gravity do the same?

Is Gravity an electromagnetic effect?

The Gravity of the Earth has a tail from the Solar Wind that is bent due to the Earth's Gravity.

Does the Gravity of the Earth bend the Gravitational effects of the Sun, Moon and the Planets Gravity collectively?

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Chronos
Gold Member
Gravity is not an electromagnetic phenomenon, it would otherwise be adequately explained by Maxwell's equations.

Yes, gravity is not a Electromagnetic Phenomenon but does it yield to the pull of another Gravitational source?

Chronos
Gold Member
Er, no, there is no observational evidence of this.

Why have you came to this conclusion ? As chronos says, where is the evidence ?

Drakkith
Staff Emeritus
The Gravity of the Earth has a tail from the Solar Wind that is bent due to the Earth's Gravity.

Does the Gravity of the Earth bend the Gravitational effects of the Sun, Moon and the Planets Gravity collectively?
The Earth does not have a tail of gravity. The Earth's magnetic field disrupts the solar wind, which being composed of charged particles produce their own magnetic field. This interaction causes the magnetic field of the Earth to develop a tail as the particles bump up against the Earth's field on the near side and get pushed outwards, leaving a "void" of solar wind behind the Earth. (At least I think that's what happens)
Gravity does not behave similar to the EM force in this manner.

So you are saying gravity is a linear since it does not form a tail.

Does Gravity act like a wave or a particle?

It must be a particle in reverse.

Drakkith
Staff Emeritus
So you are saying gravity is a linear since it does not form a tail.

Does Gravity act like a wave or a particle?

It must be a particle in reverse.
I think you have a misunderstanding of what gravity is. This ties in to the other fundamental forces of nature as well. Let me see if I can explain.

Science has come up with 4 ways to explain how two objects can "interact". Interact meaning that one object can cause a change in another object, or that one object can spontaneously change. That's the underlying theme of an interaction, change. It could be a change in position, velocity, a decay of a particle into other particles, absorption of a photon, and other things. Remember, an interaction is NOT a physical object! It is not a particle! As such none of the forces can be assigned things like spin, velocity, or even a position. These things can only be assigned to objects. (I'm going to abstain from attempting to describe QFT, where things are a little less clear cut)

Each of the 4 fundamental forces, or interactions, has specific rules that we have observed them to follow. For example, the electromagnetic force only works between two electrically charged particles. This means that a neutral particle such as a neutrino does not partake in this kind of interaction. Gravity on the other hand is believed to affect ALL particles. It so happens that our primary theory describing gravity, General Relativity, explains gravity as bending and curving of space, much like the surface of the Earth is riddled with mountains and valleys and such. (Only an analogy, don't read too far into it)

The tail in the Earths magnetic field is NOT a result of the EM force itself being stretched out, for that cannot happen. Remember, the force isn't an object! It is actually a result of charged particles having their paths changed, and the resulting interactions between those particles and the magnetic field of the Earth. The field, which is a way of describing how particles will interact with each other through a force, is determined by what particles we already have present and how they are acting. Without particles we could not have any concept of the field, as we wouldn't even be able to observe it!

Does that make sense?

http://en.wikipedia.org/wiki/Fundamental_interaction
(It's kind of advanced, so if you can't make heads or tails of it don't worry about it.)

In a sense it does. Einstein's field equations are highly nonlinear and the solution for the metric tensor depends upon its self.

There is an energy density associated with a gravitational field, and this energy density acts as a source of gravity in it's own right. So if your question is does gravity gravitate, then I believe so.

Drakkith
Staff Emeritus
There is an energy density associated with a gravitational field, and this energy density acts as a source of gravity in it's own right. So if your question is does gravity gravitate, then I believe so.
Hmm. Do you have a reference that could explain this? (Or can you explain it in a little more detail?) I haven't heard of this before.

Bobbywhy
Gold Member
In a sense it does. Einstein's field equations are highly nonlinear and the solution for the metric tensor depends upon its self.

There is an energy density associated with a gravitational field, and this energy density acts as a source of gravity in it's own right. So if your question is does gravity gravitate, then I believe so.
Two sentences from “Einstein, Albert, in “The Principle of Relativity”, Dover Publications, 1952” seem to agree with dipole’s statements:

“The energy of the gravitational field itself contributes to the space-time curvature.” and

“The total gravitating action created by the galaxy or cluster depends on its total energy, that is, the total ponderable energy plus the gravitational energy.”

Cheers,
Bobbywhy

Is Gravity an electromagnetic effect?
No.

Does the Gravity of the Earth bend the Gravitational effects of the Sun, Moon and the Planets Gravity collectively?
That's a complicated question, but the short answer is yes, which is why it's been so difficult to get a theory of quantum gravity.

Drakkith
Staff Emeritus
There you have it Philo. It doesn't work quite the way you imagine it to, but yes, gravity does in fact affect itself.

I heard that space itself is the gravity that pushes us down,

heavy mass such as the sun warped space, and planets orbit the sun

Does anyone know the resullts about the twin Grail (Gravity Recovery and Interior Laboratory)spacecrafts that went around the Moon measuring the gravity in 2011 & 2012?

Drakkith
Staff Emeritus
Here is a non-technical explanation

http://www.einstein-online.info/spotlights/gravity_of_gravity

I'm looking for something that goes into the more gory details.
Awesome, that explains things pretty well for me. Thanks Twofish.

I heard that space itself is the gravity that pushes us down,

heavy mass such as the sun warped space, and planets orbit the sun
Gravity is the result of the curvature of spacetime, which is caused by mass and energy being present.

Gravity is not an electromagnetic phenomenon, it would otherwise be adequately explained by Maxwell's equations.
There are some close analogies between gravity and electromagnetic fields though when dealing with rotating masses which is probably where the OP is getting confused.

http://en.wikipedia.org/wiki/Gravitoelectromagnetism

Very fascinating stuff.

Also, it's an interesting fact that if you write Einstein's equations in five dimensions, you get Maxwell's equations

http://en.wikipedia.org/wiki/Kaluza_Klein

This is a deep and profound fact although people aren't sure exactly what it means.

Also, it's an interesting fact that if you write Einstein's equations in five dimensions, you get Maxwell's equations

http://en.wikipedia.org/wiki/Kaluza_Klein

This is a deep and profound fact although people aren't sure exactly what it means.
That's extremely interesting, it's as if the rotating mass somehow "brings out" whatever characteristics this fifth dimension has to produce the same results.

I'd love to do research in GR if I didn't care about ever finding a job. :)

A "gravitational collective" bending another, singular gravitational force. A very good question actually when one deeply thinks about this. I immediately look towards black holes for a possibility of two gravitational forces bending each other, and/or individually yielding to another gravitational source. A gravitational collective (as I call it just to simplify the meaning), such as the sun, moon, and planets within the inner and outer solar system would effect each others individual gravitational forces, yet, the effects are shared so this would happen as one whole collective. For example, the earths gravity effects the moon, and thus the moons gravity effects the earth (Jupiters gravity effects its moons, as well as every other planet in the system to separate but certain degrees). The stability of our system is due to the forces from each body acting on one another and therefore keeping each other 'in-line', such as the orbits, and planetary rotations.
My further question towards this topic would be how did our system balance itself out, down to the tiniest fractions of earths position relative to the sun, to the position of the moon to balance earth, to the rate of it's rotation and orbital path which are the effects which caused our apparently perfect 24 hour days, and 12 month calendars?
I could get into details on how seemingly perfectly placed each planet is but then this would be too long a post, so I'm simply asking was our goldilocks position simply a result of murphys law over time or something else?

That's extremely interesting, it's as if the rotating mass somehow "brings out" whatever characteristics this fifth dimension has to produce the same results.
It's interesting you mention that. A lot of things kind of get "brought out" when looking at higher dimensions.

If you have four dimensions where three are space and one is time, a curious thing happens when you define the surface of a 4D "sphere" of constant radius where the definition is that all surface points have the same distance to the origin... d=sqrt(x^2 + y^2 + z^2 - c^2t^2).
Its three dimensional space projection, a 3d sphere, grows hyperbolically for t>0.

I'd love to do research in GR if I didn't care about ever finding a job. :)
You can get a Ph.D. and then work for an investment bank. When I interviewed for my current job, I was impressed when one of the interviewers grilled me with questions on numerical relativity. I was even more impressed when he pointed out that I flipped the names of two variables in my answer.

A "gravitational collective" bending another, singular gravitational force. A very good question actually when one deeply thinks about this. I immediately look towards black holes for a possibility of two gravitational forces bending each other, and/or individually yielding to another gravitational source.
If you try to do physics this way, you pretty quickly end up with equations that are completely unmanageable.

The way that people have worked the problem since the 19th century is to calculate things in terms of "fields." An object creates a gravitational, electromagnetic or whatever field, and the field then influences the behavior of other objects.

So if you have two objects, their gravitational fields will add up. And if you have two situations in which you have the same field, it doesn't matter what the original objects were.

My further question towards this topic would be how did our system balance itself out, down to the tiniest fractions of earths position relative to the sun, to the position of the moon to balance earth, to the rate of it's rotation and orbital path which are the effects which caused our apparently perfect 24 hour days, and 12 month calendars?
Our days are 24 hours, because 24 is a nice round number. The number of lunar cycles for one solar cycle is roughly 12, but there is enough of a difference to give people lots of head aches.

Now there *are* situations called resonances in which objects do end up in perfect synchronization. For example, one revolution of the moon is one rotation. What happens is that you end up in situations where a synchronized system happens to be the state with the lowest energy, and that happens a lot in the solar system.

I could get into details on how seemingly perfectly placed each planet is but then this would be too long a post, so I'm simply asking was our goldilocks position simply a result of murphys law over time or something else?
One possibility is the anthopic principle (i.e. we are finding that hot jupiters are pretty common in the universe, but if there were one in our solar system, we wouldn't be here to talk about it).

Chronos