# Gravity as a dimention?

1. Dec 15, 2005

### fiber

hello,

i was just wondering what the general concencus is for gravity being a dimension.

i was thinking about it the other day and seeing as the gravity "particles" would have to be traveling at faster than the speed of light for our model of things to be stable i would think this solution fits better. also, seeing as technically every object has a gravitational attraction to every other object i don't see how a particle would be able to achieve this.

thanks a lot and sorry if i seem so confused.

2. Dec 15, 2005

### Mk

Welcome to PF Mr. fiber!

Gravity is not a dimension.

Theoretical gravitons (the gravity particles) travel at ≤c, if not, you run into the whole infinite energy problem...

Gravity is caused by mass, any object with mass is gravitationally attracted to any and all other massive objects.

You ask how mere particles can cover so much volume of space? How does the electromagnetic force, with its photons cover so much empty volume of space? Well, it has a field. The electromagnetic fields arise from electrically charged particles, such as electrons.

Gravitational fields arise from masses, just as electromagnetic fields arise from charges. I don't know where to go from here...

In fact I have a few questions of my own.

How quickly do electromagnetic fields propagate? The speed of light?

Oh, that was all.

3. Dec 15, 2005

### Danger

Hey mk;
Since light is a segment of the electromagnetic spectrum, electromagnetism travels at the speed of itself.
And I join you in welcoming fiber to the crowd.

4. Dec 15, 2005

### pervect

Staff Emeritus
Gravity is not a dmension, but gravity can be thought of as being due to the curvature of space-time.

See for instance the sci.physics.faq http://math.ucr.edu/home/baez/physics/Relativity/GR/gravity.html" [Broken]

It's not true that gravity has to travel faster then the speed of light for "things to be stable". It is true that the gravity of the sun points towards the current position of the sun, not its retareded position, but the same is true for electromagnetic forces. (This is a bit technical, see for instance
http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html) [Broken].

I'm not sure why you think gravity has to travel faster than the speed of light, you have not offered any context or references for this statment.

Last edited by a moderator: May 2, 2017
5. Dec 16, 2005

### Mk

What do you mean, gravity points toward the sun, not its retarded position? Like if we moved the sun really fast, gravity would have to catch up with it?

6. Dec 16, 2005

### pervect

Staff Emeritus
This is explained more fully in the links I posted earlier. The light from the sun is subjected (like all light) to a phenomenon known as "stellar abberation". The Earth is moving at an orbital velocity with respect to the center of mass (barycenter) of the solar system, said barycenter being essentially the location of the sun because the sun is so much more massive than the planets.

Because of this velocity, the apparent direction of light from the sun is shifted from the current actual position of the sun. It's easiest to explain if you plot everything in heliocentric (sun-centered) coordinates.

...........................................Earth(later)- point C

Sun(point A)...........................Earth(now)- point B

Light emitted from the sun travels along a path form A to C because of the finite sped of light.

The actual position of the sun now is a line joining A to B.

Some people (incorrectly) take the fact that gravity points in the direction of the AB line rather than the AC line to mean that the force of gravity acts instantaneously. What's actually happening is more subtle and described in the FAQ. We know tht people are incorrect in using this argument to say that gravity must act instantaneously because the Coulomb force between charges acts in _exactly_ the same manner. (As it must, if angular momentum is to be conserved). If we replace the Earth-Sun system by a pair of charges, the Columb force points directly towards the current position of the charge, not the retarded position. But we do not conclude from this argument that the speed of attraction between charges is faster than the speed of light, i.e. we do not say that the speed of light is faster than the speed of light.

For more detail, I again refer interested parties to the FAQ on this topic

http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html" [Broken]

Last edited by a moderator: May 2, 2017
7. Sep 27, 2011

### vsfoster

"According to observations of structures larger than solar systems, as well as Big Bang cosmology interpreted under the Friedmann equations and the FLRW metric, dark matter accounts for 23% of the mass-energy density of the observable universe. In comparison, ordinary matter accounts for only 4.6% of the mass-energy density of the observable universe, with the remainder being attributable to dark energy.[2][3] From these figures, dark matter constitutes 83%, (23/(23+4.6)), of the matter in the universe, whereas ordinary matter makes up only 17%"

The above was taken from wikipedia so we have to take for what it's worth, but I thought it was an accepted notion that there is more gravity in the universe than there is mass.

I was also pondering the possibility of gravity being definable as another dimension and am still wondering.

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

8. Sep 27, 2011

### vsfoster

"According to observations of structures larger than solar systems, as well as Big Bang cosmology interpreted under the Friedmann equations and the FLRW metric, dark matter accounts for 23% of the mass-energy density of the observable universe. In comparison, ordinary matter accounts for only 4.6% of the mass-energy density of the observable universe, with the remainder being attributable to dark energy.[2][3] From these figures, dark matter constitutes 83%, (23/(23+4.6)), of the matter in the universe, whereas ordinary matter makes up only 17%"

The above was taken from wikipedia so we have to take for what it's worth, but I thought it was an accepted notion that there is more gravity in the universe than there is mass.

I was also pondering the possibility of gravity being definable as another dimension and am still wondering.

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

9. Sep 27, 2011