# B Magnetic field vs. gravity

1. Dec 6, 2016

### AshPowers

1) If the idea that gravity is a curvature of space that causes objects to move towards each other then would it be appropriate to say that magnets do the same thing to space?

2) Does it seem a bit coincidental that both gravity and magnetism operate by the inverse square law or is it possible that these two forces are fundamentally the same thing?

If the notion that space is being curved is used as what draws bodies of mass towards each other then that explanation must also define what "curving" space actually means. It also requires one to define what space is, as a start.

2. Dec 6, 2016

### phinds

No, electromagnetism is a field. Gravity is the result of the geometry of spacetime.

No, they are not the same; see above.

No, space it not curved. SPACETIME is curved, and it's "curved" only if you insist on applying Euclidean geometry to something that is not properly described by Euclidean geometry but rather by Riemann geometry. Things that we say move on a "curved" path are actually following straight lines (called geodesics) in Reimannian spacetime.

Last edited: Dec 6, 2016
3. Dec 6, 2016

### Staff: Mentor

It's not easy to construct a natural-feeling theory along those lines. The basic problem is that we'd have to explain why spacetime (not "space"! as phinds as already mentioned) is curved one way when a positive charged particle moves through it but in exactly the opposite direction when a negative charged particle moves through it (and how do you explain what happens when a positive and a negative charged particle are moving through the same region of spacetime at the same time - one of them is deflected left and one of them deflected right, so which way is the spacetime curved?). Gravity is unique in that it affects everything the same way, which is what you'd expect when everything is moving through the same curved spacetime.
So do electrical forces and the attenuation with light with distance. Heuristically, it's expected behavior in a three-dimensional space as you'll see if you google around for for why light intensity obeys an inverse square law.

4. Dec 6, 2016

### AshPowers

Ok then. When spacetime curves, how does this "curvature" interact with matter to apply a force to make it move?

To say that mass affects the spacetime around it in such ways, does this not also infer that spacetime must have a structure to it as well? And if this is the case, what is the difference between said structure vs. the concept of the aether?

5. Dec 6, 2016

### AshPowers

Nugatory, If these forces, and light, are expected to behave this way in 3-dimensional space then how does that expectation fit in with strong and weak nuclear forces?

6. Dec 6, 2016

### Ibix

General relativity doesn't say why objects follow geodesics, only observes that the behaviour of real world objects is accurately modelled by that description. Gravity is not a force in this model, it is simply the way geometry works.

The model does not assume any structure to spacetime. You can assume a Lorentz ether if you like - that doesn't have a structure either.

7. Dec 6, 2016

### PeroK

A key difference between gravity and electromagnetic forces is that gravity affects everything to the same extent. An electric force only acts on a charged particle and the direction of the magnetic force depends on the velocity of the charged particle.

Another key difference is that there is no force of gravity - no force you can measure. When you are standing on the ground there is only one force acting on you - the force of the ground pushing you up.

Space time curvature does not, therefore, produce a force. Instead, particles follow a path that maximises the proper time they experience.

Your last question is a good one. Spacetime has a metric structure rather than a physical structure. Metric means the measure of distance between points.

8. Dec 6, 2016

### Mihai_B

It's a bit more complicated but if you are interested in such a theory you could try and read (or get) this book The Meaning of Relativity, the 6th edition
(you must get the 6th edition if you want Einstein's latest or rather last revision - published in december 1955 a less than a year before his death)

In the last appendix called Relativistic Theory of the Non-symmetric Field (which in previous edition was called 'Generalized theory of gravitation') contains a revised version of the unified field.
It was released as a scientific paper also known as A New Form of the General Relativistic Field Equations, co-authored with B. Kaufman ( https://www.jstor.org/stable/2007103 )

Again some people say it's a fail, but for my part I think it's a great theory.

I think that
1) Yes it's kind like that only that if there is a unified field of gravity-electricity-magnetism-etc than the unified field has a bit more complicated properties than gravity - gravity is a only a type of state of the unified field (in analogy with closed strings and opened strings)

2) It's not coincidental - it's about our physical world and how things interact. Theories use math to describe behavior "how will that move a little to the left or right if this comes from this direction" and so on. Unifications it's about math - therefor about geometry. The physical world is another thing - much more complex, with "real" leaving things - but the interactions inside can be described to some detail with math :)

You will receive different answers depending on who you ask. It doesn't mean that others view are better or wrong. It's a matter of view - and/or - experience.

Alternatively you could look into Einstein-Yang-Mills-Dirac-Higgs or in more modern theories such as M-theory or multiverse theory (super theories of the string theory).

I'll let Einstein defend his own theory with his own words

A Note On The Sixth Edition
For the present edition I have completely revised the 'Generalization of Gravitation Theory' under the title 'Relativistic Theory of the Non-symmetric Field'. For I have succeeded - in part in collaboration with my assistant B. Kaufman - in simplifying the derivations as well as the form of the field equations. The whole theory become thereby more transparent, without changing its content.

A.Einstein, December 1954, The meaning of Relativity

There's is also a notes section in the final pages of the book which says (aka regarding other approaches):

"More complex field theories have frequently been proposed. They may be classified according to the following characteristic features:
(a) Increase the number of dimensions of the continuum.In this case one must explain why the continuum is apparently restricted to four dimensions.
(b) Introduction of fields of different kind (e.g. a vector field) in addition to the displacement field and its correlated tensor field gik or (gik).
(c) Introduction of field equations of higher order (of differentiation).
In my view such, more complicated theories and their combinations should be considered only if there exist physical-empirical reasons to do so.
"

I totally recommend you to get this book - that if you have some background in physics or other study fields (like electrical engineering, astronomy, etc).
It contains Einstein's Stafford Little Lectures, delivered in may 1921 at Princeton University.
It details the Special Relativity, the General Relativity (and tensor math stuff) and more - so you have a all in one book.
Also I recommend L. Susskind lectures on G.R.if you need to get into it (you can find them on youtube).

PS: I didn't meant to offend the others posts I just like more Einstein's version - that's all. Thank you.

9. Dec 6, 2016

### AshPowers

I appreciate the attentiveness and thoroughness to the answers from you all. I humbly admit that some of what has been described with the terminology used is a bit challenging, however, I still feel unfulfilled to a satisfactory answer.

Perhaps a different approach to my question is in order.

What I am trying to understand is how these forces, be it by way of charges or gravity, attraction or repulsion, are propagating a force through spacetime that affects surrounding matter. How is the information of attraction or repulsion force "transmitted" through the space between objects? Is it an effect that these calls to action modify the structure of space in a way so as to tell space to "push" or "pull" an object or does space simply act as a conduit for the information of push or pull between objects? If it does not serve a passive role to action and is affected by matter such to be the actor performing the work, then what must this spacetime be composed of to be able to grab ahold of matter and make it move? Conversely, if it is just the message carrier then how are the calls for action transmitted through it? What is the medium of spacetime?

10. Dec 6, 2016

### jbriggs444

Experiment tells us how gravity affects things. It does not tell us what it is made of. It only tells us how it behaves. This is a natural limitation of the scientific method. We can only measure what things do, not what they are.

As scientists, we simply leave it at that. If one cannot measure it, it is not science. It is, instead, philosophy. These forums have no truck with philosophy.

Perhaps someday we will come up with an explanation (and supporting experimental evidence) for gravity as an emergent property arising from some more fundamental interaction. Then we can tell you what gravity is. But we'll still only know how that more fundamental interaction behaves, not what it is made of.