# The force of gravity

#### Glenn

I have read about attempts such as String Theory, M-Theory, Loop Quantum Gravity Theory, etc. to unify the four forces of physics into one grand unified theory.

My question is, why is gravity considered a "force" that needs to be unified into this equation?

In General Relativity, the principal of equivalence says that the effects of gravity and the effects of acceleration are indistinguishable. Thus gravity and acceleration are equivalent.

Isn't Einstein saying that the idea that gravity as a force that attracts masses to other masses is unnecessary?

Isn't he saying that the presence of mass changes the geometry of space-time curving it. We perceive this curving as "gravity". In the absence of matter space-time is flat. The larger the amount of matter, the greater the curvature of space-time. The curvature of space-time is greatest near the massive object and progressively decreases as you move farther away.

With this is mind, how is gravity any more a "force" than centrifugal force is a "force"?

Thanks,
Glenn

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#### jcsd

Gold Member
Firstly LQG does not attempt to unify the four funamental forces it only attempts to explain garvity in quantum terms and a GUT is one that unifies the electroweak theory with quantum chromodynamics and thus explains all interactions accept gravitaional ones.

I'm no expert on string but IIRC it was orginally related to the attempt to explain electromagnetism in general relativity as the curvature of space-time by adding a fifth dimension.

#### Tom D

Thus gravity and acceleration are equivalent.
Just an idea:
Consider the assumption that space is dynamic hence a black holes gravitaional mass warps ( sucks and compresses ) local space toward the centre of it so stationary matter within that space gets carried along with it--like it experiences spacial drag. So spacetime is not just bent, it is truly warped.
On earth, local space is 'sucked' and warped less due to earth's mass, and we, stationary in this local space experience the same dragging by space toward earth's centre, but the earth is in the way so we don't go through the ground.
We 'know' that gravity pulls on each atom (or massive particle ) of any material with the same force so space, according to this conjecture, travels straight through matter so it drags on inner atoms while also exerting a drag on each atom ( particle )it passes through.
Consider acceleration from staionary state in local space of a spaceship .Furthering the conjecture, space exerts a drag on each atom (particle ) and the spaceship feels this as inertia.
But can this be correct? No: the ship turns off its engines and remains at constant velocity within local space.But now the ship does not experience inertia hence space is no longer dragging at the atoms even though the ship is no longer at rest within space.
So, if the idea of dyanamic space is 'right', then, it seems to me, gravitational effects on space contradict acceleration effects.
But if dynamic space is 'wrong', then theories of black holes, their singularities, and also expansion of universe, will need reinterpreting.
I haven't thought this very well thought through yet, but it occurred to me when reading the above post, and you gotta strike while the ion is 'ot, :D

#### Ambitwistor

As jcsd mentioned, loop quantum gravity does not attempt to unify gravity with anything else. Otherwise, your main point is correct: in Einstein's general relativity, gravity is not considered to be a force, any more than the centrifugal force is a "force".

However, we don't know if general relativity's picture of gravity is correct, when it comes to quantum gravity. It could be that gravity really is like the other interactions; then you get theories like string theory. On the other hand, maybe it is not; then you get theories like loop quantum gravity. We don't have any experiments that tell us one way or the other, at this point, so both possibilities should be pursued for now.

#### pmb

Isn't Einstein saying that the idea that gravity as a force that attracts masses to other masses is unnecessary?
No. In fact he says the opposite. He stated that the so-called "fictitious" forces from Newtonian physics should be considered as real forces. They are just forces of a different character. They are called "inertial forces."

Isn't he saying that the presence of mass changes the geometry of space-time curving it. We perceive this curving as "gravity".
That too is something Einstein never said. You can have gravity without spacetime curvature. In fact a uniform gravitational field has no spacetime curvature.

With this is mind, how is gravity any more a "force" than centrifugal force is a "force"?
Gravity is the same kind of force as this. In GR think of the centrifugal force as being a real force.

In the February 17, 1921 issue of Nature Einstein wrote the following
Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordiantes? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.
Pete

#### Zhou Yu

This not really an answer, rather another question on the same topic. We all talk about gravity and most of us know how it works and can calculate it, if we are given the masses of the two objects and the distance between them. However I ask the question,

Why does gravity occur?

We know that 2 masses are attracted to each other, but why is it that they are attracted to each other.

#### Rybo

Physical Universe places maximal speed-of-radiation limits on our observational/detecable abilities ergo electro-magnetic radiation(EMR) when not observed in its fermionic matter form/phase beomes the bosonic force of gravity by becoming maximally dispersed on the most economical, most spherically curved, faster than speed-of-radiation, geodesic pathways associtated with the --conceptaulizable-only-- static Euclidean regular polyhedral prime structure --the icosahedron-- that dynamically exists as a maximally expanded and frequenced spheric/spherical.
========
Gravity Souls
Rybo6 alias Os-jbug

Gravity is,
Those icosahedrally integral,
Inter-relationships,
Of mass-attraction.

Mass as fermions of matter,
A.k.a. baryons and leptons
A.k.a. "energy"
Bound tightly/densely by,
Other bosonic forces,
A.k.a. the gluonic strong force,
And massive weak forces W+,W-,
Zo(Z naught).

Interferes with itself,
To have its speed retarded,
And more densely formed,
As the matter of mass.

Gravity maintains the more effective,
Closed-chordal-circumferential-vectors,
Over its less effective,

Gravity is the integrity of,
A finite, space-time matrix,
Composed of thirty or more,
Single vectored icosahedra.

Gravity is the more massive,
Of the contracted Vector Equlibrium phase,
As the tetrahedron and the octahedron.

Gravity is the most spherical,
It is the most geodesic,
It is the invisble convex essence,
Just beyond our inside looking out,
And our inner concave inabilities,
To quantisize it.

Gravity is the quasi-physical buffer zone,
That eternally maintains our finite wholeness,
As a minimal concept of unity,
And all of Universes associated,
Precessionally induced motions.

Gravity is in motion,
Ergo, we are in motion.

Gravity is dynamic,
Ergo we are dynamic.

Gravity "IS",
Ergo we are,
Toroidal manifesting evolutions
As subsequent to the involutions,
Of mass-attraction .

We are metaphysially attracted,
To knowing our essence,
Ergo, we are,

The soul of gravity,
Searching to find gravity,

We are the souls of light,
Searching to find light.

Attracted to the light,
We collect in mass,

Attracted by mass,
We gravitate in wholeness.

Our involuting special-case parts,
Recursively evolve outward,
A new wholem,
A new perfection,
Eternally.

#### krab

Originally posted by Glenn
With this is mind, how is gravity any more a "force" than centrifugal force is a "force"?
Your reasoning is correct. Centrifugal force is just as much a real force as gravity.

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