Is Gravity a Force or Curvature?

[jazamm]

The Wikipedia article on Quantum Gravity reads: "The observation that all fundamental forces except gravity have one or more known messenger particles leads researchers to believe that at least one must exist for gravity. This hypothetical particle is known as the graviton"

To which... yikes. Is this true? If so, why on earth?? Should it not instead read "the observation that gravity is a curvature and not a force leads researchers to believe that messenger particles are unlikely to be related to gravity as they are to the three forces."

Is the attempt to unify the theories really happening by just eschewing the relativistic concept of gravity and forging ahead treating it as if it were indeed a "force," and Einstein's model is just wrong?

I can't imagine anyone would do that, (although I guess the old adage "to a man with a hammer, everything looks like a nail" is possible).

Please let me know if there is any other quantum paradigm with which this is being approached, that maintains gravity is the shape of space and time, and is not a force, therefore very unlikely to be mediated by any particle. (Even if ultimately it can be modeled as such through considerable extra acrobatics).

Sorry if I seem a little incredulous, not at you folks but at the Wikipedia article and its potential for misinformation.

Please let me know what I'm misunderstanding, thanks so much in advance for your help.

 

[Nugatory]

Instead of wikipedia, you might give the Stanford Encyclopedia of Philosophy a try: https://plato.stanford.edu/entries/quantum-gravity/

 

[DaveC426913]

The very short (and very layperson's) answer is that General Relativity and Quantum Mechanics are mutually incompatible theories. When we try to apply them both to the same scenario, we are inundated with infinities arising from the continuous field nature of the former and the discrete particle nature of the latter. Heisenberg's Uncertainty Principle raises its ugly head (If it's a field, then you have can two events immeasurably close to each other, which results in energy transfers that go off-the-charts).

So, our current understanding of the compatibility of the very large with the very small has to be incomplete.

 

[PeroK]

The Wikipedia article on Quantum Gravity reads: "The observation that all fundamental forces except gravity have one or more known messenger particles leads researchers to believe that at least one must exist for gravity. This hypothetical particle is known as the graviton"

To which... yikes. Is this true? If so, why on earth?? Should it not instead read "the observation that gravity is a curvature and not a force leads researchers to believe that messenger particles are unlikely to be related to gravity as they are to the three forces."

Is the attempt to unify the theories really happening by just eschewing the relativistic concept of gravity and forging ahead treating it as if it were indeed a "force," and Einstein's model is just wrong?

I can't imagine anyone would do that, (although I guess the old adage "to a man with a hammer, everything looks like a nail" is possible).

Please let me know if there is any other quantum paradigm with which this is being approached, that maintains gravity is the shape of space and time, and is not a force, therefore very unlikely to be mediated by any particle. (Even if ultimately it can be modeled as such through considerable extra acrobatics).

Sorry if I seem a little incredulous, not at you folks but at the Wikipedia article and its potential for misinformation.

Please let me know what I'm misunderstanding, thanks so much in advance for your help.

Newton's theory of gravity gave accurate predictions for the solar system. But, it relied on instantaneous action at a distance. Newton himself was critical of his own theory in that respect. GR was a major update to Newton's theory and, although it removed action at a distance, it included no explanation of how elementary particles create spacetime curvature. GR needs an upgrade in this respect. It doesn't mean GR is wrong, just that it cannot be the whole story.

I don't believe all potential theories of quantum gravity entail gravitons. In any case, QG is proving a tough nut to crack.

 

[vela]

Sorry if I seem a little incredulous, not at you folks but at the Wikipedia article and its potential for misinformation.

Please let me know what I'm misunderstanding, thanks so much in advance for your help.

I'd say you're putting too much stock in the word force.

https://www.wtamu.edu/~cbaird/sq/2022/08/05/why-is-gravity-not-a-real-force/

 

[L Drago]

Instead of wikipedia, you might give the Stanford Encyclopedia of Philosophy a try: https://plato.stanford.edu/entries/quantum-gravity/

Actually gravity is both a force and curvature. As described by Newton, gravity is a force which attracts object towards it.

F = (G m1 m2) /r²
Gravity is a force directly proportional the product of masses of two objects and inversely proportional to the square of distance between two objects.

According to Cavendish experiment, gravity is a force acts on and is also exerter by smaller objects.

Gravity is also a curvature because according to the theory of General Relativity by Sir Albert Einstein, our universe has a space time fabric. The more the mass of an object the more the curvature and more the gravity and hence slower the time. All planets are said to revolve around sun because of this phenomenon.

Hence, gravity is both a force and a curvature.

 

[PeroK]

Gravity is also a curvature because according to the theory of General Relativity by Sir Albert Einstein ...

Einstein was never knighted, as far as I'm aware. He was, however, offered the presidency of Israel in 1952.

https://www.britannica.com/story/the-time-albert-einstein-was-asked-to-be-president-of-israel

Sir Isaac Newton was knighted:

https://en.wikipedia.org/wiki/Isaac_Newton

 

[L Drago]

Einstein was never knighted, as far as I'm aware. He was, however, offered the presidency of Israel in 1952.

https://www.britannica.com/story/the-time-albert-einstein-was-asked-to-be-president-of-israel

Sir Isaac Newton was knighted:

https://en.wikipedia.org/wiki/Isaac_Newton

I know but according to the theories of these two great scientists, gravity is both a force and a curvature.

 

[jedishrfu]

One doesn't conflate the work of two scientists of different eras and make statements like this. Cavendish used classical mechanics to investigate gravity, representing gravity as a force.

However, the notion of representing gravity as a force had flaws. For example, classical mechanics couldn't explain Mercury's motion about the sun.

In addressing issues with gravity as a force in classical mechanics, Einstein developed a new theory that gravity is better represented as a curvature of spacetime. His theory of General Relativity more accurately described Mercury's motion and light bending in a gravitational field.

You can't mix these theories and say gravity is both a force and a curvature.

Be aware that even today, scientists and engineers still use the principles of classical mechanics in their work. Still, they are careful to understand its limitations and to move to either general relativity or quantum mechanics when the limitations are reached.

 

[L Drago]

But one answer to this question is not possible also. As two scientists ' theories are different. So better stated

According to Newton, it is a force.

According to Einstein, it is a curvature

 

[jedishrfu]

It's good that you know the scientists and what they are known for. We are trying to help you understand that science is more nuanced and not simply tossing out statements like gravity is a force and gravity is curvature.

In physics, words mean specific things; one must know their context, meaning, and usage.

 

[L Drago]

It's good that you know the scientists and what they are known for. We are trying to help you understand that science is more nuanced and not simply tossing out statements like gravity is a force and gravity is curvature.

In physics, words mean specific things; one must know their context, meaning, and usage.

I knew the meaning and usage earlier also but could not write it properly. Anyways, a huge thanks for reminding me to acknowledge and write work of both scientists in a proper way and not just mix these two. Again, thanks.

 

[Demystifier]

Please let me know what I'm misunderstanding

General relativity and quantum field theory.

 

[ohwilleke]

A closely adjacent question I've wondered about, but never really figured out a way to get to the bottom of, is how the physics of gravity as a gauge theory mediated by a spin-2 massless graviton that couples in proportion to a particle's mass-energy in Minkowski space would differ from GR (which is not formulated in Minkowski space and for that matter, isn't usually expressed in a particle physics based formulation), and what observational evidence exists to distinguish the two cases.

I suspect, but do not know, that there isn't any real convincing observational evidence from which to make the distinction, although there may be theoretical or consistency reasons to prefer GR. While the two would be indistinguishable in most circumstances, I think that there should be corner cases where they would be distinct.

 

[Demystifier]

A closely adjacent question I've wondered about, but never really figured out a way to get to the bottom of, is how the physics of gravity as a gauge theory mediated by a spin-2 massless graviton that couples in proportion to a particle's mass-energy in Minkowski space would differ from GR (which is not formulated in Minkowski space and for that matter, isn't usually expressed in a particle physics based formulation), and what observational evidence exists to distinguish the two cases.

I suspect, but do not know, that there isn't any real convincing observational evidence from which to make the distinction, although there may be theoretical or consistency reasons to prefer GR. While the two would be indistinguishable in most circumstances, I think that there should be corner cases where they would be distinct.

GR is formulated in terms of the metric field $g_{\mu\nu}(x)$, while the spin-2 theory is formulated in terms of the spin-2 field $h_{\mu\nu}(x)$. The two fields are related as
$$g_{\mu\nu}(x)=\eta_{\mu\nu}+h_{\mu\nu}(x)$$
The spin-2 theory is problematic when $g_{\mu\nu}(x)$ has a coordinate singularity, e.g. at the horizon, because in that case it is not clear, at least to me, how to interpret the corresponding singularity of $h_{\mu\nu}(x)$.

 

[phinds]

Is Gravity a Force or Curvature?​

Yes.

 

[FRANKENSTEIN54]

Space itself doesn't "curve" . Einstein's Equations are just a Mathematical geometric way to describe how things move through space & time .

 

[ohwilleke]

You can't . . . say gravity is both a force and a curvature.

Sure you can, as long as you use mathematically equivalent descriptions. Like the wave-particle duality in quantum physics, sometimes it is convenient to conceptualize gravity as a force (indeed, lots of the time), and sometimes it is more convenient to conceptualize gravity geometrically, as a curvature. But these are ultimately issues of style and aesthetics, not differences of substance.

Geometry-algebraic dualities are common. In the same vein, polynomials have corresponding conic sections, and trig can also be expressed in both geometric and algebraic ways.

In a more sophisticated example, the Langlands program (a set of conjectures about connections between number theory and geometry) is rooted in a geometry-algebra correspondence. Electroweak unification in the Standard Model of Particle Physics is similarly deeply immersed in parallel algebraic and geometric expressions, for example, with probabilities often expressed as "mixing angles" which is a geometric characterization of them.

Pretty much any force (that meets some minimal mathematical conditions) can be conceptualized geometrically or as a force. The choice to use one approach rather than another is primarily a matter of custom, convention, and convenience.

Indeed, Newtonian gravity and mechanics is actually easier to represent geometrically than GR. Newtonian gravity and mechanics is path independent and conserves mass-energy globally as well as locally, unlike GR with a cosmological constant. Newtonian gravity and mechanics can be represented geometrically simply as a fifth dimensional scalar field of gravitational potential in Euclidian space, relative to some arbitrarily chosen point at which you declare the numerical value of gravitational potential to be zero. Representing GR geometrically as a curvature is much more complex.

 

[DaveC426913]

Space itself doesn't "curve".

We don't talk about curved space; we talk about curved space-time.

 

[Nugatory]

Sure you can, as long as you use mathematically equivalent descriptions

That “as long as” is an important qualification - “the same” in the sense of having mathematically equivalent descriptions means something quite different than the lay understanding that this thread started with.

 

[jedishrfu]

Another distinction is that when we think of gravity as curvature of spacetime we are thinking in a general relativity way using differential geometry of spacetime.

When we are thinking of gravity as a force, we are thinking in a classical way using Newtonian theory.

In that sense, they aren't the same.