Gravity

[show_no_mercy]

As far as I'm aware of, there are still some uncertainties about the properties of gravity. Like, what is the speed of gravity (as in, how long does it take for gravity to affect another mass)? Or is gravity "instantaneous"? Is gravity a wave? A particle? Do gravitons actually exist, or are they theorized to exist and haven't been detected yet?

Does anyone have any insight into these questions, or is there no answer for them yet in the scientific community?

[g-21-lto]

As far as I'm aware of, there are still some uncertainties about the properties of gravity. Like, what is the speed of gravity (as in, how long does it take for gravity to affect another mass)? Or is gravity "instantaneous"? Is gravity a wave? A particle? Do gravitons actually exist, or are they theorized to exist and haven't been detected yet?

Does anyone have any insight into these questions, or is there no answer for them yet in the scientific community?
As far as I know, gravity is instantaneous. What we perceive as being the "force" of gravity is an effect of the warping of space around objects of mass. The curvature of space causes objects to tend to "gravitate" toward a massive body; the objects themselves are following the contours of space.

I know that gravity waves were theorized (I believe in the theory of general relativity), but I don't know if we have detected them yet. There is a complex project going on in California to detect gravity waves using precisely measured metal rods which will change length, in theory, when gravity waves strike them. (?? I'm out of my kiddie-wading-pool depth. someone else could give a much better answer.)

As for gravitons, they haven't been detected yet. They remain theory at this point.

[Dorje]

According to Einstein's General Relativity, gravity is an effect caused by curved spacetime due to the presence of mass and these effects travel at the speed of light. It is not a force nor is it instantaneous as in Newtonian Theory; it is a distortion of the geometric structure of spacetime itself. The simplest way to explain it is that "mass tells spacetime how to curve" and "spacetime tells mass how to move."

Gravity waves are predicted in General Relativity, but their magnitudes are unimaginably small. Several satellites will soon be placed in orbit in an attempt to detect gravity waves. The best source for gravity waves involves two orbiting black holes if I remember.

Since the other known fundamental forces (electromagnetic, strong, and weak) are mediated by particles on a quantum level (photons = electromagnetic, gluons = strong force, and W and Z bosons = weak force), it is hypothesized that gravity also has a mediator equivalent, the graviton. Unfortunately, the quantum nature of gravity is unknown, so there is no conclusive evidence for gravitons' existence yet.

[ohwilleke]

Almost everyone in the physics community agrees that on a solar system scale that there is close agreement between Einstein's theory of general relativity and the physical gravitational force. This differs in important ways from Newton's theory of gravity. For example, objects which are moving relative to each other do not gravitate precisely as they would as objects that were at rest with respect to each other, light as well as mass is impacted by gravity, and energy as well as matter can induce gravity.

There is a large consensus, but little definitive proof, that gravity propogates at the speed of light in a wave-like manner.

The gravitational constant (G in the Newtonian formula GM/r^2 which General Relativity simplifies to in the static objects, weak force approximation) is the least accurately known of the fundamental physical constants.

There are two main efforts underway to express gravity in a quantum formulation. One is through string theory. The other is through a theory called loop quantum gravity. (Each has numerous variations). Neither theory has been sufficiently well formulated to make predictions which have been tested and proven by experiement at this point in time. Most quantum theories of gravity propose the existence of a massless graviton, with a spin of 2, to transmit the gravitational force. Spin 2 is desirable because spin two particles in Quantum Mechanics would have their behavior described by a Rank 2 tensor and general relativity uses Rank 2 tensors in its equations. It is not clear, however, if the Rank 2 tensor produced by a spin 2 particle would be the same as the one used in general relativity. At least one scholarly paper has taken the position that it would be different.

There is also considerable dispute over whether a quantum theory of gravity must also, to work, impose a quantum structure with minimum lengths and units of time on space itself (commonly believed to be at the "Planck scale" if they exist at all). Loop quantum gravity takes the position that this is necessary and that in any model where casuality is present, that it will produce a four dimensional universe. String theory often supposes additional dimensions.

While most in the physics community hope for a quantum theory of gravity, there are many who simply believe that gravity is not mediated by a graviton and is not quantum in nature.

There are two other main controversies in the physics community involving gravity.

One is the MOND (Modified Newtonian Dynamics) v. Dark Matter controversy. Stars in galaxies do not behave as general relativity would predict if visible stars are the predominant source of matter in the universe (as they are in the solar system). They act as if gravity was stronger at the galactic fringe than it is in the galactic core in many galaxies. MOND takes the position that gravity is indeed stronger than GR and Newtonian laws would predict, once a critical strength of the gravitational force is reached. Dark Matter theory accounts for the disparity between the observed matter and the observed dynamics by supposing that there is non-visible matter which is causing this behavior. If dark matter theory is correct, about 90% of all matter in the universe must be dark and this matter cannot be made of the ordinary protons and neutrons arranged into atoms with which we are familiar, and this matter must have a strong tendency arrange itself in galactic systems in a particular way. In 2004, MOND passed an important threshold by gaining a formulation (by Jacob Bekenstein) that reduces to general relativity in cases where MOND is not expected to apply, while still being theoretically "well behaved". This resolves a number of theoretical concerns which had previously impeded wider acceptance of MOND. (There are several variations on MOND in existence).

Many quantum gravity scholars suspect that MOND may be implied by quantum gravity. This is because most quantum gravity theories look more like "Quantum Chromodynamics" (QCD) (i.e. the theory of the strong nuclear force) than they do like Quantum Electrodynamics (i.e the theory of electromagnetism) and the closely related theory of the weak nuclear force. QCD is notable because it is attractive at some distances and repulsive at others and because it is "non-abelian" which is to say that its mathematics does not always obey the communitive law of algebra. This in turn, some quantum gravity theorists suspect, could imply MOND like behavior in weak gravitational fields.

The other is often know as the "dark energy" or cosmological constant question. At cosmological scales (i.e. intergalactic scales), visible objects seem to be accellerating away from each other more rapidly than naiive general relativity theory would suspect. General relativity theory can be modified, however, to produce this result with a term called the cosmological constant. This terms controls whether the universe will infinitely expand, maintain a steady state, or collapse. Without a cosmological constant (or rather with the constant set equal to zero), the universe expands. Other values imply an accellerating expansion of the universe. A cosmological constant has an effect which is equivalent, or at least nearly equivalent, to a uniform distribution of energy throughout empty space. Some people have tried to equate this "dark energy" (believed to make up 70% of the matter-energy in the universe by the majority of physicists) to "zero point energy" from quantum fluxuations in empty space, but the amont of ZPE predicted by quantum mechanics far exceeds the amount of dark energy called for by the most accepted dark energy numbers from empirical data.

Of course, you don't even need Newton's theory of gravity to build a bridge if you aren't using GPS technology (which relies on relativity to work). The simplification of the law of gravity at Earth's surface F=mg where m is mass, F is gravitational force directed towards the center of the Earth, and g=approximately 9.8, all in the appropriate MKS units, will suffice, and you can do a just fine job of navigating the solar system and explaining the behavior of the planets with a Newtonian approximation of F=GMm/r^2 (where r is the distance between the objects, and M and m are the respective masses of the objects, with G as a fundamental constant), although Mercury would be just a hair out of place if you used that method. For most astronomical applications short of star formation dynamics and black holes (e.g. the prediction of where dark matter is located in a dark matter theory of galactic dynamics), Newtonian gravity works just fine too.

[ohwilleke]

According to Einstein's General Relativity, gravity is an effect caused by curved spacetime due to the presence of mass and these effects travel at the speed of light. It is not a force nor is it instantaneous as in Newtonian Theory; it is a distortion of the geometric structure of spacetime itself. The simplest way to explain it is that "mass tells spacetime how to curve" and "spacetime tells mass how to move."

Gravity waves are predicted in General Relativity, but their magnitudes are unimaginably small. Several satellites will soon be placed in orbit in an attempt to detect gravity waves. The best source for gravity waves involves two orbiting black holes if I remember.

Since the other known fundamental forces (electromagnetic, strong, and weak) are mediated by particles on a quantum level (photons = electromagnetic, gluons = strong force, and W and Z bosons = weak force), it is hypothesized that gravity also has a mediator equivalent, the graviton. Unfortunately, the quantum nature of gravity is unknown, so there is no conclusive evidence for gravitons' existence yet.

All of this is correct and Dorje has been a bit more precise in his language than I regarding the distinction between space-time distortion and a gravitational "force".

[ohwilleke]

As far as I know, gravity is instantaneous.

This is the one point upon which most, but not all physicists would disagree. The evidence isn't very definitive at this point, but instantaneous gravity would create lots of cause and effect paradoxes in conventional general and special relativity theory.

[ohwilleke]

While I'm at it, I'll add one more related puzzle. Where does mass come from?

The "Standard Model" of quantum physics proposes that inertial mass (i.e. the amount of force necessary to accellerate an object in a particular amount) is a function of a "Higgs field" which interacts with the particles that make up ordinary matter (of spin 1/2, i.e. neurinos, electrons and quarks) through a massive "Higgs Boson" which scientists hope to detect in the latest round of particle accellerator experiments.

This approach resolves some mathematical issues in the model which has otherwise been very successful at describing the strong force, weak force and electro-magnetic force, and all observed forms of matter.

If a Higgs Boson is not detected in the near future, theorists may have to go "back to the drawing board" to figure out why it wasn't detected.

Since a core notion of general relativity is the equivalent of gravitational and inertial mass, any mechanism related to mass is intimately involved in understanding gravity.

Other theorists, called Machians, suppose that interial mass is a sort of "background gravitational force" produced by the combined gravitational forces of everything else in the universe.

[Schneibster]

As far as I'm aware of, there are still some uncertainties about the properties of gravity. Errrrmmmmm, no. Have you heard of the Theory of Relativity? Or were you sleeping in class? Do you know how many different tests they have run checking up to see if it's right? A lot.

Now, there's a prediction we've never been able to check- it's called frame dragging. And they're checking it now, and will be for a year or two yet. But that's pretty much the last thing we haven't checked one way or another.

Like, what is the speed of gravity (as in, how long does it take for gravity to affect another mass)? Or is gravity "instantaneous"? The speed of gravity is the speed of light. Just to absolutely nail it down, they're building a gravity wave observatory- named "LIGO" (google it)- and they'll double check the speed of propagation. But nobody expects to find anything else but that it't the speed of light- Mercury's orbit kinda settled that one about 1915 or so. The old Theory of Universal Gravitation that Isaac Newton wrote back in the seventeenth century said that the speed of gravity was instantaneous- that's why its calculations were off for Mercury. They finally were able to measure it close enough to tell that they were wrong about 1850- right around the US Civil War. Took until 1915 when Albert Einstein pointed out that General Relativity predicted the correct, observed orbit of Mercury before they found out why.

Is gravity a wave? Gravity is a curve in spacetime. Space and time are all one thing- spacetime- and gravity curves it. That's why it makes matter and even energy travel in curved paths.

Now, under certain circumstances- what's required is oscillation- matter can make gravity waves. We think that when stars spiral into the giant black holes at the centers of galaxies, they will make gravity waves. But we'll have to wait until LIGO is up and running to find out for sure. If LIGO works, they're planning to put a set of satellites up to be a gravity wave observatory sometime around 2020.

A particle? Do gravitons actually exist, or are they theorized to exist and haven't been detected yet? They are theorized and haven't even been given very many characteristics yet, much less detected. We don't have a particle theory (called a "quantum" theory) of gravity yet. We know how it works for electricity, and some other forces, but not gravity. However, we know lots about the field theory for gravity, just like we do for electricity. The field theory for electricity was also discovered, just like the problems in Mercury's orbit, around the middle of the nineteenth century, by James Clerk Maxwell. And Einstein's General Theory of Relativity gives us the field theory for gravity. We know more about those two field theories than just about anything else in physics, because they've been around so long.

[ohwilleke]

Errrrmmmmm, no. Have you heard of the Theory of Relativity? Or were you sleeping in class? Do you know how many different tests they have run checking up to see if it's right? A lot.

You are too harsh. While I know of only one physicist in the world who believes that Newtonian gravity is right and that General Relativity's effects don't exist, gravity is also the least well understood of the four fundamental forces. There are signficant issues pretaining to gravity that are not univerally agreed upon, we have only really scratched the surface regarding what General Relativity equations imply outside highly symmetrical constructions where the math is easy (and haven't had much motivation to do so because there are few such structures which we observe in nature), and there are important predictions associated with general relativity that have not been proven experimentally (like the exitence of gravitational waves).

Obviously, just as Newtonian gravity was not rendered useless by General Relativity for practical purposes, any correct theory of gravity will have to reduce to be almost identical to general relativity under a wide variety of conditions, but there is more wiggle room than one might think in the details.

If gravity does flow from a quantum process, I would be surprised if general relativity were not modified to some extent under certain circumstances (some of which we haven't even had any occassion to examine rigorously).

[I. C. Unicorns]

Maybe gravity is caused by the inexact cancellation of two attractive forces, maybe it's an inexact cancellation of the electric force. Matter has an attractive gravitational force to other matter and maybe a repellant gravitational force to antimatter. Half of the universe could be composed of antimatter that was repelled from the matter of the universe and these two parts no longer interact.