What is Gravity?

[ions]

I've been always wondering what is the real explanation of gravity.

How & Why are we attracted to Earth regarding different materials, I mean Earth's gravity attracts everything(living organisms, mountains, oceans, even air) so what is it exactly the physical reason for this kind of attraction.

I couldn't even compare it with Magnets, as magnets attracts only metals, when Earths gravity attracts everything.

I would be grateful to any answer, thanks.

[ajb]

Gravity is the "thing" that makes apples fall from trees and the planets orbit the Sun.

 

To date the best theory of gravity is general relativity. The theory has a reputation of being difficult. At the heart of general relativity is the fact that gravity is understood as the local geometry of space and time.

 

Anyway, it turns out that the source of gravity is energy-momentum. So anything with mass and or energy will act as a source of the gravitational field. That is it can interact gravitationally.

 

This includes photons, which have no mass but have energy. Light is effected by gravity.

 

We can paraphrase the laws of gravity as

 

"Geometry" = "matter"

 

In the weak field limit, general relativity reduces to Newton's theory of gravity. In this limit is is mass that is the source of gravity. Now, as it is the mass and not any other property of an object that matters all massive objects interact gravitationally irrespective of their "details".

 

Note that in this limit light is not effected by gravity. Only massive things interact in Newtonian gravity.

 

So, we can compare this with electromagnetic theory. Here the sources are electric charges. So, only things that are charged interact.

 

Hope this helps a little.

[Bob_for_short]

Gravity is a particular physical phenomenon: attraction of neutral non magnetic masses to each other with a force proportional to their masses and decreasing with the distance as 1/r^2. It is not explained as a part or a residual of some other forces. That is why it is called a forth type of interaction (the strong, electromagnetic, and weak are the first three ones).

 

As any fundamental force, it is not "explained" but only described quantitatively (modeled) in physics.

[Baby Astronaut]

Anyway, it turns out that the source of gravity is energy-momentum. So anything with mass and or energy will act as a source of the gravitational field. That is it can interact gravitationally.

 

This includes photons, which have no mass but have energy. Light is effected by gravity.

Does that mean (in bold) light is also a source of gravitational fields?

[ajb]

Yes, as photons carry energy.

[Baby Astronaut]

Since light has the kinetic energy of nearly 300,000 km per second, it probably doesn't take a whole lot of it to create a gravitational pull? If so, how much light (packed into a small area) would it take to cause a 5 kg ball to revolve around it out in low-gravity space?

 

(ballpark figure. Example: like the sun's total light output for ___ days?)

[ajb]

I think the gravitational field of a photon is quite weak.

 

According to Bonnor [1] "The gravitational field of a photon on an infinite straight path is a single sheet of plane-fronted gravitational wave accompanying the photon and perpendicular to its track".

 

I am not really sure how to answer your question. You could think about two perfect mirrors placed close together and allows photons to bounce back and fore. This would have energy-momentum and create a gravitational field. I have no idea how strong this would be.

 

There are also some technical questions related to energy conditions.

---------------------------------------------------------------------------------------

[1] The gravitational field of photons

Bonnor, W. B. General Relativity and Gravitation, Volume 41, Issue 1, pp.77-85

[liarliarpof]

Think of it in terms of a sweeping generalization of Newton's Third law: The effect of the Sun's gravitational field upon a photon is indeed 'demonstrable'. Therefore, there must be an equal but opposite 'reactive force' exerted by the photon upon the Sun. But, as ajb states, it will be small to the extreme! I realize such 'semi-classical' reasoning is quite vulnerable, but if one deeply ponders the Third law, it's as if Newton, in the most vaguish of ways, foresaw extensions of his 'laws', such as GR.

[Baby Astronaut]

I'm thinking, as every bit of space if filled with photons, and throughout the universe -- even in deep space, there's virtual particles continually -- even if only briefly -- popping into existence (and sending out gravity waves), then how much gravity does their combined amount generate (en route photons + virtual particles)?

 

Enough to be a candidate for the effects of dark matter?

[Bob_for_short]

I'm thinking, as every bit of space if filled with photons, and throughout the universe -- even in deep space, there's virtual particles continually -- even if only briefly -- popping into existence (and sending out gravity waves), then how much gravity does their combined amount generate (en route photons + virtual particles)?

 

Enough to be a candidate for the effects of dark matter?

 

Photons (real particles) in the Universe do not contain sufficient energy to be a dark matter/energy. There is no popping into existence pairs in vacuum. Vacuum is a state with a certain energy.

 

Dark mass/energy is needed in some theories to make ends meet. It is not an experimental fact but an idea invented in order to save otherwise failed theories.

[michel123456]

Gravity is the "thing" that makes apples fall from trees and the planets orbit the Sun.

 

Hi AJB. Apples fall from trees hand hit the ground. Planets orbiting the sun is the consequence of gravity & something else, "initial velocity", as mentioned in another thread on this forum. Gravity all alone is not enough for orbiting. Right?

[ajb]

The apple also has an initial condition.

[StrontiDog]

I hope I don't step on any toes, here, but the actual answer to the question is. . .We're not sure. There is still some debate about it.

 

Gotta love that General Relativity.

 

Here's something I found that may shed some light on the subject. Some GR highlights:

 

1. Space and space-time are not rigid arenas in which events take place. They have form and structure which are influenced by the matter and energy content of the universe.

 

2. Matter and energy tell space (and space-time) how to curve.

 

3. Space tells matter how to move. In particular small objects travel along the straightest possible lines in curved space (space-time). (Note the above descriptions of General Relativity are due to John Wheeler.)

 

So basically, gravity might be curved space. Think of the stuff that space is made of 'leaning in' toward mass, from all directions.

 

From this perspective, photons (no mass) might not be affected by gravity, they're just traveling in a straight line through space that is curved, so they seem to be pulled by it.

 

This is a simplistic overview, but I like it.

 

Bill Wolfe

[michel123456]

Here is my point.

In GR and SR, we have matter, mass, energy. We have space, we have time. And we are working with all these. WE KNOW that matter, mass & energy exist. As about space & time, the question is still open.

So, my point is: how are we so sure that space & time are curving, expanding, accelerating, a.s.o.?

Why do we give properties to entities we don't even know if they exist?

 

Shouldn't it be more sensible to suppose that things that WE KNOW exist are curving, expanding, accelerating, a.s.o.?

 

The principle of mathematics (the = sign) is symmetric. This wonderful feature allows us to flip the elements of equations at will.

 

That make me think that we can turn some sentences upside-down without having to change anything in the theory. For example, instead of saying that matter & energy tell space-time how to curve, we could say that matter & energy inversely curve into space-time.

The only difference between the 2 statements is the equation of the curve must be the inverse. To pose it otherwise: the equation we know is exactly the inverse of what is actually happening because we are observing all phenomenas from the inside.

 

That is my point of vue.

[Bob_for_short]

Space and time are spaces of past and possible events. According to the past experience, the possibility of observing events remains so the space and time exist and taken as granted. Of course, it is an inclusive property of matter.

 

Curved space-time or GR is just a model to describe gravity. It is not the only or ultimate model. There is a gravity theory in a flat space-time with the same tested predictions but free from singularities (A.A. Logunov's RTG). I personally prefer the latter.

Edited February 11, 2010 by Bob_for_short

[ajb]

Curved space-time or GR is just a model to describe gravity. It is not the only or ultimate model. There is a gravity theory in a flat space-time with the same tested predictions but free from singularities (A.A. Logunov's RTG). I personally prefer the latter.

 

General relativity seems to be the most accepted theory when discussing classical gravity. It has passed all direct and indirect tests.

 

I don't know much about Lounov's theory, or other theories of flat space-time gravity. I do know that gravity is described by a spin-2 and spin-0 fields. Both these are ok and in fact only massless spin-2 or spin-0 fields can be used to describe gravity. (Forgetting supergravity here).

 

I was under the impression that considering the consistency of of the spin-2 field coupled to the energy-momentum of matter together with non-linear effects pretty much gets you back to general relativity.

 

Any comments on this Bob and Lounov's theory? Which I have not really looked into, so my question may be bit moot and answered in his papers.

[Bob_for_short]

General relativity seems to be the most accepted theory when discussing classical gravity. It has passed all direct and indirect tests.

In weak fields, to be exact. But how about singularities?

I don't know much about Lounov's theory, or other theories of flat space-time gravity. I do know that gravity is described by a spin-2 and spin-0 fields. Both these are ok and in fact only massless spin-2 or spin-0 fields can be used to describe gravity.

Spin states are representations of Lorentz group which exist well in a flat space-time and do not exist in a curved space time. So the Minkowski space-time is an ideal for describing spin. In particular, the gravitons are spin-2 and spin-0 massless particles in RGT.

I was under the impression that considering the consistency of of the spin-2 field coupled to the energy-momentum of matter together with non-linear effects pretty much gets you back to general relativity.

Not obligatorily. In RTG the Minkowski's metric is separated from the gravitational field in the field equations - by construction. R=0. It is not so in GR where one cannot introduce a flat metric - there is no space-time for it in the Riemann geometry! Besides, in RTG the gravitational filed is a filed indeed - with energy-momentum, so the gravity is not completely "geometrized". The energy-momentum conservation laws are fulfilled due to 10 Killing vectors.

 

On arXiv there are papers and books of Logunov's. His construction is kind of gauge gravity theory in a flat space-time. There is no black holes in his approach. Massive objects do not form singularities. Also the Universe evolution is represents oscillations.

Edited February 11, 2010 by Bob_for_short

[ajb]

In weak fields, to be exact. But how about singularities?

 

Similar statements can be made about classical electrodynamics. So, the presence singularities need not be a reason to doubt the experimental tests of a classical theory. Anyway, ok.

 

Spin states are representations of Lorentz group which exist well in a flat space-time and do not exist in a curved space time. So the Minkowski space-time is an ideal for describing spin. In particular, the gravitons are spin-2 and spin-0 massless particles in RGT.

 

Yes. Even in general relativity trying to discuss gravitons requires us to consider linearised equations and perturbations about the Minkowski metric.

 

Introducing spin fields on a general Reimannian manifold is as you know not possible.

 

 

Not obligatorily. In RTG the Minkowski's metric is separated from the gravitational field in the field equations - by construction. R=0. It is not so in GR where one cannot introduce a flat metric - there is no space-time for it in the Riemann geometry! Besides, in RTG the gravitational filed is a filed indeed - with energy-momentum, so the gravity is not completely "geometrized". The energy-momentum conservation laws are fulfilled due to 10 Killing things.

 

As it is a flat space-time theory with 10 Killing vectors the notions of energy-momentum conservation are much simpler that on a general curved space-time. This would be an advantage.

 

Is there any questions about the bending of light? I know that a pure scalar theory will not give the observed results. I have no idea what would happen to a spin-0 and spin-2 theory.

 

What problems are there with the theory?

[Bob_for_short]

Similar statements can be made about classical electrodynamics. So, the presence of singularities need not be a reason to doubt the experimental tests of a classical theory.

We know that the CE cannot explain atom stability. Falling electron radiates an infinite amount of energy in CE due to 1/r potential.

 

Attempts to take into account the radiative friction lead to runaway solutions in CE. So there are some CE unphysical predictions.

Yes. Even in general relativity trying to discuss gravitons requires us to consider linearised equations and perturbations about the Minkowski metric.

The linearized GR treatement is very similar to RTG. Factually Logunov has noted that the harmonicity conditions may be considered as field equations in a flat space-time (rather than coordinate conditions in the Riemann space-time) and then the reminder can be separated as a pure graviational field.

Is there any questions about the bending of light? I know that a pure scalar theory will not give the observed results. I have no idea what would happen to a spin-0 and spin-2 theory. What problems are there with the theory?

There is no problem with the light bending, etc. Everything is described well. The theory was developed not so long ago, hence it is just much less spread. So far no flaws have been found in RTG.

[ajb]

My only initial worry is that the graviton has to be massive. This does not fit with gauge symmetries. I'd need to read up on it more to really comment.

 

From my quick flicking through it does very much look like linearised GR.

[Bob_for_short]

From my quick flicking through it does very much look like linearised GR.

 

So it is not a problem to learn this theory. Those who know GR, can easily understand RTG.

[michel123456]

The theory was developed not so long ago

 

1986? 24 years ago? I never heard from this theory. Thank you for the information.

 

Edited February 12, 2010 by michel123456

[ajb]

1986? 24 years ago? I never heard from this theory. Thank you for the information.

 

Other people have looked at "gauge theories" of gravity before. People have discussed localising Lorentz and Poincare symmetries as well as larger groups. The generic problem as I understand it is the presence of ghosts. That is largely why "gauge theory of gravity" is not a quantum theory of gravity.

 

Another general feature of other models of gravity is that they tend to be GR + something. Often, such a Brans-Dicke theory phenomenologically they seem identical to GR, at least within the experimental errors of today. So, they often do not really offer anything other than more fields. Thus by Occam's Razor general relativity is chosen.

Edited February 12, 2010 by ajb

[Bob_for_short]

Other people have looked at "gauge theories" of gravity before. People have discussed localising Lorentz and Poincare symmetries as well as larger groups. The generic problem as I understand it is the presence of ghosts. That is largely why "gauge theory of gravity" is not a quantum theory of gravity.

 

Another general feature of other models of gravity is that they tend to be GR + something. Often, such a Brans-Dicke theory phenomenologically they seem identical to GR, at least within the experimental errors of today. So, they often do not really offer anything other than more fields. Thus by Occam's Razor general relativity is chosen.

 

Yes, Logunov mentions many previous attempts to build a gravity theory in a flat space-time by others. The trick is that the gravity is a universal force and it looks indeed as a geometry of space-time. As soon as one geometrizes the space-time, one deals inevitably with a Riemann geometry: R>0.

 

In RTG the gravity is not fully geometrized. It creates an effective Riemann space-time for matter but it is decoupled from the Minkowski metric in some field equations. So the Minkowski metric becomes the real geometry of space-time (R=0), it is observable (via laws of conservation, for example), but effects of gravity are taken into account as universal too.

 

As to "gauge principle", it is not obligatory to follow it literally but as a hint in a phenomenological approach. Any good theory is phenomenological - it is based on experimental observations first of all. Otherwise a theory becomes a mathematical topic of some mathematical constructions - it may have many consequences but not related to observations.

Edited February 12, 2010 by Bob_for_short

[liarliarpof]

ajb: The spin-2 attribute of the graviton arises from the quadrupole character of gravitational waves, as I understand it.