Gravity (again)

[Just Curious]

Having watched dozens of Discovery Channel programs on the origins of the Universe I am getting increasingly annoyed and confused with their description of Gravity. They very quickly dismiss the myth that Gravity is a force and describe it more correctly as a warping of space-time caused by massive objects. They even use the infamous rubber sheet model to illustrate this. So OK, we accept this. But later in the programs they start talking of Gravity as a FORCE again. They say at the big bang that all four forces were equal but the Gravity force broke away and became much weaker. They even postulate that it diffused into higher order dimensions which we cannot see. There is also the question of why mass warps space-time. What is the property that mass contains to do this? Have gravitons got anything to do with it or do they only exist at the quantum level. So the basic question remains. Is Gravity a force or not? Once the scientists decide then perhaps the rest of us will understand.

[BenTheMan]

Hi JC---

The point is that, a long distances, the description of gravity as warping of space-time works better. At short distances, it is likely that gravity has a particle interpretation (i.e. the graviton), and thus is more aptly described as a force.

[CptBork]

Hi JC---

The point is that, a long distances, the description of gravity as warping of space-time works better. At short distances, it is likely that gravity has a particle interpretation (i.e. the graviton), and thus is more aptly described as a force.

Ok, I hope I don't complicate the discussion too much, but I've been really curious about a certain aspect of Quantum gravity. Now in quantum field theory describing all the "forces" except gravity, spacetime fundamentally enters the description, but it's the flat spacetime in the absence of gravity, i.e. the Minkowski metric. If you want to describe Quantum physics in the presence of gravity, don't you still have to take the region's spacetime curvature into account? I thought the problem with Quantum Gravity wasn't the ability to incorporate a curved spacetime, but rather difficulties in describing how mass causes that curvature?

[Pandaemoni]

If you want to describe Quantum physics in the presence of gravity, don't you still have to take the region's spacetime curvature into account?

Yes, and there are such things as relativistic quantum field theories which are generalizations from non-relativistic quantum mechanics. The issue with quantum gravity is that nothing th quantum field theory "predicts" the existence of gravity. Rather quantum field theory tends to be a version of quantum mechanics with allowances made specifically to reflect what we happen to know about relativity.

For many real world purposes though, space-time is so close to flat that the relativistic effects don't greatly influence the outcome of experiments. Just like Newton's law of universal gravitation is is good enough for most uses, non-relativistic quantum mechanics can be used as a good approximation in most Earth based experiments.

Relativistic quantum field theory is very successful, but it is a classical theory applied in the context of quantum mechanics, rather than a true quantum theory of gravity.

I thought the problem with Quantum Gravity wasn't the ability to incorporate a curved spacetime, but rather difficulties in describing how mass causes that curvature?

I think the problem is deeper than that. What scientists want is a single theory that predicts all the fundamental forces, including gravity, rather than two separate theories that can be grafted together. It is certainly true that they do not know "why" spacetime curves i the presence of mass, but that is only one of the mysteries. Another is whether the fine structure of spacetime (even where it appears to be flat at our level) is or is not the famous "spacetime foam". Just like a billiard ball appears smooth to the naked eye, and yet is revealed to have pronounced projections and rifts under magnification, seemingly placid spacetime at the level of the Planck length may be a roiling and chaotic soup.

[CptBork]

Relativistic quantum field theory is very successful, but it is a classical theory applied in the context of quantum mechanics, rather than a true quantum theory of gravity.

I understand that. I just wanted to note that there's a very simple, natural way to incorporate curved spacetimes into quantum field theories. Just replace the Minkowski metric with a curved spacetime metric, and replace all derivatives with covariant derivatives (covariant in the GR sense). I believe this is done to some extent for Hawking radiation, i.e. you treat the edge of the event horizon as a locally flat region to do the calculation in QFT, and then convert to the coordinate system of a distant observer to add in the gravitational effects. I also know there are physicists actively studying quantum field theories in curved spacetimes, although I hear there are some strange results coming from this approach.

I think the problem is deeper than that. What scientists want is a single theory that predicts all the fundamental forces, including gravity, rather than two separate theories that can be grafted together. It is certainly true that they do not know "why" spacetime curves i the presence of mass, but that is only one of the mysteries.

Well I don't think the problem is with QFT not "predicting" gravity or curved spacetime- there's a straightforward recipe for taking the "classical" Lagrangian of GR and plugging it into QFT as a field energy term, then adding in kinetic and other interaction terms for the graviton excitations of this field. The problem I keep hearing about (and have seen sketched out mathematically) is that the bread and butter technique of QFT renormalization doesn't work with this recipe, thus perturbation expansions don't work in scattering calculations, and the resulting theory requires an infinite number of parameters.

[Just Curious]

Hi JC---

The point is that, a long distances, the description of gravity as warping of space-time works better. At short distances, it is likely that gravity has a particle interpretation (i.e. the graviton), and thus is more aptly described as a force.

Thanks Ben. So, somewhere along the way between the big bang, where gravity was a force best described using the graviton, to where we are now with the size of the universe where gravity is best described by the warping of space-time, gravity changed. The elusive theory of everything would unite these two proprties of gravity.

[kurros]

Well I don't think the problem is with QFT not "predicting" gravity or curved spacetime- there's a straightforward recipe for taking the "classical" Lagrangian of GR and plugging it into QFT as a field energy term, then adding in kinetic and other interaction terms for the graviton excitations of this field. The problem I keep hearing about (and have seen sketched out mathematically) is that the bread and butter technique of QFT renormalization doesn't work with this recipe, thus perturbation expansions don't work in scattering calculations, and the resulting theory requires an infinite number of parameters.

I also believe it is the case that if consider the resulting field theory to be simply an effective field theory then everything is fine, you can regularise everything with a cutoff at the scale the theory breaks down and off you go. In fact I have heard it said that such a field theory of gravity can be considered to be a much better theory than the standard model, which is probably an effective theory with cutoff somewhere not much higher than the electroweak scale, because the gravity qft cutoff is way way higher, the Planck scale I believe.
The standard model is still nicer though since it is renormalisable, it just starts being wrong above the electroweak scale (or so it seems) while the gravity qft really starts to go to hell above the cutoff. Or something like that, I don't really remember.

[CptBork]

I also believe it is the case that if consider the resulting field theory to be simply an effective field theory then everything is fine, you can regularise everything with a cutoff at the scale the theory breaks down and off you go. In fact I have heard it said that such a field theory of gravity can be considered to be a much better theory than the standard model, which is probably an effective theory with cutoff somewhere not much higher than the electroweak scale, because the gravity qft cutoff is way way higher, the Planck scale I believe.
The standard model is still nicer though since it is renormalisable, it just starts being wrong above the electroweak scale (or so it seems) while the gravity qft really starts to go to hell above the cutoff. Or something like that, I don't really remember.

If my understanding is correct, the Standard Model is considered by many to be an effective cutoff of an SU(5) Yang-Mills theory. The effective cutoff would make low-energy consequences such as the "strong force" artificially appear to be nonrenormalizable. Similarly, many consider GR to be an effective low-energy field theory and that the correct, complete field theory would hopefully not run into the same difficulties. Still, I don't really see any grounds to call one theory superior over another, regardless of their respective cutoff scales; I would think physicists would call both models flawed and seek them both as consequences of a single unified theory, such as String Theory, but I haven't heard of anyone being able to deduce the Standard Model just by extending GR on its own.

Anyhow what I'm really most curious about is Ben's comment that spacetime curvature might just be a convenient approximation at large scales. I'm just saying by the looks of it, from my own personal readings and understanding, it seems that even if gravity can be modelled by the exchange of gravitons, those gravitons must still have some sort of effect on the spacetime background in which everything is moving. This is what I'm most curious about.

[kurros]

Anyhow what I'm really most curious about is Ben's comment that spacetime curvature might just be a convenient approximation at large scales. I'm just saying by the looks of it, from my own personal readings and understanding, it seems that even if gravity can be modelled by the exchange of gravitons, those gravitons must still have some sort of effect on the spacetime background in which everything is moving. This is what I'm most curious about.

Ahh yes, well that is the most interesting question isn't it? At what point do the little perturbations of spacetime which are gravitons become something which has a noticable influence on the background geometry? Or how does the background geometry arise from them fundamentally?

Gravitons are really just the quanta of a linearised gravitational theory, in which one quantises perturbations about a background metric (gravitational waves) after making the approximation that the perturbations are small enough not to affect the background (linearising the theory). The theory breaks down when you go near the non-linear regime, which is where all the interesting stuff happens. I have some vague picture in my head of lots of gravitons building up into classical gravitational waves which as they become bigger become real changes to the curvature of space, but this dodges the question of what the heck the background is in the linearised theory. Maybe we'll know one day. It's also a pretty crap picture since I guess regular curvature of space has more to do with virtual gravitons than real ones. Ahh who knows.

[BenTheMan]

Thanks Ben. So, somewhere along the way between the big bang, where gravity was a force best described using the graviton, to where we are now with the size of the universe where gravity is best described by the warping of space-time, gravity changed. The elusive theory of everything would unite these two proprties of gravity.

Yes, but I wouldn't say gravity ``changed''. If you look at a single water molecule, what does it mean to talk about temperature? The point is that the same substance can be described by different degrees of freedom depending on whether you're interested in the short distance properties or the long distance properties.

[Jack_]

Thanks Ben. So, somewhere along the way between the big bang, where gravity was a force best described using the graviton, to where we are now with the size of the universe where gravity is best described by the warping of space-time, gravity changed. The elusive theory of everything would unite these two proprties of gravity.

Since gravitation mass and inertial mass are not distinguishable, then there must be some relationship between Higgs and the graviton, if all this junk is true.
Without establishing a model to explain that mass stimulates gravity and gravity operates on mass, we certainly scientifically do not yet have the answer.

In other words, a bijective function of some nature is required between the two.

[CptBork]

Since gravitation mass and inertial mass are not distinguishable, then there must be some relationship between Higgs and the graviton, if all this junk is true.
Without establishing a model to explain that mass stimulates gravity and gravity operates on mass, we certainly scientifically do not yet have the answer.

In other words, a bijective function of some nature is required between the two.

If you had actually known what you were talking about and not making stuff up off the top of your head, you would have observed that the Higgs mechanism only determines the rest masses of the particles. Since gravity depends on the relativistic energy and momentum of the particles, not the rest mass, a particle's gravity has no direct relation with the Higgs. For example, by your reasoning, radiation would produce no gravity of its own, but in fact it actually does.

[Just Curious]

CptBork. I don't want to incur your wrath like Jack but can you explain this statement
"Since gravity depends on the relativistic energy and momentum of the particles, not the rest mass."

[CptBork]

CptBork. I don't want to incur your wrath like Jack but can you explain this statement
"Since gravity depends on the relativistic energy and momentum of the particles, not the rest mass."

Jack has been making all sorts of posts about how conventional physics is totally wrong, that's why I was a little harsh in my response to him. It's ok to make mistakes, just not ok to make mistakes and then call everyone else stupid because they can recognize the mistake.

In General Relativity, the mass and energy present in a system determines the resulting spacetime curvature through a mathematical object called the "Stress-Energy tensor". The stress-energy tensor has applications in many areas of physics, and the quantities used to calculate it can depend on its usage, but in General Relativity the quantities you use are the relativistic energy density and relativistic momentum density of the particles/gases being considered. The important point is that the rest mass is not what you need to calculate the gravitational field, and the rest mass is all the Higgs mechanism determines. If I take a particle and add energy to it to make it start moving, that extra energy will be seen as an increase in the particle's relativistic mass/energy, and there will be an increased gravitational field as a result.

[BenTheMan]

Since gravitation mass and inertial mass are not distinguishable, then there must be some relationship between Higgs and the graviton, if all this junk is true.

Why do you say this?

[Jack_]

If you had actually known what you were talking about and not making stuff up off the top of your head, you would have observed that the Higgs mechanism only determines the rest masses of the particles. Since gravity depends on the relativistic energy and momentum of the particles, not the rest mass, a particle's gravity has no direct relation with the Higgs. For example, by your reasoning, radiation would produce no gravity of its own, but in fact it actually does.

In physics, mass (from Ancient Greek: μᾶζα) commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass. In everyday usage, mass is often taken to mean weight, but in scientific use, they refer to different properties.

Inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied. It is determined by applying a force to an object and measuring the acceleration that results from that force. An object with small inertial mass will accelerate more than an object with large inertial mass when acted upon by the same force. One says the body of greater mass has greater inertia.

Active gravitational mass is a measure of the strength of an object’s gravitational flux (gravitational flux is equal to the surface integral of gravitational field over an enclosing surface). Gravitational field can be measured by allowing a small ‘test object’ to freely fall and measuring its free-fall acceleration. For example, an object in free-fall near the Moon will experience less gravitational field, and hence accelerate slower than the same object would if it were in free-fall near the earth. The gravitational field near the Moon is weaker because the Moon has less active gravitational mass.

Passive gravitational mass is a measure of the strength of an object's interaction with a gravitational field. Passive gravitational mass is determined by dividing an object’s weight by its free-fall acceleration. Two objects within the same gravitational field will experience the same acceleration; however, the object with a smaller passive gravitational mass will experience a smaller force (less weight) than the object with a larger passive gravitational mass.


http://en.wikipedia.org/wiki/Inertia...#Inertial_mass


Rest mass
A constant intrinsic to a body which determines its inertial and energy-momentum properties. It is a fundamental concept of special relativity, and in particular it determines the internal energy content of a body. It is the same as the inertial mass of classical mechanics. According to the principle of equivalence, the basic physical principle of general relativity, the inertial mass of a body is also equal to its gravitational mass. See Classical mechanics, Gravitation, Relativity

The rest mass or inertial mass of a body, m, is a measure of its resistance to being accelerated at a by a force F ;

http://encyclopedia2.thefreedictionary.com/Rest+mass

[Jack_]

If you had actually known what you were talking about and not making stuff up off the top of your head, you would have observed that the Higgs mechanism only determines the rest masses of the particles. Since gravity depends on the relativistic energy and momentum of the particles, not the rest mass, a particle's gravity has no direct relation with the Higgs. For example, by your reasoning, radiation would produce no gravity of its own, but in fact it actually does.

This is curious, this article tries to unify of GR gravity and Higgs bosons.

http://arxiv.org/PS_cache/arxiv/pdf/...004.4866v1.pdf

Say, based on your strong response, I would like to see your unification of the graviton and Higgs, since I am so wrong in your view.

How long will this take?

If you are unable to do this, my statement that they have not been sucessfully unified stands.

[BenTheMan]

This is curious, this article tries to unify of GR gravity and Higgs bosons.

http://arxiv.org/PS_cache/arxiv/pdf/...004.4866v1.pdf

Say, based on your strong response, I would like to see your unification of the graviton and Higgs, since I am so wrong in your view.

How long will this take?

If you are unable to do this, my statement that they have not been sucessfully unified stands.

See my objections, here.

Just because it's on the arXiv doesn't mean it's legitimate science.

[Jack_]

See my objections, here.

Just because it's on the arXiv doesn't mean it's legitimate science.

I read your objections. Since I believe SR is false and have proven it, I believe GR is false also as a consequence. So I do not adhere to this paper anyway.

My objective was to demonstrate the mainstream has not yet proven the logical equivalence of inertial mass and gravitational mass though no experiment has refuted it.

It is mathematically still an open question which was the point of my earlier post.

[Billy T]

... by your {Jack's} reasoning, radiation would produce no gravity of its own, but in fact it actually does.

I believe a hot brick makes more gravity than the same brick when it has cooled off. Do you agree?
I believe that photons do not make gravity. Why do you say they do?

Most of this thread is over my head (educated in physic too long ago.) but here is simple argument suggesting that photons do not make gravity:

Look at the night sky (stars) with alternating eyes (only one open at a time) It has the same stellar pattern with either eye. Now consider how long a path the photons traveling to your single observing eyes have had - how close they have been together. If they mutually attract, why have they not coalesced into discrete fine streams? I.e. the right eye should see some stars the the left eye can not and conversely.

If they were hydrogen atoms and only their mutual gravity were present (no radiation pressure or other gravity fields etc.), and traveled so closely together for so long, by the time they arrived at Earth they would have coalesced into hydrogen molecules even neglecting any other mutual forces related to chemical binding.

As stars do have considerable size the photon pairs reaching either eye probably were well separated when leaving the star. Thus, replace the star with much more distant quasar or better still an early supernova - 13B years old event. If photons have mutual attraction would they not when arriving at Earth be coalesced into discrete steams smaller that the separation between my eyes?

Perhaps not true when calculated but that can be relatively easily done as you claim to know their mutual gravity. I.e. calculate how long it takes them to"fall together" under their mutual gravity. For example, start a pair of photons 100,000 meters apart and let them initially have slightly convergent trajectories, aimed towards one point on Earth and travel 13 billion light years. Will they not have a common trajectory long before reaching Earth?

PS my simple rule for when energy makes gravity and when it does not is that it does iff the energy is the same in all references frames, as is the case for the hot brick (Temperature of melting lead is not a function of the frame it is viewed from) but photon energy is frame dependent.