James: GR of course does not describe gravitational waves as excitations of a quantum field... it is not a quantum theory. That task belongs to a yet-to-be-discovered (if it exists at all) quantum theory of gravity.

A spherically symmetric source can cause a "wave" (a freely propagating change in the field) by changing its strength, changing its size, or changing its shape/location (losing spherical symmetry). Changes in size are out due to Birkhoff's theorem, as they make no difference. Changes in charge (mass) are also out, as mass-energy is conserved. So the only way a spherically symmetric source can generate a gravitational wave is by losing spherical symmetry. This is exactly analogous to electric charges: a spherically symmetric electric charge also cannot generate a free electromagnetic wave unless it starts moving or changes its shape, i.e., it loses its spherical symmetry.

I think it is important not to confuse a static field from free radiation. A static mass or a (static electric charge) has a corresponding static (gravitational or electrostatic) field that, in a corresponding quantum theory, may be mediated by virtual particles, but this has nothing to do with free radiation. To produce free radiation, a gravitating source would lose energy, just as a source of EM waves loses energy.

No, I do not believe that this observation says anything about the existence of gravitons. It is about the imprint of gravitational radiation (due, in particular, to inflation) on the CMB. The theory that describes the tensor and scalar modes and their ratio is the classical theory of perturbations.

V. Toth,

I've modified the question to highlight the recent paper, http://arxiv.org/abs/1309.5343 (sorry if you didn't notice it before) - as I understand, it argues that gravitational radiation is a manifestation of gravitons, just as EMR is comprised of photons.

I understood gravitational waves in GTR to be the oscillation of dimensional spacetime - not a propagating particle...

James: Indeed I missed the point about the Krauss and Wilczek paper, but I stand by my answer: even if you buy into their argument, according to their own paper there are a bunch of "if"-s that need to be satisfied above and beyond the mere detection of tensor modes before one can conclude that gravity is indeed quantized. To use the electromagnetic analogy, the mere existence of light is no proof that the EM field is quantized: for that, you need to find observational phenomena that cannot be explained by a classical theory. I don't think we are anywhere near that with gravity.

A propagating particle in QFT is, well, a propagating excitation quantum of the underlying field, i.e., a quantized wave, wave-particle duality and all. So (assuming of course that a yet-to-be-discovered quantum theory of gravity is a quantum field theory) oscillations of spacetime and propagating gravitons are really two ways of looking at the same thing.

V. Toth,

In the case of merging binary black holes, for example, the oscillations would be produced by the rapidly varying gravitational field strength. A single black hole would ostensibly not produce gravitational waves, yet would produce gravitational effects.

If gravitational waves represented a propagating excitation of the quantum gravity field/gravitons mediating gravitational interactions, do you have some idea how gravitational effects might be imparted in the absence of gravitational waves? Thanks...

James: In a field theory the field, in the process of mediating an interaction, also carries energy and momentum. Hence, "waves" will exist. This has nothing to do in my mind with whether or not the theory is quantized. The least exotic way that I can think of if you want to give up on the existence of waves would be to replace the field theory with an action-at-a-distance interaction, and there are many good reasons why physicists since Newton have not been fond of the concept of action-at-a-distance. So I guess my honest answer to your question is that no, I don't have any ideas that I'd consider acceptable to produce gravitational effects without gravitational waves. (That does not mean that I don't consider it incredibly important to confirm the existence of gravitational waves through observation.)

V. Toth,

If gravitational effects are not mediated by particle interactions and gravitons do not exist, then there must be some other, distinct mechanism for imparting gravitational effects.

In the case of a propagating light wave imparting EM effects, Its effects are imparted when material particles absorb the photon energy. Massless gravitons should likewise be expected to linearly propagate at the speed of light - their energy must be absorbed by matter in order to interact gravitationally.

Photons emitted by the Sun are absorbed by the Earth; when the Earth blocks the propagation of Sunlight to the Moon, it is not illuminated. However, in such cases there does not appear to be any measurable reduction in the gravitational effects imparted to the Moon. This illustrates that the photon model of gravitational interaction mediation is not valid - suggesting to me that some distinct mechanism must produce the mediation of gravitational interactions.

I do not think that any spooky action at a distance is necessary if gravitation is strictly an interaction between mass-energy and some physical property of the vacuum, distinct from any direct material force interaction described by existing quantum theory.

James: In a field theory such as classical EM or general relativity, matter interacts with the field (and it is irrelevant what we call this, be it "field" or "some physical property of the vacuum" or whatever), while the field carries energy and momentum. So the electrostatic interaction between two charges, or the gravitational interaction between two masses is described as the interaction between the charge or mass and the field, and the response of the field to the presence of these charges/masses. In the quantum version of a field theory, these interactions take place in the form of discrete quanta, which we recognize as "particles".

Don't confuse electromagnetic radiation and electromagnetic interactions. When two charged particles interact, you don't see rays of light exchanged between them. There are no freely flying photons emitted by one and absorbed by the other. Yes, we actually describe the (quantized) interaction between two charges in a quantum field theory as the exchange of _virtual_ photons, but this has nothing to do with propagating rays of light, and these virtual photons certainly don't travel in straight lines or cast shadows.

Indeed, if the Sun were, say, electrically charged as a positive charge and the Earth and the Moon had negative electric charges, they'd both be orbiting the Sun. And just because the Moon is behind the Earth at one point would not mean that it would stop responding to the Sun's electrostatic field, despite the fact that it's in the Earth's shadow.

V. Toth,

I can appreciate your view to some extent, but as I understand, the sun-side of the Moon is primarily warmed by discrete, persistent, linearly propagating photons - emitted by material within the Sun's corona - being absorbed by its surface material, not by its electromagnetic field or virtual photons.

I think it is important to consider the properties static fields as opposed to any 'action at a distance', but as I understand, electrostatic fields are composed of large numbers of electromagnetically charged particles. No such particles are detectable within gravitational fields. Gravitational fields are best described by dimensional distortions that seem to represent some unidentified physical aspect of the vacuum.

Even considering gravitation to be and interaction between mass-energy and a vacuum-energy field, I think there are major obstacles to fitting it within the existing quantum framework that describe the three material force interactions.

James: It is correct that the Earth is warmed by free photons from the Sun. This is precisely why it is NOT a good analogy with gravity, where the Sun-Earth interaction is primarily through the static gravitational field of the Sun, not through gravitational radiation. This is why I offered a more appropriate thought experiment analogy with an electrostatically charged Sun, Earth and Moon.

The electrostatic field is not "composed of large numbers of electromagnetically charged particles". The electrostatic field is, well, the field. It's not composed of anything. Its excitations are quantized, and if spatially localized, we detect these as particles, but it is wrong to say that the field is composed of excitations. It is not. And although there are many obstacles in the way of quantizing gravity, this line of thought is not one of them.

V. Toth,

"The electrostatic field [...] excitations are quantized, and if spatially localized, we detect these as particles..." The question is whether this is also true of gravitational fields.

As I understand if an electrostatic field is not considered to be composed of charged particles (despite their being readily detectable) then it must be considered to be composed specifically of electromagnetic energy.

Conversely, gravitation is described as the spatial contraction and corresponding time dilation of dimensional spacetime. The gravitational waves expected to be produced by high frequency rotating binary masses, and even those primordial gravitational waves thought to be have produced the observed B-mode polarization of light may best be described as density waves propagating kinetic energy. See http://www.preposterousuniverse.com/blog/2014/03/16/gravitational-waves-in-the-cosmic-microwave-background/.

This would be more analogous to kinetic energy being propagated through some physical medium such as water than any manifestation of quantum electromagnetic charge states. I think the question is whether such density waves could be generated at quantum scales by kinetic energy propagated through primordial spacetime.

James: I am not sure how to continue this conversation, since my contribution appears to be reduced to refuting one misconception after another. The electromagnetic field in QED is not composed of anything. It is a fundamental component of the theory. Gravitation is not spatial contraction and time dilation. Simple situations (e.g., the spherically symmetric, static solution) can be described as manifesting themselves in the form of spatial contraction and time dilation, but that is not what gravity *is*. Gravitational waves indeed carry energy and momentum, but B-modes do not; they are simply the response, imprint if you wish, of the presence of large wavelength gravitational waves on the CMB. And neither B-modes nor the gravitational waves that may be responsible for their presence are density waves. And then we are back to a discussion topic that you and I had once before, about your insistence on a medium. There is nothing I can add that on that topic that I have not said previously, except perhaps emphasizing that you can't do theoretical physics using just words. If you wish to seriously question the validity of existing theories or the direction of on-going research, you must do it through the math.

V. Toth,

I think I'm only questioning whether gravitation can be fit into the framework of existing quantum theory. I don't think its been established that the effects of gravitation, best described as spatial (length) contraction and time dilation, can be described by QFT or any other quantum theory. Thanks for your discussion - I agree that our conceptual views conflict - I don't concede that only your conceptions are correct. On-going research often takes contradictory directions...

See http://www.nature.com/news/telescope-captures-view-of-gravitational-waves-1.14876

"... Cosmologists knew, however, that inflation would have a distinctive signature: the brief but violent period of expansion would have generated gravitational waves, which compress space in one direction while stretching it along another [see below]. Although the primordial waves would still be propagating across the Universe, they would now be too feeble to detect directly. But they would have left a distinctive mark in the CMB: they would have polarized the radiation in a curly, vortex-like pattern known as the B mode."

James: the "effects of gravitation" are best described in terms of falling apples. Or the orbit of the Moon. The description involving spatial contraction and time dilation applies only to very specific cases, from the viewpoints of very specific observers. And I still sense a degree of confusion in your comment, between the gravitational field vs. free gravitational waves (the EM analogy would be electrostatic or magnetic fields on the one hand, vs. free electromagnetic radiation on the other.)

James, I am trying hard to avoid presenting subjective "conceptual" viewpoints. I do not believe that ResearchGate is the right forum for me to speculate or to present my personal opinions (unless I clearly indicate it as such), or to promote unproven ideas. I restrict my contributions to areas that I am familiar with, and to facts that I know (be they observed facts from nature, of facts about specific theories.) Basically, I am doing my best not to say anything here that I cannot backed with solid math and established theory. If you don't want my answers, please don't hesitate to say so, but please do not snub them as mere preconceptions.

V. Toth,

"Gravitation is not spatial contraction and time dilation. Simple situations (e.g., the spherically symmetric, static solution) can be described as manifesting themselves in the form of spatial contraction and time dilation, but that is not what gravity *is*."

"And neither B-modes nor the gravitational waves that may be responsible for their presence are density waves."

These are your responses to my statement:

"Conversely, gravitation is described as the spatial contraction and corresponding time dilation of dimensional spacetime. The gravitational waves expected to be produced by high frequency rotating binary masses, and even those primordial gravitational waves thought to be have produced the observed B-mode polarization of light may best be described as density waves propagating kinetic energy."

Note that I _did_not_ say what gravity "*is*" - I do not think that a physical mechanism for producing gravitational effects has been described.

I also _never_ stated or even implied that B-mode polarization of light is a density wave!

Moreover, I think that the description of gravitational waves as density waves is consistent with the referenced description and illustration in my preceding comment.

Since you expressed that you "still sense a degree of confusion" in my comments, I'll mention that I sense in you an abject inability to consider alternatives and object to your misrepresentations of my remarks in your dismissals. Please refrain from making any further personal remarks.

James: I am not sure what significant difference there is between sentences like "X is Y" vs. "X is described as Y" but I believe I was responding to the exact sentence ("Conversely...") that you yourself quoted from your earlier message. I was also responding to the sentence, "even those primordial gravitational waves thought to be have produced the observed B-mode polarization of light may best be described as density waves". And no, quadrupole radiation is really not a density wave. I believe that the Sean Carroll blog entry, which you yourself referenced, provides a clear description distinguishing E-modes (scalar modes) due to density fluctuation as "gradient without curl" vs. B-modes (tensor modes) as "curl without gradient".

James, I am not making personal remarks. My statement about confusion referred to the subject of your comments (gravity, physics), not to the person doing the commenting. Specifically, I am responding to the fact that in your comments, effects due to free radiation vs. effects due to interactions appear to be confused, to the extent that you expressed an argument that the Earth's shadow should somehow shield the Moon from gravity if gravity was mediated by a (quantum) field. This really is not the case, and this is the confusion that I was hoping to dispel. It was certainly not intended as a personal remark of any kind.

In the excellent Sean Carroll article (thanks, Bernard Jones), http://www.preposterousuniverse.com/blog/2014/03/16/gravitational-waves-in-the-cosmic-microwave-background/ - just beneath the CMB map image is the following paragraph:

"Then, of course, there are quantum fluctuations in the gravitational field: gravitational waves, or “gravitons” if you prefer speaking quantum-mechanically (sometimes called “tensor” fluctuations in contrast with “scalar” density fluctuations). It was in the early 80′s, soon after inflation itself came along, that several groups pointed out this prediction: Rubakov, Sazhin, and Veryaskin; Fabbri and Pollock; and Abbott and Wise. Just as an electromagnetic wave is an oscillation in the electric and magnetic fields that propagates at the speed of light, a gravitational wave is an oscillation in the gravitational field that propagates at the speed of light. We can detect electromagnetic waves because they would cause a charged particle to jiggle up and down; we could (in principle, though not yet in practice) detect gravitational waves because they alternately stretch things apart and then compress them together as they pass."

As I understand, what is referred to as "“scalar” density fluctuations" are those that produce the temperature variations found in the CMB map. They are not what I'd call density waves but rather "density fluctuations". As the text states, gravitational waves "alternately stretch things apart and then compress them together as they pass." I assert that this can be described as a (spacetime) density wave - rather than any propagating 'graviton'.

I make this assertion because it is the dimensions of spacetime that are oscillating - unlike electromagnetic waves, which are "an oscillation in the electric and magnetic fields that propagates at the speed of light".

IMO, it is this oscillating dimensional distortion of spacetime that polarized the CMB light as it propagated.

James: Gravitational waves stretch objects in one direction while compressing them in another. The volume (hence, the density) remains unchanged. (This is generally the case in the vacuum where the Ricci tensor is zero, as the stuff that remains, the Weyl curvature, is volume-conserving.)

I think it's important to point out that quantum theory has yet to clinch its appropriation of gravitation, so any objection to the quantum interpretation remains legitimate. The quantum-theoretical effort has been from the beginning an attempt to glom electromagnetic mathematical analogies onto GR, and it still hasn't worked.

Consider James' statement, which correctly expresses the expectation of "gravitational waves" according to quantum theory:

"We can detect electromagnetic waves because they would cause a charged particle to jiggle up and down; we could (in principle, though not yet in practice) detect gravitational waves because they alternately stretch things apart and then compress them together as they pass."

Quantum theory has somehow failed to notice that the moon generates a big, slow "gravitational wave" which jiggles the earth on a daily basis, as is most strikingly evidenced by the oscillation of the tides.

I know, that's not the sort of wave quantum theory predicts: A binary star system, for example, is supposed to generate quantum-like gravitational waves, just as the oscillation of electrons on an antenna generates electromagnetic waves. But here's the fundamental misconception: While the binaries may appear, in our field of vision, to be oscillating, they are actually involved in a mutual orbit, and any energy released due to the orbit's decay is simply a conversion of kinetic/potential energy within the system to kinetic/potential energy relative to the rest of the universe. Does a distant, imbalanced system generate gravitational waves? Sure, with each orbit, as the more massive binary moves closer then further away, the gravitational influence here oscillates (however faintly) in waves -- just as with our moon, as it moves periodically closer and further from points on the earth.

Jim: the jiggling of the Earth by tides is really not due to "big, slow 'gravitational waves'". A gravitational wave is free radiation propagating in empty space, carrying energy and momentum. In a quantum theory, gravitational waves would correspond to free, on-shell gravitons. This is distinct from the interaction (oscillating or not) between the Earth and the Moon through the gravitational field (or, in a quantum gravity theory, through the exchange of virtual, off-shell gravitons.) And I think it is misleading to suggest that quantum theory failed to take notice of this; on the contrary, attempts (not very successful, of course, but for different reasons) to quantize gravity are about quantizing precisely this, the observable effects of gravity, which includes tides.

The gravitational waves generated by a binary are not due to the fact that their distance changes. Any accelerating mass generates gravitational radiation in the theory, just as any accelerating charge generates electromagnetic radiation. So even if the orbits were circular, gravitational radiation would be produced (and the orbital radii would slowly decrease, but that's a result, not the cause, of gravitational radiation.) The periodically varying distance between the Earth and the Moon is due to the elliptical orbit, with energy constantly converted from gravitational potential energy to kinetic energy and vice versa, but very little of it is released in the form of gravitational radiation for such a small, weakly gravitating system like the Earth-Moon system.

V. Toth, you've re-stated the quantum theory of gravity clearly and succinctly, but my point is that in terms of GR, two bodies in a mutual orbit are moving uniformly in their own coordinate systems, and there is no basis for supposing that they are radiating energy. The energy generated by tidal stresses is purely local, and likewise cannot be identified with a quantum-like radiant eneergy. Hence the only jusfification for introducing a quantum component to gravitation theory is to give custody of gravitation theorizing to quantum theorists.

Jim: I am not sure I understand your comment. Coordinate systems are an abstraction: the whole point of (general) relativity is that the physics is independent of the choice of coordinate systems. In classical (non-quantum!) gravity, two interacting bodies move in spacetime which is characterized by the metric which, in turn, is determined by the position and motion of the two bodies. This metric field carries energy and momentum, some of which escapes the two-body system in the form of free gravitational radiation propagating to infinity. This part of the theory is entirely classical and does not require any quantum concepts.

The justification for introducing a quantum theory of gravity has very little to do with gravitational radiation. In fact, the only case when you really DON'T need quantum gravity is in the absence of matter, i.e., in a vacuum where gravitational radiation propagates and the theory is reduced to finding metrics for which the Ricci tensor is zero.

The need for quantum gravity arises in the presence of matter that is itself described by a quantum field theory (the standard model), resulting in a self-contradictory form of the Einstein field equations with quantum quantities on one side, and ordinary numbers on the other. One way to resolve this discrepancy is to replace the Einstein field equations with the "semiclassical" version, in which the quantum values describing matter are replaced by their expectation values which are ordinary numbers, but as far as I know, nobody really believes that this is a satisfactory resolution of the problem (even though it is a useful effective approximation.) The other resolution is to develop a quantum gravity theory. In other words, I don't think this has anything to do with custody battles between competing groups of physicists.

Gravitational waves are thought to propagate gravitational energy. Gravitational fields are described as locally (length) contracted space and dilated time progression. IMO it follows that propagating gravitational energy must represent the propagation of the contraction of space and dilation of time.

The question is: are the effects of gravitation mediated by the hypothesized graviton particle and is their existence proven by the indirect detection of gravitational waves - or is gravitation fundamentally distinct from the electromagnetic, strong and weak interactions described by existing quantum theories?

It's important to consider whether gravitational energy represents some specific quantum particle charge property or more generally external kinetic energy that imparts momentum to matter in proportion to potential mass-energy. Certainly mass-energy is a property of quantum particles and quantum particles are affected by gravitational fields, but I'm not aware of any evidence that quantum particles produce gravitational effects.

@V Toth: "the whole point of (general) relativity is that the physics is independent of the choice of coordinate systems."

That is indeed the position of quantum theory. But according to a consistent geometric theory of gravity, a body may be considered to be accelerating in a gravitational field, or not, depending on the coordinate system of a particular reference frame. Mathematical models aside, it is an empirical fact: From my reference frame orbiting the earth I can perform no experiment internal to my craft that will indicate that I am accelerating or under the influence of a force; I can observe another orbiter at a different elevation to be accelerating according to my coordinate system, and yet an observer in that craft can report by the same tests that he is not accelerating or under the influence of a force. No force: No basis for radiation.

Robert, I think I agree. I was referring to gravitational acceleration, in the context of the geometric theory. But inertial acceleration, that which is non-geodesic, isn't necessarily "local." A stream of gas being ejected from a distant star or galaxy is an example of a very non-local, non-geodesic acceleration.

The answer to the original question is: No!

And, by the way, while indirect consequences of the emission of gravitational waves have been observed for quite a long time (binary stars - Taylor), a direct observation of gravitational waves has not been made, yet.

I think Jurg Frohlich has summarised the essential points rather well. For, this is perhaps the first evidence of a direct observation of a gravitational, as against the indirect observations reported earlier (e.g., Taylor ).

asoke mitra

The equations of classical gravitational theory (Einstein's 'General Relativity') have propagating wave solutions. These gravitational waves have nothing whatever to do with quantum theory - the are ripples in the geometry of space, just in the same way that than ripples on a pond are changes in the geometry of the surface.

The proper unification of physics that would incorporate quantum theor and gravitation in a consistent way is unknown. Until we have such an all-encompassing theory, "gravitons" remain in the realm of speculation.

Eric,

Well put! That's precisely my understanding - that even primordial gravitational waves do not represent any propagating quantum particle, but rather the oscillation of spacetime dimensional distortions. That primordial gravitational waves associated with inflation could have rippled through the dense early universe at quantum scales, polarizing light waves escaping the last scattering of photons through spatial distortion, does not suggest to me that they are fundamentally like electromagnetic energy emissions.

While predicted primordial effects of inflationary gravitational waves - the polarization of initial light emissions produced by oscillating spacetime distortions - may heave been identified, not even the primordial gravitational waves associated with universal inflation have been identified as a feature of background spacetime.

I would like to introduce an entirely new perspective to the discussion. Gravitational wave equations imply that spacetime has impedance: Z = c^3/G = 4×10^35 kg/s. The implication is that spacetime is an elastic medium that is capable of absorbing energy and returning energy to a gravitational wave. I have recently written a technical paper (ready for submission) which proposes that everything in the universe – all particles, fields and forces – are all derived from the single building block of 4 dimensional spacetime. While this is a speculative idea, it is justified by deriving from first principles both the weak gravity curvature of spacetime produced by fermions and the gravitational force between individual fermions. This new way of deriving forces also results in an easily verified prediction that the magnitude of the gravitational and electromagnetic forces are related through a simple difference in exponents. The implication is that there are no gravitons. A preview of this paper is available at:

http://onlyspacetime.com/QM-MackenCB.pdf

I would appreciate any comments and/or criticism.

John,

This question posting was intended to highlight the apparent conflict among relativity and especially quantum theorists who seem to adopt very different descriptions of gravitational waves without acknowledging that doing so produces this conflict.

Gravitational waves in GR seem to clearly describe mechanical waves propagating energy as deformations in a medium of curved spacetime. Conversely, quantum theorists seems to ignore that original description and presume that gravitational waves follow the model of EMR - periodic oscillations of electrical and magnetic fields generated by charged particles - except they are oscillations of an undetected quantum gravitational field. In this context, it's also presumed that gravitational waves represent a hypothesized propagating graviton particle, just as EMR represents a propagating photon.

Graviton is fiction. Photon is gravitaing, and gravitation accompanies all quantum particles, but bare graviton is only a naive analogue. So far, there is no way to quantize gravity, and apparently it is deadlock.

FYI - also see http://physicsworld.com/cws/article/news/2014/apr/10/have-galactic-radio-loops-been-mistaken-for-b-mode-polarization.

"Graviton is fiction...bare graviton is only a naive analogue." - Alexander Burinski

Gauge fields are spin-1 fields. The electromagnetic equations are linear because there is no self-coupling; the photon doesn't contribute to its source (it has no charge). But in general the equations of a gauge field are non-linear because they contribute to their own source. There is self-coupling. Now, if you take the standard linear equations (Bargman-Wigner equations) for a spin-2 field and insert the energy-momentum tensor as source term, the contribution of the energy-momentum of the spin-2 field itself has to be taken into account. There is self-coupling. The equations become non-linear.

The resulting non-linear equations are precisely Einstein's gravitational equations!

So the analogue may not be so naive.

S N Gupta, Proc. Phys. Soc. London 1952, A65, 608

S Deser, Gen. Rel.Grav. 1971, 1, 9

E A Lord, Pramana 1987, 29, 359

Still, it seems to me that general relativity describes gravitational waves only as mechanical waves propagating kinetic energy in a medium of curved spacetime - directly analogous to energy waves propagating through a puddle of water - not as a localized excitation of a quantum energy field. See http://en.wikipedia.org/wiki/Gravitational_wave#Sources_of_gravitational_waves.

There seems to be no prediction of a gravitational wave being propagated by any non-rotating spherically symmetrical gravitational field - how is the gravitational effect physically propagated? Unlike the propagation of EM energy, there seems to be no loss of any quantum source energy. No reduction in particle mass-energy....

James: GR of course does not describe gravitational waves as excitations of a quantum field... it is not a quantum theory. That task belongs to a yet-to-be-discovered (if it exists at all) quantum theory of gravity.

A spherically symmetric source can cause a "wave" (a freely propagating change in the field) by changing its strength, changing its size, or changing its shape/location (losing spherical symmetry). Changes in size are out due to Birkhoff's theorem, as they make no difference. Changes in charge (mass) are also out, as mass-energy is conserved. So the only way a spherically symmetric source can generate a gravitational wave is by losing spherical symmetry. This is exactly analogous to electric charges: a spherically symmetric electric charge also cannot generate a free electromagnetic wave unless it starts moving or changes its shape, i.e., it loses its spherical symmetry.

I think it is important not to confuse a static field from free radiation. A static mass or a (static electric charge) has a corresponding static (gravitational or electrostatic) field that, in a corresponding quantum theory, may be mediated by virtual particles, but this has nothing to do with free radiation. To produce free radiation, a gravitating source would lose energy, just as a source of EM waves loses energy.

V. Toth,

Thanks for so clearly explaining - well put!

However, I do not think that the gravitational waves that are produced by rapidly oscillating (static) gravitational fields (i.e. merging black holes) represent free radiation that can be described as a propagating elementary particle (graviton)...

V. Toth, I second James' appreciation!

You may or may not agree (I should probably review other comments you've made), but I believe (as I think James would agree) any "free radiation" of gravitational waves would be a non-quantum propagation of a change in the intensity of the warping of spacetime -- i.e., a change in the internal potential/kinetic energy of a system corresponding to an equal and opposite change in its potential/kinetic energy relative to the universe, as has been observed in at least one binary star system.

BTW, sniffing around a bit that at http://blankonthemap.blogspot.com/2014/03/b-modes-rumours-and-inflation.html the blog author discusses primordial gravitational waves as primordial tensor fluctuations. That suggests to me that they're considered the product of rapid amplitude modulation of the stress-energy tensor describing universal spacetime - that's why they're referred as 'gravitational waves' - not that they're really produced by gravitation per se...

Also see http://www.lbreda.com/listing/fisica/Cosmologia/dodelson.pdf section 6.4 "Gravity Wave Production", (labeled) pages 155 - 162

(many thanks to Abhijit Saha for the helpful reference).

As I understand, these so-called 'gravity waves' are best described as (temporally varying) "tensor fluctuations in the gravitational metric" (spacetime dimensional coordinates). They are not the product of any strictly gravitational primordial interaction...

in relativistic terms, it seems that these fluctuations are not necessarily associated with the propagation of any quantum particle (gravitons) - although quantum theorists prefer to interpret them in that way...

Since "wave-particle duality" is well-established for the rest of physics, it seems to me unlikely that the gravitational field would be an exception. But that's just a hunch - we simply don't have a consistent physical theory at present. Science is not about what people "believe".

IMO, gravitational waves are most likely mechanical waves propagating through a condensed vacuum medium - much like energy waves propagating through a fluid medium. I agree that "science is not about what people believe". IMO, gravitation is fundamentally distinct from the 'other' quantum force interactions of matter, although many would prefer that it fit within the framework of quantum field theory.

James ~

We are in agreement that, in the present state of theoretical physics, this question remains in the realm of opinion and speculation. I understand the basis for your opinion but I am not able to agree with it, for the following reasons:

It is true that gravitation does seem different from all other physical field - it describes the arena ("spacetime") in which the rest of physics takes place. In classical physics and in relativistic quantum mechanics (which is formulated in the context of special, not general, relativity) the spacetime background is not dynamical. But in GR the metric is a dynamical field, whose source is energy-momentum. GR in its present form is a classical theory. The advent of quantum mechanics then leads to a severe anomaly. In Einstein's gravitational equations, the left-hand side describes geometry while the right-hand side is constructed from all other physical fields. Since the right-hand side is quantum-mechanical, the left-hand side must be, too, otherwise it makes no sense. The problem is simply that we don't know how to do it! The following considerations may be pointing the way:

I already mentioned the work of Gupta, that derived Einstein's equations from the self-coupling of a linear spin-2 field. Conversely, small dynamical perturbations on a fixed background spacetime (flat or curved) take the form of a linear spin-2 field. A linear spin-2 field can be quantized and we have "gravitons" in that context. Then there are consistent approaches to gravitation that extend Einstein's theory by formulating gravitation as a gauge theory. Kibble-Sciama theory and the Poincaré gauge theories that superseded it are gauge theories (the Lie group is the Poincaré group). Gauge theories for internal symmetries (Yang-Mills fields) are, of course, a fundamental part quantum physics.

Your view of gravitational waves as analogous to perturbations propagating through a material medium is a useful analogy in some contexts. But, for reasons given above, it cannot be the whole story.

Eric,

Thanks for explaining we - do agree on some points. Unfortunately, I'm not able to comprehend much of your discussion in the paragraph mentioning Gupta's work.

However, I think your point regarding the dynamical nature of spacetime in GR is crucial - that the static nature of spacetime in quantum theory suggests to me that it is quantum theory that requires substantial revision to enable integration with gravitation!

In the gravitational waves described by GR it is the alternation of distinct tensor fields - caused by the rapid motions of massive objects - that produces the oscillations of the observed tensor field presented to an observer. Unlike EMR, there is no discrete, localized wave produced.

Likewise, the primordial gravitational waves expected in association with cosmic inflation can best be described in the context of GR as "tensor fluctuations in the gravitational metric" (spacetime dimensional coordinates). As I understand, it is those fluctuations in the dimensional coordinates of spacetime that is thought to have imparted the B mode polarity of CMB light waves - there is no defined gravitational force interaction that could have produced the expected polarization of light...

I also find it instructive to consider that the accretion of massive objects is thought to begin not by gravitational interaction of particles, but rather EM charge interactions. The condensation of potential mass-energy has a seldom acknowledged opposite action - that of _extracting_ mass-energy from the vacuum. I suspect the contraction of dimensional spacetime involves the corresponding condensation of vacuum energy. In this case the accretion of massive objects may involve the emergence of an opposing potential-kinetic energy charge state directly involving dynamic vacuum energy densities - quite distinct from any inherent property of quantum particles, except that it is proportional to mass-energy... Of course this is speculation, but such possibilities should be considered in the search for an integrated theory - not just forcing gravitation into the framework of existing quantum theory.

James ~

thank you for your thought-provoking response. It is significant, perhaps, that in Einstein's theory there is no such thing as an "energy momentum tensor" for the gravitational field. At one time there were efforts to overcome this perceived problem by trying to define suitable energy-momentum "pseudo"-tensors (an approach that I always felt was inelegant and misleading...). In contrast to the case of electromagnetic radiation, there is no localised well-defined energy transported by a gravitational wave. At one time my feeling was that it is "information" rather than energy that is transported by gravity. Maybe I've misunderstood you, but perhaps all this relates to your third paragraph "...Unlike EMR, there is no discrete, localized wave produced"?

The general opinion seems to be that our understanding of the geometry of spacetime needs to be substantially revised to make it "fit in" with quantum theory. I now see that you have an opposing view: "...it is quantum theory that requires substantial revision to enable integration with gravitation!" That's a minority view but can't be ruled out. It was also Einstein's opinion! And of course there is much that is paradoxical and counter-intuitive in quantum theory. ("If you think you understand quantum mechanics, you don't understand quantum mechanics" - attributed to Feyman).

You said "Unfortunately, I'm not able to comprehend much of your discussion in the paragraph mentioning Gupta's work". Sorry about that - I was too lazy to dig out references. Try googling some of my key phrases...

Best wishes (-:

Eric,

Thanks for your helpful discussion, consideration and kind remarks!

Best wishes

A simple idea that

1) mass warps spacetime

2) motion in time is dynamic

3) the result is either curvature in motion or persistent pressure against the resistance of conglomerate masses

explains gravitational phenomena.

Unfortunately, it's not much fun for quantum physicists.

FYI - the BICEP2 primordial gravitational wave - CMB polarization results may be confirmed (or not) in Oct. See http://www.nature.com/news/milky-way-map-skirts-question-of-gravitational-waves-1.15181.

FYI - another potential source of BICEP2 error - hopefully to be determined by Plank's map of the galactic foreground in Oct. See http://news.sciencemag.org/physics/2014/05/blockbuster-big-bang-result-may-fizzle-rumor-suggests.

For the record - the bickering continues, at least until Oct... http://www.nature.com/news/gravitational-wave-discovery-faces-scrutiny-1.15248.

FYI - regarding specifically the original question, a recent essay - http://www.gravityresearchfoundation.org/pdf/awarded/2014/Porto_2014.pdf - states:

"At first glance, the (indirect) measurement of primordial tensor modes by the BICEP2 collaboration supports an inflationary paradigm for early universe cosmology together with quantum vacuum fluctuations (aka gravitons) as the origin of the spectrum. In this essay we argue the the observed signal may instead be a signature of semi-classical sources of perturbations during inflation. In this scenario, despite a large tensor-to-scalar ratio r ~ 0:2, it may be possible to write an effective field theory of a rolling scalar field without super Planckian excursions..."

I generally concur with this view - that any primordial 'gravitational' waves - perturbations of the dimensional spacetime gravitational metric - were not the product of any strictly gravitational interaction. IMO, even if it can be shown that gravitons existed and were influential in the inflationary primordial universe - that is _not_ evidence that they directly or even indirectly mediate gravitational interactions among massive objects.

In particular, there seems to be no implicit role for gravitons in the large scale gravitational waves described by general relativity - oscillating spacetime curvature produced by, for example, the rapid juxtaposition of collapsing binary neutron stars or even Venus orbiting the Sun - see http://en.wikipedia.org/wiki/Gravitational_wave#Sources_of_gravitational_waves.

James,

I would like to point out a theoretical problem of quantizing

gravity. We know that linear superposition principle is used in

quantum mechanics. This means that if psi_1 and psi_2 are

two different solutions of a differential equation then

c1 psi_1 + c2 psi_2 is also a solution. But this principle

is not applicable if the differential equation under consideration

is not linear. That is the unknown variable and all its derivatives

occur only with the first power. Unfortunately this is not the

case with differential equations describing classical

gravitational fields. No body knows how to form a quantum

theory of gravity. Hence the term "graviton" is somewhat

loosely defined. Generally we use the word "graviton" to

mean an very massive elementary object (spin 2), which mediates

gravitational force. Newton's gravitational constant is inversely

proportional to square of the mass of the graviton, implicitly

assuming that the theory of graviton is not known yet.

Biswajoy: Nonlinearity is not necessarily a problem for quantum field theory. By way of analogy, W-bosons interact with W-bosons, and Higgs bosons interact with Higgs bosons, yet they're both described perfectly well using QFT, so gravitons interacting with gravitons should not be a problem either. The problem with quantum gravity is renormalizeability.

And the hypothetical gravitons are spin-2 particles. It'd be the supersymmetric counterpart of the graviton, the (even more hypothetical) gravitino, that would be a spin-3/2 particle. And gravitons, just like the photon, are assumed massless; otherwise, gravity would have a finite range just like the weak interaction. Indeed, a "very massive" graviton would imply a gravitational interaction with a subatomic range, which is clearly not what we observe.

@ V.T.Toth

Thank you for your comment on the spin of graviton. I have corrected

by previous post.

But I could not understand your analogy of W boson interacting with

W bosons. I guess you have referred to three gauge boson vertex of

W boson and the quartic coupling of the scalar field. Triple gauge boson

vertex is contained in gauge kinetic term of the Lagrangian, whereas quartic

term for scalars is contained in the scalar potential. Will these terms

disallow the linear superposition principles in corresponding QFTs ?

Biswajoy: Actually, I was referring to the quartic term for gauge bosons (specifically, the vertex involving 4 W's) and the cubic and quartic self-interaction vertices that arise from the Higgs potential. But my main point was that we must not confuse fields and states: the field theory can be non-linear, yet the states can still obey the superposition principle, as indeed they do in the Standard Model. So if psi_1 and psi_2 represent states, then c_1 psi_1 + c_2 psi_2 is also a valid state; but if psi_1 and psi_2 represent fields (solutions) in a (quantum) field theory, then c_1 psi_1 + c_2 psi_2 is not necessarily a solution.

@ V. T. Toth

Let me understand your comment on the differences

between fields and states. Creation and annihilation operators

act on states. Fourier transform of these operators are fields.

In this context let me ask you a question because your knowledge

in this area is very deep. Schrodinger equation is a linear equation

even though it has second derivatives. By linearity I mean that only

first power of derivative is included. Now will it remain a linear

equation if I consider a complicated potential ? For example the

Yukawa potential has powers of e. Then what will be the status

of linear superposition principle.

Biswajoy: The Yukawa potential is actually the solution of a linear field equation, namely (\partial^2 - m^2)\phi = f. Given two solutions, \phi_1 with source f_1 and \phi_2 with source f_2, it is clearly true that (\partial^2 - m^2)(c_1\phi_1 + c_2\phi_2) = c_1 f_1 + c_2 f_2.

The Schroedinger equation is linear and homogeneous in the wavefunction because every term contains the first power of the wavefunction. Given two wavefunctions \psi_1 and \psi_2, if they are solutions of the Schroedinger equation, c_1\psi_1 + c_2\psi_2 is also a solution.

Another issue has to do with not confusing QM and QFT. In QM, we solve a linear differential equation (the Schroedinger equation) for the unknown wavefunction. In QFT, it is the operator-valued field that is treated as unknown, the solution of a (not necessarily linear) field equation. The operator-valued field acts on states in a linear fashion but we are not solving for the states; rather, we assume specific (particle) states and compute corresponding amplitudes.

@ V. T. Toth

In QM the potential is a function of energy; for example

for harmonic oscillator it is x^2 in Yukawa case it is e^-x

where x is a space coordinate. Therefore by changing the

form of potential does nothing to the powers of wave function

or its derivatives, Schrodinger equation still remains linear.

Now let is consider a simple scalar field theory. To write

down Lagrangian one has to use a scalar potential of the

\lambda phi^4 form. This is a difference in QFT, that higher powers

of the field variable is entering into the potential energy.

We did not see this in QM.

Returning to the main question whether the interacting

scalar field theory is linear or non linear, we should really

compare Schrodinger equation with the Klein Gordon equation,

which is (box + m^2)\phi=0 for free particles (linear).

But introduction of \lambda \phi^4 term introduces a

\lambda \phi^3 term in the equation of motion

which then becomes non-linear. Thus the practice is to do

perturbation theory for small \lambda.

For the quantisation of Einsteins gravity, what will be the

Lagrangian, and what will be equation of motion? Do you

think perturbation theory can be done in this case as

it is done for the \lambda \phi^4 theory ?

Biswajoy,

Actually, the main question is "Does the detection of primordial gravitational waves really imply that gravity is a quantum particle interaction, and the existence of gravitons?" Can you explain how your discussion pertains to the posted question - it's not clear to me that it does. If it does not, may I suggest that you either post a new question or continue your discussion via personal messaging?

Thanks for your consideration.

James,

Here is the explanation.

The point was whether one can consistently quantize

the theory of gravity and make a nice and well defined concept

of the "graviton". This is because this question is on the

"existence of gravitons". In that context the question

is whether the theory has to be a linear theory or not. In other words

whether a non-linear theory can be quantized. We all

know the main problem of quantum gravity is

that the theory is not renormalisable.

Biswajoy,

Thanks very much for explaining. That fundamental, rather theoretical question skirts the proposition posed by the primary reference http://arxiv.org/abs/1309.5343 and many quantum theorists in their discussions of the BICEP2 results that, as stated in the reference abstract:

"[gravitons may not be detectable]... We argue here, however, that measurement of polarization of the Cosmic Microwave Background due to a long wavelength stochastic background of gravitational waves from Inflation in the Early Universe would firmly establish the quantization of gravity."

So the question is, regardless of any theoretical shortcomings, whether evidence of B mode polarization in the CMB definitively establishes that gravitation is quantized as has been asserted in many news reports - please see http://www.nature.com/news/how-to-see-quantum-gravity-in-big-bang-traces-1.13834 (also see additional references included in the question posting).

IMO, important theoretical issues regarding the quantization of gravity would best be discussed in detail in some other posting, as their inclusion here dilutes and distracts from the discussion of the posted question. Thanks for your consideration.

That is a good idea to have theoretical issues on quantum

gravity in a different posting.

Biswajoy ~

Nonlinearity of the field equations is not at the root of the difficulty in finding a quantum theory of gravity. The equations for the gauge fields of non-Abelian groups are also non-linear. The difficult arises from the fact that relativistic quantum field theory in its present form is formulated in a flat space-time, not from non-linearity.

James ~

I appreciate your point that these theoretical matters are not directly relevant to the question whether recent observational data supports the existence of gravitons,

Eric, I do think that experimental data is the last word in Physics

research. In theory we may get carried away by mathematical

subtleties, but keeping our eyes firmly in experimental numbers

keeps us anchored to reality.

Coming back to the experimental side there is a

potential problem. BICEP2 being an earth based instrument

located at south pole cannot scan the full sky. Whereas PLANCK

being a satellite based experiment can. Thus combined results

of two experiments will be more interesting to analyze.

A recent paper,

M. Mortonson & U. Seljak

"A joint analysis of Planck and BICEP2 B modes including

dust polarization uncertainty"

http://arxiv.org/pdf/1405.5857v1.pdf

does that. And points out whether the results change in a

combined fit. Perhaps a combined fit will reduce the value

of r.

Again, please also see http://www.gravityresearchfoundation.org/pdf/awarded/2014/Porto_2014.pdf - which states:

"At first glance, the (indirect) measurement of primordial tensor modes by the BICEP2 collaboration supports an inflationary paradigm for early universe cosmology together with quantum vacuum fluctuations (aka gravitons) as the origin of the spectrum. In this essay we argue the the observed signal may instead be a signature of semi-classical sources of perturbations during inflation. In this scenario, despite a large tensor-to-scalar ratio r ~ 0:2, it may be possible to write an effective field theory of a rolling scalar field without super Planckian excursions..."

Regardless of whether a B mode CMB polarization signal is confirmed, the question remains whether B mode polarization of CMB photons may or may not have been caused by primordial gravitational waves expected in association with cosmic inflation - they are termed gravitational waves _only_ because they can be described in the context of GR as "tensor fluctuations in the gravitational metric" (spacetime dimensional coordinates) - and whether any such primordial 'gravitational' waves bear any relation to commonly observed gravitational interactions among objects of mass. As I understand, it is those fluctuations in the dimensional coordinates of spacetime that is thought to have imparted the B mode polarity of CMB light waves - there is no defined gravitational force interaction that could have produced the expected polarization of CMB radiation...

In particular, there seems to be no implicit role for gravitons in the large scale gravitational waves described by general relativity - oscillating spacetime curvature produced by, for example, the rapid juxtaposition of collapsing binary neutron stars or even Venus orbiting the Sun - see http://en.wikipedia.org/wiki/Gravitational_wave#Sources_of_gravitational_waves

Any primordial 'gravitational' waves - perturbations of the dimensional spacetime gravitational metric - were not the product of any apparent strictly gravitational interaction. IMO, even if it _can_ be shown that gravitons existed and were influential in the inflationary primordial universe - that is _not_ evidence that they directly or even indirectly mediate gravitational interactions among massive objects.

See http://www.nature.com/news/full-galaxy-dust-map-muddles-search-for-gravitational-waves-1.15975 and http://arxiv.org/abs/1409.5738.  Also see http://news.sciencemag.org/physics/2014/09/evidence-cosmic-inflation-wanes.

I agree with Amdir statement and  Eric  reasoning.  The problems of quantum gravity  and graviton are based on the representation gravity  as an effective field theory. The errors lie in the traditional infinite believe in Quantum theory.  Meanwhile, Quantum theory requires  flat space, or at least simple topology,  which for gravitating solutions may be very non-trivial. It shows that  gravity is geometry and topology of space-time, which can  be considered as an effective field theory only in the simplest cases, and thus, in general it cannot be quantized.   As the theory of space-time, gravity turns out  to be primary than quantum. 

The weak field approximation of Einstein’s gravitational theory, like Maxwell’s electromagnetic theory, describes “waves”. They are “ripples” in curved space − small perturbations on a flat background or even on a curved background space. The equations that govern them are the equations of a “spin-2 field” just as Maxwell’s theory is a theory of a “spin-1” field. Note that this has nothing whatever to do with quantum theory. It is purely classical − “spin-1” and “spin-2” refer only to the classification of classical field equations according to representations of the Lorentz group.

 Planck’s analysis of the black body problem, and the photo-electric effect, revealed that when electromagnetic radiation interacts energy is exchanged only in “discrete packets” (E = hν). This led Einstein to suggest that EM radiation consists of “particles”, which we call “photons”. Angular momentum is also exchanged discretely, so we now interpret that by saying that a “photon” is a “spin 1 particle”. This is the beginning of the path that led to quantum theory as we know it today.

 The question then is: when gravitational waves are detected, are discrete fundamental units of energy and angular momentum exchanged? The answer is way beyond any possibility of experimental investigation. Theoretically, it seems a reasonable analogy. We would then be justified in talking about “gravitons” in the same way that we talk about “photons”.

 But what is an “elementary particle” really, and what is a “field”? Are they not simply convenient mental constructs for thinking about, investigating, and trying to understand our observations of phenomena that happen in the physical world?

No-the detection relies on classical properties of gravity. While gravitational waves are described as coherent superpositions of gravitons, the ``shot noise'' of gravitons isn't detected this way. Just like for any wave, the detector can determine polarization properties. It's in this way that it can be confirmed that these are consistent with massless excitations of spin-2: by the absence of the dipole moment and the properties of the quadrupole moment of the wave amplitude and intensity. 

(1) There is no possibility of finding in the CMB, B-mode polarisations produced by primordial gravitational waves, because there is no CMB. The reasons why the CMB does not exist are simply stated: (a) Kirchhoff's Law of Thermal Emission is false; (b) Due to (a), Planck's equation for thermal spectra is not universal. This has been explained in detail in the following paper:

Robitaille, P.-M., Crothers, S. J., “The Theory of Heat Radiation” Revisited: A Commentary on the Validity of Kirchhoff’s Law of Thermal Emission and Max Planck’s Claim of Universality, Progress in Physics, v. 11, p.120-132, (2015),

http://viXra.org/abs/1502.0007

(2) There is no possibility of detecting Einstein's gravitational waves in any circumstances because they do not exist. The reasons why they do not exist are simply stated: (c)  in General Relativity the total energy and momentum of a closed system is always zero so that GR violates the usual conservation of energy and momentum for a closed system, putting it in direct conflict with a vast array of experiments; (d) due to (c), gravitational energy cannot be localised. This has been explained in the following paper:

Crothers, S.J., A CRITICAL ANALYSIS OF LIGO'S RECENT DETECTION OF GRAVITATIONAL WAVES CAUSED BY MERGING BLACK HOLES, Hadronic Journal, Vol. 39, 2016, http://viXra.org/abs/1603.0127

(3) Black holes, alleged to be generators of Einstein's gravitational waves, do not exist. The reason why they do not exist is simply stated: the mathematical theory of black holes violates the rules of pure mathematics. This has been explained in the following paper:

Crothers, S.J., On Corda's 'Clarification' of Schwarzschild's Solution, Hadronic Journal, Vol. 39, 2016, http://vixra.org/abs/1602.0221

The short answer to the question posed by Dwyer is: NO! Analogy: Would anybody want to argue that radio waves prove that the electromagnetic field must be quantized and that photons exist? -  Incidentally, Freeman Dyson has argued quite convincingly that it may be impossible to detect gravitons directly. In any event, any detection of gravitational waves will test general relativity in a classical regime where quantum effects do not manifest themselves. - Of course, this does NOT mean that the problem of unifying general relativity with quantum theory is one we should not take seriously and think about.

May I add a general question: Why do the discussions in all these ResearchGate forums tend to go down the drain? People should not write about every thought that crosses their minds before they have had a chance to critically assess their ideas. It really just creates confusion! I think that the concept of ResearchGate is flawed!