There are a lot of gravity related discussions on RG in which bosons (gravitons) come up. Yet I know of no way bosons can create curved spacetime, which is a measurable fact in the solar system (Shapiro delay proportional to square of time dilation factor, double light bending, and to a lesser extent extra precession). There are also some other tricks I imagine bosons having trouble with, such as increasing the relative inertia of objects in strong fields (like objects getting stuck at extreme values of time dilation, and of course all objects slowing down in coordinate velocity - not local velocity - due to time dilation), while at the same time the energy of the object is going down, not up, due to the change in gravitational potential. Adding bosons has to make the mass and inertia go up. There is also the problem of universality of the effect (all energy has gravity, there is no "charge" concept).
I probably know more about gravity than QFT, but certainly am not a quantum gravity expert. My understanding is most boson interactions in QFT are momentum based. There may be some "sticky" based interactions such as with the Higgs boson. So I thought I'd start this thread to promote some discussion and understanding, at a general physics level for people who understand the basics of relativity and quantum theory, but below the super-specialty level of strings or quantum loop gravity. The objective is to understand either how a QFT boson theory can produce curved spacetime and the required inertia, time and lightspeed effects, or why it cannot if it cannot, i.e. to amass logic and reasons on both sides. I would ask participants in advance to remain moderate and avoid long repetitive arguments that obscure the thread for later readers. I'd like readers to be able to come in and follow the thought.
Re:"Then it seems that we can just consider general relativity not as theory of curved spacetime, but just as a field theory with additional field gmu,nu(x)." ... You haven't explained how time dilation, object contraction (or spatial expansion), or coordinate lightspeed effects arise from fields, so there is no information in the comment.
Aleksei, there is a problem I believe with the Lagrangian idea you propose. I have most often worked with a Hamiltonian formulation, and they are closely related. I would find it hard to believe one would lead to the results of curved spacetime and another does not.
Below is a Hamiltonian analysis I worked out using only kinetic energy and time dilation. It is not too difficult to imagine a field which interacts in such a way as to produce time dilation according to any specific formulation. It can do this by acting on inertia (or mass) of all entities, for example. I proposed a position-momentum field in 2013, but the mechanism is not important for the present discussion.
Now in this paper using a time dilation factor that matches that of GR, it would appear that motion was completely described. At first I thought it was so, because I was able to calculate the precession of Mercury. I submitted the paper two months ago. I have not heard anything yet. I think the reviewers are struggling because they realize something is wrong, but cannot figure out what.
What is wrong is that curvature does not appear and so the coordinate lightspeed is wrong. Shapiro and double bending would not be upheld by this Hamiltonian as far as I can tell. (I mistakenly stated that double bending would be in the paper, but I now realize I was counting the same effect twice).
Now if this method will work as you suggest, I would be very glad for a detailed critique of my paper explaining what I missed that would explain coordinate lightspeed (which goes as the square of the time dilation factor usually). I would not mind at all to find that the paper is not novel and should be withdrawn. I would prefer not to fool with gravity and return to my original interest which was showing how classical inertia theories could be fit with GR. But time dilation emerges from the classical inertia theories, and gravity emerges from time dilation (without doubt, due to the Hamiltonian). If gravity emerges, it must be correct full gravity, or something is amiss. Thank you in advance for your assistance.
Robert, You asked: What are the problems with explaining gravity using bosons? I will point out one previously unknown problem with both gravity and the electromagnetic force being carried by messenger particles (gravitons and virtual photons). The problem is that a recently derived set of equations shows that gravity and the electrostatic force are closely related in a way that is impossible if gravitons and virtual photons are unrelated. It is not possible to provide the proof of this in this short post, but the equations are available in the link below. This is a preprint of a technical paper that will be published in December.
I will try to verbally describe some of the proof. If gravity is carried by gravitons and the electrostatic force is carried by virtual photons, then there is no reason why gravity should be able to be expressed as the square of the electrostatic force. This relationship in fact exists. This relationship was predicted then proven correct during an investigation of the wave properties of particles and forces. To obtain these equations, the Newtonian gravitational equation and the Coulomb force equation must be written using the natural units of particles. For example, the separation distance between two of the same particles must be expressed as the number of reduced Compton wavelengths rather than conventional distance such as meters. The reduced Compton wavelength is a particle’s natural unit of measurement. Also the forces between particles must be expressed in natural units which is dimensionless Planck units of force. Expressing force as Newtons is an arbitrary unit of force. The natural unit of force is to divide the force between particles by Planck force (c4/G) which is the maximum force that nature can exert. The paper gives 11 equations which show different aspects of the gravitational and electrostatic force relationship, but one simple equation will be given here for illustration. Suppose that two of the same hypothetical particles both had Planck charge (about 11.7 times bigger than charge e). Also suppose that these two particles are separated by their reduced Compton wavelength. Then the forces are: FG = FE2
FG is the gravitational force in dimensionless Planck units (natural units)
FE is the electrostatic force in dimensionless Planck units (natural units)
If both hypothetical particles had the mass of an electron, then:
FE = 10-45 and FG = 10-90. More general equations also retain the square relationship. This relationship is impossible if gravitons and virtual electrons are unrelated or if both are not waves which scale with the reduced Compton wavelength.
http://onlyspacetime.com/QM-Foundation.pdf
Robert,
The following two papers are on your favorite topics.
Article Gravity as a zero-point-fluctuation force
Article Inertia and Gravitation as Vacuum Effects - the case for Pas...
Ah, yes, I'm very familiar with that trio. I have corresponded with Rueda. Puthoff is trying to sell vacuum energy at a think tank in Austin. The whole thing is very, very complicated. It makes GR or even quantum gravity look like a walk in the park. They don't try to go so far as to show a full theory of gravity.
Hello Robert,
First of all we need to find that Boson (Graviton).
If you need to even under General Theory of Relativity you need to understand fabric of space.
We still do not understand empty space very clearly, without understanding empty space I doubt we will ever be able to understand gravity.
Regards,
Bhushan Poojary
I will be honest guys, the only helpful answer was given by Aleksei. He tantalized me in his second answer with the idea that quantum gravity theorists "assume" if they meet the GR Lagrangian, they have produced curved spacetime. I responded that I have not been able to find that a Hamiltonian (very similar to a Lagrangian) completely specifies spacetime, only the time part. There may be a way, but since I am pretty close to the state of the art in this area, having contributed a complete derivation of curved spacetime from equivalence recently (available on my upload page), I'm doubtful it is common knowledge. However, it could be a common assumption? However Aleksei has not responded.
The rest of you are presenting ideas that might be interesting on another day, but they are too far afield. I am asking about the traditional physics that I know, about the conflicts, or not, between a graviton-QM theory and a spacetime theory. Not for fundamental new theories of everything. These do not help me argue the point with other physicists who haven't bought into these new theories. I might even like your theory, but I cannot use it for this purpose, sorry.
Aleksei, thank you for your comments. However, it is easy to forget my original request. I already know the Hamiltonian analysis does not produce full curvature. A Hamiltonian analysis is similar to the Lagrangian you mentioned. I'm asking how it produces full curvature, which you indicated.
Consider the paper only a working paper. It accepts the time dilation factor associated with whatever theory you care to use, for example GR. Of course the frequency of photons and massive particles dilate in the same way. Why would that be a problem? I do make an argument in the beginning of the paper as to why a universally acting field (gravity) must produce time dilation, and a charge based field need not or cannot which seems to have confused you. I'm not sure why. I use mass in the ordinary sense, as both the measure of gravitational interaction and inertia, as well as the energy equivalence.
Aleksei do a very good job in answering. I will only give a short comment connected with the question: Is it possible to obtain Einsteins equation by using spin 2 gravitons coupled to the energy momentum tensor? Amazingly, the answer is not so easy also at the classical field theory level (without any QFT argumentation).
Please look into the paper "From Gravitons to Gravity: Myths and Reality" by T. Padmanabhan, available in arXiv.org (arXiv:gr-qc/0409089)
Here is only one part of the abstract: " First, we prove that it is impossible to obtain the Einstein-Hilbert (EH) action, starting from the standard action for gravitons in linear theory and iterating repeatedly. This result follows from the fact that EH action has a part (viz. the surface term arising from second derivatives of the metric tensor) which is non-analytic in the coupling constant, when expanded in terms of the graviton field."
No, the question is will someone offer an answer to the question. Obviously there are some problems doing gravity with QFT (and therefore gravitons-bosons). The question asks for some discussion, presumably by those favoring such methods and also those offering alternatives, without simply referring to the literature on quantum gravity which is undecipherable except to specialists. Let me call this the sub-specialist literature, since it could still be expected to be quite sophisticated, requiring a physics degree or equivalent. It should be conceptual level. All we will drop is the pages of math used to demonstrate particular points.
Some discussions in the sub-specialist literature have mentioned renormalization problems which may be aggravated by bosons, and other have declared this is not a problem. I would like to understand the two sides of this argument a little better, though this is not the main thrust of my question.
Relativists defend spacetime curvature the way priests defend doctrine. Yet there is no sub-specialist literature explaining how string theory, or any other quantum gravity theory, produces curved spacetime. Presumably any quantum theory would use some variant of QFT and involve interactions with a quantum particle (graviton). If not, please explain.
In particular, Aleksei offered in an early post that quantum gravity researchers assume that if their quantum interactions obey the Lagrangian of a gravitational field, then curved spacetime (or equivalent effects) automatically follow. Now he seems to be in denial, which isn't surprising, because I agree with his later post that the Lagrangian can do no such thing! That is what I mean by saying that the answers so far have not been helpful. Понамию?
Then one of the problems with the boson-gravity theories is they do not produce the properties of spacetime that cause gravity.
Let's get this clear. There is no room in metric gravity theories for ANY other force or action principle other than curved spacetime. It is allowed to have another field that affects the metric, but only the metric can affect the behavior of objects in a gravitational field. If the quantum theorists do not understand this, they are not qualified to work on a theory of gravity, quantum or otherwise, as this is most basic.
Aleksei, there is no room for any other force to produce the effects of gravity. Not talking about QED forces.
I would like to hear from someone else now. Is quantum gravity trying to explain gravity (i.e. curved spacetime) or not? If so, what kinds of quantum interactions are involved, and how do they produce curved spacetime?
Remi, that's a good start, thanks. I agree with the comments you made in the last paragraph, all of them.
Remi, I haven't heard of BPP project. It's a big agency. We've got (or had) a lot of clever people with little known under or un-funded projects, like the VASMIR engine for example (high thrust plasma, Mars in 90 days).
You are not the only one to think everything comes down to mass increase (aka inertia). I published on this in 2011 and 2013 (papers on my profile), but I did not get curved spacetime out of it until recently (see "two step" paper also in profile).
Re: "You'll wake up one morning and say, 'we used to do so and so so well'" - already there.
Questions that combine GR concepts and QM concepts cannot be answered definitively because we have no unified theory.
However, gauge bosons are a fundamental part of quantum field theory. They are particles with spin 1. At a classical level, spin-1 fields are describable by the standard linear Bargman-Wigner spin-1 field equations, together with a source term. For example the source of the Yang-Mills field is the isospin current. The YM field itself carries isospin, so it contributes to its own source. That is, it is self-interacting – its field equations become non-linear. We can, similarly, consider the standard linear BW equations for a spin-2 boson. Then introduce energy-momentum as a source term. The spin-2 field itself has energy momentum so it contributes to its own source; it is self-interacting. Its equations become non-linear.
The resulting non-linear equations are identical to Einstein’s gravitational equations.
This deductive process leading from linear spin-2 to Einstein’s theory is an iterative procedure. It deals entirely with classical equations. What if we could apply quanization at each stage of the iteration? Where would that lead? Admittedly a wild speculation about a very formidable project. (Even the quantum-field theory of ordinary spin-1 gauge fields raises notoriously difficult unsolved problems, because of the nonlinearity - a complete theory exists only in the case of electromagnetism where there is no self-interaction. Everything stays linear. We get QED - Feynman showed how to do it…)
@Eric, re: "resulting equations non-linear equations are identical to Einstein's gravitational equation." ... you lost me ... what equations? YM at http://en.wikipedia.org/wiki/Yang%E2%80%93Mills_theory doesn't look anything like the GR field equation, and though I don't understand every term in it, I'd guess there is no Rieman curvature tensor in there.
Robert ~
I was refering to the work of Gupta (Proc Phys Soc London A65 1952 608). Of course the GR equations don't look anything like YM equations, But the way the nonlinearities arises from self-coupling is identical in both theories; that to me seems amazing. I feel Gupta's approach is profound and deserves to be better known.
So it is an analogous mathematics then, not a literal presence of S-T curvature in Gupta? I'd ask for a link to it, but I probably wouldn't understand it. I am at the first course in QM level and could not follow the YM equations.
Robert ~
"resulting non-linear equations are identical to Einstein's gravitational equations." ... you lost me ... what equations"
>>The equations you get from the linear spin-2 equations when you put energy-momentum as a source term. This is what Gupta did (which, incidentally, involves no quantum theory at all, so no need to be afraid of it. I also am not at all well-versed in quantum field theory!). It has nothing to do with YM. I mentioned YM only to illustrate the analogy: a field that contributes to its own source is self-interacting and self-interaction manifests as nonlinearity.
On the other hand, there is an important structural resemblance between YM and GR: Electromagetism has a potential Ai and field-strengths Fij = ∂iAj - ∂jAi. YM generalizes this; the Ai belong to the Lie algebra of the gauge group. They don't commute and the field strengths are Fij = ∂iAj - ∂jAi + [Ai, Aj]. The resemblance to the structure of GR is already apparent. The curvature tensor has this same structure (the Christoffel symbols taking the place of YM potentials).
[If this structural resemblance between YM and GR is followed through to its logical conclusion, we get the "Poincaré gauge theories", which are precisely YM-type theories of the Poincaré group. They are extensions of GR that incorporate 'spin' as well as energy-momentum. But that's another story... In case you're interested I've attached 41.pdf where I believe I explained it clearly and simply.]
These considerations are quite unrelated to what Gupta did . I mention them here only to clarify the structural similarity of YM and GR, which you asked about.
Robert,
I think your problem is connected with the difference between a particle approach and a pure geometric approach.
Everything above (including Guptas paper and a Dessers work) is connected to the particle picture: you see the metric as a field described by a symmetric tensor. Then (according to QFT), an excitation of this field is a particle, called graviton. Now one can write down the equation of this spin 2 graviton and introduce a self-coupling (witha cubic intereaction term). Add the end one obtains the Einstein equation of GR but as a field equation of the graviton field (or metric field). Of course w.r.t. some approximation.
But as far as I understand your question how this approach is related to geometry. Here is the problem: up to now there is no connection. I see only two ways to bridge this gap:
1. Write geometry in a particle picture (Loop quantum gravity and String theory tried it)
2. Write the particles in a geometric picture (my favourite approach)
Of course exchange the word particle by quantum field.
@Eric, thanks for the PDF, I have retrieved it. I remain interested in understanding the structural similarity, but may not be able to dig deeper until after a work deadline at the end of the month.
@Torsten, I think you have summarized the question nicely, using alternate wording, and perhaps we'll get some more interesting comments as a result. It is a philosophical discussion question, not something for a quick easy answer.
There is a connection between particle and spacetime theories that has been around for a century, and advocated from time to time by various lumenaries, either as explanatory or pedagogical. I'm referring to so-called "constructive relativity." Normally you only hear this term in connection with SR. But to the extent GR can be derived from SR + equivalence + some additional assumptions (something I've got a handle on just recently), it could be applied to GR also. It would favor one of two interpretations of curvature, the contraction of objects rather than the expansion of space. As a result, it is not very friendly to esoterica such as wormholes, but produces all other effects that I'm aware of. M. Asif has recently put forward a new increment of this theory, perhaps without intending to do so, and I can link anyone interested to his paper.
Constructive relativity is still not to the point of actually enabling a QFT description of gravity, however, because QFT cannot explain special relativity, i.e. cannot explain the propagation of light. QFT is built on SR as I understand it, rather than the other way around. Humanity does not yet seem to have collective IQ enough to formulate a theory of propagation of light (constructively at about the QFT level) without getting bogged down in ether theories. Perhaps an AI in the next century will figure it out.
@Morales, there is no locality involved in entanglement. However I have read conflicting things about QFT vs. entanglement. Some writers claim QFT gives up superposition. I don't even know how to react to such a claim it seems so odd. Entanglement is normally treated in connection with ordinary QM. It may only mean something mundane, like the existence of superluminal coupling which cannot be used for communication (or in any way to invalidate SR) and a preferred frame which we therefore cannot detect. (No need to jump on me about this, I didn't say there was a preferred frame, only that "entanglement may be something mundane"). Anyway, whether QFT is on board or not I don't know, but physics generally has been forced by experiment to give up locality. This is "almost" completely proven.
@Morales, You can't really define locality. A micron may as well be a GLY. If mass is a function of global potential as many investigators suspect, then until a particle interacts with the rest of the universe it has a large uncertainty and is spread over most of the universe, so its locality includes everything. I don't anticipate you will agree. I have made this post for the record, and will not be able to engage in any long conversation on the subject due to a deadline project.
Manuel, that's right. We cannot observe entanglement until well after not only the events, but the communication of results of one measurement at lightspeed to the point of the second measurement. Then by comparing measurements we observe the interaction. We don't at that point know which events are causes and which are consequences, so we don't have enough information to apply causal logic. It might be either way. There might be an undetectable preferred frame. etc.etc.etc.. I suggest the following book:
http://www.amazon.com/gp/product/B005X3S9LQ/ref=kinw_myk_ro_title
Any new communication technology (body language, speech, writing, printing, email, the web, wikipedia, twitter, etc.) immediately provokes a corresponding censorship regime. If mind reading were developed, thoughts would be censored. It is essentially what humans, being social animals, do. Only the lead dog, err..., human, is allowed to be "correct." Face it.
I have a different take on "free will" than Bell. It cannot arise from randomness. Since the only physical processes we know of are either random or determined, then there is no human decision based on something other than predetermination or randomness. Superdeterminism would inevitably involve sensitive dependence on initial conditions (chaos) which would be barely, possibly distinguishable from random, so it is a difference which makes no difference. Superdeterminism is therefore an ideological and religious issue, not a physics issue. In fact, shades of differences in meaning between predestination, fatalism, etc. were at the roots of schisms between Presbyterian, Lutheran, Baptist, Catholic, etc. during the protestant reformation. Do you know the difference between predestination and fatalism? It is very interesting and important, even though both are theories of superdetermination.
Manuel, I scanned through your paper. We should be having this discussion under a philosophical topic, but anyway. I found the coin-in-cup experiment to be very interesting. This paper, in my opinion, is the most interesting thing you have written.
It is not necessarily novel in its conclusions. I have an old essay I wrote in 1975 which goes over some of the same ground - not identically - but by a different means it removes initial causes as irrelevant, and does so with a selection mechanism.
For others who may be reading this thread and not wanting to wade through a paper (essay), Manuel proposes that by looking at a coin in a cup, one cannot tell whether the present state is certain or uncertain. That depends on its history. If, for example, the coin fell on the rim and teetered, or if it bounced nearly out of the cup, the present state is uncertain.
One way of looking at this is that it describes entropy and the consequent lack of time reversibility. It is an increase in entropy that damps out the bouncing of a coin, and prevents us from reversing the equations of physics to see if indeed the state of the coin came from uncertain origin. In frictionless problems where there is little entropy change - the motion of planetary bodies in a vacuum - then indeed we can back up time a long way and determine prehistorical eclipses, near collisions, etc. We can even determine some things back past reasonably ordered collisions.
Another way of looking at it is by analogy to Schrodinger's cat. We usually presume we cannot do the experiment perfectly. Little quantum interactions with the environment would collapse the state function - uncertain in that it contained both live and dead cat - much the same way as the little molecular interactions increase entropy and prevent determination of the historical state function of the coin even in the classical realm.
Is this more than an analogy? I do not know. My first time to think about it.
The paper below describes how bosons could produce curved space-time. It also proposes a different mechanism, also quantum, in which orthogonal 1/r state functions based on position-momentum uncertainty could create curved space-time.
Yes I realized you were emphasizing a different point about the coin in the cup, I was just reporting what it suggested to me.
I do not have any special insight into the QM locality issues at the moment. I am prejudiced. I think it is non-local because (a) I was unable to come up with a local hidden variable theory myself, and (b) I found a way to use non-locality via the uncertainty principle to construct a very interesting theory of gravity. So I am not too interested in revisiting what's under the hood of that idea at the moment, but in seeing how far the car will go, so to speak. : )
Kassner wrote me in a private email an interesting reason physicists are predisposed to local theories. To the extent physics might be non-local, the physicist cannot isolate an experiment from uncontrolled variables and physics would not be repeatable.
Of course, random processes are only repeatable statistically, and it is a random process (wave collapse) that we suspect of being non-local, so as long as non-locality is random the physicist remains happy, but possibly does not pay attention to something important.
I'm aware I haven't addressed your clever argument. Probably because I do not fully understand it. See you.
Do Electromagnetic and Gravitational Quanta (EM Quanta and Gravitons) Gravitate from Within?
https://www.researchgate.net/post/Do_Electromagnetic_and_Gravitational_Quanta_EM_Quanta_and_Gravitons_Gravitate_from_Within
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