Quantum Gravity: Is Plank a Fundamental Limit?

[wolram]

http://arxiv.org/pdf/gr-qc/0502110
Has anyone a reason to believe that gravity is" quanta sizable", or
quantum in nature? The above paper has little to say on the subject
its more an over view of QG, but it does state in the first few lines
that there is no reason.
And is Plank a fundamental limit to anything?

 

[Locrian]

I find myself in arguments about this pretty regularly; not about lqg per se, but about the question posed in the title of this thread. For whatever reason, there is actually a very large number of people who believe that quantum mechanics fails to predict observables involving gravity and is obviously incomplete. Their arguments usually seem odd to me - which may not be their fault. I find it interesting that the article is so clear about this issue.

I wonder if any of that crew peruse these forums, and what they might have to say about it?

 

[marcus]

some people may prefer to look at the abstract before deciding whether or not to download the whole article

http://arxiv.org/abs/gr-qc/0502110
Phenomenological Quantum Gravity
Dagny Kimberly, Joao Magueijo

"These notes summarize a set of lectures on phenomenological quantum gravity which one of us delivered and the other attended with great diligence. They cover an assortment of topics on the border between theoretical quantum gravity and observational anomalies. Specifically, we review non-linear relativity in its relation to loop quantum gravity and high energy cosmic rays. ..."

the paper is mostly about what is usually called "DSR" but which in the abstract here they refer to as "non-linear" special relativity.

 

[marcus]

http://arxiv.org/pdf/gr-qc/0502110
Has anyone a reason to believe that gravity is" quanta sizable", or
quantum in nature?...

A theory of gravity (like GR) is a theory of spacetime and its geometry.

But matter interacts with spacetime and changes its geometry. If one is going to have a quantum theory of matter, with uncertain wavelike particles changing the geometry of space then one will need a quantum geometry, and a quantum theory of the spacetime that the particles inhabit. If you are uncertain about where the particles are, then the geometry they interact with is just as uncertain as the particles and just as contingent on what the observer is measuring.

I would not say "believe" because belief is a pretty strong term. As I see it the main question is whether Quantum Mechanics and classical 1915 General Relativity are satisfactory as separate theories. If they are not, then people should work on them. And one of the chief causes of their separate troubles is their incompatibility with each other.


1. General Relativity is unsatisfactory
1915 GR is unsatisfactory because it has singularities (regions where it breaks down).
A. BB singularity. LQG looks to me like the only line of research making progress these days in exploring the very early universe. this makes sense because GR (which provides the basic roadmap to the big bang) takes us right back to a place where it breaks down. the only way to make progress here, and derive observable trace quantum effects to look for in the CMB, is to quantize GR.

B. BH singularity. a "semiclassical" analysis of black holes, like Hawking did, generates paradox. semiclassical means a bastard cobbling together of classical GR with ordinary QM, and they don't match properly. So combining them (as Hawking did in predicting black hole radiation) involves guesswork.

1915 GR may have its flaws but it is also extremely successful and makes numerical predictions which have been verified to high accuracy. So it is successful enough that we can assume spacetime is not flat and rigid but on the contrary dynamic. its geometry can be assumed to interact with the rest of nature.

2. Quantum Theory is unsatisfactory.
QM and in particular the QFT basis of the Standard Model that rules particle physics is, for its part, at least as unsatisfactory because it is founded on a rigid non-interactive spacetime. Therefore Quantum Mechanics is fundamentally unrealistic.

I think it is important not to be fooled by words like "quantum gravity" and "quantizing gravity"
what I am talking about is the emergence of a General Relativistic quantum physics
As you know QED and QFT are only SPECIAL relativistic, so that are only up to 1905 special rel. But 1915 was an improvement. So QED and QFT should be brought up to date. there should be a new release which instead of being a "special relativistic quantum electrodynamics" (which we have now)
would be a "general relativistic quantum electrodynamics"
and a "general relativistic quantum field theory" that means developing a background independent theory of matter

QG research is best understood as preparing the space upon which to build a relativistic theory of matter.

 

[Locrian]

So all we need is a case where current QM fails to predict an observable, and then we can begin forming reasonable hypothesis that will produce models that predict the new observable correctly.

 

[selfAdjoint]

what I am talking about is the emergence of a General Relativistic quantum physics

Marcus, there are several programs about doing quantum mechanics on curved spacetimes, but AFAIK they don't include the mutual interaction with spacetime (what "background free" seems to really mean). I don't know if anyone really likes the mutual interaction of a classical and a quantum physics, although the Copenhagen interpretation appears to prescribe exactly that; perhaps that idea could be firmed up? (Added) Say with the renormalization group approach and Witten's asymptotically safe idea?

Of course the stringy suggestion of a quantum graviton that exactly mimics the behavior of GR gravity on matter, in a flat noninteracting background, would be another solution, if it really worked.

 

[wolram]

By Marcus
and a "general relativistic quantum field theory" that means developing a background independent theory of matter
But why a back ground independence? all these theories speculate a
beginning from nothing, why? can you not accept that
"some thing" has all ways existed. To me it is the only way forward to
accept that nothing is nonsense.

 

[Chronos]

While background independence does not forbid a beginning from nothing, it does not demand it. What it does accomplish is permit a fully self consistent description of physical processes - something extraordinarily difficult, and many would say impossible to do without background independence. Background dependence automatically introduces a preferred reference into any theory. This not only violates the most sacred tenent of GR, it would lead to systematic and inexplicable quantum effects. Since this is not observed, background dependence is one of the most undesirable features to be found in any theory. Such theories invariably end up predicting things that are not observed.

 

[marcus]

While background independence does not forbid a beginning from nothing, it does not demand it. What it does accomplish is permit a fully self consistent description of physical processes - something extraordinarily difficult, and many would say impossible to do without background independence. Background dependence automatically introduces a preferred reference into any theory. This not only violates the most sacred tenent of GR, it would lead to systematic and inexplicable quantum effects. Since this is not observed, background dependence is one of the most undesirable features to be found in any theory. Such theories invariably end up predicting things that are not observed.

Hi, I tend to agree

 

[Chronos]

Oops, caught me.

 

[marcus]

Marcus, there are several programs about doing quantum mechanics on curved spacetimes, but AFAIK they don't include the mutual interaction with spacetime (what "background free" seems to really mean)...

Isnt another term people like to use "general covariant" (which I believe in this context means diffeomorphism invariant, correct me if I'm wrong) as when they talk about longterm theoretical goals like a general covariant quantum physics.

I suppose we all have the right to say for ourselves what we'd like to see develop in the way of theoretical models.

To me personally it seems pretty clear that space is responsive to matter: it get rippled and dragged and swirled. things passing thru it bend it and so all the speeding planets in the solar system are bending light (just slightly) and the lensing is constantly changing as they move in their orbits.

so it is a ripply jello lens with everchanging geometry. (of course experiment ultimately decides, but that's my working assumption until shown otherwise)

Because i personally think space is like that I want the physics of matter to be built on that basis.
I don't expect to be satisfied with any model that is built on a rigid geometry that you chose ahead of time.
curved or flat doesn't matter. if you make a prior choice of whatever geometry then it doesn't seem realistic to me.
"background independent" could be rephrased as "independent of prior choice of geometry"

(but I don't go to extremes and want to get rid of all prior choice, there are lots of initial conditions that I feel all right about a model-builder specifying. I don't imagine a model of the universe coming from "nothing", Wolram, I just don't want it constructed on some arbitrary fixed shape.)

Actually it looks to me as if people are approaching this goal of a background independent theory of spacetime.

 

[wolram]

While background independence does not forbid a beginning from nothing, it does not demand it. What it does accomplish is permit a fully self consistent description of physical processes - something extraordinarily difficult, and many would say impossible to do without background independence. Background dependence automatically introduces a preferred reference into any theory. This not only violates the most sacred tenent of GR, it would lead to systematic and inexplicable quantum effects. Since this is not observed, background dependence is one of the most undesirable features to be found in any theory. Such theories invariably end up predicting things that are not observed.
--------------------------------------------------------------------------------------
Observations seem to change as layers of old science are peeled
away, but maybe someone can remind me what evidence supports
back ground independence, and how or why gravity is given pride
of place in all things, as it may turn out to be a minor by product of
other "powers" that govern the U.

 

[nightcleaner]

I like Marcus' jello lense idea. Images of CMBE have that ripply, almost watery effect. It is not quite background independent, since of course jello does have a definable structure, altho an elastic one. But maybe it serves the same purpose?

Then why not go all the way to water as a model? In fact, why not use a phase-shift model that includes liquid, gel, solid, even gas and plasma? The problem with this approach seems to be that we are inquiring into the fundamental question of what matter is, and using material models is not likely to get us out of the self-definition loop.

Strings seem to me to have been an attempt to get "under" the material model of matter problem. Strings are not matter, not energy, but then what are they? Something is said to be vibrating, yet it is not a thing as we know of things, and it does not vibrate (have energy) in the sense that we know of things having energy. We are left with the Tao, which says that the things which can be spoken of are not the real things. And that, of course, leads our attention directly to the unspeakable Name which is h-ly. Not a satisfactory answer for materialists.

The higher dimensional approach, following Edwin A. Abbot's 1884 Flatland, is to try to imagine what our world might look like to a higher dimensional being. Abbot does this by first trying to imagine what a two dimensional world would look like to a two dimensional being, and then imposeing a three dimensional being, like ourselves, upon it. Miraculus events occur. The two dimensional being sees objects appear from nowhere, and then dissappear into nothing. The thee dimensional being can lift the two dimensional being right off the surface it inhabits, showing it marvelous visions of a world beyond its ordinary experience.

Parallels to spiritualism abound. Some other plane of existence where ghosts of the dead may gather. Many worlds, some like heaven, some like hell. Beings that can come and go in time as easily as we enter and leave a room. All sorts of imaginary things which cannot ever be shown to have any substance. No wonder the materialists shudder and turn away.

Yet even the materialists have to admit that the question prickles at them. The best material theories have flaws. GR predicts singularities with infinite energies, which we do not observe. QM has ghostly states and teleportation and virtual particles, and has no explanation for the families of particles. The two theories, each resplendent in their own realm, break down and give contradictory answers when forced into proximity.

Pet theories abound, and everyone who asks the question gets a different answer. Everyone is talking, it seems no one is listening. It is the third period of chaos, and high amplitude noise drowns out any meaningful signals. Yet we have learned from chaos theory that this is just the condition that results in the next steady state, the next solution which, finally, will go forward alone. Pity that chaos theory also tells us that there is no way to predict which of the many answers will be The One.

But surely we can eliminate those ideas that end in appeal to infinities, like the Prime Mover. And surely we can eliminate those that rely on self definition, I Am that I Am. Those replies just close the door on the question. Then there are the many self-contradictory responses, which, like virtual particles, self anihiliate in practically the same instant in which they are born, only to replaced by another nearly identical set which does the same. These cannot lead us anywhere except into the quantum jitters.

Be patient. Wait for it. The punch line is coming soon. Meanwhile, since I have lately been seen happy as Lather playing with his toes, I think I will start a new thread on my personal adaptation. I will call it "My Big Toe, a theory of everything," and will attempt to present what I see as the underlying answer to the ultimate question. I assure those readers who have come this far that it is not some version of "G-d did it," not "That's just the way it is." And I hope some sharp critics here will probe my efforts for the self-contradictions I may easily miss.

Thanks for Being,

Richard.

 

[wolram]

The thing i find most confusing is that the U is expanding, and yet
AFAIK the fine structure is constant, where is the energy coming
from? QG seems to be able to make its own space, but surly not
all the stuff that then moves into it.
I also enjoy the analogies MARCUS gives, but i think that the detail of a theory is purely academic if the theory can not stand alone,
many brilliant people have worked on these theories for decades and it is only in the near future that we may get some testable results, but if the
unimaginable happens and these results are negative, what then?
some modifications? the theory is destroyed beyond redemption?
complete rethink?

 

[selfAdjoint]

The thing i find most confusing is that the U is expanding, and yet
AFAIK the fine structure is constant, where is the energy coming
from?

From the dark energy, but there is controversy over where THAT comes from.

QG seems to be able to make its own space, but surly not
all the stuff that then moves into it.

Yes, the QG people of all stripes confine themselves to quantizing space (not even spacetime) and put off "matter" till later. That's one reason I like Thiemann; he isn't satisfied and tackles the hard and so far frustrating task of doing QG with matter.

I also enjoy the analogies MARCUS gives, but i think that the detail of a theory is purely academic if the theory can not stand alone,
many brilliant people have worked on these theories for decades and it is only in the near future that we may get some testable results, but if the
unimaginable happens and these results are negative, what then?
some modifications? the theory is destroyed beyond redemption?
complete rethink?

Then they pick up the pieces and do something else. That is the way of science.

 

[marcus]

Chronos said:
While background independence does not forbid a beginning from nothing, it does not demand it. What it does accomplish is permit a fully self consistent description of physical processes - something extraordinarily difficult, and many would say impossible to do without background independence. Background dependence automatically introduces a preferred reference into any theory. This not only violates the most sacred tenent of GR, it would lead to systematic and inexplicable quantum effects. Since this is not observed, background dependence is one of the most undesirable features to be found in any theory. Such theories invariably end up predicting things that are not observed.
-------------------------------------------------------------------------------------
Wolram said:
Observations seem to change as layers of old science are peeled
away, but maybe someone can remind me what evidence supports
back ground independence, and how or why gravity is given pride
of place in all things...
-------------------------------------------------------------------------------------
reply:
As far as evidence confirming or refuting goes, B.I. is for all practical purposes coextensive with Gen Rel.
People have been testing Gen Rel since 1919 and it predicts numbers that other models do not and these numbers are confirmed out to many decimal places.
People have also been trying to invent alternative theories too, all this time, that would stand up as good or better to observation. But they have not been able to.
One can HOPE that experimental/observational evidence will come that refutes G.R. and one can hope that another theory will be constructed that predicts results as accurate and is also an improvement in some aspect. And i hope this. But it has not happened yet. (And I have a strong hunch that when a better theory comes along it TOO will be B.I.!)

Gen Rel was the first B.I. theory and so far it is the only major physical theory that is B.I.

the other theories are built on rigid flat spacetime, or if it is not flat then it is some simplistic rigid pure geometry analogous to schoolboy shapes like hyperbola or sphere. No bumps! No change!
Gen Rel says that flat spacetime, or simple pure shapes chosen ahead of time are not realistic.
Gen Rel says that real space, in nature, changes and has bumps.

Wolram, if you could find a theory built on flat (or equally simple rigid) spacetime that calculates the same numbers as Gen Rel, this would kill the requirement for B.I. I do not see how you would have space expand or how you would power quasars, or model black holes, or explain the time corrections in the GPS system, or observations of binary pulsars, or framedragging, or the exact angle that light gets bent, or the orbit of Mercury. These things are not explained by Special Rel (which is a fixed background theory) or the other things. So far they are explained using a free-form spacetime. I guess "dynamic free-form spacetime geometry" is a possible synonym for B.I.
the "evidence for it" is its explanatory power and accuracy. It would be convenient to get rid of it but nobody has seen a way to do this in the 90 years since 1915.

Gen Rel does not say the world begins from nothing. When you build the theory you start with space all right---but it doesn't have any geometry yet. You just do not commit to a fixed shape for it. Getting rid of initial conditions and equipment is not the point so much as getting rid of one particular type of bias at the start, the bias about shape. There is a technique for presenting space without yet specifying geometry (without yet saying how to measure distances and angles) this trick is called Differentiable Manifolds and was invented by Riemann around 1850 or perhaps a little earlier. It gives a way of talking about space without a prior commitment to some definite distances and angles---without yet having a "metric" on the space.

the gist of B.I. is you begin studying space without specifying a metric (that is the only thing you leave out) and let the metric arise of itself from the matter and stuff. the evidence for B.I. is the same as the evidence for G.R., namely that fixed metrics are not realistic, that so far nobody has seen how to match G.R. performance with a metric fixed ahead of time.

 

[wolram]

selfAdjoint , Marcus, I do enjoy these chats and appreciate your time.
If i attempt to make an analogy of a quantum of space with your
input, i come up with, an ethereal nano bot that has all the information
to replicate itself using dark energy to feed its energy requirements,
so to create space all i need is DE and a few bits of info, so may be
DE is not a back ground, but a primordial sump.

 

[marcus]

selfAdjoint , Marcus, I do enjoy these chats and appreciate your time.
If i attempt to make an analogy of a quantum of space with your
input, i come up with, an ethereal nano bot that has all the information
to replicate itself using dark energy to feed its energy requirements,
so to create space all i need is DE and a few bits of info, so may be
DE is not a back ground, but a primordial sump.

we see that wolram, despite his hardnose skepticism and insistence on the down-to-earth, is at heart a poet like the rest of us

 

[marcus]

just kidding

 

[setAI]

I have an internet friend who is very skeptical of most current physics and does not believe at all in quantized matter or space- he recently sent me this article http://www.khouse.org/articles/2003/467/ as further evidence of his argument that the whole idea of quantizing gravity- and even QM in general- is wrong from the start- I haven't had a chance to read it yet-[ I notice it is on some silly jesus site ] but you might find it interesting- I would of course be interested in any glaring flaws/lies in the article so that I can rub it in his face

to me quantization has been too sucsessful in too many areas to doubt the basic premise of quantized granular structure at the bottom

what is the strongest empirical and theoretical evidence for Planck-units?

 

[marcus]

what is the strongest empirical and theoretical evidence for Planck-units?

well of course the speed of light is one of the Planck units and there is some empirical support for measuring speed as fractions of the speed of light.

but one wouldn't have to, one could measure speed as fractions of 2c. Only the SR equations would be slightly messier.

One chooses units for aesthetic reasons and for convenience.
Instead of "empirical and theoretical evidence" what one is mainly looking for is that the equations look nice.

I believe that Einstein may have been in part guided or motivated by a fondness for natural units. His 1905 special rel discoveries look to me like finding roles for a natural unit speed c, like E = mc^2 and velocity addition etc.

then also in 1905 he took Planck's hbar (another of the Planck units namely the Planck unit of angular momentum) which Planck had only used to describe blackbody radiation and he apparently said "suppose this is a deep ratio in nature, what about finding it in light" and he came up with the 1905 explanation of the photoelectric effect that postulated photons (units of energy proportional to frequency). And then in 1915 he found that Newtons G will give you a unit of force c^4/(8piG) which relates the curvature of a region to the energy density in that region.

AFAIK there is no empirical evidence that Planck units have any physical existence beyond making the equations clean and beautiful, unless you

A. believe the speed of light has physical existence

B. believe hbar has physical existence

C. believe the force constant connecting the LHS to the RHS in the einstein equation has physical existence (because it everywhere connects the density of matter to the curvature of spacetime)

In which case of course all the Planck units have a kind of physical existence because they are the system which arises automatically when you make

A. c your unit of speed
B. hbar your unit of ang. momentum
C. the coefficient of the Einst. eqn. your unit of force

all physical quantities arise as some combination of speed ang. mom. and force--------well you can throw in the electron charge and extra goodies like that as needed -----so the Planck quantities are present in nature to the extent that c, hbar, and that const. F are present.

however one can also say that there is no empirical evidence that they are present and one uses them only to make the equations beautiful

 

[wolram]

The only way to kill a theory is to kill its roots, AFAIK no such slaughter
"much to my disgust",is showing in any "new wave" theories, i can
only guess that modern thinking is so wrong, that proof is an impossibility