GR Controversy: Proven Phenomena but Unproven Theory?

[SpiderET]

There is a mounting evidence that GR is incomplete and insufficient theory. Dark energy, Bell theorem (nonlocality), missing direct observation of gravity waves, singularities in many cases, the fact that GR is a non Machian theory and many others are pointing to the need of new and better theory, which could be radically different and still be in line with all existing experiments.

 

[wabbit]

There is a mounting evidence that GR is incomplete and insufficient theory. Dark energy, Bell theorem (nonlocality), missing direct observation of gravity waves, singularities in many cases, the fact that GR is a non Machian theory and many others are pointing to the need of new and better theory, which could be radically different and still be in line with all existing experiments.

Yes I read about that in "Social Text".

 

[WannabeNewton]

Ok so it is that GR requires infinitesimal units of spacetime and quantum gravity requires a fundamental unit of spacetime. So an object can't be described by both, right?

Space-time is emergent in both string theory and loop quantum gravity so there is no contention. GR as a theory of space-time is effective only in the IR ("infrared") regime i.e. at "low enough" energies. In other words the concept of space-time doesn't exist in Planck scale physics.

If it helps you can think of a more down to Earth analogy with, say, fluid mechanics. If I probe a fluid at very small length scales then it becomes apparent that the fluid is made up of discrete bunches of particles so clearly there is no continuum at such length scales but if I'm at large enough length scales (say I just care about modeling fluid moving through a pipe) then the fluid is effectively described by a continuous velocity field

 

[Isaac0427]

There is a mounting evidence that GR is incomplete and insufficient theory. Dark energy, Bell theorem (nonlocality), missing direct observation of gravity waves, singularities in many cases, the fact that GR is a non Machian theory and many others are pointing to the need of new and better theory, which could be radically different and still be in line with all existing experiments.

Where could I find those theories?

 

[atyy]

Where could I find those theories?

I think most of the content of that post is misleading. The singularities, when carried over to quantum general relativity, almost certainly indicate that GR is incomplete, and that has been known for a long time, so it is not "mounting evidence". Neither dark energy nor Bell nonlocality indicate that GR is incomplete, since quantum GR is compatible with both. (Dark energy can be used to argue that GR is incomplete, if we assume that the laws of physics should not be finely-tuned.) At the moment, there are not missing observations of gravitational waves.

So yes, GR is incomplete, mainly because of the singularities, which is an old problem. String theory represents our best current understanding of how this may be resolved, but it is still a matter of research. Also, it may be that quantum GR is potentially complete, but we don't understand the mathematics well enough yet to know if this is possible. Such alternative research programmes are Asymptotic Safety and loop quantum gravity.

There is also the question of dark matter (which is not the same as dark energy). It may indicate a problem with GR, but it could also be resolved by a yet undiscovered particle. Of course GR in quantum theory is also a particle, but what I mean is that dark matter be resolved by adding a new particle to the standard model, leaving GR and the equivalence principle untouched.

 

[SpiderET]

Where could I find those theories?

At the moment there is no such theory published. At least I don't know of any.

Some would say, that String theory / M Theory is likely candidate, but String theory is not falsifiable and it can make millions of different predictions, so it has zero practical use as for prediction for something. The only practical use of String theory is for cashing in research grants and royalties from books.

 

[Isaac0427]

At the moment there is no such theory published. At least I don't know of any.

Some would say, that String theory / M Theory is likely candidate, but String theory is not falsifiable and it can make millions of different predictions, so it has zero practical use as for prediction for something. The only practical use of String theory is for cashing in research grants and royalties from books.

I thought that quantum gravity was a part of M-theory? Isn't part of it based on the graviton?

 

[SpiderET]

At the moment, there are not missing observations of gravitational waves.

Im not sure, if you did understand what I ment with missing direct observations of gravitational waves. What I know, GR predicts gravitational waves and it was INDIRECTLY confirmed for example by measurement of radio signals of pulsars. But there was no direct observation in LIGO or similar experiments in many years despite millions of dollars invested. Which is kind of a dent in GR, because theoretically the direct observation is already long overdue. And it is easily possible, that it will be never measured, because the nature of gravitational waves or gravity itself could be different than predicted by GR.

 

[atyy]

Some would say, that String theory / M Theory is likely candidate, but String theory is not falsifiable and it can make millions of different predictions, so it has zero practical use as for prediction for something. The only practical use of String theory is for cashing in research grants and royalties from books.

String theory is in principle falsifiable. From the point of view of comparing quantum field theory and string theory, string theory is more falsifiable than quantum field theory. Of course, string theory is in practice not falsifiable with our current technology.

The main reason for studying string theory is not because we believe it is true, but because understanding it could help us construct a wider class of quantum theories that give rise to general relativity.

 

[atyy]

Im not sure, if you did understand what I ment with missing direct observations of gravitational waves. What I know, GR predicts gravitational waves and it was INDIRECTLY confirmed for example by measurement of radio signals of pulsars. But there was no direct observation in LIGO or similar experiments in many years despite millions of dollars invested. Which is kind of a dent in GR, because theoretically the direct observation is already long overdue. And it it is easily possible, that it will be never measured, because the nature of gravitational waves or gravity itself could be different than predicted by GR.

Yes, but the experiments do not yet falsify GR.

 

[ShayanJ]

GR is a non Machian theory

That's not an argument against GR, but an argument against Mach principle!

 

[wabbit]

I thought that quantum gravity was a part of M-theory? Isn't part of it based on the graviton?

No, M theory is a potential theory which may describe quantum gravity, but by no means the only one. There is not currently an established theory of quantum gravity.

 

[Orodruin]

But there was no direct observation in LIGO or similar experiments in many years despite millions of dollars invested. Which is kind of a dent in GR, because theoretically the direct observation is already long overdue.

When making statements like this you have to take into account whether these experiments should have seen a signal that is necessarily present within GR. A voltmeter with a resolution of 0.1 V is not going to detect potential differences of 0.001 V.

 

[Isaac0427]

No, M theory is a potential theory which may describe quantum gravity, but by no means the only one. There is not currently an established theory of quantum gravity.

But don't most of the quantum gravity theories predict a graviton? How many different versions of the graviton are there?

 

[SpiderET]

When making statements like this you have to take into account whether these experiments should have seen a signal that is necessarily present within GR. A voltmeter with a resolution of 0.1 V is not going to detect potential differences of 0.001 V.

You mean that tens of millions of dollars for these experiments were wasted because these people should have know, that they had no chance to measure it? :)

 

[wabbit]

But don't most of the quantum gravity theories predict a graviton? How many different versions of the graviton are there?

Yes they do usually predict one or more graviton, because (as I understand it - no expert here) a graviton is defined as an excitation of the quantum gravitational field - but the presence of a graviton does not distinguish very much between theories.
As to how many candidates theories, a few of them are mentionned in https://en.wikipedia.org/wiki/Quantum_gravity#Candidate_theories

 

[Orodruin]

You mean that tens of millions of dollars for these experiments were wasted because these people should have know, that they had no chance to measure it? :)

No, I mean that that money was spent on an experiment that could have detected a signal which might have been larger if there were some more exotic physics going on. In addition, it is a development of a detector technology which, when taken a step further, will allow us to probe a more interesting part of the parameter space.

To continue the analogy to voltmeters: If you do not already know how to build a 0.1 V resolution voltmeter, how can you ever hope to build one with a 0.001 V resolution?

 

[WannabeNewton]

And it is easily possible, that it will be never measured, because the nature of gravitational waves or gravity itself could be different than predicted by GR.

This is just plain wrong. aLIGO just went into operation and the estimated number of detection events are at the least one per year starting in the next couple of years. iLIGO itself just had a small parameter space to probe which is not a fault of GR. See https://www.advancedligo.mit.edu/summary.html

 

[SpiderET]

This is just plain wrong. Advanced LIGO just went into operation and the estimated number of detection events are at the least one per year starting in the next couple of years. LIGO itself just had a small parameter space to probe which is not a fault of GR.

Yes, I have been reading about Advanced LIGO and we will see what they will find in future.

 

[SpiderET]

Yes, but the experiments do not yet falsify GR.

Yes, that is true, and I have never asserted that missing direct observation of gravity waves is falsification of GR. It is just one of things which are still pending to be confirmed.

 

[Orodruin]

In all fairness, the following quote seems to indicate that you consider GR to be in trouble because of LIGO (my emphasis):

Which is kind of a dent in GR, because theoretically the direct observation is already long overdue.

 

[SpiderET]

In all fairness, the following quote seems to indicate that you consider GR to be in trouble because of LIGO (my emphasis):

GR being in trouble of LIGO null result is a bit of overstatement, but the only facts we have is that several experiments have been build for several millions and they have brought us exactly zero results. I can't imagine that people who approved budgets of these experiments many years ago have been hearing: "Hey, we can't detect gravity waves with this proposed experiment, but we would have a lot of fun building it".

 

[wabbit]

I can't imagine that people who approved budgets of these experiments many years ago have been hearing: "Hey, we can't detect gravity waves with this proposed experiment, but we would have a lot of fun building it".

So what exactly are you imagining?

 

[SpiderET]

So what exactly are you imagining?

Isnt it obvious? I am imagining that all these budget approval comissions have received presentations and calculations based on GR, which have suggested that they can detect gravitational waves. But maybe I am just naive, and they routinely spent millions without expecting to receive some results :)

 

[WannabeNewton]

I don't think you understand the difference between theory and experiment. Getting order of magnitude estimates for gravitational wave detection through GR is easy; building instrumentation that has enough sensitivity to detect gravitational wave signals given the extremely low signal to noise ratio is obviously extremely difficult. It's like finding a needle in a haystack.

The only way to proceed would be to build instrumentation that could in principle detect it given some signal to noise ratio and then upgrade the instrument to make it more and more sensitive over time, with proper calibration along the way. Do you think that people can just build instruments which are right off the bat ready to probe through all the noise involved in ground based gravity experiments?

 

[atyy]

Isnt it obvious? I am imagining that all these budget approval comissions have received presentations and calculations based on GR, which have suggested that they can detect gravitational waves. But maybe I am just naive, and they routinely spent millions without expecting to receive some results :)

I think that is an interesting question. I haven't been following, but I remember a few years ago they reported a null result, which it seems was consistent with expectations from other data.

http://www.ligo.org/science/Publication-S6CBCLowMass/
"This "null result" allows LIGO and Virgo scientists to set new limits on the rate of compact binary mergers in the universe. These limits are still about 100 times higher than expected rates from astronomical observations, so the fact that no gravitational waves were detected is consistent with expectations."

http://stuver.blogspot.com/2012/06/null-result-not-finding-what-you-were.html (I think this is written by Amber Stuver http://www.phys.lsu.edu/newwebsite/people/stuver.html.)
"My standard response (as of this date, of course) is, "None, and we didn't expect to either." And I say this with a smile on my face. Cue the confused and disappointed expressions..."

 

[wabbit]

Isnt it obvious? I am imagining that all these budget approval comissions have received presentations and calculations based on GR, which have suggested that they can detect gravitational waves. But maybe I am just naive, and they routinely spent millions without expecting to receive some results :)

I don't know, but you are suggesting the promoters of the project obtained funding under false pretenses. Maybe they did, I have no idea. But I would think some kind of evidence would not be unwarranted before assuming that.

I don't know what they said to those commissions (in the 1990s presumably), but this is a paper from 2001 outlining what was expected then.

LIGO's "Science Reach", Lee Samuel Finn

Technical discussions of the Laser Interferometer Gravitational Wave Observatory (LIGO) sensitivity often focus on its effective sensitivity to gravitational waves in a given band; nevertheless, the goal of the LIGO Project is to ``do science.'' Exploiting this new observational perspective to explore the Universe is a long-term goal, toward which LIGO's initial instrumentation is but a first step. Nevertheless, the first generation LIGO instrumentation is sensitive enough that even non-detection --- in the form of an upper limit --- is also informative. In this brief article I describe in quantitative terms some of the science we can hope to do with first and future generation LIGO instrumentation: it short, the ``science reach'' of the detector we are building and the ones we hope to build.

 

[pervect]

My $.02. There is a lot of smoke about GR and QM being incompatible, but as far as I can tell, they are both compatible in the areas where they both work properly. That is that QM works as an effective field theory at low energies, and GR does too. Neither theory can pretend to work at all energies, QM has issues with various sorts of "diveriences" which are still being debated, but most likely lead to a failure of the theory to apply to arbitrarily high energies. Similarly, GR is ill-behaved in some circumstances, which also involve high energies, such as the singularity in a black hole.

If we stick to the "low-energy" area of physics, both GR and QM live together as a quantum field theory in a curved space-time. There are some theoretical issues, such as the lack of any scheme to quantize gravity waves, but these theoretical issues don't have any experimental consequences in the low-energy regime that our current physics describes.

BTW, "low energy" may be a bit misleading, my understanding is that anything we've been able to do with our most energetic super-coliders is still under the umbrella of "low energy".

 

[SpiderET]

I think that is an interesting question. I haven't been following, but I remember a few years ago they reported a null result, which it seems was consistent with expectations from other data.

http://www.ligo.org/science/Publication-S6CBCLowMass/
"This "null result" allows LIGO and Virgo scientists to set new limits on the rate of compact binary mergers in the universe. These limits are still about 100 times higher than expected rates from astronomical observations, so the fact that no gravitational waves were detected is consistent with expectations."

http://stuver.blogspot.com/2012/06/null-result-not-finding-what-you-were.html (I think this is written by Amber Stuver http://www.phys.lsu.edu/newwebsite/people/stuver.html.)
"My standard response (as of this date, of course) is, "None, and we didn't expect to either." And I say this with a smile on my face. Cue the confused and disappointed expressions..."

Thanks for good links, which give good insight into what they expected. Official LIGO page is rather hard to go trough. I don't want to hijack this thread into gravitational waves discusion, so this is my last response to this topic.

 

[atyy]

If we stick to the "low-energy" area of physics, both GR and QM live together as a quantum field theory in a curved space-time. There are some theoretical issues, such as the lack of any scheme to quantize gravity waves, but these theoretical issues don't have any experimental consequences in the low-energy regime that our current physics describes.

The gravitational waves can be quantized, eg. http://luth2.obspm.fr/IHP06/lectures/silk-uzan/IHP_bib/bmf.pdf (sections 9 and 10).