GR Controversy: Proven Phenomena but Unproven Theory?

[Isaac0427]

Quantum gravity does not have a fundamental unit of "space"; it has a fundamental unit of spacetime. That's the point Ibix is making, which you don't seem to be grasping.

But a Planck leingth is in meters, which only measures space... Am I missing something?

 

[PeterDonis]

a Planck leingth is in meters, which only measures space

Not in relativity. In relativity, we measure space and time in the same units. We can measure them both in meters, using the speed of light as the conversion factor; for example, a meter of time is the time it takes light to travel 1 meter. The Planck length in meters measures length and time--the time it takes light to travel the length.

 

[Orodruin]

the time it takes light to travel the length.

... also known as (surprise!) the Planck time.

 

[Isaac0427]

Not in relativity. In relativity, we measure space and time in the same units. We can measure them both in meters, using the speed of light as the conversion factor; for example, a meter of time is the time it takes light to travel 1 meter. The Planck length in meters measures length and time--the time it takes light to travel the length.

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? That's why I'm wondering if a third theory is required?

 

[Finn]

Isaac,
you said:
"...just confirming my knowlage that quantum gravity has a fundamental unit of space,while GR requires infinitesimal units of space..."

yes, they do. But not likely for the reason you may think.

The 'discreteness' and 'continuous' are fundamentally different input assumptions, not some mathematical outcome that arises after calculations based on a wholly consistent theoretical set of inputs. Another theory might well be required,

Rovelli says it this way, which I already posted.

""...The present knowledge of the elementary dynamical laws of physics is given by the application of QM to fields, namely quantum field theory (QFT), by the particle–physics Standard Model (SM), and by GR. This set of fundamental theories has obtained an empirical success nearly unique in the history of science: so far there isn’t any clear evidence of observed phenomena that clearly escape or contradict this set of theories... But, the theories in this set are based on badly self contradictory assumptions....

In other words, if we have contradictory inputs no surprise we get some contradictory outputs.

You might also be interested in some previous discussions in these forums on whether spacetime, or space if you prefer, is discrete or continuous. There are many interesting, and unresolved, views of this as expressed by a wide variety of posts. Some there argue that we may need a new theory to reconcile what MAY be an apparent, superficial, difference.

Argument for the discreteness of spacetime
https://www.physicsforums.com/threads/argument-for-discreteness-of-spacetime.391989/

 

[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?