GR Controversy: Proven Phenomena but Unproven Theory?

[Isaac0427]

Hi!
So I have noticed that phenomena, like gravitational lensing, are proven to exist, and GR is the only logical reasoning for them? If those parts of it have been proven, how could GR be possibly wrong? And if it was correct, wouldn't there have to be a way to make it work on a quantum level, as classical physics needs to be consistent with quantum physics? Please help me understand why this whole controversy is going on when much of GR has been experimentally proven.
Thanks!

P.S. Please don't get mad if this question is idiotic, I am only 13 and I have found PF a great place to expand my knowledge.

 

[russ_watters]

Welcome to PF!

GR is extremely well proven and while there is some potential conflict with QM on very small scales, it isn't really a controversy. Mostly it is just an issue of approaching physics from opposite scales and trying to meet in the middle/where they overlap.

GR is pretty much universally accepted as a highly successful theory.

Note though that in science it isn't possible, even in principle, for a theory to be 100% proven. But GR is well above 99%.

Btw, It is great that at your age you are thinking about such things/asking such questions. It's the opposite of idiotic.

 

[Orodruin]

Let me just add that theories are not going to be wrong in a true or false way. Newtonian gravitation is a very useful theory as long as we are dealing with (relatively) small masses moving at velocities much smaller than the speed of light. That it fails in some scenarios does not mean that it is not still a useful theory.

 

[Isaac0427]

GR is extremely well proven and while there is some potential conflict with QM on very small scales, it isn't really a controversy. Mostly it is just an issue of approaching physics from opposite scales and trying to meet in the middle/where they overlap.

But where do they overlap? I thought that the controversy is because people say QM and GR can't be consistent at all.

 

[newjerseyrunner]

But where do they overlap? I thought that the controversy is because people say QM and GR can't be consistent at all.

GR requires continuous space, meaning no matter how small of a distance you can think of, you can still cut it in half and have an even smaller distance. In QM, space stops existing at something called the Planck length.

Usually gravity doesn't matter in QM interactions because it's really weak compared to everything else at that scale, and QM doesn't matter in gravitational interactions because of the scale at which gravity works. There are places in the universe where the super tiny QM world and the super weak GR world have to meet: black holes. Super super tiny, but super super massive.

 

[Isaac0427]

GR requires continuous space, meaning no matter how small of a distance you can think of, you can still cut it in half and have an even smaller distance. In QM, space stops existing at something called the Planck length.

Usually gravity doesn't matter in QM interactions because it's really weak compared to everything else at that scale, and QM doesn't matter in gravitational interactions because of the scale at which gravity works. There are places in the universe where the super tiny QM world and the super weak GR world have to meet: black holes. Super super tiny, but super super massive.

Then why do people say they can't coexist? It seems like they can.

 

[newjerseyrunner]

Because the mathematics of GR require infinitely small distance and mathematics of QM require lengths no smaller than 1.61619926E-35m

 

[pervect]

Hi!
So I have noticed that phenomena, like gravitational lensing, are proven to exist, and GR is the only logical reasoning for them? If those parts of it have been proven, how could GR be possibly wrong? And if it was correct, wouldn't there have to be a way to make it work on a quantum level, as classical physics needs to be consistent with quantum physics? Please help me understand why this whole controversy is going on when much of GR has been experimentally proven.
Thanks!

P.S. Please don't get mad if this question is idiotic, I am only 13 and I have found PF a great place to expand my knowledge.

Theories aren't really "proven" by science, they're tested. GR has passed all the tests so far, but there's always the possibility that it will fail some future test which will require it to be modified or extended or even replaced outright. So theories can fail a test, by which they are disproven, even though they can't be "proven" by testing. Even if a theory has passed every test that has been made so far, there is no guarantee it will contiue to meet future tests. There will always be tests that haven't been made. The concept of "domain of applicability" is relevant here, but this is a good place to stop, I think.

For some historical perspective, look how Newtonian gravity, which worked quite well, was eventually superseded by a new theory, GR. Newtonian gravity was "almost" right, but under extreme enough conditios or sensitive enough measurements, it has been disproven.

For one an example of a possible theory of gravity that is consistent with observations to date but is different than GR, take a look at http://en.wikipedia.org/wiki/Einstein–Cartan_theory, sometimes call ECKS (Einstein-Cartan-Kibble-Sciama) gravity. It's got some unique physical predictions, which, unfortunately, are very difficult to test.

For a recent example of an attempt to replace GR that failed to match experiment, see Self Creation Cosmology, http://arxiv.org/abs/gr-qc/0302026. It made some different predictions about gravity probe B results, which turned out to be false, so it's fallen by the way side.

 

[wabbit]

Then why do people say they can't coexist? It seems like they can.

Indeed they can, most of the time - it is only for some phenomena involving very short distances, etc. that there is an issue. Actually, as I understand it, it seems one difficulty in the elaboration of a theory of quantum gravity (i.e. "whatever theory is needed to fully resolve the tensions between GR and QM") is precisely that they work together too well: it is very difficult to find clear observational evidence of phenomena in the regimes where both are at play, because such phenomena are rare or extreme. So while for instance electromagnetism or other theories were developped on the basis of a large body of experimental data, and could be tested against new experiments, quantum gravity faces a rarity (not to say a total absence as things stand today) of such tests and data to help pick between alternative proposals.

 

[russ_watters]

Then why do people say they can't coexist? It seems like they can.

Depends on exactly what they mean when they say that. Seems to me that they can and do. You just pick the one most appropriate to the problem you are working on and unless it is a special problem where they overlap (like with black holes), there is no issue of "coexisting". It's like your fork and spoon "coexisting" on your dinner table: just because you used your spoon to eat your soup doesn't mean your fork can't still be useful to eat your steak.

 

[Bandersnatch]

just because you used your spoon to eat your soup doesn't mean your fork can't still be useful to eat your steak.

It's just that those lazy physicists would oh, so love to have a spork instead.

 

[vela]

Then why do people say they can't coexist? It seems like they can.

I wouldn't say the theories can't coexist. They're both useful in their domains. It's similar to the relationship between Newtonian mechanics and special relativity. In a sense, Newtonian mechanics is wrong because it doesn't correctly describe what happens at speeds close to the speed of light. It clearly conflicts with special relativity in these situations, but when you're calculating what happens when a block slides down an inclined plane, you don't pull out the full machinery of special relativity; you use Newton's laws of motion because they give answers that are good enough. Scientific theories and models persists not so much because they're right or wrong but because they make useful predictions.

Nor would I say there's a controversy. It would be better to say that the GR and quantum mechanics are known to be inconsistent with each other. In regimes where both gravity and quantum mechanical effects matter, the two theories conflict, and we don't yet know how to resolve that conflict.

GR requires continuous space, meaning no matter how small of a distance you can think of, you can still cut it in half and have an even smaller distance. In QM, space stops existing at something called the Planck length.

Because the mathematics of GR require infinitely small distance and mathematics of QM require lengths no smaller than 1.61619926E-35m

I don't know if I'd go so far as to say that space stops existing or that QM requires lengths no smaller than the Planck length. The Planck length is just a length scale. It's the scale at which we believe quantum mechanical effects and gravitation both matter.

 

[Ibix]

Then why do people say they can't coexist? It seems like they can.

This is a bit metaphorical, but think of a street map of your town. It probably treats your town as flat. Similarly, a street map of my town treats my town as flat. Both of those are fine, but if either of us try to extend our maps to cover each other's town, we will start to get very funny looking results - more so the further apart we live.

Both maps are fine and can coexist peacefully, as long as we don't try to extend them too far.

This is analogous to the QM versus GR situation. QM is a good "map" of physics on extremely small scales. GR is a good "map" of physics on very large scales. As long as we don't push either theory too far, they coexist peacefully.

The difference between the maps and the physics is that we know what we're doing wrong with the maps - ignoring the curvature of the Earth - but not with the physics. We haven't yet managed to make precise enough measurements to see exactly how it's going wrong, although we can see some things look funny.

Does that help?

 

[Isaac0427]

Nor would I say there's a controversy. It would be better to say that the GR and quantum mechanics are known to be inconsistent with each other. In regimes where both gravity and quantum mechanical effects matter, the two theories conflict, and we don't yet know how to resolve that conflict.

Is this why many sources say they can't coexist?

 

[atyy]

General relativity is consistent with quantum mechanics, in the same way that electrodynamics is consistent with quantum mechanics.

Both are excellent effective quantum field theories.

http://arxiv.org/abs/1209.3511
The effective field theory treatment of quantum gravity
John F. Donoghue

 

[russ_watters]

Crackpot posts and responses deleted. Don't mind the noise, Isaac...it happens.

 

[Finn]

Isaac, I wish I had your knowledge when I was 13. WELL DONE!

Keeping in mind all the above reasons why the two theories do 'work' in their own domains and are 'tested' rather than 'proved', here is a perspective that is somewhat different than those posted. Here are some reasons why some prominent scientists think there is a fundamental and troubling distinction betwee GR and QM...that "... we have not been able to combine these new insights into a novel coherent synthesis, yet..."

Carlo Rovelli has a non mathematical synopsis explaining why he takes this view...

"...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 fact he seemed quite concerned when he wrote this:

"...In spite of their empirical success, GR and QM offer a schizophrenic and confused understanding of the physical world. The conceptual foundations of classical GR are contradicted by QM and the conceptual foundation of conventional QFT are contradicted by GR. Fundamental physics is today in a peculiar phase of deep conceptual confusion.

You can find some specific reasons why he thinks this...for example discrete [quantized] versus continuous space, already mentioned above, and a fixed versus a dynamic space time backgroynd...

His paper is here...and while some of the terminology is over my head, the first few pages will likely give you some helpful insights why some people are concerned about having two theories.

Relational QM

http://arxiv.org/abs/quant-ph/9609002v2

 

[Isaac0427]

Isaac, I wish I had your knowledge when I was 13. WELL DONE!

Keeping in mind all the above reasons why the two theories do 'work' in their own domains and are 'tested' rather than 'proved', here is a perspective that is somewhat different than those posted. Here are some reasons why some prominent scientists think there is a fundamental and troubling distinction betwee GR and QM...that "... we have not been able to combine these new insights into a novel coherent synthesis, yet..."

Carlo Rovelli has a non mathematical synopsis explaining why he takes this view...

"...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 fact he seemed quite concerned when he wrote this:

"...In spite of their empirical success, GR and QM offer a schizophrenic and confused understanding of the physical world. The conceptual foundations of classical GR are contradicted by QM and the conceptual foundation of conventional QFT are contradicted by GR. Fundamental physics is today in a peculiar phase of deep conceptual confusion.

You can find some specific reasons why he thinks this...for example discrete [quantized] versus continuous space, already mentioned above, and a fixed versus a dynamic space time backgroynd...

His paper is here...and while some of the terminology is over my head, the first few pages will likely give you some helpful insights why some people are concerned about having two theories.

Relational QM

http://arxiv.org/abs/quant-ph/9609002v2

Thank you very much

 

[Isaac0427]

So QM requires a quantized space (which, if I am correct, means that the Planck leingth is the fundamental/smallest unit of space) and GR uses infinitesimal units of space, right? What I'm hearing is that they explain what they need to explain correctly, but something that includes both can't be explained in quantized space and not quantized space. Is there a possibility that another set of laws is needed to explain these objects?

 

[Finn]

Isaac..." another set of laws is needed"...?... great question...again!

maybe...seems it is an open question.

Rovelli says [to me] in a 2008 paper : maybe we don't understand what QM is telling us because we don't have a valid starting point.

Keep in mind his paper is a set of ideas rather than a fully formulated complete theory...but look at the 'acknowledgments' at the end of the paper: a list of prestigious people who helped him develop the ideas.

So your question in a modified form apparently interests others...

Here is what I took from his paper:

[Some people summarize the many different interpretations of QM as "Shut up and calculate". I take that to mean while many of the people doing the calculations may interpret their meaning differently, may describe differently what is going on, most all agree on the calculation methodology...because such calculations pass experimental test... as already described by others. ]

Rovelli says:
"...Special relativity is a well understood physical theory, appropriately credited to Einstein’s 1905 celebrated paper. The formal content of special relativity, however, is coded into the Lorentz transformations, written by Lorentz, not by Einstein, and before 1905. So, what was Einstein’s contribution? It was to understand the physical meaning of the Lorentz transformations... the Lorentz transformation were perceived as “unreasonable” and “unacceptable as a fundamental spacetime symmetry”, even “inconsistent”, before 1905; words that recall nowadays comments on quantum mechanics. The physical interpretation proposed by Lorentz himself ... was a physical contraction of moving bodies, caused by a complex and unknown electromagnetic interaction between the atoms of the bodies and the ether. It was quite an unattractive interpretation, remarkably similar to certain interpretations of the wave function collapse presently investigated! Einstein’s 1905 paper suddenly clarified the matter by pointing out the reason for the unease in taking Lorentz transformations “seriously”: the implicit use of a concept (observer-independent time) inappropriate to describe reality when velocities are high. ...

Here I consider the hypothesis that all “paradoxical” situations associated with quantum mechanics –as the famous and unfortunate half-dead Schrodinger cat ... may derive from some analogous incorrect notion that we use in thinking about quantum mechanics. (Not in using quantum mechanics, since we seem to have learned to use it in a remarkably effective way.) The aim of this paper is to hunt for this incorrect notion, with the hope that by exposing it clearly to public contempt, we could free ourselves from the present unease with our best present theory of motion, and fully understand what does the theory assert about the world.

Furthermore, Einstein was so persuasive with his interpretation of the Lorentz equations because he did not append an interpretation to them: rather, he re-derived them, starting from two postulates with terse physical content –equivalence of inertial observers and universality of the speed of light– taken as facts of experience. It was this re-derivation that unraveled the physical content of the Lorentz transformations and provided them a convincing interpretation. I would like to suggest here that in order to clarify the physical meaning of quantum mechanics, a similar result should be searched: Finding a small number of simple statements about nature –which may perhaps seem contradictory, as the two postulates of special relativity do– with clear physical content, from which the formalism of quantum mechanics could be derived. In other words, I have a methodological suggestion for the problem of the interpretation of quantum mechanics: Finding the set of physical facts from which the quantum mechanics’s formalism can be derived. To my knowledge, such a derivation has not been achieved yet. ... I expect that if this program could be completed, we would at long last begin to agree that we have understood quantum mechanics.

Relational Quantum Mechanics
http://arxiv.org/PS_cache/quant-ph/pdf/9609/9609002v2.pdf

 

[Ibix]

Before I start, I'll just note that some of this post is floating around the limits of what I understand - so if one of the mentors tells you I'm talking nonsense, believe them!

So QM requires a quantized space (which, if I am correct, means that the Planck leingth is the fundamental/smallest unit of space) and GR uses infinitesimal units of space, right? What I'm hearing is that they explain what they need to explain correctly, but something that includes both can't be explained in quantized space and not quantized space.

Not exactly. General relativity describes space-time (not just space) as continuous, and describes gravity as curvature of space-time. Every other theory we have sits on top of this, because they describe things like electrons and electromagnetic fields that exist in space-time. The problem is these theories only work properly when there is no curvature to space-time (when there's no gravity, or when we can pretend there's no gravity), and GR doesn't work very well with our current model of particles as points (but works fine when we can ignore the discrete nature of matter).

GR is a continuous theory; all the rest are quantum theories. All the rest work well together, but don't work when gravity is important. It's a sensible guess that we need a quantum theory of gravity to make everything tie up together.

The problem is, we can't figure out how to make a quantum theory of gravity. Every experiment we do matches the predictions of our existing theories, basically because we can't yet do experiments at the really high energies where we think we'll see problems. We have ideas what a quantum theory of gravity might look like (Rovelli's loop quantum gravity is one) but they all have some problems (so I gather - quantum gravity theories are way beyond my current level of understanding), and the lack of experimental evidence means that we're kind of shooting blind at the moment.

We believe (with good reason, I am told) that when we manage to make instruments that can look on the scale of the Planck length we will start to be able to see quantum gravitational effects. Note that this does not mean that we expect to see Planck length sized pixels. Quantised space-time is extremely unlikely to look like a 3d chessboard.

Is there a possibility that another set of laws is needed to explain these objects?

We'd say a new theory these days, rather than new laws. "Laws" kind of assume a law-giver (i.e. god), whereas "theory" doesn't assume either way. But basically, yes, we expect quantum gravity to be the key to one theory that will "look like" our existing quantum theories when we make one kind of simplification, and will "look like" general relativity when we make a different simplification - just as GR "looks like" Newton's theory of gravity if we make a few simplifications.

 

[Isaac0427]

Not exactly. General relativity describes space-time (not just space) as continuous, and describes gravity as curvature of space-time. Every other theory we have sits on top of this, because they describe things like electrons and electromagnetic fields that exist in space-time. The problem is these theories only work properly when there is no curvature to space-time (when there's no gravity, or when we can pretend there's no gravity), and GR doesn't work very well with our current model of particles as points (but works fine when we can ignore the discrete nature of matter).

I understand what GR is, but that is not what I'm talking about. I'm talking about the type of space that it requires.

 

[Ibix]

I understand what GR is, but that is not what I'm talking about. I'm talking about the type of space that it requires.

GR models space-time (not just space - the "time" bit is really important, and speaking of space-time in this context is a very good habit to get in to) as continuous. Our quantum theories work fine in continuous space-time - as long as it's flat continuous space-time. Since GR models gravity as curvature of space-time, that's a problem.

We strongly suspect that quantising gravity, and hence quantising space-time, will allow us to make our quantum theories (possibly tweaked to some extent) work in curved space-time too - but that's not certain yet.

 

[Mentz114]

I understand what GR is, but that is not what I'm talking about. I'm talking about the type of space that it requires.

If two field theories have Lagrangians that have identical symmetries there is a case for saying they are the same thing. The theory of spin-2 bosons described by the Fierz-Pauli Lagrangian has the same symmetries as GR. People are working on this and publications are freely available.

 

[PeterDonis]

Our quantum theories work fine in continuous space-time - as long as it's flat continuous space-time.

That's not actually true. We can do quantum field theories on a curved spacetime background; for example, Hawking's original prediction that a black hole should emit Hawking radiation was based on doing quantum field theory using Schwarzschild spacetime as the background.

However, doing quantum field theory on a curved background in this way still treats the background classically, as a fixed spacetime geometry. What we don't currently know how to do is to treat the spacetime geometry quantum mechanically, so that, for example, if a massive object is in a superposition of being at different positions, the spacetime geometry is also in a superposition of the different geometries that would be induced by the massive object being at those different positions.

 

[Ibix]

However, doing quantum field theory on a curved background in this way still treats the background classically, as a fixed spacetime geometry. What we don't currently know how to do is to treat the spacetime geometry quantum mechanically, so that, for example, if a massive object is in a superposition of being at different positions, the spacetime geometry is also in a superposition of the different geometries that would be induced by the massive object being at those different positions.

OK - so we can handle quantum effects where gravity is important, but not gravity generated by bodies for which quantum effects are important.

 

[PeterDonis]

we can handle quantum effects where gravity is important, but not gravity generated by bodies for which quantum effects are important.

Yes, that's a good short way of putting it. The only thing I would add is that "gravity generated by bodies for which quantum effects are important" includes any regime where spacetime curvature is large enough, even if it is within a body for which quantum effects are not important overall--for example, close enough to the singularity of a black hole, even if the hole itself has a large enough mass to be modeled accurately by classical GR.

 

[Isaac0427]

GR models space-time (not just space - the "time" bit is really important, and speaking of space-time in this context is a very good habit to get in to) as continuous. Our quantum theories work fine in continuous space-time - as long as it's flat continuous space-time. Since GR models gravity as curvature of space-time, that's a problem.

We strongly suspect that quantising gravity, and hence quantising space-time, will allow us to make our quantum theories (possibly tweaked to some extent) work in curved space-time too - but that's not certain yet.

I don't think you are understanding what I am saying... I am not talking about time at all. I am just confirming my knowlage that quantum gravity has a fundamental unit of space, while GR requires infinitesimal units of space, which to my understanding is why they can't both explain the same thing. I know that GR explains spacetime as one thing, but again, that is not what I'm talking about.

 

[atyy]

I don't think you are understanding what I am saying... I am not talking about time at all. I am just confirming my knowlage that quantum gravity has a fundamental unit of space, while GR requires infinitesimal units of space, which to my understanding is why they can't both explain the same thing. I know that GR explains spacetime as one thing, but again, that is not what I'm talking about.

GR and quantum mechanics are fine with each other at all energies we can see. GR can be rewritten as a classical field on a background spacetime, and be treated quantum mechanically.

Whether GR and quantum mechanics are fundamentally incompatible at high energies where we have not yet probed is unknown at the moment. If it is, it is because QM requires some sort of classical spacetime, whereas the theory that replaces quantum GR may have no spacetime. This is partly seen in the conjectured holographic duality of string theory, in which one of the dimensions of ordinary space is not fundamental, and only emerges as an approximation to the activity of more fundamental variables.

 

[PeterDonis]

I am not talking about time at all. I am just confirming my knowlage that quantum gravity has a fundamental unit of space, while GR requires infinitesimal units of space

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.

 

[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/