I have studied string theory, as well as various alternatives both famous and obscure, and I can say it has no serious competition as a candidate for the next stage in physics.
I’ve done the same, and my impression is different. Of course, all theories of quantum gravity are struggling to produce easily testable predictions (although we see more efforts in that direction recently).
But when one ponders quantum gravity, one conceptual question one really likes to be addressed is this: what is the right formalism to quantize space-time, its curvature, and so on, what is the way to talk about superposition of different states of space-time, especially if curvatures differ, and so on.
The “pure string theory” sidesteps those questions, it succeeds at making relevant series converge, but it does not seem to even try to shed any particular light on space-time quantization. So I am not too excited about string theory, as it does not seem to even try to answer the questions which interest me the most in quantum gravity.
Whereas many of the other approaches to quantum gravity (including the currently very prominent loop quantum gravity, and a number of less known alternatives) are trying to address space-time quantization heads-on. Which is why I am more excited about rather impressive progress in various flavors of loop quantum gravity (I have even been trying to do a bit of research in that field at some point, with moderate success).
Smolin suggests both that there appear to be serious deficiencies in string theory and that string theory has an unhealthy near-monopoly on fundamental physics in the United States, and that a diversity of approaches is needed. He argues that more attention should instead be paid to background independent theories of quantum gravity.
Of course, when Smolin has written that book, it has been a radical dissident take, and these days it is close to consensus, and the funding and hiring practices have shifted accordingly.
But we do see emergence of hybrid approaches, where motifs from string theory are combined with motifs from theories addressing space-time quantization, such as loop quantum gravity, and I do think that those hybrid approaches are promising. String theory might still play very interesting roles within those combinations.
I certainly would be interested in hearing your critique of loop quantum gravity. (I would not say that I am happy with the state of loop quantum gravity myself, although I am glad that that approach is at least trying to address questions I care about.)
I’ve helped a friend of mine with some research he has published in that and related areas over the years, and we had published one of those papers jointly in Annales Henri Poincaré in 2017 (with that one I’ve actually done enough to be a co-author). I had composed an informal write-up explaining parts of the motivation we have not risked to include into the paper itself (for reasons which are fairly obvious when one considers the dynamics of paper reviewing when there is an ideological struggle in the field).
Here is a link to my write-up, https://www.cs.brandeis.edu/~bukatin/revisiting-eprl.html, and it contains the links to the paper itself. (The last page of the paper says, “Communicated by Carlo Rovelli”, so, presumably, he has looked at it and decided that it is good enough for Annales Poincaré. I am quite happy about that, as one of my desires had specifically been for Rovelli to be aware of that result. I think there is enough ideological affinity there with some of his thoughts. So, perhaps, he would be able to use those considerations at some point.)
So, yes, among other things I’d like to see more developments with imaginary Barbero-Immirzi parameter (mostly likely just +/-i) and with non-unitary physics (I believe that in that particular case Penrose has the right intuition, both about the need for non-unitarity in quantum gravity, and about the imaginary values of Immirzi being preferable). And, yes, I’d like to see those explorations not only in loop quantum gravity, but also in other approaches, to the extent that these considerations are at all transferable to those approaches.
Anyway, I am looking forward to your critical thoughts on loop quantum gravity (or to any other feedback).
OK, thanks.… Here’s the story of loop quantum gravity in a nutshell, as told by me. There have been two periods in the history of the subject, the canonical period and the spin foam period. During the canonical period, they tried to develop the quantum theory “directly”, but used eccentric quantization methods that fatally broke the connection with classical geometry and with the rest of physics. The spin foam period is more promising because at least there’s a connection to topological field theory, but they keep getting degenerate geometries rather than a robust 4d semiclassical limit.
So it’s not devoid of interest, but it suffers in comparisons with strings, for which there are two major paradigms that work really well (perturbative S-matrix in flat space, AdS/CFT duality in negatively curved space), and demonstrated consistency with “naive” quantum gravity in various ways.
I actually think Ashtekar’s variables (as you know, one of the ingredients that launched loop quantum gravity) are a valid window on gravity, it’s just the eccentric approach to quantization taken in loop quantum gravity’s canonical period that is misguided. I think there’s also a chance that there will be a kind of spin foam representation of M theory (in which higher gauge theory has a role too), via the work of Sati and Schreiber on “Hypothesis H”.
The paper I’ve co-authored is, of course, within the spin foam paradigm (because the EPRL itself is within that paradigm).
I feel we are not close to true understanding of quantum gravity. We are seeing a variety of important glimpses from various angles, and that’s important.
Rovelli’s hints that time might be the gradient of entropy in the 4D space is another important tidbit, together with their more formal https://arxiv.org/abs/gr-qc/9406019.
Etc, etc...
But I don’t have feeling that we are getting close to integrating all those tidbits into a unified view and to figuring out what space-time really is.
It’s not certain that there’s a good reason to try to quantize gravity in the first place. The Standard Model says other forces have carrier particles, but the whole reason that’s the dominant view is because W/Z masses were successfully predicted, and I don’t think it can be definitively said that the forces exist because of the particles rather than particles of those masses being (briefly) stable because of those forces.
Should we then think that we don’t believe that result, or should we think that it is not indicative of the quantum nature of gravity despite the tight link between entropy of black holes and the number of Plank areas covering the event horizon?
The Bekenstein bound? That doesn’t make any testable predictions, it’s just a calculation of some theoretical implications of a theoretical model of black holes. I don’t see why I should count that as evidence of anything in particular.
If you don’t believe that this result is likely to be true in reality, that’s fine, it is one possible position, it’s quite self-consistent.
But if one does believe that this result is likely to be true in reality, then that position would be difficult to reconcile with gravity not being fundamentally quantum.
No, I don’t think people should start by deciding if they think that, eg, black holes have internal structure or not. That’s backwards.
I don’t consider that bound a “result”, just a “part of a hypothesis” or “implication of a speculation”. The word “result” means, to me, something that follows from the data of experiments.
If we want to discuss that, then we need to step back.
How much do we believe that black holes exist at all? Are we certain, are we not quite certain? Everyone is talking as if it is certain, but how much should we believe that?
If we believe that black holes do exist to a sufficiently large degree of certainty, when did it become reasonable to believe that, after what events?
I suppose I’d say that without astronomical observations showing accretion disks and gravitational lensing without emission within an event horizon, the existence of black holes would be theoretically justified by general relativity but we wouldn’t be able to make strong statements about GR holding in such extreme conditions.
Yeah, and if one wants to be really sure, one needs to look at raw data a bit oneself (every time I do that, I am usually taken aback by how noisy those data are, and how it must be difficult to interpret them conclusively, and how I have a very powerful built-in bias to trust the reports on experimental data and on what those data mean, and that I should try to keep updating towards higher uncertainty in order to counter my built-in bias to trust the reports).
There is evidence that, at large scales, CDT approximates the familiar 4-dimensional spacetime but shows spacetime to be 2-dimensional near the Planck scale, and reveals a fractal structure on slices of constant time.
I’ve done the same, and my impression is different. Of course, all theories of quantum gravity are struggling to produce easily testable predictions (although we see more efforts in that direction recently).
But when one ponders quantum gravity, one conceptual question one really likes to be addressed is this: what is the right formalism to quantize space-time, its curvature, and so on, what is the way to talk about superposition of different states of space-time, especially if curvatures differ, and so on.
The “pure string theory” sidesteps those questions, it succeeds at making relevant series converge, but it does not seem to even try to shed any particular light on space-time quantization. So I am not too excited about string theory, as it does not seem to even try to answer the questions which interest me the most in quantum gravity.
Whereas many of the other approaches to quantum gravity (including the currently very prominent loop quantum gravity, and a number of less known alternatives) are trying to address space-time quantization heads-on. Which is why I am more excited about rather impressive progress in various flavors of loop quantum gravity (I have even been trying to do a bit of research in that field at some point, with moderate success).
And I agree that overfunding of string theory to the detriment of other directions in theory has been a bad counterproductive thing. I specifically agree with Lee Smolin’s take, https://en.wikipedia.org/wiki/The_Trouble_with_Physics:
Of course, when Smolin has written that book, it has been a radical dissident take, and these days it is close to consensus, and the funding and hiring practices have shifted accordingly.
But we do see emergence of hybrid approaches, where motifs from string theory are combined with motifs from theories addressing space-time quantization, such as loop quantum gravity, and I do think that those hybrid approaches are promising. String theory might still play very interesting roles within those combinations.
I can tell you my critique of loop quantum gravity, but maybe I should first ask about the successful research you say you’ve done in that area?
I certainly would be interested in hearing your critique of loop quantum gravity. (I would not say that I am happy with the state of loop quantum gravity myself, although I am glad that that approach is at least trying to address questions I care about.)
I’ve helped a friend of mine with some research he has published in that and related areas over the years, and we had published one of those papers jointly in Annales Henri Poincaré in 2017 (with that one I’ve actually done enough to be a co-author). I had composed an informal write-up explaining parts of the motivation we have not risked to include into the paper itself (for reasons which are fairly obvious when one considers the dynamics of paper reviewing when there is an ideological struggle in the field).
Here is a link to my write-up, https://www.cs.brandeis.edu/~bukatin/revisiting-eprl.html, and it contains the links to the paper itself. (The last page of the paper says, “Communicated by Carlo Rovelli”, so, presumably, he has looked at it and decided that it is good enough for Annales Poincaré. I am quite happy about that, as one of my desires had specifically been for Rovelli to be aware of that result. I think there is enough ideological affinity there with some of his thoughts. So, perhaps, he would be able to use those considerations at some point.)
So, yes, among other things I’d like to see more developments with imaginary Barbero-Immirzi parameter (mostly likely just +/-i) and with non-unitary physics (I believe that in that particular case Penrose has the right intuition, both about the need for non-unitarity in quantum gravity, and about the imaginary values of Immirzi being preferable). And, yes, I’d like to see those explorations not only in loop quantum gravity, but also in other approaches, to the extent that these considerations are at all transferable to those approaches.
Anyway, I am looking forward to your critical thoughts on loop quantum gravity (or to any other feedback).
OK, thanks.… Here’s the story of loop quantum gravity in a nutshell, as told by me. There have been two periods in the history of the subject, the canonical period and the spin foam period. During the canonical period, they tried to develop the quantum theory “directly”, but used eccentric quantization methods that fatally broke the connection with classical geometry and with the rest of physics. The spin foam period is more promising because at least there’s a connection to topological field theory, but they keep getting degenerate geometries rather than a robust 4d semiclassical limit.
So it’s not devoid of interest, but it suffers in comparisons with strings, for which there are two major paradigms that work really well (perturbative S-matrix in flat space, AdS/CFT duality in negatively curved space), and demonstrated consistency with “naive” quantum gravity in various ways.
I actually think Ashtekar’s variables (as you know, one of the ingredients that launched loop quantum gravity) are a valid window on gravity, it’s just the eccentric approach to quantization taken in loop quantum gravity’s canonical period that is misguided. I think there’s also a chance that there will be a kind of spin foam representation of M theory (in which higher gauge theory has a role too), via the work of Sati and Schreiber on “Hypothesis H”.
Thanks for the write-up.
The paper I’ve co-authored is, of course, within the spin foam paradigm (because the EPRL itself is within that paradigm).
I feel we are not close to true understanding of quantum gravity. We are seeing a variety of important glimpses from various angles, and that’s important.
For example, if one assumes https://en.wikipedia.org/wiki/Scalar-tensor_theory, the experimental data seem to indicate that Immirzi is actually +/-i, or, at least, is very close to that, and this seems to be an important tidbit https://arxiv.org/abs/2005.14141.
Rovelli’s hints that time might be the gradient of entropy in the 4D space is another important tidbit, together with their more formal https://arxiv.org/abs/gr-qc/9406019.
Etc, etc...
But I don’t have feeling that we are getting close to integrating all those tidbits into a unified view and to figuring out what space-time really is.
It’s not certain that there’s a good reason to try to quantize gravity in the first place. The Standard Model says other forces have carrier particles, but the whole reason that’s the dominant view is because W/Z masses were successfully predicted, and I don’t think it can be definitively said that the forces exist because of the particles rather than particles of those masses being (briefly) stable because of those forces.
That’s an interesting idea :-)
If we ponder that, what should we think about https://en.wikipedia.org/wiki/Bekenstein_bound?
Should we then think that we don’t believe that result, or should we think that it is not indicative of the quantum nature of gravity despite the tight link between entropy of black holes and the number of Plank areas covering the event horizon?
The Bekenstein bound? That doesn’t make any testable predictions, it’s just a calculation of some theoretical implications of a theoretical model of black holes. I don’t see why I should count that as evidence of anything in particular.
You should decide if you believe it or not.
If you don’t believe that this result is likely to be true in reality, that’s fine, it is one possible position, it’s quite self-consistent.
But if one does believe that this result is likely to be true in reality, then that position would be difficult to reconcile with gravity not being fundamentally quantum.
No, I don’t think people should start by deciding if they think that, eg, black holes have internal structure or not. That’s backwards.
I don’t consider that bound a “result”, just a “part of a hypothesis” or “implication of a speculation”. The word “result” means, to me, something that follows from the data of experiments.
If we want to discuss that, then we need to step back.
How much do we believe that black holes exist at all? Are we certain, are we not quite certain? Everyone is talking as if it is certain, but how much should we believe that?
If we believe that black holes do exist to a sufficiently large degree of certainty, when did it become reasonable to believe that, after what events?
I suppose I’d say that without astronomical observations showing accretion disks and gravitational lensing without emission within an event horizon, the existence of black holes would be theoretically justified by general relativity but we wouldn’t be able to make strong statements about GR holding in such extreme conditions.
Yeah, and if one wants to be really sure, one needs to look at raw data a bit oneself (every time I do that, I am usually taken aback by how noisy those data are, and how it must be difficult to interpret them conclusively, and how I have a very powerful built-in bias to trust the reports on experimental data and on what those data mean, and that I should try to keep updating towards higher uncertainty in order to counter my built-in bias to trust the reports).
There are some other proposed approaches as well.
Yes, indeed. This one is actually new to me (thanks!).
There are actually plenty of them: https://en.wikipedia.org/wiki/Quantum_gravity#Other_theories
I am particularly fond of https://en.wikipedia.org/wiki/Causal_dynamical_triangulation for this interesting “emergent dimensionality” feature (the crazy 4D vs 2D aspect):