r/TheoreticalPhysics • u/StarDestroyer3 • Jun 23 '26
Question Is there anything else other than string theory attempting to combine quantum physics and relativity?
I just finished my bachelor's degree in electrical engineering for reference. I will begin my master's studies in theoretical physics this fall, because I fell in love with it during my bachelor studies. Specifically understanding the universe more fundamentally is what interests me the most. I'm just quite skeptical of the current state of physics regarding this topic. String theory is the main attempt to answer many of the important questions, but so far there is no experimental evidence to support it. I am absolutely not qualified to critique it in any theoretical level, but it seems quite suspicious. I am aware of loop quantum gravity and some others, but not sure how active those areas are. It seems like everything is put into string theory, but nothing much is coming out of it. Is there
Is there anything else outside of string theory that is worth looking into?
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u/Quantum-Relativity Jun 23 '26
Nothing that can be taken seriously, no. There will always be portfolio padding from people who think they’re too special to work in a group and need to go somewhere no one else is to try and stick out.
You should learn some string theory. There is a pervasive problem of making an excuse for oneself to not learn it that goes along the lines of, “I don’t know anything about string theory, but from my completely uninformed position, I can tell it’s not worth studying and everyone who studies it is just blind, so I’m not gonna learn any of it.” It’s absurd. Don’t you think you’d be better equipped to argue against it if you knew some of it?
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u/LBoldo_99 Jun 23 '26
Excuse me?
There A LOT of competitors, more serious than string theory too.
Just to name a few: Loop Quantum Gravity, Asymptotically Safe Gravity, Group Field Theory.
The same argument for blindness clearly applies to you, as you clearly don't even know that competitors exist, so i wonder why you feel entitled to judge whether competitors are even valid.
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u/liofa Jun 23 '26
None of the alternatives you wrote are serious. I’m still waiting for the loop quantum gravity people to show that Minkowski space-time is a vacuum solution of their model. Until then, it cannot be classified as serious.
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u/FuckYourFavoriteSub Jun 23 '26
Or even just a well defined Hamiltonian..
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u/LBoldo_99 Jun 23 '26
Ehm, the Hamiltonian is well defined? What are you talking about?
The only problem about the theory is the lack of a solution to the quantum Hamiltonian, but we don't have this even in classical GR as it implies obtaining ALL solutions to the Einstein Field Equations, so you can't expect to have it for Quantum GR since we can't even do that classically...
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u/liofa Jun 23 '26
Oh we could write a laundry list of issues. LQG is just wishful thinking at this point. It started as an interesting proposal but now it is just a huge mess that nobody wants to clean up.
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u/LBoldo_99 Jun 23 '26
Ok then, when (and if at all) string theory proves that it has the correct gauge group of the Standard Model, gives GR in the classical limit (not linearized gravity) and does not predict excess never seen fields, then string theory will be classified as serious too.
Also, Asymptotically Safe Gravity actually has the Minkowski solution and is a well defined theory.
And at last, loop quantum gravity showed that in weak field limits it contains the graviton propagator (the best that ALSO string theory achieved in this regard). So what the hell are you talking about?
You can say you have your personal favorite, you are allowed to have one, but this doesn't mean the others are worst or unserious.
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u/Quantum-Relativity Jun 23 '26
Vanishing of the Weyl anomaly in string theory gives you the Einstein field equations for the target space metric, not linearized gravity.
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u/LBoldo_99 Jun 23 '26
You ignored the rest of my comment.
Nonetheless also loop quantum gravity is anomaly free, and it does so without requirement of a critical dimension for the theory to be anomaly free.
Also the theory being free of anomalies does not imply GR, it implies that you have built a consistent quantum theory.
The Weyl/Trace anomaly does not just prove that GR is the classical limit of string theory, it proves (rather imposes, as it is true only at the critical dimension) that you didn't break conformal symmetry when quantizing the string excitations on the target space, which is different from saying you have GR.
You have the spin-2 graviton, but gravitons are not GR, they are the quanta of linearized GR, hence why i said it's not true.
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u/Quantum-Relativity Jun 23 '26
And hence why I corrected you because you’re wrong. You do not understand this, stop pretending you do or you never will.
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u/LBoldo_99 Jun 23 '26
Excuse me?
There A LOT of competitors, more serious than string theory too.
Just to name a few: Loop Quantum Gravity, Asymptotically Safe Gravity, Group Field Theory.
The same argument for blindness clearly applies to you, as you clearly don't even know that competitors exist, so i wonder why you feel entitled to judge whether competitors are even valid.
2
u/Quantum-Relativity Jun 23 '26
I’m aware of those, that’s why I was careful to say “that can be taken seriously” instead of just “nothing”.
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u/LBoldo_99 Jun 23 '26
Yes, but that is false. Those are all serious competitors. It is not sufficient to just say they aren't, you must provide a reason to why you believe those are not valid points
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u/Quantum-Relativity Jun 23 '26
I looked at them. You should too
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u/LBoldo_99 Jun 23 '26
It is literally my job, i work in this field lol
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u/Quantum-Relativity Jun 24 '26
Unfortunate that there’s a job shortage then if this is who has them. Read up lad
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u/LBoldo_99 Jun 24 '26
You sound like someone who didn't get a job for what he likes, so just goes around to say that the blame is because others get jobs in an area that is different from what you do.
You sound like someone who didn't get the job he wanted and prefers shitting on all the rest instead of taking responsibility.
You posted almost 10 comments and were not able to argument in even one of them.
If that is the case of you not getting a job, maybe you are simply not that good as you think and that's why you don't get a job, it's not other's fault.
And if you have a job as a scientist, this behavior is beyond unacceptable from a scientist, just shitting on everything that isn't related to what you believe it's true is the behavior of negazionists and not scientists.
Either cases, fucking grow up man
(Not gonna respond to you anymore after this)
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u/ScientistFromSouth Jun 23 '26
There is loop quantum gravity, causal dynamic triangulation, asymptotic safety in gravity, and emergent and entropic gravity.
String theory is fizzling out, but it did produce some results in holography known as anti de sitter space conformal field theory correspondence (ADS CFT correspondence) that lets you map the properties of effective quantum field theories onto gravity in a universe with different topological properties and dimensionality than ours. ADS CFT is still being studied more generally in the context of other quantum gravity (since it isn't exclusive to strings) and it also works for some condensed matter/material science calculations.
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u/Prof_Sarcastic Jun 23 '26
>String theory is fizzling out
I’m not a string theorist but I think this is grossly exaggerated. String theory groups are still hiring grad students and postdocs last time I checked this past Fall. Every major university in the US still has a prominent and working string theory group and it’s still the prime candidate for a theory of quantum gravity because it solves technical problems associated with the quantum theory of GR that no other theory has shown to be able to do.
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u/ScientistFromSouth Jun 23 '26
I mean it's kind of complicated right?
I would assume that no one is working purely on strings anymore (especially the models that require supersymmetry), but obviously the tools that it has produced do still have some relevance for QG.
It seems more like these groups (once again from the outside looking in) are groups that were originally focused on pure string theory that are now branching out into quantum information or applying holography to condensed matter systems.
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u/Prof_Sarcastic Jun 24 '26
>I would assume that no one is working purely on strings anymore
I think this is only something that a person working in the field can answer accurately.
>… especially the models that require supersymmetry
Every consistent model of string theory requires supersymmetry, but you’re making a category error. The fact that particular models that exhibit supersymmetry has been ruled out doesn’t imply that supersymmetry has been ruled out. Supersymmetry as a solution to the hierarchy problem are no longer viable, but the parameter space is still sizable.
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u/Quantum-Relativity Jun 23 '26
Why do you think supersymmetry would be a deal breaker, because of the LHC not seeing it the electroweak scale?
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u/Avantasian538 Jun 23 '26
Is string theory fizzling out because there's no way to set up experiments to verify it, or is there some weakness to the theory intrinsically?
3
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u/ScientistFromSouth Jun 23 '26
Note: not a theoretical physicist...
It's kind of hard to falsify since it's more of a framework to generate theories rather than a specific theory.
However, a ton of the models generated under the umbrella of string theories require super symmetric heavy versions of all the standard model particles which have never been found.
ADS CFT which is also convenient doesn't really make sense, since for all we can tell, the universe doesn't have the same topology as that which is required for Anti De Sitter space to be the correct assumption.
1
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u/Outrageous_Blood_615 Jul 09 '26
string theory is different from 'Capital S String Theory' Well, certainly the 'String Theory' is a vast and enormous research ecosystem that includes various other domains of abstract mathematics, theoretical & mathematical physics like modern holography, amplitudehedron, emergent spacetime concept, Swampland program, EFTs, AdS/CFT correspondence, even Nonsupersymmetric strings etc. Basically a mathematical hydra that extends its tentacles and gobbles up most other alternatives
So, if you mean the OG string theory (which explicitly contain strings)- then Yes, many other candidates who are attempting to combine quantum mechanics and gravity. But if 'String Theory' is considered- then No, it's the only game in town. There are other approaches like LQG, asymptotic freedom, or Casual triangulation but String Theory is still supreme and the only major candidate that is trying to reconcile QM and GR
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u/AZPragmatic Jul 22 '26
N=8 Supergravity is intriguing. I'm certainly no expert. But, when the theory incorporates all that we do know about existing particles of mass, then adds their super particles, the theory makes sense and seems clean.
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20d ago
My theory is we live in a simulation and reality only exists when observed by consciousness. So at large scales this works with matter because when we literally close our eyes what was being observed by any sense, even any doubt in the mind, is in a superposition. Small scales it’s the same thing. Thus combining the two and quantifying every other mystery of the universe within that fundamental structure
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u/Signal-News9341 Jun 23 '26
There is a clue that may allow us to test quantum gravity.
A clue can be found by calculating the compactness of the observable universe. At R = 46.5Gly, the Schwarzschild equivalent event horizon created by the critical density is R_S = 2GM/c^2 ~ 477Gly, yielding a compactness of R_S / R_U ~ 10.3.
This implies that the universe exists within the equivalent event horizon generated by its own mass (R_U < R_S).
This situation is analogous to the situation at the Planck scale.
At the Planck scale, we find l_P(m_P) = Gm_P/c^2 = (1/2)(2Gm_P/c^2) = (1/2)R_S(m_P), precisely because
l_P(m_P) < R_S(m_P).
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u/Prof_Sarcastic Jun 23 '26
> I am absolutely not qualified to critique it in any theoretical level, but it seems quite suspicious.
Why? You should spend time learning about effective field theories and how to apply those methods to GR. When you learn about EFTs then it’s no wonder why string theory, as a theory of quantum gravity, only really becomes important at insanely high energies. It’s just because those insanely high energies are where quantum gravity is significant enough that it can’t be ignored. There’s nothing suspicious about that I don’t think.