String Theory's Leading Contender Disproven?

Is loop quantum gravity wrong?

The leading contender to string theory, loop quantum gravity (LQG), may have just suffered a critical blow. This is very unfortunate because string theory has a near-monopoly in the field of theoretical high-energy physics, and as with anything else, a monopoly is never good. The evidence against the linear order model of LQG comes from a recent paper from the Large High Altitude Air Shower Observatory (LHAASO) experiment.

The authors claim: "We use this unique observation to place stringent
constraints on an energy dependence of the speed of light in vacuum, a manifestation of Lorentz invariance violation (LIV) predicted by some quantum gravity (QG) theories. Our results show that the 95% confidence level lower limits on the QG energy scales are EQG,1 -10 times of the Planck energy EPl for the linear and the quadratic LIV effects, respectively. Our limits on the quadratic LIV case improve previous best bounds by factors of 5–7."

Lorentz invariance, the fundamental symmetry of Einstein’s relativity, has withstood strict tests over the past century . However, deviations from Lorentz invariance at energies approaching the Planck scale are predicted in many quantum gravity (QG) theories seeking to unify quantum theory and general relativity. Although any signals of Lorentz invariance violation (LIV) are expected to be very tiny at attainable energies EPl, they can increase with energy and accumulate to detectable levels over large propagation distances.

In this deep dive, I unpack the significance of LQG, its implications for unifying quantum mechanics with general relativity, and the potential blow it recently suffered this is one such model considered. I also reflect on the broader implications for theoretical physicists like Carlo Rovelli, Eric Weinstein, Stephen Wolfram and others if string theory remains unchallenged. Lastly, obviously this is one such model considered, and other versions of LQG could still survive to fight another day.

Key Takeaways:

00:00 Intro
02:11 What is loop quantum gravity (LQG)?
06:26 New evidence from the Lasso team challenges LQG predictions
09:15 String theory vs. loop quantum gravity
10:54 Outro

Additional resources:

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[DrBrianKeating]

Is String Theory our only hope for a Theory of Quantum Gravity?

[phpn]

Flushing LQG doesn't suddenly make ST true. Let's remain reasonable.

[Achrononmaster]

Point of note: String Theory already failed far more severe tests. It is funny people think LQG is blown away by this wee test. Also, the QG field has always been wide open. Just because a model has a spin=2 representation does not make it *_the_* theory for gravity. Do you know how many algebras have spin=2? Too many for me to count!

[ikarienator]

This is not true. String theories have been tested and have all failed the teste so far. Everytime they come back with modifications to make sure it's in the range that the current experiments cannot reach

[wilsonowuor]

Science should be "discovery" not "invention." String theory is like working backwards using an answer that you are not even sure of yet.

[tayzonday]

Couldn’t Lorentz invariance violation be obscured by anisotropic medium impurity of spacetime itself? If the different wavelength frequencies each resonate slightly differently with “translucent” impurities (wayward atoms) on the way to being observed, that random effect could normalize minute speed variance toward a single quantity. Also, wouldn’t non-uniform expansion of spacetime itself on a Planck scale also be a medium impurity that is normalizing towards a single quantity?

You might just need a gamma ray burst of energy observed at a nanometer of distance for spacetime itself to not obscure the differentiation of Lorentz invariance.

[volfan]

“Evidence against LQG” does not equate to an “absence of evidence”. Not sure how the Sagan quote about the absence of evidence applies if there is in fact evidence.

[monkerud]

Only rules out lorentz invariance violations from a difference in propagation speed that depends on frequency. There are many other ways to break lorentz symmetry though :). To be fair though, thats a very good thing to constrain because a lot of ways to construct a model results in that kind of bias.

[brunocarvalho]

Some might argue, like string theorists did before the LHC failed to find supersymmetry, that the lack of direct evidence doesn't invalidate string theory. They also emphasize that the vast number (10⁵⁰⁰) of Calabi-Yau manifold compactifications is often assumed. However, it's questionable whether these manifolds can fully explain our universe's structure, even if they remain undetected. The idea that string theory is a central, testable theory, while alternative theories are dismissed based on limited indirect evidence, raises concerns. A more balanced approach is needed that acknowledges the strengths and limitations of various theories without prematurely discarding viable alternatives.

[BenAbraham-euzg]

I don't think it's a death blow to loop quantum gravity, or that is to say the absence of violations of Lorentz invariance doesn't disprove pixelated SpaceTime. The absence of violations of lorentz invariance does disprove particular models of how the photon could be interacting with a particular model of pixelated spacetime

[Dismythed]

String Theory proved one thing: Any theory can be infinitely modified unless its core tenet can be challenged. This does not defy LQG's core tenet. It only forces it to have to adapt. How many times has String Theory adapted to a so-called death blow?

[IamdeaththedestroyerofWorlds]

String theory has been beaten to crap and still hasn’t gone away.

[baptiste]

Wait, what? Why do you say that LQG predicts violations of Lorentz invariance? I'm no expert, but Carlo Rovelli has said that models that violate Lorentz invariance are going in the wrong direction, and that was years ago. In the LQG lectures freely available on YouTube and given by Rovelli, he explicitly says in the first video around 25 minutes that “nature says no [to Lorentz violation]”. So firstly, the fact that experiment suggests that there is no Lorentz violation is not a new result, secondly why do you say in the video that Rovelli “will find ways to escape the bounds” when he doesn't believe in Lorentz violations himself???

[marinielgalvao]

From what I understand, LQG does not predict Lorentz invariance violation.

[nunya]

When i saw the thumbnail I literally thought to myself I'm going straight to comment what does Sheldon Cooper think of this 😂. Didnt expect for the video start with him lol.

[SkyDarmos]

While this disproves loop quantum gravity, it does not disprove some other theories of quantized space, such as SPD-quantum gravity. Here the space quanta have variable sizes and an incredible number of connections to each other. The zigzagging is much tighter here.

[scottrichmond]

It seems obvious that space should be quantized too, but maybe we just don't need the loops. Maybe it's flake quantum gravity, or some other cereal-adjacent shape.

[rdspam]

Oh good, we can get back to all the experimental verifications of string theory.

[SiqueScarface]

This is what commonly is referred to as "Hubble Pictures are too crisp."

[karlbarlow]

When a theory is on the right track, it immediately reveals other predictions and explanations. Mysteries are solved. If its a big breakthrough, the cascade of understanding goes on for years, such as after Einstein's magnum opus. The current impasse is a huge clue that a wrong turn has been taken. We need to back up to before we got stuck, not keep tweeking the current lame duck theories.