String Theory is a very promising theory unifying all forces in nature. It is the heir apparent to being the holy grail - the Theory of Everything. It has a rich and precise mathematical structure that defines and describes everything in nature including the elusive gravity (electromagnetism, strong/weak nuclear force + gravity).

It could have been the Theory of Everything, but it has predictions that could not be experimented on - many worlds, multiple dimensions and multiverse. For it to work, it also required other things like Supersymmetry (which comes with its own problems). More problems came out of String Theory and the theorists applied bandaid solutions that didn't convince the detractors. With no experimental data, no empirical data can be obtained. Instead of being a Theory (eg General Relatively Theory, Quantum Field Theory), it remains a theoretical framework (a much lesser stature than a Theory). String Theory has many exciting predictions. These are all fascinating predictions revealed by the math. But other scientists claim that not everything that exists in math exists in the real world. The biggest blow perhaps against String Theory is the AdS/CFT correspondence pointing out that String Theory doesn't work in our universe because our Cosmological Constant is positive (that our universe is expanding). String Theory thrives in a negative Cosmological Constant.

The Emperor Has No Clothes
Considering String Theory has been around since 1970 and none of its predictions have been produced, the intrepid whistle-blowers started calling it out - Eric Weinstein being the most vocal and laceratingly vicious. Who is this guy? Eric is one of the most fascinating and colorful intellectuals that came out of social media to joust head-to-head with anyone in a public discourse from a wide variety of fields - math, physics, politics, education, AI, etc. He struts around with his intellectual swagger, a closet narcissist, hamming his articulate abstract language, almost begging for the spotlight but pretending to shun his celebrity - as if he's convincing anyone. He's a brilliant guy who oscillates between educating (when he talks like a normal guy) and impressing (he inserts long technical jargon no one understands to underscore to everyone he knows his shit). He claims to be a mathematician and not a physicist but challenges any physicist who supports String Theory (with the notable exception of Edward Witten). In every conversation, he invokes his version of the Theory of Everything which he calls Geometric Unity, but no one seems to bother.

Eric Weinstein Quotes on String Theory and its Proponents:

1. "String Theory turned murderous and the murderousness is really the problem. It's not String Theory per se that's the problem. It was the behavior patterns of the physicists who became so drunk on power and so completely thoroughly obnoxious above the obnoxious level that physicists are usually at. They became completely intolerable."
2. "If you find anything that isn't String Theory, don't worry. We'll just call it String Theory - everything that you can do, we will write a paper called 'blank whatever you did' plus its stringy origins. It is that complete intellectual dishonesty and the failure of the string theorists who embraced it."
3. "They can keep pumping out papers and saying, 'Look this is a bit of the 21st-century that fell into the 20th century.' No it's not the fact of the matter. It's a bit of the 20th century that's still hanging around in the 21st-century. They refused to ship a product (that still hasn't worked in 50 years)" (this is an attack on Edward Witten who described String Theory as a 21st-century science that somehow landed in the 20th century, suggesting String Theory was just way ahead of its time and perhaps beyond comprehension to most)
4. "More neurons have been spent exploring this Theory and failing to find a way to connect it to anything other than mathematical reality. String theorists would accomplish things in mathematics and then they would say this is exactly why string theory is correct." (again suggesting that String Theory is elegant math but does not represent reality. Other physicists say that just because the math is elegant doesn't necessarily mean it exists in the real world.)
5. "Many string theorists could not find the men's room if their life depended on it. They are not in contact with the physical world."
6. "The serious problem is that there's no one in a position to tell some of the world's most brilliant people that there's an aspect of Q Anon (a cult of conspiracy theorists) in physics. This is the cult science that Richard Feynman warned us about."
7. "Edward Witten is a guy who could not possibly have been denied a Nobel Prize. This guy is one of the most brilliant people and I hope to God you have him on your show. Despite my frustrations with him, it has been an honor to live in the same era as Ed Witten. Ed is one of the most important minds alive and he has earned a tremendous amount of respect based on what he's done for geometry but more importantly, to show that Quantum Field Theory is a canonical and necessary mathematical structure. But what he has not done is to show us that String Theory is the likely winner."
8. "We have got to have open debates in the community where the people in the holy of holies, The Institute for Advanced Study, are actually on the same stage with people who can competently disagree with them."

Leonard Susskind is one of the towering figures in Theoretical Physics and one of the founders of String Theory going back to 1970. In fact, to me, his name is synonymous with String Theory. He can explain String Theory to lay people like me, and he can go deeply technical when he lectures his students. As an exceptional educator and popularizer of science, he brought complex ideas in String Theory and cosmology to a broader audience. I find him to be a likable guy. I wish I had someone like him as a teacher.

Leonard Susskind in one of his lectures

Leonard Susskind Quotes on String Theory:

1. "We live in a wrong kind of world to be described by String Theory."
2. "I can tell you with absolute certainty that String Theory is not the real world that we live in, so we need to start over."
3. "The most important single thing about string theory is that it's a highly mathematical theory, and the mathematics holds together in a very tight and consistent way. It contains in its basic structure both quantum mechanics and the theory of gravity. That's big news."

1. 1968 - Gabrielle Veneziano developed a mathematical equation hinting at a one-dimensional structure. This eventually gave rise to the framework of vibrating strings
2. 1970 - Susskind together with Holger Bech Nielsen and others continued Veneziano's one-dimensional work and developed the idea of vibrating strings. This gave birth to String Theory
3. 1971 - String Theory predicted the existence of the graviton - a particle responsible for gravity. This gave rise to the Quantum Gravity Theory. The rest of the 70s, String Theory fell out of favor and was overshadowed by the emergence of the Standard Model
4. 1984 - String Theory was back on track when John Schwarz and Michael Green developed an elegant mathematical equation that removed all known anomalies. As the theory gained traction, researchers realized 5 flavors of String Theory instead of just one. This created confusion and String Theory hung in the balance
5. 1995 - Edward Witten, perhaps the most brilliant of all the brilliant Theoretical Physicists, introduced M Theory to address the 5 Heterotic aspects of String Theory. He explained that the 5 flavors were emergent phenomena from a more fundamental theory - M Theory. String Theory was back on track
6. Mid 90s - String Theory was linked with Supersymmetry (linking bozons [energy] with fermions [mass]). This integrated String Theory with the Standard Model giving it more credibility.
   
   Supersymmetry has not been proven yet, but it's a theoretical extension of the Standard Model. It predicts that Fermions have a Bozon superpartner, e.g. electron = selectron. Supersymmetry gave rise to Super String Theory which is more robust because by adding symmetry, it complies more about our physical universe. Supersymmetry supports M Theory which adds credibility to String Theory
7. 1997 - Juan Maldacena proposed the holographic principle by linking String Theory in Anti-de Sitter Space (AdS) to a Conformal Field Theory (CFT) on its boundary. This breakthrough extended String Theory's application beyond Particle Physics to now include Black Holes and Condensed Matter.
8. 1999 - Brian Greene’s The Elegant Universe became popular and put String Theory at the forefront of public consciousness
The Elegant Universe by Bryan Greene


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9. Mid 90s to early 2000s - String Theory was a much talked-about topic and public attention was focused on it. It dominated the field of Quantum Mechanics. It was heralded as the theory that could be the Theory of Everything
10. 2000s - Present - String Theory continues to be plagued by non-experimental evidence, not having enough high energy to conduct experiments, it predicts many worlds making it difficult to make predictions specific to our universe, and since we are in a positive cosmological constant and String Theory works in a negative cosmological constant, it's like fitting a square peg into a round hole. String Theory has precision math but for another universe, not ours. Its application seems reduced simply as a mathematical exercise for Quantum Mechanics

AdS/CFT Correspondence is perhaps the most damaging argument against String Theory. This was discovered in 1997 by Juan Maldecena. AdS and dS are mathematical models describing space/time geometry as a solution to Einstein's General Relativity with a cosmological constant which could be negative or positive.

1. Anti-de Sitter Space (AdS) - this has a negative cosmological constant, predicting a contracting universe. Here, space/time curves down like a hyperboloid, with no horizon, and an infinite edge. String Theory thrives well in this environment. We are not in this universe. This means String Theory does not describe our universe.
2. de Sitter Space (dS) - our observable expanding universe (accelerated expansion) suggests that we are in dS Space (de Sitter Space) with a positive cosmological constant, space/time is curved up with an expanding horizon - speculatedly caused by Dark Energy. The problem here is that String Theory fits well in an AdS Space - the other universe. To fit String Theory in our universe produces unstable models and mathematical inconsistencies. Thus, we had to start all over in search of the Theory of Everything.

The Cosmological Constant plays a crucial role in the failure of String Theory. What is it? This goes back to 1915 when Einstein formulated his General Relativity. His equation revealed a dynamic universe (expanding but would eventually slow down and perhaps begin to crunch). But the current thinking then was that the universe was static, neither expanding nor contracting. Not believing his math in favor of the current thinking, he fudged his equation by adding a constant (the now famous Cosmological Constant) to make the universe static (that's right, Einstein cheated on his equation to comply with the current belief). But in 1929, Edwin Hubble looked into the Mt. Wilson telescope and made a groundbreaking observation - that the universe was expanding - not static (this laid the foundation for the Big Bang Theory). Einstein admitted to making his greatest blunder because he could have taken the credit for announcing the universe was expanding had he believed his original equation. He removed his Cosmological Constant - he was right the first time after all. But in 1998, it was observed that the universe wasn't slowing down - it was expanding at an accelerated rate. Even though Einstein's original equation accounted for a dynamic universe, it did not account for an accelerated expansion of the universe. To correct this, the cosmological constant was added back. This integrates Dark Energy (the force causing the accelerated expansion) into General Relativity. But wait! Wasn't the CC supposed to make the universe static? Why was it added to account for the accelerated expansion of the universe?

The original purpose of the Cosmological Constant was to make the universe static. But this model has already evolved. The value of the Cosmological Constant is not fixed. It is to account for the expansion / contraction of the universe. Since our universe is expanding, the Cosmological Constant is positive. For a contracting universe, the Cosmological Constant is negative. This is where the problem arises. String Theory works in a negative Cosmological Constant, not in our universe where the Cosmological Constant is positive. That's why Leonard Susskind stated that String Theory does not describe our universe and therefore it's the wrong model for the Theory of Everything.

String Theory and Quantum Field Theory are complementary and align well together. In this framework, one-dimensional strings vibrate giving rise to the quantum field (while elegant and complementary, this remains speculative because there is no experimental evidence). When this field is excited, it gives rise to its corresponding particle. In the Quantum Field Theory, the fundamental force is the quantum field - it's the starting point, they don't arise from anything else and nothing else comes before that. But in String Theory, strings are the fundamental force giving rise to the quantum field. The vibrational modes of the strings determine a particle's mass, charge and spin.

1. "If AdS Space has no horizon and is infinite, but shaped like a hyperboloid, then the boundary of that shape is essentially the horizon - so it's not infinite."
   
   AI: (Honestly, I don't understand the reply of AI but I'm putting it here anyway) You're right to raise this point—it highlights a subtlety in how Anti-de Sitter (AdS) space is conceptualized and described. AdS space is a spacetime with negative curvature, and it's often visualized as a hyperboloid embedded in a higher-dimensional flat space. This hyperboloid represents a constant time slice of the AdS geometry, and it extends infinitely in spatial directions, giving the impression of "infinity".
2. "If it was already known in 1997 that String Theory works in an AdS Space which is not where our universe is, why was String Theory pursued when it would have been logical to drop it and pursue other alternatives? It seems like 27 years were wasted pursuing something that was already predicted to fail."
   
   AI: Your question addresses a key philosophical and practical tension in theoretical physics: why pursue a theory that seems mismatched with observed reality? Here’s why String Theory was pursued despite the mismatch with our universe:
   
   1) String theory is one of the few frameworks capable of unifying Quantum Mechanics and General Relativity in a mathematically consistent way. Even though it naturally fits into Anti-de Sitter (AdS) space, this was not seen as an immediate disqualification. No competing theory offered the same level of internal consistency or explanatory power.
   
   2) The discovery of the AdS/CFT correspondence by Juan Maldacena in 1997 was groundbreaking. It showed a deep connection between gravity in AdS space and quantum field theories (CFT) in lower dimensions. These practical applications motivated further study, even if AdS space didn’t directly describe our universe.
   
   3) In the late 1990s and early 2000s, theorists worked on adapting String Theory to accommodate de Sitter (dS) space, offering hope that String Theory could describe an accelerating universe. Though debated, it kept the possibility alive.
   
   4) String Theory’s mathematical framework was so rich that it led to breakthroughs in unrelated areas, such as Black holes, High-energy physics and the standard model. These "side benefits" justified ongoing investment in String Theory research.
   
   5) The history of science shows that theories sometimes take decades to mature. Theoretical physics often involves exploring mathematically elegant frameworks first, with the hope they’ll eventually align with experiments.
   
   6) While 27 years may feel like a long time, scientific progress is often non-linear. Even if String Theory ultimately fails as a model of our universe, it has already enriched physics and mathematics in profound ways. It may yet provide stepping stones toward a more accurate Theory of Everything.

Since I heard about String Theory in 2005 I was hooked. It was very promising and exciting - 11 dimensions? Multiverse? Many worlds? I would gobble up any String Theory podcast I could view. For decades, I was at the edge of my seat awaiting its predictions. I've romanticized it in my head a zillion times. Now, Leonard comes out saying we have to start all over again? NOOOOOOOOO !!!!!