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Scene 1: On the street
QG: Hello there anonymous string theorist. What are you working on these days?
ST: Hello anonymous quantum gravitist. I am working on string theory, but you knew that.
QG: Really? I thought it was M-theory now.
ST: Its true that it is now a theory about M’s but we still call it string theory because there are so many string theory departments and it is too expensive to change their names.
QG: Ho Ho Ho. The truth is that most of us have realised that string theory is a failed theory and are moving on to new areas based on reality, such as cosmology and nuclear physics.
ST: No. The truth is that string theory is so successful, that it is being applied even in cosmology and nuclear physics. string theory predicts a vast landscape of multiverses that explain why the cosmological constant is small thanks to the anthropic principle as predicted by Weinberg. In nuclear physics the A-D-S/C-F-T duality means that we can now use string theory to understand the strong nuclear force. This shows that string theory is the only game in town and you, are a crackpot.
QG: The application to nuclear physics is just an approximation scheme unrelated to the extravagant idea that string theory is a unified theory of everything in higher dimensional space-times. The problem is that the landscape makes no predictions and we have no reason to believe in it. It’s not a real testable scientific theory, rather it is an untestable end-point of a failed idea, and you are so wrong that you are not even wrong.
ST: Back on the planet where I live, the Large hadron Collider will find supersymmetry next year with a probability of 90 percent.? Once supersymmetry has been verified the correctness of supergravity is theoretically overwhelming and superstring theory follows as the unique logical completion of physics.
QG: There is nothing unique about string theory in physics. Loop Quantum Gravity is just one example of many alternatives that make testable predictions without the need for unsupported inventions such as higher dimensions and supersymmetry. Do I have to remind you that supersymmetry was predicted to be discovered at Fermilab by 2000? We are still waiting. String theory is so weak that it can be made to fit any lack of evidence and aging string theorists will continue to support it no matter has compelling its failure becomes.
ST: Even slightly retarded children know that Loop Quantum Gravity predicted a violation of Lorentz Invariance that was ruled out by the Fermi gamma ray telescope. I am afraid that your IQ has descended below the level of a retarded monkey if you believe anything can match the successes of string theory. Crackpot.
QG: Loop Quantum gravity has progressed and no longer predicts a violation of Lorentz Invariance. Well I’d love to continue with this high level of discussion but kindergarten breaktime is up so you must have to go.
ST: Yes I am sure there are more useful things you could be doing like lying down on the freeway. Crackpot.
QG: Actually, I am on my way to do a television interview on “science question time”.
ST: That is interesting. So am I.
Scene 2: In the TV studio
I: Hello and welcome to science question time. Today I have with me anonymous string theorist and anonymous quatum gravitist. Guys, you look very similar. Am I mistaken or are you twins?
QG: Absolutely not
ST: We are completely unrelated
I: Err, okay. String Theorist, let me ask you a question first. Next month the Large hey-dron colly-door will start searching for new physics. What do you expect to see?
QG: Ho Ho Ho.
I: Why are you laughing?
ST: It’s because you called it a hey-dron colly-door. I think you mean hadron collider. Never mind. We dont expect journalists like you to know much about science, so I will try to answer your question in very simple terms. Basically, we expect to see particles of supersymmetry, and after that, evidence for large extra dimensions. This will confirm the verity of string theory.
QG: If I may interrupt. The only thing that we can predict with any possible certainty is the discovery of the Higgs bose-on. The rest is pure speculation and hype from string theorists who forget to mention its complete lack of testability.
I: So then, tell me quantum gravitist, what do you work on?
QG: I work on Loop Quantum Gravity, an alternative to string theory in which space-time is built from loops.
I: Well, I understood that string theory also says that everything is made of loops of string. Are these theories related?
ST: Absolutely not! In loop quantum gravity the loops are just holonomies of space-time that form knots. In string theory we have loops that vibrate to form states of particles including gravitons. They can’t form knots because they easily pass through each other. Furthermore, we normally work in 10 or 11 dimensions where knots cannot form. So clearly the loops and strings are not the same thing.
I: But when the strings cross over, does anything happen?
ST: Sometimes they interact and split to re-join differently. Other times they just pass through. Diagramatically it looks like this…
I: And in loop quantum gravity. Can the loops pass through each other?
QG: No I agree that they are utterly different from strings. The loops form knotted states that can evolve according to mathematical equations called skein relations. Diagramatically it looks like this.
I: Is that where? quantum groups come in?
QG: Yes. These are connected with quantum groups. Good Guess!
ST: Do you use quantum groups? So do we! What an amazing coincidence!
I: I thought you could not have knots in multi-dimensional space-time.
ST: But the quantum groups are used in conformal field theory on the worldsheet which has fewer dimensions.
I: So if you use the same mathematics doesn’t that suggest there is a close connection?
QG: Not at all. It’s just an example of the unity of mathematics. The same equations often appear in different places, but there is no real connection.
I: I see. So string theorist. What would you say was the biggest weakness of string theory?
ST: string theory is very successful, but despite years of development, we still dont have a non-perturbative formulation of string theory based on sound principles which respect the background independence of space-time that we expect from Einstein’s theory of relativity. In short, we just don’t understand what string theory is yet. Without that we are limited when we want to understand what happens in extreme circumstances such as the big bang singularity.
I: And quantum gravitist. What is the main strength of Loop Quantum Gravity?
QG: Loop quantum gravity is a direct quantisation of gravity that builds on the principles of relativity as laid down by Einstein. It is fully background independent.
I: Right. What about its weaknesses?
QG: Well, we do find it hard to incorporate forms of matter and interactions other than pure gravity. It is also hard to re-cover the classical limit and understand how familiar space-time emerges from the theory.
I: That is quite interesting. So string theorist. What are the strengths of string theory?
ST: string theory incorporates all the forces and particles of physics in a unique way. Furthermore, gravity arises in string theory in a natural and inescapable way as a result of special vibration modes of the strings that act correctly like gravitons. This means that we include matter and space-time in a way that has a well-behaved continuum limit.? I hope I am not getting too technical for you.
I: Well. If I understood you correctly it means that the weaknesses of string theory are the strengths of loop quantum gravity.? And, the weaknesses of loop quantum gravity are the strengths of string theory. Couldn’t they be combined to form a better theory?
ST: No! I mean we shouldn’t even try because loop quantum gravity is clearly a failure. It can’t even reproduce the physics of ordinary space-time. Anyone who thinks it could work is a crackpot.
QG: Really, the problem is that string theory is not background independent. This is a fundamental failure. It is not even wrong, so it can’t be made to work with loop quantum gravity.
I: Are there areas where string theory is better understood?
ST: Yes. In three dimensions we have a much better understanding because string theory can be formulated as Chern Simons gauge theory.
QG: Loop quantum gravity is also much easier to understand in three dimensions. The states are described by spin networks that combine to form a spin foam model of three dimensional gravity known as chern simons theory.
I: The same chern simons theory?
ST: Yes, sort of.
I: More accidental unity I suppose. But If I may persist with this idea for a moment.? Perhaps you could look back to the origins of each theory to see if there is a common ground?
ST: Curiously, string theory actually started as an attempt to understand the strong nuclear force that binds quarks inside nucleons. Particle physicists thought this confinement could be explained by the presence of strings of energy binding the quarks. This theory did not work but we noticed that the gravitational field was included accidentally, so the idea was re-born as a unified theory of quantum gravity and other forces at higher energies.
QG: Whereas loop quantum gravity developed out of a formulation of the strong nuclear force called the loop representation. It did not help understand the confinement of quarks as has been hoped, but we noticed that it worked naturally as a description of states in quantum gravity.
I: So you are saying that both ideas came from the same nuclear physics and developed in similar ways?
ST: Superficially, yes.
QG: But that is the only similarity. Loop quantum gravity is an alternative to string theory.
I: With completely separated funding?
ST: Of course!
I: I wonder. Do you physicists ever get together to compare your ideas?
QG: Yes. We are doing it now. Silly!
I: I mean, do you have conferences on quantum gravity where you compare your different theories and look for ways to benefit from each others work?
QG: Mostly we have conferences about Loop Quantum Gravity, and they have conferences about String Theory.
ST: I remember one time we invited anonymous quantum gravitist to give a talk at our String conference. It was a complete waste of time.
I: Oh? Why?
ST: His talk was about things we had known ten years ago. There has been no progress.
QG: Actually my talk was just an overview of the subject for the young string theorists. I did not have time to talk about our latest work.
I: But I thought these conferences were to discuss the latest research.
ST: Yes but there is no recent work worth talking about in Loop Quantum Gravity.
QG: On the contrary. We have recently discovered new spin foam models such as EPRL which solve many of the old problems. These models they offer the hope of building spacetime from abstract algebra using higher category theory.
I: And what are the latest developments in string theory?
ST: We are pursueing a more algebraic approach to understanding the different p-branes in string theory. Some new understanding comes from the application of what we call higher category theory.
I: Would it be worth comparing these ideas to see if there is anything you could learn from each other?
QG: No. I am working on alternatives to string theory. It would not make sense to work with them.
ST: In fact, our approaches are completely different. There is nothing in common. I do not need to work with crackpots.
I: Gentlemen. I’m afraid that is all we have time for today. Thank you for this fascinating discussion.
ST: Goodnight crackpot.