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Today during a very "unique" study session, I might have internalized why Quantum mechanics was not enough, and Quantum field theory makes sense. It seems the reasons are that

  • When a potential is used in Quantum mechanics, we explicitly violate special relativity because information would have to travel faster than light

  • In QFT particle creation and annihilation are understood nicely as natural implications of what can happen if the kinetic energy of the combining species is larger than the rest mass of some created pair

  • Space and time are not on an "equal footing" in QM

  • Decay processes are a testy issue

These things are starting to mean something to me. What is the reason for string theory, what does it make natural and clear that QFT falls short of?

Qmechanic
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  • gravitation? UV divergences? too many free parameters? – AccidentalFourierTransform May 18 '16 at 19:46
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    QFT is QM and string theory is QM, unless you want to define QM narrowly as "fixed particle number non-relativistic models"? I don't think that's a particularly good classification. The term "quantum mechanics" should be used in a overarching way. Even string theory is quantum mechanics, it just postulates a different Lagrangian and it can't really tell you what the self-consistent effective field theories that pop out of that hellish construct would look like. At least not, yet. What does string theory fall short off? Testable predictions. – CuriousOne May 18 '16 at 19:52
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    @CuriousOne: String theory is not QM or QFT. It might look like it because it borrows many concepts and because it has to reproduce QFT in a low energy limit, but crucially the string partition function is defined differently from a QFT partition function, and the states with their vertex operator insertions on the worldsheet also are not QFT constructs. – ACuriousMind May 18 '16 at 20:02
  • Possible duplicates: https://physics.stackexchange.com/q/387/2451 and links therein. – Qmechanic May 18 '16 at 20:05
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    As for the question, this question is really too broad. What sort of "reason" are you looking for? The historical reasons people began with string theory probably are not why most string theorists today pursue it, and I suspect that the reasons for doing string theory are not uniform among current string theorists, either. – ACuriousMind May 18 '16 at 20:06
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    @ACuriousMind: How do model dependent choices make a difference to the question of what quantum mechanics is? Do measurements work differently in string theory? Do I get anything more than wavefunctions and probabilities out at the end of the day? Does this translate into real experimental predictions? If it doesn't, then string theory is quantum mechanics. I have, by the way, heard a recent talk (by Brian Greene, I believe) in which he said that string theory might turn out to be equivalent to QFT (or the other way around). – CuriousOne May 18 '16 at 20:08
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    Will you people stop closing down good interesting questions please? – John Duffield May 18 '16 at 20:39
  • @JohnDuffield: The question isn't interesting. It's not even well defined as the OP uses terms in different ways than I do. As you can see even ACuriousMind has an opinion, but then won't back it up when asked for details that get to the core of the matter (I hope he changes his mind and gives us an answer). Other than that the OP is mangling a great number of issues that have nothing to do with each other like the structure of spacetime question, which has no meaning in theories that rely explicitly on pre-geometry and "Decay processes are a testy issue" is a meaningless phrase to start with. – CuriousOne May 18 '16 at 21:14
  • @CuriousOne It is not surprising that my question might not be well posed and might even be a stitch up of trivial not exactly connected facts, at least at first sight. I am not a professional Physicists, but enjoy engaging in study of select topics in QM and QFT. String theory is a rich but hard subject to grasp. The question is really a "Why should I put in all the effort, time and money that is necessary to gain competence in String theory?" Why it makes absolute sense to go from Quantum field theory to String theory. –  May 18 '16 at 21:57
  • The only reason why you would want to learn any science is curiosity. Why do we climb the mountain? Because it's there. You certainly won't get an industry or banking job by learning string theory. Look at it this way: quantum mechanics is a framework, like classical mechanics. Orbital mechanics is an application of CM and spacecraft navigator is a job description that uses orbital mechanics. QFT and string theory are applications of QM, but the only job description that uses either is theoretical physicist. Now, if that's what you want to become, better make sure you get tenure on top. – CuriousOne May 18 '16 at 22:02
  • @AccidentalFourierTransform has already hinted at some of the things that string theory naturally and without effort describe that seem out of reach for QFT, I think I was thinking along these lines. Some specific clear paths forward only string theory could give, and things that are made absolutely clear and unambiguous. Armed with answers like these, would encourage me to keep attempting to understand QM and QFT so that I can appreciate what string theory brings to the table when I get there. –  May 18 '16 at 22:03
  • But he hasn't answered my question, yet, on how string theory differs ontologically from quantum mechanics. I have not read anywhere that it does. Unless either of you can answer that question, the distinction is purely semantic. String theory hasn't given any path forward on anything, so far. It hasn't eliminated the real thorny issue that stops us from making progress, either: pre-geometry. What string theory does is to solve a few minor convergence and free parameter issues by increasing dimensionality. It pays for that by being ambiguous to the $10^{500}$. No progress there. – CuriousOne May 18 '16 at 22:07
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    Why you even need answers from any of us here or encouragement is completely unclear to me. If you want to learn string theory you grab three dozen books and you start reading and doing the exercises and the math until you can calculate something non-trivial yourself. What you are being told here makes absolutely no difference given the scale of difficulty of achieving that intellectual goal. – CuriousOne May 18 '16 at 22:09
  • @CuriousOne I have sort of done that on and off, but I will be going back to School in the Fall and should get into it 100%. The first technical block I had to deal was the mathematical background. I think I have a few things in check now. I am not currently trying to compute any Stringy stuff, I am going through some standard texts in QFT and GR right now, I am hoping to be competent in QFT first. I have sat in some classes in the past, but should officially take it when I return to School. –  May 18 '16 at 22:16

2 Answers2

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One has to keep clearly in mind the structure of present day physics.

Quantum mechanics is the theory that started as non relativistic with the Schrodinger equation for potentials, and became relativistic with the Dirac and Klein Gordon and quantized Maxwwell equations. Quantum mechanics has postulates which used with the solutions of the differential equations describe single particle potential problems correctly and it has been validated beginning with the hydrogen atom etc.

Quantum field theory is based on the solutions of the above equations and has been developed to describe quantum mechanical interactions which are a many body problem, as can be seen in any Feynman diagram. Quantum field theory is founded on the free particle solutions of the above QM equations and obeys the postulates of quantum mechanics. The creation and annihilation operators act on the appropriate ground wave function of the problem at hand.

The success of the SU(3)xSU(2)xU(1) model of particle physics which has unified the three interactions , strong, weak, electromagnetic, lead to the holy grail of unifying gravitational interactions to the other three. That is what lead to string theory models becoming important. Quantization of gravity exists only as effective field theories, because no renormalization program can remove the singularities inherent in the spin two higher order exchanges of gravitons. It has been shown that string theory has a group structure that can accomodate the standard model and also a representation of a spin two particle that can be assigned to the graviton; with supesymmetric models calculations of higher orders give finite results which is the reason there is such an effort in string theory research. The problem is no definitive model has been found among the thousands of possible ones, up to now.

anna v
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As you said in your question, quantum field theory is very important; it takes the ideas of quantum mechanics and applies them to fields, such as the electromagnetic force (in fact, quantum electrodynamics was the beginning of quantum field theory). Quantum field theory has plenty of evidence to support it, and it is still an ongoing work. String theory, though, is very different. In reality, string theory has very little evidence to support it. Right now, it's basically an idea, and what supports it are the dualities embodied in m-theory. Now, for string theory's advantages:

  1. Some scientists believe its use of the anthropic principle is an advantage (to explain this in a nutshell, the string theory predicts so many universes some scientists believe this explains away the fine-tuning of, for instance, the cosmological constant, though there is more to the anthropic principle and this issue than that).
  2. It provides a framework for combining particle physics and general relativity.
  3. Currently, the model of physics we use is called the standard model. While this model is incredibly useful, there are some things it cannot do - for example, it fails to incorporate gravity. Scientists hope string theory might create a path toward combining quantum theory and gravity.
  4. The Big Bang doesn't fully explain everything about the beginnings of the universe and cosmic inflation is the theory believed to be our best shot at moving forward. Cosmic inflation needs a particle called the inflaton, whose properties can't be derived from cosmic inflation but might be able to be derived from string theory.

However, it must be kept in mind while reading this is that string theory isn't on nearly as strong of a footing as quantum field theory. There are many kinks to be worked out, and there isn't really any evidence for it. I hope this helps!

Here's Wikipedia's article on string theory, which goes more in-depth into some of this and also explains some of string theory's problems and mathematics: https://en.wikipedia.org/wiki/String_theory

auden
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  • There is no evidence for astrology. For string theory, the correct claim is that the experimental evidence there exists is also evidence for e.g. Standard Model+ ΛCDM. –  Jul 08 '20 at 21:54
  • Also string theory does not need to use the anthropic principle; some string theorists do. –  Jul 08 '20 at 21:57
  • ...meaning, "is also evidence for extant mainstream physics, so strings are not thereby distinguished" –  Jul 08 '20 at 22:04