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Isn't the consensus about string theory that it replaces standard model because it is fine-tuned to the knowledge we already have? I mean if you take whatever i

Isn't the consensus about string theory that it replaces standard model because it is fine-tuned to the knowledge we already have? I mean if you take whatever information, you can create infinitely many theories and equations on how to get the numbers within that particular set of data. But only some of these theories will predict numbers not in the data set yet. This seems to be the case with string theory. It kind of describes standard model in an alternative way, and the equations often hold up, but I don't remember case where it predicted anything correctly. Instead, every time new information is found, string theory itself is changed to match new data.

orbifold 12y ago
String theory much like Gauge theory is not fine tuned to any set of observed phenomena and for the most they don't make numerical predictions. What they do predict is the general shape of cross-sections for different particle species and corrections to them in the case of Gauge theories and much in the same way one can work out various String scattering cross sections. In the case of Gauge theories this is how you determine which of them fits the measured cross section best. In the case of String theories this failed when it was still believed to be a theory of the strong interaction. It eventually was realized that the string scale lS is related to the Planck lP by lP = lS * gS^{1/4} where gS is the vacuum expectation value of the Dilaton. It is believed to be of order one, so lS is ~10^35 meters. The current accessible length scale at the LHC ~10^18 meters, so you have 17 orders of magnitude between them and in particular no reason to expect the string scattering cross sections to relate to anything we can observe directly. What is found instead is that string scattering amplitudes assemble in a way that predict that the massless string excitations form Field theories already known, like type IIB supergravity, IIA supergravity, SO(32) and E(8) x E(8) coupled to supergravity and a few others. What is striking about those initial calculations, is that from field theory arguments those field theories are the only supersymmetric ones in 10 dimensions incorporating gravity. Perturbative String theory in its current form makes most of the model input data part of the "vacuum geometry", much in the same way Einstein did with general relativity. String theory in addition has the advantage that it gives a consistent perturbative description of quantum gravity. String phenomenology, that is the study of String theory with the purpose of making low energy predictions is still in its infancy. Its value at the moment is primarily in coming up with consistent extensions of the Standard Model, the alternative of conservative step-wise refinements (conjecture additional particles to exist and their coupling mechanism to the known particles, calculate cross sections and hope that they aren't ruled out by experiments yet, for example cause existing particles to decay that aren't really supposed to, like the proton) is still the majority approach. Dark matter and other theoretical considerations tell us however that the Standard Model is incomplete. The attractiveness of the string theory approach is that in some ways it is more restrictive as you have to satisfy additional geometric constraints, whereas in standard gauge theory the only consistency constraints are unitarity and locality, which predicts that fundamental particles will have spin 0,1/2,1 and 2 and renormalizability which restricts you essentially to Yang-Mills theory, plus some extensions (Supersymmetry among others). What isn't fixed is the gauge group, the precise way Fermions couple to each other etc. Some of those can't be fixed because things like the coupling constants aren't actually constant but dependent on the energy scale (they are related by the renormalization group). Most likely if we find some new particles at the LHC it will be possible to give them a low energy effective field theory description (maybe involving supersymmetry), if we don't find things like extra dimensions. In other words it is unlikely that we are forced to consider string theory in order to understand physics at the LHC scale, its value is that it potentially has better conceptual tools to come up with super-symmetric extensions of it.

orbifold 12y ago

String theory much like Gauge theory is not fine tuned to any set of observed phenomena and for the most they don't make numerical predictions. What they do predict is the general shape of cross-sections for different particle species and corrections to them in the case of Gauge theories and much in the same way one can work out various String scattering cross sections. In the case of Gauge theories this is how you determine which of them fits the measured cross section best. In the case of String theories this failed when it was still believed to be a theory of the strong interaction. It eventually was realized that the string scale lS is related to the Planck lP by lP = lS * gS^{1/4} where gS is the vacuum expectation value of the Dilaton. It is believed to be of order one, so lS is ~10^35 meters. The current accessible length scale at the LHC ~10^18 meters, so you have 17 orders of magnitude between them and in particular no reason to expect the string scattering cross sections to relate to anything we can observe directly. What is found instead is that string scattering amplitudes assemble in a way that predict that the massless string excitations form Field theories already known, like type IIB supergravity, IIA supergravity, SO(32) and E(8) x E(8) coupled to supergravity and a few others. What is striking about those initial calculations, is that from field theory arguments those field theories are the only supersymmetric ones in 10 dimensions incorporating gravity. Perturbative String theory in its current form makes most of the model input data part of the "vacuum geometry", much in the same way Einstein did with general relativity. String theory in addition has the advantage that it gives a consistent perturbative description of quantum gravity. String phenomenology, that is the study of String theory with the purpose of making low energy predictions is still in its infancy. Its value at the moment is primarily in coming up with consistent extensions of the Standard Model, the alternative of conservative step-wise refinements (conjecture additional particles to exist and their coupling mechanism to the known particles, calculate cross sections and hope that they aren't ruled out by experiments yet, for example cause existing particles to decay that aren't really supposed to, like the proton) is still the majority approach. Dark matter and other theoretical considerations tell us however that the Standard Model is incomplete. The attractiveness of the string theory approach is that in some ways it is more restrictive as you have to satisfy additional geometric constraints, whereas in standard gauge theory the only consistency constraints are unitarity and locality, which predicts that fundamental particles will have spin 0,1/2,1 and 2 and renormalizability which restricts you essentially to Yang-Mills theory, plus some extensions (Supersymmetry among others). What isn't fixed is the gauge group, the precise way Fermions couple to each other etc. Some of those can't be fixed because things like the coupling constants aren't actually constant but dependent on the energy scale (they are related by the renormalization group). Most likely if we find some new particles at the LHC it will be possible to give them a low energy effective field theory description (maybe involving supersymmetry), if we don't find things like extra dimensions. In other words it is unlikely that we are forced to consider string theory in order to understand physics at the LHC scale, its value is that it potentially has better conceptual tools to come up with super-symmetric extensions of it.