Reddit reviews Why String Theory?

https://www.amazon.co.uk/Why-String-Theory-Joseph-Conlon/dp/1482242478

I'm not a fan of string theory, so I'm biased. If you want to read a source from an advocate of string theory, I would suggest Why String Theory? by Joseph Conlon.

Yes string theory produces elegant mathematics, but it's just math. It hasn't made its connection to reality. A lot of what string theory was designed to do is not working (i.e. quantize gravity). Especially since supersymmetric particles weren't found at the LHC.

Instead of a "theory of everything," it's now thought of as a set of possible theories where quantum gravity might work. The best thing going for string theory is probably the AdS/CFT correspondence. The problem is that AdS is a spacetime with a negative cosmological constant, but our universe has a positive cosmological constant. However, there could be applications of the AdS/CFT corrsepondence to condense matter physics which is discussed in Joseph's book.

Here is a quote I took from this page where people discuss Penrose's recent book Fashion, Faith, and Fantasy.

> String Theory arose out of particle physicists attempting to quantise the linearised version of GR, normally used in make perturbations to Newton’s laws in the solar system. This produces a free spin-2 graviton at 0th-level, and then they attempt to use first order interactions to construct a full interacting quantum field theory of gravitons - in FLAT space - which has none of the full symmetries of GR, and it fails miserably by throwing up infinities at all orders. Not to be daunted, they added supersymmetry to cancel the infinities, but that did not work as expected, so they invented Strings instead of points, to remove the short distance interactions. Why would this ad-hoc procedure produce anything like Quantum GR? It gets worse, they quantise this strings in FLAT spacetime, but find that they need 10 dimensions instead of 4, so they then ‘compactify’ 6 to get something like our world. Problem is, there is about 10\^500 different ways to do this - hence the multiverse. Never before has such a failed theory taken on such a life of its own, at least the Aristotelian epicycles produced decent predictions.

> This is also the reason why the no-show of supersymmetry has no consequences for string theory. String theory requires supersymmetry, but it makes no requirements about the masses of supersymmetric particles either.

And what will become of supersymmetric string theory if SuSy is bumped off? String theory still needs the supersymmetry for to eliminate its wast landscape of predictions and for to keep the vacuum stable (since the vacuum energy for fermions is negative and for bosons positive the hope was that for every fermion there is a bosonic partner with about the same mass to cancel these contributions - see Conlon's book - review here). Thus, no supersymmetry would imply: no gravitons and no quantization of spacetime. Which is also reasonable, since gravity is not renormalizable anyway.

The Standard Model also "required" Higgs boson, but it made requirements about its mass neither. What the absence of Higgs boson would mean for Standard Model after then? The history is written by winners.

Higgs boson in Standard Model is based on different concept, than the Higgs-Anderson mechanism in boson condensates and its technical derivation consists in a mere reshuffling of degrees of freedom by transforming the Higgs Lagrangian in a gauge-invariant manner. A well known "hiearchy problem" implies, that quantum corrections can make the mass of the Higgs particle arbitrarily large, since virtual particles with arbitrarily large energies are allowed in quantum mechanics. During time, Higgs boson mass has been guessed from 109±12 GeV to 760±21 GeV, plus two unconventional theories with 1900 GeV and 10^18 GeV. There are so many comparably likely models - most of which contain continuous parameters whose values aren't calculable now - that the whole interval is covered almost uniformly.

The title of recent another NewScientist article "In SUSY we trust: What the LHC is really looking for" (full version) illustrates clearly which priorities were given before Higgs boson finding once its search started to take too long. The article could be interpreted like: "Umm, well, ... we actually don't believe, Higgs boson will be ever found at LHC - so we should concentrate to supersymmetry instead. ." Moving the goals seems to be the only option for the true believers.


For further reading: Fundamental physics is frustrating physicists , Massive failure of mainstream physics theories at the LHC and Why the LHC is such a disappointment: A delusion by name “naturalness”.