On this day, I will be investigating the status of String Theory. String Theory may not actually be a scientific "theory" , in the sense that it describes or predicts measured phenomena, but moreso acts as some sort of mathematical framework. String "Theory" started out as an attempt to describe hadrons, but interest in it exploded when it was realized (by accident) that it might be able to describe quantum gravity.

(more cynical tone ), String Theory is little more than mathematicians trying to convince each other that 11 dimensional spacetime with six extra 'compact' dimensions, is actually sensible. Doing such convincing requires "fiendishly complex mathematics" that easily covers rooms of chalkboards. When publishing, the papers come out as half-inch thick mathematical treatises mostly having something to do with "dualities" and other esoterica from algebraic geometry.

(Less cynical tone), String Theory may be the mystical gateway to a Theory of Everything or a TOE.

basic claims

String Theory initially required that spacetime have 26 dimensions. This was later reduced to 10, then bumped up to 11 with the discovery of D-branes. While we can easily confirm the 4 dimensions in regular life, we might ask where the other remaining six dimensions are. If the theory requires there be six more dimensions to space, then why can't we see them?

The question might sound like something that a high school kid might ask, who is unaware of the more nuanced aspects of the theory. This is wrong. "Why can't we see the extra dimensions?" is a rational question -- further it is in fact the entire pivot point about which String Theory turns as a discipline. The sweat and drama of String Theory is all about convincing one's colleagues that these extra six dimensions are "curled up" across distances near the Planck Scale -- i.e. that they are "compactified" dimensions curled into tiny geometrical objects called Calabi-Yau manifolds. A working string theorist is tasked with taking a piece of chalk , going to the chalkboard, and really explaining what that means, exactly. Such a task easily fills the chalkboard, fills the rest of the chalkboards in the room, and then spills across to the chalkboards in the room across the hall. (as we shall see later)

relation to existing Physics

String Theory bears little resemblance to quantum field theory or the Standard Model of particle physics. Instead, it is a framework that appears to have the power to describe a whole arboretum of various field theories. Or if, you will, a whole garden of varieties of Standard Models (plural). String Theorists bend over backwards in trying to find a limiting case which describes the universe we inhabit. In almost every scenario this involves presuming the universe we live in obeys supersymmetry, and the limiting case is invariable the low-energy limit of the theory. The test of bending, pruning, and preening String Theory to make it look like our universe is called String Phenomenology.

String Theory strongly suggests that no new physics arises between the scales of 10

^{-17}meters and down to 10

^{-35}meters. This claim is outlandish, and sours the theory with incredulity.

One might hope that a number of outstanding unsolved problems in physics would be addressed, or even laid-to-rest by string theory. Unfortunately, string theory is agnostic about which universe it is referring to in any of its 'varieties'. Thus, we don't get a straightforward answer about what Dark Energy is -- or which particle makes up Dark Matter, or why neutrinos have a tiny mass -- or what an inflaton is or the properties of the inflaton field. It says even less about what a Sterile Neutrino is. Ironically, sterile neutrinos are alleged to exist from experiment alone.

String Theory is surprisingly successful as a tool for describing the physics around black holes.

relation to Experiment

String theory has no experimental verification to date.

Worse, string theory has weaknesses even when formulating a valid experiment to falsify it. One way to test string theory (in principle) would be to discover lots of magnetic monopoles. If none of them are lighter than the Planck Mass, that would be indirect evidence that String Theory is correct. The problem is that 1. Planck Mass = 0.02 milligrams, and 2. only such monsters would be created during the Big Bang, and even then so rarely that there would be maybe 1 monopole in our local galactic group, if we're lucky. Such an experiment is duly ruled out.

relation to Mathematics

Ben Allanach is post-doctoral research physicist. He has been working at CERN for 20 years. His time there was dedicated almost exclusively to sifting the data from the apparatuses there and determining the signatures of supersymmetric particles. Learn more about him here http://www.damtp.cam.ac.uk/user/bca20/

Dr. Allanach has described string theory as being , quote "fiendishly complex mathematics". People of Mr. Allanach's education level do not toss phrases like that around for lunch. The take-away is that even the post-docs at CERN feel that String Theory is "fiendishly complex".

This ties back in to my earlier claim above, that String Theory (as a day-to-day task) is convincing your colleagues that curled-up 6-dimensional manifolds are perfectly reasonable things to be concentrating one's energies onto. Explaining that this makes sense at all is by itself difficult.

If we consider what Roger Penrose's criticism, it is possible to claim that String Theory is mathematics, period. Even its strongest acolytes admit that it's findings have bled over into pure mathematics, particularly algebraic geometry. Mathematicians work on String theory, coming particularly from backgrounds in topology and geometry. In the case of Mirror Symmetry, the physicists and mathematicians raced against each other to churn out theorems before the other 'side' found their own proof.

relation to Cosmology

We could reasonably expect that a Theory-of-Everything would go a long way in resolving aspects of our cosmos and its origins and the results of the Big Bang. For String Theory, the case is more nuanced and complex.

The universe we inhabit is excruciatingly flat, geometrically. It is also unusually uniform in direction. Cosmologists call this "Flat and isotropic". The flatness and isotropy of our universe is so bizarre that an equally bizarre explanation was floated called Cosmic Inflation. In that scenario a field called an Inflaton Field drove rapid expansion in the early universe. The situation we see now is that there are 3 very large dimensions of space, and string theory chiming in, 6 more dimensions who are all compactified at the Planck scales.

Inflation demands that the obvious 3 Ds were expanded at a rapid pace, but what of the other curled up, compactified Ds? Why were they not also expanded during inflation?

This sounds like another question asked by a high school kid who is not up-to-speed on the nuances. Again, no. This question is highly sound and rational. Like a thorn in its side, this question haunts String Theory, and heaps incredulity onto the existing pile of doubts.

The difference between a flat dimension 10

^{26}(at least) meters and a dimension curled up into a circle at 10

^{-35}meters is excruciating. This becomes even more excruciating if you accept Inflationary Cosmology at face value. In that case the second number is multiplied again by a factor of 3 x 10

^{23}

A Theory Of Everything would explain these two numbers in a natural way.

(3 regular D) 3.0 x 10

^{49}meters

(6 compact D) 1.616 x 10

^{-35}meters

No explanation is forthcoming from String Theory, even while it requires this exact scenario to actually be manifest. In recent years, some have tried to wrangle the mathematics to make this disparity seem more reasonable, but the results are wonky.

Physics, science, or ??

I've already hinted that String Theory may not be even be physics. Worse -- is it even science?

Could String Theory even be something worse, like philosophy? Sabine Hossenfelder has sneered at it calling it "multiversal math magic".

How will String Theory be looked at 100 years from now?

250 years later, Albert Einstein, who was Newton's true successor, was able to seriously suggest that in this vast ocean , all the laws of nature might be reduced to a few fundamental ideas , expressed by a handful of mathematical symbols.