Josh asks

 String theory. What the very fuck is it.

I was going to chide you for swearing on this blog, Josh; it’s a family space and a small child could happen by and have their brain infected with that nasty language. Then I remembered that I fucking hate string theory – I truly detest it – and that a sizeable percentage of the next thousand words I write describing it are going to be swear words. On this one occasion, therefore, you are forgiven.

String theory is what happens when theoretical mathematicians have too much time on their hands and decide they want to make a contribution to the world of physics. It arises from a fundamental incompatibility between our two main theories of the universe, quantum mechanics and gravity. Quantum mechanics is the physics of the very small and encompasses three of the four fundamental forces: strong nuclear, weak nuclear, and electromagnetic. General relativity is the physics of the very large and deals with the fourth fundamental force, gravity.

If the universe were a well-ordered, logical place these four forces would all be different aspects of a single unified force that could be described by a single set of rules: this is the fabled Theory of Everything. Unfortunately as things stand they really can’t. The three quantum forces coexist happily in something called the Standard Model which is essentially a basic physics toolkit for understanding the universe; if you like, we can say that we can describe those three forces using the same physical language even if the specific mechanisms governing each are different. Wee have successfully combined the weak nuclear and electromagnetic theories into electroweak theory, and while we’re some way off managing to combine the third quantum force – the strong nuclear – with electroweak it’s not something that’s fundamentally incompatible with the theory. The same, alas, cannot be said of general relativity. The Standard Model doesn’t even attempt to explain gravity or general relativity; all attempts to do so using quantum field theory have broken down long before reaching the Planck scale where quantum gravity becomes relevant¹, which is where they’d need to go in order to be compatible with both general relativity and the Standard Model².

Why is the Planck scale important? Well, we have general relativity for describing very heavy stuff. We have quantum mechanics for describing very small stuff. The Planck scale, though, is a region of physics where the two blur into each other; very heavy, very small stuff. For why this is wacky see the Wikipedia articles on Planck scales and Planck particles (this is actually a joke, I barely understand those articles and I don’t expect you guys to either) but neither general relativity nor quantum mechanics can predict what the hell is going on at these scales indicating that those theories are incomplete in some way. The reason they’ve made it this far is that the things described by quantum mechanics and the things described by general relatively are, on the whole, completely separate worlds with little direct interaction. We can use quantum mechanics to model particle behaviour because gravity is mostly not important at quantum mechanical scales. We can use general relativity to model the curvature of space time because quantum mechanical effects are mostly not relevant at relativistic scales. But saying “Well, these theories work for 99% of situations so they’re good enough” really doesn’t fly in science, hence the search for a Theory of Everything, and hence string theory.

String theory states… actually you know what, I’m not actually going to paraphrase this Wikipedia article, I’m just going to copy and paste from the source:

String theory posits that the electrons and quarks within an atom are not 0-dimensional objects, but made up of 1-dimensional strings. These strings can oscillate, giving the observed particles their flavor, charge, mass and spin. Among the modes of oscillation of the string is a massless, spin-two state—a graviton. The existence of this graviton state and the fact that the equations describing string theory include Einstein’s equations for general relativity mean that string theory is a quantum theory of gravity. Since string theory is widely believed^[7] to be mathematically consistent, many hope that it fully describes our universe, making it a theory of everything.

So everything is made up of strings that vibrate giving them the appearance of particles, right? I don’t pretend to understand anything more than the very broadest details of string theory so I can’t explain it in detail. What I can do is explain why string theory is a crock of shit.

The modern scientific method is based around a process of formulating a theory, testing it experimentally, publishing the results and then letting other people have a go to see if they can do the same thing. The experimental part is very important; we do not simply test something once and then go “Welp, guess that proves it!”, but instead we test it constantly and repeatedly to ensure that it works in all possible situations. Science works by negative proof: it is never possible to definitively prove a scientific theory is true, but you can prove it is false by turning up results that are fundamentally incompatible with the basic tenets of the theory. Then you toss it out and come up with a new one that explains both the old results of the old paradigm and the contradictory results that caused you to get rid of it in the first place. This ensures that the theories we have are as robust as they can possibly be since they have to withstand constant experimental assault.

The scientific method has worked very well, even for theories that we can’t test directly. General relativity predicted phenomena gravitational lensing and time dilation before they were observed, while the Standard Model predicted the existence of several high-energy particles that were subsequently found in particle accelerators. We know these theories are incomplete, but we keep them around because they’ve otherwise had unparalleled success in explaining the things that we see going on around us. Quantum mechanics is ridiculously counterintuitive and hurts my head to even think about, but it too has predicted many wacky experimental outcomes that were subsequently confirmed in the laboratory so I have to grudgingly concede that it’s a kosher scientific theory. Not so with string theory, however. For an explanation as to why, I’ll turn things over to noted womaniser, self-aggrandising charlatan and Nobel Laureate Richard Feynman.

I don’t like that they’re not calculating anything. I don’t like that they don’t check their ideas. I don’t like that for anything that disagrees with a n experiment, they cook up an explanation—a fix-up to say, “Well, it might be true.” For example, the theory requires ten dimensions. Well, maybe there’s a way of wrapping up six of the dimensions. Yes, that’s all possible mathematically, but why not seven? When they write their equation, the equation should decide how many of these things get wrapped up, not the desire to agree with experiment. In other words, there’s no reason whatsoever in superstring theory that it isn’t eight out of the ten dimensions that get wrapped up and that the result is only two dimensions, which would be completely in disagreement with experience. So the fact that it might disagree with experience is very tenuous, it doesn’t produce anything; it has to be excused most of the time. It doesn’t look right.

In a nutshell, every time an experimental result confirms a prediction made by string theory, string theorists claim it as a great success for string theory. Every time an experimental result doesn’t confirm a prediction made by string theory, string theorists simply rewrite string theory. It’s a classic case of post hoc ergo propter hoc. Nobody is ever going to be able to peer all the way down to the Planck length to see if these strings actually exist or not so we can’t ever say string theory is definitively not true, and I would argue that if a phenomena is not experimentally verifiable then the existence of that phenomena is ultimately irrelevant, scientifically speaking. Nobody takes the existence of God seriously as a scientific theory because it’s not something that can ever be proven or disproven, so why should I treat string theory any differently?

1. The Planck scale is a very small scale at which quantum effects dominate, and this wouldn’t be a problem except… well, you’ve heard of micro-black holes, right? Imagine a black hole so small that its Schwarzchild radius (or event horizon) is the same size as the Planck scale. This can only be probed by photons whose Compton wavelength is also equivalent to the Planck scale. However, if they had a wavelength this small their mass-energy would be so great that they would also form micro-black holes, thwarting our theoretical attempt to resolve the Planck scale. General relativity cannot describe these micro-black holes because they lie within the regime governed by quantum mechanics, but quantum mechanics can’t do it either because gravity keeps getting in the way, hence the need for a theory of quantum gravity.

2. I don’t even pretend to understand this but: dealing with quantum field theory is, mathematically, incredibly gnarly. You tend to get a lot of inconvenient terms popping out of your equations that usually can’t be dealt with using conventional mathematics, like infinity. A system of techniques have been developed to purge quantum mechanical equations of these horrible infinities, and these techniques are collectively referred to as “renormalization”. However, they do not work when applied to quantum theories of gravity. Infinities work their way into the maths and cannot be removed, and so these theories break down before they can be applied to scales where a theory of quantum gravity would be useful. Makes me glad I never studied quantum mechanics in a big way, really.

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