Dr. Christopher Bird

Let me begin by saying that I believe that the popularization of science in general and physics in particular is a very good thing. The more educated a society becomes, the more it is able to function and advance. And in the internet age in which false information is able to spread so rapidly, it is more important than ever for trained, professional scientists such as myself to promote skepticism and rational thought, and to try to communicate what is accepted scientific fact from superstition and myth.

Unfortunately popular physics over the past twenty-five years has created one very nasty and pervasive myth - that string theory is the final theory of everything and that physics is nearing its end. Even though the scientific community has been questioning such claims for nearly as long as string theory has been around, there is still a belief in society that string theory has been confirmed and that all of modern physics is based on it. This was driven home to me a few days ago when talking with some old classmates of mine who were surprised that any physicist is working on anything else.

And so for that reason I feel it necessary to give a brief overview of the major problems with string theory. This is not meant as a criticism of the serious research into string theory, and nor is it meant to minimize the advances and the new ideas created by the string theory community. Rather it is meant to demonstrate to the lay community why string theory is not quite the ideal, perfect theory that it has been promoted as by some authors.

First off there are several requirements of string theory that have not been detected in nature. The symmetries required to make the theory work can only exist in 10,11 or 26 dimensions depending on the exact model, whereas we exist in four dimensions. That means that at least six dimensions are hidden away. And they are not in the shape of a simple sphere or torus, but need to be in the form of very complicated shapes. Furthermore string theory cannot exist unless supersymmetry also exists. This means that every particle that we know about has at least one partner in nature which has the exact same mass and properties (but a different amount of spin). Unfortunately not one of these particles has ever been detected, which casts considerable doubt on the validity of both supersymmetry and string theory.

However the existence of new types of physics that have not been confirmed experimentally is not a major problem for the theory. The bulk of research in theoretical physics is based on studying what could exist without yet being detected in nature. Higher dimensional theories have been around for over a century, and supersymmetry could be broken so that the extra particles are either far heavier than we thought or somehow do not interact with our detectors. These are both active areas of research outside of string theory, and their necessity in string theory is not an insurmountable obstacle.

Next we have the claims of quantum gravity. As I have written before, quantum mechanics and general relativity are the two most successful theories in the history of science. They have been separately tested in experiments to high levels of precision, and as yet no faults have been found in either theory. Unfortunately these two theories are incompatible with each other, so one or both will have to be modified at higher energies and smaller spatial scales. This is the mystery of quantum gravity.

One of the early successes of string theory was the claim in the mid-1980s that string theory naturally included quantum gravity. This claim caused large numbers of graduate students and young faculty members to shift their research into string theory in the hopes of solving this problem. It became the theoretical physics equivalent of a gold rush. And it turned out to be a false claim.

As it turned out there were two problems with the string theory version of quantum gravity. What the researchers had found was called a perturbative quantum gravity. In order to perform calculations, classical general relativity is used to determine the large scale properties and structure of spacetime in the region. This creates classical gravitational forces that are not quantized. Once that is known, then perturbative quantum gravity can be used to add tiny, subatomic scale corrections to the gravitational interactions, but these are only corrections.

At the time it was believed that reproducing general relativity would follow soon after, but in the past thirty years no one has been able to make that next step. Later theories such as loop quantum gravity and spinfoams were able to "predict" the equations of general relativity, but so far string theory has failed in this regard. And without that, it cannot be called a true theory of quantum gravity.

And adding to the problems with string quantum gravity, some authors later claimed that the original claims of a perturbative quantum gravity were also incorrect, and that even this weaker form of quantum gravity actually fails when more complicated calculations are included. (This point is disputed by some in the string theory community, but as yet has not been disproven)

However the most important problem with string theory is that it is not falsifiable, and therefore is simply not a scientific theory. The scientific method is very simple - a scientist makes a hypothesis or a new theory, then uses it to make a prediction for how it will make nature different from the accepted models, and then build an experiment to search for this difference. It might be large groups of scientists, and it might take decades to either prove or disprove the theory, but ultimately any scientific theory must include a way to disprove it. And string theory does not. String theory involves very interesting mathematics, and has generated some very interesting new ideas in theoretical physics, but so far no one has found a way to definitively test its validity. And that means that it is not currently a scientific theory.

String theory predicts higher dimensions, but makes no claim as to how small they are and so this cannot be used to falsify it. It predicts new particles, but there are an almost infinite number of variations on the theory so that if the particles are not found, there are always more theories that do not include those particular particles. The same is true with the fundamental constants of nature - each variation on the theory has a different set of parameters, so these cannot tell us anything about the underlying theory either. (The closest string theory came to a prediction was in the late 1990s, when some of its leaders showed that all string theories contain a positive cosmological constant. A short time later astromers proved that the opposite was true in nature, and the string theorists quickly recanted their claim)

Once again I must make it clear that this is not an attack on string theory or on the serious researchers who have devoted their lives to developing it. It contains very interesting new ideas in both mathematics and theoretical physics, and it is certainly worth developing. But for the moment it is just an interesting model that tells us nothing about the real world. Maybe one day it will, maybe it won't. And it is not even the only candidate for the final theory of everything, just the most well known of them all. 

So until some genius discovers a property of all string theories which can be tested experimentally, or even better some new experiment finds definitive evidence of string theory, it will remain nothing more than an interesting academic excursion. 

It is most certainly not the final theory of everything, at least not in its current form, and despite what some in the media would have you believe, the future of physics is still wide open!