Saturday, January 13, 2018

The Reports of the Death of String Theory may be Greatly Exaggerated

In a recent podcast, Dr. William Lane Craig discusses my blog post about the book The Grand Design by Stephen Hawking and Leonard Mlodinow. In his remarks, Dr. Craig says that he is surprised by Hawking's reference to String Theory because String Theory seems to have some fatal flaws. I have heard from others that String Theory has not lived up to the promise it seemed to show about 10 or 20 years ago. In this post, I will describe what String Theory is, why some people think it may be in trouble, and some theological implications of String Theory.

String Theory is a theoretical idea in physics that proposes that the most fundamental objects in the universe are one-dimensional objects called strings. In String Theory, the fundamental particles that we currently know make up the structure of the universe including leptons, quarks, and bosons, are composed of one-dimensional objects that are described as vibrating strings. Just as different vibrational modes of a guitar string will give different musical tones, so the different vibrating modes of the strings give rise to different particles.

By the mid-1990's, five independent string theories were developed, and scientists assumed that only one of the five would turn out to be correct. However, it was demonstrated that the five theories could be describing the same phenomena from different perspectives, leading to a single overarching theory called M-theory where the meaning of the “M” is undefined but has had various interpretations including “membrane,” “mother,” and “monster.”

String Theory holds tremendous promise for solving certain known problems in physics. One of the most troubling puzzles in physics is that we understand how objects behave at very small scales, the size of atoms or smaller using quantum mechanics, and we understand how objects interact at large scales using Einstein’s General Theory of Relativity, which describes gravity. But when we try to use the General Theory of Relativity at very small scales, the equations do not work. We cannot develop a consistent and meaningful quantum theory of gravity. String Theory can reconcile quantum mechanics with General Relativity and produces a calculable quantum theory of gravity in a way that naturally produces a graviton, the proposed quantum carrier of the gravitational force. This fact alone causes many scientists to believe that String Theory is most likely true.

There are aspects of String Theory that are quite bizarre. For example, string theories require that our universe be composed of 10 or 11 dimensions, not just the three dimensions we are familiar with, length, height, and breadth, and the fourth dimension of time, but rather many more dimensions. We don’t experience these extra 6 or 7 dimensions, so where are they? One idea is that the dimensions are “curled up” or “compactified.” An illustration of what is meant by compact dimensions would be an object like a hanging power cable. If you are far from the cable, it appears that the cable is simply a line, which is a one-dimensional object. The length of the cable can be observed but not its thickness or diameter. However, an ant walking on the cable would be able to traverse not only its one-dimensional length, but would also be able to walk around the circumference of the cable. While the very small ant experiences the cable as a two-dimensional object, someone much larger than the cable experiences it is as only a one-dimensional object because the compact diameter of the cable is too small to observe. In String Theory, it is possible that the compact dimensions are as small as 10-35 meters in size. If so, it would probably be impossible for human observers to ever experience the compact 6 or 7 extra dimensions.

String Theory has other unusual implications. Within String Theory there are as many as 10500 possible, so called, landscapes. Each of these landscapes could in theory produce another universe, so it implies there may be many other universes other than ours, collectively called a Multiverse. If so, it is likely that the laws of physics would be different in every one of these different universes.

All currently proposed string theories are classified as superstring theories. That mean they require that every fundamental particle known in the universe has a partner particle that has not yet been discovered. These partner particles are described as supersymmetric particles and the theory that describes them is called supersymmetry or simply SUSY. So within SUSY, the electron would have a partner particle not yet discovered that we call a selectron. The selectron would have a property called spin that is different than that of the standard electron. A quark would have a partner particle called a squark with a different spin. Now this may sound crazy, but SUSY itself also has the potential of solving a few challenging problems in physics and so it is quite attractive.

About 10 years ago some scientists, including some of those who had helped develop String Theory began to criticize the theory for a number of reasons. It is unclear whether or not the theory can even be classified as science since it may not be experimentally verifiable. The strings themselves may be so small that they cannot be directly detected unless we build a particle accelerator about the size of our galaxy. The fact that String Theory cannot determine a unique landscape means that it doesn't make unambiguous predictions. We just happen to be in one of the very few universes that can support life out of the nearly infinite number of possible universes. Our laws of physics are random. As John Horgan wrote in Scientific American about String Theory, "Of course, a theory that predicts everything really doesn't predict anything, and hence isn’t a theory at all."1 The extra dimensions of string theory are most likely so small they can not be detected.

One of the most lethal blows to String Theory is the lack of any hint of SUSY particles at the CERN Large Hadron Collider (LHC). String theory requires that supersymmetric particles exist and almost every theory of SUSY predicted that the LHC would see SUSY particles shortly after it began collecting data, around 2010. If not then, surely SUSY would show itself with the higher energies the LHC produced beginning in 2015. However, there have been no hints of any SUSY particles, or any other predictions from sting theory, like finding extra dimensions of space.

Do all of these problems require us to write the obituary for String Theory? I don't think so.  Let's start with the SUSY problem. One of my colleagues at the University of Oklahoma, Dr. Howie Baer, has been promoting a version of SUSY that is very challenging to find at the LHC and may take about 15 more years of data from the LHC to discover. In fact, his version may require a new type of collider, called a linear collider, to really observe. Dr. Baer has consistently claimed that SUSY will be discovered, but not in the easy straightforward ways most physicists expected. With his theory, SUSY is still alive and well, and string theory along with it. And of course Dr. Baer's ideas are just one of the proposals that allow SUSY to remain a viable, testable theory.

Despite the other problems of String Theory including its many possible landscapes and inability to detect strings, it still has the great potential of combining quantum mechanics and General Relativity. It may have promised too much too fast, but I think it is far from being on life support. I don't know if String Theory will ever be confirmed or falsified so I'm not convinced it falls into the realm of science...yet. I think that with all of its promise we should continue to develop experiments that may be able to test its predictions. Until there is solid evidence for or against string theory it remains a promising idea.

Finally, let's try to connect String Theory to some ideas about God. Suppose that in the future, experiments were to show that String Theory is true and the universe is composed of 11 dimensions. Would there be any theological implications? If this universe had 11 dimensions it would mean that the creator of the universe is minimally an 11 dimensional being. Now we know God is much more than simply an 11 dimensional being, but even that description of God would put God in the realm of a being who can access more dimensions than us. In two of my recent posts, which you can find here and here, I described some of the characteristics of a multi-dimensional being, like how such a being can easily perform miracles, can be omnipresent in our universe, and is incomprehensible in his fullness to humans.

There have been many times in history when experimental discoveries have radically changed our understanding of the universe in totally unexpected ways. It is still possible that some day that could happen with String Theory.



  1. Hi Dr Strauss,
    Happy one year anniversary. I know this is off topic, but since it deals with science I think that you, none-the-less, may find the following video and comment interesting:
    Darwinian Evolution vs Mathematics - video

    Here is a bit more information on the relationship between nature and the ‘platonic world’ of mathematics

  2. You say “it is likely that the laws of physics would be different in every one of these different universes”.
    What laws do you think could vary within string or any other theory?
    Is it the mass or charge of the fundamental particles?
    Would the conservation laws such as conservation of charge, spin, barion number, momentum and energy vary?
    Many worlds theory appears to violate conservation of energy as universes are constantly duplicated

    1. How the laws would vary depend on who you talk with. Some people think that certain laws of quantum field theory would be universal. Almost everyone think that things like mass and charge and constants would all be different. Many think that certain of the conservation laws would be retained. Of course, only a unique verifiable mathematical theory will actually make unique predictions. String theory as a whole is not at that point.

  3. In reading Roger Penrose "Road to the Reality" and particularly the chapters on String Theory and its siblings (as best a mere mortal can comprehend) I came away with the impression that Penrose is rather unimpressed by the theory(s)..even so far as to deny it's ability to unify QM and GR as to gravitation.

    In my youth I was privileged to take a grad course from Dr. Carl Solloway (JPL Senior Mathematician) on Pertubation Theory. If I recall correctly any Universe/System that is stable as to its fundamental laws and physical makeup can be slightly perturbed and it will return to its status. But it is difficult to imagine that all those supposed other universes generated by Guth's expansion, ad finitum, requiring all sort of "preturbed" variations of the laws and parameters of the only universe we know exists would be stable in the slightest sense have indicated the very very small allowable variations or perturbations in the standard model for instance that permit our universe to exist. I'm with Penrose.

  4. "All currently proposed string theories are classified as superstring theories." Consider the MILGROM DENIAL HYPOTHESIS: The main problem with string theory is that string theorists fail to realize that Milgrom is the Kepler of contemporary cosmology. Please google "kroupa milgrom", "mcgaugh milgrom", "sanders milgrom", and "scarpa milgrom". I have suggested that string theory with the finite nature hypothesis implies MOND and no supersymmetry, while string theory with the infinite nature hypothesis implies supersymmetry and no MOND. Consider the following:
    Photons and gluons cannot escape from the universe in which they are located. Gravitons travel at the speed of light on average. A statistically significant few gravitons travel slower than the speed of light. These slow gravitons cause the Fernández-Rañada-Milgrom effect (replace the -1/2 in the standard form of Einstein's field equations by -1/2 + dark-matter-compensation-constant where this constant is approximately 3.9±.4 * 10^-5). A statistically few gravitons travel faster than the speed of light and escape from the boundary of the multiverse into the interior of the multiverse. These fast gravitons cause the nonzero cosmological constant and the inflaton field. Electromagnetic radiation from the inflaton field shows up as the space roar (Fredkin-Wolfram analogue of Cherenkov radiation).
    space roar,
    If the fast gravitons never escaped from the universe in which they are located, then the slow gravitons and the fast gravitons would average out, yielding Einstein’s field equations with cosmological constant = zero and dark-matter-compensation-constant = zero. I CONJECTURE THAT THE 4 ULTRA-PRECISE GYROSCOPES USED BY GRAVITY PROBE B WORKED CORRECTLY AND CONFIRMED THE PRECEDING BIZARRE THEORY.