On the way to new fundamental theories in physics, or once again just the result of measurement inaccuracies?
The last big theory in particle physics, which fundamentally advanced physics as a whole, was the integration of the quantum field theory of the weak nuclear force and quantum electrodynamics as two different sides of a single theory by Steven Weinberg, Sheldon Glashow and Abdus Salam in the late 1960s, which physicists today call the „theory of the electroweak force“. It is characterized by the gauge invariance of the SU(2) x U(1) group – an expression known by every particle physicist, some other physicists and mathematicians, and no one else (and not quite so easily explained on texts like these). All particle physics today is characterized by this theory together with the theory of the strong nuclear force (given by the gauge invariance of the eight-dimensional SU(3) group), which was developed around the same time. Correspondingly, the particle theory is characterized by the invariance group SU(3) x SU(2) x U(1). In addition, there is the gravitational force acting as a fourth force in general physics – e.g. in our life. This force is so weak for the elementary particles in comparison to the strong and weak nuclear force as well as the electrodynamics, that it does not play any role in particle physics – only with large bodies like planets and suns it comes strongly into play.
This is, in as brief a form as possible, the essence of physics today: four forces acting on bodies, from suns to elementary particles. Their theories are all more than 50 years old (the theory of general relativity even more than 100 years) and have not been changed by anything until today. One hardly exaggerates saying that they could hardly be further away from a desired elegant overall theory of physics: On the one hand with the inelegant symmetry structure in the standard model for the microcosm (the SU(3) x SU(2) x U(1) – group), on the other hand the symmetry group of the gravitation theory being completely independent of it and in principle not compatible with those in the microcosm. The latter is the symmetry which describes general relativity, the general Lorenz group (which is a local symmetry group like SU(3) and SU(2)), which includes all translations in space and time, i.e. local displacements, rotations and velocity transformations. All these symmetry structures in today’s physics represent on the whole a rather abstruse and anything but elegant unified structure and thus contradict the ideas of the physicists that their discipline must at last be elegant and uniform.
For combining the three forces and thus their symmetries for the elementary particle theory there already exists for many years a whole number of convincing theories, which summarize the symmetries all together into an overall symmetry group. In this context, physicists speak of the „grand unified theory“ (GUT). Among them, the most popular is probably the 24-dimensional SU(5) group. However, this and all others have a fundamental problem: The energy scales on which such a unity is supposed to reveal itself in an experiment are many orders of magnitude higher than what physicists are able to achieve in experimental particle accelerators today, so that GUT theories like the SU(5) group remain untestable. But might these theories not show some of their properties at lower energies? The 24-dimensional invariance transformation matrix of the SU(5) group predicts at certain energies different new particles, which could possibly be detectable already on lower energies, but remained undetected so far.
How then finally still the gravitational force is to be integrated, is still completely unknown. But also here there exist theories, which are still some more orders of magnitude of energy away from us, so that we can not validate – or falsify – them away. Moreover, the ultimate theory requires a unification of quantum theory and general relativity (a „Theory of Everything“ (TOE)), which are fundamentally incompatible in many approaches up to this day.
Thus, nothing could be added to this inelegant particle model in the last five decades. Also numerous important parameters (17 in total) do not receive their values from the model, but have to be determined experimentally, which was in detail the work of two whole generations of particle physicists. At the same time, physicists now know that their particle theory and/or general relativity must be updated as they cannot explain two types of phenomena that are already 99% certain today, specifically the presence of invisible matter in space, so-called dark matter, and the continuing acceleration of the expansion of the universe by a force called dark energy.
But now the excitement of today’s particle physicists is growing all at once. Recently, they have been speculating about whether they may have even detected a fifth fundamental force based on preliminary experimental results. Such a conjecture is not new, already Einstein and his contemporaries speculated about whether there is one (albeit a very different one than the one being speculated about today). Particle physicists are more excited than they have been for many years. Very soon, an accelerator that has just been renovated and cleaned up will be reopened at CERN near Geneva. This one does not even have the highest energy for particle collision that can be achieved at CERN today. But with respect to the measurement accuracy for the determination that is crucial here, it is particularly good. For example, we have already been hearing for about a year about measurements of certain particles that deviate from the Standard Model, albeit only to a very small extent, so that the deviation is difficult to detect. With a not insignificant probability, they could still be simple measurement errors, and therefore burst the dream of physicists to finally go beyond the present particle model. In the meantime, however, the measurements have been made in a few different experiments, including not only CERN but also Fermilab in Chicago. These appear quite consistent in their results. But still the inaccuracy of the measurement lies in the possible range of statistical fluctuation of the measured values. Only more precise measurements will show us whether it is a normal statistical fluctuation or really the desired new type of a physical force in the microcosm.
The communication of the physicists from the American Fermilab in Illinois shows a great excitement. The team there has also found that the particle known as the W-boson (responsible for the weak nuclear force) appears to be somewhat more massive than predicted in the theories. Physicists involved, such as Prof. David Toback, a co-spokesman for the project, even call the measurement „shocking. „If the results are verified by other experiments, the world is going to look different.“ he says. „There has to be a paradigm shift. The hope is that maybe this result is going to be the one that breaks the dam.“
The scientists, however, have found only a tiny difference in the mass of the W-bosons compared to what standard particle physics theory says – not even 0.1%. So it has to be confirmed by further experiments first, like just soon at the even more accurate CERN. Here, too, excitement is high: It could be the first of many new results that could herald the biggest shift in our understanding of the Universe since Einstein’s theories of relativity more than a hundred years ago.
Yet excitement in the physics community should be tempered by caution. Although the Fermilab result represents the most accurate measurement of the mass of W-bosons to date, it conflicts with two of the next most accurate measurements from two different experiments there that were consistent with the Standard Model. So now all eyes are on the particle accelerator near Geneva, the Large Hadron Collider (LHC), which is expected to resume its experiments very soon after a three-year upgrade. Here, too, one hopes for results that will lay the foundation for a new, more comprehensive theory of physics.
However, most particle physicists are still a little cautious, having been at points like this a few times before, only to be disappointed because the Standard Model could in the end explain everything in the end. But secretly we all, including non-particle physicists or even non-physicists, hope that the time has really come when this terribly inelegant Standard Particle Model can be transcended and we can finally explain new physics, perhaps even leading us to a comprehensively extended theory. But whether less than 0.1% changes in the measured values of particle physics are sufficient for this must remain open for the time being. But let us remember: The first prediction of Einstein’s general relativity was not very big either: Under the influence of the sun, the angle of the rays of other stars passing by the sun changes by eight arc seconds, i.e. in about one five-hundredth of a single degree (more precisely 1/450). This has brought Einstein overnight the crown of physics.