Knocking on Heaven's Door - Lisa Randall [132]
Bear in mind that a “theory” is different from a “model.” By the word theory, I don’t mean rough speculations, as in more colloquial usage. The known particles and the known physical laws they obey are components of a theory—a definite set of elements and principles with rules and equations for predicting how the elements interact.
But even when we fully understand a theory and its implications, that same theory can be implemented in many different ways, and these will have different physical consequences in the real world. Models are a way of sampling these possibilities. We combine known physical principles and elements into candidate descriptions of reality.
If you think of a theory as a PowerPoint template, a model would be your particular presentation. The theory allows animations, but the model includes only those you need to make your point. The theory would say to have a title and some bullet points, but the model would contain exactly what you want to convey and will hopefully apply well to the task at hand.
The nature of model building in physics has changed according to the questions physicists have tried to answer. Physics always involves trying to predict the largest number of physical quantities from the smallest number of assumptions, but that doesn’t mean we manage to identify the most fundamental theories right away. Advances in physics are often made even before everything is understood at the most fundamental level.
In the nineteenth century, physicists understood the notions of temperature and pressure and employed them in thermodynamics and engine design long before anyone could explain these ideas at a more fundamental microscopic level as the result of the random motion of large numbers of atoms and molecules. In the early twentieth century, physicists tried to make models to explain mass in terms of electromagnetic energy. Though these models were based on strongly shared beliefs on how those systems worked, those models proved wrong. A little later, Niels Bohr made a model of the atom to explain the emission spectra that people had observed. His model was soon superseded by the more comprehensive theory of quantum mechanics, which absorbed but also improved on Bohr’s core idea.
Model builders today try to determine what lies beyond the Standard Model of particle physics. Although currently referred to as the Standard Model because it has been well tested and is well understood, it was something of a guess as to how known observations might fit together at the time it was developed. Nonetheless, because the Standard Model implied predictions for how to test its premises, experiments could ultimately show it to be correct.
The Standard Model correctly accounts for all observations to date, but physicists are fairly confident that it is not complete. In particular, it leaves open the question of what are the precise particles and interactions—the elements of the Higgs sector—that are responsible for the masses of elementary particles and why it is that the particles in that sector have the particular masses that they do. Models that go beyond the Standard Model illuminate deeper potential interconnections and relationships that might address these questions. They involve specific choices of fundamental assumptions and physical concepts, as well as the distance or energy scales at which they might apply.
Much of my current research involves thinking about new models, as well as novel or more detailed search strategies that would otherwise miss new phenomena. I think about the models I originated but the full range of other possibilities as well. Particle physicists know the types of elements and rules that could be involved, such as particles, forces, and allowed interactions. But we don’t know precisely which of these ingredients enters the recipe for reality. By applying known theoretical ingredients, we attempt to identify the potentially simple underlying ideas that enter into what is an ultimately complex theory.
As important, models provide targets for experimental exploration, and suggestions