Simplicity
We live at a remarkable moment in history. Our scientific instruments have allowed us to see the far reaches of the cosmos and to study the tiniest particles. In both cases, they have revealed a surprising simplicity, at odds with the most popular theoretical paradigms. I believe this simplicity to be a clue to a new scientific principle, whose discovery will represent the next revolution in physics and in our understanding of the universe.
It is not without irony that at the very moment the observational situation is clearing so beautifully, the theoretical scene has become overwhelmingly confused. Not only are the most popular models very complicated and contrived, they are also being steadily ruled out by the new data. Some physicists appeal to a “multiverse” in which all possible laws of physics are realized somewhere since then, they hope, there would at least be one region like ours. It seems more likely to me that the wonderful new data is pointing us in the opposite direction. The cosmos isn’t wild and unpredictable, it is incredibly regular. In its fundamental aspects, it may be as simple as an atom and, in time, every bit as possible to understand.
Our most powerful-ever microscope, the Large Hadron Collider (LHC), has just found the Higgs boson. This particle is the basic quantum of the Higgs field, a medium which pervades space and endows particles with mass and properties like electric charge. As fundamental to our understanding of particle physics as the Higgs field is, it is equally important to our understanding of cosmology. It makes a big contribution to the energy in empty space, the so-called dark energy, which astronomical observations reveal to be a weirdly tiny, yet positive, number. Furthermore, according to the LHC measurements and the standard model of particle physics, the Higgs field is delicately poised on the threshold of instability in today’s universe.
The discovery of the Higgs boson was a triumph for the theory of quantum fields, the amalgamation of quantum mechanics and relativity which dominated 20th century physics. But quantum field theory has great trouble explaining the mass of the Higgs boson and the energy in empty space. In both cases, the problem is essentially the same. The quantized vibrations of the known fields and particles become wild on small scales, contributing large corrections to the Higgs boson mass and to the dark energy density and generally giving them values much greater than those we observe.
To overcome these problems, many theorists have postulated new particles, whose effects would almost precisely cancel those of all the known particles, “protecting” the mass of the Higgs boson and the value of the dark energy from quantum effects. But the LHC has looked for these extra partner particles and, so far, failed to find them. It seems that nature has found a simpler way to tame quantum phenomena on short distances, in a manner which we have yet to fathom.
Meanwhile, our most powerful-ever telescope, the Planck satellite, has scanned the universe on the largest visible scales. What it has revealed is equally surprising. The whole shebang can be quantified with just six numbers: the age and temperature of the cosmos today; the density of the dark energy and the dark matter (both mysterious, but simple to characterize); and the strength, and slight dependence on scale, of the tiny initial variations in the density of matter from place to place as it emerged from the big bang. None of the complications, like gravitational waves or the more involved density patterns expected in many models, appear to be there. Again, nature has found a simpler way to work than we can currently understand.
The largest scale in physics—the Hubble length—is defined by the dark energy. By accelerating the expansion of the cosmos, the dark energy carries distant matter away from us and sets a limit to what we will ultimately see. The smallest scale in physics is the Planck length, the miniscule wavelength of photons so energetic that two of them will form a black hole. While exploring physics down to the Planck length is beyond the capabilities of any conceivable collider, the universe itself probed this scale in its earliest moments. So the simple structure of the cosmos is likely to be an indication that the laws of physics become simple at this extreme.
All the complexity in the world, including stars, planets and life, apparently resides in the “messy middle.” It is a striking fact that the geometric mean of the Hubble and Planck lengths is the size of a living cell: the scale on which we live, where nature is at her most complex.
What is exciting about this picture is that it requires a new kind of theory, one which is simple at both the smallest and largest scales, and very early and very late cosmological times so that it is capable of explaining these properties of our world. In fact, there are more detailed hints from both theory and data that, at these extremes, the laws of physics should become independent of scale. Such a theory won’t be concerned with kilograms, meters or seconds, only with information and its relations. It will be a unified theory, not only of all the forces and particles but of the universe as a whole.
