Humans have long gazed at the starry sky, wondering how it works, what it means. We've probably pondered such matters since at least the evolution of the genus Homo, with its enlarged frontal cortex, 2.5 million years ago. This thirst to understand drives science and religion and is a big part of what makes us human. Today we know a lot about the stars, but we certainly haven't stopped pondering.
In this spirit, Fermilab near Chicago announced last month that an important detail known as the "g-factor of the muon" has been painstakingly measured, with surprising results that suggest possible new physics reaching beyond the "Standard Model" that has dominated subatomic physics for 50 years. This could shed light on such mind-boggling mysteries as dark matter, dark energy, quantum gravity, unification of all forces and the Big Bang.
To unpack Fermilab's finding, one needs some understanding of how quantum physics describes the universe. Although it was formerly said that everything is made of atoms, we know today that atoms comprise less than just 5 percent of the universe's energy. Contemporary quantum physics tells us everything is made of several "fields" that fill all space. You are probably familiar with two of these: the magnetic field surrounding every magnet and the gravitational field surrounding every planet and star. The magnetic field is one feature of the more general universal "electromagnetic field" that has been known since 1830 -- long before quantum physics was discovered. Each of these several fields is made of energy that is bundled together to make "particles" such as the electrons, protons, neutrons, and atoms that fill the universe in large numbers.
One of these fields, the "muon field," is more esoteric. Its particular kind of bundled particle, called a "muon," behaves exactly like an electron but is about 200 times more massive. Furthermore, muons are unstable: Once they are created from other forms of energy, they vanish in 2 millionths of a second by spontaneously transforming into other things; this makes them tricky to observe.
Muons have an internal magnet that acts like a bar magnet. When placed in a magnetic field, the bar magnet wants (or "tends") to rotate into alignment with the field, like a compass needle rotates to point north. But every muon also spins rapidly, and spinning objects tend to maintain the orientation of their spin axis -- think of a fast-spinning top that remains upright. The result of these two tendencies is that the spin axis rotates--"wobbles"--around the direction of the field. The strength of the muon's magnet, together with its rate of wobbling, determine a number called the muon "g-factor."
In a simple calculation based only on the electromagnetic field, this g-factor is exactly 2. But physics knows of many other fields, and these also influence the g-factor. The deviation from exactly 2 can be calculated quite precisely from quantum theory by incorporating the effects of these other fields. The result of this enormously complex calculation is g = 2.00233183620.
However, when g was measured at Fermilab, the result was 2.00233184122. The last 4 digits differ. This difference between theory and experiment is extremely small, but could be extremely significant. The experiment will continue running at Fermilab and physicists will continue improving their theoretical calculations, so a firmer conclusion will probably be reached within a few years.
Meanwhile, there is a real possibility that the difference lies in mysterious and fascinating "new physics" reaching beyond the Standard Model that has guided quantum physics for decades. One candidate is dark matter, which is known to comprise 27 percent of the universe's energy. Another candidate is dark energy, which comprises 68 percent! We know almost nothing about these phenomena beyond the fact that they exist. Surprisingly, gravity is another obvious phenomenon lying outside the Standard Model. Of course, physics knows plenty about gravity, but physicists have not yet been able to create a quantum theory of the gravitational force, so it's unknown whether gravity could have something to do with the g-factor discrepancy.
The Standard Model does incorporate the three known fundamental forces other than gravity: electromagnetic force, strong nuclear force, and weak nuclear force. It's possible that the g-factor discrepancy arises because of a fifth fundamental force, currently entirely unknown to physics, that perhaps plays a role in dark matter or dark energy.
Stay tuned. This story will continue. It's a great privilige to ponder such ideas, and I'll probably want to write about the sequel provided I'm still alive and pondering.