While possible, this rock formation is not likely to arise in nature. |

In conversations among physicists, sometimes you’ll hear someone say, “This theory fits all the observed data, but it’s too fine-tuned.” When I first heard that, it struck me as unscientific because any hypothesis that does not contradict experimental measurement is, in principle, a possibility. “Fine tuning” refers to extreme cancellations in a proposed explanation for something. For instance, a river might be lukewarm because it is the confluence of a boiling geyser and an ice slurry, but this leaves unanswered the question of why there was exactly enough boiling water to balance the ice. Near-exact cancellations are possible, but unlikely.

While we cannot reject possible theories just because they sound unlikely, a finely tuned theory is probably incomplete and should be investigated further. As another example, consider a car that breaks down in every possible way at the same time: The head gasket blows, the engine seizes up, and windshield wiper fluid squirts everywhere. One could say that each of these components has a 10-year lifespan and that it is a coincidence that they all broke at the same instant. However, there might be a deeper explanation that links them — perhaps the head gasket caused a coolant leak that caused the engine to overheat and seize, which boiled the windshield wiper fluid and made it spray. The first explanation isn’t exactly wrong, but it is missing an important insight. In the same way, a finely tuned theory like the Standard Model of particle physics isn’t exactly wrong, but it is probably incomplete.

The Standard Model is finely tuned in several ways, but the most significant is the fact that it does not explain why gravity is so much weaker than the other forces. Here’s a more technical statement of the problem: Why is the Higgs mass (now known to be 125 GeV) about 100 quintillion times less than the characteristic energy scale of quantum gravity? When physicists tried to predict the Higgs mass mathematically, they found that the largest terms in the equation are due to ultra-high energy effects, the regime of quantum gravity. Since we know very little about quantum gravity, the equation could not be solved and the Higgs mass could not be predicted. However, it is highly suspicious that some combination of these unknown yet ultra-high energy terms result in something that is known to be 100 quintillion times smaller.

Many potential explanations have been proposed, but nothing is yet proven. One long-time favorite is supersymmetry, in which normal particles and supersymmetric particles contribute to the Higgs mass with opposite sign, resulting in a near-perfect cancellation naturally. Much like the mystery of the car breakdown, the coincidence could be explained by revealing an underlying connection, if only we can discover what that connection is.

—*Jim Pivarski*