Here鈥檚 an anomaly as old as your fridge magnet: how come gravity is so weak?
Hang on, you might say: gravity is strong enough to keep my feet on the ground, and no space agency firing craft into orbit would ever describe gravity as weak. But the mystery for physicists is why that force is so puny compared with the electromagnetic force that it doesn鈥檛 rip that magnet off your refrigerator 鈥� we鈥檙e talking about the pull of an entire planet, after all.
This mismatch between gravity鈥檚 strength and that of the other forces of nature goes by the name of the hierarchy problem. Because, uniquely, gravity is not yet described by a quantum theory, it鈥檚 not easy to quantify the problem鈥檚 size, but one measure is the Planck mass, a quantity that gets bigger the weaker gravity is. In our cosmos the Planck mass is huge. It is some 10 quadrillion times bigger than the mass of the W and Z bosons that define the strength of the weak nuclear force, for example. In fact, it is huge compared with all masses that pop up in the standard model. 鈥淭he question is not why the Planck mass is big; the question is why it is big compared to the masses of all the known particles,鈥� says theorist of Harvard University. 鈥淭he puzzle is something you can phrase either as the Planck mass being large or particle masses being small.鈥�
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Explanations following the first route often invoke the idea of 鈥渇ine-tuning鈥�: that we just happen to live in an unnatural part of the universe where gravity is just right, so atoms, stars, planets and people have come to exist. Or they propose large extra dimensions of space into which gravity 鈥渓eaks鈥�, so it appears diluted to us.
Alternatively, we can focus on the Higgs field, which generates particle masses. The low mass of the Higgs boson, discovered in 2012, indicates this field is not particularly strong, keeping all particle masses on the low side. Theories such as supersymmetry and technicolor focus on as-yet-undiscovered particles or forces whose effect is to restrain the Higgs field to the observed strength of almost 鈥� but not quite 鈥� zero.
Experiments aren鈥檛 helping decide between these options as yet. Supersymmetry 鈥� or indeed anything new besides the Higgs boson 鈥� has so far failed to make its presence felt in the particle smashes going on at the Large Hadron Collider. 鈥淣ot finding anything else yet leaves us at sea,鈥� says Strassler.
The hope is that future runs of the LHC, now operating at maximum energy and generating more particle collisions than ever, could give us more of a clue. That鈥檚 why the recent appearance of blips in LHC data, indicating the existence of a particle six times as massive as the Higgs boson and not predicted by the standard model, has made many a physicist鈥檚 heart beat swifter. But it is still too early to say whether these blips will persist 鈥� or what, if any, solution to the problem of gravity they might support.
Read more: 鈥�The 6 biggest glitches in physics鈥�
This article appeared in print under the headline 鈥淭hat鈥檚 odd鈥� The hierarchy problem鈥�
