Tuesday, March 29, 2011

What we don't know

The loose ends in science are at least as interesting to me as what we (think we) know.

Take the Higgs boson, the elusive quarry of the biggest experiment in science: the Large Hadron Collider (LHC). The Higgs boson is the remaining particle predicted by the highly successful standard model of nuclear physics -- and yet to be detected.

A very weird thing, the Higgs boson, so weird/elusive/mysterious that it's developed the nickname of "The God Particle," from the book of the same name. The Higgs boson is said to give the attribute of mass to some subatomic particles.

Because the LHC's latest round of experiments has yet to detect the Higgs boson, its mass is statistically likely to be above 207 GeV / c^2. (GeV = giga [billion] electron volts.) That division by c squared, where c = the speed of light in a vacuum, comes from the mass/energy equivalence relationship, the famous E = mc^2.

For reference, a proton's mass is about 938 MeV/c^2 (MeV = million electron volts). A neutron is slightly more massive, at 940 MeV/c^2. An electron's mass is a mere 511 KeV/c^2 (KeV = thousand electron volts). These particles must get their mass from other mechanisms (e.g., from the so-called strong force) and/or only partially from the Higgs. As I say, weird.

How weird is it that physicists don't know the mass of the particle for which they're looking? Here's the not-quite secret of the standard model: it relies upon about twenty parameters that are, as far as present theory knows, completely arbitrary. These parameters are determined by measurement. So in the latest round of LHC experiments, physicists have just about convinced themselves that the Higgs boson is not to be found within the mass range of 144 - 207 GeV/c^2.

Imagine a detective saying, "I've about convinced myself that the suspect probably doesn't weigh between 144 and 207 pounds." It wouldn't sound like said detective knew who he was after, now would it?

In The Trouble with Physics (highly recommended, by the way), physicist Lee Smolin names five grand challenges that have stymied theoreticians for decades. Among the five is "Explain how the values of the free constants in the standard model are chosen in nature." As in: why do particles have the masses that they do, and why are forces between particles the strengths that they are? Basic stuff that we can measure but in no way understand.

Don't get me wrong. I think the standard model has been a remarkable bit of experimental science. It has guided many searches for subatomic detail. But if we don't soon find the Higgs, for sure something will have been proven incomplete in the standard model.

Just as Newtonian gravitation, though it answered all scientific needs for centuries, was eventually supplanted by the more complete Einsteinian model of gravity (aka, General Relativity), so, too, the standard model may have to give way to a more complete, more basic, model of subatomic reality.

And won't that be exciting ...

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