Now that we might (maybe, possibly, could be, it could go away, let’s be careful about what we say here lest we put a jinx on it…) be seeing hints of a Higgs, it’s time of some cautionary tales that a ‘discovery’ is not the end of the story, it’s only the beginning.
When I was a young graduate student, Martinus Veltman gave a talk at a summer school. He had yet to share the Nobel Prize in physics with Gerard t’Hooft for work on the Standard Model. This was 1980. Veltman said, “Right now, theorists are in the driver’s seat. In fact, for the next 30 years, with the exception of the details of the masses of some particles, we know what’s going to happen. But in 30 years, we absolutely are going to need experimental guidance to make any progress in particle physics whatsoever.”
In point of fact, I don’t think anyone recorded Veltman’s words for posterity, but it made a deep impression on me in a number of ways. First and foremost to a rookie experimentalist was the realization that I had to toil in the vineyards for thirty years until something truly of note would arrive. The second thought was “What on earth is he talking about?” We were handed the Higgs boson as an article of faith.
In the current publicity about the Higgs boson, I often worry that we simplify the goals of what we’re doing to the point of trivializing it. Veltman was right: What we call the “Higgs boson” or the “God particle” is really a surrogate for a strange mechanism that bestows mass on all particles through an interaction. We have the most precise theory of anything and, yet, this absolutely crucial piece is missing. We’ve gotten to that point, 30 years later, at which we’ve found almost all of the particles theorists predicted as part of the Standard Model and are now heading into uncharted territory in higher energy ranges.
What we don’t often talk about are the odd properties that this mysterious particle is thought to have. Often times, theorists, like Veltman, feel that the current model is horribly inelegant and must therefore be completely wrong and only a pale approximation of what nature really does to bestow mass, the most elemental of properties a particle can possess.
What makes it so inelegant? In the first place, if it is as described in our simplest model, it would be the only elementary particle we know of that doesn’t possess an intrinsic property of ‘spin’, which seems to be key in the workings of our theory of fundamental forces. For more on the inherently quantum mechanical property of particles known as spin, see this post by Flip Tanedo.
Beyond its spin 0, the Higgs has a very odd property: It gives energy to the vacuum of space. We don’t really know what this means and often just ignore it. Yet, a kind of vacuum energy has been invoked to describe cosmology on very different energy scales from the energies we’re exploring at the LHC. Astrophysicists often talk about something called the “flatness problem”.
If you took a pot of boiling water just off the stove, and dumped ice into it, eventually you would see the ice and the hot water come to some equilibrium temperature. But, in order for this to happen, the ice and hot water have to physically come in contact with each other. When we look around the universe, the temperature of everything is the same to a remarkable degree, as if it was all sitting in the same pot and came to the same temperature. That would all be well and good, but one patch of the universe cannot possibly have been in contact with another part, because they’re separated by such a large distance that light itself cannot connect the two. How could the entire universe be at the same temperature?
The answer is largely thought to lie in a period called ‘inflation’. Initially the very early universe was so dense and compact that temperatures from one part could communicate to another part: everything was sitting in the same ‘pot’. Then, a mysterious vacuum energy appeared that pushed parts of the universe out of contact, but preserved the uniformity of temperature. This happened within a very early phase of the universe when temperatures and energy densities were far hotter than the conditions we’re producing at the LHC. This vacuum energy is about a trillion times larger than what we associate with the Higgs.
Astrophysicists have also invoked a vacuum energy at another, much weaker scale. You may have heard of ‘dark energy’. Our best guess is that, like the vacuum energy of the early universe, this mysterious force that seems to be pushing the universe apart also seems to be a kind of vacuum energy. Yet, in this case the energy of the vacuum is exceedingly weaker than the energies we’re exploring at the LHC. So, there’s a vacuum energy invoked to explain both the very early universe and the very late universe. At the same time, there’s a vacuum energy associated with the Higgs, but it just sits there like an orphan, of no consequence.
To deal with some of these strange properties, theorists have come up with other ideas for how the Higgs might manifest itself:
1.) Supersymmetric Higgs – The energy scale where the three main forces other than gravity, the strong, weak and electromagnetic- join together is close to the scale associated with the cosmic inflation. This is often called the ‘Grand Unification scale.’ The fact that we see two of the fundamental forces – weak and electromagnetic – joining together at the LHC energies presents a conundrum. It is very difficult to reconcile the Grand Unification scale with the LHC scale in a natural way without having some other kinds of matter arise. The constants of the theory would have to line up just perfectly, fine-tuned to a level of precision that is equivalent to balancing a pencil on its point. With Supersymmetry, a number of Higgs-like particles arise.
2.) Composite Higgs – Rather than deal with an inelegant particle with no spin, theorists have speculated that it’s actually made of multiple objects, possible pair of top quarks, tightly bound together. The opposite spins of the objects bound together in a composite Higgs would cancel out to give it zero spin.
3.) No Higgs –According to some models, the Higgs is not a particle at all, but the result of interactions that create mass. These models are sometimes called ‘technicolor’. Although they aren’t particularly favored by theorists because they’re difficult to calculate, we cannot rule them out.
Experimentalists are checking the data for all of these possibilities.
But, what if something like our vanilla-Higgs shows up with a high degree of certainty? Are we done? Hardly! Given all the possibilities and the somewhat inelegant nature of the vanilla-Higgs model, the work has just begun. We have to ask questions like: Is there only one? What is its spin? How does it interact with all the other particles? Are there any variations in its interactions from what we expect, and if so, how does that relate to other measurements we do. These are the tough questions, the one Veltman was alluding to and my betting odds are that we’ll find deviations from our vanilla-Higgs, but it won’t be easy. It may take a decade or more of data taking at the highest beam intensities and energies before we begin to understand what’s really going on.
Science may begin with blinders and theories may run aground, but eventually we do manage to figure out what’s going on.
Here’s a cautionary tale from the 19th century. It illustrates how people can be steered in the direction of one theory, but ultimately can end up with a far more powerful idea.
A German geographer named August Petermann championed a theory of a warm Polar sea. Some expeditions to the high Arctic reported seeing vast stretches of ice-free water extending off toward the horizon. An oceanographer named Silas Bent speculated the that warmth of the Gulf Stream waters flowing north, combined with the waters of a similar ocean current, called the Kurosiwa (black current) flowing off of the coast of Japan would be sufficient to warm the Polar Ocean to the point that an expedition, if it could make it through some part of the ice pack, could sail directly to the Pole. Petermann was one of the main champions of the idea.
James Gordon Bennett Jr. was the publisher of the New York Herald and tried to boost publication by underwriting adventurous expeditions. He financed Henry Morton’s Stanley’s search for David Livingston, garnering a boost in the circulation of the Herald. Hearing of Petermann’s theory of the warm polar sea, Bennett set about to finance an expedition and purchased a British gunboat, the HMS Pandora, and refitted it. He enlisted the US Navy to find a crew. Rechristened the USS Jeanette, it was captained by Lieutenant Commander George DeLong. Hoping to repeat the publicity of the famous Stanley-Livingston meeting, Bennett sent the Jeanette north through the Bering Strait in hopes of reaching the famed open Polar Sea. The Jeanette left San Francisco in July 1879, and was last heard from in late August of that year.
After crossing the Bering Strait, the Jeanette was soon frozen fast in the icepack. Trapped there for nearly two years, it slowly drifted northwest from the coast of Siberia and was ultimately crushed by the icepack. DeLong ordered his crew to abandon ship and began a trek over the frozen icepack, hauling three lifeboats in hopes of eventually reaching settlements along the delta of the Lena River in Siberia. DeLong didn’t make it out alive, perishing in the maze of channels. Some survivors did make it to settlements and eventually made it back home.
Three years later, the wreckage of the Jeanette washed up on the coast of Greenland, some three thousand miles away. This prompted many to wonder how the wreckage could travel so far across the frozen icecap. Theories about ocean current proliferated. One adventurer, Fridtjof Nansen, constructed a polar exploration vessel, the Fram. Fram had a rounded hull that allowed it to be frozen into the icepack without being crushed. Nansen and crew sailed to roughly the point where the Jeanette had been frozen in and commenced a drift across the Polar Sea. At this point, the theory of the Open Polar Sea was completely abandoned in the face of the overwhelming data to the contrary.
Although Nansen never reached the North Pole, during the Fram’s expedition, the remaining crew made detailed observations of wind patterns, drift, the ocean depths and temperatures. On its return to Norway, the Fram had a wealth of data that took years to sift through. Vagn Ekman was a student in physics at the University of Uppsala, Sweden. He was studying fluid dynamics and heard of the data from the Fram. After exploring the mathematics of the interactions of air and water flow on the surface of the rotating earth, he developed the modern theory of surface ocean currents, which bears his name: Ekman transport.
Ekman’s work remains one of the fundamental underpinnings of oceanography.
What I’m trying to point out is this: We are on a voyage of discovery. As Veltman said, experimentalists are really now the ones in the drivers seat. The vanilla-Higgs is an easy target to fire at, as there are quite specific predictions for how it will be manifested, but there are good reasons to be suspicious that the Higgs is precisely as it is described in the simplest version of the Standard Model. Like the long, meandering progression from the theory of the Open Polar Sea to the modern theory of Ocean Currents, I suspect that we’ll have many changes and false leads. As it stands now, with the performance of the LHC, we are just beginning to penetrate the icepack, and we don’t really know what to expect.