The International Conference on High Energy Physics met in Chicago earlier this month, and some heads are exploding in the aftermath. Portions of the particle physics community have gone a bit mental, and not because of any mind-blowing discoveries. On the contrary, the culprit is the seeming perpetuity of “null results” at the Large Hadron Collider, and at smaller experiments searching for dark matter or supersymmetry.
You might say these particle theorists have developed cabin fever.
First of all, some background. For a subject with aims as broad as physics, which seeks a framework of understanding capable of explaining both the large scale structure of the cosmos as well as the microphysics of the atomic nucleus, the current paradigm of empirical exploration has become remarkably linear. In effect, we just bang particles together at higher and higher energies and see what comes popping out.
The full story behind this linearity is complicated and rests on the achievements of 20th century physicists, who were so successful at unifying what was previously known into a single framework that (in an admittedly somewhat oversimplified sense) we only have one variable left to vary.
A short version of the story might go like this. Einstein’s relativity theories relate physics at different velocities, so that if we know how to describe the experience/behaviors of slow moving objects, we know how to describe physics at any other speed as well. They also relate energy directly to mass. On the other hand quantum mechanics unifies the notion of particles and waves in such a way that exploring the universe on short distance scales amounts to doing high energy physics experiments. This is because short wavelengths are needed to probe short distances, and short wavelengths correspond to high energy, just as ultraviolet light is more energetic than infrared.
Assisting this simplicity is what Gell-Mann, the discoverer of quarks, called the totalitarian principle: “Everything that isn’t impossible is mandatory.” Essentially, the inherent randomness of quantum mechanics aides our empirical searches. As long as sufficient energy is present, any possibility is a certainty given enough trials.
So in a (somewhat oversimplified sense), “all” physicists have to do is bang particles together at higher and higher energies and record the results. Particle physicists have become less like explorers of the Earth, with four cardinal directions to head off in and many continents to explore, and more like ocean divers, striving primarily to reach greater depths.
This game has worked for a long time. For the past half century, the deeper into the depths we’ve gone, the more we’ve found, culminating in the well-publicized discovery of the Higgs Boson in 2012.
But now it looks like that era may be coming to an end. The only hint of anything new at the LHC since the Higgs discovery ultimately turned out to be a statistical fluke. This while increasing the energy scale from 8 TeV all the way up to 13 TeV. So our best divers have nearly doubled our penetration down into the depths, and in their explorations they have found nothing but nothingness. This “null result” has left many physicists in fear, staring into an emptiness of unknown extent. They’re afraid of the nothing we’ve found.
And so heads are exploding. It’s being called the “nightmare scenario.”
But it’s not a nightmare, it’s just science. And the proper response isn’t panic, its diligence, and also a bit of a return to a broader exploratory framework.
The most important comment here is that this result, and “null results” in general, are not failures. In fact, it’s been widely acknowledged that a major problem with modern science is its refusal to recognize null results for the achievements they are, which has led to destructive research incentives. I would go so far as to say that anyone claiming the recent achievements of the particle physics community are actually “failures” worth lamenting is doing a grave disservice to science. The physicists at the LHC have accomplished one of greatest scientific discoveries of all time. We should celebrate their professionalism, precision, skill, and diligence in exploring deeper into the unknown than anyone who came before them. And we should be proud that our current theories are so stubbornly resisting falsification. It means they’re good theories.
In fact, there’s even a sense in which this “null result” provides more of a hint for future physics than a normal scientific non-discovery would. This is due to a physics principle called “naturalness.” Our current theories are incredibly successful at predicting experimental results, but their mathematical structure is puzzlingly awkward, “unnatural.” Some physicists have derided the naturalness criterion as philosophical or aesthetic in nature. I’d argue that even if that were true, it’d hardly be grounds for dismissal. Absent guidance from nature, we should look to whatever arbiters we can to help us find our way, as long as we never stop yearning for a final answer from experiment.
But naturalness is not just an arbitrary preference, it’s both a sound logical concept and a very physical insight about general tendencies.
On the simplest level, it’s just a matter of having a reasonable discomfort with improbable results. If a physical theory is able to predict that a variable X must be between 0 and 100, but cannot say anything else, we expect that most likely X will be greater than, say, 10. That’s just probability. If we then measure X, and find that it’s actually .00001, that would be an “unnatural result”: not strictly speaking illogical, but improbable enough to make us think we were missing something when made our prediction for X in the first place.
The mass of the Higgs boson is a little like our variable X. In principle, the Higgs mass could’ve fallen anywhere on the range from the electroweak scale, .2 TeV, to the plank scale (the expected scale of quantum gravity), 10^16 TeV. In fact, it turns out to be .246 TeV. Very unnatural.
On a deeper, and more physical level, the naturalness principle has to do with the relationships that exist between physics at different scales. Just as one would not expect, for example, the viscosity of water to be ultrasensitive to the electron mass, even though water is made up of atoms which include electrons, one does not expect physics at the LHC to depend too dramatically on the details of physics at the plank scale. But, as it turns out, this is precisely the implication of a lightweight Higgs boson.
An analogy with physical distances is useful. The sun, though far away, is very influential for the trajectory of the Earth through space, because it is very massive. But by “very influential” we mean that it determines the general shape of Earth’s orbit, not its every tilt and wobble. Likewise, it is the general form of the structure of the sun that determines the orbit of the earth. We do not worry that weather patterns on the sun may send the Earth suddenly careening into Mars.
Likewise, we expect the Higgs mass to be influenced by plank scale physics, but in a very general sort of way. The awkward issue is that, for reasons too difficult to explain precisely here, if it were influenced by plank scale physics in this generic sort of way, we’d expect the Higgs mass to be close to the plank scale, not the lower bound at electroweak scale. We expect the Higgs mass to be influenced in a large, but not precise, way, by plank scale physics.
It’s sort of like if the Earth’s trajectory were straight rather than circular, despite the existence of the sun. How could that be, given what we know about gravity? Tinkering with our models of solar structure won’t help much, sense as we said the details don’t matter. On the other hand one could speculate that there are other, unseen, massive objects in the solar system, which exactly balance the gravitational influence of the sun and allow the Earth to move unimpeded onward in a nearly straight line. For example we could speculate that the Earth lies at a Lagrangian point of the sun and an unseen black hole. But even though that would be logically sound, it would be an incredibly awkward explanation: how did that unseen massive object get itself into the perfect location to EXACTLY CANCEL the effects of the sun? And given this precise cancellation, the Earth’s trajectory now WOULD depend precisely on the details of the structure of the sun.
This is the issue with the Higgs mass. Its lightness is unexpected given the influence of plank scale physics. We may of course postulate a full circus of effects at the plank scale that could contrive to produce this result, but it is nevertheless unlikely. Unlikely results deserve explanations.
The unnaturalness of our physical theories, then, is fairly interesting in light of their tremendous empirical success. The more we verify these theories, the weirder it gets. All the more reason to continue patiently working.
(You think Null Result sounds like a good name for a band? You’re too late apparently)
But there’s a larger point here. It might end up being that particle colliders just don’t have much more to offer us for a while. That wouldn’t be unusual. It’s rare that progress is so straightforwardly linear for so long.
Many particle physicists have expressed frustration at the increasing disconnect between theoretical physics and the empirical world it’s meant to explain. They’ve blamed this on some sort of imagined complacency on the part of theoretical physicists, especially string theorists. But it’s been fairly clear to me for a while that this disconnect is not borne of complacency but is just a reality of our time that we must contend with. The disconnect has been forced upon us by nature. We need to encourage an exploratory spirit if we’re going to overcome it. Attacking people studying String Theory or other ambitious attempts to provide a new framework of thinking isn’t helping provide focus, it’s narrowing our ability to innovate our way out of this impasse.
It might be that the immediate future of physics lies in studying condensed matter systems, or early universe cosmology, rather than particle accelerators. String theorists, for example, have had some (limited, but real) success in these areas, and with more abstract issues like relating gravitation and quantum information theory. On another front, observational astronomy is opening up whole new windows into the universe with LIGO, Icecube, etc. Obviously a need to reorient in favor of exploring these possibilities would be a disappointment to particle physicists, but this is just the reality of progress, scientific or otherwise. We don’t know in advance what will work and what won’t.
We should keep calm, keep exploring, and keep an open mind. This isn’t a nightmare, and admittedly it’s not a dream either. It’s the hard work of living and working and moving forward in the real world.