What were the biggest physics discoveries of 2015?
I asked our Nature of Reality contributors which advances they think will have the biggest impact in the years to come. And while some of their answers will be familiar from this year’s most popular headlines, others may come as a surprise.
The Large Hadron Collider didn’t deliver a Higgs-style blockbuster this year, but the December release of results from the “big bang machine’s” second, more powerful run ranked high on both Paul Halpern and Sabine Hossenfelder’s lists. “Lots of ‘maybes’ from Run 1 have gone away,” says Hossenfelder, assistant professor for high energy physics at Nordita in Stockholm. “This is very important for the field.” In his recent post on the anomalous “bump” in the data from the 2015 run, Fermilab physicist Don Lincoln called the run “a dazzling success,” adding that the data include “approximately sixty new physics results.” Most of those results agreed with previous predictions: not the stuff of flashy headlines, but the kind of thing that helps physicists sleep better at night. (And most everyone, from physicists to journalists, is withholding judgment on the significance of that strange bump.)
To Halpern, a professor of physics at the University of the Sciences in Philadelphia, the new LHC results exemplify “frontier” physics. “In the past year we have explored two longstanding frontiers: the traditional outer limits of the solar system and the high energy regime corresponding to the upper bounds of the Standard Model of particle physics,” says Halpern, referring to the New Horizons mission, which reached Pluto in July, and the latest LHC run. During its closest approach to Pluto’s surface, New Horizons revealed mountains that could be ice volcanoes and showed that vast swaths of Pluto’s surface are geologically very young. It also delivered the first high-resolution images of Pluto, and while pictures alone don’t make science, they have transformed Pluto from an abstraction—a hazy smudge—into a detailed and particular world.
While the Higgs discovery had the satisfying ring of a story’s final chapter, these advances have a middle-of-the-book quality: science is still turning over new pages. “As we probe the features of Pluto and test the properties of the Standard Model, our hope is to press even further and explore domains beyond those limits, namely the Kuiper Belt objects beyond Pluto and possible new particles beyond what the Standard Model predicts,” says Halpern.
Frank Wilczek also picked new particles and Pluto as the year’s most (potentially) groundbreaking results. “If the recent hint of a two photon resonance at the LHC holds up (which is very much in doubt), it could be a very fundamental discovery with wide-ranging impact,” says Wilczek. But with the jury still out on that, as well as other frontier work, Wilczek gives the nod to New Horizons. “It showed in a dramatic fashion that physics really works, and gave humankind some images that will inspire people for decades to come.”
Now, from the edge of the solar system to the boundary between gravity and quantum mechanics: “In my very own field the most exciting development I find is that it looks like experiments with massive quantum systems might be able to become sensitive to certain quantum gravitational effects,” says Hossenfelder. Though Einstein’s theory of gravity and the theory of quantum mechanics both have perfect records against even the most rigorous experiments, they seem to be fundamentally incompatible. Theories of quantum gravity attempt to express the laws of gravity in mathematical language that is reconcilable with quantum mechanics. Yet it is hard, if not impossible, to design an experiment that can test out these theories. Hossenfelder points to a new study, published on the arXiv preprint server (which is not peer reviewed) this month, that shows that physicists may actually be closing in on experimental tests of quantum gravity. She blogged about the paper here. “I find this totally amazing,” says Hossenfelder. “Imagine, quantum gravity might actually become a real science!”
Outside of quantum gravity, “The most interesting thing that I’ve read this year was probably this” story on the potential to use optical light for whole-body medical imaging, says Hossenfelder. In less than a decade, Zeeya Merali writes for Nature News, the field has “exploded” as researchers have developed new ways to “unscramble” light that’s scattered off opaque surfaces, like skin. Merali writes:
Researchers have now managed to obtain good-quality images through thin tissues such as mouse ears, and are working on ways to go deeper. And if they can meet the still-daunting challenges, such as dealing with tissues that move or stretch, potential applications abound. Visible-light images obtained from deep within the body might eliminate the need for intrusive biopsies, for example. Or laser light could be focused to treat aneurysms in the brain or target inoperable tumours without the need for surgery.
Also (potentially) bringing The Future just a little bit closer, says Hossenfelder:
An advance in fusion energy from Tri Alpha Energy, a private company based near Los Angeles. Daniel Clery writes that Tri Alpha:
has built a machine that forms a ball of superheated gas—at about 10 million degrees Celsius—and holds it steady for 5 milliseconds without decaying away. That may seem a mere blink of an eye, but it is far longer than other efforts with the technique and shows for the first time that it is possible to hold the gas in a steady state—the researchers stopped only when their machine ran out of juice.
Farther yet from the glare of the popular headlines, Scott Aaronson’s pick for the biggest discovery of the year is a new algorithm for solving the “graph isomorphism” problem. (The paper on the algorithm, by László Babai, is online here.) The problem: Take two “graphs”—graph meaning a sort of map that links dots, or “nodes,” via lines, or “edges”—and try to figure out if they are identical. As Erica Klarreich wrote for Quanta, “The problem is easy to state, but tricky to solve, since even small graphs can be made to look very different just by moving their nodes around.”
Theoretical computer scientists like Aaronson don’t just like to solve problems, though. They like to classify them by exactly how hard they are to solve. The new algorithm makes it more likely that the graph isomorphism problem can ultimately be shown to fall into a category called “P,” which contains problems that are relatively easy to solve. (“P” stands for polynomial, meaning that the time it takes for a computer to crack them grows like a polynomial—that is n2, n3, etc—instead of blowing up like an exponential function like 2n.
Finally, Sean Carroll points out Jaume Garriga, Alexander Vilenkin, and Jun Zhang’s new paper “Black Holes and the Multiverse,” which is packed with so many of our readers’ favorite subjects—wormholes, parallel universes, black holes, dark matter—that the abstract reads almost like an out-of-season April Fool’s parody. But the ideas, while speculative, are serious, says Carroll: “They show that quantum fluctuations during inflation can give rise to black holes, and inside those black holes can be new baby universes. It’s an interesting—though obviously very speculative—way to create a multiverse.” Read the full paper—all 47 pages of it—here.
Now it’s your turn: What discoveries wowed you this year? What do you think we’ll still be talking about in ten, twenty, or fifty years?