2016 held some wonderful physics moments—hello gravitational waves! Other moments were experimentally impressive, like shining a laser beam through antimatter, but don’t have the same oomph as colliding black holes. And some were just downright deflating: Dark matter still won’t show itself. Still, every experimental let-down opens up new avenues for inquiry. The things physicists did, and did not, find in 2016 are clues about what to expect from the science in the coming years.
Hopes for a New Particle, Dashed
In late December 2015 CERN, the European center for high energy physics research, released data showing that there might be a new particle afoot. Was it a sister to the Higgs? A kind of neutrino? Though scientists said it was possibly (even probably) a statistical fluke, excitement spread like a shockwave. Within a month, scientists had posted 500 theoretical articles related to the particle on the preprint arXiv server.
But dreams of post-Standard Model physics were dashed as days lengthened into summer. Nope. No real evidence of a particle. And, as the Large Hadron Collider went to bed this month for the rest of the year, the machine had failed to show the way to a new particle.
Yay, We Found Gravitational Waves!
A century after Albert Einstein’s prediction, physicists confirmed the existence of gravitational waves by detecting ripples in spacetime created when two black holes crashed together 1.4 billion years ago. Einstein’s theory of general relativity predicted that when anything with mass accelerates, it should create a wave in spacetime, like a rock thrown into a pond creating ripples on the water. He thought such signals would be so weak humans would never be able to detect them. Scientists at the Laser Interferometer Gravitational-Wave Observatory were pleased to prove him both wrong and right.
After announcing their find in February, team LIGO has spent much of 2016 upgrading its observatories in Washington and Louisiana. And in late November, LIGO started listening again, straining to hear new ripples. With the recent launch of their citizen science program “Gravity Spy,” you can help them tune in.
World’s Most Sensitive Dark Matter Detector Comes Up Empty
The Large Underground Xenon dark matter experiment spent nearly two years beneath a mile of rock in the Black Hills of South Dakota hoping to hear the faint ping of dark matter—specifically, the signal of a weakly interacting massive particle, one of the favored contenders to constitute dark matter. With a third-of-a-ton of cooled liquid xenon surrounded by powerful sensors, LUX was designed to emit a tiny flash of light and an electric charge if a WIMP collided with a xenon atom in the tank, making it the most sensitive dark matter detector to date.
LUX wrapped up its observations in May. But in July, it announced that it had not found any telltale signals of WIMPs. And while, yes, this is a little bit of a bummer, physicists aren’t giving up on the search. Coming up next, the LUX-ZEPLIN experiment will replace LUX at the Sanford Underground Research Facility in South Dakota. It should have 70 times the sensitivity of LUX and is expected to be up and running in 2020.
3D Map of 1.2 Million Galaxies Measures Dark Energy
Back in 2009 we told you about the start of the Baryon Oscillation Spectroscopic Survey, an ambitious project to map the 3D structure of the early universe. This summer the BOSS program released its map—the largest ever, containing more than a million galaxies, allowing physicists to make the best estimates yet of the poorly understood “dark energy” that is accelerating the expansion of the universe. What does a map of a million galaxies look like? Kind of like Jackson Pollock married a pointillist.
Identical Twin Particles Prove to be Unique
Matter and antimatter are clearly different—matter dominates the universe, while scientists can only catch snippets of antimatter. But why this is so is a mystery. The Standard Model says the two should be essentially the same, so any indications that they can break so-called charge-parity symmetry can offer clues to why the universe favored matter over antimatter.
In summer, the T2K Collaboration, based in Japan, presented one such clue. They aimed a neutrino beam at the Super-Kamiokande underground detector in Kamioka—and when they measured them, they saw more electron neutrinos and fewer electron antineutrinos than would be expected. What this means is still not clear, but neutrinos could light the path toward understanding the difference between matter and antimatter.
Color of Antimatter Seen for the First Time
Late-breaking news from CERN rounded out the year in physics. The ALPHA collaboration saw, for the first time, the color of antimatter. By comparing the optical spectrum of an antihydrogen atom to normal hydrogen they found (within limits of the experiment) that they seem to look exactly the same.
Simply managing to make such a comparison is a feat of experimental engineering. It took 20 years for the the CERN antimatter community to get this far, but now it opens up the field to higher precision comparisons between matter and antimatter … and the hope that someday, scientists will spot a key difference to help explain why matter dominates the universe and antimatter is so hard to find.
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