Despite the stereotype of a lone, wild-haired Einstein working tirelessly at a chalkboard, today’s biggest discoveries in physics come from huge collaborations of scientists working on enormous apparatuses that can cost billions of dollars, often located in the most extreme places on Earth. After all, it’s just the fabric of the universe they’re trying to understand.
In the past century, physicists have revealed a shocking amount about how our universe works, while simultaneously leaving huge questions unanswered. Some of the biggest breakthroughs have been in understanding how tiny particles interact and affect the workings of the cosmos. Everything we can detect in the universe is made of particles, whose behaviour is described by a theory called the Standard Model of particle physics. That theory divides particles into quarks and leptons (which include electrons and the ghostly neutrino particles). Each of those particles has an antiparticle that has the same mass but is essentially a mirror image, with the opposite electric charge.
These particles interact via forces that are governed by other particles called bosons. You may have heard of the Higgs boson: It was the last predicted but undetected particle of the Standard Model, and scientists finally announced its discovery in 2012. That was a huge victory for the field, the kind of headline-making science that justifies the massive price tags of particle accelerators like the Large Hadron Collider. But since then, despite plenty of interesting research, there hasn’t been another huge discovery comparable to the Higgs.
There’s so much we still don’t know. The Standard Model doesn’t explain what caused the universe to begin, or why there’s so much more matter than antimatter, or the nature of a mysterious, invisible stuff called dark matter. It doesn’t explain why the expansion of the universe is accelerating, an effect currently attributed to so-called dark energy.
These questions and other mysteries about the universe have driven physicists to build some truly mind-boggling instruments in search of answers. See some of the most amazing experiments below.
The Large Hadron Collider
View of the ATLAS detector at the Large Hadron Collider. (Photo: Claudia Marcelloni/CERN)
The Large Hadron Collider is the world’s most powerful atom smasher. It consists of a pair of intersecting rings inside a 16-mile-round tunnel on the CERN campus beneath the Swiss-French border near Geneva. The LHC takes protons (or entire atomic nuclei) from CERN’s other accelerators, adds even more energy using superconducting magnets and radio-frequency cavities, and smashes them into each other inside various building-sized detectors designed to observe the outcome of the collisions. Recently, scientists working on the LHC found some intriguing hints of a discrepancy between what the Standard Model predicts about a particle called the B0 meson and what actually occurred inside the experiment.
The XENON1T Experiment
The XENON experiment underground. Water tank (left) and three-storey service building (right). Note humans at bottom right for scale. (Photo: The XENON Experiment)
The search for dark matter has taken physicists deep underground with experiments like XENON1T. As its name suggests, XENON1T involves a container filled with a tonne of liquid xenon, placed deep underground in order to shield the sensitive detector medium from any potential background noise. The experiment lies in wait for a dark matter candidate particle called a WIMP, or Weakly Interacting Massive Particle, to pass through and produce a telltale signal. XENON1T and experiments like it have yet to discover any hints of dark matter, but they have been able to rule out certain possibilities. XENON1T is amid a transformation into XENONnT, which will contain 8 tonnes of liquid xenon.
The Alpha Magnetic Spectrometer
The Alpha Magnetic Spectrometer experiment on the back of the Space Shuttle Endeavor. (Photo: NASA)
Other experiments are looking for dark matter from space. The Alpha Magnetic Spectrometer, launched from Earth in 2011, is an experiment that’s been measuring high-energy particles from onboard the International Space Station. It’s one of a few space-based experiments that seems to observe an excess of positrons, the electron’s antimatter partner. It may also verify a strange drop-off in high-energy particles observed by a Chinese satellite called DAMPE. These observations may provide more clues into the true nature of dark matter.
Engineers inside the empty Super-Kamiokande detector in Japan. (Photo: The Institute for Cosmic Ray Research of the University of Tokyo)
The general weirdness of neutrinos – such as how they pass right through most matter without interacting with it at all – has driven scientists to construct experiments that try to measure them. Super-Kamiokande is a vat containing 50,000 tonnes of water. It sits beneath Japan’s Mount Ikeno and is lined with detectors that turn minute flashes of light into signals read into computers. Like other giant-vat-of-stuff experiments, it lies in wait for its target particles – in this case, neutrinos from the Sun, from deep space, or produced at the Japan Proton Accelerator Research Complex 186 miles away. Super-K is most famous for discovering neutrinos’ identity-switching behaviour, called neutrino oscillations, and recently made headlines for a measurement hinting at the fact that neutrinos could differ from their antiparticle partners, which scientists think is a prerequisite for explaining why the universe contains more matter than antimatter.
The IceCube Laboratory at the Amundsen-Scott South Pole Station in Antarctica. (Photo: Felipe Pedreros, IceCube/NSF)
Other neutrino observatories rely on different designs, the most shocking of which might be the IceCube neutrino observatory at the South Pole. Despite its remote location, IceCube is basically a frozen Super-Kamiokande. It consists of 86 “strings” each containing 60 light-detecting modules, placed into holes drilled 4,500 to 8,000 feet into the ice. Neutrinos create tiny flashes in the rare event that they interact with the Antarctic ice beneath the experiment, which the detectors then sense. IceCube has already made incredible measurements of neutrinos from outer space; most recently, it found the origin of ultra high-energy cosmic rays striking Earth.
The Muon g-2 magnet (Photo: Reidar Hahn/Fermilab)
Despite its successes, the Standard Model of particle physics doesn’t explain everything that physicists observe in the universe – so they are looking for ways to break the model. The highly anticipated results of the Muon g-2 experiment at Fermilab in the US state of Illinois will likely provide some insights into one area where the Standard Model could fail. Preliminary results already seem to show hints of a discrepancy between the Standard Model and actual measurements.
A muon is basically a heavier cousin of the electron, and its “g” value, a number that governs how it behaves in a magnetic field, is thought to be around 2. Scientists at Brookhaven National Lab in the US built a huge circular magnet and passed muons through it, and they measured that the g-2 value (the difference between g and 2), was slightly off from what the Standard Model predicts. Brookhaven National Lab officials coordinated with Fermilab to continue the experiment with even more muons. The newest g-2 measurements should be released soon!
Vera C. Rubin Observatory
The Vera C. Rubin observatory under construction. (Photo: LSST Project/NSF/AURA)
The universe is expanding, and that expansion is accelerating. Scientists think that something called dark energy is causing the acceleration. Calculations seem to show that dark energy makes up more than two-thirds of the total energy and mass in the universe, but we don’t know what dark energy really is. Soon, the Vera C. Rubin Observatory, also known as the Large Synoptic Survey Telescope (LSST), will be able to map around half the sky every few nights – effectively creating a movie of the visible cosmos, which will help researchers understand dark energy’s impact on the ultimate fate of the universe. The telescope is scheduled to begin its science operations in 2023.
Northern leg of LIGO interferometer in the US state of Washington. (Photo: Umptanum/Wikimedia Commons)
In 2016, scientists announced that they had observed ripples travelling at the speed of light through space-time, the result of two black holes slamming together. The experiment behind the discovery, LIGO, is a pair of tunnels arranged in an L-shape, where a laser is split, sent into both tunnels, and then re-joined in a detector. Passing gravitational waves cause the two beams to travel in-and-out of phase with each other, creating a pattern containing information about the ripples.
The two LIGO observatories, plus the Virgo observatory in Italy, have continued to measure gravitational waves and even spotted the gravitational waves accompanying a flash of light from a pair of neutron stars colliding. Ultimately, these observations might provide information on the rate of the universe’s expansion or the true nature of dark matter.
Event Horizon Telescope
The centre of the ALMA array on the Chajnantor Plateau. ALMA is a key part of the Event Horizon Telescope. (Photo: NRAO/AUI/NSF)
Few objects in space captivate us like black holes – but they’re more than just mind-boggling objects in the sky. They’re also places where gravity is so extreme that it tests the limits of general relativity, and the effects of smaller-scale quantum forces kick in. Scientists around the world have essentially turned planet Earth itself into a telescope by linking eight observing facilities (and soon more) together to get the first image of a black hole’s shadow, in this case the shadow cast by the black hole at the centre of galaxy M87. The researchers aren’t done yet: We should soon see an image and maybe a movie of the black hole at the centre of our own galaxy.
Featured image: NASA