pump the breaks
Frozen antimatter study to 'alter our understanding' of the universe
“Its most exciting applications probably yet to be dreamt of”
It’s invisible to the naked eye, will self-destruct if you touch it, and should have caused the destruction of the universe just moments after the Big Bang — enter antimatter, the bad boy of particle physics.
Thanks to its self-destructive tendencies, it has been historically challenging for scientists to catch enough antimatter — and hold onto it for long enough — to truly take a good look at it. Until now.
New research from a CERN-based ALPHA (Antihydrogen Laser Physics Apparatus) collaboration has demonstrated for the first time how lasers can be used to slow down antihydrogen atoms, cooling them to near absolute zero (nearly -460 Fahrenheit) and making it possible to finally make precise measurements of these volatile particles.
The research was published Wednesday in the journal Nature.
What’s new — Scientists have been attempting to capture and experiment on antimatter for decades, but finding the right tool to properly corral these atoms has been difficult because particle decelerators (a particle accelerator’s ice-cold cousin) and dense gas clouds that might hold other types of matter steady can easily cause the annihilation of antimatter. That’s why the research team behind this experiment turned to a neutral medium — photons — for their experiment.
Because a photon is made of pure energy, researchers can use photons to interact with antimatter particles without triggering their destruction. By using an ultraviolet laser tuned precisely to the energy of these antihydrogen atoms, the researchers were able to decelerate these atoms by 6x their initial speeds using lasers that up to now have been elusive, Makoto Fujiwara, a co-author on the study and TRIUMF scientist, says.
“Laser light at these wavelengths is extremely difficult to produce and handle, since there is no convenient lasing medium, and the light gets absorbed in air,” Fujiwara tells Inverse.
He says the results immediately open up two important areas of antimatter research:
- allow “drastic improvements in the precision of our measurements on the properties of antihydrogen”
- and “open up an entirely new class of quantum measurements with antimatter”
It may not give us a warp drive, but it could do something else important: tell us about how the universe began, and why the universe we see today is not what it should be.
Here’s the background — According to scientists, when the universe collided into existence just after the Big Bang, it should’ve contained equal amounts of both antimatter and matter, creating symmetry in the universe — an antiparticle for every particle.
But because matter and antimatter annihilate on impact by essentially canceling out each other’s charge and spin and creating a massive amount of energy in their place, this could have ended the universe as soon as it began.
The big question now is: why didn’t it, and why is our universe asymmetrically full of matter instead of antimatter? Because hydrogen is matter’s most abundant element, that naturally makes antihydrogen an attractive candidate for scientists to study in order to get to the bottom of this mystery.
To make antihydrogen, the same CERN particle accelerators that feed protons to the Large Hadron Collider (LHC) make antiprotons by slamming protons into a metal target. The resulting antiprotons are held in CERN’s Antimatter Decelerator ring and delivered to groups like ALPHA. These are then combined with positrons (the anti-electron) that can be collected from decaying radioactive sources to create an antihydrogen atom.
Why it matters — This elusive Dr. Jekyll to regular matter’s Mr. Hyde has the exact same structure and properties as matter as we know it, but an opposite charge and spin. Studying this strange breed of matter could help scientists crack open essential questions about our universe, including why antimatter and matter didn’t destroy the universe after the Big Bang and how Einstein’s theory of general relativity may be proven — or broken.
“Our demonstration will enable future precision measurements on antihydrogen,” Fujiwara says. “The measurements could, in turn, provide a clue to a fundamental question of what happened to the antimatter in the universe.”
What they did — To (literally) chill out these antihydrogen particles, the team had a few steps to take:
- Produce roughly 500 to 1,000 antihydrogen particles over the course of 4 hours by mixing antiprotons and positrons (the opposite of an electron) together in a magnetic chamber
- Use microwaves to bump out excess particle clutter from the apparatus, including antihydrogen atoms in unnecessary energy levels
- Pulse ultraviolet laser beams at the remaining antihydrogen particles, causing them to change energy states and, ultimately, slow down — kind of like how you play fetch with a puppy to tire it out
When done just right, this process can slow down highly energetic antihydrogen particles with speeds of about 186 mph to just under 30 mph — the speed you cruise through a residential neighborhood.
The team was able to hold the atoms in this state for over an hour — a huge leap from the origin of antimatter science in the 1990s, where atoms would destroy themselves in a matter of seconds.
What next — There are still a few wrinkles to iron out of the current systems, including the time and energy-intensive process of generating these laser pulses. But overall, the success of this new method offers a huge step forward to the study of antimatter particles.
“Future precision measurements will challenge some of the most fundamental concepts and principles in physics,” Fujiwara says.
In addition to investigating theories like the symmetry of matter and antimatter and general relativity, better control over these antimatter atoms could also allow scientists to one day even form antimatter molecules or adapt the system to better control ions for quantum information systems as well.
“Laser-cooled antihydrogen will be a transformative tool in antimatter studies, with its most exciting applications probably yet to be dreamt of,” write the authors.
Abstract: The photon—the quantum excitation of the electromagnetic field—is massless but carries momentum. A photon can therefore exert a force on an object upon collision. Slowing the translational motion of atoms and ions by application of such a force, known as laser cooling, was first demonstrated 40 years ago. It revolutionized atomic physics over the following decades, and it is now a workhorse in many fields, including studies on quantum degenerate gases, quantum information, atomic clocks and tests of fundamental physics. However, this technique has not yet been applied to antimatter. Here we demonstrate laser cooling of antihydrogen, the antimatter atom consisting of an antiproton and a positron. By exciting the 1S–2P transition in antihydrogen with pulsed, narrow-linewidth, Lyman-α laser radiation, we Doppler-cool a sample of magnetically trapped antihydrogen. Although we apply laser cooling in only one dimension, the trap couples the longitudinal and transverse motions of the anti-atoms, leading to cooling in all three dimensions. We observe a reduction in the median transverse energy by more than an order of magnitude—with a substantial fraction of the anti-atoms attaining submicroelectronvolt transverse kinetic energies. We also report the observation of the laser-driven 1S–2S transition in samples of laser-cooled antihydrogen atoms. The observed spectral line is approximately four times narrower than that obtained without laser cooling. The demonstration of laser cooling and its immediate application has far-reaching implications for antimatter studies. A more localized, denser and colder sample of antihydrogen will drastically improve spectroscopic and gravitational studies of antihydrogen in ongoing experiments. Furthermore, the demonstrated ability to manipulate the motion of antimatter atoms by laser light will potentially provide ground-breaking opportunities for future experiments, such as anti-atomic fountains, anti-atom interferometry and the creation of antimatter molecules.