Written By: XAQ RZETELNY
It's a new, impressive experiment, but the results are exactly what we expected.
One of the persistent mysteries about our Universe is the extreme imbalance between matter and antimatter. Antimatter and matter were both generated during the Big Bang, but the Universe is now dominated by ordinary matter, and we don't know why that should be the case. To solve that mystery, an obvious place to look for clues would be in antimatter itself. If researchers could find something different about antimatter’s behavior, it might hint at an explanation for the extreme disparity.
To that end, a team of researchers decided to test whether hydrogen and antihydrogen have the same spectrum—do they absorb and emit light at the same wavelengths. They generated the first-ever laser-spectroscopic measurement of an antimatter atom, but the results look an awful lot like a regular hydrogen atom.
Antimatter is the same as matter but has the opposite electrical charge. So, while an ordinary hydrogen atom is made of a proton (positive charge) and a much smaller electron (negative charge), an anti-hydrogen atom is composed of an anti-proton (negative) and an anti-electron, or “positron” (positive).
When matter and antimatter collide, they annihilate, leaving high-energy photons in their place, which makes antimatter hard to study in the lab. Labs tend to be filled with things like air, people, and lab equipment, all of which are matter. All it takes is a few stray atoms and you’ve lost your anti-atoms. Given that even the tiniest bit of antimatter is extremely expensive to produce, that outcome is pretty undesirable. The trick, then, is to isolate antimatter such that it can’t bump into anything else. Using electrical fields, the researchers can isolate their antimatter for about ten minutes in a device called the ALPHA-2 apparatus, which provides a chance to examine the particles.
The antimatter is also super-cooled to about 0.5 Kelvin. This cooling is necessary to prevent the individual anti-atoms from moving fast enough to escape their trap.
The researchers then used lasers to probe the anti-atoms. When light strikes an atom, that light’s energy can be absorbed by the atom’s electron if it's the right wavelength. With that extra energy, the electron rises to a higher-energy orbit. The electron will then lose the excess energy and fall back down to its original orbit, emitting that energy as light.
The researchers wanted to examine the spectrum of that light. Different elements give off light with different wavelengths, but any two hydrogen atoms should produce the same wavelengths. And, according to physics as we understand it, anti-hydrogen atoms should also produce the same spectrum.
The researchers conducted three sets of 11 trials. The first two sets had lasers in different configurations; the third had no lasers and acted as a control. This arrangement helped the researchers eliminate systematic effects that might crop up and contaminate their results. This also helped identify the cosmic ray background. Cosmic rays collide with particles in the upper atmosphere, creating secondary particles that can get into the experimental apparatus, where they're detected as if they were the light from the trapped anti-hydrogen.
Results and conclusions
While an exciting experiment, it didn't produce exciting results. So far, the spectra from the anti-hydrogen matches that of the hydrogen, confirming a cornerstone of the Standard Model. Had the work produced a different spectrum, it would have been a big deal for physics.
Overturning the fundamental symmetry between matter and antimatter particles would go beyond telling us that the Standard Model of physics is wrong. Antimatter, mathematically, is the same thing as an ordinary particle traveling backwards in time. (While that may sound science-fictiony, the reality is less exciting; it just shows that “traveling backwards in time” doesn’t actually mean much when you’re a hydrogen atom.) So such a discovery might have implications for the reversibility of time, an interesting issue physicists think about.
Another motivation for the experiment was to learn about the imbalance of matter and antimatter in the Universe. Had the antimatter provided a differing spectrum, it would hint that it operates under different physics than normal matter. This might have provided clues for resolving that mystery.
That doesn’t seem to be the case, but the road doesn’t end here. This was only the first test of its kind, and in many ways it could be the beginning of an era. “This has long been a sought-after achievement in low-energy antimatter physics,” the researchers write in their paper. “It marks a turning point from proof-of-principle experiments to serious metrology and precision CPT comparisons using the optical spectrum of an anti-atom.” (CPT is the theoretical understanding that requires symmetry between matter and antimatter).
The researchers’ methodology, including their ability to trap a higher number of anti-atoms than before, will enable future work to go further in understanding the physics of antimatter and for researchers to learn, with greater certainty, whether there’s something new to be discovered.“The current result, along with [other recent results], demonstrate that tests of fundamental symmetries with antimatter at [CERN's Antiproton Decelerator] are maturing rapidly,” the researchers write.