Field notes from space-time | Star Trek’s light speed engines may not be possible in our universe, but we are learning more about the particles that fuel them
12 June 2019
EVERY year, I attend the Star Trek convention in Las Vegas, and every year, I get asked whether warp speed will ever be possible. In the Star Trek universe, humanoid species zoom around the galaxy at speeds faster than light, using warp engines fuelled by antimatter. Travelling faster than the speed of light is unlikely, but antimatter is real. Every particle has an antimatter partner that we call an antiparticle.
So, as a particle physicist, what I really want to be asked about isn’t the likelihood of travelling long distances quickly, but instead about the particle type that underlies this fictional technology. Star Trek‘s futuristic antimatter engine touches on one of the great unsolved mysteries in particle physics: where is all of the antimatter anyway?
The best known type of antimatter is the positron, which is the antielectron. The positron has the same mass as an electron, but the opposite electrical charge. When matter collides with its antimatter partner, they annihilate each other. This isn’t simply a matter of theory: we have seen antimatter in the lab, and not just with the electron and its partner.
Positrons can be made through radioactive decay. They are also created in a pair with electrons when extremely energetic photons, better known as gamma rays, interact with atomic nuclei. Antiprotons have also been produced, and, in 1995, scientists were finally able to directly combine positrons and antiprotons to create antihydrogen.
Although antimatter is real, it is rather difficult to make in the lab. Since matter and antimatter annihilate one another on contact, one has to wonder why we are here at all. If they are each other’s complete opposites, one might expect the same amount of matter and antimatter to have been produced in the big bang, quickly leading to annihilation and an empty universe. Instead, we live in a highly asymmetric version of the universe, where the negatively charged electron is a fundamental particle that forms a core part of all atoms, hovering in their orbitals. Why did nature use only half of the building blocks available to it?
“Star Trek‘s futuristic antimatter engine touches on one of the greatest unsolved mysteries of particle physics”
Efforts to make sense of this asymmetry are under way in both theoretical and experimental physics. Many theorists believe that the lopsided bias towards matter is connected to violations of something called charge-parity symmetry, more commonly known among physicists as CP symmetry. This is a property that demands that all particles are interchangeable with their antiparticle when their spatial coordinates are flipped, a kind of mirror symmetry. Most observed particles obey CP symmetry, but it can be violated.
Though most famous for being the facility where the Higgs boson was first detected, the Large Hadron Collider is also home to experiments that are seeking to learn more about CP symmetry breaking.
The Large Hadron Collider beauty (LHCb) experiment, for example, specifically focuses on b-physics. B-physics refers not to low-budget physics, something that our governments surely dream of, but instead to the physics of beauty quarks (sometimes referred to as bottom quarks).
Beauty quarks are just one of six flavours of subatomic quarks, which are the constituents of neutrons and protons. The other five varieties have equally delightful names: top, up, down, strange and charm. The fundamental “weak” nuclear force can cause quarks to change flavours, and it also causes the quarks to break CP symmetry. This gives us an important hint that CP symmetry violations are possible, leading theorists to consider matter-antimatter models that rely on it.
In addition to beauty quarks, LHCb can also study the properties of charm quarks. Excitingly, the experiment recently found the first evidence of CP violation among them. In order to achieve this result, LHCb looked at decays of D° mesons – short-lived particles made of a charm quark and an up antiquark.
This result is an exciting affirmation of a phenomenon that scientists had expected to find for decades, but had yet to produce in the lab. The discovery doesn’t radically change our perspective on physics yet because it matches theoretical predictions – and it certainly isn’t a warp engine. But it suggests that, under the right conditions, CP violation can occur. Perhaps those conditions existed during the big bang, producing the nearly antimatterless universe we see today.
- This column will appear monthly. Up next week: Graham Lawton
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