WHAT ANTI-MATTERS

Scientists fired lasers at antimatter in an experiment that probes the deepest human question: why are we here?

Antimatter—the fictional power source in countless sci-fi stories—isn’t just a great source of energy; understanding it could tell us why we exist at all. That’s why a recent Nature paper outlining the results of an experiment at CERN with the antimatter version of hydrogen is so important to physicists.

Called the ALPHA experiment, it involves firing a laser at some atoms of antihydrogen to see if they behave the same way as normal, ordinary matter. And so far, it looks like it does. The problem with that is if matter and antimatter act that way, we shouldn’t be here.

Opposites annihilate

Matter is made up of positively charged protons, neutral neutrons, and negatively charged electrons. In any neutral atom the protons and electrons balance out. Antimatter is made of protons and electrons as well, but these have opposite charges to the ones we are familiar with. So an antiproton is negatively charged and an antielectron, called a positron, is positively charged.

Matter and antimatter annihilate when they touch, exploding into photons and high-energy particles. Yet the current models of how the universe began—the Big Bang, when the whole cosmos was crammed into a space the size of a pinprick—suggest an equal amount of matter and antimatter during that Bang. But if that was the case then the whole universe should be full of a bunch of photons and the occasional high-energy particle, and nothing else. Clearly it’s not. Most estimates of how much more matter than antimatter there was at the Big Bang are about one part in a billion—that would explain why our world is full of matter.

 Put antihydrogen and antioxygen together and you get anti-water. At the same time, physicists have no particular reason to believe that matter and antimatter should behave differently from one another, (besides when they come into contact). A block of anti-iron would look no different than a block of iron and antimatter elements should act like normal elements. Put antihydrogen and antioxygen together and you get anti-water. The Standard Model and quantum mechanics, which predict this behavior, have so far proven correct.

That said, while the concept and theory of antimatter have been around since Paul Dirac formulated them in 1928, it wasn’t until 1932 that the positron was discovered, and it took until 1955 to find the antiproton. That’s because to make antimatter you have to generate lots of energy and turn it into matter, and the only way to do that in a lab on Earth is in particle accelerators. Making a single atom of antihydrogen is therefore a big undertaking. This is why the only technology we have that involves the regular use of antimatter is PET scans—and we’re nowhere near having Star Trek-style matter-antimatter power. This means that actually studying antimatter to see if the theories about its behavior are right is nearly impossible without a lot of very sophisticated gear.

The same, so far

This is where the ALPHA experiment comes in.

Scientists working on ALPHA wanted to investigate atoms of antihydrogen, to see what happens to them when they undergo a transition from one energy state to another. Such transitions happen in nature all the time; they are the reason we see light from neon signs and why lightning glows. In both cases an ionized atom gains an electron, or an electron is moved to a higher energy state around its atom. The electron then moves to a lower-energy state and emits the photon we see.

Because making antimatter is so hard, the ALPHA experiment involved only a few atoms—14 of them—of antihydrogen. To see the transition, the researchers fired a laser at the atoms, ionizing some of them. Antihydrogen seemed to respond to the lasers the same way that ordinary hydrogen would, undergoing the same energy-state transition—at least to a precision of about two parts in 10 billion. So it would seem that antihydrogen is the same as hydrogen.

However, physicist Jeffrey Hangst, spokesperson for the experiment, cautions that all this does is tell them that such transitions can be measured. He notes the wavelength of the light ordinary hydrogen emits when it goes through energy transitions has been measured to one part in 10^15 (a thousand trillion). That means the measurement the ALPHA experiment did needs to get five orders of magnitude more precise if we’re to know that antihydrogen really acts like its matter counterpart. Future experiments in 2017 will make more precise measurements. Another set of experiments will test how antihydrogen reacts to gravity—effectively, they are going to drop atoms and watch them fall.

 Physical laws should look the same if you reverse the charges, turn something into its mirror image, and reverse time. Assuming those experiments also show no difference, it will be an incremental victory for the Standard Model, quantum mechanics, and even relativity. It will have shown that a kind of symmetry in physics called charge-parity-time, or CPT, holds up. CPT symmetry says that physical laws should look the same if you reverse the charges, turn something (a particle, for example) into its mirror image, and reverse time. It underlies a lot of physics—if it’s wrong, even by a small part, then it means that many physical laws we believe in are incomplete.

This is why the ALPHA experiment and its future iterations are so exciting to physicists. “CPT is an absolute cornerstone of all quantum field theories, not just the Standard Model,” says Salvatore Rappocio, an assistant professor of physics at the University at Buffalo. “However, it is extremely unlikely that it will ever be to be broken, although it is exciting to look, since such resounding contradictions of our assumptions is what scientists live for. This would be a smoking gun of a fundamental revolution in all of quantum physics if it were ever observed.”

If the symmetry is preserved in these experiments, it means that wherever it is broken, it’s not in the behavior of antimatter—and the physicists will have to look somewhere else for the answer to why we exist. “Symmetry breaking can come anywhere, it may not involve standard matter,” Hangst says. “It may involve exotic things we haven’t seen yet.”

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