Antihydrogen atoms are captured for the first time

THE history of physics is littered with the detritus of once-sacred assumptions. As better technology enables more exacting experiments, phenomena that were once scoffed at as impossible become the new norm. For this reason, physicists have long been searching for more sensitive means of probing the realm of antimatter, which theory holds should mirror the familiar world of matter. If precise comparisons of the two were to turn up differences, that would signal a fundamental flaw in understanding of the universe.

Now, a team of scientists working at CERN, Europe’s particle-physics laboratory, has announced a breakthrough in the quest for such tests. In the current issue of Nature, members of the ALPHA experiment report that they have been able to trap a very small amount of antihydrogen—the simplest type of anti-atom—for the first time. Since the hydrogen atom is one of the best-measured systems in all of science, this opens the door to a series of experiments testing just how similar matter and antimatter really are.

The symmetry between particles and antiparticles is woven deep into the foundations of physics. For each particle there should be a corresponding antiparticle with exactly the same mass and lifetime but with an opposite electrical charge. Bring the two together and they annihilate each other in a flash of energy. When anti-electrons (or positrons, as they are usually called) orbit antiprotons and antineutrons, the resulting anti-atoms should have the same energy levels as the common or garden variety. Furthermore, it is thought that gravity should pull on matter and antimatter in just the same way.

In reality, no one has ever been able to drop an anti-apple and watch it fall down (or up), and the antimatter produced in particle colliders is so energetic that it is hard to examine with the tools of precision physics. For decades, physicists at CERN and elsewhere have been trying to overcome these limitations with antihydrogen, which consists of a single positron orbiting a single antiproton. By shining laser light onto hydrogen or antihydrogen and observing which wavelengths are absorbed, the energy levels of the two can be compared in detail. And since hydrogen is electrically neutral, it should be possible to observe gravity’s tiny tug on it without the confounding effects of electrostatic attraction to other particles.

Antihydrogen atoms were produced in the past by several experiments at CERN. But they were so energetic that they immediately bumped into the walls of the experimental apparatus and were annihilated. Since then several teams have been trying to make colder antihydrogen and to hold on to it using clever configurations of electrical and magnetic fields. This is what ALPHA has just succeeded in doing.

Coaxing hot and bothered antiprotons and positrons to couple is quite a task. The magnetic traps employed to hold the antihydrogen are only strong enough to confine it if it is colder than around half a degree above absolute zero. The antiprotons themselves, which are produced by smashing regular protons into a piece of iridium, are around 100 billion times more energetic than this. Several stages of cooling are needed to slow them down before they can be trapped, forming a matchstick-sized cloud of around 30,000 particles. The positrons, produced by the decay of radioactive sodium, are cooled into a similarly sized cloud of around 1m particles and held in a neighbouring trap.

The antiprotons are then pushed into the same trap as the positrons and left to mingle for a second or so. In that time some of the particles get together and form antihydrogen. Next, an electrical field is used to kick out any remaining positrons and antiprotons. The electrically neutral antihydrogen atoms are left behind.

To test whether any antihydrogen was actually formed and captured in their trap, the ALPHA team turned off its trapping magnet. The antihydrogen was then free to wander towards the walls, and thus annihilation. The detectors duly observed 38 bursts of energy which the team concluded came from antihydrogen atoms hitting the wall of the trap.

Although the number of trapped atoms recorded was small, the team is optimistic. It has developed better techniques for cooling both positrons and antiprotons, which should allow it to trap more anti-atoms. Soon it will be able to see just how contrarian antimatter really is.

It takes a lot of energy to do that, so it's not going to be a solo engine source - something else will have to start it.

On the down side it may have to be space cooled and only work in space. A degree or less above absolute zero. I stand corrected.

When calculating the temperature in space, it is important to understand that most estimates must take into account the varied makeup of space. Outer space is the portion of the universe which is almost entirely empty. Unlike the small pockets of our universe which are inhabited by stars, planets, and other large sections of matter, outer space contains very, very little. Nonetheless, it is not entirely empty, and this is important to understand when considering the temperature in space.

The short answer is that the temperature in space is approximately 2.725 Kelvin. That means the universe is generally just shy of three degrees above absolute zero – the temperature at which molecules themselves stop moving. That’s almost -270 degrees Celsius, or -455 Fahrenheit.

In one sense, we can talk about the temperature in space as being 2.725 K. This shifts a bit from place to place, but not by much more than a thousandth of a degree. For all intents and purposes, this is the generally accepted temperature in space.

Antimatter