Antimatter Atom Measured for the First Time

Scientists have taken the first-ever measurement of an atom made of antimatter.

This measurement, though not very precise, represents a first step toward being able to study antimatter atoms in detail — a goal necessary for understanding why the universe is made of matter and not antimatter, its mysterious sibling.

All particles of matter are thought to have antimatter partners with the same mass but opposite charge. When these pairs meet, they annihilate each other to become pure energy.

Scientists think the universe contained equal parts of matter and antimatter just after the Big Bang, which is believed to have started everything 13.7 billion years ago. But early on, most of the matter and antimatter destroyed each other, leaving behind a slight surplus of matter that became the stars and galaxies that exist today.
Antimatter trap

In a previous study, physicists at Switzerland's CERN laboratory succeeded in trapping antihydrogen atoms for several minutes by using magnetic fields to keep them suspended in one spot.

An antihydrogen atom is the analog of hydrogen, the simplest atom among the elements. Where hydrogen contains one proton and one electron, antihydrogen is made up of one antiproton and one positron (the antimatter partner of the electron). [Wacky Physics: The Coolest Little Particles in Nature]

In the new research, physicists found they could beam microwave light of a specific frequency at an antihydrogen atom, flipping its spin. This causes the particle's magnetic orientation to change, and the magnetic trap that held it no longer works. The antiatom is free to fly off and hit the walls of its trap, which are made of matter. When it collides with an atom in the wall, the antiatom is annihilated along with the atom, creating a signature that the physicists are able to detect.

"We have made a measurement," said Jeffrey Hangst of Denmark's Aarhus University, spokesman for the CERN laboratory's ALPHA experiment. "Precision-wise, it doesn't compete with matter, but it's the only one that's ever been done on antimatter."

The experiment proves it's possible to change an antiatom's internal properties by shining a light on it. This is the first step toward applying a detailed method of measurement called spectroscopy, which involves tuning the light to a very specific frequency so that it can excite the antiatom's positron to a higher energy level, or orbit. After the excited positron jumps to a higher orbit, it will fall back and emit the extra energy as light, and scientists will measure the light's frequency.

Antimatter spectrum

"We are now in the business of doing antimatter spectroscopy," Hangst told LiveScience. "Now we just push forward to make it more and more accurate."
The best current theory of particle physics is called the Standard Model, and it predicts an identical spectrum from hydrogen and antihydrogen. Scientists must precisely measure the true spectrum of antihydrogen to compare the two and test this theory.

"We're looking for very small changes that manifest in different new physics between the two," Hangst said.

If they find them, they may be closer to solving one of the ultimate cosmic quandaries.

"We know there's something we're missing," Hangst said."We know that we don't understand everything about antimatter because we can't explain what happened to it after the Big Bang."

Physicists' best guess is that the two particles behave slightly differently, for example, by decaying at different rates.

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A Supermassive Black Hole Gets a Sumptuous Treat : A Red Giant Star

A close look at a distant cataclysm indicates that the black hole's victim was a red giant star

Once in a while, a supermassive black hole gets a sumptuous treat. A passing star wanders too close and gets caught in the black hole's gravitational pull, like a fly trapped in a spider's web. The star then becomes an easy meal for the black hole, which tears its prey to bits and ingests a good portion of it. 

Astronomers have witnessed several such disruptions before in distant galaxies, but usually only toward the end of the process. (These feedings are far too rare, however, to have been witnessed in our own Milky Wayanytime in recent human history; they occur only once every 10,000 years or so per galaxy.) Now researchers have documented a black hole's feasting in such detail that they were able to infer its size as well as the type of star that fell prey to its gluttony. 

Astronomers cannot peer inside a black hole itself; beyond the event horizon, a black hole's point of no return, even light cannot escape into the outside world. But material falling into a black hole gives off intense flares of radiation as it compresses and heats up outside the event horizon. 

Suvi Gezari, an astronomer at Johns Hopkins University, and her colleagues used a number of different telescopes to track the outburst from a supermassive black hole in a galaxy more than two billion light-years away as the black hole consumed a star that ventured too close.

"While there has been evidence of these types of flares before, there's never been enough information to say what kind of star fell victim to the black hole, and what was the mass of the black hole that destroyed the star," Gezari says. She and her colleagues published their findings online May 2 in Nature.

They first discovered the flare using the Pan-STARRS 1 telescope in Hawaii, a relatively new observatory that can scan the sky rapidly and repeatedly to identify changes in a celestial object's appearance. In May 2010 the telescope picked up a transient feature in the sky, dubbed PS1-10jh, that brightened rapidly in visible light. Around the same time, NASA's spaceborne Galaxy Evolution Explorer registered a flare in the ultraviolet at the same location, near the center of an otherwise unremarkable galaxy. Both telescopes followed PS1-10jh into 2011 as it peaked and finally faded; the researchers also commandeered the larger MMT telescope in Arizona to take spectroscopic readings of the flaring object.

"Because we [saw] the flare of radiation at so many different wavelengths and in so much detail over time," Gezari explains, the researchers were able to closely compare their observations with theoretical predictions for a star's infall into a black hole. The match between observation and theory is quite convincing, says Giuseppe Lodato, an astrophysicist at the University of Milan who wrote a commentary accompanying the research in Nature. "The shape of the light curve is very close to what is expected for such events—it's actually one of the best examples," he says. 

And with the help of the spectra from MMT, which can pinpoint individual chemical species by the wavelength of light they emit, Gezari and her colleagues were able to parse the makeup of the debris from the disrupted star. That material is notably bereft of hydrogen, the most common element in the universe. "We see no evidence of hydrogen in the system," she says. "This is very peculiar gas. It's mostly helium gas." 

The composition of the stellar debris indicates that the devoured star was the helium-rich core of a red giant—a swollen type of star that the sun will evolve into some five billion years from now. "When [a red giant] balloons up, its hydrogen envelope is very vulnerable to being stripped by the gravitational forces of the black hole," Gezari says. "What we're seeing is this sort of stripped core." 

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