What Is Antimatter? Matter-Antimatter Asymmetry
Discovery Of AntimatterAn international race is picking up speed, to see our universe for what it really is and how it came to be?
According to the standard theory, that describes the origins of the universe, its early moments were marked by the explosive contact between subatomic particles of the opposite charge.
Scientists are now focusing their most powerful technologies on an effort to figure out exactly what happened?
Our understanding of cosmic history hangs on the question: how did matter as we know it survives? And what happened to its birth twin, its opposite, a mysterious substance known as antimatter.
Space Shuttle EndeavourA crew of astronauts sent to space from a launch pad at the Kennedy Space Center in Florida. They'll enter the Space Shuttle Endeavour for the 134th, and second to the last, the flight of the space shuttle.
Little noticed in the publicity surrounding the close of this storied program is the cargo bolted into Endeavor's hold.
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Alpha Magnetic SpectrometerIt's a science instrument that some hope will become one of the most important scientific contributions of human space flight. It's a kind of telescope, though it will not return dazzling images of cosmic realms long hidden from view, the distant corners of the universe, or the hidden structure of black holes and exploding stars.
Unlike the great observatories that were launched aboard the shuttle, it was not named for a
famous astronomer, like Hubble, or the Chandra X-ray observatory. The instrument called the Alpha Magnetic Spectrometer, or AMS, is the brainchild of Samuel Ting, from Massachusetts Institute of Technology.
At the heart of the AMS is a large superconducting magnet designed to operate in the pristine environment of space. With its intensive power requirements, the final version was attached to the International Space Station. The promise surrounding this device is that it will enable scientists to look at the universe in a completely new way.
The instrument is tuned to capture "cosmic rays" at high energy hurled out by supernova explosions or the turbulent regions surrounding black holes. And there are high hopes that it will capture particles of antimatter from a very early time that remains shrouded in mystery.
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The chain of events that gave rise to the universe is described by what's known as the Standard model. It's a theory in the scientific sense, in that it combines a body of observations, experimental evidence, and physical laws into a consistent overall picture. But this picture is not necessarily complete.
The universe became hot. After about a billionth of a second, it had cooled down enough for fundamental particles to emerge in pairs of opposite charge, known as quarks and antiquarks. After that came leptons and anti-leptons, such as electrons and positrons. These pairs began annihilating each other. Most quark pairs were annihilated by the time, the universe was a second old, with most leptons gone a few seconds later.
To be sure, antimatter does exist in our universe today. The Fermi Gamma-Ray Space Telescope spotted a giant plume of antimatter extending out from the center of our galaxy, most likely created by the acceleration of particles around a supermassive black hole. The same telescope picked up signs of antimatter created by lightning strikes in giant thunderstorms in Earth's atmosphere.
A European cosmic ray satellite called Pamela detected a huge store of antiprotons in orbit around the earth, created by high-energy particles striking the upper atmosphere, then held there by magnetic fields that ring the planet.
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Scientists have long known how to create antimatter artificially in physics labs - in the superhot environments created by crashing atoms together at nearly the speed of light. Here is one of the biggest and most enduring mysteries in science: why do we live in a matter-dominated universe? What process caused matter to survive and antimatter to all but disappear?
"Large tracts of space," he wrote, "might thus be filled unknown to us with a substance in which gravity is practically non-existent, until by some accidental cause, such as a meteorite flying through it, unstable equilibrium is established, the matter collecting on one side, the antimatter on the other , until two worlds are formed separating from each other, never to unite again."
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The issue gathered dust until 1928, when a young physicist, Paul Dirac, wrote equations that predicted the existence of antimatter. Dirac showed that every type of particle has a twin, exactly identical but of opposite charge. So for every proton, there's an antiproton. For every electron, there's a positron. For every neutron, an antineutron. Within them, are quarks and their twins, the antiquarks.
In his Nobel Prize lecture in 1933, Dirac pondered a larger reality for antimatter, "If we accept," he said, "the view of complete symmetry between positive and negative electric charge so far as concerns the fundamental laws of Nature, we must regard it rather as an accident that the Earth (and presumably the whole solar system), contains a preponderance of negative electrons and positive protons. It is quite possible that for some of the stars it is the other way about, these stars being built up mainly of positrons and negative protons."
Just the year before, the physicist Carl Anderson had confirmed the existence of antimatter by shooting gamma rays at atoms, creating electron-positron pairs. It became clear, though, that ours is a matter universe. The Apollo astronauts went to the moon and back, never once getting annihilated.
If there any large concentrations in nearby galaxies or galaxy clusters, we should see gamma rays produced when particles and antiparticles found each other. It stands to reason, too, that when the universe was more tightly packed, that it would have experienced an "annihilation catastrophe" that cleared the universe of large chunks of the stuff.
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Unless antimatter somehow became separated from its twin at birth and exists beyond our field of view, scientists are left to wonder: why do we live in a matter-dominated universe?
Dirac's "symmetrical" view of matter and antimatter, which saw them as equivalent, collapsed three decades later in 1964. The American physicists James Cronin and Val Fitch examined the decay of a particle called a kaon to its antiparticle twin.
They found that the transformation back to normal matter did not occur with the same probability. That would suggest there must be small differences in the physical laws that govern matter and antimatter.
What if there is some antimatter out there, escapes from the mass annihilation of the big bang still fleeing through the emptiness of space?
The crew of Endeavour placed the AMS instrument on its perch on the International Space Station in May 2011. Since then, scientists have been combing the data for the signatures of antimatter particles striking its detector. If they manage to detect heavier elements such as anti-helium or anti-carbon, that would point to concentrations of antimatter in space large enough to have formed stars, where those elements are created and suggest that symmetry may not have been broken after all. Such heavier anti-atoms can exist.
At Brookhaven National Lab in New York, scientists recently smashed gold atoms together at nearly the speed of light. From about a billion individual collisions, its detectors recorded the presence of 18 anti-helium atoms - atoms with two antiprotons and two antineutrons.
Just to get into orbit, the space shuttle had to be loaded up with some 15 times its weight in conventional rocket fuel. The energy contained in antimatter is orders of magnitude greater. In fact, it would take just a coin-sized portion to propel the shuttle into orbit.
Because antimatter is so volatile in our matter-filled universe, the challenge for scientists is first to create it, then to hold it for enough time to study it, before it simply vanishes. Even as the shuttle Endeavour glided onto land for the last time, AMS scientists were beginning to.
CERNMeanwhile, scientists on the ground were beginning their own intensive efforts to corral antimatter in their labs. To really find out what happened in that early epoch of annihilation, scientists will have
to understand more about the properties and behavior of antimatter. They are trying to do this at the giant European physics lab, CERN.
Inside the Alpha chamber, the magnetic field holds the particles in place and isolates them from one another. An electric field separates the electrons and positrons. They are then carefully brought into contact.
When two positrons collide, one falls into orbit around an antiproton, forming anti-hydrogen. Then, the molecule is trapped by magnetic fields, like a marble rolling around in a bathtub. Now remove the bathtub, the magnetic fields. The anti-molecule smacks up against the wall of the detector and annihilates, emitting a shower of particles.
The next step is to hold onto it, see how long can we keep it around so that we can study it. After all, that's what we want to do. We want to study the antimatter, compare it to matter, and see if they're the same.
And by study, we mean to interact with lasers or with microwave radiation to see what their structure is inside. How do they behave? Do they behave exactly like hydrogen?
ASACUSA DetectorWithin the same Lab, the effort to pinpoint differences is already underway. Scientists working with the ASACUSA detector are trying to measure the precise weight of an antiproton. These oddball molecules contain one antiproton, which would normally inhabit the atomic nucleus. Instead, it orbits the nucleus in place of an electron. It survives microseconds in the detector, but that's enough for the scientists to hit it with a pair of lasers. The molecule blows apart on impact, and that enables them to calculate the weight of its components.
Masaki Hori from Max Plank Institute- University Of Tokyo said, "We have measured to a precision of nine digits. And we found that the antimatter, that the antiproton mass is exactly the same as the proton mass to nine digits of precision."
If they find there is a difference, it's bound to be subtle. Will it be enough to shed light on why matter survived and antimatter did not?
The differences may lie much deeper in the structure of matter than we've so far been able to go.
Large Hadron ColliderScientists are now preparing to throw a new generation of powerful technologies at the problem. At the Large Hadron Collider at CERN, they can send atoms whipping around a 27-kilometer tunnel and into ultra-high-energy collisions.
Looking at the zoo of particles that splatter onto the walls of the detectors, they are hoping to find differences between quarks and their antiquark counterparts.
One recent computer calculation performed at Columbia University unveiled differences between quarks and antiquarks, when it was assumed that these particles interact with dimensions beyond the four that define the universe we experience.
Still, its authors wondered whether the differences are enough to account for our matter-filled universe.
Understanding the asymmetry between matter and antimatter is one of the most important quests in modern cosmology because it would help expand, or perhaps even challenge, aspects of the Standard Model.
The clash of these opposite forms in the early universe harks back to William Blake's poem:
"What immortal hand or eye could frame thy fearful symmetry?"