‘Time Machine’ to Probe Universe’s Birth
Deep in the sandy woods of New York’s Long Island, physicists are preparing to travel back to the dawn of the universe.
In a few weeks their time machine, buried beneath the Department of Energy’s Brookhaven National Laboratory, will begin stripping gold atoms of their electrons and accelerating them to 99.995% of the speed of light. Then it will smash pairs of the atoms together with such violence that the collisions will generate temperatures 10,000 times hotter than the sun.
There’s no danger. Because the objects involved are submicroscopic, the total energy in each collision will be comparable to that of a mosquito landing on a screen door. But that energy will be released into a space one-millionth of a millimeter across, concentrated enough to tear apart an atomic nucleus.
We all learn in school that matter consists of bits called atoms, and that those atoms are made up of a nucleus of protons and neutrons surrounded by a swarm of electrons. But in recent decades, physicists have learned that atoms are more complicated than that. Inside each nucleus are even smaller particles, called quarks and gluons, that combine to make protons and neutrons.
Studying how quarks and gluons behave is the frontier of nuclear physics and may hold the key to understanding how the universe is put together.
Current theory indicates that the first atoms appeared about a second after the universe itself--so tearing them apart means re-creating what came before. Physicists picture that realm, which would have formed just microseconds after the Big Bang, as a trillion-degree caldron known as the quark-gluon plasma. Atoms did not exist. Neither did protons and neutrons.
There were just quarks and gluons, swimming in a super-hot brew. Then, before the universe was a second old, the quark-gluon plasma congealed into the protons and neutrons that make up atomic nuclei today.
“What we hope to do is to make the quark-gluon plasma and then to actually probe and understand its properties,” said John Harris, a Yale University physicist involved in the project.
The quark-gluon plasma’s brief moment of glory is lost in the past, obscured from us by 13 billion years of cosmic evolution. But if the new particle collider succeeds in re-creating the primordial substance, physicists might learn how it formed, how long it lasted and how it reconstituted itself into protons and neutrons.
They would have glimpsed the first millisecond of creation.
Since then, physicists say, the universe has gone downhill--at least on the temperature scale. As the thermometer dipped below a trillion degrees, quarks and gluons combined to form protons and neutrons. Then, when the universe had attained the ripe old age of one second, protons and neutrons began combining into atoms.
Because we live in such a cold universe compared with the one that existed 13 billion years ago, the free quarks and gluons won’t hang around long. They will “freeze out” into more prosaic particles in about one million-trillionth of a second.
Physicists compare it to studying water by smashing ice cubes together in the hope that some of the collisions will produce enough heat to melt a drop or two.
Of course, if you wanted to study those tiny droplets you’d have to catch them first. The new Brookhaven machine, known as the Relativistic Heavy Ion Collider, has four detectors designed to do that. Each is packed with electronics that will record everything about the thousands of particles created in every collision. The detectors will produce data at the rate of one petabyte a year--enough to fill the hard disks of 30,000 personal computers.
Going through that data will be quite a task. Part of the problem is that the plasma can’t be seen directly. Physicists will look for the signs it leaves behind, like trying to prove the existence of Volkswagens by collecting hubcaps and hood ornaments.
Signs that physicists expect to see in the aftermath of a collision where the quark-gluon plasma is created include a flash of powerful gamma rays and the production of rare “strange” quarks and of another particle called the upsilon.
Scientists at the CERN laboratory in Europe claim to have seen some of those signs already, in an experiment that smashed lead ions into stationary targets of lead and gold.
“We now have evidence of a new state of matter where quarks and gluons are not confined,” CERN laboratory director Luciano Maiani said in February.
But it will take Brookhaven’s new machine, 10 times more powerful than the CERN experiment, to demonstrate beyond a doubt that the plasma exists.
In addition to simulating the early universe, the $600-million collider will help physicists learn how atomic nuclei are put together the same way children figure out how their toys work--by taking them apart.
Physicists already know that two up quarks and a down quark make a proton, and two down quarks and an up quark make a neutron. But they do not understand completely how those quarks and the gluons are arranged.
“It’s not just Tinkertoys where you take these rocks and tie them together with sticks,” said Robert Jaffe, a Massachusetts Institute of Technology physicist who works on the Relativistic Heavy Ion Collider--RHIC for short.
After about two years, RHIC will supplement its experimental repertoire by substituting individual protons for gold ions about one-third of the time. By that time the experiment will have consumed only a tiny fraction of a gram of gold.
The proton-proton collisions will investigate a property known as spin--the rotation of a particle around its axis. The proton’s spin has been measured extremely accurately, and so has the quark’s.
But add up the spins of the three quarks in a proton, and they account for less than half of the larger particle’s spin. Where does the rest of the proton’s spin come from?
There are two possibilities. Some could come from the gluons, the particles that stick the quarks together to form a proton. Some also could come from quarks and gluons as they spin around each other the way orbiting planets contribute to the solar system’s angular momentum.
“The bad news is that nobody knows a reliable way to measure the orbital angular momentum,” Jaffe said.
But the good news is that RHIC can measure the spin contribution of the gluons.
Physicists will try to figure that out by colliding a beam of protons spinning in one direction with a second spinning the opposite way. RHIC is the first collider that can collide two oppositely spinning proton beams.
As with the quark-gluon plasma experiments, the trick is to examine the debris flying out of those collisions and then use them to turn the clock back--in this case to the moment just before the collision happened.
“We think we’re in great shape to make discoveries because we have this new tool,” said Brookhaven physicist Mike Tannenbaum.
What if RHIC fails to measure the components of the proton’s spin, or can’t re-create the plasma? What if it does make the plasma, but the stuff turns out to be completely different from what they expected?
Physicists will be delighted.
Nuclear physics is built on a theory known as quantum chromodynamics. Ever since it was developed in the 1960s, the theory has been predicting the outcomes of experiments with incredible precision. As proud as they are of it, physicists can’t wait for the day when they find some hole, some special circumstance where their theory fails completely.
The last time something like that happened, early in the 20th century, relativity and quantum dynamics came to the rescue. The new ideas revolutionized physics and made things like computers, lasers and nuclear weapons possible. So physicists actually welcome things they don’t understand.
“Finding and nailing the plasma would be fantastic,” said William Zajc, a RHIC physicist and Columbia University professor. “The only thing more fantastic would be some totally unexpected surprise that defies our predictions.”
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On the Net: www.rhic.bnl.gov
