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Can Time travel backwards ?
John Gribbin
Physicists have long puzzled over the fact that two distinct "arrows of time" both point in the same direction. In the everyday world, things wear out -- cups fall from tables and break, but broken cups never re-assemble themselves spontaneously. In the expanding Universe at large, the future is the direction of time in which galaxies are further apart.
Many years ago, Thomas Gold suggested that these two arrows might be linked. That would mean that if and when the expansion of the Universe were to reverse, then the everyday arrow of time would also reverse, with broken cups re-assembling themselves.
More recently, these ideas have been extended into quantum physics. There, the arrow of time is linked to the so-called "collapse of the wave function", which happens, for example, when an electron wave moving through a TV tube collapses into a point particle on the screen of the TV. Some researchers have tried to make the quantum description of reality symmetric in time, by including both the original state of the system (the TV tube before the electron passes through) and the final state (the TV tube after the electron has passed through) in one mathematical description.
Murray Gell-Mann and James Hartle recently extended this idea to the whole Universe. They argued that if, as many cosmologists believe likely, the Universe was born in a Big Bang, will expand out for a finite time and then recollapse into a Big Crunch, the time-neutral quantum theory could describe time running backwards in the contracting half of its life.
Unfortunately, Laflamme has now shown that this will not work. He has proved that if there are only small inhomogeneities present in the Big Bang, then they must get larger throughout the lifetime of the Universe, in both the expanding and the contracting phases. "A low entropy Universe at the Big Bang cannot come back to low entropy at the Big Crunch" (Classical and Quantum Gravity, vol 10 p L79). He has found time-asymmetric solutions to the equations -- but only if both Big Bang and Big Crunch are highly disordered, with the Universe more ordered in the middle of its life.
Observations of the cosmic microwave background radiation show that the Universe emerged from the Big Bang in a very smooth and uniform state. This rules out the time-symmetric solutions. The implication is that even if the present expansion of the Universe does reverse, time will not run backwards and broken cups will not start re- assembling themselves.
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THE INTRIGUING notion that time might run backwards when the Universe collapses has run into difficulties. Raymond Laflamme, of the Los Alamos National Laboratory in New Mexico, has carried out a new calculation which suggests that the Universe cannot start out uniform, go through a cycle of expansion and collapse, and end up in a uniform state. It could start out disordered, expand, and then collapse back into disorder. But, since the COBE data show that our Universe was born in a smooth and uniform state, this symmetric possibility cannot be applied to the real Universe.
Parallel Universe
B.B.C.
Scientists now believe there may really be a parallel universe - in fact, there may be an infinite number of parallel universes, and we just happen to live in one of them. These other universes contain space, time and strange forms of exotic matter. Some of them may even contain you, in a slightly different form.
Astonishingly, scientists believe that these parallel universes exist less than one millimetre away from us. In fact, our gravity is just a weak signal leaking out of another universe into ours.
For years parallel universes were a staple of the Twilight Zone. Science fiction writers loved to speculate on the possible other universes which might exist. In one, they said, Elvis Presley might still be alive or in another the British Empire might still be going strong.
Serious scientists dismissed all this speculation as absurd. But now it seems the speculation wasn't absurd enough. Parallel universes really do exist and they are much stranger than even the science fiction writers dared to imagine.
It all started when superstring theory, hyperspace and dark matter made physicists realise that the three dimensions we thought described the Universe weren't enough. There are actually 11 dimensions.
By the time they had finished they'd come to the conclusion that our Universe is just one bubble among an infinite number of membranous bubbles which ripple as they wobble through the eleventh dimension.
Now imagine what might happen if two such bubble universes touched. Neil Turok from Cambridge, Burt Ovrut from the University of Pennsylvania and Paul Steinhardt from Princeton believe that has happened.
The result? A very big bang indeed and a new universe was born - our Universe. The idea has shocked the scientific community; it turns the conventional Big Bang theory on its head.
It may well be that the Big Bang wasn't really the beginning of everything after all. Time and space all existed before it. In fact Big Bangs may happen all the time.
Of course this extraordinary story about the origin of our Universe has one alarming implication. If a collision started our Universe, could it happen again? Anything is possible in this extra-dimensional cosmos.
Perhaps out there in space there is another universe heading directly towards us - it may only be a matter of time before we collide.
Magnetic Moondust
Nasa
Thirty-plus years ago on the moon, Apollo astronauts made an important discovery: Moondust can be a major nuisance. The fine powdery grit was everywhere and had a curious way of getting into things. Moondust plugged bolt holes, fouled tools, coated astronauts' visors and abraded their gloves. Very often while working on the surface, they had to stop what they were doing to clean their cameras and equipment using large--and mostly ineffective--brushes.
Dealing with "the dust problem" is going to be a priority for the next generation of NASA explorers. But how? Professor Larry Taylor, director of the Planetary Geosciences Institute at the University of Tennessee, believes he has an answer: "Magnets."
The idea came to him in the year 2000. Taylor was in his lab studying a moondust sample from the Apollo 17 mission and, curious to see what would happen, he ran a magnet through the dust. To his surprise, "all of the little grains jumped up and stuck to the magnet."
"I didn't appreciate what I had discovered," recalls Taylor, "until I was explaining it to Apollo 17 astronaut Jack Schmitt one day in my office, and he said, 'Gads, just think what we could have done with a brush with a magnet attached!'"
"Only the finest grains (< 20 microns) respond completely to the magnet," notes Taylor, but that's okay because the finest dust was often the most troublesome. Fine grains were more likely to penetrate seals at the joints of spacesuits and around the lids of "pristine" sample containers. And when astronauts tramped into the Lunar Module wearing their dusty boots, the finest grains billowed into the air where they could be inhaled. This gave at least one astronaut (Schmitt) a case of "moondust hay fever."
Taylor has since designed a prototype air filter with permanent magnets inside. "When the filter gets dirty, you pull the magnets out, and the dust falls into a box." A later design with electromagnets works more efficiently: "You pull the plug on the electromagnet, tap it, and the dust rains down into a container." He's now working on a prototype design for a "dust brush" using permanent magnets.
Earth dust is not magnetic, so why should moondust be?
"Moondust is strange stuff," explains Taylor. "Each little grain of moondust is coated with a layer of glass only a few hundred nanometers thick (1/100th the diameter of a human hair)." Taylor and colleagues have examined the coating through a microscope and found "millions of tiny specks of iron suspended in the glass like stars in the sky." Those iron specks are the source of the magnetism.
Researchers believe the glass is a by-product of bombardment. Tiny micrometeorites hit the surface of the moon, generating temperatures hotter than 2,000°C, literally the surface temperature of red stars. Such extreme heat vaporizes molecules in the melted soil. "The vapors consist of compounds such as FeO and SiO2," says Taylor. If the temperature is high enough, the molecules split into their atomic components: Si, Fe, O and so on. Later, when the vapors cool, the atoms recombine and condense on grains of moondust, depositing a layer of silicon dioxide (SiO2) glass peppered with tiny nuggets of pure iron (Fe).
A thin coating of iron isn't enough to make sand- or gravel-sized particles noticeably magnetic, any more than spraying a thin coating of iron on a heavy basketball would make it stick to a magnet, says Taylor. But a thin coating is plenty for particles smaller than about 20 microns. They have so little mass compared to their surface area, they're easily lifted by Taylor's magnets.
Magnets aren't the only way to deal with moondust. NASA is considering a whole suite of options from airlocks to vacuum cleaners. But, if Taylor is right, magnets will prove important, and astronauts won't find moondust so troublesome the next time around.
Quark-Gluon Plasma
An international team of physicists working at the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory in the US says it has found strong evidence for the "quark-gluon plasma" -- the state of matter thought to exist in the first millionth of a second after the Big Bang. However, the researchers found that instead of behaving like a gas of free quarks, antiquarks and gluons as expected, the matter behaves more like a liquid. The results were presented at the April meeting of the American Physical Society.
A quark-gluon plasma (QGP) is believed to have existed before the universe cooled and free quarks and gluons combined into protons and neutrons, which then bound together to form light nuclei. Physicists at the CERN laboratory in Geneva claimed to have created a QGP in 2000 but the results were inconclusive because the plasma existed only fleetingly. Then in 2003, RHIC scientists said they had come closer than ever before to creating a QGP.
RHIC uses accelerators to increase the energies of gold atoms up to 100 billion electron volts inside a 4-kilometre ring and then collides them together. When a gold nucleus collides with another gold nucleus the constituent protons and neutrons are thought to melt together to form a QGP.
The new results indicate that some of the observations at RHIC agree with theoretical predictions for a QGP. But many theoretical physicists believe that the QGP should be a gas, whereas the matter formed at RHIC appears to behave more like an almost "perfect" liquid.
The new matter created at RHIC could in fact be a form of the QGP but just different from what has been theorized says Sam Aronson, a Brookhaven director. More detailed measurements are now underway at RHIC to resolve this question.
According to Ulrich Heinz, a theoretical physicist at Ohio State University in Columbus, the prediction that the QGP is a gas is not based on solid theory but is more a qualitative statement based on "folklore" that few physicists have really challenged. "I think this is the most important nuclear physics result in recent years and have been saying for the last two years that RHIC has produced a QGP," he told PhysicsWeb. "That the QGP is an almost ideal liquid instead is extremely interesting -- and definitely not expected by everyone -- but it is in no way inconsistent with previous theoretical calculations."
Johann Rafelski, a nuclear physicist at Arizona University in the US, agrees. "The RHIC experiment has much improved since the last results were published in 2003. Furthermore, the signatures are much clearer and not in conflict with earlier results at RHIC and CERN," he says.
The teams will publish their results in the journal Nuclear Physics A, and in a special 350-page Brookhaven report.
Belle Dumé Science Writer at PhysicsWeb
Negative Speed
University of Rochester & World-Science
In the past few years, physicists have found ways to make light go both faster and slower than its usual speed limit. Now researchers say they’ve gone a step further: pushing light into reverse.
As if to defy common sense, they say, the backward-moving pulse of light travels faster than light.
Confused? You’re not alone.
“I’ve had some of the world’s experts scratching their heads over this one,” said Robert Boyd of the University of Rochester in Rochester, N.Y., one of the researchers. “It’s weird stuff.”
“Theory predicted that we could send light backwards, but nobody knew if the theory would hold up or even if it could be observed in laboratory conditions.”
Einstein determined that nothing can be accelerated to a speed greater than that of light in a vacuum. That’s about 300,000 kilometers (190,000 miles) per second.
If something broke that limit, then some observers could see it reach its destination before it left, violating a universal law of causality.
But physicists in recent years have reported finding tricks to slow light to a near-standstill, or even speed it up in apparent violation of Einstein’s rule.
Now, Boyd said, he’s taken what was once just a mathematical oddity—negative speed—and shown it working in the real world. The findings are published in the May 12 issue of the research journal Science.
Boyd and colleagues sent bursts of laser light through an optical fiber laced with the element erbium. An optical fiber is a thin, transparent tube that transmits light by letting it bounce along its interior.
“The pulse of light is shaped like a hump with a peak,” Boyd explained. “We sent a pulse through an optical fiber, and before its peak even entered the fiber, it was exiting the other end. Through experiments we were able to see that the pulse inside the fiber was actually moving backward.”
To understand how light’s speed can be manipulated, think of a funhouse mirror that makes you look fatter. As you first walk by the mirror, you look normal. But as you pass the curved portion in the center, your reflection stretches. The far edge seems to leap ahead of you momentarily.
In the same way, a pulse of light fired through a special material may move at normal speed until it hits the substance, where it is stretched out to reach and exit the material’s other side.
Conversely, if the funhouse mirror were the type that made you look skinny, your reflection would appear to suddenly squish together, with the leading edge of your reflection slowing as you passed the curved section. Similarly, a light pulse can be made to contract and slow inside a material, exiting the other side later than it naturally would.
To visualize the backward-moving light pulse reported by Boyd, replace the mirror with a TV and video camera. As you may have noticed when passing such a display in an electronics store window, as you walk past the camera, your on-screen image appears on the opposite side of the TV. The image walks in the direction opposite to yours, and thus toward you. It passes you in the middle, and continues until it exits the other side of the screen.
A negative-speed pulse of light would act similarly. As the pulse enters the material, a second pulse appears on the far end of the fiber and flows backward. The reversed pulse not only propagates backward, but releases a forward pulse out the far end of the fiber. In this way, the pulse that enters the front of the fiber appears out the end almost instantly, apparently beating light’s regular speed.
It’s as if you walked by the shop window, saw your image stepping toward you from the opposite edge of the TV screen, and that TV image of you created a clone at that far edge, walking in the same direction as you, several paces ahead.
Wouldn’t Einstein shake a finger at all these strange goings-on? Not necessarily, Boyd said, because Einstein’s speed limit applies only to effects that carry some sort of information.
“In this case, as with all fast-light experiments, no information is truly moving faster than light,” said Boyd.
The hump-like pulse has long leading and trailing edges, Boyd explained. “The leading edge carries with it all the information about the pulse and enters the fiber first. By the time the peak enters the fiber, the leading edge is already well ahead, exiting. From the information in that leading edge, the fiber essentially ‘reconstructs’ the pulse at the far end, sending one version out the fiber, and another backward toward the beginning of the fiber.”
Boyd said he’s working on ways to see what will happen if he can design a pulse without a leading edge. Einstein says the entire faster-than-light and reverse-light phenomena should disappear. Boyd is eager to put Einstein to the test.
