Universe a grand tour of modern science Phần 9

I 604 n the late 1980s physicists at CERN, Europe’s particle physics lab in Geneva, began a long series of experiments aimed at simulating the Big Bang in little bangs hot and dense enough to set quarks free. These are the fundamental entities that constitute the heavy matter in the atomic nucleus. No one doubted by then that each of the protons and neutrons in a nucleus consists of three fundamental entities called quarks. Various experiments with particle accelerators had indirectly confirmed the presence of the quarks in the nuclear material. But no one had seen any free quarks. If you try to liberate a quark in ordinary reactions between particles, you unavoidably create a new quark and an antiquark. One of them immediately replaces the extracted entity. The new antiquark handcuffs the would-be escaper in a particle called a meson. This is the trick by which Mother Nature has kept quarks in purdah since the world began. To be more precise, the confinement of quarks began about 10 millionths of a second after the start of the Big Bang, at the supposed origin of the Universe. Before then, in unimaginably hot conditions, each quark could whizz about independently. Technically speaking, it was allowed to show its colour in public. By the colour of a quark, physicists mean a quality similar to an electric charge. But instead of just plus and minus, the colour charge comes in three forms, labelled red, green and blue. The quarks are not really coloured, but it’s a convenient way of thinking about the conditions of their confinement in ordinary matter. In a TV screen, a red, green and blue dot together make white, and the rule nowadays is that nuclear matter, too, must be white. That’s why protons and neutrons consist of three quarks apiece, and not two or four. One red, one green and one blue quark within each proton or neutron are held loosely together by particles called gluons. The colour force carried by the gluons operates only over very short ranges. Space is opaque to the colour force, in much the same way as frozen water is impenetrable by fishes. But at a high enough temperature space melts, so to say, and lets the colour force through. Then the quarks and gluons can roam about quark soup as freely as do the individual charged atoms and electrons in the electrified gas, or plasma, of a neon lamp. The effect of the colour force is greatly weakened because immediate neighbours screen each particle from the pull of distant particles. The resulting melee is called a quark—gluon plasma or, more colloquially, quark soup. Extremely high pressure may have the same effect as high temperatures, and physicists suspect that quark soup exists at the core of a neutron star, which is a collapsed star just one step short of a black hole. That’s what the theory says, anyway, but to set the quarks free experimentally required creating a new state of matter never seen before. I ‘A spectacular excess of strangeness’ A multinational team of physicists working at CERN set out to make quark soup by using an accelerator, the Super Proton Synchrotron, to melt the nuclei of heavy atoms. It was a matter of whirling heavy atoms up to high energy and slamming them into a target also made of heavy atoms—lead onto lead, for example. A direct hit of one nucleus on another would create a small fireball, and might briefly produce the extreme conditions needed to liberate quarks. The quarks would recombine almost instantly into a swarm of well-known particles and antiparticles, and fly as debris out of the target into detectors beyond. Only by oddities in the composition of the debris might one know that a peculiar state of matter had existed for a moment. For example the proportions of particles containing distinctive strange and charmed quarks might change. Charmed quarks are so heavy that they require a lot of energy for their formation, in the first moment of the nuclear collision. They would normally tend to pair up, as charmed and anticharmed quarks, to make a well-known particle called charmonium, or J/psi. But if conditions are so hot that plasma screening weakens the colour force, this won’t happen. The charmed quarks should enjoy a brief freedom, and settle down only later, in the company of lighter quarks. In the next moment of the nuclear collision strange quarks, somewhat lighter, are being mass-produced. By this time the colour force is much stronger, and it should corral the strange quarks, three at a time, to make a particle called omega. In short, the first signs of quark soup appearing fleetingly should be few charmoniums and many omegas. That was exactly what the CERN experimenters saw. By 1997 they were reporting a shortage of charmoniums among the particles freezing out of the supposed soup. Within a few years they had also accumulated ample evidence for a surplus of the strange omega particles. 605 quark soup ‘A spectacular excess of strangeness, with omega production 15 times normal, is just the icing on the cake,’ said Maurice Jacob of CERN, who made a theoretical analysis of the results of the nuclear collisions. ‘Everything else checks too—the relative proportions of other particles, the size of the fireballs, and so on. We definitely created a new state of matter, ten times denser than nuclear matter. And the suppression of charmonium showed that we briefly let the charmed quarks out of captivity.’ For the sake of only one criterion did the CERN team hesitate to describe their ‘new state of matter’ as quark soup, or to claim it as a true quark–gluon plasma. The little fireballs were not sufficiently hot and long-lived for temperatures to average out. It was like deep-frozen potage microwaved but not stirred, and in Switzerland no self-respecting cook would call that soup. I A purpose-built accelerator In 2000, colleagues at the Brookhaven National Laboratory on Long Island, New York, took over the investigation from CERN. Their new Relativistic Heavy Ion Collider was expressly built to make quark soup. Unlike the experiments at CERN, where one of the two heavy nuclei involved in an impact was a stationary target, the American machine brought two fast-moving beams of gold nuclei into collision, head-on. It achieved full energy in 2001, and four experimental teams began to harvest the results of the unprecedented gold-on-gold impacts. Before long they were seeing evidence of better temperature stirring and other signs of soupiness. These included a reduction in the jets of particles normally produced when very energetic quarks try to escape from the throng. In quark soup, such quarks surrender much of their energy in collisions. ‘It is difficult to know how the resulting insights will change and influence our technology, or even our views about Nature,’ commented Thomas Kirk of Brookhaven, ‘but history suggests there will be changes, and some may be profound.’ E For more about quarks and gluons, see Particle families. The supposed sequence of events at the birth of the Universe is described in Big bang. 606 I n a way, Relativitatstheorie was always a poor name for Albert Einstein’s ideas about space, time, motion and gravity. It seemed to make science iffy. In truth, his aim was to find out what remained reliable in physical laws despite confusions caused by relative motions and accelerations. His conclusions illuminate much of physics and astronomy. Taken one by one, the ideas of relativity are not nearly as difficult as they are supposed to be, but there are quite a lot of them. One of the main theories is special relativity (1905) concerning High-speed travel. Another is general relativity (1915) about Gravity. Energy and mass appear in Einstein’s famous E ¼mc2, which was a by-product of special relativity. It reveals how to get energy from matter, notably in powering the Stars and also Nuclear weapons, which were a fateful by-product. The equation implies that you can make new matter as a frozen form of energy, but when Paul Dirac combined special relativity with quantum theory it turned out that you inevitably get Antimatter too. General relativity is another box of tricks, among which Black holes dramatize the amazing effects on time and space of which gravity is capable. They are also very efficient converters of matter into energy. Gravitational waves predicted by general relativity are being sought vigorously. More speculative are wormholes and loops in space, suggesting the possibility of Time machines. Applied in cosmology, Einstein’s general relativity could have predicted the expansion of the Universe, but he fumbled it twice. First he added a cosmological constant to prevent the expansion implied by his theory, and then he decided that was a mistake. In the outcome, his cosmological constant reappeared at the end of the 20th century when astronomers found that the cosmic expansion is accelerating, driven by Dark energy. Special relativity seems unassailable, but doubts arise about general relativity because of a mismatch to quantum theory. These are discussed in Gravity and Superstrings. 607 A t bogazkoy in turkey you can still see the Bronze Age fortifications of Hattusas, capital of the Hittites. Suppiluliumas I, who reigned there for 40 years in the 14th century bc, refurbished the city. He came to a sticky end after the widow of Tutankhamen of Egypt invited one of his sons to marry her and become pharaoh. Opponents in Egypt thought it a bad idea and assassinated the Hittite prince. An ensuing conflict brought Egyptian prisoners of war to Anatolia. They were harbouring smallpox, long endemic in their homeland. The result was an epidemic in which Suppiluliumas I himself became the first victim of smallpox whose name history records. That was in 1350 bc. The last person to die of smallpox was Janet Parker of Birmingham, England, in 1978. She was a medical photographer accidentally exposed to the smallpox virus retained for scientific purposes. In the previous year in Merka, Somalia, a cook named Ali Maow Maalin had been the last to catch the disease by human contagion, but he survived. In 1980, the World Health Organization in Geneva formally declared smallpox eradicated, after a 15-year programme in which vaccinators visited every last shantytown and nomadic tribe. This was arguably the greatest of all the practical achievements of science, ever. Individual epidemics of other diseases sometimes took a high toll, including the bubonic plague that brought the Black Death to 14th-century Eurasia. Overall smallpox was the worst. Death rates in infants could approach 100 per cent, and survivors were usually disfigured by the pockmarks of the smallpox pustules, and often blinded. The historian Thomas Macaulay wrote of smallpox ‘turning the babe into a changeling at which the mother shuddered, and making the eyes and cheeks of the betrothed maiden objects of horror to the lover.’ Populations in Eurasia and Africa were left with a level of naturally acquired immunity. But when European sailors and conquistadors carried smallpox and other diseases to regions not previously exposed to them, in the Americas and Oceania, they inadvertently wiped out most of the native populations. One victim was the Aztec emperor Ciutlahuac in 1520. Nor was it always inadvertent. ‘The devastating effect of smallpox gave rise to one of the first examples of biological warfare,’ noted the medical historians 608 ... - tailieumienphi.vn