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对不起大家，今天发晚了….
这次的两篇文章来自The Economist，附件里有原版的朗读。
[attachmedia=400,64,0]103362[/attachmedia]
[attachmedia=400,64,0]103363[/attachmedia]
推荐两个小短片，可以帮助我们形象理解Higgs boson
What is a Higgs Boson?
http://www.youtube.com/watch?v=RIg1Vh7uPyw&feature=related
The Standard Model Explains Force And Matter
http://www.youtube.com/watch?v=p5QXZ0__8VU&feature=related
以上推荐来自：
http://sprott.physics.wisc.edu/pickover/physics-book.html
The Higgs boson
Science’s great leap forward
After decades of searching, physicists have solved one of the mysteries of the universe
Jul 7th 2012 | from the print edition
[计时一]
HISTORICAL events recede in importance with every passing decade. Crises, political and financial, can be seen for the blips on the path of progress that they usually are. Even the horrors of war acquire a patina of unreality. The laws of physics, though, are eternal and universal. Elucidating them is one of the triumphs of mankind. And this week has seen just such a triumphant elucidation.
On July 4th physicists working in Geneva at CERN, the world’s biggest particle-physics laboratory, announced that they had found the Higgs boson. Broadly, particle physics is to the universe what DNA is to life: the hidden principle underlying so much else. Like the uncovering of DNA’s structure by Francis Crick and James Watson in 1953, the discovery of the Higgs makes sense of what would otherwise be incomprehensible. Its significance is massive. Literally. Without the Higgs there would be no mass. And without mass, there would be no stars, no planets and no atoms. And certainly no human beings. Indeed, there would be no history. Massless particles are doomed by Einstein’s theory of relativity to travel at the speed of light. That means, for them, that the past, the present and the future are the same thing.
[203 words]
[计时二]
Deus et CERN
Such power to affect the whole universe has led some to dub the Higgs “the God particle”. That, it is not. It does not explain creation itself. But it is nevertheless the most fundamental discovery in physics for decades.
Unlike the structure of DNA, which came as a surprise, the Higgs is a long-expected guest. It was predicted in 1964 by Peter Higgs, a British physicist who was trying to fix a niggle in quantum theory, and independently, in various guises, by five other researchers. And if the Higgs—or something similar—did not exist, then a lot of what physicists think they know about the universe would be wrong.
Physics has two working models of reality. One is Einstein’s general relativity, which deals with space, time and gravity. This is an elegant assembly of interlocking equations that poured out of a single mind a century ago. The other, known as the Standard Model, deals with everything else more messily.
The Standard Model, a product of many minds, incorporates the three fundamental forces that are not gravity (electromagnetism, and the strong and weak nuclear forces), and also a menagerie of apparently indivisible particles: quarks, of which protons and neutrons, and thus atomic nuclei, are made; electrons that orbit those nuclei; and more rarefied beasts such as muons and neutrinos. Without the Higgs, the maths which holds this edifice together would disintegrate.
[234 words]
[计时三]
Finding the Higgs, though, made looking for needles in haystacks seem simple. The discovery eventually came about using the Large Hadron Collider (LHC), a machine at CERN that sends bunches of protons round a ring 27km in circumference, in opposite directions, at close to the speed of light, so that they collide head on. The faster the protons are moving, the more energy they have. When they collide, this energy is converted into other particles (Einstein’s E=mc2), which then decay into yet more particles. What these decay particles are depends on what was created in the original collision, but unfortunately there is no unique pattern that shouts “Higgs!” The search, therefore, has been for small deviations from what would be seen if there were no Higgs. That is one reason it took so long.
Another was that no one knew how much the Higgs would weigh, and therefore how fast the protons needed to be travelling to make it. Finding the Higgs was thus a question of looking at lots of different energy levels, and ruling each out in turn until the seekers found what they were looking for.
[189 words]
[计时四]
Queerer than we can suppose?
For physicists, the Higgs is merely the LHC’s aperitif. They hope the machine will now produce other particles—ones that the Standard Model does not predict, and which might account for some strange stuff called “dark matter”.
Astronomers know dark matter abounds in the universe, but cannot yet explain it. Both theory and observation suggest that “normal” matter (the atom-making particles described by the Standard Model) is only about 4% of the total stuff of creation. Almost three-quarters of the universe is something completely obscure, dubbed “dark energy”. The rest, 22% or so, is matter of some sort, but a sort that can be detected only from its gravity. It forms a giant lattice that permeates space and controls the position of galaxies made of visible matter. It also stops those galaxies spinning themselves apart. Physicists hope that it is the product of one of the post-Standard Model theories they have dreamed up while waiting for the Higgs. Now, they will be able to find out.
[171 words]
[计时五]
For non-physicists, the importance of finding the Higgs belongs to the realm of understanding rather than utility. It adds to the sum of human knowledge—but it may never change lives as DNA or relativity have. Within 40 years, Einstein’s theories paved the way for the Manhattan Project and the scourge of nuclear weapons. The deciphering of DNA has led directly to many of the benefits of modern medicine and agriculture. The last really useful subatomic particle to be discovered, though, was the neutron in 1932. Particles found subsequently are too hard to make, and too short-lived to be useful.
This helps explain why, even at this moment of triumph, particle physics is a fragile endeavour. Gone are the days when physicists, having given politicians the atom bomb, strode confidently around the corridors of power. Today they are supplicants in a world where money is tight. The LHC, sustained by a consortium that was originally European but is now global, cost about $10 billion to build.
That is still a relatively small amount, though, to pay for knowing how things really work, and no form of science reaches deeper into reality than particle physics. As J.B.S. Haldane, a polymathic British scientist, once put it, the universe may be not only queerer than we suppose, but queerer than we can suppose. Yet given the chance, particle physicists will give it a run for its money.
[234 words]
【越障】
The Higgs boson
Gotcha!
The hunt for physics’s most elusive quarry is over
Jul 7th 2012 | from the print edition
“WE HAVE a discovery.” Rolf Heuer, the director-general of CERN, was in no doubt. He left none of the wiggle-room with which physicists often hedge their announcements when he summed up the results of his organisation’s search for the Higgs boson. These were presented in detail on July 4th by Joe Incandela and Fabiola Gianotti, the leaders of the two experiments that have been looking for the elusive particle. CMS, run by Dr Incandela, and ATLAS, run by Dr Gianotti, are fitted to the Large Hadron Collider (LHC), the principal piece of equipment at Europe’s main particle-physics laboratory, near Geneva, which CERN runs. Both have found conclusive evidence for a particle of the right type and mass to be the Higgs. If it is not actually the Higgs, that will be the biggest upset in physics for a century.
It has taken five decades, billions of dollars and millions of man-hours. But, at long last, Peter Higgs, a British physicist (pictured above), and four other, less well-known individuals—François Englert, Gerald Guralnik, Tom Kibble and Carl Hagen—can crack open a bottle of champagne. They are the ones who, in 1964, plucked what has come to be known (unfairly in some eyes) as the Higgs boson from formulae they were working on to fix a niggle in quantum theory. Another co-originator, Robert Brout, died last year.
The discovery puts the finishing flourish on the Standard Model, the best explanation to date for how the universe works—except in the domain of gravity, which is governed by the general theory of relativity. The model comprises 17 particles. Of these, 12 are fermions such as quarks (which coalesce into neutrons and protons in atomic nuclei) and electrons (which whizz around those nuclei). They make up matter. A further four particles, known as gauge bosons, transmit forces and so allow fermions to interact: photons convey electromagnetism, which holds electrons in orbit around atoms; gluons link quarks into protons and neutrons via the strong nuclear force; W and Z bosons carry the weak nuclear force, which is responsible for certain types of radioactive decay. And then there is the Higgs.
[attachimg]103364[/attachimg]
The Higgs, though a boson (meaning it has a particular sort of value of a quantum-mechanical property known as spin), is not a gauge boson. Physicists need it not to transmit a force but to give mass to other particles. Two of the 16 others, the photon and the gluon, are massless. But without the Higgs, or something like it, there is no explanation of where the mass of the other particles comes from.
For fermions this is no big deal. The Standard Model’s rules would let mass be ascribed to them without further explanation. But the same trick does not work with bosons. In the absence of a Higgs, the rules of the Standard Model demand that bosons be massless. The W and Z are not. They are very heavy indeed, weighing almost as much as 100 protons. This makes the Higgs the keystone of the Standard Model. Slot it in and the structure stands. Take it out and it topples. Little wonder that physicists were getting impatient.
The 48-year itch
There are several reasons why it has taken nearly half a century to nab the Higgs. For a start, theory suggests the particle’s own mass (which it gets by interacting with itself) should be huge. Since, as Einstein showed, energy and mass are the same thing, a heavy particle takes more energy to produce. That meant bigger, more powerful and more expensive machines, like the LHC, which smashes together protons travelling in opposite directions in a circular tunnel 27km (17 miles) in circumference.
To complicate matters, the Standard Model is non-committal about what a Higgs should weigh, so physicists had to look across a broad range of possible masses. That meant having to sift through thousands of trillions of collisions.
Nor were they looking for the Higgs per se. Higgs bosons are so unstable that they can never be observed directly. Rather, ATLAS and CMS, which are located on opposite sides of the LHC’s loop, are designed to detect patterns of observable particles that theory suggests the Higgs should break down into. Unfortunately, such patterns are not specific to the Higgs; other subatomic processes produce similar traces.
The experiments could not, therefore, simply identify a Higgs signal. Instead, they looked for an excess of possible signals, amounting to a fraction of a percent over what would have been expected were the Higgs not real. They have both found it at a mass of around 125 giga-electron-volts, in the arcane units used to measure how heavy subatomic particles are. At one chance in 3m of being a random fluctuation, the findings leave no room for doubt. A new particle has been observed.
The next step is to ascertain that it really is the sort of Higgs the inventors envisaged. Although the Standard Model says little about how heavy Higgses ought to be, it is quite specific about how mass affects the ways they decay. Measuring the proportions of the different observed decay modes, and comparing them with these predictions, should show whether the newly discovered particle is the garden variety of Higgs dreamed up back in 1964, or something more exotic.
Here, the data are more equivocal. Some observed decays—in particular one where the Higgs turns into two photons—crop up more often than the Standard Model says they should, though this could still prove to be a statistical fluke.
To many physicists, an exotic Higgs would be good news. For all its explanatory prowess, the Standard Model cannot be the last word in physics. A humdrum Higgs would shore up that venerable theory but offer few clues as to what might replace it.
The constant gardener
One problem is that, as it stands, the model requires its 20 or so constants to be exactly what they are to an uncomfortable 32 decimal places. Insert different values and the upshot is nonsensical predictions, like phenomena occurring with a likelihood of more than 100%.
Nature could, of course, turn out to be this fastidious. But physicists have learned to take the need for such fine-tuning, as the precision fiddling is known in the argot, as a sign that something important is missing from their picture of the world.
One way to look beyond the Standard Model is to question the Higgs’s status as an elementary particle. According to an idea called technicolour, if it were instead made up of all-new kinds of quark held together by a new interaction, akin to but distinct from the strong force, the need for fine-tuning disappears.
Alternatively, the Higgs can maintain its elementary status, but gain siblings. This is a consequence of an idea called supersymmetry, or susy for short. Just as all the known particles of matter have antimatter versions in the Standard Model, in the world of susy every known boson, including the Higgs, has one or more fermion partners, and every known fermion has one or more associated bosons.
Crucially, both theories have a purchase on much that the Standard Model leaves unanswered—why the universe is full of matter and not antimatter, say, or why different fundamental forces have wildly different strengths. Answers to these questions flow naturally out of supersymmetric and technicolour maths. So do candidates for the particles that make up dark matter, a shadowy substance whose presence can be deduced from its gravitational pull (see article) but which does not interact much via the three Standard-Model forces. The lightest of these dark-matter particles might pop up in the LHC.
The problem with these and other proposals has been a conspicuous lack of evidence regarding which, if any of them is a good description of reality. A decade ago physicists were confidently predicting that supersymmetric particles or “techniquarks” would be discovered in short order at the Tevatron (a less powerful American accelerator which was shut down last year) even before the LHC was up and running. This hope failed to materialise. Time and again experiments conformed neatly with the Standard Model, giving no inkling of what might lie beyond it, to the chagrin of many in the field. That may at last change as they start unpicking the Higgs.
The discovery of the boson, then, is rightly hailed as the crowning achievement of one of history’s most successful scientific theories. But it is also almost certainly the beginning of that theory’s undoing, and its replacement by something better. In science, with its constant search for the truth, this is something to celebrate.
【1452 words】 |
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发表于 2012-7-17 13:58:03
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