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BOLETÍN JULIO - AGOSTO - SEPTIEMBRE 2012
TITULARES
- WHY THE HIGGS BOSON MATTERS VER +
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PHYSICS IN DAILY LIFE: Singing glasses VER +
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CHINA, EUROPE AND MORE VER +
- NOVEDAD EDITORIAL. The US Hispanic Heritage VER +
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WHY THE HIGGS BOSON MATTERS
by Steven Weinberg
The July 4 announcement that the “Higgs boson” had been discovered at the CERN laboratory in Geneva made news around the world. Why all the fuss? New discoveries of elementary particles have been made from time to time without attracting all this attention. It is often said that this particle provides the crucial clue to how all the other elementary particles get their masses. True enough, but this takes some explanation.
We have a well-tested theory of elementary particles and the forces that they exert on each other, known as the Standard Model. A central feature of the Standard Model is a symmetry between two of these forces: the electromagnetic force, and the less familiar weak nuclear force, which provides the first step in the chain of reactions that gives the sun its energy.
The symmetry means that the particles carrying these forces enter into the equations of the theory in essentially the same way. You could interchange the photon, the particle of light that carries the electromagnetic force, with some combination of the W and Z particles that carry the weak nuclear force, and the equations would be unchanged.
If nothing intervened to break this symmetry, the W and Z, like the photon, would have no mass. In fact, all other elementary particles would also be massless. But of course, most elementary particles are not massless. For instance, unlike the massless photon, the W and Z particles have nearly 100 times the mass of a hydrogen atom.
Since the early 1960s it has been known that it is possible for symmetries to be exact properties of the equations of a theory and yet not respected by observable physical quantities, like the values of particle masses. The consequences of such symmetry breaking were worked out in 1964 by Robert Brout and François Englert; by Peter Higgs; and by Gerald Guralnik, Cari Hagen and Tom Kibble, for a general class of theories that contain force-carrying particles, like the photon.
In 1967-8 the late Abdus Salam and I independently used this mathematics in formulating a specific theory, the modern unified theory of weak and electromagnetic forces that became part of the Standard Model. This theory predicted the masses of the W and Z particles, which were verified when these particles were discovered at CERN in 1983-84.
But just what is it that breaks the electroweak symmetry and thereby gives elementary particles their masses?
Salam and I assumed that the culprit is what are called scalar fields, which pervade all space. This is like what happens in a magnet: Even though the equations describing iron atoms don’t distinguish one direction in space from another, any magnetic fíeld produced by the atoms will point in just one way. The symmetry-breaking fields in the Standard Model do not mark out directions in space — instead, they distinguish the weak from the electromagnetic forces, and give elementary particles their masses. Just as a magnetic field appears in iron when it cools and solidifies, these scalar fields appeared as the early universe expanded and cooled.
This is where the Higgs boson comes in. The illustrative models studied in most of the papers on symmetry breaking from 1960 to 1964 had introduced scalar fields to break the symmetries, and had typically found that some of these fields would show up as massive particles, bundles of the energy of the fields. Likewise, Salam and I in 1967-68 found that one of the four scalar fields we introduced to break the electroweak symmetry would appear as a new kind of electrically neutral unstable particle. This is the Higgs boson, which may now have been discovered, verifying the Standard Model’s account of how the elementary particles get their masses.
There seems no doubt that a new electrically neutral, unstable particle had been discovered, but is it the Higgs boson? All of the properties of the Higgs boson except its mass were predicted in the 1967-8 electroweak theory, and since the mass of the new particle has been measured, we can now calculate the probabilities for the various ways that it can decay. So far, only a few decay modes have been observed, and though the new particle seems to decay like a Higgs boson, more must be done to pin this down. Also, if the new particle is the Higgs boson, it would have to be like a knuckleball in baseball; unlike all other known elementary particles, it would have no spin. This too must be tested.
These are the cautious words you would expect to hear from a prudent physicist. But I have been waiting for the discovery of the Higgs boson since 1967, and it’s hard for me now to doubt that it has been found.
So what? Even if the particle is the Higgs boson, it is not going to be used to cure diseases or improve technology. This discovery simply fills a gap in our understanding of the laws of nature that govern all matter, and throws light on what was going on in the early universe. It’s wonderful that many people do care about this sort of science, and regard it as a credit to our civilization.
Of course not everyone feels this way, and even those who do have to ask whether learning the laws of nature is worth the billions of dollars it costs to build particle accelerators. This question is going to come up again, since our present Standard Model is certainly not the end of the story. It leaves out gravitation; it does not explain the particular values of the masses of quarks and electrons and other particles; and none of its particles can account for the “dark matter” that astronomers tell us makes up five-sixths of the mass of the universe. You can count on physicists to ask their governments for the facilities they need to grapple with these problems.
A case can be made for this sort of spending, even to those who don’t care about learning the laws of nature. Exploring the outer frontier of our knowledge of nature is in one respect like war: It pushes modern technology to its limits, often yielding new technology of great practical importance.
For instance, the new particle was produced at CERN in collisions of protons that occur at a rate of over a hundred million collisions per second. To analyze the flood of data produced by all these collisions requires real time computing of unmatched power. Also, before the protons collide, they are accelerated to an energy over 3,000 times larger than the energy contained in their own masses while they go many times around a 27-kilometer circular tunnel. To keep them in their tracks requires enormously strong superconducting magnets, cooled by the world’s largest source of liquid helium. In previous work at CERN, elementary particle physicists developed a method of sharing data that has become the World Wide Web.
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The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator
On a longer time scale, the advance of technology will reflect the coherent picture of nature we are now assembling. At the end of the 19th century physicists in England were exploring the properties of electric currents passing through a near vacuum. Although this was pure science, it led to our knowledge of the electron, without which a large part of today’s technology would be impossible. If these physicists had limited themselves to work of obvious practical importance, they would have been studying the behavior of steam boilers.
Steven Weinberg is a professor in the physics and astronomy departments at the University of Texas at Austin, and the recipient of the Nobel Prize in Physics for his work on the unified theory of weak and electromagnetic forces.
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PHYSICS IN DAILY LIFE: Singing glasses
Everybody knows how to make a wineglass sing. Just run a wet finger around the rim of a clean glass. It is a well known after-dinner trick, but as physicists we may ask: what exactly makes the glass squeak, and what mode of vibration are we inducing? Let’s first look at the mechanism. It is the same ‘stick-slip’ motion that is used to play the violin, or the cello and the bass. Without probably realising it, when we run our finger around the rim of the glass we are making use of the fact that dynamic friction is lower than static friction. Whenever our finger sticks for a split second, the glass is slightly stretched, but once our finger gets moving again, the glass surface slides back easily, returning to its original state.
Now what kind of vibration are we inducing in the glass? Given the relatively loud noise that we can produce using this trick, we suspect that it must be a transverse vibration, so that the side of the glass transmits the noise to the surrounding air. Moreover, we guess that we are exciting the simplest possible vibrational mode: the fundamental.
Indeed, if we tap the glass at its side using a spoon, we notice that the frequency - or pitch - of the sound produced is the same as the one we get by running our finger around the rim. This strongly suggests that we are exciting the fundamental in both cases. In other words: if we tap, say, the North side of the glass, we should expect antinodes also at the South, East and West sides of the glass, and nodes at the four positions just in between. There is an elegant way to prove us right. Just replace the glass by a mug that has a handle. If there is a choice, take a thin-walled mug made of good-quality pottery. Put it in front of you on the table with the handle towards you, pointing South, so to speak. Now take a spoon and tap. If you tap at the opposite side (North), or East or West, you produce a tone that is distinctly lower than if you tap at positions in between. This confirms that it is the fundamental vibration mode that we are exciting: it is the handle’s extra mass that makes the frequency lower if it is positioned on an antinode. Look at it as a simple variation on the harmonic oscillator theme, for which we remember the frequency to be determined by √(k/m), with k the force constant and m the mass.
Further evidence for the fact that we are dealing with the fundamental is obtained by holding the mug by the handle and repeating the experiment. If we tap the mug at the side opposite from the handle, the (lower) tone that we get is distinctly more damped than the one we get if we tap in between (the higher tone).
So, should your next formal dinner turn out a bit dull, physics may come to the rescue and bring some unexpected entertainment. Provided, of course, that there is wine. And coffee cups with a reasonable quality...
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L.J.F. (Jo) Hermans, Leiden University, The Netherlands
EUROPHYSICS NEWS, 43/4, 2011
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CHINA, EUROPE AND MORE
A delegation of the European Physical Society (EPS) was invited to participate in the celebration of the 80th anniversary of the Chinese Physical Society (CPS), on August 25, 2012, at the Tsinghua University, Beijing. The official ceremony took place in the morning, in the huge University auditorium.
A formal greetings address was delivered by former EPS president Maciej Kolwas, along with other addresses by distinguished representatives of the major physical societies in the world (fig. 1). The growing role of the CPS in developing cooperation with insitutions from other countries and regions, including the EPS, was acknowledged. The first formal exchanges CPS-EPS began in 1995, at the 2nd World Congress of Physical Societies, which was organised in Japan by the Physical Society of Japan (PSJ), the Japan Society of Applied Physics (JSAP) and the Association of Asia Pacific Physical Societies (AAPPS). The American Physical Society (APS) had organised the 1st World Congress of Physical Societies in 1986 in the US.
FiG.1: Former EPS President Maciej Kolwas during the ceremony
The EPS organised the 3rd World Congress of Physical Societies in Berlin, in 2000. In addition to resolutions regarding physics education, physics communication, and the role of physical societies, more than 45 countries endorsed a resolution calling for a World Year of Physics. The CPS was active in this world wide outreach activity demonstrating the contributions of physics to cultural, economic and societal development.
More recently, the CPS has made important contributions through the AAPPS to the co-organisation with the EPS of the Asia Europe Physics Summit (ASEPS). The ASEPS meetings address issues relating to the enhancement of cooperation and collaboration in physics research, education and outreach. Following the success of the first two ASEPS meetings, ASEPS3 will be organised in Chiba, Japan in July 2013.
FiG.2: the poster of the 80th anniversary of the Chinese Physical Society celebration
In the afternoon a restricted Joint Meeting of International Physical Societies was held in the historical Science Building of Tsinghua, where CPS foundation actually occurred in 1932 (fig. 2). During the meeting, a very friendly and informal presentation of the various societies and a round table discussion took place. The participants, warmly welcomed by CPS president Zhan Wen-Long, included, in addition to Maciej Kolwas and myself for the EPS, presidents and representatives from the American Physical Society (APS), the Institute of Physics (IOP), the German Physical Society (DPG), the Physical Society of Hong Kong (PSHK), the Japan Society of Applied Physics (JSAP), the Physical Society of Japan (PSJ), the Association of Asia Pacific Physical Societies (AAPPS), and a number of CPS delegates. (fig. 3) Various matters were discussed, ranging from education, membership and recruitment, equal opportunities, international cooperation, editorial policy and open access, joint initiatives, etc. Similarities and diversities were duly pointed out, and such a wide-range comparison among societies around the world was in my view extremely useful, interesting and enriching. ASEPS and encompassing initiatives like the International Year of Light were of course vigorously encouraged. We all agreed that research, particularly in physics, is a global undertaking. Physicists know this, through their research, publications and career development. Physical societies, through occasions such as these, share information, and best practice, and work in a very real and personal capacity to improve international communications and cooperation.
So thanks to the CPS for the pleasure of having been with them in Beijing, and our best wishes for continued success, strength and vitality.
FiG.3: The Joint Meeting of the international Physical Societies representatives (see text)
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NOVEDAD EDITORIAL
>>
 THE U.S. HISPANIC HERITAGE
JULIO A. GONZALO
About half the present U.S. territory was originally explored and colonized by Spaniards, and subsequently by Mexicans -half Spanish, half American Indians–. Today, more than 40 million Spanish speaking citizens live in the U.S. They make up more than one half the Catholic population of the country. Apart from the original Spanish Americans, they came during recent decades from Mexico, Puerto Rico, Cuba, Central America, Colombia, Peru, Chile, Argentina, in other words, from all over Spanish speaking America.
The Spanish language, indisputably today the second in importance in the U.S., is co-official with the English in several Southern States and, of course, in Puerto Rico (a free Associated state) counting about 3.9 million inhabitants, plus 5 million more now living and working in the continental U.S. Puerto Ricans travel often back and forth between the island and the continental U.S.
In summary, the Spanish heritage makes up an essential component of the all important Catholic heritage of the United States of America, as important or more as the Protestant heritage.
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