| [ PhilosophyThought ] in KIDS 글 쓴 이(By): chang (장상현) 날 짜 (Date): 1999년 4월 28일 수요일 오후 12시 13분 10초 제 목(Title): Time's broken arrow 혹시 그논문을 보실 수 없으나 읽고 싶은 분을 위해 밑에 전문을 올립니다. (번역은 않겠습니다. 번역을 하기가 귀찮기도 하지만 대충 내용을 이해하실 수 있는 분들은 굳이 번역을 안해드려도 잘 이해하시리라 믿기 때문에...) TIME'S BROKEN ARROW. By Ken Peach Two experiments(1-4) studying the properties of a particle called the neutral kaon have reported clear evidence that the world is not symmetric with respect to time - that is, it does matter which direction the arrow of time chooses to take. These observations are not unexpected, but they show that the quantum world still has mysteries to be revealed. Although special relativity teaches us that time and space are intimately entwined, time is still a very special variable. For example, in space we can compare the results of an experiment that is aligned north-south in one place with those of the same experiment aligned south-north or east-west in another. The classic Michelson-Morley experiment, which showed that the velocity of light does not depend upon the direction of space, and which was so crucial in the development of special relativity, is just one example of an experiment which relies on measurements in different regions of and directions in space. Time, however, is different. We can compare the results of an experiment today with those obtained yesterday but we cannot so easily compare the results of experiments with different directions of time. The two experiments manage to circumvent this by using the properties of the neutral kaon. The neutral kaon is made from a down quark and an antistrange quark, whereas the neutral antikaon is made from an antidown quark and a strange quark. They are different particles. However, once created, some quantum-mechanical 'magic' occurs so that as time passes an initially pure kaon state becomes a mixture of kaon and antikaon states, a process known as mixing. Similarly, an initially pure antikaon state will also become mixed. In a universe in which it is immaterial which is called the kaon (matter) and which is called the antikaon (antimatter), a beam of neutral kaons would evolve into a mixed state which consisted of exactly equal amounts of kaon and antikaon, independently of the production mechanism. Such a universe would also be symmetric under the reversal of time. It has been known for more than 30 years that the properties of matter and antimatter are not completely equivalent. In particular, in any beam of neutral kaons there will eventually be a small (0.3010) excess of matter over antimatter. This is the phenomenon of CP-violation, where C is the quantum-mechanical operation of replacing all particles by their antiparticles and P is the parity operation of reversing the direction of all spatial components. There is strong evidence to support the theoretical expectation that the Universe should be symmetric if the CP operation is combined with T, the time-reversal operation which reverses the direction of time. (There is a subtlety in that this operation also changes the quantum-mechanical amplitude which describes the physical process into its complex con jugate. ) Until now, there has been no direct evidence for an asymmetry under time reversal. The CPLEAR experiment at CERN, Geneva, has reported convincing evidence for such an asymmetry(1,2). The concept is quite simple - produce kaons and antikaons and observe them, recording when they decay as antikaons and kaons respectively (Fig. I ) . If the probability that an initially produced kaon decaying as an antikaon is different from that of an initially produced antikaon decaying as a kaon, then self-evidently the process is not symmetrical under time reversal. The experiment relies on being able to 'tag' the matter/antimatter state of both the produced kaon and the decayed kaon. The kaons and antikaons are generated in proton-antiproton annihilations, where the initial state is identified by noting the strangeness of the accompanying charged kaon. A neutral kaon is accompanied by a negatively charged kaon while the neutral antikaon is accompanied by a positively charged kaon. When the neutral kaon decays into a pion, an electron and a neutrino, the electron charge is positive (that is, e+, a positron), whereas the antikaon decays to a negatively charged electron, e- . The CPLEAR team has observed a total of 1.3 million events in which a kaon demonstrably changes its nature. It turns out that there is a 0.66 +/- 0.18% greater probability that an antikaon will turn into a kaon than the other way round. Meanwhile, as reported at a workshop and described on a website(3,4), the KTeV experiment at Fermilab, Chicago, has come to a similar conclusion using a very different method. By studying the dynamics of the decay of the neutral kaons into two charged pions and an electron-positron pair, it is possible to deduce time-reversal non-invariance. In effect, the distribution of the angle between the plane containing the electron-positron pair and the plane containing the positive and negative pion pair should be symmetrical if time reversal is a good symmetry. From a data sample of 1,800 such decays, identified in a total of 130 million events, they observe an asymmetry of 13.5 +/- 3.9i%, clear evidence that time reversal is also violated in this decay. The results of both experiments are consistent with expectations based on the observed amount of CP violation in the neutral kaon system. Further measurements will be possible, for example at the experiments that begin next year at the Stanford Linear Accelerator, California, and at KEK, the accelerator facility at Tsukuba in Japan. These will follow decay of the B meson, which is similar to the kaon but with the strange quark replaced by a bottom quark. This system should have similar characteristics to the kaon, and should show similar effects. There is also the possibility that the measurements of the electric dipole moment of the neutron could reveal evidence for time-reversal non-invariance. What is exciting now is that, for the first time there is direct evidence that the direction of time's arrow does matter -- the Universe would be different if the universal clock were the other way round. 'Ken Peach is in the Department of Particle Physics, CLRC-Rutherford Appleton Laboratory, Chilton, Nr Didcot, Oxfordshire OX11 0QX, UK. e-mail: Ken.Peach@rl.ac.uk' Reference 1. http://www.cern.ch/cplear/Welcome.html 2. Angelopoulos, A. et.al. Phys. Lett. B (in press) 3. Arenton, M. Workshop on Heavy Quarks at Fixed Targets, Fermi National Accelerator Laboratory, Chicago, 10-12 Octorber 1998. 4. http://fnphyx-www.fnal.gov/experiments/ktev/pipiee/pipiee_bg.html 장상현 e-mail : schang@tuhep.phys.tohoku.ac.jp http://www.phys.ufl.edu/~schang |