Time's arrow: Particles cannot go back to the future

We have all laughed at those fragments of film run backwards, where broken vases miraculously reassemble themselves, or sprinters reverse at full speed into their starting blocks. In the everyday world these things could never happen ­- time's arrow only points forwards. Hitherto, the laws of fundamental physics have seemed to be reversible: a film of elementary particle behaviour run backwards is indistinguishable from the same film run forwards. A particle sprinter could apparently run full-speed backwards as well as forwards. But now physicists at the CPLEAR1 experiment at CERN2 have measured directly for the first time that, for the particles called kaons, time is different when it moves forwards or backwards.

The reason why the future and the past are so different in our daily lives is because the universe started off in the big bang in a smooth and organized state. However, as the universe expanded it became more irregular and disorganized. There are very few organized states, but many disorganized states. Consider the pieces of a broken vase: there is one and only one arrangement of the pieces in which they all fit together and form the vase, but there are very many arrangements in which the pieces are all jumbled up. A vase thrown on the floor will normally go into one of these disorganized states. Similarly, the universe started in an organized state and is evolving into some disorganized state, simply because there are so many more disordered states. This is the origin of the large-scale arrow of time.

However, the classical laws of physics discovered by Galileo, Newton and Einstein are time-symmetric, and do not distinguish between the future and the past. These precedents have to be questioned in the microscopic world of sub-atomic physics. Here, quantum physics has brought us antiparticles, and previous laboratory experiments have taught us that particles and antiparticles do not always behave in the same way. This led physicists to think that the equations of the subatomic world would not look the same if time were reversed. To test this, the CPLEAR collaboration at CERN collided antiprotons and hydrogen atoms to make kaons and their antimatter counterparts, antikaons. As they travel, antikaons can transform into kaons and vice versa. The team used a large detector to count the kaons and antikaons as they decayed ­ each to an electron, a pion, and a neutrino. The charge of the electron revealed which type of kaon had decayed. In a paper published on October 7, 1998, which will appear in an upcoming issue of Physics Letters B, the CPLEAR team shows that the rate for antikaons transforming into kaons is higher than that for kaons becoming antikaons, the time-reversed process. This experiment has observed a microscopic arrow of time, for the first time in the history of physics.

This experiment casts a new and sharper light on the fundamental symmetries of physics, and how they are broken. Physicists know that time symmetry (T) has to be part of a larger, more powerful package known as CPT symmetry (for charge, parity, and time reversal), which sits at the heart of modern physics. Swap antimatter for matter, view the universe in a mirror, and reverse the direction of time, and all experiments should come out the same way they do in the real world. The CPT symmetry package has been checked by many experiments, most accurately by CPLEAR. However, back in 1964, physicists found that the charge-parity (CP) symmetry combination was not respected. According to the CPT symmetry package, this means that time symmetry (T) should also be violated, in a way that compensates for the CP asymmetry. CPLEAR's achievement is to measure a small time asymmetry at just the level that compensates for the CP asymmetry observed over three decades ago. Following preliminary reports by the CPLEAR team at a number of major conferences starting in 1995 and the recent CPLEAR publication, the violation of time-reversal has subsequently been confirmed by the KTeV experiment at the Fermi National Accelerator Laboratory in the United States, which reported preliminary results obtained using a different technique on October 12th, 1998.

CPLEAR's new measurement confirms what physicists have long suspected, that T symmetry is violated by kaons, as expected if CPT is conserved. The violation of the T and CP symmetries may be responsible for the dominance of matter over antimatter in the universe today. This physics is the subject of ongoing experiments at various laboratories around the world, and it will the principal objective of one of the experiments at CERN's future Large Hadron Collider.


1. CP LEAR is an international collaboration with Universities and Institutes from 9 Countries: France, Greece, Great Britain, Netherlands, Portugal, Slovenia, Sweden, Switzerland and the United States of America.

2. CERN, the European Laboratory for Particle Physics, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. Israel, Japan, the Russian Federation,the United States of America, Turkey, the European Commission and Unesco have observer status.

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