Electronic Precinct - February 1999

The Arrow of Time

Dr Christos Touraminis (PPARC Adv. Fellow), Dr Peter Hayman, Dr Mike Carroll (Research Associate), Professor Erwin Gabathuler and Dr Ray Gamet.

A group of Liverpool physicists working at the CERN Laboratory in Geneva have carried out an experiment which shows for the first time that the arrow of time is unique and that in our universe time travels only in one direction. The simplest way of understanding time reversal is to run your video r ecorder backwards. The laws of physics have long been considered to be symmetric in time, ie moving forward or backward in time is identical, but there has been theoretical evidence that this may not be true in an important area of particle physics relating to the interchange of matter and antimatt er. This new experimental result, published in the December issue of Physics Letters provides direct experimental evidence that the time process is not identical in both directions.

It is impossible to reverse the arrow of time in the world as we know it: once something happens it is gone forever. However when you move into the sub-atomic world of particle physics, it is possible using a strange short-lived particle called the kaon and its anti-matter equivalent called the ant i-kaon to test this hypothesis. As they travel, kaons can transform into anti-kaons and vice-versa.

The CPLEAR experiment was designed in such a way that it was possible to begin with a given number of kaons and follow their existence until they decayed into other sub-atomic particles including electrons and anti-electrons (called positrons of opposite charge). The charge of the electron revealed which type of matter or anti-matter had decayed. The process was then repeated starting with anti-kaons. The CPLEAR team showed that the rate for anti-kaons transforming into kaons is higher than the time reversed process for kaons becoming anti-kaons. This means that anti-matter decays faster tha n matter.

The Big Bang occurred 1.5x1010 years ago when equal amounts of matter and antimatter should have been created. It is one of the biggest unsolved questions in Physics as to why our present Universe consists predominantly of matter. This experiment provides a clue to that puzzle.

The experiment consisted of 100 physicists from Europe and US. Liverpool was the only UK group involved in this experiment to study the fundamental symmetries in nature.

Professor Erwin Gabathuler, emphasising the importance of these fundamental measurements, stated that the CPLEAR group, including Dr J R Fry currently on leave of absence in California, was now involved in the next stage using a much heavier matter-antimatter particle combination at Stanford Univer sity in the United States in collaboration with other UK, European and US institutes.