BaBar experiment confirms time asymmetry

Mon, 11/19/2012 - 8:24am

In this illustration, two different B mesons are changing between states (represented as colors); however blue-B changes into red-B more quickly than red-B changes into blue-B (a process running in reverse-time, as shown by the backwards clock dial). For these transformations, time has proven asymmetrical; the changes are happening at a different rate as time moves forward than when it is reversed, according to measurements from the high-energy physics experiment known as BaBar. Image: Greg Stewart, SLAC National Accelerator LaboratoryTime marches relentlessly forward for you and me; watch a movie in reverse, and you'll quickly see something is amiss. But from the point of view of a single, isolated particle, the passage of time looks the same in either direction. For instance, a movie of two particles scattering off of each other would look just as sensible in reverse—a concept known as time reversal symmetry.

Now the BaBar experiment at the Department of Energy (DOE)'s SLAC National Accelerator Laboratory has made the first direct observation of a long-theorized exception to this rule.


Reported in Physical Review Letters, the results are impressively robust, with a 1 in 10 tredecillion (1043) or 14-sigma level of certainty—far more than needed to declare a discovery.







Time violation had previously been seen in particles called neutral kaons by the CPLEAR experiment at CERN, but that measurement was not direct because of the inability to distinguish T violation from CP violation, and the interpretation of those results drew some criticism. It's hard to set up laboratory conditions that can see time reversal violation, Martínez-Vidal explains. But BaBar provided just the right conditions for a clear, direct measurement.

"In the past, a true test of time reversal symmetry with unstable particles was considered to be impossible," says BaBar associate José Bernabéu, a professor at the University of Valencia and IFIC, and one of the originators of the analysis concept. "It's spectacular that the solution came from the same entanglement phenomenon used for quantum communication and computing."

Michael Roney, BaBar spokesperson and professor at the University of Victoria in Canada, says, "BaBar's data has been extremely fruitful and continues to produce important results, such as this unique and unambiguous test of quantum field theory. As we continue to work on almost 100 measurements from BaBar that investigate the fundamental nature of time and matter, we're gratified to have further validated this underlying theory."

Source: SLAC National Accelerator Laboratory


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