Even for Quantum Particles, Time Flows One Way
Claims of time symmetry at the quantum level have been discounted by a high-reliability experiment by the Department of Energy.
A press release from the SLAC National Accelerator Laboratory announced:
Time 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’s (DOE) SLAC National Accelerator Laboratory has made the first direct observation of a long-theorized exception to this rule.
Digging through nearly 10 years of data from billions of particle collisions, researchers found that certain particle types change into one another much more often in one way than they do in the other, a violation of time reversal symmetry and confirmation that some subatomic processes have a preferred direction of time.
Reported this week in the journal 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.
The BaBar experiment was an ideal test of the CPT (charge-parity-time) Theorem that states, “the three symmetries must remain in balance for any given particle system. If one of the symmetries is out of whack, at least one of the others must be, too.” Since 10 years of BaBar data already had evidence of CP asymmetry in hand, physicists thought it “was a good place to look for violation of time reversal symmetry that would serve to balance CPT as a whole.” Other hints of time reversal were harder to test. This one shows that the rate of time reversal for quantum events is outmatched: “the changes are happening at a different rate as time moves forward than when it is reversed,” a diagram caption states.
One researcher said, “It was exciting to design an experimental analysis that enabled us to observe, directly and unambiguously, the asymmetrical nature of time.” BaBar also had another goal: “BaBar, which collected data at SLAC from 1999 to 2008, was designed to tease out subtle differences in the behavior of matter and antimatter that might help account for the preponderance of matter in the universe.” The “antimatter problem” still is a puzzle for modern cosmology. See also Science Daily‘s coverage.
It appears that time asymmetry, a consequence of the Second Law of Thermodynamics, is more firmly established by this experiment (confirmation to a one part in a tredecillion is pretty rare!), but we will leave it to the specialists to explain the implications. What impact will this have on cosmology?