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.
"It was exciting to design an experimental analysis that enabled us to observe, directly and unambiguously, the asymmetrical nature of time," said BaBar collaborator Fernando Martínez-Vidal, associate professor at the University of Valencia and member of the Instituto de Fisica Corpuscular (IFIC), who led the investigation. "This is a sophisticated analysis, the kind of experimental work that can only be done when an experiment is mature."
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. It produced almost 500 million pairs of particles called B mesons and their antimatter counterparts B-bar mesons for study. BaBar scientists found that B mesons and B-bar mesons do, indeed, behave differently in ways that violate so-called CP symmetry, which incorporates the symmetries of charge (positive versus negative) and parity (which can be thought of as left-handedness versus right-handedness). This discovery of CP violation contributed to the 2008 Nobel Prize in Physics.
CP symmetry is linked with time reversal symmetry through the CPT (charge-parity-time) Theorem, which states that 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.
So the BaBar data, with its evidence of CP symmetry violation already in hand, was a good place to look for violation of time reversal symmetry that would serve to balance CPT as a whole.
BaBar's new time violation analysis was based on a concept proposed in 1999. Researchers examined a chain of particle transformations in which B mesons flipped between two different states called B-zero and B-even. Taking advantage of the quantum entanglement of the B mesons, which enables information about the first decaying particle to be used to determine the state of its partner at the time of the decay, they were able to find that these transformations happened six times more often in one direction than the other.
"This is a fresh way to understand data we had already used to measure CP violation," said BaBar physics coordinator Abner Soffer, associate professor at Tel Aviv University. "By looking at it slightly differently we were able to undeniably see time violation as well. What's nice is that the effect was there the whole time, but nobody had thought about it the right way before."
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 explained. 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," said 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, said "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."
DOE/SLAC National Accelerator Laboratory: http://www.slac.stanford.edu
This press release was posted to serve as a topic for discussion. Please comment below. We try our best to only post press releases that are associated with peer reviewed scientific literature. Critical discussions of the research are appreciated. If you need help finding a link to the original article, please contact us on twitter or via e-mail.
A laser-driven particle accelerator just 9 centimetres long is gearing up to rival heavyweights like the Large Hadron Collider
An inside look at Corning’s labs suggests what’s next for the inventor of Gorilla Glass.Someday your smartphone might be able to help you in a new way when you’re traveling: by telling you whether the water is safe to drink.
Scientists shed light on the energetic emission of radiation that occurs in thunderstorms.
NASA's Mars rover has sniffed out short-lived bursts of methane, but whether it's Martian life or just a geological by-product is still unclear
CERN's Large Hadron Collider will be turned back on in March and a few weeks later will start smashing sub-atomic particles together again at nearly double its previous power, helping scientists hunt for clues about the universe.
The flat disc shape of the Milky Way galaxy had been a mystery. Now simulations suggest it could be thanks to winds driven by charged particles
Understanding the different ways in which birds get their vivid hues could help us make coloured displays for devices such as e-readers
The water and sediment flow might have been massive enough to build a mountain, NASA researchers say.
Liquids come in many forms, from bubbles and droplets to jets and sheets. Henri Lhuissier of Paris Diderot University and his colleagues use high-speed cameras to zoom in on the physics behind all kinds of liquid phenomena, which arise throughout nature, such as in the formation of raindrops and morning dew
There’s still a lot scientists don’t know about the Higgs boson. Now, you can help make the next discovery.