November 19, 2012 | David F. Coppedge

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?


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  • snelldl says:

    While following the verification of the Higgs boson earlier this year, one of the things that was frequently discussed in the physics blogs was that the confidence level for declaring verification was 5 sigma. Here we have 14 sigma. I guess that pretty much nails it.

    Is time asymmetry a consequence of the 2nd law, or is the 2nd law a consequence of time asymmetry? Or are the two related at all? I’m just asking.

  • AnthonyMills says:

    Basically while this is an extremely impressive result, this has been predicted for a while. Cronin and Fitch won the Nobel Prize in Physics in 1980 for demonstrating Charge-Parity asymmetries, and this implied that certain Time asymmetries had to occur. But while this shows that shows that B mesons oscillate at slightly different rates depending on the time direction, the concept of T violation in physics has no bearing on why entropy increases.

  • AnthonyMills says:

    Hi snelldl,

    The two aren’t related at all. For most interactions, time is symmetric. A photon moving left to right through time looks exactly like a photon moving right to left through anti-time.

    So what does cause the 2nd Law? Think of the surface of a completely still pond. If you drop a pebble in, ripples ripple outwards and disturb the water. Now ripples move the same way; a ripple moving one direction through time is the same as a ripple moving the opposite direction backwards in time. But there’s only one way the pond can be smooth, and many ways it can be disturbed, so over time, through processes that are individually random but predictable over the long term, the system becomes more disordered.

    Basically it’s a result of the idea that you have only one past, but you could have many futures. Sorry if that’s not concise enough; it’s a difficult concept to explain.

    • Editor says:

      Anthony, the 2nd Law cannot be explained so easily. You’re looking at Boltzmann’s conjecture about time asymmetry arising from multiple states, most of which are improbable. Actually, since the laws of physics are time-symmetric, there is no reason not to expect entropy to increase in the past as well as the future. Boltzmann later changed his view to balance out the asymmetry by proposing that time ran forward in our locale, but backward in other parts of the universe.

      Many cosmologists point to the “past hypothesis” (the big bang as low entropy) as the explanation for time asymmetry, but this requires the universe being so astonishingly low in entropy as an initial condition (something like 1 chance in 10^10^43) that it would be indistinguishable from a miracle — thus the escape route into the multiverse, which is unscientific because it is unobservable even in principle. Even then, though, the past hypothesis begs the question of why entropy could not increase into the past initial conditions of the big bang instead of into its future.

      The question of why the Second Law exists and why time flows in one direction is deeper than most people imagine: thus Vedral’s interesting proposal (see 10/21 entry) that the Second Law might be the long-sought Theory of Everything, something we take as a given.

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