Symmetry: It's More Like a Guideline

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"The code is more what you'd call "guidelines" than actual rules." — Barbossa, Pirates of the Caribbean: Curse of the Black Pearl

There's some minor rumblings in the physics blogosphere about the latest research results announced this week from Brookhaven's Relativistic Heavy Ion Collider (RHIC). No, not about the record-breaking temperatures that created another quark-gluon plasma, but  about how one relatively minor tidbit about symmetry breaking was portrayed in a New York Times headline. "In Brookhaven Collider, Scientists Briefly Break a Law of Nature" the headline declared. But as pointed out at Swans on Tea, and in greater detail on Cosmic Variance, symmetry breaking is in perfect accordance with the laws of physics: 

While this result is interesting and very helpful to our quest to better understand the strong interactions, it does not represent the overthrow of any cherished laws of physics. On the contrary, it was predicted by the laws of physics as we currently understand them — and by human beings such as Dimitri Kharzeev and others. Parity is an important idea in physics, but it’s broken all the time — very famously by the weak interactions.

It's an understandable minor error: the word "violation" conveys, to non-scientists, the notion that something significant has been broken. And it has — it's just not a fundamental law of physics. The Times reporter, Dennis Overbye, is careful to mention some of the history behind the latest RHIC results, but it's a great story and worth relating in more detail.

From around 1925 until the 1950s, physicists kind of assumed that our world would be indistinguishable from its mirror image. That is, nature would make no distinction between the opposite sides of subatomic particles, or between right- and left-handed rotation. This is known as parity conservation, and numerous experiments had shown it to be true — at least when it came to strong interactions.

But what about weak interactions (those governed by the weak nuclear force)? That hadn't really been tested. By the 1950s, accelerator technology had improved to the point where physicists were seeing weird phenomena that didn't fit the existing theories, or the assumption of parity conservation — things like the unusual decay patterns in particles known as K mesons. Even though K mesons seemed identical, they decayed in two different distinct patterns. This caused physicists — specifically, Chen Ning Yang and Tsung Dao Lee of Columbia University — to question whether parity might not be conserved in weak interactions. They just needed to test that theory.

Enter their fellow Chinese-born colleague, Chien-Shung Wu, a rare woman in the halls of physics during that era. Wu specialized in experimental weak interactions, and suggested they collaborate with scientists at the National Bureau of Standards (now known as the National Institute of Standards and Technology in Maryland).

The experiment involved measuring the direction and intensity of the beta decay radiation emitted by cobalt-60 nuclei, encased in a strong magnetic field to make sure their spins were aligned in the same direction. If parity was truly conserved, there would be equal numbers of beta particles coming from each of the two poles — nature would truly be ambidextrous. If one pole emitted more beta particles than the other, then nature had a preferred "handedness."

The pivotal experiments were performed between Christmas and New Year's, 1956. In fact, Madame Wu was so caught up in the excitement of discovery that she canceled a planned cruise on the Queen Elizabeth to visit her homeland, China, with her husband — he went without her. Wu spent the holiday dashing between Columbia in NYC and Maryland for the on-site experiments. And lo and behold, one pole emitted more beta particles than the other. Parity is not conserved in weak interactions. Nature, it seems, is a bit of a southpaw.

On Christmas eve, Wu caught the last train back to New York city and told Lee "that the observed asymmetry was reprodicible and huge." It was big enough to garner Lee and Yang a Nobel Prize in Physics. It was a huge injustice that Wu's seminal contributions were not also recognized; she did not share in the prize.

So that's the backstory for CP (charge/parity) violation, and why RHIC scientists are cautiously excited about observing parity violation in their quark gluon plasma: a state of matter in which protons "melt" into quarks and gluons which can roam free, telling right from left — at least for fractions of a second. Per Overbye's article:

This happened in bubbles smaller than the nucleus of an atom, which lasted only a billionth of a billionth of a billionth of a second. But in these bubbles were “hints of profound physics,” in the words of Steven Vigdor, associate director for nuclear and particle physics at Brookhaven. Very similar symmetry-breaking bubbles, at an earlier period in the universe, are believed to have been responsible for breaking the balance between matter and its opposite antimatter and leaving the universe with a preponderance of matter.

That's right: everything we see in the universe around us — the ordinary visible matter — is the result of symmetry breaking in the early universe (including human beings). Not only is something very important being broken all the time, but we owe our very existence to that breaking.