There Is No Parity: Chien-Shiung Wu

Hold out your hands in front of you, palms forward. They look quite similar, but I’m sure you’re all too aware that they’re actually mirror images of each other. Your hands are chiral objects, which means they’re asymmetric but not superimposable. This property is quite interesting when studying the physical properties of matter. A chiral molecule can have completely different properties from its mirrored counterpart. In physics, producing the mirror image of something is known as parity. And in 1927, a hypothetical law known as the conservation of parity was formulated. It stated that no matter the experiment or physical interaction between objects – parity must be conserved. In other words, the results of an experiment would remain the same if you tired it again with the experiment arranged in its mirror image. There can be no distinction between left/right or clockwise/counter-clockwise in terms of any physical interaction.

Dr. Wu working with a particle accelerator via Biography.

The nuclear physicist, Chien-Shiung Wu, who would eventually prove that quantum mechanics discriminates between left- and right-handedness, was a woman, and the two men who worked out the theory behind the “Wu Experiment” received a Nobel prize for their joint work. If we think it’s strange that quantum mechanics works differently for mirror-image particles, how strange is it that a physicist wouldn’t get recognized just because of (her) gender? We’re mostly here to talk about the physics, but we’ll get back to Chien-Shiung Wu soon.

The End of Parity

Conservation of parity was the product of a physicist by the name of Eugene P. Wigner, and it would play an important role in the growing maturity of quantum mechanics. It was common knowledge that macro-world objects like planets and baseballs followed Wigner’s conservation of parity. To suggest that this law extended into the quantum world was intuitive, but not more than intuition. And at that time, it was already well known that quantum objects did not play by the same rules as classical objects. Would quantum mechanics be so strange as to care about handedness?

By the time the 1950’s rolled around, physicists were smashing subatomic particles into one another in high speed particle accelerators and analyzing the resulting explosion of new, sometimes previously undiscovered particles. One of these previously undiscovered particles was quite puzzling – the K meson. It appeared that there were two different versions – one would decay into 2 pi mesons and the other would decay into 3 pi mesons. Pi mesons are also called pions. All other properties of the K meson were identical, suggesting there was only one type.

Setting up the Cobalt-60 parity test via NIST.

After doing the math, it was determined that the pions in the two and three particle systems must have opposite parity. And according to Wigner’s conservation of parity, there therefor must be two types of K mesons – one that produced two pions and one that produced three pions. Conservation of parity would not allow both systems to come from a single particle.

But what if Wigner’s parity theory were wrong? The K mesons were produced during weak force interactions when protons were smashed into heavier nuclei. In 1956, physicists T.D Lee and C.N. Yang suggested that weak force interactions might not follow the conservation of parity, and that there was indeed a single K meson. That the two systems of pions were the result of a single K meson that had a definable parity, and that parity was not conserved in this particular case. Consider if the K meson had a spin, and that a clockwise spin produced the 2 pion system and a counterclockwise spin produced the 3 pion system. This would be an example of a violation of the conversation of parity. A violation that physicist were suggesting was occurring with the K meson.

An experiment was derived to put parity to the ultimate test.


As you can imagine, these kinds of experiments are a bit complicated. And it’s my mission to break complicated things down to the point that your everyday curious hacker can understand. The goal is to prove that Wigner’s conservation of parity did not hold water with weak force interactions. To do this, we’re going to need three things:

  1. Something that emits beta radiation (beta decay is caused by the weak force).
  2. Something that has two known physical states.
  3. A way to measure the radiation in each of the states.

Cobalt-60 parity test apparatus via NIST.

Conservation of parity would insist that the measured radiation be the same in either state. Because if parity is to be conserved, it should not be possible to get a different experimental outcome between different states, like spin for instance. This can be done by taking Colbalt-60, which is naturally radioactive and cooling it to a smidgen above absolute zero. Cooling it to this temperature takes away most molecular motion, and allows the atoms to arrange themselves in a crystal structure while in the presence of a very strong magnetic field. The magnet field also polarizes the Colbalt-60 nuclei, which means they spin in the same direction parallel to the magnetic field.

Now all we have to do is:

  1. Measure the beta radiation intensity.
  2. Reverse the magnetic field direction.
  3. Measure the beta radiation intensity.
  4. Compare the results.

By reversing the magnetic field, we cause the cobalt-60 nuclei to reverse polarization. Conservation of parity says there should be no measurable difference between the two physical states. And I’m sure you can guess by now what they found – the measured beta radiation intensity was greater in one direction. This was the nail in the coffin for Wigner’s parity theory. It allowed physicists to reexamine results of previous experiments involving weak force interactions and helped advance quantum theory and eventuality lead to the Standard Model of particles we have today.

Chien-Shiung Wu

As we mentioned above, Lee and Young received the Nobel Prize for the Wu experiment. Wu, unfortunately, didn’t. Wu was one of a handful of female physicists during that time whose name and research are not as well known as they should be. Gender-based injustice was widespread during her time, but she would receive worldwide recognition for her contributions to nuclear physics from the 70’s onward.

Indeed, she was awarded the inaugural Wolf Prize in Physics in 1978, partly to make amends for the Nobel slight, but also for her further work in experimental nuclear physics. Wu also worked on the Manhattan Project, held a prestigious chair at Columbia University, and was responsible for important experimental results in exploring the weak nuclear force and the first experimental confirmation of quantum photon entanglement.

Chien-Shiung Wu died in 1997 at 84. Let’s close with a fitting quote from Dr. Wu during an address at an MIT symposium in 1964:

“I wonder whether the tiny atoms and nuclei, or the mathematical symbols, or the DNA molecules have any preference for either masculine or feminine treatment.”

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