The Art of Science: Madame Wu and the Violation of Parity

Ele Willoughby (aka Minouette) calls herself a “marine geophysicist by day and printmaker by night”. She carries over this unusual combination into her artwork, which often features concepts and key figures from the history of science.

For example, she explains, “In physics, if there’s a bias which is commonly forgiven (and viewed as harmless, or even right), it’s beauty. I bet that wasn’t what you expected, but it’s true…One thing physicists invariably count as ‘beautiful’ is symmetry. The concept is invaluable in physics; symmetries are very powerful tools for figuring things out….Each symmetry leads to a conservation law.”

One of her most intriguing prints, Madame Wu and the Violation of Parity, looks at a pioneering female physicist’s research into one type of symmetry.  It’s fairly complex, so I’ll let Willoughby explain:

Chien-Shiung Wu (1912-1997, Chinese-born American physicist, whose nicknames included the “First Lady of Physics”, “Chinese Marie Curie,” and “Madame Wu”) came up with a truly beautiful experiment to test whether the weak force conserves parity (whether beta decay would be the same if reflected in the mirror).

In 1956, theoretical physicists Tsung Dao Lee and Chen Ning Yang suggested that perhaps the weak force might not be the same ‘through the looking-glass’. The idea that the “Law of Conservation of Parity” might be broken was hard to believe. The laws of physics are the same in the mirror for anything else. Face a friend, as in the mirror. If you drop a pencil from your right hand, and your friend mirrors you and drops a pencil with his or her left, the pencils will fall at the same rate. This is because Parity is conserved by the force of gravity – as it is with the electromagnetic force and even the strong (nuclear) force within atomic nuclei. Lee and Yang pointed out that no one had checked to make sure that the weak force, which controls beta decay in radioactive materials, also conserves parity. Lee convinced the brilliant experimentalist to test this.

Madame Wu did a subtle and technically difficult experiment with her collaborators which is shown schematically in the print. She took Cobalt-60 (shown as the cobalt blue sphere in the box), which is radioactive. Its neutrons spontaneously give off electrons and become protons. The electrons are the tiny blue dots. On the left, we see the Cobalt-60 in an electromagnet (a wire wrapped metal horseshoe with a source of power). Because of the spiral-wrap of the wire, we know that the North pole of the magnet will be on the bottom (you can figure this out by mimicking the curl of the wire with the fingers of your right hand and look at the direction your thumb points). It turns out that the emitted electrons are given off preferentially towards the North pole.

Next, she reversed the set-up as in the mirror. On the right you see the horseshoe and wire spiral reflected. If you use your right hand to check the direction of the magnet field, you’ll see that it is the opposite way; the North pole is now on the top. It turns out that the electrons are preferentially emitted upwards toward the North pole. Thus, beta decay IS NOT the same in the mirror! Madame Wu showed that a “Law” of physics did not hold! This result was staggering and shocked the physics world. Lee and Yang won the Nobel prize for their theoretical work. Many physicists thought Mme. Wu should have been included in this win.

She won many honors for her incredible career. Wu took part in the Manhattan Project (wikipedia states she is believed to be the only Chinese person to do so) and literally wrote the book on beta decay. She was the first Chinese-American to be elected to the U.S. National Academy of Sciences, the first female President of the American Physical Society and the first living scientist to have an asteroid named after her.  And I bet you hadn’t heard of her! I’m trying to redress that.

In my print on the left I show Mme. Wu in her lab and a schematic diagram in the box of her beautiful experiment in the box. On the right I show her reflection, as in the mirror, and in the box I show the mirror reflection of the experimental set-up and the shocking result, that the reaction is not the mirror opposite. The print is in an edition of 10 printed on ivory Japanese kozo (or mulberry) paper, 12.3 inches by 12.5 inches (31.2 cm by 31.8 cm).

The print is available from the Minouette etsy shop , where you can also find prints of other science luminaries including Ada Lovelace, Niels Bohr, Copernicus, Mercator and many more.

You can learn more about her artistic process and subjects on her blog,  and follow her on twitter @Minouette.

Author: michelebanks1

Artist and blogger

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