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Short Stories: Madame Wu

Jul 16, 2012

by minouette

Etsy.com handmade and vintage goods

Here at Etsy, we believe that the story behind an object is often just as fascinating as the object itself. Short Stories is our series dedicated to telling the tales behind extraordinary pieces found or created by Etsy sellers. 

Today, Ele shares the history of the fascinating woman whose beautiful experiment inspired this print

I’m actually a physicist by training — I like to say I’m a marine geophysicist by day and printmaker by night (though right now, I’ve been focused more on art-making than scientific research). I learned about the violation of parity as a student but I didn’t really discover Madame Wu until I was a teaching assistant for a course intended to teach physics to arts majors. Hers was one of the most beautiful experiments in the entire history of physics.

Chien-Shiung Wu (1912-1997) was a Chinese-born American physicist, whose nicknames included the “First Lady of Physics,” “Chinese Marie Curie,” and “Madame Wu.” She came up with an elegant 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 “Law of Conservation of Parity” might not be the same “through the looking glass.” This was hard to believe, as the laws of physics are the same in the mirror as for anything else.

For example, 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 Madame Wu to test this.

Madame Wu performed a subtle and technically difficult experiment using Cobalt-60 (shown as the cobalt blue sphere in the box), which is radioactive. Its neutrons spontaneously give off electrons and become protons, and 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 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 staggering result shocked the physics world. Lee and Yang won the Nobel prize for their theoretical work. Many physicists thought Madame Wu should have been included in this win.

In my print, I show Madame Wu in her lab and a schematic diagram 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.

Madame Wu won many honours for her incredible career. She 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. Among other things, she was 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.

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