Shulou(Shulou.com)11/24 Report--

Please imagine:

In the mid-20th century, a series of major discoveries in the field of nuclear physics exploded, and scientists around the world scrambled desperately to squeeze into lecture halls that no longer had seats, or even climbed pillars just to see the equations on the blackboard.

In the center of such an occasion, there stood a Chinese woman in a cheongsam.

She is the "First Lady of Physics", the "Chinese Madame Curie" and the "Queen of Nuclear Research".

She is, C.S. Wu.

1. She is not mentioned in Oppenheimer. Some people say that C.S. Wu does not appear in Oppenheimer because she does not know Oppenheimer well.

Actually, it's not. Oppenheimer affectionately called her "JieJie" as an authority on beta decay research, while she called Oppenheimer Oppie.

In 1944, C.S. Wu participated in the Manhattan Project: developing methods for enriching uranium to increase fuel supplies. The B reactor producing plutonium-239 in Hanford District, which had just started operation at that time, was often shut down to stop the reaction. C.S. Wu, based on a large number of experimental data, confirmed that xenon-135was one of the fission products, absorbing large amounts of neutrons in the reactor. so that the chain reaction lacks enough neutrons to stop.

As for whether there is no C.S. Wu in the film because it has nothing to do with the main plot, let's leave it for the time being.

What we want to say is the Wu experiment named after her, which proves the theory put forward by Yang Zhenning and T.D.Lee: parity is not conserved in weak interactions, and Yang Zhenning and T.D.Lee won the Nobel Prize in physics in 1957.

As you learn about this experiment, you will feel as if you were talking to C.S. Wu, and you can clearly sense her tenacity, wisdom and courage.

Even in the age when the great gods gathered, she was shining.

2. What is parity nonconservation? In order to understand C.S. Wu's experiment, the first question we need to solve is: what is parity?

Before we can understand parity in physics, we must first understand the concepts of symmetry and conservation laws in physics. We are all familiar with symmetry in mathematics. If you rotate the square 90 degrees along the axis of symmetry, it coincides with the original picture, and in physics, the meaning of symmetry is about the same.

Now, let's make an assumption that is not very strict:

Drop 2ml spirits into a cup of coffee and observe the time it takes for spirits to spread completely in the coffee.

Ideally, all conditions remain the same, you just move the glass from the upper right corner of the table to the lower right corner of the table, and the time required for liquor to spread remains the same. This is translational symmetry, that is, the laws of physics do not change with the position in space, corresponding to the conservation of momentum. In addition, today's drop of liquor, tomorrow's drop of liquor, will not affect the diffusion time, this is time symmetry, corresponding to the conservation of energy. Finally, if you turn the glass 90 degrees and then drop the liquor, the diffusion time will not be affected, which is rotational symmetry, corresponding to the conservation of angular momentum.

How important is symmetry to physical research?

Imagine if we put the coffee cup on two positions of the table and sum up two laws of diffusion.

Then physics really doesn't exist.

So, what exactly does parity conservation mean?

In fact, it is really not difficult, but the word "parody" is easy to make us confused.

To put it bluntly, parity conservation is the world in the mirror, which follows the same laws of physics as the real world. Mathematically, it can be simply understood as transforming the coordinate x _ m _ y _ z into-x _ m _ m _ y _ j _ z and the result remains the same, that is, parity conservation.

Now, let's take a mirror. What will happen to the spread in the mirror when we drop the liquor into the coffee? Apart from being upside down, the diffusion pattern depicted by white spirit in coffee must be the same, and you can't even tell which cup is the real white spirit coffee and which is in the mirror.

Intuitively, the world in the mirror and the world outside the mirror should really be swapped left and right.

Does God really prefer the left or the right? This has been regarded as one of the basic laws in the field of physics until. C.S. Wu proved by experiments that God really has a preference.

3. The τ-θ puzzle in the 1940s and 1950s, there was a "τ-θ puzzle":

Tau and theta particles have exactly the same charge, spin and mass, but theta decays into two pions, and tau decays into three pions. If the parity is conserved before and after decay, then the parity of theta should be + 1, the parity of tau should be-1, then τ and theta can only be two kinds of particles. But if parity is not conserved, τ and theta can be the same kind of particles.

If parity conservation, the calculation of τ and θ parity by 1956, the τ-θ puzzle has become a major problem that scientists are bound to discuss at various conferences. At this time, Yang Zhenning and T.D.Lee proposed that parity is not conserved under weak interaction, and they think that τ and θ are two different decay modes of the same particle. Because this idea breaks the law of symmetry that we have always believed in, it has been unanimously questioned by almost all famous physicists.

At that time, there was a saying among physics circles that if C.S. Wu had done an experiment, it must have been correct. Therefore, Yang Zhenning and T.D.Lee turned to C.S. Wu for help.

Wu Jianxiong gave them a book containing the results of all beta decay experiments over the past 40 years. It is found that no experimental results can prove whether parity is conserved or not.

The famous Wu experiment was born at such an emergency.

4. The theoretical basis of Wu's experiment. First of all, since we want to verify whether parity is conserved, we must first define "left" and "right".

As a result, we simplify the particle to a moving ball that is constantly spinning. The spin direction of the particle is defined as this: the right finger is wound in the direction of the particle spin (clockwise or counterclockwise), and the thumb is pointing in the spin direction. Then, the particles with the same spin direction and the traveling direction are called right-handed particles, and the particles whose spin direction is opposite to the traveling direction are called left-handed particles.

C.S. Wu chose the radioactive cobalt-60 sample, an isotope that undergoes beta decay, which involves weak interactions, which C.S. Wu is good at.

If the weak interaction is parity conserved, then the nuclear decay of cobalt-60 should produce the same results in the "real" world and the "mirror" world. But the fact is shocking: electrons are more likely to be emitted in the opposite direction to the spin of the nucleus.

Combined with this picture, we can simply understand it like this:

A simple schematic diagram in the real world, there is a spin-up cobalt-60 nucleus that, after decay, releases electrons flying down. Because the spin direction is consistent with that of the cobalt-60 nucleus, this is a left-handed electron. In the mirror world, after mirror transformation, the spin changes from counterclockwise to clockwise, and the cobalt-60 nucleus becomes spin downward. If parity is conserved, then the left-handed electron should become the right-handed electron, and the released electron should still fly down.

But then we found that the electrons flew up in the mirror world!

In other words, the left-handed electron is still left-handed after mirror transformation, but does not become right-handed.

This is parity non-conservation.

Fortunately, this only happens in weak interactions, if parity is not conserved in daily life.

You jump up, only to find yourself squatting in the mirror.

It's kind of creepy.

5. in theory, this experiment does not seem to be difficult, but in fact, many problems need to be solved, such as: microscopic particles are uncertain, and it is impossible for us to really observe the electrons emitted by a certain cobalt-60 nucleus. We can only allow a bunch of cobalt-60 nuclei to decay and analyze whether the direction of electron emission is statistically biased based on a large amount of data.

In addition, in order to minimize the thermal motion of cobalt atoms, the adiabatic demagnetization method is used to obtain ultra-low temperature close to absolute zero. She also placed the sample in a uniform constant magnetic field to make the spin direction of the nucleus consistent. These were the most cutting-edge experimental methods at that time. You can imagine all kinds of difficulties encountered during this period.

Now, let's take a look at the statistics given by C.S. Wu in a paper published in 1957:

It is mentioned in the original picture of the paper that the symbol of the asymmetry factor α is negative, that is to say, the emission of electrons is more inclined to be in the opposite direction to the nuclear spin. Because of many factors involved, it was difficult to calculate α accurately at that time.

When v / c ≈ 0.6, alpha is about 0.4. The value that can be calculated from the observed gamma ray anisotropy data is about 0.6. These two quantities give that the lower limit of the asymmetric parameter β (α = β) is about 0.7.

In addition, the observed β asymmetry does not change the symbol with the reversal of the direction of the demagnetization field, indicating that this phenomenon is intrinsic and is not caused by the residual magnetization in the sample.

This is the first time in history that parity is not conserved in weak interactions.

6. Like the young me, C.S. Wu once said: it is important for an experimental physicist to be smart, but not as smart as a theoretical physicist is the most important; for an experimental physicist, the most important thing is persistence, good judgment and some luck.

With only a few words, she told the rules of her life.

After Yang Zhenning and T.D.Lee put forward the theory of parity nonconservation, Feynman, Pauli and Landau all thought it was a crazy idea so that students didn't have to waste their time doing experiments.

It was under such circumstances that C.S. Wu expressed his willingness to fully support Yang Zhenning and T.D.Lee, immediately canceled all meetings and trips, plunged into the laboratory, and even carried out experiments with experimental equipment to the headquarters of the National Bureau of Standards specializing in gas liquefaction, in order to obtain reliable experimental results as soon as possible.

C.S. Wu's granddaughter recalled that in her later years, C.S. Wu liked to sit in an armchair with faded yellow corduroy, looking out of the window at the campus and admiring the girls who were playing basketball.

She said: look at them, how strong and fast they are. Look at them, how hard they work.

The unspoken sentence is probably:

Look at them, they look like me when I was young.

References:

[1] Wu C S, Ambler E, Hayward R W, et al. Experimental Test of Parity Conservation in Beta Decay [J]. Phys. Rev., American Physical Society, 1957, 105 (4): 1413-1415.

[2] Discovering Dr.Wu by Jada Yuan (https://www.washingtonpost.com/lifestyle/2021/12/13/chien-shiung-wu-biography-physics-grandmother/)

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop). The author: Muller's nanny.

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