I wanted to see clouds in the lab, but ended up changing history? That's physics!

When you look up at the clouds, do you know that the world has been changed by them?

When Physicists Look at Clouds

Wang Xiaobo wrote in The Golden Age: "That day I was 21 years old, in the golden age of my life, and I had many extravagant hopes. I wanted to love, to eat, and to turn into a half-bright, half-dark cloud in the sky in an instant. "

Shen Congwen mentioned in his "Travel Notes in Hunan": "I have walked across bridges in many places, seen clouds many times , and drunk many kinds of wine, but I have only loved one person at the best age."

They all mentioned the clouds, the clouds that can be seen by looking up.

Wang Xiaobo looked up at the clouds and felt that he would remain vigorous forever and would be unable to defeat anything; Shen Congwen looked up at the clouds and saw a pair of smiling eyes through the clouds.

And people who study physics, looking up at the clouds, think of...

This is the original origin of the cloud chamber.

At first, we just wanted to study the conditions for cloud formation in the laboratory, but later, we changed the whole world.

Brocken Ghost

In 1894, Scottish physicist Charles Thomson Rees Wilson observed a fascinating phenomenon on Ben Nevis:

The sunlight passed through the clouds, forming a huge rainbow ring, which seemed to surround a shadowy human figure in the middle.

He was not the first to observe such a spectacle.

As early as 1780, German pastor Johann Silberschlag observed this phenomenon on the Brocken Mountain and recorded it, calling it the Brocken spectre .

If you climb Mount Emei in Sichuan, you can often see a colorful halo with red outside and purple inside appearing in the distant sky, and your own figure is shrouded in the halo , and your shadow is dim, like the Buddha's light.

The occurrence of this phenomenon is actually related to optical illusion .

First of all, to observe this phenomenon, we need to stand on a misty mountain, with our backs to the sun and looking down.

At this time, the sun shines from behind, casting shadows on the clouds below us. Due to the perspective effect , our figures finally appear as enlarged triangles.

In addition, when sunlight passes through clouds, it interacts with the particles in them and undergoes various processes such as scattering, diffraction, interference , etc. The final combined effect is the colorful halo around the shadow.

Due to the visual illusion, we subconsciously think that the shadow and the halo have no distance difference and are on the same plane, so it looks like a giant ghost shrouded in a colorful halo.

The original cloud chamber

That said, this seemingly simple explanation for the Brocken Ghost was not so obvious in Wilson's time.

Even how clouds are formed is still a vague concept.

Scottish physicist John Aitken has been studying the conditions for cloud formation , and for this purpose he built an experimental device:

First, pour some water into the bottom of the glass container, then let the container stand and wait for the water molecules to gradually fill the entire cavity.

Then, let the entire container expand adiabatically. According to the first law of thermodynamics, we know that the temperature of the gas in the container will decrease.

Then we saw clouds in the glass container.

But if he filtered the air in the glass container to remove the dust and repeated the same experimental operation, no clouds could be seen.

So he found a way to make artificial clouds and came to the conclusion:

Clouds are droplets of water vapor condensing on dust particles . A sudden expansion in volume can create clouds in air containing dust.

This is the original cloud chamber. Its purpose was very simple, which was to study the conditions for cloud formation. However, it inadvertently left behind an unexpected easter egg.

This Easter egg remained sealed for decades until it was finally discovered by Wilson.

Wilson then won the 1927 Nobel Prize in Physics and rewrote the history of world science and technology .

Making microscopic particles visible

Now, back to the summit of Ben Nevis.

Wilson, who was shocked by the Brocken ghost, hoped to replicate this phenomenon in the laboratory and study the detailed causes behind it. Then, the first question he faced was:

How to create artificial clouds in the laboratory.

Wilson chose to stand on the shoulders of giants: he basically adopted Aitken's cloud chamber design ideas, but made his glass container have a larger expansion coefficient.

The easter egg appears here!

When the volume of the glass container continued to expand and exceeded a quarter of its original volume, a thin cloud of mist actually formed in the dust-free air.

Obviously, this is inconsistent with the conclusion reached by Aitken that year.

In this case, there must be something else that replaces the dust particles and serves as condensation nuclei for the water droplets to attach to, eventually forming clouds.

He speculated that this might be some kind of charged particles that are invisible to the naked eye , because charged particles can exist in filtered air.

In order to verify his conjecture, Wilson further improved the experimental equipment and used X-rays to irradiate the inside of the filtered glass container. As a result, the volume of the container expanded and produced a large number of droplets, which eventually formed a very obvious cloud .

Wilson's original cloud chamber

Considering that X-rays have an ionizing effect , this verifies Wilson's conjecture: X-rays cause particles in the air to be ionized into ions, and the water vapor in the container uses these ions as condensation nuclei to condense into water droplets and finally form clouds.

In other words, where the clouds form is where the X-rays pass.

Clouds and fog leave no place for X-rays to hide.

That is to say, as long as we shoot charged particles into the cloud chamber, they will collide with the gas molecules in the container. In this process, the gas molecules are ionized, and water vapor condenses on the generated ions to form clouds, showing the originally invisible particle tracks.

From now on, the cloud is no longer just a cloud.

Wilson is therefore known as:

"The last great individual experimenter in physics."

A new era in particle physics

The Wilson cloud chamber became the earliest charged particle track detector, and particle physics entered an unprecedented era.

For example, the following is the trajectory of an alpha particle with an energy of 5.3 MeV captured in a cloud chamber. It is emitted near 1, undergoes Rutherford scattering near 2, with a deflection angle of about 30 degrees, and then disperses near 3.

How do we determine that this is an alpha particle?

First, the speed of the particle can be determined based on the length of the track in the cloud chamber; based on the curvature of the track, the charge and momentum of the particle can be measured, and finally the type of particle can be determined.

A particle's mass, charge, lifespan and other characteristics together constitute its unique identity information, just like each of us has an ID number.

If you find that no known particle has the same properties as this particle, then congratulations:

You discovered a new particle!

Throughout history, many new particles were discovered in this way.

In 1928, Dirac theoretically predicted the positron . In 1932, Carl David Anderson used a Wilson cloud chamber to capture the following unexpected particle trajectory .

Anderson's positron tracks captured in a cloud chamber

Based on the particle's deflection path, it is calculated that its mass is the same as that of an electron, but its charge is opposite. Isn't this the positron predicted by Dirac?

Thus, we experimentally verified that positrons really exist .

Anderson was awarded the 1936 Nobel Prize in Physics for this.

Using the Wilson cloud chamber, we have discovered the μ meson, K meson, etc.

Next up: quarks, the standard model, quantum chromodynamics, TCP theorem…

And where did all this begin?

It's just that we want to see clouds, that's all.

We will eventually grow up and no longer be teenagers who dream at the clouds. But in the physical world, whenever I look up at the clouds, I will always have a dream to pursue.

References

[1]Ben Marsden. Counting dust anddomesticating clouds: inside the 'outdoor physics' of John Aitken,Tidsskrift for kulturforskning. 2020(9): 45-56.

[2]Halliday, EC Some Memories of Prof. CTR Wilson, English Pioneer in work on Thunderstorms and Lightning. Bulletin of the American Meteorological Society.1970(51):1133–1135.

Planning and production

Source: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop)

Author: Mueller's nanny

Editor: He Tong

Proofread by Xu Lai and Lin Lin