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

01. When the physicist saw Yun Wang Xiaobo in the Golden Age, he wrote:

"I was 21 that day.

In the golden age of my life

I have many extravagant hopes.

I want to love, I want to eat

I also want to become a half-dark cloud in the sky in an instant. "

Shen Congwen, on the other hand, mentioned in his essay on the trip to Hunan:

"I cross bridges in many places.

I have seen the clouds many times

Drank many kinds of wine.

But only loved one person of the best age. "

They all mentioned clouds, clouds that you can see when you look up.

Wang Xiaobo looked up at the clouds and felt that he would go on forever and couldn't beat anything. Shen Congwen looked up at the clouds and saw a pair of smiling eyes through the clouds.

And people who study physics look up at the clouds and think of.

This is the original source of the cloud room.

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

02. The Ghost of Brocken in 1894, the Scottish physicist Charles Thomson Rees Wilson observed a magical phenomenon on Mount Benevis:

The sun passes through the clouds to form a huge rainbow ring, which seems to be surrounded by a hidden human form.

He is not the first person to observe such a spectacle.

As early as 1780, the German priest Johann Silberschlag observed this phenomenon in the Brocken Mountains and recorded it, calling it the Brocken Ghost (Brocken spectre).

If you climb Mount Emei in Sichuan, you can often see a colorful halo of red and purple on the distant horizon, while your own figure is shrouded in a halo, like Buddha light.

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

First of all, to observe this phenomenon, we need to stand on a cloud-shrouded peak with our back to the sun and look down.

At this time, the sun shines from behind, casting shadows on the clouds below us, and finally our figure appears as a magnified triangle because of the perspective effect.

In addition, when sunlight passes through the clouds, it interacts with the particles and goes through various processes, such as scattering, diffraction, interference, etc., and the final comprehensive effect is the colored halo around the shadow.

Because of 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 colored halo.

03. Back to the original Cloud Room, the seemingly simple explanation for the Brocken Ghost was not so obvious in Wilson's time.

Even, everyone has a limited understanding of how clouds are formed.

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

First, pour some of the water into the bottom of the glass container, then rest the container, waiting for the water molecules to gradually fill the entire cavity.

Then, let the whole 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 the cloud in the glass container.

But if he filters the air in the glass container, removes the dust, and repeats the same experiment, there will be no clouds.

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

Clouds are water droplets condensed by water vapor on dust particles. Sudden volume expansion can create clouds in dusty air.

This is the original cloud room, the purpose is very pure, is to study the conditions for the formation of clouds, but inadvertently, left behind unexpected eggs.

The egg was dusty for decades before it was finally discovered by Wilson.

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

04. Let the microscopic particles manifest. Now, go back to the top of Mount Benevis.

Wilson, shocked by the Brocken Ghost, hopes to reproduce this phenomenon in the laboratory and study the detailed causes behind it. Then, the first problem we are faced with is:

How to create clouds artificially in the laboratory.

Wilson chose to stand on the shoulder of the giant: he basically adopted Aitken's cloud chamber design idea, only to make his glass container have a greater coefficient of expansion.

Colored eggs, right here!

When the volume of the glass container continues to expand, more than 1/4 of the original volume, thin clouds are actually formed in the dust-free air.

Obviously, this is not consistent with the conclusion reached by Aitken at that time.

In that case, there must be something else to replace the dust particles, as condensation nodules, let the water droplets attached to it, and finally form a cloud.

He speculates that this may be some kind of charged particle invisible to the naked eye, because it can exist in the filtered air.

In order to test his conjecture, Wilson further improved the experimental equipment and irradiated the inside of the filtered glass container with X-rays. As a result, a large number of droplets are produced after the volume of the container expands, and a very obvious cloud is formed finally.

Wilson's original cloud chamber picture takes into account the ionization of X-rays, which validates Wilson's conjecture: X-rays ionize particles in the air into ions, and the water vapor in the container uses these ions as condensation nuclei and condenses into water droplets. and finally form clouds.

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

Clouds, so that X-rays have nowhere to hide.

In other words, as long as we let a charged particle into the cloud chamber, it will collide with the gas molecules in the container, and in the process, the gas molecules are ionized, and water vapor condenses on the resulting ions to form a cloud, showing a previously invisible particle trajectory.

Since then, clouds are no longer just clouds.

Wilson, therefore, is called:

The last one in physics, the great individual experimenter.

05, the new era of particle physics Wilson cloud chamber, has become the earliest charged particle track detector, particle physics, has entered an unprecedented era.

For example, here are the trajectories of 5.3 MeV of alpha particles photographed in a cloud chamber. It is emitted from near 1, Rutherford scattering occurs near 2, the deflection angle is about 30 degrees, and then spreads around 3.

How can we be sure that this is an alpha particle?

First of all, according to the length of the track in the cloud chamber, the velocity of the particles can be determined; according to the curvature of the track, the charge and momentum of the particles can be measured, and finally the type of particles can be determined.

A series of characteristics, such as the mass, charge and lifetime of the particle, make up its unique identity information, just as each of us has an ID number.

If, in the end, it is found that no known particle has the same properties as this one, congratulations:

You found a new particle!

This is how many new particles have been discovered in history.

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

The positron trajectory taken by Anderson in the cloud chamber calculates that its mass is the same as the electron based on the deflection path of the particle, but the charge is the opposite. Isn't this the positron Dirac predicted?

Therefore, we experimentally verify that the positron is real.

Anderson won the Nobel Prize in Physics in 1936 for this reason.

Using the Wilson cloud chamber, we have found μ mesons, K mesons and so on.

And then.

Quark, Standard Model, Quantum Chromodynamics, TCP Theorem.

And the beginning of all this?

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

We will grow up eventually.

He is no longer a teenager dreaming of clouds.

But in the physical world,

Every time I look up at the clouds

There are always dreams to follow.

References:

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

[2] Halliday, E.C. Some Memories of Prof. C.T.R. Wilson, English Pioneer in work on Thunderstorms and Lightning. Bulletin of the American Meteorological Society.1970 (51): 1133-1135.

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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