A ghost in the water and a flower ended the two thousand years of atomic debate

Author: Tian Dawei

Atoms: Fantasy or Reality?

Nuclear energy, also known as atomic energy, began with people's exploration of atoms and nuclear energy. But until nearly 100 years ago, many scientists did not believe in the existence of atoms.

In fact, more than 2,000 years ago, the ancient Greek scholar Democritus believed that all matter is composed of indivisible small particles called atoms[1].

However, the atoms that Democritus believed in were infinite and had many shapes, including concave, convex, eyeglass-like, and hook-like. They were very different from the atoms we talk about today, but his view became an important ideological source of ancient Greek atomic theory[2].

(Of course, there is another saying that the first person to propose the atomic theory was Leucippus, the teacher of Democritus, but there is no clear record, so it is generally said that Democritus’s theory was more prominent).

However, atoms are too small, and no one has ever seen atoms or molecules. Therefore, for a long time, atomic theory, like other theories about the origin of matter, was just a hypothesis and speculation.

More than 2,000 years later, John Dalton once again brought atomic theory to the fore. Dalton also believed that all matter is made up of atoms, and that atoms are indivisible and indestructible.

Unlike Democritus, Dalton did not come up with this idea out of thin air. Through studying distilled water, he discovered that every water particle is the same as every other water particle, and every hydrogen particle is the same as every other hydrogen particle.

Moreover, he also determined the elemental masses of six elements through experiments and made an atomic model. Even so, the doubts over the atomic theory still exist. These are all inferences. No one has ever seen atoms, so how can we prove that they are real things?

But at that time, it was impossible to see them directly, so if we wanted to prove their existence, we had to find another way. And this method appeared in an inadvertent botanical observation.

Ghost Hiding in the Water

The first person to discover the whereabouts of atoms was a Scottish botanist named Robert Brown.

In 1827, Brown observed the pollen of Clarkia pulchella under a microscope. He found that the small particles in the pollen were shaking by themselves without any external force. This phenomenon was later called Brownian motion.

It should be noted here that many popular science articles say that Brown saw pollen trembling. But in fact, the particle size of pollen is too large to cause Brownian motion. Brown’s original text said “tiny particles from the pollen grains of flowers”. [3] Therefore, it is the smaller particles in the pollen that are trembling.

At first, people thought that this phenomenon should be some kind of life activity, that the contents of the pollen began to change when they came into contact with water. But the problem was that the pollen Brown observed was collected 20 years ago and should have been dead, but it still moved like fresh pollen.

Brown guessed that this kind of movement should have nothing to do with life activities. It was easy to verify this point. Brown found a variety of inorganic substances, from glass fragments to powder from the Sphinx.

Put all these things into water, and they all started to shake. This shows that the driving force behind the movement of these small particles is definitely not related to living things. But Brown didn't know what the real driving force behind it was at the time, he just recorded the results in his observation record.

It is this record that makes it impossible for atoms to escape the fate of being discovered.

Because Brownian motion was a remarkable discovery for those who believed in the existence of atoms and molecules. They thought that this motion was the result of particles colliding with water molecules.

In the 1870s, some people began to use the random motion of molecules to explain Brownian motion. But at that time, people could not solve a problem: the particle size of particles is much larger than that of water molecules, just like a small ping-pong ball cannot push a big ball as big as a stadium.

Finally, 78 years after Brown discovered Brownian motion, the answer came.

Einstein's miracle

It was Einstein who solved this problem. And the year when this problem was solved was also the year when Einstein broke out, "1905, the year of Einstein's miracle".

During this year, Einstein published four papers, one of which was "The Motion of Particles Suspended in a Static Fluid Required by the Kinetic Theory of Heat", which solved the problem of Brownian motion.

Einstein's point was that while a single collision wouldn't produce a kick, billions of random collisions per second would produce enough of an effect to be visible under a microscope.

More importantly, Einstein used theoretical knowledge of viscosity and diffusion rate and experimental data to predict Brownian motion. For a particle with a diameter of 1/1000 mm placed in pure water at 17°C, its average displacement within one minute is 6 microns.

In addition, in the paper, Einstein also gave a clear method for measuring the size of atomic particles. This is an extremely specific and verifiable prediction.

Soon, just three years later, French scientist Jean-Baptiste Perrin confirmed Einstein's conjecture and proved that atoms do exist. This research also earned Perrin the 1926 Nobel Prize in Physics.

At this point, the question of whether atoms exist has finally been solved. But both Democritus and Dalton guessed one thing wrong: atoms are divisible. Moreover, as scientists split atoms step by step, they will discover the power to destroy the world.

References:

[1] Kenny, Anthony (2004). Ancient philosophy. A New History of Western Philosophy. Vol. 1. Oxford, UK: Oxford University Press. pp. 26-28. ISBN 0-19-875273-3.

[2] Bernard Pullman (1998). The Atom in the History of Human Thought. Oxford, UK: Oxford University Press. pp. 31-33. ISBN 978-0-19-515040-7.

[3] Robert Brown: "A brief account of microscopical observations made in the months of June, July and August, 1827, on the particles contained in the pollen of plants; and on the general existence of active molecules in organic and inorganic bodies." In: Philosophical Magazine. Band 4, 1905, S. 161–173.

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