Why don't people go through Brownian Movement

Brownian motion

The Brownian motion was crucial to the development of the atomic theory of matter. You can find out what Brownian molecular motion is and more in this post.

If you want to learn the most important things from this post in a short amount of time, watch our video on Brownian molecular motion.

Brownian motion explained simply

In 1827, the botanist Robert Brown discovered under a microscope that grass pollen dissolved in water moves around irregularly and in a zigzag fashion. But the water seemed to be still.

Brownian motion

The irregular, zigzag-like movement of particles surrounded by tiny fluid particles is called Brownian motion (shorter: Brownian motion). 

The theoretical explanation of this observation by Albert Einstein in 1905 was crucial for the acceptance of the atomic theory of matter.

What is Brownian Molecular Motion?

Brownian molecular motion is a form of random motion of a particle that is caused by irregular collisions with other particles (atoms or molecules). The amazing thing is that the “other particles” are so extremely small that you cannot even see them with the naked eye. Nevertheless, the comparatively large particle can be set in motion by a large number of collisions. Just to give you a feeling: the size of the particles that are set in motion can be 10,000 to 100,000 times larger than the size of the atoms or molecules that cause the motion.

Illustration by fragrance in the room

If you have someone in your family who likes putting on perfumes, then you will most likely have seen Brownian molecular movement in action.

Suppose you're at your desk and someone in your family sprays some perfume on in the next room. Your door is open You won't notice any changes in your room right after the person leaves the house. But a few seconds later you will hear the typical smell of a perfume. Indeed, within a few seconds, you will notice the smell throughout the home.


What happened here At the time it was applied, many small “fragrance particles” had streamed out of the perfume bottle. Some of these particles were carried out by the person when they left the apartment. The rest is still inside the apartment. The apartment is also surrounded by air. The size of such “fragrance particles” is roughly 1000 times larger than the size of the surrounding “air molecules”. So we are in the situation where there are large particles in the vicinity of a vast number of tiny particles. The “fragrance particles” therefore spread within the apartment, among other things through the Brownian molecular movement, which is caused by collisions with the surrounding “air molecules”.

Experiment: ink in water

The illustration with the distribution of scent in the room showed you an everyday situation in which Brownian molecular motion is crucial. But you could only "smell" the Brownian molecular movement. One way of being able to “see” the Brownian molecular motion is a simple experiment with some ink (from your pen, for example) and two glasses of water.

Pour cold water into one of the two glasses and hot water into the other (after boiling in a kettle, for example). Now put some ink in both glasses. What will you see In the cold water, the ink begins to spread a little, but overall it looks as if the ink is moving as a "block" towards the ground. You can even recognize individual “ink threads”. The situation is completely different with a glass with hot water. Here the ink mixes quickly. It looks like someone stirred the ink in the water.


How can that be? The “ink molecules” are about 1000 times larger than the surrounding water molecules. In both cold and hot water, the “ink molecules” experience a huge number of collisions with the water molecules. The difference lies in the fact that the water molecules in the glass with hot water one clearly higher speed than the water molecules in the glass with cold water possess. This not only results in more frequent impacts, but the “ink molecules” also experience a stronger impact. This observation is not only an illustration of Brownian molecular motion, but also an indication that Brownian molecular motion is somehow related to temperature.

Brownian molecular motion in biology

The experiment with the ink in the water gave us an important insight: The Brownian molecular motion is somehow related to the temperature of the surrounding molecules. In this section we want to go into more detail and show you other important phenomena in which Brownian molecular motion is crucial.

Diffusion, temperature and motility

Brownian molecular motion is not only related in some way to temperature, but Brownian molecular motion is Temperature. The random movement of a macroscopic particle reflects nothing other than the movement of the particles on a microscopic level. And it is precisely this “movement of the particles on a microscopic level” that is called temperature.

The observation of the distribution of the scent in the room also falls under the name diffusion. Diffusion generally describes the movement of particles from a place of higher “particle concentration” to a place of lower “particle concentration”. This observable motion is a macroscopic example of Brownian molecular motion.

In biology it is essential to choose between the movement due to the Motility of a sample and the motion by the Brownian molecular motion. This differentiation is often possible because the Brownian molecular motion is irregular. The movement due to motility, on the other hand, acts as if the sample were following a path.

Brownian molecular motion history

Brownian molecular motion was first seen by the Roman philosopher Titus Lucretius Carus over 2000 years ago. He watched dust particles as sunlight shone into the dusty room. He noticed how the dust particles made trembling movements.

It was scientifically documented by Robert Brown in 1827 when he observed grass pollen dissolved in water under a microscope. The pollen moved irregularly and zigzagged, although the water appeared to be still. At first, Brown thought that this movement was due to a life force contained in the pollen. But he repeated the experiment with inanimate dust particles and noticed the same movement. The justification with the life force was thus falsified.

It took almost 80 years before Albert Einstein could provide a physical explanation for this observation. Independently of Einstein's work, the physicist Marian von Smoluchowski also developed a physical theory to explain the trembling movements observed.

The physicist Jean-Baptiste Perrin was able to prove the predictions of physical theory in an experiment in 1908 and received the Nobel Prize in Physics for this in 1926. Experimental evidence for Brownian molecular motion was crucial in convincing physicists of the existence of atoms and molecules at the time.