Did you know that the sunlight that shines on us actually came from 5 million years ago?

How long does it take for sunlight to reach the earth?

8 minutes, this may be common knowledge that primary school students know, because the average distance between the sun and the earth is about 149.6 million kilometers, and the speed of light is 299,792,458 meters per second, so it takes about 8 minutes. That is to say, the sun we see at this moment is actually the sun 8 minutes ago. If the sun goes out at this time, we will need 8 minutes to notice it. It takes 8 minutes for sunlight to reach the earth. These 8 minutes are calculated based on the time required for light from the surface of the sun to move to the earth. If we count from the time when sunlight is generated, how long does it take for light to reach the earth? This time is long. Where does sunlight come from? It comes from the core of the sun, because only the core of the sun has enough pressure and temperature to induce hydrogen nuclear fusion.

The sun is a huge star with a radius of 695,500 kilometers. However, for photons, this radius is not very large and it takes less than 3 seconds to escape. But the problem is not that simple.

Photons born in the core of the sun cannot rush around unhindered like in the nearly vacuum universe. The sun is a star with a density of about 1,408 kilograms per square meter. If photons want to run from the center of the sun to the surface, they will collide with various molecules inside the sun, so they cannot run straight out like a beam of light. They can only diffuse to the surface of the sun in the form of Brownian motion, which takes some time. As for how much time it takes, we have to start with the Brownian motion itself.

Brownian motion refers to the never-ending irregular motion of particles in a liquid or gas. The discovery of Brownian motion was very early, starting in 1827. The person who discovered this phenomenon was not a physicist, but a British botanist, R. Brown.

As a botanist, pollen is his regular research object. Once, Brown sprinkled pollen in the water for observation and found an interesting thing. The pollen kept moving in the water. Brown observed for a long time, but the pollen in the water did not stop, so Brown began to think about the reason. At first, Brown placed the water cup in a closed environment for observation, but the movement still did not stop, so he ruled out the cause of "air disturbance". Since it is not caused by the air, it must be caused by the pollen itself. Since plants are alive, then pollen may also be alive? Brown guessed that movement may be an active behavior of pollen.

This time, Brown fried the pollen and sprinkled it back into the water, and the pollen still moved.

Brown thought again, since it was not the pollen that was the problem, then there must be something in the water. Yes, there were microorganisms in the water, so Brown sprinkled the pollen into alcohol and kerosene, and the pollen still moved. Brown was confused, and finally he came to the following conclusion: "Active molecules are widely present in various organic and inorganic substances." Translated, this sentence means that life is everywhere, whether it is alcohol or kerosene, there is life in them. Obviously, Brown's conclusion was wrong. After nearly a century, people realized the existence of molecules and atoms, and finally solved the mystery of Brownian motion.

Whether it is gas, liquid or solid, all matter is composed of molecules or atoms, and molecules and atoms are always in motion.

Since the molecules are moving, and water is made up of water molecules, the water molecules will inevitably collide with large particles in the process of movement, such as pollen, so the pollen will also move. Since the movement of water molecules is random, the movement of pollen also presents a random trend. In addition, since the essence of temperature is movement, the higher the temperature of an object, the faster the movement of its internal molecules, so pollen moves faster in hot water than in cold water. At this point, the mystery of Brownian motion has finally been solved. Brownian motion itself is a random motion, so can the time it takes for photons that diffuse in the form of Brownian motion inside the sun to reach the surface of the sun from the center be calculated? Yes.

The distance that photons from inside the sun diffuse outward in the form of Brownian motion involves three key factors: the speed of the photons, the diffusion time, and the mean free path.

The mean free path refers to the average distance between two adjacent collisions of molecules. Specifically for the sun, the distance that photons diffuse from the center to the surface of the sun is known, which is the radius of the sun, which is 695,500 kilometers. The speed of photons is also known, which is the speed of light, which is 299,792,458 meters per second. As for the mean free path of photons in the sun, although there is no exact data, scientists estimate that it is about 0.01 mm. With these three known quantities, we can calculate the time required for photons to move from the center to the surface, which is to divide the radius of the sun by the product of the speed of light and the mean free path, and the result is about 5 million years. In other words, the photons that shine on us come from 5 million years ago.

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