Humans are diurnal creatures, on the one hand, our eyes lack rod cells that can see things at night, which mostly appear in nocturnal animals such as owls; on the other hand, humans learned to use fire in the process of evolution, gradually no longer need to be wary of other animals at night, and grew into the existence of the top of the food chain, without the need for natural selection of night vision in the long process.
But the natural night is not pitch black, it has the moon and the starry sky, human beings also began to look up at the starry sky at night, slowly, the ancients summed up some experience in the process of observation.
Humans have learned to use fire
For example, the shape of the moon changes, and the position of some stars, the first yang sail of humans to use them to guide the direction, to reach the distant continent. The earliest humans who cultivated the land learned from them to predict tomorrow's weather and achieved a good harvest.
And the romantic ancients also left countless treasures for future generations under the influence of the night sky, such as "the tide of the spring river is even with the sea level, and the bright moon on the sea is born together." "Roundabout Morning Glory Star, Kyaukgyao River Han Girl." And The poetry immortal Li Bai's "bright moonlight in front of the bed, suspected to be frost on the ground." ”
Although the night has bound human activities, it has not bound human thoughts, but has spawned a unique "night civilization" of human beings, which makes human history more mysterious and romantic.
Beautiful night sky
Ever since humans recognized the concepts of night and day, they have been pondering the question, why do the sun and moon alternate? The first people lacked astronomical knowledge, they felt that the earth was flat and the sky was a round lid.
One end of the lid is day and the other end is night, and this cover can be transformed. With the later development, human beings gradually realized that the earth is round, it will even rotate, then the sun and moon rotation is likely to be the sun, the moon revolves around the earth, this is the geocentric theory.
After a long period of exploration and struggle, the heliocentrism has gained a firm foothold, and the change of day and night is because the earth revolves around the sun, and there is a place where the sun shines, so there is no night.
Simulation diagram of geocentric and heliocentric theory
After entering the age of technology, humans have launched hundreds of satellite probes into space, and for the first time we have observed the Earth from a third perspective. It is a spherical planet that rotates while orbiting the Sun.
Because there is an inclination of the earth's central axis, so that the time to receive the sun is different, so the length of day and night in different places on the earth is different, the area in the day is called the day hemisphere, the area that envelops the night is the night hemisphere, and their dividing line is called the morning and evening line in geography.
The day and night cycle has an indelible significance for all natural things, it can allow all things to be recuperated, and it can avoid the surface temperature of the earth being too high due to the direct sun for a long time, and it will not make the side with its back to the sun too cold. It can be said that the growth of all things cannot be separated from the turnover of day and night.
Simulation of the rotation of the Earth
Why is the night sky black? One would answer that because there is no sunlight, without light we look black, what a "normal" natural phenomenon.
Later, the satellites launched by humans sent image data about the universe, and we saw the color of the universe, which was black, so everyone thought that the night sky we saw was the original color of the universe.
But the question arises, the universe is not only the sun, it has hundreds of millions of stars that will glow and heat like the sun, these stars are enough to illuminate the universe, why is it black?
This question has been pondered as early as the 19th century, and the famous German astronomer Aubus proposed that there are many galaxies in the universe, and the light emitted by these galaxies can illuminate the night of the earth, so that the earth is always in the daytime.
The black universe
At that time, this statement seemed to be a fantasy, and everyone in the dark of the night saw it in their eyes, how could the light illuminate it? And the moon's light is not emitted by itself, but the sunlight reflected by the moon.
Obers's claim did not attract attention at the time and was called the Obers fallacy. As human exploration reaches into space, we can understand the universe from a more detailed way, and astronomers have more advanced equipment to study the mysteries of the universe.
Gradually, they discover that the once-ridiculed Obersian paradox may really exist, and that there is a desperate truth hidden in the seemingly "normal" night on Earth.
The great British scientist Newton was the first to "dissect" the color of light, proving that sunlight is a mixture of seven colors, while Einstein was the first to bring another property of light to the public.
That is, light has the dual characteristics of waves and particles, which is the predecessor of the "wave-particle duality" of light. Later physicists proved these theories experimentally, and also discovered that light is an electromagnetic wave.
After unremitting research by scientists, it has been found that light is far more complex than we believe. The seven-color light obtained by Newton is physically visible, and visible light is the visible part of the electromagnetic spectrum that is visible to the naked eye.
Because the human eye can only perceive electromagnetic waves with a frequency of 380 to 750thz and a wavelength between 780 and 400 nm, electromagnetic waves other than this part are not captured by our eyes, which is called invisible light.
Triangular prism refraction of sunlight experimental simulation diagram
The range of invisible light is larger, and it is a general term, we often say ultraviolet, infrared, x-ray, etc. belong to this. The introduction of spectra has made us gradually realize that the frequency of light has no boundaries, and human perception of light is limited.
So the Obers paradox once again attracted the attention of astronomers, Obers said that the light is likely to exist, but we can't see, that is, every moment in the universe there is light to the earth, but these lights are not in the range of human beings can see, our night sky is also dark.
So why, on earth, can't we see these lights?
Beautiful view of the night sky
The phenomenon of redshift first appeared in human mechanical wave experiments, and researchers found that if the wave is constantly moving, then the farther it is from the receiver, its wavelength will become longer and the frequency will decrease, and when it is close to the receiver, the wavelength will become shorter and the frequency will rise.
By analogy, considering that light also has the nature of waves, scientists immediately set about experiments and got the same results as mechanical waves, the farther away from the receiving point of light, the light wave becomes longer, and in the spectrum, the spectral line moves towards the red light region.
Red light is the wavelength limit that the human eye can see, and the light outside it is called infrared, and the spectral line used for experiments moves towards the red light, so it is called redshift. Correspondingly, the phenomenon of the spectral line moving towards the violet spectral region is called blue shift, which is manifested by the shorter frequency of the light wave becoming higher.
Redshift and blueshift simulation plots
There are three common redshift phenomena in the universe, namely:
Doppler redshift: Caused by radiation sources moving away from us in a fixed space; gravitational redshift: due to photons escaping from the gravitational field radiating outward; cosmological redshift: caused by the expansion of space itself.
The redshift phenomenon shows that the glowing stars are moving away from our Earth, and the light they emit is also stretched in the process until the wavelength is more than 400 nm, and humans can no longer see it.
That is, the positions of stars in the universe are not set in stone. So what is the force that keeps the glowing stars away from Earth?
Doppler redshift simulation diagram
The universe has been from the explosion, and since the explosion, it has not stopped expanding. The first to propose the concept of cosmic expansion was the famous American astronomer Hubble.
He came to the conclusion that "all nebulae are moving away from each other, and the farther away they are, the faster they can leave." In fact, based on astronomers' incessant observations over the years, the universe is indeed expanding at a rate we can't imagine.
Some celestial bodies may already be moving away from Earth at a rate of one-third of the speed of light. Since the universe is expanding, why hasn't the distance between the Earth and the Sun expanded, and we can still see sunlight?
Big Bang Imagination Map
The expansion of the universe occurs between galaxies, the Earth is in the solar system, and the sun is theoretically moving away, but this speed is much slower than other stars, so the sunlight does not all redshift.
What drives the expansion of the universe is dark energy, which has existed since the day the universe exploded, and dark energy is a general term for all unknown forces in the universe, because it does not absorb, reflect and radiate light, and cannot be observed by human technology in recent days.
The concept of dark energy was proposed by the famous physicist Albert Einstein in 1917, and the equations derived on his general theory of relativity showed that the universe was in motion, and Einstein had proposed that the universe was stationary.
Simulation of the expansion of the universe
Obviously, the two theories are at odds. So Einstein introduced the "cosmological constant" to this equation. Later scientists named dark energy on this basis. The age of the universe is expected to be around 14.6 billion years.
But the diameter of the universe has reached more than 92 billion light-years, from which the rate of expansion of the universe can be seen. We see from the image that the universe is pitch black, but in fact, the universe is full of all kinds of light, which come from every star that can emit light and heat on its own.
Unfortunately, these lights are in the expanding space of the universe, the wavelengths become longer and the frequencies become shorter, and humans cannot see them, thinking that there is nothing there. If you think about it, it's not that there is no light in the universe, but that humans can't see it, and this feeling is desperate enough.
Simulation of the theme of the expansion of the universe
Although human beings know very little about dark energy, it is undeniable that dark energy will always be exhausted one day, but when dark energy is exhausted, the universe will cool down and slow down the pace of expansion, and when a balance point is reached, the average temperature of the universe will come to zero degrees, and everything will be stationary.
It sounds beautiful, but if it does happen, it will be nothing but despair for the planet. As mentioned earlier, the "black" of the night is because most creatures cannot see light beyond the visible wavelength, and not seeing does not mean that there is none, and our earth is surrounded by light waves all the time.
The little benefit of being invisible is that it can rest at night and prepare for the next day. The inability to see light is due to the phenomenon of redshift, which is one of the proofs of the expansion of the universe, that is, the night seen by human beings is the result of the expansion of the universe.
If one day this expansion stops and the redshift phenomenon does not occur again, then the light that fills the universe will be seen by the creatures on the earth, the ecology on the earth will be disrupted, and the world will be in a polar state.
Fortunately, this will not happen at this time, and it will take us a long time to figure out the dark energy that drives expansion, and scientists speculate that 99% of the dark energy in the universe is unknown, so it cannot be exhausted so quickly, and the expansion will continue.
The polar diurnal phenomenon in Northern Norway
For a long time, human beings have thought that the dark sky at night is a normal natural phenomenon because there is no light, but with the progress of science and technology and the deepening of human exploration of the universe, there is a desperate fact.
The so-called night is actually the product of the expansion of the universe, our universe is full of all kinds of light, they have different energies, but also have different wavelengths and frequencies, but we do not see these lights.
Our eyes have lost the color that should have been beautiful, as if the universe had made a joke to mankind. However, the universe is conserved, although it uses black to set off other stars, so there are stars in the sky.
These stars became the enlightenment of human cosmic consciousness, and from then on began the journey to the sea of stars. We still can't see the color of the universe, but we know the distant planets.
Dim night sky