On a clear night, we look up at the stars and see the sky full of stars. But in the gap between these stars, there is darkness, and these stars are like gems embedded in black velvet, shining.
This is already a strange phenomenon, but human beings are different from other animals in that they have a clever brain, so that many people are still thinking about this phenomenon that has been seen for thousands of years: Why is the universe and space black?
Modern scientific observation and research has gained enough understanding of the universe that it has become common sense that the universe is very large, and there are trillions or even ten trillion galaxies in the observable universe. And our Milky Way is just one of these many spiral galaxies, and the sun is just an ordinary star in the Milky Way. The Milky Way is 200,000 light-years in diameter and contains between 200 billion and 400 billion stars.
Our earth is a medium planet in the solar system, and the sun shines on the earth every day, bringing us light, so that every day, the sky is bright. But in space, since every star is the sun, then even in the Milky Way, there are hundreds of billions of suns shining, why is the space background still dark?
Ancient people did not have a deep understanding of the universe, and regarded the stars as gods, of course, they would not think about this problem. But in the age of science, intelligent people began to wonder. In 1610, the astronomer Kepler raised this question, and he is considered to be the first to raise such a problem in a scientific way; in 1823, the German astronomer Obers systematically raised this problem, so people called this problem "Obers paradox".
The core idea of the Orbers paradox (also known as the night black paradox or the luminosity paradox) is that if the universe is steady and infinite, the night should be light, not dark. This idea is based on the absolute cosmology that has always dominated human thought, which holds that the universe is born like this, that it is endless without beginnings and ends, and that the speed of light is infinite.
Ever since the germ of science emerged in ancient Greece, the absolute cosmology became the dominant idea, until Copernicus discovered the heliocentric theory, Galileo observed the vastness of the universe, and Newton discovered gravity, and this concept was not shaken. According to this view, the universe is infinite, there is no beginning and no end, then the number of stars is infinite, and the speed of light is infinite, and it fills the universe in its lifetime, so that the continuous light emitted by stars remains in the universe, so that the universe should be bright.
But the reality that the universe is dark is a paradox and therefore a paradox.
Now, science has explained this problem very well. There are three main reasons for the darkness of the universe: one is that the universe is not static, but dynamic; the second is that the speed of light is limited and constant, so the propagation of light also takes time; the third is that the universe is too large and spacey, and there is no material filling, and the light of stars is far from enough to illuminate these spaces.
At the beginning of the last century, the scientific community was in turmoil, and breakthroughs were made in the observation and discovery of the universe. The most concentrated and typical representative of this is a legendary figure who discovered and created a series of theories that completely subverted human concepts, and this person was the great scientist Albert Einstein.
Einstein's special and general relativity, based on a series of discoveries in contemporary science, such as the expansion of the universe discovered by Edwin Hubble and the constant speed of light discovered by the Michelson-Morey experiment, argue for the relativity of space-time and negate the absolute view of space-time in Newtonian mechanics.
Since then, the universe is finite, it is constantly expanding, and there is a beginning and an end, and it has gradually become the mainstream cognition of the scientific community and the world. Through various scientific methods of observation and modeling, it is believed that the universe originated about 13.8 billion years ago, went through three stages of inflation, deceleration and accelerated expansion, and is now in the accelerated expansion stage. According to the Hubble constant, the rate of expansion is much greater than the speed of light.
Based on the rate of expansion of the universe and the Hubble constant, scientific modeling has led to an observable universe radius of about 46.5 billion light-years. The observable universe is only the universe that human beings can only understand and observe in the future, and the scope of this observable universe is still beyond the universe, and its scope is unpredictable.
From the moment time began, after the Big Bang began, it took hundreds of millions of years to form stars and galaxies that were rapidly separating and moving away from us as the universe expanded. In this way, the distance between the galaxy and the star will become larger and larger, the density in space will become more and more sparse, and the light will become weaker and weaker.
This is the first reason why the light of stars cannot illuminate the entire universe. The Obersian paradox is also actually a question of the absolute cosmology, supporting a relatively dynamic cosmology.
If the assumption is that the speed of light is infinite, then the starlight will suddenly fill the entire universe, so the universe is bright. But the infinity of the speed of light has been questioned since Galileo. After hundreds of years of unremitting efforts by scientists and countless experiments, the accurate vacuum speed of light was finally reached at 299792458 m/s (meters per second).
And through the Michelson-Morley experiment, it was confirmed that light in different inertial reference frames and different directions, such as light coming from the earth and against the earth, has the same speed, so that the speed of light is constant.
Since the speed of light is finite and constant, it takes time for the starlight to reach a place after it has been emitted.
The essence of light is electromagnetic waves, the human eye can only see visible light, visible light is only a very narrow band in the entire electromagnetic spectrum, the range is between about 780 and 380 nm. And light waves have a Doppler effect. What is the Doppler effect? That is, with the movement of the wave source, the wavelength will be elongated when the high speed is far away from the observer, and the wavelength will be compressed and shortened when the high speed is close to the observer, and the Doppler effect of the light wave is to produce spectral redshift and blueshift effects.
The expansion of the universe is much greater than the speed of light, so the light observed by humans in distant galaxies will have a redshift effect, and the farther away the starlight, the more it moves out of the wavelength range of visible light and becomes invisible to the human eye. Therefore, the light waves we see with the naked eye in the universe are only a small band of electromagnetic waves, and most of the frequency bands we cannot see, so that the light of stars cannot fill the entire universe.
We know that the light seen by the human eye has the light emitted by the light source itself, such as stars, fires, lights, etc.; there are also the light reflected by objects, such as we see all objects except the light source, the moon, people, houses, trees, etc., all rely on reflected light, we can see.
In space, each star is too far apart, and in the vast expanse of space, there is a vacuum, and there are hardly any particles to carry and reflect light, so the light of the star simply cannot illuminate the whole space.
Our earth is only 150 million kilometers away from the sun, and the earth is wrapped in the atmosphere, when the sunlight comes to the earth, it is reflected by atmospheric particles, so we can see the blue sky and white clouds, and there is also a bright light in the shade; if on the moon, because there is no reflection of atmospheric particles, the place where the sun shines is a white and spicy dazzling light, and the place that is not illuminated is a dark (of course, the moon stones and so on will also reflect to the shadows, but there are no atmospheric particles and dust, it will be much darker).
In fact, the sun's light does not shine far, if you reach Pluto, the average distance from the sun is less than 6 billion kilometers, only 0.00063 light years, where the sun seen there is only the size of a star. Although it is still relatively bright, it is only 291 times brighter than the full moon, while on Earth, the sun is about 700,000 times brighter than the full moon.
Looking at the brightness of the Sun at a distance of 1 light-year, the apparent magnitude is only -2.71 (the apparent magnitude is the smaller the value, the brighter the value), and the brightest venus on Earth can reach -4.6 magnitudes, and Jupiter can reach -2.9 magnitudes when it is brightest. That is to say, if you look at the sun at a distance of 1 light-year, you have not yet seen Venus and Jupiter on Earth. If at a distance, to the nearest star to the Sun 4.3 light-years away from the South Gate II to look at the Sun, the Sun's apparent magnitude is only 0.43 magnitude, not yet we can see the South Gate II A star 0.01 magnitude bright.
How sparse are stars in space? Let's take the Milky Way as an example to calculate: the Milky Way is about 200,000 light-years in diameter, and if you calculate the volume with an average thickness of 5,000 light-years, 400 billion stars are spaced apart from each other on average by about 7.32 light-years. Such a sparse starry sky, relying on the energy of the stars is completely unable to fill the bright light.
What's more, the entire space is in a high vacuum, except for some places where dust and nebulae are concentrated, most of the space is only extremely small particles floating, and it is impossible to absorb and reflect bright light. The intervals between galaxies are even greater, calculated in hundreds of thousands to millions of light-years, so this huge space is even more vacuumed.
It is generally believed that the density of the universe averages only 1 particle per cubic meter. This density is calculated in proportion to space by adding up all the particles in the universe (including all galaxies and stars that are reduced to particles). Hence the emptiness of the universe is beyond the phenomenon of man.
Where the cosmic dust is concentrated, we can see the reflection of the nebulae, and the telescope photographs the brilliant and colorful and strange nebulae, such as the Pillar of Creation and so on (below). In fact, the density of these nebulae is also very small, only a few tens to hundreds of particles per cubic centimeter, and on the highly vacuumed lunar surface, the particles also reach tens of thousands of particles per cubic centimeter.
But these nebulae in the extremely vacuum of the universe, it is very dense, and the range is huge, often several light years or even hundreds of light years, the total number of these particles is very large, so the nebula is the mother of the birth and formation of new stars.
That's why we look up at the stars at night, the stars are dotted, but the background is pitch black. Modern cosmology believes that because the universe is constantly accelerating and expanding, the future starry sky will become more and more sparse, the background will become darker and darker, and eventually the universe will die in heat silence. Of course, this time will be very long, so long that human beings simply cannot bear it, and they have long disappeared.
I don't know if you understand? Welcome to the discussion, thanks for reading.
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