How empty is space?

“The universe is so humble. It clearly has everything, but it says it is empty.” How empty is the universe?

Speaking of the emptiness of the universe, we may have this picture in our minds: eight planets orbiting the sun in space, surrounded by the vast interstellar space.

In fact, the universe is far more empty than we imagine. Take the earth-moon system as an example. The picture that many people imagine may be like this: the moon is like a little follower, spinning around the earth.


But in fact,The average distance from the moon to the earth is more than 380,000 kilometers, which is 30 times the diameter of the earth. Even if the eight planets are arranged in a row, there will be no problem..

This is just a small earth-moon system. This system can even be simplified and approximated to a point for the earth’s orbit around the sun.For this “point”, the distance between the two closest “points” is enough100 times the distance between the Earth and the Moon. There is the planet closest to us besides the Earth—— Venusand the orbit it’s on.

100 times the distance between the Earth and the Moon is nearly 40 million kilometers, which is the closest planet.And the nearest star isearth to sunThe distance is 150 million kilometersthis distance is also called 1 “Astronomical Unit (AU)”.

In such a vast space of one astronomical unit, there are only two small points, Mercury and Venus. Even in the 1.5 AU inner solar system, including the Earth, Mars and their respective satellites, there are only 7 celestial bodies in total.

But as the eight major planets, the orbits of water, metal, earth and fire are already very close to each other. For the gas planets in the outer solar system, the distances between them are even more exaggerated.

Jupiter and SaturnThe average distance between orbits is 4.3 astronomical units, almost equal to the width of 3 inner solar systems.andSaturn and Uranus, Uranus and NeptuneBetween the tracks, it has even reached 10 astronomical unitsdistance.

So, is space emptier than you think?

Also different from imagination is the asteroid belt between Mars and Jupiter. In your imagination, is the asteroid belt an area covered with all kinds of debris? If a spacecraft passes through it, do you still have to worry about how to avoid it?

The asteroid belt is indeed considered the densest area of ​​celestial bodies in the solar system. There may be hundreds of thousands or even millions of asteroids here (discovered plus undiscovered ones). But even so, the celestial bodies here are far from being as densely packed one by one as everyone imagines, because this area is really too big.

The width of the asteroid beltalmost reached 1.5 astronomical units, almost equivalent to the width of the entire inner solar it A radius of about 3 AU, one can imagine how large the area of ​​this circular area is. Think about how small a million celestial bodies are in such a large space. In conversion,The distance between the two celestial bodies here is at least equivalent to the distance from the Earth to the Moon.. Therefore, the emptiness of the asteroid belt makes it difficult for spacecraft passing here to collide.

How about this, is the structure of the solar system different from what you thought before?

However, the area where the asteroid belt and the eight major planets are located is only a small part of the entire solar system, but it is already the densest area of ​​celestial bodies in the solar system.If withOort CloudAs a boundary, the sum of all the above mentioned is just a point.Because the radius of the Oort cloud can reachTens of thousands of astronomical unitsalmost measured in “light years”.


In other words, it takes about a year for the light from the sun to exit the solar system and shine on its nearest neighbor star (that is, located inSouth Gate 2ofProxima Centauri), it will take more than 4 years.And here 4 light yearsWithin the range, except for some wandering interstellar objects like Oumuamua, there are no other celestial bodies.

This 4 light-year distance is almost the average density of stars in the space around the solar system in the Milky Way. That is to say, within 4 to 5 light-years around a star, there is almost no second star.

How small is this density? If this space were reduced to the size of the Earth, then the only star here would be only about the size of a football.

From this point of view, even in the most densely populated places in the Milky Way, such as near the bulge in the center of the galaxy or in globular star clusters, there are only a few hundred more “footballs”, and there is no difference from a macro perspective.

However, this is only the interstellar space in the Milky Way. Compared with the intergalactic space outside the Milky Way, the density of interstellar space is already very large.

After leaving the Milky Way, it usually takes one hundred thousand or even millions of light years to reach other galaxies. For example, the nearest large galaxy neighbor to the Milky Way—— andromeda galaxyits distance from us exceeds 2 million light years.

Of course, there are also some satellite galaxies and other dwarf galaxies in the middle.But even if these dwarf galaxies are added, in Within 10 million light yearsthe total number of galaxies istens ofeverything else is empty.

However, the cosmic space we are in is already a relatively dense area of ​​galaxies.

Looking at the overall situation, in the large-scale fiber-like structures connected by galaxies and galaxy clusters, there are many seemingly empty areas between the fibers. These are called “Hollow“The place may be the real empty place in the universe.

For example, the famousBootes Hollowwe have only discovered dozens of galaxies in it so far, but its250 million light years in diameter.lie inThe Super Hollow of Canes CanisEven more exaggerated, the diameter has reached 1.0~1.3 billion light years. If there are alien civilizations living here, as long as astronomy is not interesting, they will probably think that the universe is only that big.

Of course, this kind of void is just a special case. There are many places in the universe that are densely packed with matter like the Great Wall of the Galaxy.So if all the matter in the universe is broken up and then distributed evenly, guess what?every cubic meter of the universeHow much substance will there be?

The answer is: only 6 hydrogen atoms! This is equivalent to having only one grain of sand in a space as large as the earth!

Having said that, if you observe these atoms microscopically, you will find that the atoms are also so empty. Because except for the nucleus and the electrons outside the nucleus, 99.99999…% of the space in the atom is also empty. If the atom were a football field, the nucleus and electrons combined would be no more than the size of a ping pong ball. From this point of view, the emptiness of the universe is not only reflected on the macro astronomical scale, but also on the microscopic quantum scale.


Although the void of the universe is beyond imagination, this void may not be silent. On the contrary, it may be filled with the invisible noise of quantum fluctuations. Perhaps the “emptiness” of the universe is not a lack, but the existence of another kind. It lets us know that what we see may be just the tip of the iceberg, and there are more mysteries in the universe waiting for us to discover.