# The book "Gravity. Einstein's Last Temptation »

Having solved the secret of gravity, we will be able to answer the greatest questions of science: what is space? What is time? What is the Universe? Where did all this come from? Markus Chaun, an illustrious popular science author, invites you on a fascinating journey - from the moment gravity was recognized as physical force in 1666 to the discovery of gravitational waves in 2015. A tectonic shift is approaching in our ideas about physics, and this book tells us what questions the phenomenon of gravity poses to us.

### Excerpt Moon: an attempt to escape

The tidal influence of the moon on the earth slows down the movement of our planet, reducing its rotational moment. There is a fundamental principle of physics, called the conservation of momentum during rotation, according to which the rotational moment of an isolated (closed) system never changes. So, if the Earth's rotational moment decreases, the rotational moment of another element of the system should compensate for this by increasing. In our case, only one option - the moon.

The attraction of the moon creates two tidal knolls on both sides of the earth, but one that arises on the same side as the moon attracts it with the greatest force. As we already know, this tidal hill usually overtakes the Moon in its orbit, because the Earth makes a turn around its axis faster than the Moon bypasses it. Therefore, the gravity of the Earth drags the moon forward in its orbit, giving it acceleration.

Note that the force of gravity of the Earth at a distance of the Moon has just such a value, which is necessary to bend the trajectory of a body moving at lunar speed, and give it the shape of a closed orbit, which we observe. Accordingly, if the moon moves too fast, its speed will exceed the required one and it will fly out of orbit. In relation to the Earth, “beyond the orbit” means up, and we know that if you throw a body (for example, a ball) up, gravity will delay its flight. Paradoxically, the Moon, accelerated by tidal interaction with the Earth, moves more slowly with distance from the Earth. Because of this, the torque is increased to the required value.

And this is not just theoretical reasoning. The Apollo-11, Apollo-14 and Apollo-15 piloted American spacecraft, as well as the Soviet Lunokhod-1 and Lunokhod-2 unmanned aerial vehicles, left reflectors on the lunar surface. These fist-sized mirrors are also called angular reflectors, and they can reflect light exactly in the direction from which it originates. That is, you can direct the laser beam to the moon so that it reflects from the angular reflector, and then measure the time of its return to Earth. Knowing the speed of light, you can easily calculate the distance to the moon.

Experiments show that every year the distance that the ray reflected from the moon travels increases by 3.8 centimeters. In other words, every 12 months, the Moon moves away from Earth at a distance approximately equal to the length of the thumb. If you are 70 years old, for your life it has gone a distance equal to the length of the car.

### Observation of total eclipses

The fact that every year the moon is 3.8 centimeters away from the earth means that it was once much closer to us. And this, in turn, influenced the occurrence of total eclipses - one of the most amazing natural phenomena.

As we already know, a total eclipse occurs when the moon passes between the Earth and the Sun, closing the solar disk and casting a shadow on the Earth. A total eclipse is possible because the Sun, although it is 400 times larger than the Moon, is 400 times farther from us. That is why the sun and the moon seem to us in the sky equal in size. For us, this is a very fortunate circumstance. Despite the fact that there are more than 170 moons in the Solar System, it is impossible to observe a total eclipse from any planet. Moreover, we were lucky not only with the place, but also with time.

As the moon moves away from the earth, it seemed bigger in the past and will become smaller in the future. Apparently, the first total eclipses began about 150 million years ago, and after another 150 million years they will not be at all. Observe total eclipses of the inhabitants of the Earth can only for a short period of its existence. For example, in the days of dinosaurs, they were not yet.

The fact that the moon is moving away from Earth, and in the past it was located closer to it, is perfectly combined with the theory of its origin.

### The planet that pursued the Earth

The moon is too large relative to the Earth, and its diameter is about a quarter of the diameter of our planet. All the other moons in the solar system seem tiny next to their planets. In addition to Pluto, the moon is even larger in relation to its size, but since 2006, Pluto has ceased to be considered a planet.

This size of the moon hints to us that her origin was unusual. Scientists assume that 4.55 billion years ago, when our planet was just formed, it collided with a celestial body with a mass approximately equal to that of Mars (today this hypothetical planet is called Teya). The inner layers of the Earth turned into liquid, and part of its mantle splashed into vacuum. A ring has formed around our planet, similar to those surrounding the gas giants in the solar system. From this ring, the Moon quickly formed, the orbit of which at that time was ten times closer to Earth. After that, the moon began to gradually move away from our planet.

The confirmation of the Big Burst theory was found in the course of the American space program Apollo, thanks to which we know that the composition of the moon is similar to the composition of the outer mantle of the Earth. In addition, lunar rocks contain much less water than the driest terrestrial rocks. This confirms that they were once exposed to high temperatures. The only question is: in order for a body with a mass of Mars not to destroy our entire planet, but merely create the moon, it had to pass tangentially to Earth at a very low speed. However, all cosmic bodies moving in orbits around the Sun (both within the Earth's orbit and outside it) are too fast for this.

The Big Splash Theory will only work if Theia once was in the same orbit as the Moon. It could have been formed from debris at a stable Lagrange point, that is, at 60 degrees in front of the Earth or behind it in orbit around the Sun. Today, the same debris of asteroids orbit Jupiter at 60 degrees in front of him and behind him, so Jupiter seems to be floating in the Sargasso Sea. If you believe the theory of the Big Spike, Theia pursued the Earth for millions of years, and then moved to another orbit, which caused a collision.

Since the force of attraction of the body weakens with the square of the distance from it, tidal forces, which are explained by the difference in attraction, decrease with the cube of the distance. The newly formed Moon was about ten times closer to the Earth than it is now, which means that the tidal force with which it influenced the Earth was more than once. At that time, the Earth did not yet have oceans, but if they were, the waters in them would rise twice a day not for a couple of meters, but for kilometers.

But not only the newborn moon influenced the Earth. The Earth itself also affected her with a tidal force increased 1000 times. The inhibition of the movement of the moon was so strong that, most likely, it was fixed in it quite early (approximately ten million years after its formation). Since the first microorganisms on Earth appeared much later, approximately 3.8–4 billion years ago, not a single living creature observed the opposite side of the Moon rotating in the night sky.

### The moon did not always move at such a speed.

An interesting question arises: Has the moon always moved away from the Earth at a speed of 3.8 centimeters per year? In 2013, a group of scientists led by Matthew Huber from Purdue University (West Lafayette, Indiana) found out how this situation looked 50 million years ago. They entered data on the depth of the oceans and the outlines of the continents that existed at that time into a computer simulator of tides and, based on its indicators, concluded that at that time the Moon was moving away from the Earth more slowly, probably twice.

It is all about the Atlantic Ocean, which is today wide enough to form a large tidal hump, affecting the moon and causing it to retreat quickly enough; 50 million years ago, the ocean has not yet taken its present form, so its tidal hump was less, and its influence on the motion of the moon was weaker. At that time, the Pacific Ocean was responsible for most of the tidal impact.

This example is another illustration of how complex the system of ebbs and flows. Their height and the force with which they inhibit the movement of the Earth and accelerate the retreat of the Moon, depend on how easily tidal humps can move across the ocean expanses. This, in turn, is determined by the location of the continents, which is constantly changing due to continental drift (plate tectonics, as it is officially called).

Due to the fact that it is impossible to predict the plate movement in the long term, we also cannot know when the rotation of the Earth will slow down so that it will be forever turned by one side to the Moon. We only know one thing: in order for the Earth to start making a full rotation around its axis in 47 days, and the Moon moved away from it so far that its orbital path would also take 47 days, no less than ten billion years would have to pass. We already know that this is a completely hypothetical scenario, because by this time the Sun will turn into a terrifying red giant, shining 10,000 times brighter than it is today, and will destroy (or at least substantially change) the Earth-Moon system.

Ebb and flow has another feature. Every day, when the waves roll on the coast, and then return to the sea, they pick up a lot of small pebbles. The friction between stones that constantly collide with each other generates thermal energy absorbed by the environment. It is this loss of energy that ultimately leads to a slower rotation of the Earth.

Tides heat the Earth slightly, and if you go swimming in the sea, neither sand nor stones will burn your legs. But in the solar system there is one place where the tides generate much more thermal energy. This is Io, the giant satellite of Jupiter, discovered by Galileo in 1609.

### Moon pizza

March eighth, 1979. NASA’s Voyager 1 space probe flies through the Jupiter system faster than a bullet, rushing to meet Saturn in 1980. But before the gas giant leaves the probe forever, the management team forces him to turn the camera back and take a farewell shot of Io. Navigation engineer Linda Morabito is the first to see an image that has covered a distance of 640 million kilometers from the Mission Control Center, and it takes her breath away. A tiny column of phosphorescent gas escapes from a tiny, half-visible moon.

Morabito first in the history of mankind saw Io supervolcanoes. The next day, the entire Voyager management team leaned over enlarged photographs and temperature measurement data. They found eight giant pillars of gas, throwing matter up hundreds of kilometers. It turned out that Io is the most geologically active cosmic body in the solar system, on which more than 400 volcanoes are located. The holes through which orange, yellow and brown rocks are thrown onto Io’s surface, making it look like a pizza, resemble geysers in Yellowstone Park. In a sense, Io volcanoes are really geysers. Lava in them does not break out to the surface, but heats the liquid sulfur dioxide, located directly under the bark of Io, and it turns into gas. Then the gas is ejected upwards just like steam in an earth geyser.

Each year, Io throws about 10,000 million tons of substance into a vacuum, which then drops to the surface, covering it with sulfur, like the earth around the geysers in Yellowstone. That's why in photos Io looks like a giant pizza. Bright colors are just layers of sulfur having different temperatures.

The key to understanding Io supervolcanoes is Jupiter, 318 times the mass of Earth. Io is at the same distance from him as the moon is from Earth. But because of the tremendous force of gravity, Jupiter Io revolves around him not in 27 days, like our Moon, but in just 1.7 days. Gravity, acting on the tidal humps of Io, has long since stopped its rotation, so now the moon is constantly turned to its planet on one side. Just imagine what kind of view will open to people if the spacecraft ever lands on the surface of Io: Jupiter and its colorful cloud rings will occupy a quarter of the sky.

Since Io is fixed in one position, two tidal humps, arising under the influence of Jupiter's attraction, will be directed directly at him and directly from him. They will not move in stone, as terrestrial tidal humps move in the oceans. If something like this happened on Io, the hard rocks would constantly stretch and shrink, gradually heating up due to friction (the rubber ball that you squeeze in your hand also heats up). Since this does not happen, it is logical to assume that the temperature of Io does not rise under the tidal influence of Jupiter.

But it is not.

A key role in the heating of Io is played by two other Galilean moons that move in more orbits more distant from the planet - Europe and Ganymede. Ganymede is the largest moon in the solar system and exceeds the size of Mercury. In the time it takes Io to go around Jupiter four times, Europe does it twice, and Ganymede once. Because of this, the two satellites periodically turn out to be lined up in a single line, which increases their impact on Io. They seem to be pulling Io to the side, lengthening its orbit. Thus, Io constantly moves in the direction to Jupiter, then from him. It is this movement that causes Io to warm up from the inside.

Yes, Io’s tidal humps are aimed directly at Jupiter and away from him. But when Io comes close to his planet, the tidal hump grows, and when he leaves, the hump decreases. Because of the constant movement, the rock is then compressed, then stretched, and because of this process, Io warms up so much that it is she who allocates the most heat to one pound of weight in the Solar System, and not the Sun at all.

### The Mystery of Pluto and Charon

The pair Jupiter - Io - is not the only one in the Solar System, in which two celestial bodies moving in orbits around each other, were fixed in such a position that each of them only one side of the other is visible. There is also Pluto and its huge moon Charon.

The most interesting thing about Charon is that its diameter is half the diameter of Pluto. Because of this, for some time, Pluto was considered the planet with the largest moon (relative to its own size) in the solar system. But in 2006, the International Astronomical Union deprived Pluto of the status of the planet and transferred to the category of dwarf planets. Now he is just one of many tens of thousands of ice fragments orbiting the sun on the border of the solar system.

The Kuiper belt consists of ice fragments left after the appearance of the planets. Of these, the planet did not work, because they were too sparse. The Kuiper belt is similar to the inner belt of the asteroids of the solar system - another dump of planetary debris that could not concentrate at one point under the influence of Jupiter’s gravity.

The inner edge of the Kuiper belt starts not far from Neptune (that is, the distance from it to the Sun is about 30 times greater than that from Earth), and the outer edge ends at a distance from the Sun 50 times larger than the one on which the Earth is located. Despite the name, the former existence of this belt was predicted by a former Irish soldier and amateur astronomer Kenneth Edgeworth in 1943, so he would justly have been called the Edgeworth-Kuiper belt.

Pluto meets the two criteria of the planet, formulated by the International Astronomical Union in 2006: it is round and moves in orbit around the sun. But since next to him are many objects from the Kuiper belt, he does not comply with the third requirement — a free orbit on which there are no other celestial bodies.

On July 14, 2015, the NASA New Horizons station flew through the Pluto-Charon system, like a high-speed train, having traveled just 14,000 kilometers above the celestial body, which at the time of sending the station was still considered a planet. The staff of the Mission Control Center on Earth were amazed. They expected to see a dead, motionless world bound by cosmic cold far from the Sun. Instead, nitrogen glaciers and mountains of ice appeared in front of them, the peaks of which were hidden in the swirls of thin clouds. Most surprising was the fact that the so-called Tombo region (a pink spot on Pluto that has a cloud shape and was named after Pluto discoverer Clyde Tombo) did not have a single crater, unlike the rest of the planet. This meant that ice was formed relatively recently.

Where does the energy for this unusual activity come from? The inner layers of the Earth are heated by the radioactivity of uranium, thorium and potassium, but in order to heat Pluto, this is not enough. Heating under the influence of the tidal force of Charon is also excluded, since such a process is impossible in a system where the moon moves in a circle around the planet and both celestial bodies are always turned towards each other by the same side. However, this rule only works if Charon was in orbit of Pluto at the time of the formation of the Solar System, approximately at the same time when the Moon became a satellite of the Earth. If Pluto had recently acquired its satellite (during the last half a billion years), then heating under the influence of tidal forces would have occurred and continued until until Pluto and Charon would be fixed in the current position relative to each other. No one knows how it really was. This question remains open.