Since ancient times, mankind has thought about how the world around us works. Why does grass grow, why does the Sun shine, why can't we fly... The latter, by the way, has always been of particular interest to people. Now we know that the reason for everything is gravity. What is it, and why is this phenomenon so important in today's we will consider.
Introduction
Scientists have found that all massive bodies experience mutual attraction to each other. Subsequently, it turned out that this mysterious force also determines the movement of celestial bodies in their constant orbits. The very same theory of gravity was formulated by a genius whose hypotheses predetermined the development of physics for many centuries to come. Developed and continued (albeit in a completely different direction) this teaching was Albert Einstein - one of the greatest minds of the past century.
For centuries, scientists have observed gravity, trying to understand and measure it. Finally, in the last few decades, even such a phenomenon as gravity has been put at the service of mankind (in a certain sense, of course). What is it, what is the definition of the term in question in modern science?
scientific definition
If you study the works of ancient thinkers, you can find out that the Latin word "gravitas" means "gravity", "attraction". Today, scientists so call the universal and constant interaction between material bodies. If this force is relatively weak and acts only on objects that move much more slowly, then Newton's theory is applicable to them. If the opposite is the case, Einstein's conclusions should be used.
Let's make a reservation right away: at present, the very nature of gravity itself has not been fully studied in principle. What it is, we still do not fully understand.
Theories of Newton and Einstein
According to the classical teaching of Isaac Newton, all bodies are attracted to each other with a force that is directly proportional to their mass, inversely proportional to the square of the distance that lies between them. Einstein, on the other hand, argued that gravity between objects manifests itself in the case of curvature of space and time (and the curvature of space is possible only if there is matter in it).
This idea was very deep, but modern research proves it to be somewhat inaccurate. Today it is believed that gravity in space only bends space: time can be slowed down and even stopped, but the reality of changing the shape of temporary matter has not been theoretically confirmed. Therefore, the classical Einstein equation does not even provide for a chance that space will continue to influence matter and the emerging magnetic field.
To a greater extent, the law of gravity (universal gravitation) is known, the mathematical expression of which belongs precisely to Newton:
\[ F = γ \frac[-1.2](m_1 m_2)(r^2) \]
Under γ is understood the gravitational constant (sometimes the symbol G is used), the value of which is 6.67545 × 10−11 m³ / (kg s²).
Interaction between elementary particles
The incredible complexity of the space around us is largely due to the infinite number of elementary particles. There are also various interactions between them at levels that we can only guess at. However, all types of interaction of elementary particles among themselves differ significantly in their strength. 
The most powerful of all the forces known to us bind together the components of the atomic nucleus. To separate them, you need to spend a truly colossal amount of energy. As for the electrons, they are "attached" to the nucleus only by ordinary electromagnetic interaction. To stop it, sometimes the energy that appears as a result of the most ordinary chemical reaction is enough. Gravity (what it is, you already know) in the variant of atoms and subatomic particles is the easiest kind of interaction.
The gravitational field in this case is so weak that it is difficult to imagine. Oddly enough, but it is they who “follow” the movement of celestial bodies, whose mass is sometimes impossible to imagine. All this is possible due to two features of gravity, which are especially pronounced in the case of large physical bodies:
- Unlike atomic forces, gravitational attraction is more noticeable the further away from the object. So, the Earth's gravity keeps even the Moon in its field, and the similar force of Jupiter easily supports the orbits of several satellites at once, the mass of each of which is quite comparable to the Earth's!
- In addition, it always provides attraction between objects, and with distance this force weakens at a low speed.
The formation of a more or less coherent theory of gravitation occurred relatively recently, and precisely on the basis of the results of centuries-old observations of the motion of planets and other celestial bodies. The task was greatly facilitated by the fact that they all move in a vacuum, where there are simply no other possible interactions. Galileo and Kepler, two outstanding astronomers of the time, helped pave the way for new discoveries with their most valuable observations.
But only the great Isaac Newton was able to create the first theory of gravity and express it in a mathematical representation. This was the first law of gravity, the mathematical representation of which is presented above.
Conclusions of Newton and some of his predecessors
Unlike other physical phenomena that exist in the world around us, gravity manifests itself always and everywhere. You need to understand that the term "zero gravity", which is often found in pseudo-scientific circles, is extremely incorrect: even weightlessness in space does not mean that a person or a spacecraft is not affected by the attraction of some massive object.
In addition, all material bodies have a certain mass, expressed in the form of a force that was applied to them, and an acceleration obtained due to this impact.
Thus, gravitational forces are proportional to the mass of objects. Numerically, they can be expressed by obtaining the product of the masses of both considered bodies. This force strictly obeys the inverse dependence on the square of the distance between objects. All other interactions depend quite differently on the distances between two bodies.
Mass as the cornerstone of theory
The mass of objects has become a particular point of contention around which Einstein's entire modern theory of gravity and relativity is built. If you remember the Second, then you probably know that mass is a mandatory characteristic of any physical material body. It shows how an object will behave if force is applied to it, regardless of its origin.
Since all bodies (according to Newton) accelerate when an external force acts on them, it is the mass that determines how large this acceleration will be. Let's look at a clearer example. Imagine a scooter and a bus: if you apply exactly the same force to them, they will reach different speeds in different times. All this is explained by the theory of gravity.
What is the relationship between mass and attraction?
If we talk about gravity, then the mass in this phenomenon plays a role completely opposite to that which it plays in relation to the force and acceleration of an object. It is she who is the primary source of attraction itself. If you take two bodies and see with what force they attract a third object, which is located at equal distances from the first two, then the ratio of all forces will be equal to the ratio of the masses of the first two objects. Thus, the force of attraction is directly proportional to the mass of the body. 
If we consider Newton's Third Law, we can see that he says exactly the same thing. The force of gravity, which acts on two bodies located at an equal distance from the source of attraction, directly depends on the mass of these objects. In everyday life, we talk about the force with which a body is attracted to the surface of the planet as its weight.
Let's sum up some results. So, mass is closely related to force and acceleration. At the same time, it is she who determines the force with which gravity will act on the body.
Features of acceleration of bodies in a gravitational field
This amazing duality is the reason why, in the same gravitational field, the acceleration of completely different objects will be equal. Suppose we have two bodies. Let's assign a mass z to one of them, and Z to the other. Both objects are dropped to the ground, where they fall freely.
How is the ratio of forces of attraction determined? It is shown by the simplest mathematical formula - z / Z. That's just the acceleration they receive as a result of the force of gravity, will be exactly the same. Simply put, the acceleration that a body has in a gravitational field does not depend in any way on its properties.
What does the acceleration depend on in the described case?
It depends only (!) on the mass of objects that create this field, as well as on their spatial position. The dual role of mass and the equal acceleration of various bodies in a gravitational field have been discovered for a relatively long time. These phenomena have received the following name: "Principle of equivalence". This term once again emphasizes that acceleration and inertia are often equivalent (to a certain extent, of course).
On the importance of G
From the school physics course, we remember that the acceleration of free fall on the surface of our planet (Earth's gravity) is 10 m / s² (9.8 of course, but this value is used for ease of calculation). Thus, if air resistance is not taken into account (at a significant height with a small fall distance), then the effect will be obtained when the body acquires an acceleration increment of 10 m / s. every second. Thus, a book that has fallen from the second floor of a house will move at a speed of 30-40 m/sec by the end of its flight. Simply put, 10 m/s is the "speed" of gravity within the Earth. 
Acceleration due to gravity in the physical literature is denoted by the letter "g". Since the shape of the Earth is to a certain extent more like a tangerine than a sphere, the value of this quantity is far from being the same in all its regions. So, at the poles, the acceleration is higher, and on the tops of high mountains it becomes less.
Even in the mining industry, gravity plays an important role. phenomena can sometimes save a lot of time. Thus, geologists are especially interested in the ideally accurate determination of g, since this allows exploration and finding of mineral deposits with exceptional accuracy. By the way, what does the gravity formula look like, in which the value we have considered plays an important role? There she is:
Note! In this case, the gravitational formula means by G the "gravitational constant", the value of which we have already given above.
At one time, Newton formulated the above principles. He perfectly understood both unity and universality, but he could not describe all aspects of this phenomenon. This honor fell to Albert Einstein, who was also able to explain the principle of equivalence. It is to him that mankind owes a modern understanding of the very nature of the space-time continuum.
Theory of relativity, works of Albert Einstein
At the time of Isaac Newton, it was believed that reference points can be represented as some kind of rigid "rods", with the help of which the position of the body in the spatial coordinate system is established. At the same time, it was assumed that all observers who mark these coordinates would be in a single time space. In those years, this provision was considered so obvious that no attempts were made to challenge or supplement it. And this is understandable, because within our planet there are no deviations in this rule.
Einstein proved that the accuracy of the measurement would be really significant if the hypothetical clock was moving much slower than the speed of light. Simply put, if one observer, moving slower than the speed of light, follows two events, then they will happen for him at the same time. Accordingly, for the second observer? the speed of which is the same or more, events can occur at different times.
But how is the force of gravity related to the theory of relativity? Let's explore this issue in detail.
Relationship between relativity and gravitational forces
In recent years, a huge number of discoveries in the field of subatomic particles have been made. The conviction is growing stronger that we are about to find the final particle, beyond which our world cannot be divided. The more insistent is the need to find out exactly how the smallest “bricks” of our universe are affected by those fundamental forces that were discovered in the last century, or even earlier. It is especially disappointing that the very nature of gravity has not yet been explained.
That is why, after Einstein, who established the "incapacity" of Newton's classical mechanics in the area under consideration, researchers focused on a complete rethinking of the data obtained earlier. In many ways, gravity itself has undergone a revision. What is it at the level of subatomic particles? Does it have any meaning in this amazing multidimensional world?
A simple solution?
At first, many assumed that the discrepancy between Newton's gravity and the theory of relativity can be explained quite simply by drawing analogies from the field of electrodynamics. It could be assumed that the gravitational field propagates like a magnetic one, after which it can be declared a "mediator" in the interactions of celestial bodies, explaining many inconsistencies between the old and the new theory. The fact is that then the relative velocities of propagation of the forces under consideration would be much lower than the speed of light. So how are gravity and time related?
In principle, Einstein himself almost succeeded in constructing a relativistic theory based on just such views, only one circumstance prevented his intention. None of the scientists of that time had any information at all that could help determine the "speed" of gravity. But there was a lot of information related to the movements of large masses. As is known, they were just the generally recognized source of powerful gravitational fields.
High speeds strongly affect the masses of bodies, and this is not at all like the interaction of speed and charge. The higher the speed, the greater the mass of the body. The problem is that the last value would automatically become infinite in the case of movement at the speed of light or higher. Therefore, Einstein concluded that there is not a gravitational, but a tensor field, for the description of which many more variables should be used.
His followers came to the conclusion that gravity and time are practically unrelated. The fact is that this tensor field itself can act on space, but it is not able to influence time. However, the brilliant modern physicist Stephen Hawking has a different point of view. But that's a completely different story...
