Since ancient times, mankind has been thinking about how the surrounding world works. Why grass grows, why the Sun shines, why we cannot fly ... The latter, by the way, has always been especially interesting for people. Now we know that the reason for everything is gravity. What is it, and why this phenomenon is so important on a scale of the Universe, we will consider today.
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 theory of gravity was formulated by the brilliant
Isaac Newton, whose hypotheses predetermined the development of physics for many centuries to come. Developed and continued (albeit in a completely different direction) this doctrine of Albert Einstein - one of the greatest minds of the past century.
For centuries, scientists have been observing attraction, trying to understand and measure it. Finally, in the past few decades, even a phenomenon such as gravity has been placed at the service of humanity (in a certain sense, of course). What is it, what is the definition of the term 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 call the universal and constant interaction between material bodies. If this force is relatively weak and acts only on objects that move much slower than the speed of light, then Newton's theory is applicable to them. If the opposite is the case, then Einstein’s conclusions should be used.
Let us make a reservation right away: at present, the very nature of gravity has not been fully studied in principle. What it is, we still have no idea.
Theories of Newton and Einstein
According to the classical teaching of Isaac Newton, all bodies are attracted to each other with a force directly proportional to their mass, inversely proportional to the square of the distance that lies between them. Einstein claimed 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 thought was very deep, but modern research proves it is somewhat inaccurate. Today it is believed that gravity in space bends only space: time can be slowed down and even stopped, but the reality of changing the form of temporary matter is not theoretically confirmed. And therefore, the classical Einstein equation does not even provide a chance that space will continue to affect matter and the resulting magnetic field.
The law of gravitation (universal gravitation) is known to a greater extent, the mathematical expression of which belongs precisely to Newton:
\ [F = γ \ frac [-1.2] {m_1 m_2} {r ^ 2} \]
By γ we mean the gravitational constant (sometimes the symbol G is used), the value of which is 6.67545 × 10−11 m³ / (kg · s²).
The interaction between elementary particles
The incredible complexity of the space around us is largely due to an infinite number of elementary particles. Between them there are also various interactions at those levels that we can only guess about. However, all types of interaction of elementary particles with each other vary significantly in strength.
The most powerful of all the forces known to us connect the components of the atomic nucleus. To separate them, you need to spend a truly colossal amount of energy. As for 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 common chemical reaction is enough. Gravity (what it is, you already know) in the variant of atoms and subatomic particles is the easiest form of interaction.
The gravitational field in this case is so weak that it is difficult to imagine. Oddly enough, 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 at a distance from the object. So, the Earth’s gravity keeps even the Moon in its field, and a similar force of Jupiter easily supports the orbits of several satellites at once, the mass of each of which is quite comparable with the earth!
- 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 harmonious theory of gravity has occurred relatively recently, and it is precisely according to 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 simply are no other probable interactions. Galileo and Kepler, two prominent astronomers of the time, with their most valuable observations, helped set the stage for new discoveries.
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 pseudoscientific circles, is extremely incorrect: even weightlessness in space does not mean that the attraction of some massive object does not act on a person or a spaceship.
In addition, all material bodies have a certain mass, expressed in the form of the force that was applied to them, and the acceleration obtained due to this effect.
Thus, the forces of gravity are proportional to the mass of objects. In numerical terms, they can be expressed by obtaining the product of the masses of both bodies under consideration. This force strictly obeys the inverse of the squared distance between objects. All other interactions completely differently depend on the distances between two bodies.
Mass as the cornerstone of theory
The mass of objects has become a special controversial point around which the whole modern theory of gravity and relativity of Einstein is built. If you remember Newton’s Second Law, you probably know that mass is an obligatory characteristic of any physical material body. It shows how the object will behave if force is applied to it, regardless of its origin.
Since all bodies (according to Newton), when exposed to external forces are accelerated, it is the mass that determines how large this acceleration will be. Consider a more understandable example. Imagine a scooter and a bus: if you apply exactly the same force to them, then they will reach different speeds in different times. All this explains precisely the theory of gravity.
What is the relationship between mass and attraction?
If we talk about gravity, then mass in this phenomenon plays a completely opposite role to that which it plays in relation to the force and acceleration of the object. It is she who is the primary source of attraction itself. If you take two bodies and see with what force they attract the third object, which is located at equal distances from the first two, then the ratio of all forces will be equal to the mass ratio 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 is saying 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 are talking about the force with which the body is attracted to the surface of the planet, as its weight.
To summarize some of the results. So, mass is closely related to force and acceleration. At the same time, it is she who determines the force with which attraction will act on the body.
Features of acceleration of bodies in a gravitational field
This amazing duality is the reason that in the same gravitational field the acceleration of completely different objects will be equal. Suppose we have two bodies. Assign one of them the mass z, and the other - Z. Both objects are thrown to the ground, where they fall freely.
How is the ratio of attractive forces determined? It is shown by the simplest mathematical formula - z / Z. That's just the acceleration they receive as a result of the action 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 on its properties.
What determines the acceleration 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 were given the following name: "The principle of equivalence." This term once again emphasizes that acceleration and inertia are often equivalent (to a certain extent, of course).
The importance of G
From the school physics course, we remember that the acceleration of gravity on the surface of our planet (Earth's gravity) is 10 m / s² (9.8 of course, but this value is used for simplicity of calculations). Thus, if air resistance is not taken into account (at a significant height with a small fall distance), the effect will be when the body acquires an acceleration increment of 10 m / s. every second. So, a book that fell from the second floor of the house, by the end of its flight will move at a speed of 30-40 m / s. Simply put, 10 m / s is the “speed” of gravity within the Earth.
Acceleration of gravity in the physical literature is indicated by the letter "g". Since the shape of the Earth to a certain extent resembles a mandarin rather than a ball, the value of this quantity is far from identical in all its areas. 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. The physics of this phenomenon sometimes saves a lot of time. For example, geologists are especially interested in determining the exact g exactly, since this allows the exploration and discovery of mineral deposits with exceptional accuracy. By the way, what does the gravity formula look like, in which the value considered by us plays an important role? There she is:
F = G x M1xM2 / R2
Note! In this case, the gravity formula means by G "gravitational constant", the value of which we have already cited above.
At one time, Newton formulated the above principles. He perfectly understood both the unity and universality of gravity, 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 humanity owes a modern understanding of the very nature of the space-time continuum.
Theory of Relativity, works by Albert Einstein
At the time of Isaac Newton, it was believed that the reference point can be represented in the form of 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 will be in a single time space. In those years, this provision was considered so obvious that no attempt was 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 will be really significant if the hypothetical clock moves much slower than the speed of light. Simply put, if one observer, moving slower than the speed of light, will follow two events, then they will occur for him at a time. Accordingly, for the second observer? whose speed is the same or greater, events can occur at different times.
But how is gravity related to the theory of relativity? We will open this question in detail.
The relationship between the theory of relativity and gravitational forces
In recent years, a huge number of discoveries have been made in the field of subatomic particles. The belief is growing stronger that we are about to find the final particle, beyond which our world cannot fragment. All the more insistent is the need to find out how exactly the smallest “bricks” of our universe affect 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 classical Newtonian mechanics in the field under consideration, the researchers focused on a complete rethinking of the previously obtained data. 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 field, after which it can be declared a “mediator” in the interactions of celestial bodies, explaining many inconsistencies of the old and new theory. The fact is that then the relative propagation velocities of the forces under consideration would be significantly lower than the light. So how are gravity and time related?
In principle, Einstein himself almost managed to build a relativistic theory on the basis of just such views, but 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 you know, they were just the universally recognized source of the emergence 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 more body weight. The problem is that the last value would automatically become infinite in 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 much more variables should be used.
His followers concluded that gravity and time are practically unrelated. The fact is that this tensor field itself can act on space, but is not able to affect time. However, the brilliant physicist of our time, Stephen Hawking, has a different point of view. But this is a completely different story ...