Energy is one of the fundamental concepts of physics, and it is always a core concept from classical mechanics to relativity, quantum mechanics, and cosmology.

In common terms or popular science books, energy refers to the amount that a system can release, or can obtain, which can be equivalent to doing a certain amount of work. For example, 1 kilogram of gasoline contains 12 kilowatt-hours of energy, which means that if all the chemical energy in 1 kilogram of gasoline is exerted, 12KWh of work can be done.

Energy is the physical quantity in physics that describes a system or a process. The energy of a system can be defined as the sum of the work that transitions from a state of zero energy to the present state of the system. The amount of energy in a system is not a definite value in physics, it changes with the description of the system. In the process of life activities of the human body, all life activities need energy, such as the synthesis reaction of substance metabolism, muscle contraction, gland secretion, and so on. And this energy comes mainly from food. The nutrients contained in animal and plant foods can be divided into five categories: carbohydrates, lipids, proteins, minerals and vitamins, plus water for six categories. Carbohydrates, fats, and proteins are oxidized in the body to release energy. The three are collectively referred to as "energy-producing nutrients" or "pyrogenic substances".

The law of conservation of energy states that energy does not arise out of thin air and does not disappear out of thin air, but can only be transformed from one form to another, while the total amount of energy remains the same. Energy is a scalar quantity, not a vector quantity, and has no direction. As for positive matter and antimatter, it does not mean that there are positive and negative mass, but that the electrical properties of the nucleus are opposite, and the mass is converted into energy after encountering. Any exercise requires energy. There are many forms of energy, such as light energy, sound energy, heat energy, electrical energy, mechanical energy, chemical energy, nuclear energy, etc. As an example, observe the energy of a solid with a mass of 1Kg:

In classical mechanics, its energy is the sum of the work done to accelerate from rest to the present velocity.

In classical thermalism, its energy is the sum of the work done to heat the existing temperature from absolute zero.

In physical chemistry, its energy is the sum of the work done to the raw material when synthesizing the solid.

In atomic physics, its energy is the sum of the work done on it from a state where atomic energy is zero to the present state.

It is also possible to define the energy contained in this solid in the opposite way. Here are two examples:

The internal energy of the solid is the sum of the work released by cooling it to absolute zero.

The atomic energy of this solid is the kinetic energy that releases its binding energy into a reaction product in the fission or fusion reaction of the nucleus.

Energy, while a common and fundamental physical concept, is also an abstract physical concept.

In fact, physicists didn't really understand the concept of energy until the mid-19th century, and before that it was often confused with concepts such as force and momentum.

The energy requirement of the human body refers to the energy intake necessary for individuals who can maintain good health for a long time, have a good body shape, body composition and activity level, achieve energy balance and can maintain the energy intake necessary for productive labor and social activities.

In Einstein's special theory of relativity, energy is a component of momentum in four dimensions. In any closed system, each component of this vector (one of which is energy, the other three is momentum) is conserved and does not change with time, and the length of this vector is also conserved (Minkowski modulus length), which is the stationary mass of a single mass and the invariant mass (i.e., constant energy) of a system composed of multiple mass particles.

Therefore, as long as the observer's reference frame does not change, the conservation of energy to time in the special theory of relativity still holds, the energy of the whole system remains unchanged, and the energy measured by observers in different reference frames is different, but the energy value measured by each observer will not change with time. The invariant mass is defined by the energy-momentum relationship and is the minimum amount of mass and energy of the system that can be observed by all observers, and the invariant mass is also conserved, and the value measured by each observer is the same.

In quantum mechanics, the energy of a quantum system is described by a self-adjunctive operator called the Hamiltonian operator, which acts in the system's Hilbert space (or wave function space). If the Hamiltonian operator is a non-time-varying operator, the measurement of its occurrence probability does not change with time as the system changes, and therefore the expected value of energy does not change with time. The conservation of local energy under quantum field theory can be obtained by combining the energy-momentum tensor operator with Noether's theorem. Since there is no global time operator in quantum theory, the indeterminate relationship between time and energy only holds true under certain conditions, which is different from the indeterminate relationship between position and momentum as the basis of quantum mechanics (see Uncertainty Principle). The energy at each fixed time can be measured accurately and is not affected by the uncertain relationship between time and energy, so that even in quantum mechanics, the conservation of energy is a well-defined concept.

Energy must obey the law of conservation of energy. According to this law, energy can only change from one form to another and cannot be created or destroyed in a vacuum. Conservation of energy is a mathematical conclusion drawn from the translational symmetry of time (translational invariance).

According to the law of conservation of energy, the inflow of energy is equal to the outflow of energy plus the change in internal energy. This law is a fairly basic criterion in physics. According to the translational symmetry of time (translational invariance), the laws of physics (theorems) hold true at all times.

The law of conservation of energy is characteristic of many physical laws. From a mathematical point of view, conservation of energy is a consequence of Noether's theorem. If a physical system satisfies continuous symmetry when translating time, its energy (the conjugate physical quantity of time) is conserved. Conversely, if a physical system has no symmetry in time translation, its energy is not conserved, but if it is considered that this system exchanges energy with another system, and the resulting larger system does not change with time, the energy of this larger system is conserved. Since any time-varying system can be placed in a larger, non-time-varying system, conservation of energy can be achieved by appropriately redefining energy. For the theory of physics in flat space-time, since quantum mechanics allows non-conservation for a short period of time (e.g., positive-antiparticle pairs), the conservation of energy is not observed in quantum mechanics, whereas in special relativity, the law of conservation of energy is converted into the law of conservation of mass and energy.

The law of conservation of mass and energy states that the sum of the relativistic kinetic and static energies of all particles remains constant during the interaction in an isolated system. The law of conservation of mass and energy is a special form of the law of conservation of energy.

Energy, what Lin Feng desires very much, if he doesn't have enough energy, he can't do anything, what he can do now is to constantly increase his energy. (To be continued.) )