What is the major in physics in the fourth dimension?

Actually, it's the theory of relativity.

Four-dimensional space is a concept of space-time. In simple terms, any space with four dimensions can be called a "four-dimensional space". However, most of the "four-dimensional space" mentioned in daily life refers to the concept of "four-dimensional space-time" mentioned by Albert Einstein in his "General Theory of Relativity" and "Special Theory of Relativity".

According to Einstein's concept, our universe is made up of time and space. The relationship between time and space is that there is an additional time axis in the structure of space compared with the three axes of length, width and height of ordinary three-dimensional space, and this time axis is an axis of imaginary values.

It's supposed to be theoretical physics.

I don't know if it's correct.

The mathematical concept of "space" is limited to the description of location and bearing, and does not involve time.

In classical physics, space is not related to time, so space in classical physics is actually a mathematical concept of space.

But relativistic mechanics holds that space and time are inseparably linked. Therefore, when describing the motion of an object, time can be added as an element. This is also known as the four-dimensional space.

That is, in addition to the three spatial dimensions of x, y, and z in mathematics, the dimension of time t is additionally added.

There are various multidimensional spaces in mathematics, but so far, the physical world we know is only four-dimensional, that is, three-dimensional space plus one-dimensional time. The high-dimensional space mentioned in modern microscopic physics is another meaning, which has only mathematical significance. Four-dimensional space-time is the lowest dimension that constitutes the real world, and our world happens to be four-dimensional, and as for the high-dimensional real space, at least for now, we can't perceive it.

I mentioned an example in a post where a ruler rotates in three-dimensional space (without time), its length does not change, but when it rotates, its coordinate values change, and the coordinates are related to each other. The meaning of four-dimensional space-time is that time is the fourth-dimensional coordinates, which is related to spatial coordinates, that is to say, space-time is a unified and inseparable whole, and they are a kind of "one eliminates the other" relationship. The four-dimensional space-time is not limited to this, from the relationship between mass and energy, mass and energy are actually the same thing, mass (or energy) is not independent, but related to the state of motion, such as the greater the velocity, the greater the mass.

In four-dimensional space-time, mass (or energy) is actually the fourth-dimensional component of four-dimensional momentum, and momentum is the quantity that describes the motion of matter, so it is natural that mass is related to the state of motion. In the four-dimensional space-time, momentum and energy are unified, which is called the energy-momentum four vectors. In addition, in the four-dimensional space-time, the four-dimensional velocity, the four-dimensional acceleration, the four-dimensional force, and the four-dimensional form of the electromagnetic field equation system are also defined.

It is worth mentioning that the four-dimensional form of the electromagnetic field equation system is more perfect, completely unifying electricity and magnetism, and the electric and magnetic fields are described by a unified electromagnetic field tensor. The physical laws of four-dimensional space-time are much more perfect than the three-dimensional laws, which shows that our world is indeed four-dimensional. It can be said that at least it is much more perfect than Newtonian mechanics.

At least because of its perfection, we can't doubt it. In the special theory of relativity, time and space constitute an inseparable whole – four-dimensional space-time, and energy and momentum also constitute an inseparable whole – four-dimensional momentum. This suggests that there may be a deep connection between some seemingly unrelated quantities in nature.

In the future discussion of general relativity, we will also see that there is a deep connection between space-time and the four vectors of energy momentum.