However, due to the limitations of quarks, quarks cannot exist independently, and people cannot directly observe quarks.
so how is the internal structure of atoms distributed? A macro example will be very intuitive.
suppose the atom is enlarged to the size of a football field, how big are the nucleus and electrons?
the answer is sure to surprise you. The nucleus is only the size of a broad bean and is located in the center of a football field. Electrons are smaller, smaller than a grain of dust in the stands of a football field!
the nucleus and electrons are so small, are the other parts of the atom empty except the nucleus and electrons?
It seems that the inside of the atom is indeed empty, but being empty does not mean that there is nothing, even on the contrary, there are many "activities" inside the atom!
nuclei are positively charged and electrons are negatively charged. This means that there is a strong electric field around the nucleus, and the electron itself has an electric field.
more importantly, electrons don't move in a fixed circle like the earth around the sun. They don't have a fixed orbit, but appear around the nucleus very randomly, just like a big cloud, which is called "electron cloud".
therefore, don't think that there is a vacuum inside the atom, and don't think that objects can enter the atom at will. The existence of electron cloud is like building a layer of "hard shell" around the nucleus. It is difficult for ordinary particles to enter, except neutral particles such as neutrinos, which can enter at will, and all other particles are basically rejected.
this is why an atom is very empty in theory, but it is very dense and hard. Atoms are so hard that ordinary forces can hardly compress them. To penetrate the "hard shell" around atoms, a huge object is needed, such as the core of a star, where the temperature and pressure are very high.
Although the nucleus is very small, it accounts for more than 99.9% of the atomic mass, and the density of the nucleus is very high. A cubic centimeter of nucleus has a mass of 1 million tons!
protons and neutrons that make up the nucleus rely on a strong combination. We usually say that "the mass of protons and neutrons is equal to the mass of the nucleus" is not rigorous, the former is smaller than the latter, because the combination of protons and neutrons can occupy a part of the mass.
to subdivide, protons and neutrons are composed of quarks, protons are composed of two upper quarks and one lower quark, and neutrons are composed of one upper quark and two lower quarks.
different quarks are directly combined through the strong transfer (strong interaction) of gluons. In fact, the mass of three quarks accounts for less than 1% of the mass of protons (neutrons), and the other 99% of the mass is produced by strong interaction.
gluons, like photons that propagate electromagnetic force, are communicators and have no mass (static mass) of their own.
So, can smaller quarks continue to subdivide?
at present, scientists don't know. Because quarks are imprisoned, quarks cannot exist independently, and scientists can't even observe quarks, at least not yet, let alone subdivide quarks.
a more advanced theory-string theory holds that all elementary particles such as quarks are composed of vibrating strings, and different vibration directions and frequencies constitute different elementary particles. In addition to string theory, there are membrane universe theory, high-dimensional space-time theory and multiverse theory.
However, all the above theories are difficult to be verified, and at present they are mostly in the stage of reasoning and guessing.
However, the above theory is difficult to be verified, and at present it is more in the stage of reasoning and guessing.