Is light solid, liquid or gas?

Light belongs to electromagnetic wave, and light has wave-particle duality, which is the answer of modern physics [if you are interested, please refer to the third volume of high school physics]. It's a wave, it's a particle!

Subordinate reference:

wave-particle duality

Wave-particle duality means that all substances have the characteristics of wave and particle at the same time. Wave-particle duality is an important concept in quantum mechanics.

In classical mechanics, the research objects are always clearly divided into two categories: waves and particles. The typical example of the former is light, while the latter constitutes what we often call "matter". 1905, Einstein put forward the optical quantum explanation of photoelectric effect, and people began to realize that light waves have both the properties of waves and particles. 1924, de Broglie put forward the "material wave" hypothesis, which holds that all matter, like light, has wave-particle duality. According to this hypothesis, electrons will also have fluctuations such as interference and diffraction, which was confirmed by later electron diffraction experiments.

The Mathematical Relationship between "Wave" and "Particle"

The particle nature of matter is characterized by energy e and momentum p, and the characteristics of wave are represented by frequency v and wavelength λ, which are related by Planck constant H.

history

At the end of 19, the increasingly mature atomic theory gradually prevailed. According to the atomic theory, all matter is made up of tiny particles-atoms. For example, electricity, which was originally considered as a fluid, was proved by Thompson's cathode ray experiment to be composed of particles called electrons. Therefore, people think that most substances are composed of particles. At the same time, wave is considered as another way of existence of matter. The wave theory is deeply studied, including interference and diffraction. Because of the characteristics of light in Thomas Young double-slit interference experiment and Fraunhofer diffraction, it is obvious that it is a fluctuation.

However, at the beginning of the twentieth century, this view faced some challenges. The photoelectric effect studied by Albert Einstein in 1905 shows the particle surface of light. Subsequently, the electron diffraction was predicted and confirmed. This shows the fluctuating side of electrons that were originally thought to be particles.