When we say there is no medium or there is a vacuum, we are referring to the absence of matter or substance in a given region of space. In physics, a medium typically refers to a substance or material through which waves can propagate. For example, sound waves require a medium like air, water, or solid materials to travel.
However, in the context of a vacuum, it means that there is no material or substance present in that particular space. A vacuum is a state devoid of matter, where the pressure and density of particles are extremely low or non-existent.
In classical physics, a perfect vacuum is often defined as a space completely empty of any particles, including atoms, molecules, and other forms of matter. Such a perfect vacuum is an idealized concept and is difficult to achieve in practice. However, in many situations, the term "vacuum" is used to describe spaces where the pressure and density of particles are significantly lower than atmospheric conditions.
In the realm of quantum mechanics, even seemingly empty space is not truly devoid of energy and activity. According to quantum field theory, the vacuum is filled with quantum fields that fluctuate and can give rise to virtual particles that pop in and out of existence. These quantum fluctuations are a fundamental aspect of the quantum vacuum.
In summary, when we say there is no medium or there is a vacuum, we mean that there is an absence of matter and substance in a particular region of space. This can refer to both the classical notion of a space devoid of particles and the quantum concept of the vacuum, which is filled with quantum fields and fluctuations.