Introduction
When the sky is clean and clear, a lot of stars are seen at night. We can talk a lot while watching the starlight. Not only in the present but also in the distant past, scientists have observed stars, scientifically analyzing their movements and trying to understand the principles. The related disciplines are called astronomy. The movement of stars were also used to tell the fortune of individuals and nations. People trusting in astrology are still seen. Among a lot of stars in the sky there are ones that brightly shine themselves like the Sun (we do not call the Sun a star, but it is academically a star), and there are planets such as Earth, Mars, Jupiter, etc. There are also meteors. Stars are telling us in their own language, namely light. Light is the fastest we know. (Neutrinos were once known to be faster than light, but it turned out to be erroneous.) The Sun is away from us just within 8 minutes at the speed of light (exactly 8 minutes and 19 seconds), but there are many stars very far away from us which cannot be reached after travelling for more than a decade (or a few hundred million years) at the speed of light. How can light move at the speed of light in the vacuum where there is nothing at all? A few hundred years ago, when the property of light was not yet known exactly, Newton insisted that light consisted of particles. In those days, scientists like Huygens experimentally showed that light is a kind of wave like a sound wave. If light is a wave, it can move only if there is fluid or any medium. Scientists who believed that light was a wave in the Newtonian era thought "Ether" as a medium, a fictitious material (fluid) that light can rely on. In 1887, A. A. Michelson and E. Morley experimentally tried to prove the existence of this hypothetical optical fluid. They developed a sophisticated optical device to observe the interference phenomenon expected to show up due to the speed difference of light in the presence of Ether. This is the famous "Michelson-Molly" experiment. When Earth rotates around the Sun and feels the flow of "Ether" and this flow will be felt differently in the direction perpendicular to and parallel to the direction of Earth’s rotation. When two beams of light move the same distance, but in the perpendicular directions, the speed of light of the two beams will be different from each other, so that a interference pattern had been expected to appear. However, unlike the expectation, no interference was observed. Though more sophisticated experiments had been performed since then, the results were the same. There was no such thing like "Ether" that light can move through. For this experiment denying the presence of Ether, Michelson won the Nobel Prize in Physics in 1907. As a result, light travels through the vacuum by itself and has been recognized as a dual nature of wave and particle at the same time, since the particle characteristics of light was proved in the latter half of the 20th century (light particles, namely photons, contributed greatly to the emergence of quantum mechanics, the core of modern physics). Einstein proposed the theory of special relativity from the fact that light travels through the vacuum by itself and that the speed of light is invariant regardless of the state of the observer (the same velocity is measured). From this theory, Einstein mass-energy equivalence was derived, which tells mass (m) is convertible with energy (E) via the speed of light (c): E = mc2. This formula fully explains the relationship between the mass difference before and after the reaction and the energy released during nuclear fusion or nuclear fusion, which further strengthened the conviction that the space in which the light travels is empty. Is there really no medium that mediates the movement of light in the vacuum? Is the space composed only matter and nothing? Is there no energy or substance in the vacuum that we do not know? Recent trends are approving the hypothesis that the space is composed of dark matter and dark energy in addition to ordinary matter we know. There is definitely something in the background of the universe. The vacuum also has matter or energy. Ironically, Einstein is the first person to argue that there is energy or matter in the vacuum. (Why ironic? The theory of special relativity is derived from the assumption that there is nothing to hold light in the vacuum, although Einstein presupposed that something in the vacuum does not react with light at all ...) According to him, the universe will shrink due to gravity. If it does not, then there should be energy to prevent its shrinkage, and the concept of the cosmological constant was added to his theory of general relativity. This amended theory is a cosmological theory stating that the universe does not contract or expand. However, as Hubble's observations revealed that the universe is expanding, the cosmological constant was no longer accepted. With the recent recognition that the universe expand acceleratedly, however, the cosmological constant has received renewed attention and astronomical assumptions are emerging that there will be dark energy or dark matter in the universe that we do not know. Today, the vacuum in the universe is accepted not to be an empty space, but there is only one question, how much energy does it have in itself. At present, two theories are in conflict. The vacuum has the lowest energy, namely zero point energy, 10-9 J/m3 or conversely it has enormous energy predicted to be 10113 J/m3. The difference is so large that physicists call it "the vacuum catastrophe" or "the cosmological constant problem". If these two theories are correct, we may introduce the following concept of the vacuum.
"The vacuum is made of a very dense medium (called the solid vacuum), but it contains zero or very little energy."
The above premise implies that the medium constituting the vacuum does not convert into energy via mass-energy equivalence. In addition to this premise, the second premise is that energy must be added to the solid vacuum to become matter. That is, the physical world of matter is determined by the interaction between the medium constituting the vacuum and the energy held in it. Stephen Battersby, editor of a British science magazine, wrote, "It is confirmed: Matter is merely vacuum fluctuations." October 16, 2008 in a news article of New Scientist (newscientist.com). Also, in 1985, Paul Davies noted that "knowing about vacuum is the key factor in approaching Theory of Everything." Knowing about the vacuum should be the key to the understanding our physical world.
