All this time, the field of physics did not seriously consider the possibility of existence of negative mass (energy) in a general state. The standard explanation of negative mass is that the state of low energy is stable when a negative energy level exists and that the lowest state of energy is minus infinity. Thus, this means that all positive mass emits energy and this will be transitioned to the energy level of minus infinity and the universe will collapse.
However, at the present, our universe exists without collapsing, so the explanation for this has become strong proof of the nonexistence of the negative energy(mass) and negative energy level of. Thus, we have considered this to be obvious common sense and have taught this to students.
At the center of this background, there is the fundamental prionciple(proposition) that “State of low energy is stable".
But, this principle is an incomplete, and that it is stable at a low energy state in the case of positive mass. However, it is stable at a high energy state in the case of negative mass.
Due to this, “the problem of the transition of the energy level of minus infinity" does not occur, and thus positive energy(mass) and negative energy(mass) can exist in the same space-time. Moreover, negative energy(mass) provides a qualitative explanation for dark matter and dark energy, which are the biggest issues posed to cosmology at the present.
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The acceleration of negative mass is opposite to the direction of force. Therefore, the negative mass has harmonic oscillation at the maximum point and it is also stable at the maximum point.
Motion of the negative mass :