The Middle Way of Madhyamika refers to the teachings of Nagarjuna, who, at a time when the Mahayana teachings were falling into decline, wrote his Six Treatises, four of which directly expound the doctrine of sunyata, or emptiness.
Emptiness refers to the ultimate nature of all things.
It is the key concept in this physics. One who correctly understands emptiness is described as free from the extremes of existence and annihilation. To fall to the extreme of existence is to hold that, in the final analysis, phenomena truly exist. To fall to the extreme of annihilation is to hold that phenomena don't have any kind of existence at all. According to the theory of emptiness, phenomena exist in a relative state only, a kind of 'ontological relativity'.