Description: An event with positive energy transfers this energy photons which carries it on recorders observers. Observers know that this event occurs, not before it happens. But event with negative energy should absorb this energy from observers. Consequently, observers know that this event happens before it happens. Since time is irreversible then only the events with positive energy can occur. In single-particle states events with a fermion have positive energy and occurences with an antifermion have negative energy. In double-particle states events with pair of antifermions have negative energy and events with pair of fermions and with fermion-antifermion pair have positive energy.