I want to know the answer to the following question:
Antiparticles had to exist because if particles only were of positive energy, they would not be in what?
All the background I'm provided with is that "In the same lecture, he further explained that antiparticles must exist, for if particles had only positive energies, they would not be restricted to a so-called "light cone".". Is that enough to answer the question?
Answer: yes