>>14047081General relativity:
Neutron stars and black holes change their amount of particles per the universe because it's easier than moving they are so big.
Quantum mechanics:
Empty space is not really empty but particle antiparticle pairs.
Breaking down:
Nature does what it wants regardless of our abilities. Our ability to prove a model breaks down because humans are not yet at a cosmic level. The energy needed to observe pair production or gravitational wave scattering is very big, likely larger than anything that could exist. We would need to enhance general relativity like a wave function on both sides of E=mc^2.
