Taught to me 1983. Empty space is NOT empty. It has electric and magnetic fields. Which beat back antimatter to beyond the Heliopause. A band of intense radiation, as matter and antimatter annihilate.
We sent a voyager craft into outer space, and it reported on the radiation level. And one Voyager ceased to work. The other's chronometer was wrong. Antimatter has antitime.
Sp the voyager is being abraded, by all the antimatter falling on the craft. It is anti hydrogen, where the p- particels, bounce off the electron shells of the matter. Read
Φp¯ = Z dr dEν¯ dΩν¯ 1 vν¯ Φν¯(Eν¯) dEe dΩe 1 ve Φe(Ee) (Pν¯Pe) Eν¯ Ee dσ(¯νe + e − → p¯+ n) dEp¯ dΩp¯ c
So to make a time machine, surround your craft with antimatter. But 40GW will only get you back 0.002 seconds.
So time does not vary with gravity, the electromagnetic fields do. So the maximum density of matter is as neutrons in a neutron star. The shell of electromagnetic radiation around a star, acts as lens, bending light.
But time comes in positive and negative forms, and matter surrounded by antimatter only has a slight slowing of time – not reversal.