"In 1928, Nikola Tesla registered a controversial patent. The patent claimed itself to be the “World’s First Flying Saucer”. This was groundbreaking for Tesla. This is because he had utilised the prowess of Electromagnetic force instead of Gravitational. Among the two, Electromagnetic force is said to be powerful. Hence if this Flying Saucer was to be a reality, then it might as well be the fastest aircraft. Scientifically, the Electromagnetic force is (2.2 x 10^39) stronger than the Gravitational. To put it into perspective, if the energy required for mechanically lifting a block at a distance of one-quarter of a millimetre is, say X. If the same energy given to the block using Electromagnetic force, it could lift the block (5.6 x 10^24) km. That is how powerful the aircraft propelled using Electromagnetic force would be!"
https://advancetec.co.uk/science-now/a-look-into-nikola-teslas-research-and-patenting-of-a-ufo/
"First, the main problem is one of energy. In the same way that a car needs gasoline, a time machine needs to have fabulous amounts of energy. One either has to harness the power of a star, or to find something called “exotic” matter (which falls up, rather than down) or find a source of negative energy. (Physicists once thought that negative energy was impossible. But tiny amounts of negative energy have been experimentally verified for something called the Casimir effect, i.e. the energy created by two parallel plates). All of these are exceedingly difficult to obtain in large quantities, at least for several more centuries!"
https://mkaku.org/home/articles/the-physics-of-time-travel/