UFOs

Sure, I don't mind explaining. No, we would not need near-infinite energy. We are quite capable of accelerating at 10 Gs in space right now, but eventually you will run out of fuel. So, let's say you add more fuel, well now you have more mass to accelerate so it costs more fuel per second. This becomes a balancing act which we can not overcome for long, and it's the reason space shuttles are so complicated and have multiple stages which break away to reduce mass.

This is primarily an issue because we use quite simple propulsion techniques, which rely on Newton's third law -- that forcing mass out from behind a ship will propel it in the opposite direction. It may be possible to accelerate using an Electro-Magnetic field, which would not involve burning fuel but instead some kind of depleting battery storage, or perhaps a nuclear reactor. In this case, accelerating at 10 Gs is simply a matter of matching the energy requirements to the mass of the ship, and for some perspective on the energy capabilities of nuclear fission, the Little Boy bomb reacted less than a gram of nuclear material to create the explosion in Hiroshima.

The uranium in the Hiroshima bomb was about 80 percent uranium 235. One metric ton of natural uranium typically contains only 7 kilograms of uranium 235. Of the 64 kilograms of uranium in the bomb, less than one kilogram underwent fission, and the entire energy of the explosion came from just over half a gram of matter that was converted to energy. That is about the weight of a butterfly.

So, obviously we aren't capable of converting that energy into a useful method of propulsion yet, but have some heart, because the pieces are all there -- we just need to put them together.