The problem with UFO's is that I don't think there is one credible photo where a UFO can positively be identified. That is why they call them unidentified flying objects.
The main problem with visitors from other worlds is the incredible distances between the stars. It is 26 trillion miles to the nearest star(proxima centauri). It takes light traveling at 386,000 miles a second 4&1/3 years to get there. It is impossible for a craft to build up to even a small fraction of the speed of light. As the speed of the object increases so does the mass of the craft and it requires an unbelievable amount of energy to keep increasing speed. If we do actually find a UFO it will probably be an unmanned probe. Just like we are sending to the other planets in our solar system.
Well seeing as it is proven that time and space can be bent (manipulated) would render what you said useless.