Freeman, trying to bring Aliens into this discussion doesn't really serve too much. This is scientific discussion first, conjecture second. We can
only give answers that have a base in what we know so far. As far as we know - there is no such thing as anti-gravity, nor would there be some
mythical compound or element (like "element 115" that's tossed around way too much) that would solve all the problems involved.
What do I think ETs use? Something far in advance than we can ever think of - but that's it. I would dare not guess what kind of machinery they have,
as it's likely based on principles and theorems that aren't even close to being discovered yet.
However, if you want me to take a venture at how I would build a vehicle capable of travelling the cosmos... well, here's an idea:
First, we know we can't go faster than light, so making big enough explosions isn't the answer (despite it's popularity :cool
So what kind of propulsion types could we look for? Well, there's electro-magnetic forces that can do some funky stuff. However, there's also
gravity. I think the key force we want to work with is Space-Time though. Plausibly, a Unified Theorem between gravity and EM would give us the
ground-work for some marvelous things. However, we don't have that right now - so, unfortunately, we got to work around that.
Space-Time. On one hand, there's "warping" space-time so that I could travel great displacements over little time, but have actually not achieved a
speed greater than light (even though my velocity has). I could do this by getting two black holes, and putting them close together. The curvatures of
space-time would then allow me to make a route around the black holes which would end up making me move faster than light. If I could find the point
where the time-space distortion is greatest, and keep my ship slowly moving through that, and keep the black holes (somehow) attached to my ship -
then I could likely travel at amazing velocities.
The other option would be to create a hole in space-time, much like a black hole. However, I have to be able to escape the event horizon when I come
out - and also survive the trip in. Another problem here is that I have no idea where I will come out. It could very likely be a different quantum
universe. Perhaps you could create a Quantum Tunnel between two of these space-time tears, that would make them connected. Place the black holes far
away from each other, and it could act like some sort of "jump gate".
In any case, both of these sound like they would take enourmous rsources and energies to create - likely more than we can currently manufacture on
earth. In which case, I can't see the market for them being great - although great scientific study could come from them (how particles react and so