The following section has a brief description of some ideas that have been suggested over the years for interstellar travel, ideas based on the sciences that do exist today.
Science and technology are continuing to evolve. In just the last few years, there have been new, intriguing developments in the scientific literature. Although it is still too soon to know whether any of these developments can lead to the desired propulsion breakthroughs, they do provide new clues that did not exist just a few short years ago. A snapshot of just some of the possibilities is listed below:
These emerging ideas are all related in some way to the physics goals for practical interstellar travel; controlling gravitational or inertial forces, traveling faster-than-light, and taking advantage of the energy in the space vacuum. Even though the physics has not yet matured to where "space drives" or "warp drives" can be engineered, individuals throughout the aerospace community and across the globe have been tracking these and other emerging clues. Most of this work has been fueled purely from the enthusiasm, talent, and vision of these individuals, but on occasion, there has been small support from their parent organizations.
This is a snap shot of how gravity and electromagnetism are known to be linked. In the formalism of general relativity this coupling is described in terms of how mass warps the spacetime against which electromagnetism is measured. In simple terms this has the consequence that gravity appears to bend light, red-shift light (the stretching squiggles), and slow time. These observations and the general relativistic formalism that describes them are experimentally supported.
Although gravity’s effects on electromagnetism and spacetime have been observed, the reverse possibility, of using electromagnetism to affect gravity, inertia, or spacetime is unknown.
The mainstream approach to better understand this connection is through energetic particle smashing. Physicists noticed that when they collided subatomic particles together they figured out how the "weak force" and electromagnetism were really linked. They cranked up the collision energy and learned of that this new "Electro-Weak" theory could be linked to the "strong nuclear force". SO.... just crank up the power some more, and maybe we’d understand gravity too. Unfortunately, the collision energies needed are not technologically feasible, even with the Super Conductor Super Collider that got canceled, but its still a thought.
For light waves in space, the same condition holds. For every possible color of light, that includes the ones we can’t see, there is a non-zero amount of that light. Add up the energy for all those different frequencies of light and the amount of energy in a given space is enormous, even mind boggling, ranging from 10^36 to 10^70 Joules/m3.
In simplistic terms it has been said that there is enough energy in the volume the size of a coffee cup to boil away Earth’s oceans. - that’s one strong cup of coffee! For a while a lot of physics thought that concept was too hard to swallow. This vacuum energy is more widely accepted today.
What evidence shows that it exists?
First predicted in 1948, the vacuum energy has been linked to a number of experimental observations. Examples include the Casimir effect, Van der Waal forces, the Lamb-Retherford Shift, explanations of the Planck blackbody radiation spectrum, the stability of the ground state of the hydrogen atom from radiative collapse, and the effect of cavities to inhibit or enhance the spontaneous emission from excited atoms.
The most straight-forward evidence for vacuum energy is the Casimir effect. Get two metal plates close enough together and this vacuum energy will push them together. This is because the plates block out the light waves that are too big to fit between the plates. Eventually you have more waves bouncing on the outside than from the inside, the plates will get pushed together from this difference in light pressure. This effect has been experimentally demonstrated.
Can we tap into this energy?
It is doubtful that this can be tapped, and if it could be tapped, it is unknown what the secondary consequences would be. Remember that this is our lowest energy point. To get energy out, you presumably need to be at a lower energy state. Theoretical methods have been suggested to take advantage of the Casimir effect to extract energy (let the plates collapse and do work in the process) since the region inside the Casimir cavity can be interpreted as being at a lower energy state. Such concepts are only at the point of theoretical exercises at this point.
With such large amount of energy, why is it so hard to notice?
Imagine, for example, if you lived on a large plateau, so large that you didn't know you were 1000 ft up. From your point of view, your ground is at zero height. As long as you're not near the edge of your 1000 ft plateau, you won't fall off, and you will never know that your zero is really 1000. It's kind of the same way with this vacuum energy. It is essentially our zero reference point.
What about propulsion implications?
The vacuum fluctuations have also been theorized by Haisch, Rueda, and Puthoff to cause gravity and inertia. Those particular gravity theories are still up for debate. Even if the theories are correct, in their present form they do not provide a means to use electromagnetic means to induce propulsive forces. It has also been suggested by Millis that any asymmetric interactions with the vacuum energy might provide a propulsion effect.