Let us recap on the advantages of the space drive, firstly it requires no propellant and secondly it has the potential to accelerate a spacecraft beyond the speed of light. The first point is obvious but what of the second, why can this go faster than light when no Newtonian Rocket can?
As we have said while special relativity talks of no object with mass being able to exceed light speed, it says nothing for spacetime. The idea of this drive is to actually manipulate space itself to generate the acceleration so the craft will be moving in a pocket of spacetime and we know of no limit to the speed that this pocket of spacetime could move at. Indeed there is strong evidence that spacetime expanded faster than the speed of light at the time of the big bang.

Let us use the famous analogy of the moving airport sidewalks, there is a limit to how fast you can walk (analogous to the light speed limit). You may however stand on the sidewalk that is moving faster than you can walk (analogous to a spacecraft in a moving pocket of spacetime) and the result is that you can travel faster than you can walk.

The primary theory behind the possibility of such a device is the potential coupling of gravity and electromagnetism. It is clear that there is a link between these two fundamental forces, which is described by general relativity (and at this point we see no reason for general relativity to be inaccurate).
In simple terms general relativity predicts that gravity is able to bend and redshift light (in other words an electromagnetic field measured against a gravitational field). This is not only experimentally supported but was instrumental in the acceptance of general relativity, when during an eclipse it was seen that gravity did bend light, contrary to Newton's Universal Law of Gravitation.

So the theory goes that as we are very proficient in the control of electromagnetism there is a good possibility that, with advances in understanding, we may be able to use this control to manipulate gravitational and inertial fields. Which in turn may be used as an acceleration inducing field on a space craft.

It should be noted that although gravity's effect on electromagnetism is known, the effect of electromagnetism on gravity and inertia is very much an unknown. Understanding this will be the first step in the development of a space drive.

This may seem a bit far-fetched but this concept of controlling one phenomenon with another is not science fiction, today electric fields are easily manipulated to create magnetic fields. The reason for the ease of this is that we understand the basic science behind how the two are related.

The importance of electromagnetism in the manipulation of gravitational and inertial fields is further increased with the arrival of a new theory from quantum mechanics called Zero Point Energy (or vacuum fluctuations). ZEP is an electromagnetic phenomenon and it appears to be clearly linked with gravity and inertia. Indeed finding a way to interact with ZEP could well result in the possible manipulation of gravity, therefore creating the space drive.
This interaction with ZEP would be a similar concept to the 19th century idea of an all-prevailing ' aether'. This would also overcome the problem of the fact that the space drive appears to fail to conform to the fundamental law of nature - the conservation of momentum. The problem would be overcome in this case because the reaction mass of the propulsion would be spacetime itself.

ZPE is something we shall discuss in detail in the next section but it is important to keep in mind that this is a new theory from quantum physics that is not experimentally supported yet, although the evidence is growing.

To restate, before any of this is possible new physics must be developed that describes inertia, gravity, or the properties of spacetime (ZPE) as a function of electromagnetics that leads to using electromagnetic technology for inducing propulsive forces.
It should be pointed out that if we are unable to use ZEP as some form of ether then we are back to the problems of the conservation of momentum and the situation must be addressed in terms of how to impart reaction forces against space itself. We may find that it can be satisfied if it turns out that gravitational fields have energy and momentum of their own. Other possibilities exist to, perhaps the so-called dark matter or cosmic radiation could provide reaction mass.

The GUT is the hope of many physicists and is an attempt to find a common unified theory incorporating the four fundamental forces in nature. These forces are the strong and weak nuclear forces, electromagnetism and gravity.
A discovery of a GUT would certainly allow us to manipulate gravitational forces according to desire, but would go much further, opening up many new possibilities.
The main attempts at formulating this theory have been through the use of energetic particle collisions. Scientists quickly discovered a coupling between the weak nuclear force and electromagnetism, then up increasing the collision energy they found this could also be linked to the strong nuclear force. Why not increase the energy some more and we might find the link with gravity? Well the collision energies are simply too great for us to achieve. This unified theory continues to elude us despite the best efforts of the top physicists right through the 20th and now into the 21st century.
For our spacecraft though the wait will be worth it.

Another interesting idea is the possibility of negative mass or negative energy density, and the assumption that it has negative inertia.

Given this assumption it should be possible to place negative and positive mass side by side and create a sustained acceleration of both masses. This propulsion conforms to both the conservation of momentum and the conservation of energy.
The idea of negative energy has been around since at least the late 1950's but it is unclear at this stage whether such a phenomenon exists or indeed if it even could exist. Classical physics says no, but quantum physics suggests it may exist. There is the suggestion that a relationship could exist between it and wormholes, so the first step is to seek evidence of wormholes through telescopes etc. If negative mass does exist and we are able to develop methods of collecting and handling negative mass, it seems entirely feasible that it could be used in a space drive type propulsion system.
The method of using electromagnetism to control gravitational forces is basically the creation of artificial negative mass.

The Alcubierre Warp Drive

Courtesy NASA

Alcubierre's 'Warp Drive', is a very famous design of a space drive that uses this idea of negative and positive mass to create propulsion.
The Alcubierre warp drive, would contract spacetime in front of the spacecraft and expand spacetime behind the spacecraft, and this warped space, together with the region between it, would accelerate off at warp speed. Observers would suddenly see the craft accelerate to faster than the speed of light, while the crew would feel no acceleration.

I would guess that this is the idea behind the Star Trek warp drive, though I do not know this for a fact.

The major problem here of course is making the spacetime contract and expand. For expansion we require 'anti-gravity' or in other words negative inertia, so we shall need a massive amount of such material (note the problems with this material mentioned above). Then for the contraction of the craft we will need equally massive amounts of positive mass.
There are other problems as well, for instance we will have to be able to make this matter appear and disappear at will to control the spacecraft. We also do not know if this bubble could travel faster than light, although it is suspected that it could due to the evidence that spacetime travelled faster than light at the moment of the big bang.

It does seem that developing an artificial control of gravitational and inertial forces would be the optimum method of creating a space drive, particularly if it is found that we are able to interact with ZPE. The theoretical methods of using negative matter means the operation of such a space drive will be extremely difficult, if it even exists. Another major consideration is that should such a phenomenon even be found it likely to be so far away that it would require a space drive to get there!

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