and, the shape of interstellar conveyances
In my view, the propagation rate of gravity (and consequently inertial effects) is transmitted at 1039c. Hence, it is not logically forbidden that we move forward whilst throwing the rest of the visible universe in the reverse direction. The Euclidean manifold which carries the electromagnetic force can than act as the "stationary stage" upon which the momentum exchange takes place, i.e. our craft will not have moved (relative to no standard of measure) prior to a signal being transmitted to another mass. Thus, in the standard model, no influence can be transmitted at greater than light velocity and therefore, we can't move without throwing something out behind us because everything else is too distant to provide the necessary motive feedback without which we cannot move forward.
Could it be anti-gravity?
I have an objection here which I haven't seen elsewhere. It is that UFOs allegedly move perpendicular to the Earth's gravitational field (parallel to the Earth's surface) soundlessly ... and with great acceleration. If they were doing so by anti-gravity ... how? One would expect the force to be against the direction of the field ... not 90o relative to it. And if these lateral movements were done by conventional means, i.e. throwing a mass of air in the opposite direction ... we should know about it by the air blowing in the opposite direction (with lots of noise).
Their methods smell of broken conservation laws. In fact, that is one of the prime reasons most scientists object to the existence of extra-terrestrial visitation. Their confidence in those conservation laws is unbounded ... and ... they are correct to have such confidence in them ... because ... they have never failed to prove out at any time or in any instance whatsoever. They are flawless and can be used a "tools of discovery". For in any situation, if we assume them to be true, we can deduce what nature will conspire to do ... and we will always be right. A "conspiracy of nature" is a "law of nature".
If any failure of linear or angular momentum conservation can occur (as it is presently understood), it must occur at some extreme where our science has yet to examine it.
Here is an example ...
This is a hypothetical propulsion scheme which violates linear momentum conservation. It does not work but if it did ... it would give one the power to move in any direction as UFOs do ... and traverse interstellar distances without carrying all that fuel that Tsiolkovsky's rocket equation requires (to push off on).
Here, a mass of 10 units is 1 unit distant from the center of mass with a 1 unit mass 10 units distant on the other end. Now, the moment of inertia of the smaller mass is 10 times greater than the larger mass (inertia = mr2) so that one might expect that two projectiles (a,b) ... launched by springs from points equidistant along the connecting rod ... to cause the center of mass to dislocate and ball "b" would also be travelling faster than ball "a".
In reality, this does not occur because at every increment of time, each mass is "tested" by the other as angular momentum develops giving those masses centrifugal force away from the other. Thus, the whole apparatus simply rotates at a slower rate than would be the case is both masses were of 10 units and equidistant on the connecting rod and thus conserve linear momentum while responding to the difference in moments of inertia. However, it should be noted that the initial force from the projectiles (a,b) is logically prior to the development of angular momentum by which the masses test each other for the center of mass. One wonders what might occur when the time taken for an electro-magnetic signal to travel down each arm of the rod is "monkeyed with" relative to the time of interaction involved with the spring extension. In that short time, the state of the long arm is indeterminate, i.e. is it a mass of 1 or 10 or 100 or 1/10, etc.? Can matter be fooled into applying the inertial frame (the distant universe) before the electro-magnetic frame?
This is an important point in physics. A body does not know the laws of physics ... it "feels" them, i.e. it responds not to abstractions but rather to actual forces ... then, the "laws of physics" are obeyed. The rules are not applied "de jure" by an unseen distant observer ... they are felt "de facto" by the particles which don't give a hoot about what they do. They simply respond in the only manner they can.
On the shape of interstellar conveyances
Here it is. The most practical design would be cigar shaped with a magnetic field to ward off charged particles as the Earth's does for us. The long shape means that less area will be presented in the direction of motion ... so less radiation need be dealt with. Also, the taper means that the hull is logically thicker in the direction of motion. This improves on the mass of the entire package which we would want to keep as low as possible. And it would probably rotate for artificial gravity.
Addendum 02/20/04 :
The following is an incorrect propulsion scheme (unsupported by any experimental evidence) but serves to illustrate the necessity for a dynamic (changing electro-magnetic field) propulsion system rather than a passive one (stable magnetic field).
Let us suppose that a stable magnetic field causes a "dimple" in a gravitational field and further that reversing the magnetic field reverses the dimple (concave to convex) as in the illustration.
Then dimple 1 represents an "anti-distortion" which could generate anti-gravity while number 2 would generate the opposite effect. This does not violate conservation of momentum because pushing a spacecraft carrying such an electro-magnetic field would result in a reverse push to the planet (from which the gravitational field emanated) by action-reaction through that distorted field.
However, this system would violate the 1st Law of Thermodynamics because one could reverse the polarity of the field after some suitable distance and allow the craft to plunge back to the planet from which energy could be extracted. Then the field could once again be reversed to start the process again. Since the height reached by the craft is not a function of the energy required to reverse the field, the system constitutes a perpetual motion machine of the first kind.
Therefore, to create such a reverse gravitational field distortion would require a constantly changing field such that the gravitational field would only respond to a changing electro-magnetic field ... but would do nothing in response to an unchanging one ... a situation analogous to the electric or magnetic fields changing only in response to an alteration of the other.
Thus, if it is ultimately possible to evade Tsiolkovsky's Rocket Equation, it must be through the use of an oscillating electro-magnetic field acting upon the gravitational field of some ponderous body which serves as the "push-off" material rather than the ejected and spent rocket fuel as in all other proposed types of interstellar conveyances.
I have no idea of how such a "grav-electro-magnetic effect" might be accomplished at present and believe it to be impossible due to mutually cancelling effects. However, I cannot, as yet, completely rule out the possibility. We are assuming here of course that there are no other presently unknown "forces" of nature which are available for use by human beings ... a proposition to which I adhere without reservation. For if this were not so, why is there yet no indication of such a force at any reasonable level of significance? Ultimately, answering this question determines whether interstellar travel is to be extremely difficult, impossible or incredibly easy.