Physics:  Gravity.. Space Travel.  
By mid-century, if not sooner, insights into the true nature of gravity will lead to interstellar travel at speeds vastly exceeding the speed of light.  How will this occur?  Read on.

Discovery that gravity is an all-directional "push" rather than a localized "pull".

    By mid-century, if not sooner, physicists will discover that science has been making the same opposite-nature assumption about gravity as ancient scientists made about the vacuum-- i.e., just as modern scientists discovered that what was previously thought to be the "pull" of a vacuum is instead an all-directional "push" by gas molecules, 21st Century physicists will discover that what science had thought to be the "pull" of gravity is instead the effect of an all-directional "push" by an infinite force.  This will lead them to recognize that what we call "matter" in our finite universe is really anti-gravity in a variety of forms constantly being subjected to the infinite force of gravity.

Gravity violates "action at a distance" theory.

    Mathematical recalculations based on this discovery will lead to the long-sought grand unified-field theory viewing our universe as but one manifestation of many other universes composed of discrete, localized manifestations of anti-gravity-- i.e., imperfections in the infinite, all-directional force of gravity.  They will discover that the infinite, universal force of gravity does not obey the theoretical law against "action at a distance" that seems to apply to matter (i.e., anti-gravity)

Oil & Vinegar Analogy.

    They will discover that the interaction between gravity and matter (i.e., anti-gravity) is partially analogous to the interaction between oil and vinegar.  Just as oil and vinegar in a container can appear to be evenly mixed when the container is violently shaken, soon after the shaking, the vinegar (like gravity) begins pushing the oil (like matter) into globules.

Polarized-light Analogy.

    They will also discover that interactions between matter and gravity are also similar to interactions between light and matter.   When non-polarized (randomly polarized) light waves strike a non-polarized (i.e., randomly polarized) transparent medium, the random polarization of the waves and the medium allow most light waves to find a path through the medium, but when polarized light waves strike a polarized transparent medium, no light passes through unless its polarization matches the polarization of the medium.  Thus, when polarized light waves strike a medium not polarized in a way to create a path through it, those light waves are reflected, deflected or absorbed by the medium.  Such interaction is virtually certain to change the position, speed, or energy level of the medium.  The scope and magnitude of such interaction is greater when uniformly polarized light waves strike a uniformly but incompatibly polarized medium than the aggregate effect of such interactions when some randomly-polarized light waves are reflected, deflected or absorbed by not finding compatibly polarized paths through a randomly polarized medium.

Air-Pressure Analogy.

    Matter (i.e., a discrete point emitting all-directional anti-gravity waves) exerts an outward push against the infinite, all-directional push of gravity.  The combined effect of anti-gravity waves between two points of matter weakens the all-directional push of gravity between them relative to the force of the all-directional push of gravity in all space not between them.  Just as the rapid passage of air between two sheets of paper in close proximity to each other in parallel planes weakens air pressure between them and thereby enables the stronger air pressure on the opposite sides of the two sheets of paper to push them together, the weakening of the all-directional force of gravity between two points of matter (caused by the combined effects of the anti-gravity waves between them) enables the relatively stronger all-directional push of gravity in space not between them to push the two points of matter closer together.

Gravitational Polarization to Neutralize Newton's Laws and Exceed Light Speed.

    This understanding of gravity and anti-gravity will enable physicists to learn how to "polarize" the anti-gravity waves emitted by matter to thereby weaken or strengthen their effect relative to gravity waves.  With respect to an object on or near the earth, the huge anti-gravity effect of the earth makes the force of all-directional-gravity waves between the earth and the object dramatically weaker than the gravity waves in all space not between them; therefore, the much stronger gravity waves in space not between them pushes them together.  By learning how to "polarize" anti-gravity waves emitted by matter, physicists will learn how to make an object levitate or accelerate away from earth.  This ability to "polarize" anti-gravity waves will lead to the design of space ships constructed in such as was as to permit changes in the polarization of anti-gravity waves emitted by every subatomic component of the ship and every subatomic component of everything (and everyone) inside the ship.  Using such polarization to neutralize the all-directional push of gravity coming from the desired direction of travel and simultaneously maximize the all-directional push of gravity from the opposite direction (i.e., from the direction of departure), the force of gravity will propel each subatomic component of the ship and its contents through space at speeds vastly exceeding the speed of light.  Since the force will push uniformly against every subatomic component of the ship and its contents, neither the occupants nor contents of the ship will experience any "G" force relative to the ship because all such subatomic components will be accelerating at the same uniform rate in the same direction.