One of the major problems with traveling through space is the need to carry fuel. Scientists have endlessly sought after sources of perpetual, portable fuel for spacecraft. But maybe that was the wrong approach?
The Woodward Effect indicates that we might do better manipulating the mass of the fuel that we have.
Space travel has always presented the same problem — the universe is big and mostly empty. This can make it extremely hard to provision any starship that needs to travel freely. Although technically, a spaceship can get up to speed and then jet through space on inertia alone, any change of speed or direction is going to require more fuel. If we really want to move in any direction, at will, around space, we're going to need a source of propulsion that can be accessed anywhere, without needing an outside source of matter or energy.
This is a major departure from anything we've so far used to navigate. One theoretical solution is the creation and annihilation of matter and antimatter — but antimatter-matter pairs take as much energy to create as they release when they annihilate. Solar wind, the flow of charged particles off the surface of stars and out into the universe, can be used to push a star ship, but it doesn't allow for optimal navigational freedom. There's even speculation of using giant sails to capture the momentum of spontaneously created of particles in the void. That would require large sails for a small ship.
And then we have the Woodward Effect, which proposes that we can, essentially cycle a group of particles through a loop again and again, and they will change mass. We can pull on them when they're at a light point in their mass, and push them away when they're heavy. It will be like grabbing a balloon and pulling it towards you, only to have it change into a bowling ball as it reaches its nearest point and you push it away again. You'll feel an overall push outwards.
How does this happen? According to some, it doesn't. It's based on Mach's Principle, and in fact James F Woodward himself prefers to call his idea the Mach Effect. Mach's Principle is complicated, but the best and most famous explanation for it has to do with twirling around on a lovely summer evening. Go out and look at the starry night sky. If you look up, the stars will appear to be standing still (unless you're out for hours at a time). Looking down, you will notice that your arms are dangling at your sides, as if no force is acting on them except the Earth's gravity. Now spin while keeping your arms relaxed. You'll feel your arms lifting up and moving out as if you were going to break into a chorus of The Sound of Music. Keep twirling and look up. The stars are spinning around you. Ernst Mach, a physicist, postulated that this was not happenstance. He believed that, at some level, your arms and the stars are interacting, exerting force on each other and moving in relation to each other. The arm-star objects are a physical system.
Obviously, Mach didn't believe that the stars were the only things causing your arms to move, but he did theorize that these interactions were possible. Now we skip from Mach to Einstein. Einstein explained that not only does time vary with relative speed, mass does as well. If, then, you have an object moving in a system that can include the farthest stars and the closest objects, the mass of the object will literally vary as it moves around. Snag it while it's light, and push it while it's heavy, and you can reap a push for yourself in one direction, but within an overall system in which no mass or energy is created or destroyed. A space ship could be equipped with however much mass it needs to move, spend its voyage flinging that mass around its internal systems while employing the Woodward Effect, and come back home with exactly the same amount of fuel it had in the beginning.
How likely is this? That depends on whether Mach's Principle actually works, practically speaking. Many scientists dismiss the very idea — and Einstein was one of them. Others are considering the utility of the idea, and looking into the practical benefits that we could get from it if it's true. The Woodward Effect has been tested multiple times. While some tests indicate it might be even greater than Woodward himself estimated, other tests are muddy and inconclusive. Still, its another potential avenue to the stars. We always like that.