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NASA's Breakthrough Propulsion Physics Project was a 6-year program to research feasible interstellar space travel. The fact that the project's funding was ended in 2002 because of a lack of results might lead some to be skeptical, but the program's leader Marc G. Millis still remains confident. Our current understanding of physics has thrown up several large problems for the development of interstellar travel: namely the limitations from General Relativity and the enormous distances involved. Einstein's 1915 theory of General Relativity tells us that it is impossible for matter to travel at the speed of light, or rather that for matter to travel the speed of light would require an infinite amount of energy. The fact that light itself takes centuries or millennia to travel interstellar distances doesn't give us bulky humans much hope. Luckily, exciting new research has come up with theoretical plans to overcome some of these limitations.
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Already on the Drawing Board
Bombs as Fuel
One of the earliest ideas for interstellar travel comes from the 1958 Project Orion, which proposed that a vessel could travel to different stars using nuclear bombs as its fuel. A craft would drop about five bombs every second, whose detonations would propel it forward. Initially, this idea was proposed for a ship traveling to Mars. In the 1970s, the British Interplanetary Society revised the idea to create its Project Deadalus. This hypothetical craft would use fusion explosions to propel it past Barnard's star, a six light year trip, in about 50 years. Unfortunately for NASA researchers, the Nuclear Test Ban Treaty forbids the explosion of nuclear weapons in the Earth's atmosphere and developing a bomb-based ship would be illegal.
In the 1960s, the physicist Robert Bussard proposed a craft that would pick up fuel in space rather than carry its own. The Bussard Interstellar Ramjet is a hypothetical vehicle that would collect stray protons that exist in interstellar space with a magnetic scoop and somehow create fusion reactions that would propel the craft. This craft would work in theory, but there are still many problems that would have to be overcome: drag created by the scoop, how many protons can be scooped, much less somehow getting them to fuse.
When light strikes an object, it pushes on that object with a very small smount of force. If you use large amounts of light on a large surface, that force becomes noticeable. Robert Forward's proposed light sail works on this concept. A thousand-ton vehicle with a crew could make it to the nearest star in 10 years with a thousand kilometer sail and a 10-million-gigawatt laser. This sounds pretty simple until you realise that 10-million-gigawatts is 10,000 times the amount of power used on the entire Earth today. This technology requires a massive leap forward in creating energy before it can become viable.
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Antimatter, to put it simply, is ordinary matter with its electric charge reversed. The only difference between a proton and an anti-proton is electric charge. An interesting property of a particle of antimatter is that when it comes in contact with a particle of regular matter, both particles are annihilated, releasing large amounts of energy. This process would be much more efficient than nuclear fusion or fission, and could hypothetically fuel an interstellar craft. Unfortunately for us, antimatter is very expensive. We can create antimatter in particle acceleraters, and with current methods it would cost roughly 100 billion dollars for a single milligram of antimatter.
Imagine that the universe is a piece of paper. If you were to fold that piece of paper in half, two points that were once far apart are now right next to each other. This bending of space is what is called a wormhole. Here's a hypothetical way to build one: collect a whole bunch of super-dense matter, like from a neutron star. Make a ring of this matter the size of the Earth's orbit, electrify it with a massive amount of voltage and start spinning it at nearly the speed of light. Construct another ring wherever you want the other end to be, and boom you've got yourself a wormhole. Forget the fact that constructing these rings is way beyond our capabilities, you actually have to already be where you want the womehole to end up. So while wormholes may work hypothetically, we would still need some other way to travel very far.
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Approaching Ludicrous Speed!
Einstein's theory says that an object cannot travel faster than the speed of light through space. It says nothing about how fast space itself can travel. In fact, our theory of the big bang implies that the initial expansion of the universe had space traveling much faster than light. Using this assumption, Miguel Alcubierre invented a hypothetical engine that would mimic the fictional warp drive in Star Trek. The Alcubierre Drive would create a wave in the curvature of space that traveled faster than the speed of light. This moving bubble of space-time would be created by contracting space in front of the ship and expanding space behind the ship.
The problem with the Alcubierre Drive is just how hypothetical it is. To bend space-time in this way, we would need to be able to control a ring of "negative energy" around the ship. Physicists aren't even sure that negative energy exists, much less able to control it. Classical physics tends to support the idea that negative energy cannot exist while quantum physics leans towards "maybe, yes".
Physicists believe that a propulsive force can be created by the juxtaposition of negative and positive mass, without violating conservation of momentum or energy. Negative mass, by definition, would also have negative inertia, allowing both objects to constantly accelerate in one direction. A craft using this as a means of propulsion would be able to achieve incredible speeds. But once again, the concept of negative mass is just a hypothetical construct. We are not sure that it even exists, and wouldn't have the foggiest idea of how to create it. This kind of device is relying on a breakthrough in our understanding of gravity.
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