Interstellar travel is the foundation of many science fiction universes so it’s an obvious first topic to research. At light speed, it would take almost 4.3 years to reach the nearest star. Sailing across the Atlantic ocean only took as much as 6 months. If faster-than-light travel is possible, it would definitely be worth it. With present-day science, the only two contenders are warp drives and wormholes.
Wormholes or the Einstein-Rosen bridge, could exist IF there is exotic matter with negative mass/energy. The properties of negative mass have been studied despite the general consensus of the scientific community being: that it can not exist. It wouldn’t be the first time that the popular opinion of the theoretical physics community has been wrong though. For example, the scientific community believed that the universe was stable, neither expanding nor contracting. Even Einstein dismissed the idea of an expanding universe, until new evidence surfaced that made it the most likely scenario. Even if it is possible for this exotic matter to exist, there would still need to be some process in which it could be manufactured. If the process isn’t efficient enough, it could take a million years to make enough for one trip. Even if exotic matter can exist and there is method that can manufacture it in sufficient quantities, we still don’t know of a process to actually create a wormhole or even to aim its destination point to the desired location. One possible scenario is that we can create a wormhole, with both endpoints near each other, then move one endpoint to another star, at subluminal speeds. There are scientists that are exploring the possibility of using the Casimir Effect to stabilize a wormhole, since it’s the only known phenomenon that has negative energy. The Casimir Effect isn’t exactly the same as exotic matter with negative mass/energy, since it’s “negative energy” is temporary fluctuations in the spacetime foam, like 0 temporarily becoming -1 and +1, then becoming 0 again. Wormholes could also open the door to time travel and temporal paradoxes but many scientists believe that the laws of physics wouldn’t allow such events to occur, even if it’s possible with the current (and incomplete) mathematics.
Warp drives or the Alcubierre drive would also require negative mass so it has the same problem as wormholes. One estimate is that it would require a quantity of negative mass, that is greater than the mass of Jupiter and that’s for a small spacecraft. Some have calculated that the interior of a bubble would be flooded with Hawking radiation and destroy anything inside. Others have predicted that the warp bubble would collect particles in transit and when it stops, it would release a wave of particles that would destroy everything at the destination. Another approach that was suggested was to create a “railroad” between two stars at subluminal speeds and once completed, ships would then be able to travel faster-than-light between the two stars.
If we can’t go faster-than-light or even the speed of light, that leaves us with subluminal. The physics of all existing propulsion technologies and those known-to-be possible is the principle that: exerting a force on an object, creates an equal and opposite force. Kinetic energy is 1/2mv^2 so you can increase the velocity of the particles being propelled out the back, the mass or both. All the rockets that leave the ground of Earth are chemical rockets, which use a lot of mass but in the grand scheme of things, the particle velocity is very low. Ion drives use less mass but have a higher exhaust velocity. Particle accelerators can accelerate particles to speeds higher than any other known technology but in order to be used as a propulsion drive, the number of particles would have to be increased by many orders of magnitude, which would require thousands of accelerators and power plants that produce more energy than all the energy on Earth combined. It would weigh so much, it wouldn’t be able to move. There are reactions that can accelerate particles to very high speeds, such as the annihilation of a proton and anti-proton, fusion, and fission. Torch ships are the best of both worlds, they have a high mass flow rate and high exhaust velocity.
A pion rocket or beamed core anti-matter rocket, annihilates a proton with an antiproton, which creates charged and neutral pions. About 60% of the pions are charged and you can “push off” them using a magnetic nozzle. The pions decay into gamma rays a fraction of a second later but they exist long enough to be used. There are several problems with pion rockets, such as anti-matter production efficiency, anti-matter storage density, and waste heat. Dr. Forward claimed that we could, with present-day technology, mass produce anti-matter at a 0.01% efficiency. That means that 99.99% of the electricity used would ultimately be wasted. To produce enough anti-matter for interstellar travel, the production efficiency would have to be greatly improved. The theoretical maximum is 50% since equal and opposite particles are created using the known process. If the efficiency can’t be improved, it would require a Dyson Swarm of satellites with solar panels mass producing anti-protons. The next problem is anti-matter storage density. So far, scientists have only been able to store several individual antiprotons using a Penning Trap but they all eventually collided and annihilated. The more anti-protons you pack into a storage container, the more they will repel each other and attempt to expand. If we can’t pack anti-protons densely enough then the accumulative mass of all the storage containers will cancel out the high exhaust velocity of the pion rockets. We would also need to have storage containers that can handle accelerations and with present-day methodologies, we have only stored several antiprotons for less than an hour under ideal conditions. One approach I read about was to combine the antiprotons with positrons to create anti-hydrogen then form a frozen snowball of anti-hydrogen and since its diamagnetic, it can be levitated. However, this has never been done before. The third problem is waste heat. The rocket would produce enough waste heat to melt the magnetic nozzle and cook the colonists. You could use cooling systems and a wireframe-like nozzle to handle the waste heat but their masses spiral out of control and the spaceship can’t even leave orbit.
There are also fusion rockets, both inertial confinement fusion, and magnetic confinement fusion. Unlike CERN’s ITER fusion reactor, a fusion rocket needs orders of magnitude more fusion reactions per second to create sufficient thrust, which means that the cooling system has to deal with a lot more waste heat. I tried handling the waste heat by increasing the size of the radiators, increasing the radius of the nozzle and by making the nozzle wireframe-like so heat can pass through the openings. ICF also needs lasers to crush the fuel pellets and cause fusion and the power requirements made it too massive to leave orbit. MCF has to contain fusion in a chamber, which means it has to deal with all the waste heat so the mass of the radiators spiraled out of control. With known science, I doubt there is much room for radiators to be more efficient. The best material can only handle like 8000 degrees and fusion is millions of degrees. It’s possible igniting pellets for inertial confinement fusion could improve, which would reduce the mass of the power plants.
I’ve also looked into nuclear pulse propulsion, which uses nuclear bombs and a pusher plate like a soda can and a firecracker. There are no major problems with this approach, except that it would take more than 100 years to reach the nearest star. There is also a limited amount of fission fuel in the star system.
The last approach I looked into is laser propelled light sails. The first benefit is that it uses lasers that are on Earth or Luna. It doesn’t need its own power plant or propellant onboard, which reduces its mass. The lasers could push a small spacecraft up to 10-50% of the speed of light. It could reach the nearest star system in 30 years. For a 5 ton payload, the sail would need to be 1 square kilometer, which is doable. There are also some experimental materials like a diamond film, which could be used for the sail. One concern is that, at significant percentages of the speed of light, collisions with interstellar debris could destroy the craft or sail. However, there is research on that very issue and it’s optimistic. There is also the problem of slowing it down at the destination but you could release a secondary light sail and bounce the laser off it, which slows the primary light sail while speeding up the secondary sail. The biggest challenge with that is aiming at such a small target at such a great distance.
For my universe, I decided to use laser propelled light sails which release self-replicating machines, which self-replicate and build up an industrial infrastructure. Once completed, they build laser-based communications satellites, space stations, and artificial incubators. The colonists copy their minds using nanomachines and beam their digital minds to the new star system. Once they arrive, their mind is activated in a virtual reality with all five senses and they wait for their new biological bodies to be grown. They can also download their minds into time-shared android bodies.