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 negative energy to exist, there would still need to be some process in which it could be manufactured or some place in which it could be mined. If the process isn’t efficient enough, it could take a million years to make enough for one trip. Our current method for making anti-matter is only 0.01% efficient, meaning 99.99% of the energy used is wasted. 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 sub-luminal 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 acts sort of like negative energy but that is a longshot. 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 anything at the destination. Another approach that was suggested was to create a “railroad” between two stars at sub-luminal 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 sub-luminal. 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 have a high mass flow rate but in the grand scheme of things, the exhaust 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 generated 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, millions 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. There is also quantum tunneling, where particles can wiggle their way through a magnetic field, regardless of it’s repulsive strength. 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. You could eject the spent containers out the back to reduce the mass of the spacecraft but that would be considered a propellant and the average exhaust velocity would be decreased. 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. This has been done with normal matter on small scales. The third problem is waste heat. The rocket would produce enough waste heat to melt the magnetic nozzle and cook the colonists. There are three ways to get rid of waste heat: convection, conduction and radiation. In space, you can only use radiation. You could use cooling systems and a wireframe-like nozzle, to allow most of the waste heat to escape into space without being absorbed by the spacecraft. There is also the possibility of gamma ray reflectors reducing the amount of energy absorbed by the craft.
There are also fusion rockets, both inertial confinement fusion, and magnetic confinement fusion. Unlike CERN’s ITER fusion reactor, a fusion rocket needs many 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. In my ballpark calculations, 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. Lasers are horribly inefficient, less that 50% and the rest is waste heat. Lasers also use lenses which usually have short lifespans. It’s possibly with nanotechnology approaching the sophistication of biology, you could have higher efficiencies and self-repair. ICF is still in the early days of research so it’s possible with better pellet geometry and better laser timing and accuracy, the power requirements could be reduced. MCF has to contain fusion in a chamber and directs some of the fusion plasma out of a nozzle. MCF has to deal with many types of radiation. Neutron radiation is not charged, so it can’t be contained with magnetic fields. With biology-class nanotechnology, we could have self-repairing containment chambers. You also have lots of different types of radiation which is absorbed into the chamber walls and conducted to the rest of the ship as heat. With known science, I doubt there is much room for radiators to be more efficient. It’s possible we could develop a series of reflective materials and eventually focus them out the nozzle, to reduce the load on the radiators. Assuming we are eventually able to do both ICF and MCF, which I believe is likely, the biggest challenge will be keeping the power plant and radiator masses low enough to make it worth it.
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 and new fuel cannot be created.
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 dust could destroy the craft or sail. There is some research on that very issue and it’s optimistic. There is also the problem of slowing it down at the destination. One possible solution is to release a secondary light sail and bounce the laser off it, which slows the primary sail while speeding up the secondary sail. The biggest challenge with that is aiming lasers at such a small target at such a great distance. Another option for deceleration is to use a magsail, which could use the destination star’s magnetic field to slow the craft. Once we establish a colony at another star, we can build a laser array to slow down the next wave of laser propelled light sails.
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.