The most realistic interstellar propulsion concept to date (The Bussard Collector) does not violate the theory of relativity but has other problems with physics.
There are many techniques in science fiction to colonize other planets in our galaxy, despite the great distances and short lifespans of humans. The concept of the Bussard engine from 1960 was one of the few that did not completely ignore the limits of the speed of light and the theory of relativity. In 1969, John F. Fishback estimated that theoretically, it should even be possible for humans to cross the entire Milky Way during their lifetime.
A new study by Peter Schattschneider from Vienna University of Technology and Albert A. Jackson from Triton Systems in Houston, USA, now shows that at least this hope will probably not come true.
What Is The Bussard Collector?
Bussard engines are supposed to function as jet engines with a fusion core instead of a turbine. When they fly through the galaxy, they are supposed to collect the interstellar gas, which consists largely of hydrogen and eject it again at greater speed thanks to nuclear fusion.
John F. Fishback suggested doing this with magnetic fields. The study by Schattschneider and Jackson also shows that Fishback’s principle works. However, only with a very large engine and almost perfect fusion reactors; and even then, in theory, it doesn’t work well enough to traverse the 100,000-plus-light-year Milky Way galaxy in a human lifetime.
Nobody has to exceed the speed of light
The speed of light does not even have to be exceeded for the flight through the Milky Way. Because the theory of relativity shows that it is not an insurmountable obstacle to reach distant stars and galaxies without having to spend thousands of years on the flight or at least experience it yourself. For a distant observer, the spaceship flies slowly through the galaxy and takes a little over a year per light year, but things are very different onboard the spaceship itself.
Two things happen: The time of the observers staying behind is faster than onboard, by the so-called Lorentz factor, which depends on the speed. In addition, the distance to the target for the crew of the fast-flying spaceship becomes much shorter during the flight, also by the Lorentz factor. In the end, the speed of light does not have to be exceeded in order to cover distances of 1,000 light-years in less than 1,000 years.
The Lorentz factor was found by Hendrik Lorentz, who described the principle of relativity and the change in time and space as a mathematical solution for the propagation of electromagnetic waves according to Maxwell’s equations from 1862 and was not the only one. In 1905 Albert Einstein combined them in “On the Electrodynamics of Moving Bodies” with the equations of classical mechanics, with the help of other findings from Mach, Doppler, Hertz, and others. He found the effect of the increase in mass and later derived the equivalence of energy and mass from it.
In order to be able to cross the galaxy, the principle of the Bussard engine would only have to make it possible to get close enough to the speed of light to achieve a large Lorentz factor. To do this, the Bussard engine has to accelerate continuously and collect the interstellar gas. Most of the gas is ionized. The hydrogen, therefore, consists of individual positively charged protons, without electrons. Therefore, they can be collected with a large magnetic field.
Above all, Schattschneider and Jackson took a closer look at the generation of this magnetic field. In earlier studies on the drive proposed by Robert W. Bussard in 1960, the mass for its production was only estimated by John Ford Fishback with a very optimistic formula and not calculated.
Heavy, way too long Solenoids
To collect the material, Schattschneider and Jackson assumed a parabolic magnetic funnel 200 km long and two kilometers in diameter. It collects enough material to give 13 meganewtons.
Most of the coils used to generate strong magnetic fields do not consist of electrical superconductors, but of support, material to prevent the coils from flying apart. On the one hand, the magnetic field is generated by the current flowing in the coil; on the other hand, the generated magnetic field causes an outwardly directed force to act on the current flowing in the coil.
Above all, a strong magnetic field is necessary to collect enough material for further acceleration, even at high speed, without the hydrogen flying past the spaceship through the magnetic field. Depending on the material used for the coils, it can now be calculated how heavy the coils must be at least to be able to achieve a certain magnetic field strength and thus also the highest speed at which the spaceship can accelerate significantly. That is the maximum theoretical speed.
Fishback had calculated that the maximum theoretical speeds of a Bussard drive would be 99.9996 percent of the speed of light even with simple materials such as Kevlar, which were available in 1969. Enough to shorten the flight distance by a factor of 342. With more exotic materials such as diamond or graphene, factors of up to 5,500 should be achieved. This is how a person could cross the Milky Way in 20 years of personal experience.
The drive is 40 to 600 times slower
But the calculation was not error-free — and was also limited by the almost non-existent computing technology of the late 1960s. Above all, he calculated the necessary mass of the magnet coils to generate the magnetic field, using an equation to estimate the minimum mass of a magnetic radiation shield. In doing so, Fishback decided to make some very optimistic assumptions, which he also clearly wrote. However, more precise calculations of the mass resulted in deviations of 40 to 600 times for the achievable Lorentz factors.
Even in the best-case scenario, if the spaceship only consists of the magnet coil holder and this consists of perfect graphene, only a Lorentz factor of 12.73 or 99.7 percent of the speed of light would be achieved. That would still be enough to visit neighboring stars. However, this is also an overly optimistic assumption. Because the new study only examines the mass necessary to stabilize the coils for generating the magnetic field. No weight is planned for the superconductors and their cooling, the fusion reactor, or the actual spaceship — quite apart from the fact that a reactor for the fusion of protons has not yet been feasible.
From the theory of relativity, it can be deduced that nothing can fly faster than the speed of light and that nobody has to fly faster than the speed of light to cross the Milky Way without dying of old age on the way. Here, as in most cases, it’s the rest of the physics that gets in the way of the endeavor. If there is a solution, many people will probably die of old age before it is found — and the theory of relativity does not offer a way out either.