Physics says there is no traveling through space faster than the speed of light, which is usually supposed to be a notable barrier to interstellar travel introduced by Einstein’s theory…but this isn’t quite true.
Oh, from the point of view of the Earth, you will, famously, approach the speed of light closer and closer but never quite reach it, so the absolute fastest you could ever travel from Earth to Proxima Centauri would be a hair under 4.246 years, the time it takes light to traverse the distance. You could accelerate as much as you want, but you will never reach the speed of light, only approach closer and closer. Compared to the speeds predicted by Newton’s theory, this seems a fatal limitation…but what few appreciate is that from the point of view of the ship, you keep accelerating. Relativity imposes “time dilation”, and the associated effect of “length contraction”, the net effect being that from the point of view of the spacecraft, it’s somewhat easier to reach terrific speeds than Newton’s theory would predict.
You can play around with this calculator here, which supposes a constant acceleration (and deceleration) of 1g, which would be enough to replicate Earth gravity through thrust. In Newton’s universe, on a journey to Alpha Centauri, you reach a maximum velocity of 2.12 times the speed of light halfway into the journey before decelerating, and the entire journey takes 4.115 years. In Einstein’s universe, you reach a maximum velocity of only 0.95 times the speed of light, and it takes 6 years to reach your destination…but due to time dilation, from the point of view of the spacecraft, your journey lasted 3.58 years, faster than Newton’s theory predicts. Despite your actual speed being lower; this is due to the freaky effects of length contraction.
Spaceflight using constant acceleration is powerful; assuming a sufficiently powerful energy source could be accessed, you could traverse the entire diameter of our galaxy in just 12 years worth of ship time. Which sounds like a long time, and indeed it is compared to something like Star Wars’s hyperdrive…but consider it takes light 100,000 years to traverse the same distance from the point of view of the Earth. And, as detailed above, your journey from your own point of view would be much longer than 12 years if Newton’s theory of physics applied. Einstein’s theory of relativity makes interstellar travel easier…you just can’t come home again (well, you could, but “Planet of the Apes” suggests you really wouldn’t want to; no matter what, the changes over such a long time would be distressing).
The really mind-bending part is that relativity provides us methods that are even more powerful than constant acceleration. As Robert L. Forward, a science-fiction author and actual physicist, has pointed out, gravity is analogous to electromagnetism, in as much as powerful gravitational “charges” create fields of gravity, which can be harnessed for useful purposes if you moved powerful gravitational charges along a path near the speed of light. Unfortunately, “powerful gravitational charges” means something like a mass the size of Mount Everest at the density of neutron-star material or a black hole. Which is well beyond our current technology…but, I might add, perhaps not intractable in the near future.
Consider the super-heavy nuclei in the “island of stability”, which physics predicts exists but that we have only penetrated the outermost edges of in our efforts to synthesize the super-heavy elements of the periodic table in particle accelerators. The predicted “island of stability” implies that certain radioactive elements might be much more stable than the usual millisecond or so half-lives of the super-heavy elements, with half-lives measured in days, years, or potentially even millions of years, comparable to radioactive elements we use at the macroscopic scale today like americium, plutonium, or even uranium, but with much greater mass, density, and unique properties. Useful for radiation shielding, advanced nuclear fission reactors (critical masses for these elements might be very low!), as well as any application where an ultra-high-density element is required. To date we haven’t been able to synthesize any of these isotopes since they require greater neutron flux than our particle accelerators can impart; something like the core of a high-grade neutron bomb could probably do it, though, assuming we get enough stomach to fund and build such a technology.
Despite the heady possibilities, none of these nuclei would be anywhere near the density required for a gravitic field, but there is the intriguing concept that just beyond the “island of stability”, nuclear matter undergoes a sort of phase change, and instead of atoms as we know them, matter rearranges into a soup of up quarks and down quarks, so-called “up-down quark matter”. Which might be energetically favored, and thus completely stable. To the point that in principle there is no limit to the size of the “nucleus”, and they might come in a far wider variety of charges than normal matter; “atomic numbers” could range up into several hundreds or even thousands, or even be negative. This concept is known as the “continent of stability”, and so far the theory seems to be holding up under scrutiny, though of course hard proof is lacking; it’s thought that up-down quark matter would only appear around an atomic mass of 300 daltons at the lowest, which is somewhat beyond any isotope we’ve been able to manufacture yet.
If, and its a big if, such a form of matter could be manufactured, it would be extremely dense compared to normal atoms. With quarks arranged together in a tight lattice, as opposed to the widely spaced orbitals of normal atoms, you would reach a density comparable to or even greater than neutron-star material…only it would not be dependent on a neutron star’s extreme gravitational field to be stable. Such a “nugget” would freely exist on the Earth or even in space…though if you held such an object it would be so dense it would fall through any ordinary matter instantly and eventually it would settle in the Earth’s core.
Supernovas create such extreme neutron fluxes that it’s been speculated that some tiny fraction of stellar matter might be converted into the super-heavy elements that we make in our particle accelerators, and this might include some elements on the “island of stability”, which has stimulated the search for evidence of such isotopes in nature. So far only tantalizing hints have emerged. By the same token, if up-down quark matter is stable and appears at around 300 daltons or higher, then natural phenomena like supernovas or neutron-star collisions might generate minute quantities of such matter naturally, in which case, since it’s stable against decay, it should still be there. Due to its extreme density, if Earth were born with any quark matter mixed in, it would long since have sunk to the center of the planet and be deep in the core. Utterly inaccessible to us, but still fascinating to think about.
Anyway, if up-down quark matter were generated in the laboratory, you would want to keep it in place with electromagnetic fields, so it doesn’t fall back down to earth (though frankly outer space would be a far better location for such a laboratory: zero-g means that the task of holding such dense nuggets in place is much easier!). How you would generate such matter artificially is an open question, but our current best lasers are able to reach energy densities momentarily that are only about one order of magnitude below that which occur in neutron stars. If you took a nugget of super-heavy radioactive elements that you generated via a neutron bomb core, and you then subjected it to extremely powerful lasers (the same type used in experimental fusion reactors today, but scaled up), you could liberate the quarks within. Ordinarily this would just turn it into a quark-gluon plasma and cause it to explode outward, but applying a sufficiently powerful magnetic field to confine the matter, and/or laser geometry that holds it in place in a tiny volume…you could force a phase change into up-down quark matter. Which, again, the “continent of stability” theory predicts would then be stable.
Repeat this process long enough, and you have Everest-sized nuggets that you can move around with powerful magnetic fields. Manufacture enough of these nuggets and arrange them in the proper geometry, as Robert L. Forward suggested, and you now have a ring that can generate gravitational charges. Take the nuggets of ultra-dense matter, circulate them around fast enough in the ring in appropriate fashion (presumably with electromagnetic fields), and you now have a gravitational field around the ring that can be made either attractive or repulsive. And that would act on every atom in range evenly.
The net result? Place a spacecraft inside the ring, and it could be accelerated in the appropriate direction at hundreds, thousands, an arbitrarily high number of gees, and since the acceleration is entirely even across the whole vessel, you would not experience any “g-force”. It would feel like you were not moving, but you would go in an instant from standing still to a relativistic speed, an arbitrarily high fraction of the speed of light.
Obviously if you ever wanted to slow down and arrive anywhere, you would want there to be a second gravitational catapult at your destination to decelerate you, in the reverse operation of how you were accelerated.
The key advantage here is that far higher effective accelerations can be achieved, and you can stay at maximum speed the entire journey, instead of a slow ramp-up and ramp-down.
“So”, you might ask, “could you just accelerate arbitrarily high?”. Well…yes, and no. Among relativity’s freaky effects is that the faster you go, the more you experience a sort of doppler effect: the stars in front of you will have their light “blue-shifted”, and the stars behind you will have their light be “red-shifted”. As this blog ably points out, these effects become very noticeable even at a mere 95% of the speed of light. Behind you and to your side, the starlight has been red-shifted to the point of invisibility, as the stars now shine in infrared and microwaves. But in front of you, normally invisible orange and red dwarfs have had their light blue-shifted much more into the visible spectrum, so you see a circular “window” in front of you filled with a cluster of brilliant blue stars.
At 99.9% of the speed of light, the window of stars in front of you contracts into a smaller circle but becomes brighter, denser, and bluer. This effect continues, with the sky remaining bright but the stars becoming confined to a smaller and smaller area in front of you, with the number of stars visible at all falling noticeably by the time you reach 99.999% the speed of light.
By this point, however, the normally invisible gas and dust clouds that glow in the infrared have become brilliantly visible to the naked eye in front of the spacecraft. Before they too, as you progress faster and faster, start to be blue-shifted out of the visible spectrum and into the ultraviolet and finally into X-rays and gamma rays.
The most spectacular sight of all, however, is yet to come: at 99.999% the speed of light, a reddish patch starts to become apparent to the human eye directly ahead of the spacecraft, becoming brighter and brighter, less and less red, until it reaches an apparent magnitude comparably bright to the sun as viewed from Earth, an even glow coming from directly ahead, which keeps shining long after all other light sources have been shifted to invisible short-wave radiation: this is the cosmic microwave background, only here it’s blue-shifted to become visible. Freaky, isn’t it? It will keep shining long after the stars and clouds have been blue-shifted away, and won’t start fading out from the visible spectrum until you reach 99.99999% of light speed.
After then, the light is still there, but it’s peaking in ultraviolet radiation and finally X-rays and gamma rays. And since the total illumination from this source is pretty bright — comparable to the sun as viewed from Earth! — it would eventually become akin to having a sun’s worth of X-rays shining down upon you. Quite deadly unless you have thick shielding. Admittedly this is possible, but how romantic would it be to select a speed to travel when the light from the universe’s genesis shines upon your spacecraft’s interior with the same even, warm glow that we know and love from an incandescent light bulb.
To the stars, by the light of creation itself…
Time dilation works its will, of course, and this can be calculated for a given velocity by recourse to the “Lorentz factor”. This handy calculator lets us do that in a flash, and we see for 95% the speed of light, where the stars become brighter, bluer, and in a smaller area in front of the spacecraft, your “gamma” is 3.202. Since we’re undertaking the entire journey at this velocity, a 4.2465 light-year journey to Proxima Centauri would be completed in 1.32 years ship time. Considerably more efficient than the three years’ journey constant acceleration at 1g permits. And we’re just getting started.
At 99.9% of light speed, your gamma is now 22.36, which is starting to become much more noticeable. In a flash out of the catapult, you’ll behold a smaller, denser, brighter circle containing bluish stars in front of you, which will last you 0.1899 years until you’re catapulted back to a stationary speed at Proxima Centauri. That’s traversing the distance in 69 days, a bit over two months.
Even at a speed where you’re still primarily seeing stars, though perhaps with the gas and dust clouds now brilliantly visible, 99.999% of light speed, your gamma has now reached 223.61, which means you’ll be making the journey to Proxima Centauri in a bit under 7 days. A week. For you it was a short vacation’s worth of time, but don’t forget that on Earth and Proxima Centauri, four years are passing, in any case.
At the speed when the cosmic background starts to fade out from visibility, 99.99999% of light speed, your gamma has become 2236. Which means that the 4.2465 light-year journey for you has become 0.001899 years, or 0.69 days. Yes, that’s less than a day. To be precise, 16 hours 39 minutes. Comparable to a long-haul flight across the world today. Only instead of crossing continents, you cross star systems…and the world around you has aged years during your flight, as you caught up on your favorite novel, the pages lit in the glow from the universe’s formation.
Much more powerful than constant acceleration at 1g. And a short enough trip time that the design of your spacecraft might be simplified; the lack of any differential acceleration means that the usual centrifuge form of a space habitat might be employed, but if you’re only flying for 16 hours, the designers might not even bother with simulating gravity. Old-fashioned zero-g might suffice. Instead of simulating gravity or withstanding thrust, the design might be centered around skylights to let the glow from genesis shine down upon the interior.
Which sounds like freaky classic sci-fi literature, but it’s grounded in real physics. In the future we may well have the toolkit to actually do this, and sooner than you might think. True, it would take truly tremendous amounts of energy, but out in space, energy generation is trivial. An array of nuclear reactors (fusion or even fission) could easily provide the necessary power. And if nuclear doesn’t float your boat, a sufficiently large solar array could do it (giving you the starter edition of a Dyson swarm…).
And at a Lorentz factor of 2236, Proxima Centauri looks like it might as well be across the street, but extrapolate it out: the Pleiades, at 444 light-years distance, could be reached in 72 days of ship time. Even the center of our galaxy, 26,000 light-years distant, could be reached in 12 years. A shorter time than it took our New Horizons probe to reach Pluto…though from the point of view of the Earth and your destination near Sagittarius A*, it would nevertheless have taken you 26,000 years to make that trip. But as long as you care not about who and what you leave behind on the homeworld, you could traverse our entire galaxy’s diameter in the space of one human lifetime. And since nature works its will on timescales of millions of years, not human lifetimes, if you want to see some astronomical wonder in person, chances are, even after a few thousand years, it’ll still be there waiting for you.
Your fellow space travelers would also be there waiting for you, since they “age” at the same rate you do, assuming that the genesis glow is the standard speed at which civilization travels. Indeed, one can’t help but wonder if the entire civilization might want to “accelerate” itself, synchronizing its reference frame of time with those of its travelers, so as to not have to wait a subjectively interminable time to hear back from those who travel. In this fashion entire cultures, once they master these technologies, could accelerate themselves far beyond the time we live in.
Perhaps that’s the solution to the “Fermi paradox”: where are all the advanced aliens who should have colonized everything by now? They found it much more convenient to accelerate themselves so the universe would be closer at hand…but as a result they travel into the future at an accelerated rate. Net effect: life could be everywhere, but as soon as intelligent life masters the ability to exist at a Lorentz factor in the thousands, it would seem to “disappear”…
And why not? What does terra firma have to offer compared with being living witnesses to the realm of ages, only occasionally severing yourself from the glow of genesis to check in on the realm of the mortals, your timeframe allowing you to visit a new solar system every day. The only cost? You age a day while the rest of your civilization has aged only 39 seconds.
As far as I know it’s out of left field, a premise only seldomly used in science fiction, but it could be the key to what our real future is going to be like: our children growing up bathed not in the glow of the sun, but in the light of creation itself…