On Interstellar Travel
Part 1 of 3: Can we Reach for the Stars?
“You feel so lost, so cut off, so alone. Only you’re not.”
45 years ago, Neil Armstrong took one small step for (a) man, one giant leap for mankind. Over the intervening decades, it’s interesting to see the progress that mankind has made in outer space. As a species we have continued to leap forward, placing thousands of satellites into Earth orbit and sending probes all over our Solar System. Yet we have not taken any more small steps for man, or woman. There are no more bootprints on the Moon or any other celestial body than there were in 1972.
Manned space travel is difficult and perilous, and at the moment low-reward. Earth is the only Earth-like world in the Solar System; any colony we put on the Moon or Mars would require supplies from Earth just to survive. If you’re looking for a comfortable extraterrestrial world to live on, you’ll have to go interstellar. There’s a lot of ideas for how mankind could one day walk on an exoplanet – some realistic, some less so.
As part of the celebration of the 45th anniversary of the Moon landing, I’ve written up a series of three articles on interstellar travel. Today’s article will stick to (mostly) realistic slower-than-light travel options, while the next two pieces will delve into increasingly (but not infinitely) improbable modes of propulsion.
When I’ve Been There Ten Thousand Years
Traveling Much Slower than Light
Einsteinian space-time has three space-like dimensions, one time-like dimension, and an absolute speed limit of c, approximately 300,000 kilometers per second (kps). Nothing can move faster than c without also traveling backward in time. And since arbitrary time travel causes all sorts of logic-destroying stupidity, most scientists assume that time travel is impossible. Therefore, nothing can go faster than the speed of light.
In a realistic universe, it takes an awfully long time to get anywhere. The Apollo moon missions maxed out at around 11 kps relative to the Earth. Traveling to the nearest star would take 115,000 years at this pace. Actually, you’d never get that far. Starting from the Earth, the escape velocity of the Solar System is ~42kps. You’d need a considerably faster craft to ever exit the Solar System.
In the 1960s, the Orion nuclear pulse-rocket was “designed” as a deep space exploration concept. This starship would have used repeated thermonuclear explosions to push it at extremely high velocities (compared to conventional rockets). Such a craft could accelerate up to velocities of around 3%c. This would get you to Proxima Centauri in 142 years.
With much-slower-than-light travel, a journey between the stars will either require many lifetimes, or prolonged cryogenic freezing. Either way, all of your friends at home will be long dead by the time you reach your destination. And if people live on a starship for too many generations, they may eventually forget that they are on a starship.
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Very-slow interstellar travel faces one major problem: Resource consumption. Where do you get fuel, water, and other materials while spending centuries between the stars? Every ecosystem requires light and heat, which means you have to generate energy, and energy is in short supply in interstellar space. Even nuclear reactors will run out of fuel during a thousand-year journey.
A Bussard ramscoop could gather interstellar gas for fusion power, but there’s not a lot of gas out there and it would be plain H-1. This is a much dirtier fusion fuel than He-3, and over the years would cause radiation damage to your fusion drive. You’ll burn most of the hydrogen that you collect just to create enough thrust to offset the ramscoop’s drag. And the ramscoop won’t collect any metals – if anything on your starship breaks, you can only hope that your ancestors brought a spare.
Some slow-starship designs completely bypass the energy problem by relying on laser energy beamed from Earth. This energy could be used both to propel the ship and to power its ecosystem. It’s certainly an elegant solution, as you could rely on an extremely large energy-producing infrastructure that doesn’t have to travel with your starship. But what happens when your benefactors run out of funding, are killed in a war, are destroyed by climate change or natural disasters?
The fact is that based on a present-day understanding of physics and engineering, a slower-than-light “generation ship” is really not much more realistic than faster-than-light travel. If we ignore the difficulties of energy generation and resource collection in interstellar space, we might as well ignore the rest of physics.
And let’s say someone develops a technology that allows a civilization to live forever without an external energy source – why would you even want to live on a planet at that point? Just stay in interstellar space.
In a universe where human civilization is limited to much-slower-than-light travel, there would be no such thing as an interstellar civilization. Humanity might eventually spread out to a bunch of stars, but each solar system would have its own unique way of life. The human colonies might communicate with each other, but they really couldn’t trade effectively, and no one could travel back and forth between different stars. There could be countless alien civilizations in the galaxy, but we might never encounter them because they are too far away.
Oh my God, it’s full of stars!
Traveling at near the speed of light
According to Einstein, funny things happen when you get near the speed of light. Time slows down. Distances get shorter. Mass gets more massive. A traveller moving at 99.5%c will experience 10-fold time dilation, length contraction, and mass increase. That means he experiences time passing 10 times slower than someone at rest. Relativity may sound funny, but it isn’t just empty theory – our entire telecom and GPS system is programmed with relativity in mind. If Einstein was wrong, then none of the technology you’re using to read this blog article would work.
Science fiction authors have played with the concept of time dilation for many decades, because it’s fun. An interstellar traveller may live for a normal human lifespan but witness thousands of years of galactic civilization in fast-forwards.
There’s one massive problem with near-lightspeed travel, and it’s mass. Well, it’s really energy, but we all know that’s the same thing. If you’re using time dilation to age 10x slower, that means you are also 10x as massive as you were at rest. If you were to stop moving, you’d need to shed kinetic energy equal to 9x your rest mass, a truly absurd amount. In order to get moving again, you need to gain an equally ridiculous amount of kinetic energy.
How ridiculous is this? Well, the rest mass of a 70-kg (154#) human is 6.3 exajoules. That’s equivalent to 1,500 megatons of TNT, or 3 times the total energy of every nuclear bomb ever detonated. Now imagine spending nine times that energy just to accelerate a single person to near-lightspeed. We haven’t even considered the mass of the starship yet!
Even with antimatter or black holes, it is very difficult (and highly dangerous) to come up with this kind of energy. Science fiction writers have either ignored the energy problem, or circumvented it with handwaving pseudophysics. (“It’s an inertialess drive!”) In Speaker for the Dead, Ender Wiggin wondered if a star winked out every time a starship started moving. (since the ship picked up a vast amount of energy without spending any energy)
Astute readers might wonder, if a starship can pick up energy ex nihilo, could it harness that energy to some other cause? At the very least, with enough energy you could completely destroy any planet you crashed into. Of course, if you had the technology to generate “free” energy, you may already have much more efficient ways to destroy a planet.
In a universe where travel occurs at near-lightspeed, there could be something resembling interstellar trade and travel, it would just be very difficult. If faster-than-light communication exists, it’s plausible that far-flung human colonies would stay in touch with each other, sharing the same Internet and the same entertainment and a similar culture. However, travelling to see another star system for yourself would require a major time commitment. Anyone you left behind at home would be much older by the time you reached your destination, or dead if your journey was too long.
Unless, of course, your interstellar civilization managed to dramatically extend their lifespans. Simple anti-aging and regenerative medicine techniques could keep human-like bodies alive for many hundreds of years, long enough to reach nearby stars.
However, if you wanted to tour the hundreds of billions of stars in the Galaxy, at 4 years per star you’d have to live a trillion years. Neither medicine nor mechanical prowess could keep a physical body functioning for that long. You could repeatedly switch bodies, but it’s better to transsubstantiate into an energy being. An energy being might think and act on a totally different timescale compared to biologicals. If your consciousness was slow enough, or your memory long enough, you could hold a conversation with your friends across the galaxy despite a 20,000 year lightspeed delay. At that point, you would definitely not resemble a human in any meaningful way.
Oh, what was that sound? I guess it was the rumbling boom that happens when you break the plausibility barrier. I believe that brings today’s episode to a close!
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Come back later for Parts 2 and 3, where I will delve into interstellar propulsion ideas less constrained by reality.