By James Phillips
It’s difficult to imagine that one day our Earth will be gone. We look around, and it all seems so permanent. Sure, mountains erode into watersheds, rivers meander, coastlines wander, and continents drift, but at the end of the day we still expect there to be mountains, rivers, coastlines and continents. As humans, we tend to base our expectation of the future on the experience of our past. This works well in the short term, but runs into sampling issues on any timescale longer than a human lifetime. Despite our intuition to the contrary, if we project current ongoing processes into the future one conclusion becomes abundantly clear.
Our Planet is Doomed.
The good news is that this doom is far in the future, and humankind faces much more immediate existential threats; we’re much more likely to be wiped out by a giant meteor or complete ecological collapse or nuclear war or something we didn’t even recognise as a threat before our planet’s certain doom becomes an issue. But even if we manage to overcome the ways our species is most likely to meet our demise, e.g., we regulate atmospheric carbon, diversify our food supply, fight off the alien invaders, pacify the robot uprisings, etc., the natural processes that make our planet habitable today will someday lead to our demise. Here’s how.
Our star, the Sun, is the single greatest factor in our planet’s habitability. The intensity of light we receive limits the ranges of temperatures that are possible on the surface. The spectrum of light we receive – in concert with Earth’s atmosphere and magnetic field – determines the background levels of ionising radiation we experience. As long as we are dependent on planet Earth for our habitat, we are at the complete mercy of the Sun. So long as its luminosity remains more or less steady, we can survive quite happily here.
Unfortunately, the sun is getting brighter and hotter, and has been doing so, steadily, for nearly five billion years. As the sun fuses its supply of hydrogen into helium, the core slowly contracts. Reaction rates increase and the sun glows that much hotter. This change is slow, at a rate of about 6-10% per billion years, but that 6-10% is enough to render life difficult, if not impossible, to sustain on the surface. Whether our oceans will boil away in 700 million years or 1.5 billion years is a matter of scientific debate, but it’s safe to say that we will probably not want to be here when that happens.
So we need to leave. No big deal, right? We’re humans! We sent twelve guys to the moon! We split the atom! We sequenced our genome and built the internet! We can do anything… right?
Space Travel is Hard
Make no mistake: the colonization of space, the development, construction, and settlement of permanent habitats outside of Earth will be the single greatest undertaking humans will have ever accomplished. The challenges are extraordinary. To visit space, humans need air, water, food, toilet facilities, power, propulsion, guidance, and communications. To stay there takes quite a bit more.
Any potential space colony needs to provide all the things Earth does for us that we take for granted. Earth provides the precise mixture of gases we and all our food require at the pressure required to keep those gases from boiling out of our tissues. It provides a substrate (soil) upon which we can grow the aforementioned food and reclaim nutrients from our solid waste. Its surface is replete with water, that universal solvent so key in our biology, and also provides all-natural urine distillation through a solar-powered water cycle. Harmful ionizing radiation is screened by a layer of ozone in the upper atmosphere and high energy solar ions are deflected by a powerful magnetic field. We have plenty of other humans to interact with when we desire company and plenty of acreage when we don’t.
We have learned to recycle air and water on the International Space Station (solid waste is jettisoned). Hydroponics make it possible to grow food without soil, but we haven’t yet figured out how to garden at any variety or scale (we’re working on it). Materials that make effective radiation shielding tend to be dense, which makes their application in spacecraft awkward and costly, but not impossible. Current astronauts undergo extreme psychological testing and monitoring to limit the inevitable interpersonal friction that comes from confining people in such close proximity. Many of these problems are simply ones of engineering; we only have to do the work to translate theoretical possibilities into working prototypes.
Gravity is not one of those problems. More or less, every difficulty of living in space boils down to the fact that we and all of the things we eat have evolved to be uniformly accelerated at 9.81 meters per second squared toward some surface. Without that constant pull, our bodies begin to deteriorate. Calcium gets extracted from our bones only to be deposited in our urinary tracts in the form of kidney stones. Our muscles weaken, including the heart. Upon their return to Earth, astronauts require weeks of therapy to retrain their muscles to walk. Blood and other fluids accumulate in the head and blur our vision, maybe permanently, Astronauts, who are typically selected for their excellent vision, often need glasses on their return due to deformation of the eye and optic nerve. Microgravity, in short, is bad for our health. That’s only in the short term.
To create permanent settlements in space, we will eventually need to be able to create new humans there. Mechanical and privacy issues aside, there is some question about whether or not procreation is possible in microgravity. Very little is still known about the effect microgravity has on developing embryos and fetuses, and early childhood development is likely to be severely altered. What new complications and/or risks might expectant mothers face? We simply don’t yet know.
Long term human habitats need gravity, or at least the simulation of it. Rotating spacecraft and space stations can accomplish this, but such structures will need to be large to avoid a carnival ride feel for those on board. The material requirements are enormous, with tensile strength of the kind needed for large suspension bridges. It’s not like we have any foundries or mines in low earth orbit; we’ll have to launch these materials from the surface and assemble them once they reach their destination. We’ll need either the GDP of a small European nation or some extreme advances in materials science to construct one of these.
Assume that we overcome all of these obstacles, and that humanity now has a great diaspora to the outer solar system in a fleet of rotating spacecraft. Some might make for gas giant systems like Jupiter or Saturn and extract resources from their moons. Some might mine the asteroid belt for heavy metals and fissile material. Some might attempt to reform the surface of Mars to something more hospitable. Most other planetary bodies in the solar system have lower surface gravity than Earth. Once there’s no Earth worth returning to, there might be incentive to acclimatise from earth-gravity to low-gravity environments. A craft with a lower rotation rate can be built from lighter materials; a body straining less against gravity consumes fewer calories. Different environments might incentivise this change differently. Depending on how isolated these communities are, over generations we might see humanity speciate along planetary lines. Perhaps at some point a Jovian and a Martian might be two humans too genetically different to produce viable offspring.
The Solar System is Doomed, Too
Some four billion years after Earth’s oceans are gone, the much-brighter sun will undergo another change. The hydrogen in its core will have all fused into heliumand the hydrogen-hydrogen nuclear reactions will, for a short time, cease. Nuclear reactions in the core and the heat generated by them are what holds up the rest of the star against gravitational collapse. Without them, the envelope starts to collapse and compresses the now-helium core until electron degeneracy pressure stops it. The region just outside the core piles up enough to start more hydrogen fusion in a thin shell around the degenerate core. The temperature outside the core spikes, and all that excess energy causes the envelope to expand and cool. The sun gets a lot bigger at this point, expanding into a Red Giant. It will engulf Mercury and Venus, and will quite possibly also gobble up the Earth. This will remain a stable configuration for another billion years or two.
Humanity, if we’re still around, will have some interesting changes to deal with. The habitable zone (the range of orbits that can contain liquid water at atmospheric pressure) will expand. Mars and the asteroid belt may need to be abandoned in favour of the now warmer and brighter Neptunian and Uranian systems, although these may still be too warm for liquid water. Whatever ring systems Jupiter and Saturn may possess at this time will sublimate. Europa and Enceladus will lose their oceans. Humanity will be forced out to the newly-thawed Kuiper belt. Some people, if they didn’t with the last exodus, may seek an interstellar journey and set out on a generational trek to the stars; this would probably be a good time to do so, as the situation in the solar system is more or less downhill from here. Some may find a new home on Eris, Pluto, or others, although these frozen worlds will themselves see their icy surfaces melt and boil away.
Meanwhile, in the hydrogen burning region around the core, the helium produced sinks down into the degenerate core, adding heat and density. At a critical point known as the helium flash, the core burns a lot of helium very quickly and begins to expand again. What happens next can be complicated in a low mass star like our sun, with a lot of complex dredging up and pulsating, but during these end-stages, the core will begin to eject the envelope in waves, very hot but very diffuse. The stellar material will ionise in the bright UV light from the core as it’s ejected and shimmer in vibrant colours. Eventually, the core will be exposed, a white dwarf slowly fusing helium into carbon and oxygen, surrounded by a beautiful planetary nebula. This remnant will slowly cool over billions more years.
Let’s assume this gorgeous show was seen by humans inhabiting other star systems, who can point out this nebula to their children and call it home. Just how this happens is a little difficult to say, given that it involves predicting technology seven billion years distant from our own, but there are a few things one could say about such societies and how they live. There are many factors to consider, not least of which is the question of what kind of propulsion is available to them.
Ion thrusters are the most efficient engines known to current science, but are extremely slow and low-power. Colonising space with these things will be a generational endeavour. Each ship/fleet would need to be a self contained society and ecosystem, isolated from the rest of humanity for tens of thousands of years. Population would need to be strictly controlled and even the dead would need to be recycled to fertilise crops. Tens of thousands of years of stasis by necessity may have consequences. They may become xenophobic or doctrinaire, they may suffer coups or pogroms. How will generations of humans handle a self enforced genetic bottleneck? When they arrive at a habitable exoplanet, would they want to disembark?
Let’s say that somehow future humans have found a way to gain linear momentum without shedding a significant amount of mass. This is pretty fantastical in and of itself, but as Clarke said, “any sufficiently advanced technology is indistinguishable from magic.” Coupled with a power source just as advanced – perhaps through scooping up interstellar antimatter and annihilating it with on-board matter – these humans could do away with rotating ships entirely in favor of engines that accelerate at a constant rate, perhaps the rate of Earth’s gravitational acceleration: one g. If these people could keep these magic engines running for years, then they could hop stars in a matter of years or decades instead of tens or even hundreds of millennia. With a one g engine, a trip of one hundred light years would only take ten years for those on board, but over a century would pass at their destination due to relativistic effects. The longer the trip, the more extreme the time dilation effect. This could get confusing when two groups who took different paths meet, as their subjective travel time could be centuries apart. Within eighty thousand earth years, the far end of the galaxy could be reached by a twenty year old ship. There are a few more “magical” technologies that such ships would require, like a way to deflect interstellar dust while travelling at relativistic speeds, but humans have risen to challenges that seemed insurmountable to prior generations before.
There’s also the very real possibility that we get to other star systems to find them already occupied. Any threats from this possibility should be considered non-existential, because once we’ve spread ourselves among the stars, it would take something extraordinary to wipe every one of our colonies out. A superplague that kills an entire colony fleet or a violent interstellar alien empire is just not that destructive in the scale of a galactic diaspora. Just like the nuclear weapons, robot uprisings and asteroid impacts, humanity will have to navigate this hazard, too.
There is one other hazard, however, that is certain. On the timescales we’ll be referring to, our possible descendants may no longer be “human” the way we consider ourselves to be. Future beings are likely to be as different from each other as they are to us. Humans are biological beings with self-replicating DNA and sexual reproduction; if you take populations of such beings and isolate them for generations through population bottlenecks, those populations will evolve and speciate. Even if humans maintain intermarriage with those in neighboring star systems, humanity will still change and evolve. Humans of this era will gaze upon artifacts of our era with wonder that they could have descended from such strange creatures as us.
Once we’ve settled across the galaxy, things will be pretty stable for a very long time. Stars will die, others will replace them. Every few billion years our galaxy might merge with another in our local group in a slow, dazzling display followed by a wave of star formation. The galaxies outside our local group will, over the course of hundreds of billions of years, slowly redshift out of our seeing. Eventually, trillions or even tens of trillions of years from now, star formation will end. If human beings have lasted this long, then we should consider ourselves lucky.
The supergalaxy formed from the mergers of all of the galaxies in our local group will slowly fade, star by star. The largest stars will go first, within a few million years. The Dwarf stars will be the last to go, many older than humanity itself. The last star from the stelliferous era will die within 10 trillion years after star formation ends, leaving a cold and dark universe punctuated by the occasional merger of brown dwarves forming a star for a few million years.
Over the vast stretches of time ahead, orbiting systems at all scales will decay; moons will either spiral into planets or gain escape velocity, then planets will do the same around stellar remnants, then stellar remnants around the galactic center. Eventually, nothing but black holes, ejected planets and stellar corpses, and radiation will remain. On timescales of 10100 years, even black holes will evaporate through Hawking Radiation.
If humans are still around at this point, they should probably throw in the towel. The universe will be uninhabitable, devoid of energy sources. Our energy reserves would inevitably deplete, and humans would die. As far as current science is concerned, hope is likely depleted by this time, as well. Of course, our current picture of the universe may be incomplete. Over trillions of years of technological advancement, we may find places outside our known universe and discover how to access them. Perhaps we may learn how to create universes from scratch. Whatever we may discover, the known universe has a finite lifetime.