Why the Relativistic Projectile Will Always get Through

Of all the terrifying (and yet still realistic) weapons dreamt up in science fiction, perhaps none are more devastating than the concept of relativistic bombardment. Einstein taught us that the closer you accelerate an object to light speed, the greater its resultant mass and the subsequent kinetic energy. As you approach light speed, this figure approaches infinity.

By way of an example, the Chixulub impactor (believed to have wiped out the dinosaurs) was an asteroid with an estimated diameter of about 10km. This Mount Everest-sized object impacted with the force of an estimated one hundred trillion tons of TNT—twenty thousand times the global nuclear arsenal at its peak.

Meanwhile, fully fuelled and ready to launch, a Saturn V rocket weighs about 2,800 tonnes. Accelerate that mass to 99% of light speed and you’ve already exceeded the energy of the Chixulub impactor. Fly it into some unsuspecting planet and you’ll trigger a mass extinction.

The Dinosaur Killer Will Always Get Through

Science fiction authors have been aware of the potential of relativistic weapons for some time now, as novels like The Killing Star (1995) demonstrate. Most SF universes tend to ignore them, perhaps because they introduce faster-than-light travel as a concept, which would largely negate their influence. Assuming the light barrier can never be broken however, the implications are startling.

We could quote British Prime Minister Stanley Baldwin, who declared in 1932 that ‘the bomber will always get through’ and how warfare had changed forever. By the time of the Cold War, this had evolved into the idea of Mutually Assured Destruction. Just as this spectre of aerial bombardment or nuclear war has dominated politics for the past century, the threat of relativistic weapons will likely dominate interstellar politics into the far-future. We can outline several points.

A state of mutually assured destruction will become the norm in interstellar politics

Mutually assured destruction has always been seen as something of an aberration, like an immature phase humanity is going through. As the energies at our command grow exponentially however, it seems less and less likely to go away. Robert A. Heinlein’s The Moon is a Harsh Mistress (1966) explored how a small group of lunar colonists, controlling a single mass driver, could hold the entire Earth to ransom. For a real-life example, albeit a little closer down to Earth, look no further than Al Qaeda’s commandeering of a handful of civilian airliners to slaughter thousands of people on 9/11.

If we ever sent colonists to Alpha Centauri, we better be damned sure they retain a fondness for their home planet. A single relativistic spacecraft, either gone astray or (far more likely) deliberately guided, would cause mass death and destruction upon impacting a planet. Even more mundane methods could be just as devastating. There’s no physical barrier between neighbouring star systems other than sheer distance. Attach a few rockets to an asteroid in the Alpha Centauri system, direct it towards our own, and it’s going to be entering our neighbourhood to perform its Deep Impact impression in a few thousand years’ time.

No matter how friendly you are with your interstellar neighbours, the simple fact remains that you will be able to wipe each other off the galactic map with little effort. Maintaining bonds of trust will be paramount, but several factors may work against this.

A lack of faster-than-light (FTL) communications will lead to a rapid decentralisation of humanity, with greater potential for a breakdown of trust

Our nearest star system, Alpha Centauri, is just over 4.2 light years away. If a colony had existed there at the time of Barack Obama’s election in 2008, and any American citizens present had wished to vote, we would only just now be receiving their ballots back (you have to wonder if future controversies will erupt over votes arriving late due to the constraints of the light barrier). Is there any way around the conclusion that by migrating to another star system, you resign your right to vote in local elections, if you can even retain your citizenship at all?

Even within our own solar system, it is hard to see the governments of Earth enforcing their rule over colonies established on or around other planets in the long run. Many stories have already been written about the inhabitants of Mars or the Moon fighting off their Earthly overlords, George Washington-style.

Without some form of FTL communications, it is almost inconceivable that true interstellar governments can exist at all. They must function as largely autonomous entities, though perhaps sharing standardised legal and communications systems, akin to the Universal Declaration of Human Rights or how ‘aviation English’ has become the standard language in international air travel.

During the Cuban Missile crisis, it took up to 12 hours for messages to be sent back and forth between Kennedy and Khrushchev, dangerously contributing to the situation. Although it never quite took the form of a big red telephone, the Washington-Moscow hotline was established in the aftermath of the crisis to prevent similar escalations in future. If a few hours delay was almost enough to escalate tensions to an all-out nuclear holocaust, imagine a delay of years or even decades?

At one point or another, a first strike by a single party may ignite a firestorm of unprecedented destruction

Let us imagine centuries or even millennia ahead. Humanity is starting to spread across the Orion Arm, launching countless expeditions to settle different star systems. While they remain in contact with each other, perhaps even nominal members of some sort of interstellar federation, the different systems are largely autonomous and unsupervised by any higher authority.

Somewhere out in deep space, someone hatches a plot. Driven either by madness or malice, they decide to initiate a war with another system. Building their war machine is child’s play—their local sun produces something like 10^26 watts of energy, easily harvestable with orbiting solar panels. Its retinue of planets, moons and asteroids together contains billions of cubic kilometres of useable mass. With advanced computers and machines, little labour is required. Perhaps even a single unconstrained individual could assemble a greater war machine than anything dreamt of by the old military powers of Earth.

After a few years of secret preparations, this armada is unleashed upon the galaxy. Whether it is directed against one world or a hundred, the results are the same. There is little to no warning as relativistic kill vehicles scream through the sky. There is no time to mount a defence or an evacuation, millions die. It’s a galactic 9/11.

The survivors may well rally. They may attempt to trace back the route of their attackers and hunt these relativistic terrorists down, but by the time they reach the target system, decades later, the killers have had ample time to flee into the depths of space, or may simply have aged and died altogether. Even if apprehended, the precedent cannot be ignored.

Bonds of interstellar trust are severely weakened. Say your nearest neighbour is twenty light years away, how can you know that, at no point in the next forty years, they won’t be overtaken by a similar act of insanity? Or simply evolve into a hostile regime? How long has any peace lasted here on Earth? There are currently believed to be nine powers on Earth possessing nuclear weapons. Nine parties with knives pulled at each other’s throats. Imagine if it was nine hundred? Or nine thousand?

Faced with such a possibility, the brutal logic of game theory may play out. The only way to guarantee your neighbours won’t destroy you is to strike them first. A single act of madness could spark a very logical follow-up holocaust. Planet France blows up planet Germany before the reverse becomes true, or whatever the rival factions of the future may be. The sheer scale of this disastrous scenario beggars belief, but there does seem to be a sort of brutal logic behind it.

The survivors of this conflict may adopt several strategies to prevent its repeat occurrence

1. become so powerful and indestructible that even relativistic weapons don’t bother you
2. become extremely mobile, thus minimising their threat
3. a civilization-wide retreat into cyberspace
4. very intense levels of surveillance

How’s this for a scenario? In the far-future, galactic politics revolves around humanity having basically split into several different offshoots in the aftermath of such a devastating conflict. Some build and retreat into enormous Dyson Spheres (or ‘Stellar Spheres’) which are heavily guarded and all but impervious to attack. Others live a nomadic existence, perhaps out on the galactic rim, inhabiting vast fleets of spaceships that are constantly migrating from star to star, never lingering in one place too long for fear of attack.

Yet others may decide to abandon the squishy biological bodies we currently inhabit. Given the rate at which computers are advancing, a mainframe the size of a small building may soon have more computing power than every human brain on Earth. Possibly, you could scan and upload the consciousness of every person alive and reawaken them there in a virtual reality scenario. Billions of us could ‘live’ in an area no larger than a city block. Like the TARDIS, it would be something of a ‘bigger on the inside’ situation. By choosing this option, the need for planets or other large structures disappears entirely.

For obvious reasons, these different offshoots of humanity don’t necessarily see eye to eye. Meanwhile the old middle ground—that of living on planets or other large, stationary structures—has become anathema to any advanced society. The risk of a relativistic attack is simply too great. Planets lie abandoned, at best left as nature preserves.

By way of analogy, the Stellar Spheres are the equivalent of medieval castles while nomadic fleets prowl the space lanes like Mongol hordes. The virtual societies have retreated from the wider galactic community almost entirely, like a monastery hidden up in the mountains.

Among all these groups however, intense levels of surveillance pervade. Perhaps the only thing that can destroy a Stellar Sphere, for instance, is the concentrated energy of another Sphere (weaponised into a giant mirror of sorts). This could make their construction and management a central galactic issue, akin to nuclear weapons proliferation today. While some humans remain in this vision of the far-future, inter-Sphere diplomacy is largely in the hands of intelligences far beyond ours. Humans are far too unstable to trust with their finger on such powerful triggers. Hundreds or thousands of years from now those physical humans that do remain, even if they’re flying around on starships and look very fancy by today’s standards, may well be the interstellar equivalent of the Amish. The very fact that they still eat and sleep marks them out as primitives.

The option of simply uploading your whole civilization into cyberspace has a particular appeal. By retreating into cyberspace, not only is your society much better concealed and less exposed to the threat of relativistic bombardment, your quality of life will, if anything, be vastly higher. Wasteful processes like photosynthesis, farming and digestion would be dispensed with. There is practically no limit to the population size you can support.

At the very least, this could serve as a useful method of evacuation. Perhaps the standard procedure, if a populated world is endangered, is to ‘uplift’ its people in this manner en masse as a last resort. If your technology is advanced enough, you could save billions of people this way in just a few days. Their world may die, and their bodies with it, but their minds will survive and can be stored on waiting mainframes. The philosophers will have to debate whether this would be ‘saving’ a world or merely resurrecting it, but it certainly sounds better than nothing.

In 2016 we have far more pressing issues to deal with of course, but if Elon Musk has his way, we’ll have self-sustaining cities on Mars by 2050 or so. Within another century, we could be launching interstellar missions. Well before then, we are going to have to consider these quandaries. How are we going to maintain interstellar peace in the long run, given that the circumstances seem utterly poised against it?

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