Could aliens destroy us from light years away? Mh, another day at the Kurzgesagt Labs, where we answer the most important questions with science. Today: how might civilizations wage war across light years? What kind of devastating weapons could they use, and what would they look like? Meet our two players. A yellow dwarf star system home to a species of primates. "Humans," as they call themselves, recently became a technological civilization. They have rockets, nuclear reactors and memes. How cute! The Smorpians disagree. They reside on a planet around the orange dwarf star HD 40307, 42 light years away. Smorpian civilization developed earlier than humans and they have much better technology. They've recently built a Dyson swarm around their star which gives them near limitless energy. And they noticed humanity, which is unfortunate as the Smorpians are planning a hyperspace bypass through our solar system, so they decided that humanity has to go. Interstellar war is hard though. Front lines, tactics, and logistics are meaningless at these scales. It's also fought across time. Decades will pass between firing a weapon and learning whether it hit or not. Sending an invasion fleet is futile. Even if the Smorpians travel in a large fraction of the speed of light, the journey to Earth would take decades or even centuries, and humans would have plenty of time to prepare. If you want to learn more about the mind numbing problem of war between alien civilizations, we made a video about it. Today we'll help the Smorpians construct a weapon that is not only extremely long range and as fast as physically possible, but that will totally destroy everything on Earth, so no human survivors will come to an act of vengeance on swap in the future. In interstellar war, you want to win with one shot. Our bird scientists have found three Smorpian designs: the Star Laser, the Relativistic Missile, and the Ultra-Relativistic Electron Beam. All based on real technologies that humans are using in some form already. Let's see how they work. [The Star Laser] As an advanced technological civilization, the Smorpians harness the energy of their star by surrounding it with billions of solar power satellites. This Dyson swarm collects 1% of the star's energy output, a million billion billion watts, 50 billion times more than all humanity generates. What if all the power of the Dyson swarm, all those satellites were used to create a star laser? Like any laser, the bigger it is, the longer its range. Human-built lasers use small mirrors to focus, so they have short ranges. The Smorpians could turn their entire Dyson swarm into a collective focusing element a million kilometers wide. The star laser has an insane range as a result, enough to focus on target Earth from a distance of over 2 million light years. OK, let's shoot it. Countless tiny beams combine into a single huge beam. Laser beams are normally invisible in space, but the star laser is so powerful that light scattering off bits of dust and gas in its path makes it clearly visible in the sky. A gigantic column of green light. The laser travels at the speed of light, which, oddly enough, is still pretty slow on a galactic level. It takes a whole day until the laser has left the Smorpian system, shooting into the emptiness between stars. It will travel for decades, occasionally melting the odd bit of interstellar dust or asteroid. 42 years after being fired, it arrives without warning. Humans only notice a weird green glow in the sky, and then they're gone. 1% of the energy of a star, concentrated into a beam the diameter of Earth, traveling 42 light years. It burns the exposed half of the planet with the intensity of 3 million Suns. The seas boil and evaporate, fires scour the land, and within minutes Earth's crust begins to melt into a sea of lava. As the planet rotates, it turns into a red hot hell with no trace of life. After a day, it's all over and the laser dies down. In another 42 years, the Smorpians will know if they've been successful. That's another thing about interstellar war. When you attack, your grandchildren will be the ones to find out if you won. It's like all the bombs from World War II exploding in the 80s and us only seeing the effect today. OK, the star laser's extreme range, speed of light attack, and ability to melt down any target make it a premier interstellar weapon. But is there something else? [The Relativistic Missile] What if instead of converting the energy of their Dyson swarm into a laser, the Smorpians used it to shoot a super bullet? A relativistic missile going as close to the speed of light as possible. This sort of weapon is at the limits of what the Smorpian technology can handle, as it requires loads of a highly dangerous material: antimatter. The evil twin of regular matter. Humans have only managed to produce a few nanograms of antimatter. With their unlimited energy, Smorpians can manufacture it at an industrial scale to build antimatter rockets. When antimatter and matter are mixed, they annihilate, which in more practical terms means there's a big big boom releasing gamma rays and plasma. The physics is complicated, but basically, if you have a really strong magnetic field you can deflect the plasma through a nozzle just like in the chemical rockets humans use. But it would be much, much faster. The fastest rocket possibly basically. Our relativistic missile is much bigger than a skyscraper. At the bottom is the bell shaped magnetic nozzle 100 meters wide. On top of it are 250 floors filled with antimatter and matter ready to annihilate each other. On the top floor is a 300 kilogram projectile looking quite small, about the size of a person. To stop them getting damaged on the way, the missiles have dozens of sacrificial layers that form a Whipple Shield. To make sure they do their job, the Smorpians build 1000 missiles. Let's fire them. Launching all the relativistic missiles is a spectacular event. For a moment, the antimatter engines lighting up outshine their star. Their exhaust is a long trail of brilliant white, and as they accelerate away, they appear redder and redder until they turn invisible. With the extreme amount of energy released by the matter-antimatter reactions, the missiles are accelerated to 99.9999996% of the speed of light. They have effectively infinite range, as there's nothing really to slow them down. They arrive shortly after you can see them. The light from their launch will take 42 years to reach Earth, so human astronomers might see the flash of the missile's launch, and then a few days later they'll hit. Not enough time to prepare. Each relativistic missile packs the kinetic energy of a dinosaur killer asteroid, so only one needs to hit. They never reach the ground, disintegrating instead at the edge of Earth's atmosphere. Intense blue flashes set everything on fire. Then continent-sized fireballs slam down on the surface to smash everything into dust, repeatedly, until nothing is left but rubble and smoke. So interstellar missiles with unlimited range, minimal warning, and delivering complete destruction of a planet's surface. Nice. But they are a hassle to build. Is there something else maybe? [The Ultra-Relativistic Electron Beam] Humans do funny things to their food to rid it of bacteria and make it safe to eat, like shooting electron beams at strawberries. Small particle accelerators send electrons into the food with an energy similar to the radiation from nuclear reactions. Not enough to burn the food, but deadly to bacteria. Smorpians had the same idea, but bigger. The main challenge with an electron beam is range. Electrons are negatively charged particles, so they don't want to stay near each other. A regular electron beam will quickly spread out, making it harmless. Smorpians needed to cover distances of dozens of light years, so they've used the rules of the universe to trick the electrons by building an Ultra-Relativistic Electron Beam, or UREB. What it does is accelerate the electrons to 99.99999… …99999… …99999… …99999… …9999998% of the speed of light. Phew! Faster than even the most powerful cosmic rays. The closer something travels to the speed of light, the slower time moves for it relative to the rest of the universe. And since these electrons are moving so incredibly fast, for every second of spreading their experience, over 5 million years pass in real time. A physics trick that lets the beam cross interstellar distances while remaining tightly focused on its target. The biggest particle accelerator on Earth is 27 kilometers long. The Smorpians need one that's over 100,000 kilometers long. A megastructure 8 times longer than Earth is wide. It's mostly a tube of magnets holding the beam together until the exit. Like a long trumpet of doom surrounded by an aura of deadly radiation. When it's fired, it produces a ruler straight lightning bolt pointed at Earth. Its effects on arrival are less visible than the other weapons. No flashes of light, no massive firestorms, no explosions. It doesn't destroy rocks, it destroys DNA. People get dizzy, then fall sick as their cells are pierced by radiation. You might think that a deep bunker could save a few humans, but no. The UREB is so penetrating that its effects accumulate to lethal doses even underground, over days or weeks. In the end, just like our strawberries, Earth becomes sterile. [Simulation results] Hm, another elaborate animated science explainer by Kurzgesagt where we've learned a lot, not sure exactly what. Luckily, the Smorpians don't really exist, but others… might. One major downside of all our weapons is that others around the Milky Way could see you firing them, which is not ideal because you don't want to present yourself as a dangerous species and tell everybody where exactly you are. So maybe instead of shouting or shooting out into the universe, the best course of action seems to be to stay relatively quiet for now, and observe. Maybe one day we'll witness distant stars shooting at each other and be glad we stayed out of it.