“Target acquired! Fire!” Poof the enemy ship disappears in a puff of vapor.
It’s a beloved trope but does anyone really worry about the whether a laser weapon can deliver the necessary energy to the target to destroy it? For most readers, the answer is no but as with other world-building aspects, this overlooked area of weapon design can be fertile grounds for your science fiction stories. The world of “energy density” and Petawatts (that’s 1018 watts, or about 100 trillion times more power than your average CFL light bulb might use these days) offers all sorts of interesting images, techno-babble that actually makes sense, maybe even premise and plot hooks.
For starters, while readers don’t usually worry about how to pack enough energy into a tiny space to blow something up, scientists and engineers do. For a naval laser gun, for instance, there’s no point in creating a Terawatt laser if the beam is going to create a plasma in the air that disperses the energy before it reaches the target. It would make for a pretty show and probably nice snaps and booms as well but not do much on target. Similarly, engineers have known for decades that laser confinement fusion systems can’t just pipe the laser beams around in the same short pulse and narrow beam that you intend to hit the fusion fuel pellet with: if you do, your optics are promptly going to fry, not mention the energy loss, possibly enough to blow something up, due to heating residual gas in the vacuum lines.
Engineers building real energy weapon systems or laser fusion experiments know that they have to pay attention to energy density. A case in point is Lawrence Livermore National Lab’s (LLNL) Petawatt Laser. Sounds cool, doesn’t it? Well, it is cool but the name itself is a bit of a pitfall to the unwary. This is because energy and power are not the same thing. The laser doesn’t deliver 1018 Joules of energy, I don’t think there is that much energy on the planet. It delivers that fair amount of energy for a small fraction of a second.
For instance, let’s say I fire a petawatt laser at your head. This sounds bad, but is it? Well, it probably is bad but it depends on how long I fire that laser. If it is only for an attosecond, it will impart a whopping one Joule of energy, which really isn’t much. This is because a Petawatt is 1018 Joules per second (a Watt is one joule per second) and an attosecond is 10-18 seconds, which isn’t very much time at all. Put another way, energy is power applied over time. Now, a petawatt laser is pumping out a lot of energy per unit time, but you still need to give some time to deliver the energy and an attosecond isn’t enough to deliver much. Now fire it for a whole picosecond, or 10-12 seconds, which is a million times longer and now there is plenty of time to vaporize your head. For reference, light will travel 0.3 mm in a picosecond in vacuum so that is still a tiny, tiny pulse. You could get the same effect firing a Megawatt laser for a second.
The point is that what blows things up is energy, a measure of the amount of work that can be delivered, not power, which is energy (or work) over time. You dump a lot of energy into a target and it turns to a gas (i.e., it explodes). How you get that energy there, that matters. If you take too long to do it, the energy will radiate away before it can cause the sudden vaporization of the target. Fire your petawatt laser for too short a time and you don’t deliver enough energy. But deliver the energy in too dense a form before it hits the target and you will damage or destroy your energy weapon or dissipate the energy in the media between you and the target (this media is air on the surface of the earth but could be intra-solar space, which is a near-vacuum but not a perfect vacuum.)
I was lucky enough to see the Nova inertial confinement system at LLNL in 1985. First thing that struck me was the size of the tubes carrying the light beams: they were about a meter across. Why? This is so that the energy density was not so high as to cause problems in the tubes or optics. Of course, the fusion target was not a meter across; the engineers focused it on the target but only at the very end.
With a little bit of thought, that made perfect sense but what was surprising to me was the annular ring, about 20 cm across, etched into the one of the one meter lens. This seemed liked a bad thing to do, both because it reduced the light transmission and because it might heat up and break the lens. Turns out, just the light passing through the lens could cause vibration. If they didn’t change the resonant frequency with the sand blasted ring, the lens could shatter, not from heating but from vibrations. (Remember that while it doesn’t exert much force, light does exert some pressure- enough to alter an asteroid’s course over long periods of time, also enough to keep stars from collapsing under their own gravity, at least until they exhaust their fuel.) In the case of the Petawatt laser, the team not only spread the energy out in space they also spread the pulse out in time so that power densities were lower for most of the journey through the laser, being focused in time and space at the very end for a sharp pulse.
So, where does this leave us as science fiction writers? As always, with any well used trope, it can be ignored. Readers are so familiar with energy weapons, they don’t really worry about energy density much. While they might balk if you power your super-laser-destructo-beam with a wire the size of a strand of spaghetti they probably won’t care if the laser beams isn’t piped around in one meter tubes. So, easy enough to ignore. But as with Heat Dissipation on a planet-wide city or Momentum Compensation in teleporters, if you do choose to think some of these issues through, you can create both great verisimilitude and possibly a useful premise or turning point.
First off, on the verisimilitude front, while today’s reader may not worry all that much about the physics of delivering energy on target, energy weapons are fast becoming a reality and in the next few decades your readers will probably learn more about it, enough to maybe make a story that doesn’t deal with it seem dated. But that’s the future and we all want to get published now, so not a big deal.
Instead, let’s turn to the Star Wars technical drawings. While these are lots of fun to look at, they are not usually a great repository of engineering reality. Star Wars is all about things looking cool, not about engineering. Why do we have 1600 meter long star destroyers? Because 1600 meters is a mile and Lucas thought “mile long spaceships” would look cool. (And he is absolutely correct about that, too.) But in the cut-aways for the star destroyers, you see massive conduits from the power plants to the weapons systems. Exactly what these conduits are isn’t specified but the fact that they are big is perfectly appropriate. This is actually one of the more reasonable aspects of Star Wars engineering: big guns take lots of energy which cannot be delivered through straws.
Aside from just looking cool in a drawing, this means that much of the ship is devoted to moving power around the ship. For starters, this changes what your characters see when they walk around your spaceships. Much as we notice (and maybe complain) about the hump on the floor in a car for the drive shaft in a rear-wheel drive car, your starship crew might complain about corridor detours necessitated by a straight-shot power conduit from the power plant to the weapon bays. Not a huge point but a nice detail to have when describing life on your imperial dreadnought.
There is so much more that a writer can do with this, of course. In battle, it is certainly bad to lose your power plant or your weapon bay. But think of the energy in the power plant necessary to power the weapons- if the power plant can provide the energy to a weapon that can vaporize a ship, the sudden release of that energy might vaporize the power plant and all around it. Here, though, we get back to power and energy. Maybe the weapon fires only every 30 seconds so that you can accumulate enough energy in local capacitors to fire the weapon. That energy still exists in a dangerous form in the capacitors but at least those are spread around the ship. Now your power plant need only provide the energy at a much lower power level since you let it build up at the weapon. If the power plant blows up the momentary release of energy is a lot less than the energy in the weapon beams.
Of course, when a weapon’s bay is hit, all that stored energy is released, making for a big, local explosion. And then, what happens to internal hits on your spaceship? If it puts a hole in a bulkhead, that might be unfortunate for your crew but not too dangerous for the ship. If it puts a hole in a power conduit, that might be a lot more trouble for the ship.
And think of the possibilities for sabotage: every place there is high energy density, a saboteur could try to wreck the ship: not only must the crew protect the power plant, they also must protect conduits and capacitor banks. If your conduits are beams of light, knock one out of true and you can vaporize a part of the ship. Or what if he shorts out the capacitors?
Think through these things and now your space ships aren’t just cool shapes, they have important, setting-pretty & plot-usable stuff on the inside that can make your stories sing.
One thing that got me thinking about this was the Death Star. It’s 160km across because… you guessed it, a 100 mile across space station just sounds cool. But why would it really need to be so big? Turns out, there are good energy density reasons for it to be sizable. You have to apply a lot of energy to a planet to make it explode and you probably want to spread out the energy generation and distribution systems across something as big as a 100 mile across space station and gather it all up outside the station, as they did beyond the dish. So, not terribly implausible that the station is huge.
Of course, it does beg the question where does that vast amount of energy come from? I believe the canon answer is anti-matter: fair enough, there’s a lot of energy in matter (mc2, in fact). I’m not sure how much antimatter it would take to power a weapon to destroy a planet but compared to the mass of the Death Star, a fairly insignificant amount, I’m sure. But what if you simply dropped that amount of anti-matter on the surface of the planet? That’s simple: same amount of energy, same effect. Perhaps it would be a little less efficient detonating on the surface so maybe the same amount of anti-matter would just obliterate half the planet. The planet would still be pretty much destroyed. So, why build a monster space station? Why not just build an anti-matter bomber? It might still be pretty big but probably need not be as big as a Death Star. Then again, a Death Star simply speaks to the inner geek in everyone.
Okay this isn’t a death star but I was actually in an auditorium at Goddard Space Flight Center when Voyager pictures were coming in live of this moon. All my fellow high school students immediately exclaimed, “It’s the Death star”! Maybe this is what the Death Star would look like after a big battle or 100 million years in space. (Probably not, actually, but interesting thought. What would it look like?)