Ok folks, it’s time. We’ve all asked this question, but I’ve been putting off answering it because we all actually know the answer.
How do the Zerg fly in space?
There’s a certain point in sci-fi or fantasy where you have to just suspend disbelief and go along for the ride, and I think that the Zerg ability to travel through space is a good example of this. That said, I’d like to take a look at one of the most common explanations that people give to justify the mutalisk’s ability to flap its wings and propel itself through space. As you’ll see, it’s completely implausible.
Now, we know that flapping doesn’t do anything in a vacuum: there’s nothing to provide any resistance, so you can flap all you want and it won’t move you forward. But what if mutalisks used their dragon-like wings as solar sails, catching the photons from a nearby star to cruise through interplanetary space? That might not explain the flapping, but it could explain how they can move, so let’s take a closer look.
The idea behind solar sails is the conservation of momentum. Even though photons of sunlight have no mass, they do have momentum. High school physics tells us that momentum is conserved, so if you have a bunch of photons with momentum being absorbed by a solar sail (or a mutalisk’s leathery wing) then their momentum must be transferred to the thing they’re hitting, exerting a force on it and causing it to move through space. So, how large would a mutalisk’s wings have to be to let it accelerate at a reasonable speed? To figure this out, we need to do a back of the envelope calculation, making some assumptions about how big mutalisks are.
In general, the sizes of units in the game are not reliable: I prefer to consider the cinematics as the authoritative source. So let’s take a look at this cinematic showing Jim Raynor’s battlecruiser being attacked by a swarm of mutalisks.
At about six seconds, one of the mutalisks flies over the right-hand side of the battlecruiser and crosses near a long row of windows. Based on its size compared to the set of windows, I would say that the creatures have a wingspan of around 100 meters and that their tube-like body is about the same length and maybe 5 meters across. If mutalisks are like most earth life, then they are mostly water. to get a rough idea of their weight we can calculate their volume and then multiply by the density of water. A cylinder 5 meters by 100 meters has a volume of about 2000 cubic meters. That would correspond to a whopping 2,000,000 kg or 2000 metric tons!
Now, obviously that’s too big for something to fly in at atmosphere, let alone in space by flapping its wings. But lets be generous and say that maybe mutalisks are made of some very low-density material, and maybe I overestimated their size. What if they were closer to 20 tons? How much oomph would their wings give them if they used them as solar sails. Again I’ll be generous and treat the wings as a square of material 100 meters on a side.
The momentum of a photon is given by: Momentum = Energy/Speed of Light, so near a sun-like star, where most photons are in the visible range, they have an energy of 2-3 electron volts, yielding a momentum of 1.6×10^-27 kg m/s per photon. That’s not much, but a star puts out a lot of photons, so let’s see if that balances it out and gives us a decent thrust.
Let’s say our theoretical mutalisk is orbiting the sun at the same distance as Earth, 150,000,000,000 m from the sun. The sun puts out 3.8×10^26 watts or roughly 8×10^44 photons per second, but that power is spread out in all directions. To figure out how much hits our solar-sailing mutalisk, we have to imagine spreading that power out over a sphere the size of our mutalisk’s orbit with a surface area of 4*pi*R^2 where R is the radius of the orbit. That gives 2.25×10^20 photons per square meter per second.
With a wing area of 100 m x 100 m (10,000 square meters), our mutalisk would intercept around 10^24 photons per second, corresponding to a whopping force of 0.0004 newtons! That’s enough force to accelerate a 20 ton mutalisk up to about 14 miles per hour in a year.
Edit: An astute reader points out that the size of mutalisks is described in the Starcraft novels as being much smaller than I described. They apparently have a wingspan of 20 feet and are only 7 feet long and about a meter across. to me that seems shockingly small, especially compared to the cinematics, and it also seems quite stubby compared to all of the art depicting mutalisks as having a body that is a long tube. Still, we can scale the above results to fit these new dimensions. Given a cylinder 7 feet long and 3.3 feet across, and again assuming a density like water, I get a mass of 1.7 metric tons. If we treat the wings generously as a 20 foot square, then their surface area is 37 square meters, so the thrust on wings of that size as compared to our 10,000 square meter example above would be .0004 newtons x .0037 = 1.5×10^-6 newtons. That’s enough to accelerate our 1700 kg mutalisk all the way up to 0.06 miles per hour in a year! Even assuming a much lower density, it would only accelerate a 170 kg mutalisk up to 0.6 miles per hour in a year.
You might not have followed every step of that (admittedly very crude) calculation, but that final value should give you some idea of how ridiculous it is to say that a Mutalisk’s wings could work as solar sails. Even assuming very large wings and a small body, the acceleration that you get is miniscule. Plus, solar sails are really only good for accelerating away from the star, and Mutalisks are like fighter jets: they need to be able to dodge and weave in all directions very quickly.
Bottom line, I can’t explain how the mutalisks fly in space. Heck, I can’t explain how something that big flies in air! Solar sailing certainly doesn’t cut it, so we’re left where we began. It’s magic. This is a part of the Starcraft Universe that just doesn’t fit with the laws of physics in our own universe, and that’s fine. Mutalisks are still cool, and it’s not like I’m going to stop playing Starcraft because I don’t know how they can fly in space.
Stay tuned: next I’ll take a look at all the non-winged Zerg fliers!
The other day I was playing Starcraft 2 and trying to think of the next topic to tackle here. As I watched a swarm of zerg roaches attack a group of terran marines, I realized that I needed to talk about acid. Two of the new zerg units, the baneling and the roach have acid-based attacks. The baneling is a little suicide bomber that detonates, splashing corrosive acid all over nearby units, while the roach is a nasty armored insect that spews a stream of acid on its foes from a distance.
I like both of these units in the game, but they perpetuate a myth about acid that has infested pop culture for years: it is bright neon green. I’m not sure where this idea originated. Maybe it goes back to World War I, when chlorine gas was used as a chemical weapon. It’s not an acid, but it is a nasty caustic chemical that is a sickly green color. Or maybe the green acid idea can be traced back to the movie Alien, where the alien is revealed to have highly acidic blood that looks greenish yellow.
Whatever the origin of the idea, it’s patently false. Most acids are colorless in both their pure form and in solution. Sulfuric acid? Colorless. Hydrochloric acid? Wikipedia says that it is colorless to light-yellow, but I’ve never seen an example that showed any color. Hydrofluoric acid? Colorless.
I don’t know of any acid that is brightly colored, though I admittedly not a chemist. There are probably some nice, colorful organic acids, and it’s certainly possible for acid to be mixed with other colored stuff in a solution, but in the real world acid does not conveniently advertise its corrosiveness by being bright green.
While we’re on the subject, let’s talk about the plausibility of an insect spraying acid, whatever the color may be. This is actually plausible to me. There are plenty of examples of insects that are armed with chemical-based weapons. One of the most famous is the bombardier beetle, which defends itself by blasting a nasty, boiling hot liquid at its attackers. Bee venom contains formic acid, and many ants inject or spray formic acid or other chemicals when they bite.
If we’re willing to suspend disbelief enough to allow the zerg to exist in the first place, I’m also willing to grant that they might use caustic chemicals to attack, since real-world insects do this too.
Well folks, I finally beat the campaign in Starcraft 2 last night, so I finally feel comfortable reviewing the game.
In a lot of ways, Starcraft 2 is very similar to its predecessor, and I think on the whole that’s a good thing. With the original Starcraft, Blizzard hit on a formula for a compelling game that just worked, so they didn’t need to fix it. With Starcraft 2, they tweaked it instead. There are a lot of familiar units in the new version of the game, but there are also a lot of new ones, as well as new abilities for familiar units that make things interesting. There are also a lot of tiny, careful changes to the gameplay that make everything flow more smoothly.
For me, the campaign was always the highlight of the original Starcraft and I’m happy to report that it is excellent in Starcraft 2 as well. The story focuses on Jim Raynor who has had a make-over and is now a rugged, scarred mercenary. Instead of a linear storyline with little player choice like in the original game, Starcraft 2′s missions allow a little more flexibility: at any given time there are several missions to choose from. Between missions, you can interact with other characters on your ship. Head to the cantina and share a drink with Tychus Findlay, a rough-and-tumble ex-convict marine. Or go to the lab and see what new technologies Egon Stetman, the stereotypical squeaky-voiced, lab-coat-wearing, socially-awkward scientist has cooked up for you. Or check out the armory and ask Rory Swann – the mechanic who sounds like he belongs on Car Talk and looks like a Warcraft dwarf – to upgrade your mechanical units. Other colorful plot-related characters include the awesomely stereotyped Tosh – a rastafarian rogue ghost operative, complete with voodoo charms and a “ya mon” accent – and the idealistic young ship’s captain Matt Horner, whose clean-cut look and fancy uniform were carefully designed to contrast with Raynor’s bad-boy tattooed arms and sweat-stained t-shirt.
This interesting bunch of characters send you on missions following several parallel plotlines that gradually intertwine to lead to the requisite huge battle at the end of the campaign. Each thread of missions is based on that character’s personal interests, and things get interesting when some of the characters disagree on what’s the best next step. All of these interactions are done really well using rendered-on-the-fly cutscenes. Some of these cutscenes are cinematics in their own right (at one point there’s a bar fight which was very well done) but there are also a few of Blizzard’s trademark spectacular pre-rendered cinematics for key plot points.
I really enjoyed the interplay between the missions and the “down time” on the ship: each mission gives you a new type of unit, some “research points” and some cash. The new unit is then available in all future missions and the research points and cash can be used to upgrade your units and hire mercenaries to help you. This adds another layer of customization to the game that worked really well.
I also loved the variety of the actual campaign missions themselves. There are actually very few missions where you just build an army and destroy the enemy. There’s always something more going on. In one level, lava floods the low ground every few minutes. In another level, hordes of zombies attack you at night, and you have to hold the line until daybreak, when you can take the fight to them. The “hero” levels, where you just get a handful of special units to achieve your obectives, are really excellent too.
The story itself, like the story for the original Starcraft, is passable but not great. The problem with video game stories in general is that they exist primarily to get you to the next mission, so they are often not as developed as one might hope. Compared to most video games, Starcraft 2 has a very good story. Compared to most novels, it’s pretty weak. The writing is mediocre at times as well, with plenty of cliched lines and heavy-handed character development. But it gets the job done, and the fact that there are actual characters, and that they do develop a bit, is actually a great thing to see in a game!
A final aspect that I want to mention is the overall “feel” of the campaign. There was a certain Firefly-like “space western” vibe going on that I really enjoyed. This shows in everything from the (very good) in-game music, to the characters of Jim Raynor and Tychus Findlay (roughly analogous to Malcom Reynolds and Jayne Cobb in Firefly) to the jukebox on the ship that plays down-home favorites like “Sweet Home Alabama” and “A Zerg, A Shotgun and You“.
Bottom line, Starcraft 2 is a great game. The loving attention to detail that went into every aspect of the game is obvious. The campaign is full of really well-designed missions that lead you through several strands of an intertwining story to an exciting conclusion. The ability to customize your units between missions adds a nice dimension to the gameplay, as does the ability to interact with the various colorful (if blatantly stereotyped) characters on the ship.
And of course, once you’re done with the campaign, there are limitless hours of multiplayer fun to be had. You can work your way up the competitive ladder, or take advantage of the extremely powerful map editor that comes with the game and play custom maps made by other players, some of which are good enough to be separate games in their own right.
If that’s not enough, there are two expansions in the works. The campaign in Starcraft 2: Wings of Liberty is strictly Terran (with a few protoss missions thrown in). Each of the two expansions will focus on one of the other races in the Starcraft universe. I don’t think there are release dates set for the expansions yet, but it’s pretty clear that Starcraft 2 and its expansions will be keeping us busy for a long, long time.
A couple weeks ago I took a look at railguns, how they work, and how the ones depicted in Starcraft 2 don’t look much like the real thing. This week I’d like to look at another favorite exotic gun in sci-fi video games: the gauss rifle. In Starcraft, the marines carry gauss rifles that act much like real-world assault rifles. In other games, like Fallout 3 and the Mechwarrior series, gauss rifles are a sniper weapon, used to do lots of damage at a distance with a single shot. So, what is a gauss rifle, really? And is it anything like those depicted in the games?
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Everyone has heard of supernovae, but it seems like there is some unwritten rule that they must never be depicted correctly in popular sci-fi. Starcraft 2 is, sadly, no exception. I don’t think it’s too much of a spoiler to say that there is a mission in the campaign where you have to battle your enemies to gain control of a relic on a planet that is about to be consumed by the fires of a huge blue star that is “going nova”. Or supernova. The words are used interchangeably.
As an astronomer, I always cringe when this happens, so I want to clear up what exactly supernovae and novae are.
The confusion is understandable: both are stellar explosions, and their names imply that a supernova is just a big nova. But in most cases this isn’t true. Let’s consider supernovae first, since by understanding them we will already be well on our way to understanding novae.
A supernova is when a very large star explodes. But that’s the end of a long process, so let’s rewind to the beginning: a star starts off as a ball of (mostly) hydrogen gas that is compacted into a dense sphere by its own gravity. At some point, the immense pressure of all that hydrogen is enough to force the atoms in the star’s core to collide with one another and merge into helium, releasing a huge amount of energy. This process is called fusion, and for most of a star’s life, it sits there turning hydrogen into helium and producing lots of energy. That energy makes the star so hot that it glows, and it also provides the pressure that keeps gravity from crushing the star any more. Most stars are poised in perfect balance with gravity trying to compress them and the nuclear reactions in their cores trying to expand them.
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In the Starcraft 2 campaign, there is a unit called the “diamondback”. It’s a sort of hovertank, and it is useful for hunting down other vehicles because it is armed with dual “rail guns” that can fire while it moves. In the game, the rail guns are depicted as some sort of energy weapon: they fire hot blue beams at the target. This disturbed me because rail guns actually do exist, and they most certainly don’t fire blue energy beams!
So what’s a real rail gun and how do they work? I’m so glad you asked!
Apologies for the lack of posts lately. You see, I didn’t properly plan my life around the release of Starcraft 2, so instead of being able to spend the weeks after the release playing the game and blogging here, I had to go get married and go on my honeymoon. I know, I know, I need to get my priorities straight.
Anyway, before I launch back into the Science of Starcraft, I thought I would share this post about the Computer Science of Starcraft over at Twenty Sided. You may be good enough at Starcraft to regularly beat the AI, but I’m not. I’m just too slow. So I was interested to hear the results of this experiment, where Shamus set up a map with seven AI players and a human observer, and ran it repeatedly, tabulating the results to learn a little more about how the AI ticks. He found (somewhat unexpectedly) that the computer was by far most effective with Protoss and worst with Terrans. He also has some interesting observations about how the AI uses special abilities like the Templar’s psi-storm agains human players and computer players:
I’ve come to suspect that the AI cheats a bit and detects clusters of units which have been grouped by hotkey by human players. This is very naughty if it’s true.
Anyway, it’s an interesting look at the AI of the original Starcraft. The custom map is even available to download if you want to try the experiment yourself!
In Starcraft, the biological, hive-minded Zerg can survive and even thrive in the vacuum of space, while the more fragile humans (and presumably the Protoss) require some sort of space suit. So that got me wondering: How plausible is it for living organisms to be able to withstand the vacuum, extreme temperatures and high radiation levels of space?
Fist of all, I’m going to set aside how the Zerg fly around in space. That may be the subject of a future post. I’m more concerned with just the idea of a hydralisk standing on an exposed moon-like surface and not immediately freezing or suffocating or otherwise dying gruesomely.
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I was always a fan of the Protoss: super-advanced technology, powerful units, and those awesome plasma shields protecting everything from the lowliest probe to gigantic carriers. But I always wondered: could those force fields really work?
Well… Sort of.
It really depends on your definition of a force field and what you want to prevent from passing through. Electric and magnetic fields can exert forces on charged objects, so you could call those “force fields.” If you accept that definition, then there have actually been some NASA studies of how to use force fields on spacecraft and lunar bases.
One of the biggest problems facing human space exploration (aside from the politics) is that space is a dangerous place for humans. Even if we can launch people to Mars, there is a lot of radiation out there, both from the sun and from high-energy events out in the galaxy. That’s why two studies at NASA’s Institute for Advanced Concepts (which has, sadly, been canceled due to lack of funding) looked into deflecting the radiation with electromagnetic shields.
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Well folks, it looks like I’m not the only one interested in the Science of Starcraft! The gaming site GamePro has a very interesting article taking a look at several aspects of StarCraft science, including teleportation, zerg in space, a hive mind, and hybrid species. They cover each topic pretty quickly but it’s still worth reading. But be sure to check back here! I’ll dig into those and other topics in a lot more depth.