Whoops

Apparently I haven't posted to this blog in forever. Possibly longer than that in internet terms! This has been a terrible mistake. In an attempt to remedy this, I need to watch more movies with questionable science. I suspect that Avengers might be a good place to start. Wish me luck.

Bullets and Sense

Almost every action movie, animation, TV show or anything with guns in it has something wrong with it. I'm not talking about the fact that only heroes can aim, and I'm not talking about the fact that bullets don't have that much momentum. No, we have the much simpler to fix problem that automatic firearms generally don't have that many bullets!



Take, for example, the weapon pictured above. That is an FN P90. This weapon is pretty standard for military and counter terrorist forces throughout the world today, and is very popular in movies, due in large part to it's futuristic look and wide availablity. But there is a problem with how they are used, let me show you:

Carter's P90

Watch the video, and pay attention to the number of seconds she is firing on full automatic for. By my count it's about 6 seconds. But with that firearm she only has a 50 round clip, at 900 rounds per minute, the clip would be empty in 3.3 seconds! This is the problem that I'm talking about. Even in movies where they remember to reload at all, they forget that most assault rifles and sub machine guns will run out of ammunition in less than 5 seconds with full automatic fire. Just casually glancing through wikipedia, I couldn't find a single assault rifle or SMG with more than 10 seconds of continuous fire on full auto.

So remember, if your protagonists or antagonists use their weapons at full auto, they'll probably need to reload every 3-5 seconds.

Elementary Mistakes (Spoilers ahead)

I recently saw a movie in theaters that hit a bad chord with me, because it's allegedly a movie based upon reason and logic. As has likely been deduced from the title of this post, this movie was Sherlock Holmes. Although I have few complaints with the movie overall, there was one scene in which terminology was so out of place that it distracted me from the story they were trying to tell.

There was a scene where Nefarious Villain was attempting to use a wireless device to poison the British Parliament, and take over England. Although the device itself was fantastical, there was no particular reason why it wouldn't work. It should be mentioned that the device had a magnetic "anti-tamper" field, which was probably impossible to generate at the time, but we're talking about the entire technological expertise of a country being directed at it, so we'll give them the benefit of the doubt. So, what is the problem? To discover the error, we have to take a look at the year in which the movie takes place.

During the movie, there are scenes of the Tower Bridge in London clearly being constructed. Since the Tower Bridge was opened for service on June 30th of 1894, it can be safely assumed that the movie takes place, at the latest, in 1894. The problem arises from the fact that Holmes described the device used to activate the poison dispenser as a "Radio" device. The first documented use of the word radio can be tracked to 1897, and while Sherlock is quite the detective, he never claimed to be prophetic.

Getting historical details right is just like nailing the science—there are usually details that can be corrected to avoid conflicting with the facts, without harming the story-telling.

You too could be Joss Whedon

So far I've mostly harped on things in movies or video games that are bad, but it can be important to point out examples of good in science fiction as well.

There is one common issue in science fiction that is more drawn to light by the few shining examples where it was done correctly, rather than the majority of shows and movies that flagrantly ignore reality.

This issue is sound in space.

Any person can think of three or four shows where this is done incorrectly. This is usually because some high level person on the show forced them to include it because otherwise "it just doesn't seem realistic." The reason that this doesn't seem "realistic" to these executives is that they grew up in an atmosphere. In space, where there is no medium for sound to travel in, you can't hear anything. As the saying goes "In space, no one can hear you scream."

So, what are some movies and shows that did it right? First off, as was hinted at by the name of this post, Firefly. Whenever there was an outside shot of a ship that wasn't on a planet, the only sound you heard was music. This is a perfect example. It doesn't require annoying silence, and it worked quite well, dramatically.



Another example of someone doing it right, scientifically at least, was 2001: A Space Odyssey. In the extended director's cut, there was a scene where one of the crew was outside of the ship for about 10 minutes. During the whole scene, the only sounds are the hissing of air from his space suit life support system, and some communications. It made the scene feel very desolate and creepy, but it was paced a little slow.

There are ways, however, of using the silence of space for good dramatic effect. In the most recent Star Trek movie, the first battle featured some poor red-shirt getting fwooshed out of a hole in the hull of the ship. As the sad crew member transitioned from the pressurized interior of the ship to the vacuum of space, all the crazy exploding battle sounds stopped. It made for a very clear contrast, emphasizing how hectic the inside of the ship was.

Another interesting example of sound in space was the climactic explosion of the Nostromo in Alien. Although there was sound in that scene, the sound was reasonable, given the situation. A nice ballpark amount of atmosphere needed to carry sound is that on the surface of Mars (about 1% of Earth's). Using that as a benchmark, the vaporized ship would have provided an envelope of sound approximately 15 km in radius.

Finally, if external ship sounds are necessary for the story, take a page from the movie Event Horizon. The entire film took place in the upper atmosphere of Uranus. Not only are there most of the hazards and issues of being in space, but there is also some nice creepy clouds, lightning, and most importantly, sound.

It is absolutely beyond me why movie makers continue to insist that sound in space is required by audiences. There's no dramatic reason for it that couldn't be accomplished in a more realistic manner. For most scenes, an accurate portrayal would help to highlight the stark unfamiliarity of space. Hopefully more people will be like Joss in the future.

Physics: It's not a Game (Gamer)

I recently saw a movie which reminded me why I'm doing this. The movie was Gamer. This was a pretty bad movie, but the biggest problem it had was a very serious tendency to ignore the laws of physics when it comes to certain objects. Most notably amongst these was Newton's Third Law of motion, and bullets.

Newton's Third Law is a very simply stated law: For each action there is an equal and opposite reaction. What this means is that if something is pushed on, that thing will push back on the thing doing the pushing just as hard as it is being pushed on. Overall this is a very positive thing. I imagine it would be problematic if an object pushed on the ground, but it only pushed back with half of the force. The ground would be unable to support the object, and it would sink! There are other consequences of the Third Law however. The problem is, this applies to any object feeling force, including a bullet being fired out of a gun. The bullet feels a large force as it is accelerated down the barrel of the gun, this force is then transferred through the firearm to the person holding the weapon. Which brings us back to Gamer.

In Gamer, there is a scene where the main character is attempting to rescue his lady from her job. There are people chasing him, and one of these people corners him in front of a doorway. The villain fires a gun at him, and he is propelled back with enough force that he is first knocked off his feet, then destroys the door behind him. Despite his rather epic flight through the door, the man who fired the gun barely had to brace it and recovered almost instantly to grab the lady. There are all kinds of things wrong with this scene, but we'll take them one at a time.

All of the problems stem from one thing: bullets aren't very massive objects, and people are. To propel a person back a projectile requires a lot of momentum, and the only way a bullet has that kind of momentum is if it's going very very fast. Normally a bullet going very fast wouldn't be stopped by a person, but the hero was wearing body armor, so it's assumed that the bullet transferred all it's momentum to the hero. So the question is, how fast would a bullet have to be going to pack enough momentum to throw a person back. It turns out that the equation is very simple:
Mass of the bullet multiplied by the speed of the bullet has to be equal to the mass of the person and bullet together multiplied by the speed of the bullet and person together, after the impact. This can be represented by:
Mbullet*Vbullet = Mboth*Vboth
So, by filling in the rest of the variables, the speed of the bullet before it hits the hero is known. First, assume that the hero flew back at 5 meters per second, or about 11 miles per hour. Since he was knocked off his feet and through a door, this might be on the slow side, but it will work well enough for our purposes. Then assume that, being a large man, he massed about 100 kg. Next, we need a mass for the bullet. Since a NATO standard 7.62mm round masses .01kg we can take that to be a fairly close approximation of the bullet used. Solving for the speed of the bullet we get that it would have to have been going 50 kilometers per second, or 112,000 miles per hour. This number is roughly 4.5 times the speed required to shoot and object into space and have it never come back. Now, there are objects which travel at this magnitude of speed which people see all the time, they are meteors, tiny bits of rock which occasionally fall through the atmosphere. The thing is, most meteors are very small, and at these speeds even sand grain sized meteors are visible from the ground, 50-60 miles away! So, imagine something that's emitting as much light as a bright meteor, but is only several feet away. Catching on fire due to the intense heat and being blinded by the light would be larger problems than just getting shot at.

Now that the bullet speed is known, it's time to take a look at the effect this would have on the person. The problem here is that the gun had to push the bullet up to that speed, so one of two things should have happened. Either the man holding the gun should have been thrown back at the same speed that the person hit with the round was, or (and this is much more likely) the gun would be ripped from his hands and hurled back itself. If the gun massed about the same as and M16, it would have been hurled backwards at 320 miles per hour.

The worst part about all of this is that the scene could have been easily achieved without the use of the impossible bullets. An explosive device planted in the elevator would have the same effect of being surprising and knocking the protagonist back, but it wouldn't have required breaking the laws of physics to achieve. So lets leave the really high speed stuff to meteors and keep bullets in the realm of the reasonable. It's odd though, in the scene just after the one discussed, they had a very interesting Newton's Cradle, kinda like this one, only full of scantily clad women:
Odd how you can have such a good example of physics in a movie next to a complete lack of it.

Asteroid Belt: Not as Dense as Advertised

This is a scene that is repeated over and over again in movies, books, games and any other area where science fiction has a place. Our protagonists are in a ship, it's being pursued hotly by some alien doom vessel trying to destroy them, and their only chance at escape/survival is to dive into the asteroid belt and hope that the alien pilot can't keep up with the many collisions and flying rocks! Basically, flying through a scene that looks very much like this:


This scene has one very large mistake in it: space is very, very big. There are two consequences of this; the first is that it is unlikely that one will find an entire asteroid belt that is full enough of rocks to have this scene at all, but there will be more on this later. The second consequence is that most space flight "chases" are going to be taking place with the ships light seconds apart, with the chasing ship firing, and then waiting to see if the shot hit or not. There is a small subset of people who might enjoy that type of scene, and this is the type of person who likes submarine thrillers; who waits on pins and needles to see if the shot fired hits and if the ship was detected soon enough for the enemy to get their own firing solution. But for most people, having 20 minutes of movie waiting to detect an explosion five light minutes away would be pretty boring.

The truth of the matter is, although the asteroid belt is a comparatively densely packed part of space, there just aren't that many rocks in there. The entire asteroid belt of our solar system is only 4% the mass of the Earth's moon! Even more interesting, more than half of the total mass is contained in the four largest asteroids. What this means is that a spacecraft is not very likely to find an asteroid, let alone have to dodge around one. According to Alan Stern of Space Daily, there is a less than 1 in 1,000,000,000 (billion) chance that a ballistic trajectory satellite would hit even one of them when passing through the asteroid belt. To give some perspective on the amount of rock this is, if one were to smash up all the rocky planets in the solar system and place them in the asteroid belt, there would be 10,000 times as much rock as is in the belt right now. This would only bring a ship up to a 1 in 100,000, chance of hitting something when passing through the asteroid belts, assuming that the size distribution was similar to what it is now, and that the ship doesn't dodge. So, the odds of hitting a rock in the asteroid belt with a ballistic ship are similar to the odds of throwing a rock in the ocean and randomly hitting a whale. This would seem to be an unreasonable amount of rocky material to have in a single system. Real asteroid belts are hard to tell apart from the rest of the system from the inside.

So, the moral here is that if a chase scene of this type is needed, have it be near a planet. Maybe a moon broke up, maybe they've been dragging rocks nearby for mining purposes, maybe the planet just has unusually thick rings. But having an entire asteroid belt this dense is over the top.

Dragon*Con Photos

Here are all our Dragon*Con photos and these are from our Roomate. If you got to this site because I gave you a card, try to find yourself, and have a look at my site! Science Advising will continue later this month.