Almost every science fiction movie has problems with lasers. For the most part they're all the same problems, and they all exist for the purpose of story telling. Let's see the opening scene of Star Wars: A New Hope for an example of what I mean.
This is an iconic scene, and probably one of my favorites from the trilogy (remember, there are only three films), but the movie starts out with some of the biggest science blunders.
There is one more laser blunder that is unique to Star Wars - lightsabers. The big problem is when a lightsaber reflects a laser. Photons (light) are a type of particle called bosons (you may have heard of the Higgs boson). A lightsaber could in no way reflect laser light, and if you've ever tried to recreate a lightsaber fight with a flashlight you know why this is a problem. The two beams of light just pass through eachother unaffected. Of course, Star Wars fans work around this problem by describing a lightsaber as plasma, but I'll show later why that is an even bigger problem.
- You can't see a laser beam unless it has been scattered. If you've ever used a laser pointer, you know that you can't see the path it takes. If you point it at a wall you see it on the wall, not in the air between you and the wall. If you happen to see the beam it has been scattered by particles in the air. The only way you would see a laser being fired between two ships is if the air were full of large particles (dust, water droplets, etc). If that were the case the lasers would do no damage - all the laser light has been scattered.
- Light travels at the speed of... As if the fact that you can see the laser moving through space isn't bad enough, you can see the laser moving through space. Light travels at rough 671 million miles per hour. If a laser were being fired between two ships ten miles apart the travel time of the laser light would be roughly 0.000005 seconds.
- Lasers don't make noise. The Star Wars blaster in particular is a well known sound. Every sci-fi movie has its own laser sounds. The problem is, lasers don't make sound. I've heard it argued that it's not the laser making sound, but instead the mechanism that is firing the laser. That may answer part of the problem, but then why is there such a distinct ricochet sound? Imagine waking up in the morning, turning on the lights, and hearing every photon as it bounced off your mirror. Light doesn't make sound.3
These science blunders are common in just about every sci-fi film, and there's a good reason. If it were realistic it would be boring. If the empire were shooting realistic lasers you wouldn't know that anything was happening until a ship blew up seemingly for no reason.
I'm not going to take issue with the specific idea of a force field. It's not a complete impossibility. Star Wars (and other sci-fi films) gets into trouble when they include an invisible force field.
The problem with the scene above (beside Jar Jar Binks) is the fact that the Gungan army deploys an invisible shield, but the droid army is firing visible lasers. If something is invisible that means that visible light is able to pass through it undisturbed. An easy fix would be to say that it was a two way shield (reflective from the outside and transmissive from the inside like a two-way mirror). The problem with that explanation is that no mirror is truly a two-way mirror, and that's not because our current technology doesn't allow it, but the light itself sets the limitation; there is always a possibility of reflecting or transmitting light through any surface. Also, if it were a two-way shield it would further complicate the situation by making every person inside the shield blind to everything happening on the outside.
The Kessel Run
Han Solo famously made the following statement about his ship, the Millennium Falcon (at about 0:25).
The problem with Han's statement is this: a parsec is a measurement of distance, not a measurement of time. Han bragging that his ship "made the Kessel run in less than 12 parsecs" is the same as me saying that I can run a Marathon in under 4 miles. It just doesn't make sense. Of course, Star Wars fans are well aware of the blunder and have an explanation for it. As an afterthought, they have included the Kessel run into the mythology. The Kessel run is completed by passing through an area of space full of black holes. If you have a powerful ship you are able to pass closer to the black hole than any other ship and create a more direct route through the interstellar mine field.
So Han's ship was able to make it through an area of space full of black holes using a path under 12 parsecs. Makes sense, right? The problem with this explanation is that parsec is a measurment of distance used in astronomy, and they only care about very large distances. 12 parsecs is about 40 light years. Even assuming faster than light travel that's a stretch that the fastest route through a black hole mine field is that long.4
There's no right way to argue against lightsabers. The word implies that the blade is made from light. Of course, Star Wars universe apologists will refute my arguments by saying that it's not light it's plasma. Plasma does play an important role in the bad science, but we'll get to that soon enough.
To begin, it seems that the rebels understand physics much better than the empire. After all, Darth Vader and Darth Maul both used red lightsabers, while the rebels tend to use a bluer color. Now, since energy is inversely proportional to wavelength, this means that a lightsaber like Mace Windu's (see Samuel L. Jackson on the right) is more powerful than Darth Maul's by about 145 kJ/mol if a lightsaber is made of light
The story gets better. As I said earlier, Star Wars fans have said that lightsabers are actually plasma. I think fans would have been better off sticking with the light story, and here's why. Violet light isn't a problem. I'm not going to die being near a violet light. Once you define a lightsaber as plasma, it makes more sense to treat it as a blackbody. The best example of a blackbody is your stove top. You know that when it gets really hot it glows red. That is called blackbody radiation. If you've ever described something as "white hot", you've noticed blackbody radiation. The color of light emitted by a blackbody changes depending on the temperature.
Darth Vader's lightsaber was hot, but not unbelievably so; it was only emitting red photons. Mace Windu's light saber, on the other hand, was emitting violet photons, and a large number of them. For a blackbody to emit light at that energy, it would have to be ~13,000 °F. That's about 3,000 °F hotter than the surface of the sun! So when a Jedi turns on his lightsaber, he's going to completely ionize himself, his opponent, and the air around him. This kind of temperature alone isn't enough to recreate the conditions of the sun, though. Fusion will not occur, since you need high pressures to force the nuclei together. Instead, turning on a lightsaber will just create a super hot, extending field of plasma. Kind of makes you wonder why they chose the ice planet Hoth to hide out, and why Han needed to kill a tauntaun to keep warm when a lightsaber was right there.5
So Star Wars has some scientific flaws. I don't think there was anyone that would argue otherwise. I hope you enjoyed my take on the problems though. If not, here's "How It Should Have Ended: Star Wars". Enjoy!
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If you liked this "Bad Science in the Movies" check out Bad Science in the Movies: 2012, and stay tuned for more to come!
 You heard that right. I've got myself a schedule and everything. I will not be taking another 5 month hiatus (although maybe I'll take a sabbatical).
 You know what, yes I am. Listen, the start of each of the original Star Wars begins with something to bring you into the story: The rebel ship is captured by Vader, Luke is attacked by a wampa and saved by Han Solo by cutting open a tauntaun to sleep in (not to mention the AT-AT attack), and Luke infiltrates Jabba's palace to rescue Han. How does Episode 1 begin? Trade discussions. Jedi's drinking tea while they wait for a meeting. Furthermore, what is the plot? Who is the protagonist? It's supposed to be the story of Anakin Skywalker to Darth Vader, but we don't even see Anakin until nearly an hour into the movie. What was the point of the first hour? It didn't really set up any part of the story that couldn't be written into the opening crawl. Was it to introduce us to Jar Jar Binks? That's a wasted hour if you ask me. And another thing, if Anakin built C3PO, why doesn't he clue Luke into the idea that maybe he didn't want to get too close to his sister. If C3PO had already gone through such an ordeal with R2-D2, why did he pretend not to know him later? Were they undercover? Did they give little "robot winks" when they saw each other? Maybe their memory was wiped, I don't know. And don't get me started about midi-chlorians. Seriously, the idea is stupid. Long story short, Episode I, II, and III are worse than the Star Wars Holiday Special. At least after watching the Holiday Special I could forget it and just watch The Empire Strikes back without having some major plot hole introduced ex post facto.
 An interesting exception to this statement is photoacoustic spectroscopy. Lasers are fired at a sample, which quickly heats up and sends a pressure wave (sound). By measuring how this pressure wave changes with the wavelength of the laser you can learn some cool stuff.
 Quick note: When I say that 40 light years is long I mean it is a long distance, not a long time. A light year is not a measurement of time even though it sounds like it is.
 Maybe Han secretly likes the way they smell.