Tuesday, 20 September 2011

Stop Asking for a Lightsaber!

Taken from cassiroll.com
I like to play with lasers. And when I tell people my research involves building them, 9 times out of 10 the response is ‘Can you make me a lightsaber?'. Perhaps this is an indication that I need to find new friends; ones who understand that when I say ‘No, I can't' it doesn’t give them credence to respond with ‘But if they managed to do it a long time ago, in a galaxy far far away, why can't we?!'. So, to put an end to all the late night pub debates and endless neeeeeewaw noises let me explain exactly why I can't make your lightsaber.

Before we embark on our quest for truth, it is imperative that I explain what a laser is. Light Amplification by Stimulated Emission of Radiation describes how gain media (technically any material, even jelly babies) are excited to emit synchronised packets of light called photons. This synchronisation is the fundamental characteristic of a laser that differentiates it from light from a household bulb. The gain medium can be thought of as a series of tiny ladders that electrons sit on. If a photon, let's call him Luke, meets this electron it causes the electron to jump to the next rung of the ladder. Universal law says things can't stay high; they have to come back down and in order to ‘relax' back to its original position the electron pops out a clone of Luke to balance the books. You've now doubled your photon count. Adding a feedback mechanism like mirrors to bounce the Lukes back into the medium will amplify your original signal and before you can say Millenium Falcon you've got yourself a high power laser.

The two main reasons why a laser can't be crafted into a lightsaber are:
Ridiculously high powers needed
Lasers can't be shaped in air

Back of the envelope calculations are what experimental scientists do best. And I've done one for the power needed for an infrared (non-visible) laser to shear through the average arm. The equation I used is straightforward; the amount of power is related to the energy spent raising the temperature and then vaporising the arm, the velocity of the laser going through the arm and the spot size of the beam. These factors are limited by how deep the laser can penetrate before being absorbed and the thickness of the arm. With a few assumptions which I'll declare straight away (lightsaber speed was taken to be an average baseball batting speed of 70 MPH, thickness of arm 10 cm and beam area of 5x5 cm) this gives a required power output of over 160 kilowatts, the equivalent of 250 toasters running simultaneously for one minute! So you better strap on a 4 tonne generator on your back if you want to start fencing with your lightsaber.

The second reason is a purely superficial one. It explains why you won't be able to craft a rod shaped laser, fine-tuned to stop mid-air one meter from the source. Laser radiation is not a discrete entity like rope but a flowing wave that propagates and is subject to attenuation. Attenuation causes the intensity to decrease over distance, similar to sound getting quieter as you move away from the source. The intensity is reduced because the air will absorb and scatter photons. Some photons will interact with the atoms in the air and excite them (absorption) and others will bounce off the air particles, deviating from their original course (scattering). The most detrimental to high power laser radiation is absorption that leads to thermal blooming, an effect caused by intense heating of the surrounding air. The heating effect changes the refractive index which determines how much a material bends light. As the air increases in refractive index, it begins to act like a lens and defocuses the beam to break up into separate spots like a Jackson Pollock painting.

Despite these drawbacks, the destructive power of lasers is still fascinating to the public and heavily featured in blockbuster movies or fashioned onto cute cats captioned with ‘pew pew pew'. Perhaps it's not a coincidence that Ronald Reagan proposed the Strategic Defence Initiative, fondly known as 'Star Wars' two days before the release of Return of the Jedi. Although Reagan's idea was panned at the time for many of the reasons I've stated above, technological advances such as the use of adaptive optics to counteract beam wander and the ability to string together multiple kilowatt lasers have rekindled interest. Simple laser guns set to dazzle (as in causing glare, not razzle) are already used in non-deadly operations, and the US have supposedly reached 100 kilowatts by incoherent combining of seven laser amplifier chains. So I'm sorry I can't build you a glorified glow stick, would you like a giant laser cannon instead?