Science In Real Life

If you’ve seen a science fiction movie ever in your life, you know that lasers make everything better. Ever since their invention in 1960, lasers have found applications ranging from scanning barcodes to zapping aliens from planet X. As it turns out, the laser is one of the many inventions only made possible by a generous donation from our sponsor, quantum mechanics (Atoms out of alignment? See a quantum mechanic today!). But don’t run off in fear just yet – I promise to keep the monsters at bay. So, fasten your seat belt, set your cell phone to vibrate, and keep your arms and legs inside the essay at all times.

First, “laser” is an acronym. It stands for Light Amplification by Stimulated Emission of Radiation, much like ATM stands for Automated Tell-you-you-don’t-have-enough-money Machine. The beginning and end of that phrase are already roughly comprehensible. A laser amplifies light, and light is radiation. The crux of the matter is stimulated emission, which is up there with “barrier penetration” for dirtiest physics phrase of all time. What it really means is that the photons already present guide the atoms in producing new photons in lockstep with the original ones.

One of the key discoveries of quantum mechanics is that electrons in an atom can’t have whatever energy they please. They have to occupy one of a set of distinct energy levels made available by the nucleus, like a subatomic multiple choice test with no “all of the above” option. What the available options are is dictated by how many protons are in the nucleus (i.e. which element it is), and that doesn’t change. That means that when an electron moves from a given high level to a given lower level, the energy difference is the same every single time. That energy difference is released as a single photon (Why electromagnetic radiation? Because it is the electromagnetic force that governs the electron-nucleus interaction).

Suppose you’re at a high school football game. The cheerleading squad is very good, so they get everyone in the stands pumped up to a uniform level of excitement. But then the Warriors fumble the ball, and a few of the spectators shed some excitement by letting out a sigh. Hearing that, the spectators near those people let out identical sighs, and then the people next to them and so on. If the stands were bookended by sound reflectors, you’d eventually hear one giant amplified sigh. In a laser, instead of bleachers full of middle-aged parents, we have a substance called the gain medium. Through electricity or another light source, the gain medium is pumped up so all the atoms have electrons in a common excited state, ready to fall back down.

The first few happen spontaneously, just like the fumble. Thanks to the mirrors placed on opposite ends of the gain medium, those photons bounce around triggering other photons. It’s important to emphasize that the photons generated by stimulated emission are in every way identical to the triggering photons. In addition to frequency, light also has a phase and a polarization. Briefly using the analogy of two people holding a rope and one waving it, if they both take a step to the left, they change the phase of the wave. If the waver starts waving side to side instead of up and down, she has changed the polarization. The photons generated inside a laser share all three of these attributes, as well as having a common direction. These photons are more in step than a military parade.

As a result, lasers make what is called coherent light. One of the two mirrors I mentioned will usually be only partially reflective, so that some of the photons eventually emerge. That beam, unlike other sources of light, does not spread out nearly as much. It continues in a straight line until something scatters it, like dust molecules or the back of your friend’s head. This is why one cannot see the laser beam itself unless it is reflecting off of something or it is aimed directly into your eyes. The latter is, of course, a Very Bad Idea.

Lasers at first were described as a solution without a problem. It didn’t take long for scientists, engineers, and doctors to incorporate them into all manner of techniques and devices. Medical applications include tumor removal and cosmetic surgery, including vision correction and dental procedures. One can only speculate that the laser floss is not far behind.