Coherence

Note: The following is based on recollection rather than research, so consume it with caution.

Light

Light has always seemed sorta mystical to me. When scientists finally developed the tools to measure its speed, they discovered that it didn’t obey Newton’s laws. Newton’s laws are not related to the similarly-named, durable, fruit-filled cookie, but refer to the temporal and spatial rules that describe the behavior of normal objects such as billiard balls and racing cars. So physicists had to supplement Newton’s laws with Einstein’s Theory of Relativity, who, despite this accomplishment, unfortunately does not yet have a cookie that shares his name. The point is, light is strange, fascinating, and largely inexplicable to most of us normal mortals.

Anniversary

This year marks the 50th anniversary of the invention of the laser. For some reason, I have maintained a peculiar interest in lasers, exceeding my usual fascination with things technical. The laser’s actual invention, during my elementary school years, occurred without my noticing it, perhaps due in part to Frances Sue, Nancy, and other classmates who were busily attracting my attention at the time. My first encounter with this new form of light was during my high school years. Popular Science magazine published an article describing this new invention and how it worked. I’m pretty sure I did at least one science project that involved a drawing or model of a laser, and it’s probably still in my attic at home.

Ruby

To the best of my recollection, the original laser was made of a ruby rod, carefully polished on both ends to produce parallel faces, one of which was silvered. The rod was surrounded by a twisted light source, a strobe if I remember correctly. When the external light source was pulsed, some of the emitted light entered the ruby rod and began to bounce from one polished end to the other, like a demented spring. Eventually some of the light waves busily bouncing from end to end would combine with other waves of the same phase, and they would build up enough energy to burst out of the end of the ruby rod in a pulse of coherent light.

Coordination

The characteristic that makes laser light significant is called coherence. Most naturally occurring light sources (the sun and stars, campfires, flashlights, flourescent bulbs, cartoon idea symbols) emit light waves that are scattered, out of phase, random, and disorganized, like most of my writing. Lasers, on the other hand, emit light that is organized and in phase – in a word, coherent. This characteristic means that laser light is in some ways even more amazing than “normal” light. While most automobile headlight beams, regardless of their power, scatter out to a wide, nearly invisible dimness within a few miles or so, a sufficiently powerful laser can shine its tightly-columnated beam all the way to the moon, broadening to a relatively small circle, around a mile in diameter.

Hijinx

A few years after the Popular Science article was published, I walked into a college physics lab and beheld an actual working helium-neon laser. And not just one, but one on every lab bench. Lasers were practically a commodity. That was enough to make my tiny technological head spin.

It was also enough to give clever college students, who shall remain unnamed, some crazy ideas. If you were walking across the Tech campus in the late sixties/early seventies and remember a strange, bright red light appearing on the sidewalk in front of you, just chalk it up to youthful physicists experimenting with their new toys. And if you are a campus security office who not only saw such a red light, but also shone your flashlight on it, and attempted to chase it around the corner of the building, perhaps an apology is in order. And if either of the aforementioned looked at the roof of the physics building for an explanation and saw only a faint blue glow, well, let’s just say now you have your explanation.

Of course, compared to streaking and other college adventures of that time, playing with a low-power laser was pretty innocent. No headlines were made, nor embarrassing photos taken, and no animals were harmed, etc., etc.

Brightness

Since those early days, the application of lasers and coherent light have amplified (obscure pun intended) a wide range of human endeavors, from making weapons, improving measurements, correcting vision, guiding circular saws, aiding fiber optic communication, entertaining us with CDs, DVDs, and laser light shows, all the way down to providing cat toys and adding a desperately needed colorful distraction to thousands of boring PowerPoint presentations. We now have lasers too big to fit in the average garage, and small enough to fit on your key ring. And the ruby rod was long ago replaced by electro-chemical processes that I don’t fully understand. (Perhaps it’s time for another science fair project.)

In closing I must note that laser light isn’t good for everything. You can’t use it to read by, or to get a tan. It’s not good for taking pictures of your family, or providing general illumination. The uses for ordinary, non-coherent light have not been filled by laser technology.

But I continue to be impressed by the power of tightly-focused, synchronized light to accomplish amazing tasks, and I have particularly enjoyed watching a clever invention change the world in many diverse ways.

Happy Birthday, Laser!

4 Responses to “Coherence”

  1. Carol Cole says:

    Carl, this is an incredible piece. I didn’t know it was possible to become emotionally involved with light of any kind, but your fascination with the laser technology is contagious. Thanks to Marilyn for providing the link.

    Immensely enjoyable, five stars.

  2. Cathy says:

    Charles Townes who invented the maser…. the precursor perhaps to the laser…. went to Furman!

  3. […] I find this to be an amazing story. Dirac modestly proposes a purely theoretical solution to a quantum physics problem, and over the next five decades, his theory is not only confirmed, but turned into a rather clever medical diagnostic tool. This ranks right up there with my previously-posted infatuation with the laser. […]

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