The Speed of Light

My most significant scholastic achievement in high-school was the physics project I conducted to measure the speed of light. We determined that it is very quick stuff indeed, and soon realized that our stop watches were worthless for this purpose...ta-da boom (sound of a rim shot)...but seriously folks...

Nick Allison and I were good buddies all the way through eleventh grade. After that not so much, but that has nothing to do with this story. We were in the same physics class, and we decided to team up on the mandatory semester physics project. The entire year in physics was spent trying to hammer some of the more basic elements of quantum mechanics into our recalcitrant little brains. We spent a great deal of time discussing wave theory as it applies to electromagnetism, you know, your basic E=MC squared kind of stuff. Attempting to measure the speed of light was a natural choice for a couple of space captains like ourselves. As I alluded to earlier, light is very speedy stuff indeed, as a matter of fact, as far as anyone knows, it is the speed demon champ of the universe. Until very recent history, scientists assumed its speed was infinite.

Nick was the research half of our team, and he read up on the history of measuring the speed of light. Some of the earliest experiments, were marvels of ingenuity, but were significantly lacking in accuracy. Their biggest contribution to physics was in determining that the speed of light was actually finite. Those experiments were not easy to duplicate, and we decided to do a variation on a modern experiment. Basically, the experiment was conceptually simple, you know, your basic rate equals distance divided by time test. Did you sleep through physics? Well, anyway, rate does equal distance divided by time; trust me. The idea was to time how long a burst of light took to travel a known distance and voila, you can determine its speed.

Our plan was simple...on paper that is. After setting up an eighth mile line-of-sight course, we would place a big mirror at one end and our measuring equipment at the other. Our stop watch was an oscilloscope, which is nothing more than a beam of light tracing a line on a phosphorus tube at a programmable speed. As the beam traces across the tube, it will be deflected up or down by signals you feed into it’s sensors. A really slow rate of trace on an oscilloscope might be useful for a heart monitor. Doctors use these all the time to measure heart rates and other biological functions. By measuring the distance between the signals, and factoring in how fast the light beam is sweeping across the screen, it is a simple matter to measure the time interval between signals. Light obviously moves much faster than a heart beats, so we need an oscilloscope capable of measuring extremely short intervals. My Uncle owned an electronics firm, and he just happened to have a spanking new HAL-5000 plasma-drive, state-of-the-art, warp-speed oscilloscope. Maximum sweep speed... two million cycles a second. Our calculations indicated that this would be adequate. “Oh Uncle, did I ever tell you how much I admire you and that you have always been my very favorite uncle”...he bought it!

Nick and I packed up a small fortune in borrowed electronics, and headed for my Grandfather's farm. We spent two frustrating days trying to get the main reflecting mirror aligned. Even with walkie-talkies this proved to be a formidable task. A tiny adjustment of the mirror, which was precariously balanced on a painters easel, resulted in a huge shift in the alignment.  Eventually our efforts were rewarded and we had the experiment set up to the point of trying it out on the second night.

The experiment design was to capture the round trip time of a single burst of light. As the strobe light (previously used with a fog machine at school dances) flashed at the start point, outgoing light would hit a photo sensitive cell resulting in a "bump" on the oscilloscope trace. The majority of the beam would head precisely one-eighth mile down range and bounce off a mirror. The telescope was trained on the mirror to capture the reflected light. Another photo cell was attached to the telescope eye-piece. The flash on the downrange mirror was the return trip of the original burst of light. This resulted in a secondary "bump" on the oscilloscope trace. Measure the distance between the bumps, factor in the rate the beam is sweeping across the screen, and presto, you have recorded the speed of light in that environment.

The experiment failed for two reasons. First, the oscilloscope was a very sensitive piece of electronics, and our power source was not grounded. This resulted in a huge amounts of electrical noise that completely buried the weak signal we were trying to detect. Second,  we were using standard high-school-issue photo cells to provide a signal to the oscilloscope. We were not electronically sophisticated enough to consider how they might affect the experiment. It turned out that these photo cells, in fact all photo cells of that time, have a response time that was much longer than the time interval we were trying to measure. Our HAL-5000 oscilloscope notwithstanding, the gear we had was inadequate for the job.

Nick wrote up our report and explained why the experiment had failed and I provided photo illustrations. We got an A. Physics was a two semester course, and a project was due for the second semester as well. Nick and I got permission to try again. I did some research and discovered that the only photosensitive device capable of meeting our requirements was something called a photo-multiplier. This puppy could respond to signals in a nanosecond...or one billionth of a second. It was also sensitive enough to detect star light. It turned out that your basic photo-multiplier tube only cost about seventy-five bucks. Nick and I pooled our resources, put the arm on our folks and bought one. I had to design and build a power delivery system that would feed the tube voltage on a stepped bases. That really only meant I had to follow the printed electrical diagram that came with the tube and solder some resisters in series. This resulted in the voltage for the tube increasing by tenfold for each of its eight power pins. We also needed a source of DC power in the one million volt range! No problem, the school had one. This is not so incredible as it seems because high voltage is not that hard to come by; all you need is a DC voltage amplifier. High amperage (or current) is a different story; for that you need a hydro-electric dam. Luckily, we didn’t need the amperage. Solving the ungrounded current problem was also a simple matter. We planned to drive a iron spike into the ground and hooked it up to the outlet.

Four months later, we were back at my Grandfather’s farm. Setting up the experiment went much more smoothly this time around...experience counts. We changed the experiment design to use a single light sensor (our nifty-keen photomultiplier). This accomplished two things, one was that we had a device that was responsive enough to measure the extremely short intervals involved, and two, we didn't have to factor in the response variations of multiple devices. That night, we calibrated or equipment, held our breaths, and turned on our strobe light. The oscilloscope spiked at the "triggering" outgoing flash and record a weak secondary "bump" from the reflected light off of the down range mirror. This resulted in a deviation in the smooth decay of the triggering outbound flash. This second "bump" calculates to a  speed of 187,950 miles per second. That is less than a 1% deviation from the accepted figure. The oscilloscope was set to three different speeds and the results were the same.

Nick and I had a lot of fun doing these reports. We turned both outings into three day weekends complete with friends and party accessories. It was a gas playing with all that high tech gear, and we tried to put some of the fun we were having into our reports. Aside from diagrams, calculations and textual descriptions of our experiment, there were many photographs illustrating our adventure with science. Even though the report contained the high school equivalent of ‘hard science,’ the photos and captions were definitely tongue in cheek. We borrowed white lab coats for the occasion, and all of the pictures were staged shots to look like no-nonsense-anal-retentive scientists hard at work. Through the pictures and captions, we tried to convey the sense of irreverence and fun that we were having. Lurking in the background of most of the pictures was the dark figure of a man wearing a trench coat, sunglasses, and a Fedora. Apparently our experiment had attracted the unwanted attention of agents from some unfriendly government...perhaps even our own government! We never mentioned the spy that had ‘accidentally’ been captured in our ‘candid’ pictures, but our reports became famous within our school for the sport of trying to find all of the hidden secret agents.