Wednesday, October 30, 2013

A Borepatch blog post from 1983

My favorite class as an Electrical Engineering student was Physical Measurements Lab.  It was nothing but lab work: plasma discharge, nuclear-magnetic resonance, and my favorite experiment of all time measuring the speed of light.  The professor gave us a cardboard box with a laser, a beam splitter, a rotating mirror, and a few clamps, suggested the basement of the Physics Building would be the darkest room we might find, and told us to scram - and not to come back until we'd measured the speed of light.

Last night #2 Son was doing a project for school, and in the course of looking for 3 ring binder clear plastic page holders I stumbled across my lab report from 1983.  It was a real waltz down memory lane.

Our measured value of the speed of light was high by 3.7%.  Not bad for clamping lasers and mirrors to any old railing in the basement.  I was surprised to see that I only got a B on the lab report, and some of the red pen comments were a lot harsher than I'd remembered ("This is NOT a conclusion!" about my conclusion; "Too wordy" which is sadly true today).  The report was typed (yes, on a typewriter) with hand-drawn diagrams much neater than I'd likely do today.

But what struck me was the cheekiness of my young self.  At the end of the report was a three page dissertation related to the experiment, a history lesson of sorts.  I reproduce it here as it strikes me as what may have been my first blog post, from February 3, 1983.  I see from reading it that I had a nose for junk science, even back then.  And a taste for snark, hard as that may be to believe (the seventh paragraph illustrates both of these).

In the beginning (as it is often said), we may suppose that people thought that the speed of light was infinite - when they ever thought about it at all.  The first person to try to experimentally measure this (as far as we know) was Galileo.  In his experiment, he stood on a hilltop one evening with a darkened lantern.  His assistant stood on a hill a mile away with another - also darkened - lantern.  Galileo would uncover his lantern; the assistant, upon seeing the light, would uncover his own, and Galileo would count the time from when he shown his lantern to when he saw his assistant's.  Unfortunately, the time elapsed was the time it took his assistant to think, "Hey, there's the old man's light."  The experiment was inconclusive, but did convince Galileo that the speed of light was very great indeed.

A half century later, a Danish astronomer by the name of Claus Roemer was working at the Paris observatory, observing the orbits of Jupiter's satellites.  Their periods of revolution had been carefully measured, and it was thought that the exact time of their eclipse behind the planet could be predicted; this, in fact, had been done.  Although the calculations seemed flawless, Roemer discovered that the satellites were disappearing at the wrong time.  He further noted that they were early when the Earth was approaching Jupiter and late when the two planets were moving apart.  He reasoned that the difference was the time it took the light to travel the extra distance.  The maximum distance was when the two planets were on the opposite sides of the sun, and was equal to the diameter of the Earth's orbit.  Using the best estimate of the diameter of the Earth's orbit, Roemer calculated the speed of light to be 138,000miles per second.  He announced this result, but it was burried under a storm of controversy and forgotten.

Fifty years later, in the 1720's, a British astronomer named James Bradley was hot on the trail of the stellar parallax.  This was an old dispute between Copernicus and the older Ptolemians, where the latter said that if the Earth revolved around the Sun, the stars should be seen to move; they don't, so Q.E.D.  Not so, replied Copernicus, for the motions of the stars (parallax) would be very small.  Bradley did not resolve this argument, but he did discover what is called the "aberration of light," and used it to calculate the speed of light.  His value was 188,500 miles per second, only 1.2 percent too high.

In 1849, a Frenchman by the name of Armand Hippolyte Louis Fizeau decided that the speed of light could be measured in the laboratory.  He returned to Galileo's expiment, but made some major improvements: the assistant was replaced with a mirror, and the light made to pass through a rapidly rotating toothed disk.  If the disk could be made to rotate at just the right speed, Fizeau reasoned, then a ray of light would return just as a blank space opened up on the wheel. If the speed of rotation was known, the speed of light could be measured.  Fizeau's experiment was not of high precision - only within about five percent - but it was a great success for a first try.  His assistant, Jean Bernard Leon Foucault improved Fizeau's method by replacing the disk with a rotating mirror (this was the experiment we repeated).  His accuracy was much better, and he even proved that light travels more slowly through water than through air.  This finally put to rest the particle theory of light.

The Fizeau-Foucault experiment was further improved by and American,Albert Abraham Michelson.  Michelson added some ingenious improvements to Foucault's apparatus, and measured the speed of light to within a fraction of a percent.  The importance of his experiment, however lies elsewhere, the story of which begins two centuries previously.

Sir Isaac Newton, back in the seventeenth century was firmly convinced in the particle theory of light.  He reasoned that a wave could not travel through a vacuum, such as the one above out atmosphere.  Unfortunately, light was shown to have definite wave properties very early on.  Newton struggled for the rest of his life with this problem, and almost united the two theories centuries before Maxwell.  Still, he failed, and the problem was ignored.

Later, it was shown conclusively that light was a wave.  Furthermore, it was shown that it had to be a transverse wave (by experiments with Iceland spar).  This was a terrible blow to the Physicists of the day who had postulated a medium - the "luminiferous ether" - to conduct the waves which pervaded all space.  Unfortunately, it was calculated that the ether had to be more rigid than steel to conduct the wave at that known velocity.  Nevertheless, the concept of a massless, frictionless medium, indistinguishable from vacuum yet rigid as steel was explained away by the slick, snake-oil experimenters, and became the prevalent theory (the above is, of course, not very charitable, but it does stretch the imagination to understand how the scientists who pulled down the Ptolemaic view of the cosmos for being top heavy and cumbersome could turn around and erect one of their own).

Michelson decided to try to measure the motion of the Earth through the ether (the so called "Ether Wind").  He reasoned that if the ether were motionless, then the motion of the Earth through it would be, while small, detectable.  With Edward Williams Morley, he spent a considerable amount of time and effort to eliminate all external vibration from his equipment.  When he finally measured the ether wind he found nothing.  After eliminating all possible errors, he was left with not a thing.  The experiment was a complete failure.

The results caused a world-wide furor. Some of the greatest minds in science, such as Lord Kelvin and George Gabriel Stokes, tried heroically to explain it away.  The game was up, however, and it was shown that no ether wind was detected because there was no ether.

The explanation had to wait until Max Planl in 1900 and Albert Einstein in 1905 showed the complete lack of ether which saved the day.

Michelson's experiment had failed so spectacularly that he won the Nobel Prize in 1907, the first American to win a prize in the sciences.
Too wordy.  Yes, it needed polishing, but was typed on a mechanical typewriter, not tapped into a blog text editor.  But even to this day there's sometimes a whispering about my posts, This is NOT a conclusion!

I was a pretty weird student.


chiefjaybob said...


Spike said...

I had forgot most of this from my readings in high school. Good stuff.

Dave H said...

If you want to see junk science, look up Barry Setterfield. He plotted various estimates and measurements of the speed of light made over the past few hundred years and noticed that there's a downward trend - the more precise (and recent) the measurement, the lower it tends to be. Based on this he theorized that light is actually slowing down over time, and has somehow used it as evidence of the date of Creation.

Opinionated Grump (Rich in NC) said...

ChiefJayBob took the words right out of my fingertips....
Actually, I do enjoy your 'wordyness' as you are on my 'regular' reading list.

Rich in NC

Chickenmom said...

You? Snarky? Nah! :0)

Dorsai said...

Lord, I remember this experiment. Three of us--me by no means the best student--set out to do this up in a fairly small lab. In order to get a decent baseline, we attempted to bounce the HeNe laser (yeah, I know, cheating) back and forth three times. We had excellent mirrors, and the adjustable mountings were to die for, but we couldn't do it. Just flat couldn't get the damn mirrors aligned. We tried for 2 hours, and, disgusted with each other, went home for the night.

The next morning I came in early and had the whole thing up and running and had a preliminary value for C within 15 minutes. Either of the other guys could have done the same, I'm sure; it was just a case of too many cooks.