Measuring The Speed Of Light In 1927

It is hard to remember that a lot of high tech research went on well before the arrival of electronic computers, lasers, and all the other things that used to be amazing but are now commonplace. That’s why we enjoyed [Michel van Biezen’s] two part post on how Michelson computed the speed of light in 1927. You can see the videos below.

Michelson wasn’t the first, of course. Galileo tried. He sent an assistant to the top of a hill with a lantern. When the assistant saw Galileo’s lantern, he was to uncover his lantern. They practiced near each other to account for reaction time. But when the assistant was 3 km away, it didn’t take any more time. The implication was that light traveled instantaneously, but, of course, it is actually just really fast.

By 1927, Michelson tried what was in effect the same technique but with better technology, and this time they put a reflector about 35 km away meaning the light had to go to the reflector and back for a total of about 70 km.

To get the best answer requires knowing the exact distance between the emitter and the reflector, and [Michel] explains how they did that using surveying tools. They computed the maximum error at about 8 cm which is not bad for measuring 35 km.

The other important measurement is time, as the sloppier the distance and time measurements are, the worse the estimate of the speed of light will be. Michelson used an 8-sided mirror device that rotates while bouncing light to the reflector and also collecting return light. The speed of the device’s rotation gave a way to compute the time, and he covers how that works.

The experiment was successful and the result was within .001% of the correct figure. Today we’d use lasers or any number of other techniques, of course. If you want to get some experience on something slower, try a bullet.

36 thoughts on “Measuring The Speed Of Light In 1927

    1. A really good job interview question (for e.g. an electronic designer or PCB designer) is to ask the candidate how far light travels in a ns. If they hold up their hands so far apart and say “free space” then move their hands closer together and say “optic fibre” or “FR4”, give them the job.

      1. Free space about 30 cm/ns (or one foot)
        FR4 rule of thumb, 15 cm/ns

        Typical optical fiber around 18-20 cm/ns

        Typical coax is around 22-28 cm/ns

        Sound (in air) is at roughly 330 nm/ns in comparison.

        And an Olympic sprinter is currently at about 10.4 nm/ns (or about as far as a cell membrane is at its thickest.)

        So light is indeed rather fast, unless on stuffs it down a strand of glass or plastic….

    2. I remember figuring that out when I was trying to figure out how tight the clocks needed to be for GPS to work. I wonder what the distance on google maps would give you for between the two peaks. I may have a look at that for fun.

      1. Speed of light can be summed up as either.
        “1 billion feet a second” (actually 983 571 056 feet per second, so the rule of thumb is 1.67% too fast.)

        “300 000 Km/s” (actually 299 792.458 km/s, so the rule of thumb is 0.069% too fast. (about 24 times more accurate than a billion feet a second.))

        Some people though remember it as “299 thousand km/s” but that is 0.26% too slow, ie worse then just a round 300 thousand km/s. (But still about 6.4 times more accurate than a billion feet a second…)

        So the “Suck it, metric.” is a bit of an early victory call…

  1. Albert Michelson is the other Albert- and was actually a secret hero of truth- first person in the US to win a Nobel Prize, even as late as 1920 in “Studies on Optics” he openly rejected FitzGerald’s theory of space contraction as an alternative explanation for the Michelson-Morley Experiment (first performed alone by Michelson in 1881 which help from Alexander Graham Bell), in addition to rejecting Henrick Lorenz’s theory of time dilation- both adopted as requirements for Einstein’s relativity theories. Many people have never heard about that. In the book Chandrasekhar explicitly creates a preface footnote explicitly imploring the reader to dismiss Michelson’s doubts about the space and time contraction theories. Wish people could get a basic 101 on the history of science- its full of sht like that.

    1. “Studies on Optics” is apparently a typo. I believe “Studies in Optics” is the correct title.

      After a little more googling I found:

      Truth and Beauty
      Aesthetics and Motivations in Science
      By S. Chandrasekhar · 1990

      Which mentions Michelson’s dissatisfaction with the loss of the Aether.

      1. oops yeah- “Studies in Optics”- its less important that Michelson entertained the theory of an ether than that he rejected FitzGerald’s and Lorentz’s space and time contraction and dilation theory as “artificial”. I think like Maxwell, Michelson kept an open mind about the existence or lack of existence of an ether medium for light. In his initial lesser known 1881 paper (everyone cites the 1887 paper with Morley) Michelson at least rejects the theory of a stationary ether writing: “The result of the hypothesis of a stationary ether is thus shown to be incorrect”. Of course, that leaves room for the theory of an ether that moves along with the earth, but at least it shows that Michelson flatly rejected the theory of an ether that does not move.

    2. Well, I can understand why Michelson rejected both Lorenz’s and FitzGerald’s ideas: they were quite ad-hoc ideas just for explain the experiment of the interferometer. Einstein, instead, deduced them from a framework, and it coincided with the results of the experiment.

      1. I personally, having studied the history of science, think that Einstein basically was like a negotiator who brought together the corpuscular and undulatory camps by tossing out the ether for the corpuscular camp but tossing in the theory of space and time dilation for the undulatory camp. Nothing fails in science like science by committee and the result was the Ptolemaic-like theories of relativity we struggle with today- the endless claims of supporting proof were definitely questioned and rejected for decades by upstanding scientists like William Pickering in his famous 1922 Popular Astronomy article “Shall We Accept Relativity?”…..30..199P
        Like Michelson, Tesla also rejected the theories of space and time contraction (in rejecting the theory of relativity), The astronomer Charles Lane Poor wrote a very insightful criticism of relativity in his now forgotten book “Relativity Versus Gravitation” of 1922 1922 was apparently one of the last efforts at a stand against the theories. Another great critic of the two theories was Herbert Dingle of the U of London who even as recently as 1967 rejected them in a Nature article: Dingle, H. (October 14, 1967). “The Case against Special Relativity”. Nature. 216 (5111): 119. – explaining the basic problem of the so-called twin-paradox by simply wondering how two twins could possibly have a motion that was different relative to each other that would cause a difference in their aging process- just simple and bold brilliance usually dismissed as “crackpottery” since facts cannot be drawn from in opposition.

        1. Well… I remember an anecdote: Einstein was asked him opinion about an essay titled: “one hundred scientist against relativity”, and his answer was: “Nonsense. If I were wrong, just one would be enough”.

  2. Measuring speed of anything is subjective. Even though the normal explosion (at sea level) propagates at the speed of sound, everyone knows a movie hero can and will outrun the blast. Just like you “KNOW” that all air conditioning ducts are large enough allow multiple persons to enter and crawl through them to escape the bad guys.

    1. My fav is firearms. Automatic weapons can never hit anything. They are essentially useless. Rifles are also pretty useless. By and far the most accurate firearm ever made was a short barreled revolver, but ONLY if you are shooting from horseback. Apparently they had very well trained horses back in the day, and a blind but with a colt revolver on a horse could easily take out a bad guy at 500 yards or shoot through the rope leading to a noose at 300 yards. I wonder when they stopped breading those special horses?

      1. All true.. (Mostly) Back in the day, horses were trained to “Fall On Command” and the ground was padded for safety. Some say, the horses enjoyed this process. While I was in the movie business for the better part of ten years, I didn’t have much in Cowboys and Indians lore.Today, special effects guy/gals rig the vehicles with bullet holes, fill them with bondo, and underneath, a “Squib” an electric detonated charge, roll film. (Actually, these days, roll tape or DVR.) It’s “Movie Magic” guys.. And, back in the day, the cowboys never seems to run out of ammo. Six shooters could fire for days..

  3. … or you could measure it like Ole Romer and those who followed him. Romer measured the speed of light in Earth-orbit units in 1676, by observing the moons of Jupiter with no instrument more complicated than a telescope and a clock.

    … and Michelson was using methods very similar to those used by Fizeau in 1849 and even closer to Foucault’s method from 1862. In fact, Michelson was basically just throwing money at the same type of apparatus used by Foucault.

    Also, while Michelson’s direct measurement was valuable, the speed of light was known more precisely by 1907, based on Maxwell’s equations and electrical measurements (Rosa and Dorsey).

    1. About the point of Maxwell: you can say that because you know (now) that the light is really an electromagnetic wave. But what if, after doing a precise measurement, you find that the speed of light is not the same than the speed of an electromagnetic wave, but just only “quite similar”? ;-)

    1. There’s this oddball theory I’ve heard that the speed of light is gradually slowing down, and we don’t see it in the data because we’re using atomic clocks to keep time in measuring experiments.

  4. There are two soueces of error in the M-M experiment: the distance between the fixed and rotating mirrors and the rotation speed of the rotating mirror. How did M-M get the required precision of the mirrors RPM?

    1. Get a gigantic spool of optical fiber, it need not be real thick. I had a spool I gave to someone to make fiber optic trees out of, what they did not use was supposed to come back. Right. But it must have had 10,000 feet on it. If you are board off your ass measure the length or take the sellers word it is near right. Get the velocity factor for it. Blink a light or flash a strobe into one end, also use that to to put a blip on one channel of a scope. Use a phorodiode or what not at the other end and use that to blip another channel on the scope. Play with the flash rate and the sweep rate until you see the ch 1 blip and the ch 1 blip. Measure the time between them, you know how long the fiber is, and the VF, you should be good to go. +/-

  5. Another fascinating old scientific experiment is the Schiehallion experiment, which earlier supported Newton’s theories of gravitation and provided an estimate for the density of the Earth by ‘weighing’ a mountain in Scotland – worth a Google search if you’re so inclined.

  6. Dr. van Biezen, here is an email to faculty at the LA high school where I teach that your lecture inspired me to write:
    “So, I think this team could support our students to re-create on of the coolest experiments of the 20th century, measuring speed of light. See Dr. van Biezen’s lectures, above.
    Apparently, it is a gigantic achievement in Los Angeles’ scientific history, Greg C. It requires gathering and analyzing data, Nicole. John, Janel, it involves some serious hiking to and probably camping at Mt. Baldy and Mt. Wilson in coordinated teams. Greg E, John, it would take some beautiful trig. Dave V, the device to measure the speed of light would take some cool robotics hacks. Kevork, speed of light, man! And Alyson, I think we have here a spectacular PR event for the Engineering Academies.
    Who wants to (re-) make history and inspire our students at a very scary time in their lives? I think this will be a great memory in the life of the school. I’d like to talk about how we could make this happen.”
    These faculty and staff I named have germane specialties and interests, like science history, statistics, math, physics, engineering, programming, and hiking southern California. We are all Angelinos and I think re-creating the experiment would be a matter of SoCal pride!
    If you have time to advise us, we would really appreciate the opportunity to correspond with you.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.