Testing the Speed-of-Light Conspiracy

There are a number of ways to measure the speed of light. If you’ve got an oscilloscope and a few spare parts, you can build your own apparatus for just a few bucks. Don’t believe the “lies” that “they” tell you: measure it yourself!

OK, we’re pretty sure that conspiracy theories weren’t the motivation that got [Michael Gallant] to build his own speed-of-light measurement rig, but the result is a great writeup, and a project that includes one of our favorite circuits, the avalanche transistor pulse generator.

setupThe apparatus starts off with a very quickly pulsed IR LED, a lens, and a beam-splitter. One half of the beam takes a shortcut, and the other bounces off a mirror that is farther away. A simple op-amp circuit amplifies the resulting pulses after they are detected by a photodiode. The delay is measured on an oscilloscope, and the path difference measured with a tape measure.

If you happen to have a photomultiplier tube in your junk box, you can do away with the amplifier stage. Or if you have some really fast logic circuits, here’s another project that might interest you. But if you just want the most direct measurement we can think of that’s astoundingly accurate for something lashed up on breadboards, you can’t beat [Michael]’s lash-up.

Oh and PS: He got 299,000 (+/- 5,000) km/sec.

32 thoughts on “Testing the Speed-of-Light Conspiracy

  1. I wurkt it out once. If it’s dawn in Londan at 7am, and it’s dawn in NYC at 7am, but New York is 5 hourse behind, then because it’s nearly 5,500 kilometers tween them, then that means light travels at 1,100 keelomeeters per hour. That’s pretty durned fast if you ask me, Dale Earnheart only drove up to about 300 of them keelomeeters tops.

      1. Now my Paw was a keelomeeter enthoosiast as it goes, he was in that big shooting war they yoorpeens had. They frenchies learned him that you gets 60% extra free if you reckon in keelomeeters. Being as how my grandpappy had a scots faither it was natural to regard the advantages. The fambly spread was One and Seven Eighths mile Farm back of Seven and a Half mile Holler. Which in all was a mouthful. So Paw changes it to 3K farm and 12 Keelomeeter Holler, because he says people can respect a man what has a dozen of something. This was all dandy until Mr King was making those fine speeches, and people got a mite tetchy about the name of the farm, so it went back to One and Seven Eighths Mile, but we kept the holler named right.

  2. Fascination with relativity.. check
    Metal can transistor in a socket.. check (what’s that 60s or 70s?)
    1980s Rat Shack PCB.. check
    Nice flat screen digital scope.. check

    Yup folks.. we found a time traveler!! :-)

    1. Heh, I bought one of those PCBs surplus as late as 2005. Metal can transistors were still booting around in “new” stock as late as the 90s, they still turn up in surplus and bargain bags.

    2. You forgot to mention the four color band, 5% 1/4Watt resistors from the 80’s and a KCK ceramic capacitor marked 100PF instead of plain 101 and that must be from the 70’s

      And the metal can transistor wow, I can’t see the number but I would guess that it’s a germanium transistor from the 60’s

      Looks like he ripped the parts right out of the chameleon circuit.

  3. LIGO proved that gravity waves are real and in the process proved we cannot know the exact speed of light by using measurements alone. Any device built would itself warp spacetime via gravity and thereby alter the distance it’s measuring.

      1. Well here a spanner in the works for you.

        Perhaps it’s ‘time’ itself that isn’t linear and because we use time as a reference then of course our observation of light speed will present as non-linear. I know that sounds stupid but apply it to quantum physics and all of a sudden it makes sense.

    1. Or a pellicle made with plastic kitchen wrap stretched tightly on a frame. Another common way to make a pellicle is to dip a wire loop in thin clear varnish and let the excess drip off with the loop vertical while it dries. In either case, part of the beam will pass through the pellicle with minimum offset because the film is so thin. The other part will be reflected. Very old technique. I suspect that a microscope cover glass would also work for this application.

      BTW Thanks for the avalanche link which also had lots of links to old application notes by Jim Williams. Along with Bob Pease one of my favorite EE writers.

  4. A really fun way to test light speed, if you have the equipment and license is to EME the moon with a big amp and monster yaggi.
    While you can sat-com the moon with far less power and antenna using something like WSJT and signal into a sound card using big super slow oversampling (FSK for sync and 4.375 baud BPSK for data) you don’t get that e-m-e exact lag by tapping a dit on the key and receiving the echo which you can rough time with a stopwatch. OTOH you do get use of a big super laggy stone communication satellite to message your buddies; if your buddies are into amps, radios, antennas, and sound card interfaces.

  5. Nice experiment, did similar myself years ago at summer school. In the end I got a very nice result only 0.5% larger than the accepted value with error margins of 0.75%. In my conclusion though I had to admit, like the above experimenter should have that this measurement is entirely circular and internal.

    In that you are measuring the speed of light in distance per time, where both the distance and time measuring devices you are using were manufactured / calibrated against the speed of light.

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