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How the Speed of Light Was First Measured (todayifoundout.com)
85 points by evo_9 on Dec 26, 2015 | hide | past | web | favorite | 37 comments



I do "Science Fridays" with my kids once or twice a month, and for one of them, we proved the speed of light with an old microwave and a chocolate bar.

There is a labeling plate on the microwave the specifies the frequency (~2.5ghz), and when you run it just long enough to begin melting spots of the chocolate bar, on older microwaves you can see several spots that are equidistant. Measure their separation (that's half the wavelength of the microwave), multiply by 2, multiply again by the frequency, and you have the speed of the wave.

It's poor precision (it is melted chocolate and depends on there being some standing waves, so older microwaves seem better suited) but accurate. And the math is pretty easy for kids, a ruler and a calculator.


>It's poor precision (it is melted chocolate and depends on there being some standing waves, so older microwaves seem better suited) but accurate.

That's just an excuse to do it a lot of times. :) Also a good segue into the story of the invention of the microwave oven: http://www.technologyreview.com/article/400335/melted-chocol...


A friend did a subject at university called something like "The history of science" where they studied and recreated tons of early experiments and measurements like this. I remember they did another one to calculate the mass of earth based on the deviation of a weighted string.

The idea was that if anyone is ever going to "discover" new things in the future, they'll have to use out of the box thinking and measurement techniques to get there.


Another awe-inspiring experiment that can be done in a basement or high school lab is to show that gravity really does exist between everyday ordinary objects. You'd think that any demonstration of gravity has to involve the Earth or Earth-size objects. Showing that two bowling balls attract seems impossibly difficult. But it can be done:

https://www.fourmilab.ch/gravitation/foobar/

"The [experiment above] demonstrates universal gravitation with an apparatus which could have been conceived by Archimedes and built from materials he could readily obtain [during the time Archimedes lived 2200 years ago]."


This experiment is really cool, but I'm wondering if it could be measuring any static charge on the objects instead of gravity.


That's the kind of course they should use to teach science.

Anyway, I thought the deviation of the string was done by Cavendish in his basement to find the gravitational constant.

The mass of a celestial object can be acquired by measuring the period of one of its satellites, which conveniently the Earth has naturally.


Too bad this otherwise good article thinks pop culture references are needed and funny. They are not.


Not exactly my cup of tea, either, but there's an argument to be made that these things do help make the subject more accessible, which is probably a good thing.


I did legitimately cringe at the Bieber joke. The O'Brien one didn't make enough sense to bother me. The author may want to steer clear next time.


Analogy, metaphor or just simple evocative imagery are good tools for making subject matter stick, though: https://www.coursera.org/learn/learning-how-to-learn/lecture...


I don't want to be rude, but I would like to point out that if you don't like the references, you are always welcome to invest your own time and write your own article.


Ages back I started a repo that I keep meaning to go back to - important experiments that kids can replicate - this should be one of them. Edit: https://github.com/mikadosoftware/importantexperiments4kids


This is a routine high school physics experiment.

I did a speed of light measurement in high school physics. We had a prism mirror on a 30,000 RPM motor, a slit lamp (not a laser) and some mirrors. The narrow bar of light from the slit lamp was aimed at the prism, which reflected the light to a distant mirror, then back to the prism, then onto a target. The idea is that as the motor RPM increases, the line of light projected onto the target will move slightly, and you can calculate the speed of light from this.

We didn't have a dark enough or long enough room, and aligning all the mirrors was a huge pain. We could see the line of light move slightly as the motor speed increased, but our baseline was too short to get enough movement to measure accurately.


I'm upvoting based on this sentence:

> Beeckman understood that, lacking lasers, the basis of any good scientific experiment should always involve explosions of some kind; thus, his experiment involved detonating gunpowder.

Incidentally, it's quite interesting to me that the most accurate measurements of c are actually indirect measurements. See https://en.wikipedia.org/wiki/Speed_of_light


"The first known person to question the whole “speed of light is infinite” thing was the 5th century BC philosopher Empedocles"

If you want your mind blown, there also was a physicist in the 20th century who questioned whether or not the speed of light was directionally invariant (isotropic). So the speed of light going one way versus the other might be different. This results in a different set of transformations, the Tangherlini transformations, that govern the structure of spacetime.


In many materials, especially crystals, the speed of light is not isotropic. There are a bunch of simple tools in mineralogy for making use of this fact to identify crystals in thin section, for example the optical indicatrix and Michel-Lévy colour chart.


I would expect that it would be not too hard to get some pretty severe constraints on any anisotropy.


IIRC, the only observable difference are nonlocal effects. Keep in mind that all of your measurements are subject to this anisotropy.


Why not link to the original article instead? ;)

http://gallica.bnf.fr/ark:/12148/bpt6k56527v/f234.image

Starting about halfway the page from "DEMONSTRATION TOUCHANT LE mouvement de la lumiere"


Does anyone know about the quality of telescopes in Romer's time?

Specifically, I'm interested in learning how big IO and Jupiter looked to him. I've looked up at Jupiter and Saturn etc. a few times through science class telescopes, never been able to see the moons.


The moons are easy to see because they are bright: they look like nearby stars (bright points). If you observe over time you can see that they have moved.


"Galileo, like Beeckman also suspected that the speed of light wasn't infinite and made passing references to an experiment involving lanterns in some of his work."

That's some good thinking in that time... any idea what could have made them suspect this back then? There were hardly even newtonean physics, and you can't see with the naked eye that it's a finite speed, so I'd love to know how they reasoned aobut it.


Well, it's either finite or infinite. Why would they assume either way?


How did they calculate the diameter of Earth's and Jupiter's orbit (it's mentioned in the article as known by Romer's time).


Cassini and Richter had measured the parallax of Mars (and therefore the distance to Mars) a couple of years earlier. Using Kepler's law they could then deduce the distances to the other planets.


Amazing. If you know of a comic (or graphic-heavy article) or a video that can explain to a complete astronomy-noob what parallax is and how it enables one measure the diameter of planetary orbits, please put the link below. (I can only learn science with visual illustrations, and can't mentally process equations.) Thanks!


You can buy a test spool of 25km of single-mode fiber for $130 on ebay. This is about 125 us of delay, so how can we directly observe this delay? It's easy with electronics, but if you could mount visual indicators on a rotating drum the delay would show up as a physical separation: ~1 mm for a .1 meter diameter drum rotating at 2000 RPM..


"A short history of nearly everything" is a fun read. Goes over many similar experiments/discoveries.


A brief history of time does too.


Hmm.

   Aside from the debate over whether the speed of light 
   was infinite or not, a common side debate throughout 
   history was whether or not light originated in the eye 
   itself or from something else.  Among the famous 
   scientists to believe in the “light emitted from the 
   eye” theory were Ptolemy and Euclid.  Most who thought 
   this theory correct also thought the speed of light must 
   be infinite, because the instant we open our eyes, we 
   can see a vast number of stars in the night sky and that 
   number does not increase the longer we look, unless of 
   course we were previously looking at a bright light and 
   our eyes are adjusting to darkness.
I must be missing something. How dumb would someone have to be to think that light comes from peoples' eyes? All you have do to disprove it is ask someone if an object you're both looking at seems dimmer when you close your own eyes.


You're not missing anything, but I'd say you're not clearing your mind of all the stuff you already know about light either.

https://en.wikipedia.org/wiki/Ray_tracing_%28graphics%29

It would be essentially ray tracing. "Light", from our eye, travels though stuff and ends up colliding with an object. We are informed of how the ray was affected and how it collided, perhaps because the ray is an extension of our perception (like a limb of sorts), or perhaps because the final collision always causes it to go back exactly the way it came.

It's simpler and more obvious than the idea that there exists a truly massive amount of light emitted though various processes in nature, most of it never perceived by any eye, and the minuscule sample caught by our pupil is still large enough to provide us with so much detail. In the absence of further knowledge on the subject you're studying, Occam's razor can easily mislead.


Oh, the ray-tracing-as-reality rabbit hole goes a lot deeper than that. Google "potentially visible set" and suddenly quantum mechanics will make a lot more sense. (In a pass-the-bong kind of way, anyway.)

But certainly there was no reason for any serious historical thinkers to speculate along such lines.


Well, you are assuming that the light from one person's eye can be seen by other people. How could people know it by then?

That was the beginning of the science. People had just started to question everything they knew, and discovered almost all of it was wrong. And good scientists keep assumptions at check anyway, even now that we have so much evidence.


I like to explain that ray tracing uses exactly this ancient model of vision: rays are cast from an eye point, and are coloured according to the first object they hit ;-)


Or sit in a dark room, unable to see anything. There is more to it. See, e.g., https://en.m.wikipedia.org/wiki/Emission_theory_(vision)


Sure, that's why I didn't suggest that sitting in a dark room would be enough to dispel the theory. ("If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.")

I guess they could have argued that the rays of light returning from each object were perfect specular reflections that traveled directly back to the eyes that emitted them... but wow, that's still pretty hard to fathom, even for people who never heard of Occam's Razor.


Interestingly enough, ancient Indians(south Asia) recorded the speed of light to be 186k miles per second in 1500 BC. They did not document how they got that number but that is quite accurate!




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