
How the Speed of Light Was First Measured - evo_9
http://www.todayifoundout.com/index.php/2013/08/how-the-speed-of-light-was-first-measured/
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spdustin
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.

~~~
pmiller2
>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...](http://www.technologyreview.com/article/400335/melted-chocolate-to-
microwave/)

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grecy
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.

~~~
alister
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/](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]."

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

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

~~~
deepfriedbits
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.

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lifeisstillgood
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](https://github.com/mikadosoftware/importantexperiments4kids)

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Animats
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.

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pmiller2
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](https://en.wikipedia.org/wiki/Speed_of_light)

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dnautics
"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.

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

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

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alfapla
Why not link to the original article instead? ;)

[http://gallica.bnf.fr/ark:/12148/bpt6k56527v/f234.image](http://gallica.bnf.fr/ark:/12148/bpt6k56527v/f234.image)

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

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hackaflocka
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.

~~~
jhallenworld
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.

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Aardwolf
"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.

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

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hackaflocka
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).

~~~
alfapla
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.

~~~
hackaflocka
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!

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jhallenworld
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..

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

~~~
manojlds
A brief history of time does too.

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CamperBob2
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.

~~~
pierrec
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](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.

~~~
CamperBob2
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.

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silentsea90
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!

