[ThTh1] Everyone knows that light is the fastest thing in the Universe. Nothing can move faster than light and the laws of physics get very strange when you get anywhere close to it. In fact, light moves so fast that it took us a few millennia of scientific endeavor to find that it moved at all. Aristotle, among others, thought light moved instantaneously. So how was the speed of light first measured?
Galileo had one of the more famous ideas, although it’s not clear if he ever tried to execute it. Imagine you wanted to measure the speed of sound. Here’s one way you could do it. Have two people stand a mile apart. Have one fire a starter pistol and start a stopwatch. When the other person hears the pistol, they fire theirs. When the first participant hears that second report, he stops the watch. You then divide two miles by the time it took for the sound to travel back and forth to allow the watch to be stopped (about nine seconds). Do this and you’ll have the speed of sound to reasonable accuracy. I’ve done a rough estimate of this at Penn State football games from “We Are!” cheers and…I wasn’t far off.
Galileo proposed doing this for light using covered lanterns. One person would open his lantern. When the other saw the lantern, he would open his. The problem is that light moves too damn fast for this. It takes 5 millionths of a second for light to travel a mile. So, in the end, you’d only be measuring reaction time.
In the 17th century, however, Ole Romer figured out a practical way to measure the speed of light. It depended on Jupiter’s moon Io, which passes behind the planet and into its shadow on a regular basis. The time at which it passes into Jupiter’s shadow is easily calculated from its known orbit. However, the time at which we see it pass into that shadow depends on how far away Jupiter is. Olmer timed Io’s eclipses and found that they were occurring later than expected when Jupiter was at its furthest from us — about 10-15 minutes later, depending on the time of year. This gave him the speed of light relative to Earth’s distance from the Sun and later a number of 220,000 kilometers per second, not that far off from the modern 300,000.
The Era of Classical Astronomy — from Galileo through Newton — was full of these kinds of elegant experiments to measure the shape and size of the solar system, the distance to the nearest stars and the speed of light.
[ThTh2] For the record, this was not me. Although I appreciate everyone who checked in just to be sure.
[ThT3] More evidence that plastic recycling is an ecological fiasco. At this point, you’re better off putting your plastic in a nice landfill than recycling it.
[ThTh4] So how big is the Milky Way? Depends on what we’re talking about. The stars of the Milky Way occupy a disk about 100,000 light years across. But a new paper measured the dark matter halo in which the Milky Way is embedded and found it to be 1.9 million years across, extending halfway to the Andromeda galaxy.
[ThTh5] More evidence emerges of a star being torn apart by a black hole.
[ThTh6] Was the coronavirus made in a lab? Almost certainly not.
[ThTh7] The bad news is that the oceans are taking a beating from climate change, overfishing and pollution. The good news is they can improve rapidly and some recovery is already taking place.
[ThTh8] The antarctic once hosted rain forests.
The experiment that I heard about that, now that I’m thinking about it, must not have worked.
Imagine, if you will, an octogon mirror box. In the style of a cylinder.
Now imagine two mirrors, at a 90 degree angle, a mile away. These mirrors are set up in such a way that if you shoot a laser at the cylinder at a 90 degree angle to it, offset by a few inches, the light travels a mile to hit the angle bounces 90 degrees to hit the other mirror, then bounces back and hits the cylinder again on one of the other mirrors two sides away and go in the exact same spot as if you were shining the laser without the cylinder there in the first place. That same exact spot is where you have the measuring device. It just needs to be something that says “yep, I’m being hit with a laser!”
With me?
Then start rotating the cylinder. When you find the speed at which the cylinder can turn and the initial flash of light bounces off of the first mirror and goes two miles, comes back, and hits the mirror only one side away (instead of two) and you’ve got the speed of light.
As a kid, I thought that that was elegant.
And *NOW* I’m thinking “that requires a level of precision that makes sense in comic books but they’d never be able to pull that off in real life.”
And the planet example is elegant as hell. And it predates Watt’s steam engine.Report
That experiment works just fine. We did it as undergraduates. You shine a laser onto a spinning mirror. The beam goes to a second mirror and comes back, at which point the mirror has moved slightly and creates a dot offset from the original. That was done in the late 19th century to try to characterize the ether.Report
No one is a bigger supporter of ether characterization than I, but I’m questioning the whole “0 degrees of shift over two miles using 4 mirrors” thing.Report
I don’t think the spinning mirror method is two miles.Report
I’d hope not! But my high school physics teacher explained the experiment talking about one guy on one mountain and another guy on another mountain and then did the math for us and we all sat agape at how brilliant it was.Report
Michelson-Morley Experiment. That was the word I was looking for:
https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experimentReport
That is an awesome page. Thank you!Report
Asimov once suggested repeating Michelson-Morley on the Moon. Just in case we’d misunderstood the results all these years and the earth really was the fixed point of the universe.Report
Michelson served as an officer aboard the USS Constitution (the three-masted frigate launched in 1797), when she was still an active warship in the US Navy. Later in life, long after he disproved aether theory by measuring the speed of light, he met with Einstein and Edwin Hubble at Mt Wilson to discuss relativity, red-shifts, other galaxies, and the big bang.
Nobody would ever write that as a character arc in a novel, except perhaps the old “Outlander” series about immortal beings. It’s like taking a character straight out of Pirates of the Caribbean and making him a science officer on Star Trek.Report
Trivia I am almost ashamed to know: The Cartwrights on “Bonanza” back in 1962, helped Michelson with his speed-of-light experiment.Report