November 21 - How Fast Is Light? REALLY Fast!

Posted on November 21, 2017

Scientists don't tend to be satisfied with a description like "really fast." Scientists look for a way of measuring exactly how fast something moves. Even something that moves really, really, really fast, like light.

On this date in 1676, the Danish astronomer Ole Rømer presented his data and method for calculating the speed of light.

The story of Ole Rømer's work in this field is pretty complicated. But two important points come out of this story:

1. The story of science is almost always a story of collaboration - of working together. A scientist may explore a question raised by another scientist. She or he may use a method or even an idea about a method thought up by another scientist. He or she may use the data gathered by another scientist and put a fresh analysis on it, or extend the findings by gathering more data.

2. Communication is very important. Someone may have great data - but lose it in a fire. Someone may have an important idea - but if he or she only presents it orally, and depends on others to record he idea in writing, the reporters may get it wrong. Someone may have a clear understanding of a slice of the universe but may not express it clearly to other scientists and to the general public.

Here are a few bits from the story of Ole Rømer's measurement of the speed of light - you will notice how collaborative his work was, and you will see how important records and communication are:

It was very important to people to be able to measure longitude - making accurate maps and finding the way across featureless oceans depended on it. In the early 1600s, Galileo had proposed a way to use observations of the eclipses of the moons of Jupiter as a way of creating a kind of precise clock. In other words, an astronomer looking at Jupiter's moon Io can note the precise time that Jupiter's shadow falls on Io and the precise time that Io emerges from Jupiter's shadow.

In the late 1600s, astronomers Jean Picard, Giovanni Cassini, and Rømer all used Galileo's idea; they observed 140 eclipses of Jupiter's moon Io from Copenhagen, Denmark, and Paris, France. Comparing the times of the eclipses, they were able to figure out the difference in longitude between the two cities.

Cassini noticed discrepancies in his measurements over time, and he theorized that light took time to travel to Earth. Ole Rømer took up Cassini's suggestion, and he set about to prove the idea that light traveled at a finite speed. He took all the data about eclipses, from all three scientists, and then gathered more. 

Rømer figured that there are times when Earth's orbit and Jupiter's orbit line up so that Earth is moving toward Jupiter, and there are other times when Earth is moving away from Jupiter. In the diagram below, as Earth moves around the Sun from Point F to Point G, it is approaching Jupiter and Io. As in continues to orbit the Sun, going from Point L to Point K, it is moving away from Jupiter and Io. Rømer knew that he could measure the delay in the eclipse sightings and compute light's speed from that delay.

Unfortunately, Rømer didn't write all of that up himself. Instead, he presented his work to the French Academy of Sciences, orally. The record we have of that presentation was written by a reporter. And....the reporter didn't understand Rømer's presentation, and he wrote about it with deliberately vague and wordy sentences so that his lack of understanding wouldn't be too obvious!

Another bummer is that Rømer's data and observations were all destroyed a couple of decades after he died in a huge fire. We can be very glad that one of his assistants, who later became an astronomer, described and wrote about Rømer's observations. 

Rømer himself did not calculate the exact speed of light, but other astronomers were able to take his data and make the calculation. 

Still, many scientists still thought that light traveled instantaneously. It took some time and checking and rechecking - and by 1727, science as a whole fully accepted the fact that light has a finite speed. Later astronomers were able to use Rømer's data combined with more recent data - and, as timepieces got better, scientists were able to make more and more precise observations - to calculate the speed of light. By the early 1800s, it had been calculated as just a bit more than 300,000 km/second. Now we are even more precise, with the speed of light measured as 299,792.458 km/second.

According to science fiction, we will be able to be-bop
all over the universe, traveling instantly from
one spot to another, perhaps trillions of miles away.

According to science, that may never be possible.
If you want to read an article about "faster than the speed
of light," Fact/Myth website has a good one.

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