Everything Everywhere Daily: History, Science, Geography & More - The Speed of Light

Starting point is 00:00:00 There is nothing in the entire universe faster than the speed of light. Not only is light the fastest thing, but nothing can ever be faster than light. For the longest time, humans didn't even know that light had a speed, and once they figured out that light wasn't instantaneous, it took several centuries to determine what that speed was. Learn more about the speed of light and its implications for physics and engineering on this episode of Everything Everywhere Daily. What if your perceptions about the past were wrong? throughline is a podcast that takes you back in time to uncover the parts of the story that may have gone unnoticed.

Starting point is 00:00:48 It effectively turned day into night and how it shaped the world now. Time travel with us every week on the Thurline podcast from NPR. The speed of light in a vacuum is exactly 299,792,458 meters per second. This is a universal speed limit. Nothing can go faster. It is literally impossible. Before I get into the implications of this fact and what it means, I should start with how we even know that light has a speed.

Starting point is 00:01:27 Light is so incredibly fast that from a human perspective, it seems instantaneous. For the longest time, nobody was sure if light was instantaneous or just really fast. It was a subject of debate for philosophers. In the 5th century BC, the Greek philosopher in Patechles believed that light did have a finite speed. Aristotle, on the other hand, said, quote, light is due to the presence of something, but it is not a movement. Hence, he didn't think it had a speed, but was rather instantaneous. The Aristotelian view held sway for centuries. Much of this had to do with the fact that most people thought that light emanated from the eye.

Starting point is 00:02:04 According to Heron of Alexandria, light had to be instantaneous because when you open your eyes, the stars could be seen immediately. The Aristotelian view was challenged by the Islamic scientist, Hassan Ibn El-Hatham in the 11th century. He wrote a treatise called the Book of Optics, which argued for the intromission theory of light, which said that light entered the eye from the outside, and this led him to conclude that light must have a finite speed. These debates continued for centuries with no real resolution. Even famed thinkers such as Johannes Kepler and René Descartes thought that light had to be instantaneous. In the 17th century, the age of experimentation began, and people began to develop.

Starting point is 00:02:44 experiments to determine if light indeed had a speed. One of the first proposed experiments came from Galileo Galilei in 1638. He suggested having two people with covered lantern standing on hills some distance apart from each other. One person would remove the cover on their lantern, and the moment the other person saw the light, they too would remove the cover on theirs. The experiment was finally conducted almost 30 years later by the Academia del Chimento in Florence. They had two people about one mile apart, but their results were in conclusive. The only thing that they could conclude was that if light had a speed, it had to be incredibly fast. It wasn't until 1676 when the Danish astronomer Ole Romer finally was able to

Starting point is 00:03:26 prove conclusively that light had a finite speed. He came to this conclusion through observations of the moons of Jupiter, in particular the innermost moon, Io. I.O regularly passes into Jupiter's shadow, but from the Earth we can only see it either enter Jupiter's shadow or leave Jupiter shadow. which we can see depends on the time of year and the position of the Earth with respect to Jupiter. Romer noticed that the time between eclipses wasn't constant. When the Earth was moving away from Jupiter, the time between eclipses was slightly longer, and when it was moving towards Jupiter, it was slightly shorter. He figured there were only two explanations for this.

Starting point is 00:04:04 Either the orbit of Io was somehow changing based on the location of the Earth, which was highly improbable, or light had a finite speed. From his observations, he calculated that it would take light 22 minutes to travel the diameter of the Earth's orbit. The Dutch astronomer Christian Hoygens used this estimate with his calculation for the diameter of the Earth's orbit to come up with a value of 220 million meters per second, which is about 27% slower than the currently accepted value. Newton figured out that because Io didn't change color after an eclipse, all colors of light must travel at the same speed. By the 19th century, it was known that light was a manifestation of electromagnetism. It was then thought that if light was a wave and electricity and magnetism had fields,

Starting point is 00:04:49 then it had to exist in some sort of substance, which was called the ether. If that was the case, then light should have different speeds in different directions as the earth moved around the sun. However, that was not what they found. The speed of light was exactly the same in every direction. Albert Einstein had several realizations about the speed of light and the extremely strange things that happen at light speed. Because light travels at an absolute speed, it does not change its speed relative to an observer. For example, if you are riding in a train traveling 100 kilometers per hour

Starting point is 00:05:22 and you throw a ball forward while on the train at 50 kilometers per hour, to someone outside the train, the ball would seem to be traveling at 150 kilometers per hour. However, light doesn't work like that. Regardless of your initial position or velocity, light will always go the exact same speed. There's actually a lot more he discovered, and I'll leave that to a future episode on the subject of relativity,

Starting point is 00:05:47 but Einstein also determined that it's impossible for anything with mass to travel at the speed of light. To do so would require an infinite amount of energy. This is why protons, tiny protons in a particle accelerator, can get all the way to 99.999% the speed of light, but it can never reach it, in the same way that temperature can never reach absolute zero. Also, the closer you get to the speed of light, the more time will dilate.

Starting point is 00:06:14 Time will go more slowly the faster someone is moving. This has caused all sorts of problems for science fiction writers who need to create some sort of system of interstellar travel that doesn't involve traveling faster than the speed of light. So the Millennium Falcon jumping to light speed, that can't happen. Einstein also realized that the speed of light as a fundamental constant of the universe, was tied up in the relationship between matter and energy. The letter used to represent the speed of light in equations is the letter C, and that is the C in E equals MC squared.

Starting point is 00:06:47 In the 20th century, measurements of the speed of light became ever more precise. However, it was eventually realized that because the speed of light is an absolute universal constant, it wasn't something we should even try to measure. It was something we should use to define our measurements. So in 1975, the 15th General Counsel on Wights and Measures defined the distance light traveled in one second to be exactly 299,792,458 meters. Take one over that and you get the exact length of one meter. In our everyday lives, the speed of light doesn't really factor into things. For all practical purposes, we can treat it as if it were instantaneous. However, there are

Starting point is 00:07:30 situations where the speed of light needs to be taken into consideration. The most obvious one is space travel. The distances involved in anything other than low Earth orbit spaceflight can make communications awkward or difficult. The average distance from the Earth to the moon is approximately 1.28 light seconds. When Mission Control would talk to the Apollo astronauts on the moon, there was always a slight pause because of the time it took for the radio signal to get to the moon and be sent back. A little more than one second delay isn't that big of a deal when having a conversation, but it does become a big deal if you want it to say control a drone remotely from the Earth. Once you get further than the moon, then it becomes a really big deal.

Starting point is 00:08:12 The time it takes to send a radio signal to Mars can vary depending on where the planets are in their orbits. It can be as short as 3.1 minutes or as long as 22.2 minutes. If you've ever watched footage of the landing of a Mars rover, you'll see a roomful of people at Mission Control. at the jet propulsion laboratory waiting to get the results on if it managed to land successfully. By the time they find out, they're actually watching something that might have happened 10 minutes earlier. The delay is due to the speed of light, and this is why all planetary probes and landers have to be fully automated. Likewise, if you go out even further, the effect is even more pronounced. Telescopes like the James Webb that see the edge of the universe are actually

Starting point is 00:08:53 looking into the past. The distances are so vast that it took the light emitted by distant galaxies billions of years to reach the Earth. There are parts of the universe that we may never see because they're traveling away from us at such a speed that the light will never catch us. The distant objects are not going faster than the speed of light, but the distance between us is growing at a faster rate. You don't have to deal with interplanetary distances to take into consideration the speed of light. You can also deal with very short units of time. One area where this comes into play is high-frequency stock trading. There is a category of stock traders who make very rapid trades with very small changes in price. For them,

Starting point is 00:09:36 differences of fractions of milliseconds can be the difference between making or losing money. One company called Spread Networks built a custom fiber optic cable from Chicago to New York in an incredibly straight line just to minimize the time it took light signal, to get from one city to the other. They invested tens of millions of dollars just to reduce the time it took to send a signal via a fiber optic cable from 13.1 milliseconds

Starting point is 00:10:00 down to 12.98 milliseconds. And they had people that were willing to pay for that improved service. So far, I've mentioned that the speed of light is an absolute universal constant that can't change. And that is true. However, I also mentioned at the very top of the episode that that was for light in a vacuum.

Starting point is 00:10:19 light actually can travel slower if it's traveling through some other medium. In fact, when light travels through a fiber optic cable, it goes about one-third slower than it does in a vacuum. That's still incredibly fast, but that does become a factor to consider when communicating with the other side of the Earth. The company SpaceX has a constellation of low-Earth orbit satellites, which provides internet access all over the globe called Starlink. In the next generation of Starlink satellites that they will be launching, they will be able to route data between satellites in orbit using lasers. Because light travels faster in the vacuum of space than it does in a fiber optic cable, in theory, Starlink should be able to have lower latency across long distances than fiber optic cables. While the route may be a bit longer being in orbit,

Starting point is 00:11:07 the difference in the speed of light can more than compensate for it. The speed of light is the ultimate speed limit. It determines how fast we can travel and how quickly we can communicate. And no matter how hard you try, it is the one speed limit you will never be able to violate. The executive producer of Everything Everywhere Daily is Charles Daniel. The associate producers are Thor Thompson and Peter Bennett. I just want to thank everyone, including the show's producers, who support the show over on Patreon. If you'd like to support the show, just head over to patreon.com, which is currently the only place where you can get show merchandise. Also, if you want to talk to other listeners about the show, head over to our Facebook group or

Starting point is 00:11:50 Discord server, both of which have links in the show. in the show notes.

Everything Everywhere Daily: History, Science, Geography & More - The Speed of Light

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