Why the Speed of Light is the Ultimate Speed Limit | The Physics of the Universe

in our last lesson we talked about a couple of things we talked about what the principle of Relativity means in general relativity isn't a new thing it boils down to how two people can see the world very differently however when people say the word relativity they usually mean Einstein's theory of relativity which is a special and mind-bending case as I hopefully convinced you in the previous lesson we have a real and experimental data that proves that at very high speeds are when very precise measurements are made two clocks will ticket different speeds and that observers will disagree on the length of an object depending on whether they see the object moving or not so those last two things are pretty hard to accept but they're quite true they arise from two core principles the first is that all observers no matter their speed compared to other people have an equal right to claim that they are the one unmoving center of the universe that's not so new and unusual we've accepted that since the days of Galileo the second one is much less obvious and that is that the speed of light is the same for everyone and by much less obvious I mean it sounds absolutely nuts after all if you're sitting on a high-speed train with a kitten in your lap you'll reasonably say that the kitten isn't moving on the other hand someone sitting stationary the train crossing gates watching your train fly by will say that the kitten is moving at something like 60 miles per hour it seems that velocities always depend on the frame of reference in which they're measured but it's not true for light and we know this to be true we'll get to how we know that later in the lesson for the moment it might be interesting to get a sense of how we measure the speed of light and other interesting consequences of this very bizarre fact the speed of light is very fast indeed it's the fastest thing in the universe numerically is about 186 000 miles per second that's about fast enough to circumnavigate the Earth about seven and a half times per second I said about when I gave the speed of light an old style American units it's easier to State the speed of light precisely in the metric system where it is exactly 299 million 792 458 meters per second I know that to be precisely true because that's how the meter is now defined that definition was agreed upon in 1983. that level of precision is important for scientists but it isn't so crucial for our conversation thus around the speed of light up to being 300 million meters per second or 300 000 kilometers per second so measuring something that fast takes some doing I mean it's not like sound which is slow enough that there can be a perceptible delay I'm talking about places like a baseball game where you can see the distant battery hit the ball and hear the crack of the bad a split second later or when you see a high-flying plane overhead and hear this Allen coming from a different location with a speed of 770 miles per hour just over a thousand feet per second it's easy to understand how a location determined by hearing and vision would be different after all if the speed of sound is a thousand feet per second and you're looking at a plane that is flying at an altitude of thirty thousand feet you'll hear the sound coming not from where you see the plane but from where it was 30 seconds ago though with light any possible delay in any reasonable situation is imperceptible by human senses instead of the full second it takes for sound to reach a thousand feet it takes light a millionth of a second so how can we measure the speed of light well basically you either have to utilize very fast Electronics or measure very long distances and the second method is how the speed of light was first measured the first professional attempt to measure the speed of light was made in 1676 by Danish astronomer oleiromer he was making precise measurements of the eclipse of the moon IO as it orbited Jupiter attempting to determine the exact moment in which IO slipped into Jupiter's shadow this wasn't the esoteric exercise it seemed to be it actually had applications for Sailors needing to determine their location on the globe it's a fascinating Story how determining a ship's longitude depended on having an accurate clock you should look it up what Romer knew was that IO takes 42.5 hours to orbit Jupiter from that he could calculate the moment at which the eclipse should occur for every orbit however what he found was that his calculation and measurement disagreed furthermore the disagreement changed over time and depended on where Jupiter was in its orbit compared to where Earth was in its those two locations were constantly changing which therefore changed the distance between Jupiter and Earth using precise measurements and a lot of thinking Romer realized that he could explain his observations if light had a fast but finite speed he even calculated the light would take 22 minutes to travel from one side of the Earth's orbit to the other and from that he calculated that the speed of light is 220 000 kilometers per second which is about 26 percent lower than the actual speed but still it was an incredible achievement for someone working over three centuries ago those old guys knew their stuff of course we can repeat romer's measurement today and get more accurate results but now we have fast electronics and we make different measurements they more directly measure the speed of light for instance we can set two light detectors a thousand feet apart and then shoot a pulse laser from one to the other and measure that light took a millionth of a second to travel that distance or we could use a single light detector and a dist mirror to do the same thing a millionth of a second sounds very fast but even an inexpensive modern oscilloscope can easily measure time durations a thousand times shorter however measuring the speed of light is one thing proving that it's the same for all observers is a bit harder let's look into that a bit so just how did scientists figure out the speed of light is the same for all Observers let's talk about two methods the first one to conclusively approve the conjecture and then the more modern way to do it our first Story begins in the middle years of the 1800s when researchers had an unsolved question and that question is what conducts light remember that in the early 1800s Thomas Young had demonstrated pretty convincingly that light was a wave furthermore Einstein's photoelectric effect was still decades in the future time the wave theory of light was pretty much unchallenged the problem is that waves don't move like particles do throw a baseball and the ball moves from one spot to another but if you put a rubber ducky on the surface of a lake far from the shoreline and watch the waves roll in what you see is that the ducky mostly moves up and down but doesn't actually get closer to the shore the wave is the movement of energy through the water but the water or the ducky doesn't move in the direction the wave moves a similar thing is true with sound waves with sound waves air molecules move back and forth but the air itself doesn't experience any net motion the details of how the air molecules move are different from the water example but that really isn't important here what is important is that for all waves known to 19th century scientists the waves needed a medium like water or air to pass through a second important feature is that relativity applies to most waves and by that I mean ordinary 17th century Galilean relativity that means that if you were on a fast-moving train and yelled from one end to the other you'd measure a speed of sound however a person standing on a train station watching the train Zoom by would measure a different speed of sound sounds travel through a media and when we talk about the speed of a wave we mean the speed when the medium is stationary the problem for 19th century scientists is that there didn't seem to be anything through which light could travel after all space is completely empty totally devoid of matter and light from distant Stars makes it to the Earth of course scientists of the era didn't know with 100 certainty the space was a vacuum although they were pretty sure but they certainly had made Hollow glass vessels and pumped all of the air out of them leaving only a vacuum and when they did that it could see through the glass and see whatever objects were on the other side of the glass it was clear that light could pass through a vacuum and if light could pass through a vacuum the question was what was the material through which light was transmitted there was a proposed answer that originated in the 1600s when the question of the wave or particle nature of light was raging those who held to the wave theory of light proposed that the substance that transmitted light was called The luminiferous Ether or just ether for short ether was first proposed in ancient Greek time this is a substance found above the Earth what was called the terrestrial sphere back when such names as Plato Aristotle and Ptolemy were leading intellectual luminaries and the 17th 18th and 19th century ether was thought by some to be a substance that permeated the entire universe it was everywhere and conducted light there were a few issues with ether one thing is that it wasn't something that people could directly observe so that's a problem without observation you don't know if it's real or if it's like discussing how many angels dance on the head of a pin the other problem is that in general waves travel quickly through very stiff and dense materials and slowly through ones that are low density or that can move easily for instance sound tends to travel quicker through liquids than through gases and quicker through solids than liquids and even with solids sound travels faster through stiff materials like steel and slower through materials like clay if you want a cool fact sound is 17 times faster in steel than an air light as we have seen is extremely fast that suggests that the material that carries it should be solid and very very stiff yet of course we'd notice if the universe were solid and stiff so this was a bit of a problem proponents of The Ether simply table the problem as a mystery still to be solved there was another aspect of light and The Ether that troubled the late 19th century scientist and that was the question of whether the ether was moving or whether the Earth was moving through the ether or what and this particular question was made a little more Concrete in the early 1860s when Scottish physicist James Clark Maxwell pulled together a ton of work by earlier researchers and invented what we now call Maxwell's equations Maxwell's equations describe how electric fields and magnetic fields interact and how we really shouldn't talk about electricity and magnetism independently but rather that there is but a single phenomena called electromagnetism so that was a pretty big deal but it wasn't Central to the question of light however the connection between electromagnetism and light became Crystal Clear when Maxwell used his equations and combine them taking a dash of electricity in a dollop of magnetism combined with a pinch of calculus and a generous splash of algebra he combined his equations and showed that they predicted that there existed electromagnetic waves furthermore he was able to calculate the speed of electromagnetic waves and it turned out that his equation said that they moved at the speed of light mind you this is a little different from saying that light actually was an electromagnetic wave but it would have been a huge coincidence if it wasn't and remember this was before Einstein published his work that showed that light was also a particle so people were pretty convinced that Maxwell's work showed that light was simply an electromagnetic wave however I've glossed over something here I said that Maxwell's equations said that the electromagnetic waves traveled at the speed of light but from the last lesson we need to be cautious the speed of light speed of light relative to what the obvious answer was that Maxwell's equations predicted the speed of light with respect to the luminiferous ether and if that's true then it meant that we might be able to measure the motion of the earth Through The Ether and that brings us to an effort in 1887 when American physicist Albert Michelson and Edward Morley perform what is now known as the Michelson Morley experiment at what is now Case Western Reserve University in Cleveland Ohio in lesson three we talked about how Thomas Young sent light through two different slits and made a series of bright and dark spots on a distant screen whether the spots were bright or dark depended on a shift between the waves coming from each of the two slits which I remind you physicists call a phase shift if the peak of one wave coincided with the peak of another wave the result was a bright spot if the peak of one wave coincided with the trough of another wave the result was a dark spot the important thing is the shift between two waves so Michaelson and Morley decided to do a similar thing although the details were pretty different first let me describe their apparatus they started with a beam of light that they shot at what is called a half silver mirror half silvered mirror reflects half of the light and lets the other half pass through you make a half syllable Mirror by putting a very thin reflective coating on a piece of glass so that half silvered mirror was positioned at a 45 degree angle with respect to the beam that means that half the light pass through in a straight line while the other half was reflected at an angle of 90 degrees compared to the original light after passing through the half-silvered mirror the two beams were traveling at right angles to one another they then placed two full-strength mirrors to reflect those two beams of light back to the half-silvered Mirror the reflected beams both passed through and were reflected by the half-silvered mirror and this recombined the two beams onto a screen where they could be seen here's where the phase thing comes into play depending on whether the two beams of light arrived in Phase with the Peaks enhancing one another or out of phase with Peaks and troughs canceling the results would be either a bright spot or a dark spot the pattern on the screen is just a little more complicated than I described but that's the big idea before I get into the speed of light part of the experiment I should say that this experiment was extremely sensitive to vibrations after all the wavelength of light is less than a millionth of a meter less than a thousandth of a millimeter vibrations that would make one light path a tiny bit longer or shorter would cause the pattern to flicker between bright and dark spots in fact their equipment was so sensitive that someone stomping their foot at a distance away the length of a football field would ruin their measurements so to minimize the vibrations they put their apparatus Underground that helps a lot but not enough so they mounted their entire apparatus on a heavy Sandstone block and floated the Block in their equipment in a pool of Mercury Mercury as you probably know is a very dangerous substance that can be deadly it's especially dangerous to unborn children but Michelson and Morley were most certainly not carrying unborn children and it was the 1880s so OSHA didn't exist I showed her to think what my laboratory safety board would say if I proposed their experiment today but times are different and their apparatus did suppress vibrations because the Sandstone block was floating in a pool of mercury you could also spin it and this is a critical feature so this brings us back to The Ether according to the thinking of the time The Ether permeated the entire universe but we knew that the Earth rotates and orbits the sun and it was likely that the sun moves through the cosmos and that meant that it was very probable that the Earth Moved Through The Ether alternatively The Ether could be thought of as a kind of a wind blowing across the Earth because the michelson-morley apparatus shot two beams of light at 90 degrees with respect to one another that meant that they could rotate their apparatus so one arm was in the direction of the wind while the other was perpendicular to it and if the speed of light is defined with respect to the ether that meant in Michelson and Morley's lab light would travel at different speeds through the two arms if light traveled at different speeds that would affect the phase of the Waves passing through the two arms and that would affect the expected pattern on the screen of course the experimenters didn't know the direction The Ether was passing through the Earth but here's the beauty of the experiment they could rotate their apparatus which means that they could move the arms to be parallel to the movement of The Ether or perpendicular and even if they didn't know which was which they would expect the speed of light between the two arms to change as they rotated it that means that they should expect to see changes in the patterns of brightness and darkness on the screen if they saw these changes they would have discovered the ether so what did they see in a moment totally worth of a drum roll what they found was exactly zero nothing there was no change at all in the pattern of light and dark seen on their screen and the only realistic explanation for this is that the velocity of light in both paths didn't change with orientation and from that it logically follows that the michelson-morley experiment killed the idea of The Ether now the idea of The Ether didn't die immediately people thought of possible explanations like the idea that the Earth was dragging along the ethers that orbited the Sun so The Ether in the vicinity of the Earth was stationary as far as the Earth was concerned but the michelson-morley experiment was certainly the beginning of the end a series of researchers thought about a variety of possible explanations but slowly one by one they were all falsified until we are left with Einstein's conjecture which is that The Ether doesn't exist and that all observers measure light as traveling at a single and unique speed which is about 300 000 kilometers per second physicists use the symbol C that's a lowercase C to indicate the speed of light by the way here's a bit of physics trivia the symbol C was chosen to tie it to a certain Latin word celeritus which means speed or haste now before I change topics I wanted to tell you about a different method for showing the speed of light is the same for all observers and undeniable and modern measurement demonstrates that all observers see the speed of light to be a single value this measurement involves my specialty particle accelerators first let's start with a low speed example suppose you're a pitcher for a professional baseball team if you try hard you can throw a baseball about 100 miles per hour sounds hard my shoulder aches just thinking about it but that's what they can do now let's put the same picture in a fast-moving train traveling at 100 miles per hour with respect to the track that's a fast train in the U.S but quite slow compared to for example the tgv in France if we did this in a person standing stationary next to the tracks were to measure the speed of the baseball they'd see that it was moving at 200 miles per hour that's 100 miles per hour from the pitcher's arm and 100 miles per hour from the train itself pretty straightforward you can do a similar experiment using a particle accelerator and I want to remind you that this isn't a thought experiment this is something we do every day we're talking hard data here we can start by taking an electron and inducing it to emit a photon let me remind you that photon is a particle of light we can measure the speed of light in a straightforward Way by simply setting up two stationary light detectors a fixed distance apart and measuring how long it takes for light to go that distance divide the Distance by the time and you get the velocity and you find that it's 300 000 kilometers per second now we use a particle accelerator to shoot the electron down the accelerator at nearly the speed of light and when I say nearly the speed of light I'm talking extraordinarily close in an old style television like your grandparents watched the electrons traveled at something like 20 the speedo light but even in the most modest modern particle accelerator made it can accelerate electrons to 99.99999 the speed of light in decimal form that's 0.999 etc for a total of nine nines so that is to all intents and purposes essentially traveling at the speed of light following our baseball example if we see an electron traveling at the speed of light and it emits a beam of light the the electron sees traveling at the speed of light then we should see that light is traveling at twice the speed of light and if that's the case we should expect to see this super fast beam of light arrive at the far detector and half the time it takes the electron to travel that distance traveling the same distance at twice the speed takes half the time common sense except that's not what we see the electron an emitted Photon travel the prescribed distance and essentially the same amount of time the photon and electron both travel at the speed of light and obviously this makes no sense if the speed of the electron is added to the photon no sense at all it only makes sense if the photon is traveling at the speed of light both to the point of view of the electron and to you who sees the electron zooming along at very high speeds furthermore this experiment gives the same result for any time of day and any time of year this means that if there were any effects due to traveling through what we now know to be The non-existent Ether either in the direction of The Ether flow or perpendicular to it we'd see it that's because the orientation of the accelerator changes over the course of the days and the years when you get right down to it this modern measurement is inarguable proof that light travels at one speed for all Observers so I hope that I've convinced you that the speed of light is universal to all observers which is a very counter-intuitive concept but it's experimentally true this truth leads to all of the crazy sounding predictions of special relativity which I talked about in the previous lesson those have also been tested and found to be true further validating the claim that the speed of light is the same for everyone but there are some other fascinating facts that I thought you'd be interested in one such thing has to do with the claim that the speed of light is the same for all frequencies of light or wavelengths if you prefer that way of thinking about it remember from lesson three that Max Planck and Albert Einstein deduced that the energy of a photon of light depended directly on the frequency of the light therefore low frequency light has low energy and high frequency has high energy now for particles a low energy particle travels at a lower speed than high energy particles this is something you know from common experience a high energy bullet can do more damage than a low energy bullet of the same weight and caliber so it wouldn't be foolish to imagine that perhaps low energy light might travel at lower speeds than high energy light but we know this isn't true we know it to incredible precision the way we know this is by using an epic cosmic explosion called a gamma-ray burst exactly what causes a gamma ray burst is not known there are several different hypothesized causes but what we do know is that they are brief emissions of energy that are the brightest things in the cosmos they can be seen literally from across the universe they can be as little as two seconds and as long as a couple of minutes and they emit all sorts of light from the lowest energy radio waves to the highest energy gamma rays the gamma rays can easily be hundreds of billions or even trillions of times higher energy than the lower energy light it's a huge difference as it happens we have a variety of astronomical observatories they can see all wavelengths from gamma rays x-rays ultraviolet visible infrared microwave radio waste the whole smear and that means when a gamma ray burst goes off we can time the arrival of the electromagnetic energy in all wavelengths and frequencies one gamma-ray burst occurred in 2009 with the unimaginative name grb 090510 it exploded literally a long time ago in a galaxy far far away about 7.4 billion years ago the light traveled for all that time and the different wavelengths all arrived within a few seconds of one another let's think about what that means 7.4 billion years is about 200 quadrillion seconds the initial arrival times were all within two seconds that means the various different energy photons which means frequencies of light all arrived with a difference of no bigger than one part and 100 quadrillion that's unfathomably small that's like measuring my height with a Precision of about 1 100 the size of a proton or if you're an astronomy buff the distance to Alpha Centauri with a Precision of about a foot the different wavelength light traveled at the same speed to ridiculous precision this measurement combined with the observation that the speed of light is the same to all observers so something very deep and fundamental about the universe mind you we don't know exactly what it is but it's clearly telling us something I'm going to return to the speed of light in Lesson 16 when I talk about gravitational waves it turns out that gravity also has a speed and we've measured that too that's its own level of scientific awesomeness before that though we have some other fascinating topics to talk about see you soon

Channel: The Great Courses

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Id: RcJokUVjYjU

Length: 27min 55sec (1675 seconds)

Published: Mon Oct 03 2022