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, general relativity is not something new, it boils down to how two people can see the world very differently, however, when people say the word relativity, they usually mean the theory of relativity. Einstein's relativity, which is a special and mind-blowing case, as I hope to convince you in the previous lesson, we have real and experimental data that show that at very high

speed

s, when very precise measurements are made, two clocks will tick differently.

speed

s and that observers will not agree on the length of an object depending on whether they see the object in motion or not, so those last two things are quite difficult to accept, but they are quite true, they arise from two basic principles: The first is that all observers no matter their speed compared to other people, have the same right to claim that they are the only immobile center of the

universe

that is not so new and unusual.

We have accepted that since the days of Galileo the second is much less obvious and that is that the speed of

light

is the same for everyone and by much less obvious I mean that, after all, it sounds absolutely crazy, if you are sitting on a train high speed with a kitten on your lap, you will reasonably say that the kitten is not. On the other hand, if it moves, someone sitting motionless at the train crossing gates watching their train go by will say that the kitten is moving at about 60 miles per hour, it seems like speeds always depend on the frame of reference in which they are measured. , but it is not true for

light

and we know this to be true, we will see how we know this later in the lesson, for the moment it might be interesting to get an idea of ​​how we measure the speed of light and other interesting consequences of this.

It is a very strange fact that the speed of light is very fast, in fact it is the fastest in the

universe

numerically, approximately 186,000 miles per second, which is fast enough to circumnavigate the Earth approximately seven and a half times per second. . I said it when I gave the speed. of light in old-style American units, it is easier to indicate the speed of light precisely in the metric system, where it is exactly 299 million 792 458 meters per second. I know that is precisely true because that is how the meter is now defined, that definition was agreed upon. in 1983.

That level of precision is important to scientists, but it's not as crucial to our conversation about, for example, the speed of light up to 300 million meters per second or 300,000 kilometers per second, so measuring something so fast it takes some time. I mean, it's not a slow enough sound that there could be any noticeable delay. I'm talking about places like a baseball game where you can see the distant drummer hit the ball and hear the evil crack a split second later or when you see a plane flying high and hear Allen coming from a different location with a speed of 770 miles per hour, just over a thousand feet per second, it is 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 are looking at an airplane flying at an altitude thirty thousand feet, you will hear the sound not from where you see the plane but from where it was 30 seconds ago. although with light any possible delay in any reasonable situation is imperceptible to human senses, instead of the full second it takes for sound to reach a thousand feet, light takes a millionth of a second, so how can we properly measure the speed of the light? you have to use very fast electronic devices 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 the Danish astronomer Oleiromer who was making precise measurements of the eclipse. of the moon IO as it orbited Jupiter trying to determine the exact moment IO slipped into Jupiter's shadow this was not the esoteric exercise it appeared to be it actually had applications for sailors who needed to determine their location on the globe it's a story Fascinating how determining the length of a ship 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 time at which the eclipse should occur for each orbit, however what he found was that his calculation and measurement were not agree, also the disagreement changed over time and depended on where Jupiter was in its orbit compared to where Earth was.

Those two locations were constantly changing, which therefore changed the distance between Jupiter and Earth using precise measurements and a lot of thought. Romer realized that he could explain his observations if light had a fast but finite speed, he even calculated that 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 less than the actual speed, but it was still an incredible achievement for someone who worked more than three centuries ago, those old guys knew what they were doing, of course, we can repeat Romer 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 example we can place two light detectors a thousand feet apart and then fire a laser from pulse from one to the other and measure that the light took a millionth of a second to travel that distance or we could use a single light detector and a distance mirror to do the same, a millionth of a second sounds very fast but even a modern oscilloscope and economical can easily measure time durations a thousand times shorter, however, measuring the speed of light is a bit more difficult, so how did they discover Scientists believe that the speed of light is the same for all observers?

Let's talk about two methods, the first to conclusively approve the conjecture and then the more modern way to do it, our first story begins in the mid-19th century when researchers had an unsolved question and that question is what conducts light . Remember that at the beginning of the 19th century, Thomas Young had shown quite convincingly that light was also a wave. Einstein's photoelectric effect was still decades in the future, when the wave theory of light was virtually undisputed, the problem is that waves don't move like particles throw a baseball and the ball moves from one place to another. , but if you put a rubber duck on the surface of a lake far from the shore and watch the waves roll in, what you see is that the ducky moves mostly up and down, but it doesn't actually get close to the shore, The wave is the movement of energy through the water, but the water or the duck does not move in the direction in which the wave moves.

Something similar happens with sound waves with sound waves. Air molecules move forward. and back but the air itself does not experience any net motion the details of how air molecules move are different from the water example but that's not really important here the important thing is that for all waves known to scientists 19th century, waves needed a medium such as water or air to pass through. A second important characteristic is that relativity applies to most waves. and by this I mean ordinary Galilean relativity from the 17th century, which means that if you were on a fast-moving train and shouted from one end to the other, you would measure the speed of sound, yet a person standing at a train station looking at the train Zoom by would measure a different speed of sound sounds travel through a medium and when we talk about the speed of a wave we are referring to the speed when the medium is stationary the problem for 19th century scientists is that it did not seem There was nothing through which light could travel after all, space is completely empty, totally devoid of matter and light from distant stars reaches Earth, of course, scientists at the time did not know with 100 certainty that the space was a void, although they were quite safe, but they certainly had it.

They made hollow glass containers and pumped all the air out of them, leaving only a vacuum and when they did this, they were able to see through the glass and see whatever object was on the other side of the glass, it was clear that light could pass through. of the void. and if light could pass through a vacuum, the question was what was the material through which the light was transmitted. There was a proposed answer that originated in the 17th century, when the question of the wave or particle nature of light infuriated those who held the wave theory. of light proposed that the substance that transmitted light be called Luminiferous Ether or simply ether, for short ether.

It was first proposed in the time of ancient Greece. This is a substance found on Earth. What was called the terrestrial sphere when names like Plato, Aristotle and Ptolemy. They were leading intellectual luminaries and in the 17th, 18th and 19th centuries some thought that aether was a substance that permeated the entire universe, it was everywhere and conducted light, there were some problems with aether, one thing is that it was not something that people could observe directly then that is a problem without observation you do not know if it is real or if it is like discussing how many angels dance on the head of a pin the other problem is that in general the waves travel quickly through very rigid materials and dense and slowly through those that are of low density or that can move easily, for example, sound tends to travel faster through liquids than through gases and faster through solids than liquids and even with solids , sound travels faster through rigid materials like steel and slower through materials like clay if you want an An interesting fact: sound is 17 times faster in steel than air, light, as we have Seen, it is extremely fast, which suggests that the material carrying it must be solid and very, very rigid, but of course we would notice it if the universe were solid and rigid.

This was a bit of a problem. Defenders of Aether simply present the problem as a mystery yet to be solved. There was another aspect of light and the Aether that concerned scientists in the late 19th century and that was the question of whether the aether was moving or not. whether the Earth was moving through the ether or what, and this particular question became a little more concrete in the early 1860s, when Scottish physicist James Clark Maxwell gathered a ton of work from previous researchers and invented what we now call Maxwell's equations. Maxwell's equations describe how electric and magnetic fields interact and how we shouldn't really talk about electricity and magnetism independently, but rather that there is a single phenomenon called electromagnetism, so it was a big deal, but it wasn't central. for the question of light.

The connection between electromagnetism and light became very clear when Maxwell used his equations and combined them by taking a pinch of electricity in a spoonful of magnetism combined with a pinch of calculus and a generous touch of algebra. He combined his equations and showed that they predicted that electromagnetic waves existed. In addition, 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. Note that this is a little different than saying that light was actually an electromagnetic wave, but it would have been quite a coincidence if it wasn't and remember that this was before Einstein published his work showing that light was also was a particle, so people were pretty convinced that Maxwell's work proved that light was simply an electromagnetic wave, however, I've overlooked something here.

I said that Maxwell's equations said that electromagnetic waves traveled at the speed of light, but since the last lesson we have to be careful about the speed of light in relation to which the obvious answer was that Maxwell's equations predicted the speed of light with respect to the luminiferous ether and if that is 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 the American physicist Albert Michelson and Edward Morley did what which 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, depending on whether the dots were bright or dark. in a change between the waves coming from each of the two slits, what I remind you that physicists call a phase change, if the peak of a wave coincided with thepeak of another wave, the result was a bright spot, if the peak of one wave coincided with the Through another wave, the result was a dark spot, what is important is the change between two waves, so Michaelson and Morley decided to do something similar, although the details were quite different.

First, let me describe your apparatus. They started with a ray of light shot at what is called a half silver mirror, half a silver 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 layer on a piece of glass so that the silver half-mirror is placed at a 45-degree angle to the beam, meaning that half of the light passed in a straight line. while the other half was reflected at a 90 degree angle compared to the original light after passing through the half-silver mirror, the two beams were traveling at right angles to each other, then they placed two heavy-duty mirrors to reflect those two rays of light back to the half-silver mirror.

The reflected rays passed through and were reflected by the half-silver mirror and this recombined the two rays. on a screen where they could be seen this is where the phase comes into play depending on whether the two beams of light arrived in phase with the peaks enhancing each other or out of phase with peaks and valleys canceling each other out the results would be a bright spot or a dark spot The pattern on the screen is 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 must 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, the vibrations that would make a path of light. a little longer or shorter would make the pattern flicker between bright and dark dots. In fact, his equipment was so sensitive that someone stomping his foot at a distance the length of a football field would ruin his measures to minimize the vibrations they set. their apparatus underground that helps a lot but not enough, so they mounted their entire apparatus on a heavy block of sandstone and floated the block on their equipment in a puddle of mercury.

Mercury, as you probably know, is a very dangerous substance that can be deadly, it is especially dangerous. to unborn children, but Michelson and Morley were certainly not pregnant with unborn children and it was the 1880s, so OSHA did not exist. I showed him to think what the safety board of my laboratory would say if he proposed his experiment today, but times are different and his devices suppressed the vibrations because the sandstone block was floating in a puddle of mercury, it could also be rotated and this is a critical feature, so this takes us back to the Aether according to the thinking of the time.

The Aether permeated the entire universe, but we knew that the Earth rotates and orbits the sun and the sun was likely to move through the cosmos and that meant the Earth was very likely to move through the ether. Alternatively, the ether could be thought of as a kind of wind that blows through the Earth. Because the Michelson-Morley apparatus fired two beams of light at 90 degrees to each other, that meant that they could rotate their apparatus so that 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 which meant that in Michelson and Morley's laboratory 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 did not know the direction in which the ether passed through the Earth, but here is the beauty of the experiment: they could rotate their apparatus, which means they could move arms to be parallel to the motion of the aether 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 they should expect to see changes in the patterns of light. 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 that was worth a drum roll?

What they found was exactly zero, nothing, there was no change in the pattern of light and dark seen on their screen and the only realistic explanation for this is that the speed of light in both paths did not change with orientation and from that It logically follows that the Michelson-Morley experiment put an end to the idea of ​​Aether. Now, the idea of ​​Ether did not die immediately. of possible explanations such as the idea that the Earth was dragging the ethers orbiting 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 without a doubt the beginning of the end.

A number of researchers thought of a variety of possible explanations, but little by little, one by one, they were all disproved until we were left with Einstein's conjecture, which is that the ether does not exist and that all observers measure that light it travels at a single and unique speed, which is approximately 300,000 kilometers per second physicists use the symbol C which is a lowercase C to indicate the speed of light by the way here is some

physics

trivia the symbol C was chosen to link it to a certain Latin word celeritus which means speed or haste Now, before changing the subject, I wanted to talk to you about a different method of showing that the speed of light is the same for all observers and undeniable modern measurement proves that all Observers see the speed of light as a single value.

This measurement involves my specialty. particle accelerators first let's start with a low speed example let's say you're a pitcher for a professional baseball team if you try hard you can throw a baseball at about 100 miles per hour it sounds hard my shoulder hurts just thinking about it but that's it what they can do now let's put the same image on a train traveling fast at 100 miles per hour relative to the track which is a fast train in the US but quite slow compared to for example the tgv in France if we did This is for a person standing on the side of the tracks, if you measured the speed of the baseball, you would see that it was moving at 200 miles per hour, that is, 100 miles per hour from the pitcher's arm and 100 miles per hour from the pitcher's arm. train.

It's pretty simple, you can do something similar. I experiment using a particle accelerator and I want to remind you that this is not a thought experiment, this is something we do every day. We are talking about concrete data. We can start by taking an electron and inducing it to emit a photon. Let me remind you. If we know that the photon is a particle of light, we can measure the speed of light in a simple way by simply installing two stationary light detectors at a fixed distance from each other and measuring how long it takes the light to travel that distance, we divide the distance times the time and you get the speed and you discover that it is 300,000 kilometers per second.

Now we use a particle accelerator to shoot the electron through the accelerator at almost the speed of light, and when I say almost the speed of light I'm talking about extraordinarily close. Old-style television, like the one your grandparents watched, electrons traveled at something like 20 times the speed of light, but even the most modest modern particle accelerator can accelerate electrons to 99.99999 the speed of light. light in decimal form, which is 0.999, etc., for a total of nine nines. that is, for 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 that the electron sees traveling at the speed of light, then we should see. that light travels at twice the speed of light and if that's the case we should expect to see this super fast beam of light reach the distant detector and half the time it takes for the electron to travel that distance traveling the same distance to the double speed.

Speed ​​takes half the time of common sense, except that's not what we see: the electron, an emitted photon travels the prescribed distance and essentially the same amount of time as the photon and electron travel at the speed of light. and obviously this doesn't make sense if the speed of the electron is added to the photon it doesn't make any sense it only makes sense if the photon travels at the speed of light both from the point of view of the electron and to you who see the electron approaching at very high speeds, furthermore, this experiment gives the same result for any time of the day and any time of the year, this means that if there were any effects due to travel through what we now know to be the non-existent ether, since either in the direction of the aether flow or perpendicular to it, You would see this because the orientation of the accelerator changes over days and years.

When you get down to it, this modern measurement is indisputable proof that light travels at a speed for all observers, so I hope I did it. I convinced you that the speed of light is universal for all observers, which is a very counterintuitive concept, but is experimentally true. This truth leads to all the crazy-sounding predictions of special relativity that I talked about in the previous lesson and that have also been proven. and it was 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 might be interested in.

One of those things 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 was directly dependent on the frequency of light therefore low frequency light has low energy and high frequency have 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 maybe low light Energy can travel at slower speeds than high energy light, but we know this is not true, we know it with incredible precision, the way we know it is by using an epic cosmic explosion.

It's called a gamma ray burst. It is not known exactly what causes a gamma ray burst. There are several different hypothetical causes, but what we do know is that they are brief bursts of energy that are the brightest things in the cosmos and can literally be seen from the other side. In the universe they can last as little as two seconds and up to a couple of minutes and emit all types of light, from the lowest energy radio waves to the highest energy gamma rays. Gamma rays can easily be hundreds of billions or even trillions of times more energetic than lower energy light - it's a big difference, as we just happen to have a variety of astronomical observatories that can see all wavelengths of the rays. gamma x rays ultraviolet visible infrared microwave radio waste the entire spot and that means that when a gamma ray burst goes out we can time the arrival of electromagnetic energy at all wavelengths and frequencies imaginative name grb 090510 literally exploded a long time ago in a galaxy far, far away about 7.4 billion years ago light traveled all that time and the different wavelengths arrived within a few seconds of each other.

Let's think about what that means. 7.4 billion years is approximately 200 quadrillion seconds. The initial arrival times were all within two seconds, meaning that different photons of different energy, meaning all frequencies of light arrived with a difference no greater than one part and 100 quadrillion, which is unfathomably small, It's like measuring my height to about 1,100 of the size of a proton or, if you're an astronomist, improving the distance to Alpha Centauri. With an accuracy of about a foot, light of different wavelengths traveled at the same speed with ridiculous precision. This measurement was combined with the observation that the speed of light is the same for all observers, so there is something very deep and fundamental about the universe.

I don't know exactly what it is, but it's clearly telling us something. I'm going to go back 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 genius before that, although we have other topics Fascinating things to talk about, see you soon