Can We Travel Faster Than Light? | Understanding the Misconceptions of Science

in all of science there are very few statements that are absolutely true and one of those is that you can't go faster than the speed of light that one is pretty safe except except it's not or more accurately like any absolute statement you need to be careful because such an assertion always has some assumptions that are often left unspoken in this lecture we'll talk about the various ways in which talking about the speed of light can be confusing but first let's talk about the speed of light and what it is and what people mean when they talk about note going faster than light we'll begin with what light is which is a pretty basic thing i mean you all know what light is it's a thing that lets us see light is the thing that comes out at the end of a flashlight or brightens the sky on a summer day we will speak in a later lecture about the nature of light whether it was a wave or a particle and what we're talking about here it's rarely important to make the distinction because the velocity of light doesn't depend on its nature in this lecture it's probably simplest to think of light as a wave most of the time so that's what we'll do if it becomes important to make the distinction i'll make that clear so light is an electromagnetic field traveling through space it doesn't have to be visible light for there are wavelengths that the eye can't see there are radio waves and microwaves infrared and ultraviolet x-rays and gamma rays all of these types of radiation travel at the same speed as visible light and just to be clear within the range of visible light the whole roy g biv spectrum of red orange yellow green blue indigo and violet the speed of light is identical for all of those colors now what do i mean when i say the speed of light i mean the speed of light in a vacuum indeed most of the times where i talk about variations in the speed of light it will be in material that's not 100 true but it mostly is first what is the speed of light well light moves incredibly quickly so fast that it's hard to visualize the speed of light is 186 000 miles per second or if you're a metric kind of person that is 299 million 792 458 meters per second in order to get at least kind of a feel for how fast that is light is so fast that it could circle the earth seven and a half times in a single second by the way that number i gave you for the speed of light in the vacuum in meters is exact there are no additional decimals this is because since 1983 the definition of a meter was redefined to be the distance that light travels in one over 299 million 792 458 of a second so for those of you who like trivia you only have to remember nine digits not too bad and you'll be the life of the party well depending i suppose on the kind of parties you attend when you think about it how you measure something that quick is quite a feat you'd think that it would take the most modern technology but the first successful measurement of the speed of light was accomplished in 1676. danish astronomer olaf romer calculated the orbits of the moons of jupiter down to knowing exactly when the moons would pass in front or behind the larger planet he also knew that depending on whether jupiter was closer or more distant to the earth in its orbit his calculations would disagree with his predictions by about a thousand seconds about 15 minutes he surmised that the difference was due to the amount of time it took for light to travel from jupiter to the earth knowing the distances and the time differences he could work out the velocities and determined that the speed was 214 million meters per second or about two-thirds the modern value that was a pretty respectable effort given that we're talking about almost 350 years ago there were some other attempts using astronomical approaches but the first measurement of the speed of light located entirely on the earth was french physicist armand fazeau he used a spinning gear a bright light and a mirror located about five miles away he shined the light at a distant mirror and saw when the light reflected back to him by looking through the gears as they spun his measurement was 315 million meters per second accurate to about five percent french physicist leon foucault did a modified version of the experiment using rotating mirrors that was accurate to better than a percent when what are called maxwell's equations were written down in the late 1860 by james clark maxwell it was possible to calculate the speed of light using his results to combine measurements taken of the electrical and magnetic properties of space at that point the determination of the speed of light was sufficiently accurate that it became increasingly necessary to account for the fact that light travels at different speeds and materials than it does in a vacuum [Music] so let's start with the bottom line in most transparent materials like glass water plastic etc light travels at a lower speed than it does in a vacuum and the differences is pretty large in glass and plastic light travels at about two-thirds the speed it does in a vacuum in water it's a bit faster about 75 percent the speed of light in the vacuum in diamond the effect is even greater in diamond light travels at 40 percent under half of its normal speed in the visible spectrum diamond is pretty much the record holder although there are a few obscure materials that beat it by just a bit but diamonds are both familiar and pretty so let's give them the credit in other wavelengths other materials can slow electromagnetic waves even more but let's just stick with visible light because it's familiar when light hits a transparent medium it literally slows down and then speeds up again when it leaves now i say literally and i mean it but it's more complicated than you'd think which is of course one of the topics we'll cover and even though you might have learned that light still slows down in glass if you ever took an introductory college physics class you almost certainly didn't through think through the idea of something slowing down and then speeding back up again i mean you know how to do that in the car hit the brake and then the accelerator but how do you accelerate light i better have you thinking now the term that you might have learned for this phenomena is simply the index of refraction the index of a fraction of materials make the path of light bend when it passes through the material you can see this most easily by just putting a pencil in a glass of water it's pretty easy to see that this bending is caused by the slowing of light as it hits the material there are tons of related explanations but this is probably the easiest to see let's think of light as a wave and what you're seeing here are the peaks of the wave as they travel through space as they hit the surface of water or glass or whatever they slow down we'll get to the slowing mechanism in a minute just accept that this is what's going on just for a moment light has four parameters that matter the frequency of oscillation the wavelength which is the color by the way the velocity and the amplitude or brightness classically the brightness is the strength of the electric field the frequency is the number of oscillations per second since the light exists both outside and inside the medium and further at the surface the oscillation rate must be the same that means that the frequency of the light must be the same inside and outside the medium if the velocity slows down that means that the waves move a smaller amount for every period of time since there has to be the same number of oscillations per second that means that the wavelength of light gets shorter inside the glass or water or whatever thus seeing the bending is evidence for the slowing down in water however that doesn't tell you what the mechanism that actually slows down the water there are several explanations that you can find in books some of those invoke classical physics and some of them attempt to use quantum principles it turns out that the classical mindset is more accurate although perhaps obviously all answers must eventually originate somewhere in the quantum world let's talk a bit about the classical answer because it's easiest to understand and it reflects pretty well the right quantum answer if you're a physics buff and you want to understand this in detail i recommend that you read the explanation given by richard feynman in his lectures from back in the 1960s in essence what is going on requires that you remember what light actually is light is an electric field that is changing both in position and in time visualize the peak of a water wave passing over an otherwise still pond and you got the big idea the position of the peak changes over time now inside the material you have a bunch of atoms just chalk a block full of electrons electrons are lightweight particles with electric charge particles with electric charge feel electric fields and start moving but moving electric charges make moving electric fields and so inside the material you have a very complicated situation you have the electric field of the original beam of light but you also have jiggling electric fields of the electrons these electric fields add together and what moves through the material is the mix of fields and that mix moves slower than light suppose you have two sets of electric fields that move at somewhat different speeds the combination of the two fields can move at a velocity different from either of the two fields so that's the basic reason why light moves more slowly in material than in a vacuum it's because the light in the material is a mix of electric fields including electric fields originating in the material itself and that's the basic idea i'd like to dispel a misconception that you often see on the internet about what's going on in the quantum level you find many people that talk about how light is made of photons and that photons can be absorbed by charged particles like electrons these people claim that if you shoot a single photon at a piece of material that it will move more slowly while in the material that's true but the mechanism they invoke is all wrong basically what they claim is that the photon jumps from atom to atom electron to electron getting absorbed waiting for a tiny fraction of time then getting emitted by that atom so it can travel to the next atom they describe describe it kind of like a person with a ton of friends traveling across the country the friendly traveler finds an old friend spends the night at their house then travels to the next friend and stays at their house day after day the traveler moves and then spends the night the result is that the traveler has an effective speed that is slower than they would have if they drove constantly there's only one problem this definitely doesn't happen in our next couple of lectures we'll talk about quantum mechanics and how materials absorb and emit light every atom of each element has a characteristic and specific set of energy levels that they can absorb and emit you've probably seen what happens when you take light from a glowing gas and send it through a prism that light has specific colors specific wavelengths which means specific energy that's true in a gas than in the solid but versions of it happen in both if you send light into a piece of transparent glass all wavelengths get through equally or at least pretty much so this idea of light being absorbed waiting for a bit and getting re-emitted is just wrong don't believe it the reality is more like the classical version when you mix atoms into individual molecules or even more complex into extended solids and liquids the idea of electrons and atoms and fixed energies is just not true anymore atoms share electrons the energy levels change radically instead of each atom having fixed energies molecules and more complicated structures have a continuum of energy levels the net effect is that as a photon travels through transparent matter it doesn't hit individual atoms with individual electrons instead the electric field of the photon moves the location and changes the energy of many electrons and you're basically back to the classical description [Music] so it's totally true that light traveling through transparent materials travel slower than it does through a vacuum this means that even though we say that nothing can travel faster than light we really mean only in a vacuum and matter it's actually pretty easy for particles to travel faster than light can in the same material imagine for a moment that you have a photon beam and a high energy muon beam pointed in the same direction a muon is like an electron's chubbier cousin and i picked it because it avoids complications that would bother a purist if you're more comfortable imagining an electron that works too as long as we ignore a few effects that aren't central to what i'm talking about to avoid the issues of the last topic we discussed let's assume that these beams are initially in a vacuum the photon beam of course travels at the speed of light and for the purposes of my example let's say that the muon beam is traveling at 99.99 the speed of light so to all intents and purposes you'd say that they were traveling at about the same speed what happens if you shoot both of them into a giant block of glass well we established in the last section that when light hit the glass it would slow down in ordinary glass the speed of light inside the material is about two-thirds the speed of light in a vacuum so what happens to the muon it turns out that it doesn't slow down when it passes into the glass to all intents and purposes it just barrels along still going at 99.99 the speed of light so that means in a material like water glass or even air it's possible for an object to move faster than light does in that material okay so that's the first lesson while it's impossible to go faster than light in a vacuum it's actually pretty easy for objects to go faster than light through something like glass and you want to know how we know about this when we when this happens it makes glass glow a lovely blue color this light was discovered back in the 1930s by a young russian physics student by the name of pavel cherenkov he was working with uranium salts which were dissolved in sulfuric acid when he did this he saw that the solution glowed at the time uranium research was still pretty new especially back in the soviet union it was thought that the glow was caused directly by radioactivity but as he tried to isolate the effect he found that he could see the glow in a test tube containing nothing more than pure sulfuric acid if the test tube was near a highly radioactive substance this is an analogy and shouldn't be taken seriously but you could kind of imagine it like a black light and a black light poster with the radiation being the black light and the sulfuric acid being the poster in the same way that black light will make the poster glow radiation made the sulfuric acid glow trankov eventually was able to show that even water would glow in the presence of a lot of radiation he shared his work with sergey vavlov who was his thesis advisor vavalov mentioned the effect to igor tam and ilya frank who figured out what was going on whatever you might think of the soviet union they had some awfully sharp minds cherenkov tam and frank shared the 1958 nobel prize in physics before i tell you how cherenkov radiation works let's just talk about what it looks like shall we imagine water surrounding a nuclear reactor the blue light you'd see is the cherenkov radiation being made in the water it's really quite beautiful in a terrifying kind of way without the water to shield the radiation taking that photo might have made the photographer quite ill but the water both shielded the dangerous radiation and made the shrink-off radiation there's nothing dangerous or all that special about shrink off radiation it's just blue light but how is it made basically it's analogous to a sonic boom when a jet flies faster than the speed of sound a jet flies at a high velocity but one that is lower than the speed of sound as it moves through the air it compresses the air in front of it that compressed energy moves away at the speed of sound however as the jet moves faster and faster it gets to the point where it's moving along with the compressed air right in front of it that means that more and more sound is moving along with it and that sound is moving at the single speed and if that compressed air passes over you it's incredibly loud because all of the sound arrives at once now if the plane moves faster than sound the sound waves don't stack up like they did when the plane was moving at the speed of sound but instead what happens is that each location the plane makes sound from pushing the air out of the way and that sound leaves the location at which it was made traveling in all directions at the speed of sound it makes a series of circles appearing at each point the plane passes the circles add up and make a cone of sound that surrounds the path of the plane shrink of light is about the same thing except that it occurs when a charged particle passes through a transparent material at speeds faster than light passes through the material the electric charge of the particles jiggle the molecules of the material and they give off light as they move the charge particle moves on and jiggles more molecules and you see what happens the light emitted in the material leaves where it was created traveling outwards in a circular pattern at the speed of light and the net result is another cone pattern this one being what we call cherenkov light cherenkov light is actually useful for some physicists to study what happens in some nuclear physics or particle physics experiments since churrenkov light is only emitted by electrically charged particles moving through a transparent medium faster than light does scientists can exploit this to distinguish quickly moving particles from slower ones the fast ones give off cherenkov light while the slow ones don't [Music] so far we've spoken of cases where light slows down and instances where particles move faster than that slow light there are actually two physical cases where phenomena can move faster than light even in a vacuum one of these involves quantum mechanics quantum mechanics is very tricky and i'm going to devote two entire lectures to misconceptions involving the topic since those are the next two i'm not going to describe here in detail about traveling faster than light in quantum situations it's a real thing though when you make a measurement in certain quantum mechanical situations it certainly appears that the news that a measurement was made does actually travel faster than light you can't use the phenomena to transmit information so it doesn't break einstein's theory of special relativity but it happens you have to wait until the next couple of lectures to hear more about it but there's another situation where objects can move faster than light well kind of sort of i guess i probably should explain in our last lecture we spoke about the big bang and how the universe has been expanding for eons this has an important bearing on another way in which the phrase moving faster than light is at least partially real as a reminder it was in 1929 that american astronomer edwin hubble combined the distance measurements that he made of galaxies with the velocity measurements made by american astronomer vesto sliffer and others and made a very intriguing plot on the horizontal axis is the distance of galaxy in his study as measured from earth and the vertical axis is their velocity hubble hypothesized that there was a linear relationship between the two and you can see his estimate on his graph 1929 was a long time ago when people have done better in the intervening years there are more modern versions of hubble's plot i'm showing an example here and it also can be find in the guidebook to give an idea how much better the newer plot is hubble's original data would fit in the tiny red box down in the corner his idea of a linear correlation between distance and recessional velocity seems to have been correct as we learned in the last lecture this correlation is taken to be a clear evidence for an expanding universe and b the big bang but let's think about what it means the expansion of the universe has changed over time it was originally very fast after the big bang 14 billion years ago but the attraction of gravity slowed it down for about 9 billion years about 5 billion years ago a form of repulsive gravity called dark energy came to dominate the cosmos and the expansion is now speeding up a quantitative discussion needs to take into account all of this however we can make the key points even ignoring these details using modern data and taking hubble's idea that the recessional velocity is proportional to distance we can figure out the slope of the graph we find that in round numbers the recessional velocity is 70 kilometers per second per megaparsec a megaparsec is equivalent to 3.26 million light years so in english what this says is that a galaxy that's a mega parsec away will be moving away from the milky way at a speed of 70 kilometers per second at a distance of 2 megaparsics our galaxy will be moving away at a speed of 140 kilometers per second three megaparsecs 210 kilometers per second the further and further away you go the faster the objects are moving taking hubble's linear relationship is accurate which we know isn't perfectly right but we're going to do it anyway we can find out that there's a distance where galaxies move away from the milky way at the speed of light taking a speed of 300 000 kilometers per second for the speed of light and a hubble relationship of 70 kilometers per second per megaparsec we can divide the two and we find the distance at which objects are moving away at the speed of light is 4285mhz or 14 billion light years now these numbers aren't right due to changes in the expansion history of the universe but let's use them they mean that an object that is 14 billion light years away is moving away from us at the speed of light or does it you certainly hear statements like that actually when i hear from a non-expert they often say something like the universe is expanding at the speed of light which is obviously nonsense or at least awfully incomplete for instance what the heck do they mean the universe includes the solar system and clearly pluto isn't expanding away from us at the speed of light neither is the milky way nor the andromeda galaxy which is a couple of million light years away actually andromeda is heading towards the milky way and the two of them will collide in a billion years or a few anyway supporting the people who say that the universe is expanding at the speed of light it's true that we calculate that objects 14 billion light years away are traveling away from us at that speed but objects 28 billion light years away are traveling away from us at twice the speed of light so the whole expanding faster than the speed of light assertion is really pretty mushy so what is right is the speed of light the ultimate speed well yes in a manner of speaking remember typically the first statements about not going faster than light are made within the theory of special relativity in special relativity space doesn't change shape it doesn't move it certainly doesn't expand space or more accurately space time is flat light moves in space time and there light is the maximum speed in general relativity space does bend and warp and expand to connect it to special relativity you have to treat each spot in spaces entirely flat if you know calculus that means you have to treat each point as a differential volume element and if calculus isn't your thing or if it's been a while it means that in general relativity you take the entire universe and slice it up into a near infinite number of tiny cubes you keep making the cubes smaller and smaller until in each one it's safe to say that inside the cube that space is flat and then in the cube the laws of special relativity hold so when we're talking about the speed of light through space we're talking about a flat space time however if space is expanding that's a different thing we just talked about two objects that are locally stationary in their little cube but the cubes are moving apart at the speed of light or even more now if two objects are far enough apart that the distance between them is increasing faster than light then if one of them shoots a beam of light at the other then the light will never get there in a very real sense it's like an old steam driven paddle boat that can move at five miles per hour if you try to have it move upstream but the current downstream is 10 miles per hour the paddle boat will be carried downstream this also means that at a distance beyond 14 billion light years the two galaxies can never talk to one another even in principle and remember what i said in the last lecture about how the expansion of the universe can make the numbers confusing an object whose light was emitted 14 billion years ago and is now just arriving at earth is currently 46 billion light years away so i hope you focus more on the ideas than the exact numbers here getting numbers precisely correct requires a considerably greater amount of care and nuance getting back to the question of moving faster than light are two galaxies separated by 14 billion light years moving away from each other at the speed of light well yes and no it's true that the distance between them is growing at the speed of light but relative to their little local bit of space they're not even moving the bottom line is that while light only moves through space at the speed of light the expansion of space can be faster so it's possible to be sloppy and say that objects can move apart faster than light but it's more accurate to say that objects cannot move through space faster than light but space itself can move faster than light now this doesn't help us with warp drive or hyperspeed or any of those things from science fiction but personally i think this super luminal expansion of the distant universe is just the coolest thing it makes me wonder just how big the universe really is and it makes me sad that we'll never know the answer to that simple question you

Channel: The Great Courses

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Id: CTG2v-cDUos

Length: 30min 5sec (1805 seconds)

Published: Mon Sep 12 2022