Is light a wave or a particle? | Great debates in physics

so 2020 is the 100th anniversary of something that's been dubbed the great debate in astrophysics and celebrate i figured all this month i would do a series on some of the biggest debates ever in physics history starting with the debate that saw einstein and boar facing off for most of the 20th century science after all is a debate right like the scientists are the jury and you have evidence like for and against different scenarios often that evidence isn't complete though and we're just like working with what we have at the time but over time as more evidence is found that overwhelmingly supports one idea over the other that's when we settle on accepted theory now obviously current research isn't settled the jury is still out if you will that's why it's still being researched but there are many examples of debates throughout history that we can still learn from today if we understand sort of how people thought at the different times when certain evidence was lacking now if you want me to cover you know your favorite idea or debate in physics in this series let me know in the comments because i'll be going to be doing these all month but the one i first want to start with is one of the most famous debates ever in the history of physics right it's about the nature of light itself is light a wave or is light a particle now ideas on the nature of light have been knocking around since you know ancient philosophers but we're going to start in the 1600s with rene descartes and isaac newton in 1630 descartes proposed the hypothesis that light is a wave and treating light as a wave could explain everyday things like reflection and refraction of light then 40 years later in 1670 newton who was a scientific giant at the time right famous for his ideas on gravity proposed the idea that light is actually a particle because only particles could describe the straight lines that light travel in but at the same time you've got people like hook and huygens who've come up with the maths to describe light as a wave and shown that it could perfectly characterize things like reflection and refraction and diffraction again despite that though newton's influence was just so strong at the time and in the decades to follow as well that his ideas of particles of light were just given so much more scientific weight but then there was a cat thrown amongst the pigeons with thomas young's double slit experiment of 1801 he sent a beam of light through two slits and produced this interference pattern right where light waves would add and subtract from each other in the way that only waves could the same principle is used in noise cancelling headphones today right the headphones record the sound that's coming in completely invert that sound wave and add them together so that they cancel each other out it's the same thing that's happening in the double slit experiment except it's the two waves that come through the separate slits that interfere with each other and where it's dark on the screen is where those waves have cancelled each other out and where it's bright on the screen is where they've added together to give you a stronger wave now you would think by this point that everyone would realize that all the evidence was piling up in favor of light being a wave but thomas young didn't just do his double slit experiment with light he did it with electrons as well the tiny particles that orbit the nucleus of atoms particles people knew that they were particles they had been measured in labs as particles and yet when thomas young fired these particles of electrons at the double slit they acted in the same way as light and somehow managed to interfere with each other to give that pattern of adding waves together and cancelling waves out on the screen so if electrons which were particles could make that pattern then would that mean that also light that makes that pattern is particles despite the fact that the way that the pattern has been made could only be understood if light and electrons were waves this was such a problem at the time that in 1818 the french academy of sciences actually launched a competition to try and find like the experiment to end all experiments to determine if light was a wave or a particle now a chap called fresnel so apparently it's pronounced fresnel not fresno oh universe wept becky entered who wasn't necessarily a physicist by training but he came up with a very simple experiment to test this he said if you have a beam of light and you put something in the way of that beam of light to block it for example just like a circular piece of card depending how the light behaves going around that circular piece of card will tell you if it's a wave or a particle so if light was a particle when you put the object in the way you would just block all the light there and you would get a perfect shadow of that object but if light was a wave when it hit the edges of that object it would diffract i would sort of disperse around the edges and if that happens it then disperses into the path where the shadow is blocking the light from upstream and it would actually interfere with itself there now this is the point that famous statistician poisson enters the story because poisson had heard about this entry to the competition and essentially said it was absurd because of course that if you put an object in the way it would just perfectly block the light you wouldn't get diffraction because if that was the case you would end up with a bright point of light right in the center of where the shadow of your object should be and that result would just be absurd because that's not what we see in everyday life and so therefore we shouldn't even need to conduct the experiment because if light was a wave that result would be so absurd but a member of the judging panel francoise arago who later ended up becoming prime minister of france insisted that for scientific rigor's sake this experiment had to be done and lo and behold when you do this experiment with monochromatic light i.e light that is all the same color and wavelength so it can interfere with itself properly you get this bright spot of light in the center where that object should be casting a shadow so the result of that experiment once again was against sort of the scientific legacy that newton had left behind for you know hundreds of years and it was something that scientists at the time were quite wary of doing you know it's easy for us to sort of look back with hindsight and say look at all this evidence piling up in favor of light as a wave you know how can you not you know see that and not claim that but i guess it would be akin today to you know claiming that einstein was wrong about something it's definitely possible that einstein was wrong about something but you can imagine the reaction of the scientific community and the world's media and the world if someone claims that it's a very bold and brave claim and you better be damn sure of it before you make that claim but then later in the decade in 1864 another scientific giant waded into the debate james clark maxwell so maxwell was studying electricity and magnetism and he was trying to come up with equations to describe how electricity and magnetism works as a wave and what he found was that those equations also perfectly described how light propagates through space and also how light behaves as well there's only one thing that he could conclude from that and that was that light was a wave with maxwell's results i think that was the sort of final nail in the coffin for scientists at the end of the 1800s in thinking that light was a particle it was sort of around the time that everyone had come to the general agreement okay while light is a wave and that's when sort of ever went off on a tangent trying to figure out the medium that light propagated through with the whole ether thing that i've talked on this channel before i'll i'll link it in a video somewhere so you can go watch that but ironically as soon as the scientific community had started to accept this idea of light as a wave is when the evidence started to pile up again in favor of light as a particle so at the turn of the century in 1901 max planck showed that you could only explain the spectrum of light that you get from a glowing object something we call a black body if you quantize the energy being given off i you split it up into these discrete chunks of energy and therefore you're treating light as a particle now a lot of people are very impressed with planck's work but they sort of dismissed a lot of the models that he'd come up with and said they must be incomplete in some way and if we do a proper full derivation without maybe some of the basic assumptions that planck started with then we'll end up in a scenario where we can actually explain this in terms of a wave and we don't have to maybe split everything up into chunks maybe some way our maths is limited but then in 1905 albert einstein comes along he used planck's model of these quantized energy chunks these particles of light to explain one of the most outstanding problems in physics at the time the photoelectric effect so if you shine light onto say a sheet of metal a collection of atoms then those atoms give off electrons now what's observed with the photoelectric effect is that the more light you shine on your atoms the more electrons are released and the higher frequency light you shine the higher the energy of the electrons that are released so you can shine a little bit of high frequency light and get a few high energy electrons or you can shine a lot of low frequency light like microwaves and radio waves and you can get a lot of low energy electrons and vice versa now that is not what you'd expect to see if light was a wave if light was a wave you would expect that the more high frequency light you shouldn't it the more electrons you'd get out i the higher the frequency would be proportional to the number of electrons out rather than the energy of the electrons you get out so here again was another result that suggested that light was a particle and not a wave finally after the scientific community had seemed to come to agreement that light was a wave now here's where einstein's great leap came from he said perhaps it's not a particle perhaps it's only sometimes a particle and it was this idea along with you know using planck's model to mathematically explain the photoelectric effect that won einstein his only nobel prize it was for this and not for relativity einstein also realized at the time though that if light was found in these quantized chunks of energy these particles of light sometimes at least then you should be able to measure the momentum of these particles and in 1923 compton managed to do just that and it won him a nobel prize a couple years later in 1927. he managed to experimentally measure in a lab the momentum of a chunk or a particle of light which eventually came to be dubbed photons now this was all happening against the backdrop of quantum physics and quantum mechanics so heisenberg in 1926 had proposed this idea of his uncertainty principle that the more precisely you know a particle's position the less precisely you can know its momentum and vice versa now max born then proposed the idea that underpins the entirety of quantum physics as we know it today and that is the idea that the nature of atoms or particles themselves is governed by probabilities alone and not by any property of the particle itself it basically has no say in what happens to it now einstein hated this idea and it's where his his famous quote comes from uh in 1926 he wrote a letter to max born and he said i at any rate i'm convinced that he does not play dice so at this point in the 20th century physicists were essentially at their wits end because here was all this evidence that piled up in the previous century that light was a wave and then here was all this new evidence that was coming out of the birth of quantum physics that was sort of revolutionizing every other field that said that light was a particle so they called a meeting of some of the greatest minds of the 20th century to try and get to the bottom of this the fifth solve a conference was held in october in 1927 to discuss the problem of the nature of electrons and photons now it's probably one of the most famous physics conferences ever held because 15 of the 29 people who went either had one or eventually won a nobel prize including marie curie who'd won two by that point one in chemistry and one in physics it included schroedinger poorly heisenberg bragg dirac compton de broglie born boar plank curie lorenz and einstein it's it's like a physicist's dream right you know that sort of question where it's like who would you invite to a dinner party like alive or dead you know it could be anybody or physicists just respond with like the fifth survey conference that's who i'd invite just all 29 of them that would be the most incredible room to be in so these minds gathered to try and come up with a hypothesis to explain why in some experiments light was observed to behave like a wave and in other experiments light was observed to behave like a particle and it was here that niels bohr proposed his rather controversial idea at the start anyway that the equations that we had to describe light as a wave described where you could find a particle of light yet it didn't exist until you went looking for it the very act of observing the light causes the photons to exist in both own words the photons have no independent reality in the physical sense the board didn't stop there though he said the same is true for electrons electrons only exist as particles when we observe that they are there otherwise they can be explained as a wave using the wave equations that quantum mechanics had so beautifully put to good use to explain the orbits of electrons around the nucleus of atoms this could explain why we could measure the properties of electrons in a lab and why compton could measure the momentum of a photon of light a particle of light but also that thomas young could get an interference pattern from particles of electrons and bottles of light acting like a wave when they were put through his double slit experiment and producing this interference pattern you know he didn't actually observe which slit that the photon or the electron would go through and therefore they could act like a wave it's something that's come to be known as wave particle duality i.e light or electrons or protons whatever can be both waves and particles depending on whether you observe them or not einstein was vehemently against this idea though the same way that he was about the probabilistic nature of quantum mechanics right he he said that an electron was an electron no matter whether someone was looking at it or not right that you know it was always there wherever there happened to be and so after this conference einstein and boar really sort of entered into this decade-long debate over this it was einstein's skepticism and his doubt over this idea this wave particle duality and quantum mechanics as a whole that really allowed bohr and the other sort of you know founding parents of quantum physics to really make sure that their ideas were concrete you know he einstein would point out holes and and places that you know didn't make sense in his correspondence with boar so that the ideas that they were developing through that sort of first half of the 20th century really became such a founding sort of pillar on which physics stands today you know with this accepted theory of the wave particle duality nature of light that i think there's no physics student of recent times that's gone through a physics degree and had not learned about this incredible idea it was one of my favorite parts of my physics degree and i think it's wonderful not only to know the physics but also to know the history behind this idea as well because that history can teach us so much about how we should approach science today right we should approach it with an open mind all the evidence that's available and a very healthy dose of skepticism just a quick note before i go um i'm gonna be shifting the day that i post videos from wednesday to thursday so just so you know when you know nothing pops up next wednesday that's why okay bye and managed to show that it perfectly described again things like refraction refraction and diffraction i just said refraction twice didn't i okay warm it's like the hottest day of the year so far why am i filming on this day the momentum of a particle of light which he eventually dubbed a proton no he didn't he didn't dub it a proton it wasn't even him and he didn't dub it a proton he dubbed it a photon this is an idea that has become to be known as particle wave duality duality oh so close and also wave particle no one calls it particle wave duality jesus becky all along it was some with particle duality that einstein didn't lie

Channel: Dr. Becky

Views: 83,913

Rating: 4.9622202 out of 5

Keywords: physics, science, debates, light, waves, particles, wave-particle duality, descartes, newton, thomas young, double slit, experiments, hooke, huygens, fresnel, maxwell, maxwell's equations, quantum physics, quantum mechanics, planck, born, bohr, einstein, compton, electrons, great debate, quantum, dr becky, becky smethurst, rebecca smethurst, women in stem, women in science, stem, steminist, female physicist, astrophysics

Id: HAeDBLUT9JA

Length: 19min 43sec (1183 seconds)

Published: Wed Aug 05 2020