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Current time:0:00Total duration:2:55

Video transcript

a while back I was teaching physics in California I got to class and I was all like hey class you want to hear a physics joke yeah okay totally does light hold mass I don't know does it no it's not even Catholic but weight doesn't really have mass everyone just calm the heck down you see the worst part of all this wasn't that my joke bombed it was that I actually managed to confuse people by telling it my students had read that light has momentum and they were right light really does have momentum but then I come to class and I'm all like hey guess what light has no mass now my students are thinking to themselves dude but P equals MV in other words if momentum equals mass times velocity how could light which has no mass ever have momentum well I had to break it to my students that P equals MV isn't really accurate for things that travel close to the speed of life for things going that fast you have to use special relativity I don't want to waste a lot of time talking about special relativity in this video so you're just gonna have to take my word for it that the rules of special relativity allow for a loophole this loophole provides a way for massless objects to have momentum alright so the bad news is that we cannot use P equals MV to find the momentum of a photon the good news is that the formula for the momentum of a photon is simple the momentum of a photon equals H over lambda H is Planck's constant 6.6 to 6 times 10 to the negative 34 joule-seconds lambda is the wavelength of the light in meters be careful don't use nanometers you have to convert to meters the momentum of a single foe is gonna be extremely small that's why it doesn't feel like we're getting pushed on very much when light shines on us but theoretically if you had a big enough solar sail the light bouncing off of that solar sail could propel it forward due to the impulse imparted by the momentum of the light okay so very long story short you should never ever use P equals MV to find the momentum of a photon to find the momentum of a photon you should always use momentum equals H over lambda