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Current time:0:00Total duration:8:12

Video transcript

we've seen in several videos so far that most of the ways that humans haven't have encountered in nature waves in the water or sound waves or just waves traveling along a rope they were disturbances traveling through a medium and so when light displays or has wave-like properties a very natural assumption was well light must also be a disturbance traveling through some type of a medium even if that medium wasn't so easy to detect but they conjectured that there is some type of medium that light is a disturbance traveling through they called it the luminiferous ether and much of physics in the 19th century was all around proving that the luminiferous ether existed and also figuring out what our relative velocity was relative what our relative velocity was in regards to that luminiferous ether and why do we feel confident or why did they feel confident that there was a relative velocity what we talked about that in the last video the earth is spinning and then it's spinning around the it's orbiting around the Sun at a nice clip and then the whole solar system is orbiting around the center of the galaxy at a nice clip the galaxy itself might be moving so if you have some absolute frame of reference that's defined by the ether well we are going to be moving relative to it and if you say like if we're moving relative to it well maybe you just measure the speed of light in different directions and see whether the speed of light is faster or slower in a certain direction and then that might help you identify well one validate that the ether exists but also think about what our what our velocity is relative to the ether relative to that absolute frame of reference but the problem in the nineteenth century is that we did not have any precise way of actually measuring or precise enough way of measuring the speed of light where we could detect the relative difference due to go the light going for or against or into or way from the actual direction of the ether wind and so the experiment that is usually cited with first kind of breaking things open starting to really make a dent in this whole idea of a luminous ether is the michelson-morley experiment Michael sin worldly experiment they recognize okay we can't measure the speed of light with enough precision to detect has it gotten slowed down by the ether wind or sped up by the ether wind but what we could do and this is what Michelson and Morley did do and I'm going to do a verse implication of the experiment and it said okay you have a light source you have a light source right over here so you have a light source and so that's going to send light in this direction it's going to send light just like that and what you do is you have a half-silvered mirror that allows half the light to pass directly through it and half of it to be reflected so let's put a half-silvered mirror right over here so there's a half-silvered mirror and so half of this light will bounce off like this this is just a simplification of it let me do it a little neater than that so half will bounce off like that and then the other half will be able to go through it will being able to go through it it's a half-silvered mirror and then we make each of those light rays we've essentially taken our original light rain split it into two well then we'll then bounce those off mirrors bounce those off mirrors that are equidistant and there are some adjustments when you actually have to factor in everything but just as a simple notion these things are just now going to bounce back so this one is now going to bounce back it's how silvered it can go through or part of it can go through that mirror so that's that ray and then this one is going to bounce back this one's going to go bounce back and part of it is going to bounce into this direction and then you can detect what you see you can detect what you see so this right over here is a detector and you might be saying okay Sal well what's the big deal you've taken a light source you bounced you split the light rays you've put them back together you've bounced them around a little bit but think about if there was a luminiferous if either these light waves that are going in orthogonal direction we'll be going at different velocities let's say if that luminiferous ether if that luminiferous ether was if the ether wind was doing something like let me see if the ether wind were in this direction if the ether wind were in that direction when the light wave is going that way it should be going faster and when the light wave is coming back it should be going it should be going slower and so what Mickelson and Morley did is is okay let's assume let's just if let's adjust our apparatus right over here so when these two light rays bounce off then come back together if there were no ether you would have some basic interference pattern so what do I mean by interference pattern well let's say that you have maybe this one bouncing from up here let me do that in a different color so the one bouncing from the one bouncing from up here let's say that that is looks like this I'll just draw it as a longitudinal wave just like this best I hand hand-drawn longitudinal wave and then the one coming from the other direction the one that bounces here and then comes back like this is another longitudinal wave like this and when they overlap they are going to interfere with each other either constructively interfere or destructively interfere so you could have something like you could have something like this so let me copy and then let me paste it so depending on how far how fast each of these traveled you're going to have different levels of interference and you would have a difference depending on the orientation depending on what the actual ether wind is doing but what Mickelson and Morley observed is that no matter how they oriented this apparatus and then it at different times of the year and they rotated around and they rotated in the in the vertical direction in the horizontal direction no matter what they did they always got the same interference pattern the interference pattern did not change and because the interference pattern did not change it implied that well maybe this ether isn't really having a effect on slowing down or speeding up the light waves so this was an it is often called one of the most famous failed experiments in physics so let me write this now what's powerful about it is that it was a failed experiment let me get my pin tool out it was a failed failed experiment but it made people start to question well maybe there isn't an either an luminiferous ether maybe light just somehow travels through the vacuum maybe there is not this absolute frame of reference this is defined by the luminiferous ether and I want to be clear it wasn't this experiment by itself this experiment was one of many that started to put that doubt but even after this experiment and they saw that there was no change no matter how in the interference pattern no matter how they oriented this thing whether it was going in the direction of the luminaire the hypothetical luminiferous ether are away from it or whatever when they saw no matter how they oriented that they get the same interference patterns people try to come up with other explanations that still might have been okay with a luminiferous ether maybe the length got contracted in the direction of the motion maybe other things got affected but this is a super important experiment in physics because once again it started to show that hey maybe there is no luminiferous ether that light is just going to go through that vacuum and as we'll see it's going to be traveling at the same velocity no matter what frame of reference you look at it from but we'll explore that more in future videos