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Current time:0:00Total duration:7:24

Video transcript

we've already talked about light having wave-like properties and the waves that we're familiar with in our everyday life we consider to be disturbances traveling through a medium we talked about dropping a pebble in water and the water is a medium and we see the wave travel outwards we think about sound waves which is disturbance in the air we think about we think about a wave traveling through this rope the medium there is the rope so in the mid 19th century it was completely reasonable for folks to say well look light has wave-like properties it must be a disturbance traveling through a medium and they said well what do we call that medium even though we don't observe it directly well let's call it the luminiferous ether so an obvious question that was facing folks who had this reasonable assumption they said well how do we can we somehow detect the luminiferous ether can we validate the luminiferous ether existing and a key realization is as well we must be moving quite rapidly relative to the luminiferous ether how do we know that well we just have to remind ourselves that obviously the earth is rotating but not only is it rotating on its own axis but it's rotating around the Sun and so if this is if this is the Sun right over here this is the earth the earth is rotating and these are all rough these are all rough figures the earth is moving around the Sun at approximately 30 km/s 30 km/s by our everyday standards that's quite fast but we're not done yet because the Sun is also moving around the center of the galaxy and this isn't an actual picture of the Milky Way obviously we haven't gotten this far from our own galaxy to actually get this type of a vantage point but if the if the Sun were right over there the Sun estimates our are moving with a speed of 200 roughly 200 calumny right that in a better color so you can actually see it 200 kilometres per second 200 kilometers per second around the center of the Milky Way and then the Milky Way itself could be moving so we don't know our actual kind of our orientation relative to the ether but we are we're constantly changing our orientation we're moving in these orbital patterns if there is some type of luminiferous ether if there is some type of luminiferous ether and i'm just going to draw these lines over here to kind of show our luminiferous ether we must be moving relative to it if we orient ourselves just the right way in fact the odds of us being stationary relative to the ether are pretty close to zero especially if we wait a little if we're stationary relative to the ether right now let's say at this point since we're changing our direction we're not going to be stationary relative to the ether at that point and that's just when you consider the Earth's orbit around the Sun it's even more true when you think about the the solar system's orbit around the center of the galaxy or even the movement of the galaxies so we should be moving relative to the ether or the ether should be moving relative to us so we should be able to detect some type of some type of what's called an ether wind because it should be moving relative the wind now how would you detect an ether wind well let's think about some other type of medium moving relative to us let's say that we are let's say that we are sitting on an island let's say that we are sitting on let me do this in a better color for an island so let's say that we're in an island that's in the middle of a stream so these are the shores of the stream these are the shores of the stream and there is some type of a current so the water is moving in that direction so that's the that's the medium and now let's start a wave propagating through this so if I were to just take a pebble and drop it right over here what would happen well the wave is going to propagate faster to the left than it is to the right this is from our everyday experience and that's because to the left it's moving the medium is also moving to the left so as the medium moves and then you propagate through that medium you're going to move faster the left and to the right so the wave is going to propagate is going to propagate something is going to propagate so after a small period of time the crest on the right might be there but the crest on the left might be there so it might look something something like this and then after another period of time it might look something it'll look something like this so the general point is for this little stream example you're going to see your wave propagate faster in the direction of which the medium is moving so similarly if you have an ether wind if you have ether winin and this luminiferous ether is the medium by which the light propagates is disturbance the light is a disturbance in this medium then if this ether wind has some let's say it has some speed Moses call it s if you were to if you if light were to travel in if you were to try to propagate light in that direction versus in this direction versus in that direction it should go faster we should notice it going faster if it's going along the same direction as the ether because propagating through something that's also moving relative to us and likewise if it's going in the opposite direction of the ether even though it's propagating through the actual ether at that same speed the ether is moving in the other direction so the light based on our nineteenth-century understanding of the universe the light should seem slower so you can imagine people started theorized well ok maybe we can measure light in different directions and see if relative to us if relative to us we see a different velocity for light now the problem was is that in the mid 19th century light is incredibly fast we now know that the speed of light is approximately 300,000 300,000 kilometers km/s and in the mid 19th century we didn't have good tools to measure this with a lot of with a lot of accuracy especially because the ether wind itself even if you say this is 30 km/s maybe we're moving around the galaxies you know 200 km/s maybe 300 kilometres per second that's still a small fraction of the actual speed of so if you don't have a lot of if you don't have a lot of accuracy when you're measuring the speed of light in these different directions and the ether wind is so slow relative to the speed of light well with just traditional tools in the 19th century you're not going to be able to detect this actual ether wind if it existed and that's what gets us to the famous Michelsen Morley experiment because there they didn't just directly try to measure the speed of light in one direction or another instead they thought about well let's let's let's let's split some light into two different directions and then recombine them and see the interference patterns and if the different directions traveled at different speeds that will have different interference patterns and we're going to see that in the next video