How big is the Universe? | *THE* Great Debate in Physics

so continuing my series of great debates in physics that we're doing all month here on my channel this week we are covering the one that inspired it all the great debate it was a debate that raged for decades before seeing two giants of astronomy curtis and shapley going head-to-head with each other in 1920 and it was a debate that eventually got humanity to re-evaluate its entire position in the universe it is the 100th anniversary of that debate this year and even after all that time i still think it can teach us so much especially sort of about how science is done you know on the front lines if you will you know when you have all this evidence but it's in fragments and pieces and you're not entirely sure how it all fits together you know how do you reason your way through all that until the bigger picture becomes clear and in this case the big picture was the biggest picture there is how big is the universe now i think since the dawn of time humanity has probably been asking this question of how big is the universe and what is our place in it but world views or i guess universe views if you will have changed significantly over the millennia right so you've got ptolemy saying that the earth is at the center of the universe way back sort of around 100 bc then you've got sort of in the 1500s a big shift when copernicus actually says that the sun is at the center of the solar system and you know the earth and all the planets orbit around the sun it was a hugely controversial idea at the time you know it saw galileo galilei being put under house arrest because of his support of copernicus's theory as well but once there was this sort of huge pile of evidence in favor of you know the sun being at the center of the solar system from planets motions in the sky etc you know it was readily accepted and not only was the sun kept at the center of the solar system for centuries to come but also the center of the universe which at the time was essentially all the known stars in the sky i.e the milky way now i've talked before on the show about how we know the shape of the milky way how we know that it's a flat disc and it has this spiral structure so go check out that video if you're interested because it does have a few sort of extra details that do tie in to this debate on how big was the universe because it was emmanuel kant in 1771 who was sort of pondering on the fact that you do see stars in this narrow strip across the sky that you know supposed to like spill milk which is where we get the name milky way from and he suggested that perhaps all of these stars orbit in a flat plain like the planets do around the sun and so the milky way was this system of stars that was orbiting something in the center which was assumed to be the sun but can't took it one step further as well and he said that the things in the sky that weren't stars the sort of fuzzy patches that we called nebula were also these systems of rotating stars which he dubbed island universes khan was a philosopher right not a physicist so his ideas were based on conjecture and not necessarily observations and evidence like physicists like deal with so his ideas weren't really picked up on at the time but by the turn of the 20th century astronomers had collected a lot of that evidence and so everyone generally agreed upon the rough shape of the milky way as like cancer sort of this flat disc with all the stars sort of orbiting around the center now it was thought still that the sun was at the center of that system and it was thanks to captain's work at the beginning of the 20th century that we thought that the milky way was around about 30 000 light years across because that was the rough distance to sort of the most distant star that had been found you can see that in arthur eddington's map that he made in 1912 which shows sort of the rough shape of the galaxy but as you can see the sun is very much still in the middle what people were missing to make accurate maps of the milky way and sort of the positions of stars that they saw in the sky was the distances to those stars because you could record where a star was in the sky and you could recall that there were more you know sort of along the plain of the milky way but you couldn't really know how distant they were and therefore you couldn't work out whether the sun was truly in the middle of the galaxy or not thankfully the ability to measure distances came in 1912 with henrietta leave its work who was working as a computer at harvard college observatory so she had been assigned some work cataloging variable stars in the small magellanic cloud which is a little dwarf galaxy that we now know to orbit the milky way but again back then it was considered sort of one of the sort of nebula as a cluster of stars in the milky way itself and she noticed that there was a correlation between the period that the variable star pulsed at and the star's brightness now obviously the brightness of an object that we measure here on earth is dependent on how far away that object is from us you know that from everyday life right the more distance an object is the fainter it appears and so leave it was conscious that obviously the brightness that she was recording might have something to do with the distance those stars were from us but all of these stars were in the small magellanic cloud and so if you make the assumption that all those stars are at roughly the same distance from us which was a good assumption to make astronomically speaking order of magnitude they're pretty much the same distance then you can assume there is a physical relationship between the actual brightness of the star and the period that it pulses at so not the brightness that we necessarily measure but the actual brightness of it to know the actual brightness instead of the what we call the apparent brightness that we observe because of our distance we are from it you obviously need to know the distance but if you could find the distance to one of these variable stars then you could calibrate that relationship so that you could convert the period to the actual brightness measure the apparent brightness of the star and then get the distance and that came a few years later in 1917 when the distance to a nearby variable star was measured using parallax this is when you measure how much stars shift against the fixed background stars that are much further away from us as earth moves around its orbit and you can use some clever trigonometry to figure out how far away they therefore are by how much of an angle they shift by so those parallaxes essentially calibrated this relationship that levitt had discovered that came to be known as leave its law and it meant that it could be used to find the distances to things that you couldn't use parallax for because they were so far away that they didn't shift against the background stars so this is where harlow shapley comes in in 1917 he used levitt's law to measure the distances to globular clusters in the milky way these are you know clusters of about 100 000 stars or so in these big blobs that are really easy to spot in the sky so because these globular clusters have so many stars in them it's likely that at least one of them is going to be one of these variable stars that henrietta levitt was studying and so shapley used levitt's law to work out the distances to the goblet clusters in the milky way and that meant he was able to make a much more accurate map of the milky way one in which he did not put the sun in the center of he noticed that you saw the most globular clusters in the direction of sagittarius on the sky and so he concluded that that should be the center of the galaxy that's where it was sort of densest the same way that you know the majority of material in the solar system is found in the very center in the sun and he also estimated that the milky way was about a hundred thousand light years wide much larger than the 30 000 light years that captain had estimated it was about 20 years earlier so because the size of the milky way that shaping measured was so much bigger than what captain had come up with you know decades earlier based on you know what was the distance to the furthest star known in the milky way he essentially concluded that the milky way must be so large that it was essentially the entire universe in the very same year though in 1917 herbert curtis was also trying to measure the distance to something but in his case it wasn't globular clusters it was the andromeda nebula as it was then known because it was thought to be one of these sort of clouds of gas and dust in our own milky way galaxy that perhaps stars were forming from and in andromeda he observed what he described as four brief brightenings of what appeared in telescopes to be a point source of light like a star now he said these brief brightenings were all nova so not supernova like what happens when a star dies but actually a sort of a brief brightening of a brand new star now when curtis observed these nova he said they were 10 times fainter than anything ever seen in the milky way before and assuming that they are similar to nova that happens in the milky way could you then work out a distance to andromeda and the distance he got was 500 000 light years way way bigger than even shapley's estimate for the you know diameter of the milky way a hundred thousand light years which he assumed was the entire universe so curtis came back to kant's idea of island universes or the fact that andromeda was a galaxy in its own right just like the milky way what curtis then did was said okay if andromeda is a galaxy just like the milky way if i have the distance to it and i know how big it appears on the sky then again for some clever trigonometry i can work out how big it actually is in real life and the number he arrived at was at about 30 000 light years which if you remember is how big captain had estimated the size of the milky way back at the turn of the 20th century and so curtis came up with this model that the milky way wasn't the entire universe that the sort of spiral-shaped nebula that you see in the sky are actually galaxies island universes if you want to call them in their own right but those galaxies were about 30 000 light years across or so much smaller than shapley was arguing that the milky way was cursed's argument therefore was that these spiral nebula were actually galaxies in their own right but they were much smaller than shapley had estimated at about 30 000 light years across or so and that the sun was actually still in the center of our own milky way galaxy like captain had argued at the beginning of the 20th century it was george elijah hale whose father had left a large endowment for a lecture series at the national academy of sciences who suggested that the two have a public debate on the scale of the universe so in april 1926 at the smithsonian museum of national history in washington dc the two both presented lectures arguing for their specific model of the universe shaply for the milky way is everything the sun is not in the middle and it's about a hundred thousand light years across and curtis with his the milky way is not everything you have these other galaxies in the universe and they're much smaller than shapley saying about 30 000 and the sun is in the middle of ours just to sort of paint the picture a bit for you so curtis in 1920 was 48 he was president of the royal astronomical society of the pacific he was a very well established and respected astronomer he was a bit of a traditionalist much more sort of conservative in his views than shapley who was the sort of young scrappy and hungry astronomer trying to make an aim for himself and bagging himself the top job at harvard college observatory shapley was supposedly a lot more uncomfortable with public speaking than curtis was you know curtis had many many years as sort of a school master teaching like latin and greek in his early days and i think with sort of all those facts about them i think you can really imagine you know how they sort of would have been uh personality-wise and how they would have carried themselves in a room now the thing is you can't really say like who won the debate on the day because it wasn't like sort of all the audience voted for which scientific hypothesis all of science would now be supporting after that fact because they didn't have that much evidence for or against either idea at the time as curtis was sort of famously quoted of saying in his actual lecture on that day you know more data was clearly needed so there wasn't really a scientific conclusion but what we can do you know with hindsight is look back on both the arguments that both curtis and shapley made and see which ones sort of stood the test of time and the thing is they were both kind of half right so in 1926 edwin hubble managed to observe some variable stars in the andromeda nebula and other spiral nebula as well and so using leave its law he managed to get a distance and found that they were all over a million light years away from us definitely in support of curtis's sort of island universe argument shapley apparently dismissed this very quickly as junk science and so hubble actually sort of wrote to him personally outlining all the observations he'd taken and all the conclusions he had come to and chapley just you know couldn't ignore the evidence after all that you know i think he's quoted in saying that you know here is the letter that killed my universe throughout the 20th century though more accurate maps were made of the milky way itself using you know different wavelengths of light like infrared that could see through dust and radio waves that could trace the hydrogen gas in it and it became very clear that the sun wasn't at the center of the galaxy and instead the center indeed was in sagittarius like shapley had said and then actually at the center was a supermassive black hole but we also found that the milky way had spiral structure just like andromeda and it indeed was about a hundred thousand light years across just like shapley had measured back in 1920. so curtis was definitely right about the spiral nebula being galaxies in their own right these island universe theory and so it's often curtis who's sort of given credit for winning the debate after all but it was shapley who was right about the size and scale of the milky way itself at about a hundred thousand light years across so like i said they kind of both got it half right it kind of makes me wonder like what scientific debates are raging today where two sides have got it half right you know maybe the crisis in cosmology that we've talked about on this channel before perhaps perhaps the sort of supernova camp and the cosmic microwave background camp perhaps they've got it half right and if we could just work out which half each campus got right we'd be able to solve this strange crisis in cosmology where we don't really agree on what the age of the universe is if you don't know what i'm talking about check out the video that i did with joe scott recently on his channel sort of breaking down this crisis in cosmology as much as we could but anyway for the great debate i think with hindsight we can say that shapley almost single-handedly won some battles by himself right with his work on globular clusters and arguing that the sun wasn't in the center of the milky way and showing that it was about a hundred thousand light years across but with an army of scientists behind him and all the results from the 20th century i think we can say that overall curtis won the war thanks for sticking around until the end i know you're waiting for the bloopers but first an ad break do you want to learn something new or something else new well the brilliant website or app is the perfect place to do just that brilliant has over 60 interactive courses in science maths and computer science which teach you the fundamental concepts with interactive aspects code writing and storytelling now brilliant has a brand new course all about knowledge and uncertainty which i think is perfect for this video because we talked about how astronomers went about solving the problem of how big the universe was you know with limited evidence and unlimited knowledge when their data was so uncertain so brilliance knowledge and uncertainty course teaches you the maths for how to deal with that uncertain information essentially putting a number on how much we don't know so if you want to learn something new then head over to brilliant.org forward slash dr becky that's d-r-b-e-c-k-y and sign up for free and the first 200 people that go to that link will get 20 off an annual premium subscription as well so if you like the sound of that and you want to support this channel then head over there and say a big thank you from me saw two giants of astronomy in 1920 going head to head curtis who's hurt so in 1917 hollow shapely came along shapely it's very shapely you know i'm just like my country i'm young scrappy and hungry and i'm not throwing away my science

Channel: Dr. Becky

Views: 114,375

Keywords: science, debates, great debate, dr becky, becky smethurst, rebecca smethurst, women in stem, women in science, stem, steminist, female physicist, astrophysics, physics, astronomy, universe, cosmos, galaxy, milky way, shapley, curtis, immanuel kant, philosophy, kapteyn, hubble, size of the universe, smithsonian museum

Id: 0MJCqJPMpeY

Length: 17min 45sec (1065 seconds)

Published: Thu Aug 13 2020