The Cosmic Illusion No One Talks About

thanks to brilliant for helping support this episode hey crazies we all know that things are smaller when they're farther away right the size of familiar objects gives us a way to tell distance except it doesn't always work I mean you're still good if the distance is less than like nine and a half billion light years but there are plenty of galaxies more distant than that and they look bigger than the closer ones [Music] this demonstration is an example of angular diameter it's the idea that if something is farther away it'll look smaller in our field of view imagine there's a sphere around your head there are 360 Degrees all the way around in any direction regardless of the sphere size if you see a ball at some distance it'll take up a fraction of those 360 Degrees meaning you can measure its diameter as an angle hence angular diameter if the ball is closer the angle will be larger if the ball is farther away the angle will be smaller that's why one of these clones looks farther away than the other his angular diameter is smaller of course our brains aren't precisely measuring these angles and doing calculations instinctively you only need a vague idea of distance so your brain Cheats by using visual cues from the environment like perspective if I replace this background with one that has less depth your brain gets confused it looks like I have a mini clone we call this forced perspective they took advantage of this trick and Lord of the Rings to make the hobbits look smaller we run into a similar problem in astronomy environmental cues don't exist in outer space we only have the vast emptiness between objects so it turns out that angular diameter is not a great judge of distance in astronomy but when we measure distance in other ways the pattern still holds up a kind of say we've cataloged a bunch of identical galaxies at various distances from what we just learned about angular diameter we'd expect the pattern to look like this an object's apparent size should be proportional to 1 over its distance what we actually see is this they match up at first but quickly begin to deviate the strangest part of the pattern occurs here at about 9.6 Giga light years that's where the angular diameter takes a U-turn galaxies start appearing larger again what what what I know I know it sounds unbelievable but it's true we call it angular diameter turnaround and I'll admit it's weird AF I've heard it described as watching Galaxy sink into a pool of diluted blood and given the redshift that makes complete sense are they actually being diluted though well no I I just thought it would be a nice metaphor anyways angular diameter turnaround is just an optical illusion and like any optical illusion it comes down to the behavior of light the first thing we need to know is that light has a finite speed it's about 300 000 kilometers per second or 186 000 miles per second give or take that might seem instantaneous on human scales it's even pretty fast when you consider the entire Earth but one step out and scale and we start to see the problem another step and it looks as if it's standing still light takes time to get places and space is big really big units can be misleading kilometers and miles are terrible units in outer space it would be more helpful to say that light travels at one light year per year Alpha Centauri is the closest star system to the Sun and it's a little over four light years away that means light takes over four years to travel that distance four years and that's the closest one we can certainly draw the angular diameter triangle on this image and Alpha Centauri is close enough that the simple rule applies but in general it's becoming clear that time is going to be a major factor let's incorporate that factor now before things get too heavy if you haven't guessed it already I'm going to use a space-time diagram why is time on the vertical axis I mean we had to pick a direction why not vertical I mean as Michael Stevens would say it's a it's a choice it's a choice that choice was made a long time ago it's just Tradition now tradition anyway the Alpha Centauri system won't move much in four years at least relative to Earth so in a space-time diagram they'll be on Parallel paths your first instinct might be to think Starlight moves like this but remember light travels at a finite speed one light year per year so we don't get to see this star as it is now we only get to see it as it was four years ago there's a delay light has to travel along diagonal paths in a space-time diagram we call them null paths for mathy reasons I'm not prepared to get into today Just Go With It the long sides of that triangle defining angular diameter are null paths for some perspective that same triangle looks like this in a space-time diagram I I know I snuck an extra dimension in there on you but I promise there's a payoff at the end of all this light from either side of Alpha Centauri A travels along 45 degree diagonals to reach Earth but if we consider all the 45 degree diagonals we get something special a light comb specifically Earth's past light cone every event that could possibly have affected the present-day Earth is inside it all light that could possibly have arrived at Earth travels along its Edge but this video is not about nearby Stars it's about distant galaxies which means we need to extend this light cone much further into the past in fact we'll be extending this cone so far into the past that it won't even look like a cone anymore at least not like anything a rational person would call a cone anyway this brings us to the second thing we need to know space is getting bigger as time passes it's spanding that means light isn't the only thing moving here in the time it takes that light to arrive at Earth from a distant Galaxy that Galaxy and ours will have moved farther apart we're not seeing that distant Galaxy we're seeing an image of that Galaxy from when space was smaller in other words an image of that Galaxy from when it was closer for a nearby star the space-time paths look like this parallel for distant galaxies it looks more like this the paths diverge in an expanding Universe the distances between most galaxies get larger over time but we don't see them as they are now we see them as they used to be light from those galaxies has to travel along the edges of our past light cone that's those null paths I was talking about the farther the Galaxy the further into the past we see so the reason they look closer is because they were closer yep but it's it's more interesting than that if we could extend the past light cone indefinitely like this we wouldn't actually get the pattern we see our angular diameter graph would look like this instead it would deviate from the simple one over distance rule but galaxies would still continue to get smaller there wouldn't be an angular diameter turnaround like in a real universe something else is going on here which brings us to the third and final thing we need to know light is also affected by the expansion of space yes really and I'm not just talking about redshift though that does happen light is dragged along just like the galaxies are light just happens to be so fast that it's better at overcoming that expansion than galaxies our past light cone isn't actually a cone the null paths gradually curve toward the Big Bang Singularity would you call the shape a cone I wouldn't but then again I'm not sure what else to call it either it's giving me serious Taj Mahal Vibes though anyway the turnaround point is right here light coming from Beyond that hump spent some time moving away from us at first the distance was increasing faster than the light could move what what what this is the detail that makes angular diameter turn around possible even before the turnaround Point galaxies begin appearing larger than they should but they still continue to get smaller the farther they are it's not until 9.6 Giga light years that we see them start to get larger again why this happens is perfectly clear from the space-time diagram all light currently arriving at Earth is coming from a point along the edge of the shape if it's coming from a Galaxy then it's coming from when that Galaxy crossed those null paths beyond the hump looking at further galaxies means looking at closer images of galaxies all we ever get to see are images of the distant past filled with illusions of cosmic proportion [Music] all right now I'll admit I did gloss over how we measure distance as in outer space if you want to learn more you should check out brilliant working with a problem yourself is the best way to learn skills in math science and computer science brilliant is a tool for learning stem interactively they have thousands of lessons with new content added monthly learning a little bit every day can have a huge impact if you like this video I bet you'll like their astrophysics course brilliant has several lessons on how to measure the vast distances between the stars but maybe you're looking for something a little more advanced they also have a course on special relativity where you can learn the basics of space-time diagrams brilliant is college level content for everyone if you want to get started with a free trial go to brilliant.org science Asylum or click the link in the description below the first 200 of you will get 20 off Brilliance annual premium subscription it'll also let brilliant know you heard about them from me which helps out the channel so do you know any mind-blowing facts about cosmology please share them in the comments Thanks for liking and sharing this video a special Thanks goes out to all my patreon patrons and YouTube members for their generous support don't forget to subscribe if you'd like to keep up with us and until next time remember it's okay to be a little crazy since a bunch of you were asking the Star Trek transporter does allow for cloning in extreme circumstances but a real-life transporter would not Quantum information must be conserved that information is either in the old you caught up in the entanglement or in the new you but never more than one place anyway thanks for watching

Channel: The Science Asylum

Views: 377,919

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Id: nSJtzn2H3Do

Length: 10min 45sec (645 seconds)

Published: Sat Oct 29 2022