How does a black hole make a shadow? What can we learn from it? What are we seeing when we look at a black hole? I discuss these questions and more in today’s Ask a Spaceman!

This episode is sponsored by BetterHelp. Give online therapy a try at betterhelp.com/spaceman and get on your way to being your best self. Visit BetterHelp to get 10% off your first month!

Support the show: http://www.patreon.com/pmsutter
All episodes: http://www.AskASpaceman.com

Follow on Twitter: http://www.twitter.com/PaulMattSutter
Like on Facebook: http://www.facebook.com/PaulMattSutter
Watch on YouTube: http://www.youtube.com/PaulMSutter

Read a book: http://www.pmsutter/book

Keep those questions about space, science, astronomy, astrophysics, physics, and cosmology coming to #AskASpaceman for COMPLETE KNOWLEDGE OF TIME AND SPACE!

Big thanks to my top Patreon supporters this month:   Justin G, Chris L, Barbara K, Duncan M, Corey D, Justin Z, Naila, Scott M, Rob H, Justin, Louis M, John W, Alexis, Gilbert M, Joshua, John S, Thomas D, Simon G, Erin J, Jessica K, David B, Frank T, Tim R, Tom Van S, Mark R, Alan B, Craig B, Richard K, Dave L, Stephen M, Maureen R, Stace J, Neil P, COTFM, Stephen S, Ken L, Alberto M, Matt C, Joe R, David P, Ulfert B, Sean M, Edward K, Tracy F, Sarah K, Steven S, Ryan L, Ella F, Richard S, Sam R, Thomas K, James C, Jorg D, R Larche, Syamkumar M, John S, Fred S, Homer V, Mark D, Colin B, Bruce A, Steven M, Brent B, Bill E, Tim Z, Thomas W, Linda C, David W, Aissa F, Marc H, Avery P, Scott M, Thomas H, Farshad A, Matthias S, Kenneth D, Maureen R, Michael W, Scott W, David W, Neuterdude, Cha0sKami, Robert C, Robert B, Gary K, Stephen J, dhr18, Anna V, Johanna M, Matthew G, Paul & Giulia S, Ron D, Steven M, Louis M, Michael C, Alyssa K, Lode D, Roger, Bob C, Patti H, Red B, Benjamin M, BlueDragon, Stephen A, Ian S, James R, Skip M, Robert O, Adam I, Lynn D, Jeffrey C, Allen E, Paul G, Michael S, Jordan, and Colin H!

Thanks to Cathy Rinella for editing.

Hosted by Paul M. Sutter, astrophysicist and the one and only Agent to the Stars (http://www.pmsutter.com).

 

EPISODE TRANSCRIPTION (AUTO-GENERATED)

pulling up to Mickey D's just for drinks. Oh yeah, that's me. Nothing extra. Just perfection and a straw coming in hot for the coldest cups on the block because there are drinks. Then there are drinks from McDonald's. Get a creamy Oreo fr or mccaf smoothie for less with 20% off any purchase of $10 or more only on the APP. Limited time only at participating. McDonald's valid one time per day. Visit McDonald's app for details. Shadows should be rather straightforward affairs, and for the most part, they are. I'm walking around the beach on a sunny day. I see my own shadow in the sand. How does that shadow get there? Well, my body is not transparent, at least at visible wavelengths of light. And so the sun's light doesn't pass through my body. The rest of the light surrounding me does, and it makes it to the sand. And so there's a pole shaped hole where they can't make it to the sand because they've been blocked by my body shadows, even total solar eclipses. As magical as they are, where the day turns into night and everything spooky and there's a ring of fire around the sun are really just shadows.

The shadow of the moon is crossing over the planet earth, and if you happen to line up with that shadow, you get a special treat. But it's the same deal. The moon is not transparent, at least at visible wavelengths, and so it carves out a hole of light passing around it. Sometimes astronomers use shadows to do some really cool science. This is how we find exoplanets. An exoplanet crosses in front of the face of its star blocks. A little bit of the light casts a tiny, tiny little shadow, and we see a dip in the brightness. We find an exoplanet, but it's just shadows. Shadows are well, you know, to be perfectly honest, a little bit boring. And that's coming from a guy who tends to find just about any subject fascinating. But black holes. Black holes are never boring, and neither are their shadows. In fact, black hole shadows are probably one of the strangest things we could possibly take a picture of in the entire universe. But first, how do we take a picture of a black hole? It's notoriously hard. Black holes are black kind of. By definition, space is also black. There's not exactly a lot of contrast.

Black holes absorb all the light at every wavelength, and nothing can escape. The boundary of that is called the event horizon. The event horizon is not a physical thing. It's not a wall. It's not a blob. It's not a membrane. Instead, the black hole is really the singularity. It's the point of infinite density in the center, but that gravity from the singularity is so strong that at a certain distance from that singularity, nothing can escape its gravitational clutches. That distance is the event horizon, so if light crosses the event horizon, it doesn't come out. So it it acts kind of like a perfectly absorbing surface, even though it doesn't actually exist. There's nothing for you to touch or taste or smell, but you can certainly experience it. Those are the surfaces of black holes, even though they technically have no surface. And yes, a aside for you picking nerds out there. Yes, there's hawking radiation, this exotic quantum process where black holes do emit a tiny bit of radiation.

But for a typical stellar mass black hole, that's something like one particle emitted per year, so it doesn't really count for the point I'm trying to make. Black holes are perfect absorbers. Every wavelength of light that encounters the event horizon of a black hole will not escape. So you can't ever image an event horizon directly because a event horizons don't really exist in the physical sense, since they're just imaginary lines in the mathematical sand and B, they don't emit or reflect any light at all, which is kind of the whole basis of this picture taking enterprise. So we only ever image or see black holes based on their influence on their surroundings, like gravitational lensing, strong x-rays. You know, if you get a lot of material falling into a black hole, it crams into a small volume, and it starts getting really hot and glowing, and we can see that glow even though we can't see the black hole itself. We can watch stars orbit around supermassive black holes so we can't see the black hole itself.

But we can watch the stars orbit around them. When black holes merge, we can see their gravitational waves and so on and so on. In other words, we can only detect usually black holes through their gravity. We can't detect them through the other forces of nature, which is similar to dark matter. That's how we detect dark matter. It's through its gravitational interactions, and there's no real point in me saying that. Just an interesting note. And we can also take their picture well, not a picture of the black hole because you know black but of their surroundings and their surroundings. You know, it's the usual mix of super hot plasma that always seems to be hanging around dense objects in the universe. And if things line up just right, we should be able to see the shadow carved by the black hole. If if there's a bunch of stuff around in front of and behind a black hole and the black hole is absorbing any light that's passing through, then we take. If we take a picture of the whole scene, there should be a hole carved out because none of the light from behind the black hole gets to contribute to our picture.

And there's a shadow that'd be really awesome to see one challenge. Black holes are small. I mean, they're big, they're they're super big. They They're up to billions of times more massive than the sun. But they're also compact, which is what makes them black holes. It's not really the mass of a black hole that makes them a black hole. It's their density that makes them a black hole. If you were to take the earth and compress it down to be the size of a bean, you would have a bean sized black hole with the mass of the earth. You know, a typical stellar mass black hole is stellar mass. Pretty self descriptive. It's It's a few times more massive than the sun. That's not a lot of mass. That's not crazy amount of mass. There are stars bigger than that, but what matters is its density. So because black holes are so compact, they are actually relatively tiny object, and the biggest ones tend to be really far away, which is also bad for astronomy. Astronomers have a tough time seeing things that are both small and very far away, and so, if we want to capture a black hole, shadow the hole carved out from the light of its surroundings, we have a very, very small target.

And the best solution is interferometry. If you thought this was going to be a patreon pitch, I hope I tricked you. Yeah, it's this weird thing, this massive hero in astronomy that has completely altered the way we do. Science is as revolutionary as the telescope itself, but nobody talks about it, so we're gonna talk about it. Our challenge is we want to take a picture of an incredibly tiny, tiny, tiny object tiny because it's naturally small and also tiny because it's very far away. Our solution is we need resolution. We need giant honking cameras or lenses or dishes, or whatever your instrument of choice is, the larger the dish or lens, the higher the resolution you can have. Take, for example, the supermassive black hole at the center of the Milky Way, which is annoyingly and confusingly named Sagittarius A star. Don't get me started. Sagittarius. A star is huge. It's 4.5 million solar masses, and the ring of plasma around it extends for light years.

The problem is that it's 25,000 light years away. To take a picture of it, you would need a telescope the size of the planet Earth. And since that's not going to happen any time soon for a clue as to watch, just listen to the Dyson Sphere episode. We need to do something else. Enter interferometry. This show is brought to you by better health. One of my favorite things about doing ask a spaceman is how much I get to learn about my own field. It's either stuff that I forgot from graduate school or I'm learning brand new because it's simply not a part of my training and expertise. And getting to learn new things and learn about yourself is so powerful. One way to do that is through therapy. Therapy doesn't solve all your problems, but that's not the point. It solves some of your problems, and that makes it worthwhile. You get to continuously learn about yourself through my own therapy. I learn a lot about myself, my relationship with others, how the world works. It's pretty powerful.

If you're thinking of starting therapy, give better help. A try. It's online. It's convenient, flexible, and it could be suited to your schedule. Discover your potential with better help. Visit better help dot com slash spaceman today and get 10% off your first month. That's better. Help EEB dot com slash spaceman. Instead of one giant telescope, which would be an engineering nightmare, we can just have multiple, smaller ones. We'll put them far apart. The farther apart we make them, the greater the effective diameter of a telescope we have, and then we'll carefully combine their signals. And we can do this through a technique called correlation. If we're looking at a distant source, an object on the sky and we have telescopes placed far apart, then the light from that source will arrive at the telescopes at different times very, very, slightly different times, because why should it? It's if if it's anything but dead center in the middle of the array of telescopes, then that light will hit one telescope and then a PICO second later, hit another telescope and you can correlate.

You can find those differences. 00, I see The light came here at this telescope and then at that telescope at this time, and I'll shift these around and that gives me the position on the sky of that object. We can do this a whole bunch of times with a whole bunch of telescopes. We do it over and over, we can build up an image. The upside to this technique is that you can very quickly get huge resolution gains just by putting your telescopes farther apart. The downside is that it's a really crappy telescope because most of the light from the distant source is hitting the ground between your dishes and not the dishes themselves. And it's very hard to turn photons that hit dirt into useful information. And so you waste a lot of time. Usually these telescopes are dishes. Uh, this works best in the radio, by the way, because of the long wavelengths involved are connected directly together. But we can take this idea to its ultimate extreme and put our dishes on the other side of the planet giving us well, well, to be honest, a really garbage telescope the size of the earth.

And we can do this. We can separate the elements, and as long as each element has a very accurate timekeeping device like its own atomic clock, then later we can do these correlations. Once we haul the data around and put it in the same room together and get everyone to talk. The downside is, is yeah, it's a really, really awful telescope because you don't get to use a dish with the entire collecting area of the earth. You have the resolution there, but not the sensitivity. And so you have to wait a really long time. You need to run this for a very, very long time. You need to run your interferometer for incredibly long periods of time. You need to get have as many stations scattered around the globe. So you get, you know, different pieces of information here and there, and then you have to spend an enormous amount of computational power to put the picture back together again. We can make up for this relative garbage dis of the telescope with patreon. There it is patreon dot com slash PM.

Sutter is how you can make astronomy better, or at least keep this show going. Patreon supporters do get early ad free access to episodes, direct connection to me and other perks, shout outs and episodes and on show notes and all that good stuff and my eternal gratitude. And you know what? If you don't contribute to patreon, you still have my eternal gratitude for just listening to this show. It really is a pleasure to give you these episodes year after year. I. I love doing it. Patreon dot com slash PM Sutter. How we can actually make up for the relative garbage properties of an interferometer is with lots of work and math and especially, this is true when we place our instruments on opposite sides of the globe. This is a technique called very long baseline interferometry, and the ultimate expression of this is the Event Horizon Telescope, a telescope designed to take a picture of the shadow of giant black holes. And that's exactly what the Event Horizon Telescope did it imaged the regions around the supermassive black holes at the center of the galaxy, M 87 and the one at the center of our own galaxy.

The resolution that they achieved is equivalent to taking a picture of an apple on the surface of the moon, which for you, astronomy aficionados out there is really stinking impressive. And when we see these images, if you haven't seen them already, type in the Event Horizon telescope, you'll you'll get images also on the show, notes at ask a spaceman dot com. There's an image right there. It's beautiful. We see this ring of light, this halo of fuzzy light with a hole cut out of it. We see the shadow of a black hole. Now the shadow is already a little bit weird, and this is the first taste of the strangeness of black hole geometries. The clump of material. The disc of material around these black holes looks like its face on. It looks like a doughnut, honestly, but it's actually edge on to us or nearly edge on to us. There's a part in the front of the black hole and then a part behind the black hole. But it looks like a donut to us because of the weird geometry, because the light from behind the black hole is actually following the curved space time around the black hole and going around it before hitting our eyes.

And that's super cool. And here's another weird thing. We see the shadow in these images. I was just about to say as clear as day, but that that seem seemed wrong. We see very clearly the shadow, but the event horizon, the actual black hole is much smaller than the shadow itself. When you look at those images which are haunting and you look into that void, the black hole itself is buried somewhere inside the dark region but does not fill it up. Why? Well, black holes are capable of making even shadows not boring, and they do it through their intense gravity. Black holes bend light so much that they even bend themselves. You don't just see in front of the black hole you see the back side of the black hole in the same image. The gravity is so strong that the sphere of the black hole appears larger than it should. It's like the sphere of a black hole unfolds itself, making it seem even larger here.

Here's a metaphor to picture what is going on around black holes. Let's say you're looking at me and, uh, you're looking at my head and my head is gonna be a black hole. And you well, you're just gonna be you, OK? And you're gonna take a laser pointer and you're gonna stick it right in the middle of your eyes. So this is your line of sight. This is where your eyes. Look that the laser pointer will tell you where you're looking. Where the laser pointer lands is is what you're currently looking at. Lines of sight. OK, now let's say I'm a black hole. If you're looking right directly, not an inch off a straight down right at my nose, then that laser pointer is gonna end right at the tip of my nose. If I were a black hole, it would end in blackness. It would end in nothing. And you would you would see a void. You would see the event horizon. OK, now look a little bit to the side. You're gonna look at my eye just a little bit off angle. You have your little laser pointer, and now it's shining in my eye, making me go blind, which I'm willing to do for the sake of you understanding this.

I hope you appreciate my sacrifice. If I were a black hole, you'd just be looking at a different part of the event horizon. So far, this is nothing special. And then you look all the way to the edge of my head and you see just the tip of my ear. All right, so you're looking off site and the laser pointer is coming at me and it's grazing right by my cheek. And it's just hitting the my ear lobe like right here. And you see my ear lobe so far, this is normal. Don't worry, it's gonna get more interesting. And then let's say you move just a little bit to the right, and the laser pointer no longer hits me. Instead, the laser pointer grazes right by my head and goes off into the back wall. And now you see the back wall, OK, this is basic shadow stuff. You're just following lines of sight and see where they go When they land on me, you get my shadow. When they don't land on me, you get the background pretty pretty easy. Black holes are not easy. Black holes have intense gravity. Black holes bend the path of light. They bend the path of lines of sight.

When you're looking at me, the black hole, if you're looking dead on you see the event horizon, you see you see blackness. If you see the edge of the event horizon, the outermost radius, you still see the black hole still see event horizon. Nothing is major here. But then imagine you're looking at me. And there's that laser point grazing my cheek, grazing the edge of my ear, not touching me If I'm a black hole, I have such intense gravity that I pull on that line of sight. I pull on that laser beam and it circles around my head because of the intense gravity. And then it lands, but it lands somewhere. Like back here. My occipital lobe. Well, right back here, I'm I know you can't see it, but, uh, I'm touching it right now. And so you see that when you look past my ear lobe, you don't see the back wall. You see the back of my head because of my intense black hole gravity, And then you look a little bit further away and there's another line of sight that gets a little bit further away before getting captured by the black hole by my head and it wraps around it hits right, dead center.

And you see, you know the spot on the back of my neck that I missed with my razor when I shaved right there you can see it and then you look a little bit further away from that, and that light makes it pretty far away from my head. That line of sight, that laser point before it curls in and it loops around and it sees the other side. So you get an image of my ear by looking past my opposite ear, and then you do it again and you see my face again. It's all weird and distorted. It's at the very edge of your your field of view. But it's there. If I were a black hole, I would appear to be about twice as large, or my face would appear to be twice as large, and in the middle of the image you would see my face. Then you would see the edge of my head. Then you would see the back of my head. Then you'd see the other opposite side of the head, and then you'd see the front again. You can imagine doing this with a real black hole. You follow this laser pointer any time a laser pointer lands on event horizon, you see nothing.

Eventually, you do see so far out you you're like looking all the way to the side, and that light gets bent. But it doesn't land on the black hole, and so you end up seeing the background universe. The shadow appears larger than the event horizon, because that's how powerfully black holes bend the paths of light. That means that if you approach a black hole, the event horizon looks larger than it should, which, to me is way is a way creepier fact than it should be. And there's more. Hey, space cadets, we need to take another small break because I need to mention that this episode is brought to you by the T minus space daily podcast. Yes, I'm advertising another podcast on this podcast because I believe that there is more than enough room for everyone. So what is this podcast? It is the first and only daily space podcast where you get to stay up to date with the latest space technology business governance, intelligence briefings, engaging discussions with experts from industry, academia everywhere, all to do with space and all in 25 minutes or less.

This is how space professionals and enthusiasts can separate the signal from the noise and stay at the cutting edge every single day. that is so cool for all of you space aficionados out there. And I think there are more than a few listening to this podcast. T minus is the one for you, so go check them out. Give them a listen. I would really appreciate it. Now back to the show. That's the shadow. What about the ring of light than we see in these images? Some of that light is from the accretion disk around the black hole. It's plasma, It's hot, it's glowing. It's emitting photons, and it's just Some of those photons are aimed towards the earth, and so we see it. But most of the light comes from a ring, something called the photon ring. It's much brighter. The location of the photon ring depends on the exact geometry of the situation. Sometimes that ring is actually embedded in the shadow. Sometimes it's outside of it. It depends.

That's right, a ring of light forged in darkness. Feel free to insert your own Lord of the Rings joke here. What the heck is this photon ring? Well, to understand that we have to understand the photon sphere. You see black holes bend the path of light cool. Got it. Check. We understand this, but at a very special distance from the black hole, the gravity is so strong that light gets bent so much that it circles around the black hole in an orbit. Yes, black holes can force light to orbit around them. If you were a near a black hole at this very special distance, you could literally see the back of your own head in front of you. Why? Because a photon would leave the back of your head and it would orbit around the black hole, would circle around and then come right into your eye. You would look in front of you and you would see your own back of your own head. This special distance, where the gravity is just right to make photons or light follow orbits is called the photon sphere. And for a regular plane, non rotating black hole, it's It's around three halves, the radius of the event horizon.

So about 50% bigger than the event horizon. Rotating black holes actually have two photon spheres that counter rotate against each other because they're awesome and complicated. In a different episode, so is this photon sphere too. Let's imagine a bunch of light rays coming in and encountering the black hole. The light rays could be from the accretion disk around the black hole. Could be from the wider universe. It doesn't matter. There's just photons. We're we're We're surrounded by photons all the time. That's what photons do. Some of these photons pass near the black hole, and when they do, if they do well outside the photon sphere, then they get bend a little, you know, like a like a curve bank shot, and they they go off in some random direction. They move on with their lives. Maybe those photons end up aiming for the Earth. Maybe they don't no big deal. That's some of the light that we see in these images. But some of the photons cross into the photon sphere, where they start to loop around the black hole. But the photon sphere itself is unstable. Photons will not last long. There. They are guaranteed to leave.

They can't stay in orbit forever. It's not a stable location. There will be some interaction. Some trade and energy or something will force them to leave when they leave the photon sphere some of them plunge down into the event horizon, never to be seen in this universe again. In some escape. When they do, they can be beamed at us at this special distance. There are a lot of photons that can loop around in this special way because there are a lot of photons in the universe, and they're going in all sorts of crazy, random directions, the the usual diffuse scattering of light. But the photon sphere acts like a magnifying lens. Light from all these directions end up getting beamed. So what you get at the photon sphere is an accumulation of light that all this light coming from all these different directions, instead of continuing on in random directions, instead get beamed away from the black hole scattered away from the black hole.

At this special distance, the radius of the photon sphere, the photon sphere appears to us as a ring because all of our images are two dimensional. So we flatten the whole thing and we see a ring in which case, this source this magnifying lens of like it's a new name, the the photon ring. Appropriately enough, that is light from the wider universe from the accretion disk from everywhere getting scattered in a very special place that accumulates that light and sends it on its way at a very specific distance from the black hole. In a perfect world, they should appear as a super thin, bright, uh, ring. But remember that observing black holes is really hard, and so our pictures are really fuzzy. So this super bright, thin ring gets smeared out like an out of focus picture. Because remember, in interferometer are just really garbage telescopes. And instead of of a bright, thin ring, we see a generally blobby diffuse doughnut of plasma around the black hole.

So when you see that light in the event horizon telescope pictures, you're not seeing light from the accretion disk itself. You you're seeing a little bit of it. But most of that light is coming from the photon sphere, this special place where light can orbit around a black hole. There's more. See, the light can take multiple loops around the black hole. It might just take one u-turn. You know, there's, you know, imagine there's this photon coming in from the right. It encounters the black hole at this special distance and it gets looped around, and then it scatters it just the right way and aims for us. And then there's another bit of light coming from a completely random direction, like from the top. And it's coming down and it's coming down and it gets bent around the photon sphere because it had just that right radius and it gets beamed towards us if the distances are off. If it's inside the photon sphere or outside the photon sphere, then there's nothing special about it. The light just scatters randomly, as usual. But because the photon sphere is there, the light gets scattered in a very special direction.

We see this emphasis of light buffoons can take one trip around two trips around a three trip four or 5 20 trips around the black hole before eventually leaving. Each one of these trips represents its own ring. Its own sphere takes one trip. That's the main sphere. The main ring that we see takes two trips. There's this tiny, subtle inner ring to it. A third trip, a third ring. Each ring is fainter than the one before, because the chances of photons making multiple trips gets lower and lower, and this can tell us so much about black holes. These nested photon rings, because this is an exquisite test of general relativity as close to the event horizon as we can get. Because remember, any light passing closer than the photon radius is going to end up in the event horizon. It's not leaving. This is the closest we can observation and get to a black hole. Is this photon ring like I said, the photon ring, depending on the exact geometry of the disc, the source of light the size?

Whether it's spinning or not, that ring can appear actually inside the shadow or it can appear outside the shadow. It depends, which is really weird to think about, but this is the closest we can see to a black hole. This is the greatest test we can have of general relativity, because this is the most extreme source of gravity that we can directly see. Yeah, there's hawking radiation, but one particle per year. Good luck. This is the closest we can see to the actual black hole is this photon ring. We currently do not have the resolution needed to see the sub rings let alone the main photon ring is still this fuzzy blob in all of our pictures. It's hoped that extending the Event Horizon telescope into space should allow us because we put one more element in space. That's an even longer baseline that gives us even more resolution. And but we'd have to work even harder to reconstruct the image. It's hoped that we can get better and better resolution images of these photon rings, but it's hard.

But no matter what, this is the best observations we can make of a black hole. The shadow itself, which is larger than the event horizon because of the extreme gravity. The photon ring, another expression of the extreme gravity that beams light from all sorts of random directions at this special radius for our enjoyment. No matter what, The shadows of black holes are not boring at all. Thank you to Kelsey R on email and him and W on YouTube for the questions that led to today's episode. Please keep sending me questions hashtag ask a spaceman. Ask the spaceman at gmail dot com or just the website. Ask apace man dot com, thanks to all my patreon contributors, but especially my top ones. That's patreon dot com slash PM Sutter I like to thank Justin G Chris Barbeque, Duncan M, Corey D, Justin Z, NAIA Scott M, Rob H, Justin Lewis M John W, Alexis Gilbert, M Joshua, John S, Thomas D, Simon G, Aaron J and Jessica Kay. Again, that's patreon dot com slash PM Suter Thank you so much to all the supporters, all the people who ask questions, All the people who write reviews of this podcast and share it with your friends.

I really do appreciate it, and I will see you next time for more complete knowledge of time and space.

Comment