Antarctica 02: Every Time You Turn Around

by Shane L. Larson

[Photo: M. B. Larson]

We are homeward bound from Antarctica, back in the travel pipeline, back in the bustle of everyday human life.

As the sights, and sounds, and silence, and color, and light filter around my mind’s eye, I begin to contemplate what to tell people who ask me about the journey I am now concluding.

How was it? What was it like? What did you see? What was your favorite thing? How do you feel?

I suspect there is no adequate way to capture true and authentic answers to any of those questions. Antarctica is a vast, ethereal place, unlike any other on the planet. Pictures and movies will never capture the expanse, the majesty, the grandeur, the mind- and spirit-altering experience of literally every moment you spend there.

Antarctica is unique, and would fail to move only the most brittle and wretched of spirits. I was there, immersed in only one small corner of a BIG place for only five days. In such a short time, I did everything I could to let the experience in and penetrate as deeply as possible. It is a transformative encounter. Every single moment you turn around there isn’t just more, there is new, there is heart-stoppingly different, there is something Antarctica is saying that no-where else on Earth can. 

You try to capture it through a camera lens.

You give up and just stare.

You try to describe it with the best words you have.

You give up, and just shrug, nodding knowingly at the person next to you who also failed to find adequate words.

You try with all your might to describe Antarctica, and its just not… enough. It’s not enough to use something as simple as words, or as inadequate as pictures. It makes you deeply contemplative, especially when you are immersed in it, and those contemplations beg to be expressed, however jumbled they may be when given voice.

But I will try. I have to try. Because Antarctica blew me away.

Every time I turned around.

[Photo: S. Larson]

The first day we were there, everyone would ask, “How are you doing?” And I would say, “I can’t stop smiling!” That was true. But then I’d look out a window, or turn around on deck, or be struck by the silence. I’d encounter Antarctica anew, different than just moments before, and it overwhelms me. It just spills out, bursts out more than just a smile, more than joy and fun and laughter. A deep pleasure of the spirit, a profound awe at the grandeur of this planet, a shift in the center you never knew you had. 

I think the best word to describe how I felt is “ebullient.” It was — it is — just wrong to contain such a swell of joy and not let it out.

Of course, the most frustrating thing about this is I want to be able to say and show something about the voyage that  somehow adequately captures this. I want to capture it and remember everything.

But I can’t photograph everything.

I can’t capture every detail.

I can’t describe it all.

I can’t remember it all.

Because Antarctica simply blows you away.

Every single time you turn around.

So I did what I could to capture, to record, to write what I saw and felt there. For the next few posts I’ll try to share some of that here. To inspire you, to give you a taste of what it was like. But mostly, I think, to remember.

It will be enough. It has to be enough. Because Antarctica deserves for me to try. It gave me everything I asked for. And because should I be so lucky to return, Antarctica will start all over with me again, like a clean slate, teaching me something new.

Every single time I turn around.

[Photo: M. B. Larson]

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This post is the first in a short series to document a journey I made to Antarctica with Lindblad Expeditions and National Geographic in the late days of January 2020. The other posts in this series are:

Antarctica 01: Daydreams

Antarctica 02: Every Time You Turn Around (this post)

Antarctica 01: Daydreams

by Shane L. Larson

When I was young, my idle daydreams were filled with imagining what it would be like to explore otherwheres that were far from ordinary: the far reaches of space, the deeps of the oceans, and the remote wildernesses of Earth. I’m not sure what incited such daydreams, but they were certainly fueled by a healthy dose of documentary television. There is no doubt that Carl Sagan, Jacques Cousteau, and David Attenborough deeply wired a wonder for the vast wide world into my brain — even today I hear their voices in my head when I read or imagine writing about Nature’s awesome spectacle.

(L) Carl Sagan, (C) Jacques Cousteau, (R) David Attenburough. Voices from my youth, who still echo around inside my head.

There is something deeply moving about places where Nature’s raw, awesome power and intricate beauty are on full display. The creativity of Nature is stunning, as is its ability to present us with mysteries that are beyond everyday human experience and (some days) beyond ordinary human comprehension. But there is something more about space, the sea, and the wilderness that gives one pause: Nature’s nearly unfathomable ability to be inhospitable to humans. Exploration and discovery in these realms isn’t just hard, it’s nigh on impossible. Nature could kill us dead without pause because our fragile bodies simply were never meant to be in these places. It’s like Nature is hiding secrets from us on purpose, defended by obstacles implacable and deadly; which of course, it is!

Nature is full of spectacles, places that are completely different from normal, human experience, and usually hostile to visiting huamns.

Being very much like children in the Universe, humans are not good at being told “No.” Over the years, we have thought deeply about how to venture into the deadly wilds of the Cosmos, and using some wits mixed with technology, have waded out into the danger zones. This inspires boundless joy to the young child still hiding in my brain, simultaneously inciting a deep longing to visit in the adult version of me that walks around. 

As life wears on, it becomes clear that many of my childhood dreams of exploring become more remote, though not entirely. Telescopes in my backyard have provided many long hours of deep personal connection with the stars, planets and galaxies. It seems unlikely that I’ll be an astronaut at this point, but with friends and colleagues I have sent cameras and experiments of our own design to the edge of space. 

Living in the modern world means that ordinary people, like you and I, have access to tremendous technology that allows us to explore the extremes of the Cosmos. The places that have so captivated me since my youth, can be directly explored by ordinary citizens of Earth.

Tragically, I did not learn to swim really until I was in college, and still struggle with concepts and skills like “treading water,” but I dearly love skimming over the surface of lakes and the ocean in a kayak, and I have for many years now been building amateur ROVs (“remotely operated vehicles”) — tethered robots that venture into the deep water with lights and cameras and sensors aplenty. 

Though inhospitable and possibly deadly, the wildernesses of Earth are accesible. Virtually every place on the globe is accessible to humans today. Few places are pristine and untouched by our species, no matter how remote they may be from our cities, roads, and farms. The impact humans have had on the planet and the species we share it with is undeniable. That fact mixes strongly with youthful desires to see and visit the unknown wilds of the Earth and informs my current desperate desire to go and see these places. 

So today, I am making good on one of my childhood dreams, and embarking on a journey to visit just briefly, Antarctica. The polar regions of Earth have always held a special place in my imagination — vast, desolate, remote, and fragile. They are the embodiment of something that is a common experience — winter — pushed to an extreme that is both beautiful and deadly. I’ve had the great fortune to travel northward to where the winter ice reaches its fingers southward, defining the boundaries of where polar bears roam. This is my first journey south.

Other than science bases, there are no permanent settlements in Antarctica — no roads, no cities, no infrastructure. Tourism there is limited to small, shipbourne expeditions with roughly a hundred or so passengers each. The seasonal advance and retreat of the ice around the continent limits visits to a few months during the Antarctic summer, which limits human impact and footprint (thought not entirely — here is a realization that humans are carrying new illnesses to penguins).

I’m making my journey under the care of National Geographic and Lindblad Expeditions, aboard a ship called the National Geographic Explorer. The first small tourism trip to Antarctica was conducted by a Swedish explorer and entrepreneur named Lars-Eric Lindblad in 1966. He had the express purpose of taking small groups of people into the remote, beautiful regions of Earth, seeding what today is called “ecotourism.”  He believed that giving people direct, personal, up-close experiences in the remote pristine corners of our planet would foster a deeper understanding of the need to protect and defend the natural world.

Outline of the expedition with NatGeo/Lindblad. The trek could end up on either side of the Antarctic Peninsula.

To make the journey, those of us on the expedition will first converge on Buenos Aries for a single afternoon and evening together. We’ll meet our expedition companions, have a briefing about the journey, and get to spend an evening in the city. The following day, a charter flight will take us to the southern port of Ushuaia, in Tierra del Fuego, where we will board the National Geographic Explorer and begin a two day trek across the Drake Passage.

The Drake Passage can be calm, or treacherous — the Southern Ocean that surrounds Antarctica can whip up stormy seas and choppy swells. While large and built for the ice, a ship the size of National Geographic Explorer is still going to feel the seas if they are rough and stormy; many who have made this journey before have suffered greatly during this part of the trip.

The National Geographic Explorer.

But after two days, we will emerge into the relative calm and shelter of the continental shelf along the Antarctic Peninsula. For the next 5 days or so, we will journey down the Peninsula, each day immersed in the grandeur of the ice, the rock, and the sea. 

Undeniably, there is an element of me that screams to see Antarctica before it changes, irrevocably forever. Undeniably it has changed in our lifetimes, and those changes will only continue. But a stronger drive right now is the fact that you are here reading this blog — why should that matter? In the days before the Apollo 1 fire which took the lives of astronauts Gus Grissom, Ed White, and Roger Chaffee, and eager reporter had asked them whether or not it was worth risking death to visit the Moon. Grissom eloquently replied,  If we die we want people to accept it. We are in a risky business, and we hope that if anything happens to us, it will not delay the program. The conquest of space is worth the risk of life. Our God-given curiosity will force us to go there ourselves because in the final analysis, only man can fully evaluate the moon in terms understandable to other men.”

The last part resonates deeply with me. It is one thing to read books, and surf the web, and watch documentaries about the exotic reaches of Earth. It is quite another to be regaled with stories and raw, visceral descriptions of personal encounters from a real, live person. One develops a deep sense of how transformative an encounter with Nature can be when you see it in another person’s eyes and hear it in their voice, lilting and wistful as they remember in their mind’s eye.

It also echoes the desire and sentiment that Lars-Eric Lindblad had when he founded his expedition company — a steadfast belief that if you expose people to beauty and grandeur, that it changes the spirit, inspiring somewhere deep down an innate desire to protect and defend the wild, desolate places of the Earth. As a person who is fortunate enough to have this opportunity there is a deep desire to take that inspiration and share it.

On this journey, we will just reach the continent, where the Antarctic Peninsula stretches up toward Tierra del Fuego; the entire continent is vast beyond this, but beyond the scope of any single visit. It is possible to visit Antarctica as part of scientific expeditions, and someday I hope to visit for science! But for now, I am content to go as a citizen of Earth. In the past, I went north.  I am about to embark for the first time (and I hope not the last) south.

More to report in part 2, after I return.

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This post is the first in a short series to document a journey I made to Antarctica with Lindblad Expeditions and National Geographic in the late days of January 2020. The other posts in this series are:

Antarctica 01: Daydreams (this post)

Antarctica 02: Every Time You Turn Around

Black Holes 5: Inklings & Obsessions

by Shane L. Larson

There are many exotic phenomena in astrophysics — some pervade the public consciousness, and others do not. Most folks have heard of the “Big Bang” and probably about “dark matter.” Fewer people have heard of the “Cosmic Microwave Background” or “neutron stars.” Perhaps even fewer have heard of “cosmic strings” or “radio jets.” But of all the strange and wonderful things astronomers and physicists have contemplated, the most universally known and recognized are probably “black holes.” Just about everyone has heard of black holes, and just about everyone has some cool science factoid they know about black holes and keep in their pocket — they pull the factoid out anytime the subject of black holes come up because the factoids typically MELT YOUR BRAIN.

On the left, an optical image from the Digitized Sky Survey shows Cygnus X-1, outlined in a red box. Cygnus X-1 is located near large active regions of star formation in the Milky Way, as seen in this image that spans some 700 light years across. An artist’s illustration on the right depicts what astronomers think is happening within the Cygnus X-1 system. Cygnus X-1 is a so-called stellar-mass black hole, a class of black holes that comes from the collapse of a massive star. New studies with data from Chandra and several other telescopes have determined the black hole’s spin, mass, and distance with unprecedented accuracy.

I find black hole factoids to be a curious mix of some things that are true, some things that are speculative but possibly true, and some things are are outright fiction. Where do all of the exotic facts about black holes come from, how do we all learn them, and why are some right and some wrong? Wondering about this led me to contemplate when I first heard about black holes.

Black holes have long been a passion of mine — my mom will tell you I was always kind of obsessed with them. But when did I first hear about and learn about them? I certainly can’t answer that question definitively, but I do know some things about my early exposure, so I can try to understand what strange and awesome ideas first attracted my attention.

The earliest encounter of which I am certain is during the 1979/1980 timeframe. This was the time that many people saw Disney’s epic space opera The Black Hole, replete with adorable robots, killer robots, really awful bad dialog, and archetypical mad scientists. It has often been derided for its scientific inaccuracies (most notably by Neil deGrasse Tyson [link]). I definitely saw The Black Hole. Multiple times. And I still watch it sometimes. Neil’s right, there is a lot of inaccurate science about black holes in The Black Hole, but there is a lot that I think was okay too (more on that later). There are definitely modern movies that get the science more uniformly correct (Interstellar), but I don’t mind The Black Hole — certainly not as much as Neil. The point here is this is a known anchor point in my love affair with black holes.

So what could a movie like The Black Hole teach me about real black holes? If you ask almost anyone, they know the correct fundamental thing: a black hole is an object whose gravity is so strong, not even light can escape — even The Black Hole got that right. Since nothing can travel faster than light, nothing can escape. If you fall into a black hole, your fate is sealed. It is this idea of being trapped forever, without recourse or hope of rescue, that lies at the heart of our fascination with black holes. They are strange; indubitably. But to be inescapable suggests a kind of absolute and infinite supremacy. 

Me in elementary school. I’m not sure what I’m doing, but I’m pretty sure I’m not getting into trouble! [Image: Pat Larson]

In 1979/1980 I was in fifth grade, and I was gobbling this stuff up left and right. I was a well known fixture in both my school library (Hygiene Elementary School, in Colorado), and in the Longmont Public Library, where my mom had gotten special dispensation for me to have an “adult” library card so I could prowl through all the science books in the grown-up section. My parents also exposed me to a steady diet of books at home, and while they were all nominally “family books,” some of them made it to the bookcase by my bed and never went through anyone else’s hands. In 1980 one such book was Roy Gallant’s lavishly illustrated Our Universe, published by the National Geographic Society.  I was certainly enraptured with outer space by then, steadily fed by the ongoing exploits of Viking and Voyager as they played out on the pages of National Geographic. But this book — this book. Blew. My. Mind.

It is a book filled with great pictures from an exquisite generation of space probes, and from the best telescopes the world knew in the pre-Hubble era. But the art and scientific illustrations are what sucked me in. Paintings of the surface of Venus. Speculations of what weird alien lifeforms evolution could have created. Stupendous cutaways of planetary interiors and atmospheres. All of it was linked together with Gallant’s trademark lucid storytelling.  This ode to the Universe captured my mind and imagination and never let go. That first copy my parents gave me was read cover-to-cover, and carried for miles and years everywhere I went, pulled out of my backpack in moments of wonder and curious indulgence.

Examples of the art and technical imagery in Roy Gallant’s “Our Universe.”

Near the end of the book, Gallant talks about black holes in just 4 short paragraphs, but accompanies the text with a lavish, full-page artist’s idealization of a black hole in space, tugging on a nearby star, bending the shape of spacetime, and absorbing a beam of light that was inexorably caught in its pull.

He asks in the caption of the picture, “Can you imagine a star so massive that its gravitation eventually crushes it out of existence, leaving only a black hole in the sky?” This is classic Gallant, imploring the reader to immerse themselves in the mystery, throw caution to the wind, and employ their imagination — take what little knowledge you have and simply speculate. That is where good ideas come from, and it is the basis for all science.

The artist’s representation of a black hole in “Our Universe.” [Image: Helmut K. Wimmer]

In many ways, the reason you and I are having this little blog conversation is precisely because astronomers know that black holes exist in Nature and are the central players in many astrophysical phenomena. But reading Gallant’s text it is clear that when he wrote Our Universe, the existence of black holes was still a subject of much debate among scientists. Today there are many ways that we have measured the properties of black holes and confirmed their existence, not the least of which are the many that have been detected via gravitational waves. But still, pictures of black holes remain elusive. The best we have so far is the Event Horizon Telescope picture, a silhouette of a black hole against the backdrop of stuff around it.

The picture in Gallant’s book is an attempt to show a black hole as a three dimensional object in real space, but how do you do that?  It was a noble attempt, and it is certainly not what a black hole looks like, but it served its purpose — it got my attention, it fueled my imagination, and it made me ask questions then go see if the answers were known. To this day I keep copies of Our Universe nearby — one in my office and one in my study at home. It is never far from my mind nor my fingertips, and I often pull it down and lose myself in the epic stories it tells.

The other thing I know happened to me in the fall of 1980 was my first exposure to Carl Sagan’s Cosmos. Starting in late September, every Sunday night, I sat rapt on my parents’ living room floor in front of the television, whisked away to worlds and places in the Cosmos I had only previously imagined, transported by the magic of film, the lilting and elegant soundtrack of classical music, and Sagan’s poetic and sonorous narrative. One of the most widely known episodes is Episode 9, “The Lives of the Stars” which famously begins with Sagan declaring, “If you wish to make an apple pie from scratch, you must first invent the Universe.”

Sagan uses his famous declaration about pies to introduce the concept of the chemical elements — the atoms from which all the beautiful and complex structures of Nature are built. Beyond the simplest elements — hydrogen and helium — very little was created when the Cosmos was born. Almost everything on the periodic table is created by stars during their lifetime, and a great deal of it (the heaviest elements) during the catastrophic death throes we call supernovae and gamma ray bursts. In telling us about the death of stars, Sagan uttered the magic words I had heard before — black hole. In his trademark penchant for poetic description, he called it “a star in which light itself has been imprisoned.”

Sagan’s Cosmos was the first place I was introduced to the ideas of black holes in the context of general relativity, beginning with masses curving space and affecting the motion of other masses, and also a discussion of the principles of black holes as tunnels [Images from Ep 9: “The Lives of the Stars”]

He had led us to the existence of the super-strong gravity of black holes through an imagined tea party with Alice and her friends in Wonderland, but then he hung on it all the modern picture of curved spacetime. It was, as far as I know, my first exposure to Einstein’s brilliant realization, and it has ever since dominated my destiny. Today, I have a doctorate in theoretical physics, earned for studying the magical mysteries of that self-same curved spacetime.

Me and J. Craig Wheeler. He’s one of the reasons you’re reading this blog right now!

For many of us, our interest in black holes might be piqued by these kinds of exposures, and then we go back to our lives as dental hygienists or soybean farmers or city managers. But this was all still swirling in my mind when I entered college, and in the true traditions of higher education, my exposures took those latent passions and exploded them into what would become my life. I was an undergrad at Oregon State University and at that time there was a stupendous class on campus called “Rocks & Stars,” run by the indomitable Julius Dasch. This was one of the most popular classes on campus, and had a regular stream of guest speakers who visited and talked to us about cool stuff.  I have strong memories of one visit from J. Craig Wheeler, a supernova expert from the University of Texas at Austin.

Supernovae are one of the pathways for making black holes in the Universe, and Wheeler gave us a spectacular talk that culminated with him reading to us from a science fiction book he wrote, called “The Krone Experiment.” I won’t give it away (go read it!) but what I remember from the talk was Wheeler talking us through what would happen if you were standing on a sidewalk and a micro-black hole came booming up out of the ground next to you. What would you see and experience? It’s the sort of question that just captures your brain and won’t let go. To be honest, it was the perfect question to ask a young scientist in the throes of deciding to commit their career to studying these enigmatic objects.

I think every one of these stories illustrates a key fact in my mind: it didn’t matter what I heard about black holes in my youth, only that I did hear about black holes. Exposure did what it should: it filled my head with all kinds of possibilities, all of them totally brain-melting, and made me pay attention and ask questions later.

This last point is the most important point here: we want people to ask questions. Either because they are confused, or because they are idly curious, or because they want to learn more. To that end, having mind bending movies like The Black Hole is stupendously important, and I don’t care if they get the science perfectly right! I have colleagues who often grouse about bad science in movies, complaining vociferously that the producers should have taken a basic science class, or gotten a good science advisor. They proclaim, “Is it really that hard to get the science right? The right science is just as cool!” 

But people aren’t watching The Black Hole to learn science (I certainly wasn’t) — they are being entertained, itching a part of their brain that wants to be asked “is that even possible or real?” And that serves its purpose, because eventually every one of them ends up in an audience somewhere at a public lecture and raises their hand and asks someone like me “is what happened in the movie real?”  THAT is where we get the science right. The movie’s job was to put a question in someone’s mind, to make them care enough to know what the right answer might be, and then in some other part of their lives have some discussions about science, what is known, what is not known, and what the other mysteries of the Cosmos might be.

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This post is the last in a series about black holes.

Black Holes 01: Imaging the Shadow of Darkness

Black Holes 02: What are black holes made of?

Black Holes 03: Making black holes from ordinary stuff

Black Holes 04: Singularities, Tunnels, and Other Spacetime Weirdness

Black Holes 05: Inklings & Obsessions (this post)

Black Holes 4: Singularities, Tunnels, and Other Spacetime Weirdness

by Shane L. Larson

I think one of the great things about the modern world is the propensity of information. Information is free and easy to come by, and it possible to learn about anything you want. More-or-less, the total knowledge of our civilization has been written down in books and documents, and disbursed to libraries, websites, and other mediums of communication. It is not always easy to discern what is authentic and what is not, as is clearly the case when one looks at the wild, apocalyptic wastelands of modern social media. But none-the-less, it is easy to indulge your desire to simply learn. We consume books and podcasts and documentaries, sacrificing time we could spend on woodworking or yardwork or binging TV shows in favor of trying to recapture how we felt in 2nd and 3rd grade, before school became about exams and homework and was just about how awesome all the far flung corners of the world and Nature could be. 

I think deep down all of us are lifelong learners; I’ve met many of you at public lectures or here at the blog. Some of you are quiet, and sit in the back with contemplative furrows on your brow; others of you are more exuberant and can barely contain your questions. Either way, you all show up, because you remember how cool it was when you were first learning. But I’ve noticed something interesting in my years talking to all of you: as a rough rule of thumb, I can usually triple the attendance of any talk if it is about sharks, volcanoes, dinosaurs, or black holes. Vast numbers of you succumb to your curious inner child if I talk about the right things. 

People’s minds, young or old, can be captured if we talk about science in ways that stimulate their interest and imaginations. [Image: Bill Watterson]

What is it about these topics that inspires deep interest in people? I think, at the heart, they are very real examples of the Universe’s ability to put you in mortal danger with implacable indifference. Never mind that it is unlikely you will encounter any of these dangers in your life. Pondering being faced with a highly improbably danger in the Universe allows us to ask ourselves, “what would I do?” in much the same way we watch super-hero films and imagine ourselves in the fray. 

Black holes are notable in this list because not only do they have the mystique of danger about them, but they are suffused with a long list of exotic, mind-bending phenomena that add to their mysterious nature.  

Let’s talk about some of the exotic things you have hard about black holes, and I often get asked about.

“Is everything is going to get sucked into black holes?” 

This is probably the most common question I get about black holes! The simple answer is “no” — black holes are not little Hoovers running around the Cosmos sucking stuff up. I’ve thought a lot about where this idea comes from, and I think it is a mis-extrapolation of the inescapability of a black hole. When you are far from a black hole, its gravity is exactly the same as the gravity produced by ordinary objects of the same mass. If you are orbiting far away from a billion-solar mass black hole, the gravity you feel is exactly the same as if you are orbiting around a dwarf galaxy that has a billion sun-like stars in it!  If we could magically replace the Sun with a one solar mass black hole, the Earth would continue along in its orbit as if nothing had happened because the gravitational influence is exactly the same! 

Far from the black hole, you cannot tell if you are orbiting a black hole or a star of the same mass — their gravity is identical unless you get close. [Image: S. Larson]

If you are silly (or unfortunate) and fall into a black hole, you are never going to get out. The gravity of a black hole is so strong that it can trap anything inside it; that is true. But it is not infinitely strong and able to influence everything outside it. 

“What does it mean to get spaghettified?” 

When you get close to a black hole, the gravity can become more intense than anywhere else in the Cosmos. Imagine you are jumping in feet first. The gravity is strongest close to the black hole, so your feet are pulled on more strongly than your head, which is farther away. The result of this dichotomy of gravitational strength is the black hole tries to pull you apart, much as you stretch a rubber band by pulling on opposite ends. Physicists call this difference in force a tidal force, and the process of pulling you apart is called tidal disruption. Stephen Hawking, in his famous book “A Brief History of Time” called this effect “spaghettification.”  

Some ways of falling into a black hole will feel less painful than others. [Image: S. Larson]

Somewhat paradoxically, the spaghettification effect is strongest near the event horizon of small black holes, and weaker near the event horizon of larger black holes. The spaghettification effect is also stronger when your head is farther away from your feet (so tall people will suffer more than us short people). The two rules of thumb for surviving spaghettification when you are jumping into black holes are this: 

  1. Jump into the biggest black hole you can find; million solar mass black holes are much more fun to jump into than solar mass black holes.
  2. Belly flopping into black holes is safer than jumping in feet first.

“Do black holes really bend time?” 

The movie Interstellar has revived broad interest in black holes and inspired wide-ranging conversations about what black holes are really like. One of the most common conversations we have is about time, which usually begins with “what was the deal with the guy who got old when he didn’t visit the black hole?” This plot device could just be accepted at face value, like we do with so much science fiction, but in this case it is rooted in the physics of the real world. General relativity predicts that the closer you are to a source of gravity, the slower your clock ticks compared to someone very far from the source of gravity. Here I use the word “clock” in the physics sense: anything that keeps regular time, whether it is a digital watch, a wind-up pocket-watch from your grandparents’ day, an hour-glass, or the steady beat of your heart.  

Consider two people, one close to the black hole and one farther from the black hole. Every clock ticks slower when you are close to the black hole — this could mean an actual clock that tells time, but it can also mean a regular biological clock, like your heartbeat. [Image: S. Larson]

The bending of time is definitely one of those counter-intuitive predictions of general relativity, but if space and time are one entity (“spacetime”), then bending space very strongly must necessarily also bend time. It doesn’t take much to bend time by a measurable amount — the bending of time is the central physical effect behind the Global Positioning System, which you use everyday on your phone to navigate to the nearest ice cream shop (or coffee shop — whatever). The difference between the bending of time around the Earth and the bending of time around black holes is the strong gravity near the black hole makes the effect much more pronounced. 

“Are black holes are infinitely dense? What does that mean?” 

Anything labeled infinity is, generally, an anathema to scientists. “Infinity” is a perfectly good concept in mathematics, but with respect to the natural world, it seems that the Cosmos is only filled by things that are finite and measurable. That is not to say there aren’t enormous, gigantic, mind-bogglingly huuuuuuge numbers, but they are all tiny compared to “infinity.” In the natural sciences, we have often encountered “infinity” in the mathematical ways we describe Nature, but we’ve found most of them were simply artifacts of our early poor understanding of how the world works, particularly on the microscopic scales of fundamental particles. Gravity is the last frontier in this regard, and there are many persistent “infinities” we encounter, and they often manifest themselves in the study of black holes. 

An example of your common experience with density. This cube of tungsten and this clown nose are about the same size, but the tungsten is significantly heavier. Why? because more stuff is packed into roughly the same amount of space. [Image: S. Larson]

To think carefully about this, let us be precise about what we mean. “Density” is a common concept in physics and chemistry. It is how much stuff (mass) is squeezed into a given amount of space (volume). Dense objects feel heavy in your hand, while less dense objects feel lighter.  As a matter of practical everyday experience, you most often encounter the notion of density when thinking about things floating or sinking in water (objects more dense than water, like rocks, sink; objects less dense than water, like styrofoam, float). 

So let us define the “density of a black hole” the way we define the density of any other object in the Universe: the mass of the black hole, divided by the volume of the black hole. Those of you who are practicing gravitational physicists will recognize that we should be careful when computing the “volume”, but for practical purposes here let us use the ordinary formula for the volume of a sphere where the radius of the sphere is the radius of the event horizon of the black hole. This is practical and intuitive, and will illustrate our point effectively.  

The first picture of the black hole at the heart of M87, formed by light being bent around the inner most regions of space outside the event horizon. This black hole has a diameter larger than the diameter of our solar system! [Image: Event Horizon Telescope Collaboration]

Imagine two black holes: one that is the mass of the Sun, and one that is one billion times the mass of the Sun (a bit smaller than the black hole in M87 that was the subject of the Event Horizon Telescope picture). The solar mass black hole only has a radius of about 3 kilometers, and a density of about 18 quadrillion times more the density of water (1.8 x 1019 kg/m3, for those calculating themselves). By comparison, a 1 billion solar mass black hole has a radius just under 3 billion kilometers (about the radius of Uranus’ orbit); it would have density of only 2% the density of water (numerical value: 18 kg/m3; slightly less than the density of styrofoam). If you could somehow drop it in a gigantic cosmic bathtub, its density suggests it should float. 

If black holes were solid objects and could interact with the world like ordinary “things,” a calculation of their density suggests some are less dense than water and could float in a cosmic wading pool. [Image: S. Larson]

It can’t float, of course — the event horizon is not a hard surface that water can act on and thus provide buoyancy in a ginormous cosmic pond. Water would flow right through the event horizon and disappear, so all the water in the cosmic pond would essentially flow into the black hole like some kind of drain.  But that’s not the point in the floating analogy. As a general rule, we think we understand the physics of things that have densities less than the density of water, so the idea that the density of a black hole is the same as materials that do float is a strange and discomfiting result. And it should be! Just remember your discomfiture is related to the odd nature of black holes — density defined in the classical way really doesn’t apply to black holes the way we’ve done it here, because as we’ve noted before, they are mostly empty space! This odd result has little to do with their overall size, and more with what lies at their heart… the singularity. 

“The Singularity” 

The real mystery of black holes lies at their heart, in the center of the space defined by the boundary of the event horizon. All the gravity of the black hole is concentrated there. All the matter that collapses to form the black hole is still being drawn together even after it falls through the area we call the event horizon. Gravity keeps pulling it inward, inexorably inward, squeezing it smaller and smaller with a force so great no known force in Nature can stop it. Everything that fell inward to create the black hole gets squeezed down smaller and smaller, becoming more and more dense. Eventually it gets squeezed into a space that is vanishingly small, or so general relativity predicts. This point of zero size with everything squeezed into it is infinitely dense, and is called the singularity. The laws of physics as we understand them break down before you ever really reach the singularity, at a distance away from it called the Planck length, about 10-35 meters (0.00000000000000000000000000000000001 meters). This is the length where we expect the physics is governed by quantum gravity, a description of gravity, space, and time on the tiniest scales. We have searched for such a mathematical description of Nature for many years, but so far have been unsuccessful. 

How do physicists think about the singularity? It is an infinity, and infinities are anathemas to physicists. More often than not we are trying to understand what is happening far away from the singularity when thinking about the Cosmos. This is, more or less, what astronomers do because they are observing the Cosmos outside the event horizon, which is far from the singularity. Easy peasy — you don’t even have to waste one brain cell on the singularity if you don’t want to! Sometimes physicists pretend they are okay with the singularity being infinitely dense, and use the classical laws of physics (general relativity in particular) to understand the influence of the singularity around it. Gravitational physicists often do this, in particular because they are trying to understand how the world behaves under the influence of strong gravity. All the tales and imaginings you have heard about the inside of black holes are figured out by scientists thinking about the singularity this way. The last prominent group that thinks about the singularity are the quantum gravity squad. There are many ideas about what a complete description of quantum gravity will look like — all of them are clever, and elegant, and exotic. But we don’t yet have a way of experimentally testing any of them. Someday we will be able to test them. The day we understand quantum gravity, it will tell us something about the true nature of the singularity. 

“Are Black Holes Spacetime Tunnels?”

The last and most famous example of spacetime weirdness and black holes is the astonishing idea that for some kinds of black holes, if you jump in, they may in fact be tunnels. For the movie nerds out there, this is the central plot device in many science fiction stories. For perfectly spherical black holes, there are no tunnels — if you jump in a black hole, the singularity lies in your future; you will be crushed ruthlessly and mercilessly.  But for black holes that happen to have electric charge on them (expected to be few) or are spinning (most black holes found in Nature are expected to be spinning) there are trajectories inside the event horizon that do not end at the singularity. They end… somewhere.   

Scientists struggle to visualize black holes just as much as ordinary people, so we have developed a special map called a Penrose diagram. As you go up the diagram from bottom to top, time increases from the past to the future. As you go left or right on the map, you change where you are in space. Here the white areas are the ordinary Universe, and the yellow areas are inside a black hole. The one on the left is a Schwarzschild black hole that has no tunnel; if you are inside, the singularity lies to your future and there is no ordinary Universe you can get to. The one on the right is a charged black hole, which might have a tunnel. If you are inside, you can avoid the singularities on the left and right, and possibly emerge at the top of the diagram. [Image: S. Larson]

In these kinds of black holes, if you avoid the singularity you come out of something that looks mathematically similar to a horizon. The difference is you come out of this horizon, emerging from the inside to the outside. Such things are variously called “wormholes” or more commonly “white holes.” They are, in essence, the other end of the black hole, like it is some kind of giant culvert or tunnel that connects one place to another. 

Tunnels to where? you quite astutely ask. The truth is we don’t know, but there are several possibilities. One possibility is the tunnel emerges somewhere else in our observable Universe. As an astronomer this is a very intriguing possibility, because it suggests there may be something that could be observed with telescopes. Sadly, to date, we have not seen anything exotic and unexplained that might be a white hole.  

Another possibility is that it may emerge somewhere in our Universe, but outside the observable part of the Universe. This idea is a bit harder to wrap your brain around, because it hinges on understanding that the Universe can be larger than what we can observe and could, in principle, go on forever. The “Observable Universe” are just those parts of the Universe that are close enough for us to observe in a telescope because light has had time to reach us in the time since the Universe was born. An easy way to think about this is to think about your home state where you live. Most of us can walk only a few miles per hour — let’s say 3 miles per hour. That means in one day, you could only walk 3 miles per hour x 24 hours = 72 miles. If you started walking right now, by this time tomorrow you could be anywhere in the state within 72 miles. Does that mean the rest of the state isn’t there? Of course not — it just means the parts of the state you could get to at the fastest pace you could walk (the “Observable State”) is only 72 miles away in any direction. 

If I start at Northwestern University and walk for 24 hours at 3 miles per hour (average walking speed of a human) I can reach anywhere inside the red circle. I can get no farther, but that doesn’t mean there aren’t more places outside the circle! The Universe is the same, only the time is not 1 day, but the age of the Universe, and the speed is not walking speed, but the speed of light. Inside the circle is what we call the Observable Universe, but it is not the Entire Universe. [Image: S. Larson, Map by Google]

A third exotic possibility is that the white hole may emerge not in our Universe, but in some other Universe. A Universe that is not our own, but is somehow parallel to our own. It is an interesting possibility to ponder and imagine because it opens up all kinds of possibilities. Are the laws of physics the same there, or is the other Universe some weird place that doesn’t have stars and planets and galaxies? Do all of our black holes emerge as white holes in the other Universe? Where do black holes from the other Universe go? Do they emerge in our Universe, or do all white holes in all the other Universes emerge in only one of the other Universes? Some things we can imagine within the realm of science and do calculations and simulations about, but others are mere speculation that we have yet to ponder and consider seriously. It makes your head spin, but these are the things that great speculative science fiction about black holes are made of.  

The last possibility is that tunnels through black holes simply do not exist at all, that Nature somehow closes them off, or we have not fully understood the mysteries of black hole insides completely yet. There is much we still have to learn.

Which of course is the point. Black holes, on any given day, seem completely unfathomable, especially in the context of the weird implications about what they do to the world around them. But is precisely that mystery that draws our attention time and again. Partly because we like that feeling of being completely baffled by Nature, but also because some deep part of us knows that these inscrutable mysteries hide deep and precious secrets, secrets that lie at the core of how Nature and the Cosmos work. 

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This post is the fourth in a series about black holes.

Black Holes 01: Imaging the Shadow of Darkness

Black Holes 02: What are black holes made of?

Black Holes 03: Making black holes from ordinary stuff

Black Holes 04: Singularities, Tunnels, and Other Spacetime Weirdness (this post)

Black Holes 3: Making black holes from ordinary stuff

by Shane L. Larson

If you are exploring the Cosmos, and either by design or accident, find yourself plunging toward a black hole, on a beeline that takes you directly across the horizon, you don’t encounter anything along the way; all you feel is the inexorable pull of gravity pulling you farther and farther down. When you reach the event horizon, the nominal “surface” of the black hole, what do you encounter? 

Nothing.

The event horizon is simply the invisible line in space where gravity has become so strong that even if you were travelling at the speed of light, you could not escape; the event horizon is a boundary that once crossed, Nature says you are never coming back out — the inside of the black hole and only the inside of the black hole is in your future. That’s a big statement, but you cross this point in space without even an alarm to let you know you are trapped. It’s as easy as walking across a line drawn in the sand at the beach.

A black hole with the MASS of the Sun is not even close to the SIZE of the Sun! Here the approximate size of a solar mass black hole is shown near the city of Evanston, IL — only about 6 kilometers (4 miles) across. What is it made of? Nothing tangible — it is empty space, filled with nothing except gravity itself! [Map: Google; Image: S. Larson]

Through the horizon and as you fall inward you still encounter absolutely nothing. This has previously led us to ask, “What is the black hole made of?” Based on what you experience, we had concluded “a black hole is made of pure gravity.” But gravity, as we learned early on in our thinking about the world, is a consequence of mass (or energy if you take the modern understanding of mass and energy being related). That normally would imply that the black hole was made of mass of some sort, but as our gedanken experiments have paradoxically shown us, there is no mass to be encountered when travelling toward and into black holes! 

A thoughtful cosmic explorer (or astronomer) would take that bit of confusing information and ask a very pointed question: “how do you make a black hole, then?” The motivation for such a question is built out of our common experience. If you want to make something, whether it is a gallon of dandelion wine, a guitar, or a cinnamon roll, you take other things and transform them into the new thing.  So what are the other things that can be transformed into a black hole? And how do you change them from being ordinary things into pure gravity? Those are very good questions, and to be honest with you here at the beginning, we don’t know all the answers. We only know part of the story, the penultimate explanation for how it happens. Some of the story is still unknown, and lies beyond the boundaries of what we currently understand about the Cosmos. That is part of what astronomers and physicists investigate and attempt to understand every day.

Astronomers have seen many phenomena in the Universe that are explained by black holes. The question is where do they come from? [Image: Wikimedia Commons]

How do we start? Where ever you are, look around you and pick up the nearest thing you can see. Maybe it’s a rock, a bagel, a book, a Lego brick, your cat — whatever. Here I’ve picked up a fountain pen. Why does the fountain pen, or any other object, have form? What is it that makes it a solid tangible object? If you try to squeeze the fountain pen, it may deform slightly, but generally resists any effort to squeeze it out of shape. Why? Because as you press the fountain pen, the building blocks of which it is made, the molecules and atoms, fight to hold their shape. They press against their neighboring atoms, and when the pressure from your fingers tries to force them closer together, they press back against you in tandem, resisting your attempt to move them.

If you press hard enough, you can sometimes squeeze them together or change how they sit next to each other. Sometimes you are stronger than a material and break the object. Some objects are hard to compress, but they can certainly be deformed if you apply a force to them in specific ways. It’s hard to flatten a paperclip into something like a piece of foil, but it is not too difficult to bend it back and forth into a new shape. Ultimately what you can do physically to any object depends on how the building blocks of its structure respond to forces applied from the outside.

(L) Very solid objects, no matter how hard I squeeze them, retain their shape. The atoms that they are made of resist external forces. (R) Some objects can be deformed, bent, or broken, like these paperclips — their atoms resist some external forces, and yield to others. [Images: S. Larson]

Now consider a slightly different example. Go to your kitchen and find a party balloon in your junk drawer. Blow it up and tie it off so it is maybe 20 centimeters across. Take a couple of sheets of aluminum foil, and wrap the balloon up. What happens when you try to squeeze the foiled balloon? It deforms a little bit under the force of your hands, but when you let go the pressure from inside the balloon pushes it back into its round shape.

A balloon wrapped in foil as a heuristic model of a star. The balloon presses outward against the pressure from your hands that is trying to collapse the foil.

This simple balloon and foil model is completely analogous to a star. The foil is playing the role of the outer layers of the star that we see when looking through our telescopes (the “atmosphere” or the upper layers of the star). Your hands pressing down are like gravity, trying to pull everything that makes up the star into the center. Opposing the inward press of your hands, the balloon represents something inside the star pressing outward against the pull of gravity. We know the outward press is the energy released by nuclear fusion deep in the core of the star. This balanced state, where the inward pull of gravity is precisely counterbalanced by the outward push from the energy created by fusion, maintains the round and stable size and shape of the star. Astronomers call this state hydrostatic equilibrium.

When the balloon is popped, nothing prevents your hands from collapsing the foil. This is similar to fusion ending in the core of a star — nothing presses outward, and gravity collapses the star.

Now, gently hold the foiled balloon in the palm of your hand and have a friend pop the balloon with a needle. The support from the balloon vanishes, leaving you holding an unsupported shell of the foil. You are gravity, so squeeze the foil down. It should be easy — there is nothing to fight back against you. You can, and should, squeeze the foil down into a small, aluminum ball. Ball it up, and squeeze it into the smallest ball you can. Once you’ve squeezed it as hard as you can, stand on it trying to squeeze it smaller. If a member of your family is stronger than you, ask them to squeeze it even smaller.

How small did you make it? Can you make it any smaller? The answer is “probably not.” Why? Because all of the aluminum atoms in the foil are resisting being pressed together, far stronger than you can press them together with your hands or feet. This is not dissimilar to the fountain pen we discussed above — all the atoms that make up the aluminum are pressing out, resisting being pushed any closer together than they already are.

The exact same thing happens in Nature. Gravity takes collections of stuff — stars, planets, anything round — and tries to pull it together as strongly as it can. Eventually everything gets crowded together, and through a variety of interactions resists the inward pull of gravity. For stars in the middle of their lives, they exist in hydrostatic equilibrium, with the inward tug of gravity balanced against the outward push from the fusion in the core.

Squeeze the foil as hard as your possibly can. Eventually your strength will be matched, and the ball will get no smaller.

When a star reaches the end of its life, the fusion in the core shuts down. That moment is like you popping your balloon — the star suddenly finds itself without much outward pressure at all, and the inward pull of gravity takes over — the star collapses. The collapse is the beginning of a supernova explosion. 

For our interests here we are not interested in what gets blown out, but what happens in the innermost core. There, the titanic pressures of the collapse and explosion break apart the atoms, and breaks apart the nuclei of the atoms. You may recall from school that atoms themselves are made of smaller bits — the smallest bits are called electrons which orbit around a nucleus made up of bits called neutrons and protons. Like you standing on a wine glass, the inward force of gravity during the collapse crushes every atom, breaking every one apart into these shards called electrons, protons, and neutrons.

In the soup of protons and electrons and neutrons that results, the protons and electrons are forced together and turn into a neutron plus a small particle called a neutrino. This conversion process is called “neutronization” (really — sounds like something from a superhero movie, right?) — the conversion of most of the matter of the core into neutrons. 

A neutron star (diameter 20 km) scaled to the Chicago skyline. [Image: LIGO-Virgo/Nick Gertonson/Daniel Schwen/Northwestern]

This core that remains is called a neutron star when it settles down, and its gravity is extreme beyond belief.  It has about 1.5 times the amount of stuff in as the Sun, but squeezed down into something about 20 kilometers across — the size of a city.  At the surface, the gravity is 200 BILLION time stronger than the gravity you are experiencing right now on Earth. What are the consequences of such extreme gravity? Imagine you could take a walk on a neutron star (and you could certainly NOT walk, but go with me here) and you had the unimaginable misfortune of encountering a cliff only ONE MILLIMETER high. What would happen if you fell off? On a neutron star, falling off a one millimeter high cliff means when you reach the bottom you will be travelling about 227,000 kilometers per hour (141,000 miles per hour)!

The gravity of a neutron star is extreme, but a neutron star, like its parent star, maintains its shape as a round, spherical object — it is in hydrostatic equilibrium! Gravity is trying to press down, but something just as strong is pressing back. In this case, it is the neutrons that make up the star. Neutrons do not like to be near each other and push back when they are squeezed into small spaces — this is called “neutron degeneracy pressure” (for the quantum mechanics aficionados among you, this is a consequence of the “Pauli exclusion principle”). The reason gravity could not collapse the neutron star is because the neutron degeneracy pressure is enough to stop it.

But there is a funny truth about gravity. All four of the fundamental forces of Nature have a range of distances over which they act, and their strength varies over those distances. They also each affect only certain kinds of objects in the Cosmos. Gravity, however, is completely indiscriminate — it acts on and affects everything that has mass and energy, which as it turns out is everything in the Cosmos!

The consequence of that simple fact is if you make a big pile of anything, gravity always tries to pull it closer together, and will succeed in pulling it together until it is opposed by a stronger force (for example the hydrostatic equilibrium, and the neutron degeneracy pressure examples we noted above).

We only know the masses of a few neutron stars, most between the mass of the Sun, and two times the mass of the Sun. Can heavier ones exist in Nature, or do they all turn into black holes? Explore the stellar graveyard on your own with this interactive tool at CIERA. [Image: Frank Elavsky/Northwestern University]

Most of the neutron stars we have observed in the Cosmos up to now have masses between about 1.4 times the mass of our Sun, up to around 2 times the mass of our Sun. Why aren’t their bigger ones? We certainly see huge stars, up to 30 or 40 times the mass of the Sun — when they explode, they definitely have larger cores that should leave behind bigger remnants, bigger stellar skeletons. So can a neutron star bigger than the ones we’ve found in the Cosmos exist? You can imagine taking one of the known neutron stars, and slowly piling more and more mass onto it. Each jelly-bean or rock or bit of starstuff you drop on the neutron star increases its mass, which increases its gravity, which makes gravity pull inward more strongly. Eventually, gravity will get so strong that the neutrons cannot resist any more — gravity overwhelms the degeneracy pressure, and presses the neutrons closer together. 

Concentrating all the mass of neutrons together makes gravity even stronger, which pulls all the mass of the neutron star even closer in a never-ending cycle of just making gravity stronger. At this point, there is no known force in the Universe that is stronger than gravity. Nothing can oppose gravity’s inexorable inward pull, and everything that was a neutron star gets smaller, and smaller, and smaller, until the gravity is so strong that not even light can escape. We have a name for that.

A black hole.

So at last, we arrive at the answer to our question: we make black holes by squeezing matter together. Black holes are not stuff but they are made of stuff in the beginning. 

Where is all that stuff now? It is concentrated somewhere behind the event horizon, where we cannot see. Mathematically, the laws of gravity suggest it is concentrated into an infinitely dense point called a singularity. It is… … … something. Something that completely defies our understanding of the Laws of Nature, and is the subject of much consternation and study on the part of modern physics and astronomy researchers.

Now we are curious creatures, and it is completely natural to ask “what is inside the black hole?” or “what is inside the event horizon?” The answer quite pointedly is you can NEVER know unless you jump in yourself! The emphasis really is on the word UNLESS — nothing prevents you from jumping in and looking around; the only prohibition is on your ability to come back out to visit your friends who watched you jump in. The prohibition that the speed of light is the ultimate speed limit in the Cosmos, coupled with having to travel faster than the speed of light to get out of the event horizon, means you will never hear about anything that happens on the “inside” of a black hole second hand!

But we can use our mathematical understanding of gravity to predict what you would experience if you jumped in, and the predictions are weird and disconcerting. We’ll talk about some of that gravitational weirdness next time. 

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This post is the third in a series about black holes.

Black Holes 01: Imaging the Shadow of Darkness

Black Holes 02: What are black holes made of?

Black Holes 03: Making black holes from ordinary stuff (this post)

Black Holes 2: What are black holes made of?

by Shane L. Larson

In modern astrophysics, the study of black holes grows out of general relativity, the modern description of gravity written down by Einstein in 1915. As descriptions of Nature go, general relativity is among one of the most remarkable discoveries humans have ever made.

On the surface, general relativity seems extraordinarily complex and mathematical, and it can be. The governing equations that describe what general relativity explains and predicts about the Cosmos are what scientists call non-linear. Generally this means the equations cannot be solved with simple pen and paper calculations, using algebra skills you learned early in your mathematical training. This was something Einstein recognized early on. He had in fact abandoned the idea that there would ever be “simple” or “exact” solutions to his equations, and started working on “weak gravity” (“linearized” for the aficionados) cases where the equations of general relativity can be written in ways simple enough to work on.

Karl Schwarzschild [Image: Wikimedia Commons]

But, as if often the case in science, even the smartest of us sometimes don’t see the obvious. In 1915, barely a month after the first presentation of general relativity at the Prussian Academy, a young German soldier named Karl Schwarzschild wrote a letter to Einstein, showing that if you use general relativity to consider a situation which has symmetry, the equations become simple enough to solve directly. Schwarzschild considered the case of a source of gravity that was perfectly spherical, like a planet or star or other massive body. Today, we’ve named this solution after him, and the most extreme example of what Nature can do with it is called a Schwarzschild black hole.

Most of us have heard quite a few weird and exotic things about black holes, much of which defies our common experience and common sense. That exoticness comes from the sophisticated nature of the non-linear equations of general relativity, but also from the fact that the places where the Cosmos creates or harbors black holes are the most extreme environments imaginable, far outside anything you might encounter on Earth, either on a wilderness hike or in a physics laboratory. But the discomfitting exoticness of black holes belies a subtle truth: they are exceedingly simple objects.

Cars are complicated, which is why you see lots of magazines like the one on the left. Black holes are simple, which is why you don’t see magazines like the one on the right. [Image: S. Larson]

What do I mean by simple? Consider for a moment one of wonders of the modern age: the automobile. An average vehicle has maybe 20,000 parts that make it up. If you want to understand all the details about how a vehicle works, or make one yourself, you have to understand everything about all those 20,000 parts. What is each one made of, how is it shaped, where does it go in relation to other parts, does it move or is it held still, what kinds of environment will it encounter? These and many other questions are required to have a complete understanding of a “car,” so there are books and magazines and entire industries and trades dedicated to motor vehicles.

Unlike automobiles, black holes are simple. They are completely characterized by only three numbers: the mass, the spin, and the amount of electric charge they have. Black holes that have one or more of these properties are named for the scientists who first wrote down the mathematical descriptions that describe them in the context of General Relativity.

These three properties are all ones that seem familiar and quite ordinary, because they are used when we talk about most things in astrophysics. Mass, spin, and charge are all quantities that are more or less familiar. However, in the context of black holes what we mean by them is less clear.  Consider “mass.” What do we mean by the mass of a black hole? To understand this, we have to first be very clear what we mean by black hole and confront some of its exotic properties.

Our adopted definition for “black hole” is a very practical one — it’s an object whose gravity is so strong, not even light can escape. As scientific definitions go, I like this precisely because it is very practical — it captures an immutable way to identify what an object is based on a simple observation or experiment you can do, in this case you can test the gravity of an object to see if it is, in fact, a black hole or something else.

One of the things you may remember learning about gravity early on in your science classes is that gravity gets stronger as you get closer to an object, and gets weaker as you get farther away from an object. That means if you are far away from a black hole, the gravity you feel from it does not have to be very strong at all! More to the point, if you are far from the black hole and point a laser pointer directly away from the black hole, the laser light goes flying off minding its own business without consequence; the black hole is so far away its gravity has little or no measurable effect. But that it also means is as you get closer and closer to the black hole, the gravity must get stronger until it finally becomes so strong that the laser light cannot get away. That transition point, where gravity is finally strong enough to stop light, is called the “event horizon.”  It is NOT a physical surface — it is simply that place where gravity has gotten strong enough to overcome light.

We can represent gravitational influence with a figure that shows lines along which an object feels the gravitational force. The number of lines emanating from an object is related to the mass. The strength of gravity you feel is related to how many lines are around you (how many lines cross into the little red circles). Stick Spock, far away, feels weaker gravity than Stick Picard, who is somewhat closer. Stick Geordi, who is very close to the source of gravity, experiences much stronger gravity. [Image: S. Larson]

This transition point is important, especially in terms of astrophysics, because it defines which places in space we can detect signals from (for example, light emitted by atoms which can be seen with telescopes), and which parts of space we cannot. Quite literally, the event horizon is an invisible boundary in space that defines “inside” and “outside” the black hole. It’s kind of like putting a line of tape across the bedroom you shared with your sibling when you were growing up — there’s nothing to indicate where the boundary is when you walk across it, but there are definite sides.  On the outside, atoms are free to create light which can move away from the black hole and be seen in telescopes. On the inside, the gravity has passed the tipping point, and light can no longer get out. Atoms can still go about their atomic business and make light, but that light cannot go sailing across the Universe — like everything else on the “inside,” the light is subject to the inexorable pull of the black hole, dragging it deeper into the interior. For this reason, we treat the event horizon as if it is the surface of a black hole, and for all intents and purposes it is — anything that crosses the event horizon disappears and vanishes into the black hole forever. Every black hole has an event horizon — it is what defines them.

The question that set us along this line of reasoning is “What do you mean by mass?” If the black hole has an inside and an outside, then that seems like an impossible question to answer because the mass lives inside the event horizon — how do you know it has mass? That is an eminently reasonable question to ask! It’s an important one to ask because of the way we talk about black holes. In most contexts, mass is a code word that means “the amount of stuff that makes up an object.”  For astronomers, a practical way to think about what we mean by mass is it defines “how much gravity a black hole creates.”

To imagine what we mean by that, try to think about how you measure the mass of the Sun. Has anyone ever taken the Sun and plopped it down on a scale at the doctor’s office? No. We’ve measured the mass of the Sun by observing how its gravity has influenced other objects around it. If I look at the orbit of a spaceship or a small asteroid at a known distance from the Sun, the time it takes to complete the orbit tells me how massive the Sun is. We didn’t “measure the mass” of the Sun (how much “stuff it has” in it), we inferred the mass by measuring the gravity of the Sun. When we state the mass of a black hole, we are doing exactly the same thing — we’re using a number (that we call “mass”) to express how much gravitational influence the black hole would have on things that might fly around it in space. 

One way to “measure mass” is to look at how long it takes to complete an orbit of a given size. You can do this around the Sun or around a black hole, and they will give the same answer if they “have the same mass.” Compared to the Sun, the gravity of the black hole is only extremely strong when you are close to the hole, seen here in this figure by comparing how close the lines are near each of the objects. [Image: S. Larson]

If you’re scratching you head, you’re doing okay — this is probably one of the hardest things to wrap your brain around. If the black hole has gravity, doesn’t that mean it “has mass?”  Not necessarily.  Consider trying to get as close to the black hole as you can, without crossing the event horizon. As you get closer and closer to the black hole, all you encounter is empty space. In fact, if you fly right through the event horizon to the inside of the black hole, all you encounter is MORE EMPTY SPACE. The black hole has gravity but it is comprised entirely of empty space.

Wait — what? That’s right, a black hole is completely empty space. It isn’t tangible, there isn’t stuff you can scoop up and collect in a little plastic bag. It is empty space. So what is it? It is pure gravity. Now it seems weird to think about it that way, because you are used to thinking that things in outer space are made of stuff — stars, galaxies, nebulae, comets, asteroids, and planets are all made of stuff. But black holes are not. You’ve encountered that idea before — if we go dig a big hole in your garden then lean on our shovels to admire our work, we point at a big empty space, full of nothing, and we call it a hole. This is kind of the same idea.

Now you rightfully might be inclined to ask what is making the gravity? After all, in the rest of the Universe, to make gravity you have to have stuff. Isn’t that the point in why we have orbits around the Sun? The Sun has stuff it is made of, that stuff makes gravity, and gravity is what makes stuff move in orbits. But gravity isn’t a substance that massive objects make and throw out into the Universe — gravity is an effect they have on the Universe around them. Einstein’s great realization is that what you and I think about as a “force” of gravity is really our response to the shape of the Universe around us (more properly, the shape of “spacetime”), which is forcing us to move in certain ways.  A black hole’s “gravity” is just a statement of how the black hole has bent spacetime outside of it.  Let’s imagine a simple example.

Paths ants take when walking along a flat surface. [Image: S. Larson]

Imagine I show you the paths some ants are walking along, but I’ve gotten some of my Hollywood special effects friends to remove the objects they are walking on, and all you can see is their path.  The only rule is an ant always walks on a straight line, directly where its head is pointing, never turning to its left and never turning towards its right. What do you think the ant paths in the figure above are showing?

You might have said a “table” or a “piece of paper” or the “ground” or a “wall.” It could be any of these things! You can’t tell the difference between them from the paths, only that whatever it is is FLAT.  Now consider the ant paths shown below.

If the surface has an interesting shape, the paths ants take walking across it, even though they are walking “in straight lines,” look interesting. One path will tell you the surface is interesting (left), but many paths will reveal what the surface really is (right). In this case, a sphere. [Image: S. Larson]

What is that ant walking on? The path the ant took was straight as far as the ant was concerned, never turning left and never turning right. But eventually the path came back to itself not because of something the ant did, but because of the shape of the surface it was walking on! So what was the surface? From a single path, you can’t tell, but if you have multiple ant paths you begin to see what the underlying shape might be, and there are many possibilities that include the first circular ant path we noted above, but in the end we would conclude it was a ball.

Now consider the more complicated collection of paths below. The more paths you have, the more likely it is you can understand the underlying shape of the space. The ants aren’t feeling any force that makes them change directions they are moving. As far as they are concerned they are walking freely in straight lines, and the shape of the surface they walk on determines what that path looks like. The end result shows you the shape of the space, and sometimes it is flat, sometimes it is spherical, and sometimes it looks like a bottle!

If the surface has some exotic shape, the ant paths can have a wide variety of different behaviours, but with enough paths you can understand the surface they are walking on. [Image: S. Larson]

So what should you take away from this parable of the ants? You should know that how something moves can be understood as moving along the shape of something. Gravity bends the Universe, and how we move can be understood as us moving along the bends, along the warps and weaves, of that bent Universe. When a planet or a star is sitting off by itself in the Universe, it bends spacetime around it, and black holes do the same thing. When we move along an orbit around a black hole, when we “feel its gravity pulling us in an orbit” we are really just moving along the bent Universe around the black hole. 

So what’s the difference between how a star bends spacetime and how a black hole bends spacetime? Only how strongly it does so. The Earth bends spacetime pretty strongly — if you try and jump straight up  you can’t get away. A rocket has to travel just over 11 kilometers a second (25,000 miles per hour) to get away. A black hole bends spacetime more strongly, so strongly that you’d have to travel faster than the speed of light, or 300,000 kilometers a second (671 MILLION miles per hour) to get away!  Exotic, to say the least!

But despite their exotic nature, black holes had to come from somewhere. Next time we’ll talk about how to make black holes in the Cosmos. 

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This post is the second in a series about black holes. 

Black Holes 01: Imaging the Shadow of Darkness

Black Holes 02: What are black holes made of? (this post)

Black Holes 1: Imaging the Shadow of Darkness

by Shane L. Larson

Of all the phenomena in the Cosmos, black holes evoke a special sense of wonder in most people.  What is it that captures the imagination when you hear the words “black hole?  There are countless mysterious and exotic astronomical phenomena that pique the interest of astronomers, but few of them have the evocative power over people’s imaginations that black holes do. 

There is no known portrait of the Reverend John Michell.

What are we talking about? By definition, a black hole is an object whose gravity is so strong, not even light can escape. The first person to imagine such an object was the Reverend John Michell in 1783. At the time, it was an interesting supposition since the speed of light was known to be “fast,” so such an object’s gravity had to be “strong,” but there was nothing inherently special about the speed of light itself. The same idea was later promoted quite widely by Pierre-Simon Laplace (he had a much better press machine behind him than the Reverend Michell). But the exoticness of black holes was not fully realized until the early 1900s, when two things happened.

There is an ultimate speed limit in the Universe!

First, in 1905, Einstein published special relativity and the scientific community rapidly tested many of the ideas and found them to be consistent with the behaviour of everything in Nature. The central tenet of special relativity, which is of paramount importance to our discussion here, is that there is an ultimate speed limit in the Cosmos, somehow built into the foundations of the structure of the Universe: you cannot travel faster than the speed of light. People often ask me, “what’s so special about the speed of light?” There is nothing special about light, there is something special about the speed. It happens that light was the first thing we ever discovered that can travel at the ultimate speed limit, so we call it “the speed of light.” The proper statement is really, “nothing can travel faster than the ultimate speed limit!

Second, in 1915 Einstein published general relativity, which showed how to think about gravity in a way that was consistent with special relativity. Less than a month later, Karl Schwarzschild found the first astrophysical solution to the equations of general relativity, what we today call a Schwarzschild black hole. This is a perfectly spherical black hole, an object whose gravity is so strong you have to exceed the ultimate speed limit to get away from it. Extraordinarily, but perhaps not surprisingly, the size of Schwarzschild’s black hole is exactly the size predicted by Michell in 1783.

At this point, we can begin to understand why black holes are so exotic and alluring. It’s not just that they have intense gravity. It’s that they are inescapable. If you have the sad misfortune to fall in one, you will never be able to get back out (we’ll talk about “tunnels” through black holes in a later post). That very idea of being trapped forever, incontrovertibly prevented from leaving by the laws of Nature — that’s mind boggling, and clashes with our normal sense of free-will. It very definitely has a “Nature is more dangerous than we suppose” air about it that we often only reserve for documentaries about sharks, the wild savannas of Africa, and just about all the wildlife in Australia. 🙂

It is the inescapable Nature of black holes that makes them interesting to think about, but it is also what makes them hard to study in astronomy. All things being equal, black holes are just about the hardest cosmic phenomena to observe because, for the most part, we study the Universe using light. By definition black holes emit no light! Recently, our burgeoning ability in gravitational wave astronomy (here are all the posts at writescience tagged “gravitational waves”) gives us a way to probe black holes directly, allowing us to understand them as single, big objects — how massive they are, how fast they might be spinning, what processes might form them. But trying to understand what is happening up close to the black hole, where the gravity is at its most intense and unusual things can happen, that’s hard. We would really like to know what happens if you are as close as possible to the black hole without being trapped, down near the surface of last escape, a place called the “event horizon.” Probing this region is the goal of one of the most ambitious projects in astronomy, called the Event Horizon Telescope.

With telescopes, we always observe light emitting objects, and watch what happens to them in the vicinity of black holes.  We use these observations as a way to understand the properties of black holes and what they are capable of doing to the Universe around them. Most of the time when you hear about black hole discoveries, this is what has happened — something did something weird near a black hole, and told us the story with light! For instance, at the centers of most large galaxies astronomers find massive black holes, millions or billions of times more massive than our Sun. The one in our own galaxy is called Sgr A* (pronounced “Sagittarius A-star”). Since the 1990s, astronomers have been watching stars down in the center of the galaxy, and have now watched them long enough to see them move across the sky and make closed round paths — orbits! Something you may remember from your early exposure to astronomy is that whatever is at the middle of an orbit is the source of gravity that makes and object go around in its orbit… and at the centers of these stellar orbits we don’t see anything. The size of the stellar orbits, and the speed they move, tells us that whatever is there has four MILLION times the mass of our Sun. There is no known dark phenomena in Nature that can be that tiny, that massive, and create that much gravity other than a black hole.

Two decades of observations have shown the orbits around the 4 million solar mass black hole at the center of the Milky Way. [NCSA/UCLA/Keck]

About 53 million lightyears away, in the direction of the constellation Virgo, there is a massive galaxy known as Messier 87, which can be easily seen with backyard telescopes. It has always been an object of intense interest, even before we knew there were other galaxies besides the Milky Way. In particular, in 1918, Heber Curtis, observing with the 36-inch Crossley Telescope at Lick Observatory noticed “a curious straight ray… apparently connected with the nucleus by a thin line of matter” (read a copy of Curtis’ original paper here). Today we call such structures “jets” and know they are powered by matter interacting with black holes. In the century since Curtis’ first observation, many observations of the M87 jet have been made and used to understand the properties of the black hole that drives it.

The black hole in the center of M87 powers an enormous, energetic jet of material spewing out from the galactic core. (L) We think we were one of the first amateurs to image this jet in 2001 [Image: S. Larson/M. Murray/A. Block] . (R) HST image of the jet, for comparison. [Image: STScI/Hubble/NASA] 🙂 [Click to embigen!]

The Event Horizon Telescope takes this observing strategy to the next level. The goal is to look as close as possible to the event horizon, and see what can be seen, down near the root of the jet. Can matter glowing brightly before falling into the event horizon be seen? Can light from stars and matter behind the black hole be bent by the black holes intense gravity and provide a lensed silhouette of the black hole? To even think about doing a project like this, you have to know two things: (1) how big on the sky does a black hole appear, and (2) what is the tiniest object in the distance that a telescope can see?

The apparent size of an object in the sky depends on how intrinsically big it is, and how far away it is.  For black hole sizes, we can use the size predicted by Schwarzschild to get a sense for their diameter.  If you want to tinker and get out your calculator, the diameter of a Schwarzschild black hole is given by the formula

where dbh is the diameter of the black hole, Mbh is the mass of the black hole, G is Newton’s gravitational constant, and c is the speed of light. If you put in the mass of any black hole in kilograms, then this formula tells you the diameter of the black hole in meters.  A black hole the mass of the Earth has a diameter of just over 1.5 centimeters. A black hole the mass of Neptune has a diameter just a bit bigger than a human head.  A black hole with the mass of the Sun has a diameter of almost 1500 meters (just under a mile). As the mass of the black hole gets larger, the diameter gets larger.  So consider the two black holes we’ve discussed above: Sgr A* and M87.

A black hole with the mass of the Earth is about the size of a marble. A black hole with the mass of Neptune is about the size of your head! [Image: S. Larson]

Sgr A* is 4 million times the mass of the Sun, and has a diameter of 23.6 million kilometers — more than 60 times the size of the Moon’s orbit! The black hole in M87 is even larger. Massing in at 3.5 billion times the mass of the Sun, it has a diameter of 20.7 billion kilometers, or about 3.5 times the size of Pluto’s orbit! If you plopped the M87 black hole down on the Solar System, every planet and world NASA has ever explored would be inside the black hole; of all things human, only Voyager 1 would escape, sitting just outside the event horizon.

So, what does it take to observe the Cosmos on the scales of even big black holes? Telescopes can make out things that appear very small in the sky, but how small? How big something appears depends on how far away it is. Imagine you have a friend holding a beach ball and a dime, standing at the far end of a field. In all likelihood, you can see the beach ball, but not the dime. The beach ball appears tiny, but from far away a larger object is easier to see than a smaller object. A telescope’s ability to distinguish the size of objects in this was is called its resolving power, and depends on the color of light you are using (technically, the wavelength) and the effective diameter of your telescope — the bigger the telescope, the better resolving power it has.

How large an object looks to you depends on its size and its distance. Here, you see when Michelle is very far away you can still make up the larger ball, but the smaller coin is harder to see! [Image: S. Larson]

The idea of the Event Horizon Telescope is to look for big black holes because they will have a discernible size even if they are far away. The targets of interest are the black hole at the center of our own galaxy (Sgr A*) and the black hole at the center of a M87. Both of these black holes have a size on the sky of a little more than a billionth of a degree. How big is that? Well, Sgr A* covers the same size on the sky as a quarter that is about 2/3 of the way between the Earth and the Moon. The M87 black hole covers the same size on the sky as a quarter that is about 1.5 times the distance of the Moon. Those are really small, but you and I live in the future — our telescopes are up to the task.

To make the measurement, the Event Horizon Telescope team uses radio telescopes spread across the Earth, all observing simultaneously as if they were a single giant telescope the size of the planet. This kind of astronomy is called VLBI — Very Long Baseline Interferometry. The resulting picture of M87, the first released by the Event Horizon Telescope, is shown below — a brilliant ring of light, surrounding the shadow of a massive black hole, the first of its kind.

The first picture of the black hole at the heart of M87, formed by light being bent around the inner most regions of space outside the event horizon. The teams measurements show the black hole is heavier than previous measurements, totaling almost 6.5 billion times the mass of our Sun. [Image: Event Horizon Telescope Collaboration]

This is the first time we’ve been able to accurately reconstruct a picture using light from all the small areas around a black hole, effectively imaging for us the shape and size of the event horizon. It is a tremendous leap forward and provides us a new and important way to probe black holes and their properties. As with all enigmatic things we see happen in the Cosmos, the more ways we have of measuring them, the easier it is for us to figure out what is going on!

And this is just the beginning!

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This post is the first in a series about black holes.  The complete series of posts is:

Black Holes 01: Imaging the Shadow of Darkness (this post)

Black Holes 02: What are black holes made of?

Black Holes 03: Making black holes from ordinary stuff

Black Holes o4: Singularities, Tunnels, and Other Spacetime Weirdness

Black Holes 05: Inklings & Obsessions

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For a great video describing the intricate details of the light interacting with the black hole to make images, check out this excellent video from the team over at Veritasium: