Feeling Small in a Big Cosmos 03: Proverbs

by Shane L. Larson

On 19 April 1610, Johannes Kepler wrote an open letter to Galileo Galilei, musing on possible future voyages that would allow explorers — human explorers — to see what Galileo’s telescope had shown.  He mused that some day inventors might “provide ship or sails adapted to the heavenly breezes, and there will be some who will not fear even that void.” Kepler called on Galileo to join him in preparing the way for those so0n to be travellers, and create a new science to light their way: astronomy.

Yuri Gagarin and the Vostok 1 launch on 12 April 1961.

Yuri Gagarin and the Vostok 1 launch on 12 April 1961.

It was almost exactly 351 years before Kepler’s speculations were realized — on 12 April 1961 the Soviet Union launched cosmonaut Yuri Gagarin into space. In a flight that lasted only 108 minutes, Gagarin orbited the Earth in a capsule bearing the callsign Kedr (“Cedar”), and initiated The Space Age.

Kepler’s poetic  words are a testament to our visceral desire to know the Cosmos. Gagarin was perhaps no less poetic when, in the middle of the launch, he belted out an exclamation of joy born from the same deep well of emotional longing — Gagarin’s hearty “Poyekhali!” (“Let’s go!”) ushered in a new era in the history of our species — the beginning of our quest to walk among the stars.

The Cosmos is vast, and nothing makes that point more abundantly clear than contemplating long journeys by humans into space. Trying to protect and sustain our fragile bodies for the duration of a long space voyage brings into sharp focus a single, glaring fact: we are designed for Earth, not for the void, not for alien landscapes, not for far-off icy moons. Despite all the tantalizing things we can see, it seems Nature never intended us to stray far from the small Blue Marble of Earth. We shouldn’t feel bad about that; it is also true for starfish, and seagulls, and housecats, and pine trees.

Every spaceprobe we have ever flung into space has returned remarkable pictures, and made new discoveries. Top to bottom: Mariner 4 was the first to flyby Mars, and returned the first pictures of the Red Planet's surface. The Soviet Venera 9 was the first to send pictures back from the surface of Venus; it survived for only 53 minutes. The Sojourner rover was the first spacecraft to move around Mars, in 1997. In November 2014, the lander Philae was the first spacecraft to land on a comet.

Every spaceprobe we have ever flung into space has returned remarkable pictures, and made new discoveries. Top to bottom: Mariner 4 was the first to flyby Mars, and returned the first pictures of the Red Planet’s surface. The Soviet Venera 9 was the first to send pictures back from the surface of Venus; it survived for only 53 minutes. The Sojourner rover was the first spacecraft to move around Mars, in 1997. In November 2014, the lander Philae was the first spacecraft to land on a comet.

But humans are a particularly stubborn and imaginative species. We could easily abandon the dream of travelling beyond Earth, but instead we designed and built machines to make the voyage for us. Our knowledge of the Cosmos today is largely populated by images collected by semi-intelligent robots built to be our eyes and ears. They have travelled where we cannot, and faithfully returned images which are arguably the most artistic, the most beautiful, the most stunning, the most confusing, the most awe-inspiring, and the most thought-provoking things humans have ever seen.

spacecraft_highresFor every space probe we have thrown into space, for every world they have visited, for every picture they have snapped, there is a tale to tell. All of them unique, all of them stirring. Let’s revisit the tale of one spacecraft that has been outbound now for almost 38 years; a spacecraft called Voyager 1.

Launched on a bright September morning in 1977, Voyager set sail for the outer solar system. Its mission was to visit Jupiter and Saturn and tell us what it discovered, and then to begin a long slow march into space, searching for the edge of the solar system. Voyager returned tens of thousands of pictures during its mission, but I find two particularly compelling.

In February of 1979, as Voyager was speeding toward its encounter with Jupiter, it snapped this photo: the most exquisite and detailed image of the iconic Great Red Spot ever taken. Voyager showed us the magnificent swirl and drift of the clouds on Jupiter, bands of colorful and dynamic gas driven by 600 kilometer per hour winds around the boundaries of a 400 year old hurricane twice the size of the Earth. This is a storm that,  before we turned our eyes to the skies, our species had never encountered nor imagined. But Voyager painted it for us, indelibly etching it into our memory, with a casual snap of a camera.

Voyager I view of the Great Red Spot as it approached Jupiter in 1979.

Voyager I view of the Great Red Spot as it approached Jupiter in 1979 [Image: NASA].

I like this picture because, to the unknowing eye, one might assume that it is a painting, made by an Earthbound artist, trying to capture or evoke some deep feeling or emotion about the human condition. But this was not painted by human hands. This is Nature painting, using a planet as its canvas.

After sailing past Jupiter, Voyager sped on to Saturn, where it took even more pictures and uncovered more mysteries, painting new pictures of a gentle giant bejeweled by a ring of ice. The encounter with Saturn ultimately propelled it on a course to carry it out of our solar system and into interstellar space. On Valentine’s Day in 1990, right after Voyager crossed the orbit of Neptune, we commanded it to turn its cameras inward and take one last series of pictures before they were turned off. In a sequence of 60 pictures, Voyager snapped a family portrait — a stitched panorama that contains every planet of the Solar System, the family of the Sun. This is the frame that contains the Earth. Like the Blue Marble, it is one of the most iconic images of Earth ever taken, dubbed The Pale Blue Dot.

The Pale Blue Dot -- the frame from the Voyager family portrait that includes the Earth [Image: NASA].

The Pale Blue Dot — the frame from the Voyager family portrait that includes the Earth [Image: NASA].

More than any other picture, this captures how small and tiny the Earth is. Voyager wasn’t even out of the solar system when it took this picture. In a Cosmic sense it was still close to home, and already the Earth was nothing bigger than a small fleck of light that if we weren’t looking for it, we may not have noticed.  Everything you’ve ever known is inside that dot.

This is the Earth. It’s tiny. And what I find most remarkable when I look at this image: there is nothing in this picture to indicate there is anything special about this planet. Nothing to indicate there is life there, nothing to indicate that we are there, nothing to indicate that this is where Voyager hails from. This one picture captures indelibly in a single frame the fact that we are small, in a Cosmic sense. Some days, we might feel despondent and overwhelmed by the immensity of it all.

The Voyager Golden Record.

The Voyager Golden Record.

But Voyager also carries a sign of our optimism about belonging to a much larger Cosmos. Bolted onto its side, is a Golden Record. It is a phonograph record, a message to anyone who might stumble on Voyager in the distant future, long after its electronics have died and it becomes little more than a fleck of space junk drifting aimlessly through the galaxy. Should someone find Voyager and its Golden Record,  they would find information about the record, and instructions for playing it. Included in those instructions is a map of the galaxy, pointing back to Voyager’s point of origin. On the reverse, etched in golden grooves that will survive a million year journey into the void, is a collection of data about us, and about our world. It includes greetings recorded in 55 languages of Earth. It includes 115 images from the planet Earth from the time when Voyager set sail into the Cosmos. It includes 90 minutes of music from our civilization. And engraved on the inner edge is a single sentence, in English, that reads “To the makers of music, all worlds, all times.”

This is not the kind of thing  you make and throw out into the vast sea of the Cosmos if you are hiding from the immensity of the Universe. Voyager will outlast every person alive on Earth today. It will outlast every one of us, every person who selected music or pictures to be included on this Golden Record. It will outlast our entire civilization. But some part of us can imagine — hopes — that Voyager will survive and be found, and tell the tale of who we are. Perhaps those listeners will be unimaginable alien intelligences; perhaps they will be our descendants who have utterly forgotten us and our civilization.

Voyager and all the other robotic spacecraft we have built are magnificent creations. We can look at them and be amazed that they have gone so far and seen so much. The very existence of pictures like those I have shown you, and literally millions of others like them, should convince you that we can do anything. We can solve any problem we face, we can uncover any mystery the Cosmos puts before us.

Which leads to one last, important thought. Let’s go back to where we started, thinking about the 10 billion billion grains of sand on Earth, and the 10,000 billion billion stars in the Cosmos.

youWaterSandCosmos

Consider: in just ten drops of water, splashed on your window in a summer rainstorm, there are as many molecules of water as there are stars in the entire Universe. You have heard that every one of us is made of 50%-60% water. Which means there are 100,000 times more molecules of water in your body than there are stars in the entire Universe.

And every molecule of water has two atoms of hydrogen, which is what the stars are made of. And the other atom in every molecule of water is oxygen, which was made by stars, burning hydrogen. At the end of their lives, those stars exploded and threw all that they were back out into the Cosmos to eventually become all that you and I are.

In a very real way, you are atoms the Universe has assembled to look at itself. You are atoms that have been organized to look out into the Cosmos and ask the question, “What’s the deal with all those other atoms?

You are the Cosmos made manifest.

You are a way the Cosmos has organized itself to ask those questions that humans have always asked. Where did we come from? Where are we going? Why are we here? And what is our role to play in the enormous universe all around us?

We’re not different than all those other galaxies, than all those other stars, than all those other grains of sand. We’re all made of the same star stuff.

There is a meme that floats around the internet that is a purported Serbian proverb (1). “Be humble, you are made of the Earth. Be noble, you are made of the stars.

It’s okay to feel small. We are small, so we should be humble. We don’t know all there is to know about the Universe.  But be noble, because you are made of the stars. You and I are members of the only species we know that is capable of asking the questions we ask, of figuring things out and asking new questions. It’s very empowering and an important part of who we are. It’s something I think we tend to forget; we get caught up in our problems and in our concerns every single day. But just like artists, scientists, and clergy, we are all true seekers. We’re just trying to understand what our place in the Cosmos really is.

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(1) I have been unable to indeed verify that this is a proverb from Serbian culture! I would love it if someone actually knew where this came from!

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This post is the last in a series of three that capture the discussion in a talk I had the great pleasure of giving for Illinois Humanities as part of their Elective Studies series, a program that seeks to mix artists with people far outside their normal community, to stimulate discussion and new ideas for everyone.  The first post can be found here:  http://wp.me/p19G0g-xB

Illinois Humanities taped this talk and you can watch it online;  many thanks to David Thomas for doing the videography!

Feeling Small in a Big Cosmos 02: Discovery

by Shane L. Larson

Suppose we wanted to imagine some very big numbers, to somehow develop an appreciation for how BIG the Cosmos truly is. Sitting on a the beach somewhere, one might idly wonder “how many grains of sand are there on all the beaches and in all the deserts of Earth?”  Counting is certainly out of the question, so how might you figure that out?

Bear Lake, Utah.

How many grains of sand, on all the beaches and in all the deserts of Earth?

You would do it the same way we “counted” the galaxies in the sky using the Hubble Extreme Deep Field. You count all the grains in some small amount, perhaps a handful of sand picked up off the shores of Lake Michigan. Then you figure out how long and wide all the beaches and deserts are, and how deep the shifting sands run, and figure out how many handfuls of sand would cover them all. Multiplying my the number of grains in my hand, you would find there are some 10 billion billion (1019) grains of sand on the planet Earth.  That’s a BIG number; a number that is beyond ordinary human understanding, beyond our everyday experience.

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove -- it has seen almost 30 million nights like this one, but very little has changed. The constellations change over thousands of years, but the sky is still full of stars, and the Milky Way still arches over the sky, giving the impression that the Universe is unchanging. [Image: Shane L. Larson]

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove — it has seen almost 30 million nights like this one, a sky full of stars [Image: Shane L. Larson]

But imagine for a moment comparing it to the total number of stars in all the Cosmos. The Hubble Deep Fields have convinced us there must be something like 100 billion galaxies in the Cosmos. A galaxy like the Milky Way has more than 100 billion stars in it, so multiplying those two numbers together, there are some 10,000 billion billion (1022) stars in all the Cosmos, more than all the grains of sand on Earth. An even bigger number, well beyond our everyday experience.

When there is so much we don’t understand here on our own small planet, it is easy to be overwhelmed by the immense size, the immense possibilities of what we don’t understand in a Universe far larger than our brains can easily imagine. We could very easily crawl into our shells, hide from the immensity, and turn our vision inward, with nary another glance outward into the deep vastness that doesn’t even notice we are here.

But we don’t do that. We have, for countless generations, stared into the immensity in an ongoing  (and surprisingly successful) camapign to understand and explain all we can about the Universe. But when everything is so impossibly far away, when the Cosmos is full of so many different and unknown things, how is it that we can know anything?  The answer to that question is that we ask questions.

questionMarkConsider a popular game that most of us have played since we were kids (I have a 9 year old — I get to play this A LOT).  Here is a box (with a question mark on it). You want to figure out what is under this box by asking 20 “Yes-No” questions. Go!

  • Is it alive? No.
  • Is it something made by humans? Yes.
  • Is it small enough to hold in my hand? Yes.
  • Is it edible? No.
  • Does it have batteries? No.

So there we have asked just 5 questions. The answers are nothing more than a simple yes or no. But the tremendous power of asking questions is clear. Despite the vastness of the Cosmos, despite its immense size and the mind-boggling large number of things it contains, you have eliminated almost ALL of it from consideration with only 5 simple questions. You know it is not something huge (galaxy, star, planet, white dwarf, asteroid, comet, …). You know it is not alive, so every organism on Earth — plant, animal, bacteria, fungus, protozoan — is eliminated.  Your attention is now focused on only things that humans make, and only those things that aren’t powered by batteries.

me_ndgt_legoAnd you have 15 questions left! With 20 carefully constructed questions, you will be able to figure out almost anything I wanted to hide under that question mark, with a high degree of success! If we went on and I let you ask the rest of your 15 questions, I am confident you would eventually arrive at the fact that hiding in my question mark box is a little Lego version of me and Neil deGrasse Tyson.

We could have done this with anything in the Cosmos. I could have had anything under that box — an elephant, a quasar, a piece of Pluto, the left foreleg of a carpenter ant, a circle of paper from a hole punch, a cough drop wrapper, an oyster shell, that little plastic do-hickey that holds your gas cap on your car, a Calving & Hobbes sketch, a molybdenum atom, Marie Curie’s lab notebook, a lost pawn from a Sorry game, and so on. ANYTHING!

But you can figure out what it is with only a few questions so reliably we’ve made it into a game children can play and enjoy! It’s usually called “20 questions,” but it also goes by the name science. Except when we play science, we don’t limit ourselves to just 20 questions — we ask as many as we want! You can learn a LOT with carefully constructed questions. And we have learned a lot. We have collected and gathered and recorded our knowledge of the Cosmos so effectively that much of it has passed into the communal memory of our species, integrating itself into the fabric of who and what we are so effectively that we often don’t give it a second thought. We’ve forgotten how hard it was to earn that knowledge, the struggle our forbears went through to wrest some secrets from Nature and then understand what they meant.

A 1/2 globe of the Moon, roughly 5 feet in diameter, made before spacecraft had ever flown to the far side. You can see this in the Rainbow Lobby of the Adler Planetarium in Chicago.

A 1/2 globe of the Moon, roughly 5 feet in diameter, made before spacecraft had ever flown to the far side. You can see this in the Rainbow Lobby of the Adler Planetarium in Chicago.

To understand this, consider the Moon. What do you know about the Moon? It orbits around the Earth. It is spherical, and is illuminated by the Sun. The near side always faces the Earth. It is covered with lowlands (called maria, lunar “seas”), highlands (called terrae, the brighter areas), mountains, craters, and canyons. All of this is common knowledge, which if you didn’t know it you could have found out using the electronic web that girdles our world. I’m pretty sure almost everyone reading this has not been to the Moon. In all the history of our species, only 24 humans have ever crossed the gulf between the Earth and the Moon; only 12 humans have ever walked on the Moon and seen what we know with their own eyes. The pictures of the Moon, taken by the Apollo astronauts and robotic emissaries have virtually erased from our memory what it was like to not know what the Moon was like.

Consider the globe of the Moon shown here. It is about 5 feet in diameter, and lives up to our expectations of a rugged, desolate landscape covered in mountains and craters. How far away from this globe would I have to stand, for it to look roughly the same as the Moon in the sky?  About 140 feet. The full moon in the sky, is about the size of a US dime, held at arm’s length.

When you see the Moon in the sky, it is quite small, roughly the size of a dime held at arm's length. The detail your eye can see is minimal -- mostly just dark and light shading, with no topography! [Image: Shane L. Larson]

When you see the Moon in the sky, it is quite small, roughly the size of a dime held at arm’s length. The detail your eye can see is minimal — mostly just dark and light shading, with no topography! [Image: Shane L. Larson]

When the Moon is that small, you can’t tell it has any topography at all. It is clearly shaded in some irregular pattern (which allows you to make the famous Moon shadows), but there are no craters or mountains to be seen. Go out and look, but don’t look with your brain plugged in to what you know; just look at what you can see. This is how the Moon has always look to the naked eye; it wasn’t until the  application of the telescope to astronomy that we knew anything different.

Galileo's early views of the Moon through his telescope revealed previously unknown topography.

Galileo’s early views of the Moon through his telescope revealed previously unknown topography.

In 1609, Galileo Galilei was the first person to plumb the depths of the sky with a telescope, and what he saw shook the foundations of what we thought we knew about the Cosmos. In 1610, he published one of the seminal works in astronomy: Sidereus Nuncius, “The Starry Messenger,” wherein he described all that he had seen during his first excursions in 1609.  He wrote of the Moon

“... the Moon certainly does not possess a smooth and
polished surface, but one rough and uneven, and, just
like the face of the Earth itself, is everywhere full
of vast protuberances, deep chasms, and sinuosities.”

Two things stand out to me about this passage. The first is how he initially describes the Moon: a smooth and polished surface. This is how people thought of the Moon — it is, in a very real sense, what the Moon looks like, and what you would think if you had never been taught that there were craters and mountains on its surface. The second is when he describes what he saw on the Moon: just like the face of the Earth itself. The telescope allowed us to see that the Moon had features and topography that were at once recognizable and intimately familiar, appearing just like the topography we see here on Earth. In a singular moment of discovery, the telescope deprovincialized our view of the Earth. The Moon is, in a very real sense, the first world other than the Earth that we ever discovered, and this is how it happened.

Galileo's planet sketches, while not showing the detail of his lunar observations, were no less revolutionary.

Galileo’s planet sketches, while not showing the detail of his lunar observations, were no less revolutionary.

There were many other startling revelations Galileo had looking through the telescope. In addition, he was the first person to look at the planets through a telescope. And what he found was that the planets were not stars at all, but also were other worlds. Every planet showed size, and round shape. The planet Saturn had odd protrusions; Galileo wrote “Saturn has ears.”  Turning his telescope to Venus, Galileo found that it went through phases, just like the Moon, a fact that was easily explained by the still new Copernican idea that the Sun was at the center of the solar system.  But Jupiter revealed one of the greatest secrets of all — it held in its grasp its own entourage of moons, that orbited the great world much as our own Moon orbits the Earth. Today, they are known as Io, Europa, Ganymede, and Callisto — the Galilean moons.

When I think about these momentous discoveries, my mind always wanders to the following, often overlooked fact: even though Copernicus’ De revolutionibus orbium coelestium had been published more than 60 years before Galileo’s observations, and placed the Earth in orbit around the Sun, Galileo’s observations were the first to reveal the planets were indeed other worlds. To put an even finer point on it, Galileo’s observations were the first to definitively show that the Earth was a planet, possibly not unlike the other planets that orbit the Sun. Galileo’s telescope allowed us to discover the planet Earth.

Galileo's telescopic observations of the Pleiades revealed stars that could not be seen with the naked eye. There was an unseen -- an unknown -- part of the Cosmos to discover.

Galileo’s observations of the Pleiades revealed stars that could not be seen with the naked eye. There was an unseen — an unknown — part of the Cosmos to discover.

Galileo also peered at stars. He found that when he looked at the Pleiades, the Seven Sisters, the telescope revealed stars that could not be seen with the unaided eye. When he peered at the diaphanous glow of the Milky Way, arching horizon to horizon in the dark skies of 17th Century Italy, he found it was comprised of uncountable numbers of individual stars, so far away and so dim that without the telescope their combined light looked no more than an evanescent fog in the dark.  The scale of the Universe was suddenly much larger. The structure of the Universe was suddenly more complex. Larger and more complex than humans had ever imagined. The revelation of the Cosmos had begun.

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This post is the second in a series of three that capture the discussion in a talk I had the great pleasure of giving for Illinois Humanities as part of their Elective Studies series, a program that seeks to mix artists with people far outside their normal community, to stimulate discussion and new ideas for everyone.  The first post can be found here:  http://wp.me/p19G0g-xB

The idea of describing science in the context of 20 Questions is one I was introduced to at a very young age, by Carl Sagan in “Cosmos: A Personal Voyage” (in Episode 11: Persistence of Memory).

Feeling Small in a Big Cosmos 01: Vastness

by Shane L. Larson

One of the great pleasures in my life is that I am both a professional as well as an amateur astronomer. I spend my days, like many of us do, behind a computer keyboard, staring at a computer screen. I get to think about things that are cool, like black holes and the death spiral of binary stars, and whether or not we can hear the faint whispers of gravity washing over us from some unimaginably distant cosmic shore.

There is nothing quite like standing out in the dark and seeing the Cosmos with your own eyes. [Grand Tetons, by Royce Bair; http://NightScapePhotos.com/ ]

There is nothing quite like standing out in the dark and seeing the Cosmos with your own eyes. [Grand Tetons, by Royce Bair; http://NightScapePhotos.com/ ]

But when I go home, I like to spend long hours of the night out under the stars, in deep personal communion with the Cosmos. Stand out in your backyard, or in a dark mountain meadow, and look up. The sky is deep and vast, studded by thousands of stars, tantalizing bright and inviting, but inexorably far away. If you’re lucky, you can see the Milky Way striking up from the horizon, soaring overhead into the velvet darkness, holding the sky up over your head. I find it deeply comforting to lose myself in that view, to let the Cosmos envelop me in its embrace; some part of me knows “this is home.”

The deep connectedness we often feel with the Cosmos is tempered by another realization: that we are small in the face of the vastness of the Universe. It is an ephemeral and unsettling feeling that is hard to explain and vocalize, but in the opening scene of Cosmos, Carl Sagan captured it perfectly, writing

“The size and age of the Cosmos are beyond ordinary human
understanding. Lost somewhere between immensity and eternity
is our tiny, planetary home, the Earth.”

Beyond ordinary human understanding. We can quantify the scale and age and makeup of the Cosmos, but most of the numbers we are forced to use are big — crazy big! Well outside the boundaries of our everyday experience. Numbers so far outside our everyday experience that to simply state them is almost meaningless, because when we hear them said aloud, our brains fail to process what we are really saying (or hearing). Saying and hearing the numbers fails to adequately capture what we instinctively know, but can lyrically convey one person to another with words that are poetic, but somehow deeply meaningful: somewhere between immensity and eternity.

Our understanding of the vastness of the Cosmos starts not by looking outward, but rather by looking inward. This photograph is one of the most iconic images of the Space Age, known as “The Blue Marble.” There have been many versions, updated every few years as new and better images become available. It looks, for all the world like a child’s blue, glass marble.

The Blue Marble, 2012. [NASA]

The Blue Marble, 2012. [NASA]

There are very few people who, when presented with this photograph, don’t recognize it as the Earth. But here is something to consider: to actually see the entire Earth at once, as it is presented in this picture, you have to be tens of thousands of kilometers away. In all the history of our species, there have only ever been 24 people who have seen the world this way: the Apollo astronauts who made the voyage to the Moon and back.  The rest of us have only become familiar with this image of our small, fragile world though their words, their memories, their pictures. Since that time, now approaching 50 years in the past, the picture has been updated and refined, not by human eyes, but through the lenses and electronics of robotic emissaries, cast out into the night to make voyages that we humans seldom seriously pursue.

The most common and fastest modes of transportation most of us will ever encounter.

The most common and fastest modes of transportation most of us will ever encounter.

This small, blue world is the starting point for all our voyages into the Cosmos, whether they be on ships adapted to the abyss of space, or on wings of thought, unfettered by physical separations in time and space. One way to think about the size of the Cosmos is to imagine making a voyage of exploration. In the stack of notebooks on my desk is one non-descript composition notebook marked “Destinations.” It contains within its leaves lists and notes of destinations on Earth that, given time and freedom, I would love to visit. Kind of my own personal Atlas Obscura.  Many of those destinations can be reached using an automobile, the transport du juor for most of the modern world. Most of us have been in an automobile, and have traveled regularly at a speed of say 100 kilometers per hour (about 60 miles per hour).  By contrast, many of the other destinations can only be reached using the air travel network that girdles our world, travelling by jet aircraft at about 900 kilometers per hour (about 550 miles per hour). Few of us have had the opportunity to travel faster, in a military jet or by rocket.

British astronomer, Fred Hoyle, once remarked “Space isn’t remote at all. It’s only an hour’s drive away… if your car could go straight upwards!” He’s right — the boundary of the Earth’s life-sustaining atmosphere is not that far over our heads. If our cars could drive straight up, we would be off on an epic, Cosmic road trip unlike any other before. Let’s consider a few interesting mileposts, and what their entries might look like in my Destinations notebook. My roadtrip car of choice: a Yugo.

voyages01

Consider Earth orbit — the first stop on the way to anywhere beyond the Earth. For your spaceworthy Yugo, the journey up will be a few hours, and only 23 minutes at the speeds of a passenger jet. By contrast, it took the space shuttle just under 10 minutes to reach orbit.

The Moon was 4 days away if you travelled on Apollo; to drive your car would take 5.4 months of non-stop driving, and just over 17 days by jet. Here, we begin to get the inkling of why exploring the Cosmos is hard — at the speeds of everyday life, even the closest destinations are far away.

Spacecraft take 6-12 months to reach Mars by rocket. Driving in your car would take more than 106 years — longer than a human lifetime. If you left for Mars in a jet when you entered first grade, you’d make it just in time to have your high school graduation on the Red Planet.

voyages02

Pluto has long been the outermost sentinel of the small neighborhood we call home. The New Horizons spacecraft has taken 9 years to fly there, and as of the time of this writing is less than 2 weeks away from its flyby encounter. If the ancient Egyptians had left for Pluto in a spacefaring Yugo, they still would not have arrived— the voyage by car takes almost 7000 years to complete; the voyage by jet takes 740 years.

Beyond the boundaries of the solar system, voyages by ordinary means can be computed, but they become utterly meaningless in terms of timescales. The center of the Milky Way is 26,000 lightyears away, which would take 31 billion years to reach at the speeds of a passenger jet — more than twice the age of the known Universe. The Andromeda Galaxy, the nearest spiral galaxy to the Milky Way, is 2.5 million lightyears away, but it would take us 3 trillion years to reach via jet.

We can compute these times, we can say these words, but our eyes glaze over and we let the words for the immensity of the Cosmos slip by us with little regard for what they really mean. The size of the Cosmos is beyond ordinary human understanding.

The Hubble Space Telescope. Arguably the most capable and successful telescope ever built by humankind.

The Hubble Space Telescope. Arguably the most capable and successful telescope ever built by humankind.

Despite the fact that we can’t voyage across the Cosmos, we can look. The most capable and accomplished telescope the human race has ever built is the Hubble Space Telescope. In its 25 year lifetime, it has seen farther than any telescope before, with tens of thousands of scientific papers published using its data. One of the most remarkable tasks we set before it was the creation of “deep fields.”

Consider the evening skies, shortly after 9pm in January. The constellation of Orion, the Hunter, lies just to the east of the meridian (an imaginary line, running from due north to due south in the sky). Striking out from his western knee is the long and sinuous constellation of Eridanus, the Great Sky River, that winds and wends its way around a non-descript constellation known as Fornax, the Furnace.

Location of the Hubble Extreme Deep Field, between Eridanus and Fornax.

Location of the Hubble Extreme Deep Field, between Eridanus and Fornax.

Between Fornax and one of the bends of Eridanus there is a small, blank patch of sky. Like many patches of the sky, there is nothing there visible to the naked eye. Even far from the city lights, if you stare into the void there, you will see little. To make a Deep Field, we take Hubble, the most storied telescope in history, and stare at one empty spot in the sky. For many days on end. In the case of this lonely spot on the banks of Eridanus, Hubble stared for 23 days.  The result is one of the most startling and revelatory pictures taken in human history.  It is called the Hubble Extreme Deep Field (XDF; NASA page here).

The Hubble Extreme Deep Field (XDF).

The Hubble Extreme Deep Field (XDF).

As you can see, the blank patch of sky is not so blank after all. Every fleck of light, every smear of something in this picture is a distant galaxy, a remote shoal of stars and planets and gas and dust, and just maybe, other intelligent beings staring up at the sky.  All told, in this single image, there are about 5500 individual galaxies. The faintest are 10 billion times too faint to be seen with the naked eye; it took Hubble, the most powerful telescope we’ve ever built, 23 days to see them.

And what have we learned from this picture of the Cosmos? All told, there may be as many as 500 billion galaxies in the entire known Universe. We know that the Universe is 13.7 billion years old, but the oldest galaxies we’ve seen formed soon after the birth of the Cosmos, about 13.2 billion years ago. Big numbers, huge numbers. Numbers beyond ordinary human understanding.

The Cosmos is ginormous (that’s a technical term). It is easy to be overwhelmed when faced with the enormity of it all. But you should also take heart. One of the most remarkable things about the Cosmos, one of the most remarkable things about our species, is that we can figure it out. Despite the size and vastness we have managed to see and understand remarkable and astonishing things about our home, and are capable of pondering the implications of our existence in the Universe. Next time, we’ll explore some of those discoveries and ponderings.

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This post is the first in a series of three that capture the discussion in a talk I had the great pleasure of giving for Illinois Humanities as part of their Elective Studies series, a program that seeks to mix artists with people far outside their normal community, to stimulate discussion and new ideas for everyone.

Part 1: Vastness (5 July 2015)

Part 2: Discovery (11 July 2015)

Part 3: Proverbs (20 July 2015)

Illinois Humanities taped this talk and you can watch it online;  many thanks to David Thomas for doing the videography!

Gravity 13: Frontiers

by Shane L. Larson

I grew up in the American West, where our self-identity is inexorably melded with a deep romanticism about the Frontier. My family homesteaded near Briggsdale, Colorado, where the Rocky Mountains fade into the vast expanse of the Great Plains of North America. You can still visit the old homestead site, now on my family’s cattle ranch, and see the foundations that were laid down many generations ago. I can remember crawling around those walls as a young boy, trying to imagine the world in those long forgotten days — before cars, before computers, before rockets.  What did my old-time relatives think about every morning when they got up? What did they work on every day? Did they stare at the sky, virtually identical to the sky I stare at (except theirs was probably much darker), and wonder?

At the ruins of my family's homestead, near Briggsdale, CO. Left to right: my maternal grandfather, my younger brother, my mother, my youngest brother, me, and my dad.

At the ruins of my family’s homestead (circa 1982), near Briggsdale, CO. Left to right: my maternal grandfather, my younger brother, my mother, my youngest brother, me, and my dad.

We portray the Frontier as a place full of adventure, discovery, and possibly undying fame. But Frontiers are in a very real sense the boundary between all the hard-won knowledge of our species, and our ignorance. This is perhaps captured no where better to my mind than in Ortelius’ 1570 Map of the World, known as Theatrum Orbis Terrarum.

Ortelius' 1570 Map of the World, known as Theatrum Orbis Terrarum.

Ortelius’ 1570 Map of the World, known as Theatrum Orbis Terrarum.

Made in the last decades before the dawn of the Age of Enlightenment, the map was one of the first to capture the knowledge of the true sphericity of the World. The map very clearly shows the boundaries of what was known and what was unknown in western culture. Consider: in 1520, Magellan’s expedition passed through what today is known as the Straights of Magellan, between the mainland of South America and the Isla Grande of Tierra del Feugo. Ortelius’ map captures that Frontier of exploration explicitly, showing the path around South America, but also showing complete ignorance of the nature of the Isla Grande. In fact, the entire lower part of this map shows the boundary of European knowledge of this part of the world, showing the farthest south on the globe any explorer had ever been. Even closer to Europe there are boundaries between knowing and ignorance that can be seen as well: the northern fringe of the Scandinavian Peninsula is clearly not correct, nor is the shape of the Caspian Sea.

Frontiers define those regions where explorers for the first time are discovering and documenting what has only been suspected or imagined before. Frontiers are more often than not enabled by technology. In Magellan’s day, access to the Frontier was enabled by great sailing vessels. When my family homesteaded in the plains of Colorado, their journey had been enabled by Conestoga wagons. Technology is almost always helping us push the boundaries of the Frontier outward. It is as true today as it was in the past.

Technology enables discovery. In the Age of Exploration, the development of great sailing vessels allowed European explorers to cross the oceans of the world.

Technology enables discovery. In the Age of Exploration, the development of great sailing vessels allowed European explorers to cross the oceans of the world.

Today, there are frontiers in science, both in terms of our knowledge, as well as in terms of what our technology is capable of. On both fronts, gravity is at the frontier. In the 100 years since the birth of general relativity, our understanding of the Cosmos has grown dramatically, and at each step, gravity has played a role. Einstein showed us how gravity can explain Mercury’s lagging orbit, and suggested it could bend the trajectory of light and change its color — effects that had never been measured. Since then, the frontiers have expanded well beyond those initial speculations. Modern cosmology was born less than 15 years after Einstein’s initial presentation of general relativity, and even today challenges our understanding of the Cosmos. We have explored the gravitational collapse and death of stars, and discovered the skeletons that survive the throes of death. Closer to home, we have harnessed gravity to allow us to navigate and map the world to exquisite precision. Our satellites have measured the gentle warp of the Earth’s gravity to map out the world in ways Ortelius never imagined.

For the past 100 years, gravity has been a major player at the frontiers of physics and astronomy. (L Top) Our understanding of the expansion of the Universe derives from general relativity. (L Bottom) The gravity of the Earth tells a tale of the movement of water and changing climate of our planet. (R Top) The evolution of stars, and their ultimate death, are consequences of gravity. (R Bottom) High energy astrophysical phenomena like black holes are staples of astronomy knowledge today.

For the past 100 years, gravity has been a major player at the frontiers of physics and astronomy. (L Top) Our understanding of the expansion of the Universe derives from general relativity. (L Bottom) The gravity of the Earth tells a tale of the movement of water and changing climate of our planet. (R Top) The evolution of stars, and their ultimate death, are consequences of gravity. (R Bottom) High energy astrophysical phenomena like black holes are staples of astronomy knowledge today.

Despite all these discoveries, there is still much to learn. Gravity is right on the boundary between our most exquisite triumphs and the precipice of our deep ignorance about the Cosmos. Science is about looking over that precipice and wondering what is at the bottom; we know there are still great mysteries Nature is hiding behind the facade that we call “gravity.” We have come a long way from the frontier Einstein imagined. What are the frontiers of gravity today?

Consider the interiors of black holes. A black hole has gravity so strong, not even light can escape. It’s boundary, the event horizon, forever hides the inside from the external Universe.  If you could somehow peer past the event horizon, deep down inside you would find a point of infinite density and infinitely strong gravity called the singularity.

The structure of a black hole is relatively simple to sketch out: the "surface" is the Event Horizon, and shrouded beneath it is the singularity.

The structure of a black hole is relatively simple to sketch out: the “surface” is the Event Horizon, and shrouded beneath it is the singularity.

Perhaps the greatest enigma, the greatest failing of general relativity, is the existence of the singularity. From a classic perspective, gravity is a purely attractive force that can grow without bound when matter is compressed into a small enough space. The limitless growth in its strength means if you squeeze hard enough, it can grow so large than no other known force can oppose it. When nothing can oppose it, everything collapses in a dramatic collapse not unlike the collapse at the end of a star’s lifeBut nothing can stop the collapse, and mathematically, everything falls into an infinitely small, infinitely dense point that we call “the singularity.”

Singularities — “infinities” — are perfectly fine in mathematics. They are less desirable in physics. There is a strong, prevailing belief that in the physical world, nothing can be “infinite.” Objects and phenomena can be ridiculously large or ridiculously small when compared to the scale of human experience, but never infinite.

focus-italy_singularity-outtake1The prevailing belief is that the singularity is an indicator that general relativity is a classical theory — it is good for large scale descriptions of the world, not for the microscopic landscape of the Cosmos. For that, we will need a new idea, an extension of general relativity into the quantum regime — “quantum gravity.” Where does the realm of quantum gravity become relevant? At distances separated by the Planck length (10^-35 meters = 0.000 000 000 000 000 000 000 000 000 000 000 01 meters).

What is quantum gravity? Fundamentally it is expected to be a theory that describes the nature of space and time itself at the Planck scale; many believe that using quantum gravity to describe the interior of a black hole will obviate the need for a singularity, but no one really knows how that will happen because we don’t have any working models that make predictions testable with observations. But there are many, many seductive and enticing ideas that are waiting for us to attain a state of understanding sophisticated enough to put them to the test.

Fritz Zwicky

Fritz Zwicky

There are also challenges for gravity on scales that are enormously large, spanning the size of the Cosmos. Some of these challenges are recent, some have been known for the better part of a century, but they are all unresolved. Part of the story begins in the 1930s with astronomer Fritz Zwicky.  In 1933 he was observing the Coma Cluster of galaxies, a group of about 1000 galaxies whose center lies 320 million lightyears away, in the direction of the constellation Coma Berenices. This was less than 10 years after the discovery that galaxies were in fact like the Milky Way, but enormously far away. Astronomers were still trying to learn all they could about galaxies, and studying their behaviour.

The Coma Cluster contains about 1000 galaxies (the yellow objects in this image), and is 320 million lightyears away.

The Coma Cluster contains about 1000 galaxies (the yellow objects in this image), and is 320 million lightyears away.

Zwicky made a very reasonable assumption: the light of the galaxy is made by all the stars in a galaxy, and since most of the mass is contained in stars measuring the light is a way to get a handle on how much a galaxy masses. If you could measure the mass of all the galaxies, then you can use gravitational theory to explain their motions. But when Zwicky measured the motion of the galaxies, he found they were moving faster than expected — given the speeds they were moving, the cluster should have flown apart long ago. The only explanation is there was missing matter he could not see — more matter would simultaneously make the galaxies move faster, but also provide enough gravitational attraction to hold the cluster together.

Vera Rubin

Vera Rubin

By the 1960s, the missing matter problem had yet to be resolved. Astronomer Vera Rubin was studying the rotation of individual galaxies. Stars orbiting the center of a galaxy obey Kepler’s Laws of Orbital Motion, just like planets orbiting the Sun. Kepler’s laws say that the farther you are from the center of gravity, the slower your orbital speed should be. What Rubin found was that the outer reaches of galaxies did not slow in their rotation; in fact they rotated just as fast as stars that were closer to the center. This is known as the “galaxy rotation problem” and the plot of the rotation speed versus distance from the center of the galaxy is described as a “flat rotation curve.” Just as was the case with the Coma Cluster, the galaxy should have flown apart. The only explanation is that there is unseen mass — more matter would simultaneously make the stars move faster, but provide enough gravitational attraction to hold the galaxy together.

The "galaxy rotation problem" is that the speed a galaxy rotates with is NOT what we would expect. We expect it to rotate slowly near the edges, but observations show galaxies rotate too fast near the edges.

The “galaxy rotation problem” is that the speed a galaxy rotates with is NOT what we would expect. We expect it to rotate slowly near the edges, but observations show galaxies rotate too fast near the edges.

Rubin began her investigation with the Andromeda Galaxy, but in surveys of many more galaxies found that it was always true — all galaxies appear to have enormous amounts of unseen matter. Today, we call this dark matter.

This has enormous implications for cosmology. If the Universe is expanding, then the rate it expands, and the ultimate fate as a consequence of expansion, depends on the amount of matter in the Universe. This begs some important questions, like “is there enough matter to slow the expansion?” and “is there enough matter to cause the expansion to reverse?” Gravitational physicists classify the possible futures of the Universe in three ways:

  • OPEN: There is not enough matter to slow the expansion of the Universe down at all; it expands forever.
  • FLAT: There is just enough matter in the Universe that the expansion is slowing, but it will never halt, instead coasting forever.
  • CLOSED: There is enough matter to eventually stop the expansion, and cause the Universe to recollapse in a backward version of the Big Bang that is often called the Big Crunch.
One way astronomers measure the expansion scenario of the Universe is looking at the spots on the Cosmic Microwave Background. The direction light travels to us from opposite sides of the spot depends on the expansion geometry of the Universe. (L) In a Closed Universe, the light is bent to make the spots appear larger. (C) In a flat Universe, the spots are seen at their true size. (R) In an open Universe, the spots appear smaller.

One way astronomers measure the expansion scenario of the Universe is looking at the spots on the Cosmic Microwave Background. The direction light travels to us from opposite sides of the spot depends on the expansion geometry of the Universe. (L) In a Closed Universe, the light is bent to make the spots appear larger. (C) In a flat Universe, the spots are seen at their true size. (R) In an open Universe, the spots appear smaller.

Each of these scenarios has particular signatures in observational data, and astronomers have found strong evidence that the Universe is indeed in the FLAT mode. That being the case, this has spawned a multi-decade quest to make a census of all the stuff in the Cosmos and characterize not only its gravitational influence, but also figure out what it all is!

We are aware of dark matter because of its gravitational influence on the rest of the Cosmos, but we have no idea what it is. And there is a LOT of it. Current estimates suggest that the Cosmos is 27% composed of this dark matter. The stuff you and I and planets and stars are made of — atoms — only make up about 5% of the total amount of stuff in the Universe.

So what is the other 68% of the Universe? Astronomers were perplexed by this for a long time, and began to doubt that the Cosmos was put together the way we thought it was. Maybe the Cosmos wasn’t FLAT but was instead OPEN and our observations were wrong in some way.

Supernovae, for a time, shine very brightly compared to other stars in the parent galaxy.

Supernovae, for a time, shine very brightly compared to other stars in the parent galaxy.

But in the late 1990s, there was a breakthrough. Mulitple teams of astronomers were using supernovae to measure the size and expansion of the Universe. Certain supernovae (Type Ia supernovae) are standard candles — they all explode with the same brightness. This means that the brightness of the supernova gives you a way to measure distance — the dimmer the supernova, the farther away it is. But cosmology gives us another way to measure distance, using Hubble’s law — redshift is also a measure of distance. The larger redshift an object has, the farther away it is.

But in 1998, the Supernova Cosmology Project and the High-Z Supernova Search Team discovered that these two methods of measuring the distance to supernovae did not agree — distant supernovae were dimmer than expected given the redshift distance. How can that be? The only explanation seems to be that the expansion of the Universe is accelerating.  An unknown something is accelerating the expansion of the Universe, ever so slightly, on the largest scales. Today, we call that something dark energy. Dark energy, whatever it is, makes up the remaining 68% of the expected stuff in the Universe.

A simple demonstration of the energy content of the Cosmos. Atoms are colored; all the unknown things (dark matter and dark energy) are black.

A simple demonstration of the energy content of the Cosmos. Atoms are colored; all the unknown things (dark matter and dark energy) are black.

At long last, astronomers and physicists have discovered all the stuff we expected to find in the Universe. But we still don’t know what it is. We call this stuff “dark matter” and “dark energy”, but we don’t know anything about their behaviour and properties beyond their gravitational influence. Maybe they are some new, exotic bit of particle physics we have never seen before. Maybe they are some new, exotic behaviour of gravity on large scales. Or maybe they are something completely new, completely unexpected, and completely unexplained. Whatever they are, dark matter and dark energy are clearly at the frontiers of our understanding of gravity and cosmology. The future lies on the other side.

What these discoveries will mean and how they will change the course of human history is not for us to know, just as it was not for Einstein to know how general relativity would change the world. Those are questions for our posterity, our future children, who will have moved on from the simple mysteries that confound us today, and will be challenging their own new frontiers.

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

This post concludes this long series for the GR Centennial. Thanks to everyone who read, commented, and supported this effort! We will certainly talk about gravity again at this blog… :-) This post was completed while in residence at the Aspen Center for Physics.

Gravity 12: Listening for the Whispers of Gravity

by Shane L. Larson

The Cosmos is alive with energetic happenings.  Planets barrel along their orbits, unstoppable by anything short of a collision with another planet.  There is a cluster orbiting the black hole at the center of the Milky Way, with stars being flung and slingshot around their orbits like they were nothing more than ping-pong balls. Massive stars, in a last desperate gasp for attention, explode and spew their guts all around the galaxy, leaving a dark, compact skeleton behind. Billions of light years away, the shredded remains of galaxies slowly coalesce to make a larger elliptical galaxy and their central black holes dance together in a deadly inspiral, spewing jets of energetic material outward to mark their titanic struggle.

Gravitational waves are created by the dynamic motion of mass, a common occurrence in the Cosmos: supermassive black holes mergering or eating stars, stars exploding, and compact interacting binaries are all likely sources.

Gravitational waves are created by the dynamic motion of mass, a common occurrence in the Cosmos: supermassive black holes mergering or eating stars, stars exploding, and compact interacting binaries are all likely sources.

All of these examples have one thing in common: huge masses moving in dynamic ways.  The changing gravitational structure in these systems will manifest itself as gravitational waves propagating across the Cosmos, whispering ripples in the structure of space and time.  Encoded in those waves, if we could detect them, is a previously unheard story for the reading.

The “sticky bead experiment,” worked out at the 1957 Chapel Hill conference, taught us the effect of gravitational waves on the world: they change the distance between points in spacetime. Once we knew what physical effect to look for, physicists began to ask “how do we detect it?”  It was straight-forward to compute the size of the distance change caused by gravitational waves, and it was tiny. But seemingly impossible measurements have never stopped physicists and astronomers from trying to imagine clever and imaginative ways to probe Nature’s secrets.

One of the first people to seriously consider how to measure the extremely tiny stretching effect of gravitational waves was Joseph Weber at the University of Maryland. After the Chapel Hill conference he began to think seriously about the problem of gravitational wave detection, and settled on a clever and imaginative idea: if gravitational waves change the distance between any two points in spacetime, it should stretch a physical object as they pass through it. Once the wave goes by, the inter-atomic forces that hold the object together take over, and try to snap it back into its original shape. This kind of snapback motion would set up acoustic waves — sound waves — in the object. If you could detect those tiny, faint sound waves, it would be an indicator of the passage of a gravitational wave.  Weber fashioned such an experiment from a 0.61 meter diameter, 1.5 meter long cylinder of aluminum that massed 1.5 tons. Such a device is now called a Weber Bar.

(L) Joe Weber instrumenting his bar detector with sensors in the 1960's. (R) You can visit the bar, live and in person, at the LIGO-Hanford Observatory.

(L) Joe Weber instrumenting his bar detector with sensors in the 1960’s. (R) You can visit the bar, live and in person, at the LIGO-Hanford Observatory.

There are, of course, many influences and physical effects that can set off acoustic vibrations in a large aluminum bar. Random acoustic vibrations could be mistaken for a gravitational wave, or more likely, hide the putative effect of a passing gravitational wave. Random signals like this are called noise; filtering noise is one of the foremost problems in any experiment. The solution to this difficulty is to have more than one bar; you set them up and wait to see if both bars ring off at the same time. Since noise is random, it is unlikely to influence both bars identically at the same time, so a common signal is most likely a gravitational wave. Weber’s detection program grew to include a second bar at Argonne National Laboratory that operated in coincidence with the bar he had built in Maryland.

By the late 1960’s, Weber’s analysis of his bar data convinced him he was seeing coincident events, which he dutifully reported to the scientific community.  The ensuing debate has been roundly documented (e.g. in Harry Collin’s book “Gravity’s Shadow”), but that tale is not germane to our discussion here. The important point is this: the scientific community suddenly became cognizant of the idea that gravitational waves could be detected through clever, high precision experiments, and Joe Weber set us on that path.

(Top L) The EXPLORER bar at CERN; (Top R) the AURIGA bar in Italy; (Lower L) The NAUTILUS bar in Italy; (Lower R) The new MiniGRAIL detector at Leiden.

(Top L) The EXPLORER bar at CERN; (Top R) the AURIGA bar in Italy; (Lower L) The NAUTILUS bar in Italy; (Lower R) The new MiniGRAIL detector at Leiden.

In the years following the construction of the Maryland experiment, many other Weber bars were built around the world. These included ALLEGRO at Louisiana State University; EXPLORER at CERN; NAUTILUS in Frascati, Italy; AURIGA at the INFN in Legnaro, Italy; and Niobe in Perth, Australia.  While most of the classic bars have gone offline, new efforts in bar detection technology have turned to spherical detectors, of which MiniGRAIL at Leiden University is the archetype. But still, no gravitational wave signal has been confirmed by any bar.

Given the steadfast absence of confirmed signals in our detectors, why are physicists so confident in the existence of gravitational waves? The answer lies in traditional, telescopic observations of the Cosmos.

The Hulse-Taylor pulsar is located just off the wing of Aquila.

The Hulse-Taylor pulsar is located just off the wing of Aquila.

In 1974, radio astronomers Joseph Taylor and Russel Hulse were observing on the 305 meter diameter Arecibo Radio Telescope in Puerto Rico. They were looking for new pulsars, and discovered one in the constellation of Aquila. Pulsing every 59 milliseconds, the pulsar rotates at a staggering 17 times per second. After studying it for some time, Hulse and Taylor noticed that the pulses varied regularly every 7.75 hours. The explanation? The pulsar was orbiting another neutron star (that was not pulsing)!  Masquerading under the scientific name PSR B1913+16, this remarkable system is more readily known by its common name: the Hulse-Taylor binary pulsar, or usually “THE Binary Pulsar.” We can track the arrival time of the pulses from the pulsar in the system, and precisely determine the size and shape of the orbit over time. After 40 years of observations, it is clear that the orbit of the binary pulsar is shrinking, by an amount of roughly 3.5 meters per year. This is exactly the amount of orbital decay astronomers expect to see if gravitational waves were carrying energy away from the system, sucking the energy out of the orbit. If all goes according to Nature’s plan, the orbit will decay to the point of collision in 300 million years (mark your calendars!).

The system has a neutron star that orbits with a pulsar -- the pulsar is a neutron star that sweeps a strong radio beam toward the Earth as it rotates. As they orbit, they emit gravitational waves, causing the orbit to shrink.

The system has a neutron star that orbits with a pulsar — the pulsar is a neutron star that sweeps a strong radio beam toward the Earth as it rotates. As they orbit, they emit gravitational waves, causing the orbit to shrink.

We now know of many systems like the Hulse-Taylor binary pulsar, giving astronomers confidence that gravitational waves do, without question, exist. So why haven’t we seen them?  The problem with Weber bars is they are “narrow band” — they are most sensitive to gravitational waves that are close matches to the sound waves that are made in the bar (a condition physicists call “resonant” — the gravitational waves closely match the shape and vibration time of the sound waves, so they reinforce each other). Since it is  unlikely a gravitational wave source will exactly match your bar’s vibration frequency, and because many phenomena generate gravitational waves at all kinds of different frequencies, an ideal detector should be “broad band” — sensitive to a wide range of gravitational waves. One solution is to build a laser interferometer.

Michelson (T) and Morley (B) built one of the first interferometers to make precision measurements.

Michelson (T) and Morley (B) built one of the first interferometers to make precision measurements.

Interferometers have a storied history with relativity and astronomy. The earliest scientific interferometers were made in the 1880’s by Albert A. Michelson, and used by Michelson and his collaborator Edward Morley to examine the propagation of light. The results of their experiments demonstrated to the scientific community that light was not propagated by a “luminiferous aether,” and was in fact able to propagate in pure vacuum. Their conclusions also support the founding postulates of special relativity, namely that all observers measure the speed of light in vacuum to be a constant, irrespective of their state of motion.

In the decades that followed, interferometry became a recognized technique for making precise measurements that could not be obtained in any other way. By the time the first results from Weber bars were being reported, people were thinking about other ways to make precision distance measurements, and laser interferometry was a prime candidate technology. The first laser interferometer designed for gravitational wave detection was a table-top experiment built in 1971 at Hughes Aircraft by Robert Forward, who was a student of Weber’s.

(L) Bob Forward's first gravitational wave interferometer at Hughes Aircraft. (R) Rai Weiss' initial sketch of the components and operation of a laser interferometer like LIGO.

(L) Bob Forward’s first gravitational wave interferometer at Hughes Aircraft. (R) Rai Weiss’ initial sketch of the components and operation of a laser interferometer like LIGO.

A year later, Rai Weiss at MIT published a report outlining in great detail the basic considerations for building what would evolve into modern day gravitational wave interferometers. Those initial musings came to fruition in the 1990s, when kilometer scale interferometers began to be constructed around the world with one intention: to observe the Cosmos in gravitational waves.

In the United States, there are two observatories that are called LIGO: one is in Hanford, Washington and the other is in Livingston, Louisiana. In Europe, a 600 meter interferometer called GEO-600 was built outside Hannover, Germany, and a 3 kilometer interferometer called VIRGO was built outside of Pisa, Italy. The Japanese built a 300 meter prototype in Tokyo called TAMA, but have now embarked on a much more ambitious instrument built underground in the Kamioka Observatory called KAGRA. These instruments are enormous endeavours, on the scale of large particle accelerators in terms of their physical size and in terms of the number of people required to bring the project to fruition. All of them can be seen from space (just fire up Google Earth or Google Maps: LIGO-Hanford from space, LIGO-Livingston from space, VIRGO from space, and GEO-600 from space).

(Top L) LIGO-Hanford; (Top R) LIGO-Livingston; (Lower L) GEO-600; (Lower-R) VIRGO.

(Top L) LIGO-Hanford; (Top R) LIGO-Livingston; (Lower L) GEO-600; (Lower-R) VIRGO.

For the first time, these observatories will show us a view of the Cosmos seen not with light, but with the whisper of gravity. The bread-and-butter source, the thing we expect to detect most often, are the merger of two neutron stars. Viewed from the right seats, such collisions generate tremendous explosions known as gamma ray bursts, but we only see a small fraction of the gamma ray bursts in the Universe because they aren’t all pointing toward us. LIGO and its fellow observatories will have no such difficulties — gravitational waves are emitted in every direction from these cataclysmic mergers.

What will we learn from these events? We hope to learn what the skeletons of exploded stars are like — what is their size and what are they made of? What is the matter at their cores like, and what do they become when they merge? Every detected neutron star merger is a clue in the story of stellar lives, which of course, is part of our story too, because we are all of us descended from the exploded ashes of ancient stars.

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove -- it has seen almost 30 million nights like this one, but very little has changed. The constellations change over thousands of years, but the sky is still full of stars, and the Milky Way still arches over the sky, giving the impression that the Universe is unchanging. [Image: Shane L. Larson]

Where do all these stellar skeletons come from? It’s a curious thing, looking out at the sky. The thing we see the most of are stars, and over the course of a human life, they change little if at all. Night after night, the stars wheel overhead, distant points of light that no human has ever visited, and no human is likely to visit in my and your lifetimes. But over the last few centuries, through a careful application of technology smothered under an insatiable desire to know, we have figured out their story. Like shrewd protégés of Jane Marple, we have pieced together many parts of the the puzzle to discover how stars are born, how they live, and ultimately how they die. Gravitational wave astronomy investigates these final end-states of stellar life. But when we see the stars, we are seeing the snapshot of the stars alive today — where are all the stars that have gone before?

They litter the galaxy — the Milky Way is a vast graveyard of stellar remnants, the burned out stellar husks of those stars that came before. Since only the largest stars produce neutron stars and black holes, and most stars are lighter-weight, like the Sun, astronomers think most of that stellar graveyard is full of white dwarf stars — tens of millions of them.

LIGO can’t see white dwarf stars because they are too big — they never shrink to small enough orbits to make gravitational waves that LIGO can detect. If we want to study this part of the stellar life story, we have to build something new.

lisa_astriumIn the next decade, NASA and ESA hope to fly laser interferometers in space. The LISA gravitational wave observatory will consist of three free flying spacecraft 5 million kilometers apart, using lasers to measure the distance between the three spacecraft. The first step toward flying LISA is a mission called LISA-Pathfinder that will launch in October 2015.

LISA will listen in on the gentle gravitational whispers of tens of millions of white dwarf stars — so many whispers that the galaxy will actually sound like racous party. Like any rowdy party, there will be loud contributors that can always be heard above the noise, perhaps as many as 20,000 that shout out above the cacophony.  These systems are called “ultra-compact binaries”, and orbit each other on orbits so small they would fit between the Earth and the Moon. We think of LISA’s view of the Cosmos as being complementary to LIGO’s — with observations from both observatories, we will be able to construct our first complete picture of the “decomposition phase” of stellar evolution.

But perhaps the most interesting thing LISA will detect are the supermassive black holes at the centers of galaxies. Some of the most fantastic pictures we have taken of the Cosmos show galaxies in collision. Occurring over billions of years, the graceful and delicate spirals are shredded, giving birth to a new, transformed galaxy. How often does this happen? Do all galaxies experience this at some point in their lives, or is it rare? How does it change the kinds of galaxies we see? Does it change the shapes of galaxies irrevocably, or do they return to their whirling spirals of arms?  And perhaps most interesting, what happens to the black holes that once lurked in their cores?

Examples of colliding galaxies. (T) NGC 4676 [the Mice], and (B) NGC 6621

Examples of colliding galaxies. (T) NGC 4676 [the Mice], and (B) NGC 6621

If astronomers are correct, those black holes will sink to the core of the new galaxy that forms, and eventually merge together. When they do, they will emit a wailing burst of gravitational waves that will be visible to LISA all the way to the edge of the Observable Universe. Encoded in that cry will be the birth announcement of a new, bigger black hole, as well as the threads of the story that led to its birth — where they were born, when they were born, and what the Cosmos was like at that time.

These stories and more are contained in the faint whispers of gravity that even now are washing across the shores of Earth. As you are reading this, astronomers and physicists are tuning up our technology to listen closely to those faint messages, and when we finally hear them, they will transform the way we think about the Cosmos.

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

Gravity 11: Ripples in Spacetime

by Shane L. Larson

We have travelled far in our journey to explore gravity, far from home and into the deep reaches of the Cosmos. But all that we know, all that we have learned, has been discovered from our home here, on the shores of the Cosmic Ocean. Today, let us return home.  In the words of the space poet Rhysling,

We pray for one last landing
On the globe that gave us birth
Let us rest our eyes on the fleecy skies
And the cool, green hills of Earth.

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Imagine yourself in a soft green meadow, far from the hub-bub of everyday life. What do you hear? What do you see? The gentle rustle of the trees, and the whisper of the long grass. The tall flowers of spring rocking gently back and forth, and the dark shadows of a bird of prey soaring effortlessly against the blue sky. All these sights and sounds are the signature of something unseen — the atmosphere of the Earth, the blanket of air that protects us and supports all the life around us.

How do we know the air is there? We can’t see it. All of these observations, infer the existence of the air by recognizing its influence on other things. If we want to measure the air directly, to detect it, then we need to construct controlled experiments where we understand the physical effect of the air and how it interacts with the experiment we design to elucidate its presence. Consider a simple experiment you can do right at home.

An experiment to convince yourself air exists. (TopL) If you just dip a straw directly in water and lift it out, then (TopR) all the water runs out.  (LowerL) If instead you put your finger over the straw before dipping it, then (LowerR) no water gets in the straw. Something invisible got in the way -- air!

An experiment to convince yourself air exists. (TopL) If you just dip a straw directly in water and lift it out, then (TopR) all the water runs out. (LowerL) If instead you put your finger over the straw before dipping it, then (LowerR) no water gets in the straw. Something invisible got in the way — air!

Take a drinking straw and a glass of water.  Dip the straw in the water, then place your thumb over the top of the straw, and remove it from the water.  If you take your thumb off the straw, you find that you had trapped some water in the straw.  Now do a slightly different experiment. Put your thumb over the end of the straw first, then put it in the water. If you take the straw out of the water and remove your thumb, you find that there is no water in the straw!  Why didn’t water go in the straw? There must have been something in the way, something invisible you couldn’t see. It is, of course, the air. This seems completely obvious to us now, thinking about it with 21st century brains, but two millenia ago, when we were just beginning to speculate on the nature of the world, this was a remarkable and marvelous observation of the world.

Today, astronomers find themselves in a similar brain loop with respect to gravity. One can “measure the force of gravity” through experiment. But when Einstein developed general relativity, he did away with gravitational forces in favor of motion on the curvature of spacetime. We can use this idea to describe everything we see in Newtonian gravity — objects freely falling to the ground, orbits of astrophysical bodies, and the weightlessness of astronauts in space. There have been exquisite tests of general relativity confirming its unique predictions beyond Newtonian gravity, and we rely on it every single day.

But is there a way to directly measure spacetime? Can we confirm that gravity is no more than the curvature of spacetime itself?  This is a question that has occupied the minds of gravitational physicists for a century now, and many ideas have been proposed and successfully carried out.

The most ambitious idea to directly measure spacetime curvature was first proposed by Einstein himself, and has taken a century to come to fruition. One of the motivations to develop general relativity was famously to incorporate into gravitational theory the fact that there is an ultimate speed limit in the Cosmos. If the gravitational field changes (for instance, due to the dynamical motion of large, massive objects like stars), that information must propagate to distant observers at the speed of light or less. If gravity is no more than the curvature of spacetime, then changes in the gravitational field must must be encoded in changing spacetime curvature that propagates from one place to another. We call such changes gravitational waves.

The opening pages of Einstein's first two papers on gravitational waves in 1916 (L) and 1918 (R).

The opening pages of Einstein’s first two papers on gravitational waves in 1916 (L) and 1918 (R).

If you want to build an experiment to detect an effect in Nature, you need a way to interact with the phenomenon that you can unambiguously associate with the effect. For the first 40 years after Einstein proposed the idea of gravitational waves, physicists were vexed by the detection question because they were confused as to whether the phenomenon existed at all!  The problem, we now know, was our inexperience with thinking about spacetime.

The International Prototype Kilogram (IPK).

The International Prototype Kilogram (IPK).

Scientists spend their lives quantifying the world, describing it precisely and carefully without ambiguity, as much as is possible. To this end, we use numbers, and so need a way of agreeing on what certain numbers mean. For example, we measure mass using “kilograms.” What’s a kilogram? It is the mass of a reference body, made of iridium (10%) and platinum (90%), called the “International Prototype Kilogram” (IPK). The IPK, and six sister copies, are stored at the International Bureau of Weights and Measures in Paris, France. Scientists around the world agree that the IPK is the kilogram, and can base numbers off of it. Nature doesn’t care what the IPK is; the Sun certainly has a mass, expressible in kilograms, but it doesn’t care one whit what the IPK is. The kilogram is something humans invented to quantify and express their knowledge of the Cosmos in a way other humans could understand.

Example coordinates that can be used to describe the screen or paper you are reading this on. They are all different because humans invented them, not Nature. They are not intrinsic to the surface they are describing, though they are often chosen to reflect underlying shapes of the surface.

Example coordinates that can be used to describe the screen or paper you are reading this on. They are all different because humans invented them, not Nature. They are not intrinsic to the surface they are describing, though they are often chosen to reflect underlying shapes of the surface.

In a similar way, when spacetime physicists describe spacetime, we have to have a way of identifying locations in spacetime, so we make up coordinates. Like the kilogram, coordinates are something we humans create to enable us to talk with each other; Nature cares nothing, Nature knows nothing about coordinates. But sometimes we get so used to think about Nature in terms of coordinates, that we begin to ascribe physical importance to them! This was the case during the early decades of thinking about gravitational waves. Physicists were confused about whether or not the coordinates were waving back and forth, or if spacetime itself was waving back and forth.  Arthur Eddington, who had led the 1919 Eclipse Expedition to measure general relativity’s prediction of the deflection of starlight, famously had convinced himself that the waves were not real, but only an artifact of the coordinates.

At the poles of the globe, all the lines of longitude come together, and there is no well defined value. There is nothing wrong with the sphere; the coordinates that humans invented are not well suited there!

At the poles of the globe, all the lines of longitude come together, and there is no well defined value. There is nothing wrong with the sphere; the coordinates that humans invented are not well suited there!

Sometimes coordinates behave badly, giving results that might seem wrong or unphysical. For instance, you can see one example of badly behaving coordinates at the top of a sphere — if you are standing on the North Pole of the Earth, what is your longitude? You can’t tell! Longitude is a badly behaving coordinate there! There is nothing wrong with the sphere, only our coordinates.

And so it was with spacetime. In the early 1930s, Einstein and a collaborator, Nathan Rosen, had discovered a gravitational wave solution that appeared unphysical and claimed this as a proof that gravitational waves did not exist. Their result was later shown to be coordinates behaving badly, and Einstein pivoted away from denying gravitational waves exist, though Rosen never did.

The argument of the reality of the waves persisted for decades; in the end, the questions were resolved by a brilliant deduction about how to measure gravitational waves. As with all things in science, the road to understanding is a slow and steady plod, ultimately culminating in a moment of  understanding. In the early 1950s, our thinking was progressing rapidly (or so we know now, with 20/20 hindsight). The watershed came in January of 1957 at Chapel Hill, North Carolina, at a now famous conference known as “The Role of Gravitation in Physics.” There were 44 attendees who had gathered to discuss and ponder the state of gravitational physics. It was barely 19 months after Einstein’s death, and the question of the existence of gravitational waves had not yet been resolved.

The community had slowly been converging on an important and central issue in experimental physics: if you want to detect something in Nature, then you have to know what the phenomenon does to the world around it. You then need to design an experiment that focuses on that effect, isolating it in some unambiguous way. At the Chapel Hill Conference, the realization of what to do was finally put forward by Felix Pirani. Pirani had settled on the notion that an observable effect of a passing gravitational wave is the undulating separation between two test masses in space (something gravitational physicists called “geodesic deviation” or “tidal deviation”). This idea hearkens back to the idea that the trajectories of particles is a way to measure the underlying shape of gravity, which was one of the original notions we had about thinking of gravity in the context of curvature.

The Sticky Bead argument was a thought experiment that convinced physicists that gravitational waves were real and could carry energy. (TOP) Imagine two beads on a smooth rod. There is a small amount of friction that keeps the beads from sliding freely. (BOTTOM) When a gravitational wave passes by, it pushes the beads apart. The friction stops the motion of the beads, heating the rod up. Measuring the heat in the rod constitutes a detection of the gravitational waves, since they were the source of the energy.

The Sticky Bead argument was a thought experiment that convinced physicists gravitational waves were real and could carry energy. (TOP) Imagine two beads on a smooth rod. A small amount of friction keeps the beads from sliding freely. (BOTTOM) When a gravitational wave passes by, it pushes the beads apart. The friction stops the motion of the beads, heating the rod up. Measuring the heat in the rod constitutes a detection of the gravitational waves, since they were the source of the energy.

Also present at the conference was Richard Feynman, by then a professor at the California Institute of Technology. Feynman took Pirani’s notion and extended it into what we now call “the sticky bead argument.” He imagined a smooth rod with two beads on it. The beads were a little bit sticky, unable to slide along the rod without being pushed. When the motion of the beads was analyzed under the influence of gravitational waves, they moved back and forth, but their motion was arrested by the friction between the beads and the rod. Friction is a dissipative force, and causes the rod to heat up, just like your hands do if you rub them together. In the sticky bead case, what is the origin of the heat? The heat energy originated from the gravitational waves and was deposited in the system by the motion of the beads.

This idea was picked up by Herman Bondi, who expanded the idea, fleshing it out and publishing it in one of the leading scientific journals of the day. As a result, Bondi is generally credited with this argument.

(L) Richard Feynman (C) Hermann Bondi (R) Joseph Weber

(L) Richard Feynman (C) Hermann Bondi (R) Joseph Weber

Confirming that the beads move validated the idea that gravitational waves not only carry energy, but can deposit it in systems they interact with. This was the genesis of the notion that an observational programme to detect them could be mounted.  That challenge would be taken up by another person present at the Chapel Hill conference, named Joseph Weber. Weber had spent the previous academic year on sabbatical, studying gravitational waves at Princeton, and left Chapel Hill inspired to begin a serious search. Weber’s entrance to gravitational wave astronomy happened in the early 1960s with the introduction of the first gravitational wave bar detector.  This was the foundation that led to the great experimental gravitational wave experiments of today; we will start our story there in our next chat.

I am indebted to my colleague Peter Saulson (Syracuse) who first made me aware of Pirani’s talk at the 1957 Chapel Hill Conference. That Conference is part of the folklore if our discipline, though details are often glossed over usually going directly to the Bondi Bead story. I am also indebted to Carl Sagan, who introduced me to the idea that one can detect the air with water experiments (in “The Backbone of Night,” episode 7 of Cosmos: A Personal Voyage).

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

Gravity 10: Signatures of the Big Bang

by Shane L. Larson

Science has two interlocking pieces that always work together. One part is “describing the world to predict the future and explain the past.” This is the part that many of us remember from science class, involving pen and paper and mathematics and the Laws of Nature. Another part is “observing the world and seeing what Nature is up to.” This is also a part that many of us remember from science class, involving doing experiments and recording numbers and making graphs. These two pieces are called theory and experiment respectively, and they constantly validate and reinforce each other in a never-ending cycle of upgrading and refining our knowledge of the Cosmos.

All science is a combination of theory and experiment. In Cosmology, theory is an application of general relativity embodied in the Friedmann Equations (left); experiment is captured in astronomy (right).

All science is a combination of theory and experiment. In Cosmology, theory is an application of general relativity embodied in the Friedmann Equations (left); experiment is captured in astronomy (right).

Cosmology has seldom had both theory and experiment walking hand in hand. Instead one or the other has been out in front — sometimes way out in front — waiting for the other to catch up. This was certainly the case when general relativity was announced to the world — as a description of the machinery of the Cosmos, it was perfectly capable of making predictions that were so far beyond our ability to observe and verify that we didn’t recognize the truth for what it was. This famously happened early on, in 1917 when Einstein published one of the earliest papers about relativistic cosmology entitled “Cosmological Considerations in the General Theory of Relativity.”

As city dwellers, it is often easy to forget that the sky is full of stars. This fact leads to the most natural assumptions about the Universe based on experience: the Universe is full of stars.

As city dwellers, it is often easy to forget that the sky is full of stars. This fact leads to the most natural assumptions about the Universe based on experience: the Universe is full of stars.

At that time, we were profoundly ignorant about the nature of the Universe. The prevailing view was that the Cosmos was full of stars, and that the Universe was static. It’s the most natural assumption in the world based on your experiences when you step out the door every night — the sky is full of stars and they change little, if at all, night to night.

Einstein considered a Universe simply filled with stars, and asked what general relativity predicted. He found it only predicted one thing: the Universe must collapse.  Preconceptions are a powerful force in science, and Einstein believed strongly in the static Universe, so much so that he supposed maybe he had not gotten general relativity completely correct. So he introduced a mathematical addition to general relativity that pushed back against the collapse, called the Cosmological Constant.

Before 1924, the nature of galaxies was unknown. They were grouped with the "nebulae" -- wispy, cloud like structures that could be seen through the telescope. These included Messier 31, the Andromeda Nebula (L) and Messier 51, the Whirlpool Nebula (R). The Whirlpool was the first nebulae that spiral structure was detected in, by Lord Rosse in 1845.

Before 1924, the nature of galaxies was unknown. They were grouped with the “nebulae” — wispy, cloud like structures that could be seen through the telescope. These included Messier 31, the Andromeda Nebula (L) and Messier 51, the Whirlpool Nebula (R). The Whirlpool was the first nebulae that spiral structure was detected in, by Lord Rosse in 1845.

But science is always on the move. Telescopes in that day and age were getting larger. In 1917 the 100-inch Hooker telescope first saw starlight on its mirror, as it embarked on a long and storied history of astronomical discovery. Telescopes gave us the capability to probe deeper into the Cosmos, and we had started to discover vast diaphanous complexes of light that showed no stellar qualities. These were called nebulae, Latin for “cloud.”  A few of the nebulae, the “spiral nebulae,” perplexed astronomers — some thought they were simply odd nebulae (vast complexes of gas and dust), and others thought they were island Universes (other galaxies, like the Milky Way).

The 100-inch Hooker Telescope on Mount Wilson, used to discover the expansion of the Cosmos.

The 100-inch Hooker Telescope on Mount Wilson, used to discover the expansion of the Cosmos.

The discovery of the distances to the spiral nebulae, using Leavitt’s Cepheid variable method, was a watershed moment in the history of cosmology. It put on the table two  important facts: first, the Universe was vast — enormously vast — with distances far beyond the boundaries of our own galaxy.  Second, the major constituent especially on large scales, was not stars, but galaxies (which are agglomerations of stars). These two simple facts suddenly and irrevocably changed the way we thought about the Universe. In the first years after the discovery of the nature of galaxies, Friedmann and Lemaître used general relativity to imagine a Universe filled with galaxies, and discovered the idea that the Universe was expanding. This notion, discovered on paper, was bourne out in 1929 when Humason and Hubble, once again using the 100-inch telescope on Mount Wilson, found all the galaxies in the Universe were receding from one another.

At that time, Lemaître made a great leap of imagination — it was a thought that was well outside the comfort zone of astronomers of the day, though today we may view his leap as completely obvious. From the mindset of the future, it is difficult to imagine just how hard it was to think different. Lemaître supposed that if the Universe was expanding, then in the past it would have been smaller, and hotter. The idea was met with incredulity and derision, sparking enormous debate for decades to come. But science is the blend of ideas on paper with observations of the Universe. If Lemaître’s ideas were right OR wrong, the evidence could be found by looking into the Cosmos.

There are many lines of evidence that confirm the basic idea of the Big Bang, but there are three major pillars of support. The first, is the expansion itself. The Universe is not like a sports car, starting and stopping on a whim, braking and accelerating at random. Its evolution is driven by the Laws of Nature, in a smooth and predictable fashion. If we see expansion today, that expansion started in the past. The rate and trends in the expansion are a function of the amount of matter in the Universe, and the initial conditions of the expansion. These quantities can be determined from astronomical observations, and are consistent with the Big Bang picture.

A popular T-shirt meme about atoms, a clever science pun!

A popular T-shirt meme about atoms, a clever science pun!

The second and third pillars of evidence for the Big Bang have to do with what happens to the Universe as it expands and cools, and the consequences for matter.  Everything you and I see around us here on Earth — rocks, trees, candy bars, platypuses — is made of atoms. Atoms are a composite structure. The center is a compact heavy core called the nucleus comprised of protons and neutrons. It is surrounded by a cloud of electrons, equal in number to the protons in the nucleus.

The fact that the atom holds together is a manifestation of the forces at work. The electrons are held to the atom by virtue of attractive electrical force between them and the nucleus. If you bang two atoms together hard enough, they break apart into free nuclei and free electrons.

The nuclei, built of protons and neutrons, are held together by a very strong force that acts over short distances called the nuclear force. The nuclear force is tremendously strong, but if you bang to nuclei together hard enough, they too can be broken up into free protons and free neutrons.

In the distant past, shortly after the Big Bang, the Universe was very compact: everything in it was closer together, and extraordinarily hot. As the Universe gets smaller, it’s a bit like being squished together in the mosh pit at a concert — you can’t really move anywhere without crashing into something else. The enormous temperatures mean that everything was moving extremely fast — the hotter the temperatures, the faster the motion, the harder the crashes. If you go far enough back in time, the Universe gets so hot you can’t have atoms. If you go even farther back, it gets hotter and the Universe can’t even have atomic nuclei.

The basic constituents made during Big Bang nucleosynthesis.

The basic constituents made during Big Bang nucleosynthesis. Protons (red) and neutrons (green) bind to form the simplest atomic nuclei.

The second pillar can be understood by going back to a time in the first minute after the Big Bang. Up to this point, the Cosmos was a primordial soup of free electrons, free neutrons, and free protons, all swirling around in a maelstrom of churning energy. The Universe had started its inexorable expansion, and was cooling as a result.  By the time the Cosmos was 10 seconds old, it had cooled from its hot beginnings down to a temperature of about 2 billion degrees Celsius. At this temperature, protons and neutrons begin to stick together to form the first atomic nuclei. This process of formation is called primordial nucleosynthesis — it makes hydrogen, deuterium, helium, and small amounts of lithium and beryllium.

Big Bang theory predicts how much of each of these was synthesized in the first 15-20 minutes, a delicate balance astronomers call the primordial abundances. What astronomers can see agrees with the predictions of Big Bang nucleosynthesis.

But perhaps the most important observational signature of the Big Bang has to do with light. After the formation of the atomic nuclei, the Cosmos was still too hot to form proper atoms. Every time a nucleus tried to bind with an electron, a collision would knock the electron free. So, for the next 400,000 years, the Universe remained a seething fluid of atomic nuclei, free electrons, and energy.

When light is packed in so tightly with charged particles, like the electrons and atomic nuclei, it is not free to travel about of its own free will. It travels only a short distance before it encounters an electron, and it scatters.  Light simply can’t go very far.

(L) Before recombination, light cannot travel very far because it encounters free electrons, which interact with it causing it to scatter. (R) After the electrons bind to nuclei to make atoms, the light decouples from matter, and is free to stream through the Universe unimpeded by scattering interactions.

(L) Before recombination, light cannot travel very far because it encounters free electrons, which interact with it causing it to scatter. (R) After the electrons bind to nuclei to make atoms, the light decouples from matter, and is free to stream through the Universe unimpeded by scattering interactions.

But, after 400,000 years, the Universe cools to a balmy 3000 degrees Celsius, cool enough that each time an electron bumps into a nucleus, it binds together to form an atom. This process is called recombination. From the point of view of the light, all of the charged particles suddenly disappear (atoms are neutral, having no overall electric charge) and the Universe becomes transparent. The light can travel anywhere it wants without being impeded by the matter; astronomers call this decoupling.  The Cosmos is full of freely streaming light.

A recreation of the 1965 Cosmic Microwave Background map, covering the entire sky (Penzias and Wilson could not see the entire sky from Bell Labs). The band of stronger microwave light is the signature of the Milky Way Galaxy.

A map of the Cosmic Microwave Background across the whole sky. First detected by Penzias and Wilson in 1965, this light is the signature of ever cooling Universe after the Big Bang. The band of stronger microwave light along the center of the map is the signature of the Milky Way Galaxy.

For the next 13 billion years, the Universe continued to expand to the present day. All the while the streaming light — the signature from the birth of atoms — surfed right along.  Spacetime is stretching as the Cosmos expands, and the light from that early, hot, dense state had to give up energy to fight against the expansion, shifting to longer and longer wavelengths as time progressed. By the time light reaches the Earth today, it should appear as microwave light.  And indeed, it is. In every direction we look on the sky, we see a uniform background of microwave light called the Cosmic Microwave Background. This is the third, observational pillar, the evidence, that tells us our thinking about the Big Bang is on the right track.

This is the basic picture of the Big Bang that was developed since the late 1920’s — decades of careful comparison of observations with theoretical calculations. The refinements and developments — of both theory and experiment — continue to this day.

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.