An #IceBucketChallenge

by Shane L. Larson

Nasa_blue_marbleThe planet Earth, like many planets I suppose, is a planet of wonders. Its landscapes are carved and wrought over aeons of geologic time by titanic forces that are almost beyond our comprehension. Mountains soar to altitudes so high a human can barely breathe; the ocean hides dark depths that have never seen the light of the Sun and never will. Entire continents move, shifting slow and steady, a few inches per year, until the world has utterly changed its face.

One of the greatest wonders of our small blue world, and so far as we know a unique one, is life. Life appears on this planet in myriad forms, and new forms are constantly being discovered and categorized. Over the last several centuries, we have slowly assembled our understanding of the machinery of life — how it works, how it survives, how it perishes.

At the frontiers of our investigations about the workings of life are our attempts to understand the nature of disease. For many diseases, we understand their symptoms, and in some cases what causes a particular disease. We do not in all cases know how to deal with — or how to cure — diseases.

Lou Gehrig in the dugout at Briggs Stadium (now Tigers Stadium) in Detroit, on 2 May 1939. The game marked the end of his 2,130 consecutive game playing streak.

Lou Gehrig in the dugout at Briggs Stadium (now Tigers Stadium) in Detroit, on 2 May 1939. The game marked the end of his 2,130 consecutive game playing streak.

One example is amyotrophic lateral sclerosis (ALS) disease. It is the most common of several degenerative motor neuron disorders. These diseases affect the cells in your nervous system (motor neurons) that control voluntary muscle activity (walking, speaking, breathing, etc.).  In the United States, ALS is commonly known as “Lou Gehrig’s disease,” named after the famed first baseman who played for the New York Yankees from 1923 to 1939. Gehrig was a powerhouse hitter in his day, holding the career grand-slam record (23) for 74 years until it was broken by Alex Rodriguez in 2013, and also the record for most consecutive games played (2130), a record that stood for 56 years until it was broken by Cal Ripken, Jr. in 1995.  Gehrig’s performance and health decreased rapidly in the 1938-39 seasons. He voluntarily benched himself on 2 May 1939 in a game against the Detroit Tigers, ending his consecutive game streak. The Detroit Tigers fans honored him with a standing ovation. In June of 1939, he visited the Mayo Clinic in Rochester, Minnesota, where he was diagnosed with ALS. He passed away two years later, on 2 June 1941.

1024px-Stephen_Hawking_050506In the gravitational physics community, we are at least sub-conciously aware of ALS because it affects one of our own colleagues — Stephen Hawking. Hawking was diagnosed with ALS at a very young age, in 1963 when he was only 21. The life expectancy of those afflicted with ALS is, on average, just a few years; Hawking was told he had about two years to live. Against all odds, he has survived well beyond that prognosis, now made 51 years ago. Hawking is one of a rare few who have survived for so long. It doesn’t happen often, but it does happen. In the time he has had, he has contributed immeasurably to our knowledge of gravitational physics. In 1970, working in classical cosmology he proved a singularity theorem that showed there was a point of infinite density associated with the Big Bang. In 1974 he discovered that black holes over time evaporate, fading away into nothing; we still don’t know what happens during their last moments. In 1988, he published “A Brief History of Time,” one of the best selling public science books of all time, with more than 10 million copies in print.

Few with ALS live as long has Hawking has. Most, like Lou Gehrig, die after only a few years, typically when they lose the ability to trigger the muscles that control breathing or swallowing. There is no known cure for ALS, but in the last few years medical research has begun to reveal what some of the causes are.

Diseases are a part of life. Some diseases are caused by one kind of lifeform infecting another; AIDS is a classic example, caused by a virus that infects the human body. Other diseases, like ALS, appear to be a result of a lifeform’s own machinery malfunctioning or breaking down in some fashion. In ancient times, before science and modern medicine, diseases were poorly understood. Sickness, particularly devastating and debilitating illnesses, killed quickly and were viewed with fear and superstition.  The advent of scientific research began to shed light into the dark corners of our biology, allowing us to understand, at least in part, how to avoid and combat some diseases.  The development of scientific research over the past four centuries has evolved our perceptions of diseases from superstition to knowledge.

motor_neuron2So what do we know about ALS?  It was first identified as a distinct disorder in 1869 by French neurologist Jean-Martin Charcot, who was also the first person to identify multiple sclerosis. However, after its initial identification, very little progress was made in understanding the disease. It wasn’t until 1991 that any kind of genetic connection was made.  Since then, it has also be found that abnormal proteins and neurotransmitters seem to be related to the disease, but our understanding is still evolving. In about 10% of cases, genetics are a contributing factor to the development of ALS. In the remaining cases, where there is no known family history, the causes of ALS are virtually unknown.

Like all scientific investigations, medical research takes time and resources, and progresses slowly. We cannot know what investigations will lead to a breakthrough, so work progresses on many fronts — genetic investigations, studies of degenerating nerve cells,  looking for correlations in environment or lifestyles, searches for therapies and drugs that ameliorate symptoms and prolong life, and more. Eventually, these avenues of investigation lead to treatments and perhaps, someday, cures.

ALS, like other motor neuron diseases, is incredibly difficult to understand and fight. In the United States there is only a single drug approved to use in the fight against ALS, but its efficacy is limited, extending life by only a few months at most. More research is needed; research requires resources.

The ALS Association (http://www.alsa.org/) is an organization dedicated to fighting ALS. It helps fund research internationally, aids patients and families who are living with ALS every day, and works to educate the public about this disease. As a non-profit organization, they are reliant on donations to fund their battle. This year, a viral donation campaign has started to support the fight against ALS, known as the ALS Ice Bucket Challenge. It’s the reason you are reading this post right now.

It goes something like this: I dump a bucket of ice on my head and challenge three other people to do the same within 24 hours or make a donation to ALS research.  Click the donate button the upper right of the ALSA homepage.

I have made two ALS Ice Bucket Videos for the different social media milieus where I post most often — Facebook, and Twitter.  On Facebook, I have challenged several of my friends: they are Trae Winter, Jackie Anderson, and David Zartman.

On Twitter, I have challenged some other folks whom I have not yet found have participated in the challenge. They are: Phil Plait (@BadAstronomer), Scirens (@Scirens), and Lucianne Walcowicz (@shaka_lulu). I also put out a special 4th challenge to another acquaintance of mine: the President and CEO of the Adler Planetarium, Michelle Larson (@AdlerPrez).

In addition to making my challenges, I have also made a monetary donation to the ALSA, and written this blog. Perhaps I’m sentimental because Hawking is a colleague in my scientific field. Perhaps I wonder where physics would be if Hawking had succumbed to ALS in his youth, and by simple extrapolation wonder how much the world has lost because of those that this disease — that any disease — has taken from us. And perhaps I’m just hearing my mother’s voice, telling me that everyone who can help, should help. I can help with this.

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I don’t have any personal friends that I know of who are fighting against ALS. I do however have many friends who are battling or have battled severe and life threatening illnesses — cancer, multiple sclerosis, diabetes, leukemia. This post is dedicated to them all, with much love, admiration, and hope.

Days of Summer

by Shane L. Larson

As a father, I watch my daughter scoot off to summer camp with a vaguely unsettled sense of longing for those by-gone days of my youth. As grown-ups, we don’t go to “summer camp” any more. Instead, we sometimes have “vacation,” but vacation never has quite the same care-free, no-holds barred, reckless sense of fun, adventure and freedom that summer camp always had. There’s just too much of the trappings of being a grown-up tied up in “vacation.”  Too much “enjoying the morning paper by the pool” instead of “dodge-ball.”  Too much “eating a salad with this fancy dinner” instead of “let’s blow every last penny I have in this candy store.”  Too much “looking for the Museum of Historical Art” instead of “standing on our heads to find the Zowie Rock so our cabin wins the giant popsicle tonight!

kayak

But sometimes I find myself in a kayak on a still mountain lake, my phone forgotten (or dropped overboard), and nothing on my mind except that serene fugue state of thought that whispers, “if you keep paddling, there is no telling what’s on the other shore…

As a scientist, I have the immense good fortune of doing something I love every day — probing the mysteries of the Cosmos, mentoring young (and old!) minds on their own voyages of self-discovery, and adding to the collective knowledge of our species. But a job as a scientist is still just like every job, and it has its share of interruptions and stresses. There is always another telecon to be on; there is always another deadline for book orders and class website requests; there is always a student who needs some career advice; there is always another midterm exam to write or grade; there is always another grant you should write a proposal for to support your next student on their path to knowledge. Like every job, there are good days and bad days, and many days that make you long for those by-gone days of summer camp!

aspenPhysics

This year I was able to spend three weeks in a workshop at the Aspen Center for Physics. Founded in 1962 by George Stranahan and Michael Cohen, the Aspen Center for Physics is located on a small, 3 building campus in Aspen, Colorado. It shares this idyllic setting with two other world-renowned intellectual organizations: the Aspen Institute, and the Aspen Music Festival and School. The idea of the Aspen Center for Physics is simple — bring scientists together, away from the demands of every day life, and give them freedom and opportunity to think and interact. Isolation combined with creative intellectual colleagues can and will spawn remarkable and ingenious moments of progress at the forefronts of science.

Let me tell you some tales about my few, short weeks at the Aspen Center for Physics.  If my third grade teacher (Mimi Martin) is out there reading this, you might call this my “What I Did This Summer” essay!

My office at the Aspen Center for Physics

My office at the Aspen Center for Physics

The Setting: The Aspen Center for Physics is set on a small campus with three buildings that are, for the most part, comprised entirely of offices for scientists, and small meeting “alcoves” where groups of us can gather to hash out mysteries and plot to win Nobel Prizes.  We share offices, kind of like when we were students, usually with a complete stranger, and often with someone who is not in our same discipline. This mixing of minds is an essential part of the Aspen Center for Physics’ recipe for success — exposure to new ideas and learning new things about other subjects always generates new and interesting approaches to science (I’ve written about that before).

The campus itself is pastoral and idyllic, replete with gathering spaces and benches conducive to quiet contemplation and speculation about the inner workings of the Cosmos. Again, the setting is purposeful — intended to produce an isolated and minimally distracting environment, free of the normal trappings of everyday life, in an effort to allow the mind the freedom to explore new ideas and discover new approaches to science.  All things being equal, it is a model that has succeeded admirably — over 10,000 physicists have visited the Aspen Center for Physics since its founding, including 52 Nobel Laureates. Over the years, more than 10,000 scientific publications have emerged as a result of time spent at the Center.

Campus of the Aspen Center for Physics.

Campus of the Aspen Center for Physics.

The Workshop: The workshop I came to the Center for was about “ultra-compact binary star systems.” That’s a mouthful — the kind of thing you like to tell your mother you work on because it sounds important. Whatever does it mean? Most stars you see in the sky, possibly as many as 50%, have a companion star that orbits them, like the planets orbit our Sun. We call these systems “binary stars.”

binarySystem

When stars reach the ends of their lives, they typically evolve into one of three different kinds of skeletons that mass as much as the Sun or more. These three stellar skeletons are called white dwarfs (something about the size of the Earth, made by low mass stars), neutron stars (something the size of a small city, made by medium mass stars), or a black hole (also about the size of a city, but made by much more massive stars).  Given the menu of stellar skeletons, you can imagine that long after binary stars are born, you can (and do!) end up with a binary made up of TWO stellar skeletons!

Evolutionary pathways from stellar life into the graveyard after stellar death. The three end states are white dwarfs, neutron stars, or white dwarfs, depending on the mass of the star in its life. [Image by NASA/CXC/M.Weiss]

Evolutionary pathways from stellar life into the graveyard after stellar death. The three end states are white dwarfs, neutron stars, or black holes, depending on the mass of the star in its life. [Image by NASA/CXC/M.Weiss]

Over time, the orbits of these skeletal star systems shrink smaller and smaller and smaller, until the stars are so close together they orbit at phenomenal speeds. For a pair of white dwarfs that orbit once every 15 minutes, they are separated by about half the Earth-Moon distance, and are travelling at a speed of 1 million meters per second (about 2.4 million miles per hour)!  These are “ultra-compact binary star systems.”

Ultra-compact binary systems have stellar mass objects, like two white dwarfs, orbiting in extremely small, short period orbits at extreme speeds.

Ultra-compact binary systems have stellar mass objects, like two white dwarfs, orbiting in extremely small, short period orbits at extreme speeds.

My office chalkboard after just a couple of days at the Aspen Center for Physics.

My office chalkboard after just a couple of days at the Aspen Center for Physics.

What Happens: We talk. A LOT. There are chalkboards all over the Center — in the offices, in the hallways, and outside on the patios.  There are always clusters of physicists around them — debating, deriving, teaching, learning. I know it sounds funny, but this is where a lot of science is born.

For instance, my graduate student and I have been working on a project where we need to know something about the number of neutron stars in the galaxy.  We need to know how many there might be, because we are thinking about an interesting way to observe them. If there aren’t very many neutron stars, we should abandon the idea, but if there are a lot of neutron stars, it could be important. I promised her that I would ask around at the workshop to see if anyone knew anything that could help us out.

(L to R) Me with my colleagues, Matt Benacquista and Melvyn Davies.

(L to R) Me with my colleagues, Matt Benacquista and Melvyn Davies.

So one day I was talking about this to my colleagues, Melvyn Davies (Lund University, Sweden) and Matt Benacquista (University of Texas-Brownsville) — they’re both experts in this sort of thing. They told me some very useful stuff, which I’ve passed on to my student. But at one point Melvyn asked me from how far away we could detect the gravitational waves from systems with a neutron star and a white dwarf together. I sketched out a quick calculation that suggested this was a very interesting idea to think about, and soon the three of us will publish a paper about how to study these systems with gravity, not light. It’s perhaps surprising that no one has thought about this before, but it’s a big Cosmos — there is a lot to think about! This is what the Aspen Center for Physics was designed to do — put scientists together, and let their brains roam free to make new discoveries.

And it’s not just at the Center that this stuff goes on. We are together all the time, which means we are always thinking and talking about science, usually intermixed with other enjoyable life activities.  We segue in and out of science and life the way you often segue in and out of sports and life or weather and life.  For instance, on any given evening if you are in Aspen, hanging out, eating dinner at the famous Hickory House, you might find us sitting next to you. You might be engaged in pleasant conversation about a nice hike you took earlier that day; we of course were hiking earlier that day too, but are still debating the question that occupied us on that hike, namely whether or not star systems with highly elliptical (oval shaped) orbits can be detected farther away in the Universe by LIGO than star systems with circular orbits.

When two stars orbit one another, the orbits can be perfect circles, or they can be elongated ellipses; we say these orbits are "eccentric."

When two stars orbit one another, the orbits can be perfect circles, or they can be elongated ellipses as shown above. When they are elongated, we say the orbits are “eccentric.”

Fun and Games. While it is all science all the time, it’s not all high-brow esoteric research. Physicists, as a rule, love to talk about what they do, as most of you who have a physicist neighbor or relative know. The Aspen Center for Physics hosts a regular public lecture series, intended to explain for a popular audience what physics is all about, and why and how we do physics. This summer I had the good fortune to hear K.C. Huang from Stanford talk about the evolutionary life cycles of bacterial cells and colonies, and also a talk about the dark energy in the Universe from my colleague, Bob Kirshner of Harvard (Bob has written a very nice book on this topic).

Bob Kirshner (Harvard) during his 2014 Heinz Pagels Public Lecture about Dark Energy and the Accelerating Universe.

Bob Kirshner (Harvard) during his 2014 Heinz Pagels Public Lecture about Dark Energy and the Accelerating Universe.

I also got to put my public game on, when I was asked if I could do a half-hour chat at the “Physics for Kids” picnic, hosted by the Aspen Science Center at the Center for Physics. This was a crowd of about 20 or 30 9-10 year olds and their parents, so I decided to talk to them about energy, which is and will continue to be a crucial topic of conversation during their lives.  So we talked a bit about how scientists think about energy, and then I did three demonstrations. First, we made craters in a tray of flour, showing how the size of the crater depends on the energy of the impactor — the biggest crater was made with a hollow shell shot from a paint-ball gun.

Impact crater made by a paintball shell from a distance of about 1.5 meters. Typical speed for a paintball shell is about 90 m/s (~200 mph!). Crater size is about 7 cm across.

Impact crater made by a paintball shell from a distance of about 1.5 meters. Typical speed for a paintball shell is about 90 m/s (~200 mph!). Crater size is about 7 cm across. (Click to enlarge!)

Second, we showed how energy is stored and converted using the famous “Bowling Ball of Doom” demo. You mount a bowling ball to a long cable, then hold it against your chin. When you release it, the bowling ball swings out across the room, then comes right back at your head but stops at the precise point you released it! It really looks like it is going to smash you in your face, but that is an impossibility because that would require it to obtain some energy from nowhere.

First person views of the Bowling Ball of Doom Demo. (L) The bowling ball is initially held touching your chin. (C) After release, the bowling ball swings away, then right back at you! (R) If you tie a camera to the bowling ball, you see it is moving pretty fast (about 3.5 m/s, or 8 mph!).

First person views of the Bowling Ball of Doom Demo. (L) The bowling ball is initially held touching your chin. (C) After release, the bowling ball swings away, then right back at you! (R) If you tie a camera to the bowling ball, you see it is moving pretty fast (about 3.5 m/s, or 8 mph!). (Click to enlarge!)

The last demo, as any of my students can tell you, is the Number One Physics Demo of All Time: the Bed of Nails. I lay on a bed of nails. A second bed of nails is laid on my chest. A cinder block is placed on top of that. A volunteer (in this case, my colleague, Stephan Rosswog, from Stockholm University) takes a 10 pound sledgehammer and smashes the cinder block. Obviously I survive (otherwise I wouldn’t be writing this blog!). How? The cinder block dissipates the energy of the hammer by breaking, thus sparing my life. You can see some videos of this demo: slow motion view; low, ground level view; first person head-mounted GoPro view.

(L) My Bed of Nails hammer weilder, Stephan "Thor" Rosswog (C) Matt Benacquista makes sure the GoPro is ready to capture the action! (R) Stephan works out some of the day's frustrations... :-)

(L) My Bed of Nails hammer weilder, Stephan “Thor” Rosswog (C) Matt Benacquista makes sure the GoPro is ready to capture the action! (R) Stephan works out some of the day’s frustrations… :-) (Click to enlarge!)

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

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

But probably the most important thing that happened this summer at Aspen, was I closed a loop in my career. When I was a young man, just starting out in college at Oregon State University, I was a mechanical engineering major. The reason for this was I was going to be an astronaut, and the way to become an astronaut (during the shuttle era) was to become a mission specialist, and one way to become a mission specialist was to design experiments that flew on the shuttle. At Oregon State during this time, there was a general science class taught called “Rocks and Stars,” and during my first year there they brought to campus a guest speaker: Dr. J. Craig Wheeler, from the University of Texas at Austin. Wheeler gave a great public lecture about black holes, which made me start seriously thinking about this whole astronomy business. This, of course, ultimately culminated in me becoming a physicist (a story I have written about before). As it turns out, he was at the Aspen Center for Physics this summer. We got to chat and hang out, I got to tell him the story that I just told you, and got a selfie of the two of us. :-)

My colleague, Enrico Ramirez-Ruiz, a professor in the Department of Astronomy and Astrophysics at the University of California – Santa Cruz, summarized a sojourn at the Aspen Center for Physics very succinctly: “It’s like summer camp for physicists.

And so it is. It clears the mind, it rejuvenates the soul, it connects you with people of like mind and like spirit. We argue, we debate, we eat, we laugh, we play, and we try to push science a little bit farther forward.  And like those summer camps from our youth, it is over far too soon. But you go home with new friends, with new ambitions, and a burning desire to come back again soon.

sunsetACP

 

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This post was written during, and after, a summer residency at the Aspen Center for Physics.

Dinosaurs in the Cosmos 2: Dinos without Radios

by Shane L. Larson

One of the things physicists and astronomers do very well is make simple estimates about the physical nature of the world around us. Part of this skill is (simple) math, and another part is knowing what physical quantities are important.  The most astonishing fact about this skill is that you and I do it every day, we just don’t realize it! Scientists have honed the skill — the place where science comes out is when those unconscious habits are used purposefully!  So how does it work? How is it that you and I are perfectly capable of describing complex physical phenomena, without necessarily resorting to equations we memorized in some long forgotten science class? To demonstrate, let’s consider sticking your hand (or your dog’s head) out the window of your car.

There is some serious physics behind dog ears in the wind, or flying your hand like an airplane. Despite that fact, you have some well defined and excellent intuition about what is important for this problem!

There is some serious physics behind dog ears in the wind, or flying your hand like an airplane. Despite that fact, you have some well defined and excellent intuition about what is important for this problem!

What affects how much force the wind presses on your hand (or your dog’s face) with?  With a little experimentation (something you probably did a lot as a kid, and have committed to memory but forgotten) you find there are three things:

  • how fast the car is driving. If the car is going faster, the force is stronger.
  • how you hold your hand (or, how big your hand is). If you hold your hand palm out, there is a bigger force than if you hold your hand finger tips out. The force is stronger if there is a bigger area being hit by the wind.
  • how thick the air is. Most of us don’t experience thick and thin air too often, at least not that we can tell the difference. But air is a fluid, like water, and water is much thicker than air. When you run your hand through water (a thick fluid) there is a much greater resistance than through air (a thin fluid).

That’s it — those are the three physical quantities that affect how much force you experience when you hold your hand/dog head out the window of your car. And you knew them, at least intuitively, whether you could explain it out loud or not! In a very similar way, the genesis of thinking about extraterrestrial life began with a few intuitive numbers that astronomer Frank Drake wrote down.

Frank Drake, circa 1962.

Frank Drake, circa 1962.

The serious scientific consideration of searching for extraterrestrial intelligences had started with a paper in the scientific journal Nature in 1959, by physicists Giuseppe Cocconi and Philip Morrison. This paper sparked Drake’s interest, leading up to his Project Ozma in 1960, the first human search for radio signals from an extraterrestrial civilization. By 1961, Drake decided to host a small scientific conference at the National Radio Astronomy Observatory, in Green Bank, West Virginia, where the Project Ozma search was carried out. Drake made a list of topics that should be discussed at the conference, dutifully writing down all the things that could affect how many communicative extraterrestrial civilizations there might be. When he was done, he realized he had created a Fermi problem estimate of the number of alien civilizations in the galaxy that we might communicate with — his list of topics were seven numbers that could be multiplied together.

A plaque showing the Drake Equation, hanging on the wall of the conference room where the equation was first presented. [NRAO].

A plaque showing the Drake Equation, hanging on the wall of the conference room where the equation was first presented. [NRAO].

He presented his seven number equation at the conference. It was promptly dubbed “The Drake Equation,” and has been used ever since as a baseline estimate for the kinds of discussions we are having now. A plaque of it now resides on the wall in the conference room where the meeting was held.

So what was Frank’s famous equation? Simply put, it is seven numbers — you multiply those seven numbers together, and you get the number of civilizations in the galaxy that could be communicated with, a number we denote as “N.”  It is written as:

          N = (R* x fp x ne) x FL x Fi x Fc x L

Of those numbers, the first three are matters of observational astronomy that can be verified and estimated from what we see of the Cosmos through our telescopes.  The last four numbers are quantities for which answers certainly exist, but whose values we are still uncertain about; it is playing with plausible values of these four numbers that illustrates our uncertainty about the Cosmos.

The stellar nursery, RCW 108, near the new emergent cluster of young stars (on the left), NGC 6193.

The stellar nursery, RCW 108, near the new emergent cluster of young stars (on the left), NGC 6193.

Let’s look at the first three numbers.  The first is R*, the rate at which stars are born in the galaxy.  The star formation rate is a simple way to start thinking about issues related to planets and life, because the number of planets must necessarily depend on the number of stars in the galaxy — you can’t have planets without parent stars for them to orbit!  For this number, astronomers think R* ~ 6/yr.

Young planetary systems form early on during the growth of a young star. [ESO image]

Young planetary systems form early on during the growth of a young star. [ESO image]

The second is fp, the fraction of stars that develop planetary systems. For a long time, we had no idea what this number was. For most of recorded history, no star other than the Sun was known to shepherd planets.  Then, in 1995 astronomers discovered planets around the star 51 Pegasi, a star very similar to the Sun about 51 lightyears away.  Today, we think planets may very well be common around most stars, and we are regularly discovering planets. As of the time of this writing (23 June 2014) there are 1797 planets known around other stars (visit the exoplanet catalogue here). To be conservative, let’s assume that not every star develops planets (though astronomers are beginning to think that a star without planets may be the exception, not the rule). We’ll take fp = 0.5.

Are there worlds like the Earth, orbiting other suns?

Are there worlds like the Earth, orbiting other suns?

The third number, ne, is the number of planets that could support life in a planetary system. Here, we don’t have a definitive value for this number, but any value we do use has some of our personal prejudices built into it since we have not had the opportunity to study an alien biology! One prejudice we have is that water plays an important role in the chemistry of life. Looking around the Sun, we find Venus, Earth and Mars are all at a distance from the Sun where liquid water could exist under the right conditions (this generic concept, the distance from a star where liquid water can exist on a planetary surface, is called “the habitable zone“). Venus has no liquid water, but Mars may harbor subsurface water. Based on what we know about our own planetary system then, let’s take ne = 2.

These numbers could change as we see more and more of the Cosmos, but probably not much.  So let’s multiply them all together and leave that number alone:

 R* x fp x ne = 6 x 0.5 x 2 = 6

For convenience, we now write the Drake Equation as:

N = (R* x fp x ne) x FL x Fi x Fc x L = 6FL x Fi x Fc x L

Now what about the last four numbers? These are numbers which have more uncertainty, and more speculation in them. They are absolutely numbers of importance when trying to figure out the number of civilizations in the galaxy, we just don’t have good ways to reliably estimate their values.

Prokaryotic cellular organisms were among the first forms of recognizable life to develop on the Earth.

Prokaryotic cellular organisms were among the first forms of recognizable life to develop on the Earth.

The first two are FL, the fraction of planets that develop life, and Fi, the fraction of planets with life that develop intelligent life.  These are complete unknowns; Earth is the only planet we know of with life!  Is it common for life to arise on other worlds? We know from the fossil record on Earth that simple life arose on Earth soon after its formation, in the form of single celled organisms — prokaryotic bacteria (cellular organisms with genetic material free floating in the cell, and not contained in a central nucleus), algae and the like. Given the simplicity of making the organic building blocks of life (chemical combinations called amino acids, used to build proteins), and given that self-replicating molecular systems are not uncommon, the early origin of life suggests that maybe life, in its simplest forms, may arise on planets quite often.  I’m an eternal optimist, so let’s assume FL = 1.  We’re just multiplying numbers together, so I can always go and change this number later.

If life arises, how often does that life become “intelligent?”  This is a harder question to answer, but again we can make an educated guess based on what we see on Earth. It is a fuzzy concept because you have to decide what you mean by “intelligent,” but there are many species on Earth we might consider intelligent — monkeys, dolphins, cats, even humans.  But there are many species that aren’t — oak trees, slime molds, or sea cucumbers.  How common is “intelligence?” Let’s assume Fi = 0.01 — a 1 in 100 chance.

What might life on other worlds look like? How do we define whether or not life is "intelligent?" In the Drake Equation context, we mean life capable of understanding and using science. [Image by David Aguilar]

What might life on other worlds look like? How do we define whether or not life is “intelligent?” In the Drake Equation context, we mean life capable of understanding and using science. [Image by David Aguilar]

The next number is Fc, the fraction of civilizations that can or want to communicate.  Here also, there are several extremes.  Consider humans — since the early 20th Century, we’ve been willy-nilly broadcasting our radio and television signals all over the place, blasting music videos of Eric Clapton and Chuck Berry out into the Cosmos (which I’ve written about before here).  We’ve even sent a few organized messages out, specifically with the intent of communicating with extraterrestrials; these have included radio signals, as well as physical messages.  On the another extreme, one could imagine a completely xenophobic civilization. Maybe they don’t want anyone to know of their existence, lest aliens invade and use them for food.  One could also imagine that a civilization never develops the technology to communicate. If Europe had not emerged from the Middle Ages in the Age of Enlightenment, perhaps we would have never had an Industrial Revolution; we’d all still be peasants, living off mushrooms and earthy root vegetables and not burdened by technology like smartphones or microwave ovens. Certainly the dinosaurs never developed radio communications, despite the intelligence we’d like to associate with marauding bands of velociraptors.  Let’s make a guess at this number (which we can always change) of Fc = 0.01.

We have no idea how easily civilizations rise, or how long they survive. All we can do is look at examples from Earth's history, such as the Indus Valley Civilization, which lasted for several millenia, then utterly vanished.

We have no idea how easily civilizations rise, or how long they survive. All we can do is look at examples from Earth’s history, such as the Indus Valley Civilization, which lasted for several millenia, then utterly vanished.

Now for the last number: the lifetime L of the civilization. There is enormous latitude in possible values for this number because we know absolutely nothing about it, and that is where this discussion gets interesting.  Suppose we take L to be the length of time modern humans have been on the planet.  We don’t know exactly how long that is, but our written history goes back only to about 3000 BCE, so we could take L to be the length of recorded human history, L = 5000 years.  By contrast, the dinosaurs lived on the planet for 170 million years before an asteroid obliterated them, so you could take L = 170 million.  Considering both of these cases we get:

 N = 6 x 1 x 0.01 x 0.01 x 5000 = 3

N = 6 x 1 x 0.01 x 0.01 x 170,000,000 = 102,000

That is quite a range in numbers — there could be more than 100,000 civilizations broadcasting radio; or there could be 3, with a very strong possibility that we are the only ones. The consequences of this calculation could be elating, or very depressing. Whatever the result is, the answer to this question will have profound consequences for our understanding of the Cosmos.

Which brings me back to where we started: dinosaurs and Fermi problems.  In many ways, the Drake equation is a Fermi problem.  What is different from many Fermi problems is that we don’t have a good handle on the last four numbers. But what if we didn’t care about all of these numbers?

What if all I wanted to know was "are there dinosaurs elsewhere?" [Images by S. Larson, at Eccles Dinosaur Park, Ogden, UT]

What if all I wanted to know was “are there dinosaurs elsewhere?” [Images by S. Larson, at Eccles Dinosaur Park, Ogden, UT]

What I love about the Drake equation is that it allows you to answer many related questions, by simply deciding what you think is important.  Let’s take the radical viewpoint that we don’t care about communicative civilizations; instead let’s simply ask how much life (of any sort) might there be in the galaxy?  Is the galaxy teeming with life, or is it a barren wasteland populated only by the descendants of some monkeys on a backwater forgotten world?

Suppose we don’t care about communication.  What if we only wanted to know if there were, say, dinosaurs?  We don’t keep the intelligent number or the communication number. That makes a modified Drake Equation that looks like this:

N = (R* x fp x ne) x FL x L = 6 x FL x L

Let’s keep our optimistic estimate of life developing on every planet possible, FL = 1. I’m interested in dinosaurs, and the dinos lived on the Earth for 170 million years before an asteroid whacked the Earth, erasing them utterly from the Cosmos; so I take L = 170 million years.  Multiplying this all together, I find

N = 6 x 1 x 170,000,000 = 1,020,000,000

There could be 1 BILLION worlds with advanced, but non-intelligent, lifeforms.  If you imagine those lifeforms to be something as complex as a dinosaur, then you might say it this way: there could a BILLION WORLDS with dinosaurs on them in the Milky Way!

That makes the little kid inside of me very happy. :-)

PS: As an even more interesting exercise, suppose we treat L not as the lifetime of a civilization, but simply the length of time for which life exists on a planet, and again ignore the issue of intelligent and technologically able lifeforms.  Taking Earth as the role model, life on Earth arose soon after the planet formed, and while there have been MANY extinction events, life has never been eradicated on Earth, making L ~ 3.5 billion years. If I replace the 170 million years we used with the dinosaurs with 3.5 billion years, we get N ~ 21 BILLION worlds with life.  Go stare at the stars tonight, and think about that for a little while.

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This is the second of two parts; the first part, about Fermi problems, can be read here.

This particular piece was completed while in residence at the Aspen Center for Physics.

Dinosaurs in the Cosmos 1: Enrico & Frank

by Shane L. Larson

shaneRockOne of the most profound consequences of the development of life on Earth is that the Cosmos has produced complex systems with the ability to question their own existence.  We are each of us a collection of atoms that the Universe has stirred together in such a way that we can contemplate the nature of the Cosmos itself. It is remarkable, really.  A rock is also a collection of atoms that the Universe has stirred together, but if a rock contemplates the Cosmos, I have no strong notion of what its rocky thoughts might be.  Humans, on the other hand, have been given a remarkable gift: we can ask questions, and then we can figure out the answers.  This game of questions and answers has a name.  We call it science.

Deep sea anglerfish (Monterey Bay Aquarium, E. Widder/ORCA).

Deep sea anglerfish (Monterey Bay Aquarium, E. Widder/ORCA).

There are many questions that we could use the atomic computing engine between our ears to consider, like: how can we grow enough food to feed 10 billion people? will a catastrophic shift in the San Andreas Fault change the geography of California? can I make a jetpack safe enough for sixth-graders to fly to school? anglerfish — what are they all about? why do some tissues develop cancer in the human body and others don’t?  Where did the Universe come from?

Questions about life and our own existence often dominate conversations in philosophy classrooms, research labs, and late nights around a campfire.  What is the origin of life?  Is there life elsewhere?  Is there intelligent life (on this planet or others)? These are BIG THOUGHTS — heady questions that have been asked for as long as we have been capable of asking them.  Some of them may have answers that can be figured out, and some of them may not.  Let’s think about one of these together — is there life elsewhere?  This is a question that could be answered by simply looking.  Except that looking for life elsewhere is difficult for two reasons: (1) Everywhere else is far away (as I’ve talked about before!) (2) We’re not even sure what life elsewhere might look like! We’re still discovering new life on Earth (like under the Antarctic ice, and even in the deep forests where humans have not tread before).

These points are hindrances to be sure, but that is the nature of this game. Our atomic computing engines are very good at facing down such adversity, and finding ways to answer our questions irrespective of the difficulties we face.  For big questions, it is often useful to make an estimate of what the answer could be before you embark on your quest for knowledge. This helps define the boundaries of your quest.  One of the defining traits of modern scientists is their ability to make quick, quantitative statements about extremely complex questions using only a few pieces of data that almost everyone agrees upon.  These kinds of problems often go by the name “back of the envelope calculations” because they are supposed to be simple enough as to fit on the back of an old bill envelope (though sometimes you may need a manila envelope).  They are often called Fermi Problems, after Enrico Fermi who was famous for this skill.

Enrico Fermi.

Enrico Fermi.

Enrico Fermi was born in 1901 in Rome; he rose to prominence in physics very quickly, completing his laurea (the equivalent of a Ph.D.) at the age of 21. He worked in Italy until 1938, when the Fascist regime passed the leggi razziali (“racial laws”), which threatened his wife Laura, who was Jewish. That same year, he was awarded the Nobel Prize in physics, and after acceptance in Stockholm, took his family to New York, where they applied to become residents of the United States. He famously worked on the world’s first nuclear reactor (“Chicago Pile-1”), and the Manhattan Project.

Fermi was, without a doubt, one of the giants of modern physics. When you first start studying physics, you are regaled with tales of the great minds of physics — their accomplishments as kids, their discoveries early in their careers, and the myriad ways they have transformed the way we view the world. As a young and aspiring physicist, it is incredibly intimidating and almost crippling; fortunately, I had many outstanding mentors. Each of them played a role in calming my doubts and fears; each of them helped me look at great scientists like Fermi and learn something about how to do science from their examples.

One of those things is Fermi problems. Fermi was famous for his ability to quickly estimate the answers to complicated problems. When his answers were checked against precise calculations, his results were amazingly close to the “real” answer! One of the most famous examples was Fermi’s estimate of the strength of the atomic explosion at the Trinity test. Fermi dropped handfuls of paper from a height of 6 feet before, during, and after the blast wave washed over the observation post. Based on the distance the paper spread as it fell, Fermi estimated the explosion to be the equivalent of 10,000 tons of TNT; the strength reported after the test had been fully analyzed was 20,000 tons of TNT.

The Trinity fireball, 16 milliseconds after the first human-made atomic explosion. Fermi was the first person to estimate the energy in the explosion.

The Trinity fireball, 16 milliseconds after the first human-made atomic explosion. Fermi was the first person to estimate the energy in the explosion.

Calculating the yield of an atomic bomb is definitely a big physics problem; it’s not the kind of thing most of us have to do in our lives. But all of us do Fermi problems every day. Every one of us. Things like: you’re going to watch curling with 4 other friends; how many pizzas should you order? how many bikes can fit in your garage with everything else? what time do you need to leave home to make it to work on time?

The classic problem that Fermi used to introduce this estimation concept is “how many piano tuners are there in your city?”  With Google, or the yellow pages (if you are a caveman), this question could easily be answered definitively.  But it can also be calculated by using some things you know or can estimate from your own personal experience.  Let’s try this together — I’ll do it for Logan, Utah, and you do it for wherever you happen to be right now.

The number of piano tuners in a given city is not general knowledge that most of us carry around. Despite its seemingly esoteric nature, it is a number that can easily be estimate to high accuracy using Fermi estimation methods!

The number of piano tuners in a given city is not general knowledge that most of us carry around. Despite its seemingly esoteric nature, it is a number that can easily be estimate to high accuracy using Fermi estimation methods!

The method is to ask a series of questions upon which the answer must depend, and that you may know the answers to. Questions like: how many people live in your city? how many households are there? how many households have pianos? How often do they tune pianos?  if you are a piano tuner, how many days a year do you work? how many tunings can you do in one day?  The answers to these questions don’t need to be 100% correct, nor do all of them have to be the same as someone else would guess.  All in all, the guessing and errors average out to give about the same answer for everyone.  This is a beautiful and elegant method of trying to understand the nature of the world by relying on the fact that your knowledge sometimes does better and sometimes does worse than reality, but overall combines to give something close to the truth.  In the figure below, I show the calculation for Logan, Utah. If I check the answer in my phone book, I find that I was pretty close!

Estimating the number of piano tuners in a city. Starting at the top, there is a simple series of numbers you need to estimate, most of which you probably know or can guess.

Estimating the number of piano tuners in a city. Starting at the top, there is a simple series of numbers you need to estimate, most of which you probably know or can guess. (Click to embiggenate)

The key methods to bear in mind in your quest to become a good Fermi problem estimator are:

  • Most problems seem unanswerable when posited, but usually can be broken down into simpler bits which you do know the answer to.
  • Rely on numbers you know the value of, or can estimate a reasonable value for.
  • Don’t worry about being precise!
  • Round relentlessly (“7 is about 10”)
  • Combine numbers sloppily (“15 x 6 is about 100”)
  • Use everyday experience as averages (“a human masses about 70 kg”)

This is a powerful and robust technique for answering questions. More to the point, you do this every day when you figure out how many cupcakes to make when the cousins are visiting, or when you buy rope to make a new tire swing, or when decide how long before the football game to start mowing the lawn to make sure you are done on time.  You could do it for all kinds of other things that may be important in your life and business, like: estimating how much pizza is consumed on a nearby college campus every day, or how many ball point pens are sold in your city each year, or how many car crashes there are in town each month.

Frank Drake.

Frank Drake.

For our purposes here, we are going to use this powerful technique to figure out how much life there might be elsewhere in the Cosmos.  One of the first people to think about this was astronomer Frank Drake. Drake was a radio astronomer, and made important discoveries regarding the nature of Jupiter’s magnetosphere, and in pulsar astrophysics. In the 1960’s he also began to think about how radio astronomy could be used to search for transmissions from extraterrestrial civilizations, initiating Project Ozma in 1960 to search for possibly intelligent transmissions from the nearby Sun-like stars, Tau Ceti and Epsilon Eridani.

When thinking about life in the Cosmos, and whether we are alone or members of a vast chorus of civilizations spanning the galaxy, Drake asked a specific (but plausibly unanswerable) question: how many civilizations might exist in the galaxy that we could communicate with?  The Fermi problem solution to this question is known as The Drake Equation.  We’ll examine the Drake equation and its implications next time.

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This is the first of two parts. The second part can be read here.

This particular piece was completed while in residence at the Aspen Center for Physics.

Chasing Starlight 2: Recipes for Discovery

by Shane L. Larson

When you and I were in elementary school, we were taught “The Scientific Method” (which we’ve talked about before) and told that it was how scientists do science. In a similar vein, I was once told that cooking is just stirring stuff together and heating it up. I’m sure that is how chefs do cheffing. But as any chef knows, the art of cooking is not so easily distilled to a pithy soundbite easily told to 4th graders.  So it is with science. Astronomy in particular often defies the rigid, black and white portrayal of science given by The Scientific Method.

Cooking is like science. (L) Sometimes it works perfectly! (R) Sometimes disasters happen.

Cooking is like science. (L) Sometimes it works perfectly! (R) Sometimes disasters happen.

For the most part, astronomy is not a bench science. Astronomers do not have controlled experiments that can be repeated and studied. Instead, astronomy is a spectator science — all we can do is look at the Cosmos, and see what it has thrown up on the vast canvas we call the sky. Some things persist, and can be looked at over and over and over again. Other things happen only rarely, and if we aren’t looking or aren’t prepared to look closely, the information washes across our small planet then vanishes forever.  Because of the unpredictability of Nature and because of the paucity of many astronomical events, one of the most important aspects of astronomy is squeezing every possible bit of information out of the signals we receive, so that we may best understand what created those signals. Our primary partner in this endeavour has for most of the history of modern science, been light.

This is what typical "bench science" looks like in physics: experiments that have been carefully designed to probe a physical effect, and are run over and over again until we understand the outcome. Astronomy is not usually like this.

This is what typical “bench science” looks like in physics: experiments that have been carefully designed to probe a physical effect, and are run over and over again until we understand the outcome. Astronomy is not usually like this.

It’s not surprising that light provides one of our main connections to the Cosmos. We are creatures that have evolved to depend on our sight as one of our primary senses. In addition, light is readily generated by atoms, which make up all the stuff that the stars and planets and interstellar gas are made of. There are both obvious and subtle ways that information is encoded in and carried by light, most of which are useful in astronomy.

The constellation of Orion is covered by a large cloud known as the Orion Molecular Complex. The large circular structure is known as "Barnard's Loop"; if you look closely in the center left, you'll see the Flame Nebula and the Horsehead Nebula, both part of the complex. The red star at upper left is Betelgeuse; the right star at lower right is Rigel. In the middle of Barnard's Loop, below the three belt stars, you can see the famous Orion Nebula.

The constellation of Orion is covered by a large cloud known as the Orion Molecular Complex. The large circular structure is known as “Barnard’s Loop”; if you look closely in the center left, you’ll see the Flame Nebula and the Horsehead Nebula, both part of the complex. The red star at upper left is Betelgeuse; the right star at lower right is Rigel. In the middle of Barnard’s Loop, below the three belt stars, you can see the famous Orion Nebula.

The foremost and most obvious role light plays in astronomy is that of signal beacon — light is shed from an object, and makes its way across the Cosmos to say to the astronomers of Earth, “I’m here! Look at me!” In the case of stars, that light is rather pointlike and fixed in space; often it is steady and unchanging, at least over timescales that are readily noticed. Many stars have color.  Consider the constellation of Orion, one of the most recognized constellations the world over.  This deep sky image shows both unincorporated gas as well as stars. The three belt stars lie in an almost perfect line, and are roughly the same brightness, shining with a blue-white light. They are called (from east to west): Alnitak, Alnilan, and Mintaka. Orion’s eastern shoulder is a baleful red star known as Betelgeuse; the right knee is a brilliant blue star called Rigel. These names have been passed to us down through the ages, from a time before we knew what the stars were, what they were made of, or what their colors meant. But today, we know far more than those who first named these beacons in the sky. Both Rigel and Betelgeuse are supergiant stars, and both are about 10-20 times the mass of our Sun, but their similarities end there. Their colors were different — one blue, one red — and it was later discovered their sizes were different: Betelgeuse is about 1000x the diameter of the Sun, whereas Rigel is about 74x the diameter of the Sun.

We’ve talked before about how the fingerprints of atoms impress themselves in light (read that post here) by making spectral lines, and how the color of a star is a direct measure of its temperature (read that post here). Together these two properties allow us to say much about the stars without ever visiting them up close — this is one of the most important discoveries about light that has ever been made. In the case of Rigel and Betelgeuse, it has allowed us to know that Rigel is blue because it is far hotter than red Betelgeuse. Why the color discrepancy? Because Rigel is young, and Betelgeuse is elderly.

If all we ever learned about the Cosmos was the temperature and composition of the stars, we would consider those discoveries triumphs of the human intellect.  But is there more that light can tell us? Of course there is! But all observations of Nature require us to be careful observers. Light is a tempestuous partner in the game of astronomy. Like messages on the internet (how science propagates on the Internet was famously described here), the news carried by light might be very interesting, but is subject to distortion and confusion because during the long journey to Earth, light is easily influenced. Any matter the light encounters during transit has the potential to change the light, to corrupt and distort the news it carries from the distant corners of the Cosmos.  There are many such effects that distort the stories told by light, but let me tell you two of them.

One example is called gravitational lensing. Einstein taught us that mass and energy are equivalent. What does that mean? There are many consequences, but the important thing for this conversation is this: energy feels the pull of gravity, just like mass does. That means light, a form of energy, should feel the pull of gravity as it is speeding along through the Universe. This is exactly what would happen to you or me. Suppose we are drifting along in our starship, and pass a large object to our left, say a galaxy. The gravity from the galaxy would tug on us, and pull our course inexorably to the left.  Once we were past the galaxy, the gravitational pull would fade with distance, and we’d continue to travel on a straight line, but it would be a different straight line than the one we started on!

(L) If a starship (that looks mysteriously like a Lego version of NdGT's Ship of the Imagination) were coasting on a straight line, a massive object will deflect its course. (R) Objects with strong gravity can also deflect the course of light; the effect on astronomy is to project the location of the source somewhere else on the sky from the perspective of observers on Earth.

(L) If a starship (that looks mysteriously like a Lego version of NdGT’s Ship of the Imagination) were coasting on a straight line, a massive object will deflect its course. (R) Objects with strong gravity can also deflect the course of light; the effect on astronomy is to project the location of the source somewhere else on the sky from the perspective of observers on Earth.

This is exactly the same thing that happens to light! Suppose light is speeding through the Cosmos, and passes a large object on its right, say a galaxy. The gravity from the galaxy tugs on the light, and pulls its course inexorably to the right. Once it is past the galaxy, the gravitational pull fades with distance, and the light continues to travel along a straight line, though it is now travelling a different direction than it started out in — this has important consequences for astronomy? Why? Because as astronomers, we often make the default assumption that when I see a light ray from the Cosmos, it came directly from its origin — we like to think that if we simply extend the direction all the way back through the Universe, we can determine where the light came from! But the deflection of light by gravity has made that extended line point somewhere else!

There is a remarkable consequence of this gravitational deflection that was predicted by Russian physicist Orest Chwolson in 1924: an “Einstein Ring.” Light from a distant source doesn’t just fly to one side of a deflecting galaxy — it flies on every side. If you are sitting in the right place, the gravitating galaxy that is bending the light will act like a cosmic magnifying glass, making a ring out of light that goes around every side of the galaxy!

The basic geometry of an Einstein Ring. Light that goes to the left is deflected to Earth, as is light that goes over the top, under the bottom, and to the right. From the perspective of an observer on Earth, they see bits of light in all of those directions, which appears as a ring on the sky.

The basic geometry of an Einstein Ring. Light that goes to the left is deflected to Earth, as is light that goes over the top, under the bottom, and to the right. From the perspective of an observer on Earth, they see bits of light in all of those directions, which appears as a ring on the sky.

In 1936, Einstein famously panned the idea of seeing a ring, thinking the chances of a perfect alignment would be small, and even if an alignment occurred, our telescopes would not be up to the task of seeing the small ring. But as is often the case, technology outstrips our most pessimistic predictions. In 1988 radio astronomers discovered the first Einstein Ring; many more have been found, like the one shown below, LRG 3-757.

A nearly complete Einstein Ring observed by the Hubble Space Telescope, a galaxy known as LRG 3-757 (read the Hubble page here).

A nearly complete Einstein Ring observed by the Hubble Space Telescope, a galaxy known as LRG 3-757 (read the Hubble page here).

For our second tale, remember where this conversation started last time — with Feynman diagrams. A Feynman diagram shows us how light and matter (specifically electrons and positrons) interact. One of the things we learned by drawing Feynman diagrams is that you can predict new effects, then go look for them. A classic example is called “pair production” where two photons collide and create an electron and a positron. This effect was first seen in  the lab in 1997 (read a NY Times article about the result here), and scientists are currently working on the design of a “photon collider” that will make pair creation a regular event in the laboratory.

But this begs an obvious question: could we use this effect to do astronomy? Is there some way we could observe this effect, and use it to answer some deep question about the Cosmos?  The answer of course is yes!

Consider a blazar — an active galaxy, billions of lightyears away. A blazar is characterized by the emission of extremely high energy light, created by the jet of a super-massive black hole that is pointing almost directly at the Earth. The light from blazars must travel through most of the known Universe to reach us here on Earth. During that time, it will pass through ever increasing amounts of starlight generated by the generations of stars that have come into being, evolved, and perished over the ages of time. Gamma ray astronomers call this “the Cosmic Fog” because it acts just like real fog on Earth.  If you shine your flashlight out into a fog bank, you can only see it from a certain distance away, and that distance depends on how thick the fog is.  Starlight is the same way — the more starlight the gamma rays have to pass through, the more likely it is that they will encounter a photon of light from a star.  And what can happen when two photons meet? They produce an electron-positron pair, and the photons vanish (their energy was used to make the particles)! The net result is that when viewed from Earth, the blazars don’t have as much high-energy gamma ray light as we expected — the gamma rays are absorbed by the Cosmic Fog of Starlight.

A map of the "extra-galactic background light" that comes from all the stars in the Universe, as seen by the Fermi Gamma-ray Observatory. The dots are 150 blazars whose gamma ray deficiency was measured.

A map of the “extra-galactic background light” that comes from all the stars in the Universe, as seen by the Fermi Gamma-ray Observatory. The dots are 150 blazars whose gamma ray deficiency was measured.

The most important thing about this is: scientists can measure how much of the gamma ray light has gone missing. That tells us how much starlight was encountered. Since we know something about how much starlight a star puts out (based on its temperature and size), this allows us to estimate the number of stars in the Universe whether we can see them directly in the telescope or not!

That is an astounding feat, and one that should impress upon you one thing: that we are an exceedingly clever species, and with science as a tool, we can figure out just about anything.

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This post is the second and final part in a sequence talking about clever ways to use starlight to understand the Cosmos. The first part was here.

Chasing Starlight 1: Sketching with Feynman

by Shane L. Larson

Some science concepts are so fundamental that they’ve become summarized by pithy soundbites that have invaded mainstream culture. For instance, have you ever wondered where the phrase “opposites attract” comes from? It’s a substantive scientific concept that has become so ingrained in our culture as to become a meme: from Lady and the Tramp, to Claire and John in The Breakfast Club, to dogs and cats being friends.

"Opposites attract" is such a common phrase in our culture that it is meme unto itself in the movies. (L) Disney's 1955 classic, "Lady & the Tramp". (R) Claire and John in the 1985 classic, "The Breakfast Club."

“Opposites attract” is such a common phrase in our culture that it is meme unto itself in the movies. (L) Disney’s 1955 classic, “Lady & the Tramp“. (R) Claire and John in the 1985 classic, “The Breakfast Club.”

It comes from physics, from an area of study called “electromagnetism.” Our understanding of how electromagnetism works is built around the study of fundamental particles called “charges.” Charges come in two flavors that we call positive and negative. The most common charge you and I encounter every day is called the electron, which is negative in flavor.

The phrase “opposites attract” is about charges — opposite charges are attracted to one another (and like charges are repulsed by other like charges). So who is the electron attracted to? In principle it is attracted to any positive charge it encounters. The most common positive charge that we all encounter in our everyday lives is called the proton. From our perspective both the electron and the proton are tiny particles, but the proton is about 1800 times more massive than the electron; that’s about the same as you compared to a space shuttle. So what happens when they see each other? They experience an attractive force, because they are charged.  The proton doesn’t move much, because it is so massive, so the electron falls toward the proton. Pulled inexorably together, they bind together and form a hydrogen atom — a proton, with an attendant electron captured in the cloud of the proton’s influence.

Expected forces between electrical charges. (Top) Two positive charges repel each other. (Mid) Two negative charges repel each other. (Bottom) Opposite charges attract.

Expected forces between electrical charges. (Top) Two positive charges repel each other. (Mid) Two negative charges repel each other. (Bottom) Opposite charges attract.

There are other positive particles in Nature, not as plentiful as the proton. One of those is called the positron. The positron has excactly the same properties as the electron, except for the flavor of its charge: the positron is positive. Otherwise it has the same mass, the same size, the same behaviour of the electron. The positron is the anti-matter twin of the electron. What happens when a positron and an electron approach each other? Just as with the proton, there is an attractive force, but since they are the same mass, both the electron and positron fall toward one another. They are so small they very likely don’t hit one another, but their close encounter deflects them onto new paths; physicists call this “scattering.”

If they do hit each other, they completely destroy each other, converting from particles into light. This process, where matter-antimatter particle pairs destroy one another is called “annihilation.”  If two electrons approach each other, they feel a repulsive force which deflects this onto new paths; this is also scattering.

If identical matter and anti-matter particles collide (like an electron and a positron) they annihilate, creating light with an energy equal to the mass energy of the two particles.

If identical matter and anti-matter particles collide (like an electron and a positron) they annihilate, creating light with an energy equal to the mass energy of the two particles.

So how do physicists describe what is going on between charges? As with all things in science, there are many ways of describing and talking about any physical phenomena. Just using words, telling a story that says what happens, maybe with a few pictures is a time honored way to introduce complex ideas to anyone. This is basically what we are doing here in this blog.

But eventually, people begin to start thinking about what all these awesome facts of Nature might be useful for. Charges repel and attract — can I do anything with that? Can I harness this physical effect and either make something that is entertaining, might make me money, or better yet, can be used to improve our lives? The answer is yes to all of these questions! Of course you can. But in order to harness Nature, in order to make something you need to have predictive power. What does that mean? It means if you know everything about some physical system — a pool of water, a pile of sludge, a rock on a table, or a cloud of electroms — then you can predict what will happen to that system in the future as it interacts with the world around it.

You have learned the predictive power of science at a very young age, and use it every day. What happens if you set your hot cup of coffee on the counter and leave it there for 3 hours? It cools off — you know that. What happens if you throw a baseball as hard as you can straight up in the air? It falls back to the ground — you know that. What happens if you hit an egg with a hammer? It makes a big mess (which you should clean up before your Mom/wife finds it) — you know that.

Example scenarios where your everyday experience has taught you the Laws of Physics. You understand these situations well enough that you have predictive power.

Example scenarios where your everyday experience has taught you the Laws of Physics. You understand these situations well enough that you have predictive power.

But sometimes you want to know something precise.  How long will it take my coffee to be about 105 degrees F, the perfect swigging temperature? What is the maximum height my baseball will reach, and is it enough to knock my keys out of that tree (don’t ask)?  Would that egg have still gotten on the ceiling if we had vaulted ceilings?  When you need precise predictive power, scientists turn to mathematics to express the “Laws of Nature.”

The mathematical expression of the force between two charges was first written down by a French physicist known as Charles-Augustin de Coulomb; appropriately enough that law is now called the “Coulomb Law.” His law showed something that had been empirically measured, namely that the force between two charges depended on two things:

  • The sizes of the charges involved (physicists use the symbol “q” to mean charge in an equation)
  • The force is weaker with greater distance, and in fact grows weaker with the square of the distance — if you double the distance, the force is 1/4 as large; if you triple the distance, the force is 1/9 as large (physicists use the symbol “r” to mean distance in an equation)
Inverse square laws from Nature; note the similarities. (L) Universal Law of Gravitation, describing force between two masses. (R) Coulomb Law, describing force between two charges.

Inverse square laws from Nature; note the similarities. (L) Universal Law of Gravitation, describing force between two masses. (R) Coulomb Law, describing force between two charges.

One of the most remarkable realizations that one has when seeing the Coulomb law for the first time is that it has exactly the same form as another great law of physics, Newton’s Universal Law of Gravitation. This is a hint that there are deep mysteries afoot, that the awesome machinery of Nature has a secret and is giving us a casual, teasing glimpse under the hood.  Physicists call this the “inverse square law.”

These kinds of teasing glimpses make scientists dig deeper, to try and better understand how the Cosmos is put together. Much of the middle of the Twentieth Century was spent trying to understand the structure and interactions of matter on the tiniest scales. The outcome of those herculean efforts of thought and experiment was a list of the particles that make up all matter and the rules for how they interact together — it is called “The Standard Model,” and it is one of the most successful descriptions of Nature humans have ever discovered. It is a triumph of our intellect, a testament to our ingenuity and diligence.

Richard Feynman.

Richard Feynman.

As you might imagine, the mathematical structure of the Standard Model is intimidating to witness, even if you’ve been trained in the field! But luckily for us, there is a beautiful and ingenious shorthand that summarizes the mathematical rules of how particles interact with one another, called Feynman Diagrams. Developed by the indefatigable Richard Feynman, the diagrams can quickly be sketched, and completely encode the content of the mathematical equations. The beauty of the method is that when you draw the diagrams, you can imagine the particles as if they were little ping-pong balls bouncing off of one another.

The basic vertex used to build Feynman diagrams.

The basic vertex used to build Feynman diagrams.

Let’s restrict our attention to just three particles: electrons, positrons, and photons.  In a diagram, an electron is represented by an arrow that generally points toward the top of your screen, a positron is represented by an arrow that generally points toward the bottom of your screen, and a photon is represented by a squiggly line.  The fundamental building block of a Feynman diagram is called a vertex. Interactions are represented by linking vertices together. The rules for drawing and interpreting basic Feynman Diagrams are as follows:

  • An interaction between two particles must have an even number of vertices
  • You can rotate and move the lines going into a vertex any way you like, so long as there is one arrow pointing into the vertex and one pointing out of the vertex.
  • When linking two vertices together, photons connect to photons, and arrows (electrons and positrons) cannot point against each other.

How do you draw and interpret Feynman Diagrams? Let’s consider a specific example to talk about that. From the Coulomb law, we know that opposite charges repel each other. That means if two electrons fly toward one other, the force between them should grow until it pushes them apart.  This is where the physical interpretation of the Feynman Diagrams are useful — I know initially two electrons should be rushing toward each other, they will interact somehow (this will be represented by the vertices), then at the end they should be rushing apart.  If I draw a Feynman Diagram representing this, and cover up all the bits with the vertices with a sticky note, what I should see is two arrows pointing inward at the bottom of my diagram, and two arrows pointing outward at the top of the diagram.

(L) The physical motion of two electrons that scatter off of each other. They approach, but feel a force pushing them apart, so eventually they change the direction of travel. (R) A Feynman diagram for this process looks very similar. Under the sticky note is "in the fog" --- necessary bits that construct the proper mathematical description of the scattering, but is not observable in the same sense as the particles not covered by the sticky note.

(L) The physical motion of two electrons that scatter off of each other. They approach, but feel a force pushing them apart, so eventually they change the direction of travel. (R) A Feynman diagram for this process looks very similar. Under the sticky note is “in the fog” — necessary bits that construct the proper mathematical description of the scattering, but is not observable in the same sense as the particles not covered by the sticky note.

So what’s under the sticky note? Under the sticky note is any possible way you can imagine to link the lines together with vertices.  The most fundamental interaction is two vertices, with a photon linking them.  When we see this diagram, we “read it” and say “the two electrons interacted by exchanging a virtual photon.”  In the language of the Coulomb law, we said there was an electromagnetic force between the electrons. In the language of the Standard Model and the Feynman Diagrams we say that the “photon is the electromagnetic force carrier.”

The most basic Feynman diagram showing electron scattering, with two vertices.

The most basic Feynman diagram showing electron scattering, with two vertices.

So does that mean there are photons flying all over the place each time electric charges repulse or attract each other?  Not exactly. We say that the vertices are “in the quantum fog” — the exchange between vertices is a completely unobservable aspect of the interaction. That may seem a bit disconcerting, but I usually take comfort in the fact that the diagrams are not physical representations of the interaction, but are rather a clever code for some very involved mathematics.

An example alternative Feynman diagram for electron scattering, with 4 vertices.

An example alternative Feynman diagram for electron scattering, with 4 vertices.

If you spend some time tinkering with the diagrams, you may notice that there is more than one way of representing the interaction — there is more than one way to draw the Feynman diagram! That is part of the mathematics; each diagram has a slightly different mathematical interpretation, and the full calculation of the outcome of an interaction depends on knowing all the possible different ways of drawing the diagrams! The great utility of the Feynman Diagram method is your job as a physicist is reduced to drawing diagrams. Instead of writing all the algebra out longhand, you just have to be creative and imaginative enough to figure out what every single diagram is.

So why have I told you all of this? Because one of the great truths of science is that when you write down the Laws of Nature, fiddling with those laws will often lead to the discovery of something previously unknown.  Consider the following diagram.

The simplest Feynman diagram representing "Delbruck scattering" --- the scattering of a photon off another photon.

The simplest Feynman diagram representing “Delbruck scattering” — the scattering of a photon off another photon.

If I put a sticky note over the vertices, what do I see? This is two photons approaching one another and scattering off of one another! This is called “Delbruck scattering.”

Here’s another interesting diagram where two photons come together and utterly vanish.

(L) Pair production --- two photons collide and vanish, and in their place leave behind a positron and an electron. (R) Two Feynman diagrams showing this process.

(L) Pair production — two photons collide and vanish, and in their place leave behind a positron and an electron. (R) Two Feynman diagrams showing this process.

If I put a sticky note over the vertices, what do I see? This is two photons approaching one another and vanishing! In their stead, two particles — one electron and one positron — spontaneously appear! This is called “pair creation” and is important in many different instances in physics and astronomy.

Generically, both of these diagrams are light scattering off of light.  It seems to be far outside of our normal everyday experience. If you and I shine flashlights at each other, the beams fly right through each other. But the laws of physics allow the diagrams I’ve drawn above. Amazingly, if we go into our labs and particle accelerators and look for these effects, we see them! This kind of validation gives us confidence that our understanding of how the world works is on the right track. It’s also exhilarating — it makes me a little giddy some days when it strikes me that we were able to figure this out. Such is the awesome power afforded to us by brains.

So light scatters off of light.  Of what possible use could that be? Well, as it turns out, it can be used to discover ancient starlight. This will be the subject of our next chat.

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This post is the first part in a sequence talking about clever ways to use starlight to understand the Cosmos. The second part is here.

Pluto’s Day of Reckoning

by Shane L. Larson

NB: I originally wrote this post to outline my TEDx NorthwesternU talk in 2014.  Watch the video here.  Please enjoy both!

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As an astronomer, I get to talk to people about all kinds of things. Some people want to talk about black holes, or about asteroids killing the dinosaurs, or about life on other worlds. But the one subject everyone always wants to talk about is Pluto!

How many people feel about Pluto's demotion. [Image by Mathias Pedersen, used with permission]

How many people feel about Pluto’s demotion. [Image by Mathias Pedersen, used with permission]

That’s because people have this uneasy feeling that Pluto has been treated unfairly by scientists.  We have this queasy feeling down in the pit of our stomachs because as children we were told Pluto was a planet, and now scientists have flip-flopped and are telling us it isn’t!  And we feel bad for Pluto!

The truth is astronomers are engaged in a very serious (and good-natured) debate about what it means to be a planet. It’s the kind of debate scientists have every day about all kinds of things that don’t catch the attention of the public or garner headlines. The difference is people seem to care about Pluto!

The reason you and I are even talking about this is because in 2006 the International Astronomical Union reclassified Pluto as a “dwarf planet.” To use more common vernacular, Pluto was DEMOTED.  Now that’s a word that carries a certain amount of emotional baggage with it!  That visceral response we have to describing Pluto’s demise was captured by the American Dialect Society who made their 2006 word of the year,

plutoed (PLUE-toed): to demote or devalue
something, as happened to the former
planet Pluto...

The changing of Pluto’s status clearly struck a chord with people.  But why?

Suppose I show you a picture of a “Pluto protest.”  This image doesn’t phase any of you! That’s because in the back of everyone’s mind, there is a little voice telling you which side of the street you’d be standing on and what your sign would say!  I’m right there with you. I’ve picked my side of the street, and I know exactly what my sign would say!

(L) A "Pluto Protest" staged in Seattle [Image: Wikimedia Commons] (R) My own sign, expressing solidarity with Pluto; my daughter is more vehement in her support.

(L) A “Pluto Protest” staged in Seattle [Image: Wikimedia Commons] (R) My own sign, expressing solidarity with Pluto; my daughter is more vehement in her support.

So the world has divided itself into two camps — the Pluto Apologists, and the Demoters (they sound like rock-n-roll band names, don’t they?).  Pluto inspires deep emotions in everyone, whether they are scientists or members of the public.  Even today, nearly 8 years after the reclassification, discussion of Pluto sparks vehement debate, but the debate is nuanced and subtle, even among the cognoscenti.  In one corner, there are people who are avid Pluto-philes and just would like Pluto to be back in the club. In another corner, there are people who think Pluto is clearly just a small rock, like many other small rocks, and not classifiable by the word “planet.” And in another corner there is a third group of people who think we really don’t know what the hell we mean when we say “planet,” and that our understanding of the Cosmos and what it means to be a “planet” must constantly evolve. This is the group I stand with.

The story of Pluto’s demotion and the ensuing arguments and exasperations about its status are an excellent backdrop to understand how science works and responds to ever changing knowledge about the Cosmos. The fact that people exhibit deep emotions about the entire affair is simply a very human dimension to the story, a manifestation of the fact that we long to be deeply connected to the Cosmos, that we want our perceptions and thoughts about the Universe to matter.  That emotional connection is a foothold for us to explore this tale.

For kids, and short people like me, Pluto always had a certain allure because it was the smallest planet — the runt of the Solar system.  I’ll freely admit that my love affair with Pluto started at a very young age, when I was even shorter than I am now! I got started looking at the sky because my Mom was a birder.  She had an old beat up spotting scope that she would use to watch birds at our house, but at night I would grab it and look at the sky.

(L) My mom's spotting scope, that I first used to look at the sky. (C) My friend Hazel and her telescope at a star party, circa 2000. (R) My own big scope, named Cosmos Mariner.

(L) My mom’s spotting scope, that I first used to look at the sky. (C) My friend Hazel and her telescope at a star party, circa 2000. (R) My own big scope, named Cosmos Mariner.

I saw the Moon and all the big planets — Mars and Jupiter and Saturn — but I always wanted to see Pluto. But it was impossible; that little spotting scope, for all the wonders it showed me, was just too small.  It took many years, but eventually a 70-year old friend of mine named Hazel showed me Pluto through her 20” telescope. I got to see Pluto with my own eyes, and it only validated all that long held wonder and ardor I held for this small world. One thing led to another and eventually I built my own big telescope and now I show other people Pluto!  My wife figures there are worse things I could be doing with my life.

But not all of us have had the chance to see Pluto with our own eyes. Even so, people all over the world love Pluto just the same.  Part of the reason is that people recognize the story of Pluto’s discovery as a kind of modern fairy tale that could have happened to any of us.

(L) Clyde Tombaugh and his homebuilt reflecting telescope. (R) The Lowell Observatory 13-inch astrograph, used to discover Pluto. [Images: Wikimedia Commons]

(L) Clyde Tombaugh and his homebuilt reflecting telescope. (R) The Lowell Observatory 13-inch astrograph, used to discover Pluto. [Images: Wikimedia Commons]

Pluto was discovered by a farmboy from Kansas named Clyde Tombaugh, who couldn’t go to college because hail had destroyed his family’s crops. But he loved astronomy, and in his early 20’s he built a telescope, and started sketching Mars and Jupiter from his family’s farm in Kansas.  In 1929 he sent some of those sketches to Lowell Observatory in Flagstaff, Arizona, and they were so impressed they hired him to come run one of their new photographic telescopes.  The job was only supposed to only last for three months, but he ended up working at Lowell for more than 14 years.

During his first year he was taking pictures of the night sky looking for Planet X, a proposed new world somewhere out in the dark beyond Neptune.  Very soon after he arrived, on the nights of January 23 and January 29, 1930, he captured two images that would change the world, though he didn’t know it at the time. Astronomers lead rugged lives — we’re in the observatory all the time, we stay up late, we sleep very little, and sometimes we don’t get to our data right away.  It took Clyde almost a month to go back to those images, but on February 18, he was looking at the images on a blink comparator, a machine that rapidly flashes back and forth between two astrophotos while you are looking at them. Stars stay put because they appear in both images, but things that move become very obvious.

The Pluto discovery images, blinked back and forth as they might appear in a blink comparator.   Click to animate. [Animation: S. Larson, from Lowell Observatory archive images]

The Pluto discovery images, blinked back and forth as they might appear in a blink comparator. Click to animate. [Animation: S. Larson, from Lowell Observatory archive images]

That night Clyde saw the tell-tale dot jumping back and forth across the center of these images and knew he had found a new world.  The discovery was announced on March 13, 1930 and made headlines around the world.

Headline announcing Pluto's discovery on 14 March 1930; the world had yet to be named. [Image: Chicago Tribune]

Headline announcing Pluto’s discovery on 14 March 1930; the world had yet to be named. [Image: Chicago Tribune]

At the time, the Lowell Observatory had the right to name the new planet and they were flooded by suggestions.  I’m sure if Stephen Colbert had been alive then, the Colbert Nation would have petitioned to name it after him.  But in the end, the name Pluto was suggested by an 11-year old English girl named Venetia Burney.

Venetia Burney [Image: Wikimedia Commons]

Venetia Burney [Image: Wikimedia Commons]

She was very interested in classical mythology, and suggested the name Pluto to her grandfather (Falconer Madan) who was a former librarian at Oxford. The name was passed through his professional colleagues until it arrived at Lowell, and on March 24, 1930 every employee at Lowell Observatory voted by secret ballot on the name for Tombaugh’s new world.  “Pluto” received every single vote, and the name was fixed!

The name was almost instantly embraced in our popular culture. Walt Disney is famously rumored to have named Mickey Mouse’s dog companion Pluto after the planet, and in 1941 Glenn Seaborg continued a tradition of naming new elements after planets when he named a newly discovered chemical element “plutonium.”

Today, it’s almost exactly 84 years since the discovery of Pluto. What do we know about it? And why did it take 76 years for us to start arguing about whether it is a planet or not?

solarSystemZones

We can think of the Solar System in zones.  Down near the Sun, the worlds are small and rocky.  In this zone, we call the planets “terrestrial,” the domain of Mercury, Venus, Earth and Mars.   A bit farther out, the planets get large, notable for their vast gaseous atmospheres and lack of solid surfaces. In this zone, we call the planets “jovian,” — Jupiter, Saturn, Uranus and Neptune.  Beyond Neptune is the Third Zone.  This is out where Pluto lives, and all the worlds out here are small, made up mostly of rock and ice, and are on weird orbits that don’t always line up with the solar system’s inner two zones.  These worldlets are the detritus, the flotsam and jetsam, left over from the formation of the Solar System.

We’ve always known that Pluto lived out here in this Third Zone, and that it was a bit weird. It lives on a strange orbit that is highly tilted and sometimes is closer to the Sun than Neptune.  In addition, it is smaller than other planets — it’s smaller even than the Earth’s Moon.  One of the arguments that’s made for Pluto’s reclassification is that it is more like worlds in the Third Zone, than is it is like the terrestrial or Jovian worlds. Those of us who object to Pluto’s reclassification don’t disagree with this.  What we don’t like, is the definition that is being used to define “planet.”

Here is the current definition:

  1. A planet must orbit the Sun
  2. A planet must have enough gravity to be round
  3. A planet must have cleared its orbit

Pluto fails only on this last point — it lives in a part of the solar system where there are lots of things floating around and there just hasn’t been enough time in all the 4.5 billion year history of the solar system to knock things out of the way.  So why should this definition bother anyone?

The difficulty with this definition is the first and the last points — they are dynamical qualities that depend on the interaction of the object in question with other things, not on the physical properties of the planet itself. The definition was created this way because we all knew Pluto needed reclassified, but we didn’t know enough about Pluto itself to know how to do it any other way. We made up this definition, but now we have to use it for everything. As a way of defining the world, it represents a very narrow and provincial view of the Cosmos.

So why does a definition like this matter? Can it really cause us trouble in the future?  Of course!  You see, all of us organize our thoughts about the world by sorting.  We do it every day — we decide what goes in our garden sheds, we decide where to put groceries in our kitchens, and we decide how to make different piles on our desks.

Scientists do exactly the same thing. We take things that we can see around us — rocks, flowers, slime molds, stars, galaxies — and we organize them based on what they look like. Everything you see gets sorted into a bucket, which is really useful when we’re learning about something for the first time — we quickly sort things based on how they look.  We call this TAXONOMY, and this is the game astronomers are engaged in right now when we talk about planets.

Pluto is just one of many worlds that have been discovered in the dark past Neptune. How do we classify these "trans-Neptunian objects"? [Image: Wikimedia Commons]

Pluto is just one of many worlds that have been discovered in the dark past Neptune. How do we classify these “trans-Neptunian objects”? [Image: Wikimedia Commons]

In today’s day and age, the rules for planetary taxonomy are being changed by technology. Telescopes are getting bigger, cameras are going digital and getting more sensitive, and roboticized telescopes are controlling those telescopes and cameras 24 hours a day, continuously searching the sky for things that go bump in the night. The result is a veritable bonanza of new objects being discovered, many in the dark beyond Neptune, in the Third Zone with Pluto, all of which have to get sorted into our planetary taxonomy.

Technology is also pressing us on other fronts.  Today, after spending centuries speculating on the matter, we’ve discovered that there are indeed worlds around other stars (not just the Sun!).  We know of almost 2000 planets orbiting other suns!  As of the time of this writing (19 April 2014), there are 1783 “planets” in our catalogs (the air quotes are necessary because they aren’t planets — they don’t orbit the Sun!). We’re going to have to put all of them into sorting buckets — into our taxonomy — and it is going to challenge us to think about what we mean when we say the word planet.

Let me tell you about two of those many thousands of planets.

Hot Jupiters are large, gas giant worlds that orbit far closer to their parent star than any worlds we have ever seen in our own solar system. [Image: ESA/NASA/STScI [M. Komesser]  (STScI-PRC2008-41) ]

Hot Jupiters are large, gas giant worlds that orbit far closer to their parent star than any worlds we have ever seen in our own solar system. [Image: ESA/NASA/STScI [M. Komesser] (STScI-PRC2008-41) ]

One of the first “planets” we found outside the solar system is orbiting a dim, naked eye star in the constellation Pegasus, called 51 Pegasi, a star much like our Sun about 50 lightyears away from Earth.  The planet is affectionately called “Bellerophon” after the Greek hero who tamed Pegasus, but astronomers call this planet “51 Pegasi b.”  This planet is about half the mass of Jupiter — if it were here in our solar system we’d consider this a serious planet!  But there is something odd about Bellerophon — it orbits 10 times closer to its parent star than Mecury does to the Sun!  We call planets like this “hot Jupiters” or “roasters”, and we have no idea how they get to be so close to their parent stars!

Rogue planets drift along among the stars, without orbiting a parent star. [Image: NASA/JPL-Caltech (PIA14093)]

Rogue planets drift along among the stars, without orbiting a parent star. [Image: NASA/JPL-Caltech (PIA14093)]

Here’s another “planet” we discovered only just last year.  Astronomers call this world PSO J318.5-22.  It is about six and half times the mass of Jupiter. If it were in our solar system, not only would it be “a serious planet,” it would be the most serious planet!  It would be larger than any other world in our home system.  So what’s special about this world?  It’s what we call a ROGUE PLANET.  It has no parent star it orbits. At some point early in its life, it was ejected from its home in some unimaginable gravitational battle.  Now, it will drift forever between the stars.

BOTH of these worlds, and many like them, challenge our definition of planet. Looking around the Cosmos, we have found something new, some new worlds that we have to sort into our buckets, like the hot Jupiters and the rogue planets. Maybe it will make us think about Pluto once again too.

New Horizons will fly past Pluto in July of 2015. This is the first spacecraft ever to visit Pluto and will return the first, up-close pictures of this far away world. [Image: NASA/JHU-APL]

New Horizons will fly past Pluto in July of 2015. This is the first spacecraft ever to visit Pluto and will return the first, up-close pictures of this far away world. [Image: NASA/JHU-APL]

There is one last part of Pluto’s story, still to come, and it also involves technology. In the summer of 2015, for the first time in history, a spacecraft from planet Earth will visit Pluto. It’s called New Horizons, and was launched in 2006. When it arrives at Pluto, after almost a decade of flying through the dark of space, it will blaze through the Pluto system in a single pass, measuring everything it can, and snapping every picture it can get. That data, those pictures, are precious commodities that will be sent back to Earth on the faint whisper of a radio link, and will, without fail, once again make us ask some deep questions about Pluto. For the first time, we’ll see Pluto up close, and we’ll start up this whole planet debate one more time — after the champagne is done, of course.

An artist's impression of the surface of Pluto. [Image: European Southern Observatory (L. Calcada)]

An artist’s impression of the surface of Pluto. [Image: European Southern Observatory (L. Calcada)]

The great truth in this story is this: Pluto doesn’t care what we call it. It is, more or less, the same today as it was when Clyde Tombaugh discovered it.  For that matter, it is more or less the same as it was it formed more than 4 billion years ago.  The notion that things can be sorted into “planets” and “not planets” is a human construct, something we made up to try and organize our imperfect understanding of the Cosmos. The debate gives me and you and all our astronomer friends an opportunity to chat and have fun and take a serious look at how we view the Cosmos and our place in it.

But I’m willing to make a promise: Someday, we’re going to come back to this question of “what is a planet.”  I don’t know if we’ll change Pluto’s status — I hope we do! — but what I do know is this.  We WILL change our definition of planet.

It doesn’t mean we were wrong, it doesn’t mean we were dumb, it doesn’t mean we were ignorant of the facts.  It just means that we are wiser than we once were. And that’s what the entire game of science is all about — to become wiser when faced with Nature’s awesome spectacle.

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This post is derived from a talk I gave at TEDx NorthwesternU 2014. You can see the video of that talk (17 min) here: TEDx – Pluto’s Day of Reckoning

For those of you interested in this debate, there are many great resources out there that you can take a look at. First, Mike Brown from Caltech has an excellent talk online at the Keck Institute:

  • How I killed Pluto & Why It Had It Coming, Mike Brown (Caltech) — 15 September 2011 (Video link)

There are also several books that I would recommend.

  • * “How I killed Pluto & Why It Had It Coming”, Mike Brown (Amazon link)
  •  “The Pluto Files”, Neil deGrasse Tyson (Amazon link)
  • “The Case for Pluto”, Alan Boyle (Amazon link)