PhysicsCentral Podcast: Gravitational Waves

Exciting news! I just completed my first episode for the PhysicsCentral podcast (part of an overall outreach effort by the American Physical Society), and it looks like I’ll be able to contribute regularly!

The podcast is aimed at anyone (roughly high school level and up) with an interest in physics, and this episode focuses on gravitational waves and what a direct detection will mean for our understanding of the universe.  I interviewed Dr. Chiara Mingarelli, a Marie Curie Fellow at Caltech working with pulsar timing arrays, and Dr. Kari Alison Hodge, who recently received her Ph.D. from Caltech working on LIGO.  Take a listen! There are also some really cool “sounds” modeling black hole and neutron star mergers.


LIGO observatory in Livingston, LA


Laser Interferometer Space Antenna (LISA)

Gravitational Wave Detectors and Sources

Neat interactive tool for understanding gravitational wave sources and detectors!

Posted in Extra Credit, News, Physics Central | Leave a comment

New Books in Astronomy: Lawrence Lipking, What Galileo Saw


Yesterday I had a lovely and long conversation with Dr. Lawrence Lipking about his latest publication, What Galileo Saw: Imagining the Scientific Revolution.  Although our chat was nominally for the New Books in Astronomy podcast, it’s the first one I’ve done that’s featured a humanist, and it felt really good to dive into seventeenth-century thoughts on the natural world.  The book focuses on the role of imagination in what we’ve come to describe as the Scientific Revolution, and it engages readers to consider the many different versions of this “revolution” that have been proposed and debated for decades (or longer).

One of my favorite parts of the book has everything to do with the title: what Galileo saw through his telescope, what it meant to him, and how he went about sharing it with others.  Lipking points out that the etchings of the Moon included in the published version of Sidereus Nuncius are very different from the drawings that Galileo made for himself (see below).  Notably, he places a very large crater, which doesn’t correspond to any real feature on the lunar surface and which doesn’t appear in the notebook drawings, prominently on the terminator, where the relationship between light, shadow, and topography is most apparent.  Lipking suggests that this deliberate manipulation of his observations amounts to more of a map than a likeness; Galileo is reorganizing the lunar features to structure his readers’ perusal of them and to help them to see as he does.

Galileo's notebook drawingsGalileo's etchings, Sidereus Nuncius

Another chapter that really jumped out for me deals with the distinction between life and death, and how we recognize the moment of transition, in Shakespeare’s epic tragedy, King Lear.  At that critical moment near the end of play, when Lear enters with Cordelia in his arms, no one is sure if she’s alive or dead.  Lear calls for a glass and a feather to catch a sign of her breath, and the many different ways that breath and wind come into the play showcase contemporary thoughts and theories of life and death, and how these states are related to the body and soul.

We also spent some time discussing recent historiography of science, and how the notion of genius (something to have or something to be?) has shaped a lot of stories concerned with how we got to now.  Lipking sees a rejection of this treatment of history in recent years and a turn toward microhistories rather than grand narratives.  As he says, “The stories of exceptional men can be a distraction from the heterogeneous, collaborative activities out of which the history of science emerges,” and I think many (including me) would agree with him on that.

All in all, it was a lovely discussion and a very stimulating book! You can listen to the podcast here, and if you happen to be going to the annual meeting of the History of Science Society later this week, check out Lipking’s talk on Robert Fludd, Thomas Browne, and the history of error (a.k.a. Chapter 7)!

Posted in Extra Credit, NBN Episodes | Leave a comment

#NerdsOnWheels: Road Trip to Meteor Crater!

Nerd Brigade at Meteor Crater

A little more than 48 hours ago, I piled into an RV with a few friends and fellow members of the Nerd Brigade, and we headed out to find an extraterrestrial…or at least, the remnant of one.  Meteor Crater, the result of a 50,000-year-old impact, lies in the northern Arizona desert about 500 miles from Los Angeles.  With a diameter of about 3/4 of a mile, it makes for an impressive reminder of how much energy is carried by space rocks traveling at planetary speeds.  Brought to rest all at once, the ~50-meter impactor caused a gigantic explosion equivalent to a 20-megaton bomb, excavating the entire cavity, uplifting the strata under the rim, and flinging room-sized boulders great distances — all within a few seconds.

Figure from Gilbert (1896) showing one of the large boulders displaced during the impact.

Meteor Crater is certainly an imposing geologic feature, especially when you’re standing on the rim and looking almost straight down the rugged cliff face that forms the interior wall along most of the circumference.  In size, however, it’s completely dwarfed by the immense craters on the Moon, the largest of which span more than a hundred miles (and impact basins are even larger).  Despite this difference in scale, the features share a common origin, and the connection between the little limestone crater in Arizona and the pitted lunar surface is what defines Meteor Crater more than anything else.

The process of impact cratering — the constant rain of scattered space debris leftover from cosmic collisions — is a fundamental tool of modern planetary science.  Counting up craters on surfaces of different ages allows us to piece together a timeline for lunar geology, which can be tied to absolute rock sample ages derived from radiogenic isotopes contained in the Apollo moon rocks.  This chronology gives us information about our own planet’s past to which we have no access here on Earth, having erased most of our early history through the steady recycling of the lithosphere known as plate tectonics.  Even better, by calibrating the impactor flux and timeline derived for the Moon, we can gain insights into the cratering histories of other, more remote, planetary surfaces.

Thanks to impact cratering, the lunar surface provides a window to our past and a link to the furthest reaches of the solar system, and the idea of using craters to unlock the secrets of remote surfaces lies at the heart of all kinds of current planetary research.  As essential as it’s viewed now, however, the pervasiveness of planetary impact has only been widely accepted for a few decades, and the details of when and how it came to be seen as a common process could, and has, filled books.  Meteor Crater, perhaps the best-preserved impact structure on Earth, has played a key role in the debate over lunar crater origins, which began with Galileo and the telescope in 1609 and stretched on through most of the twentieth century.  From the investigations of Grove Karl Gilbert in 1891, which led him to propose a volcanic origin for the crater but also inspired him to study the Moon, to the persuasive arguments of Daniel M. Barringer, who was convinced a fortune in meteoric iron lay beneath the crater floor, to the determined field studies of Gene Shoemaker, who kickstarted the Astrogeology Branch of the USGS and trained astronauts to think like geologists, thinking about the Moon meant thinking about the Earth, and Meteor Crater provided an analog for remote lunar features formed in a process impossible to witness.

Standing on the south rim

To visit Meteor Crater is to trace the history of this debate.  Abandoned mining equipment litters the crater floor and rim, a tangible reminder of how our understanding of impact physics has changed from Barringer’s time to today.  There is no vast hunk of iron buried beneath the crater; most of the impactor was vaporized and the rest scattered during the violence of the collision.  My own interest in the history of crater interpretation centers around this fundamental insight, that the impact process is essentially an explosive one, unlike anything you might be able to replicate by throwing rocks — or even firing bullets — into sand or clay or lead.  How do you study something you can’t observe directly? Do you make a model that’s as close as possible to the real thing, and trust that extrapolating will give you reliable information? Or do you look for an analog, something that you can study, and (again) trust that the differences between your analog and its counterpart are negligible?  These questions fascinate me, and they play out in amazingly complicated ways throughout the crater debate, as various actors grapple with the evidence available to them and try to sort out what it all means.

Nerd Brigade

It’s tempting to try to fit this history into a narrative of monotonic progression from less knowledge to more, but the complexity of the situation belies such a simplified retelling, and that’s what makes it so compelling.  It’s only one example, but all of the details of who knew what and how they reasoned it out, and whether they told anyone and if anyone listened — that’s science happening, and I want to know more.

For all of these reasons, Meteor Crater seemed like a good destination for a Nerd Brigade road trip, and it’s certainly very close to my heart.  Sharing a place so special to me with my science-minded friends felt pretty good.

Just one question remains:  where should we go next?!

Posted in Extra Credit, News, Reflections | 3 Comments

New Books in Physics: Roberto Trotta, The Edge of the Sky

This week for New Books in Physics, I spoke with Dr. Roberto Trotta of Imperial College London about his new book, The Edge of the Sky: All You Need to Know About the All-There-Is.  Inspired by xkcd’s Up-Goer Five comic, Trotta describes the current state of astrophysics and cosmology using only the ten-hundred most common words in the English language.  We had a lot to talk about:  everything from specific choices for technical terms to what a supersymmetric particle is, to what inspires people to go into science.  This little book and its creative turns of phrase is packed full of fun puzzles – at least that’s how I felt reading it.  When matter/antimatter collisions are described as “hugs” between “sister drops” and Sweden is identified as “a cold place with lots of ice-water, close to the top end of our Home-World,” reading becomes an exercise in re-thinking even familiar concepts.  The result is a refreshing and unprecedented perspective on the complexities of modern astrophysics, and it makes for a mind-stretching read.

On a minor and totally geeky history note, I was happy that Dr. Trotta brought up his choice of the phrase “tired light” to describe the redshift-distance relationship discovered by Hubble in 1929.  “Tired light” has been used before to describe a class of alternatives to the Big Bang theory, and the term was coined by none other than Richard C. Tolman (with whom I’ve been spending some time lately).  Tolman used the phrase to capture the idea that perhaps light from distant galaxies is not shifted to redder frequencies because the galaxies are moving away from us, but rather because the light is losing energy on its way to our telescopes.  This idea has since been abandoned, freeing up the phrase to be reused here to describe the redshift relationship in jargon-free language.  This reuse struck me as an interesting example of how language choices play into the process and communication of science, and I’m glad it made it into the podcast.

Anyway, you can listen to the full hour-long conversation here, and I think the book itself is definitely worth a read! Here’s the original Up-Goer Five comic from xkcd, and a text editor based on the ten-hundred most common English words, in case you’d like to try it out yourself!


Posted in Extra Credit, NBN Episodes

A Journey to the Center of the Earth


Jules Verne, Voyage au Centre de la Terre: Imaginary visit inside a diamond.

“La science, mon garçon, est faite d’erreurs, mais d’erreurs qu’il est bon de commettre, car elles mènent peu à peu à la vérité.”

“Science, my boy, is made up of errors, but these errors are worthwhile to commit, because they lead little by little to the truth.”

–Jules Verne, Voyage au Centre de la Terre

Eighteen hundred miles (2,900 kilometers) below your feet, the solid rock of Earth’s mantle gives way to a roiling core of molten iron (with a little bit of nickel thrown in for good measure).  Embedded within this liquid outer shell lies a solid inner core, squeezed enough by the immense pressure that the solid phase of Nickel-Iron is most stable, no matter the intense heat.  No one has ever directly observed these divisions deep within our planet, yet their existence is confirmed each time an earthquake rattles chandeliers and rings the Earth like a bell.  Whether the interior was solid or liquid, or even gaseous, remained a matter of debate up through the first quarter of the 20th century, after which consensus settled on a two-layer model of the Earth, solid outside and liquid within.1

Seismic Rays

Examples of seismic rays propagating through Earth’s layers. The P-wave shadow zone lies between about 103 and 143 degrees from the epicenter.

One major piece of evidence in support of this model was the behavior of seismic waves generated in an earthquake, which can be felt and recorded at locations all over the globe.  Earthquakes trigger many different types of waves that propagate either through the body of the Earth or along its surface. Of the former type, P-waves (primary) always arrive first and are longitudinal, like sound waves in air, while S-waves (secondary, or shear) arrive second and are transverse, more like a wave on a string or a ripple on a pond.  How fast each of these waves can travel is controlled by the material properties of the medium, and when a wave encounters an interface between layers with different material properties, it will bend, just as a beam of light bends when it crosses from air to water.

When a P-wave hits the core-mantle boundary (CMB), its seismic velocity drops abruptly, and it’s refracted, and its trajectory is bent.  When it reaches the other side of the core, it is refracted again (the opposite way), and as a result of these gymnastics, that particular wave is recorded somewhere at the surface far away from the location it would have reached had it taken a direct path.  The angle of refraction depends on the angle of incidence and the ratio of velocities going from the mantle to the outer core (just like Snell’s Law in optics), and it results in a shadow zone where P-waves just aren’t observed within a certain range of distances from the earthquake epicenter.


Average P-wave (orange) and S-wave (purple) velocities through the different layers of the Earth.

To see why that happens, consider two almost identical ray paths, one of which almost hits but misses the CMB and the other of which hits the core and is refracted.  The first ray will only pass through the mantle, so its trajectory is pretty direct,2 but the second ray is bent away from its initial direct path.  No P-waves can arrive between about 103° and 143° of the epicenter, leaving a donut-shaped region of absent arrivals – a shadow of the core.

Things get even more extreme when you consider S-waves, which can’t pass through liquid at all.  That’s because static liquids can’t support shear stress (which is why they conform to the shape of whatever container they’re in), so shear waves just can’t get through and aren’t seen beyond about 103° from the earthquake epicenter.

So that’s where it stands: no P-waves from 103°-143° and no S-waves at all beyond 103°.  Except…there are waves that arrive in that shadow zone, so where do they come from? Actually, those shadow boundaries are a bit fuzzy because some waves skirt around the CMB through a process called seismic diffraction, but even that can’t explain all of the supposedly forbidden arrivals in the shadow zone.  For that, there needs to be an inner core.

That’s exactly what Inge Lehmann,3 head of the department of seismology at the Geological Institute of Denmark, realized after a large New Zealand earthquake shook things up in 1929.  Examining the data from the Danish network of seismic observatories, which were at a great enough epicentral distance to address this outstanding issue, she suggested that an inner core with a slightly elevated seismic velocity could explain the problematic arrivals.4


Figures from Inge Lehmann’s 1936 paper, P’, showing seismic wave signatures at many Danish stations.


Ray paths through the Earth with an inner core.


Waveforms, 1929 New Zealand earthquake.


Within a couple of years, this explanation was widely accepted by the seismology community, although the fact that the inner core is solid, first proposed by Francis Birch in 1940 and extended by Keith E. Bullen in 1946, could not be decisively determined for several decades.  Bigger earthquakes naturally produce seismic waves that sample greater depths, and to “see” the inner core, seismologists needed a really big one…or better yet, several.  In the 1970s, analysis of recent large earthquakes that had excited the lowest, fundamental frequencies of the whole Earth provided support for a solid core.

Our understanding of the internal structure of our home planet is improving all the time, as seismic networks expand, international collaborations pool data, and computational power advances.  Huge earthquakes can be–and frequently have been–devastating in terms of human lives lost, but these deadly events also offer hints as to what lies beneath our feet and how it got to be that way.  Some say that space is the final frontier, while others reply that the oceans remain largely unexplored.  Earth’s interior offers another tantalizing frontier, and while a Vernian journey to the core is unlikely (to put it mildly), geophysics gets us a pretty good view, even from all the way up here.

  1. Historian of science Stephen G. Brush attributes most of this gathering consensus to the 1926 paper and persuasive powers of Harold Jeffreys.  While most textbooks point to the P-wave and S-wave shadows as the key evidence for a liquid core, Brush points out that the final straw came from a different argument altogether, one based on the total rigidity of the Earth as measured from tides and the observed (higher) rigidity of the mantle alone (i.e., the mantle is more rigid than the whole Earth, so the core must be less rigid to account for the average).  “The transmission or nontransmission of shear waves has not been a decisive test for solidity or fluidity, contrary to frequent statements in the literature.  Those who believe for various reasons that a certain region of the Earth is fluid have been able to find plausible explanations for the apparent transmission of shear waves through that region.  Conversely, if other evidence seems to prove that a certain region is solid, plausible explanations for failure to observe transmission of shear waves through it can be found.” Brush, 1980 “Discovery of Earth’s Core.” Am. J. Phys. 48 (9).
  2. The paths of seismic waves are not actually straight lines, because the density within each layer is constantly increasing as you go deeper and deeper.  The increasing density corresponds to increased seismic velocity, and the waves are continuously refracted to form a curved trajectory.
  3. More on Inge Lehmann over at TrowelBlazers
  4. Lehmann, Inge (1936): P’. Publications du Bureau Central Séismologique International A14(3), S.87-115
Posted in Extra Credit

New Books in Physics: Don Lincoln, The Large Hadron Collider

Last Monday, I sat down (via Skype) with Fermilab senior scientist Don Lincoln to discuss his new book, The Large Hadron Collider:  The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind.  This interview is my first contribution to the New Books Network (I’ll specifically be contributing to the Physics and Astronomy channels), and you can listen to the full hour here.

What Dr. Lincoln excels at, in the book and particularly in conversation during the interview, is calling up appropriate analogies to explain incredibly complex topics.  One of my favorite examples is when he compares the different types of magnets that keep the proton beam of the LHC on track to a team of Kindergarten teachers trying to keep an unruly group of children moving in the right direction.  I really enjoyed chatting with him about his experiences, both as a high-level researcher and an avid science communicator, and how those two passions feed back on each other.  Being able to explain a concept to others is truly a formidable test of understanding it yourself, and, as Dr. Lincoln says, a useful analogy or physical intuition–while not perfect–can help focus your attention and help you move forward as a scientist.

In addition to numerous books, Dr. Lincoln is responsible for many of the short videos on Fermilab’s YouTube channel, as well as a TedEd video explaining the Higgs field:

As a side note, I remember watching this video two years ago when the Higgs announcement was made.  As I recall, another analogy involving a snowy field was printed in the New York Times around the same time, and I remember wondering whether all Higgs boson analogies needed to trade on different meanings of the word “field” to get across (Higgs field, snowy field, field of physics…).  Just a funny coincidence, I guess.

In any case, I greatly enjoyed hearing from Dr. Lincoln and he has a real knack for telling stories and explaining concepts.  If you’re at all curious about modern particle physics or an insider’s perspective on the LHC and the Higgs, give the interview a listen (or better yet, pick up the book)!

The Large Hadron Collider by Don Lincoln

Posted in Extra Credit, NBN Episodes

MAVEN and the mystery of the Martian atmosphere

Image Courtesy of NASA

Just  a few hours ago, a visitor from Earth reached the Red Planet and traded her terra-bound orbit for a Mars-centric one.  The Mars Atmosphere and Volatile Evolution spacecraft, or MAVEN, has a specific assignment:  to characterize the upper atmosphere of our neighboring world and the processes that shape its evolution over time.  MAVEN is an exciting mission, and it’s taken 10 months and 442 million miles just to get into orbit.  It will take another five weeks to circularize that orbit and settle into the primary measurement configuration.  That may seem like a long time (to say nothing of the preparations and planning leading up to the launch last November), but MAVEN is part of a much longer tradition that puts our most recent ambassador to Mars in perspective.


Mars has long been an object of fascination for astronomers and the public alike, and its mysterious atmosphere has stubbornly remained at the center of speculations as to the planet’s past (and current potential) habitability.  The characteristics of the atmosphere – its composition and density – control the possibility for liquid water to exist on the surface.  Mars’ atmosphere is more than 100 times thinner than Earth’s, and it’s made up of different molecules.  Whereas Earth is shrouded with a thick nitrogen blanket (77% by volume) with ample oxygen thrown in (21%), Mars’ more tenuous atmosphere is 96% carbon dioxide.  Although the total atmospheric pressure at the surface (~6 millibars) seems to just barely put Mars within reach of the triple point for water (the lowest pressure at which liquid water can exist), it’s the partial pressure of water vapor that needs to be that high, and at 0.03% by volume, there’s just not enough of it there.


Telescopic claims of a Martian atmosphere date back at least to William Herschel, who reported on changes he observed on the Martian disk in 1784: “And these alterations we can hardly ascribe to any other cause than the variable disposition of clouds and vapours floating in the atmosphere of that planet.”  Several emerging similarities between the Earth and Mars were making headlines at the end of the 18th century, based on measurements of physical properties that were not far off modern values.  Our two planets have very close rotation periods, with a Martian day lasting just 37 minutes longer than our own.  The tilt of Mars’ rotation axis is 25°, just a degree and a half more than Earth’s, implying that our neighboring planet experiences seasons as it makes its way around the Sun.  And, perhaps most strikingly, Mars has polar caps that change in size and distribution throughout those seasons, evoking for some an image of Mars as another Earth.  Observations of a Martian atmosphere strengthened this analogy, and Herschel optimistically concluded that “its inhabitants probably enjoy a situation in many respects similar to ours.”

Screen Shot 2014-09-22 at 6.53.52 AM

Whether Herschel’s “clouds and vapours” really existed was a matter of debate for much of the 19th century.  Johann Schröter agreed with his mentor, writing that “the same shapes and positions develop and pass away again, as one would expect of the variable atmospheric appearances occurring above a solid surface.” In 1830, the great mapmakers Beer and Mädler denied the existence any atmosphere, having themselves observed no changes that might be attributed to clouds or mists hovering above the surface.  Nevertheless, the idea persisted, and with it the assumption that the composition of a Martian atmosphere would match that of Earth’s.  Richard Proctor’s 1870 book Other Worlds Than Ours cited early spectrographic observations from a Dr. Huggins in support of an Earth-like atmosphere for Mars.  “[W]e know that it is the aqueous vapor in our air which causes the appearance of the lines in question.  Hence there must be aqueous vapor in the Martial atmosphere.”  The detection of water vapor (or its absence) remained a hot topic throughout the remainder of the 19th century and well into the 20th.  In 1909, W. W. Campbell, director of the Lick Observatory, published results showing that the “quantity of water vapor present, if any, must be very slight,” directly contradicting observations by Slipher and Very the year before.  In the 1920s and 1930s, Walter S. Adams made several attempts to detect Martian water vapor and oxygen, to no avail.

Screen Shot 2014-09-22 at 8.13.10 AM

These observations and debates were occurring in the midst of the (in)famous Martian canal controversy, driven in large part by Percival Lowell, who founded an observatory in Flagstaff, Arizona, just in time for the 1894 opposition of Mars.  To Lowell, the seasonal polar caps and linear markings he interpreted as artificial constructions suggested a vast network of channels meant to control and conserve surface water on a planet that, due to its smaller size and greater distance from the Sun, was at an advanced stage of planetary evolution relative to Earth.  The canals were thus an expression of ecological disaster, a last attempt to manage a dwindling resource, and a dire warning for mankind as to the fate of our home planet.  The canal controversy fired the public imagination and Mars became a canvas on which to project our own fears and hopes for the future.

Today, we know that the “canals” were not built by intelligent Martians, and we have sent our envoys to orbit the Red Planet and traverse its dry and windswept surface.  In 1963, Andouin Dollfus released his observations of a very minute amount of water vapor present in the Martian atmosphere, and a few years later, Mariner 9 returned a close-up look at the surface features so long imagined from Earth.  The 1976 Viking landers established the composition of the atmosphere to unprecedented accuracy, and many planetary missions since then have returned a wealth of information about our neighbor’s current and past climates.  There is much we still don’t know, however, and by concentrating on the processes of the upper atmosphere, where molecules might escape due to simple thermal velocity or be stripped off by interactions with the solar wind, MAVEN will tell us about the planetary evolution that so captivated Lowell and many others a century ago.  It is likely that Mars once had a much thicker atmosphere than it does today, and while there are many theories and models that could explain why it disappeared, we don’t yet know the answer.  MAVEN’s seamless arrival at the Red Planet will soon help us fill in some of the gaps in our understanding, extending and upholding a long tradition of inquiry into Mars’ mysterious atmosphere.

Image credits: MAVEN spacecraft (image courtesy of NASA); MAVEN’s orbit insertion maneuver (image courtesy of NASA); phase diagram for water (Wikimedia Commons); Martian polar caps, sketches by Herschel (Phil. Trans. R. Soc., vol. 74, 1784); Sketch of Mars, Plate VII, by Lowell (Mars, 1895).

Posted in Extra Credit, Reflections, Uncategorized

A slice of Caltech history

The Tolman/Bacher House, from the 1930s (with Richard and Ruth Tolman relaxing outside their home) to today.

For the past several months, I’ve been working with the Keck Institute for Space Studies (KISS) to gather historical materials for the Tolman/Bacher House, one of the oldest buildings on campus, and with the Keck Center dedication yesterday, all of our hard work has paid off! In addition to the physical exhibits, which are tucked away in the bookcases and mantel-tops of the Oort Cloud Lounge (originally the living room) and the Black Hole Conference Room (study), we’ve built a website to introduce the house and all of its roles throughout the decades.

As a recent Caltech alum, I feel personally connected to this project, which incorporates both Caltech history and cutting-edge planetary science (KISS is a well-known think-and-do-tank for space science research).  At the same time, it marks a significant transition in my life and career, from research scientist to historian of science and curator, and I’m very grateful for the experience.  It feels strange sometimes to be making this transition on the same campus where I’ve spent several years working toward a doctorate in science, but I have to remember that it’s also the campus where I “snuck away” to take classes in history and Latin and where I’ve spent countless hours organizing rehearsals and film schedules for student theater and The PHD Movie.  In some sense, I think I’ve always been in transition.

Perhaps that’s why the Tolman/Bacher House appeals to me as much as it does:  this house, too, has played many roles and captures a transition frozen in time.  Completed in 1926, the house served as a comfortable home for two Caltech families, the Tolmans and the Bachers, before becoming part of the campus – which, by that time, had grown to surround it – in 1988.  As the Tolman/Bacher House Curator, I’ve tried to identify and juxtapose objects (photographs, newspaper clippings, letters, notes, etc.) that present the story of the house throughout the different eras of its history, the hope being that through this narrow lens, visitors and readers might view a little slice of Caltech’s past.  Standing in the original house and looking across the courtyard to the newly-dedicated Keck Center, both the past and the future feel close enough to touch.

photo 2 (1)

The study (now the Black Hole Conference Room) was added by Richard Tolman in 1936.

photo 1 (1)

The living room (now the Oort Cloud Lounge) was part of the original house, which was completed in 1926.

Posted in News, Reflections

Catching up with #TalkNerdy

Over the weekend, I sat down with Cara Santa Maria, an amazing science communicator (just check out her website) to chat for a bit on her podcast, Talk Nerdy.  To be honest, I was surprised Cara asked me to be on her show (which, aside from me, has an impressive guest list!), but we ended up having a really interesting conversation about the Moon, impact craters, history of science, women in STEM, and surviving grad school.  If you have 90 minutes to spare (sorry, we went into overtime!), you can listen here.

We touched on a lot of different topics, and I thought it would be good to gather some resources on a few of them, in case anyone wants more information and to clarify anything I wasn’t very clear on.  If anyone has additional links to send my way (this is by no means exhaustive), feel free to leave a comment or tweet @trueanomalies, and I’ll add it!

Here are some things we talked about:

Planetary Science: what is it?  My standard definition, if you can call it that, is “take whatever kind of science you do (astronomy, geology, chemistry, etc), just apply it to planets – you’re a planetary scientist!”  The interdisciplinary nature of planetary science is one of my favorite things about it, even though can end up confusing people because it encompasses so many kinds of research.  Many of my colleagues work with telescopes and go on observing runs, while others use mass spectrometers to analyze meteorite samples.  My research relies on models and spacecraft data, so I’ve spent most of my time in graduate school in front of a computer – less exciting to be sure, but it has its charms too.

The giant impact hypothesis: How did the Moon form? We don’t know, but the current best guess is that a roughly Mars-sized body collided with the Earth not too long after its core formed, spraying material (mostly from the outer parts of the Earth) into orbit, which ultimately would have coalesced to form the Moon.  Here’s a computer simulation showing the standard scenario (related paper here):

Why do we think something like this might have happened? Because this hypothesis does a better job than others at accounting for the observations we have at the moment:  the current dynamical configuration of the Earth and the Moon (and their total angular momentum – see below), the chemical properties of both bodies, and the lower density of the Moon compared to the Earth.  Here’s a recent article that goes into some detail about that and introduces a slightly different hypothesis, charmingly called the Big Splat.  This piece points out an interesting wrinkle in this whole giant impact story: there are many different variations on a theme, and the particulars of the collision scenario (how big was the impactor? what angle did it arrive at? was the Earth rotating? how fast?) produce different outcomes that may help us figure out what actually happened.

Angular momentum & the Earth-Moon system:  The Moon is getting further away, the Earth’s rotation is slowing down, and these events are directly related through the conservation of angular momentum.  Here’s astronaut Mike Fossum demonstrating this principle for a single body (his actual body):

By extending his arms, he increases his moment of inertia about the vertical axis (at least, vertical with respect to the camera) and his angular velocity slows; the spinning speeds up again when he brings his arms in.  Similarly, the Earth and the Moon have a shared angular momentum budget, so when the Earth loses rotational angular momentum (which happens because the Moon raises tides – solid body tides, not just ocean ones – and then pulls more strongly on this tidal bulge, against the direction that the Earth is rotating), the Moon has to gain angular momentum, moving outward in its orbit.

All of this  can be predicted mathematically and is not at all unique to our Moon – tidal evolution happens everywhere.  What’s really cool is that we can actually measure the rate at which the Moon is receding in its orbit.  The Apollo astronauts left several retroreflectors on the surface, which make it possible to shine a laser at the Moon and actually detect the reflection (this is a hard measurement to make, despite how easy it looks on TV).  The travel time of the laser pulse yields a very accurate measurement of the distance to the Moon, which is growing at a rate of a few centimeters per year, and this steady outward spiral is directly tied to the increase in length of day, which we tend to notice every now and then when a new leap second is added.  Everyone and everything on our planet is part of the system too, so in theory our angular momentum trades off with that of the Earth and Moon, but we’re too small to make much difference.

What is a barycenter? It’s the center of mass of an n-body system around which each body orbits.  If we consider just the Earth and Moon, the center of mass is very close to the Earth’s center, because it’s so much more massive: more than 80x!  So it’s almost true that the Moon orbits the Earth, but they actually both orbit the barycenter.  For Pluto and its moon Charon, which are much closer in mass (Pluto is ~9x more massive), the barycenter lies outside of Pluto altogether.  Here’s a handy animation:

Was there a Late-Heavy Bombardment? …Maybe? Generally speaking, there were many more collisions happening early on, when the planets were forming and sweeping up solid material, than there are now, and this impactor flux has decayed smoothly and exponentially over time.

But many of the Apollo samples that were brought back and radiometrically dated suggested that they formed at exactly the same time:  3.9 billion years ago.  This potential spike in the cratering rate (especially for the large, old lunar basins) is called the Late Heavy Bombardment, and whether it really exists or not is a topic of ongoing debate.  If it did happen, then it provides an important constraint for dynamical models of solar system formation, which have to explain the sudden cataclysm in addition to other key solar system characteristics, like the mass and structure of the Kuiper Belt.  For example, the Nice Model (based in Nice, France) proposes a scenario in which the giant planets formed somewhat closer to the Sun and evolved to their current positions (sometimes smoothly and other times abruptly) due to gravitational interactions with each other and with the planetary disk they formed in.  Here’s an animation showing a couple of potential scenarios simulated with the Nice Model:

Planetary migration can happen through several different mechanisms, and nobody knows for sure what combination (and timing) might have produced our solar system.  What’s even weirder (and more exciting!) is that other solar systems discovered so far don’t look much like ours.  The more we find out about other planetary systems and the outer reaches of our own, the more clues we’ll have to understand how our solar system got to be the way it is and how planets form throughout the universe.

That’s all I’ve got for now! Thanks, Cara, for having me on Talk Nerdy last weekend!

Posted in Extra Credit

Exciting times!

Last Friday, I successfully defended my planetary science thesis in front of my thesis committee, friends, and family – I am a doctor at last!


I say “at last” because it’s been quite a long time coming, really.  I arrived in Pasadena in August of 2007, having just graduated the previous June.  If I were to do it all over again, I would probably take some time off before starting grad school (and I would recommend it in general), but in retrospect, I’m glad that I didn’t.  I can’t regret the unexpected things that have come up over the past several years (to name a few: theater projects, movie producing, science communication opportunities, history of science research) that almost certainly would not have fallen out the same way if I’d changed my path, or even just delayed it by a year.

Looking ahead, I’m excited to remain at Caltech to write up for publication my research into the history of impact crater studies.  While I had originally intended to include this work as an addition my planetary science dissertation (since Caltech as an institution cannot grant any degrees in the humanities), after much careful thought, I have decided that this path forward will be more fruitful.  I am therefore at an interesting point in my trajectory (whatever it is).  I’m immensely proud of the work I’ve done in planetary science on cratered terrains and lunar surface roughness, and I look forward to publishing the bits that haven’t been published already in the coming months.  At the same time, I am so looking forward to concentrating on the history side of my research.  There’s plenty of work to do, but I’m ready for it – I have an exciting summer ahead of me!

Posted in News, Reflections