New Books in Astronomy: Bill Sheehan and Chris Conselice, Galactic Encounters

Galactic EncountersThis past month has been really crazy with all of the podcasts, but I’m finally catching up with my New Books in Astronomy interviews. In the latest episode, I talked with William Sheehan and Christopher Conselice, co-authors of a new book called Galactic Encounters: Our Majestic and Evolving Star-System, From the Big Bang to Time’s End.  I really enjoyed the book and I was very happy to be able to speak with both of them together.  Sheehan and Conselice bring their complementary backgrounds in history, psychiatry, and astronomy together to present both the current understanding and historical context of investigations into the nature of “fuzzy” objects in the night sky, from distant nebulae and galaxies to our own Milky Way galaxy.  A couple of my favorite chapters have to do with E. E. Barnard and early astronomical photography (Chapter 7) and W. W. Morgan and how he recognized the spiral structure of the Milky Way (Chapter 12).  But there’s a whole lot more, from the Herschels to Hubble and on to dark energy!

Posted in Extra Credit, NBN Episodes

Manh(a)ttan Brings Nuclear Physics to Primetime (Physics Central Podcast)

Of all the new television shows to premiere last fall, my favorite was Manhattan, a fictionalized retelling of the development of nuclear weapons at Los Alamos during WWII.  As a historian of 20th-century science and as the curator for the Tolman/Bacher House at Caltech (both Tolman and Bacher played large roles in the Manhattan Project and its aftermath), this show is especially exciting to me, but it also has the potential to seriously misrepresent the history I care about.  To achieve their dramatic goals while avoiding some of this risk of pseudo-history, the show creators pursued a really interesting strategy.  All of the main characters (aside from a couple of important figures, like Oppenheimer) are completely fictional, operating within the larger (mostly historical) framework.  I wanted to know how much of the history and science they get “right” on the show and what that really means when you’re dealing with a tradeoff between drama and reality, so I spoke with science writer and former Director of the Science and Entertainment Exchange Jennifer Ouellette and Executive Director of the Los Alamos Historical Society Heather McClenahan about their impressions of the show.  Both Ouellette and LAHS held weekly events to recap and discuss new episodes as they aired.  What was the consensus? Manhattan certainly doesn’t portray everything at Los Alamos as it would have been, and some exaggerations didn’t go over well with its former and current residents.  It did succeed, however, in portraying scientists at work and McClenahan noticed a decided uptick in tourism resulting from the show.  Overall, everyone seems happy enough with a show designed not as a documentary, but as a vehicle to explore the timeless issues of secrecy, privacy, and surveillance.  It’s been renewed for a second season, and I have reason to believe the show will have an excellent reference on hand when it comes to history.  I, for one, can’t wait!

These brilliant opening titles appeared in the third episode and added A LOT to the show (for me). Talk about a way to visually represent the all-too-human connotations of “implosion.”

Posted in Extra Credit, Physics Central

New Books in Astronomy: David Rothery, Planet Mercury

Planet MercuryWith one spacecraft coming to an end of its mission in March, and another set to launch in 2016, it’s an exciting time for Mercury.  I sat down with David Rothery of the Open University last month to discuss his new book, Planet Mercury: From Pale Pink Dot to Dynamic World for New Books in Astronomy.

The innermost planet in our solar system doesn’t get a lot of press compared to places like Mars and Jupiter’s moon Europa, but it’s an incredibly intriguing planet.  I’ll always have a soft spot for Mercury because my first project as a graduate student had to do with the orientations of the gigantic lobate scarps distributed across its surface.  These vast tectonic features speak to the thermal evolution of a relatively small terrestrial planet that has been cooling over time, and they’re just one interesting aspect of the first rock from the Sun.  From a wonky orbital resonance (a day on Mercury lasts two years – weird!) to an unexpected magnetic field, this little planet is full of surprises, and we’re just beginning to untangle them.

Posted in Extra Credit, NBN Episodes

Citizen Science: Answering the Call (Physics Central Podcast)

The last two months have been really busy, and I’m just beginning to catch up! Here’s a podcast I did for Physics Central on a few physics-themed citizen science projects that are enlisting the help of the public to sift through large datasets. In the case of Planet Hunters, the quarry is extrasolar planets that slip through the automatic algorithms designed to flag tiny dips in starlight recorded by NASA’s Kepler mission. According to co-founder Meg Schwamb, the program has been hugely successful so far, identifying several planets that would have otherwise escaped notice.

Stardust's aerogel detector

Stardust@home is another citizen science project, this one aimed at finding elusive interstellar particles trapped in the aerogel detectors returned by NASA’s Stardust mission in 2006. Project director Dr. Andrew Westphal emphasizes the inclusive nature of citizen science as a reason for the organization’s success, and Schwamb agrees. Through citizen science projects like these, members of the public can participate in the scientific process and work alongside practicing scientists to make a real difference.  Check out the Zooniverse page for more volunteer opportunities, and take a look at the other links we’ve compiled over at the Physics Buzz Blog.

Posted in Extra Credit, Physics Central

New Books in Astronomy: Vera Kolb, Astrobiology: An Evolutionary Approach

Astrobiology: An Evolutionary ApproachDecember has been a very busy month for podcasts! First up, I interviewed Dr. Vera Kolb, Professor of Chemistry at the University of Wisconsin-Parkside and the editor of Astrobiology: An Evolutionary Approach.  Thirty-seven authors from a multitude of disciplines provide an introduction to every aspect of the burgeoning field of astrobiology, which encompasses everything from prebiotic chemistry, to geologic and atmospheric conditions of early Earth, to life in extreme environments and the search for extraterrestrial life.

This is an entire short course in a book, and if you’re interested in catching up to the latest research on all aspects of life on Earth and elsewhere, you’ll get a lot out of reading it! I was especially impressed with Chapter 3, which deals with Education and Public Outreach (EPO) and communicating fundamental concepts to the public.  There’s a lot of research behind strategies for science outreach, and this chapter (by Timothy Slater) has some excellent advice for scientists and communicators alike.  And that’s just one of many interesting chapters!

You can listen to our conversation via the New Books in Astronomy webpage, or find us on iTunes!


Posted in NBN Episodes

Physics Central 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

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

#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

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