Best*. Photometry. Ever. I: The Problem

| 0 Comments

I was having coffee with David Hogg, and he asked, essentially, "what's so hard about photometry?"  "why can't we do Kepler from the ground?".  I gave the usual slew of answers, but he wasn't sure if this wasn't fundamentally a (solvable!) data analysis problem.  I told him about Ming Zhao's efforts at Palomar to get outstanding photometry for secondary eclipse work on hot Jupiters, and that I thought we might have achieved the highest precision ground-based O/IR photometry ever at about 3x times the photon limit on a bright star with a 5-meter.

Since then, our two groups have been exchanging tidbits and ideas and data, trying to produce the best possible ground-based photometry.  

In this first installment of a series, Penn State Research Associate (and NASA Origins of Solar Systems award recipient) Ming Zhao guest blogs "The Problem":

How to get ultra-high precision differential photometry? 

The key is to understand the systematics of your measurement and calibrate them well, and/or keep your instrument as stable as you can so that the instrumental systematics don't affect your differential measurements. The latter was essentially what the Kepler spacecraft was doing until its reaction wheels failed. Before that, Kepler had achieved <10 parts-per-million (ppm) precision on bright stars and had detected thousands of small planets by keeping its pointing extremely stable over time. 

Similarly, by calibrating the instrumental effects in addition to highly stable pointing, astronomers pushed the limits of both the Spitzer space telescope and the Hubble space telescope to allow high precision measurements of the dauntingly tiny signatures from exoplanetary atmospheres. [For more details on Ming's secondary eclipse work, how it works, and how it uses precise photometry, look at these links-- JTW]

Because of the Earth's gravity and atmosphere, it is a lot harder to do that from the ground. Gravity makes it difficult to keep the pointing extremely stable with a gigantic telescope, and induces flexures to optics that cause astigmatism. The atmosphere makes the point-spread-function highly variable with time. These effects are usually very tiny and do not affect most astronomers, but are disastrous for high precision measurements of exoplanetary signals. 

To address these issues, one common approach astronomers take for ground-based observations is to defocus the telescope so that the PSF is spread out over many pixels. This is key to mitigate the difficult-to-calibrate inter-pixel variations of a detector, as it makes the instrumental systematics more Gaussian-like. It also has the advantage of significantly improving the observing efficiencies since it takes longer to saturate the detector with a defocused image. 

Like other groups that were carrying out this type of study, we were also facing these issues when we started using Caltech's Palomar 200-in Hale telescope to measure thermal emission from exoplanets. After improving the guiding stability of the telescope, we were able to get precision of better than 200 ppm using the defocusing approach under the best conditions. However, due to astigmatism, the defocused images always have bright spots that cause highly time-correlated systematics and also damage the observing efficiency. 

Screen Shot 2014-04-18 at 9.57.09 AM.png

As a result, our best precisions could only be reached sporadically when the atmospheric seeing is consistently bad for a period of several hours. This basically means that our observations were uncontrollable in some sense and were based almost completely on luck -- we cross fingers to hope for the worst, stable seeing every time (quite an opposite to other astronomers!)

This is unsettling. So Jason and I brainstormed a few times to find ways to address this problem. We thought of dispersing the light, creating artificial dome seeings, better calibrating the optics, and Jason even thought of shaking the camera in some patterns [It turns out engineers really really really don't like it when you suggest deliberately shaking instruments -- JTW] But none of these approaches is simple and practical to implement since we didn't have the flexibility to modify an existing instrument. Jason then discussed this with our local instrumentation expert, Prof. Suvrath Mahadevan. Suvrath inspired us with a brilliant idea...

More in the next installment.

On Sensible Units for Spectra

| 1 Comment

How do you express a spectrum in a universal way, without appealing to units?

Spectra are fundamental to astronomy.  When we disperse light from stars, we are mathematically taking the power spectrum of the electric field as a function of time, but physically we are sorting the light by energy/wavelength/wavenumber/frequency/color, i.e. making a rainbow out of it.  Some colors are better represented than others:  the quantification of this observation is the spectrum.  Why do we need units for this?

Astronomers have lots of funny names and units for the spectrum.  If your spectrum is very coarse then you might call it a spectral energy distribution (SED), or just refer to the "broadband photometry" or "N-band magnitude" of the object (where N is some filter).  If you are just looking at whether redder or bluer photons are more common you might refer to its spectral index (especially in the high- and low-energy regimes).  If you have a very narrow-band or high resolution spectra of an absorption or emission line you might just refer to a "line profile".  

But it's units where we get really creative.  The fundamental unit of radiation is intensity, or specific surface brightness.  It measures the energy of a given color per unit time (power) coming from (or going into) some patch of sky as it crosses some surface.  It might be the upward-going intensity off of the surface of the Sun, or the downward-going intensity striking a telescope primary mirror from a patch on the moon.  Units are W/m2/sr/Hz or W/m2/sr/cm (with the last bit depending on whether you like to divide your light up by frequency or wavelength).

Astronomers, being astronomers, are not content to just use one set of units.  We use cgs versions, we multiply by big numbers from there (Janskys), we use brightness temperature (so Kelvins is a unit of intensity!) we express wavelength differences as velocities (so Kelvin kilometers per second (K k/s) is a flux!) we count photons instead of ergs, and so on.  And of course we love to take the base ten logarithm and multiply by 2.5, make it go backwards, and call it a "magnitude" (for back-compatibility with naked-eye Greek astronomers, of course).

What's more, the whole "surface brightness"/"per patch of sky" thing is generally glossed over in favor of just measuring "flux", the total amount of energy collected per area per second (something David Hogg disapproves of, and he has a good point).  Flux has units of W/m2.

If you disperse the light you collect, then you have to specify how big your color/frequency/energy/wavelength/wavenumber bins are to express your spectrum in physical units.  We call this a specific flux or flux density or spectral irradiance, and the units are W/m2/Hz or W/m2/cm or W/m2/eV or W/m2/Å (or W/m2/Gyr-1, I suppose).  If all you are doing is choosing between units of wavelength (cm vs. Å), then these units differ by just a constant factor, but switching to energy changes the underlying shape of the spectrum, which is annoying to deal with (as students calculating the Wien peak of the Planck function the world over have discovered).  This is because your bins are bigger for bluer photons if you use energy, but smaller if you use wavelength.  When you have uneven bin sizes, your histogram gets distorted.

This unfortunate situation is one reason that astronomers often publish spectra with νFν or λFλ for their units of flux density:  by multiplying the flux density by the wavelength (λ) or frequency (ν), they recover units of flux and become agnostic to the (arbitrary) choice of wavelength vs. frequency binning.  In fact, since λFλ = νFν, you can even switch between them in a paper (I've been guilty of this).  

Richard Wade pointed me to an interesting paper in Observatory, here, by Disney and Sparks called "On Sensible Units for Apparent Flux" published in 1982.  It begins: 

Gentlemen, --

The day must surely come when the present Babel of units to describe the apparent fluxes of astronomical objects is replaced by a more rational system....

...we feel that the sooner astronomers openly debate amongst themselves what they want the sooner action is likely to come.  Without laying claim to any originality but in the hope of stimulating such a discussion, we suggest a unit which we have presumptuously named the Hershel.

We will pause to *sigh* about he whole "gentlemen" thing and acknowledge that "ladies" (and all other astronomers) might also be interested in their discourse.

OK, moving on, Disney and Sparks go on to suggest the astronomers adopt the measure of apparent luminosity, the brilliance, B(x).  As a function, B(x) accepts as an argument the base 10 log of the frequency in Hz and returns λFλ = νFν .  Its units are Herschels, such that a source emitting one bolometric Solar luminosity per decade of frequency centered at x from a distance of 1 parsec from Earth delivers 1 Herschel of brilliance.  They define the base 10 log of the brilliance measured in Herschels to be the strength of the signal.

Screen Shot 2014-03-10 at 7.43.55 AM.png

(Above, a figure from Disney and Sparks, which confusingly and unnecessarily plots intensities with a variety of scaling factors, after arguing that we need a more uniform system!)

Now, I would quibble with their choices of parsecs, Solar bolometric luminosities, and base-10 logarithms (as they said folk would).  I don't see what's wrong with SI (or cgs) units and natural logs (does that make me a Jansky fan?).  But I appreciate the effort.

OK, with that table-setting out of the way, let me contribute to the discussion Disney and Sparks sought to have.

In a recent paper I wanted to express the spectrum of a galaxy as a composite of many underlying sources -- dust, stars, nonthermal emission, and so on -- each responsible for some fraction of the total luminosity, L.  But I didn't want to wed myself to any particular set of units -- I just wanted to express the shape of the spectrum gosh darn it.  

Plots of theoretical spectra often have something like "νFν (arbitrary units)" on the y-axis; the pedant in me says that the units of flux are not arbitrary and if you want to plot a dimensionless quantity you should just do it.  I also wanted to parameterize away the distance and luminosity as a "nuisance" term, so I wrote down:

f = νFν(4πd2/L)

and called f the "dimensionless SED" of the object, or its "dimensionless spectrum".  It's nice because it's area normalized to 1, so can be equally well applied to the flux or the intensity, and has no preference for things like bases of logarithms (except the natural one, I guess).  To plot it you still have to choose units for your x-axis, but that is unavoidable.

I like this because it conforms to our intuitive sense of what a "spectrum" is: a shape, without any appeal to arbitrary choices of units.  For instance, lasers emit (close to) delta functions, which need no normalization expressed as a dimensionless SED because they are also area-normalized to 1 (in physics, anyway).  

Now, you can't use this to express how bright an object is, but for that you can use Herschels, which simply scale the dimensionless spectrum by the apparent brightness (though I think I would prefer something like Jy Hz).

What do folks think?  Useful?  Interesting, at least?  Too obvious to even write about?  Am I missing existing jargon for the "dimensionless spectrum" of an object?

55 Cancri: It's Tricky!

| 3 Comments

As a kid I found my parents' old LP's: The Rolling Stones (Let It Bleed!), Bob Dylan (Blood on the Tracks!), Big Brother and the Holding Company (Cheap Thrills!).  As a result, I feel I have a good appreciation for the roots of modern rock and roll.

So it's good to see that the kids these days are acknowledging the classics.  New Penn State grad student Ben Nelson, working with Eric Ford, has been, as he put it "remastering the RV classics" by reanalyzing the LONG data streams of radial velocities for some of the longest-known and best-observed systems, like 55 Cancri (5 planets, one transiting) and GJ 876 (4 planets, probably, with strong mean-motion resonances).

My quick and usually-good-enough approach to fitting multiplanet systems measured with multiple telescopes is to use the RVLIN approach I developed with Andrew Howard (published here, code available here, parameter uncertainties available thanks to Sharon Wang's work here).  But this approach does not incorporate planet-planet interactions (usually not a problem -- they are too small to detect for almost all systems) and is a strictly "frequentist" chi-squared approach, which is decidedly out of fashion in astronomy these days.

Ben, as any good Eric Ford grad student will, brings to the problem a rigorous Bayesian (Markov chain Monte Carlo, or "MCMC") approach that generates parameter posteriors.  He also incorporates dynamical effects, so that planet-planet interactions are not just accounted for but can help constrain the physical parameters of the system.  His code also naturally accounts for the independent radial velocity time series not just for the four telescopes that have observed these exoplanetary systems, but for potential offsets between data streams for different detectors on the same telescope.  It also independently determines the quality of the data (the "instrumental jitter") for each detector/telescope.

Oh, and he also incorporates dynamical stability constraints, so that long term (108 years) unstable configurations are not part of the final posterior sample.

Oh, and he does the whole thing on a supercomputer with multithreading.

Oh, and the "supercomputer" in question is actually a cluster of graphics processor units (GPU's), which are cheap and fast but much trickier to hack into doing this sort of calculation than a "proper" supercomputer.

Really, the whole thing is a tour-de-force of how to do the problem "right".  

Ben is also an old-school hip hop fan.  Apparently, the coincidence of the initials of Markov Chain, Monte Carlo, and "Master of Ceremonies" has been too much to resist in astronomy. First we had "emcee: The MCMC HAMMER" (http://arxiv.org/abs/1202.3665), public code by Daniel Foreman-Mackey that samples MCMC ensembles very cleverly.

Ben's code is called RUN-DMC, for "Radial velocity Using N-body Differential evolution markov chain Monte Carlo.

Ben's paper on the 55 Cnc system is here:http://arxiv.org/abs/1311.5229

Applying RUN-DMC to 55 Cnc, Ben finds that each planet has something interesting to teach us:

  • b and c are near a mean motion resonance, but not actually in the 3:1 resonance.  They may, however, be apsidally locked at 180 degrees with a large libration amplitude (something Eugene Chiang refers to as the "metronome" formulation of the simple harmonic oscillator problem, as opposed to the usual "pendulum" formulation about 0 degrees).  Note that the period ratio incorporating planet-planet interactions (blue) differs by many sigma from the purely Keplerian solution (orange).  (The green solutions are osculating elements, I think -- you have have to average over large time intervals to determine a robust period ratio, which gives you the blue cloud).
Screen Shot 2014-03-09 at 10.39.29 AM.png
  • d's revised period and eccentricity make it one of the best Jupiter analogs known (though it has inner massive planets, so 55 Cnc is not a good Solar System analog).  For reference, Jupiter has P=4332 and e=0.05.
Screen Shot 2014-03-09 at 10.39.13 AM.png
  • The transiting, e component is probably reasonably well aligned with the other 4 planets (within 60 degrees, based on dynamical stability), and has a density of 5.5 (+1.3/-1.0) g/cc, very close to Earth's (5.5, though the mass of e is at least 8 times higher than Earth's, so it probably has more volatiles and maybe a big atmosphere).
  • f is in the Habitable Zone, but its amplitude is still to low to get a good handle on its eccentricity.

Incidentally, when Ben gave a talk in our department about this code, several of our department's freshmen were in attendance as part of an assignment in my First Year Seminar class to attend a department talk.  They said they were confused, in particular why everyone else laughed when Ben announced the name of the code was RUN-DMC.  They had never heard that term before.

Now that makes me feel old.  Run was the King of Rock!  There is none higher!  They're in the Rock and Roll Hall of Fame for goodness' sake!  

Kids just don't know their history any more.

(Though as much as I, as a classic rock fan, appreciated RUN-DMC's crossover hit Walk this Way, especially Stephen Tyler's Kool-Aid Man burst through the wall in the video, my Seattle roots make me partial to Mix -- his posse's on Broadway.)

[Update: Be sure to keep track of Ben's own explanation of the paper on his blog!]

The Amazing Cultural Force that is Groundhog Day

| 0 Comments
This is late, but I've been traveling.

Back when I was a graduate student at Berkeley, I hosted the astronomy department Movie Night, which included sending teasers of movies we would screen in the department.  I had a lot of fun with these, but one of my favorites was the one I did for Groundhog Day, arguing (tongue-in-cheek) that it is one of the most influential films ever made.  

In honor of the late Harold Ramis, who died on Monday, here it is, slightly revised:

How much wood would a woodchuck chuck if a woodchuck would chuck wood?

A:Just as much wood as a woodchuck could chuck if a woodchuck would chuck wood.

I post this insight to illuminate one of the many fascinating corners of the cultural phenomenon that is Groundhog Day. Before I explain, let's lay some groundwork.

Before the arrival of Christianity in parts Europe, many pagan cultures based their annual celebrations on agricultural events associated with the seasons. The most important of these events was often the celebration of the vernal equinox, a rebirth ceremony marking the arrival of baby crops and animals after winter. The other "quarter days", the autumnal equinox and the winter and summer solstices, were also marked and celebrated (not just by Europeans, but by cultures around the world). Perhaps the most famous example of this practice stands today in the ruins of Stonehenge where the alignment of the stones marks the position of the setting sun on the quarter days. Likewise, the ruins of Tulum in Cozumel, Mexico feature long holes in the stone which, (just like in Indiana Jones and the Raiders of the Lost Ark) allow sunlight to illuminate a chamber only on one of the quarter days.

In addition to these holidays, cross-quarter days celebrated the days midway between quarter days. Perhaps the most famous relic of the pagan cross-quarter days in Halloween, whose imagery is still totally divorced from the Christian holiday (the Eve of the Feast of All Saints or "All Hallow's Eve'n"), that attempted to supplant it. You see, the Roman Catholic Church, as it spread across Europe, associated many Christian holidays with these quarter and cross-quarter holidays in an attempt to ease pagans into the faith. Thus, Saturnalia and Yuletide became Christmas, the vernal equinox celebrations (complete with those images of fertility, rabbits and eggs) became Easter, All Saint's Day supplanted the precursors to Halloween, and on the 2nd of February, midway between the winter solstice and vernal equinox, became Candlemas.

Candlemas, or the Purification of the Blessed Virgin, marks the 40th day after the birth of Christ and the day, under Mosaic law, that Mary went to the temple to be purified after the birth of a son.  The pagan traditions and symbolism remained, however, as Candlemas offered a convenient marker than spring was six weeks away. Scottish tradition held that the weather on this day foretold whether spring would come early or late that year:

If Candlemas day be dry and fair,
The half o' winter to come and mair,
If Candlemas day be wet and foul,
The half of winter's gone at Yule.


I guess it rhymes in a Scottish accent.  

Anyway, tradition holds that Roman legions brought this rule of thumb to the Germans, who associated it with the hedgehog and its shadow (since if shadows were cast on that day then "Candlemas day be dry and fair" and winter is only halfway over).  From there, the Pennsylvania Dutch (as in "Deutsch", not as in Holland) brought the tradition to the New World, but were frustrated by the lack of hedgehogs here. To compensate, they pinned the predictive power on the local equivalent, the woodchuck (or "groundhog"). 

To this day, the Punxsutawney Groundhog Club of Punxsutawney, PA promotes their local woodchuck, Phil, and every 2nd of February gathers around him as he emerges from is hole with television crews which record the event for filler segments on news broadcasts across the country.

groundhog-day-driving-300x206.jpgThen in 1993, Harold Ramis overthrew thousands of years of reverent tradition with  "Groundhog Day", a film about a weatherman, who, disgruntled at being upstaged and out-predicted by Punxsutawney Phil, is damned by the gods to repeat his day of shame until he learns the true meaning of love and, I guess, Groundhog Day. 

As a result, a popular reference to Groundhog Day is now more likely to refer to a repetitive daily routine or eerie repeat of a previous experience than to the ancient February 2nd holiday. It is a true testament to the power of the cultural force of Harold Ramis that his film so effortlessly supplanted and all but erased millennia of Catholic and pagan tradition.

In my book, that makes "Groundhog Day" one of the most influential films ever made.

It's certainly compulsively watchable.

Water Detected in the Atmosphere of a Hot Jupiter!

| 3 Comments
It's nice when a long-running project reaches fruition!

irtf_201105_chad_head.jpg
Center for Exoplanets and Habitable Worlds Research Associate Chad Bender has been hunting down signals buried in the noise for a while now.  Way back when, I blogged about his work in the Kepler-16 system, where he used Hobby-Eberly Telescope data to dig out the very weak spectrum from the light of a faint star in this amazing binary star system (that has a giant planet orbiting both stars!)  This is tricky because the light of the bright star almost completely washes out the signal of the fainter star, but Chad exploits his knowledge of the likely spectrum of the faint star and his knowledge of its orbital motion to figure out exactly where it must be, which gives him a lot of leverage on the problem.

Back when I was running the Workshop on Precise Radial Velocities here at Penn State in my first year, I put Chad in touch with John Johnson about using this technique of his at Keck to attempt to dig out the signal of close-in planets, including tau Bootis and 55 Cancri e.  The latter planet was tough, not least because we gave Chad the wrong period for the planet (sorry, Chad!)

But now, I'm happy to report that the tau Bootis portion of the project has paid off.  The new paper, announcing the discovery of water in the atmosphere of tau Bootis b,  is starting to get traditional and social media attention thanks to press release promotion by Caltech, Penn State, Keck Observatory, and the American Astronomical Society.  

Chad has written up the details over on his website, and has given me permission to reproduce that post here:

Astronomers have been measuring the molecular chemistry of exoplanet atmospheres for more than a decade.  But most of those detections require a very specific geometry that requires a planet to pass in front of a star, as viewed from Earth, (commonly referred to as "transiting") so the total number of planets that have been probed is still very small.

Today, colleagues and I announced the detection of water in an exoplanet that does not transit its parent star.  tau Boötis b is a gaseous planet that is slightly larger than Jupiter, has a surface temperature of more than 2000 degrees Fahrenheit, and orbits its parent star in just over three Earth days!  Our solar system does not possess any planets remotely similar to this, and so understanding how these planets form and evolve is very difficult.  

ogle-planet-small.jpg
An artist's conception of a hot-Jupiter extrasolar planet orbiting a star similar to tau Boötes. Credit: Image used with permission of David Aguilar, Harvard-Smithsonian Center for Astrophysics

Our measurement of water in the atmosphere of tau Boötis b helps to constrain the chemical and physical processes that occur in the planet's atmosphere. At the same time, it allowed us to "weigh" the planet, using Kepler's Laws to determine its mass.   

We used the NIRSPEC spectrograph on the Keck II telescope, located on Mauna Kea in Hawaii to obtain high resolution spectroscopy of the planet and measure the water in the spectrum.  This measurement was exceedingly difficult because the planet is about 10,000 times fainter than its parent star, but they are so close together on the sky that the data we received at the telescope contains the blended light from both the planet and the star.  

Only after advanced processing were we able to separate out the planet's signal.

This difficult endeavour was carried out by Caltech graduate student Alexandra Lockwood and Penn State graduate student Alexander Richert.  Also integral were John Carr, from the Naval Research Lab, and Travis Barman, from the University of Arizona, who provided computer models of the star and planet spectra, and Geoff Blake & John Johnson who provided access to the Keck Observatories

You can access the full paper, which appeared in The Astrophysical Journal Letters on February 24, 2014.

If you can't get through the paywall, download the pre-print from arXiv.org.

Glimpsing Heat from Alien Technologies

| 0 Comments
This post originally appeared on Centauri Dreams.  I'm reproducing it here because, well, it seems like this blog should have a copy!

My colleagues and I have begun the Glimpsing Heat from Alien Technologies (G-HAT) SETI program, which has been written about here on Centauri Dreams and in other places, like in this nice summary article. I describe some of the foundations of the search here on my blog, but I have written up this short primer for Centauri Dreams to collect much of what is there into a single post.

"Dysonian" SETI

The benefits of expanding beyond "communication" SETI have been discussed on Centauri Dreams before (for instance, here) and the argument was made forcefully by Bradbury, Ćirković, and Dvorsky here.

The essence of Dysonian SETI is that one is searching for the passive signs of an alien civilization, instead of the deliberate communication from them. Freeman Dyson's original articulation of this principle remains the simplest: search for the energy that a civilization has used for its own purposes after it expels that energy. The disadvantage to this approach is that it may be difficult to distinguish such waste heat from natural sources, or it may be that advanced technologies do not emit large amount of waste heat.

Energy supply as sign of intelligent life

The term "energy supply" in the context of humanity refers to the total annual production of energy for human use. We generate this primarily through fossil fuel extraction and combustion, but it also includes energy generated through the collection of solar power.

Most species on Earth collect energy passively (through photosynthesis or collecting heat from the environment) or through consumption of other species. Intelligence is sometimes defined in terms of tool use, which involves the application of energy to objects to achieve some goal. More generally, we might use a "physicist's" definition of intelligence to be the capacity of an organism or species to apply energy to its environment to collect additional resources for survival or other purposes. For instance, an intelligent species like humanity can build solar panels to heat its homes or burn gasoline to run farm equipment to harvest more food.

This is not to say that there cannot be other forms of intelligence, but this specific form of intelligence would allow a species to overcome resource limitations and grow. This intelligent application of energy can, in principle, be used to expand a population, and an energy supply, to fundamental physical limits (see for instance, my "TED-style" talkhere.)

If this has ever happened -- if an alien civilization has ever used its intelligence to create an energy supply that rivals the output of stars -- then their waste heat would probably be detectable with today's astronomical instrumentation.

"Waste heat" does not imply inefficiency or "waste"

Conservation of energy means that when one is done using energy for some purpose, one must expel it or else store it (though in the long run you can't keep storing more and more energy). One might object that an arbitrarily advanced alien civilization could overcome this limitation, and it's true that if alien civilizations inevitably violate conservation of energy, then our search will fail. But conservation of mass-energy is as fundamental a physical law as we have, and if we cannot assume that then we cannot have a meaningful, physics-based discussion about advanced civilizations at all. So it is reasonable from a physics perspective to search for the energy in waste heat, which should exist if alien civilizations do.

The term "waste heat" may seem to imply some sort of unnecessary inefficiency that an advanced civilization would be able to overcome. Not so. The confusion here is that when most people say or hear "energy" they are really thinking of "free energy" -- the amount of work that can be done with a certain amount of energy.

For instance, when you drive to the supermarket the energy stored in the chemical bonds of your gasoline is converted to useful work that accelerates your car. When you are done with this energy -- when you are ready to slow down at the supermarket parking lot -- you press the brakes which dissipate the energy into the brake pads as heat, which then ultimately gets radiated away as mid-infrared radiation. This energy coming out of the brake pads now has a higher entropy than the energy that was in the gasoline-- this means the energy has less "free energy" than before, so you can't use that energy to make your car move again. In a regenerative braking system (like in electric or hybrid cars) your car attempts to collect this energy and put it back into the battery, but the second law of thermodynamics puts an upper limit on how efficiently this can be done -- some of the free energy is lost with each braking cycle. Also, losses to friction with the ground and the air during your trip cannot be recovered efficiently.

So conservation of energy says that on the whole, an alien civilization that has a very large energy supply must expel as much energy as it collects or generates, and the second law of thermodynamics says that this expelled energy will have high entropy (very little free energy). We call this high-entropy expelled energy "waste heat", even if the alien civilizations that uses it is very efficient and not at all "wasteful". In fact, the more efficient the civilization is, the higher the entropy of the expelled energy, and the more it will have the properties of the sort we expect to see from waste heat.

One way around this limit is to emit the heat at a lower temperature. This is not possible on the surface of the Earth, where you cannot radiate heat away at a temperature lower than your surroundings (if you try, the opposite occurs -- your surroundings heat up your apparatus). But in principle we could build huge, cold radiators in space that could operate as part of a heat pump, extracting more free energy from our waste heat to do more useful work. The difficulty here is that the radiators must be huge to get even a small benefit -- the size of the radiators scales as the fourth power of the efficiency you gain, so improving the maximum theoretical efficiency of sunlight collection on Earth from 95% to 99.5% would involve building radiators with a surface area 10,000 times the size of that of the Earth, which hardly seems worth the effort. This means that we should expect alien waste heat from starlight to never be orders of magnitudes cooler than the surface of the Earth, because the engineering difficulties make the task pointless.

Detecting waste heat with telescopes

Waste heat at these temperatures will be apparent at mid-infrared wavelengths. The IRAS mission in the 1980's surveyed the sky at these wavelengths, but did not have the sensitivity to detect most galaxies or stars because of the higher-than-expected background emission from dust in the Milky Way. The WISE satellite has much better resolution and sensitivity, and so does not suffer from this problem over most of the sky.

Most galaxies and many stars have "infrared excesses" -- they give off much more mid-infrared emission than one would expect from stars alone. Today, we understand that this is because of astrophysical "dust" -- a very fine smoke of organic molecules that is produced from the ashes of supernova explosions, in the atmospheres of giant stars, and in the disks of forming planetary systems. This dust glows brightly in mid-infrared wavelengths when it is illuminated by starlight -- just as we expect alien civilizations to do. Now that we have sensitive mid-infrared surveys, distinguishing mid-infrared emission from dust and alien civilizations is the primary obstacle to detecting alien waste heat.

For now, the best we can do is to put upper limits on these civilizations. We can show, for instance, that there are no nearby galaxies filled with alien civilizations using all of their starlight -- and we can do this for about 1,000,000 galaxies! We can also rule out civilizations using about 50% of the starlight -- even the dustiest galaxies do not have so much dust that half of the starlight is being reprocessed by it. Going forward, we will continue to lower this limit down to around 20% (or even lower for some galaxies, such as ellipticals which host almost no dust).

Going any lower will require careful observation to see if the mid-infrared morphology or spectrum of a galaxy is characteristic of dust, or if it is anomalous in some way. Looking to individual stars in the Milky Way will actually be somewhat difficult, because many things that look like mid-infrared-bright stars are actually distant galaxies that are red for other reasons, dusty giant stars on the other side of the Galaxy, or young stars still forming planets in dusty disks. When the GAIA satellite finishes its survey, it will give us distances to most of the stars in the mid-infrared surveys. This will allow us to search for those that are mid-infrared bright, not giants, and not associated with star-forming regions filled with dusty young stars. If we find a star that is very mid-infrared bright, about the luminosity of the Sun, and not part of a stellar nursery, that will be a dead giveaway that something very strange is going on with that star.

A process of exclusion

Even if we find something anomalous, as scientists we must always reach for the naturalistic explanation first. Finding mid-infrared-anomalous objects is scientifically interesting in its own right and so a worthwhile scientific endeavor. If we can find no scientific explanation for an anomalous object, we must continue to search for new explanations and not immediately jump to the conclusion of "aliens," lest we commit an "aliens of the gaps" fallacy. Only if we see an unambiguous sign of intelligence -- if the Allen Telescope Array, for instance, detects complex and obviously meaningful radio signals from the object -- will we be able to say SETI has succeeded. The G-HAT search, then, will have two implications for SETI: we will put an upper limit on the size of energy supplies being emitted as waste heat in nearby stars and Galaxies, and our best candidates will inform a target list for communication SETI efforts. In this way, the Dysonian and communication SETI approaches are strongly complementary.

Dock

| 0 Comments
Well, lucky me!  For Christmas I got a great big oil on canvas, "Boardwalk" by artist Karen Amato*:

Boardwalk.jpg
It's a commissioned variant of her earlier small painting "Dock":
Dock for cargo.jpgI've always liked the perspective that the value and meaning of art lies in the eyes of the beholder (which is not to say that artists' intent doesn't matter -- it can certainly help the beholder see what is there).  I've read that Melville was unaware of the depth of allegory in Moby-Dick until others pointed it out to him, which doesn't detract from the book's greatness at all.  

I know it wasn't Karen's intention, but I always liked "Dock" because to me it's a great allegory for... SCIENCE!

Stick with me.

First of all, we're outside, in nature.  There's water, and there's sky: real, fundamental nature (very Genesis 1:6-8, very "Mother, Mother Ocean"). 

But there's also the work of humanity.  Engineering, mathematics, purpose.  Someone used their brains, tradition, and the products of nature (wood) not just to illuminate nature, but to extend into it, and explore. 

What's out over the water?  Is it a narrow river, a lake, an ocean?  If you walk out to the end of the dock, maybe you could tell.  

The planks and the pylons are rigidly, regularly spaced, a hallmark of simple, robust engineering.  The pylons get closer together as they recede into the distance according to mathematics of perspective (the discovery of which was, as I understand things, a turning point in art history and a good example of the interplay between science and art).  

Because of the perspective of the painting is true, the dock doesn't just head out over the water, but heads straight into the distant, unseen horizon.  As if extending the dock just a little bit farther you wouldn't just reach the distant shore, but the sky itself.

Maybe that's silly -- even if you never reach the sky, you couldn't possibly extend the dock to the other side of an ocean.  But in trying, you'd learn a lot about the ocean; and about how you could do it.  

After all, a dock isn't just for walking on; it's a launching point for boats.  More complex engineering that allows you to cross rivers, lakes, and even oceans.  So maybe you can see what's on the other side.  With a dock you can explore this fundamental nature, maybe modestly (just fishing for minnows), maybe with big plans (let's sail to the Galapogos!), or grand ambition (is this the Shore of the Cosmic Ocean?)

The new version, "boardwalk" steps back.  The dock is longer now, and we can see that it extends back into the tall grass (much like the dock at our parents' house).  This adds whole new dimensions of biology to the allegory.  The grass is starting to come up through the planks and lean over the edges.  Nature is going to reclaim its own; but that's a false dichotomy.  We use nature to learn about nature.

But in both versions, the dock isn't in very good shape.  Our engineering isn't perfect, our attempts at illuminating the darkness sometimes thwarted by nature itself.  Or, old ways of doing things become obsolete and discarded.  Maybe this body of water isn't interesting any more; maybe we found better launch elsewhere; maybe someone built a better dock.

But I bet it still works.  

Someone built this dock so humans could walk on water.  Don't you want to wander out to the end and see what's there? 

Stargazers for cargo.jpg
*OK, so Karen might be my sister.

Update: Apparently, the links I had to Karen's paintings broke. The link to the final words of the post was to her work "Stargazers", which I now reproduce here.  All paintings © Karen Amato

AstroWright Lab Science at #aas223

| 0 Comments
aas223logo_0.png
Following up on last year's summary of AstroWright lab science at AAS Long Beach, here are the exciting posters and talks you can expect from my group and collaborators at the National Harbor Meeting:

Monday:


Tuesday:

Library - 3467-filtered.png
  • Talk #207.06 A Survey of the Hottest Jupiter Atmospheres via Secondary Eclipses. Ming Zhao updates on his work at Palomar, including the installation of a holographic diffuser in WIRC and custom detector calibration to perform precise NIR photometry from the ground.


Wednesday:




  • Talk #325.01 Remastering the RV Classics: Self-Consistent Dynamical Models for the 55 Cnc and GJ 876 Planetary Systems. Ben Nelson shows how to combine dynamical effects with large RV sets his work on two tricky dynamical systems, and fans of the "MCMC Hammer" will appreciate that he is including the results of his "RUN-DMC" algorithm.

Thursday:

  • Poster #411.03. Limits on Stellar Companions to Exoplanet Host Stars With Eccentric Planets AstroWright collaborator Stephen Kane searches for the Kozai perturbers of eccentric planets.  AstroWright lab member Katherina Feng contributed RV analysis, and bagged a planet!

  • Katherina.jpegPoster #441.09 Coronal heating of M dwarfs: The flare-energy distribution of fully convective stars Not really AstroWright lab science, but lab member Katherina (Ying) Feng presents her CfA REU work on flares on M dwarfs.




  • image_normal.pngPoster #442.08 Rotation and activity at 3 Gyr with Ruprecht 147 Jason Curtis shows that Ca II H&K metrics are significantly and consistently stronger at 3 Gyr than at 5 Gyr for single, Sun-like stars.  The dream of getting good ages for unevolved single stars gets a little closer...

  • Eunkyu.jpegPoster #442.06 Is Loden 1 an old and nearby star cluster? Or, "Is Ruprecht 147 Unqiue?" Eunkyu Han presents her investigation in to whether the purported cluster Lodén 1 is real, really nearby, and/or really old.

Who to Trust? Difficulties in Science Journalism

| 1 Comment
As a father of a three year old and as a scientist, the article this year that really stuck with me and encapsulated my frustration with medicine (and "medicine" and medical reporting) was this one in Slate about giving "alternative" medicine to your children.  I hope its popularity helps a lot of people avoid harming themselves and their family with "natural" and "alternative" remedies (pass it on!).

But it's not the "chemophobia is bad for you" thesis that struck me so much as the way that it illustrates how easily we come to swear by unproven and even untested remedies and dietary / medical / parenting advice.  

But even avoiding "chemophobia" and the naturalistic fallacy is not enough.  When there is no clear scientific consensus on something, it's easy even for scientists to grab on to something that is pitched just perfectly to make you think "yeah, that's probably right, I'll go with that."  I certainly follow unproven diets and parenting ideas: this article in the New York Times Magazine led me to dramatically decrease my added sugar intake to almost nothing (I've eased up now that I'm in the habit of avoiding it) and this book about baby sleep habits has ruled my daily schedule for over three years now.

But I feel I do this knowing and even expecting that these sources are imperfect, and maybe even wrong;  I have various rationalizations for following them regardless.  Most people don't apply a scientist's skepticism to big decisions like that, and that leads to things like people giving unknown doses of powerful, untested drugs with unknown side effects to their children and then bragging about it in the New York Times Magazine.

Part of my frustration is that science journalism is hard, and as a result, a lot of it is pretty bad.  Many science journalists can't distinguish between firmly established fact and plausible but untested hypothesis.

To give an example of how even good journalists can fall prey to this, the (excellent) Lee Billings had this twitter exchange with (NASA/Library of Congress Astrobiology chair) David Grinspoon upon the launch of Maven:


Here is a very good reporter on astrobiology basically getting caught mistaking a plausible but untested story about Mars's atmosphere for a well-established conclusion. And if Lee Billings can get it wrong about his area of expertise, then imagine the poor science journalists tasked with covering all of health issues reporting on a new scientific development in diet or medicine, interpreting puffed-up press releases of research that is notoriously plagued with publication bias, sloppy statistics and subsample analysis, and uncontrolled experiments.

For instance, it's common to read that a certain form of cancer or other disease is on the rise.  But it's actually hard to do a proper study of this: old people tend to get cancer and other diseases.  Cancer could be on the rise because of environmental effects (bad) or because medicine is making people live long enough to get cancer (yay science!).  How can a science journalist be sure to cover bases like those on every story they cover?

Similarly, once often reads that modest liquor intake is salubrious, in that people who drink lightly and occasionally have better health than tee-totallers.  But how many of those studies were meta-analyses or sub-sample analyses?  Did they consider why the tee-totallers didn't drink?  If they are Mormon or Muslim they probably represent a different population from the drinkers demographically and dietarily, so aren't a good control sample.  If they don't drink because they have liver damage or are recovering alcoholics then their poorer health is even easier to explain.  How did the studies control for these effects?

Even worse, how did these studies control for all of the effects that haven't been considered yet?

In short, medicine and journalism are hard, and we all need to be skeptical about medical results, especially when interpreted through the lens of journalism.  But also, medicine is awesome, and makes our lives better, and we should heed its advice, which it communicates through good and important journalism.  Trans-fats are a good example of the process apparently going very right.

It makes me glad I'm an astronomer, and not a medical researcher or journalist:  it's much easier to do well, and no one really gets hurt when I'm wrong.

So, have a Happy New Year, lay off the sweets during the celebration, and don't let your younger kids stay up to see the ball drop.  Or maybe not, what do I know? ;)






Astronomers and UFOs

| 0 Comments
One question astronomers sometimes get from the public is about UFO's.  These questions usually come from an honest place: the public doesn't really have a good sense of what astronomers do all day (or night).  Do we look through our telescopes hoping to discover a new star or galaxy?

One angle I've tried is to point out that (some) astronomers study big swaths of the sky all the time, and with much more sophisticated equipment than the cameras that have captured those iconic images of extraterrestrial "UFOs."  I tell them we don't see any UFOs.

Just today on Twitter via @jegpeek I saw this fantastic tweet:


Brilliant!  The link is to the Astronomer's Telegram, a forum where astronomers can quickly disseminate information about new, interesting, or strange things they discover with their telescopes and cameras (so, yes, some astronomers do just "look through" telescopes all night trying to discovery new "stars").

In it, an astronomer (D. Denisenko) reports an update on a previously reported unidentified object in their survey.  Denisenko couldn't figure out what a particular object was. It was moving too slowly to be in a circular orbit around the Earth within the distance of the Moon, and too quickly to be an asteroid in the main asteroid belt.  They reported it to the Telegram and the Near Earth Object Confirmation page so other astronomers could help them track it. 

After consulting with some other astronomers, they discovered that it was a man-made object in an unusual orbit, called TA29DCF.  They Googled this strange code name, and discovered it was part of the rocket that launched the RadioAstron satellite, tumbling in space after having done its job.

RadioAstron is a fascinating project -- it is a 10 meter radio telescope in an elliptical orbit around the Earth.  This orbit is not commonly (ever?) used, so the discarded upper stage of the rocket that got it there was in an unusual orbit, which caused the confusion.  Incidentally, the PI of the RadioAstron project is Nikolai Kardashev (yes, that Kardashev).

But the point here is that astronomers discovered a spacecraft orbiting the Earth that wasn't in their database and quickly informed the world about their discovery.  Just as quickly they determined that it was actually an object that just happened to have been overlooked by their database, and announced the resolution to the issue.  

What did not happen is NASA sent its goons to quiet the astronomers, or phone calls to the President sent national security officers to red alert 5, or astronomers quickly opened up Photoshop to destroy the evidence.  Indeed, most astronomers would not have even heard about this incident but for Schroeder's tweet (the Telegram is very useful for some fields, but not the sort of thing most astronomers read daily).

Now, I don't expect this example to convince hard-core UFOlogists who engage in highly developed conspiratorial thinking (such thinking can dismiss or incorporate ANY contrary evidence), but I hope it sheds some light for others on the chasm between popular misconceptions of how extraterrestrial UFO's might be real, and the reality of our understanding of all those lights in the sky.

Search This Blog

Full Text  Tag

Subscribe

  • Subscribe to feed Entries
  • Subscribe to comments feed Comments

Search This Blog

Full Text  Tag