Tuesday, November 24, 2015

The Extraordinary Success of General Relativity

(Note: this blog post was first published at the Chandra X-ray Observatory blog.)

This month, people around the world are celebrating the hundredth anniversary of Albert Einstein’s theory of General Relativity (GR). Although this theory can seem esoteric, it has an important practical application: the accuracy of Global Positioning System (GPS) relies on corrections from GR.

The GPS satellites orbit about 20,000 km (12,000 miles) above the Earth and experience gravity that is four times weaker than found on Earth’s surface. GR tells us that clocks traveling in this weaker field tick more rapidly, at a rate of about 40 thousandths of a second per day. This may not sound like much, but if these differences were left uncorrected they would cause navigational errors to accumulate faster than 10 km (6 miles) per day, as physicist Clifford Will explains in this article about GPS and relativity. By using GPS to successfully navigate around unfamiliar roads, people are inadvertently testing and retesting the accuracy of GR.

In astrophysics, GR has been tested and applied in multiple ways, including many that involve Chandra observations. Several years ago scientists used Chandra to test GR over distances that are much greater than those of Earth-orbiting satellites. They showed that GR correctly predicts the rate of growth of galaxy clusters and that GR performs better than an alternative model of gravity.  They have also provided a new way to study the accelerating expansion of the Universe.
Figure 1: Known officially as Abell 2744, this system has been dubbed “Pandora’s Cluster” because of the wide variety of different structures found. Data from Chandra (red) show gas with temperatures of millions of degrees. In blue is a map showing the total mass concentration (mostly dark matter) based on data from the Hubble Space Telescope, the Very Large Telescope (VLT), and the Subaru telescope. Optical data from HST and VLT also show the constituent galaxies of the clusters. Astronomers think at least four galaxy clusters coming from a variety of directions are involved with this collision. Credit: X-ray: NASA/CXC/ITA/INAF/J.Merten et al, Lensing: NASA/STScI; NAOJ/Subaru; ESO/VLT, Optical: NASA/STScI/R.Dupke

In a kind of cosmic GPS, GR has had a profound effect on our ability to map the location of matter in the Universe. In particular, gravitational lensing – the bending of light caused by massive objects curving space – has allowed astronomers to “see” the invisible by making maps of dark matter. The best-known example of this work is the Bullet Cluster, but other galaxy clusters have been studied using similar techniques, including MACS J0025.4-1222, Abell 520 and Abell 2744 (see Figure 1). A critical feature of these results was combining X-ray data from Chandra with optical data from observatories like the Hubble Space Telescope, to see separations between normal, visible matter and dark matter.

Gravitational lenses can also sometimes act as magnifying glasses, increasing the light from distant objects so they can be studied in much greater detail than would be possible without the lens. This was the case with the direct measurement of a black hole’s spin in the quasar RX J1131-1231.
Figure 2: Chandra data (above, graph) from observations of RX J0806.3+1527 (or J0806), show that its X-ray intensity varies with a period of 321.5 seconds. This implies that J0806 is a binary star system where two white dwarf stars are orbiting each other (above, illustration) approximately every 5 minutes. The short orbital period implies that the stars are only about 50,000 miles apart, a fifth of the distance from the Earth to the Moon, and are moving in excess of a million miles per hour. According to GR, such a system should produce gravitational waves - ripples in space-time - that carry energy away from the system at the speed of light. Energy loss by gravitational waves will cause the stars to move closer together. X-ray and optical observations indicate that the orbital period of this system is decreasing by 1.2 milliseconds every year, which means that the stars are moving closer together at a rate of about 2 feet per day. Credit: Light curve: NASA/CXC/GSFC/T. Strohmayer; Illustration: GSFC/D. Berry

Another prediction of GR is that certain objects, such as close pairs of white dwarfs, neutron stars or black holes, should produce gravitational waves. These are ripples in spacetime that travel outward at the speed of light. The shrinking separation of double stars (see Figure 2) has been explained by energy lost with the emission of gravitational waves. On much larger scales, astronomers have found evidence for a supermassive black hole that is being ejected from its host galaxy. The black hole may have collided and merged with another black hole and then received a powerful recoil kick from gravitational waves.  If this mechanism is indeed the correct explanation, then as theorist Avi Loeb explained in a blog post for us, this black hole provided the “first observational validation of Einstein's equations in the unexplored regime of dynamical strong gravity, which is responsible for gravitational wave kicks.”

The direct detection of gravitational waves would be one of the biggest advances in astrophysics in decades, and strenuous efforts to achieve this are ongoing. One major project, called Advanced LIGO, is US-based and in the next few years may start making detections of gravitational waves from dramatic events like mergers between two neutron stars. Several other projects are using pulsars to search for gravitational waves from much slower events, such as orbiting pairs of supermassive black holes. These projects include the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and the Parkes Pulsar Timing Array.

One of the most spectacular applications of Einstein’s theory is the use of GR to describe the behavior of matter near the event horizon of a black hole. For example, the spin rate of matter rotating around a black hole, and the spin rate of the black hole itself can be estimated using GR and X-ray observations, as with the result for RX J1131-1231 mentioned earlier. This result, therefore, is notable for involving two major applications of GR, the strong gravity around a black hole and gravitational lensing.

Despite the incredible successes of GR, no theory is complete. Ultimately, black holes represent a key shortcoming of GR, as this theory breaks down at the very center of a black hole, in a tiny region of extraordinarily high density called a singularity. There needs to be a melding of GR with another extremely successful theory – quantum mechanics – to treat black hole singularities. As this challenging work continues, we expect that GR will continue, for some time, to be our best theory for mapping the Universe.

Friday, June 5, 2015

Retractions and High Profile Journals

Another headline-grabbing study in a major journal has fallen. At the end of last year a paper in Science reported that people could change their minds about same-sex marriage after talking to a gay person for only 20 minutes. The New York Times was one of many news organizations to pick up this story. However, thanks to the fine work of David Brookman, Joshua Kalla and Peter Aronow, a number of “statistical irregularities” in the study have been reported, along with problems with the survey incentives and sponsorship, as explained in the retraction posted on the Science website. The prolific website Retraction Watch was the first to publicly report problems with this study.

The founders of Retraction Watch, Adam Marcus and Ivan Oransky, wrote a perceptive Op-Ed in the New York Times reacting to the same-sex marriage study’s problems. Early in the article they make the excellent point that:

“Retractions can be good things, since even scientists often fail to acknowledge their mistakes, preferring instead to allow erroneous findings simply to wither away in the back alleys of unreproducible literature. But they don’t surprise those of us who are familiar with how science works; we’re surprised only that retractions aren’t even more frequent.”

I think that retractions would be more common if both scientists and journals were less embarrassed about finding and acknowledging them, but these sorts of reactions are understandably very difficult to overcome.

Marcus and Oransky go on to note that journals with high impact factors – a measure of the frequency with which the average article in a journal has been cited in a particular year – retract papers more often than journals with low impact factors. Commenting on this correlation, they say:

“It’s not clear why. It could be that these prominent periodicals have more, and more careful, readers, who notice mistakes. But there’s another explanation: Scientists view high-profile journals as the pinnacle of success — and they’ll cut corners, or worse, for a shot at glory.”

Both of these explanations sound plausible, but it’s also important to note the severe screening process applied by journals like Nature and Science. According to a talk given by Leslie Sage, astronomy editor at Nature, only about 7% of submissions to Nature are published. Sage says a Nature paper should:

“report a fundamental new physical insight, or announce a startling, unexpected or difficult-to-understand discovery, or have striking conceptual novelty with specific predictions” and “be very important to your field”.

The general information for authors of Science papers states:

“Science seeks to publish those papers that are most influential in their fields or across fields and that will significantly advance scientific understanding. Selected papers should present novel and broadly important data, syntheses, or concepts. They should merit the recognition by the scientific community and general public provided by publication in Science, beyond that provided by specialty journals.”

Given the extraordinarily high standards that both Nature and Science set for their papers it’s not surprising that their retraction rates would be higher than average. Consider, for example, the “startling” or “unexpected” discovery that Nature seeks. Scientists can legitimately make such a discovery by, for example, developing unprecedented analysis tools or mining archival data in a novel way. However, they may also break new ground by making errors in their analysis or interpretation. Like any task with a high degree of difficulty, it’s inevitable that a larger number of mistakes will be made. Unfortunately, because the prestige of these journals is so high, a larger amount of cheating is also expected.

Where does peer review fit into this story? Marcus and Oransky go on to explain:

“And while those top journals like to say that their peer reviewers are the most authoritative experts around, they seem to keep missing critical flaws that readers pick up days or even hours after publication — perhaps because journals rush peer reviewers so that authors will want to publish their supposedly groundbreaking work with them.”

Rushed peer review may be one factor, but I think it’s also important to acknowledge why post-publication peer review is so powerful. Nature and Science papers usually only have 2 or 3 peer reviewers. For post-publication peer review, dozens or even hundreds of scientists with relevant expertise might review a paper. Therefore, just from a statistical viewpoint there’s a good chance that post-publication peer review will catch problems that traditional peer review missed, no matter how good the initial reviewers are. Nobody is perfect.  

Several lessons can be taken from this discussion. First, all of the different parties involved in research and the dissemination of it – scientists, peer reviewers, publishers, press officer and journalists – should be more careful and more skeptical. Second, although traditional peer review still has value, it’s important to stop deifying the peer review of journal papers, as Jonathan Eisen has said. Third, it’s important to pay more attention to post-publication peer review.

Some people may claim that the rise in the number of scientific retractions represents a worrying trend for scientific research. I would argue instead that it represents a triumph of the scientific method. In the case of the same-sex marriage study, careful statistical analysis helped confirm problems with it, as explained by Brookman, Kalla and Aronow and by Jesse Singal in his terrific article in New York Magazine. Debunking like this also gives a warning to others who are tempted to commit fraud.

Science is an incredibly successful endeavor, but it can also ruthlessly expose our human shortcomings. Retractions can reveal both of these sides to us.  

Thursday, September 18, 2014

How a Planet Can Mess Up a Star's Looks

Recently, beautiful photos of auroras have been in the news. These colorful light shows were generated by solar storms, and provide a vivid demonstration of activity on the Sun affecting the Earth. The pummeling experienced by our home planet is an example of our one-way relationship with the Sun: it can have a noticeable effect on the Earth, but the Earth has a negligible effect on the Sun. Further afield in the galaxy, this isn't always the case. In a few other systems planets can have a big effect on their star, changing their looks in surprising ways.

A spectacular picture of auroras by photographer Mike Taylor taken over Unity Pond in Waldo County, Maine on September 12, 2014. Credit: Mike Taylor photography.

As explained in the latest press release from NASA's Chandra X-ray Observatory, an exoplanet called WASP-18b appears to be causing the star it orbits to act much older than it actually is. WASP-18b is an example of a hot Jupiter, with a mass about ten times that of Jupiter and an orbit that is less than 24 hours long. The host star, WASP-18, is estimated to have an age that lies between about 500 million and 2 billion years, relatively young by astronomical standards.

Younger stars tend to be more active stars, with stronger magnetic fields, larger flares, and more intense X-ray emission than their older counterparts. Magnetic activity, flaring, and X-ray emission are linked to the stellar rotation, which generally declines with age. However, when astronomers took a long look with Chandra at WASP-18, they didn’t detect any X-rays. Using established relations between the magnetic activity and X-ray emission of stars and their age indicates that WASP-18 is about 100 times less active than it should be at its age.

The researchers argue that tidal forces from the gravitational pull of the massive planet – similar to those the Moon has on Earth’s tides but on a much larger scale – may have disrupted the magnetic field of the star. The strength of the magnetic field depends on the amount of convection in the star. Convection is the process where hot gas stirs the interior of the star.

The planet’s gravity may cause motions of gas in the interior of the star that weaken the convection, causing the magnetic field to weaken and activity to decline. This causes the appearance of premature aging in the star. WASP-18 is thought to have a shallow convection zone, making it unusually susceptible to tidal effects.
Shown in the main part of this graphic is an artist's impression of the star WASP-18 and, in the foreground, its hot Jupiter WASP-18b. The insets show the star in the optical image and its non-detection in X-rays with Chandra. Credit: X-ray: NASA/CXC/SAO/I.Pillitteri et al; Optical: DSS; Illustration: NASA/CXC/M.Weiss

What about other hot Jupiters that are relatively massive and close to their star? In some cases - where they orbit a different type of star to WASP-18 - the effect of hot Jupiters can be flipped and they can make a star appear younger than it really is. In the cases of HD 189733 and CoRoT-2a the presence of the planet appears to have increased the amount of activity in the star. In these cases the stars have much deeper convection zones than WASP-18 and tidal effects have little influence on convection and hence on the star's dynamo. Instead, the planets may be speeding up their star's rotation, leading to a more powerful dynamo and more activity than expected for the star's age. In these cases having a companion makes the star act younger than it really is. That makes sense for people and, in a few cases, for stars.
An artist's impression of the star CoRoT-2a and its hot Jupiter exoplanet, CoRoT-2b. Credit: NASA/CXC/M.Weiss

I've discussed the effects that extreme hot Jupiters can have on their host star. In such systems, what effect does the star have on its planet? In the cases of HD 189733 and CoRoT-2a, strong X-rays and ultraviolet radiation from the active star are evaporating the atmospheres of the planet. For HD 189733, astronomers estimate the planet is losing 100 to 600 million kilograms per second, and for CoRoT-2a astronomers estimate the planet is losing about 5 billion kilograms per second. For WASP-18, with much weaker X-ray emission and ultraviolet radiation, there is much less evaporation of the nearby planet's upper atmosphere than there would be if the star was more active. In effect, the planet is protecting itself. Its gravity causes the nearby star to be less active, and that causes the planet to be struck with less damaging radiation. HD 189733b and CoRoT-2b, on the other hand, are behaving in a self-destructive manner. 

Talk of planet destruction isn’t necessary for our present-day solar system, where the planets are much further away from the Sun than hot Jupiters are. However, that won’t always be the case. A few billion years in the future, the Sun will dramatically expand in size when it becomes a red giant. Our oceans will boil away, never to return and what’s left of the Earth may end up spiraling in towards the Sun. We don't know the exact fate of our home planet, but it is clear that our aurora-viewing days are numbered.

Wednesday, August 13, 2014

Astronomy for Everyone

The push for diversity in science comes in many different areas. Gender and race are perhaps the most familiar examples, and there remains a great deal of room for improvement in these areas. This blog post covers an area − neurological makeup − that receives less attention, partly because the differences are often not obvious. A push for diversity in this area can be framed by the following important question: how can we show children that people can have successful careers in science despite experiencing some neurological challenges?

An excellent answer to this question was provided by a neurodiversity workshop for high school students held at the Harvard-Smithsonian Center for Astrophysics (CfA), my home institution, at the end of April 2014. The workshop was called Astronomy for Everyone and was superbly organized by Smadar Naoz and Matt Schneps. An invitation was extended to high school students with Dyslexia, Attention Deficit Hyperactivity Disorder (ADHD) and/or Autism Spectrum Disorders.
The goal of the workshop was to "encourage neurodiversed high schoolers that the academic path is open for them, and to share tips to help overcome obstacles that they may encounter in their way", as stated on the workshop’s webpage. The workshop involved a full day visit to CfA to learn about careers in astronomy. An outgrowth of the work Schneps and his colleagues initiated at the Laboratory for Visual Learning at CfA, with funding from the Smithsonian Youth Access Grant program and the National Science Foundation, this was the second in a series of such programs.

Matt Schneps introducing the Astronomy for Everyone workshop. Credit: Nina Zonnevylle.

Inspired by the people I know with neurological challenges, I volunteered to help and was assigned to lead one of the four subgroups the students were divided into, along with their parents. My main job was to make sure the group made it to the different talks and sessions scattered throughout the maze-like building that is CfA. 

After registration and refreshments, the program began with a talk by Josh Grindlay, a Harvard professor, about the plate digitization program that he has led, called the "Digital Access to a Sky Century @ Harvard" project, or DASCH. This is a terrific program enabling astronomers to look at the variability of astronomical objects over more than a century, a much longer timescale than usually available. Josh’s talk was a perfect way to start the day and it quickly became clear from the question and answer session that the students and their parents were enthusiastic about astronomy and astrophysics. Later in the day other groups were able to visit the DASCH lab to see where the astronomical plates are scanned, but this was the one activity my group missed (because of time-limits each group missed one activity).

Josh Grindlay talking about the DASCH program he has led.  Credit: Nina Zonnevylle.

Also included in the program were a couple of short talks by graduate students in the Harvard astronomy department. Sarah Willis and Wen-fai Fong both gave interesting and enthusiastic presentations about their lives as students, including details about their background and what they enjoyed about doing research.

The final session before lunch involved a visit to the solar lab located in the basement, a hidden gem at CfA. Henry (“Trae”) Winter gave a great talk that included stunning movies of flares on the Sun, using a video wall containing 5,760 by 3,240 pixels. The data he showed was from the Atmospheric Imaging Assembly (AIA) suite of telescopes on the Solar Dynamics Observatory satellite. You can see some examples of AIA images and movies on their gallery webpage.

Trae's talk inspired a bunch of questions from the students, causing the session that had already started late to run over time. This resulted in a minor dilemma for me. Although I was very happy to see so many questions from the students, I noticed that the window for lunch was getting shorter and shorter, and I reluctantly interrupted the Q&A session to point out that we needed to eat and drink.

Trae Winter talking about our active Sun, using his lab's video wall.  Credit: Nina Zonnevylle.

In the afternoon, Bruce Ward gave a very good talk about the Great Refractor Telescope at CfA, located just a few feet down the hall from my office. After being installed in 1847, the Great Refractor was the largest telescope in the United States for 20 years and was “the most significant American astronomical instrument and equal to the finest in the world”, according to the Harvard College Observatory.

The Great Refractor Telescope at CfA.  Credit: Harvard College Observatory.

Afterwards, Smadar and another astronomer gave presentations about their own personal stories. These were private discussions and I didn't attend, but I'm sure that they provided valuable insight into the challenges that neurodiversed astronomers can face, and the successes that they can achieve.

Later in the afternoon, Matt Schneps gave an excellent presentation about how technology can be used to help overcome neurological challenges. Examples he gave included the use of voice recognition software to compose text, and use of the "reader" option in the Safari web browser to simplify the information presented in a webpage.

The day finished with group discussion, where the kids and their parents met separately to discuss their experiences and provide encouragement to each other.

I could tell from my own observations and my conversations with students and parents that the workshop was a success. This impression was confirmed by the feedback that the students and parents provided via a survey and email. Here is some feedback from the students:

"I have never seen a workshop like this specially paying attention on children like me supporting and guiding them throughout the day. Spending a day with such brilliant people was not only a great experience for me, but I learned a lot from it."

"I always live this hope that my future is getting built up for a purpose trying to stay happy always, but I have to say this workshop really boosted up that spirit as now I am confident for sure that my future is awesome."

and some feedback from the parents:

"Very useful in that it gave another example of how the challenges discussed can be overcome, struck a cord with my daughter due to several connections/similarities."

"I left feeling very inspired and encouraged about my son's future."

It was gratifying to help, even in a small way, with a program that was clearly inspiring to students and their parents. Much of the work I do in public affairs for NASA’s Chandra X-ray Observatory is less personal, involving press and image releases that indirectly reach large audiences, so it’s rewarding to work with a small group and see that a few hours of outreach can make a big difference to a student’s life.  

Smadar Naoz introducing the Astronomy for Everyone workshop.  Credit: Nina Zonnevylle.

What does the future hold for the Astronomy for Everyone program? Smadar Naoz has moved to the University of California, Los Angeles and plans to run the program there. Her long-term plans are bigger than that, as she wants to influence as many people as possible. She would like to expand the program in scope so it is run at multiple institutions in different parts of the country. Her really long-term goal is to do an integration day across the country and even across continents.

Matt Schneps has also relocated from CfA and is now at the Harvard Graduate School of Education and at UMass Boston, focusing his energy full time to programs designed to support cognitively diverse communities of learning. There, he and his team are doing research to identify new ways technology can be used to broaden access, and increase the inclusion of people with neurocognitive differences in challenging careers such as science.

I’ll finish by thanking Smadar and Matt for doing such a good job at organizing the workshop and making it easy for volunteers like me. Special thanks go to Avi Loeb for hosting and funding the workshop through the CfA’s Institute for Theory and Computation, and Nina Zonnevylle for helping out with the logistics. I’d also like to thank all of the other people who volunteered to help out, whether by giving talks or helping shepherd people through CfA or helping with the lunch and refreshments.