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.