We’ve all wondered at one point or another if intelligent life exists elsewhere in the universe. “I think it’s very unlikely that we are alone,” says Eric Korpela, an astronomer at the University of California Berkeley’s Search for ExtraTerrestrial Intelligence (SETI) Research Center. “They aren’t right next door, but they may be within a thousand light years or so.”
Korpela is project director of the [email protected] project. For more than two decades, that project harnessed the surplus computing power of over 1.8 million computers around the globe to analyze data collected by radio telescopes for narrow-band radio signals from space that could indicate the existence of extraterrestrial technology. On 31 March 2020, [email protected] stopped putting new data in the queue for volunteers’ computers to process, but it’s not the end of the road for the project.
Now begins the group’s next phase. “We need to sift through the billions of potential extraterrestrial signals that our volunteers have found and find any that show signs of really being extraterrestrial,” says Korpela. That task is difficult, he adds, because humans “make lots of signals that look like what we would expect to see from E.T.”
Last April, a research team that I’m part of unveiled a picture that most astronomers never dreamed they would see: one of a massive black hole at the center of a distant galaxy. Many were shocked that we had pulled off this feat. To accomplish it, our team had to build a virtual telescope the size of the globe and pioneer new techniques in radio astronomy.
Our group—made up of more than 200 scientists and engineers in 20 countries—combined eight of the world’s most sensitive radio telescopes, a network of synchronized atomic clocks, two custom-built supercomputers, and several new algorithms in computational imaging. After more than 10 years of work, this collective effort, known as the Event Horizon Telescope (EHT) project, was finally able to illuminate one of the greatest mysteries of nature.
Within weeks of our announcement (and the publication of six journal articles), an estimated billion-plus people had seen the picture of the light-fringed shadow cast by M87*, a black hole at the center of Messier 87, a galaxy in the Virgo constellation. It is likely among the most labor-intensive scientific images yet created. In recognition of the global teamwork required to combine efforts in black-hole theory and modeling, electronic instrumentation and calibration, and image reconstruction and analysis, the 2020 Breakthrough Prize in Fundamental Physics was awarded to the entire collaboration.
Now we are adding more giant dish antennas to sharpen our view of such objects. Thanks to an infusion of new funding from the U.S. National Science Foundation and others, we have set an ambitious goal: to make a movie of the swirling gravitational monster that forms the black heart of our own Milky Way galaxy.
The existence of black holes was a dubious prediction of the general theory of relativity, which Einstein developed a little over a century ago. Astronomers debated for decades later whether massive stars would create black holes when they collapse under their own weight at the end of their lives. Even more mysterious were supermassive black holes, hypothesized to lurk in the hearts of galaxies. Astronomers observed extraordinarily bright, compact objects and powerful galactic-scale jets beaming from the centers of many galaxies—including Messier 87—as well as stars and gas clouds orbiting hidden central objects. Supermassive black holes could account for such observations, and no other explanation seemed very plausible.
By the turn of the 21st century, astronomers had come to believe that black holes must be common. Most black holes are probably far too small and distant for us ever to observe. These “ordinary” black holes form when a star 10 times as massive as the sun or heavier collapses into something so dense that it bends the very fabric of space-time enough to form a spherical trap. Any matter, light, or radiation crossing the edge of this trap, known as the event horizon, disappears forever.
But the gargantuan black holes that we now think inhabit the centers of most galaxies—like the 4-million-solar-mass Sagittarius A* (Sgr A* for short) cloaked behind a veil of dust in the Milky Way and the 6.5-billion-solar-mass M87*—are different beasts altogether. Fed by matter and energy spiraling in from their host galaxies over eons, these are the most massive objects known, and they create around them the most extreme conditions found anywhere in the universe.
So says the math of general relativity. And the Nobel Prize–winning detection in 2015 of gravitational waves—essentially, a chirp of rippling space-time created by the whirling merger of two ordinary black holes more than a billion light-years away—offered yet another piece of evidence that black holes are real. It took an incredible feat of technology to “hear” the vibrations of that cosmic crash: two laser interferometers, each 4 kilometers on a side and able to detect a change in the lengths of their arms less than 0.01 percent the width of a proton. But hearing is not seeing.
Encouraged by the success of initial pilot studies in the late 1990s and early 2000s, an international team of astronomers proposed a bold plan to make an image of a supermassive black hole. They believed that advances in electronics now made it possible to see these bizarre objects for the first time and to open a new window on the study of general relativity. Teams from six continents came together to form the Event Horizon Telescope project.
Creating such a picture would require another giant leap in astronomical interferometry. We would have to virtually link observatories across the planet to function together as one giant virtual telescope. It would have the resolving power of a dish almost as wide as Earth itself. With such an instrument, it was hoped, we could finally see a black hole.
While M87* itself is black, it is backlit by a blinding glow of radiation emanating from the material swirling around it. Friction and magnetic forces heat that matter to hundreds of billions of degrees. The result is an incandescent plasma that emits light and radio waves. The massive object bends those rays, and some of them head our way.
A shadow of the black hole, just bigger than the object and its event horizon, is imprinted on that radiation pattern. Measuring the size and shape of this eerie shadow could tell us a lot about M87*, settling arguments about its mass and whether it is a black hole or something more exotic still. Unfortunately, the cores of galaxies are obscured by giant, diffuse clouds of gas and dust, which leave us no hope of seeing M87* with visible light or at the very long wavelengths typically used in radio astronomy.
Those clouds become more transparent, however, at short radio wavelengths of around 1 millimeter; we chose to observe at 1.3 mm. Such radio waves, at the extremely high frequency of 230 gigahertz, also pass through air largely unimpeded, although atmospheric water vapor does attenuate and delay them somewhat in the last few miles of their 55-million-year journey from the periphery of M87* to our radio telescopes on Earth.
The brightness and relative proximity of M87* worked in our favor, but on the other side were some formidable technical challenges, starting with that wobbly signal delay caused by water vapor. Then there was the size problem. Although M87* is ultramassive, it is comparatively small—the shadow this black hole casts is only around the size of our solar system. Resolving it from Earth is like trying to read a newspaper from 5,000 km away. The resolving power of a lens or dish is set by the ratio of the wavelength observed to the instrument’s diameter. To resolve M87* using 1.3-mm radio waves, we would need a radio dish 13,000 km across.
Although that might seem like a showstopper, interferometry offers a way around that problem—but only if you can collect pieces of the same radio wave front as it arrives at different times at all our telescopes in far-flung locations, stretching from Hawaii and the Americas to Europe. To do that, we have to digitally time-stamp our measurements and then combine them with enough precision to extract the relevant signals and convert them into an image. This technique is called very-long-baseline interferometry (VLBI), and the members of the Event Horizon Telescope project believed it could be used to image a black hole.
The details proved truly devilish, however. By the time the radio signals from M87* bounce into a receiver connected to one of our 6- to 50-meter dish antennas on Earth, the signal power has dropped to roughly 10-16 W (0.1 femtowatt). That’s about a billionth of the strength of the signals a satellite TV dish typically picks up. So the very low signal-to-noise ratio posed one major problem, exacerbated by the fact that our largest single-dish telescope, the 50-meter Large Millimeter Telescope, in Mexico, was still being completed and wasn’t yet fully operational when we used it in 2017.
The 1.3-mm wavelength, considerably shorter than the norm in VLBI, also pushed the limits of our technology. The receivers we used converted the 230-GHz signals down to a more manageable frequency of around 2 GHz. But to get as much information as we could about M87*, we recorded both right- and left-hand circular polarizations at two frequencies centered around 230 GHz. As a result, our instrumentation had to sample and store four separate data streams pouring in at the prodigious rate of 32 gigabits per second at each of the participating telescopes.
Interferometry works only if you can precisely align the peaks in the signal recorded at each pair of telescopes, so the short wavelength also required us to install hydrogen-maser atomic clocks at each site that could sample the signal with subpicosecond accuracy. We used GPS signals to time-stamp the observations.
On four nights in April 2017, everything had to come together. Seven giant telescopes (some of them multidish arrays) pointed at the same minuscule point in the sky. Seven maser clocks locked into sync. A half ton of helium-filled, 6- to 8-terabyte hard drives started spinning. I along with a few dozen other bleary-eyed scientists sat at our screens in mountaintop observatories hoping that clouds would not roll in. Because of the way interferometry works, we would immediately lose 40 percent of our data if cloud cover or technical issues forced even one of the telescopes to drop out.
But the heavens smiled on us. By the end of the week, we were preparing 5 petabytes of raw data for shipment to MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy, in Germany. There, researchers, using specially designed supercomputers to correlate the signals, aligned data segments in time. Then, to counter the phase-distorting influence of turbulent atmosphere above each telescope, we used purpose-built adaptive algorithms to perform even finer alignment, matching signals to within a trillionth of a second.
Now we faced another giant challenge: distilling all those quadrillions of bytes of data down to kilobytes of actual information that would go into an image we could show the world.
Nowhere in those petabytes of data were numbers we could simply plot as a picture. The “lens” of our telescope was a tremendous amount of software, which drew heavily on open-source packages and now is available online so that anyone can replicate or improve on our results.
Radio interferometry is relatively straightforward when you have many telescopes close together, aimed at a bright source, and observing at long wavelengths. The rotation of Earth during the night causes the baselines connecting pairs of the telescopes to sweep through a range of angles and effective lengths, filling in the space of possible measurements. After the data is collected, you line up the signals, extract a two-dimensional spatial-frequency pattern from the variations in amplitude and phase among them, and then do an inverse Fourier transform to convert the 2D frequency pattern into a 2D picture.
VLBI is a lot harder, especially when observing with just a handful of dishes at a short wavelength, as we were for M87*. Perfect calibration of the system was impossible, though we used an eighth telescope at the South Pole to help with that. Most problematic were differences in weather, altitude, and humidity at each telescope. The atmospheric noise scrambles the phase of the incoming signals.
The problem we faced in observing with just seven telescopes and scrambled phases is a bit like trying to make out the tune of a duet played on a piano on which most of the keys are broken and the two players start out of sync with each other. That’s hard—but not impossible. If you know what songs typically sound like, you can often still work out the tune.
It also helps that the noise scrambles the signal in an organized way that allows us to exploit a terrific trick called closure quantities. By multiplying correlated data from each pair in a trio of telescopes in the right order, we are able to cancel out a big chunk of the noise, though at the cost of adding some complexity to the problem.
The longest and shortest baselines in our telescope network set the limits of our resolution and field of view, and they were limited indeed. In effect, we could reconstruct a picture 160 microarcseconds wide (equivalent to 44 billionths of a degree on the sky) with roughly 20 microarcseconds of resolution. Literally an infinite number of images could fit such a data pattern. Somehow we would have to pick—and decide how confident to be in our choice.
To avoid fooling ourselves, we created lots of images from the M87* data, in lots of different ways, and developed a rigorous process—well beyond what is typically done in radio astronomy—to determine whether our reconstructions were reliable. Every step of this process, from initial correlation to final interpretation, was tested in multiple ways, including by using multiple software pipelines.
Before we ever started collecting data, we created a computer simulation of our telescope network and all the various sources of error that would affect it. Then we fed into this simulation a variety of synthetic images—some derived from astrophysical models of black holes and others we had completely made up, including one loosely based on Frosty the Snowman.
Next, we asked various groups to reconstruct images from the synthetic observations generated by the simulation. So we turned images into observations and then let others turn those back into images. The groups all produced pictures that were fuzzy and a bit off, but in the ballpark. The similarities and differences among those pictures taught us which aspects of the image were reliable and which spurious.
In June 2018, researchers on the imaging team split into four squads to work in complete isolation on the real data we had collected in 2017. For seven weeks, each squad worked incommunicado to make the best picture it could of M87*.
Two squads primarily used an updated version of an iterative procedure, known as the CLEAN algorithm, which was developed in the 1970s and has since been the standard tool for VLBI. Radio astronomers trust it, but in cases like ours, where the data is very sparse, the image-generation process often requires a lot of manual intervention.
Drawing on my experience with image reconstruction in other fields, my collaborators and I developed a different approach for the other two squads. It is a kind of forward modeling that starts with an image—say, a fuzzy blob—and uses the observational data to modify this starting guess until it finds an image that looks like an astronomical picture and has a high probability of producing the measurements we observed.
I’d seen this technique work well in other contexts, and we had tested it countless times with synthetic EHT data. Still, I was stunned when I fed the M87* data into our software and, within minutes, an image appeared: a fuzzy, asymmetrical ring, brighter on the bottom. I couldn’t help worrying, though, that the other groups might come up with something quite different.
On 24 July 2018, a group of about 40 EHT members reconvened in a conference room in Cambridge, Mass. We each put up our best images—and everyone immediately started clapping and laughing. All four were rings of about the same diameter, asymmetrical, and brighter on the bottom.
We knew then that we were going to succeed, but we still had to demonstrate that we hadn’t all just injected a human bias favoring a ring into our software. So we ran the data through three separate imaging pipelines and performed image reconstruction with a wide range of prototype images of the kind that we worried might fool us. In one of the imaging pipelines, we ran hundreds of thousands of simulations to systematically select roughly 1,500 of the best settings.
At the end, we took a high-scoring image from each of the three pipelines, blurred them to the same resolution, and took the average. The resulting picture made the front page of newspapers around the world.
A decade or two from now, astronomers will no doubt look back at this first snapshot of a black hole and consider it a milestone, but they’ll also smile at how indistinct and uncertain—and unmoving—it is compared with what they will probably be able to do. Although we are confident in the asymmetry of the ring and its size—roughly 40 microarcseconds in diameter—the fine structure in that image should be taken with a grain of salt.
But we did see a ring and the shadow of the black hole! That in itself is astonishing.
Measurements of that shadow add a lot of weight to the argument that M87* has a mass equal to 6.5 billion suns, consistent with the estimate astronomers had set by measuring the speed of stars circling the black hole. (In contrast, estimates made from the complicated effects of M87* on nearby gas were much lower: around 3.5 billion solar masses.) The size of the ring is also large enough to rule out speculation that M87* is not a supermassive black hole but rather a wormhole or a naked singularity—even stranger objects that appear to be consistent with general relativity but have never been observed.
Perhaps equally important, our initial success gives us good reason to believe that with further improvements to both the telescope network and the software, we will be able to image Sgr A* at the center of the Milky Way. Our nearest supermassive black hole is only a few hundred times as bright as the sun, and it is less than a thousandth as massive as M87*. But because it is 2,000 times closer than M87*, it would appear a little larger to us than M87*.
The biggest challenge in imaging Sgr A* is the speed at which it evolves. Blobs of plasma orbit M87* every couple of days, whereas those around Sgr A* complete an orbit every few minutes. So our goal is not to snap a still image of Sgr A* but to make a crude movie of it spinning like a billion-degree dervish at the center of the galaxy. This could be the next milestone in our quest to further constrain Einstein’s theory of gravity—or point to physics beyond it.
And there could be practical spinoffs. The methods we are developing to make movies of Sgr A* are strikingly similar to those needed to make a medical MRI of a child squirming in a scanner or to image subterranean movements during an earthquake.
For future observations, we expect to use 11 or more telescopes—including a 12-meter dish in Greenland, an array of a dozen 15-meter dishes in the French Alps, and one of Caltech’s radio dishes in Owens Valley, Calif.—to increase the number of baselines. We have also doubled the data-sampling rate from 32 Gb/s to 64 Gb/s by expanding the range of radio frequencies we record, which will strengthen our signals and eventually allow us to connect smaller dishes to the network. Together, these upgrades will boost the amount of data we collect by an order of magnitude, to about 100 petabytes a session.
And if all continues to go well, we hope that in the years or decades ahead the reach of our computational telescope will grow beyond the bounds of Earth itself, to include space-based radio telescopes in orbit. Adding fast-moving spacecraft into our existing VLBI network would be a tremendous challenge, of course. But for me, that is part of the appeal.
This article appears in the February 2020 print issue as “Portrait of a Black Hole.”
About the Author
Katherine (Katie) Bouman is an assistant professor in the departments of electrical engineering and of computing and mathematical sciences at Caltech.
The first targets will be the sun and Jupiter, which are expected to have strong emissions at low frequencies. But the team also hopes to pick up much weaker signals from the ‘Cosmic Dawn’—when the first stars lit up around 12 billion years ago—and even ultra-faint signals from the preceding Cosmic Dark Ages. Detections would give unprecedented insights into these formative periods of the universe.
New supercomputer simulations have successfully modeled a mysterious process believed to produce some of the hottest and most dangerous solar flares—flares that can disrupt satellites and telecommunications networks, cause power outages, and otherwise wreak havoc on the grid. And what researchers have learned may also help physicists design more efficient nuclear fusion reactors.
In the past, solar physicists have had to get creative when trying to understand and predict flares and solar storms. It’s difficult, to put it mildly, to simulate the surface of the sun in a lab. Doing so would involve creating and then containing an extended region of dense plasma with extremely high temperatures (between thousands of degrees and one million degrees Celsius) as well as strong magnetic fields (of up to 100 Tesla).
However, a team of researchers based in the United States and France developed a supercomputer simulation (originally run on Oak Ridge National Lab’s recently retiredTitan machine) that successfully modeled a key part of a mysterious process that produces solar flares. The group presented its results last month at the annual meeting of the American Physical Society’s (APS) Plasma Physics division, in Fort Lauderdale, Fla.
On Earth, no natural phenomenon is quite as dependable as gravity. Even a child playing on a beach knows that the sand she is excavating will just sit there in her trowel, pulled downward by this powerful force.
But on small, low-gravity celestial bodies like asteroids, the rules of gravity that we know so well no longer apply—at least, not in the ways that we’re used to. And that’s a problem for the scientists who collect samples of regolith, the dusty or pebbly material found on the surfaces of these bodies.
Asteroids are remnants of the early solar system: essentially chunks of material that did not become planets. Regolith samples from asteroids and other small celestial bodies are critical for researchers to better understand how the solar system began, and how it has evolved since.
In the absence of strong gravitational influences, even electrostatic forces that would be considered weak to negligible on Earth may hold outsized importance in space. Hartzell, a participating scientist on the OSIRIS-REx mission currently orbiting the asteroid Bennu, studies these electrostatic forces. A better understanding of electrostatic forces on particles improves understanding of the natural evolution of asteroids and helps inform the design of sampling methods and instruments on future asteroid exploration missions.
Electrostatic forces occur when oppositely charged particles interact with each other. This causes regolith particles to behave curiously in three ways.
First, they cause dusty particles that rub against each other to stick together, or clump. Second, dust exposed to the flow of charged particles from solar wind plasma can detach, or loft away from the surface, drawn to opposite charges in the solar wind flowing past. Third, particles can levitate after being kicked up by a small meteorite impact or blasted by a visiting spacecraft, because the electrostatic forces on those particles cancel out any gravitational pull.
And it’s possible that it’s not just tiny dust particles that may behave unusually—but larger grains, due to the extremely weak effects of gravity on asteroids, as well.
The catch, however, is that none of these behaviors have been directly observed in space, nor the forces causing them to occur measured there. Though Hartzell’s work has demonstrated these forces in laboratory experiments, many questions remain about what they look like on an asteroid, to what degree electrostatic forces affect dust behavior, how strong those forces are, and how the presence of a spacecraft in close proximity to an asteroid’s surface might change the environment.
Whether or not lofting occurs depends on the strength of the forces causing particles to stick together and, by extension, to other objects, such as spacecraft surfaces and optics. Hartzell is developing an experimental method to measure this cohesion.
How the method will work: an electrically charged plate is placed at a set distance above a surface with dusty particles, in an area of known gravity. By controlling the height and electrical charge of the plate, the electrostatic forces on the dust grains can be controlled. A camera is used to observe the size of dust grains and when they begin to be drawn to the plate. By controlling the electrostatic force and knowing the gravity, the unknown, cohesive force can be mathematically derived.
Hartzell’s method could potentially be used for actual sampling, as well. She suggests that charged plates could be used to attract dust samples, then drop them into sample collectors or directly onto analysis instruments by removing the plate’s charge.
More likely, however, is that the method might be employed to better characterize the surface of a site intended for longer-term use by, for example, an asteroid mining mission. Early planning stages would involve understanding the chemistry and behavior of any dusty surface, including how its cohesive properties may affect the function of tools like drill bits.
Harnessing electrostatic forces to control dusty particles might also mean cleaner, better functioning solar panels on Mars. An electrostatic dust shield could use coils embedded in solar arrays to “bounce” dust grains off the surface via alternating electrical charges.
But for now, Hartzell’s work involves a lot of creative lab experimentation and lab-based modeling, but with one goal in mind.
“We want to keep the spacecraft safe during operations,” she says.
A newly developed graphene-based telescope detector may usher in a new wave of astronomical observations in a band of radiation between microwaves and infrared light. Applications including medical imaging, remote sensing, and manufacturing could ultimately be beneficiaries of this detector, too.
Microwave and radio wave radiation oscillate at frequencies measured in gigahertz or megahertz—slow enough to be manipulated and electronically processed in conventional circuits and computer systems. Light in the infrared range (with frequencies beginning around 20 THz) can be manipulated by traditional optics and imaged by conventional CCDs.
But the no-man’s land between microwaves and infrared (known as the “terahertz gap”) has been a challenging although not entirely impossible band in which astronomers could observe the universe.
To observe terahertz waves from astronomical sources first requires getting up above the atmosphere or at least up to altitudes where the Earth’s atmosphere hasn’t completely quenched the signal. The state-of-the-art in THz astronomy today is conducted with superconducting detectors, says Samuel Lara-Avila, associate research professor in the Department of Microtechnology and Nanoscience at Chalmers University of Technology in Sweden.
Observatories like the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the South Pole Telescope might use such detectors combined with local oscillators pumping out reference signals at frequencies very close to the target signal the astronomers are trying to detect. If a telescope is looking for radiation at 1 THz, adding a local oscillator at 1.001 THz would produce a combined signal with beat frequencies in the 1 GHz (0.001 THz) range, for instance. And gigahertz signals represent a stream of data that won’t overwhelm a computer’s ability to track it.
Sounds simple. But here’s the rub: According to Lara-Avila, superconducting detectors require comparatively powerful local oscillators—ones that operate in the neighborhood of a microwatt of power. (That may not sound like much, but the detectors operate at cryogenic temperatures. So a little bit of local oscillator power goes a long way.)
By contrast, the new graphene detector would require less than a nanowatt of local oscillator power, or three orders of magnitude less. The upshot: A superconducting detector in this scenario might generate a single pixel of resolution on the sky, whereas the new graphene technology could enable detectors with as many as 1000 pixels.
“It’s possible to dream about making [THz] detector arrays,” Lara-Avila says.
Probably the most famous observation in THz or near-THz astronomy is the Event Horizon Telescope, which earlier this month won the Breakthrough Prize in Fundamental Physics. (Pictured) Some of the frequencies it operated at, according to Wikipedia, were between 0.23 and 0.45 THz.
The graphene detector pioneered by Lara-Avila and colleagues in Sweden, Finland, and the UK is described in a recent issue of the journal Nature Astronomy.
The group doped its graphene by adding polymer molecules (like good old 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane, or F4-TCNQ ) atop the pure carbon sheets. Tuned just right, these dopants can bring the ensemble to a delicate quantum balance state (the so-called “Dirac point”) in which the system is highly sensitive to a broad range of electromagnetic frequencies from 0.09 to 0.7 THz and, they speculate, potentially higher frequencies still.
All of which adds up to a potential THz detector that, the researchers say, could represent a new standard for THz astronomy. Yet astronomical applications for technology often just represents the first wave of technology that labs and companies spin off for many more down-to-earth applications. That CCD detector powering the cameras on your cellphone originated in no small part from the work of engineers in the 1970s and ‘80s developing sensitive CCDs whose first applications were in astronomy.
Terahertz technologies for medical applications, remote sensing, and manufacturing are already works in progress. This latest graphene detector could be a next-gen development in these or other as yet unanticipated applications.
At this point, says Lara-Avila, his group’s graphene-based detector version 1.0 is still a sensitive and refined piece of kit. It won’t directly beget THz technology that would find its way into consumers’ pockets. More likely, he says, is that this detector could be lofted into space for next-generation THz orbital telescopes.
“It’s like the saying that you shouldn’t shoot a mosquito with a cannon,” Lara-Avila says. “In this case, the graphene detector is a cannon. We need a range and a target for that.”
Astronomers need a quiet place to observe the cosmic dawn
The far side of the moon offers a unique opportunity to radio astronomers: an observatory built there could peer into the early universe, shielded from electromagnetic interference from Earth. Illustration: Peter Sanitra
Early designs for far-side radio observatories envisioned large parabolic antennas nestled in craters, much as Earth’s Arecibo telescope is nestled into a sinkhole in Puerto Rico. Illustration: Peter Sanitra
But modern plans for moon-based astronomy focus on the low-frequency signals from the cosmic dawn, when the first stars and galaxies formed. These frequencies, which are below 100 megahertz, can best be detected by a large array of antennas. Illustration: Peter Sanitra
In one construction approach, dipole antennas would be attached to spools of flexible film. Then a teleoperated rover would unroll the spools on the lunar surface. Illustration: Peter Sanitra
The lunar regolith doesn’t conduct electricity, so antennas won’t short out on the ground, as they would on Earth. But scientists still need to study how the regolith might otherwise affect radio waves. Illustration: Peter Sanitra
The central electronics box would sift and compress signals from the antenna before transmitting data back to Earth via a relay satellite. The equipment will have to withstand extremes of heat and cold during the moon’s month-long day/night cycle. Illustration: Peter Sanitra
Because the relay satellite’s radio uses much higher frequencies than 100 MHz, it won’t interfere with the observatory. Illustration: Peter Sanitra
Thousands of dipole antennas would be attached to the film, along with the wires carrying the signals they pick up. (An alternate approach would deposit many individual pizza-box-size antennas across the surface.) Illustration: Peter Sanitra
For decades, astronomers have gazed up at the moon and dreamed about what they would do with its most unusual real estate. Because the moon is gravitationally locked to our planet, the same side of the moon always faces us. That means the lunar far side is the one place in the solar system where you can never see Earth—or, from a radio astronomer’s point of view, the one place where you can’t hear Earth. It may therefore be the ideal location for a radio telescope, as the receiver would be shielded by the bulk of the moon from both human-made electromagnetic noise and emissions from natural occurrences like Earth’s auroras.
Early plans for far-side radio observatories included telescopes that would use a wide range of frequencies and study many different phenomena. But as the years rolled by, ground- and satellite-based telescopes improved, and the scientific rationale for such lunar observatories weakened. With one exception: A far-side telescope would still be best for observing phenomena that can be detected only at low frequencies, which in the radio astronomy game means below 100 megahertz. Existing telescopes run into trouble below that threshold, when Earth’s ionosphere, radio interference, and ground effects begin to play havoc with observations; by 30 MHz, ground-based observations are precluded.
In recent years, scientific interest in those low frequencies has exploded. Understanding the very early universe could be the “killer app” for a far-side radio observatory, says Jack Burns, an astrophysics professor at the University of Colorado and the director of the NASA-funded Network for Exploration and Space Science. After the initial glow of the big bang faded, no new light came into the universe until the first stars formed. Studying this “cosmic dawn [PDF],” when the first stars, galaxies, and black holes formed, means looking at frequencies between 10 and 50 MHz, Burns says; this is where signature emissions from hydrogen are to be found, redshifted to low frequencies by the expansion of the universe.
With preliminary funding from NASA, Burns is developing a satellite mission that will orbit the moon and observe the early universe while it travels across the far side. But to take the next step scientifically requires a far larger array with thousands of antennas. That’s not practical in orbit, says Burns, but it is feasible on the far side. “The lunar surface is stable,” he says. “You just put these things down. They stay where they need to be.”
This article appears in the July 2019 print issue as “The View From the Far Side.”
Technological enhancements to the Nobel Prize-winning detectors include ultra-efficient mirrors and “squeezed” laser light
At 4:18 a.m. Eastern time on 25 April, according to preliminary observations, a gravitational wave that had been traveling through deep space for many millions of years passed through the Earth. Like a patient spider sensitive to every jiggle in its web, a laser gravitational wave detector in the United States detected this subtle passing ripple in spacetime. Computer models of the event concluded the tiny wobbles were consistent with two neutron stars that co-orbited and then collided 500 million light-years away.
Next came scientific proof that when it rains it pours. The very next day at 11:22 am ET, the Laser Interferometer Gravitational-Wave Observatory (LIGO) picked up another gravitational wave signal. This time, computer models pointed to a potential first-ever observation of a black hole drawing in a neutron star and swallowing it whole. This second spacetime ripple, preliminary models suggest, crossed some 1.2 billion light years of intergalactic space before it arrived at Earth.
In both cases, LIGO could thank a recent series of enhancements to its detectors for such its ability to sense such groundbreaking science crossing its threshold.
LIGO’s laser facilities, in Louisiana and Washington State,are separated by 3002 kilometers (3,030 km over the earth’s surface). Each LIGO facility splits a laser beam in two, sending the twinned streams of light down two perpendicular arms 4 km long. The light in the interferometer arms bounces back and forth between carefully calibrated mirrors and optics that then recombine the rays, producing a delicate interference pattern.
The pattern is so distinct that even the tiniest warps in spacetime that occur along the light rays’ travel paths—the very warps of spacetime that a passing gravitational wave would produce—will produce a noticeable change. One problem: The interferometer is also extremely sensitive to thermal noise in the mirrors and optics, electronics noise in the equipment, and even seismic noise from nearby vehicle traffic and earthquakes around the globe.
Noise was so significant an issue that, from 2006 to 2014, LIGO researchers observed no gravitational waves. However, on September 14, 2015, LIGO detected its first black hole collision—which netted three of LIGO’s chief investigators the 2017 Physics Nobel Prize.
Over the ensuing 394 days of operations between September 2015 and August 2017, LIGO observed 11 gravitational wave events. That averages out to one detection every 35 days.
Then, after the latest round of enhancements to its instruments, LIGO’s current run of observations began at the start of this month. In April alone, it’s observed five likely gravitational wave events: three colliding black holes and now the latest two neutron star/neutron star-black hole collisions.
This once-per-week frequency may indeed represent the new normal for LIGO. (Readers can bookmark this page to follow LIGO’s up to the minute progress.)
Most promisingly, both of last week’s LIGO chirps involve one or two neutron stars. Because neutron stars don’t gobble up the light their collisions might otherwise emit, such an impact offers up the promise of Earth being bathed in detectible gravitational and electromagnetic radiation. (Such dual-pronged observations constitute what’s called “multi-messenger astronomy.”)
“Neutron stars also emit light, so a lot of telescopes around the world chimed in to look for that and locate it in the sky in all different wavelengths of light,” says Sheila Dwyer, staff scientist at LIGO in Richland, Wash. “One of the big goals and motivations for LIGO was to make that possible—to see something with both gravitational waves and light.”
The first such multi-messenger observation made by LIGO began in August 2017 with a gravitational wave detection. Soon thereafter came a stunning 84 scientific papers, examining the electromagnetic radiation from the collision across the spectrum from gamma rays to radio waves. The science spawned by this event, known as GW170817, led to precise timing of the speed of gravitational waves (the speed of light, as Einstein predicted), a solution to the mystery of gamma-ray bursts, and an overnight updating of models of the cosmic source of heavy elements on the periodic table. (Studies of the collision’s gravitational and electromagnetic radiation concluded that a large fraction of the universe’s elements heavier than iron originate from neutron star collisions just like GW170817.)
When the S190425z and S190426c signals came in, telescopes around the world pointed to the regions of the sky that the gravitational wave observations suggested. As of press time, however, no companion source in the sky has yet been found for either.
Yet because of LIGO’s increased sensitivity, the promise of yet more observations increase the likelihood that another GW170817 multi-messenger watershed event is imminent.
Dwyer says LIGO’s latest incarnation uses high-efficiency mirrors that reflect light back with low mechanical or thermal energy transfer from the light ray to the mirror. This is especially significant because, on average, the laser light bounces back and forth along the interferometer arms 1000 times before recombining and forming the detector’s interference pattern.
“Right now we have a very low-absorption coating,” she says. “A very small absorption of that [laser light] can heat up the optics in a way that causes a distortion.”
If the LIGO team can design even lower-loss mirror coatings (which of course could have spinoff applications in photonics, communications and optics) they can increase the power of the laser light traveling through the interferometer arms from the current 200 kilowatts to a projected 3 megawatts.
And according to Daniel Sigg, a LIGO lead scientist in Richland, Wash., another enhancement involves “squeezing” the laser light so that the breadth of its amplitude is sharper than Heisenberg’s Uncertainty Principle would normally allow.
“We can’t measure both the phase and the amplitude or intensity of photons [with high precision] simultaneously,” Sigg says. “But that gives you a loophole. Because we’re only counting photons, we don’t really care about their phase and frequency.”
So LIGO’s lasers use “squeezed light” beams that have higher noise in one domain (amplitude) in order to narrow the uncertainty in the other (phase or frequency). So between these two photon observables, Heisenberg is kept happy.
And that keeps LIGO’s ear tuned to more and more of the most energetic collisions in the universe—and allows it to turn up new science and potential spinoff technologies each time a black hole or neutron star goes bump in the night.
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