by Paul Gilster | Apr 12, 2010 | Astrobiology and SETI |
by James and Gregory Benford
Interstellar beacons continue to draw discussion, and the Benford brothers now return with further thoughts on the matter in response to reader comments here. How to distinguish a beacon from a natural source, and why consider it in terms of cost? The answer is below, as is an interesting twist on the Fermi paradox.
We proposed making cost a useful, perhaps universal, standard because it is a quantitative constraint. Our aim is to help observers look for plausible beacons that may exist. Using transient events seen by observers is an economic way to ask these next, exploratory questions. Speculations always yield to data, and at its 50th anniversary SETI needs a vital data point: first detection.
In our latest work, we point out that researchers should be aware of the likely properties of beacons. In particular, beacons may mimic pulsars in repetition rate. But they would distinguish themselves in some way, such as amplitude modulation, varying pulse interval, frequency modulation, etc. And they may provide ‘enough’ pulses on a given target area for their artificial origin to be noticed.
While discussions of many social issues, particularly motivations, do provoke – they seldom illuminate. Are aliens beyond economic arguments? We call this the Altruistic Alien argument — that aliens of great ability, near-infinite resources and benign intent will transmit to us without taking any consideration to the cost (which would be very high in our terms, especially for long range). They will make everything easy for us. And an omni-directional Beacon, radiating at the entire galactic plane, for example, would have to be enormously powerful and so very expensive.
On this site, several comments speak of robot manufacture, future economic miracles, etc. – but forget that the idea of exponentially falling costs in future technologies has an observable test: If vastly rich aliens can easily afford beacons of the sort we seek – microwave or IR or even visible – then where are they? If there are Altruistic Aliens, where is the evidence? They certainly haven’t made beacons that are apparent in our sky. So, the rich aliens argument fails the test.
This Fermi question limits useful speculations. Whether alien motivations will resemble ours we’ve dealt with, but think secondary – all motivations meet constraints, and biological evolutionary theory suggests that cost (economic cost, environmental, etc.) is a universal constraint.
We think that, if they are social beings interested in a SETI conversation or passing on their heritage, they will know about tradeoffs between social goods, and thus, in whatever guise it takes, cost. But what if we suppose, for example, that aliens have very low cost labor in the form of automata? With a finite number of automata, i.e., slaves, you can use them to do a finite number of tasks. And so you pick and choose by assigning value to the tasks, balancing the equivalent value of the labor used to prosecute those tasks. So choices are still made on the basis of available labor. The only case where labor has no value is where labor has no limit. One might think that might be if aliens have limitless armies of self-replicating automata, but such labor costs something, because resources, materials and energy, are not free.
Our point is that all SETI search strategies must assume something about the beacon builder, and that cost may be a near-universal driver for alien attempts at interstellar communication.
by Paul Gilster | Apr 9, 2010 | Outer Solar System |
A new instrument that lets us look deeper into things almost always changes the game. Such an instrument is CRIRES, the Cryogenic High-Resolution Infrared Echelle Spectrograph. Now operational at the Very Large Telescope, CRIRES has already done yeoman work on Pluto, and has now been used to study the atmosphere of Neptune’s large moon Triton in more detail than ever before. The result: A new understanding of Triton’s carbon monoxide, whose existence in its upper surface layer is now confirmed and shown to be an icy ‘film’ that, over time, adds to the atmosphere.
Image: Artist’s impression of how Triton, Neptune’s largest moon, might look from high above its surface. The distant Sun appears at the upper-left and the blue crescent of Neptune right of centre. Using the CRIRES instrument on ESO’s Very Large Telescope, a team of astronomers has been able to see that the summer is in full swing in Triton’s southern hemisphere. Credit: ESO/L. Calçada.
It should come as no surprise that astronomers have also identified seasonal variation in the atmosphere, given that we see similar changes on Pluto. The team, led by Emmanuel Lellouch, estimates that the atmospheric pressure on the distant moon may have risen by a factor of four compared to what Voyager found during its 1989 flyby. The spacecraft found a nitrogen and methane atmosphere at a pressure of 14 microbars, roughly 70,000 times less dense than the atmosphere of Earth. The pressure is now measured at between 40 and 65 microbars, or 20,000 times less than on Earth.
“We have found real evidence that the Sun still makes its presence felt on Triton, even from so far away,” says Lellouch. “This icy moon actually has seasons just as we do on Earth, but they change far more slowly.”
Slowly indeed. It’s hard to talk about summer on a place where the average surface temperature is minus 235 C, but Triton’s southern hemisphere is currently enjoying that season, causing a layer of frozen nitrogen, methane and carbon monoxide to sublimate into gas. It’s a long, slow process, with seasons on Triton lasting for forty years. The new findings will doubtless lead to a reassessment of atmospheric models on Triton.
You may remember a 2009 ESO study using CRIRES that revealed a temperature inversion on Pluto. The instrument helped scientists find more methane than anticipated in the atmosphere of the dwarf planet, a place where the thin envelope of nitrogen, methane and (most likely) carbon monoxide freezes out when Pluto moves away from the Sun during its 248-year orbit. Pluto’s atmospheric pressure is five times less than what is currently found on Triton. The Triton news means that the hunt for carbon monoxide on Pluto will now intensify.
The paper is Lellouch et al., “Detection of CO in Triton’s atmosphere and the nature of surface-atmosphere interactions,” Astronomy & Astrophysics Vol. 512, L8 (March/April 2010). Abstract available.
by Paul Gilster | Apr 8, 2010 | Culture and Society |
Project Ozma’s Anniversary
It was just fifty years ago today, April 8, 1960, when Frank Drake launched Project Ozma by turning the Green Bank, WV dish toward Tau Ceti. In a reminiscence of the project written for Cosmic Search magazine, Drake recalls the initial sense of anticipation, followed by examination of the chart recorder, which returned nothing but noise. When Tau Ceti set in the west, Drake and team pointed the telescope at Epsilon Eridani. Let Drake tell it:
A few minutes went by. And then it happened. Wham! Suddenly the chart recorder started banging off scale. We heard bursts of noise coming out of the loudspeaker eight times a second, and the chart recorder was banging against its pin eight times a second. We had never seen anything like this before in all the previous observing at Green Bank. We all looked at each other wide-eyed. Could it be this easy? Some people had even predicted that the most rational extraterrestrial signal would be a slow series of pulses, as that would be evidence of intelligent origin. (No one had any idea about the existence of pulsars then.)
An exciting moment, and one that led to further debate:
Suddenly I realized that there had been a flaw in our planning. We had thought the detection of a signal so unlikely that we had never planned what to do if a clear signal was actually received. Almost simultaneously everyone in the room asked “What do we do now?” Change the frequency? Well, the most likely source of a spurious signal was the earth, and we could check that out by moving the telescope off the star and seeing if the signal went away. So we proceeded to do that, and as we moved off the star, sure enough the signal went away. So we pointed back at the star. The signal did not come back. Wow. Was it really from the star, or had it been from earth and had it turned off about the time we moved off the star? There was no way to know. And there was all that adrenaline flowing and no way to apply all that excitement and energy in a useful way.
Image: The Howard Tatel Radio Telescope at the National Radio Astronomy Observatory, Green Bank WV. SETI pioneer Dr. Frank Drake used this 85 foot diameter antenna for Project Ozma, the first modern microwave search for intelligent extra-terrestrial signals, in 1960. Credit: SETI League.
It was about ten days later, with newspaper reporters already asking about the unusual signal, that the Project Ozma team was able to determine that the noise was man-made radio interference. Drake persevered, and from April to July the 26-meter NRAO radio telescope studied the 21-centimeter emission line (1420 MHz) of cold hydrogen, looking for the kind of patterned pulses that would reveal intelligence. No unusual signals turned up, but Drake had set the era of modern SETI detection attempts in motion. A second Ozma project was managed by Benjamin Zuckerman and Patrick Palmer starting in 1973, also at Green Bank, surveying about 650 stars during a four-year period with the same result as the original.
Interlacing Art and Science
Those of you in the New York area will want to take advantage of a panel on art and astronomy that will be held tonight at Central Booking in Brooklyn (111 Front St., Gallery 210). Interstellar flight specialist Greg Matloff and the artist C Bangs will be on the panel, along with Denton Ebel (American Museum of Natural History) and Ari Maller (New York City College of Technology). I’m just finishing Matloff and Bangs’ new title Paradise Regained: The Regreening of Earth (Copernicus, 2009), written with NASA’s Les Johnson. I’ll have more on the book in a subsequent post, but for now I’ll simply mention how C Bangs’ elegant artwork complements the book’s central argument, that space resources can help us revive our tired planet.
Image: Message Plaque Rainbow Hologram, by C Bangs.
How do art and science interact? From Central Bookings’ news release:
Human history has been greatly influenced by our collective image of the cosmos. Today, art and astronomy continue to interact, witness the uproar of the American public when NASA planned to prematurely de-orbit the Hubble Space Telescope. Many artists have utilized Hubble images and those taken by other modern telescopes. Conversely, art also influences astronomy. After the success of the recent movie Avatar, an on-going search for Terrestrial planets circling our near stellar neighbors Alpha Centauri A and B has been dubbed “the search for Pandora.” The universe has been considered to be many things: a divine construction, a machine and a mathematical exercise. But in his 1937 vintage science-fiction classic Star Maker, British author/philosopher Olaf Stapledon speculates that our cosmos (and all others) might be the work of a divine artist.
Bangs’ work has graced books like Matloff’s The Starflight Handbook and has appeared in permanent collections including the Library of Congress, NASA Marshall Space Flight Center and the Chrysler Museum. Matloff continues to explore our prospects for travel both within and without the Solar System in books and scientific papers. The panel will convene at 6:30 this evening and should be well worth the modest $5 entrance fee.
Universe in a Black Hole
Theoretical physicist Nikodem Poplawski describes the gravitational field of a black hole in a new paper in Physics Letters B, modeling the radial geodesic motion of a massive particle into such an object. Both Schwarzschild and Einstein-Rosen black holes are considered legitimate mathematical solutions of General Relativity, but in both cases, we can see only the outside of a black hole, leading Poplawski to question whether our universe might be itself inside a wormhole associated with a black hole within a much larger universe.
This is mind-bending stuff, and I can only quote the author:
“This condition would be satisfied if our universe were the interior of a black hole existing in a bigger universe. Because Einstein’s general theory of relativity does not choose a time orientation, if a black hole can form from the gravitational collapse of matter through an event horizon in the future then the reverse process is also possible. Such a process would describe an exploding white hole: matter emerging from an event horizon in the past, like the expanding universe.”
Are astrophysical black holes concealing universes that formed when the black holes themselves did? The concept is that a white hole is connected to a black hole by a wormhole — an Einstein-Rosen bridge — and the new paper suggests that all black holes in our cosmos may contain such wormholes. One intriguing possibility is that such a theory might help us resolve problems with black hole information loss, the notion that all information about matter is lost as it passes over the event horizon. And it might just help us explain cosmic inflation.
The paper is Poplawski, “Radial motion into an Einstein-Rosen bridge,” Physics Letters B, Vol. 687, Issues 2-3 (12 April 2010), pp. 110-113.
Geoff Marcy on Habitable Worlds
With 205 planets already discovered, Geoff Marcy and the California Planet Search team have plenty of accomplishments to look back on, and as Marcy told a recent interviewer, Kepler is working beautifully, offering unprecedented measurements of the stars it’s monitoring. The short interview adds to the excitement of future Kepler discoveries, but Marcy is cautious about getting too far out in front of actual science. Will we find an Earth-like planet soon? Marcy:
It’s presumptuous to predict that Kepler will find Earth-like planets. We may find that Earths are a dime a dozen or we might find that they are a rare treasure. We might even find zero Earth-like planets, which would be a spectacular result and suggest that our planet is an extraordinary contradiction to the norm.
If we do find Earth-like planets, it will take more than a year to identify them. Kepler needs to observe a planet passing in front of its star three times in order to confirm its existence. For a planet with the same orbit as the Earth, that’s over three years to wait.
Image: Planet hunter Geoff Marcy. Credit: University of California at Berkeley.
It’s good to see this dash of realism, along with the reminder to keep our personal biases out of the search:
I think it’s important to remain unbiased. As scientists, we need to make sure to keep an open mind so that we can faithfully report our findings and avoid over-interpreting our data. We can make mistakes if we want a certain answer badly. Quality data are most important, so I try to stay calm, vigilant, and self-critical.
A brief look at interplanetary exploration offers plenty of evidence for the value of scientific detachment. How many times have we been confounded by what our space probes have found, from the volcanoes of Io to the bizarre geysers of Enceladus? The good news about Kepler is that the recently reported CCD problem is minor (Marcy calls it an ‘inconvenience,’ nothing more), and that we’re within a few short years of finding answers that will flesh out our understanding of the cosmos and the incidence of terrestrial worlds.
by Paul Gilster | Apr 7, 2010 | Deep Sky Astronomy & Telescopes |
Our knowledge of brown dwarfs is expanding rapidly, and with the help of the WISE mission, we will be able to build a much more complete catalog of such stars in our neighborhood. But look what the Hubble Space Telescope, in conjunction with the Gemini Observatory, has produced: A companion to a brown dwarf that gets us right back into the debate about how to define a planet. When Pluto was in question, we were faced with a true imbroglio. Now the question involves not a small but a large object, and forces us to consider whether its origins can make an object of acknowledged planetary mass something else instead.
But first, the imagery (this is, after all, a direct detection). The primary brown dwarf is 2M J044144, images of which were obtained as part of a survey of 32 young brown dwarfs in the Taurus star-forming region 450 light years away. Both objects are visible below:
Image: Hubble Space Telescope (top) and Gemini North (bottom) images of the 2M J044144 system showing the smaller companion at 8:00 position. The companion has an estimated mass of between 5-10 times the mass of Jupiter. In the right panel of both the HST and Gemini images the brighter light from the brown dwarf has been removed to show the companion more clearly. Credit (top): NASA, ESA, and K. Todorov and K. Luhman (Pennsylvania State); (bottom): Gemini Observatory/AURA and K. Todorov and K. Luhman (Pennsylvania State University).
The primary brown dwarf weighs in at about 20 Jupiter masses, separated from the smaller body by 3.6 billion kilometers, which would put the latter between the distances of Saturn and Uranus in our own system. The question about the planetary status of the smaller object comes up because while its mass is consistent with other gas giants we’ve discovered, its age tells us that it may not have formed like a planet. Kevin Luhman (Pennsylvania State) has this to say:
“This is the youngest planetary-mass companion that has been found so far, and its extreme youth provides constraints on how it could have formed. The formation mechanism of this companion in turn can tell us whether it is truly a planet.”
Core accretion models of planet formation have a planet gradually forming within a circumstellar disk, a rocky core forming before the accumulation of a gas envelope. An alternative model relies on instability within the same disk, causing a clump of gas to collapse and form a gas giant on a short time-scale. But neither of these methods seems to apply here. The companion object did not have time to form in this young system by core accretion, and gravitational instability relies on having enough material to make an object of this mass.
We’re left with a third option, that this smaller object formed from the collapse of the cloud of dust and gas in much the same way that the primary star did. This is from the paper on this work, and refers not only to the system in question, but to the earlier discovered companion of the brown dwarf 2M J12073346, which was found in 2004:
This result is consistent with the rapid formation expected from both gravitational instability in disks and fragmentation of cloud cores. The latter is more likely to have produced these two secondaries given their relatively large masses compared to the primaries.
The formation models are shown below:
Credit: NASA, ESA, and A. Feild (STScI).
This would mean that the same process that makes binary stars can produce planetary-mass objects. If we define planets as objects that build up inside disks, then 2M J044144’s companion is not a planet. The idea gains weight by the observation that a nearby M-dwarf seems to have a brown dwarf companion of its own. Again quoting the paper:
Indeed, if 2M J044144 A and B are members of a quadruple system, then its hierarchical configuration further suggests that 2M J044144 B formed by cloud core fragmentation. Additional data are needed to determine more definitively whether 2M J044144 A/B and 2M J044145 A/B comprise a quadruple system.
In this presumed quadruple system, all four objects would have formed in the collapse of the same cloud. Objects that push up against our categories are cause for celebration. They force us to ponder the nature of our definitions, and to modify them to revise theory. Meanwhile, brown dwarfs will remain much on the minds of those interested in the prospects of future interstellar missions. They seem to exist in abundance, and we may yet find one or more closer to us than the Alpha Centauri stars. And who knows what kinds of companions we may turn up around them?
The paper is Todorov et al., “Discovery of a Planetary-Mass Companion to a Brown Dwarf in Taurus.” In press at Astrophysical Journal Letters (preprint).
by Paul Gilster | Apr 6, 2010 | Astrobiology and SETI |
Recently we looked at James and Gregory Benford’s thoughts on interstellar beacons, noting that using cost as a likely constraint allowed the authors to discuss how cost would affect design, and therefore the parameters of any beacon we would be likely to observe. But what is it about interstellar beacons that sets them apart from transient phenomena? After all, it was no longer ago than 1963 that Nikolai Kardashev proposed that the radio source CTA 102 could be evidence of a Type II or III extraterrestrial civilization (i.e., one that is able to use the entire energy output of its star, or in the most extreme case, of its entire galaxy).
When Gennady Sholomitskii announced his observation that CTA 102’s radio emission was varying, something of a sensation ensued. Those of us of a certain age can recall Roger McGuinn’s song ‘CTA 102,’ written and performed by McGuinn’s group The Byrds. It was on their Younger Than Yesterday LP, released in 1967. A sample:
CTA 102
Year over year receiving you
Signals tell us that you’re there
We can hear them loud and clear
and so on. We soon learned, of course, that the source of these emissions was a quasar, one that has since been observed by a huge range of instruments. But the question that lingers is how we would separate out an atypical pulsar that might be producing odd transients from a genuine interstellar beacon. It’s a problem James Benford attacks in a new paper.
A Puzzling Transient Analyzed
Take the case of PSR J1928+15, a transient bursting source observed in 2005 near the galactic disk at 1.44 GHz. A two-minute observation by the Arecibo dish noted the signal but did not find it again despite 48 minutes of revisits. Three pulses were received, according to Benford’s paper, the first and third down a factor of ten from the 0.180 Jy central pulse. The source is roughly 24,000 light years away, putting it close to galactic center.
A pulsar? Pulsars are marked by radiation from a rotating neutron star’s magnetosphere. One explanation for this event is an asteroid falling into the neutron star from a circumpulsar disk, perturbing its magnetosphere. But because we’re trying to learn how to distinguish a beacon from a natural source, Benford looks at how we might analyze the observational data in ways that would allow us to deduce a beacon’s parameters. It’s a fascinating exercise:
We make two working assumptions:
1) The Beacon is a ‘lighthouse’ scanning the galactic plane. The source is a scanning beacon and, as it swept past, Arecibo caught the central pulse, the true beam. The first and third pulses are at the edges of the antenna’s acceptance angle, which is 3.5 arcmin=1 mrad.
2) The beam bandwidth covers all channels of the 100 MHz span of the detector array. (The channel BWs are 0.39 MHz, with total BW 100 MHz.) This assumption drives the Beacon power estimate.
If this is the case, then we can start to plug in values to make sense of the signal. Benford tries out a beacon antenna diameter of 100 kilometers, working out a total power of 190,000 TW. Think of this as a beacon and you are dealing with a civilization much more advanced and powerful than our own. It’s one that ranks above Kardashev Type I but falls far short of Kardashev Type II. Benford would rank it at Kardashev 1.13 (Earth is 0.73 on this scale).
Moreover, if the beacon is scanning the disk at a thickness of 1300 light years (roughly what the disk thickness is at our distance), then the signal cycle can be estimated. Benford works out a cycle around the galactic circumference of roughly fifteen hours, noting “It’s understandable that 48 minutes of revisits hasn’t seen it again, as that is only 5% of the revisit time. Of course, it could be scanning a smaller area, so that the revisit time would be sooner.”
Playing with Beacon Assumptions
Assume an antenna diameter of 1 kilometer and interesting changes to the conclusions occur:
The beamwidth is reduced by a factor of 10 to 5 x 10-4 rad. Spot size diameter falls to 12.2 ly, As~117 ly2. Power in the spot falls to 1900 TW, Kardashev scale falls to K=0.93. This is a civilization intermediate between ours and the planetary scale civilization of the previous example.
The spot moves at the same rate, 30 ly/sec. But since the spot is smaller, the number of strips in the scan increases to 1350ly/12.2ly = 110. So the Beacon will return in 110 x 5 x 103 sec = 5.5 x 105 sec = 150 hours. Observers have revisited the site for 48 minutes, only 0.5% of the revisit time, and haven’t seen it again.
So a civilization lower on the Kardashev scale, i.e., K= 0.93 will have a narrower beam, revisit less frequently, be harder to observe…
To reach a civilization at this level given the energy growth rate we see on Earth during the 20th Century would require about 2000 years, a small time on the cosmic scale. This corresponds to a civilization of Kardashev 0.93, not yet a full Type I.
The broader principles: We can begin to distinguish beacons from pulsars by bandwidth, for pulsars have large bandwidths. But bandwidth by itself is not definitive, because advanced methods of microwave generation might allow very broadband emissions with huge data transmission rates. We can also add in pulse length (“[c]ost optimized Beacons will likely be pulsed to lower cost, with a preference for shorter pulses due to source physics”) and frequency. Benford notes about the latter that pulsar searches cluster in the lower end of the microwave, but beacons are more likely to appear at higher frequencies “due to the favorable scaling of cost with frequency.”
Learning how we would set about observing a candidate beacon signal is not only an ingenious exercise in itself, but a necessary warm-up in case of future detections of even more puzzling transients than PSR J1928+15. The fact that natural phenomena can produce some of the same observables as an interstellar signal behooves us to sharpen our tools for analyzing and differentiating between such signals. The paper is James Benford, “How can we distinguish transient pulsars from SETI beacons?” (preprint available).
