by Paul Gilster | Mar 24, 2010 | Outer Solar System |
Live by the cloud, die by the cloud. At least, that’s the way it felt this morning when I realized Gmail was down, and along with it, several emails with pointers to stories I had planned to look at for possible use today. But let’s talk about a different kind of cloud, for we still have the interesting news out of UC-Berkeley about what some reports are calling ‘helium rain’ on Jupiter. That’s a colorful way to describe an exotic process, but it may not be the best analogy given the difference between Earth’s comparatively gentle rainfall and the hellish conditions where neon ‘rain’ might fall.
After all, this is work aimed at creating models of planetary interiors, in this case a gas giant where helium forms droplets between 10,000 and 13,000 kilometers below the tops of Jupiter’s hydrogen clouds. Down in that realm pressures and temperatures reach absurd levels and both hydrogen and helium act like fluids. What we’re calling ‘rain’ then is actually made up of drops of fluid helium mixed with neon falling through another fluid, metallic hydrogen. Let me quote UCB’s Hugh Wilson on the matter:
“Helium condenses initially as a mist in the upper layer, like a cloud, and as the droplets get larger, they fall toward the deeper interior. Neon dissolves in the helium and falls with it. So our study links the observed missing neon in the atmosphere to another proposed process, helium rain.”
Wilson has been working on these matters with colleague Burkhard Militzer, whose 2008 computer simulations depicted Jupiter’s rocky core as surrounded by a layer of methane, water and ammonia ices that is twice as large as previously thought. The modeling takes into account what happens to hydrogen and helium as temperatures and pressures increase. This kind of work has to be modeled on a computer because we can’t reproduce conditions this extreme in a laboratory. The current work grows out of an observation by the Galileo probe as it plunged to its death in Jupiter’s atmosphere in 1995. The Galileo data showed that neon was only about one-tenth as abundant as found on the Sun.
Image: A slice through the interior of Jupiter shows the top layers that are depleted of helium and neon, the thin layer where helium drops condense and fall, and the deep interior where helium and neon again mix with metallic hydrogen. Credit: Burkhard Militzer.
Getting neon out of the upper atmosphere involves having it mix with helium (the two mix easily), and the new work shows that temperatures of 5,000 degrees Celsius and pressures up to 2 million times the atmospheric pressure of Earth can turn hydrogen into a conductive metal. Not yet a metal at this point, helium does not mix with the metallic hydrogen and thus forms drops. Helium and neon again mix with hydrogen in Jupiter’s deeper interior. Says Militzer:
“As the helium and neon fall deeper into the planet, the remaining hydrogen-rich envelope is slowly depleted of both neon and helium. The measured concentrations of both elements agree quantitatively with our calculations.”
So we’re learning more about what conditions may be like deep within the largest planet in our Solar System, and that’s helpful because the Juno mission is scheduled to be launched to Jupiter next year. Among other things, Juno will be making maps of the giant planet’s gravity, magnetic fields and atmospheric composition from a polar orbit, revealing the mass of its core and deepening our understanding of how such planets form. In turn, that gives us insights into the gas giants we’re discovering around other stars.
The paper is Wilson and Militzer, “Sequestration of Noble Gases in Giant Planet Interiors,” Physical Review Letters 104, 121101 (2010) (abstract).
by Paul Gilster | Mar 23, 2010 | Exotic Physics |
Yesterday’s look at black holes and their potential role in generating energy for advanced civilizations flows naturally into newly released work from Xavier Hernandez and William Lee (National Autonomous University of Mexico). The astronomers have been studying how dark matter behaves in the vicinity of black holes, simulating the way early galaxies would have interacted with it. Current theory suggests that clumps of dark matter drew together gas that eventually became the stars and galaxies we see around us in the cosmos.
How to study material that is invisible save for its gravitational influence? Its effect on gravitational lensing is one way, but Hernandez and Lee have found another. The duo looked for clues in the massive black holes now thought to be at the center of most large galaxies. Assuming such black holes are common, then large haloes of dark matter have coexisted with massive black holes over most of the history of the universe. It follows that part of the growth of these central black holes has come at the expense of captured dark matter particles. By studying how central black holes grow through accretion, the scientists place upper limits on the maximum density of dark matter at the center of haloes.
Image: Artist’s? schematic ?impression ?of ?the? distortion? of? spacetime ?by ?a? supermassive ?black?hole? at? the ?centre? of ?a ?galaxy. ?The ?black? hole? will? swallow ?dark ?matter? at ?a ?rate ?which ?depends? on ?its ?mass? and ?on? the? amount ?of? dark ?matter ?around ?it. Credit: ?Felipe ?Esquivel? Reed.?
From the paper:
We find the process to be characterised by the onset of a rapid runaway growth phase after a critical timescale. This timescale is a function of the mass of the black hole and the local density of dark matter. By requiring that the runaway phase does not occur, as then the swallowing up of the halo by the black hole would seriously distort the former, we can obtain upper limits to the maximum allowed density of dark matter at the centres of haloes.
Concentrate dark matter greater than 250 solar masses per cubic parsec, the authors find, and you get a runaway black hole, one that engulfs so much dark matter that the galaxy involved would be fundamentally changed.
Says Hernandez:
“Over the billions of years since galaxies formed, such runaway absorption of dark matter in black holes would have altered the population of galaxies away from what we actually observe.”
We don’t see that result, which implies that dark matter is distributed evenly through dark matter haloes rather than found in clumps of the sort that could lead to the runaway scenario. That’s a useful result because some dark matter models, analyzed via computer simulation, assume that dark matter is clumpy, following what dark matter researchers call ‘cuspy density profiles.’ The new work gives weight to dark halo density profiles that are constant, a sign that earlier modeling of these haloes is suspect.
This is how science often progresses, one tweak at a time, as we deepen our understanding of the dark matter that observation suggests must shape the galaxies around us. The paper is “An upper limit to the central density of dark matter haloes from consistency with the presence of massive central black holes,” accepted by Monthly Notices of the Royal Astronomical Society (preprint).
by Paul Gilster | Mar 22, 2010 | Exotic Physics |
by Adam Crowl
Louis Crane’s work at Kansas State University caught my eye some time back, but I was uncomfortable trying to explain it when I knew polymath Adam Crowl had so much better insight into Crane’s thinking than I did. One thing led to another, and now we can get an overview of Crane’s thoughts on black holes and starships from Adam himself. As a source of power, an artificially created black hole dwarfs alternatives, but the most intriguing possibility here is that a sufficiently advanced civilization might be able to use such a power source to propel a starship. Is forty years to Alpha Centauri a reasonable expectation with such technology? Read on.
Infinities in physics are usually a sign that something has gone wrong with theory. Towards the end of the 19th Century classical physics when applied to the heat emission from a uniformly heated cavity predicted an infinite amount of ultraviolet emissions – the so-called “ultraviolet catastrophe.” In 1900 Max Planck solved the puzzle by assuming energy was emitted in discrete amounts – quanta – and thus quantum theory was born. Similarly the gravitational collapse of stars, according to classical Newtonian gravity and 19th Century thermodynamics, should allow the release of an infinite amount of energy as the mass became infinitely small – an infinity eliminated by the theory of relativity.
Thus when infinities arise from attempts to combine quantum theory and general relativity (the modern theory of gravity) many physicists believe the theory is in error and a new quantum gravity is needed to eliminate the messy infinities. String theory is one such quantum gravity theory, but several other approaches exist. Loop-quantum gravity, for example, has been explored extensively by theorists like Lee Smolin. No clearly correct resolution to the puzzle has yet been found, but many researchers are hopeful in their quest. See this recent New Scientist story on approaches to ‘theories of everything.’
Intelligent Life and the Cosmos
One researcher in quantum gravity is professor Louis Crane, at Kansas State University, who has been working on quantum gravity theory his whole professional life. One question prompted by his investigations is of particular relevance to interstellar travel – what is the relationship between intelligent life and the Cosmos? Lee Smolin proposed a theory in which different kosmoi (the plural of ‘kosmos‘) reproduce and evolve via producing slightly different daughter kosmoi – the daughter kosmoi being generated by the formation of black holes. Black holes form via stars, and stars are friendly to life, thus there is a connection, albeit accidental. Louis Crane wasn’t convinced an accidental connection was direct enough, so he wondered: Could intelligent life make black holes? And if so why?
Image: Will future civilizations be able to create black holes for use in propelling a starship? Credit: Ute Kraus.
As Einstein’s relativity has taught us, mass is equivalent to energy, immense amounts of energy. A kilogram of mass is equivalent to 90,000 trillion joules of energy. However matter, in the form of particles, is incredibly difficult to turn into energy. Several conservation laws are observed to operate in the many interactions and transmutations that particles can undergo in the high energy collisions studied in particle accelerators, and these laws prevent merely dissolving matter into energy. However in 1974 Stephen Hawking showed a link between gravity and quantum mechanics in the form of the entropy and implied temperature of the event horizons that wrap around the insides of black holes. This means that black holes radiate energy with a purely thermal output, seemingly violating the conservation laws that give particles their identities, effectively making black holes a means for converting matter into pure energy.
Harnessing a Black Hole’s Power
Thus civilizations might create artificial black holes in order to convert matter into pure energy. This process is immensely more powerful than any other known energy source in the Universe aside from the mutual annihilation of matter & antimatter. The difference is that Hawking radiation needs only regular matter to be fed into a black hole, not the inefficient creation of equal amounts of matter and antimatter from energy. To create useful amounts of energy a black hole has to be relatively small – the immense black holes created by the implosion of stars produce less than attowatts of power as the luminosity of a black hole is inversely proportional to its mass squared. To produce as much energy as a 100 watt light-bulb a black hole needs to mass 1.9 trillion tons. A million ton black hole would produce almost 360 trillion watts – in fact the energy output computation gets complicated as the black hole temperature becomes high enough to produce heavy particles via pair-production.
According to the high-energy equations used by Louis Crane the actual energy produced by a million ton black hole is 56 petawatts, the equivalent of 0.62 kilograms of mass-energy radiated away per second, meaning a lifespan of less than 17 years. To keep the black hole operating it has to be continually force-fed new mass, else it will eventually decay, exploding in an immense blast of high energy particles.
Such an immense power, 56 petawatts, is roughly 4000 times the total energy usage on planet Earth, thus black holes are over-kill as terrestrial power sources, but ideal for a different purpose – propelling starships. Very big ones. A million tons of starship surrounding a million ton black hole could use the black hole’s power as a photon drive and accelerate at about 0.01 gee. Such acceleration would allow a trip time to Alpha Centauri of about 40 years. If the starship could draw in mass from the interstellar medium its range would be effectively infinite. Such a “mass annihilation Bussard ramjet” would be the ultimate sub-light starship.
To Colonize the Universe
Louis Crane is sceptical of the suggested alternative space-drives – for example wormholes and warp-drives – since the negative energy needed to keep such distorted space-time stable has yet to be demonstrated in such large amounts. To colonize the Universe, Crane believes, intelligent life will need to create and control black holes, thus completing the circle which created that life in the first place. Kosmoi in which black holes can be created by intelligent activity and used to expand intelligence’s reach across that kosmos, would be favoured by the process of cosmic natural selection, since they produce more daughter kosmoi friendly to intelligent, star-faring life.
Some species of jelly-fish, the medusids, live a two-stage life-cycle – in one stage they’re immobile polyps that produce the second stage, the mobile medusa stage. One creates the other, and the other in turn recreates the first, in a never-ending cycle. Crane likens the process of cosmic creation by intelligent life, and the creation of life by those kosmoi, to the two-stage life-cycle of the medusid jelly-fish, what he calls the Meduso-Anthropic Principle.
Crane’s current research is to apply quantum gravity theory to the finer details of the process of creation and decay of low-mass black holes. The possible creation of miniature black holes by the LHC will help refine and constrain quantum gravity theory, perhaps even give us clues towards creating matter annihilating black holes. Another important area of research is the question of how to convert the intense radiation output from a black hole into a useful exhaust stream. The finer practicalities may well take centuries to achieve, but the implications go well beyond even the lifetime of our Universe, affecting the evolution of countless more daughter universes we may create in the aeons ahead.
Three Crane papers covering these ideas are “Possible Implications of the Quantum Theory of Gravity” (preprint), “Starships and Spinoza” (preprint) and “Are Black Hole Starships Possible?” (preprint).
by Paul Gilster | Mar 19, 2010 | Deep Sky Astronomy & Telescopes |
Type Ia supernovae have become important ‘standard candles’ in judging cosmic distances, telling us how far away the host galaxy of a given supernova is. The idea here is that this kind of supernova produces a consistent luminosity because the white dwarfs that explode in the process are of uniform mass. The Type Ia supernova happens like this: A white dwarf gathers material from a companion star, growing in pressure and density so that the dwarf approaches the Chandrasekhar limit, beyond which it cannot support its own weight.
The result is a violent explosion that, like Cepheid variable stars, offers astronomers a way to gauge distances, and thereby to probe the shape of the cosmos at various distances and eras. Just how fast is the universe expanding, and in what ways? It was in 1998, prompted by supernovae of this kind, that the High-z Supernova Search Team discovered that the universe was not only expanding, but that its expansion was accelerating. Suddenly we were talking about ‘dark energy,’ which evidently operated against the gravitational force and seemed to offer a cosmic future in which the universe expanded without end.
Image: Cosmologists use Type Ia supernovae, like the one visible in the lower left corner of this galaxy, to explore the past and future expansion of the universe and the nature of dark energy. Credit: High-Z Supernova Search Team, HST, NASA.
It’s significant, given the importance of Type Ia supernovae in this analysis, that we’re now talking about exceptions to the Chandrasekhar limit, which is a critical mass of about 1.4 times that of the Sun. Four supernovae since 2003 have been discovered whose white dwarfs evidently surpassed the limit, the events being dubbed ‘super-Chandrasekhar’ supernovae. Richard Scalzo (Yale University) and a team of American and French physicists have measured the mass of one of the white dwarfs involved in the supernova SN 2007if, finding that it did indeed exceed the Chandrasekhar limit. Now work intensifies on modeling its structure.
Measuring the mass of the central star along with a shell of material ejected in the explosion and a surrounding envelope of pre-existing material, Scalzo and crew determined that the star had a mass of 2.1 times that of the Sun, significantly above the limit. So what’s going on here? In the paper on this work, Scalzo speculates that SN 2007if is the result of the merger of two white dwarfs, but other possibilities are also discussed in a paragraph that seems quite significant for our studies of dark energy:
The single-degenerate scenario… ensures that the white dwarf slowly approaches MCh [the Chandrasekhar mass limit] via accretion from a non-degenerate companion. In contrast, the double-degenerate scenario…, in which two white dwarfs in a binary system merge and explode, provides a way for SN Ia progenitors to exceed MCh and to give rise to more luminous events… There may therefore be a population of SNe Ia with a distribution of masses greater than MCh, with different explosion physics that interferes with luminosity standardization [italics mine].
And here is the nub of the problem:
The relative rate of such events among SNe Ia in general may also depend on redshift, and unless they can be identified or their luminosities accurately calibrated, they need not be common to produce significant biases in reconstructions of the dark energy equation of state.
Let me quote Scalzo as well from this Yale University news release on the same point:
“Supernovae are being used to make statements about the fate of the universe and our theory of gravity. If our understanding of supernovae changes, it could significantly impact our theories and predictions.”
And that’s why a paper studying a rare type of star that seems to ignore the Chandrasekhar limit swims into focus as we discuss a much broader issue. We’ve been speculating about the nature of dark energy for some time in these pages, even wondering if it might offer a clue to a far-future propulsion system by harnessing a force that appeared to operate against gravity. Now we find evidence that at least in some rare instances, the standard candles used in this work may be suspect. We need to find out how such supernovae can exist above the Chandrasekhar limit and learn whether they exist in sufficient numbers to compromise our measurements of the universe’s expansion.
The paper is Scalzo et al., “Nearby Supernova Factory Observations of SN 2007if: First Total Mass Measurement of a Super-Chandrasekhar-Mass Progenitor,” to be published in the Astrophysical Journal (preprint available).
by Paul Gilster | Mar 18, 2010 | Deep Sky Astronomy & Telescopes |
Watching how exoplanet news hits the press is always interesting, but I was surprised at how the discovery of CoRoT-9b (discussed here yesterday) was received. The scientific reward could be significant, which is why one scientist referred to the find as a ‘Rosetta stone,’ but the fact that we had a gas giant that was both analyzable through transits and not a ‘hot Jupiter’ evidently needed to be ginned up in some media circles. What emerged were headlines seeing similarities to our Solar System (New Exoplanet Like One of Ours) and making bizarre extrapolations: Corot-9b: Extra Solar Planet Proffers Hope of Inhabitation.
I suppose CoRoT-9b is like a planet in our Solar System in being a gas giant in a stable orbit not hugging its star, but it’s hardly alone in that regard. What makes it special is that we can study it both by radial velocity and transit methods, gaining insights into the composition of such ‘temperate’ gas giants. I suspect the headlines left many readers disappointed when they read the ensuing story and realized it wasn’t about a terrestrial world just like the Earth. As for colonizing a gas giant, well, I leave that to your imagination.
And what to do about Gliese 710? Here is a dwarf star that may, in approximately 1.5 million years, pass through the Oort Cloud, with all the disruptive effect that seems to describe. A rain of comets moving into the inner system? Headlines like Rogue Star to Hit the Solar System are lively, to be sure, and send a chill up the spine of that dedicated band that is convinced we’re all about to be destroyed by runaway celestial objects in 2012. News on a million-year time cycle is not something that sells papers, but ‘rogue stars’ just might.
As to Gl 710, Vadim Bobylev (Pulkovo Astronomical Observatory, St Petersburg) used revised Hipparcos data to make the call on its future near-miss. Looking at stars within 30 parsecs of the Sun, Bobylev found nine new candidates to add to previously known close encounters (the astronomer defines a ‘close encounter’ as passing less than 2 parsecs from the Sun). And here’s the scoop on the closest of these:
For the star GL 217.1, a well-known candidate for a passage close to the Sun, the new observational data were shown to change noticeably its previously known encounter parameters with the Sun. The encounter parameters found here are: dmin = 1.28 ± 0.06 pc and tmin = ?(861 ± 40) thousand years. Improving the radial velocity for the white dwarf WD 0310–688 (HIP 14754) whose orbit passed at a distance dmin = 1.61 ± 0.19 pc from the solar orbit about 300 thousand years ago is of current interest. Our statistical simulations showed that the star GL 710 has not only a high probability of penetrating into the Oort cloud, P1 = 0.86, but also a nonzero probability, P2 = 1 × 10?4, of penetrating into the region where the influence of the passing star on Kuiper Belt objects is significant.
Gliese 710, then, may well penetrate the Oort Cloud, potentially causing the kind of disruption there that could bring comets into an Earth-crossing orbit. Of all the stars the astronomer studied, this is the only one with a high probability of entering the Oort region. Interestingly enough, apart from what Bobylev has given us, we know about 156 Hipparcos stars within a radius of 50 parsecs that either have or will encounter the Solar System within a distance of less than 5 parsecs in a window from 10 million years in the past to ten million years in the future. One revised study shows the frequency of encounters closer than one parsec to be roughly 11.7 events per million years (plus or minus 1.3).
A New Scientist article goes to work on the Gl 710 encounter (Hurtling Star on a Path to Clip Solar System), saying the star will almost certainly send comets toward the Earth, and speculating on possible changes to Neptune’s orbit, which could occur if the one in ten thousand probability of such a close penetration comes to pass. So Gl 710, now ‘hurtling’ 63 light years from us in the eastern part of the constallation Serpens, becomes an object of concern for our remote posterity.
Does interstellar travel occasionally get such nudges for those species fortunate enough to have the technology to exploit them? A star closing to these distances is potentially in range for a civilization able to make a 10,000 AU journey (roughly the distance between Proxima Centauri and Centauri A and B). It’s useful to remember that the vast distances between the stars are themselves changeable as the galaxy continues to evolve and as our Sun continues its passage around it. It’s also useful to keep a cosmic perspective, one that reckons that on a galactic scale, civilizations may find innumerable ways to spread.
There’s your headline: ‘Civilization Jumps onto Passing Star.’ If we wait long enough, we may get to read it. The paper is Bobylev, “Searching for Stars Closely Encountering with the Solar System,” Astronomy Letters, Vol. 36, No. 3 (2010). Abstract available.
