by Paul Gilster | May 12, 2006 | Exotic Physics |
If you thought the trillion-year crunch was mind-boggling, how about light that moves backwards, and does so at speeds faster than c? From the University of Rochester comes word that Robert Boyd, a professor of optics there, has slowed light to negative speeds. To do this, the experimenter sent a pulse of laser light through an optical fiber laced with the element erbium. Leaving the laser, the light pulse was split, with one pulse going into the fiber and the other left undisturbed for reference.
The remarkable result: The peak of the pulse emerged from the other end of the fiber before it entered the front of the fiber, and ahead of the reference pulse. “Through experiments we were able to see that the pulse inside the fiber was actually moving backward, linking the input and output pulses,” says Boyd, who acknowledges “I’ve had some of the world’s experts scratching their heads over this one.”
Centauri Dreams hardly qualifies as an expert, but head-scratching does seem in order here. Boyd is quick to note that there is no violation of Einsteinian principles involved. What Einstein said was that information cannot travel faster than light, and no information is required to do that in these experiments. Boyd again:
“The pulse of light is shaped like a hump with a peak and long leading and trailing edges. The leading edge carries with it all the information about the pulse and enters the fiber first. By the time the peak enters the fiber, the leading edge is already well ahead, exiting. From the information in that leading edge, the fiber essentially ‘reconstructs’ the pulse at the far end, sending one version out the fiber, and another backward toward the beginning of the fiber.”
Can a pulse be designed without a leading edge? Einstein’s work would imply that if it can, the reverse light phenomenon will disappear. Reverse light thus becomes an interesting test of Einstein, though one that still confounds this writer. Animations of fast light, slow light and backward light may help. The paper is Gehring, Schweinsberg, Boyd et al., “Observation of Backward Pulse Propagation Through a Medium with a Negative Group Velocity,” in Science 12 (May 2006), pp. 895-897.
by Paul Gilster | May 11, 2006 | Exotic Physics |
How to explain dark energy, which is pushing distant galaxies away at an accelerating rate? The cosmological constant that would account for the phenomenon — originally conceived but then rejected by Einstein — is far smaller than one would expect from conventional Big Bang scenarios. In fact, the observed vacuum energy (a possible explanation for the repulsive force) is smaller by a factor of 10120 than it would need to be to do the job. But if the universe were older than today’s estimate of 13.7 billion years, and I mean a lot older, then this tiny value might make sense.
So say Paul Steinhardt (Princeton) and Neil Turok (Cambridge, UK), who put forward a startling concept: there was indeed a time before the Big Bang. There is remarkable solace in this for all of us who grew up asking what happened before the Big Bang, only to be told that the question made no sense because it was unanswerable. So said a kindly astronomy professor in a long-ago college course, raising his shaggy eyebrow at me: How you can talk about something before the advent of spacetime?
But now these two researchers argue in Science that the universe may be at least a trillion years old. It may, in fact, be eternal. That sound you just heard is Fred Hoyle spinning in his grave — how I wish I could buy him a drink! But, of course, this new theory isn’t Hoyle’s ‘steady state’ notion, either; it’s a remodeling based on possibly testable gravity wave and vacuum energy discoveries, and it adjusts to quantum theory demands. Its eternal universe is anything but serene…
Couple an eternal universe with what we know of Big Bang physics and you get a universe that is cyclical. Each new Big Bang replenishes the universe and sets up the next collapse on a trillion-year cycle. To pull this off, Steinhardt and Turok argue that the vacuum energy responsible for the cosmological acceleration is mutable. It started big but has continued to decline to present values, with each change in values taking exponentially longer than the previous one. Each cycle of growth and collapse needs a trillion years or so, enough time for the cosmological constant to have decayed close to zero.
The abstract of “Why the Cosmological Constant Is Small and Positive” argues for this principle: “…a cyclic model of the universe can naturally incorporate a dynamical mechanism that automatically relaxes the value of the cosmological constant, taking account of contributions to the vacuum density at all energy scales. Because the relaxation time grows exponentially as the vacuum density decreases, nearly every volume of space spends an overwhelming majority of the time at the stage when the cosmological constant is small and positive, as observed today.”
Ingenious, and a solid reminder that we are far from a comprehensive theory about what got us here. The difference between a 13.7 billion year old universe and an eternal one is, to say the least, thought provoking. And if this post seems light-hearted, consider it a celebration of that universe, a place so inexplicably odd that it produces theories like these.
Addendum: Larry Klaes passes along the arXiv link for this paper. Also be aware of Alexander Vilenkin’s thoughts on the paper, published first in Science as a perspective essay for the Steinhardt/Turok discussion in the same issue.
by Paul Gilster | May 10, 2006 | Interstellar Medium |
Can we say anything definitive about organic materials in the early Solar System? Perhaps so, judging from recent news from the Carnegie Institution. Researchers there have found organic particles from the days of Solar System formation inside meteorites. The material is similar to what is found in interplanetary dust particles believed to have come from comets, and gives us a view of the complexity of the organic mix that may have been available as the planets formed.
Studying six carbonaceous chondrite meteorites, the researchers looked at different isotopes of hydrogen and nitrogen associated with insoluble organic materials, which are extremely difficult to break down chemically. The relative proportion of these isotopes can reveal much about how the carbon was formed, and the meteorite samples show in some cases even higher amounts of the relevant isotopes than those found in interstellar dust.
“We have known for some time, for instance, that interplanetary dust particles (IDP), collected from high-flying airplanes in the upper atmosphere, contain huge excesses of these isotopes, probably indicating vestiges of organic material that formed in the interstellar medium,” says Larry Nittler, a co-author on the paper that was published in the May 5 issue of Science. “The IDPs have other characteristics indicating that they originated on bodies — perhaps comets — that have undergone less severe processing than the asteroids from which meteorites originate.”
But interplanetary dust particles provide only tiny samples; the new work makes it possible to examine much larger amounts of these materials from meteorites. What stands out to Centauri Dreams is a comment by another co-author, Conel Alexander: “…the parent bodies – the comets and asteroids — of these seemingly different types of extraterrestrial material are more similar in origin than previously believed.” And the study of early system organics just received a powerful boost.
Image (click to enlarge): These tiny particles, from carbonaceous chondrite meteorites, are just a few millionths of a meter wide and have different proportions of nitrogen (N) and hydrogen (H and D) isotopes. These isotopes are chemically bonded to meteoritic organic matter and can reveal a lot about what happened to the meteorite as it made its way through the solar system over billions of years. The two images show the regions with high levels of 15N and heavy hydrogen (deuterium or D)—indications that the associated carbon is very old and originated from interstellar matter or the outer regions of the solar system. Credit: Henner Busemann.
The paper is Busemann, Young, Alexander et al., “Interstellar Chemistry Recorded in Organic Matter from Primitive Meteorites,” Science (5 May 2006), pp. 727-730. Abstract available here.
by Paul Gilster | May 9, 2006 | Exoplanetary Science |
I remember wondering, while still getting acclimated to the odd existence of ‘hot Jupiters’ in those amazing first years of exoplanet discovery, what the view from a terrestrial world in one of those systems might be like. After all, a Jupiter-sized mass in close solar orbit must make for some unusual visual effects. Do terrestrial worlds exist around these stars? For that matter, what are the constraints on terrestrial planet formation in systems where gas giants orbit farther out, well past the habitable zone?
These questions are occasioned by the work of Sean Raymond (University of Colorado), whose paper on the subject will soon run in the Astrophysical Journal Letters. Raymond looks at how the presence of gas giants would affect the late stages of terrestrial world formation and presents the results of his simulations on same. Bear this in mind: gas giants, it is now thought, must form within the first few million years of the early protoplanetary disk. Whereas terrestrial worlds take tens of millions of years to finally assemble themselves. Obviously, the two engage in a powerful gravitational dance.
Let’s look at outer gas giants first. Raymond’s simulations say that putting them inside 2.5 AU inhibits the growth of small rocky worlds (0.3 Earth-mass and above) in the habitable zone of Sun-like stars and also prevents the appearance of water-rich habitable planets. In some cases, water-planet formation can only occur when the gas giant is beyond 3.5 AU. That leaves us Sol system dwellers in the clear with Jupiter at 5 AU, and at first blush it puts the brakes on all those hot Jupiter scenarios I was imagining. This is just for Jupiter-mass planets, mind you; more massive planets create even stronger perturbations.
Most exoplanets detected are, of course, in close orbits around their star. Raymond finds that only seven out of 153 planetary systems he worked with meet the criteria for small terrestrial worlds, while only two allow for water-planet formation in the habitable zone.
But here’s the good news: inner system gas giants may not totally rule out terrestrial worlds. In earlier work, Raymond determined that low-eccentricity gas giants inside 0.5 AU might permit habitable planets to form outside their tight orbits. The problem is, most of the detected gas giants display significant orbital eccentricity. Those in orbits closest to their stars (within 0.1 AU) have the lowest eccentricity; those beyond 0.15 AU show an average eccentricity of 0.32.
How you juggle eccentricity thus tells the story. As Raymond writes: “…if we arbitrarily assume that habitable planets can form in systems with giant planets interior to 0.5 AU with eccentricities less than 0.1…then the number of known extra solar systems that could harbor habitable planets increases to 45 (29%).” And my imagined view from the terrestrial world of such a system takes on life once again, though that gas giant will practically be hugging its star as it whips around the stellar disk.
And ponder this: Some recent studies suggest that planetary systems with habitable worlds need not contain gas giants at all. On that one, we have all too little information and must await the findings of future missions like Terrestrial Planet Finder, whose budgetary woes have allowed for serious reassessment to determine the best technologies for the job. All too often we’re shooting in the dark and extracting data on a very long thread — we need hardware in space to build our datasets, but making sure it’s the right hardware (and more on this soon) will pay off even if the continuing delays are frustrating.
by Paul Gilster | May 8, 2006 | Sail Concepts |
Rudolph Meyer’s work on solar arrays and ion propulsion elicited quite a few e-mails asking for further information. I don’t yet have the Acta Astronautica paper that spells out the details — nor do I know just how detailed Meyer gets — but I’ll try to provide some answers soon. In the interim, I was startled to realize that Geoffrey Landis, who commented on the Meyer design for New Scientist, had actually gone into this concept at some length as long ago as 1989.
In fact, Landis’ key paper “Optics and Materials Considerations for a Laser-propelled Lightsail” (available here) was presented at the 40th International Astronautical Federation Congress in that year. Landis speculated on a lightweight sail that focuses power on a small solar array, noting that a basic problem with laser-propelled lightsails is their low energy efficiency:
The energy efficiency may be greatly improved, at the cost of a reduction in specific impulse, by combining the laser sail with a photovoltaic powered electric (ion) engine. Ion engines in principle have no physical limits on the specific impulse, although extremely high specific inpulses require proportionately high energy consumption. Such a laser-powered rocket would have the ability to decelerate at the target star (with some loss of efficiency), and could also greatly decrease the amount of power required from the laser.
Landis then presents a schematic for a rocket like this, with solar array mounted so that the sail acts as a mirror to focus light on it. A little later in the paragraph, he presents a related idea (internal references deleted):
An alternate version would be to form thin-film solar cells directly on the sail. The specific impulse of such a system can be extremely high as long as the mass flow rate of reaction mass is low; but even with extremely low mass flow rates the energy efficiency of the sail can be greatly improved…
Landis examined the idea again at an American Physical Society meeting and presented further thoughts at the legendary “Interstellar Robotic Probes: Are We Ready?” conference hosted by Ed Belbruno in 1994. At the latter, he discussed a “…laser-powered rocket, where the laser is converted into electrical energy, which is used to power an electric propulsion system.” Here’s the conceptual figure Landis used then, with his original caption:
Image: Fixed laser, at left, illuminates a light-weight solar array, shown here as a centrifugally-tensioned thin-film membrane supported by tension wires. Power from the array is fed to an ion engine.
Clearly, the combination of solar sail and electric propulsion has been around for a while. Indeed, one of the fascinations of interstellar propulsion studies is the sheer range of brainstorming they generate. Landis’ papers drew on Robert Forward’s ideas about laser-propelled lightsails, and it was Landis who did key early work on refining our ideas about the best sail materials for that job. How Meyer’s work advances our knowledge of lightweight arrays and their potential uses is something we’ll talk about again once more facts become available.
