What conditions would you say are 'congenial to life'? For physicist Robert Jaffe and colleagues at MIT, the phrase refers to places where stable forms of hydrogen, carbon and oxygen can exist. Jaffe explains why: "If you don't have a stable entity with the chemistry of hydrogen, you're not going to have hydrocarbons, or complex carbohydrates, and you're not going to have life. The same goes for carbon and oxygen. Beyond those three we felt the rest is detail." It's an important issue in Jaffe's work because he wants to see whether other universes could harbor life. We know that slight changes to the laws of physics would disrupt the evolution of the universe we live in. The strong nuclear force, for example, could have been just a bit stronger, or weaker, and stars would have been able to produce few of the elements needed to build planets. Remove the electromagnetic force and light would not exist, nor would atoms and chemical bonds. Nudging Nature's Parameters Run through the...

It was in 1900 that mathematician David Hilbert created a list of the most significant unsolved problems for mathematics at a conference in Paris. The list would eventually be fleshed out to reach a total of 23 problems. Hilbert's Paris talk, "The Problem of Mathematics," began this way: Who among us would not be happy to lift the veil behind which is hidden the future; to gaze at the coming developments of our science and at the secrets of its development in the centuries to come? What will be the ends toward which the spirit of future generations of mathematicians will tend? What methods, what new facts will the new century reveal in the vast and rich field of mathematical thought? The Wikipedia entry on Hilbert notes that the 23 problems, fewer than half of which were presented at the meeting, have gone on to be discussed throughout the following century, with some remaining unresolved to this day. I look at Hilbert's introduction and think about how apropos the idea of gazing at...

Paul Titze, who somehow finds time to write the excellent Captain InterStellar blog when not preoccupied with his maritime duties in Sydney, passed along a 2009 paper on warp drives yesterday that I want to be sure to consider before the year is over. Warp drives as in Miguel Alcubierre's notion of a method of reaching speeds that are faster than light. The Star Trek echo in the choice of names was playful and intentional on Alcubierre's part, and the physicist kicked off a cottage industry in exotic spacetimes and their geometries when he used it in a 1994 paper on superluminal flight. Specifically, Alcubierre noted that although nothing can move faster than the speed of light through spacetime, spacetime itself has no such restriction. That notion is more or less built into the theory of inflation, which demands a vast expansion of the infant cosmos that would have far outstripped any lightspeed restriction. And Alcubierre saw that if spacetime could be made to contract in front of...

With WISE now on its way (a spectacular launch in the dark at Vandeberg Air Force Base), we now turn to the realm of exotica. Specifically, can we find ways to exploit the quantum vacuum to produce propulsion? I've seldom had such a flurry of interested emails than what followed the appearance of a paper by Alex Feigel, recently put up on the arXiv server. Feigel (Soreq Nuclear Research Center, Israel) discusses modifying the momentum of the quantum vacuum, an idea dear to that segment of the interstellar propulsion community that focuses on 'propellantless' propulsion. Some background: Heisenberg's uncertainty principle implies that it is impossible to achieve an absolute zero electromagnetic energy state in the vacuum of space. The measurement of the Casimir effect in 1997 demonstrated that a force would be exerted between two narrowly separated conducting plates. Indeed, at the micron scale, such plates are squeezed together as longer wavelength waves are excluded. The possibility...

I had intended to start the week with a look at Charlie Stross' ideas on the 'starship' metaphor, but I'll hold the Stross essay until tomorrow because I want to finish up Marcus Chown's piece in New Scientist. On Friday we talked about the idea of using Hawking radiation from a man-made black hole to propel a starship. That's outrageous idea number one, but Chown actually began the article with a look at Jia Liu's ideas on using dark matter to propel a ship, something along the lines of a Bussard ramjet without the hydrogen. The Uses of Speculation I enjoy looking at speculative concepts, even when they're so far out on the edge that they've attained a kind of intellectual redshift, but New Scientist's squib for the story surprised me: "We could reach the stars if we built a black hole starship or a dark matter rocket - we've got the physics to do it." Er, no, we don't have the physics to build a dark matter rocket. We don't know what dark matter is. The case for its existence seems...

When you consider that conventional chemical rockets extract a mere 10-8 of the energy locked up in their fuel, the attraction of antimatter becomes undeniable. Could we build an engine that extracts 100 percent of the energy created by matter-antimatter annihilation? Louis Crane (Kansas State University) is dubious, pointing to problems of storage and the difficulty of making enough antimatter to get the job done. Black Holes as a Propulsion Option Working with colleague Shawn Westmoreland, Crane has been exploring a different and far more speculative option for upping the energy extraction levels. What about using black holes for propulsion? Specifically, Crane and Westmoreland ask whether Hawking radiation from black holes can power a starship, calculating that a black hole of about a million tons would be just the right size, small enough to generate the needed Hawking radiation, while large enough to survive for the duration of a century-long star crossing. Adam Crowl has...

If you're going to snare dark matter, you'd better have incredibly accurate detectors. So the thinking goes at Case Western Reserve, where researchers are planning the most sensitive experiment yet to go after WIMPs (weakly interacting massive particles). WIMPs are almost impossible to detect because they don't give off radiation and pass through normal matter unimpeded. The CWRU group has received a three year $3.2 million National Science Foundation grant to design a new WIMP detector. The existence of dark matter is a theory that received support in 2006 when the collision of two distant galaxies was analyzed in ways that seemed to show the effects of dark matter on a cloud of galactic gas. Dark matter could provide the needed mass that keeps galaxies like the Milky Way from flying apart, but we still need a direct detection. The new experiment is a 20-ton liquid xenon detector called LZD. The Case Western group proposes LZD as an experiment for the Deep Underground Science and...

By Larry Klaes We have much to do as we scramble to explain the universe's continuing acceleration. Dark energy seems to be demanded by the data, but there are holdouts who argue for a reinterpretation of General Relativity. Tau Zero journalist Larry Klaes looks at one proponent of a revised GR who sees exceptions to the rule in a far earlier era. Albert Einstein's work created one of the biggest revolutions in the history of science and radically changed our perceptions of the Cosmos. One of his later breakthrough ideas is the Theory of General Relativity, or GR for short. Really massive objects such as the Sun literally warp space and time around them as they move through the heavens. Since Einstein first published his ideas on GR in 1915, scientists have been able to use the theory to understand the behaviors of even more massive celestial bodies and the very beginning of the Universe itself. Image: This key Einstein paper included the effect of gravitation on the shape of space...

Gamma ray bursts (GRBs) are much in the news. GRB 090423 turns out to be the most distant explosion ever observed, an event that occurred a scant 630 million years after the Big Bang. We're detecting the explosion of a star that occurred when the first galaxies were beginning to form. Current thinking is that the earliest stars in the universe were more massive than those that formed later, and astronomers hope to use GRB events to piece together information about them. GRB 090423 was evidently not the death of such a star, but more sensitive equipment like the Atacama Large Millimeter/submillimeter Array (ALMA) will soon be online (ALMA within three years) to study more distant GRBs and open up more from this early epoch. And then there's all the fuss about Einstein. The Fermi Gamma Ray Space Telescope, which has already captured more than a thousand discrete sources of gamma rays in its first year of observations, captured a burst in May that is tagged GRB 090510. Evidently the...

Can we calculate the gravitational field of a mass moving close to the speed of light? Franklin Felber (Starmark Inc) believes he can, with implications for propulsion. Back in 2006 we looked briefly at Felber's work, describing what the physicist believes to be a repulsive gravitational field that emerges from his results. Felber discussed the matter at the Space Technology and Applications International meeting that year, where he presented his calculations of the 'relativistically exact motion of a payload in the gravitational field of a source moving with constant velocity.' Above a certain critical velocity, Felber believes, any mass will gravitationally repel other masses, an effect that is twice as strong in the forward direction of motion, but also works in the backward direction. An object lying in the narrow beam thus produced could be accelerated quickly and with little stress. He described the effect in a paper he submitted in 2005 to the arXiv site: At radial approach or...

It's odd to think that there would be a connection between the large-scale structure of the universe and what we hope to achieve with deep space propulsion. But figuring out how things work on the largest scale may offer us valuable clues about what is possible and what is not. If we understand correctly how gravity works at the macro scale, then the evidence for 'dark energy' seems persuasive. Something is causing the universe not only to expand but to accelerate its expansion, and that something must operate against the force of gravity, which ought to be slowing the process down. Which brings us to BOSS, the Baryon Oscillation Spectroscopic Survey, now beginning its operations after taking first light on the night of September 14-15. A part of the Sloan Digital Sky Survey III, BOSS will use the 2.5-meter telescope at Apache Point Observatory in New Mexico to measure the spectra of 1.4 million galaxies and 160,000 quasars by 2014. Out of this we should derive the most accurate data...

Point a Voyager-speed spacecraft at Alpha Centauri and the travel time would be on the order of 73,000 years. Those of us obsessed with the idea of interstellar journeys are forced to hope for profound breakthroughs in physics and engineering. The word 'breakthrough' is, if anything, an understatement. An Alcubierre-style 'warp drive' would, so far as we know, require energies that would tax even a Kardashev Type III civilization, as physicist Richard Obousy points out. Hence the acknowledged 'giggle factor' that plagues serious discussion of these matters. Writes Obousy: The giggle-factor is a consequence of using a name for a cutting edge propulsion concept that is taken straight from science fiction. In reality the name is a double-edged sword. When one mentions a 'warp drive' it should be immediately obvious (one would hope) that what is being dicussed is a hypothetical propulsion mechanism that utilizes an asymmetric manipulation of the fabric of spacetime to generate an exotic...

Sometimes what you don't detect tells a scientific story just as important as what you do. In the case of LIGO (Laser Interferometer Gravitational-Wave Observatory) and the VIRGO Collaboration, we're talking about setting limits to the amount of gravitational waves that would have been produced by the Big Bang. Those waves, predicted by Albert Einstein in 1916 and consistent with his theory of General Relativity, should be traceable and quite valuable to us, carrying as they do information about the earliest stages of the universe. Image: Modeling gravitational wave complexity. Laser interferometers should be able to detect the gravitational waves produced by the most violent astrophysical events, such as the merging of two black holes. Credit: MPI for Gravitational Physics/W.Benger-ZIB. The gravitational waves ought to be out there (General Relativity predicts that all accelerating objects should produce them) but they have yet to be observed directly. In fact, the so-called...

Anomalies in scientific data can sometimes lead to a richer understanding of the underlying principles involved. Einstein was able to explain the difference between the Newtonian description of Mercury's orbit and subsequent observations by applying his developing theory of General Relativity. Add the curvature of spacetime to the Newtonian picture and the problem of a tiny discrepancy in Mercury's perihelion precession can be resolved. This anomaly briefly changed our view of the Solar System. Originally, the astronomer Urbain Le Verrier had thought it could be explained by the presence of another planet -- Vulcan -- closer than Mercury to the Sun, but reported sightings of Vulcan were found to be spurious. Einstein's work solved the precession problem. In a letter to his close friend Michele Angelo Besso, Einstein would write: "In these last months I had great success in my work. Generally covariant gravitation equations. Perihelion motions explained quantitatively… you will...

Tibor Pacher has been kind enough to publish the text of my public lecture in Aosta, Italy on his PI Club site. The lecture took place at the Aosta town hall and wasn't part of the ongoing conference just down the street, although some conference participants attended. It's a broad overview of earlier work on interstellar flight. My intention was to acquaint non-scientists with the fact that the subject has been under study for decades in ways that do not violate the laws of known physics. A major challenge is how to scale some of the colossal engineering involved down to realistic levels. Although I only touched upon it in the lecture, I often talk about the twin tracks of interstellar studies. The first track comprises work that tries to scale current technology up for an interstellar mission. The second track is oriented toward examining physical laws in hopes of finding potential breakthroughs that current theory doesn't allow. No one knows if such breakthroughs are possible, but...

A testable prediction about dark energy? Such is the promise of a new formulation from Sourish Dutta and Robert Scherrer (Vanderbilt University), whose dark energy model interacts with normal matter and has observable results, including a prediction about the expansion rate of the universe. Astronomical surveys in the next decade should be able to detect the slowdown in the expansion rate predicted by this model, if it exists. Think 'quintessence,' a new field with the unique property that it can act like antigravity, forcing nearby objects to move away from each other rather than pulling them together. The quintessence field as developed by Dutta and Scherer likely went through a phase transition somewhere around 2.2 billion years after the Big Bang. 'Freezing out' as the universe cooled creates a scenario where the energy density of the field remained high until, with the phase transition, it dropped abruptly to a level it retains to this day. Another result: The release of some of...

How can the space between the stars be so full of stuff? So commented a friend who chanced upon this site, reading our discussion of interstellar gas and dust and the troubling fact that moving through it at high speeds bathes a spacecraft in radiation. Not an issue for our current generation of spacecraft, dust and gas rise in significance as we reach velocities that are an appreciable fraction of the speed of light, creating the need for various kinds of shielding. So what exactly is that stuff in outer space? Break down the interstellar medium and you get almost 90 percent hydrogen, with ten percent or so helium and trace elements like carbon, oxygen, silicon and iron accreted in dust particles. Oleg Semyonov, in his recent Acta Astronautica paper, examines all this, noting that the concentration of interstellar gas varies greatly between 104 cm-3 in galactic clouds to less than 1 cm-3 in the regions between the clouds. Our own Solar System lies in a cavity of low-density gas,...

Cosmic inflation, first proposed by Alan Guth (MIT) in 1979, seems about as intractable a subject as dark energy. How to study it? Inflation does something mind-bending to spacetime by making it expand far faster than the speed of light. Oddly, this doesn't contradict anything Einstein said, because while nothing we know can travel faster than light through spacetime, there is no restriction implied in these equations on the expansion of spacetime itself. This is why Miguel Alcubierre's 'warp drive' notions can fit within an Einsteinian universe. After all, what Alcubierre proposed in his 1994 paper was that a spacecraft that could create the right kind of spacetime distortion would at no point in its journey go faster than the speed of light. Compressing spacetime in front while expanding spacetime behind, it would itself remain within a 'bubble' of normal spacetime. Of course, the amount of energy required to achieve this feat (and it's negative energy, at that) may render the...