by Paul Gilster | Aug 17, 2007 | Exotic Physics |
If you had a device that could manipulate the expansion of spacetime, would you have the makings of a stardrive? Miguel Alcubierre’s ‘warp drive’ concept is based on something like this. The physicist’s 1994 paper points out that the speed of light constraint applies to objects moving within spacetime, but makes no prediction about how fast spacetime itself can move. Inflation theories draw on the same idea, with the early universe suddenly expanding at rates far surpassing light speed.
So think about contracting space in front of your vehicle while creating more space behind it. Distorted spacetime carries the starship along at fantastic velocities while violating no principle dear to Einstein. The trick, of course, is energy, the necessary amount of which has been discussed in various papers. As far as I know, no one has been able to get the figure down below the total energy output of a Sun-like star, but that’s a big reduction over earlier views that it would take all the energy in the universe to make a warp drive work.
Which brings us to dark matter and dark energy, the two elephants in the room of contemporary physics. We seem to be looking at only a fraction of the universe when we gaze into the sky, with perhaps as little as four percent being made up of the things we see. Whatever dark energy does that causes the acceleration of the universe’s expansion, it’s not something that we can observe other than through its apparent effects. But you can see why in the dreams of propulsion theorists dark energy comes into play. Anything that causes the expansion of spacetime’s acceleration plays into our concepts of a genuine star drive.
Dark matter, too, is of great propulsive interest. Understanding it will give us new insights into mass, and since we are interested in accelerating objects to appreciable fractions of lightspeed, we need to know as much as possible about how mass operates (not to mention the mysteries of inertia). We can only detect dark matter through its gravitational effects, one of which is its ability to cause the bending of distant light sources, an effect familiar through our discussions of gravitational lensing.
The recent dark matter news is challenging. Scientists using data from the Chandra x-ray observatory have been studying the galactic cluster system called Abell 520, mapping the Chandra data against optical data from Earth-based telescopes. A dark matter core could be identified, but mysteriously enough, it contains hot gas but no bright galaxies. The galaxies and the densest dark matter nearby are separated, the dark matter core in one place, a group of galaxies with no dark matter in the other. “This would be the first time we’ve seen such a thing,” says Hendrik Hoekstra (University of Victoria) and could be a huge test of our knowledge of how dark matter behaves.”
Image: An artist’s illustration of the Abell 520 system shows where the bulk of the matter (blue) is found compared to the individual galaxies (yellow) and the hot gas (red) in the aftermath of a massive galaxy cluster collision. The material shown in blue is dominated by dark matter. As with the Bullet Cluster there are large separation between the regions where the galaxies are most common (peaks 2 and 4) and where most of the hot gas lies (peak 3). However, unlike the Bullet Cluster, a concentration of dark matter is found (peak 3) near the bulk of the hot gas, where very few galaxies are located. In addition, there is an area (peak 5) where there are several galaxies but very little dark matter. These observations conflict with the general understanding that dark matter and the galaxies should remain together, despite a violent collision. This raises questions about the current understanding of how dark matter behaves. Credit: CXC/M. Weiss
So just what are we dealing with here? Gravitational ‘slingshot’ effects that could separate the dark matter don’t seem powerful enough to do the job. Is there an interaction between dark matter particles of which we had no previous idea? And if so, how can we make our observations of other galaxy clusters gibe with what we’re finding in Abell 520? We’ve begun to get used to the notion that a galactic cluster should include large amounts of dark matter. Indeed, dark matter was first hypothesized to explain why individual galaxies look as they do, when visible matter isn’t sufficient to account for the gravitational effects that shape them.
No one can say whether we’re on the edge of new physics or a more conventional explanation may yet emerge. What the universe persists in reminding us, though, is that the frontiers of deep sky astronomy provide potential clues that may one day help us derive a new understanding of mass. Meanwhile, ongoing work on dark energy probes the very substance of spacetime, the manipulation of which may remain forever beyond our grasp. Or perhaps not. A universe that conceals 96 percent of what makes it work surely holds surprises galore for future engineers.
The paper is Mahdavi et al., “A Dark Core in Abell 520,” accepted for publication in the Astrophysical Journal (preprint available). And those who collect classic papers, as I do, may want the reference for Miguel Alcubierre’s “The Warp Drive: Hyper-Fast Travel Within General Relativity,” Classical and Quantum Gravity 11 (May 1994): L73-L77.
by Paul Gilster | Aug 16, 2007 | Exotic Physics |
Can photons move faster than the speed of light? You wouldn’t think so, not if the name ‘Einstein’ has resonance, but Günter Nimtz and Alfons Stahlhofen (University of Koblenz) have been working on so-called quantum tunneling, joining two glass prisms and feeding microwave light into them. Tunneling occurs when a particle jumps an apparently uncrossable gap, and that’s just what the team’s microwave photons appear to have done, at least a few of them, when the prisms were separated. The bulk of the microwaves were reflected by the first prism.
New Scientist will soon be reporting on this story, which picks up on the German researchers’ recent paper. The tunneling photons seem to have reached the detector at the same time that their non-tunneling cousins did, suggesting movement far beyond the speed of light. The tunneling time evidently did not change when the prisms were pulled further apart.
Is this a violaton of relativity? Perhaps not. Note this from the New Scientist story, discussing Aephraim Steinberg’s views on the matter as an expert in quantum optics at the University of Toronto:
Steinberg explains Nimtz and Stahlhofen’s observations by way of analogy with a 20-car bullet train departing Chicago for New York. The stopwatch starts when the centre of the train leaves the station, but the train leaves cars behind at each stop. So when the train arrives in New York, now comprising only two cars, its centre has moved ahead, although the train itself hasn’t exceeded its reported speed.
“If you’re standing at the two stations, looking at your watch, it seems to you these people have broken the speed limit,” Steinberg says. “They’ve got there faster than they should have, but it just happens that the only ones you see arrive are in the front car. So they had that head start, but they were never travelling especially fast.”
The paper (short, dense and containing a diagram of the experimental set-up) is Nimtz and Stalhofen, “Macroscopic violation of special relativity,” available online. I’ll post a link to the New Scientist story as soon as it goes online. In the interim, here’s the Telegraph‘s brief coverage.
Addendum: The New Scientist story is here, though only available in its entirety to subscribers.
by Paul Gilster | Aug 16, 2007 | Outer Solar System |
We always knew the surface of Enceladus was cold, but those tantalizing plumes breaking out of the Saturnian moon’s south polar region gave hope of warmer things within. Liquid water fits with one model, pockets of which could account for the occasional geysers of ice crystals mixing with methane, nitrogen and carbon dioxide that Cassini has measured. Fill Enceladus with an internal ocean and the possibility of some kind of biology becomes an attractive study.
But an alternate take on the plumes has been on view for some time now. Coming out of the University of Illinois, it’s based on the idea that stiff compounds of ice called clathrates may cover Enceladus to a considerable depth. Whereas the warm interior model could produce such geysers, coined ‘Cold Faithful’ out of analogy to Yellowstone National Park’s Old Faithful geyser, so could the clathrate model. But the latter, dubbed ‘Frigid Faithful,’ could operate far below the freezing point of water, with obvious implications for the possibility of life.
Indeed, if Susan Kieffer and Gustavo Gioia are right, their approach does a better job of accounting for what Cassini sees in the ‘tiger stripes’ region of Enceladus, where large and evidently deep fractures wound the terrain. A mild heat source (perhaps only a few tens of degrees warmer than the surrounding shell) could cause the clathrates above to become expanded and stretched. Tensile stresses become compressive, forming the ridges that encircle the tiger stripes, and then tensile again, forming further fractures that radiate northwards.
Image: The ‘tiger stripes’ of Enceladus’ south polar region. Does liquid water lurk below this surface, or can the ‘Frigid Faithful’ model explain the plumes Cassini has observed without it? Credit: NASA/JPL/Space Science Institute.
According to this model, the tiger stripes should be about 35 kilometers deep. Clathrates on their cracked surfaces would undergo decompression and dissociate explosively, thus exposing more clathrates to decompression and so on. Gaseous plumes transport heat to the surface, but this is a kind of heat advection that can take place at a nearly uniform temperature. Says Gioia, “This is indeed a frigid Enceladus. It appears that high heat fluxes, geyser-like activity and complex tectonic features can occur even if moons do not have hot, liquid or shifting interiors.”
The paper is Gioia et al., “Unified model of tectonics and heat transport in a frigid Enceladus,” Proceedings of the National Academy of Sciences (published online in advance of the print edition, with abstract here).
by Paul Gilster | Aug 16, 2007 | Culture and Society |
Carnival of Space #16 is now available at Brian Wang’s Advanced Nanotechnology site. Particularly recommended is an essay we also looked at recently here, Alex Bonnici’s discussion of Dandridge Cole and his visionary outlook on using asteroids for the good of mankind. And you’ll also want to read Mark Whittington’s look at what the next fifty years may bring in space travel. If fifty years rings a bell, it may be because you’re thinking of the upcoming anniversary of Sputnik. Let’s hope the next fifty years manage a more consistent pace of development…
by Paul Gilster | Aug 15, 2007 | Deep Sky Astronomy & Telescopes |
GALEX — the Galaxy Evolution Explorer — was an interesting mission to begin with, a space-based observatory conducting an all-sky survey of distant galaxies at ultraviolet wavelengths. Now it’s come up with a real newsmaker, a star moving at an unusually fast 130 kilometers a second and sporting a comet-like tail. The material blowing off the red giant Mira is, in fact, forming a wake some thirteen light years long. No such phenomenon has ever been seen around a star before.
Image: Mira appears as a small white dot in the bulb-shaped structure at right, and is moving from left to right in this view. The shed material can be seen in light blue. The dots in the picture are stars and distant galaxies. The large blue dot at left is a star that is closer to us than Mira. Credit: NASA/JPL-Caltech.
From what GALEX is telling us, the elements Mira is leaving behind, including carbon, oxygen and other building blocks for future star and planet formation, have been shed over a period of approximately 30,000 years. Although similar to our own Sun billions of years ago, the star has now swollen to variable red giant status, periodically growing bright enough to become visible to the naked eye. And as will happen to the Sun, its distant future involves its transformation into a white dwarf.
Nor does Mira travel alone. Mira B is itself a white dwarf [but see comments below] that orbits Mira A as the duo move through the constellation Cetus, some 350 light years from Earth. Interestingly, a bow shock has formed in which hot gases build up in front of the onrushing star, and astronomers have noted two streams of material that emerge from the star itself. Evidently the hot gas in the bow shock heats up the gas blowing off the star, causing it to fluoresce with ultraviolet light as it forms the wake.
Researchers admit to a sense of surprise. Here’s Mark Seibert (Carnegie Observatories, Pasadena), a co-author of the paper on this work:
“This is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved. We hope to be able to read Mira’s tail like a ticker tape to learn about the star’s life.”
And from the discovery paper, this comment about putting the Mira findings to work:
The discovery of a two-degree-long wind wake emitting only in the far ultraviolet provides an unprecedented fossil record of post-main-sequence stellar evolution and mass loss, a laboratory for the study of astrophysical turbulence and the complex physics of a multiphase hydrodynamical flow, and suggests a new cooling process for hot gas that entrains a cool molecular phase. After 400 years of study, Mira continues to astound.
Indeed. A 30,000 year passage through the cosmos is now on display, discovered through a mission conceived to study much different things. If surprise is in the air, it’s understandable. A 13-light year long tail is not exactly standard issue, and who would have predicted that a star as well studied as Mira would turn out to have a wake that glowed only with ultraviolet light? How energizing it is to reflect that the pace of discovery is only accelerating, capable of blindsiding us at almost every turn.
The paper is Martin et al., “A turbulent wake as a tracer of 30,000 years of Mira’s mass loss history,” Nature Vol 448 (16 August 2007), pp. 780-783.
