by Paul Gilster | Dec 22, 2005 | Breakthrough Propulsion, Deep Sky Astronomy & Telescopes |
Wormholes make for great science fiction because they get us around the speed-of-light conundrum. Taking a shortcut through spacetime, they connect one part of the universe to another, though where and when you would come out if you went in a wormhole would be an interesting experiment, and not one for the faint of heart. But do we have any evidence that wormholes exist, and if they did, what could we look for that might reveal their presence?
Perhaps it’s time to revisit a fascinating 1994 paper called “Natural Wormholes as Gravitational Lenses.” The authors are a compendium of names known to anyone with an interest in the physics of interstellar flight or its depiction in science fiction: John G. Cramer (whose columns in Analog set high standards for science writing); Geoffrey A. Landis (Mars Crossing and innumerable short stories); Gregory Benford (whose bibliography of novels is too long to list); Robert Forward (the leading proponent of interstellar studies) and two other physicists whose work deserves a wider audience: Michael Morris and Matt Visser.
It was Visser (Washington University, St. Louis) who suggested a possible configuration for a wormhole that frames it with ‘struts’ of exotic material, the struts having a negative mass density that could result in an interesting object indeed, what the paper describes as ‘…a flat-space wormhole mouth framed by a single continuous loop of exotic cosmic string.’
Geoffrey Landis calls cosmic strings ‘flaws in geometry,’ but you can also think of them as flaws in the structure of spacetime itself. They’re quite useful in imagining wormholes because to preserve a primordial wormhole formed at the beginning of the universe, you need to wrap it in negative energy, and a negative mass cosmic string could do the trick. There are plenty of conditions here, but Landis put it this way in an interview I did with him back in 2003: “If one of these hypothetical negative mass cosmic strings got wrapped around a hypothetical primordial wormhole, you could have a hypothetical stable primordial wormhole, one that could still exist.”
Detecting such an object becomes a fascinating exercise in itself. We know how to look for the signature of gravitational lensing, as in imagery of a distant galaxy that has been shaped by the gravitational influence of an intervening galaxy. A wormhole should show a negative mass signature that, instead of focusing light, does the opposite. The signature of that ‘defocusing’ is characteristic. Landis again:
“If the wormhole is exactly between you and another star, it would defocus the light, so it’s dim and splays out in all directions. But when the wormhole moves and it’s nearer but not in front of the star, then you would see a spike of light. So if the wormhole moves between you and another star and then moves away, you would see two spikes of light with a dip in the middle.”
As theoretical as all this sounds, it’s actually quite useful. As the authors of the wormhole paper put it: “…the negative gravitational lensing presented here, if observed, would provide distinctive and unambiguous evidence for the existence of a foreground object of negative mass.” It makes sense, then, given the interest among astronomers in observing normal gravitational lensing with positive mass objects, to keep open the possibility of finding such a signature, which would provide our first solid evidence of wormholes.
And it also corresponds with how science works today. Much of the analysis of the voluminous data collected by our instruments is performed by computer software. And the value of a paper like this one, beyond its purely scientific interest, is that it identifies a pattern that such software should be written to recognize among all the other patterns that clue researchers in to interesting findings. It would be absurd to have solid evidence of a wormhole in data that was never analyzed in precisely the right direction.
As for that wormhole itself, an ancient one from the earliest days of the cosmos wouldn’t be of much use from a transportation perspective — you have to get to it first, after all, and it might be millions of light years away. And then there’s that problem of figuring out where it comes out on the other side. But it may be that a Kardashev Type II civilization, able to use all the energies of a star, could create artificial wormholes using negative energy, and if that is the case, the universe may have shortcuts galore through the Einstein barrier.
The paper is John Cramer, Robert L. Forward, Gregory Benford et al., “Natural Wormholes as Gravitational Lenses,” Physical Review D (March 15, 1995): pp. 3124-27, also available on the arXiv site (and thanks to Gregory Benford for forwarding a copy). Be sure to read Robert Forward’s novel Timemaster for his fictional take on negative energy and many other ideas from the outer limits of science. In addition to being a fascinating speculative romp, it’s a rich and funny book.
by Paul Gilster | Dec 9, 2005 | Antimatter, Breakthrough Propulsion, Sail Concepts |
In an article on interstellar propulsion options at Physorg.com, writer Chuck Rahls focuses on three technologies that have been proposed to make a trip to Alpha Centauri possible. Of the three, laser-pushed lightsails are indeed in the running, and have been since Robert Forward realized the implication of the laser while working at Hughes Aircraft. Also employed by Hughes in the company’s research laboratories was Theodore Maiman, who had shown how to make a functional laser in 1960. Forward wrote the concept up as an internal memo at Hughes in 1961, and later went public in the journal Missiles and Rockets. In the same year (1962), he described the idea in an article in Galaxy Science Fiction.
Rahls writes about a laser-driven craft weighing 16 grams making it to the Centauri stars in ten years. It’s a grand concept — Forward came up with it, too, and gave it the wonderful name Starwisp, though he used not lasers but microwaves to drive it — but Geoffrey Landis has convincingly shown that Starwisp could never fly, the intensity of the microwaves needed to accelerate it being sufficient to vaporize the entire spacecraft. Forward knew this and was working on other solutions at his death.
Centauri Dreams also has serious reservations about the second concept addressed here, the Bussard ramscoop. One problem is that enormous speeds are needed just to ‘light’ the ramscoop’s engine. But a more profound issue is that physicists have shown the ramscoop idea to be unworkable because of drag. In fact, Dana Andrews and Robert Zubrin demonstrated in the late 1980s that a spacecraft of Bussard’s design would experience more drag from its enormous electromagnetic ‘scoop’ than thrust. The real beauty of the ramscoop concept is that it generated an equally interesting — and workable — notion: use an electromagnetic sail tens of kilometers in diameter that could be pushed by particle beam, or used in the destination solar system for braking upon arrival.
The third, and perhaps most exciting in today’s terms of Rahls’ technologies is antimatter. Here the options are proliferating, and because we know how to harvest only the minutest quantities of the stuff, we’re finding ways to make fast propulsion systems that use antimatter only as a catalyst, igniting fusion, perhaps, or using it to interact with an uranium-coated sail. The latter concept is Steve Howe’s (I referred to it in these pages just the other day), a proposal so ingenious that any star-minded reader should make haste to the NASA Institute for Advanced Concepts site to download Howe’s “Antimatter Driven Sail for Deep Space Missions.”
NASA’s John Cole told me at Marshall Space Flight Center back in 2003 that the power released by Howe’s design is on the order of 2000 kilowatts per kilogram. “It’s just an enormous figure,” Cole said. So even if Howe’s figures are an order of magnitude off, even two orders of magnitude off, a factor of 100, he is still in realm of where we can have human exploration of the outer planets.”
The Andrews/Zubrin article mentioned above, by the way, is a key work in the development of interstellar concepts. It’s titled “Magnetic Sails and Interstellar Travel,” found as International Astronautical Federation Paper IAF-88-5533 (Bangalore, India, October 1988). If I had to put money on the proposition, I’d bet a particle-driven magnetic sail will be our first true star mission, a robotic probe launched around 2100. But that’s the last bet you’ll get out of me.
by Paul Gilster | Nov 24, 2005 | Breakthrough Propulsion, Missions |
Consider two hypothetical spacecraft. The Orion vehicle would have worked by setting off low-yield nuclear devices behind a massive pusher plate, driving forward a payload attached at a safe distance from the pusher (and protected by mind-boggling shock absorbers). Even if we had the nuclear devices at our disposal, agreed to use them for such a purpose, and found the political will to construct an Orion craft for deep space exploration, a problem still remains: most of the energy from the nuclear blasts is dissipated into space, and the craft thus requires a huge critical mass of fission explosives.
Orion, in short, is not efficient in using its energies. Now consider Project Daedalus, the hypothetical mission to Barnard’s Star designed by members of the British Interplanetary Society back in the 1970s. Daedalus was designed to use fusion microexplosions instead of fission. One of the reasons the Daedalus craft demanded as much fuel as it did is that the ignition apparatus, whether laser or particle beam, to ignite fusion is massive, adding unwanted heft to the vehicle. Daedalus would have massed an overwhelming 54,000 tons.
But there is a third option, discussed by Friedwardt Winterberg (University of Nevada/Reno) in a new paper in Acta Astronautica. Winterberg describes so-called ‘mini-nukes,’ which are devices in which the mass of the fission explosive is hugely reduced by a reflector in which a deuterium-tritium (DT) fusion reaction takes place — the fusion neutrons thereby increase the fission rate, and the increased fission rate, in turn, increases the fusion production rate. The implosive compression to ignite the process is provided by chemical high explosives.
We wind up with a fission-fusion assembly with serious advantages: 1) we eliminate the weight of a laser or particle beam generator to ignite fusion, and 2) we add to the specific impulse and thrust of the exhaust, which is composed of combustion products both from the nuclear and chemical reactions.
And no longer do we, Orion-style, lose the bulk of our energies into space. “With mini-nukes,” Winterberg writes, “the situation is much better because the explosions can take place there in the focus of a parabolic reflector positioned in close proximity to the spacecraft. Since still smaller mini-nukes appear possible, with their feasibility depending only on the technical perfection to focus the chemical energy for ignition, the mini-nukes can be detonated inside a large combustion chamber…”
The paper is Winterberg, F., “Mini-fission-fusion explosive devices (mini-nukes) for nuclear pulse propulsion,” Acta Astronautica Vol. 57 (2005), pp. 707-712. Some of Winterberg’s earlier work was studied, incidentally, by the Daedalus designers as they set about examining various ways of making an inertial confinement fusion system that could power their starprobe.
And incidentally, check this Fusion News Update at the Star Spangled Cosmos site for news of a possible development in inertial confinement fusion in South Korea.
by Paul Gilster | Nov 7, 2005 | Breakthrough Propulsion |
What could inflation theory have to do with the Fermi paradox? Quite a lot, if at least one recent paper is to be believed. The question ‘where are they’ about extraterrestrial visitation becomes even more pointed when faster-than-light technologies move out of the realm of the impossible to something that may be seriously investigated by physicists. Inflation theory, which holds that the early universe underwent a vast expansion as spacetime itself stretched far beyond the velocity of light, opens the door to technologies that might use this effect to create spacefaring civilizations spanning entire galaxies.
Just how fast did inflation occur? In a space of time lasting about 10-35 seconds, the universe could have expanded by a factor of 1030 to 10100. As Brian Greene puts it in The Fabric of the Cosmos:
An expansion factor of 1030 — a conservative estimate — would be like scaling up a molecule of DNA to roughly the size of the Milky Way galaxy, and in a time interval that’s much shorter than a billionth of a billionth of a billionth of the blink of an eye… In the many models of inflation in which the calculated expansion factor is much larger than 1030, the resulting spatial expanse is so enormous that the region we are able to see, even with the most powerful telescope possible, is but a tiny fraction of the whole universe.
Enter Bernard Haisch (National Aviation Reporting Center on Anomalous Phenomena), Hal Puthoff (Institute for Advanced Studies at Austin) and colleagues. The authors argue in a recent issue of the Journal of the British Interplanetary Society that while General Relativity is valid, there are several approaches within it that may permit bypassing the speed of light limit (even if any civilizations that could build them might have to be more advanced than ours by millions of years):
The use of wormholes, engineered through the use of exotic matter. The mathematical requirements for creating a traversable wormhole have entered the scientific literature in the works of, among others, Kip Thorne.
The Alcubierre warp drive concept, which notes that there is no limit to the speed at which space itself might stretch. Faster than light relative motion is built into inflation theory. Alcubierre’s work shows that a spacecraft could theoretically make use of expanding spacetime behind and a similar contraction in front of the vehicle to overcome the restrictions of General Relativity.
Superstring theory suggests the possiblity that adjacent universes could be all around us. The added dimensionalities of M-brane and superstring theory might allow a sufficiently advanced technology to move into an adjcent universe where the speed of light limit is different than our own.
From the paper:
Clearly when it comes to engineering warp drive or wormhole solutions, seemingly insurmountable obstacles emerge, such as unattainable energy requirements or the need for exotic matter. Thus if success is to be achieved, it must rest on some yet unforeseen breakthrough about which we can only speculate, such as a technology to cohere otherwise random vacuum fluctuations. Nonetheless, the possibility of reduced-time interstellar travel by advanced extraterrestrial (ET) civilizations is not, as naive consideration might hold, fundamentally ruled out by presently known physical principles. ET knowledge of the physical universe may comprise new principles which allow some form of FTL travel. This possibility is to be taken seriously, since the average age of suitable stars within the ‘galactic habitable zone’ in which the Earth also resides, is found to be about 109 years older than the Sun…
Is the answer to the Fermi paradox, then, that we are even now being visited by extraterrestrial spacecraft whose contact strategy is based on a sense of interstellar ethics we do not yet understand? The paper argues that such visitations must be considered more likely than a ‘we are alone’ answer to the Fermi question, and goes on to consider what ethical considerations might motivate a culture investigating our planet from outside Earth. The paper is Deardorff, Haisch, Maccabee and Puthoff, “Inflation-Theory Implications for Extraterrestrial Visitation,” in the Journal of the British Interplanetary Society Vol. 58, No. 1/2 (January/February 2005), pp. 43-50.
by Paul Gilster | Nov 5, 2005 | Breakthrough Propulsion |
When physicist Geoffrey Landis reviewed interstellar concepts at the American Association for the Advancement of Science’s 2002 meeting, his wide-ranging presentation considered where we stand on nuclear propulsion, solar and lightsail technologies, and particle-pushed sails. He also addressed the question of the Bussard ramjet, which would use an electromagnetic scoop to collect atoms from the interstellar medium to fuel a fusion reactor. Finding serious problems here (he cites, among other things, the fact that the scoop technology acts more like a brake than an accelerator), Landis went on to consider an alternative:
“These problems can be alleviated if, instead of using the ambient interstellar medium, fuel is deliberately emplaced in the path of the spacecraft before flight. In this way, the fuel (probably in the form of small ‘pellets’) can be chosen to be the optimum composition…
The ‘runway’ of fuel pellets could be emplaced, for example, by a dedicated craft which drops fuel pellets at predetermined locations along the flight path. Each ‘fuel pellet’ would have to contain more than the fusion fuel; at a minimum the pellets would have to contain passive locator beacons. The interstellar probe would adjust its trajectory as required to ingest and utilize each successive fuel pellet. Alternately, and possibly more realistically, the fuel pellets themselves would each have a small amount of propulsion capability, enough for them to station-keep in a predetermined location, for example, maintaining position along a laser used as a guide-beam.
[Jordin] Kare noted that, at the velocities proposed for interstellar flight, nuclear fusion can be accomplished at the temperatures produced by impact. If a small pellet carried on the vehicle impacts a stationary pellet of fusion fuel, the result of the impact will be ignition of the fusion reaction, and potentially the liberation of a considerable amount of energy…”
When Centauri Dreams talked to Landis in 2003 about the fusion runway concept (Kare calls the idea the ‘Bussard Buzz Bomb’), the physicist elaborated on why he found the concept so intriguing. Kare’s work had indicated that it would take a velocity of 200 kilometers per second to light the starship’s main engines using these techniques. Landis pointed out that a spacecraft making a close pass by the Sun (virtually skimming the solar surface) could reach 600 kilometers per second, with the fuel pellets lined up so that the vehicle could start using them immediately after the solar pass.
How long would a fusion runway have to be to function? The answer depends on your spacecraft. An unmanned robotic probe that could endure intense gravitational forces would require a much shorter runway. If the craft is manned, the pellets would have to be strung out over a tenth of a light year to allow the probe to reach a cruising velocity of 30,000 kilometers per second. That translates into a forty-plus year journey to the Centauri stars.
Landis’ AAAS presentation is available as “The Ultimate Exploration: Approaches to Interstellar Flight,” in Y. Kondo, ed., Interstellar Travel & Multi-Generational Space Ships (Apogee Books, 2003).
by Paul Gilster | Oct 14, 2005 | Breakthrough Propulsion |
What a pleasure to discover that Robert Freitas’ Kinematic Self-Replicating Machines is now available online. The 2004 book (from Landes Bioscience of Georgetown TX) is the most comprehensive study of nanotechnology yet written, a compendium of information on self-replicating systems both proposed and experimentally studied. Moreover, it contains a survey of the historical development of nanotechnology, 200 illustrations and over 3000 references to the technical literature.
That nanotechnology (and self-replicating systems in particular) could change our ideas of interstellar flight now seems obvious, but not so long ago ago the concept of one machine building another was studied only at the macro-level. Thus Freitas’ previous work on a self-reproducing spacecraft he called REPRO. The scientist wrote the concept up in a 1980 issue of the Journal of the British Interplanetary Society, conceiving of a mammoth Daedalus-style spacecraft built in orbit around Jupiter and, like Daedalus, using helium-3 from the giant planet’s atmosphere in its fusion engine.
REPRO was a vast and ambitious project, equipped with numerous smaller probes for planetary exploration, but its key purpose was to reproduce. Each REPRO probe would create an automated factory that would build a new probe every 500 years. Probe by probe, star by star, the galaxy would be explored.
Just 25 years later, Freitas still ponders galactic exploration, but he now concentrates on the world of the very small, studying nanotechnological methods that could allow space probes the size of sewing needles. Containing the computing power of thousands of human brains, such probes could be sent out by the millions. If just one of them reached a planet, moon or asteroid around a nearby star, it could begin to reproduce, and in much shorter time frames than allowed for the earlier REPRO probe.
When I interviewed him for Centauri Dreams in 2003, Freitas put it this way:
“The fastest known bacteria I’m aware of is e. coli that can replicate in fifteen minutes at ideal temperature with an excess of nutrients. It is approximately one to two microns in size, which is roughly the same size as the sophisticated assemblers that will one day manipulate matter at this level. If such assemblers landed on an asteroid that was a great distance away from the suns of the Alpha Centauri system, where perhaps there would not be the best energy density, and where materials would have to be scavenged, this would not be an ideal ‘petri dish’ kind of environment. So you might have to add two or three orders of magnitude of time. But you’re still looking at replication times on the order of weeks.”
So nanotechnology’s implications for interstellar flight may be profound, particularly as they enable the vision of a survey of the entire galaxy within a million-year time frame. Some (Frank Tipler among them) have argued that the lack of evidence for extraterrestrial probes within our own Solar System demonstrates that no technical civilizations exist in our part of the universe, but if probes are built with nanotechnology, they may have little trouble avoiding detection in the systems they survey.
We need, then, to give nanotechnology a good look as we plan an interstellar future. Freitas’ Kinematic Self-Replicating Machines is an essential reference for anyone hoping to see how breakthrough methods may make robotic probes possible not just to the nearby stars but throughout the Milky Way.
Freitas’ REPRO paper is “A Self-Reproducing Interstellar Probe,” Journal of the British Interplanetary Society 33 (1980): 251-64. Also available in revised form on the Web. See also F. Valdes and Robert A. Freitas Jr., “Comparison of Reproducing and Nonreproducing Starprobe Strategies for Galactic Exploration,” Journal of the British Astronomical Society 33 (1980): 402-406.
