In the final part of this book review series we look at the last chapter where the author gives his conclusions and further thoughts on many of the topics discussed so far and gives some assessments on how far it would be feasible to push technology and possible scientific breakthroughs needed: "If prospects for mastering the ultra-technical problem of interstellar travel in the near future seem somewhat discouraging, this chapter tries to show where we must try harder." so let's have a look.
To understand Nature, theories are developed to help us describe her and to verify these theories experimental evidence is acquired to see if Nature agrees with them. Many theories are developed however few standup to observational evidence or experiment. [CI: a good quote from Planck below].
The author goes on to mention that: "Whether our science is slowing for lack of startling new evidence, or for lack of deeper, more clever human thinking, or from reaching final barriers inherent in nature cannot be determined. The situation is perplexing. Some would say that scientific progress continues to accelerate, not just applied science but basic science." It is also pointed out that as far as interstellar travel is concerned, this is partly an engineering problem and building a starship would involve assembling materials of the right size, with reliability, performance and cost. This may also mean pushing known materials to their limits. Several examples are then given with a discussion on the current state of affairs regarding superconductors, magnetic fields and materials sciences and their applications to previously discussed power systems, shields and solar sails. The author then mentions some of the current problems we face in science related to interstellar travel:
"Scientifically, the crucial advances are needed first in physics. If powerful propulsion is developed, along with solutions to some of the operations problems, then biological, social, and economic problems of travel would eventually be solved. Those who study interstellar travel in a fascinating blend of fantasy and real physics have found a large number of different ways of getting there, almost all meriting careful theoretical study, none yet leading to affordable hardware even if scientists had ready access to space now. Unfortunetly, there is very little indication in general relativity or elementary particles of new higher energy sources, forces, or effects that could be used in propulsion. If there were, motion in the context of general relativity (curved space-time) would also merit more study and be part of this book."
[CI: Research is ongoing.
Regular conferences are also held. For those who wish to study the
propulsion aspects further, a good starting point is:
Frontiers of Propulsion Science. If you need to brushup on your
physics try this
free book, these
free lecture notes and read this
other free book while you're at it. Understanding the physics of the vacuum (and how it affects matter/energy and vice versa) may be an important step in making some progress in this area. Here are some interesting items to get started:
1,
2,
3. Regular news in this field can also be found from
Centauri Dreams and keep an eye for the latest papers on
arXiv and
viXra. Here's a useful guide to sort out the
crackpots.]
The author then discusses further the enormous energy requirements for interstellar travel and even with anti-matter we still have the fundamental energy problem: "how to get more energy from less inertial mass." It is pointed out that some studies for eg. have looked into the vacuum with its varying amounts of fluctuating energy mostly as particle/anti-particle pairs and the possibility of harnessing these quanta of energy. Nature allows one to borrow energy without penalty provided this loan is over a very short period of time: "Planck's constant is involved and permits only very tiny effects [see Appendix D]. One could borrow 10^24 J, the whole KE requirement of a star journey, from space itself for about 10^-58 second, an inconveniently short-term loan." with longer time loans one could borrow smaller amounts of energy but for 1 J the loan period would still only be 10^-34 second and this quantum borrowing doesn't appear feasible however notes: "A way to cheat nature on this law is yet to be found. Perhaps it will be overthrown in some subtle, unforseen, dramatic way."
Other areas worth a look where the Stardard Model has left doors open include the possibility for a "false vacuum" field or
Higgs field which is thought to be left over from the Big Bang and might be producing the masses for all particles by "resisting" their motion until they enter the TeV energy domain. The Higgs field would make itself known by its Higgs bosons over 1 TeV [CI: one of the goals of the
LHC is to detect these particles] and the author gives a rundown of what we think of this Higgs field looking at the
cosmological constant considerations and
inflation theories. The author suggests further studies into the higher energy domains using space based accelerators may provide some solutions for interstellar drives and notes that: "space has many "vacuum" fields: gravitational, electro-magnetic, hypothesized false vacuum (Higgs), nuclear, and others that may be discovered. All should be ruled by the law of quantum physics in that the amount of energy that temporarily appears is determined by Planck's constant." Studies in the the electromagnetic field of the vacuum have also been ongoing with the
Casimir effect. The force on the plates can be made to
[CI: Image: When two metal plates are a few micrometers apart, an attractive Casimir force develops due to an inbalance of the electromagnetic fluctuations in the vacuum between the plates.]
do work that is yield energy however no useful amounts of energy are expected to be extracted using this method. Some consideration is given to "
Dark matter" and the author gives a rundown of what is and isn't known so far however notes that: "More discouraging is that the bulk of dark matter, if it exists, seems to be present not within galaxies but in intergalactic space where we cannot use it." All avenues for a possible "ultra-energy" source need investigating, this is directly linked to any future prospects for ultra-relativistic interstellar travel.
A discussion is then given on
Back Holes and other interesting phenomena that follow from General Relativity that "greatly distort, disconnect, or connect space-time" such as hypothetical
wormholes and
whiteholes. Some studies have looked into the possiblities of using these unusual spacetime topologies for interstellar travel and all are highly speculative. The author notes that: "Black Holes are probably real although finding sufficient evidence to be convincing has been a major task. The nearest partly-confirmed one
Cygnus X-1 is thousands of lightyears away." Approaching a black hole alone would be very dangerous because of the intense radiation due to infalling material and the large gravitational tidal effects ripping objects apart. Various authors have considered black holes as a possible power source for starships however these appear unrealistic.
Some mention is also given to so called "
anti-gravity", "
hyderdrives", "
spacewarp drives" and
Faster-than-light (FTL) ideas which are thought to be either physically unrealistic or highly speculative however notes that General Relativity as currently understood does not prohibit particles moving FTL locally but they cannot move slower than light, these hypothetical particles are called
Tachyons. [CI: Many authors have written
papers on various spacetime warping schemes however
some authors have also shown that these may not be physically possible when one considers quantum effects as well. My point of view is that the highly distorted spacetime topologies are unfeasible when one considers quantum vacuum effects and are at best interesting exercises in General Relativity for considering the various solutions it can offer, not all of which can be used or are allowed in nature.]
In chapter 8 the author discussed the problem of space transport from the starship to the surface of the new planet and vice versa with a discussion of why this would be a problem as there would presumably be no facilities on the new planet (for shuttle launches for eg) and the use of chemical rockets for moving large amounts of materials and personel is expensive and unsuitable for planets larger than Earth (with a deeper gravity well) because of the large amounts of fuel (preferably hydrogen/oxygen) to be carried on the interstellar mission and this would need to be found at the destination. It is also noted that with new planets with an atmosphere, shuttles could glide their way to the surface however for some interesting airless planets these will require rocket landings (and probably would be deficient in fuel). Capsule landings via parachute are not recommended because they only carry small loads and are less likely to be reusable. Abundant hydrogen would still be required even if anti-matter provided the energy.
Mention is given to new designs of spaceplanes which were considered such as the
Hermes however these may be difficult to design for unknown atmospheres. All the previous other ideas pointed out on
non-rocket space launches (space elevators, launch tracks, balloon supported launches etc) also have their problems and notes that: "As with spaceplanes, most of these methods seem to use materials to their limits in strength, toughness, and/or integrity. At an interstellar destination, most of these involve massive structures which cannot easily be carried or built on site." It is also pointed out that: "At present massive emigration from Earth to local space seems impossible due to the cost and complexity of lifting just the people, never mind their equipment, into space. Transport using cables and the like would change that." It would be necessary to build an extensive infrastructure in the local space environment (for fuel and structures) before landing people on the planet is considered.
[CI:
Image: One alternative to using rockets on an airless moon or planet: shoot your
precious cargo off the planet using the
eletromagnetic catapult concept?]
Some considerations are also given to the "wilder ideas" such as stronger materials using only neutrons (such as the closely packet matter in
neutron stars where the electrons have combined with protons and notes that: "If neutrons did "touch" and bond, nuclear matter would be a trillion times stronger than ordinary everyday matter." and with physicists starting to collect very cold and slow neutrons in bottles the possibility could be explored.
Several interesting conclusions are then given by the author on the various topics discussed throughout the book and makes a note again that: "The key practical step toward the stars is propulsion, but technology is inseparable from purpose." and points out that a trend in the articles published on starship propulsion ideas seem to lean more and more towards lower speeds from the high gammas of 0.9c down to 0.1c (Daedalus) to 0.01c for some fusion drives. The author gives his assessment on propulsion on p303:
"The best guesses here for propulsion at present are sunbeam-pushed sail or pulsed fusion, with a separate braking system. Farther down the list because they are still slower are an ion-drive powered by a fusion reactor, or a solar sail using direct local sunlight."
and notes further along that: "shielding, artificial gee, operating power (especially if sails are used), and landing remain to be solved as discussed earlier." pointing out that a starship with no artificial gee is likely to deny the crew to land on a planet due to the permanent changes this would cause to their bodies.
Further conclusions on the sociopoliticial context are also given. It is pointed out that although there was initial great enthusiasm for the space age, progress has become very slow and: "public support and money seem to have diminished, although US NASA's budget increases year by year. Less and less is done, while some billions of dollars are spent on mistakes." and further along asks:
"Political problems aside, is this the trend of our future in space, less and less over longer and longer time, with perhaps bigger confusion and failures? Have we reached technical limits already? Delays on new propulsion would seem to corroborate this... A yet unimagined drive would drastically change the situation."
It is also pointed out by the author that over the several chapters, many great problems were presented and this all might seem discouraging to future prospects for successful interstellar missions:
"the difficulty of determining a habitable destination before going there, the difficulty of returning, the long ordeal of many generations traveling to a near star, the hazards of breakdowns, radiation, and zero gee, the long time commitment to find out if a mission yields results, and the lack of visitors here who have accomplished such travel. But it is too early in our social and technological history to draw final conclusions about this futuristic goal, and no single paramount conclusion can be presented here. To reach the expected scientific and technical adventures and achievements, we simply may need to try harder."
Given the expected enormous costs of an interstellar mission (given at 100 T$ upwards for a 100 person mission), the author gives his point of view of why interstellar travel should be pursued despite the costs and great technical difficulties:
"The energy, environmental, and monetory costs of launch from Earth already seem almost prohibitive. Some say that we can no longer afford space, that we must work on problems of population control, environment, energy, housing, education, equality, crime, and health at home. Even if these problems were fully and cooperatively addressed, one should still ask, toward what end? Is this all a young civilization can do for the rest of its time, try to hold onto a high quality of life for some and establish a mediocre but adequate quality for the rest? Restricted to one planet, humans at most can try for an ecologically stable or "sustainable" set of "lifestyles", more complex, based on different economics, probably more austere than present varied styles. At best, a garden planet with a lower stable population would result. New frontiers in space can provide direct and symbolic outlets for untamable instincts, places for adventurers to fulfull themselves and initiatives for homebound populations to support at modest levels."
and also makes several conclusions based on human nature as it appears now. Contrary to what is portrayed in science fiction, because of the many difficulties mentioned earlier, very few people would actually get to travel to the stars and interstellar travel cannot also be used to "save" Earth. Sending any useful practical results back to Earth could also take centuries (if one considers a 1000 year mission) and hence interstellar missions would be very difficult politically to implement and most people would have forgotten of the mission over time however: "While the problem of interstellar travel is closely intermeshed with our future, it is an open question whether we can expect a study of interstellar travel to make a difference in the general human condition."
The author concludes by mentioning that the goal of his study and review on interstellar travel was: "to leave the case in the hands of present and future readers with a wide range of interests and let them judge what the prospects are now and later. All options should be set out for contemplation and planning... Interstellar studies as a complex branch of science and technology could serve as one of many inspiring vehicles to teach the spectrum of sciences and applied math, to introduce systems and design, to teach integrated and creative thinking, to show the interaction of technology and society, and much more. Grasping and exploring the problem can engross a student for years. Education in the broadest ways possible is one key to a future that permits interstellar missions."
[CI: In the Appendix sections of the book, the author gives a concise summary of classical physics and relativistic travel with an explanation of the physics behind the various concepts discussed in the book with more detailed mathematical treatment and numerical output tables of the included BASIC programs comparing various values of acceleration, time, gammas, distance traveled etc. The author has also put together an impressive bibliography section.
Although somewhat dated, this was one of the best books on Interstellar Travel I've read and thoroughly enjoyed reading it. I was impressed by the level of detail and information given by the author and highly recommend the book to anyone.]
