Alien and human astronauts alike must conform to the dictates of Relativity when traveling at velocities near the speed of light. Einstein’s theory, generally accepted today by the scientific community, predicts a host of fascinating consequences of near-lightspeed voyages.

First, we should briefly mention some of the jargon commonly employed by physicists and writers in this field. Relativity predicts that no material object can be accelerated up to the speed of light without an expenditure of an infinite amount of energy. Since the entire universe contains only 10⁸¹ joules of energy, attaining the speed of light becomes a practical impossibility. The velocity of light, designated as "c", this is a kind of "cosmic speed limit" imposed on all material objects within the physical universe. (Other jargon for c includes "100 psol" or percent-speed-of-light, and "Mike 1.0" after Dr. Albert A. Michelson.) Velocities below c are referred to as "suboptic" or "subluminal"; those faster than light are called "FTL," "hyperoptic" or "superluminal." The speed of light itself is "optic velocity."

Now back to the fascinating consequences. According to Relativity theory, time passes more slowly at near-optic velocities than at low suboptic ones. This apparent breach of common sense is traditionally presented in the form of a paradox. Imagine twin brothers A and B. A becomes an astronaut and flies away in a relativistic starcraft capable of a peak velocity of 98%c. B stays behind on Earth. A travels 12 light-years out into space, and then 12 light-years back to Earth. Because A has been moving slower than light, B must wait a total of 28 years for his return. B is thus 28 years older than his age when the brothers parted. But when he meets A in the Debarkation Area, A has only aged 10 years. A is 18 years younger than his identical twin.

This unusual consequence of near-optic flight, often called the Twin Paradox, has been confirmed indirectly by scores of experiments over the past half-century. There is little doubt in the minds of most physicists that the Paradox is a correct prediction of the consequences of traveling close to the speed of light. The contraction of time at high velocities is known as the phenomenon of time dilation.

Table 17.1 Effective FTL Starflight Using Relativity Theory

The frame of reference of the observer is of critical significance here. Those observers who remain at rest with respect to the universe at large (such as the twin who stays on Earth) will always observe a relativistic starcraft to travel at suboptic velocities. But to the astronauts on board the spaceship, the contraction of distance between the points of origin and destination (another peculiar consequence of Relativity theory) will make the trip seem shorter. They will, from their frame of reference, actually be moving at a faster apparent velocity than that perceived by stationary observers (say, back on Earth). In fact, when the starship reaches exactly 70.7%c as measured by stationary observers, the astronauts calculate their own effective velocity as 100%c! As acceleration continues still further, shipboard-determined speed increases to seemingly hyperoptic velocities (which Earthbound observers still see as suboptic from their frame of reference).

Table 17.2 Duration of Interstellar Travels, Using a Standard Flight Plan at One Gee Acceleration

What does all this mean in plain English? Simply this: An astronaut, provided his starcraft has sufficient energy, can effectively travel faster than the speed of light relative to a stationary frame of reference! (See Table 17.1 for details.)

Time dilation permits very long journeys within a single human lifetime. Consider a starship that accelerates uniformly to the midpoint of the trip and then decelerates uniformly at the same rate the rest of the way to the destination — called the Standard Flight Plan.

With an acceleration of 1 gee — appropriate for inhabitants of terrestrial worlds similar to Earth — only a few years of ship-time are required to reach the nearest stars. (See Table 17.2.) Only 21 years are spent reaching the Galactic Core, and in 28 years of shipboard time the intrepid explorers can visit Galaxy Andromeda in person. Since Andromeda is about 1.7 million light-years distant, this works out to a mean effective velocity of 61,000 times the speed of light.

Of course, there is no time dilation on the home planet — since it went nowhere. If our intergalactic astronauts turned around at Andromeda and immediately returned to Earth, they would have aged a total of 56 years. The Earth and all of its inhabitants, however, would have aged 3.4 million years. This is a "twin paradox" with a vengeance!

Table 17.3 Acceleration Required to Complete Journey in One Decade Shipboard Time, Using Standard Flight Plan

When science fiction writers and others speak of "FTL" they are usually referring to true (rather than "effective") FTL — that is, faster-than-light travel from the point of view of both astronauts and stay-at-homers. We shall discuss the theoretical possibility of true FTL later in this chapter. But for now it is important only to realize that Relativity does permit effectively hyperoptic interstellar journeying, at least from the standpoint of an astronaut setting forth to explore the universe.

This fact is highlighted by the data shown in Table 17.3. It is assumed that an astronaut wishes to travel a certain distance out into space, but he doesn’t want to use up more than 10 years of his life in getting there. Table 17.3 lists the starship accelerations that must be sustained throughout the entire trip in order to arrive at a destination at the specified distance within exactly one decade as measured on the astronaut’s own wristwatch. A Standard Flight Plan is assumed.

Note that any point in our Milky Way galaxy can be reached in ten years of shipboard time, at accelerations tolerable to human beings for long periods of time. Accelerations of 2-4 gees, perhaps sustainable by inhabitants of jovian or heavy subjovian worlds, or by bioneered former inhabitants of terrestrial worlds, would put the entire known universe within 10 years reach.

Naturally, the faster a starship is pushed the more energy is required. The levels of power expenditure needed to achieve the benefits of relativistic time dilation are enormous by today’s standards, even for fairly small vehicles. (See Table 17.1.) But as we shall see presently, this by no means bars interstellar or intergalactic commerce. Indeed, such commerce should be commonplace among Type II and Type III civilizations.

Let us consider three illustrative classes of starflight missions:

• 1. Interstellar personnel transport;
• 2. Intergalactic personnel transport;
• 3. Intergalactic cargo transport.

Too many writers have succumbed to the Fallacy of the Big and the Costly. That is, if it’s big and it costs a lot, it must be impossible. The Fallacy lies in the simple observation that what seems big and costly to one culture may be negligible and cheap to another.

The relative costs of missions to the stars may perhaps best be appreciated by a comparison with the familiar. Humanity has launched about ten Saturn V rocket boosters to date. Each of these blustering behemoths developed 1.3 × 10¹¹ watts of power for about 150 seconds each. This is the equivalent of harnessing the entire human energy output for exactly 4 seconds. Keep this quantity in mind as we work through the following examples: One Saturn V equals 4 seconds of humanity’s aggregate power output.

First we consider the interstellar personnel transport mission. We assume a flight distance of 100 light-years, appropriate for short hops between neighboring Type II civilizations. To make things comfortable for the pilots and passengers, we further assume a Standard Flight Plan at a constant 1 gee acceleration. Using the equations of Special Relativity, total trip time works out to a mere 9 years.

How much energy is required? If we take the mass of the vessel to equal that of the Starship Enterprise of original-series Star Trek fame (190,000 metric tons), then about 9 × 10²⁶ joules of energy are required for the mission. A mature Type II civilization, having 10²⁶ watts (joules/sec) at its disposal, should have no trouble with this at all. The interstellar personnel transport mission uses only nine seconds of the culture’s total power output, a feat equivalent in stature to the launching of two Saturn V rockets by modern human engineers. For a Type III civilization, this mission is inordinately trivial. In fact, roughly 44 billion such starliner missions could be dispatched if only 4 seconds of the galactic society’s aggregate energy output were utilized — a feat comparable to the launching of a single Saturn V from Earth today.

What about our second class of starflight mission — the intergalactic personnel transport? These are somewhat more difficult, probably well out of reach of a lone Type II civilization. Again, using a Standard Flight Plan at 1 gee acceleration to keep crew and passengers at ease, a trip of 1.7 million light-years to Galaxy Andromeda (the nearest giant spiral) would require only 28 years shipboard time. Approximately 1.5 × 10³¹ joules would be consumed making the journey.

Such a mission would tax the resources of Type II culture to the breaking point. More than 41 hours of the stellar society’s power output would be needed to launch a single intergalactic starliner, a feat analogous to the firing of 38,000 Saturn V’s by present-day humankind. Such an enormous sacrifice and commitment of resources would require some overwhelmingly compelling purpose to justify it.

For a mature Type III galactic civilization with 10³⁷ watts under its control, however, the intergalactic personnel transport mission would again prove utterly trivial. Such a culture could launch more than 2.5 million such sorties to Galaxy Andromeda for a mere 4 seconds worth of its total power output — again, a feat comparable to our launching a single Saturn V rocket.

Finally, we consider the case of the intergalactic cargo transport mission. Since these ships can be unmanned, far higher accelerations may be tolerable. We assume a robot-controlled, 190,000-metric-ton cargo ship, dispatched to Andromeda at an acceleration of 10⁶ gees (probably the upper limit for normal physical materials) on a Standard Flight Plan. Fortunately, the shipboard time of flight is only 1724 seconds, a bit under half an hour, so the ship and its contents are subject to extreme forces only for a very brief period of time. A total of 1.5 × 10³⁷ joules are required for the mission, about 1½ seconds of the aggregate power output of a galactic culture. To a Type III civilization, the launching of two such high-acceleration cargo vessels requires a resource commitment equivalent to the launching of one Saturn V by human technologists.

So we see that galactic and intergalactic commerce and tourism are very real possibilities for advanced extraterrestrial societies. The dispatch of an interstellar personnel carrier by a Type II culture, or a high-acceleration intergalactic cargo transport vehicle by a Type III culture, represents about the same allocation of energy and resources as the launching of a Saturn V rocket by human Type I civilization.

Three fundamental requirements must be satisfied if starflight is to be considered plausible for alien or human civilizations. First, the kinematics must be right. Second, the energy available for the mission must be sufficient. Third, there must exist an astronautic technology capable of performing the requisite kinematics using the available energy. In the preceding section we saw that the kinematic and energetic requirements are satisfied by Type II and Type III civilizations operating within the bounds of Einsteinian Relativity theory. For the remainder of this chapter the third requirement is examined in more detail — the technological aspects of interstellar voyaging.

Consider the nature of technical progress. There is an orderly progression from the emergence of new ideas, further research and development, then finally a reduction to practice and economic exploitation. Thus, the realization of new technology normally proceeds in four stages:

• Stage 1 — IDEA — the basic philosophical idea, discovery of a new physical law, a new branch of mathematics, a new possibility within the existing framework of science.
  (See Hogan²⁹¹⁶ for an excellent example of this in science fiction.)
• Stage 2 — THEORY — development of the mathematical/physical theoretical framework which ties the new idea to other known phenomena.
• Stage 3 — PRACTICE — research and engineering; building a device which utilizes the new idea in its manufacture or operation.
• Stage 4 — PROFIT — social, political, and economic acceptance and exploitation; who pays for it, is it worth the effort, who will benefit, etc.?

Matching this progression against current human achievements, we find that radio, television, computers, automobiles, and chemical rockets all have arrived at the profit stage here on Earth. Such advanced technologies as nuclear power planets, SSTs, and fission propulsion rockets are at the practice stage, awaiting full societal and economic commitment to move them into the profit stage. Laser fusion and satellite solar power stations are presently in transition from theory to practice.

The five conventional interstellar propulsion systems described in this section are all at the theory stage in human technological development. Each has received sufficient study to tell us that they have the basic physical and energetic feasibility to be considered for missions to the stars. All retain a number of technical uncertainties which will require significant engineering effort to overcome. Nevertheless, xenologists expect that all five propulsion systems are likely to advance at least to the practice stage within a century or less on this planet. So it is likely that these same systems also accurately represent the interstellar transport technologies of many extraterrestrial sentient races in the universe.

A sixth conventional interstellar propulsion system employing paired high-energy mass drivers for efficiently transporting massive cargoes between distant stars is briefly described in Section 21.4.1.

Figure 17.1 Project Orion (from Dyson478) Figure 17.2 Nuclear Pulse Vehicle (from Forward718,2812)

In the 19th century Hermann Ganswindt proposed the use of a series of gun powder charges behind a vessel to propel it into space.²⁷⁶² The concept of nuclear pulse propulsion, the modern extension of this basic idea, was developed under USAF contract at Gulf General Atomic during the years 1958-1965.

Dubbed "Project Orion" by the scientists who worked on it, the system operates by tossing out a nuclear bomb, exploding it astern of the ship, and absorbing part of the momentum of the resulting debris. The rocket thus consists of at least five distinct parts: An hemispheric ablation shield or "pusher plate," an enormous shock absorber, a mechanism for ejecting the bombs, the bomb magazine, and finally the payload at the front end. Small test models using steel pusher plates and TNT charges were successfully flown during the experimental phase of Project Orion, but the work was shelved after the Nuclear Test Ban Treaty entered into force late in 1963. (The Treaty prohibits the explosion of nuclear devices in the atmosphere or in space.)

Freeman J. Dyson of the Institute for Advanced Study at Princeton, New Jersey, who worked on Project Orion, described a prototype model of a space vehicle utilizing the principle of nuclear pulse propulsion.⁴⁷⁶ His "Ablation Space Ship" has a total mass of 400,000 metric tons, consisting of 300,000 tons of one-megaton H-bombs (weighing 1000 kg each), 50,000 tons of structure and payload, and 50,000 tons of ablation shield. The pusher plate construction is such that about 30 meters/second forward velocity are imparted to the vessel with each explosion. A smooth 1-gee acceleration is maintained by the detonation of one bomb every three seconds, which requires a shock absorber stroke length of about 75 meters (Figure 17.1).

Dyson’s nuclear pulse vehicle exhausts its bomb supply in ten days, having reached a final velocity of 10,000 km/sec (about 3.3%c). The flight time from Earth to Proxima Centauri would then be 130 years for a simple flyby mission. For an encounter-capture mission the payload must be decelerated at the target, which more than doubles the time of flight for a vehicle of this size.

More efficient variants of the nuclear pulse technique have been suggested in recent times (Figure 17.2), particularly the idea of igniting tiny deuterium pellets instead of huge bombs behind the ship. Ignition would be achieved using pulsed laser beam fusion^2751,2752 or heavy ion or electron beam fusion.²⁷⁶⁴ This latter technique has been selected for use in the Daedalus starprobe, an interstellar vehicle designed as part of a feasibility study sponsored by the British Interplanetary Society during 1975-1977.²⁹⁵³ In the Daedalus system, a 54,000-ton craft is propelled up to 12%c using a stream of frozen deuterium/helium-3 pellets which are zapped by a megavolt electron gun 250 times a second. The designers claim that the probe should be able to reach Proxima Centauri in 35 years with a 500-ton payload.²⁷⁶¹

Montage of fusion-powered interplanetary spacecraft concepts from 1987-2004 (from Wikipedia)

Another highly promising starship propulsion system is the technique of using a controlled, continuous thermonuclear fusion reaction as the main power source. Because of the tremendous temperatures involved (upwards of 10,000,000 K), no known material can physically contain a fusion reaction. Magnetic fields therefore must be used to contain, compress, and heat the plasma fusion fuel (hydrogen, deuterium, tritium, etc.). Scientists are currently engaged in designing and testing various "magnetic bottle" configurations strong enough to hold such an energetic plasma, in connection with electrical power generation at major fusion research facilities around the world.

The biggest problem is to make the magnetic bottle leakproof enough so that fusion reactions occur in sufficient abundance for the process to become self-sustaining. In terms of propulsion, however, a leaky bottle is exactly what is required. Hot plasma, energized by fusion energy, streams rapidly from the site of the "leak" and produces the desired rocket thrust.

According to Dr. Robert L. Forward, Senior Scientist at Hughes Research Laboratories, a deuterium fusion rocket capable of a steady 1-gee acceleration and consisting 90% of fuel (by mass) could reach a final velocity of 10%c.⁷¹⁸ This would mean a travel time to Proxima Centauri of 45 years. Theoretically, a fusion-powered starship could be fueled by hydrogen isotopes drained from the atmosphere of Jupiter or from the icy rings of Saturn, and writer Alan Bond has estimated that a 10 light-year mission could be completed in 60 years.¹¹⁵⁹

One major problem to be overcome from a practical standpoint is to learn how to deal with the various forms of energy released by a fusion engine. Only about 20% of the energy liberated by nuclear reactions appears as kinetic energy — direct propulsive thrust — in the leaking plasma stream. Ten percent is thrown off as heat and ultraviolet radiation, but the lion’s share (70%) is released as X-rays. G.L. Matloff and H.H. Chiu have suggested that this energy may be reclaimed by using an auxiliary laser thruster surrounding the fusion reaction chamber.²⁷⁵⁴ Waste X-rays, absorbed by, say, xenon-doped gas in the laser, are converted into a collimated light beam which serves as a photon thruster.*

* A wide variety of related but "futuristic" propulsive energy systems may be possible. These may include:

• Condensed cold neutron reactions
• Pion fusion (which has been demonstrated experimentally)
• Muon catalysis fusion,²⁷³⁷
• Hawking black hole induced fusion¹⁹⁴⁷
• Compact monopole fusor and energy storage devices or magnetic monopole metamatter.¹²²⁴
• Fission rockets are not ruled out²⁷⁵⁸
• High-energy superpropellants such as monatomic hydrogen, metallic hydrogen,²⁶⁸⁴ cryogenic metastable triplet helium (stores 0.5 megajoule/gram) may be available.²⁷³⁶

Figure 17.3 Bussard Interstellar Ramjet2812

The interstellar ramjet, first proposed by Dr. Robert W. Bussard at Los Alamos Scientific Laboratory in 1960, is a propulsion system which acquires energy and reaction mass from the surrounding medium.²⁷⁶⁶ Using some combination of electric and magnetic fields, the ramjet scoops up ionized interstellar gases to fuel its fusion rockets. This eliminates the need to carry large masses of fuel on board. Conventional chemical or nuclear rockets must be used to accelerate the starrammer up to about l-5%c, the threshold velocity at which the ramscoop mechanism becomes reasonably efficient.

The forward scoop would be immense (Figure 17.3). Bussard originally calculated that a 1000-ton vehicle would require a funnel diameter of 3600 kilometers to achieve a one-gee acceleration in normal interstellar space (~1 H-atom/cm³). In regions of dense hydrogen clouds, with perhaps 103 atoms/cm³, the ramscoop diameter could be as small as 120 kilometers. Theoretically, the acceleration could be maintained indefinitely, making possible the circumnavigation of the entire universe in less than a human lifetime.²⁷⁵⁵

Such monstrous scoops, of course, need not be constructed of physical materials. Most probably electromagnetic fields will suffice. To generate such fields, A.J. Fennelly of Yeshiva University and G.L. Matloff of the Polytechnic Institute of New York proposed in 1974 an annular copper cylinder coated with a layer of superconducting tin-niobium alloy (Nb₃Sn).¹⁴⁵⁴ The device would be rather modest in size (as starships go), measuring 400 meters in length and 800 meters in diameter. Energized with electrical current, an electromagnetic scoop with an effective diameter of 10⁴ kilometers would be generated. For braking at the destination, a drogue chute made of boron (noted for its high melting point) about 10 km in diameter is recommended. (An electrically charged wire mesh would give sufficient drag without being destroyed by erosion.^1066,1061)

As if to underscore the tremendous engineering difficulties involved in scoop design, Fennelly and Matloff announced in 1976 their original device was simplistic and probably would not work:

It is not possible, we have found, to design such a scoop. The {forces} induced … stress a scoop beyond the elastic limit of the substrate material and shear the superconductor to such an extent that it will be driven to a normally resistive state, with a subsequent catastrophe from the almost instantaneous Joule heating.¹⁶¹⁵

Nevertheless, assert the authors gamely, "we have hope that further analysis will lead to feasible scoop designs with some type of electromagnetic field to give a large scoop effective radius." It is now believed that a mixed electrostatic/electromagnetic field design will give the best results. (See Matloff,²⁷⁵⁹ Matloff and Fennelly,²⁷⁶⁶ and Powell.²⁷⁶⁰)

By adding "wings" to the starrammer, travel times may be cut in half.²⁷⁸² Explains one writer:

The wings are two great superconducting batteries, each a kilometer square. Cutting the lines of the galactic magnetic field, they generate voltages which can be tapped for exhaust acceleration, for magnetic bottle containers for the power reaction, and for inboard electricity. With thrust shut off, they act as auxiliary brakes, much shortening the deceleration period. When power is drawn at different rates on either side, they provide maneuverability — majestically slow, but sufficient — almost as if they were huge oars.²¹⁸⁰

The Bussard ramjet is perhaps the most intensively scrutinized potential interstellar propulsion system. As a result, scientists are beginning to call attention to collateral problems involved in the design and operation of ramscoop vehicles.¹¹⁵⁵ One objection voiced by John Fishback in 1969 is fundamental.¹⁴⁶¹ He points out that the section of the starship which contains the sources of the magnetic scoop fields must be strong enough to withstand the forces generated by those fields. As the starcraft goes faster and faster, the required field strengths also increase. Since materials are limited by their maximum tensile strength, at some point the acceleration of the vehicle will have to be reduced to avoid the breakdown of its structure caused by the pressure of magnetic forces.¹⁴⁶²

For realistic building materials, this cut-off velocity at which further acceleration must be drastically curtailed occurs at about 99.999998%c. This is high enough to be of no practical significance for galactic travel at 1 gee, but may prove restrictive for higher acceleration rammers or for starships on intergalactic missions.

Another major difficulty, noted by Bussard and many others since, is that the proton-proton nuclear reaction is a poor candidate for fusion rockets. Most of the gas likely to be scooped up by the interstellar ramjet will be ordinary hydrogen, and hydrogen is very finicky when it comes to fusion. Deuterium reactions have a cross-section roughly twenty orders of magnitude greater, but this heavy isotope of hydrogen is relatively rare in the interstellar medium.

Recently, Daniel P. Whitmire has suggested the concept of a catalytic nuclear ramjet to overcome this problem.¹⁴⁷¹ In Whitmire’s scheme, the starship would carry on board a supply of "nuclear catalyst" consisting of carbon, nitrogen, and oxygen atoms. This fuel additive should catalyze a vastly increased reaction rate among ordinary hydrogen atoms without itself being consumed. Calculations indicate that this technique will yield a rate of fusion more reasonable from the standpoint of interstellar missions. For this scheme to succeed, of course, a workable heavy ion fusion reactor must be developed, but, in Whitmire’s words, "the difficulty seems to be of a technological rather than fundamental nature." (He also proposes the use of a bank of forward lasers to ionize neutral atoms approaching the rammer’s maw, thus greatly increasing the reaction mass available for the starship’s engines.)

An interesting hybrid variation of the basic ramscoop technique involves a vessel that carries its own nuclear fuel supply and exhausts the reaction products for thrust, much like a conventional fusion rocket. However, this Ram Augmented Interstellar Ramjet, or RAIR as Alan Bond of the British Aircraft Corporation calls his device, enhances its performance by scooping up atoms from the interstellar medium and using them as reaction mass rather than for energy generation.¹⁴⁵⁵

In other words, fusion fuel is carried by the spaceship and additional reaction mass is collected from gas clouds through which the vessel passes. Preliminary calculations show that the RAIR design may save at least an order of magnitude of fuel savings at speeds up to 50%c, and as much as two orders of magnitude of fuel savings up to 70%c. Performance characteristics of RAIR starships have been worked out by Bond¹⁴⁵⁵ and Powell.^{1117,1115,2769}

Two other fascinating variations on the interstellar ramjet have been proposed by Whitmire and A.A. Jackson IV.²⁷³³ The first of these is called the Fusion Ramjet Runway. Micron-sized frozen deuterium pellets are accelerated electrostatically or electromagnetically out into space several years prior to the launching of a standard Bussard ramjet having a comparatively small scoop cross-section (perhaps it would be just a simple physical structure, such as a giant funnel). The starrammer could then collect a more concentrated fuel en route simply by staying on the "runway."

The other suggestion, rather bizarre and considerably less likely, is the Stellar Ramjet. This vehicle accelerates up to near-optic velocity across the photosphere of a star. Whitmire and Jackson propose that the envelope of a red giant or a large protostar would be ideal for this technique. Accelerative forces would be large but not prohibitive, and biological crews should survive if they are somehow immobilized or "frozen" during the starship’s relatively brief period of acceleration.

Figure 17.4a Beamed Power Propulsion Laser-Pushed Vehicle718 Figure 17.4b Beamed Power Propulsion Laser-Pushed Vehicle718

We have seen that it may be best to use an external source of energy to achieve near-optic speeds. The interstellar ramjet discussed above is a good example of this technique. Another possibility is the Laser Pushed Vehicle, or LPV.^122,2767

The LPV obtains its energy and momentum from a solar-system-based laser network which pushes the ship by photon reflection from an onboard mirror. Calculations indicate that a power of roughly 10¹⁴ watts delivered to a 100-ton starship should be sufficient to impart a one-gee acceleration to the craft. Focusing would be of critical importance, and it may turn out that only x-rays will have a small enough wavelength/diameter ratio to forestall gross energy wastage. Upon arrival at a destination, LPV deceleration is effected in reverse fashion with the active assistance of the receiving civilization.²²

What if there is no receiving civilization? Is the Laser Pushed Vehicle strictly limited to one-way flyby missions of exploration? The answer, apparently, is no.

According to Philip Norem, a space-based laser system could be used to accelerate a starprobe up to relativistic velocities. After a while, the craft extends long wires and charges them up to high voltage. These would interact with the Galactic magnetic field, swinging the LPV around in a slow, giant arc (Figure 17.4a). The course is chosen to aim back through the target star system, but on a general heading towards Earth. The wires are discharged and reeled in; the orbital laser network is turned on again, this time functioning to decelerate the starcraft.²⁷⁵⁶

Very large space-based laser arrays would be required to carry out such a mission, perhaps as big as 250 kilometers in diameter. These should be parked in close solar orbit, drawing their power directly from the high solar flux available there. For maximum efficiency, the LPV’s mirrored "sail" should be of a size comparable to that of the laser array — perhaps 250 km wide, weighing thousands of tons even if it is very thin. Surprisingly, the laser array energy flux need not be very high to push a vehicle to relativistic speeds. Typically the beam should be no more powerful than ordinary sunlight.

A proposed hybrid system combines the best qualities of the interstellar ramjet and the laser pushed vehicle and avoids many of their disadvantages. Called the Laser Powered Ramjet, or LPR, the starcraft obtains its power from a space-based laser network and its reaction mass from the interstellar medium using an electromagnetic ramscoop. The propulsion system is greatly simplified because the LPR does not require an onboard hydrogen-burning fusion reactor motor — since all power is furnished by laser beam. Calculations suggests that the LPR may be superior to the LPV under virtually all conditions. It should also outperform the Bussard ramjet at speeds below 14%c during the acceleration phase and
at all speeds during the deceleration phase of the mission.²⁷³³

Whitmire and Jackson propose two additional alternative propulsion systems that appear promising. The first of these is the Laser Powered Rocket, which differs from the LPR because it carries along its own reaction mass onboard instead of gathering it from the interstellar medium. Extraordinary energy efficiency may be possible because the exhaust velocity is controllable.²⁷³³

The second possibility may be called the Particle Powered Ramjet, which obtains its reaction mass, its fuel, or both from space-based particle accelerators: After acceleration, the particles could be neutralized by the addition of electrons or positrons to avoid coulomb spreading of the beam. The neutral particle current required would be relatively modest — for antimatter, about 10⁵ amps to produce 10¹⁴ watts on board at low velocities. The problem seems to be collimation since there is no particle analogue to the laser.²⁷³³

Table 17.4 Black Hole Power Generation and the Spontaneous Evaporation of Hawking Black Holes

The basic idea of using antimatter to power starships has been discussed in the technical and fictional literature for many decades. The most common system is the "photon drive"*: Simply bring together equal quantities of matter and antimatter, allow annihilation or "total conversion" to take place, and then convert the products to useful thrust. Gamma rays as well as high energy electrons and positrons are thrown off, but about half of the energy liberated is in the form of neutrinos which escape isotropically and are wasted.

The usual antimatter propulsion scheme thus amounts to no more than "partial conversion" with an efficiency well below 50%. Better results may perhaps be obtained by using cold or "frozen" positronium gas as fuel. Positronium consists of pseudo-atoms in which a negatively charged electron orbits a positively charged positron (the electron’s antiparticle) — this has been observed experimentally. As the fuel is warmed, electron and positron annihilate, producing a pure beam of gamma radiation. Eugene Sänger proposes using an electron gas reflector to focus and direct the photonic jet.²⁸⁴⁰ Alan Bond estimates that a million-ton starship with a mass ratio (fueled/unfueled, by weight) of 7.4 and an acceleration of one gee could reach 60%c.¹¹⁵⁹

Dr. D.D. Papailiou at Jet Propulsion Laboratory in Pasadena claims that for missions to nearby stars optic exhaust velocities are not necessary. A far more efficient technique is to use a small amount of antimatter to energize a large amount of ordinary matter. Preliminary calculations by Papailiou show that a mass ratio of 5.0 and a 2% charge of antimatter (by weight) is optimal to achieve a probe coast velocity of 33%c.²⁷⁵⁷

So if we wanted to launch a 10-ton starship on an encounter-capture mission, we must built a top stage of 51 tons — the 10-ton probe, 40 tons of ordinary matter, and 1 ton of antimatter — to obtain a mass ratio of 5. This stage serves to decelerate the probe at its destination. To get it there, we will need a 205-ton bottom stage — 200 tons of ordinary matter and 5 tons of antimatter — in order to accelerate the top stage up to 33%c. Delivery of a 10-ton space probe with a coasting speed of 33%c to another star system thus requires a total ship mass of about 256 tons, of which 6 tons are antimatter.

A related scheme is the Antimatter Ramjet, which gathers normal matter in the forward scoop primarily as reaction mass. In a manner similar to Papailiou’s antimatter drive outlined above, interstellar matter would be commingled with bits of antimatter stored onboard. In this case the probe need only carry the requisite 6 tons of antimatter fuel when it leaves the planet of origin (perhaps stored in the form of frozen antihydrogen maintained a few degrees above absolute zero), and can pick up the remaining 240 tons of ordinary matter en route to its destination.

Many physicists object to the feasibility of total conversion drives such as those mentioned above because of the difficulty today of generating macroscopic quantities of antimatter. But the problems of creation and control should not prove insuperable. Writes Dr. Forward:

The present methods for producing antimatter involve the use of large accelerators which can produce a proton beam of 10¹⁵ protons per second. When such a beam collides with a target, antiprotons are produced as part of the debris. The antiproton yield of present machines is very low. However, the presently used methods are not designed for antimatter production but rather for studies in the physics of elementary particles. Rough calculations assuming special purpose high amperage colliding beam accelerators indicate that the generation of kilograms of antimatter per year is not out of the question. The containment and control of the antimatter, once made, should not be too difficult since we have a number of ways of applying forces to the antimatter without touching it. Electric fields, magnetic fields, rf fields and laser beams are all used in present day technology to levitate and control small amounts of regular matter that we do not want to contaminate. These would all be equally effective on antimatter.⁷¹⁸

Another considerably more speculative total conversion system involves so-called Hawking Black Holes (HBHs). According to Dr. Stephen H. Hawking at the University of Cambridge, all black holes (if any exist) radiate energy due to quantum mechanical "tunneling" effects. This is equivalent to mass loss, so eventually the entire corpus of the black hole "evaporates." A stellar-mass BH is very "cold" — in fact, close to absolute zero — but a low-mass HBH is extremely "hot" and prone to explosive evaporation.²⁰²¹ For instance, a million-ton HBH should radiate about 10¹⁸ watts at a temperature of about 10¹⁵ K, and will take about one year to finish evaporating.

The HBH becomes a total conversion engine when we start "feeding" it. Evaporation can be indefinitely postponed simply by shoveling in raw matter — any matter — at an appropriate rate (Table 17.4). Such a device could be used to construct an extremely high-efficiency photon drive propulsive system. The main problem is how to construct an HBH in the first place. Dr. John A. Wheeler estimates that a black hole of mass 10⁹ kilograms might be generated artificially by the controlled thermonuclear fusion implosion of approximately 5 × 10¹³ kg of deuterium.²⁰²² This involves the handling of some 5 × 10²⁸ joules of energy, which looks like a job for an ambitious mature Type II civilization or an early Type III galactic society.

* Acceleration A (m/sec²) of a perfect photon rocket of mass M (kg) and power output P (watts) is given by: A = 2P/Mc, where c is the speed of light (3 × 10⁸ m/sec).

The exotic interstellar propulsion systems discussed in this section are all at the "idea" stage in human technological development. There is no guarantee that any or all of them can be made to work. A few have some theoretical support, but many do not.

Nevertheless, xenologists deem them important because of the likelihood that some may have advanced to the "profit" stage of exploitation in at least a few extraterrestrial Type II or Type III technological civilizations.

It is possible to conceive of "machines" that are capable of pushing a body, using gravitational force, up to fairly impressive suboptic velocities. These devices violate no known basic laws of physics, but it's difficult to see how to do the required engineering.

The most "conventional" of these, first discussed by Freeman Dyson, has been called the Contact Binary Catapult.¹⁰²³ A starship approaches a binary star system whose members are both white dwarfs orbiting each other at close range. By swinging in very near to one star on the proper trajectory, the starcraft benefits from gravitational slingshot or Newtonian "gravity whip" effect. The vessel withdraws orbital energy from the stellar pair.

Dyson estimates that a system involving two 1 Msun white dwarfs could accelerate delicate and fragile objects to a velocity of 0.7%c — at about 10,000 gees. Since there are no engines or propellants, there are no physical stresses on the payload — gravity acts on all parts of a material body equally. (Tidal forces, on the order of D/80 gees where D is starship diameter in meters, should not prove troublesome.)

Dyson also considered the possibility of using a pair of orbiting neutron stars as a gravity catapult. Unfortunately, they cannot exist! A neutron star binary would radiate away all of its orbital energy as gravitational radiation in less than 2 seconds. The two objects would coalesce almost immediately with a spectacular "gravity flash" at a frequency of about 200 Hz.

A more exotic technique is the Black Hole Catapult. This scheme requires that and find a rotating black hole (stellar mass) somewhere in space, then travel around it in the direction of spin very near to the equator. Besides losing a lot of time because of General Relativistic time dilation effects, some of the BH’s rotational energy would be converted to linear kinetic energy of the star ship. There would be a substantial increase in velocity. Tidal forces during transit will be rather extreme — about 500 million gees per meter — but apparently there are tricks with dense masses in a space vehicle that can be used to cancel the tides.²⁰¹⁴ A Neutron Star Catapult is also possible, with similar effects.¹⁰⁹⁹

The most speculative gravity machine of all is the Smoke Ring Catapult, which consists of a rotating torus of dense matter (such as neutronium) turning inside out like a smoke ring.²⁷³⁹ Relativity theory predicts a force in the direction of the rotation, so a starship fired through the center of the massive hoop could be kicked up to very high velocities — depending upon the rotational energy of the torus.

Over the years the theme of antigravity and inertialess propulsion systems have been widely discussed, mostly in the fictional or pseudoscientific literature.

• From H.G. Well’s cavorite gravity screen in his First Men in the Moon,
• To such questionable propositions as the Biefield-Brown effect,¹³⁷
• Blackett’s Spin-Magnetic Coupling Theory¹¹⁹⁴ (which apparently inspired the late science fictioneer James Blish to write Cities in Flight with its "spindizzy drive"²⁸⁰⁹),
• F. B. Hli’s Theory of Electrogravitics (see the lively debate in Cashmore and Gordon,^1197,1199 Hli,¹¹⁹⁶ Hli and Okress,¹²⁰¹ Johnson and Hli,¹¹⁹⁸ and Okress¹²⁰²),
• Leonard G. Cramp’s G Field Theory,⁷⁵⁵
• And of course the infamous Dean Drive (see Adams,²⁸⁵⁹ Campbell,¹³⁶¹ Cuff,²⁷⁷³ Davis,¹³⁷¹ Jueneman,²⁸⁰⁷ Klotz et al,²⁷⁷² Pournelle,²⁸⁰⁶ and Stine²⁷⁷¹), the idea has enjoyed a colorful and vituperative history.
• A number of U.S. patents have been issued on supposed antigravity machines.²⁷⁷³
• Even Einstein himself spent the last thirty years of his life searching for a unified field theory that would relate gravity and electromagnetism, and the search continues apace today.²⁷⁷⁴

To construct a gravity screen would theoretically require the ability to achieve gravitational polarization of matter.¹⁶ This would imply the existence of two very different kinds of matter — positive mass, which is attracted towards the Earth, and negative mass, which is repelled. At one time it was believed that antimatter might turn out to have negative gravitational mass,²⁶⁹² but most physicists would dispute this today.^1314,2952

Yet the search for negative mass continues. So far as we know there is no experimental evidence for negative matter, although it does appear in several solutions to the field equations in General Relativity. Papers by A.K. Raychaudhuri¹⁵²¹ and R. Mignani and E. Recami¹⁵¹⁹ suggest that tachyons may experience a gravitational repulsion to ordinary mass and thus may be interpretable as "negative matter," although because of their imaginary masses they will still fall towards a positive mass.

Besides gravity shields, negative masses, if they exist, could be employed directly for propulsion. A negative mass at rest beside a positive mass would begin to accelerate. Why should this be so? The negative mass (which repels all matter) would push on the positive mass, but the positive mass (which attracts all matter) would pull on the negative mass¹¹⁹⁰ If the two objects "weigh" the same, they will chase each other and will neither separate nor collide. Energy and momentum is conserved, since they each sum to zero.

Presumably the negative mass could be created out of empty space if a positive mass of equal "weight" was created simultaneously. The net energy cost would be zero, since (-m)c² + (+m)c² = 0. To achieve reasonable starship accelerations, compact masses with densities like black holes should be used. Dr. Robert Forward elaborates:

What we really want to do is make a dense negative mass and a dense positive mass down in the engine room. We’d just pull them out of empty space, put a charge on the positive one and couple it with the spacecraft with electric fields. Now we have the two masses down in the engine room; they’re probably about 10^-23 cm across and they weigh a little more than the spacecraft. The positive one is coupled to the spacecraft and the negative one pushes the positive one which pushes the spacecraft. Our vehicle’s acceleration can be as high as we can tolerate.²⁰¹⁴

A related concept is the idea of inertia control. Gravitational mass represents the force of gravity, and inertial mass represents the force of physical acceleration. The Eotvos experiment demonstrated that the two kinds of mass are identical out to eleven decimal places, under normal terrestrial conditions. But suppose we (or clever aliens) could arrange abnormal conditions which would allow the inertial mass of a chunk of matter to vary. When inertia is decreased, the same force imparts a higher acceleration; as inertial mass is brought close to zero, tiny forces would be able to produce huge accelerations. Lowering the inertial mass of fusion rocket propellant tanks would eliminate most of the normal constraints on lengthy interstellar missions for such vehicles.

Figure 17.5 Three Possible Antigravity Machines2014 Special General Inertia Redistribution Special Relativistic Antigravity Machine Newton’s law of gravity does not obey Special Relativity. However, a "linearized" version of Einstein’s gravity theory (General Relativity) does. In this version, a mass flow gives rise to a "protational field" much as a charge flow gives rise to a "magnetic field" in conventional physics. This is called the "Lense-Thirring Effect." The Special Relativistic Antigravity Machine is a torus wrapped in helical tubing. A dense mass flow is rapidly circulated through this tubing, causing a protational field to arise along the circumferential axis of the torus. Just as a changing magnetic field generates an electric field, a changing protational field should generate a unidirectional gravity field. This field, directed upwards, would cancel planetary gravity and provide vertical propulsion. General Relativistic Antigravity Machine According to Einstein’s General Relativity theory, the presence of mass in a flat spacetime will cause that space to become curved. Mass is intimately connected with space — a rotating mass causes spacetime to "rotate" too. The General Relativistic Antigravity Machine consists of a torus of dense mass which is turning inside-out like a smoke ring. A mass with this motion, according to Einstein’s theory, "drags the metric" through the center of the torus. Unidirectional general relativistic forces arise which are equivalent to gravity, and may be used to neutralize planetary surface gravity or as a space propulsion system. Inertia Redistribution Antigravity Machine While not widely accepted by theoretical physicists, there is a theory that the inertia possessed by all matter is a "tensor" quantity — that is, a multidimensional vector. In other words, inertia is not just a simple quantity but rather must be measured in the three separate directions of normal space. The Inertia Redistribution Antigravity Machine does not destroy inertia or convert it into energy, but merely redistributes it in different directions. The starship to which the device is attached becomes "heavier" in the horizontal plane (thus imparting navigational stability) and "lighter" in the vertical direction. An object with minuscule inertial mass in one direction can be accelerated to fantastic velocities with negligible force.

Three other antigravity machines have been discussed by Robert Forward which involve no violations of the basic and established laws of physics (Figure 17.5).

The first of these, which he calls the Special Relativistic Antigravity Machine, involves a mathematical analogy between gravitational and electric fields.²⁷⁴⁰ A "linearization" of General Relativity gives a version of Newtonian mechanics which obeys Special Relativity. (Classical Newtonian mechanics does not.) In electromagnetism, something called charge is surrounded by a spherically symmetric electric field. In gravitation, something called mass is surrounded by a spherically symmetric gravity field. It may be said that the simple Newtonian gravity field is the gravitational analogue to the electric field.

The linearized General Relativity theory provides a similar analogy to magnetic fields. Much as a magnetic field is due to the motion of an electric charge or current flow, the linearized theory suggests that a moving mass, or "mass current," will give rise to a new kind of gravity field by a mechanism known as the "Lense-Thirring Effect."²⁸⁹⁰ Scientists plan to try to measure this field as it is produced by the rotating Earth in future satellite experiments.³³²⁰

Forward calls this new field a "protational field." He claims that, based on the existence of the field, an antigravity machine might theoretically be constructed in the shape of

…a torus with a tube wrapped around it, filled with very dense matter. If we started accelerating that mass flow through the tube around the torus, we would get a constantly increasing protational field, inside the torus. A changing protational field will create a gravity field just as a changing magnetic field will create an electric field. If we did it right, we would have an upward gravity field that could be used to cancel the field of the Earth.²⁰¹⁴

A second kind of antigravity machine suggested by Dr. Forward couples directly to "the fabric of space-time." His General Relativistic Antigravity Machine makes use of the notion that the presence of mass in a flat space-time causes a curvature, and that a rotating mass causes space-time to rotate too:

Imagine a rotating torus of dense mass, turning inside out like a smoke ring. An inside-out turning ring of very dense mass will create a force in the direction of the motion — a "dragging of the metric" as it is sometimes called. There will be general relativistic forces in the direction of the velocity of the mass. These forces are equivalent to a gravity field which again, theoretically, can be used to cancel the gravity field of the Earth.²⁰¹⁴

And, as Willy Ley once pointed out, such a weightless body "would be squeezed out of the atmosphere by the weight of the air around it."²⁸⁰⁸

Finally, there is the Inertia Redistribution Antigravity Machine. The main principle behind inertia redistribution is the idea that inertia is a "tensor" quantity.²⁷⁴⁰ A tensor is just a multidimensional vector, so all this means is that we are accepting for the sake of argument the hypothesis that inertia may be a quality of matter that can be resolved into distinct directional components. That is, in our normal three-dimensional world, inertial mass becomes a three-dimensional quantity. While current experimental evidence does not support the tensor theory of inertia, if it is correct it leads to an interesting possibility for propulsion.

The Redistribution Machine does not get rid of inertial mass, but rather redistributes it so that some of it is pointing in new directions. If the machine makes the starship’s mass heavier in the horizontal plane and proportionately "lighter" in the vertical direction, a relatively tiny amount of force applied vertically would cause relatively large accelerations in that direction. The benefits are similar to those achieved using inertia control, discussed above, with the added advantage that inertia is conserved. (Note that since gravitational mass is unchanged, spacecraft will still feel the same attraction to planets and other massive bodies.)

Inertialess starships would have a number of interesting performance characteristics. Such a system must necessarily act upon every atom of the vessel in order to be effective. Far from any planet (so gravitational mass can be ignored), the inertialess craft could start and stop almost instantaneously. Since passengers have almost no inertia in the direction of flight, hideous accelerations can be tolerated (in that direction) with equanimity. For instance, if vertical/forward inertia is cut to 1% of normal and the ship accelerates at 100 gees, passengers would feel only an effective 1 gee of force. Inertialess starcraft would be virtually crashproof, since with no forward inertia people would not be thrown from their seats if an obstacle was struck. And, depending on how fast inertia can be suddenly redistributed, right-angle turns and hairpin bends should also be quite possible.

We have already mentioned the possibility of using tachyons for faster than-light communication. But could aliens use them for FTL space travel too?

Gerald Feinberg of Columbia University has divided all matter into three general classes:

• Tardyons — particles which can move at any velocity less than the speed of light (normal matter)
• Luxons — which can travel only at 100%c (photons and neutrinos)
• Tachyons — particles restricted to superluminal velocities¹⁴⁹²

All three classes may exist on the basis of Relativity theory.

If we desire to travel at hyperoptic velocities and achieve "true" FTL, somehow our starship must be converted from tardyon matter into tachyon matter at the start of the journey and then back again at the destination. While "conversion" may sound a bit like magic, actually it violates no laws of physics to presume it can be done. In fact, such conversion between classes of matter, to a limited extent, has already been verified experimentally.

Of course, no tachyons have been discovered yet. But nuclear physicists long have known that an electron and a positron, both tardyons, undergo "annihilation" when brought together with the release of two or three gamma-ray photons, which are luxons. Another example is the decay of the neutron, a tardyon, into among other things an antineutrino, which is a luxon. Still another example is "pair production," in which a gamma-ray (luxon) striking an atomic nucleus gives rise to an electron/positron pair (both tardyons). Conversion between tardyons and luxons, and back again, may be regarded as a verified physical phenomenon. There would seem to be no theoretical objection to conversion from tardyons to tachyons and vice versa, although the process may have to be moderated by luxon intermediaries.

The real technological trick will be to discover a procedure for interconversion which leaves undisturbed the essential molecular relationships upon which life and physical structure are based. Dr. Gregory Benford, nuclear physicist and science fiction writer, has suggested one highly speculative possibility in his story "Seascape":

In the laser the problem was simply to produce a coherent state — to make all the excited atoms emit a photon at the same time. Okawa reasoned that the same problem appeared in the faster-than-light drive. If all the particles in the ship did not flip into their tachyon states at the same time, they would all have vastly different velocities and the ship in one grinding instant would tear itself apart. Okawa’s achievement lay in finding a technique for placing all the ship’s atoms in excited tachyon states so that they could all be triggered at the same instant; the particles of the ship Jumped together, coherently. … All this was accomplished by maximizing the cross section for transition from real particle to tachyon. Complex modulated electromagnetic waves controlled the transition through microelectronic components, which operated on the scale of atomic dimensions.

Another conversion technique involves the use of the concept of quantum mechanical "tunneling." Quantum mechanics predicts that it is possible for particles to pass through energy barriers which would otherwise be "too high" for them to surmount. (This, in fact, is Hawking’s explanation for the evaporation of black holes.) Using the tunnel effect, a particle with insufficient energy to pass "over" a barrier instead passes "through" it. This peculiar behavior has been verified in the laboratory, and is exploited in modern electronic devices (e.g., tunnel diodes) as components in computer circuits.

Presumably, if we can get up close enough to the lightspeed barrier, it may be possible to "tunnel" across into tachyonland. The starship will then have to lose energy to go faster.

A third technique for achieving superluminal tachyonic starflight has been suggested by Dr. Forward. Mathematicians have long known that certain wave phenomena can be represented by "complex numbers." A complex number is a two-dimensional quantity, having a real dimension and an "imaginary" dimension. (Recall our earlier discussion of the imaginary mass of tachyons.) Electronics engineers regularly use complex numbers to describe the behavior of alternating current (AC) circuits.

If physical mass can be represented mathematically as a complex "something" in "mass space," then objects in that space may be able to rotate to new orientations along the real and imaginary coordinate axes. Since tachyons have imaginary mass, a full 90^o rotation would correspond to a conversion to tachyons. Says Forward:

Since we want the mass to be imaginary, we would have to get our spaceship off the real-line in that complex space and onto the imaginary-line. This would require that we make a right angle turn in mass space. When we do that, our ship becomes a tachyon.²⁰¹⁴

Finally, what would tachyonic astronauts see during their hyperoptic journey? The tardyon and tachyon universes seem to possess symmetric equivalence, an intuitive observation borne out by careful mathematical reasoning.²⁷⁷⁰ Consequently, travelers in tachyonland should observe what appears to be another tardyonic universe (assuming there is as much tachyonic mass as tardyonic mass in all the cosmos). That is, a tardyon starship moving at 50%c in our universe which suddenly converts into tachyons will be traveling at an effective 200%c relative to our universe; but in the tachyon regime, where the vessel really is, it will still appear to be moving only at 50%c.

Terran scientists already know that electricity and magnetism are interchangeable, based on the work of James Clerk Maxwell in the 19th century. Mass and energy are too, according to Albert Einstein, as well as space and time. It is not unreasonable to hypothesize that angular and linear momentum may be similarly related.

If such interconversion were possible, theory has it that some fundamental physical constant would be required, having the dimensions of length, to balance the mathematical equation involved. According to John Wheeler,²⁷⁴¹ one of the fundamental constants of nature is the Planck length, written Λ, based on the Planck constant (h), the universal gravitation constant (G), and the speed of light (c).*

Robert Forward suggests that if we use the Planck length a suitable interconversion formula may be derived: Angular momentum (L) equals linear momentum (p) times the Planck length, or L = pΛ. Using this purely hypothetical relationship, it is simple to calculate that the destruction of a unit spin from a single tiny atom (about 10^-34 kg-m²/sec) would yield 6.6 kg-m/sec of linear momentum. This means 6.6 kilograms of mass moving at 1 meter/second. The interconversion of only 45 billion atoms — about 0.1 picograms of hydrogen — would thus be sufficient to propel a 100-ton starship at l%c.

The above scheme, of course, requires the violation of conservation of linear and angular momentum. This difficulty may be made somewhat more palatable in the following way. Physicists recognize that the source of gravity in General Relativity theory is what is called the "stress-energy-momentum-mass tensor." That is, mass alone isn’t the only source of gravity. Kinetic energy, stress energy, linear and angular momentum also contribute to the field.

We already know that mass and energy can be interconverted, according to the relation E = mc². It may turn out to be possible to interconvert everything in the stress-energy tensor, perhaps according to an equation like E = mc² = pc = Lc/Λ. Vast amounts of propulsive energy would become available. Says Forward:

If we took one unit of spin which is 10^-34 units of angular momentum (very small), we would get 6.6 kg-m/sec of linear momentum or 10^-8 kg of mass or, equivalently, 10⁹ joules of energy — all from one atom.²⁰¹⁴

* The formula Wheeler gives is: Λ = (hG/2πc³)^1/2 = 1.6 × 10^-35 meters.

The mysterious, often apparently "magical" results of modern quantum physics have enticed many writers to try their hand at devising propulsion systems based on the principles of quantum mechanics and statistical thermodynamics. Most of these fall reliably into two general categories: Bootstrap Drives and Heisenberg Drives.

To visualize a Bootstrap Drive, consider a cylindrical vessel filled with ordinary gas. Although each of the molecules is rushing madly about at speeds in the kilometer/second range, the net effect of trillions of particles darting in random directions is a wash-out. The molecule motions are averaged out over the walls of the container, yielding a net system velocity of zero.

About half a century ago, the late John W. Campbell, Jr. suggested that it might be possible to devise an external field that would induce the molecules subject to its influence to assume the highly improbable state of collective upward motion.¹¹¹⁰ Much of scientific experimentation involves the judicious rearrangement of probabilities to achieve desired results, so this idea is certainly not impossible. The result would be a reactionless Bootstrap Drive that could be used to propel spaceships to other worlds.

It is fairly clear that the technology to achieve a bootstrap effect will not be trivial. The magnitude of the difficulty may be made plain with a fairly simple example. Imagine a pan containing a liter of water, placed on a stove that can bring it to a full boil in 15 minutes. Our experiment consists simply of heating the liter of water to the boiling point over and over again. According to the laws of statistical thermodynamics, there is a very small but nonzero probability that during one experimental cycle the molecules will spontaneously arrange themselves in crystalline form — that is, freeze instead of boil. But calculations show that this event is so improbable that it is expected to occur only once in every 10^{10,000,000,000,000,000,000} years, assuming 15 minute cycles. Clearly, to enhance such miniscule probabilities will not be easy.

But if it could be done, the Bootstrap Drive would permit a container of gases to move on its own without the ejection of any reaction mass. The ship would rise up, so to speak, by tugging on its own bootstraps. In normal operation, the Bootstrap Drive would give up thermal energy and become very cold. To maintain the propulsive force, it should be necessary to supply additional energy to the system in the form of heat.

In the 1940s Campbell also came up with a number of starship propulsion system designs operating on Heisenberg’s Uncertainty Principle in quantum mechanical theory. Arthur C. Clarke describes the functioning of the Heisenberg Drive:

In the Uncertainty Theory, a particle cannot be said to have a fixed position in space but has a very small, though finite, probability of being anywhere in the universe. All you had to do, therefore, to get an instantaneous mode of transport was to manipulate the Heisenberg equations until you were more likely to be somewhere else than where you started, and — presto!¹¹¹⁰

In essence, starships using the Heisenberg Drive "move" by going from maximum probability of existence at one position in the universe, through universality, to maximum probability of existence at another position elsewhere.²⁶⁴²

Figure 17.6 Using Black Holes for Interstellar Travel Wormhole Rotating Black Holes Folded diagram The Wormhole In the Einstein-Rosen model of the nonrotating black hole, a wormhole connects portions of two separate flat-space-time universes. This wormhole, called the Einstein-Rosen Bridge, may be interpreted either as connecting two distinct universes or as joining two distant points in the universe of origin. According to this theory, starships passing through the event horizon of a black hole would find the curvature of space-time growing less severe, eventually emerging through a "white hole" in distant space.2954 The diagram on top shows the Einstein-Rosen Bridge as it might connect two distant points in the same universe. The event horizon lies at the center of the wormhole. The diagram on the bottom is an "unfolded" version of the Bridge, for those who would rather view universal space-time as essentially flat. The two are mathematically equivalent Kruskal Diagram for Rotating Black Holes The figure at right represents a possible space-time configuration of a rotating black hole, using "Kruskal coordinates." There are now two event horizons instead of one, so there are three classes of space./p> It is possible to reach other universes like ours by selecting the correct trajectory upon entering the black hole. (Trip C) A collision with the ring singularity is possible. (Trip B) Cowardly astronauts can avoid the black hole altogether. (Trip A) Some "Type I" universes cannot be reached at all until tachyonic star travel is permitted .(Trip D) Folded Kruskal diagram The Kruskal diagram of the Kerr solution to the equations describing rotating black holes may be folded cylindrically as shown in the diagram at right. Using this interpretation of the wormhole caused by a rotating BH, a starship is theoretically capable of circumnavigating the entire Kruskal space-time and returning to the universe of origin anywhere in space and anywhen in time. This would appear to violate causality and permit time travel.

Severe gravitational distortion of space-time offers several scientifically plausible mechanisms for extremely fast interstellar communication and travel.²⁷⁹⁸ Dr. John A. Wheeler of Princeton University has predicted the existence of "wormholes" — a warpage of free space — based on his own version of General Relativity which has come to be known as geometrodynanics.²⁷⁴¹

Wheeler wormholes should be exceedingly small (subatomic dimensions, say, about 10^-35 meters). They would allow point-to-point linkages of all locales in the universe; pointlike particles, such as electrons, could be used to communicate without traversing the intervening space. Wheeler draws an analogy to the sea:

Space is like an ocean which looks flat to the aviator who flies above it, but which is a tossing turmoil to the hapless butterfly which falls upon it. Regarded more and more closely, it shows more and more agitation, until … the entire structure is permeated everywhere with worm-holes Geometrodynamic law forces on all space this foam-like character.⁷⁷

J.C. Graves and D.R. Brill at Princeton have shown that electric field lines threaded through the throat of a wormhole may prevent it from closing.²⁷⁷⁷ Such a tunnel in space should stay open indefinitely, allowing particles of matter to pass through to known destinations. Wormholes may connect a vast number of alternative universes at the subatomic level.²⁷⁷⁸

Black holes, predicted in 1939 by Oppenheimer using General Relativity theory, are concentrations of mass so dense that even light cannot escape the tremendous gravitational pull. Stellar-mass BHs are typically several kilometers in diameter. The standard black hole model predicts a "singularity" at dead center, a point at which density becomes infinite, This collapse of physical laws as we know them, fortunately, is hidden from view behind an event horizon — the surface below which photons cannot escape.

However, in the case of a rotating black hole things are quite different. According to relativistic solutions first obtained by R.P. Kerr in 1963, the singularity is no longer a point but has expanded into a ring.²⁷⁴² Many theorists believe that the region lying in the disk of the ring singularity may be a gateway to other universes or to our own universe at a different point in space and time. Rotating BHs have two event horizons instead of one, passage through which by a starship may involve such peculiar phenomena as negative mass effects and time running in reverse.²⁷⁴⁶ The Kruskal Diagram illustrated in Figure 17.6 shows one interpretation of possible trajectories through a rotating black hole assuming a Kerr spacetime metric.²⁷⁴⁷

A few writers have offered fabulous accounts of galactic commerce among sentient extraterrestrial races, using black holes as the entry gates to a kind of hyperspatial subway system.* Says astronomer Carl Sagan:

I can imagine, although it is the sheerest speculation, a federation of societies in the galaxy that have established a black hole rapid-transit system. A vehicle is rapidly routed through an interlaced network of black holes to the black hole nearest its destination. … Great civilizations might grow up near the black holes, with the planets farthest from them being designated as farm worlds, ecological preserves, vacations and resorts, specialty manufacturers, outposts for poets and musicians, and retreats for those who do not cherish big city life. In such a galaxy the individuality of the constituent cultures is preserved but a common galactic heritage established and maintained. Long travel times make trivial contact difficult, but the black hole network makes important contact possible.¹⁵

There are a number of practical difficulties associated with the use of BHs as an interstellar rapid-transit system. First, there is the problem of navigation. Until you jump into a black hole, you don’t really know where you will end up. Once you emerge at your destination (from a "white hole"), it is difficult to know how to get back. Many theories predict that it may be impossible to return. If you have entered another universe, the journey is probably irreversible and strictly one-way; if you have remained in your own universe, it would be an extraordinary stroke of luck to find another rotating BH in the immediate vicinity whose exit terminus happens to lie near the original starting point.

Tidal forces are yet another problem. Astronauts venturing even within a few hundred kilometers of a stellar-mass black hole would be savagely ripped apart by the simultaneous squeezing and stretching forces which would amount to hundreds of gees. While it may be possible to compensate for these effects by using special arrangements of ultradense matter within the starship structure, it may be better instead to search for supermassive black holes. Several theoretical physicists have proposed that gigantic BHs may exist at the center of many galaxies — possibly even our own — with masses ranging from 10⁶-10¹⁰ solar masses. Surprisingly, such monstrous objects need be no more dense than air, and tidal forces would be measured in milligees rather than megagees at the event horizon.

Catastrophe theory, first devised by French mathematician Rene Thom more than a decade ago,²⁸⁰⁰ offers a totally new conceptual variation on the theme of space warp drives. Catastrophe theory is a controversial new mathematical tool for examining highly discontinuous events — such as bridges buckling, sudden economic depressions, rapid emotional changes from fear to rage, and a host of other abrupt alterations in physical structures, lifeforms, and societies. The theory explains how slight changes in the initial state of a system can result in major divergence in the course of its evolution and subsequent behavior.

Since the Big Bang was a highly discontinuous event, creating a universe out of nothingness, it is quite conceivable that a catastrophe cosmology may eventually be devised. Perhaps it would involve catastrophe surfaces rather than simple spherical space-time geometry. Indeed, the coordinates need not all be spatial or temporal, but could include axes representing energy, momentum, spin, mass, charge, angular velocity, or whatever else is appropriate. If this is so, then the proper combination of fields and physical parameters could cause a packet of photons (a message) or a chunk of matter (a starship) suddenly to assume new parameters in a single, "catastrophic" leap. These new parameters might correspond to changes in energy, spatial position, or time.

* We will not here discuss the fascinating possibility of tachyonic black holes, although these have already been investigated theoretically.¹⁵²⁰

The first type of matter transfer system we’ll consider may be called the Teleportation Booth. To travel, the subject is seated in the transmission chamber. Complete data on the composition, position, and energy states of each atom in his body are read out by means of a sophisticated scanning device, and recorded in computer memory. The original may or may not be destroyed. The data is transmitted at the speed of light (radio waves) or faster (using tachyons) to a distant receiver, which picks up the data and places it in the memory banks of a second computer. A new human body, an exact duplicate right down to the last atom, is then reconstructed using the information taken from the original. The replica emerges from the booth, indistinguishable from the original in every way.*

If you think about this scheme, what has been accomplished here is not true matter transmission but rather transmission of information about matter. There is nothing fundamentally impossible about this process. In fact, in one dimension — sound — the problem may be regarded as solved by human technology. With the very finest audio equipment, duplicated sounds can no longer be distinguished from the original variations in air pressure that caused them. Further, these "replicas" can be transmitted over vast distances by radio waves.

Television represents the solution in two dimensions. In a typical system, a sophisticated TV vidicon scanner reads the information from the surface of, say, a human face, transmits the data to distant receivers, and the image is reproduced on the picture tube. Admittedly the visual reconstructions are far from perfect, but the electronics engineers are busily working to correct that defect.

Figure 17.7 Teleportation Booths: Precursor Technology on Earth Biostereometrics CAT Scanner X-ray Holography Biostereometrics Biostereometrics is a new scientific discipline which allows the three-dimensional measurement of living things using techniques similar to aerial mapping. The figure at right is the result of a low-resolution computer-generated optical replica of a male human body. The data stored in the computer may be used to rotate the replica to any angle or position, or to "dissect" the figure and examine any single part in greater detail.2835 CAT Scanner As for the present state-of-the-art in examining the insides of human bodies, the "CAT Scanner" is about the best there is. Essentially an automated x-ray machine, the Scanner looks at the body in very thin slices and can detect variations of only a few percent in transmitted intensity. The photos BELOW are a Scanner record of a human female across the chest area, showing the spine, aorta, and even a small pancreatic cyst just to the right of the spine and aorta.2835 X-ray Holography At right are the first direct images of atoms of magnesium, oxygen and carbon in a section of crystal, using a technique called x-ray holography in conjunction with a digital computer system.2810

The Teleportation Booth is the answer to information transfer concerning three dimensions. It operates in such a manner as to transmit a 3-D image through space. As with radio and television, the original goes nowhere.

It is entirely possible to imagine the construction of the Booth by making a direct extrapolation of currently-foreseeable human technology (Figure 17.7). Our present techniques of x-ray diffraction scanning, neutron-beam crystallography and field-ion microscopy easily permit resolutions at the atomic level (say, 1 Angstrom),^419,2801 and subatomic scanning is already available using large cyclotrons and linear particle accelerators. Indeed, high energy neutrino beams have been used to examine details of subnuclear structure as small as 10^-18 meter, or about 0.00000001 Angstroms.²⁸²⁵

How about computer memory? Ten terabit (10¹³ bits) memories are already available for use in Booth construction.⁵⁸³ Will this be enough? The human body consists of roughly 3 × 10²⁷ atoms, so at first blush we might expect that at least 10²⁸ bits of information should be needed to completely specify the human transmittee. Fortunately, the vast majority of these data are redundant. Our genes, a considerably more compact specification or "blueprints" for our bodies, represent only about 10¹⁰ bits. Our brains, however, contain at least 10¹³ bits of information — so this turns out to be the limiting factor.

According to Tim Quilici of Collins Telecommunications Systems Division of Rockwell International, a fairly new technique called electro-optical modulation may soon permit transmission rates through space of 10¹⁰ bits per second per channel.²⁷⁷⁹ The information detailing the construction of the human body thus could be completely transmitted, perhaps using a 1 mm infrared space-based laser beam, in just one second — although it would require another twenty minutes for the subject’s entire brain-map to arrive, if only one data channel is utilized.

Once the information has been received, the subject could be physically reconstituted using an extended and more exact version of the present-day techniques of molecular beam epitaxy, electron beam microfabrication, or some similar process.²⁸⁰⁴ The living subject would probably have to be assembled cold, close to absolute zero dissociative and degenerative chemical reactions, and would later be warmed and reanimated. It would seem that the Teleportation Booth is a great way to scatter copies of one’s self throughout the universe or across a planet, but it is a lousy way to travel — because you don’t go anywhere.**

Booth technology would also make possible a device which Arthur C. Clarke has called the Replicator.⁵⁵ The Replicator has access to a vast library of information which specifies all known physical objects and consumer goods, and is able to reproduce any number of exact copies of them at will. If practical matter transmutation is also available (see Chapter 19), bags of sand could be dumped in at one end and Univacs and Mona Lisas would emerge from the other.

The second major class of matter transmission techniques is called the Transporter Beam. This system, familiar to viewers of the television series Star Trek, involves what computer specialists refer to as "destructive readout." The transmittee is somehow converted into patterned electromagnetic radiation which is fired across space, reassembling itself back to normal matter at a predetermined "focus." The original is destroyed during the conversion into photonic radiation. Only a transmitter is required and the subject is self-assembling at the destination without the assistance of a receiver mechanism.

Such a procedure, while seemingly improbable, is not wholly inconceivable in terms of modern science. We know that electrons can be converted into patterns of gamma rays by the addition of positrons. Furthermore, optics theory tells us that unsynchronized light waves give rise to regions of destructive and constructive interference. If the phase and frequency of electromagnetic radiation could be forced to enter into constructive interference in a compact volume of space, pair production might be initiated along with other related processes giving rise to structured matter. Or, if the theory held by a few physicists that mass consists of "standing light waves" has any plausibility, then it might be possible to induce the spontaneous conversion of energy into matter at remote points. The real trick would be to retain the complex structure of the living organism throughout the process of beamdown, and to handle the nearly 10¹⁹ joules of interconversion energy without mishap.

A third major class of teleportation technology, known as Matter Transposition, involves the passage of physical objects from point A to point B without traversing the intervening space (and without being destroyed or merely duplicated). In this case it is the original who completes the journey, unharmed. Similar in concept to the idea of space warps discussed earlier, transposition depends on the proposition that space is not only curved, as predicted by General Relativity, but is also wrinkled and discontinuous.

To make a trip, some mechanical or electronic device is used to render two points in space — say, where the passenger is and where he wants to go — contiguous. The subject is then fixed in the new position, and space allowed to snap back to its original configuration. As Donald Wollheim describes the process:

Two segments of space may be separated by thousands of light-years traveling along the visible three-dimensional continuum of space, yet may be touching each other like two pages of a book. The Gate then is merely an extradimensional means of cutting across this touching point and thereby avoiding the problem of having to travel those thousands of light-years inch by inch.⁹⁸⁴

If it turns out to be theoretically possible to selectively bend space-time locally by artificial means, the technological problems will be immense. For one thing, the energy required to adequately bend space would probably be prohibitive over distances of more than a few kilometers. A stellar-sized black hole, the best space-warper known to human science, has a mass-energy on the order of 10⁴⁸ joules. This represents the entire power output of a mature Type III civilization for ten seconds, and yet the BH causes severe distortion of space-time over distances of less than 100 kilometers. Also, as Larry Niven has pointed out, the simultaneous operation of two or more Matter Transpositors in close proximity could prove embarrassing.²⁷⁴⁴ At best, space would be bent in some unanticipated way, causing transmittees to arrive at some arbitrary and unscheduled destination. At worst, passengers could wind up gravitationally collapsed.

Two other teleportation schemes also involve the idea of point-to-point transmission without crossing the intervening space. The first of these, and easiest to understand, is the Tunnel Transporter. This hypothetical device operates on the same principle of quantum mechanical tunneling discussed earlier in connection with tachyon starships. Explains Larry Niven:

Apparently physics students are now taught that a tunnel diode takes an electron here and puts it there without allowing it to occupy the intervening space. If you can do it with quantum physics, why not with larger masses? With people? The theory looks good, and it hasn’t been used much in science fiction.²⁷⁴⁴

The second teleportation scheme, called the Fourier Transporter, requires a bit more explanation.²⁷⁸⁰ In the early 19th century, a brilliant French mathematician and physicist by the name of Baron Jean Baptiste Fourier determined that almost any function of a real variable could be mathematically represented as a sum of sine waves, each of whose wavelengths are integral multiples of the variable. Any three-dimensional function can be transformed mathematically into "Fourier space," a coordinate system which uses inverse wavelength for the axes rather than spatial position.

An object sitting in space near Sol has a representation in Fourier space (say, S) that is distinctly different from the representation of that same object in orbit around the star Arcturus (say, X). Terrestrial electronics engineers already know how to build digital "filters" that will accept any input S and output any response X in one dimension. The Fourier Transporter works as follows.

A device near Sol transforms a passenger, by means unspecified, into Fourier space as S. A universal filter, driven by a computer which calculates what X must be in accordance with the traveler’s itinerary, almost instantly converts S into X. The device then performs a reverse Fourier transform on X-and the passenger is gone! He emerges from Fourier space to find himself in orbit around Arcturus.

The validity of Fourier transforms out to mathematical infinity is crucial to the successful operation of the Transporter. If, as some have suggested, Relativity only limits the velocity of transfer of mass-energy and not information, then it might be possible to transmit information very quickly without using any form of mass-energy to do it.²⁰¹⁴ The Fourier Transporter would thus provide virtually instantaneous travel, at little or no cost in energy.

* Some of the natural philosophy and pragmatic aspects of matter transmission may be found in Cleaver,¹¹⁶⁷ Elliott,¹¹⁶² Gooden,¹¹⁶⁵ Lawden,^1170,1164 and Niven.²⁸⁰² [Ed. Note: And the Saga of the Cuckoo.]

** There is an interesting side issue respecting the accuracy of transmission. It is well-known among radiation scientists that the random alteration of one out of every 10⁸ atoms in the human body will produce death. That is, if more than one atom in every 100,000,000 transmitted by the Booth is erroneous, the passenger will get sick and probably die of symptoms resembling acute radiation poisoning.³⁸⁰ A Teleportation Booth with an error rate of 10⁹ bits/error would allow people to make only ten successive trips through the machine before death ensued.

Citing the tremendous difficulties involved in high speed interstellar journeying, many writers have turned in desperation to the concept of the "generation ship" or "interstellar ark."*

• These mammoth vessels would contain self-sufficient communities of the sentient spacefaring species and their offspring.²⁷⁵⁹
• Farfetched, sophisticated space drives would be unnecessary, since the revolving crew could tolerate trips lasting hundreds of years to the nearer stars.
• While the original explorer-colonists might never live to see the New World, their great-grandchildren and successive generations would survive to carry the great mission on to its conclusion.

Science fiction authors have described the social and genetic backsliding that could occur in such closed ecologies.²⁷⁹⁴

• The degree of discipline that each individual would have to accept would be more demanding than that of any present totalitarian regime here on Earth.
• Birthrates must be strictly controlled, psychological interactions skillfully managed, epidemiological and eugenics rules absolutely enforced.
• All aberrant ship-threatening behavior severely punished.
• From the human point of view the restrictions on personal freedom would be well-nigh intolerable.

Still, as Dyson points out, "we have no right to impose our tastes on others."²⁷⁹²

* See especially Bernal,²⁸²⁰ Clarke,²⁷⁸⁹ Gelula,²⁷⁹⁰ Haldeman,²⁸³⁹ Heinlein,²⁸⁵⁴ Macvey,⁷³² Panshin,²⁵⁷⁹ and Strong.⁵⁰

The generation ship would provide a pathway to the stars for any patient extraterrestrial species that either could not or would not build relativistic propulsion systems.

• Speeds from 1-5%c should be ample to commute between neighboring star groups in periods of only a few centuries.
• This can be done using relatively primitive space hardware.
• Our own Pioneer 10 probe, which departed the solar system a few years ago, is now heading out into the interstellar void at 0.004%c — a respectable velocity, considering that the craft was designed solely for interplanetary travel.

Of course, during the long journey the technology of the home planet will not stand still. In more than one science fiction story, the crew of the first interstellar ark arrives at their destination only to find that they had been passed many times en route by superior starcraft of more modern design, and that the target system had long since been colonized by others.²⁷⁹¹ James G. Strong suggests that "such action will only arouse bitterness among the pioneers, and it would be kinder to avoid their destination — certainly never to come within hailing distance of their ship."⁵⁰

Note that there is no absolute requirement that the full complement of consumables be carried on board the Ark at launch. According to C.A. Cross, "its materials balance could be sustained by the return to it of unmanned vehicles carrying raw materials on long stern chase trajectories."²⁷⁹³  Like the interstellar ramjet that picks up its fuel along the way, an "Ark runway" would enable a generation ship to pick up supplies positioned earlier across its flight path by unmanned cargo vessels dispatched from the home planet.

There are several other alternatives to the generation ship.
As Freeman Dyson once remarked:

"Interstellar travel is essentially not a problem in physics or engineering but a problem in biology."

• For instance, if medical science can learn how to prolong life indefinitely and create immortal astronauts, then the crew which began a lengthy voyage would live to see the end of it.
• The perspective of immortal beings would doubtless be quite farsighted, and it is not unreasonable to suppose that such creatures would have a leisurely, relaxed outlook on life.
• A century or two spent out of a 2000 year lifespan would seem no more painful than a 3-7 year voyage would to a human.
• Sea voyages of this length were common among sailors and traders of centuries past: Magellan’s global circumnavigation required 2 years, Sir Francis Drake’s 3 years, and Marco Polo’s excursion to China totaled 24 years.

Another possibility is the "sleeper ship," a well-worn science fiction theme during the last half-century.

• There are basically three classes of sleepers: Ectogenetic astronauts (discussed in the previous chapter), hibernauts, and cryonauts.
• Hibernauts are starship crew members whose metabolisms have been greatly slowed. Like bears, woodchucks, bats and many rodents, these interstellar travelers would "hibernate" for most of the trip.
• For starfaring hibernauts, objective centuries would melt into subjective hours, creating a delightful illusion of near-instantaneous travel.
• Experiments with small mammals here on Earth indicate that it may be possible to induce artificial hibernation in humans using a variety of specific metabolic inhibitors such as the antabolone found in aestivating lungfish.²⁷⁸⁵
• The addition of antabolone to other anti-metabolic ingredients should permit metabolism to be reduced to 1% of normal at temperatures near 0 °C.
• Hibernation for extended periods should be possible, although as in natural hibernation it will probably be necessary to periodically raise the temperature to normal for a few hours each month to allow certain cellular restorative functions to take place.⁶⁷

Cryonauts are interstellar travelers whose bodily functions have been entirely shut down. Placed in "suspended animation" at cryogenic temperatures, these starfaring passengers cross the Galaxy in compact, refrigerated vessels equipped with ultrareliable supercomputers designed to patiently wait out the lonely centuries and then automatically reanimate the frozen travelers when the target star system is reached. Robert Prehoda, a science writer and technology forecaster, speculates that:

Cryonauts may travel for many centuries between the stars in fully automatic self-repairing spacecraft controlled by immortal super-computers — descendants of "HAL" in 2001, A Space Odyssey. When they are revived, the journey will seem to have been only one night long. perhaps they will stay alert for a few years to explore new planets and transmit information about them back to Earth. Then they could be frozen again for another mission to a more distant star, repeating the freeze-revive-freeze cycle several times before circling back to Earth after an odyssey of many millenia.⁶⁷

Suspended animation will require only a modest-level biotechnology, in all probability.

• The main problem with freezing organic tissues is the extracellular formation of ice crystals which cause tissue damage, making reanimation impossible.
• Nevertheless almost every kind of mammalian tissue has already successfully been frozen and thawed under appropriate conditions.³⁶⁹⁷
• For instance, rabbit skin has been frozen to -196 °C in liquid nitrogen for 7½ months and is still viable when thawed.²⁷⁸⁶
• Successful freezing and thawing of rat pancreas cells down to -196 °C has been shown.²⁷⁸⁸
• Rat heart tissue can be reanimated after indefinite storage in liquid nitrogen.¹⁶⁸⁷
• Low temperature preservation of human blood cells, sperm and ova is now routine.

Whole organs have also been reanimated, though this is more difficult

• Marshal Shlafer and Armand M. Karow, Jr. successfully restored isolated rat hearts cooled to -30 °C and perfused with a commonly-used biological antifreeze called "dimethylsulfoxide," or DMSO.^1685,1692
• Dr. Isamu Suda of Kobe University in Japan froze a whole cat brain to -20 °C for more than 6 months using a DMSO cryoprotectant perfusate. After thawing, the brain was shown to have nearly normal electroencephalic wave tracings.^3694,3695

What about whole-body freezing?

• Many experiments during the past few decades have demonstrated that laboratory rats can be cooled to -10 °C and later restored to life in apparently normal condition.
• However, the period of "cold sleep" must not exceed a few hours and no more than 50% of the body fluids may be solidified.²⁷⁸⁷
• Lower lifeforms are much easier to freeze.
• Nematode worms, for example, survive suspended animation at liquid nitrogen temperatures indefinitely when subjected to a specific DMSO perfusion treatment.¹⁶⁸⁹

What about whole humans?

• The well-known mammalian diving reflex, together with the body’s lowered oxygen needs at temperatures close to freezing, have permitted people to survive drownings in icy river waters during literally hours of submersion.
• Dr. Ivan W. Brown, Jr. of Duke University Medical Center has successfully revived a human child whose body-temperature was reduced to 5 °C.⁶⁷

Following the successful cryopreservation experiments on dog kidneys performed by Dr. M.D. Persidsky at the Institute of Medical Science in San Francisco and Ronald Dietzman of the University of Minnesota, Robert Prehoda has tentatively suggested a hypothetical medical procedure for placing human beings into a state of reversible suspended animation:

The body temperature is lowered to 0° C and a heavy water (D₂O) perfusate now circulates through the body. Soon most of the H₂O molecules have been replaced with D₂O. A 5%-by-volume addition of fluorinated DMSO is now added to the perfusate. This chemical will partially protect the cells during freezing, but its main function is to act as a biological carrier, insuring that the powerful metabolic inhibitors will reach adequate levels within all the cells. Salt-free albumin and ATP are added to the perfusate, allowing the level of dissolved salts within the cells to be reduced. The pressure is greatly increased, and large quantities of dissolved xenon gas begin to be circulated through the body. The fluid-filled cavities around the brain, spinal cord and in the eyes are properly protected by separate perfusion systems.

The heavy-water-based perfusate is replaced with a liquid fluorocarbon which can hold large quantities of dissolved xenon. The pressure is slowly raised as the temperature is reduced. At 5000 psi, an optimum quantity of xenon can be perfused through the body, thoroughly penetrating every cell. No more xenon is needed. The pressure is slowly increased to 30,062 psi. The perfusate pump is shut off. The body temperature is -24 °C. The pressure is then lowered to 5000 psi and rapid solidification begins to take place through the body. Cooling continues as the pressure is again increased to 30,062 psi, allowing the heat of fusion energy to be dissipated. The up-and-down pressure cycle is repeated four more times during continuous cooling, permitting the body to be uniformly frozen. Xenon hydrate protects every cell against freezing damage. The body temperature continues to be lowered at a controlled rate until it is only 4.2 degrees above absolute zero — the temperature of liquid helium.

You are now in a state of complete suspended animation.⁶⁷

These steps may be followed in reverse order to achieve reanimation.*

Cryonauts may not be immortal.

• Every minute 10⁶ atoms of radioactive K-40 and 2 × 10⁵ atoms of C-14 decay in a human body, spraying bones and organs with beta particles and other low-level radiation.
• Accumulation of trace amounts of radionuclides is virtually inevitable for any carbon-based lifeform, and these minute natural emissions may produce genetic mutations and other irreversible cellular damage over millennia of cold storage.
• Prehoda estimates that the natural decay of radioactive isotopes in body tissues will cause a lethal accumulation of radiation damage in human cryonauts in roughly 35,000 years.
• This difficulty can perhaps be avoided by raising potential astronauts from birth in a carefully controlled radiation-free environment.
• Food, water, and air must be purified and made free of the harmful isotopes.
• This accomplished, cryonauts should last for millions of years in suspended animation (assuming adequate shielding from the cosmic ray background) and would be prime candidates for long-term low-velocity galactic exploratory and colonization missions.

* Already about fifty people have been frozen, using somewhat more primitive perfusal techniques, by several Cryonics Societies around the country. These methods may not prove successful, but we won’t know for sure until reanimation is attempted on one of the many frozen patients, perhaps a century from today.