In the past, practical interstellar travel has been held to be an impossibility. It will require fantastic amounts of energy and superluminal speeds, and it was commonly understood that meeting those requirements is not possible if we accept Einstein’s relativistic model of the universe. However, examination of the problem based on the most current understandings of theoretical physics offers optimistic potentials that have never been considered before. These potentials, though radical, are consistent with a relativistic viewpoint.
Although interstellar travel is a technical feat that will occur far in the
human future, it is important to remember that we are a young race. Humanity
has had functional, technical science for several hundred years. Humans have
enjoyed powered flight for one hundred years, computers for fifty years, solid-state
electronics for fifty years, space flight for 40 years, lasers for 30 years,
semi-conductors for 20 years, nanotechnology for a decade, etc.
While we understand a great deal about the nature of reality, there is a greater amount of knowledge that has yet to be acquired. It is the epitome of arrogance to assume that something as complex as interstellar travel is impossible simply because we can’t yet imagine how such a feat could be achieved. Specific knowledge has arisen which will overturn that sense of implausibility and humanity will find a means to do what was once suspected to be impossible. Science often progresses because of an examination of the unexpected. So, scientific progress is not entirely predictable. The perspective that interstellar travel is impossible is a prediction that the universe is quite likely to disprove.
A true scientist is aware that there are leading edges in every field of endeavor and will calibrate his position against what he considers to be the most current information. This section offers of white papers that specifically address the problem of interstellar travel through a variety of viewpoints. These documents are compiled to provide scientists with perspectives and specific information to make a better assessment of the possibility of both human interstellar travel and extraterrestrial visitation.
SETI, the Velocity-of-Light Limitation and the
Alcubierre Warp
Drive: An Integrating Overview
Dr. H.E.Puthoff, 1996
In SETI (Search for Extraterrestrial Intelligence) conventional wisdom has
it that the probability of direct contact by interstellar travel is vanishingly
small due to the enormous distances involved, coupled with the velocity-of-light
limitation. This paper shows the naiveté of this
assumption. It provides details which provide yet further support for the
concept that reduced-time interstellar travel, either by advanced extraterrestrial
civilization at present or ourselves in the future, is not fundamentally constrained
by physical principles.
Dr. Harold E. Puthoff is Director of the Institute for Advanced Studies at Austin. He has published over 30 technical papers in the areas of electron-beam devices, lasers and quantum zero-point-energy effects and has patents issued and pending in the laser, communications, and energy fields.
SETI, the Velocity-of-Light Limitation, and the Alcubierre Warp Drive:
An Integrating Overview
H.E. Puthoff, Ph.D.
In SETI (Search for Extraterrestrial Intelligence) conventional wisdom has it that the probability of direct
contact by interstellar travel is vanishingly small due to the enormous distances involved, coupled with the
velocity-of- light limitation. Alcubierre's recent "warp drive" analysis [Class. Quantum Grav. 11, L 73
(1994)] wit hin the context of general relativistic dynamics, however, indicates the naivete of this
assumption. We show here that Alcubierre's result is a particular case of a broad, general approach that
might loosely be called "metric engineering," the details of which provide yet further support for the
concept that reduced-time interstellar travel, either by advanced extraterrestrial civilizations at present or
ourselves in the future, is not, as naive consideration might hold, fundamentally constrained by physical
principles.
Key words: SETI, velocity of light, general relativistic dynamics, space-time metric, interstellar travel,
vacuum energy, Casimir effect, vacuum engineering, warp drive, superluminal travel.
SETI researchers routinely subscribe to the view that interstellar travel between civilizations is
exceedingly improbable due to the velocity-of-light limitation, with but few dissenting views offered.(1,2).
Hence there has evolved, on the one hand, the emphasis on searches of the electromagnetic spectrum for
information-bearing signals and, on the other, the reasoned dismissal by the scientific community of any
evidence purported to be a signature of extraterrestrial visitation. (3)
As shown recently by Alcubierre, however, rejection of the concept of hyperfast (superluminal) travel is
not justified when one takes into account the possibility of engineered dynamic space-times within the
context of general relativity. (4) Specifically, Alcubierre showed by example that by distorting the local
space-time metric in the region of a spaceship in a certain prescribed way, it would be possible to achieve
motion faster than the speed of light as seen by observers outside the disturbed region, without violating
the local velocity-of- light constraint within the region.
Furthermore, the Alcubierre solution shows that the proper acceleration along the spaceship's path would
be zero and the spaceship would suffer no time dilation, features presumably attractive in interstellar
travel. We present here a supporting viewpoint that further explicates the Alcubierre approach as a special
case of an overarching concept of metric engineering that can be stated in an especially compact form,
fully incorporating general relativistic dynamics.
To elaborate the metric engineering perspective, we begin with the apparent velocity-of- light limitation.
As a physical concept this limitation is based on the fact that mass and energy find mathematical
expression in a form proportional to 1/[l-(v/c)2]1/2, which implies that an infinite amount of energy would
be required just to accelerate a mass to the velocity of light v = c. A hidden assumption in the argument that this constitutes a practical limitation with regard to interstellar travel, however, is the idea that the
value c is a fixed, immutable constant of nature, understood in a straightforward, natural way. It is this
crucial assumption that is called into question and redefined, however, by the metric engineering
approach.
In engineering terms the velocity of light in free space c is given by the expression c = 1/(μ0 0)1/2, where
in mks units μ0 = 4 x 10-7 H/m and 0 = 8.854 x 10-12 F/m, are, respectively, the magnetic permeability
and dielectric permittivity of the vacuum. Therefore, the argument that c is fixed is, at base, an argument
that μ0 and 0 are fixed and not subject to manipulation by technological means. If, on the other hand,
these vacuum constants were subject to change such that within a localized region the value c could be
made to assume a new value, say c' = 10c, then, without violating the governing equations of physics,
travel at speeds greater than the conventional velocity of light would be possible; it is just that a new
restriction would apply involving the (elevated) local velocity of light, and travel inside the local light
cone would still obtain, a point demonstrated in detail in the Alcubierre example.
Although perhaps surprising to the nonspecialist, within the context of general relativity and vacuumenergy
physics, such variability of the free space velocity of light c (as seen from a distant frame) under
certain conditions has long been part of the literature. For the case of propagation near a massive body, for
example, we have a reduction in the velocity of light by an amount proportional to the gravitational
potential, a result first noted by Einstein himself. (5) For the case of propagation between closely spaced
conducting boundaries as in discussions of the Casimir effect, we have an increase in the velocity of light
which is associated with the reduction of vacuum fluctuation energy between the plates. (6) In short, as
emphasized by Wesson, the speed of light c is context-dependent and not as fundamental as widely
believed.(7)
Such variations in c, considered in terms of its subcomponents μ and , are routinely treated in a compact
form that recommends itself for simplicity of concept, the so-called "TH μ" formalism used in
comparative studies of gravitational theories.1 This approach has its foundation in the recognition that the
covariant Maxwell equations in a Riemannian space with arbitrary metric are identical in form with the
usual vector Maxwell equations for a material medium with variable and μ, where these parameters are
themselves now a function of the metric.(8) This concept can be extended to nonmetric theories as well,
and in the TH μ context goes under the name "gravitationally modified Maxwell equations. "1 The
formalism is then completed by casting the Lagrangian for particle motion under the influence of
electromagnetic and gravitational fields into a canonical form involving two additional metric-dependent
functions, T and H. Such a formalism leads naturally to the concept of metric engineering in which one's
familiarity with variable -μ media can act as an intuitive guide. 3,(8) Although under ordinary conditions
effects involving variations in vacuum values of μ, , and hence c typically are vanishingly small, they
nonetheless indicate the possibility under extraordinary conditions of "vacuum engineering," as Nobel
Laureate T. D. Lee put it.(9) The Alcubierre warp drive example, which can be reframed within the TH μ
context, is an especially pithy example of such, and additional space-times with desired properties can be
derived at will within this context. 4
Therefore, the proper conclusion to be drawn by consideration of engineered metric/vacuum-energy
effects is that, with sufficient technological means to appear 'magic" at present (to use Arthur C. Clarke's
phrase characterizing a highly advanced, technological civilization), travel at speeds exceeding the
conventional velocity of light could occur without the violation of fundamental physical laws. And, we might add, this could in principle be done without recourse to concepts as extreme as wormhole traversal.
(10) (However, clearly, exotic matter/field states, e.g., macroscopic Casimir- like negative-energy-density
vacuum states, would be required.) As a result, the possibility of reduced-time interstellar travel, either by
advanced extraterrestrial civilizations at present or ourselves in the future, is not fundamentally
constrained by physical principles.
Acknowledgment
I wish to acknowledge the Fetzer Institute in Kalamazoo, Michigan, for their support of this work.
The Physics of Interstellar Propulsion was presented at the Science Symposium at the George Washington University on Nov 8, 2002 and is located in the CFi site. Dr. Kaku is a Professor of theoretical physics at the City College at the City University of New York.
Physics of Interstellar Propulsion
by Dr. Michio Kaku
Henry Semat Prof. of Theoretical Physics, City Univ. of New York
The speed of light is a daunting but not necessarily insurmountable challenge to any
star-faring civilization.
The physics of interstellar propulsion depends crucially on the available energy
source.
Physicists, when cataloging potential civilizations in outer space, often rank
civilizations according to their energy output on the Kardashav scale:
· Type I (which harnesses the energy output of an entire planet)
· Type II (harnessing the energy output of a star)
· Type III (harnessing the energy output of a galaxy).
Each civilization type is separated from the previous one by a factor of about 10
billion.
By contrast, our present civilization is Type 0, which harnesses the energy output of
dead plants. Growing at a modest 3% per year, we are about 100-200 years from
becoming Type I, several thousand years from attaining Type II status, and perhaps
several hundred thousand years to a million years from reaching Type III).
On this energy scale, various civilizations might exploit the following modes
of exploration.
Type 0
Chemical rockets
Ion rockets
Atomic rockets
Type I
Fusion/Ramjet propulsion
Laser propulsion
Type II
Anti-matter propulsion
Nano/von Neumann probes.
Type III
Planck energy propulsion.
According to Special Relativity, no usable information can locally exceed the
speed of light, hence Types I and II civilizations can only explore near-by stars.
However, according to General Relativity, objects can exceed the speed of light globally if we use global distortions of space and time (e.g. the Big Bang expanded
much faster than light) and multiply connected spaces (e.g. wormholes). Although
there are hundreds of solutions of Einstein's equations which allow for such
distortions, there are two limiting factors:
a) quantum corrections may destabilize these anomalies
b) the energy source necessary to drive these systems is the Planck energy
(i.e. 10^19 billion electron volts, the energy at which space and time becomes
unstable, and virtual universes might become real).
In the first case, superstring theory (M-theory) is on the verge of giving us
a calculable quantum theory of gravity and may settle this question once and for all.
But in the second case, only a Type III civilization may be able to harness this
mode of propulsion (using large quantities of either positive or negative energy in
the form of zero-point energy).
Thus, if we ever encounter a civilization capable of leaping into 11 dimensional
hyperspace or punching a hole in space-time, we can safely conclude that they
are a Type III civilization, hundreds of thousands to a million years ahead
of us in technology.
UFO Propulsion Systems
By: Stanton T. Friedman
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(1) How can a spaceship travel from a nearby solar system to earth in a reasonable time?
(2) Once here, how can flying saucers behave the way they are observed to behave? How do they achieve their reported high speed flight in the atmosphere (thousands of miles per hour), their ability to stop and start abruptly, to move up and down and back and forth seemingly with none of the limitations of conventional aircraft?
Typically there are no visible external engines, wings, or tails. Usually the objects are relatively silent compared to conventional craft. Often unusual colored glows are seen adjacent to the craft, and a variety of physical and physiological effects are produced on living and inanimate objects in the vicinity. These are the truly technological challenges we face.
The problem must be divided into two parts because there is no good reason to assume that the same propulsion system is used for both the long haul and local portions of the trip. It seems reasonable to assume that the huge cigar-shaped “mother ships,” into and out of which the smaller disc-shaped craft fly, are the interstellar vehicles and the others are Earth Excursion Modules for local travel only. Mother ships are rarely observed cavorting or flying close to ground level. In Ted Phillips’s huge collection of trace cases more than 90 percent of the low-level vehicles are disc-shaped. A useful analogy here is the aircraft carrier Enterprise, which is nuclear-powered and operates at low speed for many months or years on the surface of the ocean. The much smaller aircraft it carries cannot operate on the ocean but can fly at high speed and altitude for short periods and are highly maneuverable. But they are not nuclear-powered. Neither craft could replace the other.
The problem of traveling to the stars must also be viewed from an entirely different perspective than is useful for understanding our recent flights to the moon and flights of instrument packages to other planets. Distances within the solar system can be measured in light-seconds, light-minutes, or at most in a few light-hours. Stars are at least several light-years away. Our chemical rockets carry astronauts to the moon in about sixty-nine hours, and the Viking spacecraft to Mars took about ten months to reach its destination. But they are propelled by forces other than gravity for only seventeen minutes or one hour respectively. The rockets are coasting and slowing down until they are close to the target for almost the entire trip. The Apollo spacecraft, at an altitude of two hundred thousand miles, is going only two thousand mph although it left the vicinity of earth at twenty-five thousand mph. If it had been able to accelerate at just one G (a twenty-one-mph increase every second) for just one hour, the final velocity would have been 79,000 mph; for just one day it would have been 1.9 million mph! Peak acceleration during an Apollo launch is actually close to eight Gs (a 168-mph increase every second). To understand the foregoing a bit better, note that an acceleration of one G at the surface of the earth equals 32.17 feet per second, which in turn means that as each second passes velocity is increasing by an additional 32.17 feet. Translated into miles per hour one-G acceleration means that velocity is increasing at the rate of 21.9 mph every second! At the end of two seconds it is 21.9 mph plus 21.9 mph, or 43.8 mph, and at the end of three seconds it is 64.7 mph, and so on.
In just one day at one-G acceleration a velocity of almost two million mph would be reached and the craft would be far out of the earth’s gravitational field. For each minute of operation near the earth, gravity effectively pulls the craft at 1260 mph. While in space there is practically no gravitational or atmospheric friction. It is extremely important to recognize that it takes only approximately one year at one G to approach the speed of light--about 670,000,000 mph---and we can speculate that any space travelers may have refueling or rest and relaxation centers at locations between the stars, so that our earth visitors need not have come directly from their home planet.
Unfortunately, chemical rockets such as we have been using are by their very nature extremely limited in their ability to provide high velocities in their limited operating times because of their great inefficiency.
Starship and Earth Excursion Module designers thus face two obvious questions: (1)How much acceleration can people stand for how long? (2)What method can provide more miles per hour than chemical rockets, either by operating for longer times or at higher accelerations?
The amount of acceleration a person can stand depends on many factors. The three most important are the duration of the acceleration (the greater the force, the shorter the time it can be tolerated), the direction of the force in relation to the body (back to front acceleration is much easier to handle than head to foot acceleration, and for this reason Apollo astronauts have their backs perpendicular to the direction of thrust, rather than along it, as in an elevator), and body environment is important (a person immersed in a fluid can withstand greater acceleration than one not so immersed).
Let’s consider some of the variables. A trained and highly motivated pilot can perform a tracking task while being accelerated at fourteen Gs (about three hundred mph increase every second) for two minutes. Starting from rest he would be moving at three hundred mph in one second, at three thousand mph in ten seconds and at thirty-six thousand mph at the end of two minutes! Obviously conventional propulsion systems such as airplanes, trains, buses, and cars cannot provide fourteen Gs. A drag racer achieving 210 mph in ten seconds would have an average acceleration of only one G. A trained person properly constrained can stand thirty Gs for one second without damage. data suggest that much higher accelerations could be withstood for shorter times. Reports of EEM (Earth Excursion Module) flight often indicate that the high acceleration--as when making a nearly right-angle turn or changing altitude--takes place in an extremely short period of time. In modern physics and technology the primary method for providing very high forces for relatively short periods of time is the use of electromagnetic forces such as with lasers, magnetoforming of complex shapes, and the acceleration of nuclear particles to velocities close to that of light.
In the mid-1960s and electromagnetic submarine designed by Dr. Stuart Way, who was on leave from Westinghouse Research Laboratory, was successfully tested. It made use of the fact that electric and magnetic fields at right angles to each other produce a (Lorentz) force at right angles to both. The force pushes against the surrounding electromagnetically conducting fluid (seawater) which pushes back and moves the submarine. It is possible to envision an airborne analog in which seawater is replaced by ionized electrically conducting air, and conventional electromagnetic fields are produced by superconducting magnets which need little space, very little power and weight, and generate very high magnetic fields. Substantial research, much of it classified, has been done showing that a magnetoaerodynamic system would be capable of solving all the problems of high-speed flight by controlling lift, drag, heating, and sonic-boom production--all electromagnetically rather than mechanically or chemically. The resulting system would be symmetric, highly maneuverable, relatively silent, often have a glow around it, and be capable of sudden starts and stops. It could carry its own power supply or be charged up on board its mother ship in much the same manner as a golf cart which carries only a storage battery.
The reason much of the research on MAD propulsion systems is classified is that the nose cones of ballistic missiles create an ionized air region around them as they reenter. Modifications of the nose cones can be used to vary the radar profile, lift, drag, and light direction and other important parameters without carrying along fuel or propellant which would normally be required. It should be stressed that such systems work by interacting with their surroundings and not by carrying along something thrown out the back end. A real benefit is derived from producing very high magnetic fields since afield ten times as great produces one hundred times as much force.
For the interstellar trip the obvious first choice, although undoubtedly not the ultimate choice, for replacing primitive chemical rockets is a nuclear rocket. Although most people are unaware of nuclear propulsion systems other than those the Navy developed for submarines and surface ships, there have been several other programs for the development of airborne or space-propulsion systems. Jet engines were successfully operated on nuclear power for the Aircraft Nuclear Propulsion program. A nuclear ramjet was successfully ground-tested as part of the Pluto program. An entire family of nuclear rockets were successfully ground-tested during the NERVA (Nuclear Engines for Rocket Vehicle Applications) program. Most of the work involved in these multimillion-dollar-a-year programs was classified and conducted by industrial contractors in conjunction with national laboratories under the direction of NASA, the Air Force, and/or the old Atomic Energy Commission. All of the above systems utilize nuclear fission of the uranium-235 nucleus to produce huge amounts of heat by the conversion of a small amount of mass into a large amount of energy. Millions of times more energy per pound can be produced in this way than by burning rocket fuel.
The design and development of nuclear flight-propulsion systems requires the solution of very real problems associated with complex nuclear physics, sophisticated hardware operating at very high temperatures, and the lethal radiation produced by the fission process. Similar problems, although not as difficult, were solved first for nuclear weapons and then in the production of a large, relatively low-temperature submarine and stationary nuclear-power plants. The primary difficulty in employing fission for space or atmospheric propulsion systems is associated with the weight and high performance limitations of such systems. Large ships weight more than a hundred thousand tons. Airplanes weigh fewer than four hundred tons, and even the Saturn 5 rocket weighed only three thousand tons. Despite the problems, the NRX A-6 nuclear-rocket-reactor propulsion system was successfully tested in December 1967 by Westinghouse Astronuclear Laboratory at a power level of 1.1 billion watts in a package less than ten feet long and under five feet in diameter. In June 1968 the Los Alamos Scientific Laboratory successfully tested the Phoebus-2B at a power level of 4.4 billion watts; it had a diameter under six feet. The old Grand Coulee Dam produced 2.2 billion watts by comparison. All the NERVA (and preceding KIWI and Rover) systems used solid fuel, through which was pumped liquid hydrogen which changed to a gas and was exhausted through a nozzle. Because hydrogen has the lowest weight of any molecule, for the same energy expended it will achieve the highest exhaust velocity. The weight of the oxygen and its associated tankage is also eliminated. More advanced systems have been designed in which the U-235 is in a very high-temperature gas-plasma form and thus provides far higher exhaust temperatures for the hydrogen. Reactors actually have operated with the fuel in a gaseous form.
Of considerably greater interest from a long-term viewpoint would be fusion propulsion. Fusion is the nuclear process involving the combining of light nuclei to make heavier nuclei and, as in fission, convert a small amount of mass into a huge amount of energy. It is the primary process by which energy is produced in most stars and in so-called hydrogen bombs. Every civilization--even on distant stars--would become aware of the fusion process as it reached a minimal level of scientific maturity. There are many different reactions and processes which can be used in both fission and fusion devices. One of the most attractive for a space-propulsion system would be to cause the reaction of just those particles which, when made to fuse, produce only charged rather than neutral particles. These very high-energy particles then could be directed out the back of the rocket, using appropriate electric and magnetic fields. Neutral particles come off in all directions and cannot be directed or controlled, only slowed down and their heat absorbed . . . a very inefficient process. Using the right reactions in the right way, a space fusion-propulsion system could be designed to exhaust light ions having more than ten million times as much energy per particle as they can receive in a chemical rocket. A second advantage of considerable interest is that the fuel or propellant for a fusion rocket would be isotopes of hydrogen and helium, which are not only the lightest elements but are also by far the most abundant in the universe. Thus one could be certain of finding the raw materials for a fusion fuel stockpile in any star system to which one traveled.
There have been a number of studies published showing that staged fission and fusion deep-space propulsion systems are capable of round trips to nearby stars in a shorter time than an average life span. Chemical rockets would be used to launch starships into orbit or to the moon for relaunching from there because of the greatly reduced energy requirements on the moon. Clever design would be employed such as was used by the lunar landing program. Full advantage would be taken of every “free loading” possibility just as the Apollo vehicle takes advantage of the earth’s high rotation to the east near the equator and of the gravitational field of the moon and of staged rockets which fire in programmed succession on the way and by counting on the earth’s atmosphere to slow it down rather than carrying and firing retrorockets to slow it down on the way back. The final weight and cost depend almost entirely on the design assumptions rather than (as academic calculations so often assume) being independent of those design features. An early study of the required launch weight of a chemical rocket capable of sending a man to the moon and back concluded that the launch weight would have to be a million million tons. The launching was accomplished less than thirty years later with a chemical rocket weighing three hundred million times less.
Stars and planets along the way also would be used both for their fuel and solar energy and for gravitational assistance, just as the Pioneer spacecraft, which was without propulsion systems after leaving the vicinity of the earth, used the gravitational field of Jupiter to hurl itself past Saturn and eventually out of the solar system.
Earthlings are capable of building both fission and fusion deep-space propulsion systems if they are willing to spend the tens of billions of dollars required. However, these are not the only possibilities for interstellar travel. Other possibilities include:
1.Lasers based on the earth, or in orbit, or on the moon, to be aimed at the back of the rocket, spilling off material which would exhaust toward the laser and push the rocket forward. This has the advantage of putting the power supply elsewhere than on board the rocket.
2.Systems producing energy by some as yet unknown process power the strange stellar beasts known as quasars. Watts per gallon of fuel are enormously greater in a quasar than in a typical fusion-powered star like the sun.
3.Systems utilizing whatever type of force holds subnuclear particles together are also a possibility. In the nucleus involved in fission and fusion the amount of energy per particle is much greater than in the larger atoms involved in chemical processes. Going inside the nucleus should also decrease the size of the particle but greatly increase the amount of energy available per particle.
4.Systems using some means of bending space and time so as to “pop” from one place to another without having to really travel along the path between the points would do the trick. Picture a flat sheet of paper and then bend it so that diagonally opposite corners touch each other. Obviously travel between these touching corners would be more rapid than travel across the paper had it remained flat.
5.We also must remember there undoubtedly are systems that we cannot yet imagine--just as fusion as the primary energy-producing process on the sun wasn’t understood until 1937 although it had been going on for five billion years. Any study of technological progress clearly shows us that progress comes from doing things in an unpredictable way. The future, technologically speaking, is not an extrapolation of the past.
An important aspect of the design of any interstellar propulsion system involves taking full advantage of Albert Einstein’s theory of relativity. Theory and experiment have both clearly demonstrated that as things having mass such as people, particles, and starships approach the speed of light (c), time slows down for them as compared to those not moving so rapidly. The extent of the time slowdown depends on how close one approaches c, the speed of light. For example a one-way trip of thirty-seven years (the distance to Zeta 1 or 2 Reticuli) at 99.9 percent c would take only twenty months’ crew time; at 99.99 percent c it would take only six months’ crew time. Thus even a trip to a distant galaxy such as Andromeda, two million light-years away, would take under sixty years’ crew time if the intergalactic ship somehow could manage to keep accelerating at one G, using some yet unknown technique.
An important point to bear in mind in any discussion of interstellar travel is that it would be done in a systematic fashion. Observations would be made, unmanned craft would be sent, followed by orbiters, the installation of refueling stations, manned craft, colonizers, travelers, and all the rest. It took only twelve years from the time the first small satellite was launched before we accomplished a manned landing on the moon.
Considering that there are stars in our local neighborhood that are billions of years older than the sun, it would not be surprising if interstellar travel has been commonplace for billions of years. Several published papers have concluded that our Milky Way galaxy already has been colonized. Furthermore, it must be noted that travel between star systems is more likely to occur the closer the next system is. Zeta 1 and Zeta 2 Reticuli are both sunlike stars that are less than three light-weeks apart. Observers on a planet around one of them could easily observe planets around the other. One would certainly expect interstellar travel to develop earlier there than in our isolated corner of the neighborhood, where the nearest star to us is one hundred times farther away than the Zeta Reticulans are from each other.
