* The ultimate challenge for space propulsion systems is interstellar flight. Science-fiction stories treat the concept casually, but in reality the practical obstacles are enormous. This chapter outlines the prospects for interstellar flight.
* The concept of interstellar flight is a 20th-century invention. The stars were not known to be distant suns until a few centuries ago, and up to the end of the 19th century, even the idea of flying to the Moon required a great leap of imagination.
By the first decades of the 20th century, the basic elements of practical spaceflight, such as rocket propulsion and life support, were understood. Planetary exploration, and by extension interstellar flight, could be discussed in technical terms. However, the obstacles to interstellar flight were understood as well: the distances involved; the absolute speed limit imposed by Einstein's theory of special relativity; and the overwhelming energy requirements.
* The distances to even the nearest stars are staggering. The Earth is roughly 150 million kilometers from the Sun. Light travels at a speed of 300,000 kilometers per second, and so takes light a little over eight minutes to travel from the Sun to the Earth. The Earth can be said to be eight "light-minutes" from the Sun.
The most distant planet in the solar system, Pluto, is about 30 times farther away from the Sun than the Earth, and so is a little over four light-hours from the Sun. In contrast, the nearest star, the Alpha Centauri system, is 4.3 light-years away. To visualize this difference in scales, consider a model of the Solar System where a meter represents a million kilometers. In this model, the Sun is a sphere about the size of a chair, and the Earth is a marble down the street, 150 meters away. Pluto is a seed about 4.5 kilometers away, across town. Alpha Centauri is over 40,000 kilometers away, about the same as a trip all the way around the Earth.
A trip to even the nearest star, then, is a tough proposition. Furthermore, according to Einstein's theory of relativity, there is no possibility of making the trip any faster than the speed of light, and so a voyage to the nearest star would take years under the best circumstances. Albert Einstein's theory of special relativity, which was published in 1905, states that as an object accelerates, its mass -- technically its momentum, not mass, but that's fine print ignored here for simplicity -- increases by the factor:
1 / SQRT( 1 - fraction_of_lightspeed^2 )
This is a small increment at low speeds. For example, at a tenth the speed of light, the object's mass only increases by half a percent. However, at large speeds, the mass increase is considerable. At 90% of the speed of light, the mass increase is 229%, and it keeps increasing, with the mass climbing toward infinity as the speed of light is approached. There is no finite amount of energy that will push the object to the speed of light.
In compensation, the theory of special relativity also says that as an object accelerates toward the speed of light, time on the object slows down by that same factor. This "time dilation" effect means that even if a voyage to a nearby star takes decades as seen from Earth, the time it takes a star traveler moving at near the speed of light is much less. Although it would take a starship moving at 90% of the speed of light 4.78 years to reach the Alpha Centauri system, for the crew the trip would only last 2.09 years.
Unfortunately, the energy requirements for pushing a spaceship to any substantial fraction of the speed of light are also staggering. Consider accelerating a small space probe weighing 100 kilograms to 10% of the speed of light. Since relativistic effects can be ignored at this velocity, the kinetic energy of the probe is given by the classical equation:
energy = (1/2) * vehicle_mass * velocity^2
Disregarding losses, this is the same as the amount of energy that has to be pumped into the probe to get it to that velocity. This is obviously a large value, and so for a standard of comparison, let's arbitrarily define an "energy unit (EU)" as the total power output of a 100-MW power plant for a year:
EU = 100,000,000 * 60 * 60 * 24 * 365 = 3.15E15 joules
In terms of this unit, the energy needed to accelerate the probe to a tenth of lightspeed would be:
((1/2) * 100 * (0.1 * 300,000,000)^2 ) / EU = 14.3 EUs
This is substantial, but not beyond imagination. However, consider accelerating a manned vehicle weighing a thousand tonnes to a tenth of the speed of light. Since this vehicle is 10,000 times heavier than the unmanned probe, it requires 10,000 times more energy, or 143,000 EUs. In comparison, the Earth's entire current power generation of about a terawatt is equivalent to 10,000 EUs, meaning the energy requirements for accelerating the probe are an order of magnitude greater than all the electrical power we are now capable of generating. Slowing the manned vehicle down again at its destination would require the same amount of energy, and the return voyage would double the total amount of energy required once more.
* Such back-of-envelope calculations don't prove that interstellar flight is impossible, only that it is challenging: the energies required are enormous, but not completely ruled out. The flight times are very long, meaning that only nearby stars can be seriously considered, but within 12 light-years there are 19 star systems with 25 visible stars and likely hundreds of planets. Of particular interest are two Sun-like stars, Epsilon Eridani at 10.8 light-years and Tau Ceti at 11.8 light years.
In 1929, British crystallographer J.D. Bernal published a short speculative document titled THE WORLD, THE FLESH, AND THE DEVIL. In this document, he discussed a number of far-sighted and, for the time, revolutionary ideas, such as space colonies and a related concept, the "space ark", a self-sustaining space colony that could be sent on an interstellar voyage lasting for centuries, with successive generations of people born on board.
The space ark was plausible, but still a big job. Assuming that the space ark weighs about 100,000 tonnes, about the same order of magnitude as an aircraft carrier, then getting it to, say, 1% of the speed of light and decelerating it again would require 285,000 EUs. At 1% of lightspeed, a voyage of 12 light-years would require 1,200 years. Building machinery that could last over a millennium would be difficult, and anybody who's ever had a fishtank knows it can be difficult to sustain an artificial environment for a long period of time. Assuming that the space ark would need 100 MW of power to sustain itself, that would mean the consumption of one EU a year for the duration of the journey, or 1,200 EUs in all -- a modest quantity in comparison to the propulsion requirement, but still large.
A smaller craft could be sent to the stars at greater velocity, if there were no need to build it as a self-sustaining ecology. One way of doing this that has long been used in science-fiction stories is "suspended animation", where human beings hibernate in "cold sleep" and survive the voyage of centuries. However, although some mammals can survive by sleeping through the winter with their metabolism reduced to a very low level, none are known to be able to survive in this state for a period of years, much less centuries. At present, there is no way to say if suspended animation is either possible, or that it is ruled out. Bioengineering might be able to produce humans capable of surviving for centuries in a state of hibernation. Certainly sentient machines could make the trip, and if desired, they could fabricate humans and other organisms from scratch at the destination.
BACK_TO_TOP* The biggest critical path for interstellar flight is the vehicle's propulsion system, and this is a difficult matter. Chemical rocket propulsion systems do not conceivably have the energy to do the job.
One of the most potentially powerful means of obtaining energy is to combine matter and antimatter. The two forms of matter annihilate each other, converting all their mass into energy. The EU defined in the previous section is equivalent to the conversion of 17.5 grams of matter and 17.5 grams of antimatter. Unrealistically assuming perfect efficiency in converting the energy released into propulsive force, the energy required to get a 100 kilogram probe to a tenth of the speed of light could be provided by the conversion of a sum of 250 grams of matter and 250 grams of antimatter. The 100,000 tonne space ark could be powered to 1% of lightspeed and sustained in flight by about 10 tonnes of matter and 10 tonnes of antimatter.
Unfortunately, matter-antimatter conversion is a troublesome prospect, for two reasons. First, there are no sources of natural antimatter of any quantity near at hand, and synthesizing it or gathering it from space particles would be extremely expensive and time-consuming. Second, although the mutual annihilation of matter and antimatter would generate a shower of charged particles that could be funneled by a magnetic field out the exhaust, the conversion efficiency wouldn't be close to 100%, and the radiation would demand heavy shielding for crew protection, plus a way of getting rid of excess heat.
An Orion-type spacecraft, propelled by ejecting small nuclear explosives from a heavy shield, is another plausible candidate, but the shield would be heavy, and even nuclear explosives don't have enough energy to move a starship across interstellar space at acceptable speeds.
A more sophisticated approach towards fission propulsion was proposed by a group under George Chapline of Lawrence Livermore National Laboratory, based on a "fission fragment" reactor. It involves a stack of disks coated with a radioactive material such as plutonium or americium, with the disks successively rotated into a reactor core containing more radioactive material that forces the fuel on the disk to go critical into a controlled fission reaction. The energetic products of the reaction are funneled out the exhaust by strong magnetic fields. The scheme still involves a lot of shielding and mass, and obtaining the huge amounts of fissionable material required would be difficult. A fission fragment reactor would be able to get a starship moving up to 6% of the speed of light, or 10% if two stages were used; of course, a comparable number of stages would be needed to slow down, and the sum of stages would have to be doubled to get back home again.
Rocket engines powered by controlled nuclear fusion processes have also been considered, with the fusion produced through magnetic compression or laser ignition. However, nobody has yet come up a continuous controlled fusion process that generates more energy than is needed to initiate it. Some researchers have speculated that even if matter-antimatter annihilation is not a useful approach to interstellar propulsion in itself, it could be used as a "trigger" to initiate controlled fusion reactions.
* All these proposed methods of propulsion still have the limitation that the mass of fuel for an interstellar voyage has to be carried on the starship. In the 1960s, a physicist at what is now the US Los Alamos National Laboratory named Robert Bussard came up with a variation on a hydrogen-fusion powered starship that he called the "interstellar ramjet" and is now often called the "Bussard ramjet".
Interstellar space is not empty. It contains tenuous traces of hydrogen, mixed with a small percentage of helium and a very slight sub-percentage of other elements. Bussard proposed that a starship could be built that generated a magnetic field about 3,200 kilometers across to funnel interstellar hydrogen into the intake of a fusion rocket, eliminating the need for the starship to carry its own fuel supply.
The idea is appealing, but beyond present technologies. As noted, nobody has any good idea of how to perform controlled fusion, particularly with the ordinary hydrogen that makes up most of the interstellar medium instead of deuterium or tritium, the "heavy hydrogen" isotopes, and nobody has ever generated magnetic fields of the magnitude needed by a Bussard ramjet.
The most detailed schemes for interstellar flight were proposed by the late Robert L. Forward, previously mentioned in the chapter on space tethers. Forward worked as an engineer for Hughes and then established his own space technology firm, modestly named Forward Unlimited. Forward proposed robotic and manned interstellar missions based on microwave or laser beamed power.
Forward described an ultralight flyby probe that he called "Starwisp" that could be sent to nearby stars using microwave beamed power, acting as a pathfinder for manned missions. Starwisp would fly across interstellar space at 30% of lightspeed, allowing it to reach any star within 12 light-years in 40 years or less.
In Forward's scenario, Starwisp is a wire-mesh "sail" with microcircuit modules at the junctions of the mesh. Starwisp has a diameter of hundreds of meters and a total weight of 16 grams, with a quarter of that mass allocated to payload. The Starwisp is propelled and powered by an intense microwave beam, formed by a segmented transmitter lens made of alternating sparse metal mesh rings and blank rings.
Total power provided by the microwave transmitter is 10 gigawatts, which accelerates Starwisp at 115 gees to a cruise velocity of 0.2 lightspeed in a few days. Starwisp then sails across space for decades. On arrival in the target star system, Starwisp is flooded with microwave energy from the distant transmitter to power the probe's microcircuits. The Starwisp's microcircuit modules act as synthetic-aperture sensors to obtain images and other data from the target star system, while the mesh forms itself into a transmitting antenna to send the imagery back home.
The microwave transmitter system's output power is comparable to that of proposed solar-power satellites. If such power satellites ever become common, the hardware for sending Starwisp to the nearby stars will be available.
* Forward envisioned a much more ambitious manned interstellar mission, also based on beamed power, that could visit a nearby star and then return to Earth, within the lifetimes of the crew.
Starwisp is propelled by a microwave beam. There is no obvious way to use beamed microwave propulsion to slow the probe down at its destination, and so Starwisp is necessarily a flyby probe. The manned mission uses a lightsail driven by powerful laser beams in orbit around our Sun for propulsion, and this scheme can support both the acceleration and deceleration phases of the mission.
The lasers are required because lightsails driven solely by the Sun's rays are only really useful in the inner solar system. In Forward's scenario, the lasers orbit Mercury, using the planet's gravity to keep them anchored against the reaction force of their output beams. The lasers obtain their energy from solar power systems, and their output beams are combined into a single coherent laser beam. Total beam power is 43,000 TW, or about 43,000 times greater than the Earth's current power generation capabilities.
This enormous laser beam is sent to a segmented transmitter lens in orbit around the Sun between Saturn and Uranus. The lens is 1,000 kilometers in diameter and made of rings of very thin plastic film, arranged concentrically with spacing between them. The total mass of the lens is relatively small, 560,000 tonnes. The construction of the lens is crude and it only works at the wavelength of the laser beam, but it is capable of keeping the beam in focus for a distance of over 40 light-years.
The lightsail attached to the interstellar spacecraft is made of aluminized film and has an initial diameter of 1,000 kilometers. The sail must be highly reflective, since the laser beam is so powerful that it would vaporize the sail if any substantial fraction of beam power was dissipated in it. The spacecraft itself must also have a reflective shield both fore and aft to protect itself from the beam and its reflection from the sail. Total mass of the entire assembly is 80,000 tonnes, with 3,000 tonnes of that being the actual spacecraft, and the rest of it being the lightsail itself.
The laser beam accelerates the spacecraft at 0.3 gees for 1.6 years, bringing it to a velocity of half lightspeed. The spacecraft cruises towards Epsilon Eridani for 20 more years Earth time, though only 17 years pass for the crew. Then the spacecraft begins its deceleration maneuver. Once the spacecraft comes to within 0.4 light-years of its destination, the outer regions of the lightsail are cut free, except for a central "rendezvous sail", 320 kilometers in diameter. The spacecraft is turned around, and the laser light reflected from the discarded outer sail onto the rendezvous sail slows the spacecraft down for another 1.6 years.
Once in the target system, the crew navigates from planet to planet using the lightsail as a solar sail. After five years of exploring, the crew prepares to return home. The outer regions of the rendezvous sail are discarded, except for a "return sail" 100 kilometers in diameter. Laser light reflected off the discarded rendezvous sail accelerates the spacecraft to half lightspeed for the return journey. As the spacecraft approaches the solar system, it turns around, and the laser beam gives it a final deceleration. When the crew arrives back on Earth, 51 years have passed, though they have aged only 46 years.
* None of the technology envisioned by Forward is ruled out in principle, though it requires a certain ability to think really big. To build such a system is beyond the sensible resources of an Earth-based economy, and generating such tremendous energies on Earth would lead to environmental catastrophe.
An interstellar mission would have to be conducted by a space-based economy, where robotic systems could dismantle asteroids and convert them to megascale structures. These structures would include solar-power systems with collectors 1,000 kilometers in diameter, weighing up to 100,000 tonnes and collecting gigawatts of power. Microwave lenses would be as heavy, and laser lenses much heavier.
The material requirements would not be inconceivable for a space-based economy. The microwave lens could be built from a nickel-iron asteroid only 25 meters in diameter. The aluminum for a solar collector could be obtained from a stony-metal asteroid 100 meters in diameter. The plastic for a laser lens could be synthesized from a sooty "carbonaceous chondrite" asteroid about a kilometer in diameter.
Even accepting such a scale of thinking, there are still many technical obstacles to such interstellar missions. For example, how could a huge lens be built that not only kept the beam focused light years into space, or could tolerate the flood of power driven through it? In addition, building sprawling space structures out of filamentary wires or ultrathin aluminized films is problematic. It may be impossible to lay them out and keep them from wadding up or tearing, and minor damage from space dust could rip them to shreds.
A probe like Starwisp is a headache as well, due to sensor and communications requirements. While microcircuits and micromachines continue to shrink, fundamental physics dictates that many sensors grow less capable as they grow smaller, until they become useless. The light-gathering ability of a telescope or telescopic camera, for instance, is strictly dependent on the size of the aperture. Similarly, to send information back from a nearby star at reasonable data rates requires a reasonably powerful laser, augmented by a focusing and targeting system. A large orbiting telescope, possibly ten meters in diameter, would be needed back home to pick up the laser beam. These difficulties do not rule out an interstellar mission, but they make the idea more troublesome.
* There have been other proposals for interstellar propulsion systems that are almost entirely speculative. Forward himself has suggested propulsion systems based on "negative matter", a concept that was suggested by astrophysicist Hermann Bondi in 1957. Negative matter is not antimatter. Antimatter is normal matter with reverse electrical charge, while negative matter has negative mass.
Electromagnetic and gravitational forces are similar, but have several differences. The electromagnetic force between two objects depends on their electric charge, while the gravitational force between them depends on their mass. However, there are two "polarities" of charge, positive and negative. Electromagnetic forces are repulsive if the two objects have the same charge, and attractive if the two objects have a different charge.
Gravity, as far as anyone knows, is always attractive, or in other words there is only one "polarity" of mass. Bondi speculated on the properties of a hypothetical "negative matter" that had a reversed mass polarity. In 1988, Forward described a starship that included enough negative matter to nullify the mass of its positive matter. Since electromagnetic forces are much more powerful than gravitational forces, Forward suggested that a charge imbalance might be set up between the positive and negative matter to accelerate the starship. Nobody has found any trace of negative matter yet.
Another speculative suggestion for interstellar propulsion is to make a starship that obtains energy from the structure of space itself. Modern quantum physics proposes that what we think of as empty space is actually a seething sea of high-energy particles that come into existence and vanish too quickly to be detected, in what are referred to as "quantum fluctuations". Although quantum physics specifies that these "virtual particles" cannot be detected directly even in theory -- the more energetic the particles, the shorter the time they can exist -- some suggest that if there were some way of tapping into this "zero-point energy", a starship could propel itself across deep space without carrying any significant fuel.
Unfortunately, current experiments strongly hint that the actual energies present in the vacuum are very feeble, and certainly nobody has figured out a way to extract any useful energy from the vacuum. After all, there's a lot of energy in the ambient heat of the atmosphere, but there's no way even in principle we can actually get at it -- at least if all we've got is uniform ambient heat. Extracting energy from a source at a high energy implies an energy transition to a sink at a lower energy level; with zero-point energy, it's hard to see where the sink is.
BACK_TO_TOP* Negative matter and zero-point energy stardrives are entirely speculative. They are, however, more conservative than faster-than-light, or "superluminal" drives, which flatly contradict contemporary physics. As noted, Einstein's theory of relativity implies that the speed of light is an absolute barrier, due to the fact that an object's mass increases towards infinity as the object approaches the speed of light.
History is littered with flat statements that proclaimed things impossible, such as manned flight, that turned out to be commonplace later, and now people are reluctant to say anything is impossible. It is still interesting to consider the actual prospects for superluminal flight.
* Many of the objections raised to Einstein's theory of relativity are easily dismissed. Some are simple misunderstandings that point out seeming contradictions that are specifically addressed by the theory. Others point out examples of things that seem to exceed the speed of light, but don't actually do so in practice.
For example, consider a searchlight beam at the center of a huge ring wall with a radius of, say, one light year. If searchlight is rotated around in five seconds, then one year later, the beam spot will swing around the entire 6.28-light year wall in five seconds as well. The movement of the beam along the wall will be tens of thousands of times faster than the speed of light. This is useless, because nothing has actually traveled from point to point on the wall. Light has traveled from the searchlight to the wall at lightspeed, no more and no less, and though the terminal point of the light moves much faster than that, absolutely nothing can be transferred along from one endpoint of the beam to the next.
Another misleading example is Cerenkov radiation, in which electrons in a solid can be faster than light propagating through the medium. However, the speed of light in a solid is slower than the speed of light in a vacuum due to interactions between light and the medium, and the electrons are still slower than the speed of light in a vacuum. Yet another illusion of superluminal travel is provided by the huge cosmic jets emitted by active galaxies, which can appear to be moving faster than light when the jets are flying toward us along our line of sight. However, this was immediately known to be an illusion, and the actual speed of such "superluminal" jets is slower than light.
Physicist Raymond Chao of the University of California at Berkeley devised another, much subtler superluminal illusion, involving the "tunneling" of a light photon through a thin barrier. According to a rule quantum physics known as "wave-particle duality", a particle can be mathematically described as a "wavepacket" function that gives the probability of the detection of the particle at various locations in space. If confronted with a thin barrier, the wavepacket can extend through the barrier, meaning the particle has a certain probability of being detected beyond that barrier even though it shouldn't be there by the rules of classical physics.
If two photons race each other along a parallel path in free space and one encounters such a barrier, if that photon tunnels through the barrier it will have a high probability of being detected before the one that doesn't hit the barrier. However, this only really means that the tunneling event has modified the wavepacket that describes the photon, shifting the wavepacket forward. This increases the probability that the photon that tunnels will be detected before the one that does not, and in no way violates relativistic physics. In any case, the "correspondence principle" establishes that although the behavior of reality at the level of individual particles follows the probabilistic and often non-intuitive laws of quantum physics, in the macroscale the probabilities of the actions of many individual particles average out, and the laws of macroscale physics apply. They have to, because otherwise we would observe something different in the macroscale.
Roughly comparable "superluminal" experiments have been performed more recently with light pulses sent through refractive media, such as cesium gas. The refractive medium causes a shift forward in the peak of the light pulse such that the peak may be detected even before it is emitted! However, once again this is only due to shifts in the wavefunction of the light pulse, and people who have conducted these experiments flatly deny they violate relativistic physics.
* Another quantum physical scheme that seems to hint that superluminal communications are possible is known as "quantum nonlocality", where "nonlocality" means "remote interactions faster than the speed of light".
Quantum nonlocality is based on a quantum physics quandary, the "Einstein-Podolsky-Rosen (EPR) paradox", which was invented by Albert Einstein, his assistant Nathan Rosen, and physicist Boris Podolsky in the 1930s as a "thought experiment". According to quantum physics, the act of measurement influences the behavior of a particle at the microscale level, and in fact it is not possible to claim that the properties of a particle are defined in the absence of a measurement. It's misleading to even call it a "particle", since that implies some well-defined object resembling a tiny ball; some prefer the term "quantum entity" instead. In other words, the measurement actually establishes the properties of the particle, at least from the range of possible values the properties might have under given circumstances.
The EPR paradox imagines the generation of, say, two photons from an event that creates them with polarizations at right angles to each other, and sends them in opposition directions. In quantum physics terms, the two photons are said to be "entangled". Entangled photons with a right-angle polarization relationship can be generated, for example, by shining a laser beam through certain types of optically-active crystals. According to quantum physics, until the polarization of at least one of the photons is measured, the polarization of the photons remains indeterminate. Once the polarization of one photon is measured and "resolved", then the polarization of the other entangled photon is also implicitly resolved, even though it is far away by that time.
The paradox is that this seems to imply instantaneous communications between distant points, which is ruled out by relativistic physics. Einstein and his colleagues were trying to mock contemporary quantum physics by pointing out that its logic led to a contradiction. Einstein didn't live long enough to be confronted with experiments conducted beginning in the 1970s that demonstrated that quantum nonlocality is a provable fact, like it or not.
It is tempting to think that quantum nonlocality can be used for faster-than-light communications, but in fact it can't. The trick is that although a measurement will establish the polarization of the photons, there is no way to specify a particular polarization by the measurement. As an analogy, suppose two different trinkets are individually gift-wrapped and each given to two friends, named Alice and Bob. They both know what the trinkets are, but they don't know which one they have. If Alice opens her box and finds out what trinket she has, she will know instantly that Bob has the other trinket. Big deal.
This is not quite the same as quantum nonlocality, which implies that until Alice opens her box, its contents are actually undefined, and could actually be either trinket. However, from a practical point of view, the end results are little different. If Alice opens her box and finds one trinket, she has to inform Bob that he has the other trinket is by lightspeed communications. If Bob opens his box before Alice's message reaches him, he figures out the score, just the same as Alice does.
* Quantum physics also implies that there is a degree of ambiguity in measuring the polarization of a photon to begin with, and to banish that ambiguity Alice needs to inform Bob of how she performed the experiment, which has to be done using normal speed-of-light-or-less communications. This leads to one of the more bizarre aspects of quantum nonlocality, the phenomenon of "quantum teleportation". In this scheme, a particle is allowed to interact with one of an entangled pair of particles; the interaction is performed in such a way that the same properties of the original particle are transferred exactly to another particle interacting with the other member of the entangled pair at a remote location. In essence, the particle has been "teleported".
Use of the term "teleportation" in this matter has led to some confusion, with starflight enthusiasts believing it means that some form of instantaneous travel has been discovered in principle. That's not the case. The process of quantum teleportation also involves ambiguities, and to get the scheme to work right the proper instructions need to be transmitted from one side of the experiment to the other by conventional means. Even if that were not the case, quantum teleportation only really works for simple quantum systems, and it is hard to see how it could be scaled up to more complicated things. As some of those working in the field have suggested: "It's teleportation, Jim, but not as we know it."
* One last example of superluminal flight is actually true, though it's still not useful. Modern cosmology indicates that the entire Universe is expanding, causing objects in the Universe to move away from each other. The rate of movement increases with the distance between two objects, and in principle that rate may exceed the speed of light.
Once more, this does no good, because the speed of light is still an absolute within any locality in the Universe. Even if two distant objects are receding from each other at more than the speed of light, no object in the entire Universe can proceed from point to point at any more than the speed of light. In fact, if the expansion of space between two planets exceeded the speed of light, the two planets would then be completely out of communication with each other in every respect, being effectively in separate universes.
* Since nothing has been observed to exceed the speed of light in practice, does theory leave open any theoretical possibility of superluminal travel?
Einstein's equations specify that no normal matter may exceed the speed of light, but they also have as a solution particles that can travel faster than the speed of light. These superluminal particles are known as "tachyons", after the Greek "tachys", for "swift". They have interesting properties, which mirror those of the "slow" particles ("tardons") of which we are made:
Researchers have tried to detect them in the particle showers generated by "cosmic rays", the somewhat mysterious high-energy particles that smash into the upper atmosphere. A tachyon created in a cosmic-ray shower would appear before the beginning of the shower, but so far nobody's spotted one.
Nobody's made much effort. Simply because they are possible solutions to Einstein's equations doesn't imply they exist. Suppose, as an analogy, we walk 6 kilometers due north and then 4 kilometers due west. We can figure the straight-line distance from start to destination using the Pythagorean theorem:
SQRT( 6^2 + 4^2 ) = SQRT( 36 + 16 ) = SQRT( 52 )
Of course, this gives the distance as 7.21 kilometers -- but from a mathematical point of view, the answer could also be -7.21 kilometers. Nobody thinks twice about discarding the useless negative answer. Tachyons are at best just an interesting speculation, at worst just an irrelevant artifact of computations, and even if anyone could find any proof that they really exist, there's no reason to believe that we could make any use of them.
* The discovery of collapsed matter objects, including black holes, in the 1960s and 1970s led to another theoretical possibility, known as the "wormhole". If a large stellar object collapses into itself, once the collapse reaches a certain critical radius, the collapse will never stop, creating a "singularity". The gravitational field around the singularity will be so intense that below a certain radius known as the "event horizon" light will not be able to escape, and the region around the singularity will appear as a "black hole" in space.
Einstein's theory of general relativity states that gravitational forces are due to the distortion of space and time by mass. It is conceivable that the distortion caused by a pair of singularities at different locations in space might create a linkage between them -- the wormhole.
Wormholes provide a tempting possibility for a shortcut between distant regions of space. The first problem is the simple one that nobody's ever proven one exists, though on the other hand physicists have not been able to prove them impossible, and some physicists believe them plausible. The second problem is that even if they do exist, they may not be more than a quantum-level phenomenon, possibly a virtual phenomenon that pops in and out of existence too rapidly to be detected directly even in principle, and in any case nothing that could support a transportation scheme -- though communications might be possible. The third problem is that even in theory going into a black hole is a fatal one-way trip, with no possible escape once a traveler falls below the event horizon, and wormholes have event horizons as well.
Physicists also have noted that wormholes open up a possibility of traveling backward in time, or put in another way violating the laws of cause-and-effect. Of course the Universe operates by its own rules without concern for our likes and dislikes, but so far nobody's ever found any evidence of a violation of causality, and nobody's expecting to see one any time soon.
Theorists have still toyed with wormholes to see if something can be made of them, considering the modifications of the basic theory allowed by rotation, charge, and other parameters. In 1988, the well-known American physicist Kip Thorne postulated that if a small amount of "exotic matter" with "negative energy" were put down the throat of the wormhole, it would become passable.
Thorne's exotic matter would have less energy than the zero-point energy of the vacuum, giving it negative energy. Negative energy does actually exist, at least in theory, during the continuous quantum fluctuations of space. However, as noted such quantum fluctuations can cannot be detected directly, and in any case the amount of negative energy they create at any one time is on the quantum scale. Some theorists have suggested that only a tiny amount of exotic matter would be required to make a wormhole passable, but the matter remains in debate, and even the most optimistic don't believe they are hot on the trail of a faster-than-light drive. It's just an exercise in pure theoretical physics at this time.
There have been similar speculations on superluminal drives that involve modifying the structure of space, but these concepts are comparable to the wormhole idea and suffer from similar limitations. In short, theory hints that there may be doors to superluminal flight, but so far nobody knows where they might be found, or if they would really go anywhere if they were.
BACK_TO_TOP* I had fun putting together this document. There's a bit of a joke in the subject matter, too. Sometimes I get a compliment on one of my documents. I shrug and answer: "It's not rocket science." In this case, however, I can't use that dodge.
I need to add, in closing, that I see the obstacles to interstellar flight as so great as to say confidently there is no prospect of it in the foreseeable future. In particular, at least given the state of our knowledge, superluminal flight is completely out of reach. However, a small sublight robot probe is within reach of technical imagination, if hardly practical at the present time. Although it can't be done now, consideration of what is needed to make it happen is an interesting and worthwhile exercise. A manned interstellar mission is not ruled out in principle, but would be an undertaking of unprecedented scale. It cannot be seriously considered at present, and will have to be put off for a society that thinks in larger scales than our own.
In a sense, I do not believe that humans will ever travel to the stars. Current work in bioengineering and machine intelligence may well lead to new and improved "post-human" intelligences that have vastly longer lifespans and the ability to hibernate or go into a "standby" state for centuries. Our descendants may perform interstellar missions, but they may not be what we would recognize as humans.
I have to add that every rare now and then I have had someone email me to insist that faster-than-light flight really is possible. OK, for the record: am I going to say that faster-than-light flight is absolutely impossible forever? Of course not, I'd feel silly and I'd deserve to be abused for saying it. But I will draw the line at being told that there is a factual basis for it at present, since in fact our current understanding of physics flatly says exactly the opposite. There are speculations about loopholes, but pure speculations are all they are for now.
Of course, any faster-than-light advocate will immediately respond that we may someday acquire an understanding of the laws of physics that will show our current view to be too restrictive. Absolutely. If pigs had wings, they might fly; if we had some ham, we could have ham and eggs -- if we had some eggs.
I have to make one last comment: I ran across a reference to this document on an internet forum, in which a reader noted my statement that interstellar flight was "challenging" and observed that "the author deserves a Pulitzer prize for understatement." I found that flattering.
* Sources:
I found useful materials on the Web. Valuable organizational websites included Hughes Space & Communications and the NASA JPL Advanced Propulsion Concepts site. Paul Woodmansee's ROCKET SCIENCE site provided many of the details on conventional liquid propellant rocket engines discussed in the first chapter of this document. His site came as something of a shock to me: I had no idea a rocket engine was that elaborate.
In particular, Richard K. Graf's online materials on the HARP project are extremely detailed and fairly clear. The HARP project and Gerald Bull are surrounded by misinformation and mythology, but the detail of Graf's material, which is as much or more than anyone would want to know, gives it a strong air of authenticity. It would hard to understand why anybody would go to the trouble to fabricate such extensive materials.
Some materials were even obtained from TV shows. The History Channel's SWORN TO SECRECY series had an installment on PROJECT ORION. I had heard about the idea when I was a kid back in the 1960s, and it has been used in science-fiction novels, such as Larry Niven and Jerry Pournelle's FOOTFALL. I always had the impression that it was strictly a blue-sky scheme that nobody ever took very seriously, but the program described in fair detail how it had been seriously investigated for a few years. It had interesting interviews with Ted Taylor and Freeman Dyson, plus entertaining footage of the "Hotrod" demonstrator. It was no surprise that it caught von Braun's interest.
Another SWORN TO SECRECY installment on SUPER GUNS discussed the Wilhelmgeschuetze, the V-3, Atomic Annie, and Bull's superguns. An entertaining TV movie titled DOOMSDAY GUN was also made about the Babylon Gun, with Frank Langella playing Gerald Bull. The SWORN TO SECRECY show was weak on some of the details, while DOOMSDAY GUN played a little fast and loose with the facts, though it was still entertaining.
* This document started life as a set of independent articles, released in the following order:
The LIGHTCRAFT document originally only discussed lightcraft, but it was expanded in January 2000 to cover lightsails and related spaceflight concepts.
The original INTERSTELLAR EXPLORATION & SETI document, as its title implies, included materials on the "search for extra-terrestrial intelligence (SETI)". This particular document was revised a number of times, with the SETI materials split off into an independent document in July 2000.
The SPACE TETHERS document, ironically, began life as a set of unreleased notes on beanstalks with some comments on space tethers, and in release ended up being a document on space tethers with some comments on beanstalks.
The first chapter, on liquid and solid propellant rockets, was actually the last written. It is also the least satisfactory, in that it has to try to survey a wide range of engine technologies, and figuring out how to come up with something not too long and still readable has proven very difficult.
* Revision history:
v1.0.0 / 01 mar 02 v1.1.0 / 01 feb 04 / Went to seven chapters from six. v1.1.1 / 01 sep 04 / Cleaned up some comments on quantum physics. v1.1.2 / 01 nov 05 / A few minor tweaks and cleanups. v1.1.3 / 01 jul 07 / Review & polish. v1.1.4 / 01 jun 08 / A few updates. v1.1.5 / 01 apr 10 / Review & polish. v1.1.6 / 01 mar 12 / Review & polish. v1.1.7 / 01 feb 14 / Review & polish. v1.2.0 / 01 jan 16 / Cleaned up liquid-fuel propulsion writeup. v1.3.0 / 01 dec 17 / More materials on liquid / solid rockets. v1.4.0 / 01 nov 19 / Various minor updates. v1.4.1 / 01 nov 19 / Update, review, & polish.BACK_TO_TOP
