... between stars, what distances.
Rainer Maria Rilke, Sonnets to Orpheus, Part 2, XX
Perhaps the most straightforward solution to the Fermi paradox is that the distances between stars are too great to permit interstellar travel. Perhaps, no matter how technologically advanced a species becomes, it cannot overcome the barrier of interstellar distance. (This might explain why ETCs have not visited us, but not necessarily why we have not heard from them. But let us put this criticism to the side for the next few sections.)
That the stars are far away does not in itself make interstellar travel unattainable. it is not impossible to build a vessel that can leave a planetary system and then travel through interstellar space. Take our Solar System as an example: its escape velocity, starting at Earth's distance from the Sun, is only 42 km/s. In other words, if we launch a vessel traveling at 42 km/s relative to the Sun, then it can escape the grip of the Sun's gravitational influence. It can become a starship. No problem: NASA has already built several such vessels! (With our present technology we have to cheat a little and use the gravity assist offered by the planets. The so-called "slingshot effect" is quite sufficient to boost a slow-moving craft to escape velocity.)
Voyager 1, launched on 5 September 1977, toured the outer planets and then headed out into space. On 17 February 1998 it became the most distant man-made object, and it is now farther from the Sun than is Pluto. Unless alien probes pick it up, as happened to the fictional Voyager 6 in Star Trek: The Motion Picture, it will eventually make its closest approach to a star — it will drift within 1.6 light years of an unprepossessing M4 star called AC +79 3888. The trouble is, Voyager will take tens of thousands of years to reach its closest encounter with the star. And that is the difficulty with interstellar travel: unless you travel fast, the transit times are long.71
The best way to rate a starship's speed is in terms of c, the speed of light, since c is a universal speed limit.72 The speed of light in a vacuum is 299,792.458 km/s. So Voyager 1, which as I write is traveling at 17.26 km/s away from the Sun, travels at a mere 0.000058c. Now, the stars are so widely separated that a favored method of presenting interstellar distances is to use the light year: the distance light travels in one year. For example, the nearest star to our Sun is Proxima Centauri, which is 4.22 light years distant.73 So the fastest possible "craft" — photons of light — would take more than 4 years to reach the nearest star; Voyager 1, were it traveling in that direction, would take almost 73,000 years to complete the same journey. The huge travel time involved when traveling at sub-light speed leads many commentators to conclude that interstellar travel, while perhaps not theoretically impossible, is impracticable.
But perhaps exploration of the Galaxy, even at Voyager speeds, is possible. As we have seen (page 45), the notion of directed panspermia supposes that the Galaxy could be seeded with life using slow-moving probes. And as long ago as 1929, John Bernal proposed the idea of the "generation ship" or "space ark": a slow-moving self-contained craft that would effectively constitute the whole world for its passengers. After setting off from the home planet, many generations of passengers would live and die before the craft arrived at its destination.74 Bernal's idea was wonderfully dramatized in Heinlein's story Universe.75 Another possibility would be to put the passengers into suspended animation, as in the film Alien, and revive them upon arrival. It has even been suggested that frozen embryos could be transported on slow-moving craft, and then grown in artificial wombs at journey's end.
Clearly, though, if we wish to reach the stars in a reasonable time, we need to build craft that can travel at a substantial fraction of the speed of light. Even then, the travel times involved may be long on an individual human scale. For example, ignoring the acceleration and deceleration times at either end of a journey, a craft traveling at the enormous speed of 0.1c would take 105 years to reach Epsilon Eridani, which is one of the nearest Sun-like stars. Few crew members seeing their new star for the first time would remember the star their ship left behind. (When talking about travel times, we tend to assume that people will choose not to spend so many years of their life away from home. But we base this assumption in terms of the present human lifespan. After gaining their degrees, several of my more adventurous contemporaries chose to spend a year — which is roughly 2% of their adult life — simply traveling around the world. If human lifespans were increased by a factor of ten, say, then perhaps an adventurous soul would be quite willing to spend a mere decade of his life traveling to the stars. Perhaps even a century-long journey would not be uncommon. Who knows? As always, it is difficult to argue about future activities based on present technology.)
The journey time mentioned above — 105 years to reach Epsilon Eri-dani, at 0.1c — is the time that Earthbound observers would measure. People on the ship would measure a slightly smaller interval due to the special relativistic effect of time dilation.76 We are justified in ignoring time dilation effects for on-board observers traveling at 0.1c, since the effect is only about 0.5%. The closer the speed is to c, however, the more noticeable the effect. A craft traveling to Epsilon Eridani at 0.999c would take 10.5 years to complete the journey as measured by Earthbound observers, but to a crew member the journey would take only 171 days! If it were possible to travel at speeds infinitesimally smaller than c, then for the traveler the journey would take a mere fraction of a second. A trip to the farthest galaxies would be possible within a human lifetime — though to Earthbound observers the trip would take so long that Earth itself would be consumed in the Sun's death throes.77
What is the likelihood that an intelligent species could develop techniques for interstellar travel at reasonable speeds? (By "reasonable" I mean any speed that enables a mission to reach nearby stars on a timescale of hundreds rather than tens of thousands of years. Highly relativistic speeds would be nice, of course, since they would put the stars within reach of individuals living a human lifespan. But a craft leaving the Solar System traveling at 0.01c will reach the nearest star in about 430 years, which puts the stars within range of generation ships.) To answer this, we need to consider the various space-travel technologies that have been suggested. I give only a brief overview here; the notes in Chapter 7 point to further resources.
Although I concentrate here on propulsion methods, it is worth bearing in mind that there are other factors to consider. For example, a starship traveling at high speeds would suffer a ferocious bombardment — tiny dust particles from the interstellar medium would deposit large amounts of energy into the starship structure. Protecting the structure against such erosion, and protecting the crew from the more insidious problem of cosmic-ray bombardment, would require sophisticated shielding. There is also a navigation problem: the stars move with different velocities in three dimensions, making it difficult for a slow-speed mission to rendezvous with a particular star.78 Nevertheless, these problems are moot if no systems exist that can propel a ship to the stars. If interstellar travel is impossible, then maybe we have a solution to the Fermi paradox.
Most people's initial idea for a starship propulsion mechanism is the self-contained rocket. NASA's familiar chemical rockets obtain all their energy and expellant mass from on-board reserves. Consider the Apollo missions, for example. The multi-stage Saturn V rockets burned liquid propellants: a mixture of kerosene with liquid oxygen for the first stage, and liquid hydrogen with liquid oxygen for the second stage. The exhaust from these chemical reactions was sufficient for reaching the Moon, but this approach is simply not feasible for interstellar travel: the nearest star is more than 100 million times more distant than the Moon. The kerosene tanks would be enormous!
Nevertheless, it may be possible to employ variations on this theme. For decades, scientists have considered alternatives to chemical rockets. An ion rocket, for example, would expel charged atoms to generate thrust; a nuclear fusion rocket would generate high-speed particle exhaust by means of controlled thermonuclear reactions. Perhaps the boldest possibility is the antimatter rocket, first suggested in 1953 by Eugen Sanger. When a particle of matter comes into contact with its antiparticle, both particle and antipar-ticle mutually annihilate and produce energy. Choose the initial particles correctly and it might be possible to channel the annihilation products into a directed exhaust. Although further analysis showed that Sanger's initial design could not succeed, advances in antimatter physics made in recent decades have stimulated proposals that may one day lead to an antimatter rocket.79
The whole concept of using a self-contained rocket — which has to carry the energy source and the payload — may be impractical for interstellar travel. Are there propulsion systems that do not require the ship to carry its own fuel? In 1960, Robert Bussard suggested that a fusion ramjet might power its way to the stars.80
The space between stars is not empty. There exists an interstellar medium, comprised chiefly of hydrogen. A ramjet would use an EM field to scoop up this hydrogen and funnel it to an on-board fusion reactor, which in turn would "burn" the hydrogen in thermonuclear reactions to produce thrust. As with Sanger's antimatter rocket design, Bussard's fusion ramjet proposal suffers from a host of practical difficulties. It is unlikely that Bussard's initial idea could be made to work. Nevertheless, several studies have proposed methods to improve the design. Perhaps one of these designs could eventually form the basis of a working starship. Enthusiasts remain enticed by the possibility of the ramjet, because in theory it could attain speeds close to c after just a few months.
At about the same time that Bussard proposed the fusion ramjet, Robert Forward proposed the laser sail as a means of reaching the nearest stars.81 Imagine a vast "sail" attached to a spaceship; and imagine a giant solar-powered laser aiming a narrow beam of radiation toward the ship. Photons from the beam would cause a tiny pressure on the sail, and the ship would be gently pushed toward the stars. A laser sail could accelerate to extremely high velocities; hitting the brakes would be more difficult, although deceleration mechanisms have been proposed. Forward's idea has been refined over the past four decades, and enthusiasts have designed schemes to use laser sails for both a one-way colonization mission and a round-trip to the stars.82
In 1958, Stanislaw Ulam considered the possibility of accelerating a ship to high velocity using its gravitational interaction with a system of two much larger astronomical bodies in orbit around each other. (It is a trick similar to the gravity-assist trajectories that gave Voyager 1 sufficient velocity to leave the Solar System.) A few years later, Freeman Dyson considered more realistic (though still, of course, speculative) scenarios. Using Dyson's approach, an advanced technological civilization might employ two orbiting neutron stars to accelerate spaceships to near light speed.83
The technologies mentioned above are based on established physics. The construction of starships using these ideas are, of course, way beyond our present capabilities; indeed, engineering considerations may make it impossible in practice to construct starships. But there seems to be nothing wrong with these ideas in theory. They break no physical laws.
For many years, people have wondered whether it is possible to travel really fast. If we could travel at speeds greater than c, then the stars would no longer be grindingly distant. Faster-than-light (FTL) travel would bring the ends of the Galaxy within reach. Nearly all ideas for FTL travel can immediately be discounted, since they clearly violate established physical principles. A few suggestions, however, have not yet been ruled out.
Tachyons. The special theory of relativity does not absolutely forbid su-perluminal travel. Rather, it states that massive particles cannot be accelerated to light speed, while massless particles (like photons) always travel at the speed of light. Particles with imaginary mass must always travel faster than the speed of light. Such imaginary-mass particles are called tachyons.
There is nothing particularly unusual about imaginary quantities: we represent several physical quantities by imaginary numbers. But it is difficult to understand what an imaginary mass represents. We have no problem understanding the idea of a positive mass; nor is there any difficulty with the idea of a zero mass; we can even ascribe meaning to negative mass (and note that, if negative mass existed, we might be able to use it in a propulsion device).84 But imaginary mass? Whatever it might mean, physicists have searched for signs of it. So far, the tachyon remains hypothetical. There is no evidence such particles exist, and our theories work fine without them. Even if we found tachyons, how could we harness them for FTL travel? We are clueless, here, and it seems reasonable to strike tachyon drives from the list of propulsion possibilities.
Wormholes and warp drives. Most of us are familiar with the Newtonian picture of gravity. We are taught in school that massive objects attract one another by exerting a mysterious influence through empty space. Einstein's general theory of relativity presents a very different picture of gravity. In this view, space — or rather, spacetime — plays an active part in the gravitational interaction. In the words of John Wheeler: mass tells spacetime how to curve, and curved spacetime tells mass how to move.
We can think of special relativity as a particular case of general relativity. It applies locally to any region of spacetime small enough that its curvature may be neglected. The interesting point to consider here is that general relativity permits FTL travel — so long as the local restrictions of special relativity are obeyed. The speed of light is a local speed limit, but general relativity permits ways to circumvent this limit. Although this may seem peculiar, there are well-established examples of FTL phenomena in general relativity. For example, standard cosmological models suggest that, due to the expansion of the Universe, distant regions of space recede from us at FTL speeds. Only if the expansion slows will those regions appear over the light speed horizon and be visible to us.
So far, general relativity has passed every experimental test. It correctly predicts the bending of light rays near the limb of the Sun, the orbits of binary pulsars, and the arrival of signals in GPS systems. However, most tests of the theory occur in situations where spacetime curvature is small. Sometimes, the distribution of matter can cause a large curvature of spacetime. At the singularity of a black hole, for example, the density of matter is infinite; the very fabric of spacetime is punctured.
figure 24 If space folds over on itself, then a wormhole linking A to B might allow travelers to move between these points without having to traverse the "normal" spacetime between the points.
It is difficult to interpret the results of general relativity in the extreme situations that occur near the singularity of a black hole. Perhaps the theory cannot be applied in such situations; we may require a quantum theory of gravity to describe what happens there. But in an attempt to understand these extreme regions of spacetime, physicists have pushed the theory. One speculation is that the formation of a black hole can lead to the formation of
a wormhole — a "bridge" that links two separate black holes. The two holes may link two quite separate points of spacetime, or two different regions of the Universe. Enter one black hole and you might emerge from the other hole moments later, thousands of light years from your starting point. As you traveled through the bridge you would have observed the local speed limit and moved slower than c; yet your effective speed could be millions of times greater than c. Sagan used this idea in his SF novel Contact.85
Although based on solid work, the wormhole remains a hypothetical creature in the theoretical physicist's bestiary. Wormholes may not exist. Even if they do exist, we may be unable to travel through them: calculations suggest that they are likely to be small and wildly unstable. Nevertheless, there remains a tantalizing possibility that an ETC in possession of "exotic" matter (matter with a negative mass-energy) could take a microscopic wormhole, stabilize it, inflate it to a large size — and then use it to traverse huge distances. Recently, the Russian physicist Sergei Krasnikov has shown that a certain class of wormhole might be constructed using "normal" (positive mass-energy) matter. Perhaps a K3 civilization could use such Krasnikov wormholes for interstellar travel.
figure 25 The figure shows the curvature of space in the region of Alcubierre's warp. Space expands at the rear of the warp and contracts at the front; the flat region is pushed forward.
—> x figure 25 The figure shows the curvature of space in the region of Alcubierre's warp. Space expands at the rear of the warp and contracts at the front; the flat region is pushed forward.
There is another way in which general relativity might permit superlu-minal travel (and in the style to which Star Trek has accustomed us). Imagine a spaceship — one as large and luxurious as the QE2 — inside a flat region of spacetime. Everything on board the ship would behave as it does in the flat region of spacetime we are accustomed to here on Earth. Now imagine that, at the rear of the volume, space expands (in the same way that the Universe itself expands). And at the front of the volume, space contracts
(as would happen if the Universe were to collapse into a Big Crunch). The result of this particular warp in space is that the flat-space volume, containing the spaceship, would move forward — propelled by the expansion of space at the rear and the contraction of space at the front. The ship effectively surfs a spacetime wave.86
The warp can travel at arbitrarily large speeds, perhaps many times faster than c, and it carries the ship with it. With respect to the local volume of flat space, however, the ship is at rest. There is no relativistic mass increase, and no time dilation. For the crew, everything is as normal. As they speed toward the stars at a speed of 100c, the passengers are free to enjoy the hospitality of the Spaceship QE2.
The properties of this peculiar solution to Einstein's equations were first analyzed by Miguel Alcubierre while he was at Cardiff University. I have a soft spot for the Alcubierre warp drive, since I was wasting time in the office opposite Miguel while he was working on his idea. Nevertheless, the Alcubierre drive, at least as first proposed, is unlikely to work. First, we have no practical idea of how to produce the required curvature of space. Second, the energy density within the warped region is very large, and negative. (Some theorists would argue this second problem kills the whole idea of a working Alcubierre drive. However, quantum theory provides circumstances in which a negative energy density can occur. If we ever advance to the stage where we can produce large quantities of exotic matter, then perhaps we could make an Alcubierre drive. Even this seems unlikely, though. A warp large enough to carry the Spaceship QE2 would require a total negative energy that is ten times larger than the positive energy of the entire visible Universe!) The Belgian physicist Chris Van Den Broeck may have found a way around some of the problems of the Alcubierre drive. The construction of a microscopically small warp bubble would require just small amounts of exotic matter; combine this with some topological gymnastics, which are allowable in general relativity, and you can end up with an interior volume of the warp bubble that is large enough to hold a spaceship. It would be rather like the Tardis in Dr. Who: microscopically small on the outside, but roomy enough for passengers on the inside. We may find, when we have a full quantum theory of gravity, that the Van Den Broeck drive is ruled out; in any case it is worth emphasizing that the drive is speculative and possesses unrealistic features (unreasonably large energy densities are required, for example).87
Perhaps wormhole and warp-drive transportation will never be practical. But they have not yet been shown to be impossible. Maybe one day.
Zero-point energy. The quantum uncertainty principle tells us we cannot know simultaneously both the position and the momentum of a particle.
Therefore even at absolute zero a particle must jitter, since if it were at a perfect standstill we would know both its position and momentum. Energy and time also obey the uncertainty principle; similarly, then, a volume of empty space must contain energy (since to establish that the energy was zero we would have to take measurements for eternity). The Casimir effect88 — a small attractive force that acts between two uncharged parallel conducting plates brought into close proximity — is the clearest example of the existence of zero-point energy (zpe). The effect can only be explained in terms of quantum fluctuations of the electromagnetic field.
Some writers suggest there is an infinite supply of energy in the vacuum and that some day we will tap into this ZPE. Perhaps we can use ZPE for a propulsion system. Recently, NASA even sponsored a meeting on innovative propulsion systems in which ZPE was identified as a potential breakthrough technology. If it works, then we have limitless cheap energy. Personally, I remain highly skeptical of the idea; we never get something for nothing. But it is yet another suggestion of how an advanced ETC might use the possibilities inherent in the laws of physics to develop technologies that seem almost magical to beings at our level of development.
I have only touched on the various proposals for interstellar propulsion systems. At present, we could not build one of the devices mentioned above and use it to reach the stars. With our present level of technology, we would find it almost impossible to send people safely to Saturn and back, let alone Sirius. There is a host of problems — economic, political, scientific and technical — that we (and presumably an ETC) would have to overcome in order to travel to the stars. What is remarkable, though, is the number of methods that reputable scientists have proposed for starflight. The methods range from the slow to the essentially instantaneous; from the tried-and-tested to the exotic. Although the human race cannot build a starship in 2002, what about in 2102? What about in 3002? Remember that 1000 years corresponds to only 2.5 seconds of the Universal Year. Other civilizations might be millions, even billions, of years older than our own. Is it likely that none of them have the requisite technological skill (or, if relativistic travel is impossible, simply patience) for space travel?
The stars are indeed distant. This fact alone may explain why we have not been visited (though it does not necessarily explain the "great silence" — the absence of signals from ETCs — nor why we see no other evidence of advanced civilizations). However, for those who are optimistic about the reach of science and technology, the distance barrier can be overcome. For those people, the size of the Galaxy alone does not explain the Fermi paradox.
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