AN ANALYSIS OF THE TECHNICAL CHALLENGES OF INTERSTELLAR TRAVEL

by Michael Harkness

Of the most intriguing scientific challenges that we face today, the prospect of interstellar travel is among the most fascinating. The capability for humans to visit everything from neighboring star systems to the farthest corners of our galaxy is one that offers many discoveries and countless possibilities. Before it is possible for humanity to reach even the nearest of galactic destinations, we must make advancements in nearly every field of science. We will need to make new discoveries, create new technologies, and improve our existing understanding of the Laws of Physics.

Based on our present technological and scientific understanding, we must make three major technological advances necessary before interstellar travel is possible. We need to be able to travel at speeds far greater than those currently attainable, we need to be able to travel through space without the use of a propellant, and we need to find a more bountiful energy supply. Before we can hope to overcome such challenges, we should completely evaluate what these technological obstacles are and what possibilities have been theorized to overcome them.

To avoid the numerous problems that arise on a very long journey--one that is much longer than a human lifetime--we must be able to attain very high speeds. If we cannot, then we must either take enormous supplies of resources (food, energy, medical supplies, tools and materials to repair the craft, etc.) or be able to generate them on the way. Taking sufficient resources for a journey that would last 1000 years--assuming we are able to travel at speeds roughly 50 times those currently possible--would consume tremendous amounts of volume and require even greater resources to take along. Creating food is not beyond our control, but being able to create enough matter to refine into parts for our ship, build necessary tools, etc. is not at within our current capabilities.

Furthermore, we have to consider that the party making such a journey would need to survive for 1000 years on a spaceship. At this point in the human civilization, we cannot completely predict the effect 1000 years of space travel would have on our species. Clearly, we are better off waiting until we can attain speeds sufficient to make the journey in a much more reasonable time.

Assuming that we have discovered a way to accelerate our spacecraft to speeds near or even greater than that of light, we must find a way to do so without "traditional methods." By "traditional methods", I am referring to the use of some sort of propellant fired from one end of the craft to accelerate it in the opposite direction. The amount of propellant needed for such a journey would be incredible--even assuming that we only travel one way. As Marc Mills points out (Warp Drive When?, 1997), even the most efficient propellant that could be constructed in the near future (Ion engine or Anti-matter rockets) would require roughly 10⁵ kg of propellant simply to get to the Centauri cluster. Of course this does not include the propellant required to stop when we get there or to return to Earth if we so desire. As a result, there is little choice but to find a method of space flight that does not require a propellant.

The third technological obstacle that we must hurdle is that of an energy source. Tremendous amounts of energy would be required to power the band of pioneering humans and their spaceship. Almost everything that we see and use in our environment requires some sort of energy supply; our futuristic pioneers would be no different. Everything from navigation and communication to heating, lighting, and repairs will require some form of power.

Such a voyage to other stars would likely end in the colonization of new worlds. In doing so, our colonizers will need to take the necessary resources to start a new civilization. For example, we cannot assume that a world will be found with a suitable food supply for humans. We must assume that none will be available; therefore, we must take seeds, plants, animals, etc. to colonize on our new world. We must also take these additional resources into account when considering the technological requirements for such a journey. In other words, better energy and propulsion systems will be required to transport the extra colonization resources that we will need.

There are many possible solutions to all of the problems I have mentioned, but we cannot yet discern which possibilities are viable solutions. Therefore, we will consider multiple solutions to the aforementioned problems.

To meet our travel time limitations, we clearly need the ability to travel at greater speeds than are currently possible. Two potential resolutions to this are a "warp drive" and "wormholes". Fortunately, these solutions would also satisfy our need for a method of travel that is not dependent upon a propellant.

A warp drive is a drive that functions on the principal of warping (or bending) spacetime to move an object. There are two main types of warp drives: a simple warp drive and Alcubierre's warp drive.

We will first consider a simple warp drive. For this drive, as spacetime is stretched and deformed, the apparent distance to one's destination can be altered, thus shortening the trip involved. For example, if we wish to journey to Alpha Centauri (approximately 4.3 light years from Earth), we could use a warp drive to distort the spacetime in between. In doing so, we could make the Alpha Centauri appear to be only a fraction of its normal distance (say, 0.01 light years) so that travel by conventional methods takes us there in a reasonable time. In this example, a speed of only 150 km/s would get us to Alpha Centauri in less than 20 years.

For an Alcubierre warp drive, the spacetime behind the object is given a negative curvature--resulting from negative mass, while the spacetime in front of the object is given a positive curvature--as caused by normal mass. This means that spacetime is given a slope of sorts with "downhill" facing the direction that we wish to move our object. As our object moves through space, the area of spacetime that is curved is moved as well so as to provide continual force in whatever direction we desire.

A warp drive can offer speeds equal to or greater than light speed because spacetime itself is moving the object. As a result, the object itself does not have to be accelerated and therefore would not violate the light speed limit. One analogy for this type of travel would be a surfer riding an ocean wave. The surfer may make some movement with respect to the wave, but it is the wave that causes the majority of his motion with respect to the shore. Likewise, an object using a warp drive may make some motion relative to its local spacetime, but it is spacetime itself that causes the majority of its motion relative to its source and destination.

If we are able to exercise great control over the curvature of spacetime, this could promise to be a powerful method of interstellar transportation. However, curving spacetime will affect everything in our vicinity and along our path; it will also require a vast supply of energy and an advanced understanding of gravitational forces. We must be cautious of our effect on our celestial environment; we should prohibit actions that would have sizable effects on the motions of other planets, stars, or even galaxies and galactic clusters. Also, we simply do not have the means to bend spacetime as we wish, both because we do not yet understand how and because we do not have the energy resources to do so. As a result, we may need to look elsewhere for our transportation solution.

Wormholes are another possible solution to the problem of interstellar travel. A wormhole is essentially a link or a tunnel between any two points in spacetime. In theory, these points may or may not be in the same universe and do not require a specific relative location. If they exist, wormholes are believed to connect two singularities in spacetime to each other through "hyperspace". Hyperspace is theorized to be the portion of reality beyond the three dimensional world which we comprehend.

The underlying concept of travel using a wormhole is quite simple. We find (or create) a wormhole that connects the point in space where we are and the point in space where we want to go; we then travel through the wormhole and arrive at our destination without having to travel through the known spacetime.

One way to understand a wormhole is to think about traveling along a piece of paper. Instead of traveling from point to point across the sheet, why not fold the piece of paper so that a hole poked through the paper connects your present location with your destination? Similarly, instead of flying through our spacetime to Alpha Centauri, we would use a wormhole to connect our present location to Alpha Centauri. We then only need to fly through the wormhole.

This fantastic form of transportation seems to be far beyond our understanding and control in the near future. First, although wormholes are valid mathematically according to general relativity and the Laws of Physics, it is not believed that a wormhole would be stable. In general, scientists believe that the presence of an object trying to pass through a wormhole would disturb the wormhole enough to destroy its link between singularities. Second, it is not known how anything passing into a singularity would be able to survive intact. The gravitational tidal forces near a singularity would destroy any object that we could currently manipulate. Third, the ability to create a singularity is beyond our ability and would require immense amounts of energy. With this ability, we would also need the ability to destroy the singularity. If we could not, then our spacetime would become littered with singularities. Fourth, even with the ability to create and control a singularity (and presumably create a wormhole in the process), we would not yet be able to connect two singularities in spacetime. Surely there are many other technological and scientific challenges that must be met before we can utilize wormhole-based travel.

Of the two hypothesized solutions to our problem of propellant and travel time, I believe that a warp drive (specifically, the Alcubierre warp drive) is the more promising. To achieve warp drive, we only need to be able to control the curvature of spacetime, while the use of wormholes entails the ability to control at least two singularities and the ability to control which singularities connect to each other. Obviously controlling spacetime curvature is not a trivial task, but in theory it is fairly straightforward. Although a warp drive seems far beyond our present grasp, the technology required for a warp drive could be fundamental to other advanced forms of interstellar travel.

Although farther beyond our current technology, wormholes could theoretically be the most effective form of interstellar travel yet conceived. The ability to travel to nearly any region of our universe (and possibly beyond) with very little travel time is difficult to imagine. The affects of such ability on science, society, and humanity in general would be staggering. As with other forms of advanced travel, wormhole technology could offer tremendous amounts of scientific understanding; scientists would have the ability to visit other worlds, stars, civilizations, and galaxies to study biology, chemistry, physics, astronomy, and any other field of science.

Even with our transportation problem solved, we still must face the problem of an energy supply that is immense enough to power our hypothesized methods of transportation, sufficient for a lengthy interstellar journey, and still capable of supporting whatever needs we have upon our arrival. There are many theories that could provide more abundant forms of energy, not only for interstellar travel, but also for all of our energy needs. Of these proposed energy sources, we will discuss three: "cold fusion", vacuum fluctuations, and matter-antimatter reactions.

The first form of energy, "cold fusion", is the fusion of lighter elements into heavier elements (hydrogen to helium, helium to carbon, etc.) without the extreme pressures and temperatures present in solar fusion. The mass of the initial elements is greater than the resulting product; the difference in mass is released as energy according to Einstein's equation, E = mc². As a result, the ability to utilize "cold fusion" could provide huge amounts of energy from small amounts of matter.

There is no doubt that fusion is possible and occurs every day in every star in the universe, but being able to control fusion in an environment closer to ours than to the sun's core remains a question. Despite multiple claims to the contrary, humans have yet to discover the elusive "cold fusion" power source. Fusion clearly releases energy, but with our current understanding of the process of fusion, we must commit more energy into causing atoms to fuse in a controlled reaction than we gain from the fusion reaction. Until we can find a way to fuse atoms without using large amounts of energy to do so, we must continue to search for other forms of energy.

One such possible energy alternative is the practical use of vacuum fluctuations. Vacuum fluctuations are waves of electromagnetic radiation that radiate from spacetime itself in all directions and from every region. Theoretically, this radiation can be used as a source of energy by what is known as the Casimir Effect. The Casimir Effect is a phenomenon where two closely spaced plates are pushed closer together by the vacuum fluctuations. These plates are pushed together because the pressure due to vacuum fluctuations is not equal between the plates and outside the plates. Since the electromagnetic spectrum is vast and because the plates are close together, only a portion of the vacuum fluctuations (from only part of the electromagnetic spectrum) can exist between the plates because no wavelength greater than the distance separating the plates can fit between these plates. If only short wavelengths of electromagnetic radiation can exist between the plates, then the pressure due to longer wavelengths of electromagnetic radiation is only pushing from the outside--thus pushing the plates together.

As the plates are pushed together, work is done on them by the radiation, so any method of manipulating this work into "useful" work will provide an energy supply. That is, if we can find a way to use the work done on the plates by vacuum fluctuations, then we can use this as a supply of energy. Since vacuum fluctuations are believed to exist nearly everywhere and have little or no limit in their source, they could provide a potentially enormous supply of energy.

It is clear that if the work done by Casimir Effect could be harnessed, we would have a vast supply of energy. However, we do not yet know if "useful" work can actually be gained from the Casimir Effect. This scientific anomaly may prove to be nothing more than a curiosity of our universe and the Laws of Physics. One key to making the Casimir Effect a source of energy is whether or not a positive amount of energy can be drawn from using vacuum fluctuations to push plates together, then pulling the plates apart and repeating the process. We may find that more energy is required to keep pulling the plates apart than we get out of them being pushed together by the radiation. If this were the case, then we would need to look to other supplies of energy.

The third potential energy resource that we may be able to make use of is matter-antimatter reactions. Antimatter is simply normal matter with the opposite charge of what we expect for that matter. For example, an "anti-electron" (called a positron) has the same mass as an electron, the same possible quantum states, and the same magnitude of charge; however, the charge is positive instead of negative. Antimatter is not limited to subatomic particles; each element has an antimatter partner, and consequently, every known form of matter has a corresponding antimatter partner (with equal mass, quantum states, etc., but opposite charge).

Antimatter does exist and can be created in laboratories today. However, we can not yet produce it efficiently enough (and on a large enough scale) to be able to use it for an energy source. Antimatter could provide a powerful source of energy, because when a matter-antimatter "pair" (electron and positron, proton and antiproton, etc.) react, they annihilate each other and release a tremendous amount of energy. Since the energy of a system is related to the mass of the system as defined by Einstein's famous relation of mass and energy, the energy released in the reaction is incredibly large for reactants of considerable mass.

One key today to utilizing this potential form of energy is being able to readily and efficiently produce antimatter. Using current methods (particle accelerator), more energy is used to produce a particle of antimatter than can be gained by the resulting antimatter particle.

Of these three energy possibilities, I believe that matter-antimatter reactions may prove to be the most promising. "Cold fusion" has been a mystery for decades and seems to be a dead end. Even if we are able to make use of "cold fusion", matter-antimatter reactions would still provide greater amounts of energy. Both processes function on the same principal--convert mass into energy. However, all fusion reactions only convert a small portion of the system mass into energy; as a result, the energy released is much less than that of a matter-antimatter reaction.

Vacuum fluctuations also could provide incredible sources of energy, but less is known about the required technology and understanding than is about matter-antimatter reactions. Because vacuum fluctuations do not destroy matter (and thus are not limited by the amount of matter available), they have the potential to provide the greatest amounts of energy. Unfortunately, it seems that matter-antimatter reactions are much closer to being within our reach as a practical form of energy (although much remains to be done regarding matter-antimatter reactions).

In summary, although the human race faces many challenges to accomplishing interstellar travel, three major technological challenges exist: travel speed, propellant limits, and an energy source. There are abundant theorized solutions to all of these issues, but we have considered only a few for each. The problems of travel speed and the limits imposed by the use of a propellant would both be solved by the use of an Alcubierre warp drive or a wormhole. The problem of a large enough energy supply could be resolved by "cold fusion", the Casimir Effect, or matter-antimatter reactions.

After examination of these possible solutions, an Alcubierre warp drive seems much closer to a practical means of travel than a wormhole. Likewise, it appears that of the three energy sources we have discussed, matter-antimatter reactions are the best combination of energy potential and probability success. We are left to wonder, could humans someday travel to the far reaches of the galaxy in an Alcubierre warp drive ship making use of matter-antimatter reactions?

Einstein, Albert. Relativity. Translated by Robert W Lawson. Random House: New

York, 1961.

Hawking, Stephen. A Brief History of Time. Edited by Peter Guzzardi. New York:

Bantam Books, 1996.

Herbert, Nick, Ph.D. Faster Than Light: Superluminal Loopholes in Physics. New York:

Penguin Books, 1988.

Kaku, Michio. Hyperspace. New York: Anchor Books, 1995.

Millis, Marc G. "Warp Drive When?" http://www.grc.nasa.gov/WWW/PAO/warp.htm.

Curator: David M. DeFelice. NASA Lewis Research Center, 1997.

Millis, Marc G. "Breakthrough Propulsion Physics Research Program."

http://www.lerc.nasa.gov/WWW/bpp/BPPWrkshp_STAIF_PrePrnt.htm. NASA Lewis

Research Center, 1998.

Serway, Raymond A. Physics for Scientists and Engineers. Harcourt Brace College

Publishers: Orlando, 1996.

Thorne, Kip S. Black Holes & Time Warps: Einstein's Outrageous Legacy. Edited by

Lewis Thomas, Ph.D. New York: W. W. Norton & Company, 1994.

Yenne, Bill. The Encyclopedia of US Spacecraft. Edited by Susan Garratt. Greenwich,

CT: Brompton Books, 1990.