Summary: The ideal interstellar propulsion system would be one that could get you to other stars as quickly and comfortably as envisioned in science fiction. Before this can become a reality, two scientific breakthroughs are needed: discovery of a means to exceed light speed, and discovery of a means to manipulate the coupling between mass and spacetime. This article explains why these breakthroughs are needed and introduces the emerging possibilities that may eventually lead to these breakthroughs.
Originally published in the Interstellar Propulsion Society Newsletter, Vol. I, No. 1, July 1, 1995.
Marc G. Millis
Space Propulsion Technology Division
NASA Lewis Research Center
The ideal interstellar propulsion system would be one that could get you to other stars as quickly and comfortably as envisioned in science fiction. Before this can become a reality, two scientific breakthroughs are needed: discovery of a means to exceed light speed, and discovery of a means to manipulate the coupling between mass and spacetime. This article explains why these breakthroughs are needed and introduces the emerging possibilities that may eventually lead to these breakthroughs. It should be noted that either of these breakthroughs by itself would have revolutionary consequences which would be of enormous value.
The need to exceed light speed: Simply put, the universe is big. The fastest thing known is light, yet it takes over four years for light to reach our nearest neighboring star. When NASA's Voyager spacecraft left our solar system is was traveling around 37- thousand mph. At that rate it couldn't reach the nearest star until after 80-thousand years. If we want to cruise to other stars within comfortable time spans (say, less than a term in Congress), we have to figure out a way to go faster than light.
The need to manipulate mass and spacetime coupling: This need is less obvious than the light speed issue. The problem is fuel, or more specifically, rocket propellant. Unlike a car that has the road to push against, or an airplane that has the air to push against, rockets don't have roads or air in space. Rockets have to carry along all the mass that they'll need to push against. To circumvent this problem, we need to find a way to interact with spacetime itself to induce propulsive forces without using propellant. This implies that we'll need to find a way to alter a vehicle's inertia, its gravitational field, or its connectivity to the structure of spacetime itself.
Just how limited are rockets for interstellar travel? Although rockets are reasonable for journeys into orbit or to the moon, they become unreasonable for interstellar travel. If you want to deliver a modest size payload, say a full Shuttle cargo (20,000 kg), and you are patient enough to wait 900 years for it to just fly by the nearest star, here's how much propellant you'll need: If you use a rocket like on the Shuttle (Isp~ 500s), there isn't enough mass in the universe to get you there. If you use a nuclear fission rocket (Isp~ 5,000s) you need about a billion super-tankers of propellant. If you use a nuclear fusion rocket (Isp~ 10,000s) you only need about a thousand super-tankers. And if you assume that you'll have a super-duper Ion or Antimatter rocket (Isp~ 50,000s), well now you only need about ten railway tankers. It gets even worse if you want to get there sooner. (Based on mass fractions from ref 1, p. 52)
There are other ideas, like using laser pushed light-sails that don't need propellant, but these have limitations. The biggest limitation is their dependence on the laser that remains near Earth. To make an unplanned course change they need to radio back for the laser to track their new course and wait for it to do so. At interstellar distances this is prohibitive. At one light-year from Earth, for example, it would take two years for the command to be sent and the new pointing received.
Is there hope?: Science continues to advance. In addition to the continuing refinements of general relativity and other attempts to better understand mass, space, and time, there have been some recently published theories which provide new perspectives, theories which have been reported in various news articles (Refs 2-6). Each of these theories has some relevance to propulsion and presents new avenues from which to start searching for the breakthrough physics. These recent theories are summarized next.
A theory about "warp drive": Using the formalism of general relativity, it has been shown that faster than light travel may be possible (ref 7). All you need to do is contract spacetime in front of your ship and expand spacetime behind your ship. This "warped" space and the region within it would propel itself "with an arbitrarily large speed" (ref 7). Observers outside this "warp" would see it move faster than the speed of light. Observers inside this "warp" would feel no acceleration as they zip along at warp speed.
So what's the catch? First, to expand spacetime behind the ship you'll need matter having a negative energy density like negative mass, and lots of it too. It is unknown in physics whether negative mass or negative energy densities can exist. Classical physics tends toward a "no," while quantum physics leans to a "maybe, yes." Second, you'll need equal amounts of positive energy density matter, positive mass, to contract spacetime in front of the ship. Third, you'll need a way to control this effect to turn it on and off at will. And lastly, there is the debate about whether this whole "warp" would indeed move faster than the speed of light. To address this speeding issue, the theory draws on the "inflationary universe" perspective. The idea goes something like this: Even though light-speed is a limit within spacetime, the rate at which spacetime itself can expand or contract is an open issue. Back during the early moments of the Big Bang, spacetime expands faster than the speed of light. So if spacetime can expand faster than the speed of light during the Big Bang, why not for our warp drive?
Just prior to the publication of the above theory, there was a workshop held at JPL to examine the possibilities for faster-than-light travel (ref 8). Wormholes, tachyons, and alternate dimensions were just some of the topics examined. The conclusions from this informal two-day workshop are as follows:
(1) Faster-than-light travel is beyond our current horizons. Not only is the physics inadequately developed, but this physics is not oriented toward space propulsion or toward laboratory scale experiments.
(2) Causality violations (where effect precedes cause) are unavoidable if faster-than-light travel is possible, but it is uncertain whether causality violations are themselves physically prohibited.
(3) A few experimental approaches are feasible to address the science associated with faster- than-light travel, including:
(b) Measure the velocity of light inside a Casimir cavity (between closely spaced conductive plates) to search for evidence of negative space energy. This pertains to wormholes, tachyons, and the negative energy density issue.
(c) Resolve the rest mass issue of the Neutrino, determining whether the unconfirmed experimental evidence of imaginary mass is genuine.
(d) Study cosmic rays above the atmosphere, using scattering targets of know composition to look for characteristic evidence of tachyons and more general particle physics events.
New ways to think of inertia and gravity: As mentioned earlier, the ideal interstellar drive would have the ability to manipulate the connection between mass and spacetime. One approach is to look for ways to use electromagnetism, a phenomenon for which we are technologically proficient, to control inertial or gravitational forces. It is known that gravity and electromagnetism are coupled phenomena. In the formalism of general relativity this coupling is described in terms of how mass warps the spacetime against which electromagnetism is measured. In simple terms this has the consequence that gravity appears to bend light, red-shift light and slow time. These observations and the general relativistic formalism that describes them have been confirmed (ref 9, 10). Although gravity's affects on electromagnetism have been confirmed, the possibility of the reverse, of using electromagnetism to affect gravity, is unknown.
New perspectives on the connection between gravity and electromagnetism have just emerged. A theory published in February 1994 (ref 11) suggests that inertia is nothing but an electromagnetic illusion. This theory builds on an earlier work (ref 12) that asserts that gravity is nothing other than an electromagnetic side-effect. Both of these works rely on the perspective that all matter is fundamentally made up of electrically charged particles, and they rely on the existence of Zero Point Energy.
Zero Point Energy (ZPE) is the term used to describe the random electromagnetic oscillations that are left in a vacuum after all other energy has been removed (ref 13). This can be explained in terms of quantum theory, where there exists energy even in the absolute lowest state of a harmonic oscillator. The lowest state of an electromagnetic oscillation is equal to one-half the Planck constant times the frequency. If all the energy for all the possible frequencies is summed up, the result is an enormous energy density, ranging from 1036 to 1070 Joules/m3. In simplistic terms there is enough energy in a cubic centimeter of the empty vacuum to boil away Earth's oceans. First predicted in 1948, ZPE has been linked to a number of experimental observations. Examples include the Casimir effect (ref 14), Van der Waal forces (ref 15), the Lamb-Retherford Shift (ref 10, p. 427), explanations of the Planck blackbody radiation spectrum (ref 16), the stability of the ground state of the hydrogen atom from radiative collapse (ref 17), and the effect of cavities to inhibit or enhance the spontaneous emission from excited atoms (ref 18).
Regarding the inertia and gravity theories mentioned earlier, they take the perspective that all matter is fundamentally constructed of electrically charged particles and that these particles are constantly interacting with this ZPE background. From this perspective the property of inertia, the resistance to change of a particle's velocity, is described as a high- frequency electromagnetic drag against the Zero Point Fluctuations. Gravity, the attraction between masses, is described as Van der Waals forces between oscillating dipoles, where these dipoles are the charged particles that have been set into oscillation by the ZPE background.
It should be noted that these theories were not written in the context of propulsion and do not yet provide direct clues for how to electromagnetically manipulate inertia or gravity. Also, these theories are still too new to have either been confirmed or discounted. Despite these uncertainties, typical of any fledgling theory, these theories do provide new approaches to search for breakthrough propulsion physics. Their utility and correctness remains to be determined.
Another viewpoint on gravity and spacetime: As mentioned earlier, the ideal interstellar drive must not use propellant. Instead the ideal drive would have to use some means to push against spacetime itself. One of the major objections to this notion is the issue of conservation of momentum (ref 19). In order to satisfy conservation of momentum, something must act as a reaction mass. For rockets it is the expelled propellant; for aircraft it is the air. If one considers propelling against spacetime itself, then one must entertain the possibility that the fields of spacetime have an energy or momentum that can serve as a reaction mass. Although existing physics does not provide this perspective, a recent theory has emerged that might. A news article published in December 94 (ref 6) introduced a theory (ref 20) that is challenging Einstein's general theory of relativity. The theory is generating a bit of controversy because it claims that the Einstein field equations need a slight correction. Without this correction it is claimed that the Einstein equations can only predict the behavior of simple one-body problems (where only one gravitating mass exists whose affect on an inconsequential test particle is described). For two-body or n-body problems, this new theory shows that the Einstein equations are inadequate. The required correction is that another term must be added to the matter tensor, specifically a term for the stress-energy tensor of the gravitational field itself. This suggests that gravitational fields have an energy and momentum of their own. This may be a foundation to address the issue of a reaction mass for the ideal space drive.
Like the previously mentioned theories, it is uncertain whether this theory is correct or not, but it is certain that this theory adds yet another research path to search for breakthrough propulsion.
But wait, there's more: Another avenue to explore pushing against space is to examine the contents of the vacuum that may be indicative of a reaction mass. In addition to the items mentioned above, consider the following phenomena: Cosmic Background Radiation (ref 21), Virtual Pair Production (ref 22), and Dark Matter (ref 23). Whether any of these may constitute a reaction mass or may be evidence for a reaction mass is uncertain.
In addition to these recent events, there have been occasional surveys by the Air Force and others to examine science that may be applicable to propulsion technology (refs 24-29). The options identified by these studies include assessments of the technological status of many popular ideas, such as light-sails, nuclear rockets, and antimatter rockets, plus they include mention of more speculative work. Many of the more speculative ideas, from alternative theories of gravity and electromagnetism through unconfirmed anomalous effects, would be relativity simple to test. Very few of these possibilities have been rigorously investigated.
As you can see, there are a number of dangling loose ends in physics that may prove to be fruitful paths to the goal of creating the breakthroughs for practical interstellar travel. Pick your favorite idea and let us know what you discover.
Note: An annotated bibligoraphy is available.
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