Is Interstellar Travel "Preposterous"?

It occurs to me that a solid statement of the case against the feasibility of interstellar travel is not easily available, and hence is not well-known to the public. Following on my recent posting Is There a Warp Drive in your Future?, which considers the question of what technologies are or are not likely to exist in the future, let us now examine the general question of the feasibility of interstellar travel. In this inquiry, we are not concerned with technological difficulties or breakthroughs, but with fundamental laws of physics. Even if the only limits we faced were those of physics, not technology, what are the prospects of making interstellar travel a reality?

Stanton Friedman, the “Flying Saucer Physicist,” is confident that interstellar travel is not only possible, but likely. In his essay UFO Propulsion Systems, Friedman writes,

a one-way trip of thirty-seven years (the distance to Zeta 1 or 2 Reticuli) at 99.9 percent c would take only twenty months’ crew time; at 99.99 percent c it would take only six months’ crew time. Thus even a trip to a distant galaxy such as Andromeda, two million light-years away, would take under sixty years’ crew time if the intergalactic ship somehow could manage to keep accelerating at one G, using some yet unknown technique.      

various proposals for fusion-powered rockets

Ah, that pesky little “yet unknown technique.” Now this is all perfectly true, but it blithely ignores some very fundamental problems that are not related to any level of technology. A trio of “classic” papers written in the 1960s by physicists examine the fundamental physics involved in proposed interstellar travel, and explain the formidable obstacles: obstacles imposed by fundamental laws of physics, not by limits of technology. Note that nothing here rules out the possibility of travel within our solar system, even to its edges, or rules out non-relativistic interstellar travel, taking thousands of years to reach one's destination. But the notion that we will someday travel between stars the way we now sail between seaports is pure fantasy.

These articles sufficed to convince the scientific community that the concept of interstellar travel is utterly implausible, and explanations for UFO sightings must be sought elsewhere, in psychology and sociology, not in physics. However, in recent years these articles have largely been overlooked, so I think it’s very important to examine each one in some detail and explain its consequences. 

            1. Radioastronomy and Communication Through Space by Edward M. Purcell. (U.S. Atomic Energy Commission Report BNL-658, reprinted in Cameron, A.G.W. (editor), Interstellar Communication. New York: W.A. Benjamin, Inc., 1963.) Purcell (1912-1997) was in the physics department at Harvard University, and shared in the 1952 Nobel Prize for physics. He was a pioneer in radio astronomy, the first to detect the famous 21-cm radio emission line from neutral hydrogen in the galaxy. He also is credited with the discovery of Nuclear Magnetic Resonance.

Edward M. Purcell

            Most of the paper is uncontroversial and explains then-recent discoveries in radio astronomy. But in the section titled Space Travel, Purcell examines claims that someday we will travel to the stars at almost the speed of light. “The performance of a rocket depends almost entirely on the velocity with which the propellant is exhausted,” he notes. Thus, “the elementary laws of mechanics – in this case relativistic mechanics, but still the elementary laws of mechanics – inexorably impose a certain relation between the initial mass and the final mass of the rocket in the ideal case… It follows very simply from conservation of momentum and energy, the mass-energy relation, and nothing else.” (Emphasis in original.)

            “For our vehicle we shall clearly want a propellant with a very high exhaust velocity. Putting all practical questions aside, I propose, in my first design, to use the ideal nuclear fusion propellant… I am going to burn hydrogen to helium with 100 percent efficiency; by means unspecified I shall throw the helium out the back with kinetic energy, as seen from the rocket, equivalent to the entire mass change. You can’t beat that, with fusion. One can easily work out the exhaust velocity; it is about 1/8 the velocity of light. The equation of Figure 13 tells us that to attain a speed 0.99c we need an initial mass which is a little over a billion times the final mass.”

            A billion times the final mass?????!!!!!!! In fact, the exact figure is 1.6 X 10^^9. So in the ideal case, where you had somehow mastered nuclear fusion with 100% efficiency and could control and direct the energy in whatever way you choose, you still will need 1.6 billion tons of fuel for each ton of payload! Surely, such a rocket has never been built, and never will be built, in our solar system, or any other. Thus Purcell has demonstrated, beyond any possibility of doubt, that all proposals to reach near-light speed using nuclear fusion propulsion are complete absurdity.

            But supposing some other, more energetic reaction could be found? Nuclear fission produces an even lower exhaust velocity than fusion, so it’s less plausible still. Is there any reaction more energetic than nuclear fusion? “This is no place for timidity, so let us take the ultimate step and switch to the perfect matter-antimatter propellant…. The resulting energy leaves our rocket with an exhaust velocity of c or thereabouts. This makes the situation very much better. To get up to 99 percent the velocity of light only a ratio of 14 is needed between the initial mass and the final mass.” That sounds very much better. If I can “somehow” procure sufficient antimatter, “somehow” store it, and “somehow” control its reaction with matter, and “somehow” direct the resulting energy where I want it to go, I need only 7 tons of matter, and 7 tons of antimatter for each ton of payload. That sounds almost possible. But Purcell points out that all that buys you is a one-way ticket out of the galaxy: you have no way to slow down and stop when you get where you want to go. So to stop when you reach your destination requires a fuel-to-payload ratio of 196. And if you want to someday return, unless you know of a convenient matter-antimatter fueling station at your destination, you will need to square that again, for a fuel-to-mass ration of almost 40,000.

            And even if you could “somehow” construct such a vehicle, your problems are not over. “If you are moving with 99 per cent the velocity of light through our galaxy, which contains one hydrogen atom per cubic centimeter even in the ‘empty spaces,” each of these hydrogen atoms looks to you like a 6-billion-volt proton, and they are coming at you with a current which is roughly equivalent to 300 cosmotrons per square meter. So you have a minor shielding problem to get over before you start working on the shielding problem connected with the rocket engine.” Also, “In order to achieve the required acceleration our rocket, near the beginning of its journey will have to radiate about 10^^18 watts. This is a little more than the total power the earth receives from the sun. But this isn’t sunshine, it’s gamma rays. So the problem is not to shield the payload, the problem is to shield the earth.”

            “Well, this is preposterous, you are saying. That is exactly my point. It is preposterous. And remember, our conclusions are forced on us by the elementary laws of mechanics.” Nothing else needs to be written about the possibility of relativistic travel – Dr. Purcell has shown it to be completely preposterous. Purcell concludes his paper, however, by demonstrating that interstellar communication using radio waves is perfectly possible. His final words are, “All this stuff about traveling around the universe in space suits – except for local exploration, which I have not discussed – belongs back where it came from, on the cereal box.”

Sebastian von Hoerner

            2.The General Limits of Space Travel by Sebastian von Hoerner (Science 137, 18, 1962; reprinted in Cameron 1963). Immediately following Purcell’s paper in the Cameron volume is this related paper by von Hoerner (1919-2003), a German radio astronomer who was influential in early discussions and proposals for SETI. He examines the physical difficulties of propulsion for space travel, including possibilities not covered by Purcell. Von Hoerner considers ion thrust propulsion, but concludes that “nuclear reactors and all the equipment needed to give a strong ion thrust are so complicated and massive, as compared with the relatively simple combustion equipment, that there is no hope at present of reaching, with reactors, the value of P [engine power to mass ratio] already attained with combustion rockets.” He also considers proposals for a huge “scoop” or funnel for a rocket to fuel itself as it goes along, scooping up galactic hydrogen. But he notes that interstellar matter has very low density, and “in order to collect 1000 tons of matter (10 times the fuel of one Atlas rocket) on a trip to a goal 5.6 parsecs away, one would need a funnel 100 km in diameter; we will rule out this possibility.”

            After several pages of equations covering much the same ground as Purcell, Von Hoerner concludes, “there is no way of avoiding these demands [for power], and definitely no hope of fulfilling them…space travel, even in the most distant future, will be confined completely to our own planetary system, and a similar conclusion will hold for any other civilization, no matter how advanced it may be. The only means of communication between different civilizations thus seems to be electro-magnetic signals.”

 

William Markowitz

 3.Physics and Metaphysics of Unidentified Flying Objects by William Markowitz (Science 157, 1274, 1967). Markowitz (1907-1998) was an Austrian-born astronomer who worked at the U.S. Naval Observatory, and also taught astronomy and physics at Pennsylvania State University and Marquette University. He was a pioneer in the use of atomic clocks for astronomy, and specialized in precision time measurement issues. Markowitz wrote, “Aristotle wrote on natural phenomena under the heading ‘physics’ and continued with another section called ‘metaphysics’ or ‘beyond physics.’ I use a similar approach here. First I consider the physics of UFO’s when the laws of physics are obeyed. After that I consider the case where the laws of physics are not obeyed. The specific question to be studied is whether UFO’s are under extraterrestrial control.” By the laws of physics, he is concerned with only the simplest and best-known ones, like those of motion, gravitation, conservation of energy, and the restrictions of special relativity. He points out an obvious but seldom-noted problem: “Apart from propeller and balloon action, a spacecraft can generate thrust only by expelling mass.” And something that uses propellers or balloons is an aircraft, not a spacecraft.

            UFOs are sometimes reported to land, and take off again. “If an extraterrestrial spacecraft is to land nondestructively and then lift off, it must be able to develop a thrust slightly less than its weight on landing… if nuclear energy is used to generate thrust, then searing of the ground at 85,000 deg C should result, and nuclear decay production equivalent in quantity to those produced by an atomic bomb should be detected. This has not happened. Hence, the published reports of landing and lift-offs of UFO’s are not reports of spacecraft controlled by extraterrestrial beings, if the laws of physics are valid.”

            “We can reconcile UFO reports with extraterrestrial control by assigning various magic properties to extraterrestrial beings. These include ‘teleportation’ (the instantaneous movement of material bodies between planets and stars), the creation of ‘force-fields’ to drive space ships, and propulsion without reaction. The last of these would permit a man to lift himself by his bootstraps. Anyone who wishes is free to accept such magic properties, but I cannot.”

            To those who were following the controversy at that time over the proposal championed by J. Allen Hynek and Jacques Vallee for a “scientific study of UFOs,” an ‘ulterior motive’ for the Markowitz article was immediately apparent. The previous year Hynek had a letter published in Science, arguing that UFOs were worthy of scientific study (Science 154, 329, 1966). Markowitz carefully notes several instances where Hynek and his colleagues were contradicting themselves in their statements about UFOs. For example, in his letter in Science, Hynek wrote, “Some of the very best, most coherent reports have come from scientifically trained people.” But Markowitz noted that Hynek had written quite the opposite in his article in the Encyclopedia Britannica in 1964: “It appears unreasonable that spacecraft should announce themselves to casual observers while craftily avoiding detection by trained observers.” Markowitz further noted that Vallee’s 1966 book Challenge to Science presents the “classic” 1948 sighting of pilots Chiles and Whitted, who reported a dramatic close encounter with a huge metallic object while flying a DC-3; “the book fails to mention that Hynek had identified the object as an undoubted meteor in his report of 30 April 1949 to the Air Force… This omission is curious because Hynek wrote a foreword to Challenge to Science.” These and other self-contradictions, carefully noted by Markowitz, showed that the Hynek/Vallee case for the UFO was utterly lacking in intellectual rigor. Markowitz unmasked the real Hynek: disorganized, indecisive, and confused. This revelation, published in the peer-reviewed pages of Science, was fatal to the credibility of Hynek’s proposed “scientific study of UFOs.” There were, and still are, a few scientists who took Hynek’s UFO theorizing seriously, but they have always been a tiny minority.

                                

What About “Wormholes”?

             Some theorists of interstellar travel are quite aware of the extreme difficulties involved in actually traveling to interstellar destinations, in the sense of going from Point A to Point B. So they hypothesize easier ways to reach interstellar destinations, without the pesky problem of traversing every point between them. Maybe we can warp space so that the distance between earth and the Andromeda galaxy is not two million light years, as in ordinary space travel, but far, far less? Suppose there is a wormhole with one end where we now are, and the other where we want to go?

            The “Bohemian physicist” Jack Sarfatti of San Francisco is a colorful figure. He has written papers claiming that wormholes can be used not only to travel through space, but through time as well. (He has also studied, and promoted, Uri Geller.) He suggests that UFOs are real, and travel through wormholes to reach us from some other place or time.

Unfortunately for Sarfatti, according to Wikipedia,

 Wormholes which could actually be crossed, known as traversable wormholes, would only be possible if exotic matter with negative energy density could be used to stabilize them. (Many physicists such as Stephen Hawking, Kip Thorne, and others believe that the Casimir effect is evidence that negative energy densities are possible in nature.) Physicists have not found any natural process which would be predicted to form a wormhole naturally in the context of general relativity, although the quantum foam hypothesis is sometimes used to suggest that tiny wormholes might appear and disappear spontaneously at the Planck scale, and stable versions of such wormholes have been suggested as dark matter candidates. It has also been proposed that if a tiny wormhole held open by a negative-mass cosmic string had appeared around the time of the Big Bang, it could have been inflated to macroscopic size by cosmic inflation.

supposed travel through a wormhole

So yes, a wormhole is something that might theoretically exist, although their actual existence is frankly extremely dubious. There is no reason to think that they could occur naturally, and no observational evidence that they actually do exist (unlike Black Holes). Even if they do exist, they may exist only on the Planck scale (subatomic quantum size). It seems extremely dubious that traversable wormholes exist in nature, and even if they do, we still have seemingly insurmountable problems. How do we find wormholes? How do we determine whether they are stable? How do we know where their destination is? If we go into one, is it possible to return? There is also the problem of simply getting to the wormhole’s mouth. If a wormhole were near our solar system, we would already detect its disturbing effects of warped space. And if it is far from our solar system, we need to develop interstellar travel simply to travel to the wormhole’s mouth!

            Can we create a wormhole to go from where we are to where we want to be? Perhaps in theory we might, but the reality of a recipe for creating a wormhole will undoubtedly be something like this:

Take 100 solar masses. Bake at one million degrees for ten thousand years. Stir in 100 solar masses of exotic matter with negative energy density. Stretch out the mix from desired source to destination. Let cool for one million years.

             So the idea of using wormholes as a convenient transportation network to wherever in the universe we want to go is, well, fanciful and implausible in the extreme. We can’t proclaim it completely “impossible,” but the person who proclaims it as a reality had better have extraordinarily good evidence that such a thing exists.