The “engineering in medicine and biology” community pays lip service to mental problems; typically, the clinician “talks” to the patient with the aid of unglamorous equipment such as tape recorders. But many people suffer mental stress associated with the war on terrorism, and much of their anxiety can be traced to an irrational belief in unidentified flying objects (UFOs). Our job is not to treat mental problems with high-tech equipment, but to educate the public with the following message: “Yes, let’s keep on the lookout for suicide bombers, but we need not worry about UFOs.” It is a next-to-impossible task because the public does not read the scientific literature; nevertheless, here is my “point of view”:
The “intelligent layperson” is now being told that we may have to colonize our Milky Way galaxy. The implication is that UFOs are, after all, a realistic possibility. The following message comes, not from a religious group seeking immortality, but from a well-known scientific publication:
“Assuming a typical colony spacing of 10 light-years, a ship speed of 10 percent that of light, and a period of 400 years between the foundation of a colony and its sending out colonies of its own, the colonization wave front will expand at an average speed of 0.02 light-year a year. As the galaxy is 100,000 light-years across, it takes no more than about five million years to colonize it completely. Though a long time in human terms, this is only 0.05 percent of the age of the galaxy. Compared with the other relevant astronomical and biological time-scales, it is essentially instantaneous. The greatest uncertainty is the time required for a colony to establish itself and spawn new settlements. A reasonable upper limit might be 5000 years, the time it has taken human civilization to develop from the earliest cities to space-flight. In that case, full galactic colonization would take about 50 million years.”
(The above, reinforced by sensational illustrations, is taken from the publication.)
Scientists may view this as a betrayal because, since the first “sightings” of UFOs several decades ago, they have been pointing out, to those who will listen and are not emotionally captured by it, that interstellar travel is physically unrealistic. It is contrary to the most basic laws of physics, and is therefore an impossible dream. (Some would call it a nightmare — imagine populating the universe with a creature that is clever enough to plunder and devour everything on its own planet.)
Let’s look at a realistic scenario: The starship has just (somehow) escaped from the earth’s gravitational pull, and is ready to blast off for the nearest planet that can be colonized. Inside are six people (three Democrats and three Republicans, male and female couples, would be fine as a starter). How much does it weigh? Well, the Mars Lander is a hermetically-sealed vehicle (of necessity, of course), in which the crew would live for 260 days (that is how long it takes to get there, one-way). It has been carefully planned and designed in considerable detail, and will weigh some 130 metric tons.
Let’s take a wild guess and assume that the starship will therefore weigh, say, 200 tons (200,000 kilograms or 440,000 pounds). After all, it has to carry its own fuel (surprise!). But the best part is yet to fly: As the colonizers imply above, without any touch of humor, the nearest candidate planet is 10 light- years away; that is, it is so far away that light, traveling 186,000 miles per second, takes 10 years to get here. No problem at all; we simply travel at one-tenth the speed of light, 18,600 miles (or 30 million meters) per second, so the ship will take only 100 years to get there! Since this is a one-way trip, we only have to open the hatch very infrequently, to discard (delete is a more up-to-date expression) a crew member who has died of old age (if not of boredom). Now we know why galactic colonizers can’t have a sense of humor.
But this is just the beginning. The conventional fuel is liquid hydrogen and liquid oxygen, which accelerate a ship by means of jet propulsion (a high-speed exhaust) as they burn. The space-ship addicts insist that more exotic fuels can be used. A summary of various propulsion systems appears in an article by G. Musser and M. Alpert ; these are presented in Table 1 and briefly considered as follows:
A space vehicle has nothing to push against, in the vacuum outside the earth’s atmosphere, in order to accelerate. The vehicle has to push against its own high-speed exhaust jet that is generated from the fuel that it carries. The simple equation that governs this is that the change in speed of the vehicle, multiplied by its mass, is equal to the speed of the exhaust jet relative to the vehicle, multiplied by its mass. In other words, momentum change = momentum change.
(Vehicle minus fuel)/Fuel.Obviously, it is desirable to have a high jet exhaust speed, and also a high “(vehicle—fuel)/fuel” ratio.
Comments with regard to each row of Table 1 follow:
1. Variable Specific Impulse Magnetoplasma (1) Hydrogen is first ionized, then heated by a radio-frequency field to 10 million °C.
2. Pulsed Inductive The fuel is first ionized, then accelerated using a pulsed capacitive-inductive circuit.
3. Magnetoplasmadynamic The fuel is first ionized, then heated by a magnetic field.
4. Ion The fuel is first ionized, then accelerated by an electric field.
5. Hall Effect The fuel is first ionized, then accelerated using the “Hall” effect.
6. Nuclear Thermal A nuclear reactor heats hydrogen to over 2500°C.
7. Variable Specific Impulse Magnetoplasma (2) In the system of row 1, a choke is released, which allows the thrust to increase while reducing the jet exhaust speed.
8. Chemical This is the usual system, in which liquid hydrogen reacts with liquid oxygen.
How much fuel do we need to get up to a speed of 18,600 miles per second (and we need not worry about polluting the atmosphere)? I don’t have to answer that, because there is an easier way to look at it: Kinetic Energy. The phrase “kinetic energy” turns UFO people off; the reasoning section of their minds becomes vaporized, and they simply walk away. This is the part they don’t want to hear and/or cannot understand: One of the first and easiest equations in a physics course is that kinetic energy equals one-half mv squared. Plug 200,000 kilograms in for m and 30 million meters per second for v and — voila — we get around 1 followed by 20 zeros (watt-seconds). So what does that mean in down-to-earth terms? Well, the total capacity of the United States electrical power system is, very approximately, one trillion (1012) watts. If we could somehow use all of this trillion watts (or its equivalent in one of the propulsion systems of Table 1) to drive the starship, it would take 100 million seconds, or three years, to get up to speed! Negligible, of course, on a 100-year trip.
As I have said, scientists have been annoying UFO-believers with these astronomical figures for decades. Their response is something like “A superior intelligence could somehow get here; they have resources we don’t have.” Charles P. Snow was certainly correct about there being two cultures! Well, perhaps I exaggerated above; we could in principle recruit resources equivalent to that of the entire U.S. power grid and give the colonizers a 3-year shove into the galactic void. Or ten times as much power for a 100-day shove, and so forth.
The consensus of recent reports is that life may be commonplace, but intelligent civilizations are extremely rare. The best evidence for this is provided by the earth itself (and I don’t mean to deprecate our species), and its solar system. There are lots of planets in the galaxy, but intelligent life is restricted to a relatively narrow range of temperatures and planet ages. (For Earth, which formed 4.5 billion years ago, the ratio thus far for intelligent occupancy, versus the earth’s age, is minuscule.) A corollary to this is that the resources of a habitable planet would not be much different from those of Earth. Our present-day technology (which is quite sophisticated), cannot find any star and habitable-planet combination that is less than 10 light-years away (in miles, 6 followed by 13 zeros). The conclusion is that we definitely do not have any competitors within UFO striking distance.
The interstellar travel article comes from the “pen” of Ian Crawford in the July 2000 issue of Scientific American . Also consider this gem: “If we find no evidence for other technological civilizations, it may become our destiny to embark on the exploration and colonization of the galaxy.” The spirit here is reminiscent of England’s destiny to colonize the rest of the world. Why was this published? Perhaps the editors couldn’t resist some sensationalism in the hope of attracting new subscribers. Perhaps the editors were ignorant of the huge amount of kinetic energy involved. They know about kinetic energy = mv2/2, of course, but I’ll bet that they never plugged numerical values into the equation to discover interstellar nonsense.
If scientists are befuddled, is there any wonder that our youth turn to creationism, extrasensory perception, astrology, ectoplasm, and the like?
If we do contact an extraterrestrial civilization, what will we tell them? They will undoubtedly believe that some kind of deity designed the Universe. One message we can send is that our deity is the true God, and that their idol is a phony; this will serve as a crash Introductory Course on Earthlings. [This had better be a powerful, irresistible endeavor; since we may have to wait 20 years round-trip for a reply, at a cost of $100 million a minute to Consolidated Edison (much less if a highly directional antenna can be used), time will be of the essence.] According to recent news reports, the John Templeton Foundation will financially support the sending of religious messages to any extraterrestrial intelligence that shows up.
But there is still some important unfinished business: The colonization proposal is closely tied to Search for Extraterrestrial Intelligence (SETI), as the above quote implies. The search is conducted by many enthusiastic hopefuls listening to galactic “voices,” at appropriate frequencies, using sensitive receivers. We are looking for Morse-code-like signals that could only be sent by an extraterrestrial civilization (rather than a cellular-phone signal sent by somebody walking down Broadway). We are certain that there are thousands of intelligent enclaves out there but, after several decades of listening, nothing that makes sense has been heard. The listeners are very disheartened. Why don’t we hear from somebody out there? One reason is very simple: If every extraterrestrial is passive, like we are, then nobody is sending any messages. Let’s send messages; if “somebody” picks them up, he/she/it may respond. The round-trip may take 20 years, but the sooner we get started, the sooner the response.
Alas, insufficient energy also dooms SETI. As George W. Swenson, Jr. beautifully explains, also in the July 2000 issue of Scientific American , the signal we send has to be stronger than the electrical noise at the receiving station. A 100-light-year reach requires a transmitter power that is “… more than 7000 times the total electricity-generating capacity of the U.S.” In other words, “they” may possibly be transmitting to us but, with only a locally-reasonable amount of transmitter power, the message is lost in the vastness of interstellar space. We feel sorry that Carl Sagan died prematurely, but it appears that his favorite dream will never be realized.
A four-million-watt transmitter will make our presence known out to a distance of one light day; if we get an answer, we can communicate with a sharp beam at much less power.
Figure 1 is a plot of transmitter power needed versus distance, using Swenson’s figures. It dramatically illustrates the discouraging facts, from a distance of 100 light-years to a next-door neighbor located only 1 light-day away. Because power spreads out as the square of distance, the straight line of Fig. 1 has a slope of 2. At the upper-right end is a dot representing Swenson’s value of 5.8 X 1015 watts necessary to overcome noise 100 light-years away. At the lower-left we have a dot that corresponds to the 4,400,000 watts needed for a distance of only one light-day. That is doable because, if we ever locate that neighbor, we can greatly reduce the power requirement by using a highly directional antenna beam.
Fig. 1- Transmitter power, versus distance, needed to overcome electrical noise at the receiver. The dot at the upper-right end represents 5.8 X 1015 watts at a distance of 100 light-years. The dot at the lower-left corresponds to 4,400,000 watts at a distance of 1 light-day.
At one time (1961), radio astronomer Frank Drake came up with a simple formula for estimating the number of civilizations in the Milky Way, N, that could be as technically sophisticated as we are. Michael Shermer wrote a column about the Drake equation, “Why ET Hasn’t Called” . In my opinion, discussions of Drake’s equation are interesting, but useless, mental gyrations because the answer is N = zero insofar as we on Earth are concerned. We can’t prove any conjectures regarding N for any other planet.
Some civilizations are undoubtedly lucky enough to be within one light day of each other. Just think of all the excitement that would cause if it happened here: The New York Times with a special section devoted to All the News That’s Fit to Print from Outer Space.
My conjecture, that the editors of Scientific American were not aware of the impossibility of interplanetary travel, is not too far-fetched. For example, a book has come out written by a physicist, Stephen Webb, titled If the Universe is Teeming with Aliens … Where is Everybody? The subtitle is Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life . “The Fermi paradox” refers to a question that Enrico Fermi asked in 1950: “Where is everybody?,” meaning “Where are the extraterrestrials?” Now we know that there are two parts to the answer: 1)Unidentified Flying Objects always were and always will be physically impractical, and 2)For Earth, at least, any scheme for communicating with extraterrestrials is, also, physically impractical. But I suppose that it is more fun to write a 300-page book about it.
To summarize: The message to SETI people is: Keep listening, but don’t expect to ever hear anything intelligent. The message to UFO-watchers is: There is not the slightest chance that a starship will ever make it to Earth; but if it does, after traveling for 100 years, the crew would be most appreciative of a cup of coffee and a hot bath.
With regard to the kinetic energy (KE = mv2/2) of a space ship brought up to 10% of the speed of light, substituting m = 200,000 kg and v = 30 X 106 m/s, we get
KE = (2 X 105)(3 X 107)2/2 = 9 X 1019 joules.
Since a joule is equal to a watt-second, we end up with approximately 1020 watt-seconds. This is equal to one trillion watts applied for 108 seconds. But
108/(60 X 60 X 24) = 1157 days = 3.17 years.
To convert light-years into meters: The velocity of light is 2.998 X 108 m/s, so
2.998 X 108 X 60 X 60 X 24 X 365 = 9.46 X 1015 meters.
Ten light-years = 9.46 X 1016 meters = 5.88 X 1013 miles.
The power needed at a receiver to equal electrical noise is given by
P = kTBA
where P = power, watts,
k = Boltzmann’s constant = 1.38 X 10-23 joules/kelvin,
T = noise temperature, kelvins
B = receiver bandwidth, hertz,
A = effective area of receiving antenna, square meters.
Swenson  made three very reasonable assumptions: The noise at the receiver corresponds to a temperature of 15°K; the bandwidth of the received signal is reduced to 2.5 Hz; and the area of the receiving antenna is one square meter. Then
P = (1.38 X 10-23)(15)(2.5)(1) = 5.18 X 10-22 watt.
This extremely small value is the secret behind the tremendous success of wireless communication. Nevertheless, it cannot overcome a distance of 100 light-years: Assuming that we have to search in all directions, so the antenna is omnidirectional, the power is spread out over the surface of a sphere whose area is 4πr2. For r = 100 light years = 9.46 X 1017 m, the area is 1.125 X 1037 m2. To get 5.18 X 10-22 watt at the receiver, then, we have to transmit
P = (1.125 X 1037)(5.18 X 10-22) = 5.83 X 1015 watt. This is the value indicated in Fig. 1.
To “talk” to an organism 10 light-years away, we have to transmit, according to Fig. 1, 5.83 X 1013 watts. At 10 cents/kilowatt-hour, to talk for one minute, the bill would be
5.83 X 1013/(1000)(60)(10) = 100 million dollars.
To talk to an organism one light-day away: The distance is
2.998 X 108 X 60 X 60 X 24 = 2.59 X 1013 m.
The “sphere” has a surface area of
4π(2.59 X 1013)2 = 8.43 X 1027 m2, so that we need
P = (5.18 X 10-22)(8.43 X 1027) = 4,400,000 watts, as indicated in Fig. 1.
 George Musser & Mark Alpert, “How To Go To Mars,” Scientific American, March 2000.
 Ian Crawford, “Where Are They?,” Scientific American, July 2000.
 George W. Swenson, Jr., “Intragalactically Speaking,” Scientific American, July 2000.
 Michael Shermer, “Why ET Hasn’t Called,” Scientific American, Aug 2002.
 Stephen Webb, If the Universe is Teeming with Aliens … Where is Everybody?, Copernicus (Springer-Verlag), 2002.
(A shorter version of this essay was published in IEEE Engineering in Medicine and Biology Magazine, Jan/Feb 2003.)