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?
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 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.
