Unfortunately, things are not as nice as they seem. There is a much bigger piece of science fiction that must be introduced before the McAndrew drive can exist as a useful device. We look at that next, and note that it is a central concern of the third chronicle.
Suppose that the drive mechanism is the most efficient one consistent with today’s physics. This would be a photon drive, in which any fuel is completely converted to radiation and used to propel the ship. There is certainly nothing in present science that suggests such a drive is theoretically impossible, and some analysis of matter-antimatter reactions indicates that the photon drive could one day be built. Let us assume that we know how to construct it. Then, even with this “ultimate” drive, McAndrew’s ship would have problems. It’s not difficult to calculate that with a fifty gee drive, the conversion of matter to radiation needed to keep the drive going will quickly consume the ship’s own mass. More than half the mass will be gone in a few days, and McAndrew’s ship will disappear from under him.
Solution of this problem calls for a lot more fictional science than the simple task of producing stable condensed matter. We have to go back to present physics and look for loopholes. We need to find inconsistencies in the overall picture of the Universe provided by present day physics, and exploit these as necessary.
The best place to seek inconsistencies is where we already know we will find them — in the meeting of general relativity and quantum theory. If we calculate the energy associated with an absence of matter in quantum theory, the “vacuum state,” we do not, as common sense would suggest, get zero.
Instead we get a large, positive value per unit volume. In classical thinking, one could argue that the zero point of energy is arbitrary, so that one can simply start measuring energies from the vacuum state value. But if we accept general relativity, this option is denied to us. Energy, of any form, produces space-time curvature. We are therefore not allowed to change the definition of the origin of the energy scale. Once this is accepted, the energy of the vacuum state cannot be talked out of existence. It is real, if elusive, and its presence provides the loophole that we need.
Again, we are at the point where the science fiction enters. If the vacuum state has an energy associated with it, I assume that this energy is capable of being tapped. Doesn’t this then, according to relativity (E =mc2), suggest that there is also mass associated with the vacuum, contrary to what we think of as vacuum? Yes, it does, and I’m sorry about that, but the paradox is not of my creation. It is implicit in the contradictions that arise as soon as you try to put general relativity and quantum theory together.
Richard Feynman, one of the founders of quantum electrodynamics, addressed the question of the vacuum energy, and computed an estimate for the equivalent mass per unit volume. The estimate came out to two billion tons per cubic centimeter. The energy in two billion tons of matter is more than enough to boil all Earth’s oceans (powerful stuff, vacuum). Feynman, commenting on his vacuum energy estimate, remarks:
“Such a mass density would, at first sight at least, be expected to produce very large gravitational effects which are not observed. It is possible that we are calculating in a naive manner, and, if all of the consequences of the general theory of relativity (such as the gravitational effects produced by the large stresses implied here) were included, the effects might cancel out; but nobody has worked all this out yet. It is possible that some cutoff procedure that not only yields a finite energy for the vacuum state but also provides relativistic invariance may be found. The implications of such a result are at present completely unknown.”
With that degree of uncertainty at the highest levels of present-day physics, I feel not at all uncomfortable in exploiting the troublesome vacuum energy to service McAndrew’s drive.
The third chronicle introduces two other ideas that are definitely science fiction today, even if they become science fact a few years from now. If there are ways to isolate the human central nervous system and keep it alive independently of the body, we certainly don’t know much about them. On the other hand, I see nothing that suggests this idea is impossible in principle — heart transplants were not feasible forty years ago, and until this century blood transfusions were rare and highly dangerous. A century hence, today’s medical impossibilities should be routine.
The Sturm Invocation for vacuum survival is also invented, but I believe that it, like the Izaak Walton introduced in the seventh chronicle, is a logical component of any space-oriented future. Neither calls for technology beyond what we have today. The hypnotic control implied in the Invocation, though advanced for most practitioners, could already be achieved. And any competent engineering shop could build a Walton for you in a few weeks — I am tempted to patent the idea, but fear that it would be rejected as too obvious or inevitable a development.
Life in space and the Oort cloud.
Most chronicles take place at least partly in the Halo, or the Outer Solar System, which I define to extend from the distance of Pluto from the Sun, out to a little over a light-year. Within this radius, the Sun is still the primary gravitational influence, and controls the orbits of objects moving out there.
To give an idea of the size of the Halo, we note that Pluto lies at an average distance of about 6 billion kilometers from the Sun. This is about forty astronomical units, where the astronomical unit, usually abbreviated to AU, is defined as the mean distance of the Earth from the Sun. The AU provides a convenient yardstick for measurements within the Solar System. One light-year is about 63,000 AU (inches in a mile, is how I remember it). So the volume of space in the Halo is 4 billion times as large as the sphere enclosing the nine known planets.
By Solar System standards, the Halo is thus a huge region. But beyond Neptune and Pluto, we know little about it. There are a number of “trans-Neptunian objects,” but no one knows how many. Some of them may be big enough to qualify as planets. The search for Pluto was inspired early this century by differences between theory and observation in the orbits of Uranus and Neptune. When Pluto was found, it soon became clear that it was not nearly heavy enough to produce the observed irregularities. The obvious explanation is yet another planet, farther out than the ones we know.
Calculations of the orbit and size of a tenth planet needed to reconcile observation and theory for Uranus and Neptune suggest a rather improbable object, out of the orbital plane that all the other planets move in and about seventy times the mass of the Earth. I don’t believe this particular object exists.
On the other hand, observational equipment and techniques for faint objects are improving rapidly. The number of known trans-Neptunian objects increases almost every month.
The other thing we know for sure about the Halo is that it is populated by comets. The Halo is often called the Oort cloud, since the Dutch astronomer Oort suggested thirty years ago that the entire Solar System is enveloped by a cloud of cometary material, to a radius of perhaps a light-year. He regarded this region as a “cometary reservoir,” containing perhaps a hundred billion comets. Close encounters between comets out in the Halo would occasionally disturb the orbit of one of them enough to divert it to the Inner System, where it would appear as a long-period comet when it came close enough to the Sun. Further interactions with Jupiter and the other planets would then sometimes convert the long-period comet to a short-period comet, such as Halley’s or Encke’s comet, which we observe repeatedly each time they swing by close to the Sun.