The third in a series of articles on science and science fiction

In the series pilot of Enterprise, "Broken Bow," Lt. Malcolm Reed made this observation.

"Pardon me, but if I don't realign those deflectors the first grain of space dust we come across will blow a hole in this ship the size of your fist."

This is a recognition of one of the problems that will be encountered at near light speed as well as super-luminal velocities that must be addressed for any practical interstellar mission.

Interstellar space is not completely empty. Atomic hydrogen and helium densities of one to five atoms per cubic meter have been measured and there are indications that other particles of dust and heavier materials also exist. This may not seem like much, but at velocities of hundreds of thousands of kilometers per second, the impact rate on a starship will be in the billions of events per second. Heavier particles—say, the size of a small grain of sand—would strike the ship with an energy release of a small nuclear bomb. These collisions would incapacitate or destroy a starship if appropriate measures were not taken.

The problem has been recognized by many writers. Arthur C. Clark, in his story Songs of Distant Earth, protected his quantum fluctuation powered starship with a huge block of ice.

In Star Trek stories, the ships use a deflector system that is seen as a large dish on the front of the spacecraft. Actually, this idea is borrowed from one of the earliest serious concepts for interstellar vehicle design. The Bussard ramjet method of spacecraft propulsion was proposed in 1960 by the physicist Robert W. Bussard and popularized by Carl Sagan (see "Bussard Ramjet" in Wikipedia for a detailed description of this design concept).

This idea takes advantage of the presence of material in interstellar space by using it as fuel. Bussard's notion was that if interstellar hydrogen could be collected and fed into a fusion reactor, there would be sufficient energy and reaction mass to propel the vehicle to speeds close to that of light. If it can be made to work, this idea solves two problems: first, it provides a source of fuel and reaction mass necessary to propel the vehicle, and second, it acts to clear out any material in front of the vehicle that may cause damage by collisions. This is the deflector shield concept in Star Trek.

Unfortunately, the magnetic shield encounters problems resulting from relativistic effects when velocities approach light speed, but it is an example of a serious idea proposed by a serious scientist.

A couple of ideas for even more exotic interstellar drive systems are the "Black Hole Launcher" and the "Warp Drive" made popular in the Star Trek series.

In last month's article, we discussed a black hole launcher that could provide a kind of "Stargate" for travel to nearby stars. While the physics of gravitational exchange propulsion is well understood, at least for non-relativistic velocities, building a system of this sort is a non-trivial task.

So what kind of material do we use to build a "Stargate"?

There are actually several choices: Metallic Hydrogen, White Dwarf Matter, Neutronium, a Black Hole, and Quark Matter. Each of these represents increasingly dense stages of quantum fluids.

The ideal material to use is neutronium, difficult to obtain unless there is a neutron star in your local neighborhood. Neutronium is condensed matter that is so dense that a spoonful weighs thousands of tons. It is kept at this incredible density by its own gravitation. Atoms are compressed into a soup of protons, neutrons, and electrons and while falling short of being a black hole it is still a good candidate for generating moving gravitational fields that approach the speed of light.

The trick is to get a couple of planet size masses and compress them into degenerate matter, no small feat. The earth, for example, if compressed to the density of a black hole would be about nine millimeters in diameter, about the size of a large pea, but it would still weigh the same as it does now. If the earth were compressed to the density of white dwarf matter, it would be about two kilometers in diameter. If the earth were compressed to the density of quark matter, it would be slightly larger than a hydrogen atom and still weigh the same as it does now. This is how strange degenerate matter is.

However, we don't need to squash the Earth because a degenerate matter launcher does not need to be that massive, just a few trillion tons. It is conceivable that if we gathered up all the rocks in the asteroid belt there would be sufficient mass to build a launcher.

Another approach is warp drive. It takes a bit of explaining to describe how degenerate matter may play a role in the development of such a system but here is one possible approach. Let me begin by describing a machine that could be built with today's technology.

Mechanical gravitational wave generator

The concept is derived from a device that I proposed a number of years ago: a means of producing measurable gravitational waves. The basic idea is as follows:

A pair of wheels loaded with a pattern of very heavy pellets, tungsten or heavier, located at the periphery. These wheels are rotated in close proximity to one another. The wheels rotate in opposite directions so that the pellets form minute gravitational perturbations as they pass one another. Because all of the pellets on one wheel encounter their corresponding fellows on the other wheel simultaneously, there is constructive interference that produces a very weak oscillating gravitational dipole that propagates along the spin axis of the wheel. Now if we build several such wheel assemblies, arrange them along their common spin axis and synchronize their rotation rates constructive interference again acts to strengthen the magnitude of the gravitational wave and to narrow the width of the beam. Calculations indicate that if tungsten were used it would take several thousand such wheels in an assembly a kilometer or so long to produce minimally detectable waves. If depleted uranium were used those numbers could be cut approximately in half. Detection could be performed using a Forward Gravitational Gradiometer (Robert Forward's invention) operating at the same frequency as the generator. The advantage of this device is that the frequency and orientation of the gravitational waves are known in advance, making detection much easier.

What does such a device buy you? First, it would demonstrate that gravitational waves do exist. Then, if it worked, it would permit the direct measurement of the propagation velocity of gravitational waves. The general relativity theory predicts that G waves propagate at C but this has never been demonstrated. This device would provide yet another confirmation of Einstein's GRT. It would also provide a general-purpose tool for analysis of G waves and a means to explore their properties.

The closest analog to this device in electronics is a traveling wave amplifier. I like to think of it as a "GRAZER," or Gravitational Laser.

So what does all this have to do with a space drive? Consider this.

Let's assume that we can build one of these things but instead of using pellets of dense ordinary material like tungsten or uranium we use relativistically degenerate matter or even quantum black holes. Now I admit that the components have now moved from "hardtogetium" to "unobtainium" but calculations indicate that if such a device were to be built very powerful gravitational waves could be produced. Powerful enough to warp space-time and to create a warp bubble as described by Alcubierre (See "Alcubierre drive" in Wikipedia and elsewhere).

Interestingly, a GRAZER built to the specifications described above would look almost exactly the same as the warp nacelles on the Enterprise NX01.

Next month, we will talk about teleportation and faster-than-light communication systems.
Prof Chuck