Twinkle, Twinkle Laser Star !
What if you could control the direction of photons emitted by a star ?
Instead of spreading out in all directions some could be projected in
the form of a highly coherent and collimated laser beam.
An artificial
laser star
could be constructed to
send an extremely well focussed and concentrated beam of light towards
a space station or planet to sustain a biosphere or propel an interstellar
light sail.
From such a planet, the sun would appear like a bright twinkling point rather
than a small disk. This could be accomplished with mirrors with total area
smaller than the surface of the earth, they would span a single pixel
element on the picture at left, too small to see.The artificial laser star approach to collecting solar energy is more efficient in terms of materials and requires a lower level of technology than the Dyson sphere or RingWorld approach. Since the laser beam is highly directional, only a minuscule portion of the star's total light output is required. This web site covers several schemes to construct artificial lasers on stars with rapidly cooling stellar atmospheres.
However, life around a very young white dwarf is possible on an outer planet orbiting at a safe distance to avoid the strong solar flares, x-rays and ultraviolet radiation. The flux of ultraviolet radiation varies as the inverse square of the distance from the sun, therefore ten times farther means 100 times lower radiation exposure. However it also means that optical radiation or white light is also diminished by this factor.
The solution is to construct an artificial laser star on the surface of the sun, then channel a portion of the energy produced by the star into a directed beam shining straight towards the outer planet. The highly collimated radiation directivity of artificial laser stars is in sharp contrast to Dyson spheres which must capture all the radiation from a star emitted isotropically. i.e. There would no longer be the need to cover the entire spherical solid angle. Multiple beams could be directed to individual planets.
The laser intensity growth equation is very different than spontaneous emission plasmas where the intensity is linearly proportional to distance travelled. In a laser medium the light intensity is proportional to the distance travelled in an exponential relationship:
| I | Laser output intensity (after leaving the gain medium) |
| I0 | Initial intensity (usually spontaneous emission) |
| a | Exponential gain coefficient (a > 0 for a gain medium) |
| x | Distance travelled trough the gain medium (single pass - no mirrors) |
For example say your gain was a=1 per meter and the laser medium was 10 meters long by 1 meter wide, the intensity on axis would be I0 exp(10) or about 22,000 I0. Now the intensity transverse to this axis would be I0 exp(1) or about 2.7 I0. This turns out to be a ratio of about 8,000 to one, rather than 10 to one which you would expect for spontaneous emission only. This translates to an extremely strong asymmetry in the radiance of an extended laser plasma.
Another mechanism which increases this ratio is gain saturation : The longitudinal photon intensity become so large that it effectively 'depletes' the transverse photon intensity, which means that virtually all the laser radiation can be made to travel along the direction in which the plasma is longest. (the longest possible photon path length). Saturation is equivalent to one laser mode triggering via strong stimulated emission all the other excited atoms to emit their photons 'early' into one giant macroscopic coherent wavefunction. A photon 'monopoly' thus ensues cutting off the 'competition'.
Thus a highly directional artificial stellar laser could be created if a stellar plasma gain medium could be generated which was very elongated. There are several solutions to this problem:
| Orbiting Mirrors | Create a 'virtual' elongation by surrounding the gain medium with a pair of orbital mirrors from which the laser photons would bounce off and make multiple passes through the gain medium. |
| Laser Amplifier | An orbiting laser could fire a beam through the gain medium to enormously amplify the beam. Elongation is created in the cylindrical region through which the beam passes. |
| Meteor Streaks: | Create elongation by rapidly transferring a large amount of coolant to an elongated region deep within the stellar atmosphere, temporarily enhancing its gain. |
The mirror and laser amplifier schemes are non-invasive but requires that the stellar plasma already sustain a population inversion. The meteor method is more invasive and attempts to directly modify the stellar plasma itself. Although the meteor method can be applied to a gain medium, it can also be used to actually create a gain medium in stellar plasmas near the threshold of a population inversion; thus triggering potential laser stars on the verge of lasing. We will review each of these three methods in the following text:
Sherwood (1988) described a platoon of orbital mirrors around mars or venus to extract a coherent and directed CO2 laser beam from the gain medium. He foresees the use as means of transmitting/propagating our entire culture to a future nano-civilisation established in various other solar systems throughout our galactic neighborhood.
Mirrors external to the stellar atmosphere would reduce the problems of erosion and orbital deflection by the violent and corrosive stellar winds. Although pairs of mirrors would be ideal to extract directed energy from the gain medium, there are power limitations due to the finite reflectivity of the mirrors (which absorb a small but damaging fraction of the radiation). To overcome this problem larger mirrors could be constructed to distribute the heat load, and reduce the Airy diffraction pattern (beam divergence). However large mirrors are extremely vulnerable to orbital debris such as an encounter with a handful of sand from an intersecting orbit etc...
Large mirrors will act like solar sails, this side-effect could be used to maintain a stationary satellite (statite) over the desired location near the solar limb. (see papers on 'statites' by R.L. Forward) The photon pressure from the laser photons and the photosphere could be adjusted to exactly compensate for the solar gravitational force and prevent the hovering statite from falling onto the star's surface. Conservation of momentum would impart a momentum of 2p every time a photon bounced off the mirror. For a multiple pass optical cavity the photon travels back and forth many times stimulating emissions at every pass. The force on the mirrors from this photon pressure in combination from the photon pressure from the star's photosphere acts like a solar sail and can be designed to exactly balance the inward gravitational force pulling the mirror onto the star. These three counterbalancing forces could be harnessed to maintain the mirror stationary against the gravitational pull of the star. ('statites' or stationary satellites by R.L.Forward). Very near the star there may be deviation from 1 over r squared law by dilution of radiation effect of finite angular width of solar disk.
If more than two mirrors are used a ring laser could be produced, the radiation would make numerous quasi-circular trips around the sun; in other words, a photon merry-go-round spinning at the speed of light !
The mirror scheme will produce an artificial
laser star
which would
appear as an extremely bright spot on the limb of the parent star.
The disk of the star would be difficult to discern in the
glare from this 'star-like' spot.
If we take photographs of the sun and are
careful to mask out the bright point, we would notice a faint disk,
corresponding to the light from the underlying 'natural' sun.
Although most stars are too far away to resolve their disk,
amateur astronomers could easily observe the unusual
spectra of laser stars.
The major disadvantage is that the beam only makes a single pass though the laser gain medium and can only be used in high gain plasmas or large low gain plasmas. To overcome this restriction a hybrid scheme involving mirrors and seed-lasers could be used to create a multi-pass cavity. (see previous section).
The orbital mechanics of the laser amplifier method could be greatly simplified if the seed laser was placed in same orbit as the target planet but on the opposite side of its orbit; the star's limb would thus always be in the line of sight to the planet (unless the orbit's eccentricity was too great). In this particular case, only one seed laser would be required.
The seed beam doesn't have to be parallel, it could be slightly diverging as long as the final beam spot size is of the order of the target planet size. A diverging beam seed laser would be cheaper because it would require much smaller optics but it must orbit closer to the star, complicating alignment because these objects orbit faster. In this case, multiple seed lasers would have to be constructed in order that at least one of them have its line of sight intersecting the star's limb at any given moment. A diverging seed laser augmented with the mirror scheme would require concave mirrors to create a confocal resonant cavity.
If the orbit is very near the star, the seed laser could be optically pumped by broadband photospheric radiation, with abundant UV photons, there should be no problem getting materials to fluoresce and lase in the optical region of the spectrum. Successful designs for solar pumped lasers have already been published elsewhere and will not be covered here. In any case, preliminary mapping of the laser star's plasma gain medium will almost certainly involve an orbital seed laser probe locked in a synchronized orbit with a detector satellite.
The laser power of this scheme would depend on the amount of asteroidal material which was vaporized, i.e. more material means more plasma can be cooled increasing the gain.
Special launchers located within the asteroid belt could send into the sun a precisely timed sequence of asteroids. They impact the sun on the side directly facing us. Although they don't exactly 'hit' the sun, as the sun has no solid surface, the parts we can see are actually regions of hot gas that have become transparent enough that we can see through the thin plasma. Before each asteroid enters the solar photosphere it encounters strong frictional forces in the denser outer layers of the sun, and just like the meteors you see at night they usually burn up before the reach the sun's hot surface. The energy required to vaporize the asteroid comes from its own potential energy high up in the solar gravitational well.
We don't want to send matter from the planet, as we would have to expend too much energy to raise it out of its gravitational well. Even from the lighter moons, the effectiveness of matter sent into the sun would be 10 times less than from the asteroid belt.
Amplified spontaneous emission, or laser action occurs, and since the intensity increases exponentially, the beam emerges mostly along the direction where the laser medium is the longest; the laser light emerges with high efficiency as a narrow precisely aligned with the meteor streak. Since the emission can occur in the forwards as well as in the reverse directions, half of the energy is lost because it is beamed right back into the sun ! The other half of the laser beam is transmitted directly to the planet. It must be aimed slightly ahead of where the planet would be by several minutes, compensating for the light travel transit time.
The beam from the meteor scheme has the flexibility to emerge from any location on the surface of the star. The longest possible path length through the gain medium is tangential to the surface of the star, therefore the meteor should reenter the stellar atmosphere at a very shallow angle for maximum gain. Also, since the wavelength dependence of the laser gain in naturally occurring laser stars has a maximum at the center frequency of the quantum transition, Doppler gradients in the motions of the plasma wind could reduce the brightness of the laser line. Natural laser emissions will thus preferentially occur along directions perpendicular the radial stellar wind direction. The longest possible path lengths which are tangential to the stellar surface should also be the preferred directions of laser emission. From the point of view of an external observer far from the star, the brightest natural laser intensities would appear to emerge from the limb of the star, creating a colored ring surrounding a much fainter white stellar disk. Artificial stellar lasers are in direct competition with these natural modes, which have the potential to deplete the gain medium before it can be 'coaxed' artificially. This 'competition' should be less important in stellar atmospheres just below the threshold of laser action.
The orbit of a meteor within the gravitational well of one large mass and outside the influence of drag or other external forces must be a conic section. Newton's laws restricts the possible orbits to a line, an ellipse, a parabola or a hyperbola. The problem therefore arises as to how to create an extremely linear and straight laser channel from possibly an initially curved path, the laser photons will not 'curve' and must travel in a straight path. The reentry curvature and velocity is therefore critical, it must create a channel as linear as possible while still matching the intended target direction.
Another related problem is the meteor's initial kinetic energy, if it is too high it will explode like the Jupiter Shoemaker-Levy comet collision. There would be no problem if it exploded after the coolant streak was created. Too little kinetic energy and the cooling will not be uniform along the entire channel, i.e. a gain medium will be created near the meteor's plume but will rapidly decay reducing the effective channel length. Also, the gain channel must be created faster than the laser photons can deplete the medium by gain saturation. Also, at even lower velocities, the meteor could not penetrate far before being vaporized by the corrosive stellar winds, shortening the channel, and not reaching the higher density regions more favorable to laser action.
The composition of the meteor could be tailored in such a way that the outer shell could be made of a more heat resistant material which would ablate more slowly until it was burned off revealing an inner core which could be made of solid hydrogen or some other light element with a greater cooling capacity. The reason is because inelastic collisions among atoms are more effective at transferring heat when the particles involved have roughly of the same mass (Stellar plasmas are predominantly hydrogen, helium with traces of other light elements). Although heavier atoms such as iron take longer to cool the plasma they have a higher heat capacity and are able to cool more plasma increasing the gain per injected coolant atom.
While examining astrophysical spectra of natural laser stars we often find more than one wavelength at which laser action can occur, sometimes three or more strong emission lines are present. Given the sensitivity of the population inversion to initial conditions such as density, temperature and cooling rate, the gain of the lasing medium can in principle be tailored to operate in the multi-wavelength regime.
If a continuous barrage of such meteors are sequenced to reenter the stellar plasma, it would be possible to direct a somewhat steady beam to a planet too far from the sun to receive enough ordinary light because of the inverse square law. An artificial stellar laser would be just the thing to start growing potatoes on Pluto ! Their 'sun' would appear as an extremely bright star which was twinkling and perhaps even monochromatic (one color) Life would indeed seem strange to such a civilization, spoon fed with a steady diet of solar laser photons !
The closest idea to artificial laser stars is the thermonuclear fusion warhead pumped x-ray laser conceived to detonate in low earth orbit and shoot down ballistic missiles in flight with multiple x-ray beams. This thermonuclear plasma laser is based on collisional rather than recombination ion pumping as in laser stars, however the principle of amplified spontaneous emission is the same as laser stars. A low yield thermonuclear warhead was designed to create a million degree plasma which striped away most of the electron from atoms around the warhead. As the electron density rose, it was enough to pump by electron collisions, certain highly stripped ions from the ground state into upper levels which decay to intermediate metastable states eventually creating a population inversion. It was tested in underground nuclear detonations, however the published results indicated insufficient laser intensity for military purposes.
Artificial laser stars could also be used to propel a solar sail to another star. The military applications are limited by extremely simple countermeasures for a mirror scheme, consult publications relating to space based laser weapons planned for the Strategic Defence Initiative. The meteor scheme could be thwarted by simply deflecting the meteor by an infinitesimal amount or by destroying the meteor. The possibilities of an artificial stellar laser are limitless.
Laser Stars
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