Astrophysicists M. Lingam and A. Loeb of Harvard have recently speculated that we may be observing evidence that aliens in other galaxies are powering their starships with beams of microwaves.
To explore this amazing idea, we'll begin by considering microwave beaming as a means of space propulsion. Presently, the most aggressive effort to reach nearby stars is the Starshot Project, started in 2016 by billionaire science philanthropist Yuri Milner and described in my AV column in the October-2016 issue of Analog. The basic Starshot strategy is to use a large bank of coherent lasers to send a tiny but intelligent probe to Alpha Centauri, boosting it to 1/5 of the speed of light with light pressure acting on its reflective sail. This scheme has the advantage over conventional rocket propulsion that no reaction mass or energy-generating fuel needs to be carried on the propelled space vehicle, which can be very small and light.
A similar scheme involving microwave beaming was proposed in 1985 by physicist and SF author Robert W. Forward. It involved a large space-based solar-powered maser that beamed microwaves to push a space vehicle with a large low-mass superconducting "sail". Forward named his scheme the "Starwisp, and several of his SF novels used the concept. In 2000 another physicist and SF author, Geoff Landis, investigated Forward's original scheme and improved on it with a sail design involving carbon fibers.
The basic underlying propulsion concept used by these schemes is that electromagnetic radiation, including microwaves and visible light, carries a quantity of momentum proportional to its energy that can be used to push a sail. Unfortunately, that push is rather small, about 3.3 micro-newtons of force per kilowatt of power if the radiation is completely absorbed by the sail and twice that if the radiation is perfectly reflected backwards. In other words, each 10 newtons (or 2.25 pounds) of thrust generated for propulsion requires that between 1.5 and 3 gigawatts of electromagnetic power be beamed to the sail. Also, the sail's reflection efficiency must be nearly perfect, because if any significant fraction of the huge quantity of beamed power is absorbed and transformed to heat, the vehicle will overheat and be destroyed. The beam-driven sail propulsion scheme must also deal with wavelength-dependent aperture diffraction effects, which limit the system's ability to bring the beam to a tight focus on the sail from a long distance away.
Nevertheless, such beam-driven sail schemes presently represent our best technological opportunity for sending small probes to nearby stars systems. And it may be that intelligent aliens in other galaxies have reached the same conclusion and are using beam-driven sail technology to reach their own nearby stars.
In 2016 physicists James Benford and his son Dominic pointed out that one of the best ways for SETI searches to identify a high-technology alien civilization among the stars might be to look for leakage radiation from such power beaming. And this kind of SETI signals may have already been observed.
In 2017 two Harvard astrophysicists, Manasvi Lingam and Abraham Loeb, have ventured a very interesting speculation about the origin of the unexplained radio-astronomy phenomenon known as "fast radio bursts" (or FRB). Fast radio bursts were first reported in 2007 by D. R. Lorimer and colleagues, based on Australian radio telescope data that was recorded on June 21, 2001. Since that time, more FRB events have been observed. As of this writing there is a growing FRB catalog that presently lists 20 observations.
Because radio waves of different frequencies travel through intergalactic space at slightly different speeds due to interactions with free electrons along their path, radio astronomers can use the "chirp" of arrival time vs. frequency of an FRB to estimate how far it has travelled from its source. This analysis shows that all of the observed FRBs originate well beyond our galaxy, at estimated distances on the order of three billion light years.
An FRB is fast and bright, lasting for only a few thousandths of a second. Their high brightness at such vast distances of origin implies that they are the result of a huge release of non-thermal radio energy. While two of the catalogued FRB have been observed to repeat, most of them are single non-repeating events. The FRB times of arrival have never been observed to correlate with bursts of either visible light or gamma rays coming from the same direction, and there is no verified observation of any "afterglow" following an FRB. There is no known physical phenomenon that can account for these bizarre characteristics. It is not physically plausible that such a vast release of energy in the radio domain would be "pure" and not accompanied by electromagnetic radiation in other forms. This raises an important question: If no natural phenomenon can make an FRB, what does that leave?
Lingam and Loeb (L&L) have speculated that FRBs may be the leakage of radio waves coming from massive works of advanced engineering built by intelligent aliens in other galaxies, aliens who are beaming high-intensity radio waves at sails in order to power their starships.
Exploring the full implications of this scenario, L&L have calculated the characteristics of such a beam-driven sail launch facility. They assume that the radio emission is broadband, that stellar energy is used to power the beams, and that water is used as a coolant. The observed FRBs are consistent with a beaming apparatus located about 3 billion light years from the Earth and beaming radio waves with a center frequency around one GHz. L&L calculate that the radiating surface would have to be of planet-size, about 3,000 km in diameter and would have a water-cooled surface temperature of about 100° C. It would be beaming radio waves at a total power level of 1015 kilowatts. (For comparison, our Sun has a power output of 3.8 x 1023 kilowatts.) The 3,000 km in diameter size of the radiator was calculated in two different ways, and the two results agree. A frequency of about 1 GHz was calculated to be the most efficient for beam propulsion, and this agrees with the FRB observations.
For the launched vehicle, L&L assume a large sail-plus-payload that is roughly the same size as the radiator and has a mass of about 107 tons. (For comparison, at present the Earth's largest ocean-going cargo ship has a mass of about half a million tons.) L&L calculate the acceleration and the final velocity of this sail ship following launch. Beam diffraction considerations limit the distance over which the vehicle can be accelerated to about 1/3 of a light year. They assume an acceleration of around one gee over that distance, and they calculate that the final speed of the vehicle would be close to the speed of light.
They also consider the implications of the few-millisecond duration of the Earth-observed FRBs. The sail and beamer would presumably both be in orbit around their parent star, and the beamer would need to rotate as it orbited in order to keep the beam centered on the sail. This means that to an Earth observer the leaked beam will rapidly sweep across the sky and will only be visible for a few milliseconds. The millisecond time-width of the FRBs implies that the rotation of the beamer is about 10-5 radians per second, implying a full 360 degree rotation in about a week. This is consistent with what might be expected for the orbiting beamer scenario.
L&L then consider the implications of the rate at which FRBs have been observed for the probability of finding a highly advanced civilization that is using power-beaming launch technology on a given Earth-like planet or in a given galaxy. Based on the rate of observation of FRBs, they conclude that the probability of finding an advanced civilization on a given Earth-like planet is less than one chance in a million. However, since some estimates put the number of habitable Earth-like planets in our galaxy as high as 10 billion, there might be up to 10,000 such advanced civilizations in our galaxy alone. We note, however, that no FRB has ever been observed to originate in our galaxy, and the numbers indicate that such an observation might occur only once in 300 years.
To conclude, L&L consider how more detailed measurements of FRBs might show a signature of their beam-driven sail origin or might falsify that scenario. Following the Benfords, they point out that radio wave leakage around the edges of an opaque sail should produce diffraction edge-effects that would be observable as interference "wiggles" in the detected intensity. Present FRB measurements have reported neither the presence nor absence of such modulations. L&L also suggest that radio astronomers should note the sites of the major FRB bursts and look for repeat events, because "astrophysical explosions tend to be single bursts, while artificial beacons can repeat".
Finally, we note that in their paper on beam leakage as an SETI signal, James and Dominic Benford speculate that an intelligent civilization using beamed-power sail technology should realize that they are sending out a beacon that is likely to be detected by other advanced civilizations far away. That might motivate them to modulate the frequency, intensity, or polarization of their beam so that it contains information that would be of interest to civilizations detecting their beam-beacon. This suggests that Earth-based radio astronomers should devote some effort to carefully examining the frequency and polarization structure of FRBs detected in search of such encoded information.
From one science fiction point of view, these L&L calculations are very exciting. Our galaxy may be peppered with other civilizations far more advanced than ours, and they may be regularly sending payloads to their nearby star systems. However, from another point of view, I rather hope that a better explanation for the FRB phenomenon emerges, one that doesn't require an advanced alien civilization. That is because as a physicist, I am hoping that there are yet-undiscovered physical phenomenon that will provides a means of space propulsion and interstellar travel that don't require mega-engineering and planet-size sails and radiators of radio waves. It will be a long time before the human race can build such vast works of mega-engineering, and I would prefer that we get to the stars much sooner, if possible.
First Observation of FRB:
"A bright millisecond radio burst of extragalactic origin", D. R. Lorimer, et al, Science, Vol. 318, Issue 5851, pp. 777-780 (02 Nov 2007); preprint arXiv:0709.4301v1 [astro-ph].
Power Beaming as an SETI Signal:
"Power Beaming leakage radiation as a SETI Observable", James N. Benford and Dominic J. Benford, Astrophysical Journal 825, 101-107 (2016); preprint arXiv:1602.05485v2 [astro-ph.IM].
"Fast Radio Bursts from Extragalactic Light Sails", Manasvi Lingham and Abraham Loeb; preprint arXiv: 1701.01109v2 [astro-ph.HE].
John Cramer's new book: a non-fiction work describing his Transactional Interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available for purchase online as a printed or eBook at: http://www.springer.com/gp/book/9783319246406 .
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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