≡ Menu

SETI: Power Beaming in Context

Thinking that we can understand the motivations of an extraterrestrial civilization seems like a fool’s gambit, but we have to try. The reason is obvious: We have exactly one technological society to work with — we’re all we have — and if we want to look for SETI signals, we have to interpolate as best we can. An alien culture, it is assumed, will do the same. This was the procedure outlined by Giuseppe Cocconi and Philip Morrison in their classic 1959 paper “Searching for Interstellar Communications,” that began the modern era of SETI.

If there are civilizations around stars like the Sun, the paper reasons, then some will be motivated to reach out elsewhere. From the paper:

To the beings of such a society, our Sun must appear as a likely site for the evolution of a new society. It is highly probable that for a long time they will have been expecting the development of science near the Sun. We shall assume that long ago they established a channel of communication that would one day become known to us, and that they look forward patiently to the answering signals from the Sun which would make known to them that a now society has entered the community of intelligence. What sort of a channel would it be?

What results is an explanation of the factors needed to overcome signal attenuation and the frequencies most likely to be used. Thus we get what Cocconi and Morrison called “a unique, objective standard of frequency, which must be known to every observer in the universe.” This is 1420 MHz, the 21 centimeter wavelength of neutral hydrogen. Thus we know where to look assuming a civilization is trying to make contact with us. The authors conclude “… the foregoing line of argument demonstrates that the presence of interstellar signals is entirely consistent with all we now know, and that if signals are present the means of detecting them is now at hand.”

The Hunt for SETI Observables

We’ve looked frequently in these pages at how SETI has changed since the Cocconi and Morrison days, with so-called ‘Dysonian SETI’ invoked as a way of looking for observable evidence in our astronomical data. The unusual star KIC 8462852 has elevated the method to wider attention because of the possibility that some kind of astroengineering could explain its unusual light curves (search the archives here for numerous articles on the star).

In any case, Dysonian SETI assumes, contra Morrison and Cocconi, no intent to contact anyone. We are simply looking for activity, albeit on a colossal scale. We must be the ones to figure out what that activity might be.


Which brings me to a new paper from James Benford and son Dominic (NASA GSFC). “Power Beaming Leakage as a SETI Observable,” submitted to The Astrophysical Journal, asks whether we might detect the use of power beaming to transfer energy and accelerate spacecraft within a distant solar system and, perhaps, beyond it if the technology is being used to accelerate starships. KIC 8462852 is also considered here because there have been two attempts to study it, one optical and one at radio frequencies, in the context of a SETI search.

As the paper notes:

The most observable leakage from an advanced civilization may well be from the use of power beaming to transfer energy and accelerate spacecraft, both within and beyond the star system where the civilization is located. In future, such applications may make the Earth’s radiation in the microwave, millimeter and visible/near-IR parts of the electromagnetic spectrum be very intense…. The power levels are high, focused, and transient and could easily dwarf any of our previous leakage to space. These are not SETI signals so much as leakage, a detectable aspect of advanced civilizations.

The kind of SETI observable the Benfords are examining is essentially leakage, but of a different kind than the widely repeated trope of picking up alien TV signals, just as extraterrestrial cultures are presumably now enjoying “I Love Lucy” from our signals. As the Benfords argue, the leakage of radio and television signals is essentially undetectable between stars not only due to the weakness of the signal but to its lack of coherence. Planetary radars are much more likely to be detectable, but signals like these are also extremely transient.

Intense beams of radiation being used to move power about in a distant solar system or accelerate spacecraft on long journeys should be more readily detected. James Benford argued as much in 2008, and more recently James Guillochon and Abraham Loeb have quantified the leakage to be expected from space propulsion-related beaming, showing that a beam being used to drive a large sail would be observable. The duo write: “…for a five-year survey with ~10 conjunctions per system, about 10 multiply-transiting, inhabited systems would need to be tracked to guarantee a detection” using our existing radio telescope infrastructure.

Fig-1-Benford (1)

Image: Taken from the Guillochon and Loeb paper, this diagram shows a Mars mission using microwave beaming for propulsion. In the schematic, the dashed line represents the sailcraft. You can see the prospects for signal leakage here. Notice the inset showing the beam profile as it overlaps the sail of diameter Ds. Credit: Guillochon & Loeb.

But remember the notion we started with: If we are trying to understand what an extraterrestrial civilization might do, we have to look at the uses we ourselves would make of these technologies. We try to be as flexible as possible while acknowledging the fundamental gap in our knowledge of an alien culture, but we have to start somewhere. And this kind of reasoning takes us down an interesting path, as the Benfords note in their paper. For possessed of powerful beaming technologies, such a culture should be aware of SETI implications:

It has previously been noted that such leakage from other civilizations could be observable (Benford 2008). Guillochon & Loeb (2015) have quantified leakage from beaming for interplanetary space propulsion, its observables, and implications for SETI. Extraterrestrial Intelligence (ETI), having done the same thinking, could realize that they could be observed. Hence there may be a message on the power beam, delivered by modulating it in frequency, amplitude, polarization, phase, etc., and broadcast it for our receipt at little additional energy or cost. By observing leakage from power beams we may well find a message embedded on the beam.

That’s a fascinating notion in its own right. It’s a kind of METI signal sent out by an alien civilization that is, like a beacon, broadly targeted rather than aimed at a specific solar system. And it operates on the assumption that another culture might in the course of its SETI investigations notice high-powered, focused and transient beams and analyze them. Or put another way, if our own civilization has figured this out, our extraterrestrial counterparts working the SETI various concepts must surely have come up with the same implications.

Uses of the Beam

Detecting a message embedded in a power beam would, of course, be a breakthrough of historic proportions, but so would the simple detection of power-beaming itself — no message attached — which would signal the presence of an advanced civilization. One of the things recently looked at by the Allen Telescope Array was whether the intriguing star KIC 8462852 showed any signs of activity, and it turns out that the observations, while finding nothing, do allow us to set some limits on power-beaming in that system. KIC 8462852 became, in other words, a useful exercise even though the observations were short-lived. More tomorrow on this, and also see the Benfords’ Quantifying KIC 8462852 Power Beaming in these pages.

And just what might a technological civilization do with power-beaming methods that we could observe? One answer is obvious: We’ve been talking about beamed sails for well over ten years on Centauri Dreams, keying off Robert Forward’s fascination with the subject as it applied to interstellar missions. Here the requirements are enormous, with a space-based solar power station beaming to a 1000-kilometer sail in some of Forward’s missions, although subsequent work found ways to trim the sail down to the 1-10 kilometer range.

The idea of a power station orbiting in the inner part of the Solar System where the photon-flow is maximized has dual use, as using microwave beams to transport power to a planet’s surface is a major driver. As a SETI observable, power beaming to a planet is not a likely target. The paper notes that the beam would have to be tightly controlled, with side lobes maximally reduced. In contrast, power being beamed to a spacecraft should show increasing leakage as the craft is accelerated — the beam increasingly leaks around the edges of the accelerating vehicle — making transportation applications a realistic SETI observable.

The Benfords’ paper quantifies the various kinds of power beaming missions and applications and their observable parameters, looking at twenty concepts in terms of power radiated, duration and likely time for the radiation to repeat. More on this tomorrow as we continue looking at the paper and the question of how an advanced technology could use power beaming.

The Benfords paper is “Power Beaming Leakage Radiation as a SETI Observable,” submitted to The Astrophysical Journal and available as a preprint. The Guillochon and Loeb paper is “SETI via Leakage From Light Sails in Exoplanetary Systems,” The Astrophysical Journal 811, No. 2 (23 September 2015), with abstract here. Jim Benford also discusses the Guillochon and Loeb paper in Seeing Alien Power Beaming.


Comments on this entry are closed.

  • NS February 25, 2016, 14:26

    Don’t current SETI standards reject transient signals though? Are we in a situation where the type of signal we are likely to detect won’t last long enough to be confirmed, and therefor won’t be considered a valid detection?

  • Jim Strom February 25, 2016, 17:13

    Great stuff, and thanks to all involved. Intuitively, energy leakage seems unlikely to be detected. Unless a beam was aimed directly from PlanetX to Earth (yet intercepted by a vehicle), the 1/r^2 rule would apply, on average. In other words, the leakage would radiate uniformly. So we are talking about detecting a leakage of a planet-to-vehicle energy beam. Wouldn’t the power of the leaked signal be quite low?

    Also, why should we expect leakage at all? How hard is it to get ~100% absorption efficiency? Consider a beam from PlanetX to Earth. Suppose the direction and source of the beam is roughly constant, perhaps changing gradually over time, yet staying close to its mark of Earth. Precise beam maintenance might not be possible, but the vehicle itself would be able to alter its course to stay perfectly aligned with the beam. Why wouldn’t the receiving craft just make small adjustments to maintain a constant 100% beam blockage? Any leakage would seem to be undesirable, assuming that there is no acceleration limit. Not only that, from estimates I’ve seen of proposed human beam-based travel, not much energy needs to be supplied. The accumulation of energy is considerable given sufficient time. Seems to suggest the power of a beam could be small even before possible 1/r^2 diminution.

    One last thought. Wouldn’t all distant scientific societies capable of beam-based travel have given interstellar radio communication some thought? Has anyone considered the probability that a craft would be sent prior to any attempt to communicate at the speed of light? An exchange could take place in the fraction of the physical one-way travel time.

    Last last thought. What if a vehicle must decelerate using beam-acquired energy? Is direct beam “push” the most efficient form of photon-to-motion conversion? Is it more efficient than using the beamed energy indirectly for some other means of propulsion? I.e., is it more efficient to to send “wind” to push a sail, or to turn a generator? How does the deceleration requirement factor into a search for signal leakage?

    • Harold February 26, 2016, 11:13

      To leakage….The beam would increase in diameter as it traveled away from the its source with the target sail covering a smaller and smaller share of that diameter. To prevent leakage, the source would need to constantly re-focus, not just move.

  • DJ Kaplan February 25, 2016, 17:23

    We send out transient signals, yet we have a valid existence.

  • Geoffreey Hillend February 25, 2016, 18:09

    his is” 1420 MHz, the 21 centimeter wavelength of neutral hydrogen.” This is the frequency we use to detect the location and movement of hydrogen dust clouds. I don’t see the logic in using a frequency that could be confused with a natural phenomenon like the little green men and the discovery neutron stars. Why not make it a unique frequency.

  • Geoffreey Hillend February 25, 2016, 18:10

    I mean the discovery of pulsars

    • Mark Zambelli February 29, 2016, 13:11

      The key thing here is the phenomena of “The Water Hole”… the atmosphere is transparent between a narrow range around Hydrogen and Hydroxyl emissions so we look there.

  • Andrew Palfreyman February 25, 2016, 18:11

    Lubin, with his recent DEEP-IN system descriptions in the news this week, asserts that his phased laser arrays are far superior to microwaves for propulsive purposes. I transcribe from the Q&A at the end here:
    Paraphrasing, 1mm microwaves vs. 1 micron laser puts 10^6 less power on the target.

    Would Dr. Benford or others care to comment?

  • Larry Kennedy February 25, 2016, 19:27

    @Jim Strom
    I too was curious if anyone had looked, for instance into converting a beam to electricity and then using just the drive portion of a bussard ramjet.

  • David February 25, 2016, 20:57

    I saw that too. NASA Gave him a grant. I have not read the whole paper yet but it looks like has a project that is so near term we might add it to Moon Mars or Interstellar space ship. I go with …Interstellar.I am a biologist. I would like to put off Mars for a while. I am afraid if we put humans and our microbiome on Mars to do on we will never know if life evolved independently there.

  • Gregory Benford February 25, 2016, 21:14

    NS: Yep, averages over minutes. But the Breakthrough Initiative aims to go for short pulses. That’s the Benford strategy.

  • Tom Baty February 25, 2016, 21:37

    “A Mote in Gods Eye” is a SF story about a discovery of an alien civilization based on this premise. The story involved alien propelling an interstellar craft using lasers instead of microwave beaming but the concept is the same. Interesting study.

  • DJ Kaplan February 26, 2016, 13:36

    Which wavelength of laser would be the most efficient/effective for this purpose?

    Physics doesn’t change from planet to planet, and it would give us a window to start looking at – assuming that we could distinguish this relatively weak signal from that of the ETC’s home star.

  • Steve Bowers February 26, 2016, 15:12

    Power beaming is such a good idea that at least a fraction of all alien civilisations would use it, unless there is a better alternative we don’t know about (like reactionless/warp drive). Even if reactionless drive is possible, not all civilisations might be expected to discover it, so some might still use power beams.

  • Michael.million February 26, 2016, 15:37

    I personally do not favour microwaves although cost wise they are better they have a poor energy transfer density. If we used a laser system although less efficient than the microwave concept the heat could be used to thermalize a large habitat to offset the inefficiency and give a very good energy transfer to a high speed craft. With a laser as I have discussed before we could use a tube that could accelerate materials to very speeds by high acceleration by recycling photons.

  • Andrew Palfreyman February 26, 2016, 17:12

    Laser power beaming is the only viable technology for rapid transport around our solar system and eventually out towards the stars. To that end, the vision must encompass a web of beamers around our system for directed acceleration and braking for a multiplicity of routes. We will weave a transportation web over our solar system made of light. What a wonderful vision that is.

    But we are absolutely going to need cheap launches to make it work such that we are lofting tons of equipment into space for deployment of this beamer web. Reusable rockets take us partway towards that goal, but the real cost savers are fully reusable technologies like StarTram and Skylon. We should be building StarTrams right now if we are serious about launching our dreams. With cheap launches we can get heavy equipment onto the Moon and Mars and take serious first steps at establishing permanent presences there.

    At least, that’s the roadmap as I see it.

    • Eniac February 27, 2016, 23:10

      Laser power beaming is the only viable technology for rapid transport around our solar system and eventually out towards the stars.

      That sounds awfully presumptive to me. How can you be sure it is indeed viable? And how can you be sure that we (or ETI) would not come up with something better, in time?

      The answer, I am afraid, is: You can’t

  • DJ Kaplan February 26, 2016, 19:35

    Transitioning from ETC power-beaming to ours…

    Andrew, do you think it would be cost-effective to install lasers on the moon, just to keep Earth’s atmosphere from robbing optical power? Or would it be easier to build a big set of Earth-bound lasers to power through it?