The Forgotten Challenge: Pulsars

As stated earlier, one of the two most obvious choices for an electromagnetic beacon would be a pulsing signal with a fixed repetition rate. A fixed pulse rate would optimize a receiving civilization's possibility of finding the beacon through the use of adaptive techniques requiring minimal a priori knowledge or assumptions. In situations with moderate signal-to-noise ratios (SNR), the signal would be noticable even without advanced receiving techniques. In these cases, the fixed repetition rate would serve to call attention to the pulse sequence and possibly even suggest artificiality.

It would be left to the receiving society to aim some directive antenna in the direction of the signal source in order to maximize SNR, either as part of an intentional search or accidentally.

In fact, this is exactly what happened in 1967 when Cambridge University radio astronomers Ms. Jocelyn Bell and Dr. (now Professor) Anthony Hewish discovered first one, and then a second regular pulsing source in two widely-separated parts of the sky. Since no pulsing signal sources other than terrestrial man-made ones had ever been seen before, a strong possibility of ETI-origin was suspected. The scientists decided that, if this proved to be correct, they could not make a public announcement without checking with higher authorities. There was even some discussion about whether it might not be in the best interests of mankind to destroy the evidence and forget it! (Sturrock, 2000)

For Jocelyn Bell's own story of the events, see Little Green Men, White Dwarfs or Pulsars?

The pulsing signal finding was not published until an initially-plausible non-ETI intepretation had been constructed: highly dense compact stars (white dwarf stars) that were somehow contracting and expanding or dimming and brightening (Hewish et. al., 1968). In classic scientific tradition, the sources were labeled "LGM1," "LGM2," etc., the term 'LGM' standing for "Little Green Men"!²¹

But the idea of pulsars (and other newly-discovered astrophysical objects and phenomena) as ETI beacons must have been circulating among astronomers. In a note added to his published proceedings of a 1971 USSR conference on Communication with Extraterrestrial Intelligence (CETI),¹⁹ Sagan (1973) wrote,

"The very serious current energy problems both in quasar and in gravity wave physics can be ameliorated if we imagine these energy sources beamed in our direction. But preferential beaming in our direction makes little sense unless there is a message in these channels. A similar remark might apply to pulsars. There are a large number of other incompletely understood phenomena, from Jovian decameter bursts to the high time-resolution structure of x-ray emission which might just conceivably be due to ETI. Perhaps, in the light of Doctor Marx's presentation, we must ask if the fine structure of some fluctuating X-ray sources is due to pulsed x-ray lasers for interstellar spaceflight. But Shklovsky's principle of assuming such sources natural until proven otherwise, of course, holds. Extraterrestrial intelligence is the explanation of last resort, when all else fails.³¹
"The pulsar story clearly shows that phenomena which at first closely resemble expected manifestations of ETI may nevertheless turn out to be natural objects--although of a very bizarre sort. But even here there are interesting unexamined possibilities. Has anyone examined systematically the sequencing of pulsar amplitude and polarization nulls? One would need only a very small movable shield above a pulsar surface to modulate emission to Earth. This seems much easier than generating an entire pulsar for communications. For signaling at night it is easier to wave a blanket in front of an existing fire than to start and douse a set of fires in a pattern which communicates a desired message."

At about that time, Oliver and Billingham published the influential Cyclops Report (1972) containing what I claim to be a flawed justification for dismissing pulsed signals as probable ETI beacons in place of a search for monochromatic signals.¹⁵

Sagan's suggestion was not taken up by the astronomical community. Astronomers were unwilling to (publicly) consider an ETI-based source for the signals they were receiving. One reason they gave (Jastrow and Thompson, 1977) was that the pulse type of beacon was too wasteful of energy and wouldn't be the method they would choose.

That was an echo of Oliver's argument. But Oliver aside, refusing to examine evidence of ETI because the putative ETI behaves oddly is a commonly-encountered, and thoroughly-unsound, rationale. Here, the astronomical community was projecting our own contemporary resource limitations onto unseen and unknown ETI civilizations. Furthermore, pulsing beacons (as the Russians knew) are no more wasteful of energy than the monochromatic kind, given that the civilization on the receiving end employs matched filters and synchronous detection techniques, as discussed earlier in this essay. Such receivers would gather energy from the beacon that had been dispersed throughout the spectrum.

We have often noticed that perfectly-competent scientists lose their capacity for rational thinking when it comes to the subject of ETI actually encountered, as opposed to ETI theoretically considered. In this, scientists reveal their common humanity, and this human race has a deep fear of such an encounter.

On the subject of a civilization's resource limitations, it would be well to consider here the classification of civilizations according to the scale of their access to energy, as proposed originally by Russian astronomer Nikolai Kardashev and taken up more recently by Michio Kaku. Kardashev and Kaku visualize societies capable of harnessing the entire energy output of its planet (Type I society), its star (Type II), and its galaxy (Type III). (We would be a Type 0.) For Kaku, a Type III civilization has access to physics that we would not only not comprehend, but would not even be able to perceive. One would not have to look very high in this hierarchy of civilizations to find some for whom the efficiency of beacons would not be a consideration.

Recently, Sagan's speculations about pulsars as ETI beacons have been revived in a fascinating book, The Talk of the Galaxy, by Paul LaViolette (2000). With the benefit of years of observations made since that CETI conference in 1971, LaViolette's analysis makes an excellent case for seriously reconsidering Sagan's idea.

We will draw a bit from the history of pulsar research that he has conveniently provided, and outline some of his reasoning and key points.

The Neutron Star Lighthouse Model

After the initial two pulsars, many more were discovered, and continue to be discovered. More than 1100 are known today.

Quite early, the radially-pulsating white dwarf model had to be discarded as unrealistic when two pulsars with periods less than one tenth of a second were found in the Crab and Vela supernova remnants. Out of some twenty different theoretical models that had been proposed to explain pulsars, astronomers settled on the "neutron star lighthouse" model proposed by Thomas Gold (1968). This would be a neutron star emitting two narrow opposed beams of "synchrotron radiation" (see any astrophysics text or LaViolette). The pulses are our perception of the beams as they sweep by us, if we happen to be in the plane or cone that they sweep out.

Pulsar Diagram

The Neutron Star Lighthouse Model

Listen to The Sounds of Pulsars

The fact that some of these neutron stars would have to be spinning at a rate of many revolutions per second has not deterred astronomers from continuing to accept and develop the model, even after pulsars with millisecond periods were discovered.¹⁶

Challenging Behaviors

But the short periods have not been the only challenge to the neutron star lighthouse model. Pulsars have been found to exhibit a large number of interesting and quite intricate behaviors - behaviors that (though this may be called post hoc reasoning) fit much more easily with a model of an ETI beacon carrying information than they do with any natural-origin model that has been proposed. Astronomers and astrophysicists have been pushed to the limit as they contrive more and more intricate neutron star models to explain what they are seeing, and for some behaviors they have no explanation.

Here, then, is a brief listing of some of the key behaviors:

Time-Averaged Regularity: Time-averaged pulse contours do not change over days, months, or years. Timing of averaged profiles is similarly precise.
Single-pulse Variability: Timing and shape of individual pulses vary considerably.
Pulse Drifting (certain pulsars): Individual pulses occur successively earlier and earlier within the averaged profile. For certain drifiting pulsars, drift rate abruptly shifts in value. Or drift may be random with occasional recurring patterns.
Polarization Changes: Polarization parameters vary within individual pulses, but time-averaged profile of polarization is constant.
Micropulses: About half of observed pulsars exhibit micropulses within individual pulses. Micropulses typically last a few hundred microseconds. Or they may have oscillatory periods.
Pulse Modulation: Signal strength may wax and wane over a series of pulses. Or this may be seen only when sampling every other pulse. Or maybe only at particular times in the profile.
Pulse Nulling: Pulse transmissions may be interrupted for seconds or hours. When resumed, varying parameters continue from where they had left off!
Mode Switching: More than one stable pulsation mode, with sudden switching between them. (Bartel et. al., 1982)
Pulse Grammar: "Grammatical" switching rules.
Glitching: Pulse periods grow at a uniform rate (as though spinning pulsar is slowing down), but occasionally the period abruptly changes to a smaller value (pulsar instantaneously assumes a higher rotation rate?) and the sequence continues from there.

As the reader can imagine, the above is an extremely brief compilation of the complex behaviors of pulsars. But it is key to keep in mind that, when averaged over several minutes or so, these complexities disappear, leaving only extreme regularity.

That is important when considering pulsars as ETI beacons, because the regularity over time supports the detection of weak pulsar signals using matched detection techniques, yet the signals actually can carry information in the small-scale variations. Once the gross pulsar signal has been acquired, the receiving civilization can add resources to bring out the details.

Pulsar Spatial Distribution

The neutron star lighthouse model predicted that pulsars would be formed in supernova explosions and in fact several of them have been found near supernova remnants. If that were truly how they were formed, one would expect to find pulsars concentrated toward the center of the galaxy where most supernovas occur. However, LaViolette has noticed that the distribution of observed pulsars in the galactic plane differs markedly from that. (He also cites studies of neutron stars associated with supernova remnants showing that the stars were not formed in the supernovas.) In fact, there is a clumping of them near a point one radian to the "north" of the galactic center. There is a sharp fall-off of pulsars just beyond that point. He also noticed that some of the most unusual pulsars are found right at that edge in the distribution.

Now that is very odd because the distribution of pulsars appears to clump at that point only when seen from exactly where we are, and there is nothing special about the place where we are, except for the fact that we are in this place.

Furthermore, the clumping appears at a position that is very special. A radian is, by definition, that angle subtended by the arc of a circle whose length is equal to the radius of the circle. That makes the radian a natural (i.e., not arbitrary) unit of angular measure. A one-radian angle would be meaningful to an intelligent entity such as a human or a human society or other entity that thinks the way we do. Entities who think about geometry would most certainly have thought about this way of measuring angles.

This strongly implies that the pulsars appear where they are by design, and furthermore that the design is intended to get the attention of a society that lives exactly where we are.

Does that get your attention? It does for LaViolette and he devotes a large part of his book to it.

Shall we go on? Consider that the two fastest known pulsing pulsars are located at the two one-radian points. These pulsars have other unique features that are listed by LaViolette.

Yet there is even much more in LaViolette's thesis. As he shows, the constellation Sagita (The Celestial Arrow) is located adjacent to that point. The arrow of Sagittarius' bow (and the stinger of the Scorpion) designate the Galactic center, and the cross of Crucis marks the southern Galactic one-radian point! These are all marker images.

As we know, what we call constellations are usually not actual three-dimensional groupings of stars but the projections of star positions seen from our location, and they are supposedly the arbitrary designation of primitive observers who, among other things, wouldn't have known where the Galactic center was. (It can't be seen optically because it is obscured by interstellar dust.)

So what is going on here? The strong implication is that we are not only surrounded by signs meant to attract our attention - our attention and not just the attention of any intelligence in the galaxy - and that, sometime in the past, somebody knew it and set up the definitions of constellations to help show us.

At this point we might pause to reflect on the SETI community once again and their dogged determination to find their idea of an ETI beacon.

Pulsar Technology

Unlike Sagan, who accepted the conventional model of a pulsar but wondered if ETI could be adding fine-grained modulation, LaViolette proposes a way in which the steady emissions of stars could be focused into the pulses we see. He explains that ETI might be using a nearly-collimated beam of synchrotron radiation, applying technology that we actually are developing today. This dramatically offsets the effect of distance in the equations governing the detectability of a beacon over interstellar distances

Although we may now have or soon will have the capability to transmit focused synchrotron beams, LaViolette's transmitting society has access to energy on a scale far exceeding ours. Although pulsars are probably not neutron stars, they are still stars - white dwarfs modified to produce the pulsar signals. The short of it is that we are observing a Kardashev/Kaku Type II civilization in terms of its ability to harness the total energy of a star.

Why would we balk at such a proposition? If our physicists can propose it, should we not accept that we might have found it?

Consider further. The pulsars we detect seem to be intentionally directed to our location (not just in our direction). But might there not be beams we don't see that are directed at others?

On the other hand, perhaps they are pointing out our position to others! (Suggested by Dan Drasin, private communication.)

Whatever may be its purpose, one visualizes at this point a Galactic-scale communications network that may have been in place and functioning for what to us would be geologic time. It would be operated by a society for whom stars are playthings and galaxies are villages.

Messages

We have already referred to a kind of message given us by the pulsars: their meaning by association with constellations named and defined by we know not who. But surely if these are intentional transmitters, they themselves must be transmitting information.

In a sense, the zero-order information is that they are there and they are intentional. This information tells us that there is a Galactic society.

LaViolette pursues the issue further and discerns a first-order message as well. But at this point, we will leave the unfolding story to LaViolette and his book. We will give one hint, however. One would expect the first-order message to be of Galactic significance, and this, as divined by LaViolette, it surely is. LaViolette has looked into the astrophysical aspects of the association of several pulsars with supernova remnants and seen something that would be of critical importance to all civilizations in the Galaxy. Critical, meaning critical to their survival.

To learn more about this, see Dr. LaViolette's website.

Conventional Searches for Pulsing Signals

For the past several years, the SETI Institute's Targeted Search System (TSS) has included algorithms for detecting a variety of pulsing signal in the final analysis stages of its receivers. This is not, of course, a search for pulsars. Nor is there even any need at this time to search for pulsars, for purposes of SETI, as so many of them are already known.

TSS searches for sets of three regularly-spaced "pulses", where "pulse" is defined as energy in a frequency bin exceeding some threshold value. This constitutes a very crude pulse receiver that would not be sensitive to a wide variety of "type 2" signals as described in The SETI Search Space. For one thing, the generic pulsing signal might consist of very short pulses with concommitant broad spectra (such as pulsar pulses) whose energy would mostly be missed by the TSS algorithm. For another, the TSS algorithm can respond to only a few discrete pulse repetition rates.

The TSS pulsed signal search algorithm is best seen as an afterthought, tacked onto a receiver designed for monochromatic signals, and without even any rationale being offered in support of it.¹⁷