The Forgotten Challenge: Pulsars



Note: much of the information on this page is based on material published by Dr. Paul LaViolette in 2000. Since that time, additional pulsars have been catalogued, including additional eclipsing binary pulsars. This required a recalculation of the spatial distributions of pulsars and a review of any conclusions drawn from these distributions.

In November 2005, LaViolette published on his website an Update on conclusions presented in The Talk of the Galaxy. In his update, LaViolette noted that with 433 newly cataloged pulsars, the "clump" originally seen adjacent to the Galactic one-radian point is no longer supported. However, in new discoveries made in September 2005, the positions of the two fastest pulsars in the sky, the Millisecond Pulsar and the Eclipsing Binary Millisecond (EBM) Pulsar, as well as relations between the period of a new eclipsing binary pulsar which lies in the vicinity of the one-radian longitude, and that of the Millisecond Pulsar, all encode the constant p in remarkable ways, supporting the interpretation that "pulsar beacons" have an extraterrestrial origin.

LaViolette sends a link to current discussion (2011) at his Starburst Foundation here.

"More than 30 years after the discovery of pulsars, we still don't know how the radio waves are produced....

Explaining pulsar radiation is one of the most difficult problems of astrophysics"

- Prof. Janusz Gil
J. Kepler Astronomical Center, Zielona G6ra, Poland

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) Antony 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".35 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


Gold's neutron star pulsar model was a congenial development for astronomers because until then no neutron star had ever actually been observed. Now there was, provisionally, an explanation for pulsar signals and confirmation of the existence of neutron stars, provided subsequent pulsar observations did not cause any problems with the model. The fact that some neutron stars would be spinning at a rate of many revolutions per second did not deter astronomers from continuing to accept and develop the model, even after pulsars with millisecond periods were discovered.

Pulsar tangential velocities are very high, ranging up to 1/7 c, which is nearly a relativistic speed and which corresponds to a monumental centrifugal force. Can mere gravitational forces be sufficient to keep the object from flying apart? Well, possibly they can, if the object's mass density is sufficiently high, as the ratio of gravitational to centrifugal force is proportional to mass density. Perhaps the need for an extremely high mass density is what drove Gold to consider a neutron star explanation in the first place.


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.

In 2002, two years after the publication of LaViolette's book, Kramer et. al., in a paper summarizing the results of high-resolution single-pulse studies of the Vela Pulsar open with the statement:

"After more than thirty years of pulsar observations, the emission mechanism of pulsars is only poorly understood.”

The paper itself details a taxonomy of Vela pulsar signal characteristics, with suggestions for trying to adjust the model to accomodate everything. I think it would be fair to say, however, that the authors acknowledge unresolved difficulties in making all the details of the pulsar's microstructure fit a single model.

For another example of continuing problems in understanding the physics of pulsars, see the article headlined Pulsars "Lying About Their Ages," Astronomers Say, Throwing Theories Into Doubt from the National Radio Observatory, July 12, 2000. Also (Seiradakis 2000).

These articles describe a supernova remnant now thought to be from 39,000 to 170,000 years old, that has an associated pulsar whose age, based on the standard method of age determination, is only 16,000 years. As a news brief in Scientific American puts it, "The discrepancy implies that theories of pulsar formation and the physics of neutron stars need to be rethought." Indeed.

Dr. LaViolette enumerates the many problems faced by astronomers in understanding pulsar signals in terms of their model. He goes into some details, showing unresolved contradictions, etc. On this page I can give only a listing of the key behaviors discussed by LaViolette. My purpose is to try to convince the reader that there is good reason for considering an alternative.

Here, then, is the listing:

Time-Averaged Regularity; Single-pulse Variability
Time-averaged pulse contours are unchanged over days, months, or years. Timing of averaged profiles is similarly precise. But timing and shape of individual pulses vary considerably.16
Frequency-Dependent Pulse Profiles
In some pulsars, time-averaged profile is invariant with observing frequency. In others, shape and/or number of components in profile changes radically with frequency (e.g., Crab pulsar).
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 with time during individual pulses, in a pattern that itself changes from pulse to pulse, but the variation of polarization in the time-averaged profile is constant.
Micropulses (ultra-short intensity variations within individual pulses)
About half of observed pulsars exhibit micropulses within individual pulses. Micropulses typically last a few hundred microseconds. Or they may have oscillatory periods.
Pulse Amplitude Modulation
Signal strength may wax and wane over a series of pulses with period 2 to 20 times longer than the primary pulse period. The period of this variation can be a function of the "phase" (position) in the profile or can correlate with a pulse at a different phase and time lag. This may be seen only when sampling every other pulse.
Fixed Characteristics
Each pulsar’s specific characteristics of pulse modulation and drifting remain fixed for years.
Pulse Nulling and Freezing
Pulse transmissions may be interrupted for seconds or up to eight hours. Some studies show that the pulsar continues to transmit, but at a very low intensity. During nulling, drift rate becomes exceedingly slow. When normal transmission is resumed, pulses continue from almost the exact position in the profile where they had left off!
Mode Switching
More than one stable pulsation mode, each highly stable for ~10-10000 periods. Abrupt switching between modes occurs in as little as one pulse period.

Mode switching can

Alter shape of time-averaged pulse profile

Restructure pulse drifting and modulation

Alter polarization properties

Change how profile component intensities vary as function of radio frequency

But the exceedingly precise primary period and period derivative remains unchanged. (Bartel et. al., 1982)
Frequency-Dependent Mode Switching
Pulsars with mode switching have different switching modes, or different numbers of available modes, at different observing frequencies
Mode Switching Grammar.(Example: PSR 0031-07)
There are "bursts" of pulses separated by null periods. Three pulsation modes are identified: A, B, and C, with quantized drift rates in the proportion 1:2:3. Within a burst, mode A may switch to mode B, and B may switch to C.
Period glitching in 21 pulsars including Crab, Vela
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, but relaxes over several weeks to the previous period.

As the reader can imagine, the above is an extremely brief compilation of the complex behaviors of pulsars. Each of these behaviors is described in full detail in the literature. But a key point to keep in mind is 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, while 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.



Positions and Unique Features

Milky Way with 1-Radian Markers

This is an equal-area projection of the Milky Way in galactic coordinates. North galactic longitude is to the left. The view shows stars and constellations, not pulsars. The distribution of pulsars drops abruptly near 1 radian north of the Galactic Center - as seen within 53 l.y. from our location. The profile of supernova remnants does not show this drop-off. There is an anomalous concentration of pulsars at the south 1-radian point. The two fastest known pulsars are located at the two points.

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 (2-1/2 fold) 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 concentrate at that point only when seen from near where we are (within 53 l.y of our location), and there is nothing special about the place where we are, except for the fact that we are in this place.

Furthermore, the concentration 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? Here is the all-sky view with four pulsars shown. These are four of the six known eclipsing binary pulsars - binaries in whose orbital plane we are located. Notice that one each of these pulsars is located at a 1-radian longitude position; the other two are at the Galactic Center!

Eclipsing Binaries

It also happens that the pulsars at the two 1-radian positions are the two fastest-known-pulsing pulsars! These pulsars have other unique features that are described by LaViolette, some of which are detailed below. And there is still much more in his thesis. To continue, take a closer look at the north 1-radian position:

1 Radian North Longitude

Pulsar 1937-21, the "Millisecond Pulsar", is the fastest known, flashing 642 times per second. Its period is extremely constant at 3.3 x 10-12 sec/yr (better than our best atomic clocks). This is the most luminous of all millisecond pulsars with 10-100 times more energy than typical ones. It is one of only three optical pulsars and the only millisecond optical pulsar. 1937-21 is one of only two pulsars that emit giant pulses. And it has the lowest proper motion of any pulsar.

1957-20 is also a millisecond pulsar as well as being the eclipsing binary mentioned above that is stationed at this point. It is the second fastest millisecond pulsar, with period just 3% longer than that of the Millisecond Pulsar. Its period is even more constant, at 0.5 x 10-12 sec/yr.

A third pulsar appears in this little cluster: 1930-22 in the constellation Vulpecula.

And there is yet another interesting astronomical feature at this position. Sagitta, the Celestial Arrow constellation, is here, and Gamma Sagittae, the point of the arrow, is right at 1 radian north galactic longitude. Do you think there is any meaning in that? Well, hold your opinion until we have discussed the Galactic Center and 1 radian south.

Galactic Center

Two of the six known eclipsing binary pulsars are here: 1718-19 and 1744-24A. Here is another "arrow point": that of Sagittarius' arrow, as well as the "sting" of Scorpius.

1 Radian South Longitude

At 1 radian south longitude we find Pulsar 1259-63, another of the four eclipsing binary pulsars located at these special positions. (Is there any doubt at this point that they are special?) The constellation located here is Crux, the Southern Cross.

Consider now the symbols represented by the constellations or the particular stars associated with these three positions: Arrow Point, Arrow Point, Sting, Cross. Each has meaning as a focal point. Now how might that have come about? Clearly, "constellations" are arbitrary groupings of stars in their two-dimensional positions on the celestial sphere. The symbolic meanings they have been given are seen by our modern culture as quaint and fanciful products of simpler and more naive times. How odd, then - how very coincidental that these particular ones have been given symbolic meanings associated with pointing out something - something quite invisible to the stargazers of old. That is, pulsar with unique qualities, the (hidden) center of the Milky Way Galaxy, and 1-radian displacements therefrom, a "radian" itself being a unit of angular measure not yet thought of, at those times.

What are the implications of all this? How does it happen that very special objects are located at very special positions, as seen only by observers located in the vicinity of our solar system? LaViolette concludes, and at this point I have no difficulty in supporting him in this, that the pulsars are there due to someone's intervention. And the almost inescapable other conclusion is that the constellations were named by beings who understood what was going on, and who had to mark those postions so as to draw the attention of a future technological society. Those are strong words, but Open SETI is dedicated to paying attention to evidence of extraterrestrial civilizations, and here we have surely found such.

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

Is it possible that pulsars could be engineered objects? 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 white dwarf or X-ray stars could be focused into the pulses we see. He explains that ETI might be using projected magnetic fields to focus the particle flux from these stars into a nearly-collimated beam of synchrotron radiation. He points to rumors of present-day military technology that projects force fields and aerial plasmoids. Sagan's "smoke signal blanket" is retained after all.

Note that even the neutron star lighthouse model invokes standing wave magnetic field patterns to modulate the particle flux.

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.

For those interested, a concise review of what Dr. LaViolette believes may be the message of critical importance to civilizations in our Galaxy (or in any galaxy) can be found here. Of course, the ultimate reference to Dr. LaViolette's ideas is his own website.


Updates

In November 2005, LaViolette updated his discussion of pulsars as possible ET technology by incorporating data on an additional 433 pulsars that had been located in the Parkes Multibeam Pulsar Survey. In the large, LaViolette finds that the new data support his thesis of ET origin. See his Update on conclusions presented in The Talk of the Galaxy.

LaViolette sends a link to current discussion (2011) at his Starburst Foundation here.

Conventional Searches for Pulsing Signals

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

TSS searched for sets of three regularly-spaced "pulses", where "pulse" was defined as energy in a frequency bin exceeding some threshold value. This constituted a very crude pulse receiver that would not be sensitive to a wide variety of "type 2" signals as was described in TSS. For one thing, the generic pulsing signal might have consisted 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 could 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

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