Science fiction dealt with interstellar navigation issues early on. In fact, Clément Vidal’s new paper, discussed in these pages yesterday, notes a George O. Smith story called “Troubled Star,” which originally ran in a 1953 issue of Startling Stories and later emerged as a novel (Avalon Books, 1957). Smith is best remembered for a series of stories collected under the title Venus Equilateral, but the otherwise forgettable Troubled Star taps into the idea of using an interstellar navigation network, one that might include our own Sun.
The story includes this bit of dialogue between human and the alien being Scyth Radnor, the latter explaining why his civilization would like to turn our Sun into a variable star:
“We use the three-day variable to denote the galactic travel lanes. Very effective. We use the longer variable types for other things – dangerous places like cloud-drifts, or a dead sun that might be as deadly to a spacecraft as a shoal is to a seagoing vessel. It’s all very logical.”
“…you’re going to make a variable star out of Sol, just for this?”
Well, why not, in Scyth Radnor’s view — after all, what’s one star in a galaxy-spanning navigation network? From our point of view, distant pulsars make for less local disruption, and as we saw yesterday, navigation by X-ray millisecond pulsars is already undergoing testing.
Image: Our early experiments, described yesterday, explore how we might use millisecond X-ray pulsars (MSPs) to provide autonomous spacecraft navigation. Credit: Astrowatch.net.
Visualizing a Pulsar Navigation Network
Using millisecond X-ray pulsars (MSPs) for galaxy-spanning navigation raises more than a few questions, especially when we try to predict what an artificial pulsar navigation system might look like to outside observers. If we are willing to posit for a moment a Kardashev II-level civilization moving between stars at relativistic velocities, then we would make as one of our predictions that such a system would be suitable for navigation at such speeds. In following the predictive model of Vidal’s paper, we would then check through our voluminous pulsar data to see how such a prediction fares. The answer, in other words, is in our datasets, and demands analyzing the viability of pulsar navigation at high fractions of c.
To my knowledge, no one has yet done this, making Vidal’s paper a spur to such research. The key here is to make predictions to see which can be falsified. But a quick recap for those just coming in on the discussion. What Vidal (Universiteit Brussel, Belgium) offers is an examination of millisecond X-ray pulsars as navigational aids, of the sort we’re already beginning to exploit through experiments via NASA, Chinese efforts and studies at the European Space Agency.
Specifically, the idea here is to develop a methodology for studying cases where astrophysical phenomena may have as one proposed explanation an extraterrestrial technology. Vidal also wonders whether we might find SETI implications even if a fully natural network like this were simply put to work by civilizations more advanced than our own, using it as we might wish to do.
Image: From Vidal’s paper, Fig. 4. Caption: A three-dimensional position fix can be obtained by observing at least three pulsars. Given three well-chosen pulsars, there is only one unique set of pulses that solve the location of the spacecraft (SC). Figure adapted from Sheikh (2005, 200).
Vidal is hoping to make predictions that are testable against our accumulating data to assess the likelihood of natural and artificial explanations, with pulsars as the case in point. The paper examines the kinds of predictions we would want to weigh against available data in a program the author calls SETI-XNAV. Whatever conclusions it reaches, such a program would improve our knowledge of pulsars themselves and our techniques at using them, possibly leading to our augmenting existing resources like the Deep Space Network with XNAV capabilities.
The author’s approach assumes that subjecting decades of data to analysis will teach us much about pulsar navigation as well as future SETI efforts:
Scientifically, SETI-XNAV is a concrete ETI hypothesis to test. The data is here, the timing and navigation functionalities are here. Historically, the suspicion of artificial canals on Mars triggered space missions to Mars and developed knowledge about Mars. Similarly, the project to try to decipher any potentially meaningful information in pulsar’s signals… could lead to the development of tools and methods that can be used for any future candidate signal.
That, of course, would augment the SETI effort as we expand into Dysonian SETI and the examination of possible engineering as the explanation for enigimatic astrophysical observations. If we assume a galaxy with a completely natural navigational system of this power, then we can imagine other civilizations putting it to use. Thus MSPs are likely to be standards in timekeeping and navigation for all putative civilizations in the Milky Way.
The Landscape of Prediction
Millisecond pulsars account for perhaps 10% of known pulsars, and as I mentioned yesterday, they appear to be distributed isotropically in the galaxy, a contrast to the rest of the pulsar population, which appears more concentrated in the galactic disk. MSPs offer numerous advantages from a navigational standpoint given that, according to Vidal, they are more than 100,000 times more stable than normal pulsars. Timing noise, an irregularity found in normal pulsars, and so-called ‘glitches’ (abrupt changes in rotation speed) are less frequent in MSPs. The latter are also associated with lower velocities than the other 90 percent of the pulsar population.
From the standpoint of artificiality, Vidal breaks the possibility terrain down seven ways (this is drawn from the paper’s Figure 1):
0 – Natural. All pulsars are natural. We are just lucky they provide stable clocks and an accurate navigation system
1 – Pulsars as standards. All pulsars are natural, but ETIs use them for timing, positioning and navigation purposes. Communication is galacto-tagged and time-stamped with a pulsar standard
2 – Natural and alterable. Some ETIs have the technology and capability to jam, spoof or interfere with a natural pulsar positioning system
3 – Artificial MSXP for navigation. Only a few millisecond X-ray pulsars have been modified by ETI for galactic navigation and timing purposes
4 – Artificial MSXP for navigation and communication. Only a few MSXPs have been modified by ETI, for navigation, timing and communication purposes
5 – Artificial pulsars. All pulsars are artificial. ETI build them, even the new ones, by intentionally triggering supernovas
6 – Artificial pulsars for us. All pulsars are artificial. ETI build them and they are currently sending us Earth-specific messages
The point here is telling for Dysonian SETI in general. We have established pulsar formation models that seem to work. To establish a program of XNAV-SETI, examining our storehouse of pulsar data, we do not need to challenge it.
But as we have learned more about pulsars over the years, we have learned that there is no unified pulsar model that explains the variety we have seen among this population. We can look toward understanding what MSPs are doing by asking what new hypotheses explain this rich set of observations.
The wide range of Vidal’s seven scenarios makes his case straightforward: “…we do not necessarily need to contradict existing pulsar models to entertain the possibility that ETI might be involved.” The issue then becomes, Vidal adds, to make and validate new predictions.
Yesterday I mentioned a recent paper examining radio pulsars in a SETI context. It was Chennamangalam, Siemion, Lorimer & Werthimer, “Jumping the energetics queue: Modulation of pulsar signals by extraterrestrial civilizations,” New Astronomy Volume 34, January 2015, pp. 245-249 (abstract). The paper examines the possibility of pulsars as ‘naturally occurring radio transmitters’ onto whose emissions information has been encoded. Vidal likewise thinks about millisecond X-ray pulsars in the context of possible information content, noting that Carl Sagan pondered studying pulsar amplitude and polarization nulls as far back as 1973.
It might be argued that communications signals would likely be compressed, making decoding extremely problematic, but Vidal’s point here is that navigational systems differ in fundamental ways from communications systems. Navigational signals should be more regular and easier to process than highly modulated signals with communications intent. If we are looking for content grafted onto the navigational signal, we can bring to bear the entire SETI toolkit, perhaps examining pulsar data in light of delay-tolerant networking and discontinuities in connectivity.
We move back into the area of predictions. World clocks on Earth are regularly re-synchronized, just as the time on global positioning satellites is synchronized through methods Vidal discusses, using a control segment that communicates with a satellite segment. Can we observe anything like this in our pulsar data? The author frames the matter this way:
The fastest and most stable MSPs might constitute such a control segment, to which the other pulsars would synchronize. Concretely, we could look for time correction signals broadcasts (that exist in GNSSs [Global Navigation Satellite Systems]), or synchronization waves. For example, synchronization might occur first on pulsars nearest the putative control segment and then diffuse to further away pulsars. This could be investigated via rare MSP glitches, or other remarkable features, such as giant pulses in MSPs.
Synchronization between MSPs would be evidence for a distributed solution on an interstellar level.
Other questions to explore: Do we find that MSPs further away from the galactic plane are more powerful than those closer in, potentially designed for low-density regions of the galaxy? Is MSP distribution random or does it show a pattern fitting the needs of galactic navigation? Estimates of the number of MSPs needed to navigate the entire galaxy might be contrasted with astrophysical predictions of the MSP population, currently estimated to be between 30,000 and 200,000. This one, of course, is tricky: We can only derive a theoretical lower boundary.
How MSPs form and evolve is fruitful ground for inquiry, given that some scientists have argued that the most commonly cited scenario for MSP evolution does not produce the X-ray MSP population we see. It is hard to see how an MSP in a non-binary system can maintain its spin without degrading over time, making the single MSP ground for study. Thus another round of prediction is possible. Single MSPs, those without an energy source, may simply be non-working parts of the network. Do we see redundancy between single and binary MSP coverage, given that binary MSPs are likely more reliable over long time periods?
What Vidal calls SETI-XNAV makes a significant departure from conventional SETI in the sense that it is not localized around a single star, but rather involves a search for a distributed signal that exists in the form of a navigation system, one either established by extraterrestrial engineering or simply relying on a natural phenomenon to pursue its own activities. That we can begin to use millisecond X-ray pulsars as navigation standards implies that more advanced civilizations have done so. Thus SETI-XNAV as constructed in this paper intends to survey the testable predictions against which we can run our expanding dataset on pulsars.
…all pulsars could be perfectly natural, but we can reasonably expect that civilizations in the galaxy will use them as standards… By studying and using XNAV, we are also getting ready to receive and send messages to ETI in a galactically meaningful way. From now on, we might be able to decipher the first level of timing and positioning metadata in any galactic communication.
But I would also emphasize that making testable predictions about pulsar navigation also exercises our skills at analyzing future astrophysical data that may prove enigmatic. That, in and of itself, is a useful contribution in this era of KIC 8462852 and ‘Oumuamua.
The paper is “Pulsar positioning system: a quest for evidence of extraterrestrial engineering,” published online in the International Journal of Astrobiology 23 November 2017 (abstract / preprint). See also Vidal, “Millisecond Pulsars as Standards: Timing, Positioning and Communication,” Proceedings IAU Symposium No. 337, edited by P. Weltevrede, B. B. P. Perera, L. Levin Preston, and S. Sanidas. Jodrell Bank Observatory, UK (2017). Preprint available.