Astrobiologists believe that if extraterrestrial life exists, it could have started 2 billion years before life appeared on Earth. Given the amazing rate of technological progress we have been experiencing in the past century, it is entirely conceivable that an extraterrestrial civilization far more technologically advanced than us could navigate the galaxy. How could they do it? Could ET have a GPS adapted to the galaxy?
Navigation techniques
Navigation systems such as GPS work by comparing the arrival times of signals sent by different satellites orbiting the Earth. Our smartphones then combine them to give our position with a geometric technique called trilateration (see fig. 1).
Fig 1. An example of trilateration in two dimensions. Imagine that you are lost in Europe. A friend tells you that you are 264 km from Paris. You can therefore be anywhere on the circle that has Paris as its center and 264 km as its radius (black circle). Fortunately, you have a second friend who states you are 764 km from Berlin (blue circle). You can now only be at one or the other intersection of these two circles. A third friend tells you that you are 2369 km from Rome (green circle). Now there is no ambiguity, you are at the only intersection of the three circles, i.e., in Brussels.
GPS is revolutionary because it can serve an unlimited number of users. Since radio signals can go through clouds, rain and snow, it works in all weather conditions. The system is autonomous and passive: our smartphones only receive data from GPS satellites, such as distance and time. This is why navigation works even in airplane mode.
However, GPS no longer works as you move away from Earth. Instead, human-made probes are guided via Earth-based stations, mainly through NASA’s "Deep Space Network". These transmission stations offer active communication between the probes and three huge radio antennas located in Spain, California, and Australia. Unfortunately, this method results in larger and larger errors as the probes move further away from Earth. At the distance of Pluto, the uncertainty on the position of the probe is 200 km. Beyond the solar system, the terrestrial signals become too weak to be detected. Moreover, only one probe can be guided at a time by radio antenna. It can guide more if it switches from one probe to another.
The ideal would be to have a kind of GPS for the galaxy, which would allow all the probes and spacecraft to navigate in an autonomous, reliable and accurate way.
Pulsar Navigation
Pulsars - short for “pulsating stars” - are formed when some giant stars explode into a supernova at the end of its life. This core is incredibly dense: a matchbox of matter that composes it would weigh 3 billion tons! They emit a regular electromagnetic radiation beam from their poles that we observe on Earth as a pulsation that corresponds with each rotation of the pulsar.
The idea to use pulsars for positioning and navigation was first proposed in the 1970s. Carl and Linda Sagan with Frank Drake famously composed a message for hypothetical extraterrestrials who would find Pioneer 10, the first probe to leave the solar system (Fig 2).
Fig. 2. The plaque of the Pioneer 10 probe. On the left, the Sun is located in relation to 14 pulsars and the center of the galaxy.
As Carl Sagan half-joked in a TV interview, the right side of the plaque would probably be quite enigmatic for an extraterrestrial, while the left part would be perfectly clear: it is the position of our Sun in relation to 14 pulsars and the center of the galaxy. However, the use of such normal pulsars to navigate is limited because the positional errors are of the order of 1500 km. In 1993, the discovery of a new class of pulsars, millisecond pulsars, renewed the interest for pulsar navigation. Indeed, these pulsars rotate about 500 times per second, which is 1000 times faster than normal pulsars. Millisecond pulsars are also 100 000 times more stable than normal pulsars, a stability comparable to atomic clocks.
Today, interest in pulsar navigation is lively. Recently, China and the United States have sent missions into space to test it. In November 2017, NASA's SEXTANT (Station Explorer for X-ray Timing and Navigation Technology) mission demonstrated a system aboard the International Space Station that uses pulsars to navigate in space, or PPS for “Pulsar Positioning System”. The mission was considered a success, as engineers were able to determine the position of the space station with an accuracy of 5 km thanks to four millisecond pulsars.
Pulsar navigation works similarly to GPS: one compares arrival times of the signals of at least four millisecond pulsars to determine one’s position in the galaxy (see fig. 3).
Fig 3. Determination of position with 4 pulsars. Similarly to trilateration, by choosing well 4 pulsars whose position is well known, only one possible solution remains, the intersection of the different signals of these pulsars. These intersections are represented by the black points. (Figure from Becker, Bernhardt, Jessner 2013, arXiv:1305.4842).
Unlike guided navigation from Earth, pulsar navigation is autonomous and passive, and can be used by an unlimited number of probes. Navigation also works in all galactic weather conditions, because millisecond pulsars emit X-rays that penetrate the gas clouds and nebulae of the interstellar medium.
Pulsars as standards
If pulsars have allowed us to signal our position with the Pioneer 10 plaque, by symmetry, could extraterrestrials also use them to signal their own position? It would indeed be natural that civilizations in the galaxy have converged on a space and time standard based on pulsars.
Moreover, in any transmission, communication is divided into two parts, one is about the communication itself, and the other is the metadata. For example, even if I send a friend a secret letter containing a coded message, there will always be my friend's address on the envelope (space coordinates), and the postmark (time coordinates).
This logic suggests that even if it turns out to be impossible to decode an extraterrestrial message - because it is encrypted - we would still have a chance of extracting its space and time coordinates. The proof of the existence of extraterrestrials could therefore come from an analysis of the metadata of a message, not from the message itself.
In 1973, Francis Crick, the co-discoverer of the DNA molecule, speculated that an advanced civilization might decide to deliberately send living organisms to other planets, a hypothesis called “directed panspermia”. Today we know that this project is potentially greatly facilitated by the Pulsar Positioning System which allows, as soon as a civilization has the means and motivation, to send very precisely ships, organisms or artifacts to any habitable planet in the galaxy. A chilling idea for sure!
Galactic engineering?
But we can go even further and ask: could these millisecond pulsars be modified, accelerated, or moved by extraterrestrials for the purpose of galactic navigation? To try to answer this question, we must use the fields of astrophysics, astrobiology, and space navigation to test whether we are dealing with extraterrestrial engineering or not. Let's look at two possible tests.
First, what is the distribution of millisecond pulsars in the galaxy? An important property of time-of-arrival navigation systems is the homogeneous distribution of information sources. In the trilateration example (Fig. 1), if your three friends had been in three different districts of Paris, their information would have been redundant and insufficient to determine your position. The same is true for GPS, where the satellites are carefully positioned to be both sufficiently far apart and to service the entire surface of the Earth.
For navigation with pulsars, remember that only millisecond pulsars are useful for navigation because they are much faster and more stable than normal pulsars. Their historical formation is less simple than that of normal pulsars, because they acquired their speed by absorbing matter from a companion star. Their distribution in the galaxy appears more homogeneous than the one of normal pulsars (Fig. 4). Is their distribution in the galaxy, their emission power and their number close to an optimal configuration for a galactic navigation system? If so, could this be an indication of extraterrestrial engineering?
Fig. 4. Distribution of pulsars in galactic coordinates. Normal pulsars are concentrated in the Galactic disk (top); millisecond pulsars are more evenly distributed (bottom). Acknowledgements: Duncan Lorimer.
A second test could be based on the search for clock synchronization signals. The atomic clocks in GPS satellites are regularly synchronized by a control base located in Colorado. Could such clock correction signals also be broadcasted towards millisecond pulsars?
It has long been said that pulsars are the lighthouses of the galaxy. Today we know that they act also as a Pulsar Positioning System, perhaps even engineered by extraterrestrials. In any case, our galaxy does have its GPS, and all that remains is to explore it.
Dr. Clément Vidal
Vrije Universiteit Brussel, Center Leo Apostel &
University of California Berkeley, Berkeley SETI Research Center
Text originally published in French in Le Figaro Science, Hors Série,
November 2018, pp 66-69.
Acknowledgements
Thanks to Evan Sneed for comments and edits on the English version.
References
McNamara, G. 2008. Clocks in the Sky: The Story of Pulsars. Springer-Praxis Books in Popular Astronomy. Berlin: Springer.
Vidal, C. 2019. “Pulsar Positioning System: A Quest for Evidence of Extraterrestrial Engineering.” International Journal of Astrobiology 18 (3): 213–34. doi:10.1017/S147355041700043X. https://arxiv.org/abs/1704.03316.