Centauri Dreams
Imagining and Planning Interstellar Exploration
How Far Can Civilization Go?
Robert H. Gray, author of The Elusive Wow: Searching for Extraterrestrial Intelligence, has searched for radio signals from other worlds using the Very Large Array and other radio telescopes. You’ll find numerous links to his work in the archives here. In today’s essay, Gray takes a look at a classic benchmark for assessing the energy use of civilizations, introducing his own take on Earth’s position in the hierarchy and how these calculations affect the ongoing SETI effort. His article on the extended Kardashev scale appeared in The Astronomical Journal https://iopscience.iop.org/article/10.3847/1538-3881/ab792b. Photograph by Sharon Hoogstraten.
by Robert H. Gray
Human civilization has come an amazingly long way in a short time. Not long ago, our major source of energy was muscle power, often doing hard work, while today much more energy is available from fuels, fission, hydro, solar, and other sources without breaking a sweat. How far can civilization go?
It’s probably impossible to say how far civilizations can go in areas like art or government, because such things can’t be measured or forecast, but energy use is measurable and has trended upward for centuries.
The astrophysicist Nikolai Kardashev outlined a scheme for classifying civilizations according to the amount of energy they command, in order to assess the type of civilization needed to transmit information between stars. He defined Type I as commanding the energy available to humanity in 1964 when he was writing, Type II could harness the energy of a star like our Sun, and Type III would possess the energy of all of the stars in a galaxy like our Milky Way.
Harnessing the energy of stars might sound like science fiction, but solar panels are already turning sunlight into electricity at a modest scale, on the ground and in space. Gerald O’Neill and others have envisioned orbiting space settlements soaking up sunshine, and Freeman Dyson envisioned something like a sphere or swarm of objects capturing all or much of a star’s light.
Carl Sagan suggested using Arabic numerals instead of Kardashev’s Roman numerals, to allow decimal subdivisions, and he suggested more uniform power levels. He re-defined Type 1 as 1016 watts—very roughly the Sun’s power falling on the Earth—and he rounded off Type 2 and 3 levels to 1026 and 1036 watts respectively, so planetary, stellar, and galactic categories increase in steps of 1010 or ten billion. A simple formula converts power values into decimal Types (the common logarithm of the power in megawatts, divided by ten). In the recent year 2015, human power consumption was 1.7×1013 watts, or Type 0.72—we’re short of being a Type 1.0 planetary civilization by a factor of roughly 600. In 1800 we were Type 0.58, and in 1900 we were Type 0.61.
The 2015 total power consumption works out to an average of 2,300 watts per person, which is 23 times the 100 watts human metabolism at rest, but it’s not many times more than the 500-1,000 watts a human can produce working hard. Maybe we haven’t gone all that far, yet.
I recently extended the scale. Type 0 is 106 watts or one megawatt, which is in the realm of biology rather than astronomy—the muscle power of a few frisky blue whales or several thousand humans. That seems like a sensible zero point, because a civilization commanding so little power would not have enough to transmit signals to other stars. Type 4 is 1046 watts, roughly the power of all of the stars in the observable Universe.
One use for the scale is to help envision the future of our civilization, at least from the perspective of energy. If power consumption increases at a modest one percent annual rate, we will reach planetary Type 1 in roughly 600 years and stellar Type 2 in 3,000 years—roughly as far in the future as the Renaissance and ancient Greece are in the past. That simplistic growth rate would put us at galactic scale Type 3 in 5,000 years which is almost certainly wrong, because some parts of our galaxy are tens of thousands of light years away and we would need to travel faster than light to get there.
There are, of course, many limits to growth—population, land, food, materials, energy, and so on. But humans have a history of working around such limits, for example producing more food with mechanization of agriculture, more living space with high rise buildings, and more energy from various sources. It’s hard to know if our civilization will ever go much beyond our current scale, but finding evidence of other civilizations might give us some insight.
Another use for the scale is to help envision extraterrestrial civilizations that might be transmitting interstellar signals, or whose large-scale energy use we might detect in other ways.
If we envision ET broadcasting in all directions all of the time, they would need something like 1015 watts or 100,000 big power plants to generate a signal that our searches could detect from one thousand light years away using the 100-meter Green Bank Telescope. That means we need to assume at least a Type 0.9 nearly planetary-scale civilization—and considerably higher if they do anything more than broadcast—a civilization hundreds or thousands of times more advanced than ours. That seems awfully optimistic, although worth looking for. If we envision civilizations soaking up much of a star’s light with structures like Dyson spheres or swarms, then unintentional technosignatures like waste heat re-radiated in the infrared spectrum conceivably could be detected. Some infrared observations have been analyzed along those lines, for example by Jason Wright and associates at Penn State.
If, on the other hand, we envision ET transmitting toward one star at a time using a big antenna like the 500 meter FAST in China, then we need to assume only something like 108 watts or one-tenth of one big power plant, although the signal would be detectable only when the antenna’s needle beam is pointed at a target star. To catch intermittent signals like that, we will probably need receiver systems that monitor large areas of sky for long periods of time—ideally, all-sky and full-time—and we can’t do that yet at the microwave frequencies where many people think ET might transmit. A modest prototype microwave receiver system called Argus has been monitoring much of the sky over Ohio State University in Columbus for a decade with very low sensitivity, and an optical system called PANOSETI (Panoramic SETI) is planned by Shelly Wright of UCSD and Paul Horowitz of Harvard to potentially detect lasers illuminating us.
Detecting some signature of technology near another star would be a historic event, because it would prove that intelligence exists elsewhere. But the U.S. government has not funded any searches for signals since Sen. Richard Bryan (D-NV) killed NASA’s program in 1993, even though thousands of planets have been discovered around other stars.
Both Kardashev and Sagan thought civilizations could be characterized by the amount of information they possess, as well as by energy. An information scale much like the energy scale can be made using 106 bits or one megabit as a zero point—roughly the information content of one book. Sagan thought that 1014 or 1015 bits might characterize human civilization in 1973 when he was writing on the topic, which would be Type 0.8 or 0.9 using the power formula (he used the letters A, B, C… for 106, 107, 108… bits, but letters don’t allow decimal subdivisions). More recent estimates of humanity’s information store range from 1018 to 1025 bits or Types 1.2 to 1.5, depending on whether only text is counted, or video and computer storage are included.
Nobody knows what information interstellar signals might contain. Signals could encode entire libraries of text, images, videos, and more, with imagery bypassing some translation problems. What might motivate sending information between stars is an open question; trade is one possible answer. Each world would have its own unique history, physical environment, and biology to trade—and conceivably art and other cultural stuff as well. Kardashev thought that the information to characterize a civilization could be transmitted across the Galaxy in one day given sufficient power.
Whether any interstellar signals exist is unknown, and the question of how far civilization can go is critical in deciding what sort of signals to look for. If we think that civilizations can’t go hundreds or thousands of times further than our energy resources, then searches for broadcasts in all directions all of the time like many in progress might not succeed. But civilizations of roughly our level have plenty of power to signal by pointing a big antenna or telescope our way, although they might not revisit us very often, so we might need to find ways to listen to more of the sky more of the time.
Additional Resources
N. S. Kardashev, Transmission of Information by Extraterrestrial Civilizations, SvA 8, 217 (1964).
C. Sagan, The Cosmic Connection: An Extraterrestrial Perspective, Doubleday, New York (1973).
V. Smil, Energy Transitions: Global and National Perspectives, 2nd edition, Praeger (2017).
R. H. Gray, The Extended Kardashev Scale, AJ 159, 228-232 (2020). https://iopscience.iop.org/article/10.3847/1538-3881/ab792b
R. H. Gray, Intermittent Signals and Planetary Days in SETI, IJAsB 19, 299-307 (2020). https://doi.org/10.1017/S1473550420000038
A Chronological Look at a Transiting Earth
Call it the Earth Transit Zone, that region of space from which putative astronomers on an exoplanet could see the Earth transit the Sun. Lisa Kaltenegger (Cornell University) is director of the Carl Sagan Institute and the author of a 2020 paper with Joshua Pepper (LeHigh University) that examined the stars within the ETZ (see Seeing Earth as a Transiting World).
While Kaltengger and Pepper identified 1004 main sequence stars within 100 parsecs that would see Earth as a transiting planet, Kaltenegger reminds us that stars are ever in motion. Given the abundant resources available in the European Space Agency’s Gaia eDR3 catalog, why not work out positions and stellar motions to examine the question over time? After all, there are SETI implications here. We study planetary atmospheres using data taken during transits. Are we, in turn, the subject of such study from astronomers elsewhere in the cosmos?
Thus Kaltenegger’s new paper in Nature, written with Jackie Faherty (American Museum of Natural History), which identifies 2,034 nearby star systems (within the same 100 parsecs, or 326 light years) that either could have seen the Earth transiting by observing our Sun within the last 5,000 years or will be able to within the same span of time going forward. Kaltenegger takes note of the dynamic nature of the dataset:
“From the exoplanets’ point-of-view, we are the aliens, we wanted to know which stars have the right vantage point to see Earth, as it blocks the Sun’s light. And because stars move in our dynamic cosmos, this vantage point is gained and lost.”
Image: With the plane of the Milky Way galaxy seen stretching from the top to the bottom of the image, this artistic view of the Earth and Sun from thousands of miles above our planet, shows that stars (with exoplanets in their own system) can enter and exit a position to see Earth transiting the Sun. Credit: Kaltenegger & Faherty/Cornell University.
Looking at the results more closely, we find 117 stars over the 10,000 year window that are within 100 light years of our Solar System, while 75 of these stars have been in the Earth Transit Zone since the advent of commercial radio roughly 100 years ago. From the paper:
Among those sources, 29 were in the ETZ in the past, 42 will enter it in the future, and 46 have been in the ETZ for some time. These 46 objects (2 F, 3 G, 2 K and 34 M stars and 5 WDs [white dwarfs]) would be able to see Earth transit the Sun while also being able to detect radio waves emitted from Earth, which would have reached those stars by now… Seven of the 2,034 stars are known exoplanet host stars… Four of the planet hosts are located within 30 pc of the Sun.
It’s intriguing to look at some well known systems in this context. Trappist-1, for example, with its seven transiting worlds, will not enter the Earth Transit Zone for 1,642 years, but once within it, will have the ability to see a transit for 2,371 years.
In fact, even the closest stars tend to spend a millennium or more in the ETZ once there, plenty of time for extraterrestrial astronomers, if such exist, to take note of biological and/or technological activity on our world. Ross 128 is another interesting system. Here we have the second-closest known possibly temperate exoplanet after Proxima b, orbiting a red dwarf in Virgo about 11 light years out. Denizens of this world had a 2,158 year window they entered about 3,057 years ago but moved out of 900 years ago.
As I’m curious about unusual venues for potential life, I found this interesting:
109 of the objects in our catalogue are WDs, dead stellar remnants. Whereas most searches for life on other planets concentrate on main sequence stars, the recent discovery of a giant planet around a WD opened the intriguing possibility that we might also find rocky planets orbiting evolved stars. Characterizing rocky planets in the HZ of a WD would answer intriguing questions on lifespans of biota or a second ‘genesis’ after a star’s death.
It should come as no surprise that Kaltenegger and Faherty find M dwarfs dominating the spectral types, given their wide distribution n the galaxy; 1,520 of these stars are M-dwarfs. They also find 194 G-class stars like the Sun and a range of other stellar types, of which 102 are K-class stars like Alpha Centauri B. At the present time, 1,402 stars within the 100 parsec bubble can see Earth as a transiting world. Early observations of stars in the ETZ have begun via Breakthrough Listen as well as the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China.
While technological activity may be difficult to observe, the authors point out that Earth’s biosphere has been at work on its atmosphere for billions of years, meaning observations of Earth transits would have identified it as a living world ever since the Great Oxidation Event. A transiting Earth’s biosignature should be hard to miss.
The paper is Kaltenegger & Faherty, “Past, present and future stars that can see Earth as a transiting exoplanet,” Nature 594 (23 June 2021), 505-507 (abstract).
Mixing and Growth in the Sun’s Protoplanetary Disk
The Allende meteorite is the largest carbonaceous chondrite meteorite ever discovered. Falling over Mexico’s state of Chihuahua in 1969 and breaking up in the atmosphere, the object yielded over two tons of material that have provided fodder for scientists interested in the early days of the Solar System. The meteorite contains numerous calcium-aluminum-rich inclusions (CAIs), which are considered to be the first kind of solids formed in the system 4.5 billion years ago.
Samples of the Allende meteorite are considered ‘primitive,’ which in this parlance means unaffected by significant alteration since formation. Now a team led by Tom Zega (University of Arizona Lunar and Planetary Laboratory) has gone to work on a dust grain from this object, in order to simulate the conditions under which it formed in the Sun’s protoplanetary disk. The grain was drawn from one of several CAIs discovered in the Allende meteorite sample. Analysis of the sample’s chemistry and crystal structure provides a look at the conditions that produced it.
The work appears in The Planetary Science Journal and is, according to Zega, something of a ground-breaker:
“As far as we know, our paper is the first to tell an origin story that offers clues about the likely processes that happened at the scale of astronomical distances with what we see in our sample at the scale of atomic distances.”
Image: A slice through an Allende meteorite reveals various spherical particles, known as chondrules. The irregularly shaped “island” left of the center is a calcium-aluminum rich inclusion, or CAI. The grain in this study was isolated from such a CAI. Credit: Shiny Things/Wikimedia Commons.
To examine the CAI, the researchers used a scanning transmission electron microscope located at the University of Arizona as well as a twin instrument at the Hitachi factory in Hitachinaka, Japan. The microstructure of the sample was examined at varying scales down to the level of individual atoms, in a process co-author Venkat Manga (UA) likens to opening a book recording what happened 4.567 billion years ago in the nebula that gave birth to the Sun.
It’s a book with an intriguing plot. The investigation revealed types of minerals called spinel and perovskite. Examined at the level of atomic-scale crystal structure, the sample creates a puzzle, with Zega noting that the chemical pathways that produced it seem to contradict current theories on the processes at work in protoplanetary disks. I like the way Zega puts this: “Nature is our lab beaker, and that experiment took place billions of years before we existed, in a completely alien environment.”
The paper recounts the team’s effort to create new formation models that could produce the sample CAIs found in the Allende object, simulating their chemistry and cutting across the fields of materials and mineral science as well as microscopy in the kind of multidisciplinary challenge that the study of planetary systems involves. Just how wide-ranging such work can be is made clear in the paper:
Our motivation is to understand the micro- and atomic-scale structure of the materials reported herein, which defy existing thermochemical models of the early solar protoplanetary disk. To this end, we combine state-of-the-art nanoscale to atomic-scale characterization of several CAIs with quantum-mechanical, thermodynamic modeling and dust-transport calculations to explain their origin. We propose that such an integrated approach is an important step toward a more comprehensive paradigm for reverse engineering the microstructures observed within primitive meteorites and extracting from them the thermal and dynamic histories that they have recorded.
The history of the particle at the heart of the model is worked out in detail, showing us a dynamic journey moving from a region of the disk near Earth’s current orbit to close proximity to the Sun and later transport to cooler regions further out. The sample winds up being part of an asteroid which, as it fragmented, became the parent body of meteorites like the Allende object. Crucial here is the history of movement and mixing within the disk, which challenges earlier, more static models.
It is clear from the above observations and computational effort that some assemblages within the oldest solar system solids deviate from long-standing assumptions and, rather than remaining in contact with the same parcel of gas in a monotonically cooling system, are admixed into other regions, possibly warming once or multiple times before cooling down in the disk. Indeed, this mixing and transport are necessary for the preservation of CAIs for millions of years and the delivery of CAI-like grains to outer regions of the solar nebula to be accreted by comets as revealed by the Stardust mission (Brownlee et al. 2006).
Image: Illustration of the dynamic history that the modeled particle could have experienced during the formation of the solar system. Analyzing the particle’s micro- and atomic-scale structures and combining them with new models that simulated complex chemical processes in the disk revealed its possible journey over the course of many orbits around the sun (callout box and diagram on the right). Originating not far from where Earth would form, the grain was transported into the inner, hotter regions, and eventually washed up in cooler regions. Credit: Heather Roper/Zega et al.
The authors say that their work is part of a longer-term effort to “deconstruct CAIs phase by phase,” so we should be hearing more about these methods. It will be interesting to see how asteroid sample return missions provide further insights.
We are digging here into the nature of planet formation as we try to understand how material moves around inside a protoplanetary disk. These are presumably processes common to the emerging systems we also see with the Atacama Large Millimeter/submillimeter Array (ALMA), whose images show the earliest stages of stellar system growth. But examining that commonality with a critical eye will be part of the continuing study of evolving disks and the movement of materials within them.
The paper is Zega et al., “Atomic-scale Evidence for Open-system Thermodynamics in the Early Solar Nebula,” Planetary Science Journal Vol. 2, No. 3 (17 June 2021). Full text.
Email Subscribers Take Note
Google will no longer be supporting its email distribution service as of July 1, and I am preparing for this through the work of my friend Frank Taylor, who is fine-tuning a replacement. However, I’ve had a few reports already of emails not being delivered. So if you are an email subscriber to Centauri Dreams, please bear with us as Frank gets the new service up and running. This may take a few more days. There will be no need to re-subscribe, as the existing subscription list will be transferred to the new feed.
An AI Toolbox for Space Research
Let’s take a brief break from research results and observational approaches to consider the broader context of how we do space science. In particular, what can we do to cut across barriers between different disciplines as well as widely differing venues? Working on a highly directed commercial product is a different process than doing academic research within the confines of a publicly supported research lab. And then there is the question of how to incorporate ever more vigorous citizen science.
SpaceML is an online toolbox that tackles these issues with a specific intention of improving the artificial intelligence that drives modern projects, with the aim of boosting interdisciplinary work. The project’s website speaks of “building the Machine Learning (ML) infrastructure needed to streamline and super-charge the intelligent applications, automation and robotics needed to explore deep space and better manage our planetary spaceship.”
I’m interested in the model developing here, which makes useful connections. Both ESA and NASA have taken an active interest in enhancing interdisciplinary research via accessible data and new AI technologies, as a recent presentation on SpaceML notes:
NASA Science Mission Directorate has declared [1] a commitment to open science, with an emphasis on continual monitoring and updating of deployed systems, improved engagement of the scientific community with citizen scientists, and data access to the wider research community for robust validation of published research results.
Within this context, SpaceML is being developed in the US by the Frontier Development Lab and hosted by the SETI Institute in California, while the UK presence is via FDL at Oxford University and works in partnership with the European Space Agency. This is a public-private collaboration that melds data storage, code-sharing and data analysis in the cloud. The site includes analysis-ready datasets, space science projects and tools.
Bill Diamond, CEO of the SETI Institute, explains the emergence of the approach:
“The most impactful and useful applications of AI and machine learning techniques require datasets that have been properly prepared, organized and structured for such approaches. Five years of FDL research across a wide range of science domains has enabled the establishment of a number of analysis-ready datasets that we are delighted to now make available to the broader research community.”
The SpaceML.org website includes a number of projects including the calibration of space-based instruments in heliophysics studies, the automation of meteor surveillance platforms in the CAMS network (Cameras for Allsky Meteor Surveillance), and one of particular relevance to Centauri Dreams readers, a project called INARA, which stands for Intelligent ExoplaNET Atmospheric RetrievAl. Its description:
“…a pipeline for atmospheric retrieval based on a synthesized dataset of three million planetary spectra, to detect evidence of possible biological activity in exoplanet atmospheres.”
SpaceML will curate a central repository of project notebooks and datasets generated by projects like these, with introductory material and sample data allowing users to experiment with small amounts of data before plunging into the entire dataset. New datasets growing out of ongoing research will be made available as they emerge.
I think researchers of all stripes are going to find this approach useful as it should boost dialogue among the various sectors in which scientists engage. I mentioned citizen scientists earlier, but the gap between academic research labs, which aim at generating long-term results, and industry laboratories driven by the need to develop commercial products to pay for investment is just as wide. Availability of data and access to experts across a multidisciplinary range creates a promising model.
James Parr is FDL Director and CEO at Trillium Technologies, which runs both the US and the European branches of Frontier Development Lab. Says Parr:
“We were concerned on how to make our AI research more reproducible. We realized that the best way to do this was to make the data easily accessible, but also that we needed to simplify both the on-boarding process, initial experimentation and workflow adaptation process. The problem with AI reproducibility isn’t necessarily, ‘not invented here’ – it’s more, ‘not enough time to even try’. We figured if we could share analysis ready data, enable rapid server-side experimentation and good version control, it would be the best thing to help make these tools get picked up by the community for the benefit of all.”
So SpaceML is an AI accelerator, one distributing open-source data and embracing an open model for the deployment of AI-enhanced space research. The current datasets and projects grow out of five years of applying AI to space topics ranging from lunar exploration to astrobiology, completed by FDL teams working in multidisciplinary areas in partnership with NASA and ESA and commercial partners. The growth of international accelerators could quicken the pace of multidisciplinary research.
What other multidisciplinary efforts will emerge as we streamline our networks? It’s a space I’ll continue to track. For more on SpaceML, a short description can be found in Koul et al., “SpaceML: Distributed Open-source Research with Citizen Scientists for the Advancement of Space Technology for NASA,” COSPAR 2021 Workshop on Cloud Computing for Space Sciences” (preprint).
Finding the Missing Link: How We Could Discover Interstellar Quantum Communications
Six decades of SETI have yet to produce a detection. Are there strategies we have missed? In today’s essay, Michael Hippke takes us into the realm of quantum communication, explaining how phenomena like ‘squeezed light’ can flag an artificial signal with no ambiguity. Quantum coherence, he argues, can be maintained over interstellar distances, and quantum methods offer advantages in efficiency and security that are compelling. Moreover, techniques exist with commercially available equipment to search for such communications. Hippke is a familiar face on Centauri Dreams, having explored topics from the unusual dimming of Boyajian’s Star to the detection of exomoons using what is known as the orbital sampling effect. He is best known for his Transit Least Squares (TLS) exoplanet detection method, which is now in wide use and has accounted for the discovery of ~ 100 new worlds. An astrophysics researcher at Sonneberg Observatory and visiting scholar for Breakthrough Listen at UC-Berkeley, Michael now introduces Quantum SETI.
by Michael Hippke
Almost all of today’s searches for extraterrestrial intelligence (SETI) are focused on radio waves. It would be possible to extend our search to include interstellar quantum communications.
Quite possibly, our Neanderthal ancestors around the bonfires of the Stone Age marveled at the night sky and scratched their heads. What are all these stars about? Are there other worlds out there which have equally delicious woolly mammoths? Much later, about 200 years ago, the great mathematician Carl Friedrich Gauß proposed to cut down large areas of Siberian forest, in the form of a triangle, to send a message to the inhabitants of the Moon. At the end of the 19th Century, many canals were built, including the Suez and Panama canals. Inspired by these engineering masterpieces, astronomers searched for similar signs of technology on other planets. The logic was clear: What the great human civilization can build must reflect what other civilizations will inevitably build.
Clearly, Martians must equally be in need of canals. Indeed, the Italian astronomer Giovanni Schiaparelli discovered “canali” on Mars in 1877. Other observers joined the effort, and Percival Lowell asserted that the canals exist and must be artificial in origin.
Something similar happened again a short time later when Guglielmo Marconi put the first radio into operation in December 1894. Just a few years later, Nikola Tesla searched for radio waves from Mars, and believed he had made a detection. It turned out to be a mistake, but the search for radio signals from space continued. The “Search for Extraterrestrial Intelligence,” or SETI for short, received a boost in 1960 from two publications in the prestigious journal Nature. For the first time, precise scientific descriptions were given for the frequencies and limits of interstellar communication using radio waves [https://www.nature.com/articles/184844a0] and optical light [https://www.nature.com/articles/190205a0]. Between 1960 and 2018, the SETI Institute recorded at least 104 experiments with radio telescopes [https://technosearch.seti.org/]. All unsuccessful so far, which is also true for searches in the optical domain, for X-rays, or infrared signatures.
Photons? Neutrinos? Higgs bosons?
Particle physics radically changed our view of the world in the 20th century: It was only through the understanding of elementary particles that discoveries such as nuclear fission (atomic weapons, nuclear power plants) became possible. Of the 37 elementary particles known today in the Standard Model, several are suitable for an interstellar communication link. I examined the pros and cons of all relevant particles in a 2018 research paper [https://arxiv.org/abs/1711.07962]. The known photons (light particles) were the “winners”, because they are massless and therefore energetically favorable. In addition, they travel at light speed, can be focused very well, and can carry several bits of information per particle.
Photons are not only known as light particles – they are also present in the electromagnetic spectrum as radio waves, and with higher particle energies than X-rays or gamma rays. In addition, there are other particles that can be more or less reasonably used for communication. For example, it has been demonstrated that neutrinos can be used to transmit data [https://arxiv.org/abs/1203.2847]. Neutrinos have the advantage that they effortlessly penetrate kilometer-thick rock. However, this is also one of their disadvantages: they are extremely difficult to detect, because they also penetrate (almost) every detector.
Incidentally, the particle that is the least suitable of all for long-distance communication is the Higgs boson. It was predicted by Peter Higgs in 1964, but was not observed for the first time until 2012 at the Large Hadron Collider (LHC) at CERN – it also won a Nobel Prize.
The Higgs boson decays after only 10-22 seconds. To keep it alive long enough to travel to the next star, it would have to be accelerated very strongly. Due to the Lorentz factor, its subjective time would then pass more slowly. In practice, however, this is impossible to achieve, because one would have to pump so much energy into the Higgs particle that it would become a black hole. It thus disqualifies itself as a data carrier.
Photons and quanta
Quanta, simply put, are discrete particles in a system that all have the same energy. For example, in 1905 Albert Einstein postulated that particles of light (photons) always have multiples of a smallest amount of energy. This gives rise to the field of quantum mechanics, which describes effects at the smallest level. The transition to the macroscopic, classical world is a grey area – quantum effects have also been demonstrated in fullerenes, which are spheres of 60 carbon atoms. So although quantum effects occur in all particles, it makes sense to focus on photons for interstellar communication because they are superior to other particles for this purpose.
Four advantages of quantum communication
1. Information efficiency
Classical communication with photons, over interstellar distances, can be well illustrated in the particle model. The transmitter generates a pulse of particles, and focuses them through a parabolic mirror into a beam whose minimum diameter is limited by diffraction. This means that the light beam expands over large distances.
For example, if an optical laser beam is focused through a telescope measuring one meter and sent across the 4 light years to Alpha Centauri, the light cone there is already as wide as the distance from the Earth to the Sun. So a receiver on a planet around Alpha Centauri receives only a small fraction of the emitted photons. The rest flies past the receiver into the depths of space. On the other hand, photons are quite cheap to buy: You already get about 1019 photons from a laser that shines with one watt for one second.
In the sum of these effects, every photon is precious in interstellar communication. Therefore, one wants to encode as many bits of information as possible into each transmitted photon. How to do that?
Photons (without directional information) have three degrees of freedom: their arrival time, their energy (= wavelength or frequency), and the polarization. Based on this, an alphabet can be agreed upon, so that, for example, a photon arriving at time 11:37 with wavelength 650 nm (“red”) and polarization “left” corresponds to the letter “A”. The number of bits, which can be encoded per degree of freedom, scales unfortunately only logarithmically: 1024 modes result in 10 bits per photon. In practice, one still has to take losses and noise into account, so that with this classical communication it is rarely possible to transmit more than on the order of 10 bits per photon.
Quantum communication, however, offers the possibility to increase the information density. There are several ways to realize this, but a good illustration is based on the fact that one can “squeeze” light (more on this later). Then, for example, the time of arrival can be measured more accurately (at the expense of other parameters). There are analytical models, and also already practical demonstrations, which show that the information content can be increased by up to 50 percent. In our simple example, about 15 bits per photon could be encoded instead of only 10 for the classical case.
2. Information security
Encryption of sensitive data during data transmission is an important issue for us humans. Of course, we don’t know if this is the case for other civilizations. But it is plausible that future colonies on Mars (or Alpha Centauri…) will also want to encrypt their communications with each other and with Earth. In this respect, encryption is quite relevant for transmissions through space.
Today’s encryption methods are mostly based on mathematical one-way functions. For example, it is easy to multiply two large numbers. However, if the secret key is missing, you have to go the other way around and calculate the two prime factors from the large number. This is much more difficult. However, the security of this and similar methods is “only” due to the fact that no one has yet found an effective method of calculation. We have in no case the mathematical proof available that such a calculation is not possible. There is always the danger that a clever algorithm will be found which cracks the encryption. Quantum computers could also be used in the future to attack some encryption methods.
In contrast, there is quantum cryptography. The best-known method uses a quantum key exchange, which has also been used in practice over long distances, for example via satellite. This is based on quantum mechanics and is unbreakable as long as no mistake is made during transmission – and as long as no one disproves quantum mechanics.
3. Gate-keeping
If there really is a galactic Internet, how to protect it from being spammed by uneducated civilizations? This problem has already occupied Mieczys?aw Subotowicz, a Polish professor of astrophysics, who wrote in a technical paper on neutrino communication in 1979 that it was: “so difficult that an advanced civilization could intentionally communicate only through it with aliens of its own level of development”.
Now, as mentioned above, neutrino communications are very inefficient. It would be much more elegant and energy efficient to use photons instead. As an entry barrier, it seems plausible not to allow classical photons, but to require quantum communications. This would leave out young technological civilizations like ours, though we would have a good chance of joining in the next few decades.
4. Quantum computing
Konrad Zuse built the Zuse Z3, the first Turing-complete computer, in his Berlin apartment in 1941. This was a single computing machine. It took several decades until the first computers were connected (networked together) in 1969 with the ARPANET. This gave rise to the Internet, in which billions of computers of all kinds are connected today: PCs, cell phones, washing machines, etc. All these devices are classical computers exchanging classical information (bits) on classical paths (for example via photons in optical fibers).
In the future, quantum computers may gain importance because they can solve a certain class of problems much more efficiently. This could give rise to a “quantum Internet” in which quantum computers exchange “qubits,” or entangled quantum bits. These could be intermediate results of simulations, or even observational data that are later superimposed on each other [https://arxiv.org/abs/2103.07590].
Likewise, it is conceivable that quantum-based observational data and intermediate results will be exchanged over larger distances. This is when interstellar quantum communication comes into play. If distant civilizations also use quantum computers, their communications will consist of entangled particles.
Excursus: The (im)possible magic Pandora quantum box
The idea of using quantum entanglement to transmit information instantaneously (without loss of time) over long distances is a frequent motif in science fiction literature. For example, in the famous novel The Three Body Problem by Chinese author Liu Cixin, the “Trisolarans” use quantum entangled protons to communicate instantaneously.
This method sounds too good to be true – and unfortunately it actually contains three fundamental flaws. The first is the impossibility of exchanging information faster than the speed of light. If that were possible, there would be a causality violation: one could transmit the information before an event happens, thus causing paradoxes (“grandfather paradox” [https://arxiv.org/abs/1505.07489]). Second, quantum entanglement does not work this way: one cannot change one of two entangled particles, thereby causing an influence on the state of the partner. As soon as one of the particles is changed, this process destroys the entanglement (“no communication theorem”).
Third, an information transfer without particles (no particle flies from A to B) is impossible. Information is always bound to mass (or energy) in our universe, and does not exist detached from it. There are still open questions here, for example when and how information that flew in with matter comes out of a black hole again. But this does not change the fact that the communication by quantum entanglement, and without particle exchange, is impossible.
But wait a minute – before we throw away the “magic box of the entangled photons”, we should once more examine the idea. For there is, despite all the nonsense that is written about it, an actually sensible and physically undisputed possibility of use: known under the term “pre-shared entanglement” [https://arxiv.org/abs/quant-ph/0106052].
To perform this operation, we must first assume that we can entangle and store a large number of photons. This is not so easy: the current world record for a quantum memory preserves entanglement for only six hours. And even that requires considerable effort: It uses a ground-state hyperfine transition of europium ion dopants in yttrium orthosilicate using optically detected nuclear magnetic resonance techniques [https://www.nature.com/articles/nature14025]. But it is conceivable that technological advances will make longer storage possible. Conditions are particularly good for interstellar travel, because space is dark and cold, which slows decoherence caused by particle interactions.
So let’s assume such a quantum memory is available – what do we do with it? We take one half of the magic box on board a spaceship! And the counterpart remains on earth. Now the spaceship flies far away, and wants to communicate home. The trick is then not to send the bits of the information transmission simply on a photon letter to the earth, but to superpose each classical signal photon first with one (or more) stored entangled photons. The result is one classical photon per superposition, which is then sent “totally normally” to the receiver (for example the earth). Upon arrival, the receivers opens their own magic box and bring their part of the entangled particles with it to superposition. This allows the original message to be reconstructed.
The advantage of this procedure is increased information content: The amount of information (in bits per photon) increases by the factor log2(M), where M is the ratio of the entangled to the signal photons. Even a very large magic box is therefore of limited use, because unfortunately log2(1024), for example, is only 10. Losses and interference (due to noise, for example) also have a negative effect on the amount of encodable information. Nevertheless, “pre-shared entanglement” is a method that can be considered, because it is physically accepted – in contrast to most other ideas in popular literature.
Quantum communication in practice
But what does quantum communication look like in practice? Is there even a light source for it on earth? Yes, for a few years now this has actually been the case! When gravitational waves from merging black holes were detected for the first time at the Laser Interferometer Gravitational-wave Observatory (LIGO) in 2016, “squeezed light” was used. This is laser light traveling through a very precisely controlled crystal (an “OPO” for “Optical Parametric Oscillator”). This converts one green photon into two entangled red photons, to what is called a squeezed vacuum. This reduces phase uncertainty at the expense of amplitude fluctuation. And it is the former that matters: One would like to measure the arrival time of the photons very precisely in order to compare the length of the path with and without gravitational waves. The brightness of the photons is not important.
Such a squeezed light, with lower fluctuations compared to classical light, also improves interstellar communication. It still remains unresolved what is the best way to modulate the actual data. Signal strength is also still low, with just a few watts of squeezed light in use at LIGO. By comparison, there are classical lasers in the megawatt range. So the development of quantum light is several decades behind classical light. But more powerful quantum light sources in the kilowatt range are already planned for next-generation gravitational wave detectors. This would also mark the entry threshold for meaningful interstellar quantum communications.
Detection of quantum communication
Entangled photons are also just photons – shouldn’t they already be detectable in optical SETI experiments anyway? In principle this is correct, because for a single photon it is in principle not determinable who or what has generated it. If it falls on the detector at 11:37 a.m. with a wavelength of 650 nm (color red), we cannot possibly say whether it came from a star or from the laser cannon of the Death Star.
However, a photon rarely comes alone. If we receive one thousand photons with 650 nm within one nanosecond from the direction of Alpha Centauri in our one-meter mirror telescope, then we can be sure that they do not come from the star itself (the star sends only about 32 photons of all wavelengths per nanosecond into our telescope). Classical optical SETI is based on this search assumption. It is thus very sensitive to strong laser pulses, but also very insensitive to broadband sources.
Quantum SETI extends the search horizon by additional features. If we receive a group of photons, they no longer have to correspond to a specific wavelength, or arrive in a narrow time interval, for us to assume an artificial origin. Instead, we can check for quantum properties, such as the presence (or absence) of squeezed light. Indeed, there is no (known) natural process that produces squeezed light. If we receive such, it would be extremely interesting in any case. And there are indeed tests for squeezed light that can be done with existing telescopes and detectors. In the simplest case, one tests the intensity and its variance for a nonlinear (squared) correlation, which requires only a good CCD sensor [https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.113602].
There are numerous other tests for quantum properties of light that are applicable to starlight. For faint sources from which only a few photons are received, one can measure their temporal separation. Chaotic starlight is temporally clustered, so it is very likely to reach us in small groups. Classical coherent light, i.e. laser light, is much more uniform. For light with photon “antibunching”, in the extreme case, the distance between every two photons is identical – so their arrival times are perfectly uncorrelated. This quantum mechanical effect can never occur in natural light sources, and is thus a sure sign of a technical origin. The technique is used from time to time because it is useful for determining stellar diameters (“intensity interferometry”).
For a few stars we can already deduce on the basis of existing data that they are of natural origin: Arcturus, Procyon and Pollux [https://academic.oup.com/mnras/article/472/4/4126/4344853]. In the future, however, the method can be applied to a large number of “strange” objects to test them for an artificial origin: impossible triple stars [https://academic.oup.com/mnras/article/445/1/309/988488], hyperfast globular clusters [https://iopscience.iop.org/article/10.1088/2041-8205/787/1/L11], or generally all interesting objects listed in the “Exotica” catalog by Brian Lacki (Breakthrough Listen) [https://arxiv.org/abs/2006.11304].
Current status and outlook
The idea to extend SETI by quantum effects is still quite new. However, one can fall back on known search procedures and must adapt these only slightly. Thus, dubious light sources can be effectively checked for an artificial origin in the future. We can be curious what the next observations will show, and ask the question: “Dear photon, are you artificially produced?”
The paper is Hippke, “Searching for interstellar quantum communications,” in press at the Astronomical Journal (preprint). See also the video “Searching for Interstellar Quantum Communications,” available at https://www.youtube.com/watch?v=Kwue4L8m2Vs.
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