Response to Schwartz and Townes (1961)

The authors propose the use of optical/near-IR masers as an alternative to radio transmissions for the purposes of searching for extraterrestrial intelligences (ETIs). In that way, they are suggesting new search methods.

As of 1961 (when this paper was published), the “[development of maser oscillators in the optical/near-IR spectral region which would allow transmission across several light-years]” was on the horizon. Interestingly, the authors state that such masers could have been thoroughly developed even 30 years earlier, suggesting that other ETIs may have pursued such avenues (as opposed to radio transmission).

Technology in 1961 suggested that the continuous operation of high power masers was entirely within the realm of possibility, and the outlook has only brightened in the years since then. One issue relates to the directability of such a maser, and the authors suggest that the problem can be overcome by employing masers in tandem with optical systems. They recommend a coordinated system of 25 masers as the optimal configuration.

There are two important factors that must be considered when evaluating the detectability of a maser signal: (1) it must produce a sufficiently large photon flux and (2) it must be distinguishable from the astronomical background. The authors argue that the first condition can be easily satisfied, and, again, the outlook has only become more optimistic over the past half century. The second condition requires more consideration. Due to the small separation between Earth and the Sun, it is likely that a maser signal cannot be spatially separated from the light of the host star around which the signal is originating. The authors suggest transmitting far away from the peak energy output of the Sun (~5000 Angstrom), i.e., either the extreme violet (shortward of ~2000 Angstrom) or in the near-IR. The former choice suffers from limited atmospheric transmission, while the latter suffers more from the diffraction limit at longer wavelengths (if such a limit is applicable for the system being employed). The authors also suggest transmitting in strong absorption features, e.g., the Ca II H or K lines. Since 1961, our knowledge of stellar populations and corresponding exoplanet systems has greatly improved, so perhaps it is more useful to optimize the transmission interval for a typical M dwarf (as opposed to the Sun, which is a G dwarf).

This study proposes a very novel idea. When considering how to find an ETI (and by extension, in trying to envision how they might attempt interstellar communication) it is important to broaden our perspective and consider all possibilities.

Masers are the Future

Cocconi and Morrison initially proposed interstellar communication using radio waves, particularly near the hyperfine transition of hydrogen. The first SETI observations, conducted by Frank Drake, followed this suggestion of where to look. The focus in the microwave was a result of technological limitations, as observations at other frequencies were unimaginable when Cocconi and Morrison initially presented their work. With the discover of the maser in the 1950s, a new vector for communication became available. The authors postulated that “maser oscillators and other appropriate apparatus in or near the optical region [will] allow detectable light signals to be beamed between planets of two stars separated by a number of light years”. The creation of the laser in 1960, a more practical device than a maser, served as further justification for this claim.

Townes and Schwartz note the physics behind the maser was first described by Einstein. There was no theoretical deficit precluding its discovery or delaying development. The authors use this to argue that there may exist an extraterrestrial society comparable to Earth that discovered and developed masers before radio waves. They boldly state:

We propose to examine the possibility of broadcasting an optical beam from a planet associated with a star some few or some tens of light-years away at sufficient power-levels to establish communications with the Earth. There is some chance that such broadcasts from another society approximately as advanced as we are could be adequately detected by present telescopes and spectrographs […]

A maser, much like its optical counterpart, could theoretically operate continuously at high power and would be almost monochromatic. Townes and Schwartz note the limit to producing an ideal maser would be the technological problems in mirror accuracy and control of any optical distortions. They considered two masers (1) one energetic maser and (2) twenty-five masers pointing in the same direction and specified two criteria for the detectability of either maser:

  • it must produce enough photons per unit area at the receiver to be detectable with a lens of practical size and in a reasonable time and
  • it must be distinguishable from the background stellar light.

They argued that the intensity of radiation from the group of masers would produce a beam of high intensity capable of being observed by the naked eye or binoculars out to 0.1 or 0.4 light years, respectively. A larger telescope of long integration would be required for masers further than 10 light years. The authors note that, from the work of Cocconi and Morrison, there were ten Sun-like stars within a distance of 10 light years, making masers very applicable to SETI. Spectra would be another useful diagnostic for a maser. A grating spectrograph in the 1960s could have resolved the energetic maser as a signal equal to the stellar background. Noting this, Townes and Schwartz propose that “[a] spectral line sought can be expected to be exceptionally narrow, at an abnormal frequency for the type of star in question, and varying in intensity [o]bservation of any of these characteristics should lead to closer examination” of an object.

In less than a decade after discovery, masers were already being considered for SETI. The advantages include the coherence of radiation over a very large aperture and the theoretical possibility of obtaining coherence among several maser sources. Given that nay plausible atmosphere would prevent emission of masers, the authors propose utilizing a “very high-altitude balloon, a space platform, or natural Moon”. Only two years passed after Cocconi and Morrison published and people began considering where to look. This is an on-going discussion, but the authors correctly argue that charged particles would be deflected while UV and IR emission would be absorbed by an atmosphere. This blogger considers this to be an important discussion. When this was originally published, SETI was still in its infancy and the authors emphasized the need to consider other wavelengths before all of SETI focused on microwaves. While the development of masers may have stymied after the discovery of the more practical laser, optical SETI now exists. Recent progress in masers (see Movie 1) suggest their applicability will soon increase. Perhaps the masers of the future will become useful for SETI as Schwartz and Townes initially proposed.

Movie 1. Mainstream Masers Coming Soon™
Laser are everywhere, but masers came first! They are like lasers but in the microwave. This video shows the latest applicability of masers. Who knows, perhaps the suggestion by Schwartz and Townes to use masers for communication is not too far off.

The Gangrenous Limb of Science: Hard Science Fiction

***It should be stated the author is not a fan of science fiction in general. But there is nothing inherently wrong with the genre until scientists begin to use fiction to address scientific problems***

Is it fiction, science, or an unholy amalgam of both? That is the question this blogger tried to address when reading “Gravity’s whispers”. Gregory Benford is both an astrophysicist and a writer of hard science fiction. Hard science fiction attempts to lead the reader to a fictitious world with an emphasis on scientific accuracy. On his Amazon page, it states:

Often called hard science fiction, Benford’s stories take physics into inspired realms. What would happen if cryonics worked and people, frozen, were awoken 50 years in the future? What might we encounter in other dimensions? How about sending messages across time? And finding aliens in our midst? The questions that physics and scientists ask, Benford’s imagination explores. With the re-release of some of his earlier works and the new release of current stories and novels, Benford takes the lead in creating science fiction that intrigues and amuses us while also pushing us to think.

This piece hardly makes one think about the science and more about the literary elements forgotten in science fiction. The story begins with a date and a quote popularized by Voltaire: perfection is the enemy of good. It should be noted this piece is neither. An unnamed scientist (this is left unclear, for all we know it could be part of the janitorial staff at the VLA) has tried to decipher a signal received from their date, Sam the Slow. The mysterious protagonist purportedly spent a day trying to decipher a noisy pattern. Their work paid off and revealed “a string of numbers, […] the zeroes of the Riemann zeta function”. Some exposition later, the reader learns Sam is a scientist working on LIGO and this first gravitational wave detection, thought to be a neutron star crust vibration, actual contains a message. Real talk follows:

‘What? A tunable gravitational wave with a signal? That’s im—’ ‘—possible, I know. Unless you can sling around neutron stars and make them sing in code’

The progenitors of the signal even provide a proof for one of the unsolved problems in mathematics. There is talk of a Nobel prize and relief in that humanity cannot answer the SETI signal.

There were various moments of drivel, notably in the discussion of romance between both parties. It neither adds to the plot nor to the purported science. What should have been discussed more was the signal processing. To this blogger, the mysterious protagonists might as well be ETI. They were somehow able to decipher a gravitational wave chirp to reveal a solution to a Millennium Prize Problem. These individuals will apparently win a Nobel for the detection of SETI and a Millennium Prize. Sam notes “the rest of them”, presumably scientists, would laugh at this assertion and this serves to emphasize the delicate nature of the topic.

The actual content of the message should not be too important as it could have been random prime numbers, albeit the discussion of the Riemann hypothesis gives ETI high intelligence. Greater scientific accuracy could have been invoked by using eLISA instead of LIGO and positioning the scientsits somewhere other than the VLA. This particular piece was neither amusing nor particularly thought provoking. The only moment of connection between the reader and scientists would have been at the end (not because the story completed…) with the relief that humanity cannot contact this extremely intelligent form of life. This blogger thinks writings such as this are dubious at best. It is the height of folly to presume scientific accuracy on completely fictitious topics, and melding the two somehow gives disappointment a tangible form.

SETI at Different Wavelengths

Charles H. Townes, the inventor of the maser and laser, wrote a paper in 1983 to discuss the appropriate wavelength for the search for extraterrestrial intelligence (SETI). He argued that, while SETI developed, it was important to consider which wavelengths to conduct searches. Beginning with Drake’s Project Ozma, most SETI experiments have used the radio region of the electromagnetic spectrum with particular emphasis on the water hole. With the discovery of masers and lasers, it was possible to consider using them for interstellar communications at optical wavelengths. Schwartz and Townes in a 1961 letter first presented this idea of using masers to communicate over long distances, assuming we used a narrow-band receiver. In this article they proclaimed:

We propose to examine the possibility of broadcasting an optical beam from a planet associated with a star some few or some tens of light-years away at sufficient power-levels to establish communications with the Earth. There is some chance that such broadcasts from another society approximately as advanced as we are could be adequately detected by present telescopes and spectrographs, and appropriate techniques now available for detection will be discussed. Communication between planets within our own stellar system by beams from optical masers appears a fortiori quite practical.

They concluded “the frequency of the hydrogen line in the micro-wave region is not the only reasonable place at which to search for possible interstellar communications, and […] the optical region also seems a logical one”. This paper can largely be viewed as a continuation of his initial work.

Townes begins by motivating SETI at other wavelengths. An extensive search focusing on one regime in the electromagnetic spectrum would be a large endeavor and potentially a waste of resources. The communication capabilities of ETI were assumed to be analogous to our capabilities. The principals for SETI are described via various strategic questions and under the assumption ETI wishes to minimize costs of any technology they use. The first question addressed is the nature of the signal, primarily if ETI signals would be isotropic or directive. It is preferred that a civilization broadcast narrow band. Townes uses the excessive power an isotropic signal would require (9 ordered of magnitude more) to suggest ETI would favor sending a beam. Other assumptions regarding ETI and its capabilities:

  • regarding power sources, there is no necessary choice as a function of wavelength from the radio region down at least into the ultraviolet,
  • there are detectors of sensitivity close to the ultimate limit dictated by the quantum properties of radiation over the whole range of wavelengths, and
  • if needed, the use of space for the beacons is to be expected.

Townes consider numerical evaluations of the signal-to-noise ratio (SNR) for different wavelengths. One potential observation scheme involves using longer wavelengths with linear detection of all wavelengths and a constant antenna area but solid angles corresponding to the diffraction limit only for wavelengths >1 cm. The other observation scheme involves short wavelengths with a quantum counting detector and an antenna with a fixed diameter for long wavelengths down to 1 cm and then decreasing linearly in size to 10 m in the infrared.

These were but two examples discussed. Townes concludes that, depending on the assumptions, other regions, such as the infrared, should be considered. This was a marked departure from what was initially proposed by Cocconi and Morrison. While Townes initial suggest of using the infrared may not be used today, the discussion regarding where to look is still ongoing. Experiments in optical SETI have since been conducted (e.g. Reines & March, 2002), Laser SETI is a thing (see Movie 1), and it optical SETI is one of the projects of the SETI Institute. Recent papers have scrutinized both the wavelength of photons and even the nature of the particle observed by SETI. It may have taken over forty years since the first publication from Townes discussing masers, but at least proponents of SETI are no longer latching onto the microwave.

Movie 1. Laser SETI Wants Your Money
Laser SETI is an example of the types of searches Townes proposed – something not tied to the microwave region. The optimal wavelength to observe is an important discussion that is still ongoing.

Is Love Really Stronger Than Gravity?

In fictional story Gravity’s Whispers by Gregory Benford, we follow a nameless data analyzing protagonist (whom I shall call Alex) and a romantically apathetic LIGO scientist named Sam during the discovery of the unambiguous SETI signal.

This story is an example of the science fiction background that has heavily influenced SETI thought. This piece can be seen as using the medium of fiction to communicate new SETI ideas, and is one of the few pieces of common literature with the suggestion of SETI messaging via gravitational waves. While incredibly difficult, gravitational waves could be one of the best ways to send a SETI beacon over the largest distances because the amplitude of gravitational wave signals only decreases as a function of 1/r instead of 1/r^2 like most other methods.

In summary: Sam gives Alex a noisy signal which they decode and find a prominent mathematical sum (the Riemman Sum) and a highly sought after mathematical proof (to the Riemann Hypothesis). On the way to the bar for beers, Alex kisses Sam and he remarks that maybe its a good thing we can’t communicate back to this ETI.

The story was a bit confusing to me. It is hard for short stories to pull the reader into the characters and make them investing, but I feel like the whole romance angle wasn’t well put together. It honestly gave off a creepy vibe for me. Alex has been interested in Sam for a long time, but it has so far been unrequited interest up until this point.

“I wondered whether Sam the Slow had finally decided to make a date with me, in his odd way. I’d been waiting half a year.”

“‘I gave him a smile he didn’t notice'”

The picture of pining, unrequited lover. Even after they discuss the absolutely world-changing signal, Alex’s comment is

“Maybe, just maybe, this could be more important than at last getting Sam to date me.”

Then, they proceed to kiss Sam without solicitation in a car ride where it is “a long drive back to Socorro”. While there is a line where “He kissed back, his eyes flickered, he grinned”, that is the only indication that this type of behavior was okay. It seems like a strange time to make a move on a collaborator and longtime friend (when they are trapped in a car and coming off such a big discovery). The quote continues “but he didn’t look happy. He grasped the steering wheel and peered ahead into the starlit darkness.” Alex believes he is thinking about the aliens, and the author suggests this as well, but the whole romantic interplay throughout the story felt unnecessary and seems to encourage harassment-y behavior.

What’s the Best Way to Reach You?

To follow up on the end of my last post, what if by optimizing for photon communication, we’re just making a giant planet-sized wheat triangle that’s primitive, quaint, and functionally useless because no ETI in their right mind uses wheat triangles anymore?

The readings for this week, especially Hippke’s 2017 paper about other information carriers for SETI, actually settled my mind on this score.

To skip to the punchline, Hippke finds that everything that we know of so far is inferior to photon transmission (specifically 1 nm X-rays, based on the argument in his previous paper), except perhaps physical artifacts (which might be preferable if you don’t care about speed). This is exciting, and puts my mind at ease about the wheat field thing.

He looks at the following methods, and generally finds the following flaws:

You will notice a lot of ???s in the Pros categories, and I think that’s interesting. Hippke does a good job of going through a lot of messaging options that seem ridiculous at face-value (and not excluding them for that), working out some actual physics behind them, and doesn’t jump onto being a proponent of any “new thing!”. I appreciate this.

The assumptions that he makes with regards to point-to-point communication are interesting. He assumes that more information transmitted is preferable to less, information arriving earlier is preferable to later, and more efficiency is preferable to less. He then discusses, at the very end of the paper, how the landscape would change if any of these assumptions were incorrect (which is very cool!), or incorrect and stacking.

I would like to point out that Hart’s sociological argument probably stops any of this assumption-fiddling from mattering too much. Just because one ETI actually doesn’t care how fast information arrives (Because they’re very long-lived? Because they’re post-biological?) doesn’t mean that another won’t. Just because one ETI is naturally incurious and doesn’t care about actually transmitting their entire “encyclopedia” (if you will), doesn’t mean that another is.

If anything, I think that the “more efficiency / less efficiency” might be the easiest one to break without running afoul of this sociological argument. If you have access to enough energy, you won’t care whether a big METI project takes 10^-100 or 10^-95 of your energy budget. And, with the assumption that virtually all ETIs should have been around for far longer than we are (and that they care about energy/resources in the first place!), they’ll all probably have a much larger energy budget, and might not care too much about efficiency. Just a thought!

 

OMG WHY IS SETI ALWAYS IN THE RADIO WHY HAVEN’T THEY THOUGHT TO DO SOMETHING ELSE… They did?

In Townes (1983), it is proposed that the commonly accepted view that SETI should operate in the microwave might not be as robust as it seems.

Before this paper, most searches were proposed to be performed in the microwave region, but other regions of the EM spectrum can be shown to be more valuable when other considerations and conditions are used. For example, IR may be better than the microwave region if one considers the use of photon counting instead of linear amplification.

We do not know what design parameters ETI considers important for METI, so we should be very cautious about limiting what frequencies we search for. The microwave region is good in that it can be searched in “right now” (in 1980s time).

Shorter wavelengths could also be better if the geometric directivity of their telescopes can be utilized.

The paper is notable for looking at previous assumptions in SETI and trying to remove ones that may be unnecessary or ill-motivated. I also appreciate that it stresses that we should not get too confident in our guesses for what frequency ETI will use.

The paper still has practical considerations which will always be dated. It does not do a deep dive into the physical upper limits of transmission. This is not strictly a problem, but we now have technology that far surpasses that of this time, so the arguments for search recommendations are now outdated.

X-Rays = Best Rays

This paper contains an argument for X-ray SETI. I will admit that I was skeptical when I read the abstract – after all, X-ray photons are much higher energy than radio photons and thus the typical logic of energy efficiency (of the transmitter) does not apply.

The paper speaks about the “streetlight effect” – an observational bias that causes you to “look where the light is good” aka. to search where it’s easiest (cheapest, already available data, good quality data given your technology, etc. etc. etc.). So, to try to preempt the gradual growing of our “streetlight” and cut to the chase, so to speak, the authors wanted to derive a physical optimum instead of just looking at the current technological “sweet spot”.

The Streetlight Effect in comic form

In this paper, the authors only considered photons, which is a choice that I’ll comment on at the end of this post.

They decide that a minimum wavelength for communication is probably about an atomic width; they adopt 1 nm as their order of magnitude value. The choice of this wavelength is dictated by how smooth we could possibly make a physical receiver surface.

The actual focusing of wavelengths of this scale is typically done with X-ray grazing mirrors that involve multiple mirrors in the design, but they are expensive to build. Because of this, the authors also discuss as-of-yet unknown alloys that could be used in a single mirror design and focusing with EM fields (not possible now, needs too much energy, but maybe in the future?).

The authors make the point that the advantage of X-rays is the amount of data that you can send and the tightness of the beam that you can create (both functions of the shorter wavelength). With a tighter beam, the pointings that you choose have to be proportionally more precise, even down to having to account for a planet’s position around a star.

Another benefit of the 1 nm wavelength choice is that it works at all distances, even when extinction is considered. Gamma rays are even better in this regard (they are barely extinct by anything), but they also would require instrument precisions that exceed the physical limitations described in the early part of the paper.

Finally, if you assume that each photon carries 1 bit of information, the authors find that you can get reasonable data rates in the megabit per second-year range, which would be sufficient for substantial communications. They propose searching for intentional communications in the existing Newton-XMM X-ray data. They note, however, that we have the technology to create pulses that are orders of magnitude shorter than we can detect with current technology, and that the time domain constraint might make us miss potential signals.

A Few Final Thoughts

  • “Ephemerides sharing is likely to be a small but significant component of all interstellar communications” – how best to share ephemerides in a general way, with the least assumptions possible (Schelling Points?) might be an interesting topic in CETI.
  • Choosing to look at only photons is a fair choice, but I can’t help but wonder if this is analogous to placing a physical constraint on the giant wheat triangle proposed by Gauss by setting it at an Earth diameter. It’s a physical limit for the method, but it means nothing if the method itself is (in hindsight) quaint/silly/outdated. Maybe photons will be a quaint/silly/outdated mode three centuries from now, and this is just a pointless thought experiment. I have a little bit more faith in EM communication than that, but it is something to consider.

Optimal Frequency According to Hippke

Although this seems like unfair criticism, I found this paper to be dense, boring, and unnecessarily long. Given that, this post will be drier and terser than my other posts (apologies to my big fans).

The authors try to find the optimal frequency for an ET civilization (or I suppose possibly even Earth) to communicate with for long distances. In particular, they look to maximize the data rate. While this is all well and good, and is an interesting thing to think about, I’m not going to bother going into their analysis or even their conclusions because I frankly find them to be useless. All of the analysis assumes Earth technology and Earth knowledge and Earth communication. It irks me. While these assumptions are fair to a degree for when you are designing a search (with current technology we could produce this signal and detect this signal, etc), I feel that expanding them into such a deep analysis is not fruitful. I feel there is merit into looking into most wavelengths, and since a lot of SETI will turn into parasite searches or hopefully get its own funding to do its thing, most wavelengths will be analyzed, especially if time stretches on for a while without a detection. So I don’t think this paper is very useful for SETI.

That being said, there is merit to this for mankind and our possibly inevitable expansion in the solar system and maybe beyond. If there is a specific wavelength that, with our knowledge and technology, works best for long distance travel in terms of data rate, then maybe we should keep this in mind as an alternative to radio (if it’s better). I’m not sure our technology is currently advanced enough to communicate with x-rays, but in the near future I wouldn’t be surprise if it becomes feasible.

Townes 1983

This article by Charles Townes from 1983 discusses the optimum frequency we should use to listen at for ETI. It discusses how all the previous SETI searches have focussed in the microwave due to the obsession with the water – hole and the potential ‘fallacy’ of this obsession. It must be noted that this article was written in 1983, before many of the technological advancements of the 21st century.

The author talks about how the diffraction limit affects observations, and the advantage of directed observations over a beamed observation. It also suggests that the transmitter might want to correct for barycentric movement to ensure that the signal is in the ‘rest frame’, and then the receiver could correct for their respective motion. I believe this drift is an important factor that needs to be taken into account when we decide the bandwidth of future SETI searches (cue: Sofia’s project).

To differentiate between the microwave and the infrared, the author compares the detectors available and the background contributions in the two domains. Since the diffraction limited solid angle is different in the two regimes, the background plays a higher role in the microwave and radio.

The suggestions of the paper have aged though since photon counters are no longer the most sensitive means of detecting optical and IR light. It does not extrapolate into the future for potential telescope primary sizes, however the detector assumptions made have fallen way by the side due to advancements in the field.

All that being said, I believe this paper is important as it is the first one which goads us to not be myopic and consider looking outside the water-hole (21cm) for communication with ET.