Month: March 2018

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.

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.

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!

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.

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.