- Paper Title: The First Very Long Baseline Interferometric SETI Experiment (arxiv ID: 1205.6466)
- Authors: H. Rampadarath, J. S. Morgan, S. J. Tingay, C. M. Trott
- First Author’s Affiliation: Curtin Univeristy, Perth, Australia
- Journal: The Astrophysical Journal (Accepted)
This paper describes the first-ever application of the technique of Very Long Baseline Interferometry (VLBI) to the Search for Extra-Terrestrial Intelligence (SETI).
The SETI project, born in 1960 as Project Ozma, seeks to find evidence of technological civilizations elsewhere in our cosmos. Now a private initiative supported by donations after public funding was cut by Congress, SETI aims to detect electromagnetic (EM) transmissions from extraterrestrial (ET) life. These transmissions may either be of an intentional nature (i.e. a civilization is broadcasting its existence) or leakage, e.g. radar broadcasts, radio transmissions, and the myriad other forms of radio noise a civilization like ours puts out. SETI searches across the EM spectrum for such transmissions, but its most extensive and most mature efforts are in the radio.
The dominant technical challenge faced by radio SETI is screening out terrestrial Radio Frequency Interference (RFI), the EM detritus of human activity on Earth. Signals from satellites, television broadcasts, cell phone towers, and other terrestrial sources can easily creep into the lines-of-sight of SETI radio telescopes. The worst part is that, unlike astrophysical noise, RFI signals exactly mimic the characteristics of a successful SETI signal: they display the structure and encoding expected for the transmissions of a technological species because, well, they are the transmissions of a technological species – us! Many of us are familiar with the SETI@Home project, one of the first Citizen Science initiatives. In this initiative, individuals can install a program that donates idle time on their computers to help SETI run algorithms to process out RFI from their observations, in search of that ever-elusive signal from ET (see Figure 1).
The Role of VLBI
VLBI offers a powerful new tool in discriminating RFI transmissions. The technique of interferometry involves phasing simultaneous observations from multiple radio telescopes together so the telescopes essentially act as one large dish with an effective diameter equal to the distance between the most distant telescopes. Not only does this let you collect a lot of photons, this also enables you to have higher resolution. Resolution at a particular wavelength goes as 1/d, where d is the diameter of your telescope, and so the resolution of an interferometer goes as 1/D, where D is the distance between telescopes. VLBI takes this concept to the limit by separating the dishes by distances as large as thousands of kilometers, enabling extremely high-resolution observations down to the sub-milliarcsecond (mas) level.
In addition to dramatically increasing resolution, the high spatial separation of dishes enables the authors to more readily separate out RFI in their observations. The most direct RFI discriminant is simple correlation: If a signal is detected along one baseline and not another, it’s probably local and hence not an ET signal. Even if signals pass that test, it turns out that interferometric observations phase variances that make it obvious when a source is far from where the array is pointed. Hence VLBI offers a ready method of discriminating RFI that is inaccessible to more conventional radio observation techniques.
This Paper: Observing Gliese 581
This paper tested out this method by using three telescopes of the Australian Long Baseline Array to observe the star Gliese 581 (Gl581), which is 20 light years distant. Gl581 is orbited by at least two planets, of which one and possibly both are in the habitable zone. As such, it is a ripe candidate for SETI investigation. In addition to looking for extraterrestrial transmissions from this particular star, the authors aimed to validate the VLBI method for SETI observations.
The authors detected a number of radio sources at the 5-sigma level across all dishes (5 times the ambient noise). However, the frequencies of these sources aligned suspiciously closely with the frequencies used by certain satellite constellations in the field of view. Follow-up work on the characteristic phase structure showed that the signals were due to sources located far from where the array was pointed, meaning they were local. While no source was detected for the Gl581 system, the VLBI approach to SETI was validated: simple methods were readily able to screen out RFI. Furthermore, the study established an upper bound of 7 MegaWatts per Hertz (MW/Hz) for an isotropic transmitter in the Gl581 system. For comparison, the Arecibo radar transmitter puts out about 1 MW/Hz. Were a transmitter like Arecibo in operation in the Gl581 system and beaming in our direction, we would expect the signal to be about 650 millijanskies (mJy) at 21 cm, well above this study’s detection limit of 1.55 mJy. Had there been a civilization beaming us an analog of our own SETI Message, we would have been able to detect it! (see Figure 3)
Conclusions & Future Work
This work validated the use of VLBI for SETI. VLBI permits simple, robust, and even programmatic screening of local RFI, one of the great challenges of SETI. It also established an upper limit of 7 MW/Hz for an isotropic transmitter in the Gl581 system.
This paper also pointed out the remarkable prospects for SETI observations in the age of the planned Square Kilometer Array, a gigantic radio interferometer that will be built in South Africa and Australia. With projected sensitivities below a microjansky, this instrument would probe sources far less luminous than radars and satellites, opening up a whole new regime of SETI exploration. For SKA studies, we first need a target list. Planet hunters, over to you!