AstronomyOnline.org
home observation science solar system stars our galaxy cosmology astrobiology exoplanets astrophotography
SETI Alternatives, Not Just Radio - an Essay by Ricky Leon Murphy:

This is an essay I wrote that describes additional methods that could be used to detect extra-terrestrial intelligence. As of right now, the primary method for determining this is detection of radio signals.

Back to Astrobiology

In 1960, recently graduated Frank Drake began the search for extra-terrestrial intelligence (ETI) using a radio dish in Green Bank, West Virginia. It was speculated by Frank Drake and two other scientists, Guisseppe Cocconi and Philip Morrison, that an ETI would communicate using the frequency of Hydrogen as a call sign (Shostak, page 151, 154). Given the relative low cost of radio, it is widely believed that this would be the preferred media. After all, a technology advanced civilization like ours would have already realized that objects within the Universe emit radio frequencies and would have already begun studying these objects using radio astronomy. There are two other methods of possible detection of an ETI, but they have not been put into use in the search for an ETI. The reasons may be as simple as cost, but the main reason is the relative difficulty in detecting and decoding such a method of communication. The beaming of neutrinos is one possible alternate method of communication. More advanced civilizations may have the ability to produce artificial streams of neutrinos, or develop some control over the streaming of natural occurring neutrinos. The other method is the modulating of gravity waves. We have already detected gravity waves on Earth; however, gravity waves as a part of Einstein’s Theory of Relativity are the gravity waves of interest. The science of neutrinos and astronomical sources of gravity waves is relatively new. Only as we continue studying neutrino and gravity wave sources can we improve detection methods so we will be able to use these detectors to look for alternate sources of these particles and waves.

Neutrino detection is fairly new; however since the 1930’s, the idea of neutrinos was used to explain the proton-proton chain reaction within our Sun’s core. The release of what was thought to be a mass less, powerless, half-spinning by-product happened to fit the model fusion within the Sun (Kitchin, page 149). The existence of neutrinos has since been proven several times over by numerous experiments, and John Bahcall and Ray Davis led the way. John Bahcall’s groundbreaking theory and Ray Davis’ groundbreaking method of detecting neutrinos paved the way for more improved versions of the ‘neutrino telescope.’ However, with detection came further questions. There was a very low number of detected neutrinos from the Sun than initially predicted thereby creating the ‘Solar Neutrino Problem’ (Kitchin, page 150). Theoretical work at Stanford already shows three types of neutrinos: the electron neutrino, the muon neutrino, and the tauon neutrino (Quinn); however, only the electron neutrino was detected using the early ‘neutrino telescopes.’ Speculation gave rise to a theory that the Solar Neutrino Problem was not a problem at all. The Sudbury Neutrino Observatory (SNO) in Canada laid this speculation to rest. Using a much more sensitive method of detection, it was discovered that a neutrino can oscillate – that is change from electron neutrino to muon neutrino to tauon neutrino at random. This of course gives rise to new questions as to why the oscillations occur in the first place (Physicsweb.org, http://physicsweb.org/article/world/14/7/10).

Detection of neutrinos has improved much since the first experiments in 1968. Since the neutrino was thought to interact with particles already here on Earth, a detection method was devised using tetrachloroethene as a base. Because a large amount – 600 tonnes – was required, a relatively inexpensive chemical was to be used, and tetrachloroethene fit because of its availability as an industrial solvent. Other requirements were to be met. Believing neutrinos pass unhindered though Earth and to avoid interactions with other particles, the ‘telescope’ had to be underground. As the neutrino ‘hit’ a chlorine atom, the added energy resulted in an argon atom. As the argon atom looses energy, helium is released and bubbles to the surface. These reactions were counted to indicate the number of neutrinos reacting in the chamber. This is called a Chlorine 37 detector (Kitchin, page 151). It wasn’t until two decades later than an improved version of the neutrino telescope would be built. While initially constructed to examine proton decay, the Kamiokande and the IMB (Irvine-Michigan-Brookhaven) water-based detectors were very effective in detecting neutrinos (Kitchin, page 153). It was the Kamiokande detector that found a neutrino source other than the Sun when a shower of neutrinos was emitted from a supernova. The water-based detectors examine neutrino interactions by studying electron scattering and inverse β-decay (Kitchin, page 153). By increasing the size of the detector, the sensitivity in neutrino detection would improve. However, the Super-Kamiokande telescope was un-able to detect other sources of neutrinos, nor solve the ‘Solar Neutrino Problem.’ Using an ultra-pure heavy water as the detection media, the SNO would prove to be very sensitive. It was the SNO that revealed the solution to the Solar Neutrino Problem by identifying neutrino oscillation. Other elements are believed to be even more sensitive in detecting neutrinos. While such telescopes are yet to be constructed, Iridium, Lithium, and Potassium-Hydroxide are thought to provide a more sensitive media, but cost and availability could hinder such projects (Kitchin, page 155 to 157). So far, our current ability to detect neutrinos is only possible from the Sun, supernova, failed supernovae, and know Gamma-Ray Bursts. Naturally we are able to detect neutrinos in a controlled environment, such as a particle accelerator, however detecting neutrinos from astronomical sources pose a challenge until we increase sensitivity.

Gravity waves were initially an entirely theoretical idea first stated by Albert Einstein’s Theory of Relativity in 1917. While atmospheric gravity waves have been discovered and measured, the mechanisms behind atmospheric waves differ greatly. For example, atmospheric gravity waves exist by the propagation and wave “breaking of wind over terrestrial landforms” (“Atmospheric Gravity Waves,” http://sprg.ssl.berkeley.edu/atmos/gj_science.html). Astronomical gravity waves are thought to appear by the oscillation of a massive object resulting in the alteration of the gravity the object creates (“Gravity Waves,” http://www.physicscentral.com/action/action-02-8.html). Russell Hulse and Joseph Taylor were the first two scientists to actually measure gravity waves, although not directly. In studying pulsars for regularity in radio energy, they discovered one with an oscillating irregularity. By studying the Doppler changes and pulse arrival times, this indicated a closely orbiting companion of this particular pulsar. For discovering this as a source of gravity waves, they won the Nobel Prize in 1993. Based on this work, a known source of gravity waves are Neutron Binary systems (Barish, slide 8). Using laser interferometers will reveal other sources as well.

Gravity wave detection is a new science, and there are currently two methods of detection: the resonant mass detector and the interferometer. The resonant mass detector is designed to detect the theoretical resonance of a double-star system. This frequency, based on math, is thought to be at 1600Hz. Much broader in its detection ability is the interferometer such as the Laser Interferometeric Space Antenna (LISA) due to orbit the Earth in 2011, and the ground based Laser Interferometer Gravitational Wave Observatory (LIGO) which has just now began to make observations. LIGO works by combining the observational data from two or more sources combined into one result. The two observatories that make up LIGO are the Hansford Observatory in Washington State and the Livingston Observatory in Louisiana. The great distance between these observatories greatly improves the accuracy of measurements from a distant source by confirming readings that have been picked up by both interferometers. In addition, the two separate installations will rule out any localized seismic events that can lead to false readings. Long tubes are constructed in an L-shape and large masses are placed on each end. Lasers are reflected off these masses. Fluctuations in the laser bean are measured by photo detectors. The current detection ability of LIGO is binary stars, periodic sources (like Pulsars), gravity wave background (Stochastic Background Sources), supernovae and Gamma Ray bursts (Lazarini, slide 4). Planned detection from LIGO will be possible gravity wave sources such as spinning neutron stars, birthing neutron stars, and black holes (Shoemaker, slide 4). Such detection would require improved suspension of thermal, internal, and background noise as measured from the current LIGO.

There are limitations to our current detection ability of gravity waves and neutrinos. Gravity wave detection must be followed up with alternate methods of confirmation. For example, when detecting gravity waves from neutron stars, that information will be compared to visual confirmations of known neutron stars. Supernovae and GRB’s are to be confirmed within one hour by neutrino detectors (Barish, Slide 12). However, with the international network of interferometers from LIGO, GEO, Virgo, TAMA, and AIGO improve detection confidence and allow for the pinpointing of sources (Barish, slide 15). Only in its second scientific stage, the LIGO detector is still gathering gravity wave data. On the other hand, neutrino detection is moving slowly forward ever since the solution of the Solar Neutrino Problem. Now new answers must be found as to why neutrinos oscillate. Do the oscillations occur because of interactions with other neutrinos? Or because of interactions with dark matter? There are many other questions to be answered. Once we answer these questions, we may find a way to test these experiments using particle accelerators and possibly find a way to oscillate neutrinos artificially.

Radio astronomy is a proven method of observing radio sources in our universe. The radio frequencies are also used to search for artificially made signals from sources of extra-terrestrial intelligence. It is fair to say that if we discovered radio frequencies within the Universe, so have other technologically advanced civilizations. How advanced would a civilization have to be to use neutrinos and gravity waves as a form of communication? Our own civilization is just now beginning to detect gravity waves. There is still theory regarding alternate sources of gravity waves – like black holes. They have yet to be observed. Only as we continue to detect gravity waves will our confidence level of looking for stray sources improve. Until then, we must compare results of known gravity wave sources with other known observational styles – like neutrino detection and visual observations. Neutrino detection has improved over the past few years, but our improved detection ability has raised more questions. Before we can even speculate on how extra-terrestrial civilizations can use gravity waves and neutrinos for communication, we must first test and improve our current methods of detection. Once the ideal match with successful observation and improved detection methods are met can we use these methods to explore our Universe, and search for the unusual sources of neutrinos or gravity waves. There is one last point to make. Thanks to Seth Shostak’s book Sharing the Universe, I can offer one final opinion: an advanced civilization will also have the ability to detect neutrinos and gravity waves. By studying sources of neutrinos and gravity waves, like supernovae’s and GRB’s, an ETI can use that event to relay a signal in the direction of the event using standard radio. Since our instruments are pointed in the direction of the event, it is the possibility that our radio dishes will also point in the same direction in order to listen to a possible signal. That will no doubt eliminate the random searches of our very large Universe.

References:

Barish, Barry. “Gravity – Studying The Fabric of the Universe.” Presentation. AAAS Annual Meeting. Denver, Colorado, 17 February 2003

Foust, Jeff. “Solar Neutrino Problem Solved.” Internet. Spaceflightnow.com. June 20, 2001. http://spaceflightnow.com/news/n0106/20sno/.

Kitchin, C R. Astrophysical Techniques. Third Edition. Institute of Physics Publishing, Bristol and Philadelphia. 1998.

Lazzarini, Albert. “LIGO: First Results from the S1 Science Run.” Presentation. AAAS Symposium: “Looking Beyond Earth.”

Pennicott, Katie. “Solar Neutrino Puzzle is Solved.” http://physicsweb.org/article/world/14/7/10. Physics World. July 2001

Physics Central. “Gravity Waves.” Internet. http://www.physicscentral.com/action/action-02-8b.html

Quinn, Helen. “Theory. Leptons.” http://www2.slac.stanford.edu/vvc/theory/leptons.html  05 May 2003.

Shoemaker, David. “The Future – How to make a next generation LIGO.” Presentation. AAAS Annual Meeting, 17 February 2003.

Shostak, Seth. Sharing the Universe: Perspectives on Extraterrestrial Life. Berkeley Hills Books. Berkeley California. 1998.

 

Back to Top | Back to Astrobiology

Search | Site Map | Buy Stuff - Store | Appendix
©2004 - 2013 Astronomy Online. All rights reserved. Contact Us. Legal. Creative Commons License
The works within is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.