Lecture #26: Searching for Other Civilizations SETI = Search for Extra-Terrestrial Intelligence Searching for signals from other beings
Searching for Other Civilizations Searching for signals from other beings. The big problems are: is a signal artificial? where to look? what to look for?
What is an artificial signal? Signal with beats corresponding to prime numbers or some other mathematical theme (= universal language) Where to look? Each star one at a time can take a long time. What to look for? Searching for Other Civilizations Radio, optical, X-ray? What specific frequency of radio, optical, or X-ray?
What is Expected? Signal of Abnormal Behavior Codes Prime Numbers, Patterns Jodie Foster in 1997 movie Contact
Prime Numbers Numbers divisible only by themselves and 1 Advantage: no regular pattern, and cannot be duplicated by natural processes If signal repeats 2, 3, 5, 7,... a few times, it has to be artificial http://ocw.mit.edu/nr/rdonlyres/cbcc8193-3af9-4fd2-a5aa-97df6659a77c/0/chp_prime_numbrs.jpg
Photon (light) Strategy The Electromagnetic Spectrum But Which Waves?
What is the Best Frequency / Wavelength? Frequency = (speed of light) / (wavelength) = # of crests in one second Considerations include energy to send signal and bandwidth allowed Lower-energy (longer wavelength) are more energy efficient However, shorter wavelength has higher frequency, thus higher bandwidth, thus can send more information Also, what about host star interference? Compromise is not clear
Consider Visible Infrared Near-infrared wavelengths cut through the dust
The stars disappear in the mid-to-far infrared à less interference!
The Power of Wavelength Conclusion: Use Long Wavelengths! But Which Ones?
Nature Provides A Clue Galactic Noise (Synchrotron Radiation) H OH Atmospheric Emission The Water Hole http://www.jb.man.ac.uk/distance/life/sample/seti/microwave.jpg
Searching for Other Civilizations The water hole is a minimum in galactic emission and a pointer to our origins.
The Cosmic Water Hole H + OH H 2 O H 21.1 cm 1420 MHz OH 18.3 cm 1638 MHz These are in the radio region!
Searching for Other Civilizations The hydrogen emission line at 21 cm comes from the spin flip transition of hydrogen à best frequency? The first searches by Frank Drake were done towards stars at that frequency (21 cm à 1420 MHz = 1.4 GHz): Project Ozma
Project Ozma, 1960 National Radio Astronomical Observatory Green Bank, West Virginia Two Stars observed by Frank Drake during one month
Project Ozma, 1960 Observed at 1.420 GHz Total bandwidth: 400 KHz Single-channel receiver with bandwidth of 100 Hz
Little Green Men? 1967, Cambridge, England Doctoral student Jocelyn Bell Constructed radio telescope (interferometer) Found super-regular signals Could these be aliens?! http://homepage.mac.com/dtrapp/people2.f/jocelynbell.jpg
Little Green Men? 1967, Cambridge, England Doctoral student Jocelyn Bell Constructed radio telescope (interferometer) Found super-regular signals Could these be aliens?!
Searching for Other Civilizations The first pulsars were called LGM 1, 2 and 3 where that stood for Little Green Men as a half-joke. The signals have extremely fast and regular periods which are very unusual we now know that this arises from the spins of pulsars. à 1974 Nobel Prize to Ryle (telescope design) & Hewish (Bell s supervisor) but not Bell!
Project Ozma II, 1973-76 National Radio Astronomical Observatory Green Bank, West Virginia 650 stars observed by B. Zuckerman & P. Palmer 4 years
November 15,1988
Today: National Radio Astronomical Observatory Green Bank Telescope, West Virginia Same observations as Ozma in under 1 sec!
How Many Channels in Cosmic Water Hole? 1638 MHz = 1,638,000,000 Hz (OH) 1420 MHz = 1,420,000,000 Hz (H) 0.1 Hz / channel (1,638,000,000 1,420,000,000 ) / 0.1 2 billion channels
Math ~ 2 billion radio channels Modern computers measure Band ~ 250 million channels at once Examine each band for 1 sec need 8 seconds for total coverage of one star
~ 2 Billion Candidates in Milky Way?
More Math ~ 2 billion radio channels in the Cosmic Water Hole Each star needs ~ 8 seconds for coverage For ~ 2 billion stars will need about 500 years for total coverage!
Computing Power Doubles every ~2 yrs Moore s Law In ~20 years, can study all candidate stars in ~ 1/2 year, not 500 years!
Searching for Other Civilizations The Allen Telescope Array: 42 six-meter antennae in Northern California Privately funded by Paul Allen (Microsoft co-founder; >$30M), Franklin Antoniio (QUALCOMM, $3.6M), others SETI observations as of 2016 Breakthrough Listen: In July 2015, Russian billionaire Yuri Milner has agreed to donate $100M in 10 yrs to fund SETI projects Stay tuned! Breakthrough Starshot: Milner donated another $100M to prove the principle of sending multiple tiny crafts to Alpha Centauri (planet Proxima B)
Searching for Other Civilizations There are no detections to date at any wavelength. The researchers are very motivated and committed to making their results public! So what does this mean? What would it mean if we continue to detect nothing for 100 years?!
ASTR 380 Searching for Other Civilizations Two critical questions in interpreting the lack of detections are: How likely is it that ET wants to communicate with us? -- very hard to guess this. -- perhaps our only hope is accidental emissions How long does a civilization broadcast freely into space? -- perhaps after 100 years we will use optical fibers, or something new and the Earth will be radio quiet (no losses / leakage to space).
The Drake Equation A Fun Exercise: the Drake Equation (simplified) where: N I = N * x f P x f L x f I N I is the number of planets in our Galaxy that contain intelligent life N * is the number of suitable stars in our Galaxy f P is the fraction ( 1) of these stars that have a habitable planet f L is the probability ( 1) of life forming on such a planet is the probability of intelligent life forming (now) given that life had formed f I What are your guesses for these factors???
The Drake Equation A Fun Exercise: the Drake Equation (simplified) where: N I = N * x f P x f L x f I N I is the number of planets in our Galaxy that contain intelligent life N * is the number of suitable stars in our Galaxy (~10 10 ) f P is the fraction ( 1) of these stars that have a habitable planet (~0.1) f L is the probability ( 1) of life forming on such a planet (~0.01) f I is the probability of intelligent life forming (now) given that life had formed (~0.001) N I = 10 10 x 0.1 x 0.01 x 0.001 ~ 10,000 à Not Bad!
However: If one other civilization exists in our Galaxy, AND Searching for Other Civilizations 1. It started 1,000,000 years before us. 2. And does space travel 5-10 light-year distances 3. And colonizes a new stellar systems every 20,000 years 4. And is long lived.. Exponential growth à It would span every stellar system in our Galaxy with suitable Earth-like planets right now! But then where are they?! This is often called the Fermi Paradox
Summary Finding intelligent life probably means patiently waiting until we receive some signal No detections yet What does it mean? Fermi Paradox? (take ASTR 380 to find out more!)
Reminders ASTR101 course evaluations: 75% response rate! There is still time: courseevalum.umd.edu Review session: Today, 6-8 pm (PHYS 1412) Final Exam: Saturday, 8-10 am (PHYS 1412)
Final Saturday 8:00 10:00 am (PHYS 1412) Be there early (~7:45 am)! Material: Everything! ~50% on material covered after midterm #2, ~50% on material before 40 multiple choice questions + 6 problems No need to remember any formula Bring your student ID card Write on the scan sheet: your section #, student ID # Use #2 pencil only No books, calculators, phones, computers, hats, Wait before sitting for the exam Remain seated until told to leave