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1 | The Search For Extraterrestrial | 17 | waterhole”. Alien civilizations will know |
Intelligence (SETI). AST 309 part 2: | that these two lines are from the | ||
Extraterrestrial Life. | dissociation products of water, whatever | ||
2 | The Drake Equation: N = N* fpl nhab fL | they call H and OH. Not taken too | |
fC fT L/T. Stars? Planets? Habitable | seriously, But convenient range to strive | ||
Origin Complex Intelligence, Lifetime | for. (Allen Telescope Array uses this | ||
planets? of life? life? technology? of | range, and more.). OTHER ‘MAGIC” | ||
civilization. | FREQUENCIES Some frequency based on | ||
3 | Another way to see what the Drake | combinations of fundamental constants of | |
equation represents: What is the | nature? (e.g. speed of light, Planck’s | ||
likelihood or probability that the | constant, …) The combination can be | ||
sequence of events shown to the right will | expressed without referring to “our” units | ||
occur, ending up with the peculiar group | (e.g. meters) ”Intergalactic” frequency | ||
of phenomena listed at the bottom | standard based on temperature of cosmic | ||
branching? And are there many other | background radiation? Many others have | ||
possible branchings that we haven’t | been suggested. Too many! None in use | ||
thought of, or are incapable (for, say, | today. | ||
biological reasons) of imagining? | 18 | SETI projects: partial historical | |
4 | Intelligence, technology. | list. Ozma (1960) In 1960, radioastronomer | |
Intelligence—If we want to communicate | Frank D. Drake, then at the National Radio | ||
with aliens, they must have something like | Astronomy Observatory (NRAO) in Green | ||
what we usually call “intelligence.” What | Bank, West Virginia, carried out | ||
does this mean? Are there different | humanity's first attempt to detect | ||
“types”? What about other animals? Other | interstellar radio transmissions. The | ||
cultures? Especially oral tradition | stars chosen by Drake for the first SETI | ||
cultures. Why did humans evolve such big | search were Tau Ceti and Epsilon Eridani. | ||
brains? Why think that extraterrestrials | From April to July 1960, for six hours a | ||
would share our forms of cognition? Factor | day, Project Ozma's 85-foot NRAO radio | ||
in Drake equation: What is probability | telescope was tuned to the 21-centimeter | ||
that life elsewhere develops | emission (1420 MHz) coming from cold | ||
“intelligence”? Call it fI . | hydrogen gas in interstellar space. A | ||
Communication, technology—If we want | single 100 Hz channel receiver scanned 400 | ||
signals, the aliens must have some kind of | kHz of bandwidth. The astronomers scanned | ||
system of representation, like a language. | the tapes for a repeated series of | ||
How likely? Other forms? Musical? | uniformly patterned pulses that would | ||
Unimaginable? And no matter what kind of | indicate an intelligent message or a | ||
language they have, they have to be | series of prime numbers such as 1, 2, 3, 5 | ||
technological civilizations if they are to | or 7. With the exception of an early false | ||
send signals across the Galaxy. We could | alarm caused by a secret military | ||
enter these topics as two more | experiment, the only sound that came from | ||
probabilities, fC fT . N(planets with life | the loudspeaker was static and no | ||
that also developed intelligence and | meaningful bumps superimposed themselves | ||
technology) = N* fpl nhab fL fC fT. | on the formless wiggles on the recording | ||
5 | Lifetime of the communicating phase. | paper. | |
We left out one crucial factor: The | 19 | Arecibo: message sent to M13 (1974): | |
fraction of the Galaxy’s age T that a | Globular cluster ~25,000 lyr away Message | ||
civilization is in the technologically | written by Drake, Sagan, et al. Frequency: | ||
communicating phase. We know the age of | 2380 MHz 1679 binary digits. SETI | ||
our Galaxy is T ~ 10 billion years. | projects: partial historical list. | ||
Consider our chances if civilizations only | 20 | Ohio State University SETI (1977-1997) | |
last for 100 years… Average length of time | With the Big Ear fixed radiotelescope | ||
spent in the communicating phase L. | (used Earth’s rotation to scan the sky) | ||
Sometimes referred to as lifetime of a | August 15, 1977: detected a strong | ||
technological civilization. Call it L. Our | narrow-band signal for 72 secs: SETI | ||
final Drake equation is then: N = N* fpl | projects: partial historical list. | ||
nhab fL fC fT (L/age of Galaxy) This | Duration consistent with extraterrestrial | ||
represents the number of communicating | source Strength: 30 times above background | ||
civilizations in our Galaxy. We have been | noise Frequency: 1420 MHz (neutral H) | ||
“this way” for only ~ 100 yr. If this were | Bandwidth: <10 kHz. | ||
L, then N (number of civilizations for | 21 | SETI projects: partial historical | |
communication in our Galaxy) would be | list. The location of the signal in the | ||
around 1. Even if N ~ 100, there is no | constellation Sagittarius, near the Chi | ||
hope for two way communication, and | Sagittarii star group. RA= 19h25m31s ± 10s | ||
probably very difficult to locate them | or 19h28m22s ± 10s declination= ?26°57? ± | ||
among a 100 billion possibilities. And too | 20? There were ~50 follow-up searches | ||
far for biosignatures. In order for our | performed in this area, but nada => | ||
Drake equation “N” to come out reasonably | Origin of the Wow! signal is still | ||
large (say a million), the average | undetermined. | ||
lifetime L must be extremely large, | 22 | SETI projects: partial historical | |
something like 100,000 to a million years. | list. NASA asks for SETI proposal, | ||
? This means we can expect any nearby | astronomers propose “Project Cyclops”, | ||
civilizations to have had a long lifetime, | 1000 100 meter radio telescopes on back | ||
i.e. be very advanced. I think this is the | side of moon, costing $10 billion (1970s). | ||
only thing we can know with any certainty | NASA asks for more moderate plan, planning | ||
about extraterrestrial civilizations: If | for next ~ 17 years. Ohio State SETI: | ||
there are enough of them to make it likely | 1977-1997 (replaced by golf course). Best | ||
we will find them, then they must be very | known for the “wow” signal. Harvard, Paul | ||
advanced (using beacons)! | Horowitz and Project META (millions of | ||
6 | SETI web sites. SETI at Space.com | bands in frequency), Project BETA (billion | |
http://www.space.com/searchforlife/index.h | bands in frequency). Horowitz and Sagan | ||
ml SpaceRef.com | 1993 Astrophysical Journal summarize | ||
http://www.spaceref.com/Directory/Astrobio | results. One of first SETI papers in | ||
ogy_and_Life_Science/seti/ Ongoing SETI | refereed journal. Harvard and Horowitz now | ||
searches: SETI Institute/Allen Telescope | converted to Optical SETI, largest in | ||
Array http://www.seti.org/ Project | world. UC Berkeley’s Project SERENDIP. | ||
SERENDIP, UC Berkeley | Since 1977! Part of data analyzed by 5 | ||
http://seti.ssl.berkeley.edu/serendip/sere | million home computers through SETI@home. | ||
dip.html SETI@home | Dec. 1991. NASA funds $100 million SETI | ||
http://setiathome.berkeley.edu/ Sourthern | effort (“MOP”). Detailed plan for combined | ||
SERENDIP, Univ. Western Syndey | targeted and sky survey searches. 1993: | ||
http://seti.uws.edu.au/ SETI Italia | Funding removed by senate amendment | ||
http://www.seti-italia.cnr.it/ Optical | Project Phoenix (SETI Institute) rises | ||
SETI at Berkeley | from the ashes. Piggy-backs off various | ||
http://seti.ssl.berkeley.edu/opticalseti/ | radio telescopes, mainly Arecibo. 2001: | ||
Harvard http://seti.harvard.edu/oseti/ | Paul Allan and others fund the Allen | ||
Amateur SETI: Project BAMBI | Telescope Array, 350 6meter telescopes. 42 | ||
http://www.bambi.net/ SETI League | complete by Oct. 2007. | ||
http://www.setileague.org/ Project ARGUS | 23 | SERENDIP. One of oldest operational | |
http://www.setileague.org/argus/index.html | SETI searches--since 1979, UC Berkeley. | ||
7 | STRATEGIES FOR COMMUNICATION WITH | 1997--installed as piggyback at Arecibo | |
EXTRATERRESTRIAL CIVILIZATIONS. ?SETI is | radio observatory (picture below), largest | ||
concerned with searches for signals from | single-dish radio telescope in world (but | ||
extraterrestrial civilizations, not | can only point in one direction). SERENDIP | ||
spectral “biomarkers” we discussed earlier | = Search for Extraterrestrial Radio | ||
in course, or actual travel to other star | Emissions from Nearby Developed | ||
systems. ? Except for 1974 signal to | Intelligence Populations. SERENDIP IV is | ||
globular cluster M13 (thousands of light | the fourth instrument of the project, | ||
years away), we only try for reception, no | collects data by 'piggybacking' on top of | ||
transmission. (“What if they are all | the Arecibo radio telescope. SERENDIP IV | ||
listening?”) ? “Signals” could be | instrument is basically a 200 billion | ||
intentional (they are trying for contact) | operations per second supercomputer that | ||
or nonintentional (we eavesdrop, one way | scans 168 million narrow (0.6 Hz) channels | ||
or another). A few unintentional | every 1.7 seconds for signals that are | ||
candidates listed on next slides, but we | significantly 'louder' than the background | ||
are most concerned with intentional | static. Some of its data is analyzed | ||
signals, and with establishing a two-way | through SETI@home for desktop | ||
conversation. ? Remember: Distance to even | computers--so far millions of users, | ||
the nearest 1000 or so stars ~ 50 light | largest distributed computing project in | ||
years, and we expect only a tiny fraction | world, led to ~ 100 other distributed | ||
of them to have life, let alone | computing projects, e.g. folding@home, | ||
intelligent communicating life, so we are | prime@home, climatenet@home, … (details on | ||
necessarily asking whether to undertake a | next slide). The Arecibo radio telescope | ||
search with a guaranteed very low | in Puerto Rico, used by both SETI Insitute | ||
probability of success, and, even if | for Project Phoenix, and by UC Berkeley | ||
successful, communications will involve | for their SERENDIP IV. | ||
decades or even centuries. You can see why | 24 | SETI@home: Now searching for pulses. | |
funding for SETI is sparse! | August 2008: SETI@home switches to search | ||
8 | Four types of nonintentional types of | for pulsed radio signals. Observations are | |
signals: Leakage radiation from radio, TV, | from SERENDIP piggy-backed on radio | ||
or other radio broadcasts. Earth as | telescope at Arecibo that is built into a | ||
example: Many TV stations broadcasting | mountain. This dish only points in one | ||
different shows, or same shows at | direction as sky drifts across this | ||
different times, radio waves emanating | direction--the drift takes about 12 | ||
from Earth have always been incoherent, | seconds for a given point in the sky. | ||
completely scrambled. It is not true that | SETI@home searches for signals that rise | ||
alien ETI could be seeing our early TV | and fall in 12 seconds--any object will do | ||
shows! Alien communications, e.g. between | this, but most will be broad-band sources | ||
home planet and colonies. We would have to | (top image). Narrow search by requiring | ||
be almost exactly in the line of sight | narrowband signal (2nd image). Will check | ||
between home planet and colony, and guess | for several different bandwidths. | ||
the frequency range. Seems very unlikely. | Information in image? Search for pulsed | ||
Dyson spheres -- hypothetical constructs | signal (3rd image). If from planetary | ||
built by advanced civilizations in order | system, should also be Doppler shifting | ||
to collect nearly all the energy of their | (“chirped” signal), as in 4th image. Home | ||
parent star. Spherical shell at same | computers look for various combinations of | ||
distance from star as the home planet. The | frequencies, bandwidths, and chirp rates. | ||
intercepted energy is somehow channeled to | See if you can understand why the white | ||
planet. But shell is heated by the | “thing” in the illustration below might be | ||
incident radiation: What will its | a signal… SETI@home screensaver Can you | ||
temperature be? At what wavelength would | see the alien signal?? | ||
you conduct a search for Dyson spheres? | 25 | SETI@home and distributed computing. | |
Products of technological activity – e.g. | By 2007 over 40 projects had joined the | ||
gamma rays from their (hypothetical) | BOING distributed computing family, using | ||
fusion propulsion systems, … CFC molecules | the software provided by SETI@home. | ||
from their air conditioners… | Protein folding: folding@home, | ||
9 | TV leakage radiation. World TV | predictor@home, Rosetta@home, | |
spectrum. World TV power vs. time. | Proteins@home Primegrid.com: privately run | ||
10 | Geographical distribution of TV | mathematical project that searches for | |
stations. Geographical distribution of TV | very large prime numbers and has already | ||
stations. As Earth rotates, this “pattern | found more than 100 new primes. | ||
of populated areas” is the only evidence | Einstein@home is based at the University | ||
for TV broadcasts. | of Wisconsin in Milwaukee and searches for | ||
11 | How would an alien civilization try to | pulsars in the sky based on data from the | |
communicate across many light years of | gravitational wave detectors LIGO and GEO. | ||
space? The only thing that is almost | climateprediction.net: Oxford Univ. UK | ||
certain is that they will use | high-profile climate simulator. Jan 2005: | ||
photons—fastest and cheapest way to | First paper in Nature, 2,570 simulations | ||
transmit information that exists (as far | of Earth. By 2007: 50,000 simulations. | ||
as we know). Even though photon signals | Goal is several million, to explore 23 | ||
are the only choice we can think of, that | parameters of the climate model. | ||
still leaves many considerations that we | 26 | Allen Telescope Array (ATA). | |
need to guess about: Where to point our | Eventually 350 6 meter antennas, | ||
telescopes? What kind of stars should we | equivalent to 100 meter single dish. 42 | ||
point our telescopes toward? Or would it | dishes saw “first light” in Oct 2007. | ||
be better to survey the whole sky? | Entered hibernation due to funding | ||
Wavelength: What wavelength region should | problems this year! => Maybe restart in | ||
we expect is optimum for sending | Sep 2011 due to private donations > | ||
interstellar signals? Radio? Optical? | $200,000 Unique features: ? Large field of | ||
Other? Bandwidth: What range of | view, so can scan sky faster in survey | ||
wavelengths? Broadband or narrow-band? | mode. ? Large range of frequencies (1-10 | ||
WHY? Recognition and Interpretation: How | GHz for targeted search, five times range | ||
would a message, or some sign that it is | of Phoenix), and small bandwidth (~ 1Hz), | ||
not a natural phenomenon, be | using more than a billion channels. ? | ||
distinguished, and how would a meaningful | Finally offers SETI 24/7 monitoring | ||
message be encoded? We’ll discuss each of | (Phoenix had Arecibo for only about 3 | ||
these in turn. | weeks per year 1998-2004) Goals: ? | ||
12 | WHERE TO POINT? a. Sky survey. Survey | Targeted search: Survey 106 stars with | |
entire sky with telescope’s “beam” – this | good sensitivity between 1 and 10 GHz for | ||
might involve millions of directions for | weak non-natural transmitters. ? Sky | ||
typical radio telescopes. If you want to | Survey ~ monitors inner Galactic plane in | ||
finish in your lifetime, you could only | “water hole” range 1420-1720 MHz for very | ||
spend a very brief time on each direction, | strong non-natural transmitters. | ||
so you could only detect very strong | 27 | Optical SETI (OSETI). When faced with | |
signals. But at least you won’t miss any | the question “What kind of signals would | ||
of them. And the method doesn’t make any | alien civilizations transmit, the | ||
assumptions about what the most likely | traditional answer has been: Continuous | ||
stars are for signal reception. ? This is | narrow-band radio transmissions An | ||
a low-sensitivity method, but complete for | alternative: Maybe they would send | ||
strong signals. b. Targeted search. Point | distinct broad-band pulses. They would | ||
at the nearest (less than about 50 to 1000 | stand out against background noise not | ||
l.y.) stars roughly like the sun and | because they are precisely centered on a | ||
cooler (recall conditions for habitable | particular wavelength, but because they | ||
planets). Could detect weaker signals, | are very short and punctuated bursts of | ||
i.e. would have higher sensitivity. But | energy—unlike most other natural | ||
you will only cover a tiny fraction of the | phenomena. This is the world of Optical | ||
sky. ? This is a high-sensitivity method, | SETI, which searches for signal in the | ||
but severely incomplete. Most current | visible (or infrared in future) light | ||
searches have shifted to a sky survey mode | range, usually looking for nanosecond | ||
(a) However plans change rapidly—the Allen | pulsed laser radiation. Reasons: Radio | ||
Telescope Array (largest current project) | 21cm line has huge noise problem with | ||
combines both approaches. And some | interstellar gas. Laser-like signals are | ||
“optical SETI” searches (see below) are | tightly beamed, so can be sent over very | ||
targeted searches. | large distances (no loss due to | ||
13 | What frequency should be used to | inverse-square law). With current | |
listen or send interstellar messages? From | equipment can send out pulsed laser beam | ||
the Earth’s surface, most radiation is | 5000 times brighter than the Sun | ||
blocked by the atmosphere. The exceptions | Unidirectional--can pinpoint direction | ||
are optical (visual) and radio photons. | with high precision. Higher frequency | ||
Earth’s atmosphere also blocks out most of | --can encode more information. There are | ||
the infrared part of the spectrum due to | two OSETI programs at UC Berkeley, and now | ||
water vapor in our atmosphere. From the | ongoing or about-to-launch OSETI searches | ||
highest mountains or a jet plane, the | at Harvard, Lick Observatory, and Columbus | ||
infrared is barely accessible, but not for | Observatory. Current projects: Berkeley: | ||
the continuous kinds of surveys we have in | Monitors 2500 stars as part of exoplanet | ||
mind. Note that if we could do such a | search. Searches for ultra-narrow band | ||
survey from Earth orbit (expensive), or, | signals. Harvard: since 1998, using 61 | ||
if we only had about $100 billion dollars | inch telescope. Nearly 100,000 | ||
so that we could build a facility for SETI | observations. Dec. 2000: new 72 inch | ||
on the far side of the moon (“Project | telescope dedicated to an all-sky survey. | ||
Cyclops”), our considerations might be | Can detect nanosecond (billionth of a | ||
different. Why have most SETI searches | second) pulses and cover entire sky in 200 | ||
concentrated on radio wavelengths instead | nights. Lick Observatory Targeted search | ||
of optical? A single amazingly influential | began in 2000. First results published in | ||
paper by Cocconi and Morrison (1960 | Stone et al. 2005 Astrobiology. 14 | ||
Nature) set the stage. Their arguments for | candidate events. | ||
radio SETI are on next slide. | 28 | Encoding a message. ?SETI researchers | |
14 | Reasons for radio SETI: 1. | focus on a signal anyone could comprehend. | |
Interstellar dust selectively blocks | Not clear this is sensible! ? It is very | ||
shorter wavelengths (higher frequencies) | sensible to expect digital, binary, not | ||
strongly suggests we use the IR or radio | analogue signal. ? How to encode a picture | ||
parts of the spectrum (long wavelengths or | into a string of binary (0,1) signals? The | ||
small frequencies). But IR is dominated by | simplest and most efficient way to encode | ||
Earth’s atmospheric molecular emission if | a message (we think) is binary code. Use | ||
search from surface, so that leaves radio. | only 2 characters, e.g. a 1 and a 0, or a | ||
Notice that radio SETI allows reception | + and a - , or "on" and | ||
from the entire galaxy, but optical isn’t | "off", ... Each 1 or 0 (or | ||
that bad, since we can see stars out to ~ | whatever) is a "bit". Then the | ||
1 kpc. Besides, most radio searches are | message can just be sent as a series of | ||
concentrating on nearby stars anyway. (We | pulses. Expect the message to be a | ||
don’t want 1000-year “conversations.”) 2. | two-dimensional picture that is encoded in | ||
Radio photons are cheaper to send than | a one-dimensional binary string that | ||
optical photons (because energies are ~ | factors into prime numbers. e.g. 551 = 29 | ||
100,000 times smaller for radio). 3. The | x 19 (or 19 x 29); 1679 = 23 x 73 (the | ||
main consideration is noise: Here “noise” | 1974 Arecibo transmission). Example: We | ||
means anything that is not an alien | receive signal 1111100000101011010110101. | ||
signal--any kind of interference. We | This factors into 5 x 5, giving a picture | ||
should listen (or send) where the noise is | of the greek letter "pi". Or try | ||
minimized, so that we can recognize the | the letter "E", etc. But why | ||
(probably weak) signal. Noise is minimum | would ETI send out signals that anyone | ||
in a region of the radio part of the | could decode? Perhaps they send out | ||
spectrum. This is summarized in a classic | signals which could be understood only by | ||
graph shown on next slide. Arecibo Home of | others who are already "at the same | ||
SERENDIP, SETI@home, Phoenix. Allen | level" as they are. What would be a | ||
Telescope Array, SETI Institute, N. Calif. | difficult signal for us to recognize? | ||
15 | Alien signaling: Choosing a wavelength | Maybe the test would be to recognize some | |
range that minimizes “noise” -- anything | sort of "meaning" in the | ||
that is not an alien signal. Avoid very | message. (Think about musical signals. At | ||
low frequencies (wavelengths too large), | present, there is no viable theory of | ||
because synchrotron radiation from | musical meaning in music analysis, | ||
supernova remnants dominates there. (Far | philosophy, cognitive science, pattern | ||
left in figure) Avoid frequencies higher | recognition, or any other field that has | ||
than about 10 GHz because of H2O and O2 | approached the problem.) Deeper questions: | ||
emission from Earth’s atmosphere. Cosmic | Will symbolic communication systems be | ||
microwave background radiation sets | universal among intelligent creatures? Is | ||
“floor” at intermediate frequencies, and | “grammaticity” hard-wired into our brains? | ||
that is where the noise is minimum, and | Another example of single mutation? | ||
where we should search. A message will | 29 | The Eerie Silence….. ? SETI has been | |
arrive in a narrow wavelength band or | on-going for about 50 years and NOTHING | ||
bands, not spread over the whole 1-10 GHz | has been detected ? Conclusion: are we | ||
region. There are 10 billion 1 Hz bands in | alone? Are there no ETIs in our galaxy? ? | ||
this range. How to decide which ones to | No, not yet. But they are probably no | ||
pick? First, must understand bandwidth. | nearby ETIs similar to us, nor a nearby | ||
16 | The importance of bandwidth. Basic | strong radio beacon intended to notify us | |
idea: Can pack more power in a narrow | of ETI’s presence. There are many stars | ||
frequency range (narrowband signal) than | not observed yet, so many frequencies not | ||
spreading out over a large range | explored and many other possible ways for | ||
(broadband signal). So can distinguish a | ETIs to communicate. | ||
narrowband signal from the background more | 30 | What IF The Eerie Silence is broken! | |
easily. Think of the everyday radio | What then? There is actually a so-called | ||
analogy again, and it should be clear! | “SETI Post Detection Task Group” (a | ||
SETI@home: Each vertical “band” is a 10 | permanent study group of the International | ||
kHz “slice” of the 2.5 MHz wide SERENDIP | Academy of Astronautics). It’s only a | ||
data. There are 250 such “slices.” But | think tank, no legal status, no | ||
search is for signals much narrower than | governmental involvement. In case a signal | ||
these bands. | is detected the task group would advise to | ||
17 | Which frequency? If it is true that | perform the following steps: Verify | |
narrow-band signal is the only sensible | authenticity of ETI signal Contact the | ||
approach, how will we decide which band to | International Astronomical Union => | ||
use? Suggested “beacon frequencies” (or | which will contact UN. inform the | ||
“hailing frequencies” or “magic | government of the country (countries) | ||
frequencies”): HI (neutral hydrogen) 21cm | where radio telescope is located Public | ||
(wavelength) line? The frequency is 1420 | announcement Such a discovery will have | ||
Megahertz = 1.42GHz Natural, abundant, but | unprecedented and enormous scientific, | ||
lots of interference by interstellar gas. | social, cultural, political and spiritual | ||
(Latter has apparently been forgotten.) OH | consequences for humanity. Nobody knows | ||
line at 1.7 GHz? H + OH = H2O, so maybe | what will happen (depends on nature of | ||
region between these two --> “the | signal, e.g. distance to source). | ||
The Search For Extraterrestrial Intelligence (SETI).ppt |
«English for you» - Все слова и выражения озвучены носителями языка. Ты сможешь совершенствовать своё произношение. Может ли компьютер заменить учителя? Ты убедишься насколько интересным и захватывающим может быть обучение языку. Твои школьные учебники. Ты научишься правильно строить предложение. You are welcome! Узнать насколько хорошо ты усвоил материал тебе помогут:
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