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“We seek him here, we seek him there, Those Frenchies seek him everywhere. Is he in heaven? —Is he in hell? That dammed, elusive Pimpernel. ” Sir Percy Blakeney, Chapter 12, The Scarlet Pimpernel
Searching for ET § A Scientific Recipe
Birth of SETI Search for Extra-Terrestrial Intelligence § § Searching for Interstellar Communications Nature vol. 184, no. 4690, pp 844 -846 Sept. 19, 1959 Giuseppi Cocconi and Philip Morrison “… The reader may seek to consign these Few will deny wholly the profound importance, practical speculations to the domain of scienceand philosophical, the detection of fiction. We submit, which rather, that the foregoing line interstellar communications would We, of of argument demonstrates that thehave. presence therefore, that aisdiscriminating search for all interstellarfeel signals entirely consistent with intelligent signals a considerable effort. we now know, anddeserves that if signals are present, the The probability of success difficult to estimate; means of detecting them isisnow at hand. but if we never search, the chance of success is zero. ”
Communication … the Basis • The building blocks of nature and the fundamental laws of physics that describe their behavior are universal, i. e. , they are the same for ET as they are for us. • Universal constants such as c (the speed of light), e (the unit of electrical charge), h (Planck’s constant) etc. characterize these laws. • Mathematics is the language in which these laws are expressed. • Communication cannot take place unless ET has discovered those laws and developed a mathematics in which to express them.
Operational Intelligence § In other words, ET must have reached the stage of “operational” intelligence, i. e. , they must have discovered these laws and built a radio telescope. • Radio waves—the optimum way to transmit information through space. Why? 2. Fast—they travel 1. photons areat the 3. Radio They are penetrating, speed of easily light. cheap…you cantravel make i. e. , they 10 millionintergalactic of them for through the cost… asand 1 dustsame and gas visible photon. planetary atmospheres.
The Milky Way Galaxy ET Here? The distance to ET is on the order of 1000’s of light years!
Space is Unimaginably Vast • Milky Way can be approximated as a giant disk. – Radius = 50000 LY – Thickness = 2000 LY • Volume = p(Radius)2 x Thickness • V ~ 1. 6 x 1013 LY 3 = 16 trillion cubic light years!
How Far to Our Neighbors? • If N = L/103 and L = 106 years, then there are 1000 civilizations in 16 trillion cubic LY … or – 1 civilization in every 16 billion cubic LY • Imagine every civilization surrounded by a bubble in which it is alone. • • V = 4/3 π R 3 V ~ 16 billion cubic LY R ~ 1, 600 LY Distance between civilizations: D = 2 R ~ 3, 200 LY! R R 21
2 – Way Communication? I’m here! Cool! 3200 LY Send message … Receive response 6400 years later! Somewhere in the galaxy … far away In the inimitable words of Captain, Road Prison 36 (Strother Martin, Cool Hand Luke, 1967) …“What we got here … is failure to communicate. ”
Question Assume that all the multiplicative factors in the Drake equation lead to the conclusion that N = L/10 3 (L in years). If the lifetime of humans is typical of the lifetime of other species—we estimate L = 106 years and thus N=1000. Given this, roughly how long would it take to exchange a single message with one of those civilizations? A. about 10 years B. about 100 years C. about 1 – 10 thousand years D. at least 1 million years E. a time longer than the age of the universe
Better to Receive Than To Give If there are lots of alien civilizations … how many of them are older than ours? • Our galaxy is about 10 billion years old. • Life emerged on Earth at least 3. 5 billion years ago. • We became operationally intelligent only 50 years ago. So…if, blindfolded, you pick at random one kid out of all the For if N = LKand L = 106 kidsexample, in SLC, grades — 12+, yrs if yoube pick ET at what…would thean odds of random, what areathe chances of… picking kindergartner who one is <? 50 yrs? just whose learned‘age’ to read It’s about 50/106 = 1/20, 000! Yup … we’re the new kid on the block … better that we “listen” and try to “hear messages” from those “ancient” civilizations now broadcasting.
The Optimum Frequency to Tune In? … somewhere between 1420 and 1720 mhz on your radio dial. The Water Hole 1420 mhz 1720 mhz
The “water hole” radio frequency region between Question 1420 MHz generated by neutral hydrogen atoms and 1720 MHz generated by hydroxyl (OH) radicals is a good choice for interstellar communications because _____. A. they are the easiest frequencies to generate B. all radio transmissions eventually fall into this hole so that’s where all aliens should look C. all operationally intelligent civilizations would be aware of its significance and would think of sending and receiving signals within this spectral region D. radio telescopes cannot be tuned to frequencies outside this region E. —it is not a good choice! Intelligent civilizations would use lasers that produce visible light that can be targeted much better
Where Look for ET and How? § Where? § Well, we’ve discussed this … essentially we should target F 7—K 2 stars. § How? 1. A Telescope with a wide angle field of view targets lots of stars at once but picks up a lot of unwanted radio noise so signal has to be very strong! 2. A Telescope with a narrow field of view targets a particular star … but this strategy eliminates background radio noise so signal doesn’t have to be so strong … but a much smaller number of targets can be examined at a given time!
Wide Angle Field of View Target star Wide Angle FOV picks up radio waves from the entire region shown in the figure, but the circled star is G 2 V and that’s ET’s home star! Obviously, if ET is far away, his signal will be weak when it arrives at Earth. If it is to be detectable it must ‘stand out’ above background noise…so
Narrow Angle Field of View Narrow angle FOV picks up radio waves only from small region ─ therefore less interference from region surrounding the target and a ‘weaker’ signal can be detected.
Narrow Angle FOVTarget Wide Angle FOV
How Do We Carry Out a Search of Selected Targets? A very daunting task … • What When“bandpass” language do we “listen? ” will the we aliens use? frequency dodo we “set our dial to? ” • (Wellcan can’t … let’s easily scan take pick all athe look upfrequency stars a at lotan at of example once frequencies so of we what at don’t know what the … aliens use • Where do we search for a signal? once might with miss expect. ) a their wide transmission bandpass … but when wewe’re also pick not for their radio station. ) • (There about 250 billion stars in the Milky up looking. ) a lot of noise. We can eliminate much of the Way Galaxy. ) … but we’ll look at F 7 ─ K 2 and no noise by using a narrow bandpass, but then we binaries! only pick up one particular frequency. )
When Do We Listen? • We want to look for ET’s as far from us as possible. That implies that we adopt a narrow angle field of view targeting strategy… but … • We could be ‘scanning’ a solar system where ET resides but suppose his transmitter …is ‘shut down for maintenance’ …
… Or our receiver is shutdown when ET is trying to ‘talk to us! 21
What is ET’s “Channel? ” • We’ve guessed that ET is likely transmitting at a frequency somewhere in the “water hole” … somewhere between 1420 and 1720 Mhz but • Suppose she’s not! • Well … maybe we punt! • Even if she is, there’s a 300 Mhz frequency band in the water hole! … (1420 ─ 1720 Mhz) • How do we scan that band? 21
Wide or Narrow Bandpass? • What is bandpass? Filter Amplitude Frequency Wide Bandpass Narrow Bandpass
• Wide bandpass picks up a lot of frequencies at once … good! • But … it also picks up a lot of background noise … this means signal has to be strong to be detectable … bad! • Narrow bandpass picks up only the desired frequency with minimal competing bacground noise … good! • ET’s signal will be about 0. 1 – 0. 5 Hz wide because of an effect called interstellar broadening. • … But, there are roughly 600 million 0. 5 Hz wide ‘channels within the 300 Mhz region of the water hole … so you need a receiver capable of picking up 600 million channels at the same time when you scan a target … bad!
BETA ─ SETI BETA – SETI has been in operation since 1996. It receives 250 million channels simultaneously with a bandpass of 0. 5 hertz per channel. It scans through the water hole from 1400 to 1720 Megahertz in eight hops, with two seconds of observation per hop. BETA automatically re-observes candidate signals by observing the sky simultaneously at two different directions with two separate BETA receivers, one scanning slightly to the east and the other slightly to the west. A successful candidate signal would first transit the east FOV and then the west FOV with a speed determined by Earth's rotation rate. A third BETA receiver observed the horizon to ‘veto out’ signals of obvious terrestrial origin.
The ALFA Multibeam Receiver Scans 7 directions simultaneously at 1 Hz bandwidth frequencies ranging from 500 ─ 11000 Mhz!
The Allen Telescopic Array ─ 350 Telescopes!
The Arecibo Message • The opportunity arose in 1974 to send a message from the giant Arecibo radio telescope … Drake sent it towards the globular cluster, M 13. It consisted of a repeating string of 1679 binary bits (“ 0” or “ 1”).
0000001010000001010000000100100010010110010101001001000000000000000000011000000000 00110100000000000000000010101000000000111 1100000000000000001100001110000110000000011 00100001101000110000110101111101111100000000001000000000000000000000000000011 11110000000111110000000000001100001110001000001000011000111001101011111011111000000000100000011000000000001100000000100000110 0000011111100000011111000000000000010000001100000001100000010000011000011000 00000011000000001100001100000110000001000000001000000011000000001000100000100000001000000010000001100000110000010001110101100000010000000000100000111110 00000010000101110100101101100000010011111110000111000001 101110000010100000111011001000000101000001111110010000001010000011000 0001000001101100000000000000000011100000000001 110101000101010011100000101000000001010000000 011111000000001111100000011100000000011000000011010000010110000011001100001010000010 1000010001001000000001010000000100001000 000000000100000001001010000001111001111101001111000 … repeating … Why 1679?
Decoding the Message Arrange the 1679 bits into a matrix of boxes of 23 columns of 73 rows (23 and 73 are the two prime numbers when multiplied together equal 1679). Color the box with a “ 1”— black and a box with a “ 0” — white . The image is “parity reversed, ” i. e. , it was generated by scanning right to left, i. e. , the first bit in the message occupies the upper row, rightmost cell. The next bit is the in the top row, next cell proceeding to the left. … could be that aliens are left-handed? … but the "handedness" of the image is completely irrelevant.
How to Count in Binary… Decimal 10 101 0 0 0 0 0 1 1 100 0 1 2 3 4 5 6 7 8 9 0 Binary 8 23 0 0 0 0 1 1 1 4 22 0 0 1 1 0 0 0 2 21 0 0 1 1 20 0 1 0 1 0 1 0 Decimal Number 0 1 2 3 4 5 6 7 8 8 10
The Arecibo Message Explained The boxes have been colored for clarity and “right to left parity” restored • Numbers 1 -10 • Atomic numbers for H, C, N, O, P • • • Formulas for sugars, bases and phosphate in DNA nucleotides Double helix structure of DNA wrapped around a binary number that represents the number of nucleotides in the human genome Height of human being marked on the left and human population marked on right • • Schematic of solar system (human stands on 3 rd planet) Arecibo transmitting telescope • Diameter of telescope
The Numbers • The first bit pattern tells how humans count in binary from 1 -10. The Atoms of Life Atomic Number 1 6 7 8 15 Element Hydrogen Carbon Nitrogen Oxygen Phosphorus • The numbers provide the key for interpreting the next bit pattern … a list of the atomic numbers of the five elements that make up DNA.
The DNA Molecule Sugar C 5 OH 7 A T C 5 H 4 N 5 C 5 H 5 N 2 O 2 C 5 OH 7 PO 4 C G PO 4 C 5 OH 7 C 4 H 4 N 3 O C 5 H 4 N 5 O C 5 OH 7 PO 4 Phosphate PO 4
The Structure of DNA and Size of the Human Genome The structure of DNA is a double helix. If the aliens synthesize the molecule using the previously decoded components, they would see that DNA did form a double helix, which would confirm that they had decoded its chemical makeup correctly. A 32 -bit number sits in the middle of the double helix that gives the approximate number of nucleotides in the human genome, about six billion.
Human Shape, Size, and Population This bit pattern depicts a human… at least it does to another human. What an alien would make of it is less obvious. To the left of the “the human” are the binary digits 0111 (14) in the middle of a “dimension bar” that denotes the height of the human … the human is 14 tall … 14 what? The only possible scale is based on the universality of the speed of light c. In SI units, it is c=3 x 108 m/s. The signal was broadcast at the frequency f = 2380 Mhz. The recipient knows the wavelength of the received message is: Another 32 -bit number, this one about four billion, is located on L = c / f = 12. 6 centimeters in our units (--- some other number of the right … gives the human population (back in 1974 --- it’s 7+ “alien” units). This provides a length scale for the alien. 14 of these billion now). It tells aliens how large an army they need to conquer units is 176 cm, which is Frank Drake’s height. Earth.
The Solar System This schematic represents the Sun and planets of the solar system. The gas giants are all larger than the terrestrial planets, with Jupiter and Saturn indicated as larger than Uranus and Neptune to their right. Earth appears directly below the figure of the human and is offset in that direction, identifying it as the planet where humans live --- third rock from the Sun.
The Arecibo Telescope The pattern represents the Arecibo radio telescope used to transmit the message, with dimension bars and a number … binary 100101111110 … at the bottom giving its diameter … decimal 2430 … the number of units of the 12. 6 centimeter wavelength scale. Thus, the size of the antenna is 30, 618 centimeters, or 306 meters.
The Arecibo message consisted of 1679 Question bits since _______. A. that's all the bits that could be sent before funding for the project ran out B. 1679 is one more than 1678, the frequency of the radio wave that the water molecule emits C. Sir Isaac Newton found a pulsar in M 13 on Nov. 16, 1679 D. The transmission frequency was set to 1679 Mhz E. 1679 is the product of two prime numbers, 23 and 73 and if the bits were arranged in 23 columns and 73 rows, a message appeared in picture form
Other Possible Methods of Communication • High intensity, short (1 nsec) laser light pulses.
Other Possible Methods of Communication Berkeley 0. 76 m Berkeley Lick Observatory 1 m Harvard 1. 8 m
[email protected] • What is [email protected]? • [email protected] is a scientific experiment that uses Internetconnected computers in the Search for Extraterrestrial Intelligence (SETI). You can participate by running a free program that downloads and analyzes radio telescope data. • Go to www. setiathome. berkeley. edu