CommunicationCommunicationMuch cheaper than travelEnergy needed for Mass (M) at speed (v)E = 1/2 Mv2 if v much less than ce.g., travel to nearest star (4 ly) in 40 yr⇒ v = 0.1 c ⇒ E = 4.1 × 10–9 ergsfor M = M (electron)Photon E = hν h = 6.6 × 10–27ν = frequency= 6.6 × 10–18 ergs if ν = 109 HzRatio ~ 109 (and photon gets there in 4 yrs)100 M watt transmitter - 1 yr$ 40 × 106Spacecraft to nearest star~ $ 5 × 1016(some recent analysis questions this conclusion)Light is an Electromagnetic WaveElectric Field: Indicates force on charged particleE fieldForceForceMagnetic field: created by changing electricfield. At right angle to electric field.Electromagnetic WaveMotion of waveEBEEBBVertically PolarizedHorizontally PolarizedCircularly PolarizedWave PropertiesSnapshotDistance or PhaseA = Amplitudeλ = Wavelength–AA0λ…Wave PropertiesLook at one point along wavetimeν = frequency = 1period# of cycles per second (hertz, Hz)1 kHz = 103 Hz 1 MHz = 106 Hz1 GHz = 109 HzSpeed of light c = λν ⇒ λ = c–AAPeriod…νA Wave DemoElectromagnetic Spectrum (Light)Gamma raysX raysUltravioletlightVisiblelightInfrared lightRadiowavesWavelength ( !)Frequency ( ")Require more energyper photon (E = h ν)Penetrate BetterNoise: Any unwanted signalArtificial, Natural1011021031041021101310Frequency (GHz)Noise (K)Quantum limitUniverseGalaxyTotalTotal with atmosphereH O2O2RadioInfraredNoiseν = (GHz)109 HzMagic Frequencies1. Morrison & Cocconi 1959ν = 1.42 GHz λ = 21 cmH atoms2. Water “Hole”OH 1st molecule discovered at Radio λν = 1.6 GHzH + OH H2OLow Noise “Hole”1.4 1.6 GHz3. Kuiper - MorrisUse fundamental constantsν = c all very high ν length Most plausible is electron “radius”Scale by powers of “fine structure constant”~ 1 137 (if multiply 5 times, get to radio) ν = 2.5568 GHzRadio Telescope PrincipleGreen Bank Telescope (GBT)Arecibo TelescopeVery Large Array (VLA)Very Long Baseline Array (VLBA)Caltech Submillimeter Observatory(CSO)Atacama Large Millimeter Array (ALMA)Allen Telescope Array (ATA)Recognizing the MessageDistinguishing from natural “signals”:Expect: Variation with time, narrow band(small range of freq.)Not random noiseIf not random, it is artificial (ETI or Human)Examples of natural signals that might have been ETI1. Pulsars (LGM)2. OH MasersBoth are random noise (no coded information)CrucialCoding the MessageChange the signal with time1. Amplitude modulation (AM)AM RadiotCoding the Message2. Frequency Modulation (FM Radio)tsidebandscarrierνCoding the Messagehttp://www.chem.tamu.edu/rgroup/north/FM.htmlAnalog vs. Digital1. Analog - need accurate amplifiers, etc.to avoid distortione.g. radios, tv, records, analog tapes2. Digital “digitize” signalRepresent by Base 2 NumberBase 10 Base 20 01 12 103 114 100……Analog vs. DigitalSend one digit at a time so electronics just need toDistinguish 1 from 0Can use 2 very different voltagesNeed fast digital electronicse.g. CD’s, DVDs, Computers, Digital Tapes, DigitalTV, …Decoding the MessageAssume DigitalRepeat to Establish PatternImage? 1 dimension (string of bits)2 dimensionsRows + columnsMake product of # rows + # of columnseach a prime numbere.g., 23 × 73 = 1679 so 23 rows, 73 columnsor vice versaSemanticsCan we understand the
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