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Six Degrees to Harry Lewis On Friday, January 23, 2004, at 05:09 AM, Mark Elliot Zuckerberg wrote: [MEZ] Professor, I've been interested in graph theory.

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Presentation on theme: "Six Degrees to Harry Lewis On Friday, January 23, 2004, at 05:09 AM, Mark Elliot Zuckerberg wrote: [MEZ] Professor, I've been interested in graph theory."— Presentation transcript:

1 Six Degrees to Harry Lewis On Friday, January 23, 2004, at 05:09 AM, Mark Elliot Zuckerberg wrote: [MEZ] Professor, I've been interested in graph theory and its applications to social networks for a while now … [HRL] Can I see it before I say yes? It's all public information, but there is somehow a point at which aggregation of public information feels like an invasion of privacy … [HRL] Sure, what the hell, seems harmless … March 22,2011Harvard Club of Cincinnati1

2 Wireless and Wiretapping Harry Lewis April 12, 20112Harvard Bits

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7 Fourth Amendment The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized. April 12, 2011Harvard Bits7

8 Semayne’s Case (1603) [T]he house of everyone is to him as his castle and fortress, as well for his defence against injury and violence [I]n all cases when the King is party, the sheriff (if the doors be not open) may break [into] the party's house, either to arrest him or to do other execution of the King's process, if otherwise he cannot enter. But before be breaks [into] it, he ought to signify the cause of his coming and to make request to open the doors. April 12, 2011Harvard Bits8

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14 What if the only data turned over is the telephone numbers? April 12, 2011Harvard Bits14

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17 What about web sites visited? April 12, 2011Harvard Bits17

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19 What about your cell phone’s location? April 12, 2011Harvard Bits19

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21 Electromagnetic Spectrum is a Continuum with Variety April 12, 2011Harvard Bits21

22 Radiation and Matter Waves interact with matter in various ways depending on both the wavelength and the matter April 12, 2011Harvard Bits22 Transmission: Refraction: Reflection:

23 Radiation and Matter, cont’d Absorption: April 12, 2011Harvard Bits23 Scattering: Diffraction:

24 Photons and their Energy Electromagnetic radiation behaves like particles = photons, as well as like waves Energy = h · Frequency, where h is a proportionality constant (Planck’s constant) High frequency photons carry enough energy to knock electrons from molecules, causing chemical changes High frequency = short wavelength = UV, x-ray, gamma ray Radio waves are low frequency, long wavelength, low energy April 12, 2011Harvard Bits24

25 Ionizing and Nonionizing Radiation April 12, 2011Harvard Bits25 IonizingNonionizing

26 Communication with Visible Light April 12, 2011Harvard Bits26

27 French Semaphores ca. 1800 April 12, 2011Harvard Bits27

28 Communication with Visible Light Pros: Human beings have detectors (eyes) Short wavelength => can discriminate small features Air is mostly transparent Cons: Not so good at night or in fog Can’t go around hills or over the horizon Signals confused by light from other sources Guided light (fiber optic cables) a recent invention April 12, 2011Harvard Bits28

29 April 12, 2011Harvard Bits29 Radio: The Incredibly Complicated Story

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32 Old Style Radio Phones Broadcast tower covering a large area Each phone call uses a separate frequency band over entire broadcast area Both high power usage and high bandwidth usage April 12, 2011Harvard Bits32

33 Inverse Square Law Signal strength at distance D is proportional to 1/D 2 April 12, 2011Harvard Bits33 So a radio signal is only 1/10,000th as strong at distance 100 miles as it is at distance 1 mile

34 Cell Phone Technology Cells are about a mile across Tower in each cell Phones can have low power because distances are small More frequency bands can be allocated to simultaneous calls because the “guard bands” can be narrower Frequencies can be reused in different cells because with fading, calls on the same frequency will not interfere As phone moves from cell to cell frequency may have to be switched Phone that is turned on constantly reports its location to nearby towers so incoming calls can be directed to it April 12, 2011Harvard Bits34

35 Cell Phone Technology Cells are about a mile across Tower in each cell Phones can have low power because distances are small More frequency bands can be allocated to simultaneous calls because the “guard bands” can be narrower Frequencies can be reused in different cells because with fading, calls on the same frequency will not interfere As phone moves from cell to cell frequency may have to be switched Phone that is turned on constantly reports its location to nearby towers so incoming calls can be directed to it April 12, 2011Harvard Bits35

36 Global Positioning System 21-29 satellites 12,000 mi (~2 orbits/day) Orbit @ 55 o to equator From anywhere on earth, ≥ 4 at least 15 o above horizon Each transmits – Its own ID – Clock time – Its own location April 12, 2011Harvard Bits36

37 Global Positioning System GPS receiver calculates its distance to 3 or 4 satellites (T gps -T sat )*c Infers its own location (trigonometry) Passive device! Civilian use April 12, 2011Harvard Bits37

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