Six Degrees to Harry Lewis

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 … Harvard Club of Cincinnati

2. Wireless and Wiretapping Harry Lewis Harvard Bits

3. Harvard Bits

4. Harvard Bits

5. Harvard Bits

6. Harvard Bits

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. Harvard Bits

8. Semayne’s Case (1603) • [T]hehouse of everyone is to him as his castle and fortress, as well for his defence against injury and violence • [I]nall 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. Harvard Bits

9. Harvard Bits

10. Harvard Bits

11. Harvard Bits

12. Harvard Bits

13. Harvard Bits

14. What if the only data turned over is the telephone numbers? Harvard Bits

15. Harvard Bits

16. Harvard Bits

17. What about web sites visited? Harvard Bits

18. Harvard Bits

19. What about your cell phone’s location? Harvard Bits

20. Harvard Bits

21. Electromagnetic Spectrum is a Continuum with Variety Harvard Bits

22. Reflection: • Transmission: • Refraction: Radiation and Matter • Waves interact with matter in various ways depending on both the wavelength and the matter Harvard Bits

23. Diffraction: • Scattering: Radiation and Matter, cont’d • Absorption: Harvard Bits

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 Harvard Bits

25. Ionizing and Nonionizing Radiation Nonionizing Ionizing Harvard Bits

26. Communication with Visible Light Harvard Bits

27. French Semaphores ca. 1800 Harvard Bits

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 Harvard Bits

29. Radio: The Incredibly Complicated Story Harvard Bits

30. Harvard Bits

31. Harvard Bits

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 Harvard Bits

33. Inverse Square Law • Signal strength at distance D is proportional to 1/D2 So a radio signal is only 1/10,000th as strong at distance 100 miles as it is at distance 1 mile Harvard Bits

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 Harvard Bits

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 Harvard Bits

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

37. Global Positioning System • GPS receiver calculates its distance to 3 or 4 satellites • (Tgps-Tsat)*c • Infers its own location (trigonometry) • Passive device! • Civilian use Harvard Bits

38. Harvard Bits

39. Harvard Bits

40. Harvard Bits

41. Harvard Bits

42. Harvard Bits

43. Harvard Bits

44. Harvard Bits

45. Harvard Bits

46. Harvard Bits

47. Harvard Bits