Harvard Club of Cincinnati 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 …

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Wireless and Wiretapping

Harry Lewis

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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.

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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.

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What if the only data turned over is the telephone

numbers?

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What about web sites visited?

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What about your cell phone’s location?

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Electromagnetic Spectrum is a Continuum with Variety

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Radiation and Matter

• Waves interact with matter in various ways depending on both the wavelength and the matter

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• Transmission:

• Refraction:

• Reflection:

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Radiation and Matter, cont’d

• Absorption:

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• Scattering:

• Diffraction:

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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

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Ionizing and Nonionizing Radiation

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IonizingNonionizing

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Communication with Visible Light

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French Semaphores ca. 1800

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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

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Radio: The Incredibly Complicated Story

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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

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Inverse Square Law• Signal strength at distance D is proportional

to 1/D2

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So a radio signal is only 1/10,000th as strong at distance 100 miles as it is at distance 1 mile

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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

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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

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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

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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

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