Six Degrees to Harry Lewis On Friday, January 23, 2004, at 05:09 AM, Mark Elliot Zuckerberg wrote:...

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 …

March 22,2011

Harvard Bits 2

Wireless and Wiretapping

Harry Lewis

April 12, 2011

Harvard Bits 3April 12, 2011

Harvard Bits 4April 12, 2011

Harvard Bits 5April 12, 2011

Harvard Bits 6April 12, 2011

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.

April 12, 2011

Harvard Bits 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, 2011

Harvard Bits 9April 12, 2011

Harvard Bits 10April 12, 2011

Harvard Bits 11April 12, 2011

Harvard Bits 12April 12, 2011

Harvard Bits 13April 12, 2011

Harvard Bits 14

What if the only data turned over is the telephone

numbers?

April 12, 2011

Harvard Bits 15April 12, 2011

Harvard Bits 16April 12, 2011

Harvard Bits 17

What about web sites visited?

April 12, 2011

Harvard Bits 18April 12, 2011

Harvard Bits 19

What about your cell phone’s location?

April 12, 2011

Harvard Bits 20April 12, 2011

Harvard Bits 21

Electromagnetic Spectrum is a Continuum with Variety

April 12, 2011

Harvard Bits 22

Radiation and Matter

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

April 12, 2011

• Transmission:

• Refraction:

• Reflection:

Harvard Bits 23

Radiation and Matter, cont’d

• Absorption:

April 12, 2011

• Scattering:

• Diffraction:

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

April 12, 2011

Harvard Bits 25

Ionizing and Nonionizing Radiation

April 12, 2011

IonizingNonionizing

Harvard Bits 26

Communication with Visible Light

April 12, 2011

Harvard Bits 27

French Semaphores ca. 1800

April 12, 2011

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

April 12, 2011

Harvard Bits 29April 12, 2011

Radio: The Incredibly Complicated Story

Harvard Bits 30April 12, 2011

Harvard Bits 31April 12, 2011

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

April 12, 2011

Harvard Bits 33

Inverse Square Law• Signal strength at distance D is proportional

to 1/D2

April 12, 2011

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

April 12, 2011

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

April 12, 2011

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

April 12, 2011

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

April 12, 2011

Harvard Bits 38April 12, 2011

Harvard Bits 39April 12, 2011

Harvard Bits 40April 12, 2011

Harvard Bits 41April 12, 2011

Harvard Bits 42April 12, 2011

Harvard Bits 43April 12, 2011

Harvard Bits 44April 12, 2011

Harvard Bits 45April 12, 2011

Harvard Bits 46April 12, 2011

Harvard Bits 47April 12, 2011