1 Week 11 - Wednesday CS 113
2 Last time What did we talk about last time? Exam 2! But before that?Turing machines Halting problem
3 Questions?
4 Project 4
5 Science!
6 What is science? It is not truth! It is a search for factsScience is an attempt to build a model that explains the Universe The model is always wrong But we are working to make it better!
7 The Solar System What does our solar system look like?Planets circling the sun, right? How do you know? How does anyone know? The model with the sun in the center is called the heliocentric model Also called the Copernican model because Copernicus gave compelling evidence for it What did we have before?
8 Geocentric model Before Copernicus, the geocentric model was popularThe earth was at the center of everything Sometimes called the Ptolemaic system Was the geocentric model accurate? Yes! It was a complex model that involved planets revolving on a smaller circle which was placed on a larger circle around Earth But once they worked out the details, it could accurately predict the motions of the planets
9 Which is better? Nowadays, (nearly) everyone agrees that the heliocentric model is a better model for the solar system It agrees with what we know about gravity It is simpler than the geocentric model It more accurately predicts the movement of the planets Is geocentrism is wrong? It's just a bad model, and we're always trying to find a better one
10 Falsification It is central to any scientific theory that it can be demonstrated Eventually, it might be possible to find a better explanation for some phenomenon Any scientific theory can be refuted if there is enough evidence to the contrary This is called falsification In fact, it's not science if it's impossible to make an argument otherwise In the 2005 Kitzmiller v. Dover Area School District case, one of the central arguments that Intelligent Design was not science is that it would be impossible to falsify
11 From xkcd:
12 Scientific method Today, science is done through a method of inquiry that has developed over time It is generally agreed that this method includes: Observations My brother got mad when he fell into a puddle of water Hypotheses My brother gets mad whenever he gets wet Prediction If I throw a bucket of water into my brother's face, he'll get mad Experiment Get a bucket of water… Strangely enough, sharing (publishing) your findings with others is usually considered central as well If other people get the same results, it carries more weight
13 Theories vs. laws Some people deride, for example, the theory of evolution because it is only a "theory" In science, a theory is a hypothesis that has been well-demonstrated A scientific law is a statement about what happens under certain conditions They're sort of distilled versions of theories They don't explain why, just how Both theories and laws are as close to facts as science gets Exceptions to both can still found
14 Ideal Gas Law
15 Ideal gas law The ideal gas law describes the behavior of a gas in a container It is usually stated as PV = nRT P is pressure V is volume n is the number of moles of atoms R is a constant T is the absolute temperature
16 Moles I'm going to take a moment to talk about molesOne mole is a number, like one dozen, or four score It's a huge number, 6.02 x 1023, essentially 6 with 23 zeroes after it Also called Avogadro's number A mole of paperclips, end to end, would stretch from the center of our galaxy to the center of the Andromeda galaxy In chemistry, they talk about moles of atoms because atoms are so small You have to have a lot of them to be worth talking about 1 mole of hydrogen is a little over a gram
17 More on the ideal gas lawThe ideal gas law was developed based on observations about the behavior of gas When temperature goes up (and everything else stays the same), pressure goes up When there are fewer moles of gas at the same temperature, the pressure goes down Pressure is force per unit area I can poke your cheek with my thumb, and you'll just get annoyed If I use the same amount of force but stick a knife into your face, the pressure is much higher, because the area of the knife's point is much smaller than my thumb
18 Why is it ideal? The ideal gas law is based on an ideal model of gas molecules bouncing off the walls of a box Each molecule pushes against the wall If they have random directions, the wall gets pushed constantly all over in a predictable way If the pressure and the temperature is low, the gas molecules won't hit each other very much When the pressure or temperature are high, the gas molecules bounce into each other all the time, changing the results
19 History of the Internet
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21 What is the Internet? The network of hardware and software systems that connects many of the world’s computers Typically, people say the Internet and capitalize the “I” because there is only one Until we meet aliens Or decide to break off from the rest of the world The World Wide Web is the part of the Internet that is concerned with webpages The Internet also includes: FTP VOIP Bittorrent Multiplayer video games Much, much more…
22 Packet switched The Internet is a packet switched systemIndividual pieces of data (called packets) are sent on the network Each packet knows where it is going A collection of packets going from point A to point B might not all travel the same route C B 1 A 2 D
23 Circuit switched Traditionally, phone lines have been circuit switchedA specific circuit is set up for a specific communication Operators used to do this by hand Now it is done automatically Only one path for data A C B 1
24 Circuit vs. packet switchingWhich one is faster? Circuit switching Which one is more predictable? So, why is the Internet packet switched? More adaptable
25 Birth of the Internet The Advanced Research Projects Agency was created in 1958 to respond to the Russians launching Sputnik The ARPANET connected its first two major nodes over 10 years later Packet switching was used so that the network could still communicate after a nuclear strike
26 What the Internet isn't A big truck A series of tubesInvented by Al Gore He never claimed it was But he did create the High Performance Computing Act of 1991 which funded many efforts that brought the Internet to the masses faster
27 IP addresses Computers on the Internet have addresses, not namesGoogle.com is actually [ ] Google.com is called a domain The Domain Name System or DNS turns the name into an address
28 IPv4 Old-style IP addresses are in this form: 4 numbers between 0 and 255, separated by dots That’s a total of 2564 = 4,294,967,296 addresses But there are 7 billion people on earth…
29 IPv6 IPv6 are the new IP addresses that are beginning to be used by modern hardware 8 groups of 4 hexadecimal digits each 2001:0db8:85a3:0000:0000:8a2e:0370:7334 1 hexadecimal digit has 16 possibilities How many different addresses is this? 1632 = 2128 ≈ 3.4×1038 is enough to have 500 trillion addresses for every cell of every person’s body on Earth Will it be enough?!
30 Other failures in designY2K bug 2 bytes for the date is not enough It’s all just going to get messed up in Y10K Y2038 bug Unix and Linux machines often use a signed 32-bit integer to represent seconds since January 1, 1970 Zip codes Vehicle identification numbers
31 Quiz
32 Upcoming
33 Next time… More about the Internet The World Wide Web Lab 11
34 Reminders Finish Project 4 Read Blown to Bits Chapter 4Don't wait until the last minute! Due this Friday Read Blown to Bits Chapter 4