23Mar/1227

Formal Logic

by Jeff

Image text: Note that this implies you should NOT honk solely because I stopped for a pedestrian and you're behind me.

The key part of this comic is "IFF", which in formal logic means "if and only if".  "If and only if" represents a formal logic connector that the result is that the truth of either one of the connected statements requires the truth of the other, i.e., either both statements are true, or both are false.  In the comic, if someone honks at this car, it means they like formal logic, there is not any other possible reason per this bumper sticker. (As we see in the image text.)

Have a great weekend everyone!

Filed under: Cars, Math 27 Comments
15Feb/1223

Backward in Time

by Jeff

Image text: People tell me I have too much time on my hands, but really the problem is that there's too much time, PERIOD.

Here's the spreadsheet which Randall used to calculate the times and dates for the comic.  It also has a lot of other percentages and dates, so take a look if you are interested.

This comic has a lot of formulas that I'm not going to try to explain because I'm not a math person and it will probably make you dumber if I try.  Any math people out there?  Have at it in the comments.  The reason I'm not going to try is that understanding the math is not a prerequisite for understanding the comic.

Cueball/Randall creates this formula which helps him wait for long stretches of time which goes increasingly faster into the past as more time goes by, which gives him the effect of looking like the time goes by quickly.  Which assists in the waiting process.

And of course, there is a particularly obvious pun in the image text.  He makes a play on the phrase "time period", which is used to describe large blocks of time throughout history.  And of course, the other side is the punctuation mark: period.

(Also, the workout website, Fitocracy has been mentioned previously in xkcd.)

Filed under: History, Math 23 Comments
2Dec/1155

Percentage Points

by Jeff

Image text: Grayton also proposed making college scholarships available exclusively to sexually active teens, amnesty for illegal immigrants who create room for themselves by killing a citizen, and a graduated income tax based on penis size. He has been endorsed by Tracy Morgan, John Wilkes Booth's ghost, and the Time Cube guy.

Senator Grayton is a fictional name, made up for this comic, which is a bit surprising to me as the names used in xkcd are usually references to something.

I'll see if I can do a line on each of the issues that Grayton fictionally supports:

Tax Breaks to Drunk Drivers - Grayton proposed to give money back on their taxes for people who have been convicted of a DUI or DWI.  Driving Under the Influence or Driving While Intoxicated

Predator Drones and the War on Christmas - The War on Christmas is a not a real war, it is simply Christians who believe that non-Christians are trying to get rid of Christmas trees, Christmas music and the endless barrage of Christmas-themed nonsense we get from Thanksgiving to Christmas.  I don't know how his support would go down if he supported unmanned Predator Drones to destroy people who hate Christmas, knowing America.  (Jokes!)

College Scholarships to Sexually Active Teens - Not sure how they would test for this on the scholarship application, but it is funny none the less.

Illegal Immigrants Who Murder - This is an argument for immigration, but only if they make "room" and possibly an open job for themselves by getting rid of another person.  This one is pretty evil.

Income Tax Based on Penis Size - Wow, this one would never pass!

On To the Names:

Tracy Morgan is an actor in the TV Show 30 Rock, who plays Tracy Jordan, he is known for his outlandish statements on politics and everything else in the world.  He has been reprimanded for controversial comments on homosexuals and Sarah Palin (in separate incidents).

Josh Wilkes Booth is the person who assassinated Abraham Lincoln.  He was a sympathizer with the Confederate cause and was for slavery and all its trappings.

Time Cube Guy is Gene Ray, also known as Otis E. Ray.  He has a website known as Time Cube (Thanks Wikipedia) where he sets out his personal model of reality, which he calls Time Cube. He suggests that all of modern physics is wrong, in addition to claiming religion to be evil, specifically Christianity, and the idea of family is poisoning children.

Ok, finally to the numbers:

If his polling is at 20% and drops by 19%, without specifying percentage points, that means that his support has only dropped 1% (EDIT: I'm bad at math.  Thanks commentors, it is 3.8% percentage points), because 19% of 20% is only 1% 3.8%.  That would mean after all his outrageous statements, his support dropped from 20% to 16.2%.  However, if they had said his 20% polling dropped 19% (removed percentage sign for redundancy, thanks again commentors) percentage points, that would mean his support has dropped to 1%.  Therefore, the distinction is important.

26Nov/1114

Set Theory

by Jeff

Image text: Proof of Zermelo's well-ordering theorem given the Axiom of Choice: 1: Take S to be any set. 2: When I reach step three, if S hasn't managed to find a well-ordering relation for itself, I'll feed it into this wood chipper. 3: Hey, look, S is well-ordered.

This comic is a pun on the phrase "Proof by Intimidation" which normally means (thanks Wikipedia) a jocular term used mainly in mathematics to refer to a style of presenting a purported mathematical proof by giving an argument loaded with jargon and appeal to obscure results, so that the audience is simply obliged to accept it, lest they have to admit their ignorance and lack of understanding.

However, in the comic, instead of using "Proof by Intimidation" in that way, the narrator's math teacher takes regular proofs and axioms and places threatening words into them so that the students will learn better because she threatens them with death or harm.

The axiom of choice (which has been referenced in previous xkcds) says that given any collection of bins, each containing at least one object, it is possible to make a selection of exactly one object from each bin.

In the image text, the well-ordering theorem states that every set can be well-ordered. A set X is well-ordered by a strict total order if every non-empty subset of X has a least element under the ordering. This is also known as Zermelo's theorem and is equivalent to the Axiom of Choice.

Filed under: Math 14 Comments
26Oct/11116

Delta-P

by Jeff

Image text: If you fire a Portal gun through the door of the wardrobe, space and time knot together, which leads to a frustrated Aslan trying to impart Christian morality to the Space sphere.

This comic was posted late and now I'm late and I'm at work so I can't do as much explaining as I usually do, but I'll do my best.  That's also why we have the best comment section on the Internet.

The basic idea of the formula and the comic are based on the books and movies of the Lion, the Witch and the Wardrobe in which a giant wardrobe contains a portal to a world known as Narnia. In the comic, someone connects an anchor to the wardrobe and throws it into the ocean which means that a steady stream of water at a velocity of 200 m/s will flow into Narnia.

The White Witch, the antagonist in the books and movies apparently won't know what hit her according to the caption.

The image text references the video game "Portal" in which you fire a portal gun into walls and etc to make "portals" that can open holes in other places so that you may portal through.

The image text also references the fact that CS Lewis wrote the Lion, Witch and Wardrobe books as a thinly veiled allegory of Christ's crucifixion with Aslan, the Lion in the title, playing Christ's part.  (Spoiler alert! Sorry!)

This is a classic xkcd based on the intersection of literature, math and video games.

19Sep/1157

1 to 10

by Jeff

Image text: If you get an 11/100 on a CS test, but you claim it should be counted as a 'C', they'll probably decide you deserve the upgrade.

This one is a bit confusing, so let me see if I can sort it out.  Please let me know if you interpret something else in the comic.  Megan says "On a scale of 1 to 10", but if you are using binary numbers that is "On a scale of 1 to 2" because 10 in binary equals 2.

When Cueball answers '4', not using binary, Megan has no idea what a four is because in fact she was using binary in her question.

In the image text, 11/100 would be 3/4 or 75% which is why you could ask for a 'C' grade.

14Sep/1137

Working

by Jeff

Image text: And if you drive a typical car more than a mile out of your way for each penny you save on the per-gallon price, it doesn't matter how worthless your time is to you--the gas to get you there and back costs more than you save.

Ok, let me see if I can iron this math out.  In this comic, gas is 10 cents cheaper per gallon 5 minutes away.  That's 1 dollar saved on a 10 gallon fill-up.  (The car in the comic is small, so I'm going to assume a 10 gallon fill-up.  Sorry to all our international friends, this is all in American measurements.)

$1 saved/earned for 5 minutes comes out to $12/hour which is more than the US Federal minimum wage of $7.25/hr.

$1 saved/earned for 9 minutes comes out to $6.66/hour which obviously comes below that threshold.

And then the image text adds on the double whammy of having to pay extra money in gas if you go more than a mile out of your way to get the cheaper gas.

Filed under: Cars, Math 37 Comments
5Sep/1146

Investing

by Jeff

Image text: But Einstein said it was the most powerful force in the universe, and I take all my investment advice from flippant remarks by theoretical physicists making small talk at parties.

Compound interest is a type of interest in which the interest earned is added to the total amount, so that the interest itself then begins to gain interest.  In the opposite type "simple interest", the amount used to calculate the interest will always be, for example, $1000.

Snopes has its doubts about the quote about compound interest being actually said by Einstein.

Filed under: Math, Money 46 Comments
10Aug/11101

Password Strength

by Jeff

Image text: To anyone who understands information theory and security and is in an infuriating argument with someone who does not (possibly involving mixed case), I sincerely apologize.

This comic is saying that the password in the top frames "Tr0ub4dor&3" is easier for password cracking software to guess than "correcthorsebatterystaple".  And this is absolutely true that people make passwords hard to remember because that means that they are "safer".

The important thing to take away from this comic is that longer passwords are better because each additional character adds much more time to the breaking of the password.

Steve Gibson from the Security Now podcast did a lot of work in this arena and found that this password "D0g....................." is harder to break than this password "PrXyc.N(n4k77#L!eVdAfp9".  Steve Gibson makes this very clear in his password haystack reference guide and tester:

"Once an exhaustive password search begins, the most important factor is password length!"

That's what xkcd is trying to get through here.  Complexity does not matter unless you have length in passwords.  Complexity is more difficult for humans to remember.  Length is not.

18May/1136

Religions

by Jeff

Image text: But to us there is but one God, plus or minus one. --1 Corinthians 8:6±2.

This comic is a reference to the increasing coverage of the prediction by Harold Camping that the Rapture will be on May 21, 2011.  The Rapture is the event prophesied in the Bible in which Christians believe that Christ will return to Earth and take all true believers up to Heaven.

The first frame is a reference to raptors in Jurassic Park, if you haven't seen it, the raptor dinosaurs get much more dangerous once they learn how to open doors.  Cueball mishears Megan, which is why he thinks she said "Raptors" instead of "Rapture".

In the 2nd frame, Cueball references three major religions.  Christian and church for Christianity, Mosque for Islam and Rabbi for Judaism.

The 3rd frame is a math joke in which Megan references error bars which are used on graphs to indicate the uncertainty.  So, Megan believes in one God (monotheism), as she says in the comic.  But if she is still trying to find the error bars, she is uncertain if there could be many more Gods as well.

The image text is a supposed excerpt from the holy text of experimental monotheism.  Corinthians is a real chapter book of the Christian Bible.

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