explain xkcd - User contributions [en]

Talk:936: Password Strength

2016-09-16T13:10:28Z

162.158.150.231:

''Fix the software first.'' If you double the time it takes to enter each repeated password attempt you make brute force attacks pointless. Imagine you allowed a hurried user who screws up their own password entry w/ frozen fingers. If their system starts out with a 1 second delay, then doubles to two, then to four, etc. the time it takes to wait is 2^n. Six screw ups cost you a minute, twenty errors and you are waiting 291 hours before your next log-in attempt.... kmc 2015-05-10 {{unsigned ip|108.162.229.124}}

: That's not how brute force attacks work. They steal the hashes of the passwords and then brute force them locally. [[Special:Contributions/198.41.235.107|198.41.235.107]] 23:43, 10 January 2016 (UTC)
:: Both are brute force. It is specified in the comic that we assume an attack against a weak remote web service though. --[[Special:Contributions/162.158.150.231|162.158.150.231]] 13:10, 16 September 2016 (UTC)

You still have to vary the words with a bit of capitalization, punctuation and numbers a bit, or hackers can just run a dictionary attack against your string of four words. '''[[User:Davidy22|{{Color|purple|David}}y²²]]'''[[User talk:Davidy22|<tt>[talk]</tt>]] 09:12, 9 March 2013 (UTC)
: Several discussions around the internet around this -- the consensus [ http://www.explainxkcd.com/wiki/index.php/936 looks like] that once this scheme is published it is fairly simple to run a dictionary attack on the password. My advise to most people is to use a password manager like lastpass or onepass that can generate pure random password. [[Special:Contributions/162.158.253.6|162.158.253.6]] 23:52, 10 March 2016 (UTC)

No you don't. Hackers cannot run a dictionary attack against a string of four randomly picked words.
Look at the number of bits displayed in the image: 11 bits for each word.
That means he's assuming a dictionary of 2048 words, from which each word is picked randomly.
The assumption is that the cracker knows your password scheme.
[[Special:Contributions/86.81.151.19|86.81.151.19]] 20:17, 28 April 2013 (UTC)
Willem

:I just wrote a program to bruteforce this password creation method. https://github.com/KrasnayaSecurity/xkcd936/blob/master/listGen936.py Once I get it I'll try coming up with more bruteforcing algorithms such as substituting symbols, numbers, camel case, and the like. Point is, don't rely on this or any one method. I wouldn't be surprised if the crackers are already working on something like this. [[User:Lieutenant S.|Lieutenant S.]] ([[User talk:Lieutenant S.|talk]]) 07:03, 8 September 2014 (UTC)
:It took 1.25 hours to bruteforce "correcthorsebatterystaple" using the 2,000 most common words with one CPU. [[User:Lieutenant S.|Lieutenant S.]] ([[User talk:Lieutenant S.|talk]]) 07:09, 9 September 2014 (UTC)
:: 1) ... as compared to 69 milliseconds for the other method. 2) Since you are able to test 3,9 billion passwords as second (very impressive!) I am guessing that your setup is not performing its attack over a ”weak remote service”, which is breaking the rules of the #936 game. 3) five words and a 20k-wordlist would get you 9400 years (still breaking the weak remote service rule).--[[User:Gnirre|Gnirre]] ([[User talk:Gnirre|talk]]) 09:13, 14 October 2014 (UTC)
:: 2) Two thoughts: You use itertools.permutations, which only covers non-repeating words, but mainly you don't actually hash the password. If you have a plain-text password, there no need to crack the password because you could just look at it. Example of an actual crack for this type of password: https://github.com/koshippy/xkcd_password/blob/master/password_crack.py My computer gets 10,000,000 guesses in ~16 seconds (non-hashed takes ~2 seconds), meaning it would take almost a year to try every combination. (2048^4 total password space). Even optimizing by using c++/java or JtR, you wouldn't see huge improvement since most of the time is from the SHA hashing. Point being: a typical user can't crack this type of password in a short amount of time, even if they know your wordlist. [[Special:Contributions/199.27.128.212|199.27.128.212]] 12:05, 17 February 2015 (UTC) Koshippy

Sometimes this is not possible. (I'm looking at you, local banks with 8-12 character passwords and PayPal) If I can, I use a full sentence. A compound sentence for the important stuff. This adds the capitalization, punctuation and possibly the use of numbers while it's even easier to remember then Randall's scheme. I think it might help against the keyloggers too, if your browser/application autofills the username filed, because you password doesn't stand out from the feed with being gibberish. [[Special:Contributions/195.56.58.169|195.56.58.169]] 09:01, 30 August 2013 (UTC)

The basic concept can be adapted to limited-length passwords easily enough: memorize a phrase and use the first letter of each word. It'll require about a dozen words (you're only getting 4.7 bits per letter at best, actually less because first letters of words are not truly random, though they are weakly if at all correlated with their neighbors -- based on the frequencies of first letters of words in English, and assuming no correlation between each first letter and the next, I calculate about 4 bits per character of Shannon entropy). SteveMB 18:35, 30 August 2013 (UTC)

Followup: The results of extracting the first letters of words in sample texts (the {{w|Project_Gutenberg|Project Gutenberg}} texts of ''The Adventures of Huckleberry Finn'', ''The War of the Worlds'', and ''Little Fuzzy'') and applying a {{w|Entropy_(information_theory)|Shannon entropy calculation}} were 4.07 bits per letter (i.e. first letter in word) and 8.08 bits per digraph (i.e. first letters in two consecutive words). These results suggest that first-letter-of-phrase passwords have approximately 4 bits per letter of entropy. --[[User:SteveMB|SteveMB]] ([[User talk:SteveMB|talk]]) 14:21, 4 September 2013 (UTC)

Addendum: The above test was case-insensitive (all letters converted to lowercase before feeding them to the [[http://millikeys.sourceforge.net/freqanalysis.html frequency counter]]). Thus, true-random use of uppercase and lowercase would have 5 bits per letter of entropy, and any variation in case (e.g. preserving the case of the original first letter) would fall between 4 and 5 bits per letter. --[[User:SteveMB|SteveMB]] ([[User talk:SteveMB|talk]]) 14:28, 4 September 2013 (UTC)

I just have RANDOM.ORG print me ten pages of 8-character passwords and tape it to the wall, then highlight some of them and use others (say two down and to the right or similar) for my passwords, maybe a given line a line a little jumbled for more security. [[Special:Contributions/70.24.167.3|70.24.167.3]] 13:27, 30 September 2013 (UTC)
:Remind me to visit your office and secretly replace your wall-lists by a list of very similar looking strings ;) --[[User:Chtz|Chtz]] ([[User talk:Chtz|talk]]) 13:53, 30 September 2013 (UTC)

Simple.com (online banking site) had the following on it’s registration page:

“Passphrase? Yes. Passphrases are easier to remember and more secure than traditional passwords. For example, try a group of words with spaces in between, or a sentence you know you'll remember. "correct horse battery staple" is a better passphrase than r0b0tz26.”

Online security for a banking site has been informed by an online comic. Astounding.
[[Special:Contributions/173.245.54.78|173.245.54.78]] 21:22, 11 November 2013 (UTC)

The Web service Dropbox has an Easter egg related to this comic on their sign-up page. That page has a password strength indicator (powered by JavaScript) which changes as you type your password. This indicator also shows hints when hovering the mouse cursor over it. Entering "Tr0ub4dor&3" or "Tr0ub4dour&3" as the password causes the password strength indicator to fall to zero, with the hint saying, "Guess again." Entering "correcthorsebatterystaple" as the password also causes the strength indicator to fall to zero, but the hint says, "Whoa there, don't take advice from a webcomic too literally ;)." [[Special:Contributions/108.162.218.95|108.162.218.95]] 15:17, 11 February 2014 (UTC)

The explanation said that the comic uses a dictionary[http://www.explainxkcd.com/wiki/index.php?title=936:_Password_Strength&oldid=59309]. In fact it's a word list, which seems similar but it's not. All the words in the word list must be easy to memorize. This means it's better not to have words such as ''than'' or ''if''. Also, it's better not to have homophones (''wood'' and ''would'', for example). The sentence ''dictionary attack'' doesn't apply here. A dictionary attack requires the attacker to use all the words in the dictionary (e.g. 100,000 words). Here we must generate the 17,592,186,044,416 combinations of 4 common words. Those combinations can't be found in any dictionary. At 25 bytes per "word" that dictionary would need 400 {{w|tebi|binary terabytes}} to be stored. [[User:Xhfz|Xhfz]] ([[User talk:Xhfz|talk]]) 21:37, 11 March 2014 (UTC)

This comic was mentioned in a TED talk by Lorrie Faith Cranor on in March 2014. After performing a lot of studies and analysis, she concludes that "pass phrase" passwords are no easier to remember than complex passwords and that the increased length of the password increases the number of errors when typing it. There is a lot of other useful information from her studies that can be gleaned from the talk. [http://www.ted.com/talks/lorrie_faith_cranor_what_s_wrong_with_your_pa_w0rd Link]. What she doesn't mention is the frequency of changing passwords - in most organizations it's ~90 days. I don't know where that standard originated, but (as a sys admin) I suspect it's about as ineffective as most of our other password trickery - that is that it does nothing. Today's password thieves don't bash stolen password hash tables, they bundle keyloggers with game trainers and browser plugins.--[[Special:Contributions/173.245.50.75|173.245.50.75]] 18:14, 2 July 2014 (UTC)
:: Lorrie Faith Cranor gets the random part of #936 word generation correct, which is great. Regarding memorizability, this study (https://cups.cs.cmu.edu/soups/2012/proceedings/a7_Shay.pdf) does not address #936. The study uses no generator for gibberish of length 11. Most comparable are perhaps two classes of five or six randomly assigned characters. None of the study's generators has 44 bits of entropy – its dictionary for the method closest to #936 – noun-instr – contains only 181 nouns. The article contains no discussion of the significance of these differences to #936. In her TED Lorrie Faith Cranor says ”sorry all you xkcd fans” which could be interpreted as judgement of #936, but there is no basis in the above article for that. It does however seem plausible that the report could be reworked to address #936. --[[User:Gnirre|Gnirre]] ([[User talk:Gnirre|talk]]) 10:42, 14 October 2014 (UTC)

:Password-changing frequency isn't about making passwords more ''secure'', but instead it's about ''mitigating the damage'' of a successfully cracked password. If a hacker gets your password (through any means) and your password changes every 90 days, the password the hacker has obtained is only useful for a few months at most. That might be enough, but it might not. If the hacker is brute forcing the passwords to get them, that cuts into the time the password is useful. --[[Special:Contributions/173.245.54.168|173.245.54.168]] 22:22, 13 October 2014 (UTC)
::However, brute-forcing gets much ''easier'' that way.
::Say the average employee is around for 10 years, which is reasonable for some companies , absurdly high for others, and a bit low for a family business. That's 40 password changes.
::Now if you have to remember another password every now and then, you sacrifice complexity, lest you forget it. A factor of 40 is like one character less. But how much shorter will the password be? It's more likely that it's gonna be 3 or 4 characters less. Congrats, you just a factor of 1000's for a perceived "mitigation", which doesn't even work. Pro attackers can vacuum your server in a DAY once they have the PW. [[Special:Contributions/141.101.104.53|141.101.104.53]] 13:03, 4 December 2014 (UTC)

Just because you are required to have a password that has letters and numbers in it doesn't mean you can't make it memorable. When caps are required, use CamelCase. When punctuation is required, make it an ampersand (&) or include a contraction. When numbers are required, pick something that has significance to you (your birthday, the resolution of your television, ect.). Keep in mind that, if your phrase is an actual sentence, the password entropy is 1.1 bits per character (http://what-if.xkcd.com/34), so length is key if you want your password to be secure. (Though no known algorithm can actually exploit the 1.1 bits of entropy to gain time, so it might be more like 11 bits of entropy per word. Even then, my passwords have nonexistent and uncommon words in them, (like doge or trope), which also adds some entropy.) [[Special:Contributions/108.162.246.213|108.162.246.213]] 22:18, 1 September 2014 (UTC)
:Flip side of the story, the "capital plus small plus other char" policy doesn't make your password any safer.
:The German company T-online had an experimental gateway with the password, "internet". Now that sucked. No problem, tho, because that gateway wasn't accessible from outside. When they went live, they "improved" the password to "Internet1". There are still lots of these passwords around: first letter is a Cap, and the only non-alphabetic char is a 1 at the end. This doesn't add any entropy. [[Special:Contributions/141.101.104.53|141.101.104.53]] 13:03, 4 December 2014 (UTC)
::[http://ask.metafilter.com/193052/Oh-Randall-you-do-confound-me-so#2779020 This] shows that about one third of all digits in a sample of passwords was "1" . [[Special:Contributions/141.101.104.53|141.101.104.53]] 13:14, 4 December 2014 (UTC)
You can also troll the brute-force engine by using words from other languages, fictional books and video games.--[[User:Horsebattery|Horsebattery]] ([[User talk:Horsebattery|talk]]) 03:04, 3 November 2014 (UTC)
:That's a good idea; it adds to the entropy bits per word. If you really want to throw them off, mix different languages. Just don't use very well-known words; I'm sure the hackers have ''cojones'' and ''Blitzkrieg'' in their dictionaries. [[Special:Contributions/141.101.104.53|141.101.104.53]] 13:03, 4 December 2014 (UTC)

Also, passwords that are 'hard to remember' are themselves a security vulnerability. A password reset scheme (or even a lockout scheme) is a vulnerability. The more it needs to be used, the harder it becomes to police that vulnerability. Relatedly, hard-to-remember passwords leave users uncertain whether their password has been changed by someone else or they've just forgotten it. [[User:Ijkcomputer|Ijkcomputer]] ([[User talk:Ijkcomputer|talk]]) 15:32, 18 December 2014 (UTC)

Hi there, this comic gave me the idea for a password generator that can (optionally) use dictionary words. Have a look if you're interested: https://wordypasswords.com Use your common sense though about what is and isn't secure! Hope someone finds it useful. [[User:Mackatronic|Mackatronic]] ([[User talk:Mackatronic|talk]]) 08:23, 9 January 2015 (UTC)

I have not read all of the replies and in truth most of the detail is boring to me but it has occurred to me that with this sort of problem and since the Snowden affair, serious security devices will have to make the keyboard redundant.

At the moment all I can imagine is a series of pictures like hieroglyphs but even using a rolling code of ever changing font glyphs would do. When the security required by money minders reaches the stage of development possible with keyboards that can supply that sort of security, we will have some idea which banks have some idea about security.

Tip:
Not Barings. They have an history of intransigence and stupidity. (Still revered in banks though as able to cure colon cancer with poor investment strategies.)

[[User:Weatherlawyer| I used Google News BEFORE it was clickbait]] ([[User talk:Weatherlawyer|talk]]) 13:46, 23 January 2015 (UTC)

The D0g..................... (24 characters long) is NOT stronger than PrXyc.N(n4k77#L!eVdAfp9 (23 characters long). The reason why, is that the later password is random. There is no pattern. The former, "padding" technique can be very easily cracked. You just need to assume that each character be repeated 1~30 times. Then the first password would become : 1(D)1(0)1(g)21(.), which, is then of complexity 30^4 + 96^4, versus 96^23 for the random password. And that is assuming that any character can be repeated 1~30 times, so DDDDDDDDD0000000ggggggg...... also would be cracked extremely quickly. If you limit yourself to only last character padding, your password now becomes 30*96^4 possibilities. {{unsigned ip|108.162.222.235}}

And that's why it is stupid to explain this kind of joke : it depends on many (MANY) parameters such as brute-force method and encryption/hash algorithm. Giving this kind of (wrong) explanations about "pass cracking" (as if it was always the same way to process ...) is ridiculous. And they talk about entropy .......... Holy shit, go back to school and stop screwing cryptography up. zM_

I just use a password with a ␡ character or two, and ␇ for banks. [[Special:Contributions/108.162.242.21|108.162.242.21]] 08:33, 18 August 2015 (UTC)

I'am astonished that even someone like Schneier don't get 936 right immediately after reading it. So, I think I know what was going on in Munroes mind conceptually. Maybe there are some grans of salt, but I don't have a problem with these. But I do have one (or two) quantitative problem(s) with 936:
* I was not able to find out, how Munroe get the value of about 16 bits of entropy for the "uncommon" nine letter lower case "non-gibberish base word". This would mean: On average, a letter of such a word will have about 1.8 bits of entropy. May be, but how do we know? "Citation needed!" ;-)
* (Secondly: The "punctuation" should have 5, not 4 bits of entropy. There are 32 (2^5) ASCII punctuation characters (POSIX class [:punct:]). But I assume this is a lapse.)
Can someone enlighten me? --[[Special:Contributions/162.158.91.236|162.158.91.236]] 17:31, 19 September 2015 (UTC)
:I have missed the sentence "Randall assumes only the 16 most common characters are used in practice (4 bits)". Hm. There is a huge list with real world passwords out there, leaking from RockYou in 2009. After some processing to remove passwords containing characters that are not printable ASCII characters (ñ, £, ๅ, NBSP, EOT, ...), the list contains about 14329849 unique passwords from about 32585010 accounts (there are some garbage "passwords" like HTML code fragments). The following are the number of accounts using a password containing a particular printable character (one or more tokens of a particular type):
<nowiki>
226673 .
186883 _
179264 !
125846 -
104224 @
95237 *
92802 (space)
60002 #
36522 /
31172 $
28550 &
27686 ,
23905 +
18704 =
18268 )
17927 ?
16401 (
16074 '
14407 ;
11819 <
11118 %
10723 ]
8975 \
7718 [
7209 :
5815 ~
5673 ^
4995 `
2847 "
2741 >
1050 {
939 }
502 |

(NB: 1222815 accounts were using a password containing at least one of these.)
</nowiki>
:Sorry, I have no "citation". But you can play with the leaked RockYou password list yourself. Here is a way to reach that playground:
<nowiki>
$ # Download the compressed list (57 MiB; I have no idea what "skullsecurity"
$ # is, it was simply the first find and I assume it's the said list):
$ wget http://downloads.skullsecurity.org/passwords/rockyou-withcount.txt.bz2

$ # Decompress the list (243 MiB), or, to speak more exact, it's a table:
$ bzip2 -dk rockyou-withcount.txt.bz2

$ # The content of the table is: "How many accounts (first row) were using that
$ # password (second row)?" Let's take a peek:
$ head -n5 rockyou-withcount.txt
290729 123456
79076 12345
76789 123456789
59462 password
49952 iloveyou

$ # The following command processes the table to remove lines with passwords
$ # containing characters that are not printable ASCII characters (14541
$ # lines/passwords, 18038 accounts), and lines insisting that there were some
$ # accounts with no password (1 line, 340 accounts). Moreover, the command
$ # removes every space character not belonging to a password, makes the rows
$ # tab-delimited and writes the result in a file called "ry" (161 MiB; many
$ # bloating spaces removed).
$ LC_ALL=C sed -n 's/^ *$[1-9][0-9]*$ $[[:print:]]\{1,\}$$/\1\t\2/p' rockyou-withcount.txt >ry

$ # The following are shell functions to build commands. They will be explained
$ # below using examples (I can not express myself well in this language).
$ counta() { LC_ALL=C awk 'BEGIN { FS = "\t"; p = 0; a = 0 } { if ($2 ~ /'"$(printf %s "$1" | sed 'sI/I\\/Ig')"'/) { p++; a += $1 } } END { print a " (" p ")" }' "$2" ;}
$ countap() { LC_ALL=C awk 'BEGIN { FS = "\t"; p = 0; a = 0 } { if ($2 ~ /'"$(printf %s "$1" | sed 'sI/I\\/Ig')"'/) { p++; a += $1; print $0 } } END { print a " (" p ")" }' "$2" ;}

$ # We have reached the playground. Here are some examples for how to use the
$ # toys:

$ # Count how many accounts were using a password containing the string love:
$ counta 'love' ry
671599 (188855)

$ # The first operand of the above command is a extended regular expression
$ # (ERE). The second operand is a file, namely the previously generated file
$ # called "ry", that is the (processed) table. The first number of the output
$ # means: "That many accounts were using a password matching the ERE." The
$ # second number inside parentheses means: "That many unique passwords matching
$ # the ERE." If the first number is greater than the second number, some
$ # accounts sharing the same password (we will see this clearly in one of the
$ # examples below).

$ # Count how many accounts were using a password containing at least one
$ # character:
$ counta '.' ry
32585010 (14329849)

$ # Count how many accounts were using a password containing exactly one
$ # character:
$ counta '^.$' ry
144 (45)

$ # Count how many accounts were using a password containing exactly one numeric
$ # character:
$ counta '^[0-9]$' ry
55 (10)

$ # Let's have a look at the distribution:
$ countap '^[0-9]$' ry
29 1
6 7
6 3
3 9
3 2
2 6
2 5
2 0
1 8
1 4
55 (10)

$ # Obove we see the second command at work. You see what it does and what it
$ # does different. And here we see clearly the meaning of the first number and
$ # the second number inside parentheses.

$ # Count how many accounts were using a password containing at least one
$ # numeric character:
$ counta '[0-9]' ry
17609065 (9761364)

$ # Count how many accounts were using a password ending with a numeric
$ # character:
$ counta '[0-9]$' ry
15728238 (8313698)

$ # Count how many accounts were using a password beginning with a numeric
$ # character:
$ counta '^[0-9]' ry
6409397 (3283946)

$ # Count how many accounts were using a password containing only numeric
$ # characters:
$ counta '^[0-9]+$' ry
5192990 (2346744)

$ # And, last but not least, count how many accounts were using a password
$ # containing that "uncommon non-gibberish base word" in 936, with an upper
$ # or an lower case first letter, with or without some of the "common
$ # substitutions":
$ counta '[tT]r[o0]ub[a4]d[o0]r' ry
3 (3)

$ # Yes, there are some. 14 million unique passwords are a lot. Let's see what
$ # exactly was used:
$ countap '[tT]r[o0]ub[a4]d[o0]r' ry
1 troubador1
1 troubador
1 darktroubador
3 (3)
</nowiki>
:[[Special:Contributions/162.158.91.236|162.158.91.236]] 06:23, 21 September 2015 (UTC)

Interesting read about the generated password streangth: https://www.schneier.com/blog/archives/2016/01/friday_squid_bl_508.html#c6714590 [[Special:Contributions/162.158.91.190|162.158.91.190]] 08:09, 8 January 2016 (UTC)

: That person sounds confused. [[Special:Contributions/198.41.235.107|198.41.235.107]] 23:43, 10 January 2016 (UTC)

;You've Already Memorized It

Originally I logged in to report a local xkcd related phenomenon, and ask if anyone else had experienced it. The destiny, seemingly inescapable, that at once became my own upon seeing that last panel; the effect of the self-fullfilling combination of the very specific look of inquiry -- one I recognize immediately and associate with the words ''"interesting, Captain"'' -- and the insidiously performative ''"You've already memorized it."'' At first I doubted this was actually the case, but soon I could no longer, since not only did the phrase readily come to the mind and out the mouth, it also came up often. The ''"correct"'' soon replaced the word ''"right"'' in everyday conversation, then ''"right you are"'' and ''"yes"'' and so forth, then its opposite (with a ''"no"'' in front), then replacing the direction, the verb involving pen and paper (the most recent development was merely a quick under the breath aside of an acronym of the remaining words). All followed by the rest of the absurdly perfect password. '''Now here's the kicker: I logged on to tell you all this for some reason, only to find, I had memorized ''"correct horse staple battery"'' instead of ''"correct horse battery staple."'''''[[User:A female faust|A female faust]] ([[User talk:A female faust|talk]]) 03:58, 31 July 2016 (UTC)

54: Science

2016-09-05T09:35:31Z

162.158.150.231: /* Explanation */

{{comic
| number = 54
| date = January 18, 2006
| title = Science
| image = science.jpg
| titletext = Bonus points if you can identify the science in question
}}

==Explanation==
The solid line represents the theoretical {{w|blackbody radiation|radiation for a blackbody}} at 2.73 K according to {{w|Planck's Law}} (derived as early as 1900 by {{w|Max Planck}}). The formula, almost as written in the graph, can be found {{w|Black-body radiation#Planck's law of black-body radiation|here}}. The only changes are that on Wikipedia, the frequency f is represented by the Greek letter ν (nu) and the temperature T is included as an independent variable, so I(f) becomes I(v,T). However, I(v,T) still represents the {{w|Radiance#Spectral radiance|spectral radiance}} (similar to energy density). In this formula, h is the Planck constant, c is the speed of light in a vacuum and k is the Boltzmann constant. The frequency (f or v) along the X-axis is measured in {{w|GHz}} (Giga (or billion) Herz). The curve peaks at 160.4 GHz. There is no scale or unit on the {{w|energy density}} on the Y-axis.

The theory is that the blackbody in question was in fact the universe at the point when it had cooled down enough {{w|Decoupling (cosmology)|to allow photons to escape}}, 0.38 billion years into the universe's {{w|Big Bang|13,800 billion years}} history. The photons that reach us today are the ones that have been travelling to us at lightspeed since then. As the light from astronomical objects suffers from {{w|redshift}} due to the expansion of the universe, and this shift becomes more pronounced with distance from the observer, this light displays in the infrared range.

The title text praises viewers who can identify where this equation and corresponding graph come from (without consulting this wiki, of course).

===Official T-shirt explanation===
This comic was made into a T-shirt.

On the xkcd store there is both an '''explanation for the title:'''

Science: We finally figured out that you could separate fact from superstition by a completely radical method: observation. You can try things, measure them, and see how they work! {{w|Bitch (insult)|Bitches}}.


And specifically an '''explanation for the graph:'''

The graph on the back of the shirt is data from the{{w|COBE|COBE mission}} which looked at the background microwave glow of the universe and found that it fit perfectly with the idea that the universe used to be really hot everywhere. This strongly reinforced the Big Bang theory and was one of the most dramatic examples of an experiment agreeing with a theory in history -- the data points fit perfectly, with error bars too small to draw on the graph. It's one of the most triumphant scientific results in history.


The above is a direct copy paste, with errors. The current wiki page of the COBE mission can be found at {{w|Cosmic Background Explorer|http://en.wikipedia.org/wiki/Cosmic_Background_Explorer}}

This comic is available on a T-shirt from the [http://store-xkcd-com.myshopify.com/products/science-works xkcd stores].

==Transcript==
:[A graph with a curve that begins at zero, then peaks at a given frequency, indicated via a thin vertical line, and then fades down towards zero. It is possible to see the data point, which the curve fits perfectly. The Y-axis is labeled. Along the X-axis the zero point and the frequency where the peak has its maximum are labeled and close to the arrow the unit of this axis is written.]
:Y-axis: Energy Density
:Along the X-axis:
::0
::160.4
::GHz
:[Above the graph to the right is the following formula, with the last inner parentheses only included to make the formula clear, since in the drawing the fractions are written above and below horizontal lines:]
:I(f) = (2hf3/c2)(1/(ehf/kT-1))
:[Below the graph is written the following:]
:'''Science.'''
:It works, bitches.

==Trivia==
*This was the 48th comic originally posted to [[LiveJournal]].
**The previous was [[50: Penny Arcade]].
**The next was [[51: Malaria]].
*This comic kept its original title: "Science"
**It is part of the last six comics on LiveJournal which all had a title without the word "Drawing" in it.
**Five of these had exactly the same title on both sites.
**Only 11 comics have the same title on both sites.
**Apart from the [[:Category:First day on LiveJournal|thirteen first comics]] posted to LiveJournal, there were only three other comics without the word "Drawing" in the title before these last six.
*Original [[Randall]] quote: "Bonus points if you can identify the science in question."
**Only difference between this and the title text on xkcd is the last period: "."
**It was rare that these two texts were so similar.
*This comic was one of the last 11 comics posted on LiveJournal.
**These 11 comics were [[:Category:Posted on LiveJournal after xkcd|posted both on LiveJournal and xkcd]] after the [[xkcd]] site opened on the 1st of January 2006.
**The first six comics were posted on both sites on the same day. But not this one.
*For some reason this comic was first posted a week later on xkcd (25 January 2006), on the day that [[53: Hobby]] was released on LiveJournal.
**On the day 54: Science was released on LiveJournal (18 January 2006), another comic ([[51: Malaria]]) was released on xkcd.
**[[51: Malaria]] also became the next comic released on LiveJournal, but this meant that three comics in a row were posted a release day earlier on xkcd than on LiveJournal.
**Only with the last comic released on LiveJournal, [[55: Useless]], did the two sites release the same comic on the same day again.

{{comic discussion}}

[[Category:Comics posted on livejournal| 48]]
[[Category:Posted on LiveJournal after xkcd]]
[[Category:Line graphs]]
[[Category:Math]]
[[Category:Physics]]
[[Category:Science]]

Talk:1162: Log Scale

2016-03-18T01:19:29Z

162.158.150.231: Uranium Energy content

The fictional notation MAY BE a parody of Knuth's up-arrow notation - and uranium MAY BE an effective energy source. By the way, labeling the energy sources just with material name is insufficient: how good energy source is hydrogen? -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 09:17, 18 January 2013 (UTC)
:It has a calorific value of about 150 kJ/gm(much higher when compared to coal,etc.) but is too explosive[[User:Guru-45|Guru-45]] ([[User talk:Guru-45|talk]]) 14:24, 18 January 2013 (UTC)
:That is for burning it I assume? But what if you use it as fuel in a fusion reactor? Or an H-Bomb for that matter?
The calorie standard is defined by burning. So comparison doesn't fit with the graph as written. [[User:DruidDriver|DruidDriver]] ([[User talk:DruidDriver|talk]]) 20:46, 24 January 2013 (UTC)
:is it really a parody? (well, probably arrow notation grows much more, here there is just a log log log etc) --[[User:.mau,|.mau.]] ([[User talk:.mau,|talk]]) 14:10, 18 January 2013 (UTC)
It's true that uranium has an extremely high energy density, which is of great importance for mobile power plants; however, nuclear fission has a lot of safety issues, especially for mobile power, which is why it is used only for stationary power plants and large military vessels, such as aircraft carriers and subs.

Hydrogen is pretty good when highly compressed so as to get high energy volume density as well, but that leads to problems too. Also, hydrogen leaks more easily than almost anything else. That is especially a problem for an extremely flammable gas. On the plus side for hydrogen, nothing burns more cleanly.

:"The log scale can also be abused to make data look more uniform than it really is, so on a log scale sugar and other materials would look largely equal energy density when they clearly are not."
I think this is missing the point, which I take to be that displaying the data on a log scale would understate the vast difference between ''uranium'' and the hydrocarbons/carbohydrates:
E/m log(E/m)
sugar 19 1.3 *
coal 24 1.4 *
fat 39 1.6 **
gas 46 1.7 **
uranium 76e6 7.9 ****.***
Uranium is clearly larger than the others, but only by a factor of 4, so the real magnitude of the difference may not be appreciated.
With the stack of paper, he's proposing a way to show linear values for the data without having the uranium column simply shooting off the top of the page, with an arrow and the number. [[User:Wwoods|Wwoods]] ([[User talk:Wwoods|talk]]) 17:26, 18 January 2013 (UTC)
: or, he could just print at a scale that allows 76,000,000 to fit on the page, with the other values shown as near-infinitesimally thin lines. [[Special:Contributions/67.51.59.66|67.51.59.66]] 18:23, 18 January 2013 (UTC)

A googolplex in Knuth's paper stack notation (based upon 3818 chr per page, and 25,824 pages to fill up a typical 8ft tall room), would be:
96.41816408 with a 2 pinned on it.

The algorithim is:

KnuthPaperStack(N):

y = log10(N)/3818

If y >= 25824
Z = Z + 1
z = KnuthPaperStack(y)
Return z,Z
Else
Return y,Z
End if

--[[User:Markozeta|Markozeta]] ([[User talk:Markozeta|talk]]) 15:25, 20 January 2013 (UTC)

I think the name "Knuth paper-stack notation" sounds like "'Nuff paper-stack notation", meaning that it is a notation in which you need "enough paper" to stack up.

--[[User:NiccoloM|NiccoloM]] ([[User talk:NiccoloM|talk]]) 00:46, 21 January 2013 (UTC)

Isn't there a pun on Log which is itself an energy source as well as being the source of any reams of paper used to record values.
[[Special:Contributions/192.11.175.219|192.11.175.219]] 06:58, 22 January 2013 (UTC)

Am I the only one not seeing the glaring mistake on the comic? First thing I thought was "that stack of paper is not high enough!". Please someone double check my math: If the height has to be 6.6e6cm (stated above) at 29.7 cm each A4 (vertical), that would mean 222,222 sheets of paper one on top of another. Each stack of 100 pages is aprox 1cm high. That would represent the stack to be 2222cm high, ergo 22m, roughly a 7 story building. Unless there is the equivalent of 6 stories in the waving paper, or the length of the folding 7x that of an A4, or the stick figure is 7 times closer to the camera than the stack of paper is... '''THE HEIGHT OF THE PILE IS OH SO WRONG'''!!!!!! Please prove me wrong!

[[Special:Contributions/87.238.84.65|87.238.84.65]] 14:45, 28 January 2013 (UTC) Guest, 2nd time posting :)

Assumption #1) the graph is drawn on an 8.5 x 11 sheet of ordinary paper in landscape orientation.

Assumption #2) the graph is drawn in normal (linear) scale.

Assumption #3) Cueball is 6 feet tall.

Trusting MSPaint with the conversions, I read the first four bars to have about 5 units (megajoules per kg) per pixel. 76 million units divided by 5 units per pixel is a 15.2 million pixel tall bar.

Looking again to MSPaint, I read the 8.5" dimension of the paper to be about 193 pixels. 15.2 million pixels of graph bar divided by 193 pixels per page is 78756 pages.

Looking above, I read that 100 pages is 1cm, so our stack is going to be 787.56cm tall.

On this side of the pond, that's 310 inches, or about 25 feet.

So, the stack Cueball is looking at is too short to house an accurately long enough bar....

...IF the stack's footprint's longer dimension is only 8.5 inches. While the original graph paper appears to be 8.5x11, the ribbon of paper continuing the bar does not appear to be segmented. Again looking at MSPaint, it would seem the ribbon is about 4.75" wide. The stack is clearly much longer than it is wide. If the stack is 30" long and 4.75" wide, the stack would be whittled down to just over 6 feet tall.

So, making a gang load of assumptions, and scaling from an drawn image, it's reasonable to say the stack in the image could be accurate enough.

The explanation's assumption above that the gasoline bar is 4cm tall makes the piece of paper 96.5cm (38") tall, and that's just not practical. Using the scale I've based my statements on makes the gasoline bar just about 9mm.

-psychoboy[[Special:Contributions/70.164.66.64|70.164.66.64]] 19:57, 2 February 2013 (UTC)

What is the energy density of gasoline if it undergoes nuclear fusion? {{unsigned ip|173.245.48.91}}

This is similar to the iterated logarithm function, right? --[[Special:Contributions/199.27.128.192|199.27.128.192]] 21:28, 18 January 2015 (UTC)

I actually had to present this chart when I was ten years old, but I needed to use a split level to save paper due to the theme of environmentalism.[[Special:Contributions/141.101.104.186|141.101.104.186]]

The stated value of 76'000'000 MJ (76 Terajoules) per kg Uranium corresponds roughly to the energy released by the fission of U-235 nuclei of 202.5 MeV or 3.24·10-17 MJ per nucleus [[https://en.wikipedia.org/wiki/Nuclear_fission#Output]]. One kilogram of pure U-235 would release about 83 Terajoules. In a real nuclear power plant more energy is generated due to breeder reactions, so that per kg U-235 about 128 TJ are produced. One kilogram of natural uranium has about 0.71% U-235 and therefore the potential to produce about 910'000 MJ in a usual nuclear power plant. A fuel rod as used in a nuclear power plant (enriched to 3.5% U-235) has the potential to produce about 4.5 TJ. --[[Special:Contributions/162.158.150.231|162.158.150.231]] 01:19, 18 March 2016 (UTC)

743: Infrastructures

2016-02-20T18:55:15Z

162.158.150.231: /* Explanation */

{{comic
| number = 743
| date = May 21, 2010
| title = Infrastructures
| image = infrastructures.png
| titletext = The heartfelt tune it plays is CC licensed, and you can get it from my seed on JoinDiaspora.net whenever that project gets going.
}}

==Explanation==
[[Cueball]] has sent an essay to a friend. While the essay itself was good, his friend was worried because the essay was in the {{w|Doc (computing)|.doc}} format, the {{w|Proprietary Software|proprietary format}} that old versions of {{w|Microsoft Word}} used. The friend advises Cueball to use a format based on an open standard. He may be thinking of the ODF format (file extensions <code>.odt</code>, <code>.ods</code>, <code>.odp</code>, etc.) used in {{w|OpenOffice}} and {{w|LibreOffice}}, which are both {{w|free software}}. ODF was not standardised until 2005, but was based on OpenOffice XML which has existed since about 2000, and there were other open markup languages such as Docbook, LaTeX and HTML.

Cueball, who does not appreciate his friend's contradicting him, argued that the friend is making petty fights about the details of software instead of simply bothering that the software works (which is, in essence, a primordial purpose of software). Given that it can be a challenge to move from a familiar proprietary application to an open-source rival which may lack compatibility, features, support and popularity, Cueball has *some* justification for his stance.

The bearded fellow brings up that he is just concerned about the current proprietary software infrastructure that forces users to use software in a specific way, penalizing them for sharing the software or even preventing looking at the source code in order to learn what the program actually does or how it works. Cueball makes a retort that his friend has [http://www.gnu.org/philosophy/no-word-attachments.html an arrogance that crowds out his perspective] while claiming that he is {{w|Autism|autistic}}. (Autistic people do have a tendency to have intense fixations to things, even things that other people would find mundane or even odd. They also tend to have trouble knowing the problems of the world outside of themselves, having them lack perspective of things at times. Even so, Cueball's remark suggests that he thinks that "autistic" is just another word for "retarded" which is another word for "stupid", a double-fallacy.)

Seven years later, Cueball runs to the friend, alarmed about Facebook's heavy policies about its complete control about the information its users submit. Since Facebook is like Microsoft in its lack of transparency about their services and taking away a lot of control from the user, the fellow retorts with playing "the world's tiniest open-source violin." This is dubious since "[http://tvtropes.org/pmwiki/pmwiki.php/Main/WorldsSmallestViolin playing the world's smallest violin]" is a [http://www.urbandictionary.com/define.php?term=World%27s+Smallest+Violin sarcastic expression] that denotes that the speaker will not give pity to the recipient.

In response to this comic people released [http://www.thingiverse.com/thing:3193 3d printer specifications for tiny violins], as open source files. This one was designed by Allan Ecker.

The title text references the following pieces of infrastructure that are compatible with the "free software" ideology:
* [http://www.creativecommons.org Creative Commons licenses] (CC licensed) use existing copyright law to permit someone to share a creative work anywhere so long as the sharer attributes credit to the creator of the work. The particular CC license chosen may also allow for modification, derivative works, and/or commercial usage. The fellow's phrase "you can get it" in the title text is ambiguous: is he offering to share the code for the violin, or the tune that the violin plays? But since CC licenses are not used for software, we can assume "it" refers to the tune: either an audio recording of it, or perhaps source material from which to make modified versions.
* [http://joindiaspora.com joindiaspora.com] (formerly joindiaspora.net) is the central host of {{w|Diaspora (social network)| Diaspora*}}, an open-source alternative to Facebook which puts the user in control of how their information is used. Of course this sort of use of Diaspora would eliminate Cueball's concern over how Facebook handled his information.
* a Diaspora "seed" is a personal web server that interacts in a Diaspora "pod" of servers. It stores all of your information (such as the tune in this case) and shares it with your friends, in a way that respects your preferences around privacy, etc.

The problem with the lack of open source and Facebook is also the subject of [[1390: Research Ethics]].

==Transcript==
:2003:
:[Cueball approaches a bearded fellow.]
:Cueball: Did you get my essay?
:Bearded Fellow: Yeah, it was good! But it was a .doc; You should really use a more open-
:Cueball: Give it a ''rest'' already. Maybe we just want to live our lives and use software that ''works'', not get wrapped up in your stupid nerd turf wars.

:Bearded Fellow: I just want people to care about the infrastructures we're building and who-
:Cueball: No, you just want to feel smugly superior. You have no sense of perspective and are probably autistic.

:2010:
:Cueball: Oh my God! We handed control of our social world to Facebook and they're ''DOING EVIL STUFF!''

:Bearded Fellow: Do you see this?

:[Inset, the bearded fellow rubs his index and middle fingers against his thumb.]
:Bearded Fellow: ''It's the world's tiniest open-source violin.''

{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Programming]]
[[Category:Social networking]]

673: The Sun

2016-02-20T00:43:18Z

162.158.150.231: /* Explanation */ Core movie

{{comic
| number = 673
| date = December 9, 2009
| title = The Sun
| image = the_sun.png
| titletext = Obligatory bad guy: This operation is sheer foolishness, and it's not happening on my watch! Mainly because I can't figure out how to adjust the time.
}}

==Explanation==
This comic makes fun of {{w|science fiction}} {{w|disaster movies}}, especially the 2003 film "{{w|The Core}}", where a group of scientists travel through the Earth's mantle to place a series of nuclear devices in order to speed up the slowing rotation of the Earth's core and prevent a complete collapse of Earth's magnetic field.

The movie described by the comic shows a scenario where the "sun's fusion is failing". In fact the 2007 British science fiction film "{{w|Sunshine (2007 film)|Sunshine}}" was about a group of astronauts sent on a mission to the reignite a dying {{w|Sun}} with a battery of nuclear bombs.

The {{w|sun}}'s energy comes from nuclear fusion reactions among the extremely hot dense hydrogen plasma in its core. The idea of the sun's fusion failing is rather ridiculous from a scientific perspective, because the fusion reactions are well understood and the sun has enough hydrogen to fuel it for about 5 billion more years. Even if the sun's hydrogen was getting low it would {{w|Star#Post.E2.80.93main_sequence|start fusing helium}} and begin expanding into a {{w|red giant}}, we later be destroyed by it. Even in the nearly impossible event of the sun fusion is {{w|Supernova#Core_collapse|failing in the traditional sense}}, the sun would {{w|Star#Collapse|collapse}} causing a {{w|supernova}}. In other words if the sun stopped fusing we wouldn't have to worry about less sunlight, we would have to worry about more.

In any case, it appears to be failing and the solution is to send a team of astronauts to the sun to restart the fusion (which is analogous to sending an ant to the US Senate to break a budget deadlock). The team leader is motivated by concern that if the sun's fusion stops, there will be no more light, and so the earth will be in perpetual darkness.

The final panel gives the movie's name and subtitle. "Daylight Saving Time" refers both to the policy of changing clocks, which is intended to "save" daylight for a more useful part of the day; and the scenario in this movie in which it is time for the team to literally save the sun's daylight from being extinguished. "Never fall back" is an additional word play on the {{w|mnemonic}} used (in the States at least) to remember the direction to change clocks. The mnemonic goes, {{w|Spring_forward,_fall_back#Terminology|"spring forward, fall back"}} to indicate that in the spring season, clocks get set ahead by an hour, while in the fall the clocks are set backwards an hour. The phrase "fall back", however, can also mean to retreat from a battle.

The comic makes fun of these disaster movies in a couple of ways. The characters in the first panel acknowledge that the scenario doesn't make sense scientifically, but are prepared to sacrifice scientific value for the plot. Also, in the second panel the team is to be composed of {{w|NASA}}'s "hottest astronauts", which makes fun of the fact that the characters in movies are much more attractive than average, and the fact that they will be much hotter when they reach the sun. The team leader expresses his concern with a few buzz phrases often used in such films.

The title text continues the lunacy (solacy?) of the situation with the cliche of the "obligatory bad guy" -- a person in the plot who acts antagonistic, often for the flimsiest of reasons. There is also the common complaint, especially among the technologically inept, that he can't figure out how to change the time, punning again on Daylight Saving(s) Time.

==Transcript==
:Coming this March from the makers of The Core...
:[Ponytail is looking through a telescope in an observatory. Two men are nearby.]
:Ponytail: The sun's fusion is failing!
:Cueball: Does that make sense?
:Friend: Whatever.

:Ponytail: If we don't send a ship to restart it, it could go out completely!
:Cueball: Call NASA!
:Friend: (on the phone) Assemble our hottest astronauts.

:[Four astronauts stand at the other end of the phone. The one holding the handset has the helmet of a space suit under his arm.]
:Astronaut: The earth bathed in eternal darkness? A night without a dawn? Not on my watch!
:Astronaut: Saddle up.

:[The four astronauts are shown in silhouette on gray, casting huge shadows towards the bottom of the panel from the sun in the center.]
:It's '''''DAYLIGHT SAVING TIME.'''''
:Never fall back.

{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring Megan]]
[[Category:Physics]]

681: Gravity Wells

2016-02-06T22:13:19Z

162.158.150.231: /* Cut outs and sketches */

{{comic
| number = 681
| date = December 28, 2009
| title = Gravity Wells
| image = gravity_wells.png
| titletext = This doesn't take into account the energy imparted by orbital motion (or gravity assists or the Oberth effect), all of which can make it easier to reach outer planets.
}}
{{TOC}}

The xkcd page links to [http://xkcd.com/681_large/ a much larger version].

==Explanation==

The comic shows the gravitational potential (energy transferred per unit mass due to gravity) for the positions of each planet in the solar system -- including some moons and Saturn's rings. An object traveling along an upward slope loses energy, while an object traveling along a downward slope gains energy.

Escaping a planet or moon's orbit requires enough energy (e.g. by walking, jumping, or rocket) to reach the top of either peak that defines the edge of the well. The peak to the left indicates the minimum energy required to exit orbit. The peak to the right indicates the maximum energy required to exit orbit. In order to exit orbit with the minimum amount of energy, you would have to travel towards the center of the solar system; to exit orbit with the maximum amount of energy, you would have to travel away from the center of the solar system (the Sun). In reality, the strength of gravity decreases with distance from the planet. However, a comparison of energy expended to escape the gravitational pull allows for a simpler comparison between the objects.

The height of the graph is scaled to kilometers via the gravitational potential an object has at the given height assuming at a constant acceleration due to Earth's surface gravity. The {{w|Sun|Sun's}} gravity well is not shown in its entirety, but is just indicated on the far left as ''"Very very far down"''. Had it been shown in its full extent it would have made the rest of the drawing so small in comparison that it would have been unreadable. As the gravitational potential increases with distance from the sun the graph has a general upward slope. To rise out of each well on the diagram, and therefore escape the planets gravity, it would require the same energy required to rise out of a physical well of that depth at Earth's surface gravity.

The length of each gravity well is scaled to the diameter of the planet and the spacing between the planets is not to scale with distance from the sun. This is necessary to make the graph readable. Because the distance between the planets are condensed the gravitational potential, from the gravity pulling toward the sun, accumulates quicker. This is the reason for the large peaks between the planets. The moons shown in the chart are at the appropriate distance from their respective planets' gravity wells for their orbits.

Each planet is shown cut in half at the bottom of its well, with the depth of the well measured down to the planet's flat surface.

==== Inner Planets ====
* {{w|Mercury_(planet)|Mercury}} -- no facts listed
* {{w|Venus_(planet)|Venus}} -- no facts listedc
* {{w|Earth}} and {{w|Moon}}: The listed depth of the gravity well at Earth was originally listed at 5478 km rather than the correct value of 6379 km seen in the cutout. Randall has since corrected it. The Moon's is 288 km.
* {{w|Mars}}: The listed depth of the gravity well of Mars is 1286 km.

==== Outer Planets ====
* {{w|Jupiter}}: Jupiter is so massive and dense that it is comparable in mass to a {{w|Brown dwarf}} which is the smallest kind of star. Saturn, while similar in size, is composed of much lighter gas material. Hence Saturn's mass and therefore its gravitational pull are much smaller. Had a few dozen times the mass of gasses contained in Jupiter condensed in that location, the gravitational pull would cause the pressure and temperature to increase to a level that is sufficient to ignite {{w|Nuclear fusion|nuclear fusion}}. Had that happen during creation of our solar system, we would have two {{w|Sun|Suns}} and our solar system would be a {{w|Binary system (astronomy)|Binary system}}. Jupiter has {{w|Moons_of_Jupiter|67 moons}} of which 3 are shown;
** {{w|Ganymede_(moon)|Ganymede}} -- moon of Jupiter, no facts given
** {{w|Io_(moon)|Io}} -- moon of Jupiter, no facts given
** {{w|Europa_(moon)|Europa}} -- moon of Jupiter, no facts given

* {{w|Saturn}}: The diagram shows the position of the {{w|rings of Saturn}} in Saturn's gravity well. Saturn's rings start fairly near the planet and extend out quite far, therefore multiple stripes are shown in the figure. The rings are also shown in multiple colors and roughly match the observed colors from photos take by the {{w|Cassini–Huygens|Cassini spacecraft}} expedition as it passed Saturn. All of the colors of the planets and moons represent the predominant color of that object as observed from earth. Saturn has {{w|Moons_of_Saturn|62 moons}} of which one is shown;
** {{w|Titan (moon)|Titan}}, a moon of Saturn. The figures on Titan are sirens, a reference to Kurt Vonnegut's ''{{w|The Sirens of Titan}}''.

* {{w|Uranus}}: Notably absent is any "your-anus" jokes.
* {{w|Neptune}}: Megan's quote is a paraphrase of {{w|Carl Sagan|Carl Sagan's}} quote, "...but from a planet orbiting a star in a distant globular cluster, a still more glorious dawn awaits, not a sun-rise, but a galaxy rise." [http://www.youtube.com/watch?v=zSgiXGELjbc Video here]

==== Cut outs and sketches====
The following items are listed from top to bottom and left to right.

* Mars moons: The Mars cutout shows the Mars moon system, including the moons Deimos and Phobos. The depth of the Mars gravity well is listed at 1286 km.
** {{w|Deimos (moon)|Deimos}}: The gravity on Deimos, a moon of Mars, is so weak that a bike jump would be sufficient to escape its gravity.
** {{w|Phobos (moon)|Phobos}}: The gravity on Phobos, a moon of Mars, is so weak that you could launch a baseball into space simply by throwing it.

* Your mom and a local football team: The sketch next to Jupiter is playing on the classic "Yo Mama" joke. It combines "Yo Mama is so fat" and "Yo Mama is so horny". The sketch implies that she has a huge gravitational pull because she is very fat, and has sex with an entire football team by demonstrating a football team falling into her very deep gravity well. A "Yo Mama" joke also appears in comic [[89: Gravitational Mass]].

* Earth's Moon: The cut out shows the significant difference in strength between the {{w|gravity well}} of the Earth and the Moon. Cueball comments that the {{w|Apollo Lunar Module|Apollo Lunar Module}} was very small and the {{w|Saturn V}} rocket was very large because escaping the Earth's gravity well takes much more energy than escaping the Moon's. The cut out also shows objects like the {{w|International_Space_Station|International Space Station}}, the {{w|Space shuttle|space shuttle}}, {{w|GPS satellite|GPS satellites}} and {{w|Geostationary orbit|geo-stationary satellites}} at their respective positions within Earth's gravity well. The depth of Earth's gravity well is listed correctly at 6 379 km (note the difference from the non-cutout number). The depth of the Moon's gravity well is listed at 288 km.

=== How to calculate gravity wells ===
The text near the bottom of Jupiter's gravity well explains that the depth of the well is mass-of-planet over radius-of-planet with Newton's constant and 9.81 m/s² as constants, where 9.81 m/s² is the acceleration of a free falling body at Earth's gravity.

The calculation for a gravity well is:
:depth = (G * Planet-mass ) / (9.81 m/s2 * Planet-radius)
::where G is {{w|Isaac_Newton|Newton}}'s {{w|Gravitational_constant|gravitational constant}}, and
::9.81 m/s2 is the {{w|Acceleration|acceleration}} rate of a {{w|Gravity_of_Earth|free falling body on earth}} at sea level (g).

====Title text====
The title text indicates that the planets motion can affect the amount of energy for escape velocity. It is possible to change speed by using the planets orbital speed and gravity. This is know as a performing a slingshot or a {{w|Gravity assist|gravity assist}}, and is done to gain speed or to brake when needed. The use of rocket engines are more effective when used at a high speed slingshot maneuver, which is know as the {{w|Oberth effect}}, where most energy is going into moving the rocket as opposed to moving the exhaust -- conserving the maximum useful energy. On earth the same principle is used when launching rockets. Rockets are always launched in a eastward direction to make maximum use of the rotational energy of the earth. Launching rockets in a westward direction would require significant additional energy. Because of this most artificial satellites are flying east around the globe.

The size of the gravity-well as described in this comic is not accounting for these factors. Therefore leaving the solar system (or any of the gravity wells of the planets) could require less energy than described by the graph, assuming that the launch and slingshots are properly designed and executed.

==== Escape Velocities ====
The following table was adapted from the table in {{w|Escape velocity#List of escape velocities|Escape velocity}}, using ''h'' = ''V_e''^2 / 2''g'':
{| class="wikitable" border="1"
| Location || with respect to || Ve (km/s) || Well depth (km) ||
| Location || with respect to || Ve (km/s) || Solar well (Mm) || Total depth (Mm)
|-
| on the Sun, || the Sun's gravity: || 617.5 || 19,435,000 || || || || || || 19,435
|-
| on Mercury, || Mercury's gravity: || 4.3 || 942 ||
| at Mercury, || the Sun's gravity: || 67.7 || 233.6 || 235
|-
| on Venus, || Venus' gravity: || 10.3 || 5,407 ||
| at Venus, || the Sun's gravity: || 49.5 || 124.9 || 130
|-
| on Earth, || the Earth's gravity: || 11.2 || 6,393 ||
| at the Earth/Moon, || the Sun's gravity: || 42.1 || 90.3 || 97
|-
| on the Moon, || the Moon's gravity: || 2.4 || 294 ||
| at the Moon, || the Earth's gravity: || 1.4 || || 91
|-
| on Mars, || Mars' gravity: || 5 || 1,274 ||
| at Mars, || the Sun's gravity: || 34.1 || 59.3 || 61
|-
| on Jupiter, || Jupiter's gravity: || 59.5 || 180,400 ||
| at Jupiter, || the Sun's gravity: || 18.5 || 17.4 || 198
|-
| on Ganymede, || Ganymede's gravity: || 2.7 || 372 || || || || ||
|-
| on Saturn, || Saturn's gravity: || 35.6 || 64,600 ||
| at Saturn, || the Sun's gravity: || 13.6 || 9.43 || 74
|-
| on Uranus, || Uranus' gravity: || 21.2 || 22,907 ||
| at Uranus, || the Sun's gravity: || 9.6 || 4.7 || 28
|-
| on Neptune, || Neptune's gravity: || 23.6 || 28,400 ||
| at Neptune, || the Sun's gravity: || 7.7 || 3.02 || 31
|-
| on Pluto, || Pluto's gravity: || 1.2 || 73 || || || || ||
|-
|at Solar System galactic radius, || the Milky Way's gravity: || 525 || 14,000
|}

==Transcript==
:'''Main Text'''
:Gravity Wells scaled to Earth surface gravity
:This chart shows the "depth" of various solar system gravity wells.
:Each well is scaled such that rising out of a physical well of that depth — in constant Earth surface gravity — would take the same energy as escaping from that planet's gravity in reality.
:Each planet is shown cut in half at the bottom of its well, with the depth of the well measured down to the planet's ''flat'' surface.
:The planet sizes are to the same scale as the wells. Interplanetary distances are not to scale.
:Depth = (G × PlanetMass) / (g × PlanetRadius)
:G = Newton's constant
:g = 9.81 m/s2

:'''Planetary Descriptions'''
:To Sun, very very far down
:Mercury
:Venus
:Earth - 6379 km [originally 5,478 km]
:Moon - 288 km
:Mars - 1,286 km
:Ganymede
:Io
:Jupiter
::[A drawing of a "very deep" gravity well, "Your mom" at the bottom, several member of "local football team" falling down towards her.]
::Jupiter is not much larger than Saturn, but much more massive. At its size, adding more mass just makes it denser due to the extra squeezing of gravity.
::If you dropped a few dozen more Jupiters into it, the pressure would ignite fusion and make it a star.
:Europa
:Titan
::Two alarms: Weeoooeeoooeeooo
:Saturn
::Rings
:Uranus
:Neptune
::Megan: An even more glorious dawn awaits!

:'''Mars Inset'''
:[Mars gravity well, the Pathfinder probe on its surface, with its moons Deimos and Phobos as smaller gravity wells.]

:[Figure of a man (to scale) in Deimos's gravity well.]
:You could escape Deimos with a bike and a ramp.

:[Figure of a man (to scale) in Phobos's gravity well.]
:A thrown baseball could escape Phobos.

:'''Earth Inset'''
:[Zoomed-in view of Earth/moon gravity well, featuring the relative locations of the atmosphere, Low Earth Orbit, the International Space Station, the Space Shuttle, GPS satellites, and satellites in geosynchronous orbit.]
:Cueball: This is why it took a huge rocket to get to the moon but only a small one to get back.

:It takes the same amount of energy to launch something on an escape trajectory away from Earth as it would to launch it 6,000 km upward under constant 9.81 m/s2 Earth gravity.

:Hence, Earth's well is 6,000 km deep.

{{comic discussion}}
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