explain xkcd - User contributions [en]

2118: Normal Distribution

2019-03-01T15:11:41Z

108.162.241.166: /* Transcript */

{{comic
| number = 2118
| date = March 1, 2019
| title = Normal Distribution
| image = normal_distribution.png
| titletext = It's the NORMAL distribution, not the TANGENT distribution.
}}

==Explanation==
{{incomplete|Created by an ANNOYED STATISTICIAN. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

Midpoint - 52.7%

"Remember, 50% of
the distribution
falls between
these two lines!"

{{comic discussion}}

2118: Normal Distribution

2019-03-01T15:10:15Z

108.162.241.166: /* Explanation */

2118: Normal Distribution

2019-03-01T15:09:46Z

108.162.241.166:

{{comic
| number = 2118
| date = March 1, 2019
| title = Normal Distribution
| image = normal_distribution.png
| titletext = It's the NORMAL distribution, not the TANGENT distribution.
}}

==Explanation==
{{incomplete|Created by an ANNOYED STATICIAN. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

{{comic discussion}}

2107: Launch Risk

2019-02-04T18:42:51Z

108.162.241.166: /* Explanation */

{{comic
| number = 2107
| date = February 4, 2019
| title = Launch Risk
| image = launch_risk.png
| titletext = Don't worry--you're less likely to die from a space launch than from a shark attack. The survival rate is pretty high for both!
}}

==Explanation==
{{incomplete|Created by an ASTRONAUT STRUCK BY LIGHTNING. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}

This comic deals with statistics based on a large population, such as all Americans, when the people in question are in a smaller group with vastly different statistics, such as astronauts.

In a capsule about to be launched into space, one astronaut asks another how they are feeling. The second one admits to feeling nervous. The first one offers the supposedly reassuring observation that they are more likely to be struck by lightning than to be selected as an astronaut. Such "more likely to be struck by lightning" comparisons are commonly used to illustrate that a particular risk is very remote, and thus should not be considered particularly frightening.

The second one agrees with the first one for a moment, but then realizes that something is wrong with the argument. Presumably, they realize that the likelihood of being ''selected as an astronaut'' is a moot point -- they are there because they ''already have'' been selected as an astronaut. The relevant concern is the risk level faced by an astronaut, given that they already hold that position. Unfortunately, the historical record shows that this risk is somewhat high, certainly far above the minuscule risk of being struck by lightning.

The lifetime odds of being struck by lightning are approximately 1 in 14,600.[How Dangerous is Lightning? https://www.weather.gov/safety/lightning-odds]

The title text refers to another common comparison, the risk of a shark attack. In addition to shark attacks being rather rare, they are also not as likely to kill the victim as is commonly assumed. Most people attacked by sharks, and most people launched into space, live through the experience; however, it remains true that both are considerably riskier than many if not most common activities.{{Citation needed}}

A tall rocket, such as depicted would be more likely to be struck by lightning than nearby structures. However launch controllers monitor weather carefully to reduce the chances of attempting to launch when lightning is likely.

A space craft launch can trigger lightning, by creating a conductive path through charge bearing clouds, which would normally not produce lightning. Apollo 12 was struck by triggered lightning twice during launch phase, resulting in the entire operational platform shutting down from overload. Backup systems allowed the flight to proceed.[NASA: Lightning and Launches https://www.nasa.gov/audience/foreducators/9-12/features/F_Lightning_and_Launches_9_12.html]

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

:[A rocket is about to launch.]
:Astronaut 1: How you feeling?
:Astronaut 2: Honestly, pretty nervous.
:Astronaut 1: I know it seems dangerous, but just remember: you're more likely to be struck by lightning than to be selected to become an astronaut.
:Astronaut 2: Oh that's a good-
:Astronaut 2: ...Wait.
:Countdown: T-Minus 20...19...

{{comic discussion}}

2107: Launch Risk

2019-02-04T18:39:56Z

108.162.241.166: /* Transcript */

{{comic
| number = 2107
| date = February 4, 2019
| title = Launch Risk
| image = launch_risk.png
| titletext = Don't worry--you're less likely to die from a space launch than from a shark attack. The survival rate is pretty high for both!
}}

==Explanation==
{{incomplete|Created by an ASTRONAUT STRUCK BY LIGHTNING. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}

This comic deals with statistics based on a large population, such as all Americans, when the people in question are in a smaller group with vastly different statistics, such as astronauts.

In a capsule about to be launched into space, one astronaut asks another how they are feeling. The second one admits to feeling nervous. The first one offers the supposedly reassuring observation that they are more likely to be struck by lightning than to be selected as an astronaut. Such "more likely to be struck by lightning" comparisons are commonly used to illustrate that a particular risk is very remote, and thus should not be considered particularly frightening.

The second one agrees with the first one for a moment, but then realizes that something is wrong with the argument. Presumably, they realizes that the likelihood of being ''selected as an astronaut'' is a moot point -- the are there because they ''already have'' been selected as an astronaut. The relevant concern is the risk level faced by an astronaut, given that they already holds that position. Unfortunately, the historical record shows that this risk is somewhat high, certainly far above the minuscule risk of being struck by lightning.

The lifetime odds of being struck by lightning are approximately 1 in 14,600.[How Dangerous is Lightning? https://www.weather.gov/safety/lightning-odds]

The title text refers to another common comparison, the risk of a shark attack. In addition to shark attacks being rather rare, they are also not as likely to kill the victim as is commonly assumed. Most people attacked by sharks, and most people launched into space, live through the experience; however, it remains true that both are considerably riskier than many if not most common activities.{{Citation needed}}

A tall rocket, such as depicted would be more likely to be struck by lightning than nearby structures. However launch controllers monitor weather carefully to reduce the chances of attempting to launch when lightning is likely.

A space craft launch can trigger lightning, by creating a conductive path through charge bearing clouds, which would normally not produce lightning. Apollo 12 was struck by triggered lightning twice during launch phase, resulting in the entire operational platform shutting down from overload. Backup systems allowed the flight to proceed.[NASA: Lightning and Launches https://www.nasa.gov/audience/foreducators/9-12/features/F_Lightning_and_Launches_9_12.html]

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

:[A rocket is about to launch.]
:Astronaut 1: How you feeling?
:Astronaut 2: Honestly, pretty nervous.
:Astronaut 1: I know it seems dangerous, but just remember: you're more likely to be struck by lightning than to be selected to become an astronaut.
:Astronaut 2: Oh that's a good-
:Astronaut 2: ...Wait.
:Countdown: T-Minus 20...19...

{{comic discussion}}

2107: Launch Risk

2019-02-04T17:51:56Z

108.162.241.166:

{{comic
| number = 2107
| date = February 4, 2019
| title = Launch Risk
| image = launch_risk.png
| titletext = Don't worry--you're less likely to die from a space launch than from a shark attack. The survival rate is pretty high for both!
}}

==Explanation==
{{incomplete|Created by an ASTRONAUT STRUCK BY LIGHTNING. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}

This comic deals with statistics based on a large population, such as all Americans, when the people in question are in a smaller group with vastly different statistics, such as astronauts.

In a capsule about to be launched into space, one astronaut asks another how he's feeling. The second one admits to feeling nervous. The first one offers the supposedly reassuring observation that he's more likely to be struck by lightning than to be selected as an astronaut. Such "more likely to be struck by lightning" comparisons are commonly used to illustrate that a particular risk is very remote, and thus should not be considered particularly frightening.

The second one agrees with the first one for a moment, but then realizes that something is wrong with the argument. Presumably, he realizes that the likelihood of being ''selected as an astronaut'' is a moot point -- he's there because he ''already has'' been selected as an astronaut. The relevant concern is the risk level faced by an astronaut, given that he already holds that position. Unfortunately, the historical record shows that this risk is somewhat high, certainly far above the minuscule risk of being struck by lightning.

The title text refers to another common comparison, the risk of a shark attack. In addition to shark attacks being rather rare, they are also not as likely to kill the victim as is commonly assumed. Most people attacked by sharks, and most people launched into space, live through the experience; however, it remains true that both are considerably riskier than many if not most common activities.{{Citation needed}}

This comic is especially funny considering Apollo 12 was in fact struck by lightning twice during launch phase, resulting in the entire operational platform shutting down from overload.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

:Astronaut 1: How you feeling?
:Astronaut 2: Honestly, pretty nervous.
:Astronaut 1: I know it seems dangerous, but just remember: you're more likely to be struck by lightning than to be selected to become an astronaut.
:Astronaut 2: Oh that's a good-
:Astronaut 2: ...Wait.

{{comic discussion}}

2107: Launch Risk

2019-02-04T17:51:02Z

108.162.241.166: /* Transcript */

{{comic
| number = 2107
| date = February 4, 2019
| title = Launch Risk
| image = launch_risk.png
| titletext = Don't worry--you're less likely to die from a space launch than from a shark attack. The survival rate is pretty high for both!
}}

==Explanation==
{{incomplete|Created by an ASTRONAUT STRUCK BY LIGHTNING. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}

This comic deals with statistics based on a large population, such as all Americans, when the people in question are in a smaller group with vastly different statistics, such as astronauts.

In a capsule about to be launched into space, one astronaut asks another how he's feeling. The second one admits to feeling nervous. The first one offers the supposedly reassuring observation that he's more likely to be struck by lightning than to be selected as an astronaut. Such "more likely to be struck by lightning" comparisons are commonly used to illustrate that a particular risk is very remote, and thus should not be considered particularly frightening.

The second one agrees with the first one for a moment, but then realizes that something is wrong with the argument. Presumably, he realizes that the likelihood of being ''selected as an astronaut'' is a moot point -- he's there because he ''already has'' been selected as an astronaut. The relevant concern is the risk level faced by an astronaut, given that he already holds that position. Unfortunately, the historical record shows that this risk is somewhat high, certainly far above the minuscule risk of being struck by lightning.

The title text refers to another common comparison, the risk of a shark attack. In addition to shark attacks being rather rare, they are also not as likely to kill the victim as is commonly assumed. Most people attacked by sharks, and most people launched into space, live through the experience; however, it remains true that both are considerably riskier than many if not most common activities.{{Citation needed}}

This comic is especially funny considering Apollo 12 was in fact struck by lightning twice during launch phase, resulting in the entire operational platform shutting down from overload.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

Astronaut 1: How you feeling?
Astronaut 2: Honestly, pretty nervous.
Astronaut 1: I know it seems dangerous, but just remember: you're more likely to be struck by lightning than to be selected to become an astronaut.
Astronaut 2: Oh that's a good-
Astronaut 2: ...Wait.

{{comic discussion}}

Talk:2039: Begging the Question

2018-08-29T22:19:06Z

108.162.241.166:

First[[Special:Contributions/162.158.74.231|162.158.74.231]] 17:17, 29 August 2018 (UTC)

Of course it's also possible that the food made them so Nauseated that they also became Nauseous (i.e. they could have started vomiting or smell horrible due to eating the food, causing people around to feel unwell as well). [[User:NormanR|NormanR]] ([[User talk:NormanR|talk]]) 21:13, 29 August 2018 (UTC)

I believe the reason the two words have become confused is due to the word "noxious". which means "very unpleasant". So, someone who is "nauseated" could feel "noxious", and when wires end up crossed in the brain, they associate it with "nauseous" rather than "noxious".

2034: Equations

2018-08-20T00:11:08Z

108.162.241.166: /* Simplified Explanations */

{{comic
| number = 2034
| date = August 17, 2018
| title = Equations
| image = equations.png
| titletext = All electromagnetic equations: The same as all fluid dynamics equations, but with the 8 and 23 replaced with the permittivity and permeability of free space, respectively.
}}

This comic gives a set of equations supposedly from different areas of science in mathematics, physics, and chemistry. To anyone not familiar with the field in question they look pretty similar to what you might find in research papers or on the relevant Wikipedia pages. To someone who knows even a little about the topic, they are clearly very wrong and only seem even worse the more you look at them. In many disciplines, the mathematical description of a large area is summed up in a small number of equations, such as Maxwell's equations for electromagnetism. In similar fashion, the equations here purport to encompass the whole of their given field.

==Simplified Explanations==
{{incomplete|Created by a mere human. Do NOT delete this tag too soon.}}

;All kinematics equations
Kinematics is the study of the motion of objects. More specifically, it describes how the location, velocity, and acceleration of an object vary over time. The equation shown contains two of these standard kinematic variables, velocity ''v'' and time ''t'', in addition to several quantities (''E'', ''K0'', and ''ρ'') that are completely unrelated to kinematics.

;All number theory equations
Number theory is a branch of mathematics concerned primarily with the study of integers. However, the equation shown contains the non-integer number ''e'' (approximately equal to 2.718...), and uses the Greek letter ''π'' as an integer, even though ''π'' is almost exclusively used in mathematics to denote the well-known, ''non''-integer number 3.14159.... It also treats ''π'' as a variable component in a summation, rather than as a constant.

The non-integer ''e'' is to the power of (π-∞), for most cases causes ''e'' simply gets reduced to 0. However, as π approaches infinity, it will result in an undefined value.

;All chemistry equations
This shows a parody of the common example chemistry equation of burning Methane and Oxygen (with added heat), to form water and carbon dioxide. However in this form "HEAT" is an actual molecule, rather than simply indicating the presence of heat to start the reaction. Thus the equation is modified to incorporate the fictional "HEAT" into the reaction. While the H in "HEAT" is the chemical symbol of the element hydrogen, none of the letters E, A, or T are symbols of any actual elements.

TODO: other simplified explanations.

==Technical Explanations==
{{incomplete|Created by an EQUATION. Do NOT delete this tag too soon.}}

;All kinematics equations
:<math>E = K_0t + \frac{1}{2}\rho vt^2</math>
{{w|Kinematics}} describes the motion of objects without considering mass or forces.

This equation here literally states: "Energy equals a constant <math>K_0</math> multiplied by time, plus half of density multiplied by speed multiplied by time squared".

The first term here is hard to interpret: it could be correct if <math>K_0</math> is a constant power applied to the system, but this symbol would more normally be used to denote an initial energy, in which case multiplying by <math>t</math> would be wrong. Alternatively, the term is similar to <math>k_B T</math> (sometimes written as ''kT''), a term that often appears in {{w|Statistical_mechanics|statistical mechanics}} equations, where ''kB'' (or ''k'') is {{w|Boltzmann_constant|the Boltzmann constant}}, and ''T'' is the {{w|Thermodynamic_temperature|absolute temperature}}. In this latter case, the term would have units of energy, consistent with the left side of the equation.

The second term looks similar to the kinetic energy term <math> \frac{1}{2}\rho v^2 </math> in [http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html the Bernoulli equation] for fluids. (More properly, this is the kinetic energy ''density'' in the fluid).

The whole equation appears to be a play on the kinematics formula: <math>s = v_0t + \frac{1}{2}\ at^2</math>, where distance travelled (''s'') by a constantly accelerating object is determined by initial velocity (''v0''), time (''t''), and acceleration (''a'')

Kinematics is often one of the first topics covered in an introductory physics course, both at the high school and freshman college levels. As such, mixing in material from more advanced topics like statistical mechanics and the Bernoulli equation, even if done correctly, would be very confusing for a typical student learning kinematics.

;All number theory equations
:<math>K_n = \sum_{i=0}^{\infty}\sum_{\pi=0}^{\infty}(n-\pi)(i-e^{\pi-\infty})</math>
{{w|Number theory}} is a branch of mathematics primarily studying the properties of integers.

Taken literally the equation says: "The nth K-number is equal to: the sum for all i from 0 to infinity, the sum for all pi from 0 to infinity; subtract pi from n, and multiply it with i minus e to the power of pi minus infinity". A twofold misconception can be seen here. The first is the reassignment of pi as a variable instead of the constant (3.14...). This might be a jab at how in number theory letters and numbers are used interchangeably, but where some letters are all of a sudden fixed constants. The second misconception is the use of infinity in the latter part of the formula. Naively this would signify that (with the reassigned pi values) the part in the power would range from minus infinity to zero. However, infinity is not a number and cannot be used as one without using a limit construct.

;All fluid dynamics equations
:<math>\frac{\partial}{\partial t}\nabla\cdot \rho = \frac{8}{23}
\int\!\!\!\!\!\!\!\!\!\;\;\bigcirc\!\!\!\!\!\!\!\!\!\;\;\int
\rho\,ds\,dt\cdot \rho\frac{\partial}{\partial\nabla}
</math>
{{w|Fluid dynamics}} describes the movement of non-solid material. In particular for gases, the density <math>\rho</math> is often the most interesting quantity (for liquids, this is often just constant). A unique feature of fluid-dynamic equations is the presence of {{w|Advection|advection terms}}, which take the form of often strange-looking spatial derivatives. This equation turns this up to a new level by differentiating with respect to a differential operator <math>\nabla</math>, which does not make any sense at all. Also it has a contour integral which seems reminiscent to a closed-circle process like in a piston engine, but this does not really fit in the context (differential description of a gas), and it has a pair of {{w|Magic number (programming)|unexplained numbers}} <math>8</math> and <math>23</math>, probably alluding to the {{w|Heat capacity ratio|specific heat ratio}} which is often written out as the fraction <math>\tfrac{7}{5}</math>, whereas most other physics equations [[899: Number Line|avoid including any plain numbers higher than 4]].

The title text stating that the electromagnetism equation is the same as the fluid dynamics equation, but with the arbitrary 8 and 23 replaced with the permittivity and permeability of free space is likely because electromagnetism equations often have relations to fluid dynamics, and because those two constants appear in the vast majority of electromagnetism equations.

;All quantum mechanics equations
:<math>|\psi_{x,y}\rangle = A(\psi) A(|x\rangle \otimes |y\rangle)</math>
{{w|Quantum mechanics}} is a fundamental theory in physics which describes nature at scales of atoms and below. It typically uses the {{w|Bra–ket notation|bra–ket notation}} in its formulae.

This equation takes a state psi in the dimensions of x and y and equates it to an operator A performed on psi multiplied by the same operator performed on the tensor product of x and y. Since the state psi is already the tensor product of the states x and y, this is equivalent to performing the same unknown operator twice on psi, and unless this operator is the identity or is its own inverse such as a bit-flip or Hermitian operator, this equation is therefore incorrect.

;All chemistry equations
:<math>\mathrm{CH}_4 + \mathrm{OH} + \mathrm{HEAT} \rightarrow \mathrm{H}_2\mathrm{O} + \mathrm{CH}_2 + \mathrm{H}_2 \mathrm{EAT}</math>
A {{w|Chemical equation|chemical equation}} represents a chemical reaction as a formula, with the reactant entities on the left-hand side, and the product entities on the right-hand side. The number of each element on the left side must match those on the right side. The energy produced or absorbed in this process is not included in that formula.

This is a modification of the combustion of methane. The correct form is often taught and a good example problem but obviously there are more chemistry problems.<math>\mathrm{HEAT}</math> is normally shorthand for {{w|activation energy}}, but in Randall's version it's jokingly used as a chemical ingredient and becomes <math>\mathrm{H}_2\mathrm{EAT}</math>, taking the hydrogen atom freed by the combustion equation shown. The proper methane combustion equation would be: <math>\mathrm{CH}_4 + 2 \mathrm{O}_2 \rightarrow 2 \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2</math>

;All quantum gravity equations
:<math>\mathrm{SU}(2)\mathrm{U}(1) \times \mathrm{SU}(\mathrm{U}(2))</math>
This is more similar to expressions which appear in {{w|Grand_Unified_Theory|Grand Unified Theory}} (GUT) than general quantum gravity. Unlike some of the other equations, this one has no interpretation which could make it mathematically correct. This is similar to the notations used to describe the symmetry group of a particular phenomena in terms of mathematical {{w|Lie_Group|Lie Groups}}. A real example would be the Standard Model of particle physics which has symmetry according to <math>\rm{SU(3)\times SU(2) \times U(1)}</math>. Here, <math>\rm{SU}</math> and <math>\rm{U}</math> denote the special unitary and unitary groups respectively with the numbers indicating the dimension of the group. Loosely, the three terms correspond to the symmetries of the strong force, weak force and electromagnetism although the exact correspondence is muddied by symmetry breaking and the Higgs mechanism.

Of course, an expression missing an "=" sign, is difficult to interpret as an "equation", because equations normally express an "equality" of some kind. Nobody knows whether Randal refers to a horse, zebra, donkey or other equine here.

Randall's version clearly involves some similar groups although without the <math>\times</math> symbol it is hard to work out what might be happening. A term like <math>\rm{SU(U(2))}</math> has no current interpretation in mathematics, if anyone thinks otherwise and possibly has a solution to the quantum gravity problem they should probably get in touch with someone about that.

;All gauge theory equations
:[[File:All gauge theory equations.png]]
In physics, a {{w|Gauge theory|gauge theory}} is a type of field theory which is invariant to local transformations. The term gauge refers to any specific mathematical formalism to regulate redundant degrees of freedom.

This equation looks broadly similar to the sorts of things which appear in gauge theory such as the equations which define {{w|Yang–Mills_theory#Quantization|Yang-Mills Theory}}. By the time physics has got this far in, people have normally run out of regular symbols making a lot of the equations look very daunting. The actual equations in this field rarely go far beyond the Greek alphabet though and no-one has yet to try putting hats on brackets. The appearance of many sub- and superscripts is normal (this links to the group theory origins of these equations) and for the layperson it can be impossible to determine which additions are labels on the symbols and which are indices for an {{w|Einstein_notation|Einstein Sum}}.

The left-hand side <math>S_g</math> is the symbol for some {{w|Action_(physics)|action}}, in Yang-Mills theory this is actually used for a so-called "ghost action". On the right-hand side we have a large number of terms, most of which are hard to interpret without knowing Randall's thought processes (this is why real research papers should all label their equations thoroughly). The <math>\frac{1}{2\bar{\varepsilon}}</math> looks like a constant of proportionality which often appears in gauge theories. The factor of <math>i = \sqrt{-1}</math> is not unusual as many of these equations use complex numbers. The <math>\eth</math> symbol looks similar to a <math>\partial</math> partial derivative symbol especially as the {{w|Dirac_equation#Covariant_form_and_relativistic_invariance|Dirac Equation}} uses a slashed version as a convenient shorthand.

The rest of the equation cannot be mathematically correct as the choice of indices used does not match that on the left-hand side (which has none). In particle physics subscripts (or superscripts) of greek letters (usually <math>\mu</math> or <math>\nu</math>) indicate terms which transform nicely under Lorentz transformations (special relativity). Roman indices from the beginning of the alphabet relate to various gauge transformation propetries, the triple index seen on <math>p^{abc}_v</math> would likely come from some <math>\rm{SU(3)}</math> transformation (related to the strong nuclear force). Since <math>S_g</math> has none of these (and is thus a scalar which remains constant under these operations), we would need the right-hand side to behave in the same way. Most of the indices which appear are unpaired and so will not result in a scalar making the equation very wrong. For those not familiar with this type of equation, this is similar to the mistake of messing up units, for instance setting a distance equal to a mass.

;All cosmology equations
:<math>H(t) + \Omega + G \cdot \Lambda \, \dots \begin{cases} \dots > 0 & \text{(Hubble model)} \\ \dots = 0 & \text{(Flat sphere model)} \\ \dots < 0 & \text{(Bright dark matter model)} \end{cases}
</math>
This is a parody of equations defining the {{w|Hubble's_law#Derivation_of_the_Hubble_parameter|Hubble Parameter}} <math>H(t)</math> although it looks like Randall has become bored and not bothered to finish his equation. Such equations usually have several <math>\Omega</math> terms representing the contributions of different substances to the energy-density of the Universe (matter, radiation, dark energy etc.). In this context <math>G</math> could be Newton's constant and <math>\Lambda</math> is the cosmological constant (energy density of empty space) although seeing them appear multiplied and on the same footing as <math>H</math> is unusual (the dot is entirely unnecessary). Choosing to make <math>H</math> a function of time <math>t</math> and not of redshift <math>z</math> is also unusual.

The second section looks like the inequalities used to show how the equation varies with the shape of the Universe, based on the value of the curvature parameter <math>\Omega_k</math>. A value of 0 indicates a flat Universe (this is more or less what we observe) while a positive /negative value indicates an open /closed curved Universe. Randall's choice of labels further makes fun of the field as both a flat sphere and bright dark matter are oxymoronic terms which would involve some rather strange model universes.

;All truly deep physics equations
:[[File:All truly deep physics equations.png]]
<math>\hat H</math> is the Hamiltonian operator, which when applied to a system returns the total energy. In this context, U would usually be the potential energy. However, there is also a subscript 0 and a diacritic marking indicating some other variable. Much of physics is based on Lagrangian and Hamiltonian mechanics. The Lagrangian is defined as <math>\hat L = \hat K - \hat U </math> with K being the kinetic energy and U the potential. Hamiltonian mechanics uses the equation <math>\hat H = \hat K + \hat U </math>. The Hamiltonian must be conserved so taking the time derivative and setting it equal to zero is a powerful tool. The "principle of least action" allows most modern physics to be derived by setting the time derivative of the Lagrangian to zero.

==Transcript==
:[Nine equations are listed, three in the top row and two in each of the next three rows. Below each equation there are labels:]

:E = K0t + 1/2 ρvt2
:All kinematics equations

:Kn = ∑i=0∞∑π=0∞(n-π)(i-eπ-∞)
:All number theory equations

:∂/∂t ∇ ⋅ ρ = 8/23 (∯ ρ ds dt ⋅ ρ ∂/∂∇)
:All fluid dynamics equations

:|ψx,y〉 = A(ψ) A(|x〉⊗ |y〉)
:All quantum mechanics equations

:CH4 + OH + HEAT → H2O + CH2 + H2EAT
:All chemistry equations

:SU(2)U(1) × SU(U(2))
:All quantum gravity equations

:Sg = (-1)/(2ε̄) i ð (̂ ξ0 +̊ pε ρvabc η0 )̂ f̵a0 λ(ʒ̆) ψ(0a)
:All gauge theory equations

:[There is a brace linking the three cases together.]
:H(t) + Ω + G⋅Λ ...
:... > 0 (Hubble model)
:... = 0 (Flat sphere model)
:... < 0 (Bright dark matter model)
:All cosmology equations

:Ĥ - u̧0 = 0
:All truly deep physics equations

{{comic discussion}}

[[Category:Science]]
[[Category:Physics]]
[[Category:Math]]
[[Category:Chemistry]]
[[Category:Astronomy]]

Talk:2034: Equations

2018-08-18T07:24:12Z

108.162.241.166: Fluid dynamics eqn.

Is the joke that all of the equations are actually wrong/malformed/meaningless but they sort of look like typical equations for that field? {{unsigned ip|172.68.133.66}}
:Sort of. A bit of dimensional analysis would have helped. ;-) --[[Special:Contributions/162.158.91.221|162.158.91.221]] 07:28, 17 August 2018 (UTC)

He's nerd sniping us all.. ([[Special:Contributions/162.158.167.120|162.158.167.120]] 03:30, 18 August 2018 (UTC))

Should we add a column with examples of similar correct equations from the respective fields? Sebastian --[[Special:Contributions/172.68.110.4|172.68.110.4]] 09:33, 17 August 2018 (UTC)
:That would definitely tidy up my attempts to provide context for Randall's versions. The challenge then is working in explanations for the correct equations as well as arguing over which examples should be used. [[User:Exxi|Exxi]] ([[User talk:Exxi|talk]]) 09:45, 17 August 2018 (UTC)

I don't think the part in parentheses about OH in the Chemistry equation explanation is correct. OH- would mean that it's negatively charged and has nothing to do with unpaired electrons of Oxygen. It would add another horror to the equation, though, as it wouldn't be charge preserving anymore. [[Special:Contributions/162.158.88.230|162.158.88.230]] 09:58, 17 August 2018 (UTC)

"Redshit". Best typo ever. Please keep it. [[Special:Contributions/172.69.54.177|172.69.54.177]] 10:13, 17 August 2018 (UTC)

;Deep physics equations
The transcript is wrong here, the last letter is not a <math>\mu</math>, but a "u" with a cedilla: u̧. The math parser refuses to render it, though. [[Special:Contributions/162.158.88.230|162.158.88.230]] 05:54, 17 August 2018 (UTC)
:Looks like it. But I don't think that letter exists even. --[[Special:Contributions/162.158.91.221|162.158.91.221]] 07:28, 17 August 2018 (UTC)
:Is this equation a sort of nod to a Theory Of Everything which unifies quantum mechanics and gravity... H-hat (a Hamiltonian, which in quantum mechanics describes the total energy of a system, and usually runs in to problems describing large systems - such as the entire universe - where gravity or spacetime curvature effects matter) *minus* u0 (the relativistic mass of the whole system at time zero ie. the big bang) gives 0 (no energy everywhere always). Since mass is energy (e=mc^2) and mass is also the sole cause of gravity the two theories cleanly collapse together when mass is zero, and figuring out how to extend the theory to other less clean points on the mass axis is obviously a job for less profound physics? I've no ideas to explain the cedilla. [[Special:Contributions/141.101.98.28|141.101.98.28]] 08:49, 17 August 2018 (UTC)
:It looks to me a little like a parody of the {{w|Wheeler-DeWitt_equation#Hamiltonian_constraint|Wheeler-DeWitt equation}} which (in theory) describes a wavefunction for the entire Universe. [[User:Exxi|Exxi]] ([[User talk:Exxi|talk]]) 09:06, 17 August 2018 (UTC)一
: I'm just thrilled someone found the right character for it. I spent 20 minutes looking for the right u symbol without any luck at all. {{unsigned ip|172.68.143.132}}
Is this poking fun at equation-filled blackboards in movies and cartoons? {{unsigned ip|172.68.254.42}}
:Doesn't seem like it. These equations actually do look like the kinds of equations you would see in these fields. On blackboards in movies you tend to get equations that are pure nonsense. {{unsigned ip|172.68.143.132}}
I think this may also be a reference to Feynman's unworldliness equation, http://www.feynmanlectures.caltech.edu/II_25.html#Ch25-S6 . [[Special:Contributions/108.162.219.220|108.162.219.220]] 17:02, 17 August 2018 (UTC)

;Table layout at the explanation
That oversized table is really bad layout. We've had this discussion many times before - tables should only be used for small contents. Right now I would run into too many edit conflicts but I'll change it to a proper floating text with small headers for each section. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 11:51, 17 August 2018 (UTC)
:Done, looks much more like a real paper... --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 12:58, 17 August 2018 (UTC)

;All number theory equation
The explanation for math doesn't seem entirely correct. You can in fact extend the ring of integers (as well as rational and real numbers) with positive and negative infinity, but it won't be a ring anymore. Specifically, the infinities don't have an additive or multiplicative inverse (but 1/infinity = 0); and addition of positive and negative infinity, as well as the product of 0 and either infinity is undefined. However, these properties are not used in the above equation. What we ''can'' use is that <math>\forall n < \infty: n - \infty= -\infty </math>. We would thus have <math>K_n = \sum_{i=0}^{\infty}\sum_{\pi=0}^{\infty}(n-\pi)(i-e^{\pi-\infty}) = \sum_{i=0}^{\infty}\sum_{\pi=0}^{\infty}(n-\pi)(i-0) = \sum_{i=0}^{\infty}\sum_{\pi=0}^{\infty}(n-\pi)i= \sum_{i=0}^{\infty}i\sum_{\pi=n}^{\infty}-\pi= \sum_{i=0}^{\infty}i\cdot(-\infty)=-\infty</math>. Also, how often does one use e and pi in number theory? --[[User:Ycthiognass|Ycthiognass]] ([[User talk:Ycthiognass|talk]]) 12:11, 17 August 2018 (UTC)
:Pi (or any other number) minus infinite is just absurd. You can use the infinite symbol only as a limit but NOT as number in calculations. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 12:33, 17 August 2018 (UTC)
::It is not absurd. Adding the rules <math>n+\infty=\infty\text{ for }n>-\infty,n-\infty=-\infty\text{ for }n<\infty, \pm n\cdot\infty = \pm\infty\text{ for }n>0, \pm n\cdot(-\infty) = \mp\infty\text{ for }n>0,\frac1{\pm\infty}=0</math> gives you a consistent theory that is especially useful when talking about infinite sums and integrals. Would you say the term <math>n-\sum_{i=1}^\infty i</math> is absurd? --[[User:Ycthiognass|Ycthiognass]] ([[User talk:Ycthiognass|talk]]) 14:35, 17 August 2018 (UTC)
:::Of course it's absurd. It is <math>\infty-\infty \neq 0</math> because it could be everything between <math>\infty</math> and <math>-\infty</math>. {{w|Infinity}} is a concept describing something without any bound... And, as you can't divide by zero you can't do the same for infinity. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 17:24, 17 August 2018 (UTC)
:::One more: It is
::::<math>\sum_{i=1}^\infty a_i = \lim_{n\to\infty} \sum_{i=1}^n a_i.</math>
:::When this limit exists, one says that the series is ''convergent'' or ''summable''. Otherwise it's called ''divergent'' and has no solution like this one:
::::<math>\sum_{i=1}^\infty i</math>
:::Infinite is NO number! --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 17:33, 17 August 2018 (UTC)

Completely separate from the above, it's probably worth noting that i is also a constant, and as such has the same misconception as <math>\pi</math>. Computer scientists are happy using i for loops/summations, but mathematicians prefer using n. Based off that, it's probably another misconception/joke that n is treated as a constant, while known-constants are used as variables. [[Special:Contributions/108.162.246.149|108.162.246.149]] 17:28, 17 August 2018 (UTC)
:There is nothing non-standard about using i as an index variable. Often as part of the series i,j,k. Searching for summation convention will give plenty of examples.
:There are fewer letters than mathematical concepts in need of letters, so most letters are used for multiple purposes. Occasionally this causes difficulty. You can be halfway through a linear algebra problem before you discover you need i for an imaginary number despite already using it as an index. Hilarity ensues. [[Special:Contributions/162.158.74.105|162.158.74.105]] 19:57, 17 August 2018 (UTC)

;Chemistry equation
OH should have a charge symbol: OH-. The actual reaction would be:

CH4 + OH- + heat -> CH3- + H2O

The methyl group can dissolve in water, and this is presumably happening in water, so this equation can work, just not the one provided by Randell. Reacting longer alkanes with bases is a way to make soaps, but the methyl group would be too reactive to be used this way. [[User:Nutster|Nutster]] ([[User talk:Nutster|talk]]) 13:13, 17 August 2018 (UTC)

;Fluid Dynamics equation
I believe the fraction 8/23 in the Fluid Dynamics equation is a Randallesque reference to the fractional approximation of pi = 22/7. It's probably not a coincidence that you get 8/23 from 22/7 if you invert it and add 1 to both the numerator and denominator. [[User:Ianrbibtitlht|Ianrbibtitlht]] ([[User talk:Ianrbibtitlht|talk]]) 15:19, 17 August 2018 (UTC)

:I think that is a bit of a stretch. If Randall wanted to reference the 22/7 approximation, I think he would simply use 22/7 and not 8/23. [[User:Redbelly98|Redbelly98]] ([[User talk:Redbelly98|talk]]) 00:40, 18 August 2018 (UTC)

::It might be a stretch, but maybe Randall wanted to be more clever than just inserting 22/7, since pi really has no place in that equation. I don't see anyone else suggesting any reasonable source for 8/23 in the equation. The current explanation is an even bigger stretch, since it has nothing in common with 8/23 beyond being just another fraction - it seems to suggest he picked two random numbers, 8 and 23, for the fraction! How unsatisfying! But if nobody else agrees, I'm not losing any sleep over it. (Sometimes I wish Randall would chime in to clear things like this up for us. Randall, where are you?) [[User:Ianrbibtitlht|Ianrbibtitlht]] ([[User talk:Ianrbibtitlht|talk]]) 04:54, 18 August 2018 (UTC)

:Fluid dynamicist here -- strange looking numbers and fractions come from multiplying tensors. 2/3 is a common one, but you also get numbers like 1/7 and 8/27.[[Special:Contributions/108.162.241.166|108.162.241.166]] 07:24, 18 August 2018 (UTC)

I changed a "p" to a Greek "rho". [[User:Redbelly98|Redbelly98]] ([[User talk:Redbelly98|talk]]) 00:40, 18 August 2018 (UTC)

;Gauge theory equation
<s>I think the transcript is missing a left superscript 0 before the turned xi. [[Special:Contributions/172.68.226.16|172.68.226.16]] 16:50, 17 August 2018 (UTC)</s> Ah no, sorry. False alarm. It's just that Randall writes the xi with a funny tail. The same tail is on the non-turned xi earlier. [[Special:Contributions/172.68.226.10|172.68.226.10]] 16:52, 17 August 2018 (UTC)

If explainxkcd.com is to make XKCD comics more understandable then this explanation is failing that. I assumed from the beginning that the joke was about the equations being wrong, but the description of the joke is making my head hurt. {{unsigned ip|162.158.106.216}}
:Read the first paragraph: "To someone who knows even a little about the topic, they are clearly very wrong and only seem even worse the more you look at them." Nevertheless the rest sometimes does hurt. See below. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 20:28, 17 August 2018 (UTC)

;All truly deep physics equations
In the description paragraph, the last sentence starting "The principle of least action says allows..." does not scan. If someone can fix this (copy&paste?) error, please delete this comment. [[Special:Contributions/162.158.58.171|162.158.58.171]] 19:33, 17 August 2018 (UTC)
:I edited the sentence slightly to address this issue. [[User:Ianrbibtitlht|Ianrbibtitlht]] ([[User talk:Ianrbibtitlht|talk]]) 20:51, 17 August 2018 (UTC)

It looks like there is already a section for this above, "Deep physics equations", or am I missing something? [[User:Redbelly98|Redbelly98]] ([[User talk:Redbelly98|talk]]) 00:40, 18 August 2018 (UTC)

Note that this comics also emphasizes that Randall is more familiar with physics than with chemistry : while most of the equations here require college-level education to grok, the chemistry one is at the very most high-scool-grade. {{unsigned ip|141.101.69.33}}

;Explanations in general
“Nobody knows if Randall references a horse here” - what?! Because the expression lacks an equal sign; doesn’t represent an equality, it might mean Randall is referencing equines, aka horses?! Is this vandalism, an attempt at a joke, or what? This explanation clearly still needs quite a bit of work! [[User:PotatoGod|PotatoGod]] ([[User talk:PotatoGod|talk]]) 20:14, 17 August 2018 (UTC)
:I've put a header on top here. It's not vandalism but every explanation looks still highly unscientific. I've gave real sources to the most topics at the beginning but the following explanations are mostly bad. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 20:22, 17 August 2018 (UTC)

;Kinematics equations
I fixed an error: Randall's Greek "rho" ''ρ'', a common symbol for mass density, was incorrectly shown here as ''p'', the common symbol for momentum. The term with the ''ρ'' is very similar to a term in the Bernoulli equation, and I have changed the explanation to reflect this. [[User:Redbelly98|Redbelly98]] ([[User talk:Redbelly98|talk]]) 00:40, 18 August 2018 (UTC)

1567: Kitchen Tips

2017-08-02T22:52:51Z

108.162.241.166: /* Explanation */

{{comic
| number = 1567
| date = August 21, 2015
| title = Kitchen Tips
| image = kitchen_tips.png
| titletext = Household tip: Tired of buying so much toilet paper? Try unspooling the paper from the roll before using it. A single roll can last for multiple days that way, and it's much easier on your plumbing.
}}

==Explanation==
In this comic, [[Cueball]] appears to be hosting a show (or be in an ad) giving out kitchen advice. He starts with a reasonable tip to use a meat thermometer instead of guessing when meat is cooked. His later tips, though, are little more than telling how to complete normal kitchen activities performed using common sense. Moreover, in most cases he repeats "If you're anything like me," suggesting he's actually ''done'' these things in his kitchen. This is a parody of many commercials and infomercials that {{tvtropes|TooIncompetentToOperateABlanket|imply their consumers have no basic motor skills or common sense}} in order to make their product more appealing.

The first tip he gives is reasonable because, though the use of a meat thermometer is fairly well known, not everybody goes to the trouble of using one. To determine if meat is done cooking, one can either guess or use a meat thermometer to check that the internal temperature has reached the correct level to render meat safe for consumption. Many people don't own a meat thermometer and rely on an alternative solution that doesn't require special equipment (such as testing by feel, cutting the meat open to check its doneness, checking the color of the juices after pricking the meat with skewer, or simply guessing).

The second panel shows that Cueball throws away dishes and buys new ones every time they are used. This is perfectly normal if the plates are disposable plates made of paper or Styrofoam, but we see his trashcan is filled with chipped glasses and ceramic plates. Naturally, this would be a very expensive practice. The virtually universal chore of "washing the dishes," is one Cueball presumes the audience is heretofore unaware of.

Cooking on a stove is typically done placing the food into a pot or pan which is placed on the burner. Cueball seems to suggest that the use of a pan is a tip most people would be unaware of, suggesting that most people cook eggs directly on the burners themselves, a method that is likely to burn the food and create a great mess. Cueball's stove has T-shape raised burners (probably gas, but might be electric), making the task very impractical, though owners of glass-top electric stoves could conceivably cook directly on the glass surface.

Ice is usually made by filling an ice cube tray with water and leaving it in a freezer for several hours. Cueball, however, sprays a hose directly into his freezer compartment and quickly slams the door shut to trap some water inside. (This would work somewhat better in the type of freezer that has a door on the top, so it could be filled with water and the door would not need to be closed to trap the water inside.) While this unorthodox method ''will'' make ice, it will result in a large sheet of ice on the bottom of the freezer. More importantly, it will also make it impossible to actually use the freezer to hold anything else (unless you were to put anything in beforehand and you don't mind breaking through a block of ice to get it out). Also, ice expands as it cools (it is one of the few substances with a negative coefficient of thermal expansion), and its expansion might push the freezer door open.

The title text, a '''household tip''', suggests using toilet paper a few sheets at a time, which is how most people use it. Cueball, however, seems to suggest that most people use the entire roll as a single object without unspooling it and then flushing it whole, using at least one roll each time they use the bathroom. This is economically impractical, and is prone to clogging the toilet and the plumbing if you throw the toilet paper away by putting it into the toilet and flush it.

For more '''household tips''' like the one in the title text, see the sequel to this comic: [[1715: Household Tips]].

''Kitchen tips'' are yet another type of [[:Category:Tips|tips comic]].

==Transcript==
:[Cueball at a kitchen counter, holding a meat thermometer.]
:Cueball: If you're anything like me, you may have trouble telling when meat is fully cooked.
:Cueball: Instead of guessing, try a meat thermometer!

:[Cueball at a sink, holding a dirty dish, with a trashcan next to him full of broken ceramics and glasses.]
:Cueball: If you're anything like me, you probably throw away your plates and glasses when they get dirty. But if you clean them, they can often be used again!

:[Cueball cracking an egg over a pan on a hot stove.]
:Cueball: Making scrambled eggs? Put a pan under them!
:Cueball: It's easier, and it keeps your burners clean.

:[Cueball holding a garden hose, spraying it into the freezer compartment of a freezer.]
:Cueball: If you're anything like me, you make ice by spraying a hose into your freezer and then slamming it shut.
:Cueball: But there's a better way...

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Tips]]
[[Category:Food]]

Talk:572: Together

2015-07-24T19:17:24Z

108.162.241.166:

Definitely one of the sweetest xkcd strips ever. [[User:Alpha|Alpha]] ([[User talk:Alpha|talk]]) 07:30, 1 March 2013 (UTC)
:Or one of the most sad - as he just maried for a game and left when he was finished with his project... [[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 00:41, 7 April 2014 (UTC)
U
Could "building a treehouse" refer to an adult activity that is frequently postponed and possibly never finished, like items in a to do list? [[Special:Contributions/108.162.212.196|108.162.212.196]] 17:34, 1 January 2014 (UTC)
:No, those are shark teeth. [[User:Weatherlawyer| I used Google News BEFORE it was clickbait]] ([[User talk:Weatherlawyer|talk]]) 04:04, 30 January 2015 (UTC)

I feel maybe the wife was concerned that he was wandering off, and wants him to come back. [[Special:Contributions/108.162.241.166|108.162.241.166]] 19:17, 24 July 2015 (UTC)