explain xkcd - User contributions [en]

Talk:3068: Rock Identification

2025-03-28T21:59:52Z

172.71.146.78:

I made a transcript [[User:New editor|New editor]] ([[User talk:New editor|talk]]) 21:03, 26 March 2025 (UTC)

It might also imply that the geologist doesn’t know but just wants the $5 so comes up with an answer [[Special:Contributions/198.41.236.163|198.41.236.163]] 00:01, 27 March 2025 (UTC)

I doubt they're lying about what rock it is since both mica schists and garnets are visually obvious and even more obvious if a scratch test is performed.[[Special:Contributions/172.69.186.157|172.69.186.157]] 04:36, 27 March 2025 (UTC)

The funny part is it works the other way round: you first identify minerals by sight (also Mohs scale and polarizing microscope), then conclude what the geologic context is. [[Special:Contributions/172.71.232.25|172.71.232.25]] 11:01, 27 March 2025 (UTC)

I think this random "it was wrapped in the bill, no idea where the money came from" also implies corruption and destroying the traces of the money, instead of just payment or tip. --[[User:Lupo|Lupo]] ([[User talk:Lupo|talk]]) 11:29, 27 March 2025 (UTC)

Unfortunate missed chance to call this comic a “Geologist Tip” [[Special:Contributions/172.70.176.56|172.70.176.56]] 13:04, 27 March 2025 (UTC)

Surely the joke is that it only takes $5 to bribe a Geologist.
:Hey, if you want to pay me $5 to pay attention to you for a minute, I would take the deal. That's a great hourly rate! Though I guess my perspective might be skewed, since I'm not a professional scientist but just a (somewhat broke) grad student[[User:Dextrous Fred|Dextrous Fred]] ([[User talk:Dextrous Fred|talk]]) 17:38, 27 March 2025 (UTC)

wonder if the comic is referring to the (somewhat popular) whatisthisrock subreddit --172.68.210.176 19:29, 27 March 2025 (UTC)

Whoever entered "If you're good at something, never do it for free" should be congratulated for doing quality voluntary work [[User:Kev|Kev]] ([[User talk:Kev|talk]]) 22:27, 27 March 2025 (UTC).

"the $5 bill would be covered with dirt and no longer be usable." - technically, as long as 50% or more of the bill is left, it's still legal tender. if it's less than 50% you can [https://www.bep.gov/services/mutilated-currency-redemption / ask the US government to exchange it] for intact money. --[[Special:Contributions/172.71.146.79|172.71.146.79]] 21:59, 28 March 2025 (UTC)

1606: Five-Day Forecast

2025-02-24T02:27:54Z

172.71.146.78: Reverting - The main comic image was replaced with sick emoji - Undo revision 366561 by 162.158.178.74 (talk)

{{comic
| number = 1606
| date = November 20, 2015
| title = Five-Day Forecast
| image = five_day_forecast.png
| titletext = You know what they say--if you don't like the weather here in the Solar System, just wait five billion years.
}}

==Explanation==
{{w|Weather forecasting}} is an extremely difficult task, even if it is only for five days. In numerical models, extremely small errors in initial values double roughly every five days for variables such as temperature and wind velocity. So most {{w|Meteorology#Meteorologists|meteorologists}} provide us with only a five-day forecast.

In this comic [[Randall]] takes this to the extreme by first showing a '''Five-Day Forecast''' and then progressing to five-month, year, million, billion and finally trillion-year forecasts, leading to {{tvtropes|WeirdWeather|weather patterns that we don't regularly see.}}

Since the first weather symbol is the same in all six rows, we can assume it indicates the weather today and not tomorrow, in a trillion years, etc. It is only in the second panel of each row that time has passed per the row's label. Consequently, the last column gives the predictions for four days, four months, ..., four trillion years from today.

When moving past the five-day prediction, the forecast is just a qualified guess based on the time of year. In a month it is Christmas as shown in the second panel of the second row. Then it is January and February so snow is likely, but certainly not something that happens on all days of a winter month.

Looking at the five-year forecast, guesses are made as to what the weather will be like at the same time of year. For these first three predictions the weather symbols are all of the same three types: Sun, clouds and some kind of {{w|precipitation}}, rain or snow, with the temperature ranging from 21 to 44 °F (-6.1 to 6.6 °C) - winter temperatures.

Then we go into the far future, jumping a million years from panel to panel. But still the weather symbols stay the same. In 3 million years, however, aliens (or advanced humans) attack with energy beams from {{w|flying saucers}}. They are gone a million years later. The temperature range remains the same across the panels except that it rises to 52 °F (11 °C), a possible reference to global warming, in one panel, and while the attack is going on it rises to 275 °F (135 °C).

Once we get to the billion-year mark it actually becomes more meaningful to try to predict the "weather", because now we reach the times when the {{w|Sun}} begins to change. Although the Sun will continue to burn hydrogen for about 5 billion years yet (while in its {{w|Sun#Main sequence|main sequence|}}), it will grow in diameter as it begins to exhaust its supply of fuel. The core will contract to increase the temperature, and the outer layer will then compensate by expanding slightly. This is what is indicated in panels two and three, where the color of the Sun changes towards red as the surface becomes cooler as it expands away from the center of the Sun. The temperature will rise on Earth as indicated in the panels (105 °F = 40.5 °C and 371 °F = 188 °C). The temperature will get hot enough in about [http://phys.org/news/2015-02-sun-wont-die-billion-years.html a billion years] that the Earth's oceans will boil away.

Once it {{w|Sun#After core hydrogen exhaustion|no longer has enough hydrogen}}, the Sun will expand into a {{w|red giant}}. This should not happen until around {{w|Sun#Composition|five billion years from now}}, but in the forecast it is indicated to happen in only three. Maybe this is Randall taking liberties to show what happens during this phase, which would not fit into a four-billion-year forecast. Alternatively it just indicates how uncertain these kinds of forecasts are, or a statement that we may not know for certain that it will take five not three billion years.

In any case, the fourth panel shows the temperature at Earth's position inside the red giant Sun. The color of the panel indicates that we are inside the Sun. The temperature is 71,488,106 degrees Fahrenheit (39,715,597 degrees Celsius). The current temperature of the center of the Sun is "only" 27 million degrees Fahrenheit (15 million degrees Celsius), and although that may rise by a factor of ten during {{w|Stellar nucleosynthesis|helium fusion}}, that will only be at the very core and not out in the solar atmosphere reaching out to Earth. Here the temperature would only be of the order of thousands of degrees Fahrenheit, since the Sun's outer temperature decreases as it increases its diameter. So this panel's temperature also makes little sense. It may involve some ambiguities regarding what the forecast means; the edge of the red giant Sun is predicted to be somewhere near the current orbit of Earth, but the position of the Earth could change. The most likely prediction at the moment is for Earth to move outward, but if the planet is engulfed by the Sun, it would spiral inward, and at some point fall apart. So in some sense "here" for the forecast could become a position deep inside the Sun, where core temperatures could reach 100 million Kelvin. The temperatures shown are unreasonably precise; they probably should have only two or at most three significant figures.

The red giant phase lasts only half a million years, so a billion years after the Sun has been a red giant its outer atmosphere will definitely have disappeared, leaving only a dim, cool {{w|white dwarf}} to cool down. Given Randall's version of this time schedule, then it will have had about a billion years to cool down, but would still likely be the brightest object in the sky as seen from where the Earth once was. It is not shown in the last panel, where we just see other stars of the Galaxy. The temperature is down to that of the {{w|Cosmic microwave background|background radiation}}. Today this radiation has a temperature of 2.72548 kelvin = -270.4245 °C = -454.7641 °F. That is a few degrees F colder than what is shown in the comic, which states the temperature is -452 °F = 4.26 kelvin. This higher temperature may have been chosen to reflect that even the light from other stars would increase the actual temperature.

In the last panel with trillions of years, we jump right past the Sun's red giant phase to a panel looking much like the one after five billion years with only other stars. Over the next three trillion years the stars become fewer and fewer and dimmer and dimmer as they run out of fuel and fewer new stars form. After four trillion years the background temperature decreases one degree to -453 °F as the universe keeps expanding and the wavelength of the radiation does the same, thus decreasing its temperature.

The title text is a play on comments referring to fast-changing weather on a more ordinary human timescale, such as Mark Twain's quip, "If you don't like the weather in New England now, just wait a few minutes."

A ten-day forecast was used in [[1245: 10-Day Forecast]]. In [[1379: 4.5 Degrees]], Randall looked at the weather over long periods of time as well. in [[1643: Degrees]] he addressed Celsius vs. Fahrenheit for measuring temperature.

===Image using Celsius===

There is a different user-made version for the picture, using [[3001|Celsius]] instead of Fahrenheit, [[:File:five_day_forecast_Celsius.png|in this image link]].

==Transcript==
:[A grid with six rows of five columns, where each row is labeled to the left. For each of the 30 squares a temperature is given in Fahrenheit at the top left. The rest of the square represents the weather as in a weather forecast (or some other relevant items for the comic), mainly in bright colors. Below are the six labels given above each of their five weather symbols with temperature given below these symbols description.]

:'''Your 5-day forecast'''
:[A bright yellow sun.]
:38°F
:[A grey cloud.]
:41°F
:[A grey cloud with six lines of blue raindrops below.]
:36°F
:[A grey cloud in front of a yellow sun.]
:40°F
:[A bright yellow sun.]
:44°F

:'''Your 5-month forecast'''
:[A bright yellow sun.]
:38°F
:[A green Christmas tree with red presents beneath it.]
:29°F
:[A grey cloud with four snowflakes below.]
:21°F
:[A grey cloud with four snowflakes below.]
:24°F
:[A grey cloud.]
:35°F

:'''Your 5-year forecast'''
:[A bright yellow sun.]
:38°F
:[A grey cloud.]
:25°F
:[A bright yellow sun.]
:36°F
:[A grey cloud with six lines of blue raindrops below.]
:37°F
:[A bright yellow sun.]
:41°F

:'''Your 5-million-year forecast'''
:[A bright yellow sun.]
:38°F
:[A bright yellow sun.]
:52°F
:[A grey cloud.]
:40°F
:[Two red flying saucers (with bright domes) are shooting energy beams downwards. One of the beams seems to impact with something at the bottom of the panel, which then explodes. Two plumes of smoke rises up from below, drifting to the right.]
:275°F
:[A grey cloud in front of a yellow sun.]
:40°F

:'''Your 5-billion-year forecast'''
:[A bright yellow sun.]
:38°F
:[A larger orange sun.]
:105°F
:[A very large red sun.]
:371°F
:[A pale yellow panel with no drawing.]
:71,488,106°F
:[A night sky with many bright stars.]
:-452°F

:'''Your 5-trillion-year forecast'''
:[A bright yellow sun.]
:38°F
:[A night sky with many bright stars.]
:-452°F
:[A night sky with many stars.]
:-452°F
:[A night sky with fewer not so bright stars.]
:-452°F
:[A night sky with few dim stars.]
:-453°F

{{comic discussion}}

[[Category:Comics with inverted brightness]]
[[Category:Comics with color]]
[[Category:Science]]
[[Category:Space]]
[[Category:Astronomy]]
[[Category:Weather]]
[[Category:Aliens]]

Talk:3048: Suspension Bridge

2025-02-10T15:43:45Z

172.71.146.78:

What if a car doing one of the fun jumps lands on a car that didn't do a fun jump? Should we mandate that every car does a fun jump for this reason (and yay, fun jump!)? [[Special:Contributions/198.41.227.115|198.41.227.115]] 22:08, 7 February 2025 (UTC)
:Radar speed management, vehicles are only let on the bridge if their expected land point is between the vehicles around them. [[Special:Contributions/172.68.55.80|172.68.55.80]] 22:48, 7 February 2025 (UTC)
::Carrot/stick question. If they do a jump, they get their toll refunded on the far side of the bridge. Maybe give them double refund if they do a flip. [[Special:Contributions/172.69.246.135|172.69.246.135]] 04:07, 8 February 2025 (UTC)
:::The same would happen as on a normal road. If you drive faster than the car in front of you and do not break you will run into it. Only difference here is that you cannot break after jumping so if the one in front of you did make a small jump and then hit the brakes you could hit him. But if you drive very fast up towards a car on a normal road, and they hit the brakes, you will likely also hit them. Even though you do have a chance to brake as opposed to after a jump. So I do not see this as any different than a regular road. Only problem is someone will find it so fun to drive fast towards the top, they might forget to think about safety. But that is already a real problem on normal roads... You cannot stop someone from entering the bridge based on their speed before thet enter the bridge... So the radar comment makes no sense to me!? --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 08:20, 9 February 2025 (UTC)

Do suspension bridges still work when built this way? Those pilings look much easier to sway to me. Maybe they are just buried a little more securely. [[Special:Contributions/172.68.55.80|172.68.55.80]] 22:48, 7 February 2025 (UTC)
:Yep! With "classic" suspension bridges (like the Golden Gate Bridge), there is no fixed connection between the deck and the towers. With differring loads and temperatures the deck can move up and down about 30cm at tower level (and almost 2 netres at midpoint). Some modern bridges do ise a fixed connection or hinge between deck and tower. [[User:IIVQ|IIVQ]] ([[User talk:IIVQ|talk]]) 12:58, 8 February 2025 (UTC)

This looks a lot like a {{w|stressed ribbon bridge}}. [[Special:Contributions/172.69.64.132|172.69.64.132]] 22:59, 7 February 2025 (UTC)

This ''exact'' proposal, the [https://poly-bridge-2.fandom.com/wiki/Dangling_Road 'dangling road'], is actually The Meta in Poly Bridge 2. Poly Bridge 3 tried to tweak the numbers to make this strat less effective, but a variant of it continues to persist to this day. (This probably goes in the trivia section, because it seems like Randall came to this concept from first principles.) [[Special:Contributions/172.71.151.65|172.71.151.65]] 01:07, 8 February 2025 (UTC)

:I came to the comment section to see if somebody already said that. The other thing I would have said is I wonder if RCE reads XKCD… if not, I think someone should ask him to do a blind reaction to this comic. I'd love to see his response :) -- [[User:Angel|Angel]] ([[User talk:Angel|talk]]) 19:18, 8 February 2025 (UTC)

It's funny to me, there's a level in Jak II for the PS2 that's literally just this idea: for whatever reason the city's central palace is held up by five suspension bridge-esque wires, and one level has you crawling along wire #4 to get inside the palace (the catch? The wires are covered in security robots) [[Special:Contributions/172.71.159.7|172.71.159.7]] 01:54, 8 February 2025 (UTC)

I like the fact that vehicles are clearly on flexible cables, not some rigid material formed into a catenary shape. There's a truck between the two towers that is visibly depressing the cable it's riding. [[User:Nitpicking|Nitpicking]] ([[User talk:Nitpicking|talk]]) 03:57, 8 February 2025 (UTC)
:Happy happy joy joy. {{w|Tacoma_Narrows_Bridge_(1940)|No dogs allowed on this bridge}}. [[Special:Contributions/162.158.42.87|162.158.42.87]] 04:35, 8 February 2025 (UTC)

If you add a downward pressure to a rope that's significantly stronger than it's normal tension, you end up with a [https://en.wikipedia.org/wiki/Catenary#Suspension_bridge_curve parabola] [[Special:Contributions/162.158.137.212|162.158.137.212]] 17:47, 8 February 2025 (UTC)

Related: The Wooden bridge Tatzlwurm" in Essing Germany by Richard Johann Dietrich: [https://bsky.app/profile/jamonir.bsky.social/post/3lddc7rae4s24]. Maybe it could be included in the explanation. [[User:Rps|Rps]] ([[User talk:Rps|talk]]) 20:46, 8 February 2025 (UTC)
:Not sure I think this is really related. It is also possible as opposed to Randall's proposal here ;-) --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 08:20, 9 February 2025 (UTC)

It would seem that Randall has re-invented the https://en.wikipedia.org/wiki/Simple_suspension_bridge {{unsigned ip|172.68.54.207|15:17, 9 February 2025}}
:More so the mostly 'simple'-derived {{w|Stressed ribbon bridge}}. Unless you consider that development more a version of the non-simple type, with the descenders degenerated completely into the deck structure through high tension, i.e. a step beyond the "structure hiding" levels of other {{w|Millennium Bridge, London|low-profioe suspension bridges}}. [[Special:Contributions/172.71.178.78|172.71.178.78]] 15:37, 9 February 2025 (UTC)

I seem to recall a bridge on the overseas highway (Florida Keys) that was originally an arch bridge above a grade-level rail line. When they wanted to "upgrade" the route for cars, cars wouldn't fit into the narrow rail bridge, so they planked up and over the arches, and it was apparently quite a scary drive. I can't seem to locate it via google, although I'm sure I recall seeing it on my drive to key west about 20 years ago. I suspect it's gone now. [[User:RandalSchwartz|RandalSchwartz]] ([[User talk:RandalSchwartz|talk]]) 20:34, 9 February 2025 (UTC)
:Ahh, it was the Bahia Honda Railroad Bridge. A few pictures of the structure (mostly torn down now) can be seen [https://www.abandonedspaces.com/uncategorized/bahia-honda-rail-bridge.html here]. [[User:RandalSchwartz|RandalSchwartz]] ([[User talk:RandalSchwartz|talk]]) 22:55, 9 February 2025 (UTC)
:: Find it on Google Earth at 24°39.3142'N 81°17.5182'W. Mostly still there as of image date (2023, if I read correctly). [[Special:Contributions/172.71.146.78|172.71.146.78]] 15:43, 10 February 2025 (UTC)