explain xkcd - User contributions [en]

2932: Driving PSA

2024-05-17T09:00:49Z

Fod: /* Alternate interpretation of the waving gesture */ + Denmark

{{comic
| number = 2932
| date = May 13, 2024
| title = Driving PSA
| image = driving_psa_2x.png
| imagesize = 414x538px
| noexpand = true
| titletext = This PSA brought to you by several would-be assassins who tried to wave me in front of speeding cars in the last month and who will have to try harder next time.
}}

==Explanation==
{{incomplete|Created by a CLUELESS BOT DRIVING AT 72.42048 km/h (20.1168 m/s) (outpaced by raptor) - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

A PSA is a {{w|Public Service Announcement}}. Some drivers, when having {{w|Traffic#Passage_priority_(right_of_way)|priority}} by the rules of the road (termed "right of way" in US legal statutes), will let others take it before them. However, yielding the right of way when it is not required does not legally grant the other driver the right of way -- they may still be required to yield to other vehicles on the road. (Hence the title of the PSA: "Random drivers can't grant you the right of way as a gift.")

At a 4-way stop, giving another driver the right of way is usually safe and courteous, but in other cases it can be dangerous. This comic is saying that people who exhibit this behavior dangerously can be assumed to be {{w|The_Terminator|Terminator}}-style assassins, sent to kill you by sending you into contention with other traffic to make it look like an accident, and thereby prevent some future act on your part that is not to their liking.

In this comic, the deferential driver is holding up a queue of vehicles (including a large tractor-trailer truck) that is obscuring the immediate view of oncoming traffic. But instead of simply turning left and reducing the queue, the deferential driver is waving Randall's car into traffic, perhaps because they forgot that the other lanes have priority over the crossing driver. The effect could be to wave them through right into the path of another car traveling at full speed, a clever way for a time-traveling assassin to take down one's target without arousing suspicion.

Not pulling into traffic when your view is obstructed is good advice, and Randall's comical exaggeration may make the advice more memorable. Always check for yourself that your way is clear, and if your view is blocked, sit tight.

However, Randall seems to be assuming that the waving gesture can only ever mean one thing: Pull all the way into traffic. It may be that a "waving out" gesture is intended to give the waiting car a chance to turn into the median strip (see details below). Viewing courteous behavior as conclusive evidence of a temporal assassination conspiracy is humorously ego-centric and improbable.

The title text explains that Randall made this PSA because he has experienced this multiple times in the last month, and that the assassins should try harder next time.

===Alternate interpretation of the waving gesture===

Note that in this comic, as currently illustrated, there appears to be sufficient room in the median strip for the waiting car to pass the first set of lanes and stop in the median strip, protected from passing traffic on both sides, to legally wait for the second stream of traffic to safely subside. Waiting in this median turning area is a normal maneuver in suburban and exurban US areas where these types of non-signaled intersections are common. There's no reason to assume that the supposed would-be assassin is not simply waving the waiting car to the safety of the median strip. Randall's message of caution is still sound, but he accidentally illustrated an intersection diagram that fails to optimally support his case.

[[File:2932 Alt.png|frameless|Illustration showing room to safely turn left halfway, stopping in the median strip]]

'''Legality'''

Median strips are typically used to allow vehicles to safely make left turns without impeding the flow of traffic. When emerging from a side road, vehicles can cross the first lane(s) of traffic and wait in the median strip until it is safe to merge into the opposite lane(s).
* In Texas, using the median strip to wait for a gap in traffic is generally acceptable. The Texas Department of Public Safety suggests that drivers use the median strip when appropriate. Source: [https://www.texashighwayman.com/laws.shtml Texas laws]
* [https://le.utah.gov/xcode/Title41/Chapter6A/41-6a-S801.html?v=C41-6a-S801_2015051220150512 The state of Utah also allows this to happen.]
* In {{w|Denmark}}, it is considered bad practice to pause in the median strip when crossing; left turns should be carried out in a continous manuveur.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:Driving PSA:
:Random drivers can’t grant you the right of way as a gift.
:[A T-intersection with a major road separated by a concrete median going from top to bottom, and a minor road coming from the left]
:[A car is stopped at the end of the minor road]
:First car [arrow pointing to car]: You, waiting to turn left
:[A second car is stopped in the dedicated left turn lane of the right major road, with a third car, a truck hauling cargo, and a fourth car lined up behind it]
:Second car [in a speech bubble]: You go ahead! I’m feeling generous.
:Second car [arrow pointing to car]: Time traveler pretending to be polite
:[In the second lane is a black arrow pointing upwards, with text below it reading 45 MPH, and a fifth car below the text]
:Fifth car [arrow pointing to car]: Car that they are waving you into the path of
:If someone waves you out, assume that they are an assassin sent from the future to kill you and make it look like an accident.

{{comic discussion}}

[[Category:Time travel]]

1179: ISO 8601

2021-01-20T14:05:32Z

Fod: + Denmark

{{comic
| number = 1179
| date = February 27, 2013
| title = ISO 8601
| image = iso_8601.png
| titletext = ISO 8601 was published on 06/05/88 and most recently amended on 12/01/04.
}}

==Explanation==
When abbreviating the date into numerical form, {{w|Date format by country|various areas of the world}} tend to list the year, month, and day in different orders (as well as with different delimiting symbols), which can cause confusion particularly when the day value is 12 or lower allowing it to be easily interpreted as the month and vice versa. As a {{w|public service announcement}}, this comic states that there is in fact one international standard for writing numeric dates, set by the {{w|International Organization for Standardization}} in its {{w|ISO 8601}} standard: YYYY-MM-DD.

The comic then proceeds to list several discouraged ways of writing out the date of the comic's publication, as they do not match the standard. It begins with several commonly used ones in countries around the world, but then begins to list increasingly uncommon ways, ranging from strange (Roman numerals) to quirky (binary, Unix time) to essentially impossible (painting the numbers onto a black cat).

The title text provides a perfect example of the kind of ambiguity that can arise when non-standard formats are used. The ISO standard was in fact published on 1988-06-05 and amended on 2004-12-01. This is mentioned in the title text in MM/DD/YY format; however, there is no way to naturally figure this out, particularly with the second date. With the year truncated to two digits and all three numbers at 12 or lower, the date referring to December 1, 2004 may well be interpreted as 12 January 2004, or as 2012-01-04.

Date formats were again the subject in [[1340: Unique Date]].

The other mentioned formats are:

{| class=wikitable
! Date !! Explanation
|-
| 02/27/2013
| MM/DD/YYYY, used mostly in the United States.
|-
| 02/27/13
| MM/DD/YY, same as above but with the year shortened to two digits.
|-
| 27/02/2013
| DD/MM/YYYY, used variously in South America, Canada ({{w|Date_and_time_notation_in_Canada|officially uses ISO 8601}}), Australia, New Zealand and Europe.
|-
| 27/02/13
| DD/MM/YY, same as above but with the year shortened to two digits.
|-
| 20130227
| YYYYMMDD, same as ISO 8601 without delimiting punctuation. Allowed by the standard. Technically not ambiguous but is hard to read as a date at first glance.
|-
| 2013.02.27
| YYYY.MM.DD, used in Japan, South Korea and Hungary. Same as ISO 8601 except with different punctuation.
|-
| 27.02.13
| DD.MM.YY, used in Germany, Russia, and others.
|-
| 27-02-13
| DD-MM-YY, used in Denmark, Netherlands, Indonesia, India, Bangladesh, and others.
|-
| 27.2.13
| D.M.YY. It is common in several areas to abbreviate the month or day to a single digit and drop the leading zero when possible.
|-
| 2013. II. 27.
| YYYY. MM. DD., with month as {{w|Roman numerals}}, used in Hungary.
|-
| 27⁄2-13
| D⁄M-YY, traditional format in Denmark and Sweden
|-
| 2013.158904109
| Year and decimal fraction of year. 0.158904109 is a decimal approximation of 58/365, with February 27 being the 58th day of the year. This format may be easier to read for computers/programs in some contexts, but is difficult for humans to interpret.
|-
| MMXIII-II-XXVII
| The ISO 8601 standard but written in Roman numerals. Never used as a traditional standard anywhere as it is hard to read, parse, and interpret for no benefit.
|-
| MMXIII LVII⁄CCCLXV
| Year followed by its partial fraction 57/365, all in Roman numerals. Equally useless as the above. As a note, apparently this 'standard' is different from the decimal fraction two rows above, as the decimal fraction notation uses the ''end'' of the day (first day of the year is 1/365 while the last is 365/365), while this uses the ''beginning'' (first day is 0/365 and last is 364/365).
|-
| 1330300800
| {{w|Unix time|UNIX Timestamp}}, a standard method of storing absolute time in many computer systems and defined as the number of seconds since 00:00:00 on 1970-01-01 (UTC). The Unix time listed here appears to mistakenly be for '''2012'''-02-27, which is also mentioned by [[Randall]] in the original transcript. The Unix Timestamp for 2013-02-27 would be 1361923200.
|-
| ((3+3)×(111+1)-1)×3/3-1/33
| A useless format where the numbers 2013, 2, and 27 written as needlessly long arithmetic expressions using just the digits 1 and 3. For additional confusion, the values are delimited by slashes, enabling confusion with the fraction bar. (If evaluated literally, the entire expression evaluates to 670.963, or 671 minus 1 divided by 27.)
|-
|    2  272013
| A nearly impossible to read date "format" that can be considered a parody "compromise" between different formats: rather than argue about the order in which the year, month, and day should be, they are simply all written on top of each other. As a "bonus", there is also no arguing over which separator character to use.
|-
| 10/11011/1101
| The US mm/dd/yy format in {{w|Binary number|binary}}, corresponding to 2/27/13. Never used for obvious reasons.
|-
| 02/27/20/13
| MM/DD/CC/YY, where CC stands for century. This format is never used. Note that while months and days count starting from 1, centuries and years in this format count from 0 for extra confusion. But the CC value is widely used on many operating systems to distinguish between the 20th and 21st century, represented by the values "19" and "20" because 1950 belongs to the 20th century.
|-
| <ruby><rb>0</rb><rb>1</rb><rb>2</rb><rb>3</rb><rb>7</rb><rt>2</rt><rt>3</rt><rt>1</rt><rt>4</rt><rtc style="ruby-position: under"><rt>5</rt><rt></rt><rt>67</rt><rt></rt><rt>8</rt></rtc></ruby>
| An obfuscated format where the small numbers indicate the positions where the large digits should be placed. In this reading, 0 is used at positions 2 and 5, 1 is used on position 3, etc.; the result being 20130227
|-
| [A hissing black cat with "2-27-13" painted on it]
| In Western cultures, black cats and the number 13 are associated with bad luck. The cat might also just be angry that someone covered it in paint.
|}

==Transcript==
:Public Service Announcement:

:Our different ways of writing dates as numbers can lead to online confusion. That's why in 1988 ISO set a global standard numeric date format. This is '''''the''''' correct way to write numeric dates:

::2013-02-27

:The following formats are therefore discouraged:
:*02/27/2013
:*02/27/13
:*27/02/2013
:*27/02/13
:*20130227
:*2013.02.27
:*27.02.13
:*27-02-13
:*27.2.13
:*2013. II. 27.
:*27⁄2-13
:*2013.158904109
:*MMXIII-II-XXVII
:*MMXIII LVII⁄CCCLXV
:*1330300800
:*((3+3)×(111+1)-1)×3/3-1/33
:*   2  272013 [the numbers 2013, 02, and 27 written overlapping each other]
:*10/11011/1101
:*02/27/20/13
:*2053112674378
:*[A black cat with 2-27-13 scrawled across its body in dripping white paint.]
:**Cat: ''Hissss''

{{comic discussion}}
[[Category:Math]]

Talk:2292: Thermometer

2020-04-13T16:25:06Z

Fod: + Rømer Trivia

First non-Covid post other than April fools?[[Special:Contributions/162.158.107.167|162.158.107.167]]
23:04, 10 April 2020 (UTC)
:Since a fever is a common symptom of Covid-19, I'd say this is as much about Covid-19 as all the previous comics on the topic. [[User:Ianrbibtitlht|Ianrbibtitlht]] ([[User talk:Ianrbibtitlht|talk]]) 02:59, 11 April 2020 (UTC)
::I'd disagree. Fevers aren't inherently related to COVID-19, and while it's certainly easy to draw a connection based on current events, at no point is the connection made explicit. [[Special:Contributions/172.69.34.104|172.69.34.104]] 10:29, 11 April 2020 (UTC)
:::Seriously? Fever is associated with 88% of COVID-19 cases! I'd say that's inherently related, and I'm drawing a connection based on that fact. [[User:Ianrbibtitlht|Ianrbibtitlht]] ([[User talk:Ianrbibtitlht|talk]]) 12:59, 11 April 2020 (UTC)
::::Fevers are associated with almost all infectious diseases. By that logic, this could be about the flu, mono, or a hundred other conditions. [[User:Shamino|Shamino]] ([[User talk:Shamino|talk]]) 17:24, 11 April 2020 (UTC)
:::::I, too, think calling this a Covid-19 comic is excessive. Sure, thermometers for measuring body temperature are sold out at my local drugstore, and pandemic likely inspired the comic, but if it had been published a year ago, we wouldn't infer any connection to a specific disease or global epidemic. - Ada in New Hampshire, USA [[Special:Contributions/172.69.6.22|172.69.6.22]] 07:56, 12 April 2020 (UTC)
::::::I would assume anything that can be linked, even loosely, is probably part of this chain. I have been assuming since the 6th one that Randel would aim for 19 of these just because. Though perhaps he will keep going till the hype is over. Either way, requiring that it directly mentions the topic it was inspired by would be way overkill. Mentioning things that likely inspired a comic is something we have done for a long time, and the virus seems like the most likely inspiration, especially when taking the full comic chain into account[[Special:Contributions/172.69.198.52|172.69.198.52]] 21:33, 12 April 2020 (UTC)
::The comic doesn't mention a fever. For all we know Cueball is trying to measure the outside air temperature, or how hot his coffee is. We can rule out the idea that he is trying to measure the temperature of some liquid helium only because he skipped past the kelvin scale. [[User:Jeremyp|Jeremyp]] ([[User talk:Jeremyp|talk]]) 18:39, 12 April 2020 (UTC)

A common practice in schools and the like prior to quarantine was temperature taking upon arrival. So it's like that this comic continues that to the home setting. [[Special:Contributions/162.158.78.112|162.158.78.112]] 23:19, 10 April 2020 (UTC)

A pessimist would guess that this means someone in Randall's household has a fever. [[Special:Contributions/108.162.219.52|108.162.219.52]] 23:26, 10 April 2020 (UTC)

> The Physician Ducks[[Special:Contributions/172.69.62.94|172.69.62.94]] 23:32, 10 April 2020 (UTC)

Personally I'd welcome a home thermometer marked off in Kelvin, avois all the "twice as cold" sort of confusion you can get with an arbitrary zero as used in Celsius and Fahrenheit. [[Special:Contributions/162.158.34.210|162.158.34.210]] 23:21, 10 April 2020 (UTC)

I might have enjoyed a "Degrees of Kevin Bacon" joke in this comic somewhere. :-) [[Special:Contributions/172.69.68.143|172.69.68.143]] 23:42, 10 April 2020 (UTC)

Double-plus-dissapointed we didn't get the Delisle measure referenced at all... [[Special:Contributions/162.158.34.202|162.158.34.202]] 01:17, 11 April 2020 (UTC) ...and now added. It would be better in any Trivia section, but we don't have one so hoping it's no more out of place in the explanation as Fahrenheit. [[Special:Contributions/162.158.34.202|162.158.34.202]] 02:02, 11 April 2020 (UTC) ...''aaaand'' someone removed it (as pure trivia, of course), fair enough. Anticipated. Anyone still interested in what I put just needs to check this IP, at about this timestamp, in Page History, though, so not going to argue the point. [[Special:Contributions/162.158.34.202|162.158.34.202]] 02:08, 11 April 2020 (UTC)

No temperature scale is defined using melting or boiling points of water anymore. Since 2019 Kelvin is defined via the Boltzmann constant, and all other temperature scales have been (re-)defined relative to the Kelvin scale for quite a while. --[[Special:Contributions/172.69.63.103|172.69.63.103]] 01:24, 11 April 2020 (UTC)

Randall forgot the Réaumur scale.[[Special:Contributions/162.158.123.97|162.158.123.97]] 03:00, 11 April 2020 (UTC)

I'm not sure why some people seem to look for any opportunity to take a dig at the US, but I removed the line in the explanation about US-based readers not being familiar with the Celsius temperature scale. I'm sure most Americans are familiar with it but prefer the Fahrenheit scale instead. I don't understand why anyone holds that against us. [[User:Ianrbibtitlht|Ianrbibtitlht]] ([[User talk:Ianrbibtitlht|talk]]) 03:04, 11 April 2020 (UTC)
:Hey, let's assume good faith. Chances are, some rando just genuinely had no idea how that kind of stuff works here. [[Special:Contributions/172.69.34.104|172.69.34.104]] 10:22, 11 April 2020 (UTC)

Regarding USA Fahrenheit and non-USA Celsius preference, I was in Niagra Falls a few years back, listening to a Canadian station on the radio (ok, more than a few years ago...) and the DJ gave a weather report, saying “The current temperature is 25 degrees, that’s 77 on the understandable scale.” [[Special:Contributions/173.245.54.201|173.245.54.201]] 04:22, 11 April 2020 (UTC)

I guess if you wanted to use the Newton scale you'd need to have Newton's original "degrees of heat" measuring device. [[Special:Contributions/108.162.250.67|108.162.250.67]] 04:31, 11 April 2020 (UTC)

Nitpicking alert : the correct writing is "kelvin", not "Kelvin".

100°F is "really hot"? Maybe on a stripper... [[Special:Contributions/162.158.190.106|162.158.190.106]] 13:00, 11 April 2020 (UTC)

:Also nitpicking: "Kelvin" is correct, as it is a name like Fahrenheit (https://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin or https://en.wikipedia.org/wiki/River_Kelvin as you like...)[[User:Tier666|Tier666]] ([[User talk:Tier666|talk]]) 14:59, 13 April 2020 (UTC)

Randall, as a physicist, should know about the equipartition theorem. It states that all degrees of freedom will carry the same average amount of energy in thermal equilibrium, not only the translational kinetic ones (but also rotational, and potential energies). It is technically not false to exclude some of these, but an arbitrary choice. I guess he just wanted to include the terms “translational” and “kinetic” to make sure it sounds ridiculously over-specific (which works well). [[Special:Contributions/162.158.91.213|162.158.91.213]] 15:07, 11 April 2020 (UTC)
:No, it's still an important distinction. Many Thermometers can only 'measure' the average Translational energy and the rotational and elastic energy is just assumed to match that. (The only Thermometers that measure rotational and elastic Energy are the ones who only measure their own temperature... which is 99.5 of all consumer Thermometers.) And it probably does except in some very specific cases with ultra high speed pressure changes.

+Using the average Translational Energy would would sidestep all the problems with the different units of temperature and would also eliminate the necessity of using the Boltzmann constant, simplyfying a lot of physics. But nobody wants to make the transition since most everyday temperatures would be between 5 and 8zJ, with 5 being freezing, six being tolerable and seven a desert at noon. The Unit, Zeejays would sound cool though.[[Special:Contributions/162.158.92.70|162.158.92.70]] 09:30, 13 April 2020 (UTC)
:Alternatively, use molar mean disordered translational kinetic energy per mole, making the numbers nicer by a factor of Avogadro's number, and bringing the scale to 2-3kJ/mol. Or add in a factor of 1.5 as well to make the gas K.E. formula simpler. [[User:Sqek|Sqek]] ([[User talk:Sqek|talk]]) 10:27, 13 April 2020 (UTC)

Well, it has Fahrenheit after a fashion. Just substract 460 from Rankine. It's even easier than converting Kelvin to Celsius!
:I find it much quicker to subtract 0.01C° 27,315 times than to subtract 0.01F° 45,967 times, personally. I think you're quite barmy to suggest otherwise, Unsigned... :P [[Special:Contributions/162.158.34.222|162.158.34.222]] 16:17, 11 April 2020 (UTC)
::Now that I, the formerly Unsigned, think of it, I must agree with you - but for an entirely different reason. 273.15 in binary is a nice, round 100010001.001(00101) with 3 1's in the integer and 4+2n 1's for every 3+5n fractional digits, whereas 459.67 is much messier: 111001011.10110001111110... , with 6 1's in the integer alone. The more 1's there are in a number, the more operations you have to do for each addition or subtraction. So in binary, Kelvin-to-Celsius is much easier to convert than Rankine-to-Fahrenheit. Yet another point in favor of the glorious metric master system, da? [[User:Osato|Osato]] ([[User talk:Osato|talk]]) 19:57, 11 April 2020 (UTC)

I removed the weasel words, indicating that Fahrenheit is "generally appreciated" because 0 means very cold and 100 very hot. I adjusted it to "some claim" and adjusted the text to fit.

Make the scale in Celsius 0 to 200, and I think you would have a system much more relatable to Fahrenheit users.

I can imagine a worse scaling system! Base it on Cat-Scratch-Fever, Hot-Blooded, Yellow Snow, SpringTime in Alaska, Beds are Burning, Burning Down the House . (not in that order) [[User:Cellocgw|Cellocgw]] ([[User talk:Cellocgw|talk]]) 12:49, 13 April 2020 (UTC)

By a strange coincident, I went for a walk to the real {{w|Ole Rømer|Rømer}}'s {{w|Ole Rømer#The Ole Rømer Museum|Observatorium Tusculanum}} today {{w|da:Observatorium Tusculanum|(full details in the Danish wiki)}}. If you like Historical Science, it's worth a visit next time you're in {{w|Denmark}}... --[[User:Fod|Fod]] ([[User talk:Fod|talk]]) 16:25, 13 April 2020 (UTC)

1938: Meltdown and Spectre

2018-01-07T12:44:17Z

Fod: + reference to comic 538

{{comic
| number = 1938
| date = January 5, 2018
| title = Meltdown and Spectre
| image = meltdown_and_spectre.png
| titletext = New zero-day vulnerability: In addition to rowhammer, it turns out lots of servers are vulnerable to regular hammers, too.
}}

==Explanation==
{{incomplete|Looks good, but something seems to be missing. Could someone recheck this again? Do NOT delete this tag too soon.}}
This comic was inspired by the {{w|Meltdown (security vulnerability)|Meltdown}} and {{w|Spectre (security vulnerability)|Spectre}} bugs in certain processors. These vulnerabilites were disclosed to the public the week of this comic. The bugs made big news because they broke the "walls" between programs executing concurrently on the same computer, in some circumstances allowing malware to steal secrets from normal, bug-free programs.

{{w|Speculative execution}} is a technique used to speed up computer processors. Processors {{w|Instruction pipelining|handle instructions in a series of steps}}, like an assembly line. The processor works on several successive instructions, each at a different stage in the assembly line. It may start speculatively executing instructions that follow a decision before the logic on whether or not they should be execute is finished. Once the decision is made, it keeps results from the selected path, and discards unnecessary processing. This allows it to keep doing useful work while some slower decision is made. [[Ponytail]] uses {{w|Tram|trolley (tram)}} tracks as an analogy for streams of instructions in a program.

The {{w|Trolley Problem}} is a thought experiment where an out-of-control trolley is heading to a switch which you control. Leaving the switch as-is will cause it to kill multiple people stuck on the tracks, but switching the track will cause it to kill one person; this creates the ethical dilemma of passively causing multiple deaths versus actively causing one. The Trolley Problem has gained significant memetic traction, helped in no small part by its frequent inclusion in “introduction to philosophy” type courses. Also, the problem has seen revitalized interest with the emergence of {{w|autonomous car}}s, which may be faced with essentially {{w|autonomous car#Moral issues|such choices}} in emergency situations.

The "quantum" aspect of this is that in some {{w|interpretations of quantum mechanics}}, quantum-level particles can be viewed as taking every possible path at once and the result is the sum of all of them. Or it might be related to some interpretations of {{w|Schrödinger's cat}}, in that the system can be regarded as being in both states until some event results in one of the states being selected.

Meltdown and Spectre result from flawed implementations of speculative execution, where the effects of speculatively executed instructions are not completely discarded. The phantom trolley driving through walls is an analogy for the computer instructions being able to access protected areas.

Contrary to what the comic implies, in many cases both paths are not simultaneously taken during speculative execution. A {{w|branch predictor}} may be used to select the most likely path, and the effects should be completely erased if the predicted path is incorrect. Both branch prediction and taking both paths, also known as eager evaluation, are considered speculative execution and are affected by these bugs.

The {{w|Row Hammer}} problem has been known for many years before this cartoon. A {{w|Dynamic random-access memory|common form of computer memory}} is constructed from tiny capacitors organized in a two-dimensional grid of rows and columns. Capacitors store charge to represent information. By applying a pattern of memory access that rapidly changes a row of capacitors, you can cause charge to overflow to nearby rows and incorrectly change their states.

Ponytail mentions that we suck at building "shared computers" because Rowhammer, Spectre, and Meltdown all break down the security divisions built between programs and between users. A hacker running a separate program in a separate account shouldn't be able to access your secrets or change the behavior of your program, but these problems allow them to. This is particularly dangerous for time-sharing, servers and {{w|Cloud computing|the cloud}}, where different programs, websites, or even companies can be sharing the same hardware.

[[Cueball]] took her explanation literally, and came to the conclusion that the cloud "is full of phantom trolleys armed with hammers". Instead of correcting him, Ponytail decides to accept his interpretation (perhaps because she found this idea to be kinda cool).

The title text humorously states that as well as row hammer, computer servers also can be harmed by regular hammers. A {{w|zero-day (computing)|zero-day}} vulnerability is an attack that takes advantage of a vulnerability that hasn't been published yet, and so it's not patched in any vulnerable system. This would imply that the Rowhammer vulnerability is what inspired someone to try taking a hammer to a server. One might "patch" a server against this attack by plating it with stronger metal. (The fact that compromising IT systems is sometimes easier done physically than logically is also mentioned in [[538: Security]].)

==Transcript==
:[Cueball and Ponytail are walking.]
:Cueball: The Meltdown and Spectre exploits use "speculative execution?" What's that?
:Ponytail: You know the trolley problem? Well, for a while now, CPUs have basically been sending trolleys down '''''both''''' paths, quantum-style, while awaiting your choice. Then the unneeded "phantom" trolley disappears.

:[Ponytail framed alone, facing left. They have stopped walking.]
:Ponytail: The phantom trolley isn't supposed to touch anyone. But it turns out you can still use it to do stuff.
:Ponytail: And it can drive through walls.

:[Cueball and Ponytail are standing, facing each other.]
:Cueball: That sounds bad.
:Ponytail: Honestly, I've been assuming we were doomed ever since I learned about Rowhammer.

:Cueball: What's ''that''?
:Ponytail: If you toggle a row of memory cells on and off really fast, you can use electrical interference to flip nearby bits and—
:Cueball: Do we just suck at ... computers?
:Ponytail: Yup. Especially shared ones.

:[They resume walking to the right.]
:Cueball: So you're saying the cloud is full of phantom trolleys armed with hammers.
:Ponytail: ...Yes, that is exactly right.
:Cueball: Okay. I'll, uh... install updates?
:Ponytail: Good idea.

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Computers]]