explain xkcd - User contributions [en]

1547: Solar System Questions

2024-03-07T19:38:12Z

172.68.94.150: Expanded Ceres explanation.

{{comic
| number = 1547
| date = July 6, 2015
| title = Solar System Questions
| image = solar_system_questions.png
| titletext = My country's World Cup win was exciting and all, but c'mon, what if the players wore nylon wings and COULD LITERALLY FLY?
}}

==Explanation==
{{incomplete|Some questions have received more complete answers in the past decade, but are not explained in much detail here. What have we learned since of all these things? Particularly: "What's Ceres like?" "Why is Iapetus weird-colored?" "Which of the other moons have seas?" "Why does Iapetus have a belt?" Gimme some science!}}

This comic is a list of questions which [[Randall]] has about the Solar System, which at first glance may appear to be things that Randall would like to learn about. In actuality, most of the questions have not been satisfactorily answered or proven by anyone in the {{w|List of unsolved problems in physics#Astronomy and astrophysics|scientific community}}. These open questions may serve to intrigue readers and prompt further interest in astronomy and austronautics.

{| class="wikitable"
|-
! Question given
! Answered?
! Answer given by Randall (in red in the original)
! Comments
|-
| Why is the the Moon so blotchy?
|style="background: lightgreen" | Yes
| Lava
| The {{w|Moon}} is in synchronous rotation with Earth, which means that we always can see only one half of the surface of the Moon. And on that side we can see large {{w|lunar maria}} formed by lava from big volcanoes. This surface is very different from all other celestial bodies we know in our Solar system. The double "the the" could be a Randallism — intended or unintended.
|-
| Why are all the blotches on the near side?
|style="background: salmon" | No
| ...
| The nearside of the Moon is dominated by the blotchy 'seas' or maria, the far side by craters. {{w|Far side of the Moon#Differences|Several explanations}} for this have been proposed, including an overabundance of impacts obliterating the blotches on the more exposed far side, different compositions of heat-producing elements, large collisions, or heat produced by the still-cooling Earth.
|-
| Did Mars have seas?
|style="background: lightgreen" | Yes
| Yes (briefly?)
| Recent explorations have confirmed there was once standing (and also flowing) water on {{w|Mars}}. Many rovers and orbiters on Mars give us the evidence on this early development of that planet, but it is still unknown how long such conditions existed in its history. Two of these probes have been the subject of comics before: [[695: Spirit]] and [[1504: Opportunity]].
|-
| Was there life on Mars?
|style="background: salmon" | No
| ...
| One of the big mysteries, {{w|Life on Mars|not yet answered}}.
|-
| What's Titan like?
|style="background: lightyellow" | Partly
| Cold, yellow, lakes + rivers (methane)
| The {{w|Cassini–Huygens}} mission confirmed the presence of {{w|Lakes of Titan|lakes and rivers}} on {{w|Titan (moon)|Titan}}. The {{w|Huygens (spacecraft)#Findings|Huygens}} lander itself returned some very yellow images of a dry lake bed from Titan's surface. The possibility of life on Titan was mentioned in [[829: Arsenic-Based Life]].
|-
| What was Earth like during the Hadean?
|style="background: lightyellow" | Partly
| ...
| The {{w|Hadean}} was the first geologic era on Earth, the planet had just formed and not much is known of that period of Earth. But since it was the time when Earth was formed it was mainly very hot with extreme volcanic activity, with the entire surface melted. This is why the era is named after {{w|Hades}} the ancient Greek god of the underworld, even though Hades was never associated with fire.
|-
| Is the Oort Cloud a real thing?
|style="background: salmon" | No
| ...
| The {{w|Oort Cloud}} is a theoretical spherical cloud of icy planetesimals, maybe dust, and also larger objects at a distance of up to around 100,000 {{w|Astronomical units|AU}} from our Sun. We can see similar clouds at other stars, but there is still no evidence that this cloud exists in our Solar System.
|-
| Why is the Sun's corona so hot?
|style="background: lightyellow" | Maybe
| Something about magnets?
| The {{w|corona}} of the Sun is hotter than it theoretically should be. Tiny solar flares called {{w|nanoflares}} might be responsible. The Sun's {{w|Stellar magnetic field|magnetic field}} is almost certainly relevant.
|-
| What are comets like?
|style="background: lightgreen" | Yes
| Precipitous
| A comet can be dangerously steep: the {{w|Philae (spacecraft)|Philae lander}} finally stopped tumbling when it ran into a cliff.
|-
| Where's Philae, exactly?
|style="background: lightgreen" | Yes
| ...
| ...but we were not sure ''which'' cliff until September 5, 2016, when the European Space Agency announced that Philae had been found and photographed by Rosetta on the previous Friday (September 2). The landing of Philae was depicted in real time in the dynamic comic [[1446: Landing]]. This lasted for several hours. Later the comic was updated with a new image where Philae is resting on the edge of a cliff.
|-
| What's Pluto like?
| rowspan="2" style="background: lightyellow" | Partly
| rowspan="2" | [Soon!]
| rowspan="2" | {{w|Pluto}} is so far from Earth ''and'' so small that traditional telescopes couldn't discern much about it. When this comic was released, the probe {{w|New Horizons}} was eight days away from its closest approach to Pluto and its moon {{w|Charon (moon)|Charon}}; Randall was naturally excited about it. The probe was the subject of the comic [[1532: New Horizons]].
|-
| What's Charon like?
|-
| Why don't we have in-between-sized planets?
|style="background: salmon" | No
| ...
| There is a size-gap between the rocky {{w|terrestrial planets}} up to Earth size and the {{w|gas giants}} very much larger than Earth in our Solar System.
There are many known {{w|exoplanets}} (planets in other solar systems) filling in the range between our rocky planets and our gas giants, known as [https://upload.wikimedia.org/wikipedia/commons/2/2c/Exoplanet_Mass-Radius_Scatter_Super-Earth.png Super-earths] However, there is an observed but unexplained {{w|Small planet radius gap|scarcity of planets of this size category}} even among exoplanets.
|-
| What's Ceres like?
|style="background: lightyellow" | Partly
| [Working on it!]
| The {{w|Dawn (spacecraft)|Dawn probe}} was currently exploring the {{w|dwarf planet}} {{w|Ceres (dwarf planet)|Ceres}} at the time that this comic was written and released. Since the spacecraft's mission, it was determined that Ceres's surface is a mixture of ice and hydrated materials, like clay. The crust is at most thirty percent ice by volume, and though it likely lacks an internal ocean of water, highly-concentrated saltwater can still reach the surface from the outer mantle, allowing {{w|Cryovolcano|cryovolcanoes}} to form.
|-
| Why is Europa so weird-looking and pretty?
|style="background: lightgreen" | Yes
| Ice over a water ocean
| {{w|Europa (moon)|Europa}} is a moon of {{w|Jupiter}} and the surface is basically thick pack ice covered in {{w|lineae}}.
|-
| Why is Io so weird-looking?
|style="background: lightyellow" | Partly
| Sulfur volcanoes (? in the wrong places?)
| The moon {{w|Io (moon)|Io}} is also orbiting Jupiter and is close enough that {{w|tidal forces}} make it the most volcanic object in the solar system. The moon is mainly yellow but there are several other colors on the surface, for instance spots and streaks of bright red that comes from {{w|sulfur}} ejected by the volcanoes. The "wrong places" refer to some volcanoes discovered by the {{w|Voyager program|Voyager missions}} and believed to erupt sulfur. But more recent measurements showed that the temperature inside those volcanoes is about 2000 °C where this element is not liquid anymore but gas.
|-
| Why are so many Kuiper Belt objects red?
|style="background: lightyellow" | Maybe
| ...
| Many objects in the {{w|Kuiper Belt}} have a reddish hue. A possible explanation is that they are [http://www.space.com/9418-icy-red-objects-solar-system-edge-point-life-building-blocks.html covered in organic molecules] formed by the irradiation of their surface ices. The New Horizons probe might also shed light on this.
|-
| What are those spots on Ceres?
|style="background: salmon" | No
| ...
| The Dawn probe found some mysterious spots on Ceres. These [http://photojournal.jpl.nasa.gov/jpeg/PIA19568.jpg white spots] are still not understood, but the mission is still running and we may figure out the source of the glowing white features. These spots became the punch line of the joke in [[1476: Ceres]].
|-
| What's in the seas under Europa's ice?
|style="background: salmon" | No
| ...
| The {{w|ESA}} selected the mission {{w|Jupiter Icy Moon Explorer|Jupiter Icy Moon Explorer (JUICE)}} to Jupiter. The moon Europa is one target for that mission. But we have to wait, because, even though it has already launched on 14 April 2023, its arrival at Jupiter is planned for 2031. But that's not uncommon for missions like this. New Horizons and {{w|Rosetta (spacecraft)|Rosetta}} also traveled approximately ten years to reach their targets. And before such a mission can start many preparations have to be done. {{w|2010: Odyssey Two}} is a 1982 science fiction novel by {{w|Arthur C. Clarke}} in which he envisions life under the ice on Europa. This life becomes a major plot point both in this and in the two sequels.
|-
| Which of the other moons have seas?
|style="background: lightyellow" | Partly
| Several
| Randall has already mentioned above that there are liquids on two moons. The moon Titan at Saturn has lakes on its surface formed by liquid ethane, methane, and propane and the Jupiter's moon Europa has a sea of water covered by a thick sheet of ice. Depending on the definition of 'sea', other less obviously 'frozen water world' moons such as {{w|Ganymede (moon)|Ganymede}} at Jupiter may have {{w|Ganymede (moon)#Subsurface oceans|subsurface}} [http://www.newscientist.com/article/dn27151-aurora-reveals-jupiter-moons-secret-subsurface-sea.html oceans] of liquid water and on other moons it could be other substances that are liquid at the relevant temperature, like on Titan.
|-
| What are the big white things in Titan's lakes?
|style="background: lightgreen" | Yes
| ...
| This is a joke about some gaps in the radar measurements as shown in [https://commons.wikimedia.org/wiki/File:PIA10008_Seas_and_Lakes_on_Titan_full_size.jpg this image].
|-
| What do Jupiter's clouds look like up close?
|style="background: salmon" | No
| ...
| The Jupiter mission {{w|Galileo (spacecraft)|Galileo}}, operated by NASA and the German Aerospace Center (DLR), arrived at Jupiter in 1995 and was sent to impact the planet at the end of that mission in 2003 to eliminate the possibility of contaminating local moons with terrestrial bacteria. Several measurements were done on the atmosphere but no pictures were sent back to Earth. So there is still no answer on this question.
|-
| What's all that red stuff in the Great Red Spot?
|style="background: salmon" | No
| ...
| The {{w|Great Red Spot}} is a storm south of Jupiter's equator. Observations from Earth show a lifespan of more than 150 years. It's unknown why it's stable for that long a time and it's also not clear why the color is red. The probe {{w|Juno (spacecraft)|Juno}} has arrived in July 2016 at Jupiter but has not answered Randall's question.
|-
| What's pushing the Pioneer probes?
|style="background: lightgreen" | Yes
| Heat from the RTG
| Discussed as the {{w|Pioneer anomaly}}. RTG stands for {{w|Radioisotope Thermoelectric Generator}}. This effect was mentioned (and explained a little differently) in the title text of [[502: Dark Flow]].
|-
| What pushes spacecraft slightly during flybys?
|style="background: salmon" | No
| ...
| Several spacecraft experienced unexplained speed increases during Earth flybys. This is called the {{w|flyby anomaly}}.
|-
| Where are all the Sun's neutrinos?
|style="background: lightyellow" | Maybe partly
| Oscillating
| There are fewer observed electron neutrinos from the Sun than the standard model predicts. This is called the {{w|Solar neutrino problem}}. Since the sum of all the neutrinos, regardless of type, that come from the Sun add up to the predicted number of electron neutrinos it is theorized that neutrinos can change their type. This is called {{w|Neutrino oscillation}}, and can occur only if {{w|neutrinos}} have mass. Neutrino oscillation is considered a proof that the mass of a neutrino is non-zero. The {{w|Neutrino#Mass|mass of a neutrino}} is not yet measured and is one of the problems on the {{w|List of unsolved problems in physics#High-energy physics.2Fparticle physics|list of unsolved problems in physics}} on Wikipedia.
|-
| Why is there so much air on Titan?
|style="background: salmon" | No
| ...
| {{w|Titan (moon)|Titan}} has an atmospheric pressure 1.45 times that of Earth, but only 1/7th of the surface gravity which is less than Earth's own airless Moon has, hence the confusion. In fact, Titan actually has almost 20% more atmosphere by mass than Earth, and ''seven times'' more atmosphere across a given surface area! Less influence from the more distant Sun probably helps retain more of the atmosphere's gases (for instance, Mars saw most of its atmosphere blasted away by the Sun), and {{w|cryovolcanoes}} may replenish the methane fraction which should by now have ''all'' been converted into the other hydrocarbons present from subsurface reservoirs. Further studies are required to properly answer this question.
|-
| Why does the Kuiper Belt stop?
|style="background: salmon" | No
| ...
| A reference to the {{w|Kuiper Cliff}}. Most Kuiper Belt Objects are found between 42 and 48 {{w|astronomical units|AU}}; calculations predicted that there would be more and larger KBOs beyond 50 AU, but instead very few objects have been found in that region. The reason for this is still unknown.
|-
| Why is Iapetus weird-colored?
|style="background: lightyellow" | Partly
| ...
| {{w|Iapetus (moon)|Iapetus}} is an icy moon of {{w|Saturn}} and always keeps the same face towards Saturn. The trailing hemisphere is bright, with the leading one notably darker.
|-
| Why does Iapetus have a belt?
|style="background: salmon" | No
| ...
| Iapetus has a 13 km high ridge around most of the equator, and a number of 10-km-high mountains where the ridge is interrupted.
|-
| What's the deal with Miranda?
|style="background: salmon" | No
| ...
| {{w|Miranda (moon)|Miranda}} is the smallest of {{w|Uranus}}' five round satellites, and it's {{w|Verona Rupes|a bit rough around the edges}} and also has an unusually high orbital inclination that is difficult to explain. Also possibly a [[Category:Firefly|''Firefly'' reference]] since {{w|List of Firefly planets and moons#Miranda|Miranda}} is also the name of a planet in {{w|Serenity (film)|''Serenity''}}, a film based on the {{w|Firefly (TV series)|''Firefly''}} TV series.
|-
| Did Uranus and Neptune change places?
|style="background: salmon" | No
| ...
| The {{w|Nice model}} is a theory of how our solar system formed, which suggests the possibility of Uranus and {{w|Neptune}} having swapped places before reaching their current positions. Work by Professor S. Desch [http://dusty.la.asu.edu/~desch/publications/2007/Desch2007.pdf also came to this result].
|-
| Did the Late Heavy Bombardment happen?
|style="background: salmon" | No
| ...
| The {{w|Late Heavy Bombardment}} is the name given to a theorized bombardment of the planet Earth during its early history, along with the other rocky planets around the Sun. It's believed that during that time many large objects still existed and are likely to have impacted the planets. On Earth the evidence for those impacts would have been destroyed, but on the Moon or Mercury some evidence may be available.
|-
| Did life start before it?
|style="background: salmon" | No
| ...
| For some speculation on this topic, see [http://www.livescience.com/5426-life-survived-earth-early-bombardment.html Life Could Have Survived Earth's Early Bombardment]. It is still a mystery if life was formed on Earth first or if it came from outer space.
|-
| Is Europa covered in ice spikes?
|style="background: salmon" | No
| ...
|Dr Daniel Hobley has put forward a [http://www.bbc.co.uk/news/science-environment-21341176 theory] that Jupiter's icy moon {{w|Europa (moon)|Europa}} has the right conditions to form ice spikes called {{w|Penitente (snow formation)|penitentes}} of up to 10m in height.
|-
| Why haven't we built a big inflatable Extreme Sports Complex on The Moon?
|style="background: lightyellow" | Partly
| ...
| The only silly item besides the "white on Titan's lakes" question, this question is less about science than about human priorities. It would be fun to watch sports in such a stadium - see the title text. Building a sports complex on the Moon would be prohibitively expensive in the context of government budgets, and transporting athletes to such a venue regularly would be logistically and financially complex. But it would be extremely cool.

See also [https://what-if.xkcd.com/124/ What If # 124], which gives great detail to the topic of lunar swimming. ''{{w|The Menace From Earth}}'', a 1957 short story by Robert Heinlein, describes another potential moon-based extreme sport.
|}

The title text refers to the {{w|2015 FIFA Women's World Cup}} which was won by the USA the day before. The nylon wings and flying may be a reference to two passages from 3001: The Final Odyssey, one where Frank Poole tries out various wings while in an extremely low gravity environment, and one where he remarks while watching Swan Lake that Tchaikovsky could never have imagined a performance where the dancers were actually flying (due to aforementioned low gravity). This is also a reference to the last point on the list, because if we had such a stadium on the Moon, maybe it would be possible to use such wings to make very long floating leaps.

==Transcript==
:'''Questions I have'''
:'''about the solar system'''
:(some answered)

:{| class="wikitable" style="border: 0px;"
|-
| style="border: 0px; text-align: right" | Why is the the Moon so blotchy?
| style="border: 0px; color:red;" | Lava
|-
| style="border: 0px; text-align: right" | Why are all the blotches on the near side?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Did Mars have seas?
| style="border: 0px; color:red;" | Yes (briefly?)
|-
| style="border: 0px; text-align: right" | Was there life on Mars?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | What's Titan like?
| style="border: 0px; color:red;" | Cold, yellow, lakes + rivers (methane)
|-
| style="border: 0px; text-align: right" | What was Earth like during the Hadean?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Is the Oort Cloud a real thing?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Why is the Sun's corona so hot?
| style="border: 0px; color:red;" | Something about magnets?
|-
| style="border: 0px; text-align: right" | What are comets like?
| style="border: 0px; color:red;" | Precipitous
|-
| style="border: 0px; text-align: right" | Where's Philae, exactly?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | What's Pluto like?
| style="border: 0px; color:red;" rowspan="2" | [Soon!]
|-
| style="border: 0px; text-align: right" | What's Charon like?
|-
| style="border: 0px; text-align: right" | Why don't we have in-between-sized planets?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | What's Ceres like?
| style="border: 0px; color:red;" | [Working on it!]
|-
| style="border: 0px; text-align: right" | Why is Europa so weird-looking and pretty?
| style="border: 0px; color:red;" | Ice over a water ocean
|-
| style="border: 0px; text-align: right" | Why is Io so weird-looking?
| style="border: 0px; color:red;" | Sulfur volcanoes (? in the wrong places?)
|-
| style="border: 0px; text-align: right" | Why are so many Kuiper Belt objects red?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | What are those spots on Ceres?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | What's in the seas under Europa's ice?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Which of the other moons have seas?
| style="border: 0px; color:red;" | Several
|-
| style="border: 0px; text-align: right" | What are the big white things in Titan's Lakes?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | What do Jupiter's clouds look like up close?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | What's all that red stuff in the Great Red Spot?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | What's pushing the Pioneer Probes?
| style="border: 0px; color:red;" | Heat from the RTG
|-
| style="border: 0px; text-align: right" | What pushes spacecraft slightly during flybys?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Where are all the Sun's Neutrinos?
| style="border: 0px; color:red;" | Oscillating
|-
| style="border: 0px; text-align: right" | Why is there so much air on Titan?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Why does the Kuiper Belt Stop?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Why is Iapetus weird-colored?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Why does Iapetus have a belt?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | What's the deal with Miranda?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Did Uranus and Neptune change places?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Did the Late Heavy Bombardment happen?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Did life start before it?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Is Europa covered in ice spikes?
| style="border: 0px;" |
|-
| style="border: 0px; text-align: right" | Why haven't we built a big inflatable extreme sports complex on the moon?
| style="border: 0px;" |
|}

{{comic discussion}}

[[Category:Comics with color]]
[[Category:Charts]]
[[Category:Soccer]]
[[Category:Space probes]]
[[Category:Astronomy]]
[[Category:Firefly]]

Talk:955: Neutrinos

2023-03-24T22:35:35Z

172.68.94.150: reply to Yfmcpxpj's comment summarizing an external article

When the news about the neutrino speed thing first came out, all these people I knew were asking if everything they knew was wrong because of the new discovery, and panning me for not believing the word of god that is the scientific journal. I don't know how these people get through life blindly believing everything that they're told. '''[[User:Davidy22|{{Color|purple|David}}y²²]]'''[[User talk:Davidy22|<tt>[talk]</tt>]]

: It actually isn't too hard to live like that. Fortunately for me, though, I didn't have all these people I know asking if everything they knew was wrong. Inversion? --[[User:Quicksilver|Quicksilver]] ([[User talk:Quicksilver|talk]]) 18:34, 17 August 2013 (UTC)

"...arguing with people and preaching caution is futile. Cueball realizes that it is more satisfying and profitable to place bets with them instead." Does anyone else notice the similarity between this sentiment and mortgage security? Instead of preaching caution in lending and trying to convince people that they shouldn't be taking on loans, hedge funds were created - much more satisfying and profitable. Sorry if this has nothing to do with the comic, it was just an observation that struck me. -naginalf[[Special:Contributions/108.162.216.40|108.162.216.40]] 19:58, 11 March 2014 (UTC) {{unsigned ip|108.162.216.40}}

The discussion on this page is incorrect. Relativity does not say that nothing can travel faster than the speed of light, it says that objects with mass cannot travel at the speed of light. Massive objects can either travel below or above the speed of light. {{unsigned ip|108.162.237.183}}
:Not as I read the wiki page on this: {{w|Speed_of_light#Upper_limit_on_speeds|Upper limit on speeds}}. Maybe if you could have negative mass could you travel faster than c. But according to the page, nothing with zero mass or any finite (positive) mass can move faster than c. And yes if it has mass, then the speed will have to be sharply smaller than c. (But can get as close to c as you like by pouring in more energy).--[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 14:15, 30 September 2015 (UTC)
It is actually that you can't accelerate to the speed of light if you're already going faster you won't automatically have a immeasurable speed that is almost C you will keep going at that speed theoretically. [[Special:Contributions/108.162.246.72|108.162.246.72]] 07:38, 22 September 2016 (UTC)

Cueball mentions Galileo. Is it because he went against the then-consensus on scientific knowledge and was punished, and others are accusing Cueball (and other "thought police") of attempting to do the same thing? [[Special:Contributions/108.162.210.196|108.162.210.196]] 11:35, 12 November 2016 (UTC)

Could someone explain to me why a GPS wouldn’t work if relativity didn’t exist? {{unsigned ip|162.158.186.156}}

:[http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html This is a good read.] [[Special:Contributions/172.69.210.28|172.69.210.28]] 06:43, 13 September 2018 (UTC)

::It is a good read. To summarize the main points of that article:
::* GPS satellites contain {{w|atomic clock}}s to keep time, and they broadcast their current time. A GPS receiver measures the amount of ''time'' it took for the signals to arrive from the satellites' known positions, using ''{{w|trilateration}}'' to determine the receiver's position. (This is different from ''{{w|triangulation}}'', which determines position by measuring ''angles''.)
::* GPS satellites orbit at a speed of 14,000 km/h relative to Earth's surface. One effect of this is {{w|Time dilation#Velocity time dilation|velocity time dilation}}: time passes ''slower'' for GPS satellites than it does near Earth's surface, drifting 7 microseconds ''behind'', every day.
::* GPS satellites orbit at an altitude of 20,000 km, which is far enough away from Earth's mass that {{w|spacetime}} curvature is different there. One effect of this is {{w|Time dilation#Gravitational time dilation|gravitational time dilation}}: time passes ''faster'' for GPS satellites than it does near Earth's surface, drifting 45 microseconds ''ahead'', every day.
::* Combining those, time passes faster for GPS satellites, by a net 38 ''microseconds'' (10-6) every day, relative to near Earth's surface.
::* But GPS precision depends on ''nanosecond''-range accuracy (10-9), so 38,000 nanoseconds drift would cause GPS precision to accumulate an ''additional 10 km of error every day''. GPS would still ''function'' – the satellites would still transmit, and the receivers would still calculate a position – but that position could be out by miles and miles; so if the purpose of GPS is to provide a reasonably precise position, then by that standard, GPS wouldn't ''work''.
::* {{w|Albert Einstein}}'s theories of {{w|special relativity}} and {{w|general relativity}} enable GPS to predict ''velocity time dilation'' and ''gravitational time dilation'' (respectively), and thereby compensate for the error so that GPS can ''work''!
::[[User:Yfmcpxpj|Yfmcpxpj]] ([[User talk:Yfmcpxpj|talk]]) 21:58, 5 September 2020 (UTC)
:::I'm sorry, but to me, your summary seems to make it sound like relativity is a problem that we are correcting for, rather than enabling anything. If relativity didn't exist, we wouldn't need to correct for velocity and gravitational time dilation. Mind you, that's only what I get from your summary, haven't actually read the article.--[[Special:Contributions/172.68.94.150|172.68.94.150]] 22:35, 24 March 2023 (UTC)