explain xkcd - User contributions [en]

Talk:2596: Galaxies

2022-03-23T15:19:38Z

Owengwynne:

I bet this was inspired by the image from the James Webb Space Telescope after fine alignment was complete: https://www.nasa.gov/sites/default/files/thumbnails/image/telescope_alignment_evaluation_image_labeled.png. That image shows the one star that JWST was focused on, along with a bunch of galaxies in the far background demonstrating that there are a lot of galaxies. [[User:Orion205|Orion205]] ([[User talk:Orion205|talk]]) 04:52, 22 March 2022 (UTC).
:My thoughts exactly - definitely a JWST thing.[[Special:Contributions/172.70.218.75|172.70.218.75]] 07:09, 22 March 2022 (UTC)
::I do not really think so. Not directly. The image is some weeks old now, and also the knowledge of the number of galaxies comes from the Hubble Deep Field. Which Randall has mentioned before, at least in his Thing explainer. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 13:43, 22 March 2022 (UTC)

Speaking of JWST, isn't the "''only a few stars, and probably no planets.''" just a nod to the fact that one of the missions of its infrared astronomy is to [[wikipedia:James_Webb_Space_Telescope#Infrared_astronomy|"see back in time to '''the first galaxies forming''' just a few hundred million years after the Big Bang."]]? i.e.: given the conical shape of the sector of universe covered by the pin-hole, most of the galaxies in it will be the furthest back, thus those of which we see the "oldest" picture and thus which much earlier in their process of forming stars and planets? [[User:DrYak|DrYak]] ([[User talk:DrYak|talk]]) 09:17, 22 March 2022 (UTC)
:Maybe but we cannot see them before the stars have formed and begun shining light. And by that time most of the planets would have formed. And also many many stars. So I do not think there is anything in the title text other than to make you think that what he says there is just plain wrong, and thus become even more overwhelmed! --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 13:43, 22 March 2022 (UTC)
::I sort of thought it might be an(other) H2G2 reference, to whit...
Population: none.
It is known that there are an infinite number of worlds, simply because there is an infinite amount of space for them to be in. However, not every one of them is inhabited. Therefore, there must be a finite number of inhabited worlds. Any finite number divided by infinity is as near to nothing as makes no odds, so the average population of all the planets in the Universe can be said to be zero. From this it follows that the population of the whole Universe is zero, and that any people you may meet from time to time are merely the products of a deranged imagination.
::...but worlds-to-galaxies, by the same 'logic', rather than beings-to-worlds. But I'm not sure enough to mention it in the main article, because it disagrees on the basic 'number of planets' issue (indeed, whether the ((observable)) universe is infinite). But putting it out there because it's perfect Adamsonian logic. ;) [[Special:Contributions/172.70.90.173|172.70.90.173]] 14:09, 22 March 2022 (UTC)

While space contains contains a vast number of things, it's still mostly empty. Like, on average, few atoms in whole square meter. Atoms of hydrogen, usually. Imagining so much of practically empty space is even harder than imagining all the stuff in it. -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 05:50, 22 March 2022 (UTC)

It took me half a minute to understand it: not galaxies within the circle, but within the region of the sky with the same apparent size from your position as the circle. [[Special:Contributions/162.158.22.175|162.158.22.175]] 10:19, 22 March 2022 (UTC)
:And you can move that circle anywhere you like. Also looking down at Earth surface. Then there will still be 50,000 galaxies within that circle on the other side of the Earth. As long as you hold it at arms length. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 13:43, 22 March 2022 (UTC)

Another way to consider this is that the moon is a privacy screen for 1.5 million galaxies at any one time. [[Special:Contributions/172.70.131.122|172.70.131.122]] 16:55, 22 March 2022 (UTC)

The size of the circle could be more accurately estimated by noting that the majority of phones have between a 4.7 to 5.8 inch diagonal (https://deviceatlas.com/blog/viewport-resolution-diagonal-screen-size-and-dpi-most-popular-smartphones). While aspect ratios vary, that seems a decent approximation for the diagonal of the image, and from that, the size of the circle could more accurately estimated from its pixel size relative to the image diagonal. Overkill, though, I know. [[Special:Contributions/172.70.131.122|172.70.131.122]] 16:55, 22 March 2022 (UTC)

"Most are undetectable by even our most powerful astronomical instruments today," - surely the point is that they ''are'' all detectable by the most powerful instruments, otherwise we could not count how many (typically) there are. It's just that we ''need'' more than your 'average' telescope, i.e. Hubble/Webb. The HDF image shows 3,000 items in a solid-angle approximately 1/24millionth of the sky, by the way, which is (at 50cm radius) approximately an eighth of a mm². 50,000 <s>stars</s>galaxies would be in ~2mm² of such an image, which is a circle almost 0.8mm in radius - if I haven't slipped up somewhere in the mental arithmatic (someone will tell me that I have, probably!). But that seems to relate well to the scenario suggested. [[Special:Contributions/141.101.98.145|141.101.98.145]] 20:29, 22 March 2022 (UTC)

:"Surely the point is that they ''are'' all detectable by the most powerful instruments" - no, not necessarily. I figure that we can guess at the number of galaxies out there without being able to detect them all - by estimating the density of galaxies and considering the size of the universe, for example; and it may be that some galaxies are so far away and that the universe is expanding at such a rate that energy from those galaxies has not yet reached us and might never. - [[User:Brian Kendig|Brian Kendig]] ([[User talk:Brian Kendig|talk]]) 20:47, 22 March 2022 (UTC)
::Both those points are (differently) addressed by the fact that we're talking about the ''Observable Universe''.
::We're already seeing to the edge of where galaxies are. In reality, as far back in time as we can, when galaxies and their composition of stars were about as young as they could be, meaning that there's not many things yet to be seen beyond (though whatever we ''do'' end up seeing is going to be interesting to science).
::Anything whose image will never reach us is beyond the limit of the O.U. and is, by definition, unknowable and uncountable.
::(Maybe the interface between the matter and antimatter remnants of the original ''actually'' balanced universe is beyond this effective limit? Or we may indeed have an infinite universe that, if we could see it all, means there's ''no'' patch of dark sky, because there's always something bright in whatever bit of the sky we look, at whatever scale or level of zoom.)
::Actually, I tend to suspect that the real problem is that the universe has a wrap-around topology which means that looking far/long enough in any direction (barring expansion or even big-rip limits to what we could see) means we start seeing things from the opposite side of the entire field of view but from the other direction... Then everything closer to us (beyond that) then eventually, ourselves. And beyond us (*hand-wave away what happens if some strangeness of the situation means we're effectively the thing we see 28.5 gigaparsecs or more away ''in every direction at once'' and so can't be seen beyond... ;) *) a smaller copy of everything we's been peering between to see our current view....
::Not that we could (speed of light + age of universe + continuing expansion of universe + any acceleration of expansion + something else we probably don't know of yet), but I have an unhealthy affinity for the concept of a boundless but finite universe, like the 3D surface of a hypersphere where looking in any direction (including a time-component!) eventually wraps-around on a Great Circle analogue. ;-p
:: ...anyway, the point being that if we have a problem with counting visible objects, it's more likely that we can be counting some things twice or more (cosmos-wide gravitational lensing?). Anything truly beyond visibility is irrelevent, and probably won't become any more relevent any time soon. But we can develop means to better see things that aren't that easy to see, yet can be seen with enough effort. [[Special:Contributions/172.70.162.147|172.70.162.147]] 23:18, 22 March 2022 (UTC)

:Also note that the Hubble Deep Field depicts approximately 50,000 <i>galaxies</i>, not just stars. See [https://en.wikipedia.org/wiki/Hubble_Deep_Field]. - [[User:Brian Kendig|Brian Kendig]] ([[User talk:Brian Kendig|talk]]) 20:50, 22 March 2022 (UTC)
::My error, now corrected. I was refering to the galaxies that both the comic and the HDF were enumerating, but I ''wrote'' stars - for reasons best known to my dithering mind at the time. But nice to see you're reading my waffle and actually paying attention. ;) [[Special:Contributions/172.70.162.147|172.70.162.147]] 23:18, 22 March 2022 (UTC)

I measured the size of the circle compared to the frame width (it's 1/40 of the frame width). I left the other calculations alone, but someone should probably re-scale them, as they presume that the frame is 40mm wide, which seems small for "fullscreen on your phone". Or maybe the calculations are wrong; if I had to bet between Randall and some random Internet editor (myself being included in that category) I'd bet on Randall. -- [[User:Dtgriscom|Dtgriscom]] ([[User talk:Dtgriscom|talk]]) 10:29, 23 March 2022 (UTC)

I question the comment about the title text being 'sarcasm' on the basis that "we know that galaxies contain billions of stars and most of them will have planets" (I paraphrase). My view is that most of the 50,000 galaxies are likely to be very young, and we believe that the first generation of stars were large, few and short lived.
As to whether they would have had multiple planets, this is an open question. Certainly, there wouldn't have been rocky planets, as the stars will have formed from the elements formed in the original big bang, which was almost exclusively Hydrogen and Helium, with essentially no heavier elements to form anything like rocky planets.
Gas giant planets may have formed alongside the first massive stars, but we just don't know.[[User:Owengwynne|OwenGwynne]] ([[User talk:Owengwynne|talk]]) 12:00, 23 March 2022 (UTC)

Adding to the discussion about planets in galaxies: Most galaxies in the universe are [[wikipedia:Dwarf galaxy|Dwarf Galaxies]] (see also the external sources of the Wikipedia article). DG have very distinct physical properties than our Milky Way, they contain older and/or bigger (=hotter) stars, less heavy elements (>He) and are more compact, i.e., there are more stars in less space. They also seem to have more dark matter per visible mass than larger galaxies. All of these properties make it more improbable for the stars to form planets, and even more so small rocky ones. And it probably also makes it more difficult to create life, as the cosmic vicinity of each star is much more unstable.

Not relevant, but a point about the numbers involved. It's estimated that there are 200 billion galaxies in the observable universe - undoubtedly a large number. It is also estimated that there are 3 ''trillion'' trees on Earth, so maybe 15 trees for every galaxy.

Talk:2596: Galaxies

2022-03-23T12:02:42Z

Owengwynne:

Talk:2596: Galaxies

2022-03-23T12:01:49Z

Owengwynne: apply personal tag

I bet this was inspired by the image from the James Webb Space Telescope after fine alignment was complete: https://www.nasa.gov/sites/default/files/thumbnails/image/telescope_alignment_evaluation_image_labeled.png. That image shows the one star that JWST was focused on, along with a bunch of galaxies in the far background demonstrating that there are a lot of galaxies. [[User:Orion205|Orion205]] ([[User talk:Orion205|talk]]) 04:52, 22 March 2022 (UTC).
:My thoughts exactly - definitely a JWST thing.[[Special:Contributions/172.70.218.75|172.70.218.75]] 07:09, 22 March 2022 (UTC)
::I do not really think so. Not directly. The image is some weeks old now, and also the knowledge of the number of galaxies comes from the Hubble Deep Field. Which Randall has mentioned before, at least in his Thing explainer. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 13:43, 22 March 2022 (UTC)

Speaking of JWST, isn't the "''only a few stars, and probably no planets.''" just a nod to the fact that one of the missions of its infrared astronomy is to [[wikipedia:James_Webb_Space_Telescope#Infrared_astronomy|"see back in time to '''the first galaxies forming''' just a few hundred million years after the Big Bang."]]? i.e.: given the conical shape of the sector of universe covered by the pin-hole, most of the galaxies in it will be the furthest back, thus those of which we see the "oldest" picture and thus which much earlier in their process of forming stars and planets? [[User:DrYak|DrYak]] ([[User talk:DrYak|talk]]) 09:17, 22 March 2022 (UTC)
:Maybe but we cannot see them before the stars have formed and begun shining light. And by that time most of the planets would have formed. And also many many stars. So I do not think there is anything in the title text other than to make you think that what he says there is just plain wrong, and thus become even more overwhelmed! --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 13:43, 22 March 2022 (UTC)
::I sort of thought it might be an(other) H2G2 reference, to whit...
Population: none.
It is known that there are an infinite number of worlds, simply because there is an infinite amount of space for them to be in. However, not every one of them is inhabited. Therefore, there must be a finite number of inhabited worlds. Any finite number divided by infinity is as near to nothing as makes no odds, so the average population of all the planets in the Universe can be said to be zero. From this it follows that the population of the whole Universe is zero, and that any people you may meet from time to time are merely the products of a deranged imagination.
::...but worlds-to-galaxies, by the same 'logic', rather than beings-to-worlds. But I'm not sure enough to mention it in the main article, because it disagrees on the basic 'number of planets' issue (indeed, whether the ((observable)) universe is infinite). But putting it out there because it's perfect Adamsonian logic. ;) [[Special:Contributions/172.70.90.173|172.70.90.173]] 14:09, 22 March 2022 (UTC)

While space contains contains a vast number of things, it's still mostly empty. Like, on average, few atoms in whole square meter. Atoms of hydrogen, usually. Imagining so much of practically empty space is even harder than imagining all the stuff in it. -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 05:50, 22 March 2022 (UTC)

It took me half a minute to understand it: not galaxies within the circle, but within the region of the sky with the same apparent size from your position as the circle. [[Special:Contributions/162.158.22.175|162.158.22.175]] 10:19, 22 March 2022 (UTC)
:And you can move that circle anywhere you like. Also looking down at Earth surface. Then there will still be 50,000 galaxies within that circle on the other side of the Earth. As long as you hold it at arms length. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 13:43, 22 March 2022 (UTC)

Another way to consider this is that the moon is a privacy screen for 1.5 million galaxies at any one time. [[Special:Contributions/172.70.131.122|172.70.131.122]] 16:55, 22 March 2022 (UTC)

The size of the circle could be more accurately estimated by noting that the majority of phones have between a 4.7 to 5.8 inch diagonal (https://deviceatlas.com/blog/viewport-resolution-diagonal-screen-size-and-dpi-most-popular-smartphones). While aspect ratios vary, that seems a decent approximation for the diagonal of the image, and from that, the size of the circle could more accurately estimated from its pixel size relative to the image diagonal. Overkill, though, I know. [[Special:Contributions/172.70.131.122|172.70.131.122]] 16:55, 22 March 2022 (UTC)

"Most are undetectable by even our most powerful astronomical instruments today," - surely the point is that they ''are'' all detectable by the most powerful instruments, otherwise we could not count how many (typically) there are. It's just that we ''need'' more than your 'average' telescope, i.e. Hubble/Webb. The HDF image shows 3,000 items in a solid-angle approximately 1/24millionth of the sky, by the way, which is (at 50cm radius) approximately an eighth of a mm². 50,000 <s>stars</s>galaxies would be in ~2mm² of such an image, which is a circle almost 0.8mm in radius - if I haven't slipped up somewhere in the mental arithmatic (someone will tell me that I have, probably!). But that seems to relate well to the scenario suggested. [[Special:Contributions/141.101.98.145|141.101.98.145]] 20:29, 22 March 2022 (UTC)

:"Surely the point is that they ''are'' all detectable by the most powerful instruments" - no, not necessarily. I figure that we can guess at the number of galaxies out there without being able to detect them all - by estimating the density of galaxies and considering the size of the universe, for example; and it may be that some galaxies are so far away and that the universe is expanding at such a rate that energy from those galaxies has not yet reached us and might never. - [[User:Brian Kendig|Brian Kendig]] ([[User talk:Brian Kendig|talk]]) 20:47, 22 March 2022 (UTC)
::Both those points are (differently) addressed by the fact that we're talking about the ''Observable Universe''.
::We're already seeing to the edge of where galaxies are. In reality, as far back in time as we can, when galaxies and their composition of stars were about as young as they could be, meaning that there's not many things yet to be seen beyond (though whatever we ''do'' end up seeing is going to be interesting to science).
::Anything whose image will never reach us is beyond the limit of the O.U. and is, by definition, unknowable and uncountable.
::(Maybe the interface between the matter and antimatter remnants of the original ''actually'' balanced universe is beyond this effective limit? Or we may indeed have an infinite universe that, if we could see it all, means there's ''no'' patch of dark sky, because there's always something bright in whatever bit of the sky we look, at whatever scale or level of zoom.)
::Actually, I tend to suspect that the real problem is that the universe has a wrap-around topology which means that looking far/long enough in any direction (barring expansion or even big-rip limits to what we could see) means we start seeing things from the opposite side of the entire field of view but from the other direction... Then everything closer to us (beyond that) then eventually, ourselves. And beyond us (*hand-wave away what happens if some strangeness of the situation means we're effectively the thing we see 28.5 gigaparsecs or more away ''in every direction at once'' and so can't be seen beyond... ;) *) a smaller copy of everything we's been peering between to see our current view....
::Not that we could (speed of light + age of universe + continuing expansion of universe + any acceleration of expansion + something else we probably don't know of yet), but I have an unhealthy affinity for the concept of a boundless but finite universe, like the 3D surface of a hypersphere where looking in any direction (including a time-component!) eventually wraps-around on a Great Circle analogue. ;-p
:: ...anyway, the point being that if we have a problem with counting visible objects, it's more likely that we can be counting some things twice or more (cosmos-wide gravitational lensing?). Anything truly beyond visibility is irrelevent, and probably won't become any more relevent any time soon. But we can develop means to better see things that aren't that easy to see, yet can be seen with enough effort. [[Special:Contributions/172.70.162.147|172.70.162.147]] 23:18, 22 March 2022 (UTC)

:Also note that the Hubble Deep Field depicts approximately 50,000 <i>galaxies</i>, not just stars. See [https://en.wikipedia.org/wiki/Hubble_Deep_Field]. - [[User:Brian Kendig|Brian Kendig]] ([[User talk:Brian Kendig|talk]]) 20:50, 22 March 2022 (UTC)
::My error, now corrected. I was refering to the galaxies that both the comic and the HDF were enumerating, but I ''wrote'' stars - for reasons best known to my dithering mind at the time. But nice to see you're reading my waffle and actually paying attention. ;) [[Special:Contributions/172.70.162.147|172.70.162.147]] 23:18, 22 March 2022 (UTC)

I measured the size of the circle compared to the frame width (it's 1/40 of the frame width). I left the other calculations alone, but someone should probably re-scale them, as they presume that the frame is 40mm wide, which seems small for "fullscreen on your phone". Or maybe the calculations are wrong; if I had to bet between Randall and some random Internet editor (myself being included in that category) I'd bet on Randall. -- [[User:Dtgriscom|Dtgriscom]] ([[User talk:Dtgriscom|talk]]) 10:29, 23 March 2022 (UTC)

I question the comment about the title text being 'sarcasm' on the basis that "we know that galaxies contain billions of stars and most of them will have planets" (I paraphrase). My view is that most of the 50,000 galaxies are likely to be very young, and we believe that the first generation of stars were large, few and short lived.
As to whether they would have had multiple planets, this is an open question. Certainly, there wouldn't have been rocky planets, as the stars will have formed from the elements formed in the original big bang, which was almost exclusively Hydrogen and Helium, with essentially no heavier elements to form anything like rocky planets.
Gas giant planets may have formed alongside the first massive stars, but we just don't know.[[User:Owengwynne]] ([[User talk:Owengwynne|talk]]) 12:00, 23 March 2022 (UTC)

Talk:2596: Galaxies

2022-03-23T11:59:54Z

Owengwynne: Question the comment about the title text being 'sarcasm'

2447: Hammer Incident

2021-04-08T14:57:07Z

Owengwynne: /* Explanation */ change "lower wavelengths" to "longer wavelengths"

{{comic
| number = 2447
| date = April 7, 2021
| title = Hammer Incident
| image = hammer incident normal.png
| titletext = I still think the Cold Stone Creamery partnership was a good idea, but I should have asked before doing the first market trials during the cryogenic mirror tests.
}}

==Explanation==
{{incomplete|Created by: a WAFFLE CONE MIRROR. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}

The {{w|James Webb Space Telescope}} (JWST) is a {{w|space telescope}} created to be the successor of the {{w|Hubble Space Telescope}} under construction at time of publishing and expected to launch in October 2021.

It's implied that Cueball dropped a hammer on the mirror of the JWST, and breaking mirrors in superstition causes seven years of bad luck. But since the mirror panel is not made of glass it is likely that a dropped hammer would dent and distort the panel rather than shatter it.

NASA is quite angry as the telescope was very expensive (current estimate US$10 billion). While Cueball appears to only damaged or destroyed one mirror panel (The mirror is made of eighteen separate panels, so if he had destroyed the entire telescope, he would have been facing 7×18=126 years of bad luck.), even breaking a single panel would likely be very expensive because it would have required extremely accurate machinery to make and extensive calibration tests. Another factor making the mirror panels more expensive is the fact that they were coated with gold (While most mirrors are coated with silver or another reflective gray metal in order to reflect all of visible light, the designers of the James Webb Telescope wanted it to be able to view redshifted galaxies, which would have required the mirror to be able to reflect infrared light. While silver is good at reflecting visible light, it is a poor reflector of infrared light, as are most other metals, but relativistic effects shift gold's spectral range of reflectivity to longer wavelengths so that it reflects near- and mid-infrared light but not the shorter wavelengths of visible light.) While Cueball hasn't destroyed the individual gold atoms, so the gold at lest theoretically can be melted down, salvaged, and reused, expensive measures must be taken in order to ensure that no gold is lost during the process, and it's possible that a little gold will be lost even then, in which case replacing it would be expensive because gold is very valuable. Also, NASA would have to redo a bunch of background checks and other security precautions for the goldsmiths, and those would likely be expensive. However, Cueball is more concerned about the fact that he has 7 years of bad luck. Cueball seems be on trial for breaking a very expensive piece of equipment. The trial seems to be conducted by NASA, given the caption below.

The title text refers to the ice cream chain {{w|Cold Stone Creamery}}. Since the cryocooler of the JWST cools things down to 7.00K (-266.15°C, or -447.07°F), it would be a bit of an overkill. Cold Stone Creamery mixes ice cream with various other ingredients, such as fruit or candy, in front of the customer before serving it to the customer. The usual surface for mixing is a piece of metal or marble which is kept quite cold (about -10 C), but nowhere as cold as components of the JWST. If Cueball had tried mixing his ice cream and flavourings in the style of Cold Stone Creamery on the JWST, it would have messed up the surfaces on the telescope and perhaps have gotten into the instrumentation. If it was the finely polished reflective surfaces, he may end up scratching and/or staining them beyond usability. Possibly, due to the localised temperature differential from ice-cream hundreds of degrees ''warmer'' than the material, promoted damaging distortions/fractures. If Cueball was mixing ice cream within a 7K cryo chamber, it also raises the question of how he could have survived. Also, if such an irresponsible person was able to enter the cryo chamber where the expensive telescope was being kept, it raises the question of whether NASA has any sort of security.

The JWST has previously been mentioned in [[2014: JWST Delays]], [[1730: Starshade]], and [[1461: Payloads]]

For some reason, the image for this comic is very large (4332×4838px). This is probably a mistake, as the double-resolution version [https://imgs.xkcd.com/comics/hammer_incident_2x.png] is significantly smaller at 578×645px. This appears to have been fixed.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:[Cueball stands before a seated panel of four people (Ponytail, Hairy, a second Cueball and Hairbun).]
:Cueball: Yes, I know you're mad that I dropped that hammer.
:Cueball: But think about me—
:Cueball: '''''Seven years of bad luck!'''''
:[Caption below the panel]:
:Man, NASA is really on my case about the James Webb Space Telescope.

{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring Hairy]]
[[Category:Comics featuring Hairbun]]
[[Category:Multiple Cueballs]]
[[Category:Telescopes]]