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May 31

Is redshift calculated differently for different spectra?

I ask because I came across an article, TXS 1545-234, in the course of regular gnoming. The article claims this radio galaxy to be one of the most distant known objects, but its redshift is only around $z$ = 2.754 (which I take to be measured from radio emissions). It does appear in this source (|date= at least 2006), and I'm wondering whence the claim of such great distance paired with such pedestrian redshift. I'm not able to understand our article Redshift. Also, if anyone has any ideas about how to de-orphan the article linked, please do have at. Folly Mox (talk) 11:31, 31 May 2024 (UTC) edited 11:52, 31 May 2024 (UTC)[reply]

It is certainly not one of the most distant galaxies known. The author of the article, @Galaxybeing:, should explain why they think it is. --Wrongfilter (talk) 12:12, 31 May 2024 (UTC)[reply]

They appear to have made a similar claim at MRC 0406-244 (

z

= 2.44) although thankfully not at another recent creation, QSO J0100-2708 (

z

= 3.52). Folly Mox (talk) 12:57, 31 May 2024 (UTC)[reply]

The redshift, a dimensionless quantity, is the same for the whole spectrum emitted by an object. Compared to TXS 1545-234, JADES-GS-z13-0 is thought to be more than three times as far away from us. --Lambiam 12:37, 31 May 2024 (UTC)[reply]

Thanks both for your answers, and confirming my suspicion that the claim was merely incorrect. Folly Mox (talk) 13:00, 31 May 2024 (UTC)[reply]

A redshift of 2.754 is pretty far away, but stating it's one of the most distant known objects is overstating it a bit. Distances to far-away galaxies are normally expressed in redshift, as redshift is directly observable, in contrast to distance, which depends on a model of the expansion of the universe. But when using redshift as a distance measure, one has to keep in mind that it's highly non-linear. Also, distance is a bit of a strange concept when dealing with these cosmologically distant objects. Are we talking about the distance today, or at the time the light was emitted, or the distance travelled by the light? That last at least has some relevance as it translates to the time that the light has travelled and therefore when it was emitted. JADES-GS-z13-0 may be several times farther away today than TXS 1545-234, but most of that is thanks to the expansion of the universe after the light was emitted. When the universe was young and small (although still infinite), it expanded fast in absolute numbers (percentage per year). In light travel distance, the difference isn't so much.

These objects can be studied to learn more about the early universe. For that, knowing the distance to us is not so important; we want to know about the distance (or time) to the Big Bang. At some point, distances (times) to the Big Bang are known more accurately than distances (times) to us. In any case, the redshift tells us immediately that the universe expanded by a factor of five between the times when the light of JADES-GS-z13-0 was emitted and when the light of TXS 1545-234 was emitted. That puts JADES-GS-z13-0 a lot closer to the Big Bang, although only a small fraction farther from us. PiusImpavidus (talk) 11:05, 1 June 2024 (UTC)[reply]

On a tangent but an amusing use (abuse?) of "red shift": chemists use red shift generically to mean "moves to lower energy". Even in the IR part of the spectrum, the term red shift would be used to describe the shift of a band to lower E, say 2000 to 1950 cm-1. This language is of course strange because, formally speaking, a shift toward red for an IR band would mean a shift to higher E. Just sayin'.--Smokefoot (talk) 17:11, 1 June 2024 (UTC)[reply]

Radioastronomers apply the same "abusive" terminology. We see this linguistic phenomenon also in uses of the verb "to dial", as in the advise to "dial 9-1-1 for any emergency" given to users of smartphones with touch screens. (Using the original rotary sense of the verb in connection with the casual parlance of "butt dial" results in the unfortunate mental image of Giuliani twerking.) Other examples are referring to cotton bed sheets as "linens", or (in the US) to stainless-steel knives, forks and spoons as "silverware", and the computer-graphics terminology calling a screen region a "canvas". I think there is a learned term for this phenomenon if the sense of a word getting abstracted from the physical embodiment after which it was originally named. --Lambiam 05:29, 2 June 2024 (UTC)[reply]

Astronomers talk about redshift if it goes to longer wavelengths, in radio, IR, UV etc., and about blueshift if it goes to shorter wavelengths, in radio, IR, UV etc. Things can also redshift past red. See how red that distant object in this picture is? It's a feature coming from UV, shifted to IR. The Lyman-alpha absorption line is at 121.6 nm, here broadened into a Gunn–Peterson trough, redshifted to 1.6 μm, between the F150W and F200W filters of the camera in JWST. It's how they make a first estimate of the redshift, based on broadband images. A precise number follows later from spectroscopy, but takes far more observing time, so this is only done for the most promising targets. PiusImpavidus (talk) 10:18, 2 June 2024 (UTC)[reply]

Physics problem

A shop sign is made of a panel that protrudes slightly from the wall on which it is hung, forming an angle of 5° with it. It is 0.74 m tall and has a mass of 8.9 kg. The upper side of the panel is attached to the wall by two cables, one from the right side and one from the left side. Find the tension of the 2 cables 78.211.54.11 (talk) 19:40, 31 May 2024 (UTC)[reply]

Original in Italian, now edited by IP to English.Translates as: "A shop sign consists of a panel that protrudes slightly from the wall on which it hangs, forming an angle of 5° with it. It is 0.74 m high and has a mass of 8.9 kg. The upper end is attached to the wall by two cables, one on the right side and one on the left side. Find the tension in the 2 cables."

IP editor: as it says at the top of this page, we don't answer homework questions and what we do answer should preferably be asked in English. Mike Turnbull (talk) 19:46, 31 May 2024 (UTC)[reply]

Asking homework questions also causes tension. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:15, 1 June 2024 (UTC)[reply]

Doesn't the tension depend on the angle the cables (idealized as straight line segments) make with the panel? --Lambiam 02:53, 1 June 2024 (UTC)[reply]

Correct. The answer to the question as asked is a curve of tension vs cable length or vertical location. Greglocock (talk) 22:56, 1 June 2024 (UTC)[reply]

In my interpretation of the problem – which is not necessarily the intended one – the lower edge of the panel is attached to the wall and the panel can rotate along that edge, like in the "or is it like this?" diagram, which has an exaggerated thickness for the panel. Unlike that diagram, the panel does not stick out at a 90° angle but is standing almost upright. Also, the cables are attached to the upper edge. So it is more like the situation here to the right, but instead of a bottom 60° angle we have a 5° angle. Not enough info has been given to determine the other angles. --Lambiam 04:45, 2 June 2024 (UTC)[reply]
P.S. Another way to frame the problem could have been in terms of a drawbridge. --Lambiam 04:53, 2 June 2024 (UTC)[reply]

The answer is 42 (glorps). I'll leave it to you to figure out the conversion factor to mks. Clarityfiend (talk) 12:06, 3 June 2024 (UTC) [reply]

                |
      		|'.
		|  '.
		|    '.
		|      '.   
		|        '.
		|          ::
		|         ::
		|< 5deg >::
		|       ::
		|      ::
		|     ::.
		|    :: .
		|   ::  .
		|  ::   .
		| ::    .
		|::     V
		|*      8.9kg

Equate turning moments at *
                      clockwise  =  anticlockwise
 8.9 kg x sin(5 deg) x 0.74m / 2 = t/2 x 0.74m
                         where t = tension in each of 2 cables
                                 = 8.9 sin(5 deg)
                                 = 0.775686... kg

Philvoids (talk) 13:16, 3 June 2024 (UTC)[reply]

Shouldn't there be a sine or cosine factor in the anticlockwise term? Imagine the cables being attached to the wall very close to the *. Consider dU/dL, where U is potential energy of the panel and L is the length of the cables. --Lambiam 15:27, 3 June 2024 (UTC)[reply]

Although not accurately shown in the ASCII sketch my calculation presumes that the two cables are at right angles to the sign, are parallel and are fastened to the wall separately at two points. This minimises the cable lengths and tensions. Philvoids (talk) 10:28, 4 June 2024 (UTC)[reply]

The homework problem did not specify where the cables are attached or that the tension should be minimized, making it unsolvable for the average student. For an old-fashioned drawbridge over a castle moat, as in the animation, a better choice is to place the attachment point of the cables at a distance from the hinge equal to the length of the bridge, making the triangle isosceles. Otherwise, the tension grows without bounds as the bridge near a vertical position. --Lambiam 15:37, 4 June 2024 (UTC)[reply]

I could not find an ASCII symbol for a hinge but installing one at the base of the sign is a well thought mechanical improvement that can stop the thing flapping about in a wind. This reference desk cannot grant permission to construct a drawbridge with walkway at this location and the shop owner of the wall would likely protest at your plan to cut a hole for passage. Unless he is a herder of sheep or other small animals and himself less than 74cm tall and sees some advantage. The OP doesn't ask for the cable tension when the sign/bridge is vertical but we can say there are two cases: Case #1: The hinge is broken or absent. t = 8.9/2 = 4.45 kg plus distributed weight of the cable itself; Case #2: The hinge takes the weight, t = 8.9 sin (0) so both cable lengths and tensions are zero. In that case it would be simpler just to nail the sign to the wall. Or put out a call to Banksy who can save us the cost of the sign, usually upset someone and greatly increase the value of the wall. Philvoids (talk) 18:39, 4 June 2024 (UTC)[reply]

For all we know, there is already an opening behind the sign for letting the proprietor's homing pigeons in. If they are equipped with an RFID chip, the sign can be let down automatically on their arrival. --Lambiam 05:39, 5 June 2024 (UTC)[reply]

Pigeon RFIDs can be compromised by malicious Yinpterochiroptera and Yangochiroptera. Members of this notorious "YinYangBat Gang" equipped with ultrasound-to-UHF converters can mount Spoofing attacks on the shared ID frequency. An ad hoc aposematic remedy such as a wall painting of a cat with the sign text "I EAT BATS" does not alone give security. To this end all pigeons must be urged to register a pass-coo that is less obvious than the too common "Coo Coo". Philvoids (talk) 10:28, 5 June 2024 (UTC)[reply]