This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
This article is within the scope of WikiProject Measurement, a collaborative effort to improve the coverage of Measurement on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Under normal circumstances, no material object can travel faster than the speed that light propagates in a vacuum. Particles routinely move faster than light in some media, such as the water used as coolant in nuclear reactors(see Cherenkov radiation). However, even the general light-speed rule seems to be abrograted by cases of quantum tunneling, and several laboratory experiments have suggested that light can, in some cases, move faster than the standard 299,792,458 m/s. See Theory of relativity.
Yes, it belongs in speed of light, although the last sentence has to be qualified; these experiments show a high speed of light for some suitable definition of "speed of light" and in no case can you transmit information that fast. AxelBoldt 00:29 Jan 9, 2003 (UTC)
The observable universe is bigger than 13 billion light years
Due to expansion of space, the Hubble Volume is about 46 Billion light years. I've corrected the paragraph. (jcl July 17 2005)
I mean, a kilometer is already a derived unit from the meter using a metric prefix, and the trillion word just makes things more confusing than they should, imho. What's the point of having all those prefixes if we never use them? I suggest we write that one light-year is about 9.5 Pm.--Grondilu (talk) 12:40, 28 February 2014 (UTC)
Two reasons. First, we are able to give the metres value precisely, which isn't actually possible for the others. That gives people a base from which to perform precise conversions for other scales. Second, petametres aren't really a unit that people can relate to in the same way they can relate to metres. There are dozens of situations in which one of the various metric prefixes could be used but aren't (eg. earth's circumference = 4 myriametres; Earth-Moon distance = 380 megametres). This is no different. Rhialto (talk) 22:09, 28 February 2014 (UTC)
I agree with Grondilu that using the ambiguous word "trillion" is confusing because the word means something different to me (I would use "billion" for a million million). However, the note clarifies the meaning sufficiently, and the "peta" prefix is seldom used in everyday English, so I'm happy to leave the article as it is. Dbfirs 11:17, 1 March 2014 (UTC)
I'm going to argue that the high prefixes Mega, Giga, Tera, Peta and so on are getting more and more known even by the layman. This is due to their relatively recent use in consumer electronics. At some point in the future, the fact that Wikipedia does not use those prefixes despite the fact that they are available and well known, is going to be seen as an oddity and will fail under the principle of least astonishment. I believe this time has pretty much come.--Grondilu (talk) 22:16, 3 March 2014 (UTC)
At this stage, those only see regular usage within the field of consumer electronics. Once they see regular usage in the wider world in relation to units of length, then yes, WP should follow suit. But WP should follow current trends, not lead them. The current trends in measuring distance (or anything really outside of consumer electronics) do not yet include widely using those prefixes. Rhialto (talk) 09:47, 4 March 2014 (UTC)
Did Bessel really introduce the word light-year?
The article currently says: "Bessel ... announced that the distance to 61 Cygni was 10.3 light-years. This was the first appearance of the light-year as a unit of distance." No reference is given to support that claim. To me it seems doubtful that the meticulous scientist Bessel would have introduced the confusing name light-year as a unit of distance. In his 1838 article he mentioned that it takes 10.3 years for light to travel from 61 Cygni to us, but he did not use the word light-year. Ceinturion (talk) 12:25, 29 March 2014 (UTC)
I have removed Bessel's connection with the light-year unit from the article. While the source (National Geographic) did say "10.3 light-years" in one place, it immediately qualified the use of the unit as sometimes being attributed to Bessel, but noting that that is not a well-accepted view. It was a casual mention only, and lacked any implication of scholarly backing even then. Evensteven (talk) 23:50, 29 March 2014 (UTC)
Ok. Using Ngram viewer I came across an early use of the light-year unit in the Monthly Notices of the Royal Astronomical Society from 1864. The article was a review of new parallax data from the Königsberg Observatory, where Bessel had worked until his death in 1846. The distance of two stars is given in light-years (61 Cygni 5.77 ± 0.017 light-years, and another star 12.01 ± 0.50 light-years). The reviewer did not pay attention to the light-year unit, which suggests that it already was a common unit for star distances before 1864. Ceinturion (talk) 21:00, 30 March 2014 (UTC)
That's useful to the article in itself. Would you like to put it in? Evensteven (talk) 21:13, 30 March 2014 (UTC)
Let's have Ngram search for German sources first. The first appearance of Lichtjahr is in 1851, in an article by Otto Ule (Was wir in den Sternen lesen, in Deutsches Museum: Zeitschrift für Literatur, Kunst und Öffentliches Leben, Volume 1): Wo aber finden wir ein größeres Maß, ein Maß, das unserer Vorstellung noch Raum fur seine Vervielfachung gibt? Wir wollen uns auf eine Reise begeben und auf Reisen pflegt man nicht nach Meilen, sondern nach Stunden zu fragen. Wie, wenn wir auch am Himmel nach Stunden, oder vielleicht nach Jahren die Entfernungen messen, wenn wir uns aber nach einem etwas schnellern Läufer als den langsamen menschlichen Fuß, oder selbst der schleichenden Locomotive umfähen? So muß also wohl das Licht diesen Läufer abgeben, das Licht, das in einer Secunde 42,500 Meilen .. zurücklegt. Das Maß, mit dem wir den Himmel zu messen wagen, sei also das Lichtjahr, den das Licht in einem Erdenjahr durchlauft .. Das sei unsere himmliche Wegstunde! Another interesting explanation is given in 1855 by Adolph Wilhelm Diesterweg (in Populäre Himmelskunde und astronomische Geographie): Man hat diesen Raum (seltsamer Weise) ein Lichtjahr gennant. .. Die Natur hat uns ein absolutes Zeit-, nicht ein absolutes Maaß fur den Raum gegeben. Jenes besitzen wir in dem sich immer gleichbleibenden Sterntag. Die Franzosen glaubten in den zehnmillionsten Theile eine Meridian-Quadranten, den sie Metre nannten, ein sich immer gleich bleibendes Naturmaaß fur den Raum zu finden; aber die Unregelmäßigkeit des Erdkörpers vereitelte die Hoffnungen. Somit bleibt uns nur die Wahl unter künstlichen (conventionellen) Raummaaßen übrig. Wünschenswert aber bleibt die Wahl eines Raummaaßes fur alle Länder der Erde.Ceinturion (talk) 20:56, 31 March 2014 (UTC)