Talk:Light year
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The page light year has been moved around a bit. For some reason, the page history is now scattered over two pages. A part of the page history is in light-year/more (history (http://en.wikipedia.org/w/wiki.phtml?title=Light-year/more&action=history)).
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1 year equals 365.25 days
JHKJHArticle said light year was defined using the time the Earth takes to orbit the sun. It isn't -- its defined in terms of the Julian year (365.25 days of 86400 SI seconds each). The reason for this is that the time it takes for the Earth to orbit the sun can be known only to limited accuracy, and varies over time. TJHhus if the light year was defined based on that, it would not be a stable nor accurate measurement. (Although arguably the amount of inaccuracy and stability would be insignificant over the distances the light year is used to measure.)
- In german Wikipedia the light year is said to be based on the tropical year. Is that definitely wrong? Could anyone please point out a reference where the internationally accepted definition (if any exists) of a light year is shown?--SiriusB 13:50, 20 May 2005 (UTC)
Move to speed of light?
Axel removed this:
- 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.
Is it incorrect, or just in the wrong place? Can it be be moved to speed of light? -- Tarquin
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)
Spelling: lightyear?
Would there be any objection to using the compound form lightyear? It actually gets about 7 times as many yahoo! returns. Pizza Puzzle
- At least I don't object. But is it necessary? User:Wshun
Nothing is necessary. Pizza Puzzle
- please could someone check with the OED. My dictionary gives "light year" in two words. -- Tarquin 21:34, 22 Sep 2003 (UTC)
Google gives a ratio of 5:1 in favour of lightyear. Dictionary.com (http://dictionary.reference.com) also gives preferences to a combined form. Pizza Puzzle
- Regardless of what's more popular among the general public, practicing physicists and astronomers always use "light year" or "light-year", never "lightyear". So the page should be moved back. -- BenRG 21:49, 22 Sep 2003 (UTC)
I agree with BenRG. Most of books I read use "light year" not lightyear. A number of Google hits hardly matter because articles on the wikipedia should sound authentic, not popular, common or ordinary. -- Taku 21:55, 22 Sep 2003 (UTC)
- I moved the page back. -- BenRG 22:15, 22 Sep 2003 (UTC)
(Incidentally, regarding the 5:1 ratio on Google, notice that all of the top 10 hits for "light year" point to astronomy-related sites, while none of the top 10 hits for "lightyear" point to anything remotely astronomy-related. I went as far as the eighth page of hits without finding any astronomy sites. Buzz Lightyear appears to be mostly responsible for the large total hit count.)
So how about light-year? Pizza Puzzle
hyphen
Why the modern hatred of the standard (traditionally standard, anyway) use of hyphens? Anyone who doubts their utility should consider the difference in meaning between a headline that says
- New Age-Discrimination Rules Released
and one that says
- New-Age Discrimination Rules Released.
I found "light-year" redirecting me to "light year"; I have interchanged the two pages. -- Mike Hardy
- But since you're not a logged in user, you've done a cut & paste job -- so we've left the article history behind. I've seen your name on many talk pages -- have you any plans on creating a login for yourself here? I'm restoring the old way round -- my dictionary (Collins) has "light year" and doesn't mention a hyphenated form -- Tarquin 00:02 Dec 17, 2002 (UTC)
- It should be "light year". Hyphens have a grammatical function when they connect nouns in English in phrases such as the new-age example above and are not part of the English-language spelling itself. It's still "new age" without hyphen. One could write: "A light-year-long distance", but "a distance of one light year". Due to the talk-page discussion under "light year", I cannot move this quite-well-written page to "light year", so I'll leave it as it is. - Hankwang 13:24, 24 Mar 2004 (UTC)
time dilatation
Caveat: since our galaxy is 100,000 light years across, a hypothetical spaceship travelling close to the speed of light would need somewhat longer than 100,000 years to cross it. However, this is only true for an observer at rest with respect to the galaxy; the space ship's crew will experience the trip across the galaxy in a matter of minutes. This is because of the time dilation of moving clocks explained by special relativity.
- I am not a physicist, but this conclusion of Einstein's theory sounds wrong. The space ship's crew should take 100,000 years to cross the galaxy in their own time, yes? Otherwise, relative to their own time frame, the ship's crew would have travelled far faster than the speed of light if they traverse the distance in only a few minutes. Alex.tan 03:47, 16 Feb 2004 (UTC)
- I'm no physicist either, but I'm pretty sure that explanation sounds right. Or at least it gibes well with what I remember the example being from my physics textbook. Meelar 03:50, 16 Feb 2004 (UTC)
- The quoted text is correct. Kip Thorne made some use of this fact in Black Holes and Time Warps, where he explained the principles of relativity with fictional accounts of people travelling across the galaxy in a few years of their time. It can be understood by observing that the galaxy is shortened by length contraction, in the spaceship frame. The crew observe themselves to be travelling near lightspeed, it's just that the galaxy is smaller than it used to be. To bring my current favourite physics link into it, in the frame of reference of the Oh-My-God particle (http://www.fourmilab.ch/documents/ohmygodpart.html) before it hit the atmosphere, the earth was only half the thickness of a human hair. -- Tim Starling 04:04, Feb 16, 2004 (UTC)
Yottametres
How much would a ly be in Ym or similar SI prefixes?--Sonjaaa 21:02, Nov 9, 2004 (UTC)
It's right on top of the page: 9.46 Pm. However, I find it silly to use mega, giga, tera, peta in front of a meter. I added a section about that to SI prefix: grams, liters, meters, and seconds are never prefixed by M and bigger (not even k for seconds and liters). It is legal to do so, but scientists dealing with such quantities (almost) never do, except a few purists here on Wikipedia. (I removed petameter from light year once, but somebody put it back). Apart from convention -- we have cubic meters, tons, and hours/years, prefixes of peta and above, and zepto/yocto occur so rarely that most scientists will need to look them up, which kind of defeats their purpose. Han-Kwang (talk) 18:55, 11 Nov 2004 (UTC)