Talk:Laser/Archive 4

Usage of Laser
I hate youvisually appealing effects either here or "Laser Applications" section, which ever applicable. --87.194.3.52 23:19, 6 August 2007 (UTC)

Archives
Old discussions with no (big) contributions after Jan 1, 2006 have been archived to Talk:Laser/Archive1. A couple of additions after January 2006 are also present.

The verb 'to lase'
The article says: 'By back-formation, the verb "to lase" has also been created, meaning "to produce coherent light through stimulated emission".'

Although my dictionary agrees with this definition, perhaps it should be noted that "to lase" is often used to mean, "to apply a laser beam to".


 * Hmm...I'd never heard it that way. Where'd you hear that usage?  &mdash; Laura Scudder &#9742; 00:18, 27 June 2006 (UTC)

Could you please cite a reference for the usage "to apply a laser beam to" I've never heard and can't find a reference to this. Graemec2 14:07, 31 October 2006 (UTC)

lase (past and past participle lased, present participle las·ing, 3rd person present singular las·es)

intransitive verb

Definition: emit radiation: to emit the type of single-wavelength radiation produced by a laser --Handmedown 05:04, 24 February 2007 (UTC)


 * That is a definition, alright, but where did it come from? Without that, it is not a reference as requested some time ago by Laurascudder
 * Pzavon 22:16, 24 February 2007 (UTC)

Patently False Statement
"Popular misconceptions...a laser beam is never visible in the vacuum of space." Just as in air, a laser beam is visible in "the vacuum of space" according to the dust (whatever particulate) content of the space. If one argues that it's not a vacuum if there is dust content, then they are not talking about the "vacuum of space," but some theoretical vacuum. 12.144.20.254 18:50, 9 August 2006 (UTC)


 * The question is not whether there is some dust to illuminate in space, but whether there is enough dust for a laser beam to be visible. Molecular clouds get up to 106 atoms per cubic centimeter, most of them hydrogen.  I'd think that even if it was largely dust a laser beam wouldn't be visible.  &mdash; Laura Scudder &#9742; 20:26, 10 August 2006 (UTC)


 * Oh, come on. 12.144 is right. We should just get rid of the whole stupid section on lasers in space. It's not educational. Birge 15:43, 11 August 2006 (UTC)


 * I don't care one way or another if the space discussion stays or not. I just  think it isn't patently false.
 * I do like some of the other popular misconceptions, like the finite propagation speed of laser guns. &mdash; Laura Scudder &#9742; 00:01, 12 August 2006 (UTC)


 * he is not "right" he is simply being pedantic to the point of absurdity. the overwhelmingly enormously huge majority of space is so empty that the statement "a laser beam is invisible in space" is correct for probably oh I don't know 99.999999999% of the time. --Deglr6328 04:50, 16 August 2006 (UTC)


 * I agree it's pedantic, but science is pedantic. We should write a science article with as much precision as possible, I think. (Also, Laura I completely concur that saying it's patently false is going way overboard.) Birge 06:52, 16 August 2006 (UTC)

Ok, I've taken a crack at cleaning up the section. It was actually very poorly written in a few places, with meandering explanations that seemed out-of-place and only designed to show off the writers' knowledge of laser facts. (For example, this section is not the place to explain diffraction except to say that all beams spread to some degree or another. A link to the diffraction page suffices.) Birge 07:07, 16 August 2006 (UTC)


 * Much improved. &mdash; Laura Scudder &#9742; 14:59, 16 August 2006 (UTC)


 * I think that that's fine. I don't think my "patently false" characterization was overstated, because "never visible in the vacuum of space," for one thing, implies that it's the lack of atmospheric pressure that makes the laser invisible.  Even if it had said "never visible in outer space" it would be better.  But "never?"  Obviously, there will some dusty conditions in space, like any time a rock impacts the moon... the fact that it's in space or in a near vacuum is obviously not going to make the laser invisible in the presence of dust.  Someone who didn't know better, and read the former statement, might have been convinced otherwise.

12.144.20.254 21:14, 19 August 2006 (EDT)


 * While I think you are right about the need to be technically correct, I think you are patently wrong about what it means for something to be patently false. When you have to point out that a statement is false in extreme circumstances that would only matter to a scientist, that's not exactly "openly, plainly, or clearly" false. Birge 08:31, 20 August 2006 (UTC)

In the misconceptions section, the intensity at which a laser's beam becomes visible in air should be an energy density (mW/cm²) instead of a power (~ 10 mW, as now). I'll change it, but I don't know what a realistic value is. A few milliwatt green laser pointer, focused to a few square millimeter spot, doesn't produce a visible beam in a darkened room, so the threshold is at least tens of mW/cm². Rob Mahurin 18:13, 4 October 2006 (UTC)

Misconception?
Someone added the following to the "misconceptions" section:
 * Another common misconception involves the explanation of coherent light production. Many authors claim that laser light is coherent because stimulated emission is in phase with the stimulating wave.  However, in-phase emission only produces gain (since superposed in-phase waves produce a wave of higher amplitude) and incoherent light is only amplified without losing its incoherence.  In reality, both spatial and temporal coherence is due to the optical cavity.  The cavity produces temporal coherence by excluding any light with wavelength which is not contained an integer numer of times in the cavity length.  The cavity produces spatial coherence by acting as a spatial filter with gain; by reflecting only light with a spherical wavefront of a particular radius.

I don't think this is completely accurate. Coherence is more than simply cavity resonance. In particular, the linewidth of a laser above threshold is orders of magnitude narrower than the width of a cavity mode of the same cavity.--Srleffler 02:14, 8 February 2006 (UTC)


 * Right, it's not simply cavity resonance, yet the narrow linewidth still is caused by the cavity. We need to ask ourselves WHY the laser output linewidth is so much narrower than the cavity linewidth.  As I understand it, the narrowness arises because the gain of the laser medium, minus the cavity-dependent losses through the partial mirrors, together puts the peak of the system gain vs. frequency right at 1.0.  So, as light repeatedly passes through the system, the cavity's frequency curve is repeatedly multiplied by itself.  Repeatedly multiplying a mathematical function by itself will produce a delta function, if the original function has a single maximum with a value of one.  "The laser's coherence is caused by the cavity" is a compressed explanation.  Also, my intent was to point out the common and widespread misconception: the wrong idea that stimulated emission explains laser coherence.  Any explanation of the actual cause of coherence should be put elsewhere in the article.  --Wjbeaty 04:31, 29 March 2006 (UTC)

The width of a cavity mode is usally smaller than the linewidth of the gain above threshold, but the original statement is very misleading. Let's take an example: in an Argon ion gain profile is about 4-12 GHz wide, while the linewidth of a one meter long cavity has a free spectral width of 150 MHz and thus a much smaller linewidth (1.5 MHz assuming mirrors of reflectivity 96.8%). But the cavity does not reduce the spectral width, it only clears some spectral regions. To reduce the laser linewidth to the linewidth of the cavity (monomode operation), an etalon is necessary, which is an additional very narrow cavity. And secondly, even without cavities the coherence would have been already excellent. Third, if the cavity becomes very narrow (as in VCSEL diode lasers or photonic crystals) the gain width perhaps becomes smaller than the width of the cavity mode. --danh 16:40, 8 February 2006 (UTC)


 * I guess I should have qualified what I was saying better. If you constrain a laser to operate in a single mode (longitudinal and transverse), it will have a linewidth much narrower than the cavity linewidth. For the example of your argon ion laser, the laser initially operates in multiple longitudinal modes. The role of the etalon is to force the cavity to operate in a single longitudinal mode. If you got the laser working with the etalon in, and then turned down the pump source so that the laser was a little below threshold, the on-axis stimulated emission would have linewidth equal to the cavity mode linewidth, as you would expect. If you then raise the current above threshold, you would find that the laser's linewidth gets orders of magnitude narrower. This is a well-known phenomenon that you can read about in any laser physics text. Look for the equation for the Schawlow-Townes limit. This gives the theoretical linewidth of the laser, above threshold. In practice, real lasers always have much larger linewidth than the S-T equation predicts, because vibrations and thermal fluctuations cause the effective cavity length to fluctuate on very short timescales. This introduces an extra source of broadening to the laser output. The laser will still typically be significantly narrower than the cavity modes, though. --Srleffler 00:24, 9 February 2006 (UTC)

I propose to just keep it out. Mode selection is done in several ways and we don't want to go in the details in the Misconception section. --danh 14:14, 9 February 2006 (UTC)


 * This misses my point. Many intro textbooks claim that the coherence of laser light arises only because the stimulated emission produces a special type of light called "in phase light."  This is wrong, since the in-phase emission process only explains why the laser media is transparent and why it has gain.  The light within the cavity is initially incoherent via spontaneous emission, and stimulated emission isn't a recipe for removing this incoherence.    To explain the coherence properly, we have to view lasers as acting as amplified narrowband filters, with the light passing repeatedly through the "filter."  The partially-mirrored cavity in a conventional laser supplies the "narrowband filter" effect.  The Schawlow-Townes limit equation shows why the linewidth isn't zero, but doesn't clarify why the linewidth is so small.    --Wjbeaty 04:31, 29 March 2006 (UTC)

I disagree
I take issue with Srleffler over the assertion that "Football field sized Nd:glass lasers have only two significant applications" (namely ICF and weapons design). The applications of giant Nd:glass lasers have expanded widely in the past couple decades. HEDP (high energy density physics) done at these facilities now includes sophisticated astrophysical jet modeling, supernova shockwave physics, liquid metallic hydrogen research, megagauss regime investigations relevant to neutron star environments, shock Hugoniot data for silica to better understand geophysics of conductivity in the earth's mantle and now with the ultra-high energy chirped pulse Nd beams: laser wakefield particle accelerators(!!), fast fusion ignition, visible wavelength photo-fission or photo-transmutation of radionuclei, attosecond regime x-ray spectroscopy. Really it goes on and on. I would even say that some of this research is MORE significant than the fusion stuff. There really should be a HEDP article.--Deglr6328 07:05, 21 February 2006 (UTC)
 * I'll adjust the caption. I haven't been following this field for a while and forgot about the other applications.--Srleffler 12:48, 21 February 2006 (UTC)

Error
"These lasers have particularly narrow oscillation linewidths of less than 0.01 cm-1 "

Shoudn't this be 0.01 nm?

Sorry if I misplaced this comment, but I'm in a hurry-


 * No error. Whoever wrote it was expressing the linewidth in terms of wavenumber instead of wavelength. -- The Photon 05:12, 3 March 2006 (UTC)

It's me again. The linewidth can of course be represented in both ways but 0.01 cm-1=1000000000 nm which doesn't make any scense for a linewidth. The laser would not have a narrow linewidth at all. I may be wrong, which is also the reason that I didn't correct it. (Hauberg 13:31, 3 March 2006 (UTC))


 * Yes, but the computation gives a funny result. He's talking about a center wavelength of, for example, 224 nm, which is 280,499 cm^-1. Similarly, 224.01 nm would be 280,486 cm^-1; so a laser line spanning these wavelengths would have a linewidth of about 12 cm^-1. So 0.01 cm^-1 linewidth is actually about 1200 times narrower than 0.01 nm in this wavelength range. -- The Photon 00:19, 5 March 2006 (UTC)

That makes sense for me. Thanks for the explanation :-) I am sorry for the inconvenience. Feel free to delete this section. Hauberg 17:08, 10 March 2006 (UTC)


 * Let's not use wavenumbers at all, I think they're confusing sometimes too and they're conventionally used in the IR range. changed to Hz. --Deglr6328 16:38, 11 March 2006 (UTC)

Pulsed laser plasma
Anything you laser experts could contribute to Volumetric display would be appreciated. The press releases are rather short on details. In Chirped pulse amplification, it mentions "self-focusing". I assume this is something involving a change in the index of refraction from heating up the air, related to blooming? (needs a real article) It also says 700 gigawatts/cm² will ionize the air. Is that just an arbitrary example that happens to ionize air, or is it the actual breakdown threshold of air? (I'm more familiar with breakdown voltage, which happens at ~30 kV/cm, for instance.) If so, could we get a rough guess of the display's output power? (1 nanosecond pulses at 100 Hz.) — Omegatron 06:19, 14 March 2006 (UTC)


 * That 700 GW example is just an example. I'm not sure of the exact intensity level where the kerr effect becomes important in air. Anyway I doubt the laser used in that 3d thingy uses CPA at all. really all it needs to do is use a q-switched high power laser with a conventional lens and a couple mirrors to control the Z and X Y axes respectively. It's virtually certain that the technique used is essentially identical to that used to make those "laser crystal" things. Focus a 1 GW IR pulse down to a few micron spot (easy) and your talking intensities around the quadrillion W/cm^2 range (if I added right). Everything breaks down to plasma at those intensities! no cpa needed. --Deglr6328 07:30, 14 March 2006 (UTC)

Wow. Doesn't sound that difficult at all, now. This page has people creating the same effect with $200 YAG lasers off ebay, apparently. (Though without the mirrors or any long distance.)

"I saw a bright pinpoint flash of light at 1.5 inches from the lens in mid air! Very very cool (first time I have seen this phenomenon, though I have heard of it). I guess this gives another data point to the output power level... Air sparks at 200 to 400 mJ? :)"

"I have also sparked the air using a short focal length (about 1.5 cm) lens."

Hmmm... — Omegatron 01:42, 15 March 2006 (UTC)

Metal ion lasers
It states the the metal ion lasers have a particularly narrow linewidth of 3GHz. Maybe for that UV wavelength region. but not generally is it? Most gas and fiber lasers have kHz linewidths.

Military Uses
Has the use of lasers to burn the enemy troops' retinas at a distance been outlawed? Anon user 12:30, 20 March 2006 (UTC)
 * Yes. One of the later Hague Conventions, I believe. --Carnildo 05:20, 21 March 2006 (UTC)
 * Although tank laser rangefinders might accidentally have that effect when simply ranging...Midgley 15:46, 26 March 2006 (UTC)

Why...
"Some types of lasers, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for the generation of extremely short pulses of light, on the order of a femtosecond (10-15 s)." It is not apparent to me (I know little of lasers) why this property makes them ... Midgley 15:45, 26 March 2006 (UTC)

Vibronic lasers are often used in a technique called modelocking to produce ultra short light pulses. A large gain bandwidth is needed in order to lock an high number of longitudinal modes. Lasah 15:55, 26 March 2006 (UTC)


 * The "why" is the Heisenberg uncertainty principle, which says that the product of our uncertainty in knowing the momentum and the position of a particle is bounded by a constant, e.g. $$\Delta x \Delta p > \hbar / 2$$. A short pulse requires the position be very accurately known, since the duration of the pulse is related to the position of the constituent photons by the speed of light. But the wavelength of the photons is closely related to the momentum, so for the pulse to be very narrow, it must contain a wide range of wavelength components. -- The Photon 06:05, 28 March 2006 (UTC)
 * Thank you. Makes perfect sense.  Is it gettable intot he article without moving from encyclopaedic to physics textbook I wonder?  I suppose the best way might be as a footnote to the para at which I went "Why?".  Midgley 18:45, 29 March 2006 (UTC)


 * Hmm, it happens that the Heisenberg uncertainty applies to light as well, but I wouldn't add that as an explanation. It is just a property of the Fourier transformation (from which you can derive the Heisenberg relation): $$\Delta t \Delta \omega > 1/2$$ without any fundamental constants. The minimum for a time-bandwidth product also applies to e.g. sound waves, where Planck's constant doesn't play a role. Han-Kwang 12:49, 31 March 2006 (UTC)


 * Maybe we should just add a link to another article, perhaps one dealing with the fourier transform or the uncertainty limit. The beauty of a hypertext encyclopedia is that even if you don't have scope in an article to fully explain something, you can probably link to an article that does. Birge 03:18, 16 August 2006 (UTC)

Lasers categorized
FT2, I'm not sure what is added to the article by changing from "Types of lasers" to "Lasers categorized" and adding the "by output power" section. Do you plan to add any further means of categorizing lasers, for example "by wavelength", "by average power", "by efficiency", "by pulsed or cw", "by cost", "by weight", ... The article is already near the maximum recommended article length, and its difficult to imagine adding all these different ways of sorting out lasers without going far beyond the recommendation.

The information added seems to belong more in the "Uses of lasers" section. For example, the list of important laser characteristics currently in the history section might be moved in following some phrase like "Laser characteristics that determine the best laser for a particular application include ..." And the CD, DVD, and DVD-R articles might be updated to include the information on the power of the laser involved in those systems. The information on the most powerful laser claim from Livermore could be moved under "Recent innovations" in the history section (with a citation).

--The Photon 03:51, 18 April 2006 (UTC)

Just out of curiosity...
Why does Xaser redirect to laser? The tiny part that mentions a possible X-ray cousin doesn't seem to justify the redirect, especially when uaser/uvaser aren't redirects and graser has an article, even if it is just a stub. BioTube 01:34, 15 May 2006 (UTC)

Just out of curiosity, one thing that I'm unable to find out is the shape of the gain medium of solid state lasers, is this shape fixed or does it vary depending on the mediunm used? This has been bugging me for a while now.65.93.199.170 03:26, 6 December 2006 (UTC)

cold laser
I've recently been hearing a lot about cold lasers. I came to this site expecting to see more information about various types of lasers than I see here.69.6.162.160 01:03, 24 May 2006 (UTC)Brian Pearson


 * heavens! so sorry to fail you by not including every last form and use of laser ever to exist on earth. you are looking for information on "cold laser therapy" which is bunk and hence does not belong here. maybe if you actually tried searching the internet you would've found the ten billion sites already out there on the subject. .--Deglr6328 06:02, 24 May 2006 (UTC)

Star Wars
Just wanted to note that the ships in Star Wars do not actually fire lasers. The term blasters is consistently used for their energy weapons, and I believe the mechanics involve kinetically firing highly energized packets of tiberian gas. Thus we have glowing, relatively slow-moving, laser-like weaponfire. This still doesn't excuse the blatantly called "Death Star Super-Laser", but it would have been dealing with extremely high levels of energy which may cause a visual effect even in the near-vaccuum of space. All this aside, I will admit that the whole argument smacks of retroactive explination, but it's canon now, so...

-John (AIM: LordRobertQwert for discussion)


 * ummm, wow, just.....wow.--Deglr6328 04:16, 25 May 2006 (UTC)


 * This is mostly true, but i seem to recall at least one scene where lightsabers were referred to as a "laser sword" in the original English versions, and the French versions of the movies call it something which translates to "cut off laser sword". That just REALLY calls into question the validity of Star Wars technology. - Lucy
 * There are lasers and there are beams... A lightsaber is a 'frozen' beam, not laser. - Adler

Tunable laser needs some serious attention
If anybody feels like doing a little work on it, Tunable laser is currently IMHO a notably ugly stub. (No dig intended at the folks who have already done some work on it!) -- 201.78.233.162 16:54, 10 June 2006 (UTC)

Vanadite laser
Nd:YVO3, called 'vanadite,' has the same IR line as a Nd:YAG laser but is more efficient, as the absorption bands are better matched to the pump.


 * YVO also has a weaker line at 1342 nm.--Deglr6328 06:03, 14 July 2006 (UTC)

Grain
A point of light from a laser pointer appears grainy in a particular way. Can anyone explain why? —Ben FrantzDale 23:17, 26 June 2006 (UTC)


 * It's called speckle and is common when you shine monochromatic laser light on a rough surface: light scattered from one point will interfere with light scattered from elsewhere nearby. &mdash; Laura Scudder &#9742; 00:07, 27 June 2006 (UTC)


 * By the by, I mention monochromatic because it doesn't happen with broad bandwidth lasers (like Donnelly and Saeta used at HMC), only in lasers with high coherence. &mdash; Laura Scudder &#9742; 00:16, 27 June 2006 (UTC)

How do they work?
How do lasers actually work?


 * why do you keep putting this here? Maybe if you actually took the ten seconds you used on posting such an obvious dumb question to actually read the fucking article whose talk page you're posting to you mightn't be in such a clueless state?--Deglr6328 06:23, 15 July 2006 (UTC)

Classes of Laser
Could someone explain the different classes of laser. i.e. My CD-ROM says it is a Class A laser device. 212.2.17.234


 * I'm not sure about the details but I found this Laser Fact Sheet. —Ben FrantzDale 15:42, 7 September 2006 (UTC)


 * Heheh, reminds that of the time that one of my friends was reassembling a laser I had worked with the previous two years. He walks in all casual and asks me whether I thought the pump laser could set anything on fire.  I think about it but say no, it doesn't even hurt the skin.  He gives a smirk and then gets up and turns around and there's a huge hole burned in the back of his navy shirt, about six inches across.
 * As to your question, we've got a pretty sizable article on laser safety. A CD-ROM laser probably is type II, class A.    &mdash; Laura Scudder &#9742; 16:57, 7 September 2006 (UTC)

Medical uses of lasers
Resspected sir, I'am very pleasant to gather information about lasers in your web.Also mention various medical applications of lasers.


 * zomg!! c'mon people! see laser applications --Deglr6328 01:43, 17 September 2006 (UTC)

Laser Physics
The purpose of an encyclopaedia is to inform and educate.

However, there are many ‘science’ articles where it seems the author (Scientist?) is more concerned with revealing their titanic intellect by proxy through these articles than educating.

The article on how a laser works is relatively complete, i.e. describing the phenomenon of light amplification. However it does not clearly show how the device works (the purpose of the resonant cavity) and too many caveats are introduced into the main body of work; this clouds the understanding of the reader.

Knowledge is not envy nor secretive but should be shared freely whenever gained. I have lost count of how many times I have read the discussion page where contributors cannot resist adding a pedantic piece of information flaunting their knowledge (generally wrong) and how this is refuted infinitum until the point being discussed is neither relevant nor interesting.

All well and good but a forum for this one-up-man-ship an encyclopaedia is not. One would do well to remember the opening line of this article and if you dislike it go to one of many physics chat rooms where you can engage in as much intellectual male member jousting as you wish.

A disgruntled Scientist


 * gosh! it's good thing you didn't bother to give a single example of your complaints! that'd've been almost helpful! where are people "flaunting thier knowledge"? I swear, this page's talk attracts some of the most bizzare comments. --Deglr6328 22:46, 27 September 2006 (UTC)

Uses of lasers
I merged Laser into Laser applications because yet again what was intended to be a brief summary pointing to a separate page had evolved into major content forking. Anthony Appleyard 09:29, 7 October 2006 (UTC)

Popular misconceptions
The paragraphs about popular misconceptions are useful and should stay. They show how members of the public often misunderstand lasers due to how they are portrayed in popular culture. The scene in Goldfinger is the most famous example of this.--Ianmacm 15:58, 19 October 2006 (UTC)

Vandilization fixed
Someone just vandalized this page with stuff like "peanut butter and jelly" I restored an older version.

Feature article
The Македонски version has became a feature article. Would anyone who understand Македонски be able to compare that with the english version to see what improvement we can make? --Cyktsui 00:42, 21 November 2006 (UTC)


 * I don't know Makebenbfs/ekfh~apsbn but it doesn't look like there's much there that this page doesn't already have.--Deglr6328 03:51, 21 November 2006 (UTC)

Vandalism
I removed some vandalism, but it seems some other content was lost as well. I'm not sure what is factual, and what is not; could someone review the last few edits? | A ndonic O Talk 16:21, 1 December 2006 (UTC)

Optical laser
The article is not clear about what an optical laser is. I would correct it, but I have not been able to find any sources that give a good definition. From some sources, I get the impression that optical lasers are those that emit visible light. However, other sources make it seem like it includes any laser that emits electromagnetic radiation of any type, as opposed to things like atom lasers. -- Kjkolb 07:55, 14 January 2007 (UTC)

Picture Caption
How encyclopedic (or helpful, for that matter) is it to say that the laser in the first picture is "likely" an argon laser? Why not just plain "laser?"--72.48.22.30 08:36, 21 January 2007 (UTC)

I have issue with that picture as well, alltough I may be being really thick. Why can we see the beam? Doctorp9999 (talk) 23:23, 23 November 2007 (UTC)


 * They traditionally blow a light smoke over the table to make a beam visible for photographs, although there are many other tricks involving a long exposure time with only a quick flash illuminating the background so that everything else stays sharp. The amount of glow off the mirrors makes me think they did the latter.  And I can say that it really is likely an Argon laser based on the color.  &mdash; Laura Scudder &#9742; 13:46, 24 November 2007 (UTC)


 * For instance, this photo clearly uses a long exposure time, moving some material through the beam to make it visible. Notice that the beam is brighter just before and after an optic like the material was held there longer.  The blue has the advantage of being pretty easy to see with a little dust or smoke.   &mdash; Laura Scudder &#9742; 13:52, 24 November 2007 (UTC)

Oh yes, that makes more sense. I thought it was the goldfinger thing again. Thanks Laura. 152.78.254.81 (talk) 15:50, 25 November 2007 (UTC)