Talk:Speed of light/Archive 3

message: Rømer observed that Io revolved around Jupiter once every 42.5 hours when Earth was closest to Jupiter. He also observed that, as Earth and Jupiter were moving apart, Io's revolution seemed to take longer.
This statement is ambiguous because it implies that the movement of the Earth away from Jupiter is what makes Io's observed orbital period longer.  The Speed of Light in a vacuum is constant and not affected by the movement of the observer or the subject.
Rømer made his observations by taking a previously measured time for Io's orbit about Jupiter, and making a schedule so he could observe the eclipses.  He noted that Io was eclipsed earlier than the schedule predicted when the Earth was close to Jupiter, but was late when it was far away.  Therefore, whether the Earth was moving toward or away from Jupiter was largely irrelevant, it was their distance apart and its affect on his schedule that he observed and measured.
:Good point, we should clear up that ambiguity. Please feel free to adjust the text accordingly if you have not already done so. Regards & happy editing, Wile E. Heresiarch 14:11, 12 Apr 2005 (UTC)
replies: text: Not entirely. In the paper (which was translated into english from the orginal french), what matters is whether we are on the approaching or the receding segment of our orbit. It says explicitly that it is the motion, relative to Jupiter that matters, and this agrees with standard physics today (Doppler effect in the non-relativistic limit for ligth). Ulcph 17:59, 29 August 2006 (UTC)
thread_title:  Measurement of the Speed of Light - Ole Rømer 

message: From the article (about Roemer's detection of a variable time-lag in the occultations of Io): "These observations are akin to what today is known as the Doppler effect."
This statement is not correct.  It is not the relative velocity of the two systems (Earth and Jupiter/Io) that is causing the time offset, but instead simply the distance between them, combined with the finite speed of light.  
Reference to a Doppler effect is undesirable in this astronomical situation because of the potential for confusion with red shift (which is precisely a Doppler effect) or with the aberration effect that Bradley used for this purpose (which could reasonably be said to be akin to the Doppler effect, since both are velocity-based).
The effect is more like echolocation, or perhaps a closer analogy would be the determination of the speed of sound by measuring the differences in thunder delays between lightning flashes to points at known distances.  But I do not see an analogy close enough that I think it enhances the article. Hunter 23:10, 17 Apr 2005 (UTC)
thread_title:  Roemer time-lag not similar to Doppler effect 

message: I think with as many references to studies as there are in the subluminal section, there should be some citation... --HantaVirus 14:05, 21 July 2006 (UTC)
replies: text: Since this is a two-year-old Featured Article, it doesn't have the inline citations one would expect. I can look for a couple for that section... Melchoir 23:58, 21 July 2006 (UTC)
thread_title:  Citation for Subluminal studies? 

message: Dumb question, but I do really want to know (can also be included in the A.)
Scenario:
You are traveling at the speed of light (though it might be impossible, but imagine), in your "spacecar". You then turn the front lights on. What happends?
Ok, I'm no physican but heres ny therory, please correct me:
Since Mass is another form of energy, energy is another form of mass. Therefore light have mass. We also know that the speed of light is constant, does not change if the car moves. And we are moving at the speed of light. Since the light can't go faster than we are, the light will be "trapped" inside the lightball. The light will continue to come, and get "stuck". As the amount of light increases, so does the mass. When the lightball is as full as it can be the light will create a preassure inside the lamp. When the preassure exeeds the force the lightball can take it will explode.
So, what do you think of a 14yr olds therories? Cybesystem 00:22, 2 August 2006 (UTC)
replies: text: This doesn't seem like such a dumb question; it's actually very popular. My own answer is above at #Vehicle. For a longer answer, try this link. Melchoir 00:40, 2 August 2006 (UTC)

text: Relativity does not require events to happen simultaneously for both observers, it is possible for the light to be observed hitting at different times. There are even situations where the order of events can be altered for different observers raising interesting questions about cause and effect.Jameskeates 10:45, 17 August 2006 (UTC)
thread_title:  Going at the speed of light and turn on the lights 

message: The opening paragraph seems redundant:
:"According to standard modern physical theory, all electromagnetic radiation, including visible light, propagates (or moves) at a constant speed (or velocity) in a vacuum.  This physical constant is commonly known as the speed of light and denoted as c. This speed c is also the speed of the propagation of gravity in the theory of general relativity."
Most of this was said right before, or will be right after. I suggest to delete it, in the interest of readability (but haven't).
The author of the last sentence probably was referring to a linearized approximation, for the propagation of a "small" wave-like disturbance in otherwise gravitation-free space. Of course, in general relativity coordinates are freely chosen, and speeds can come out as nearly anything, merely as a consequence of this. It is not covariant. To obtain the constant, go to a locally inertial frame (free fall), but you only get it locally, and light will still bend globally. This would be rather much to saddle the reader with right up front. Besides, it is not directly testable (so far), and only established indirectly in the binary pulsars - but convincingly, of course. Besides, this is, after all, an article about light. Ulcph 23:41, 8 September 2006 (UTC)
thread_title:  Overview 

message: Although there is much reasoning over this, lets say for now, that light does have a mass. Would it therefore be possible to move something using light, if the 3 factors which make up light were strong enough?
:I'm not sure what you mean by "3 factors."  But light has momentum, so you can use it to move things anyway, in principle.  You'd need very high total energy, but very low individual photon energies to avoid blowing the object apart; but low energy means long wavelength and a tendency to pass through things, so this isn't practical in the slightest.
:In the future, can you please ask science questions at Wikipedia:Reference desk/Science?  Article talk pages are for discussing changes to the articles. -- SCZenz 15:55, 12 September 2006 (UTC)
replies: text: You may want to consult the article Radiation Pressure. Ulcph 04:19, 13 September 2006 (UTC)
replies: text: It's entirely practical.  That's how solar sails work. 67.87.115.207 09:48, 30 December 2006 (UTC)
thread_title:  Question. 

message: Hi all,
The Photon article is now in peer review, in preparation for its Featured Article candidacy.  Could you please give us some tips on reaching FA?  Your article is already FA (congrats!) and it's very close to ours in its subject.  Thanks! Willow 11:40, 15 September 2006 (UTC)
thread_title: Photon in Peer Review

message: Why is the speed of light 1,079,252,848.8 km/h and not something else?
:One answer is that we don't know.  Another answer is that the speed of light is the universe's fundamental "conversion factor" between space and time, and we define our unit systems around it. See meter#Timeline of definition. -- SCZenz 02:42, 20 September 2006 (UTC)
replies: text: You can look at it this way: if the speed of light were twice as great, then everything, from atoms to cells on up to humans, would be twice as big -- so you wouldn't notice. Melchoir 02:51, 20 September 2006 (UTC)

text: Because the meter and the second were historically defined in terms of the size of the Earth and the length of a day, both totally arbitrary pieces of data from a physical perspective.  In the most universally natural set of units, the speed of light is simply 1. 67.87.115.207 09:47, 30 December 2006 (UTC)

text: Yep, our basic units of measurement were all made up before anyone had the slightest notion about the definition of the speed of light, never mind it's a constant, so the distance/time value we use seems arbitrary. In truth, we've defined it using arbitrary (in effect random) units. Gwen Gale 04:29, 24 February 2007 (UTC)
thread_title:  Speed of Light 

message: Why isn't the International System of Units (SI) upheld in this article?
Wikipedia is international.
:A quick search of the article shows that, except for one historical reference, the speed of light is always given in SI first and imperial units are given in parentheses.  Unless one has an ideological need to "kill off" the imperial units that is strong enough to mitigate against putting difficult matters into somewhat more familiar terms for people who do not use SI in daily life, there is no need to restrict an article to mention of one set of units. P0M 17:40, 7 October 2006 (UTC)
thread_title:  SI 

message: it's was mentioned in the Qur'an (Arabic script: يدبر الأمر من السماء إلى الارض ثم يعرج اليه في يوم كان مقداره الف سنة مما تعدون) (سورة السجدة الآية 5).
Translation: (Allah) Rules the cosmic affair from the heavens to the Earth. Then this affair travels to Him a distance in one day, at a measure of one thousand years of what you count. Quar'an 32:5
Check the following link:
http://www.speed-light.info/angels_speed_of_light.htm
::This link is so full of logical holes its hard to even know where to begin.  Just the initial statement that what the Qur'an is saying relates in any way to the speed of light is quite a stretch.  I still cannot understand where the 12000 they are using initially comes from.  It is true that there are approximately 12 lunar months in a year (actually somewhere between 12 and 13), but why does that matter?  Why should we use the distance the moon travels, and not the distance the Earth travels in 1000 years?  Most of the article is just complete nonsense, but I'll just pick an argument at random that is vital to the author's reasoning and debunk it, thus invalidating his argument:
::"When we vectorially remove the energy gained from this force we can calculate the total energy and hence the length of the lunar orbit outside gravitational fields."  Not only is the "length" of the lunar orbit not defined with no gravitational field - it would not orbit at all in this case - but how can you "vectorially" remove energy when energy is a scalar quantity? Grokmoo 19:34, 1 November 2006 (UTC)
thread_title:  Speed of light in the Qur'an 

message: The third paragraph of the overview says:
If information could travel faster than c in one reference frame, causality would be violated: in some other reference frames, the information would be received before it had been sent, so the 'effect' could be observed before the 'cause' is. Due to special relativity's time dilation, the ratio between an external observer's perceived time and the time perceived by an observer moving closer and closer to the speed of light approaches zero. If something could move faster than light, this ratio would not be a real number. Such a violation of causality has never been observed.
This passage is confusing. It should be rewritten to reflect the fact that there are at least three reasons why faster than light travel is believed to be impossible. Reason one involves the space and time coordinates of two events as observed from two reference systems moving at substantial fractions of c with respect to each other, and the possibility of arranging a series of transmissions among observers in the two reference system so that information could be received before it was sent by the observer who makes the first transmission. The second reason involves the physical issue of how any interactions that depend on speed of light propagations (e.g., exchange of photons) could be perceived to operate by an observer in a reference system moving at a speed greater than c with respect to the reference system in which the (clock or other) physical operation is occurring.  The third reason is that once we have set up the equation that represents the time of one system of reference and the time it observes in the second system of reference, we can calculate meaningful results as long as the speed between the two systems of reference is less than c. If the speed presumed between the two systems of reference is set to c then we get a divison by zero in our result and cannot continue the computation.  If the speed is arbitrarily set to some value greater than c we get an equation that involves the square root of a negative number.  P0M 19:43, 14 November 2006 (UTC)
thread_title: Problem in the overview?

message: Ok. So the article says that the speed of light is exactly 299,792,458 meters per second, and 1,079,252,848.8 kilometers per hour. But I know for a fact that the speed of light is a few trillion miles per hour, and measured in kilometers, the number should be higher. So I multiplied the 299,792,458 m/s by 3600 for the number of seconds in an hour, and by 1000 because we're doing kilometers, not meters. I got this number: 1079252848800000 km/s. Apparently someone had forgotton to calculate a few more zeros into the number. Could an expert on this tell me if this is the correct number before I edit the page? Stevv 21:43, 19 December 2006 (UTC)
replies: text: The article as it stands is correct. Remember that if I am going at 1000 m/s then that is the same as 1 km/s. i.e, you must divide by 1000 to convert to km/s. Then multiply by 3600, for seconds->hours. The correct answer is: 1,079,252,848.8 LeBofSportif 08:57, 21 December 2006 (UTC)
thread_title:  Error Found