Talk:Speed of light/Archive 4

mnemonics
To be honest, I'm not convinced the mnemonics section is very encyclopaedic; do people feel that it enhances the quality of the article, or should we get rid? Thanks &mdash; Alan✉ 21:15, 23 January 2007 (UTC)


 * I don't know, maybe leave just one, or maybe not. I'm for removal too, since people who wants to learn by heart the speed of light are rare (personal opinion). Certainly, this shouldn't be section 2 of the article. Dravick 01:42, 24 January 2007 (UTC)


 * It's kind of neat to have such a mnemonic. That being said, when I was a physics major in first-year physics we used rounded numbers. People who need the exact number probably have it programmed into their desktop Crays by now. But I think any mnemonics should be preserved for posterity in a footnote or other brief mention near the end of the message. P0M 02:28, 24 January 2007 (UTC)


 * Okay, thanks for the responses. I'll relegate them (or one of them) to a footnote.  &mdash; Alan✉ 20:48, 26 January 2007 (UTC)


 * On second thoughts -- even the existing footnote references are to serious material; a 'mnemonic' reference would be conspicuous among them by its comparative lack of seriousness. I'll just delete, as I can't see any way to include them that fits in with the content and style of the rest of the article.  &mdash; Alan✉ 20:58, 26 January 2007 (UTC)

The actual Speed of Light
It says in the main article that the speed of light is 299,792,458 metres per second... When in fact it's that many KILOmeters per second... Someone should really fix that.... Seriously.


 * Erm, not it isn't. The majority of physics undergrads knwo that the speed of light ~ 3E8 m/s. As the article says, 299,792,458. Which is the same as 299,792.458 km/s. Note comma in one, decimal point in the other. In localities where the comma is the decimal separator, then it would be written as 299.792.458 m/s or 299'792'458 m/s or 299.792,458 m/s or 299'792,458 m/s. To avoid decimal separator confusion, the speed of light is 299792458 m/s. Three hundred thousand kilometres per second, or three hundred million metres per second. Google Speed of light and look at Google Calculator if you don't believe me. Stannered 23:24, 28 March 2007 (UTC)


 * Um, you're right, sorry about that. For some reason I thought the second comma was a decimal point. Feel free to delete this comment

Einstein's metaphysics implications HELP PLEASE !!!
In previous questions it is mentioned that if we assume that light has some kind of awareness, or if we place our selfs in the place of a photon (or simply if we would by some means travel at the speed of light) the speed would be not c but infinite. The Maths works out right i think. Light has infinite speed in it's own perspective (and maybe infinite mass as well...?). And time is 'stooped' at the speed of light. So, if for instance a photon leaves the Sun towards the Earth, get here and is reflected back, it would arrive at the Sun in time to see itself leave? Doesnt this mean that for light the Sun and the Earth are actually in the same place? Isnt the universe all in the same 'place' then? And can time stop and still exist at all? Doesnt all this mean that light is actually outside time and space? Meaning completely outside time and space as we know it, not in 3D or 4D but actually in zero dimensions. Should we (or can we) think of all this as some kind of Einstein's metaphysical implication of omnipresent and immortal light? Is one photon all the photons there are? Does all this make any sense? (Uanbiing 00:10, 7 February 2007 (UTC)).


 * Sorry, but your main idea of time is still tied to the age-old way of thinking about it. If you were rendered unconscious, and your wrist watch was stopped temporarily, and then you were put in the cargo compartment of a bus going from LA to NYC, (subjective)time would not have passed for you because your brain was effectively put in neutral just as your watch was turned off. When you were revived and your watch was restarted and put back on your wrist you would think that it was only a few minutes since the knock on your door in LA, but when you "went back outside" you would discover that a week or so had gone by, somehow. That's only an analogy. If travel were possible at the speed of light, not even in the dustiest cargo bin would dust settle on you. You would think that the trip had gone real fast, but only because you clock had been stopped. Your clock being stopped doesn't affect all the other clocks in the universe. They keep ticking, and when you wake up you have to deal with all that has changed in the interim.


 * Time is private and personal to the timekeeper. A perfectly good and highly accurate clock and its twin keep perfect time with each other while they are both on earth. Send one into orbit above the earth and, because it is whizzing around up there, it will slow down. The only way you can make sense of the words "slow down" is "slow down in comparison to its twin clock back on earth."  So if it went so fast that it stopped then you could only say that "it slowed down to the point that it stopped -- from the standpoint of the twin clock on earth." The guy who was wearing this very expensive timepiece gets off the space ship after its return and says, "My, that trip hardly took any time at all, so why have my hamsters starved to death in my absence?"  (Well, I hope we educate our astronauts well enough that they don't have such a naive reaction.)


 * Light is not inside and not outside of time. That is because time is not like a flowing river -- even though that is how we think of it most of the time. Being "in" time means being in motion, and being in motion can only happen if something is in space. If something were "not moving" at a radical level, if electrons don't even move around in that thing, then there would be no time for it because there would be no movement, no change, so it would be as though that thing were something in a motion picture and somebody stopped the projector.  The clock showing on the screen is stuck at a certain hour, minute, and second until somebody starts the projector again.  (Again, that's an analogy. Obviously we have no evidence that we are part of a movie. Some Buddhists have a kind of idea in which the universe exists as a sequence of little slices analogous to the frames of a movie but in 3-D, however.)


 * Going faster in space causes you to go slower in time. That's the key observation. We see that fact by observing the clocks on any GPS going slower, and we can formulate an equation that relates the time seen on that clock with the time seen on one's clock here on earth, and we now have made these measurements over and over and over again and always get the same answer, so we regard it as a very good bet that it will work the next time we try it. That kind of thing is the reality that we all see, and we have to take this basic and well-confirmed observation and theory and work out its consequences until we get used to it.


 * Probably if we all had to program GPS location finders and systems and had to deal with the time dilation on a regular basis, then it would cease seeming funny after a while. I suppose that the first human ever to regard the horizon as "the end of the world" and then try to get to the end of the world must have had some trouble explaining things to himself/herself. Even as kids we may have had the same impression. Eventually we all get over it.  P0M 02:25, 7 February 2007 (UTC)

Huygens' and Newton's estimations
According to data in the article Huygens' value is "better" than Newton's. If we use equatorial Earth radius 6378135 m and assume data from article - Huygens (?): 2000 Earth radii per minute, we get about 212,600 km/s, Newton (Optiks, 1704): 33.2 Earth radii per second, we get about 211,750 km/s. Obviously Huygens' value is better, or am I missing something? Also Boscovich in 1758 gave estimated value 20,000 Earth radii per 1/8 hour (~ 450 s) and 283.470 km/s, what is even better. --xJaM 03:35, 8 February 2007 (UTC)


 * Newton and Huygens's values are virtually identical, according to the article. I don't think it is worth distinguishing one as "better" than the other, when they are within 1% of each other, yet more like 30% from the true value.  You are right though, Huygens' value is slightly better.  In regards to Boscovich's value, keep in mind that it was determined about 80 years after Newton's value.  I have removed the incorrect wording from the article. Grokmoo 04:11, 8 February 2007 (UTC)


 * Yes, I uderstand that both values are almost identical - but just a sentence in the article said differently. I've added Boscovich's value not regarding directly to Huygens's or Newton's estimations, but just for an information (that might be included in the article). Yes, I know that it was estimated so late, but it is still a very good one, since a.u. at that time was not yet measured so good. Cassini measured a.u. in 1672 (109.8&times;109 m - parallax of Mars at oposition, Cassini, Flamsteed, 1672, 138.4&times;109 m - declination of Mars), and the first pretty satisfying measurement of a.u. was made in 1769 during the transit of Venus (153.9 and 148.2&times;109 m) - 11 years after Boscovich's estimation, which gives for a.u. around 127.6&times;109 m. --xJaM 01:56, 10 February 2007 (UTC)


 * Ok, everything looks good. Thanks for the heads up on the error. Grokmoo 17:13, 16 February 2007 (UTC)

Question on definition
Does anyone know why the definition went with 299792458 instead of a nice round 300000000? was it just to keep everyone from having to junk their exisiting meter sticks? —The preceding unsigned comment was added by 216.70.247.242 (talk) 22:42, 12 February 2007 (UTC).

You're exactly right. The metre was originally defined way back by the French. It was only in 1980s that the current (much more precise) definition was adopted as being a 1/299792458 of the distance light travels in a second. But they needed to choose the right number so that all existing distances were close to the new definition, so avoiding the need to remeasure everything and replace all rulers.Mralph72 11:45, 15 February 2007 (UTC)

Contradictory (?) sentence in Sec. 3.2 - Technical impossibility of hyper-light-speed travel
"If this is true, an object may travel at the speed of light, but it will not travel through space when this is done, as no time will pass while it is at the speed of light."

The above sentence, which appears in the article, is either contradictory in itself, or implies some kind of speed-of-light travel of objects which occurs, but not through space. If an object is traveling at the speed of light, but it is not traveling through space, where is it traveling? If "no time passes", it should be specified FOR WHAT or WHOM, or for the entire universe. Is this non-passage of time a way of saying that the event of faster-than-light travel doesn't actually occur? Or does it mean the TRAVELER or TRAVELING OBJECT experiences no passage of time? Also, if "no time passes", would not this object be permanently frozen in time, as no events could occur to it to change its circumstance? If no force can act upon it, since it is outside of time, would it therefor be an irresistable force? An unmoveable object? Is this an accepted theoretical phenomenon? Does the sentence mean that the object will in fact travel at the speed of light, but for a period of time exactly equal to zero? Does this make sense, or is it sophistry, or is it a kind of thought experiment? This sentence needs clarification, correction, or removal. --4.239.0.189 18:12, 23 February 2007 (UTC)


 * Ugh. I think this wording should be removed from the article entirely.  If you look at the equations for the Lorentz transformations, you see that they indicate that no time will pass for an object traveling at the speed of light.  This, in some sense, implies that for  a light particle, all events actually occur simultaneously.  Note that this is only for the light particle itself, not for an outside observer traveling at less than the speed of light (relative to what he is observing).  The light particle is not really "outside of time" in the sense you are thinking, but yes, the wording in this section is not really clear.  In fact, I think the whole section could really use a good rewrite.  I'll mull this over for a couple days, and then hopefully have something better to put down. Any other contributions will be very welcome. Grokmoo 02:14, 24 February 2007 (UTC)


 * I agree. The idea of "traveling" when delta T = 0 and delta x,y,z = 0 is a total distortion of the idea of moving or traveling. If one observes cases of things that move at speeds closer and closer to c, one will observe their clocks going slower and slower, but their perception will be that less and less time passed while they were going a very great distance.  P0M 02:39, 24 February 2007 (UTC)

Double Ugh! When I read the whole thing I realized that it is one of those pieces of writing that is "correct" when you know what the guy is trying to say, but is terribly confusing if you are coming at it from the outside. The basic idea is very simple, but gettint it out using everyday concepts like "velocity" to talk about time is bound to lead to problems because, for one thing, it hypostaticizes time. If you talk about clocks and how many ticks a clock makes between here an Alpha Centauri it is much clearer than saying that "the velocity of time slows" during this trip. I rewrote this section off the top of my head, depending on memory and visualization. I think I have not misrepresented the math, but this is one of the cases where more is less and less is more, i.e., if you take the reader through the math it's clearer than if you act as a "simplifier." Here is one case where good visual aids would be invaluable. Actually seeing the measured time bar shrink as the measured velocity bar increases would make things lots clearer. A little sweep second hand accompanying the measkured time bar that would slow down as the speedometer needle crawls up toward C would make things even clearer. And an accompanying equation could show the relevant calculations. Is anybody good at writing simulation software that will work on a website?

As it is, this section is still something that will make the unprepared reader conclude that relativity theory is incomprehensible. P0M 03:37, 24 February 2007 (UTC)

Is there an easy way to transfer an Excel spreadsheet to a Wiki page? I can do the simulation in numbers and static graphics, show the math, etc., but the easy way is to use a real spreadsheet. I don't feel like copying it cell by cell into a table. P0M 03:45, 24 February 2007 (UTC)

If what you are saying is true-- and "everything happens at once" from the point of view of a light particle, then is perhaps light a window on eternity?

Sean7phil 17:14, 17 June 2007 (UTC)

Another imperfection
The current text says that light moving through a medium other than a complete vacuum is actually a "light-like hybrid of electromagnetic waves and mechanical oscillations of charged or magnetic particles such as electrons or ions, whereas light in the strict sense is a pure electromagnetic wave." What is the reader supposed to make of this odd formulation? "Hybrid" is a biological term and is not helpful in guessing at what the writer was trying to say. The picture it paints in my mind is that of light in empty space hitting the window of a space craft (for instance) and suddenly turning into some sort of conjugal combination of electromagnetic waves traveling along with mechanical oscillations of various kinds of subatomic or atomic entities.

Somebody who has a thorough grounding on this subject should rewrite the quoted material so that it accurately communicates the correct understanding of the change in measured speed.P0M 05:21, 24 February 2007 (UTC)

Gravitational Lensing
"Changes of gravity, however, warp the space the light has to travel through, making it appear to curve around massive objects." Is it more accurate to say it "appears to curve" or simply that "it does"?69.119.13.218 20:59, 12 March 2007 (UTC)


 * The problem is that light always goes straight, and what "curves" is the space it travels through. Or, to put it another way, light always takes the shortest trip between two points. If we graph the positions of Earth and stars we think we know where to set our "gunsight" so that at a certain time of the day or night the gunsight will exactly line up on the star. Then we find that when the light has to skim by our sun on the way to earth the distant star is no longer where we expected it to be.  Do we say that light in this case has selectively changed its nature so that it curves through empty space? Or do we say that light goes in a "shortest distance" path, but what is the "shortest distance" changes in the presence of a massive object like the sun? Light does not have mass, so we cannot say that the sun "pulls" the light out of its normal path.

The idea that gravity is an attraction explains why Mars stays in orbit around the sun, but it does not explain why something massless would be "attracted" by the mass of the sun. The idea that "gravity" is a misconception may be a very productive idea because space curvature explains what was previously called "mass attraction" but it also explains "curving" of light's path. P0M 03:46, 24 March 2007 (UTC)
 * In classical optics you have fermat's principle stating that light chooses the path that takes the least amount of time. I think this works for gravitational lensing to. As in coordinate time (although not in proper time) light goes slower close to massive objects. I would not try to change the article though, since the current formulation is the standard one.Agge1000 00:38, 12 November 2007 (UTC)
 * "Massless" means that were the particle stationary, it would have no mass (see rest mass). You're not going to find a stationary photon, hence why it is massless (crudely). However, it has an energy, and that energy can be translated into a mass by E=mc^2 (or more accurately, E^2 = m^2 c^4 + p^2 c^2). So you can think of gravity applying to this equivalent mass, hence bending the light. Or, more accurately, you can indeed think of space-time curving. Mike Peel 05:14, 19 April 2007 (UTC)
 * In this case, treating photons as massive particles influenced by strictly newtonian gravitation will only yield half the measured bending (that Eddington found in his famous experiment)Agge1000 00:56, 12 November 2007 (UTC)

Error in the main text
Because of my purr English I do not dare to mess with the original text of the article. However, somebody should take a look at the following erratic paragraph:

"At velocities at or approaching the speed of light, however, it becomes clear from experimental results that this rule does not apply. Two spaceships approaching each other, each travelling at 90% the speed of light relative to some third observer between them, do not perceive each other as approaching at 90% + 90% = 180% the speed of light; instead they each perceive the other as approaching at slightly less than 99.5% the speed of light.

This last result is given by the Einstein velocity addition formula:


 * $$u = {v + w \over 1 + v w / c^2} \,\!$$

where $$v$$ and $$w$$ are the speeds of the spaceships as observed by the third observer, and $$u$$ is the speed of either space ship as observed by the other."

According to the explanation of the last sentence the formula should be:


 * $$u = {v - w \over 1 - v w / c^2} \,\!$$,

since in the third observer's eye the two velocities are 0.9c and -0.9c. The formula results in the correct valule (0.995) only this way. If the formula does remain then the last sentece is incorrect:

"where $$v$$ and $$w$$ are the speeds of the spaceships as observed by the third observer, and $$u$$ is the speed of either space ship as observed by the other."

The right description is

"where $$v$$ is the speed of, say, the first paceship and $$w$$ is the speed of the other one if observed by the first spaceship."

Please consider my suggestion.
 * Since no one had reacted I did the correction by myself.


 * I'm sorry that nobody responded to your comment before. I just checked the first textbook of relativistic physics I found on my bookshelf, Introduction to the special theory of Relativity, by Claude Kacser, Department of Physics and Astronomy, University of Maryland. Published by Prentice-Hall.


 * That textbook gives:


 * $$v = {v' + V \over 1 + v' V / c^2} \,\!$$ (4.8)


 * The text explains: "Here v, v', and V are all parallel velocities, and Eq. (4.8) refers to their magnitudes.  If V and v' are both positive, v is less than their algebraic sum. That is, parallel velocities do not add according to the laws of arithmetic.  Of course, if both v and V are much smaller than c, the relativistic correction is negligible.


 * According to this writer's scheme, v' and V are velocities of two travelers as they experience their velocities. In other words, v' and V are velocities relative to each other or as measured by the two travelers, and v gives us the velocity relative to a "motionless" observer.


 * The author notes, "We have assumed that both v' and V are positive. All our equations hold independently of the signs of v' and V, and the modifications necessary for our discussion of negative v' or V are straightforward."


 * However, if you work through the formulas and use 0.9c and -0.9c you will indeed come up with a mathematical problem because you get a zero in the numerator.


 * A negative velocity is just a representation of motion in the opposite direction of some other motion given a positive velocity. Depending on where they are, the two are either approaching, passing, or departing from each other.  Two positive or two negative velocities descriptive of some situation would indicate two ships going in the same direction. They could still have a  velocity relative to each other, and they could still pass.   On the other hand, what would happen if the two spaceships were both moving in the same direction at 0.9 c (or -0.9 c) according to an Earth observatory? They wouldn't have any velocity relative to each other. If we write the formula your way, then two identical velocities would calculate out as a 0 velocity between ships, which is what you want.


 * Now I'm puzzled as to why two different (?) sources have preferred a formula that looks like it needs a good spin doctor. The equations may "hold" regardless of the signs of v' and V, but the answers are different. P0M 07:07, 16 March 2007 (UTC)

I'll come back to this later. P0M 22:28, 16 March 2007 (UTC)

The equation works when you have a spaceship traveling at 0.9 c, and a very fast crew member runs at 0.9 c in the direction the ship is traveling and we get the 0.995c. If he runs in the other direction then it is at -0.9c, he appears stationary, and the zero numerator makes sense. These final values are seen by the fixed observer watching the ship come directly toward him. Now if the observer is on the ship, he appears fixed and the rest of the universe is coming directly toward him at 0.9c, in that universe is another ship moving at 0.9c wrt the universe. This is the same as observing a ship (the universe) and a crew memeber (the other ship.) Plugging the numbers in from that perspective makes the equation work as written, particularly since the original text said 0.995c was what the spaceships observed. I claim no knowledge of relativity, so correction would be welcome. John N. 00:29, 17 March 2007 (UTC)


 * The problem wasn't with the formula. I doubt that two reputable physics textbooks would get it wrong, and get it wrong the same way.


 * The problem was with applying formulae without thinking things through. The textbooks that give the formula that uses plus in the numerator and in the denominator talk about a situation wherein somebody is sitting in a train station, measures the speed of a departing train, knows somehow that some guy is running toward the locomotive at a speed relative to the floor of the train. He wonders how fast the guy is "really" going. In classical physics, if you knew the train was going 50 mph and the guy was running 15 mph you would conclude that his speed is 65 mph. The relativity formula follows that general scheme with a correction factor. (Either way, you have to do some addition.)


 * The situation involving two spaceships approaching each other is different. It's the guy sitting in the caboose who considers himself immobile, sees the train station receding in one direction and the runner receding in the other direction. Imagine that you are in the middle of the ocean on a boat. As far as you are concerned, you are immobile. You see one boat approaching you from the direction of your stern and another boat approaching from the direction of your prow.  You would calculate the speed that they were moving toward each other by summing the absolute values of their apparent speeds (known by your handy radar speedometer).  But if you were being a physicist about it, you would assign one of the velocities a positive number (because you've graphed its position along the x axis and it's going from lower to higher values of x, and you would give the other of its velocities a negative number (because it is moving from a higher position on the x axis to a lower position.  If you add a positive number and a negative number you get something in between, but you want to get a larger result instead. (We all know what happens when cars going 70 mph have a head-on collision.)  So you have to subtract the negative number.  If you work a few examples you'll see. Three ships, Alph, Bell, and Cass are adrift. Bell sees Alph approaching at 7 mph. Bell also sees Cass approaching at -7 mph (i.e., it's going 7 mph in the opposite direction. They are approaching each other at 14 mph, not at 0 mph.  If Alph is approaching at +7 mph and Cass is receding at +7 mph, they are not moving with respect to each other. (They way they see it, they are really "still" and Bell is moving at -7 mph between them.)  7 - 7 = 0.

P0M 03:58, 17 March 2007 (UTC)

Error in the main text
You completely misunderstand the velocity addition formula. The correct interpretation goes like this:

Take three observers. A, B, C. Let A observe the speed of B. Let this speed be VB. Also, Let A observe C, too and let the result be VC. Now, at the same time let B observe C and let the result be V. The velocity addition formula describes the relationship between these measurements:

VC=(VB+V)/(1+VB*V)(for the sake of simlicity c is 1 now.)

Your greatest mistake is that you identify the velocities of the right side of the equation with the measuremnts of A, the observer "in rest". No, only one of these variables belongs to A's observation, the other one belongs to B.

Let us now work out yor problem: Two space ships, B and C approach each other. Their speed measuremnts have to be the same, only the sign is and has to be different: Say V=-0.9, that is B sees C approaching him. The sign is negative because the direction of C is opposite. What does a A see? It depends on his relative motion. We may assume that VB=0,9. That is A sees B running towards C at 0.9. The velocity addition formula tells now that VC=0. Which is correct. This means that if A sees B running away at a speed 0,9 and B sees C approaching him at a speed of 0.9, then A must see C standing still.

But this is not what you wanted. What you wanted was: A sees B running in the same direction as C. A's measurement on B is VB=0.9 and B's measurement on C, V=0.9 then VC is not VB+V but (VB+V)/(1+VB*V)
 * That's not what the article wants.

Please reconsider your interpretation or I will change the whole paragraph. Then you can appeal to the saint moderators. —The preceding unsigned comment was added by Zgyorfi (talk • contribs) 10:06, 18 March 2007 (UTC).

Here is what the article says: "wo spaceships approaching each other, each travelling at 90% the speed of light relative to some third observer between them, do not perceive each other as approaching at 90% + 90% = 180% the speed of light; instead they each perceive the other as approaching at slightly less than 99.5% the speed of light." So there are three spaceships. What the text wants is two spaceships, A and C that are approaching each other. B is between them. B measures the velocity of A as 0.9c and measures the velocity of C as -0.9 c. The article asks the question, what velocity will A and C measure relative to each other. (You assume that they measure their velocity, which is another matter.) So the formula says that to compute the speed of two things relative to each other based on measurements of both made by an observer, we use the formula given in the article. With your letters, that's asking to compute VC when you know VB and V. That result is why the person who started this section wanted to change the formula in the article, claiming, rightly, that the universe is not so crazy that if I stand between two onrushing trains and measure a high velocity toward me for both of them it does not mean that the locomotives are not moving. I explained that we have to be careful about what we are doing with our numbers and not just apply formulae as they fall into our hands. (It turns out that the person who wanted to change the formula in the text was you.)
 * VC = 0.9c + (-0.9 c)/(1PVB*V) = 0

What do I do if I stand between locomotives and calculate their closing speed relative to each other? To save the remainder of my hair I am going to choose a coordinate system that helps keep the plusses and minuses easy to deal with. Somewhere is the station in Moscow. I am somewhere on the new train line heading due east. Behind me comes a locomotive. My little switching engine is stalled on the track. No fuel. My radar sees the train from Moscow approaching at 300 kph. Bad. Then I turn around and see a train approaching on the same track from the east, at 300 kph. Even worse. I get out of my little engine and stand on a hill some distance away. I wonder how fast their closing speed is. If they have radar speedometers, they will see each other as closing with a speed of 600 kph. If one of them is a physicist he may say that the other guy is going at +300 and he is going at -300, so adding their velocities their closing speed is 0. He should not be reassured. The reality is that with signed numbers in this case you have to subtract. It doesn't matter whether they hit at +600 kph or at -600 kph. It's all in their point of view.

What if on another day I am standing by the same track and observe one train heading east at 300 kph and a while later I see another train coming along at 600 kph. To know how fast they hit, I will have to subtract again.

So even when the speeds get up to large fractions of c the basic procedure has to be the same. The calculation that the article is looking for if it wants to get the answer it calculates is VC = VB-V/(1-VB*V)

Do the math. VC = 0.9 c - (-0.9 c)/ 0.9c - (-0.9c)/(1 - (-0.81)) = 1.8c/1.81 = .994 c

Please sign your postings. ( ~ ) P0M 04:56, 19 March 2007 (UTC)

The citation form of the formula will calculate a 0, so it's not the right one to use in the word problem given in the article so I have added a little more explanation to the article. The formula the article quotes is the "citation form" of the formula, i.e., it is the way the physics textbooks give it. (It is not a mistake.) But when physics textbooks give this formula they explain what kind of a physical situation it is to be used to analyze. If we simply change the formula in the article then somebody with a physics textbook will change it back again. Probably what really needs to be done is to fix the article that is linked to that formula (assuming it doesn't make it clear that you can get in trouble by following the formula blindly). An easier way might be to advise people to use the absolute values of velocities. Physicists would probably get creeped out by that approach. The formula is already a simplification of a formula used when one cannot assume that the velocities are parallel. Maybe using absolute values would cause errors under some circumstances. I don't know off the top of my head. P0M 05:29, 19 March 2007 (UTC)


 * I don't know the history of the article, but it currently uses the word "speed" to describe all the variables, and speeds are always positive. That's more than enough for an article on the speed of light. Melchoir 02:03, 20 March 2007 (UTC)


 * The way you you fixed it looks fine to me. There are multiple occurrences of the word "velocity" in that section, but your way of phrasing things should make what is going on clear enough for the general reader. P0M 02:42, 20 March 2007 (UTC)

Dear fried, Patrick
I do not want to mess with your article but you'd better listen to me. You are wrong. Physics is not something that you quote, physics is something that you are supposed to understand.

As far as the cited article: That article is wrong too inasmuch it does not explain which velocity (vector) is observed by whom.... Now I go and organize a scandal over there too. But you understand quotations only the here is one from the article "Special relativity" from wikipedia:

Composition of velocities
If the observer in $$S\!$$ sees an object moving along the $$x\!$$ axis at velocity $$w\!$$, then the observer in the $$S'\!$$ system, a frame of reference moving at velocity $$v\!$$ in the $$x\!$$ direction with respect to $$S\!$$, will see the object moving with velocity $$w'\!$$ where


 * $$w'=\frac{w-v}{1-wv/c^2}.$$

This equation can be derived from the space and time transformations above.

Now, this is the formula that applies to your situation. You'd better try to understand rather than referrring to authorities.

Signature: I am not and never will be politically correct enough to follow your rules. My signature is my username, which is not hidden. Report to the inquisition if you like.

My, we've all expended quite a bit verbiage, and a wee bit of vitriol, to address something that didn't need correction. The main text clearly states (stated) that these are speeds. Signature: I am not politically correct, but that has nothing to do with my being civil, courteous, or considerate, or with keeping the agreement I made to abide by the guidelines when signing onto Wikipedia so I'll sign John N. 23:26, 26 March 2007 (UTC)

Speed of light no longer a constant
I'm rather new to contributing to wikipedia and i'm not quite bold enough to edit the article directly but there are recent studies that the speed of light has in fact been changing. While this is still a new thing, this has wild implications all over the physics world. While this is still too new of research to be redoing everthing, I think it at least merits a mention on the page and wanted to bring it up for discussion with more experienced editors. I'm sure anyone reading is skeptical so I'll refer to you an article from newscientist which can explain it far better than I could. http://www.newscientist.com/article.ns?id=dn6092  Dcpirahna 02:44, 27 March 2007 (UTC)


 * I understand why you (and, of course, lots of other people) are interested in the new debate that has opened up over the constancy of the speed of light over time. If the speed of light changes over long spans of time then that fact has implications for the history of the development of the universe in its present form.


 * I suspect that bringing up this contentious issue in the context of a beginning article on the speed of light for the average well-informed reader risks destabilizing the whole enterprise. It's hard enough keeping up with the consequences of speeds at substantial fractions of c not being additive, the consequences of time dilation, etc. For most people it is conceptually very challenging. For people like me, going over and over the calculations while making as sure as I can that I am not just blindly applying formulae is the only way to keep my head on straight.


 * If we bring in the idea that measurements of c taken at great time distances from each other could be different, then the reader may get confused and wonder if the speed of light could be changing or could be different for two observers living in close temporal proximity to each other. The greatest reason that c is important in modern physics is that anybody who measures it while moving at some constant velocity is going to get the same answer. People need to be given as much help as possible to see the consequences of this fact. Bringing in some finding that is possibly of great importance to cosmology but incidental to the things we are trying to inform readers about is likely to do a disservice to that reader.


 * It's an analogy, but possibly a helpful one, to say that the speed of a wave phenomenon is a function of the density of the medium through which the wave propagates, and if the density of the medium changes then the speed of wave propagation will also change everywhere in that medium. If we had to say something it might be best to say that there are indications, in line with this analogy, that the speed of light may have gradually changed everywhere as space has changed. P0M 04:10, 27 March 2007 (UTC)


 * Fair enough. It is a fairly confusing topic as is.  Hopefully more research will be done on the changing speed over time and can always be included later once the ramifications of the research are better understood.  Thank you for the response. Dcpirahna 05:59, 1 April 2007 (UTC)

Speed of Light and mathematical Notation
I am considering posing the following paragraph to the main page, so please reply with constructive criticism.

In mathematical notation we write concerning speed v=d/t where v, d and t are v velocity, distance and time respectively. v=d/t Implies that t=d/v. Since according to the Principle_of_relativity it is forbidden to travel faster than the speed of light, instead of writing t=d/v we could write t=d/(c-vn). We define vn =c-v. This would produce a singularity for time at the speed of light. This would imply that we could write (c-vn) =d/t. This would symbolize that Velocity an object having Invariant mass is undefined at the speed of light. Where appropriate, c=299,792,458 m/s. 71.156.91.18 00:20, 3 April 2007 (UTC)


 * Hi, and thanks for posting this here first before trying to edit the article. Note that according to your derivation, with t = d / (c - vn), and vn = c - v, we end up with t = d / v, which is the same as the Galilean definition you use above.  This does not produce a singularity for v = c.  For the correct derivation of how this goes, you should check out Lorentz transformation.  Also, note that there are many possible transformations that satisfy the singularity property you are talking about.  However, the standard Lorentz transformation is picked, because it is in fact derived from the underlying Minkowski space metric. Grokmoo 18:14, 3 April 2007 (UTC)

Weber's Constant
Why does Weber's constant redirect here? It is part of psychology...used to determine the JND of various stimuli... while it can deal with light as a stimulus it has little to do with the speed of light and is not exclusive to light.

Speed of light in your top section is wrong
"In metric units, c is exactly 299,792,458 meters per second (1,079,252,848.8 km/h)."

hi, i was going over your page and i noticed that this part is wrong. your information on the speed of light is correct at the m/s stage, but your km/s calculation in wrong. it should be about 17,987,547.48 km/h not 1,079,252,848.8 km/h.

299,792,458 X 60 /1000 = 17,987,547.480

Ochosi 23:22, 4 April 2007 (UTC) Ochosi
 * You forgot to multiply by 60 again (times 60 for seconds to minutes, then again for minutes to hours). – mcy1008  ( talk ) 23:35, 4 April 2007 (UTC)

Removal of "Technical impossibility of travel faster than the speed of light" section
See this edit.

I removed the section because I found it to be highly unclear, and to the extent that I understood it a novel approach to the issue. As any novel synthesis is a violation of WP:NOR, I felt obliged to remove it. Note that my disagreement is with the apporoach, not the conclusion. If this approach is known and valid, then a reference should be provided.

If anyone stongly disagrees with this action, then please feel feee to reinstate it pending a decision on the issue by the editors of this article. In the meantime, I will be adding a note to the section of light as a limiting speed showing how the relativistic velocity addition formula gives this result. --EMS | Talk 04:21, 21 April 2007 (UTC)

Here is the section that EMS removed. In general I think it is better to discuss things first rather than making a drastic edit. P0M 00:45, 24 April 2007 (UTC)

To understand why an object cannot travel faster than light, it is useful to understand the concept of spacetime. Spacetime is an extension of the concept of three-dimensional space to a form of four-dimensional space-time. Having the classical concepts of height, width, and depth as the first three dimensions, the new, fourth dimension is that of time. Graphically it can be imagined as a series of static,three-dimensional 'bubbles', positioned along an arbitrarily chosen line, each bubble representing a separate position along one of the four dimensions. That graphical approach is analogous to using a sequence of two-dimensional cross-sections taken at some standard interval along the third dimension to represent a three-dimensional object on a two-dimensional surface. (Imagine a map of a multi-story building that is created by giving the floor plan for each story of the building on a new page.) The mapping of space and time can be rotated so that, e.g., the x dimension is replaced by the t dimension, and each "bubble" represents a cross-section taken along the x dimension. Supposing that travel is occurring along the y and or the z dimension, what one will observe is that change along the t dimension will decrease from "bubble" to "bubble" as change across the y-z plane increases from "bubble" to "bubble." With this understood, there is a clear implication that an object has a total velocity through space-time at any instant, and for all particles of matter this velocity is equal to the speed of light. While this result may seem contradictory to the idea of speed-of-light travel being impossible, it in fact proves it, taking into account the fact that faster-than-light travel was a spatial, or three-dimensional concept, not a four-dimensional concept. In the case of four-dimensions, all of the total velocity of an object not accounted for in three-dimensional space is in the fourth dimension, or time. To go back to our bubble picture, if an object is remaining at the same x, y, z positions will make maximum progress in the t dimension. And that is just to say that any clock associated with whatever we are watching at x, y, z is ticking away at its maximum rate according to a static observer in the same frame of reference, e.g., somebody at x+3, y+4, z+5 or any other position that is not changing with respect to x, y, and z. But the greater the changes of x, y, and z according to the clock of the other observer, the smaller will be the changes in t. But using the Pythagorean theorem to calculate the distances between a point at x,y,z,t and some later point x', y', z', t', then those distances will always be the same. While this may seem confusing, it shows that as displacement through space increases, measured time will decrease to maintain the overall space-time velocity. If this is the case, it makes speed-of-light travel impossible, since when as an object approaches the speed of light spatially, it will have to approach zero velocity temporally. Another implication is that an object might be said to travel through four-dimensional space-time at the speed of light, but only in cases wherein its velocity through space is zero. That statement is just a counter-intuitive way of expressing the idea that when one is motionless (according to another observer) one's clock is ticking away most rapidly, and that as one moves faster and faster (according to the other observer) one's clock is ticking at slower and slower rates that approach zero.

I noted at the end of February that I didn't know whether this part was worth saving. However, I think that if provided with some drawings it could help some readers, for whom other ways of explaining things are not easy to absorb, see the way things work. P0M 01:04, 24 April 2007 (UTC)


 * As someone who understands special relativity (SR), I find this to be a very complex bunch of handwaving which asserts a lot and shows the reader almost nothing. IMO, there are plenty of ways to show why faster-than-light travel cannot occur in SR which will make the point cleanly and concisely.  One is what I did in the "constant velocity ..." section of the article, in which I showed that the velocity addition formula prevented it.  The energy and momentum equations also show much the same thing, which infinite energy and momentum being needed for massive objects to reach the speed of light.


 * The point is certainly legitmate (which is why I preserved it), but the means are not. Do keep in mind that the topic of this article is the speed of light in general.  Related topics such as the theories of relativity are better covered in their own articles. --EMS | Talk 04:02, 24 April 2007 (UTC)

Did you want others to not discuss this passage? I thought you had already made your point about not finding it helpful in an admirable way. Perhaps a third, fourth... opinion may be useful. P0M 04:15, 24 April 2007 (UTC)


 * LOL! (but at myself). You have a point there.  I would like more opinions, in spite of the fact that I have my own, and am too ready to express then. --EMS | Talk 04:40, 24 April 2007 (UTC)

Earth/Moon photo makes no sense
The photo captioned "A line showing the speed of light on a scale model of Earth and the Moon" does not make any sense. Can someone come up with a better photo? Davidwr 00:29, 29 April 2007 (UTC)
 * I think it would be better if it showed a 3D globe of the earth with atmosphere, dark space and a 3D moon. Then it showed a finite beam of light leaving the earth's domain and hitting the moon. It also would be nice to have a clock running in micro-seconds. --Marvin Ray Burns 02:24, 29 April 2007 (UTC) P.S. I see that you almost have it that way.
 * I stole the existing photo and added a distance-scale so at least it makes sense now. Feel free to do a 3D one.  Davidwr 04:50, 29 April 2007 (UTC)
 * It wasn't a photo, it was an animation. The new diagram isn't an animation (as far as I can tell). Stannered 09:47, 29 April 2007 (UTC)
 * I restored the old animation. EdwardLockhart 12:27, 29 April 2007 (UTC)
 * Thanks. I've reverted the other article that used this same image. Davidwr 21:52, 29 April 2007 (UTC)

299,792,458 m/s, but about 299 338 km per second?
''the speed of light [is] precisely 299,792,458 m/s ... the speed of light is about 186,000 miles (about 299 338 km) per second'' This looks very odd. I suggest just saying about 186,282 miles per second. --Occultations 23:41, 18 May 2007 (UTC)
 * Good point. Done. Dravick 23:51, 18 May 2007 (UTC)

Dead Links
Footnotes 6 and 7 are dead links. Thus they should either be fixed or removed. --Trakon 10:46, 31 May 2007 (UTC)
 * Actually it looks like I meant 7 and 8. --Trakon 09:27, 7 June 2007 (UTC)

Footnote 9 is also a dead link. SpaceTym 12:44, 23 October 2007 (UTC)

c is for celeritas
I deleted language to the effect that using c to denote the speed of light derives from constant. The one cited essay speculates that c may alternatively derive from constant, but acknowledges grudgingly that celeritas is the widely accepted derivation. Weber's constant isn't the speed of light. Further, with all the constants floating around, the idea that any one constant would appropriate the initial letter of constant as a symbol is far fetched. We should go with the consensus derivation. Finell (Talk) 04:49, 1 August 2007 (UTC)


 * I've reverted your change, although it might be reworded yet again to note that c for constant was only the original usage and now generally means celeritas. — Joe Kress 07:49, 1 August 2007 (UTC)

Standard nomenclature: speed of light in vacuum.
The standard physics nomenclature for this physical constant is "speed of light in vacuum", see for exampel NIST. I think wikipedia should adhere to to standardized physics nomenclature, to avoid endless discussions and redirects. I propose to rename this article. /Pieter Kuiper 18:11, 8 August 2007 (UTC)


 * Please do not rename the article. Wikipedia's naming convention on en:WP is to use the name that the typical English language Wikipedia user would be most likely to use. In this case, that name is Speed of light. Only a specialist might ever think to add "in vacuum". The current, very long-standing name of this WP:FA is also shorter, and therefore easier to type into a search box, than what you propose. A redirect from "speed of light in vacuum" is not necessary (although it wouldn't hurt); if someone were ever to enter "speed of light in vacuum" in the search box, this article tops the list of articles that is returned with 100% relevance. Also, the article in not exclusively or even primarily about the named physical constant; it is more broadly about the phenomenon, its history, underlying theory, etc. On the other hand, correct terminology should be used in the body of the article. Finell (Talk) 21:41, 8 August 2007 (UTC)


 * OK, I understand your points. I will create the redirect then, which will make a name change of these article more difficult. Thank you for taking the time to respond to my suggestion. /Pieter Kuiper 21:58, 8 August 2007 (UTC)

The Light Cone image does not show up
I'm using Linux & Firefox so it may be a upper-/lowercase problem. Does it look OK in Windows? Johnmuir 09:44, 11 August 2007 (UTC)

I've fixed it myself. It was a) wrongly spelt and b) marked as a thumbnail Johnmuir 09:49, 11 August 2007 (UTC).


 * I have no clue what your technical problem is. The image works fine as a thumb (I use Opera on a winXp - also works with Netscape) I'll try it on the computer lab machines w/ie later. Without thumbing the image the caption dissappears. Do you have any problems with other thumbed images? Does anyone else have the same problem? Thumbed images are preferred as it allows user pref to control the size when set at default. Vsmith 02:48, 15 August 2007 (UTC)


 * It's a common problem that .svg images don't display thumbnails. Unfortunately, the issue is hard to predict, reproduce, or fix. The move to a .svg in this article is fairly recent, which probably explains why more readers haven't complained. I recommend using raster files until the issue is solved (and I'll do so here). Melchoir 03:29, 15 August 2007 (UTC)


 * It looks fine now - the last change was successful. I'd be pleased to help on the general problem if I can. (Testing, programming or similar, but I'm not a graphics expert.) Johnmuir 19:25, 15 August 2007 (UTC)


 * That would be great! Unfortunately, I don't know if there's any centralized discussion yet. I'm sure the problem must lie in the MediaWiki software, not Wikipedia's implementation, and I don't do work outside of Wikipedia myself, but you could always ask Village pump (technical) for guidance. Melchoir 21:59, 15 August 2007 (UTC)

graphic doesn't time 1.2 seconds
Using Opera, it's more like 3 seconds. IE timing seems more plausible. Since this is apparently browser dependant, I move that the comment should be corrected, or the graphic removed. —Preceding unsigned comment added by PoorLeno (talk • contribs)


 * I agree. Apart from the timing problems, the graph is annoying and distracting. Jaho 01:30, 23 August 2007 (UTC)

Possible to travel faster than speed of light
I found this article. Maybe we can add something like that into the article? Oidia (talk) 12:14, 21 August 2007 (UTC)

Even with this change there's still the matter of whether information can travel faster than light. According to this article it is "impossible", though I've heard of both situations in which there has been data transfer across large distances that exceeded c and people who're studying the issue. If it's at all in dispute then I think it should probably be made a little less definite than the current wording. I'll try to find an article or journal to back my claims. Tom Joyce1 03:56, 4 October 2007 (UTC)

There must be a Disambiguation
Speed of Light is a song by Stratovarius. I'm not english and i've not time enought to make the desambiguation page. Please, could someone else do it? Thanks. —The preceding unsigned comment was added by Special:Contributions/ (talk)


 * Done. — Joe Kress 02:14, 25 August 2007 (UTC)
 * Speed of Light is also a song by OMD, Teenage Fanclub, and probably dozens of other bands. So it seems like a disambiguation page is better than linking to every single song with the name by every minor band who's made one (not to mention other possible uses).WildElf (talk) 17:31, 28 November 2007 (UTC)

It may be a dumb question but..How come?
Why does light travel at a constant speed? What governs this? Structure? Kazuba 16:16, 10 November 2007 (UTC)

I am not sure we know any fundamental reason or structure for why light travels at c. John (talk) 18:03, 27 November 2007 (UTC)


 * The structure of spacetime within general relativity requires that there be one speed that all massless particles travel at. Why general relativity is true (as far as we can tell), and why light is massless (as far as we can tell), are questions I don't think there are good answers to at the moment. -- SCZenz (talk) 18:51, 27 November 2007 (UTC)

Evidently faster than the speed of light . . . And how.
Look to the left and you will see galaxies at point 'A' moving away from you at three quarters of the speed of light. Look to the right and you will see galaxies at point 'B' moving away from you at three quarters of the speed of light. The fact that we are in the middle is immaterial. But because we can see them simultaneously, it means that the two sets of galaxies existed simultaneously at those points 'A' and 'B' all those years 'ago'. Galaxies at 'A' were therefore moving away from galaxies at 'B' at 1.5 times the speed of light. How come?

The medium of 'space', even as a vacuum, is not 'nothing'. It is a structure or medium which (apart from being the single ingredient of which Universe forms) conducts light 'at the speed of light'. Galaxies at 'A' emit light and the space there conducts the light 'at the speed of light'. When the light passes us it is still travelling at the speed of light, and when it arrives at point 'B' it will still be travelling at the speed of light. Why? - Because 'space' itself extends with distance, the galaxies at both 'A' and 'B', as do we, seem to be stationary in their own locality of space, with the space at 'A' also travelling away from the space at 'B' at 1.5 times the speed of light.

The light from galaxies at 'A' is red-shifted as it passes us, and at some point between us and point 'B', perhaps where the speed of recession of 'A' reaches the speed of light, the light wave will become null. As it travels further the light wave can only become an inverse wave and return to be progressively less red-shifted, though it may be difficult to detect the fact of the wave inversion. It is supposed that when the recession of galaxies at 'A' reaches more than twice the speed of light, the light wave will become progressively blue-shifted, and it has been suggested that a search should be made for greatly blue-shifted light to indicate bodies moving away from us at many times the speed of light.

In reality, instead of an entity in itself, the apparent existence of 'time' is only an impression afforded as a consequence of observation or measurement of progressively differing positions in space. In reality the Universe is already complete; we are simply an integral part of the process of its completion, so, from here and. . . here and. . . here and as we are chemical organisms capable of observation during 'time', for us the Universe necessarily seems to be incomplete and 'in action'.

Universal 'expansion'? - due to evident long-range inter-galactic repulsion, as understood and suggested more than forty-five years ago. Of course, it may rather be that in reality this is long-range space repulsion, with the space carrying the galaxies along with it, but that may or may not be better appreciated and understood in the future.

Comments?

Geraldomanchego (talk) 19:11, 23 November 2007 (UTC) 18:20, 17 November 2007 (UTC)


 * See the explanation in Hubble's law. — Joe Kress (talk) 09:34, 18 November 2007 (UTC)

Super. Thanks. The problem is that I don't believe in the 'Big Bang' any more than does Fred Hoyle, who invented the term. And questionable equations about questionable equations are about as reliable as whispers about whispers to a simple chap like me, so if it's all the same to you I'll stick with my simple observable evidence and my simple - and to me credible - understanding of it.

But kind regards, and repeated thanks for your comment anyway,

Geraldomanchego (talk) 11:06, 18 November 2007 (UTC)

In fact, re-reading 'Hubble's law#Interpretation', so far as it goes, I don't see that much difference from what I am saying, except that I seem to be saying it without a fuzz of arguable formulae or arcane language and from a position a bit further out. No?

Geraldomanchego (talk) 11:16, 18 November 2007 (UTC)


 * I fail to see why light waves should become null. Why should they?
 * I don't think we can assume that because galaxy A recedes from us at 3/4 of the speed of light and B at the same speed in the opposite direction it means that they recedes from each other at 1.5 the speed of light, because to mesure that we would have to change our frame of reference (e.g. go to galaxy A and mesure) Dravick (talk) 22:00, 22 November 2007 (UTC)

Dear Dravick, with the greatest respect, I fail to understand your incomprehension but will work on it and come back to you when I have a moment. Kind regards, Geraldomanchego (talk) 08:51, 23 November 2007 (UTC)


 * Your question is based on a false premise: “Galaxies at 'A' were therefore moving away from galaxies at 'B' at 1.5 times the speed of light.” is false so questions about that conclusion cannot be addressed.  If A is moving out at 3/4c and B is moving in the opposite direction at 3/4c (relative to you) the rate of change between them is not 3/4c +3/4c as you assume.  Your math is wrong.  It is more complex than that.  See Length Contraction at []


 * It helps to have an intuitive understanding of what the speed of light represents. You have to abandon the incorrect earth-familiar way of thinking.  C is a little like 'infinity,' for speed, it is almost as if hypothetical things traveling at c take no time to get places, they would be in multiple places at once.  The only difference is that c is not infinite in this universe.


 * If you use the formulas that represent the way the universe really works, not what seems intuitive and consistent with our slow earthbound experiences, then you’ll see that the rate A moves away from B can never exceed c. John (talk) 17:04, 23 November 2007 (UTC)

Well, if you consider that space itself also extends with distance, then the light travelling through extending space never does travel faster than the speed of light, does it, though the galaxies are receding from each other at faster than the speed of light relative to each other. Geraldomanchego (talk) 19:12, 23 November 2007 (UTC)


 * You have stated another false premise: "...though the galaxies are receding from each other at faster than the speed of light relative to each other." You don't give up on that V1 + V2 view easily, do you?  Have you come to grips with the fact that two people walking away from you, each at 1 MPH are not leaving each other by quite 2 MPH?  Almost, but not quite.  In truth, this universe just doesn't work the way you expect it to, relative velocities just don't add like that.  You probably don't believe me, or think it is just a mathematical trick, but it is actually real.  John (talk) 01:02, 24 November 2007 (UTC)

"Have you come to grips with the fact that . . ." I'm afraid not. I accept the Universe as it seems to be, but appreciate it overall as a done completed thing, a very simple unit formed of one ingredient, and without 'time', and this hampers me in seeing it from your point of view in having to accept any particular 'established' credo.


 * To have an answerable question, I suggest you try to state the question so it isn't asking about anything going faster than c relative to anything else. John (talk) 01:07, 24 November 2007 (UTC)

I haven't any questions. I invited comments on the possibility of looking for greatly blue-shifted light to indicate multiple-c receding galaxies in multiple-c receding space, and I have received one or two, for which many thanks, even though they don't entertain my notion. Are you going to tell me that you honestly believe that remote galaxies are not receding from us at greater than the speed of light, and that they all tamely conform to mathematical considerations and pile up out there when they achieve a particular distance from us? Sorry, but consider the acceleration of galaxies when they were at a distance of three-quarters of c, and consider their 'present' velocity that this indicates. I'm afraid that I for one decline to jump through any theoretical hoop, no matter with how many equations you try to lead me to it. Is it the concept of expanding space that bugs you? as you will see that according to the expansion of this medium then galaxies can recede at many times c but still be 'at rest' within their own locality of space.Geraldomanchego (talk) 11:56, 24 November 2007 (UTC)


 * Exactly… I believe galaxies are not receding from us faster than c.  Certainly the universe doesn’t tamely conform to math; math (tries) to model what we know about the universe behavior.  What we know is that we’ve been looking, and found nothing moving faster than c.  Again, you are inviting comments on false premises; you assume delta velocity from acceleration simply adds to current velocity, it is more complex than that in this universe.  You seem believe the approximations that always worked in your community as a kid will generalize at fast speeds, but you are mistaken about that.  Physics cannot work the way you envision, your view leads to unworkable contradictions in time and space.  Even if something were traveling at (0.999999…)c, it would nevertheless take infinite acceleration to push it up to c.  There is a shortage of galaxies out there with that much acceleration.  It isn't the math you reject, you reject reality and substitute your own.  If you find something traveling faster than c it will be a really big deal.  If you will take a look at general relativity, you would be better able to make sense out of this.   John (talk) 19:28, 24 November 2007 (UTC)

"Even if something were traveling at (0.999999…)c, it would nevertheless take infinite acceleration to push it up to c." Light travels at c voluntarily, naturally conducted by the medium of space. I understand it can spiral even faster, then emitting a blue glow, the name of which I forgot forty-odd years ago. And if - as I suppose - space is extending with distance, then the light is naturally travelling faster than c with respect to its point of origin, as indicated in its red-shifted wave-length.

"There is a shortage of galaxies out there with that much acceleration." I refer you to the Hubble Ultra Deep Field in Wikipedia. Plenty of candidates there must now have accelerated to C++ by now. I don't know how blue-shifted light from c+ galaxies will be differentiated, most probably beyond normal visibility, other than by looking for greatly blue-shifted light, but it will then probably be too remote or attenuated to capture. Happily it is not my pidgin.

May I comment that your view of the possibilities of the Universe will be limited by the commonly-accepted doctrinal platform that "nothing can travel faster than c". That galaxies visibly receding from us at 90% of c 13 billion years ago are not now receding faster than c 'at rest' in their own locality of space seems to me to be improbable. We, 'at rest' in our locality of space are most probably travelling away from them at c+ and accelerating, but we don't notice it because our space is travelling and accelerating with us.

I said fifty years ago that distant galactic repulsion was an aspect of the unified field force, and a couple of years ago some other scientists started to say it was a possibility and that it would increase with distance, thus accelerating those remote galaxies - beyond c? You betcha.

"Physics cannot work the way you envision". In my opinion reality is immutable. Human 'physics' is not.Geraldomanchego (talk) 20:35, 24 November 2007 (UTC)


 * Re: "...reality is immutable. Human 'physics' is not."
 * But which is which?   John (talk) 08:08, 25 November 2007 (UTC)

I think all sane people would admit that the ontological reality of Universe, predating frail humans and their 'physics' by evidently 13 billion years and even tolerantly permitting their presumption and their errors would have the edge in reliability, wouldn't you? Geraldomanchego (talk) 10:27, 25 November 2007 (UTC)

Woops! I've done it now. And someone is trying to send me viruses.84.78.91.152 (talk) 19:21, 23 November 2007 (UTC)


 * If you sign in at the upper right corner of any Wikipedia page, and sign your entries with four tildes (~), your entries will be automatically signed with your signature "Geraldomanchego (talk)" as well as the time/date. Clicking "remember me" when you sign in places a cookie on your computer which automatically signs you in every time you access any Wikipedia page for the next two months or so before the cookie expires. — Joe Kress (talk) 21:05, 23 November 2007 (UTC)

- Or I expire and stop sending, right. It wouldn't sign for me, and then it did that - Or my extra mental fingers got in the way. Thanks for sorting it out. I was expecting at least the Wiki police at any mo.Geraldomanchego (talk) 11:56, 24 November 2007 (UTC)


 * I think it is quite reasonable for me to mention that I am totally lost. From what I understand, User:Geraldomanchego reasoned that galaxies receding at 0.75c on each side from our frame of reference must be receding from each other at 1.5c, then pushed his reasoning further to come to some conclusions (which I don't quite understand, maybe because I've missed a step somewhere in the logic :S). However, John pointed out that velocities do not add up like that, therefore the whole reasoning is based on false premises, and probably leads to false conclusions. Did I get that right? Dravick (talk) 04:31, 25 November 2007 (UTC)


 * Pretty much. & I think I missed that step too.  The premise that is false (to the best of our current knowledge) is that things can go faster than c with respect to each other.  Going faster than c seems reasonable enough on the surface until you think about what that implies in terms of things like how  simultaneous events in different places can be observed.  The premise that V can exceed c leads not only to false conclusions but the necessary and simple implications of it lead to obviously unworkable contradictions in space-time relationships.  If all the theories are wrong, and things actually can exceed c, it must by some other scheme besides the obvious.John (talk) 08:04, 25 November 2007 (UTC)

Gentlemen, please note that I said in my initial note, and have said since, that the medium or structure of space also extends with distance, and so light - or galaxies - contained within that space may well be moving faster than c with relation to us, but not travelling faster than c with respect to local bodies within their own locality of space.Geraldomanchego (talk) 12:25, 25 November 2007 (UTC)


 * You stated in your initial post that "the two sets of galaxies existed simultaneously at those points 'A' and 'B' all those years 'ago'." This means that both existed in space having the same characteristics. The expansion of the Universe that we observe causes both to move away from us at the same velocity. But they did not move away from each other because they themselves were in the same space. Even we were in their space long ago, so at that time even we were motionless relatve to both galaxies A and B. It is our great age relative to those young galaxies (1/4 of our galaxy's age) which caused them to move away from us at 3/4c. The current ΛCDM (Lamda-Cold Dark Matter) model of the Universe, which was invented to account for the observation that its expansion is accelerating, nevertheless has space that is nearly flat throughout—it was not stretched at any time in the past.


 * No doubt, your 'blue glow' is Cherenkov radiation. But it is only generated by a subatomic particle of matter moving through some transparent matter faster than the transparent matter's speed of light. By definition, light itself must move through transparent matter at its speed of light, just as it must move at c through the vacuum of space. The two speeds of light are related by their indices of refraction. — Joe Kress (talk) 09:16, 26 November 2007 (UTC)

I am saying that they were already that far apart then and already travelling at that speed all those years 'ago'. At that time they did not occupy the same locality of space, but the locality of space of each was already that far greatly separated and accelerating. Look, is this difficult? From one single ingredient, space, discrete concentrations give the effect of different sub-atomic particles, which interact to give the effect of atoms, which interact to give the effect of molecules etc. Aggregations of these generate the forces of binding energy, weak local repulsion, gravity and distant galactic/space repulsion as harmonics of the same force - But it is still only space. The Universe is already complete, but as we are part of its process of completion for us it seems incomplete and in action. - Surely it doesn't need any more than that, and all the mathematics and special provisions and exceptions and inventions etc. are only trying to support this or that limited opinion about the workings of its tripes. As to what it is, it is itself. What is it? I don't know what a grain of sand is. Oh, I know what I'm told, but I do not and cannot know what it is any more than I know what Universe is. So we don't need exclusions and provisions and so on. . . All we have to do is to look and appreciate, and if we can, perceive. I suppose that cosmological speculation provides comfy jobs for the boys, but I don't think it is so complicated or difficult as they would have us believe.Geraldomanchego (talk) 13:06, 26 November 2007 (UTC)

"they themselves were in the same space" - They weren't in the same locality of space even then - See above.Geraldomanchego (talk) 13:06, 26 November 2007 (UTC)

"Even we were in their space long ago" - if you subscribe to the Big Bang. . .Geraldomanchego (talk) 13:06, 26 November 2007 (UTC)

"The current ΛCDM (Lamda-Cold Dark Matter) model . . ., which was invented" Ah-hahGeraldomanchego (talk) 13:06, 26 November 2007 (UTC)

". . . its expansion is accelerating" - and if you look at how far it must have accelerated from that already remote position in similarly expanding space and that speed of already 3/4 c - then see where and how fast it is now Geraldomanchego (talk) 13:06, 26 November 2007 (UTC)

"space that is nearly flat throughout" - That's what they say now, but you must admit almost all human 'science' is little more than a biography of human error Geraldomanchego (talk) 13:06, 26 November 2007 (UTC)

"No doubt, your 'blue glow' is Cherenkov radiation. . ." - Right. I hope you don't mind me having had my little say, but I do appreciate that we are of differing viewpoints from which neither of us needs or is likely to shift. But interesting. Geraldomanchego (talk) 13:06, 26 November 2007 (UTC)

Perhaps it could be simply expressed that "Within the activity of Universe light is conducted by empty space at a velocity of c and in a straight line until or unless that space is distorted by such phenomena as gravity or distant space-repulsion." Geraldomanchego (talk) 10:15, 28 November 2007 (UTC)


 * That is already included in the various theories. Light always travels at a velocity c along a geodesic, which is the shortest path between two points, regardless of how the space between those points is distorted. It space is not distorted, than a geodesic is a straight line. — Joe Kress 04:56, 1 December 2007 (UTC)

"It space is not distorted, than a geodesic is a straight line." . . . Yerrss. And if the space - containing that geodesic and conducting light along it - is extended from point A then the light progressively will be extended. Therefore the light at its head will be travelling faster than the tails of both geodesic and light still being emitted at A. If the geodesic and the light are slightly extended, then the head will be travelling slightly faster than the tail. And if greatly extended, then it will be proportionately greatly faster.

The acceleration of galaxies with distance has been apparent for more than fifty years. Surely it is not too much to suppose that the space containing those galaxies is also expanding in proportion, and that the light emitted by them is conducted by space both there and here 'at the speed of light', even when the galaxies are receding 'FROM US' at > c, as, considering their distance in 'time' and space, arithmetically they have to be. - I fail to see any difficulty or inconsistency in this supposition. Happily the problem of reconciling other views with what seems to be an ever-straighforward and already satisfactorily-completed Universe is not mine.

But why should you suppose that the structure or entity of space should be accelerated any less than the more visible galaxies and their acceleration that we perceive? If you admit that space expands, then surely you must admit that its conducted light extends. . . and the rest of the argument follows. —Preceding unsigned comment added by 84.78.91.152 (talk) 12:28, 8 December 2007 (UTC) 84.78.91.152 (talk) 13:20, 8 December 2007 (UTC)


 * Einstein showed that space cannot be separated from time, so space alone does not expand in our expanding universe—spacetime expands. The speed of light is distance divided by time. Because both space and time expand, they maintain their ratio, so the speed of light is constant even in an expanding universe. Even the observed acceleration of expansion would produce a corresponding accelerating expansion in time, and again the ratio is maintained, so the constancy of the speed of light is maintained. — Joe Kress (talk) 09:03, 10 December 2007 (UTC)

Well, I'm just a simple soul who has believed for the last fifty-odd years that the Universe is composed of just one ingredient, which I think of as 'space', and that time doesn't exist, If you and Einstein want to consider it as 'spacetime', then go ahead, but for me it is still the same single ingredient of which everything is formed, and I am not separating it into space and time just to satisfy any clever-clever mathematical equations that folks have dreamed up to seem particular. For me, be it 'space' or 'spacetime', it expands with distance, and so light conducted by it is extended, with the leading point therefore travelling faster than the point of emission, even though the leading point is still travelling at the regulation speed within its own locality of space. Agreed or not? I'm not trying to be clever-clever; I just think the simple subject is fogged in terminology and extraneous maths. Geraldomanchego (talk) 12:06, 10 December 2007 (UTC)

Do we need a section on "Problematic Implications of V > c"
But Geraldomanchego (above) shouldn't be so sorry, he is not alone. I have talked to many people who labor under this misconception. I wonder if we need to add a Problems With Speed Exceeding c section to this article to gently usher or transition readers who steadfastly adhere to Newtonian physics into Relativistic physics. Can someone clearly articulate the problems with speed beyond c? John (talk) 19:44, 24 November 2007 (UTC)

Supposing that space itself extends, perhaps there aren't any. Best wishes anyway, Geraldomanchego (talk) 13:14, 26 November 2007 (UTC)

speed
a sprinter running 100 meters in 10 secondds —Preceding unsigned comment added by 209.254.12.73 (talk) 21:38, 26 November 2007 (UTC)

If there is a meter each metre and he is operating all of them in ten secondds, that is speed indeed! Why, if they are light meters, he must be operating them almost at the speed of. . . Well, obviously. . . of a sprinter running light meters at 10 light meters per secondd. Or is anyone of another opinion? Geraldomanchego (talk) 12:59, 27 November 2007 (UTC)


 * Gents, This page is intended for discussion of approaches to improve this article, not general questions and discussions about speed of light. The[| Reference Desk] is available for that.John (talk) 17:58, 27 November 2007 (UTC)

About the Earth to Moon animation
This is from something that we talked about today in my physics class. Please correct me if I'm wrong, or making incorrect assumptions.

If we use the time dilation equation:

T = T ' / sqrt(1-(v2/c2))

And assume that you are traveling at the speed of light. In this case, I am talking about light, so you can consider v=c. This makes the equation inoperable. So could you say that light itself does not have observe time? And that it does not move over time, but instead moves instantaneously? If this is the case, then the little animation showing light moving from the earth to the moon is incorrect. If this assumption that has been made is incorrect, then what really happens with the time dilation for light? —Preceding unsigned comment added by Kurt1288 (talk • contribs) 19:47, November 30, 2007


 * The animation is fine. No matter what you would perceive if you could ride a photon, the light's speed in the reference frame of the animation is c and not infinity. Melchoir 01:31, 1 December 2007 (UTC)

I'm not suggesting that the speed is infinity. It stays c. But that's what I'm saying. Isn't it that as you approach the speed of light while traveling, your time in transit (say from point A to point B) decreases? So at the speed of light, wouldn't the time spent in transit be instant? And then as you go faster, you'd go back. This is all talking about a photon, and nothing else. —Preceding unsigned comment added by Kurt1288 (talk • contribs) 22:05, November 30, 2007


 * It would be helpful if you could phrase your thoughts in more concrete terms. What should the animation depict? Melchoir 09:26, 1 December 2007 (UTC)


 * A couple of things, first, the animation is just an animation. Just as if you were to turn on a flashlight and shine it to a wall, that's the same concept as this little animation.  Second, as far as I know, it is when matter approaches the speed of light that these so called "rules" (originally from special realativity I think) apply. Light itself is Not matter, so light is beyond this restriction that is placed on matter.  Third, special relativity is just a strong human theory.  From other sources, I have heard that it is possible for matter to travel faster than light, and it is done constantly in other places in the universe.  One strong credible source (in my humble opinion) is Mr. L. Ron Hubbard.  Remember, our technology is only a few millineums old,  And we have yet to actually test Einsteins thoery.  Good luck in your physics class.  --DreHectik (talk) 20:47, 27 December 2007 (UTC)

Speed of light in interstellar space
Locally the speed of light is always c but what is the average speed of light for light reaching us from distant stars (if earth based clocks are used)? Far away from stars and galaxies the speed should be higher than in our solar system due to the higher gravitational potential. (I am not interested in effects due to space-curvature or expansion of space just the effect due to gravitational potential) Perhaps there is some number on this speed that could be included in the article.Agge1000 (talk) 15:42, 8 December 2007 (UTC)