Talk:Speed of light/Archive 2

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current speed of light

there doesn't seem to be any information in the history section about how the current (exact) speed of light was obtained. Is there any reason for this? I have also heard that the reason that a meter is defined as a fraction of the speed of light and not the other way around is because the speed of light was infact known more accurately than the length of a meter, is this worth putting in? --LeakeyJee 13:42, 3 April 2006 (UTC)

The meter is currently defined in terms of the speed of light. In sensible units, the speed of light is 1. However, humans don't use sensible units; they like to use units that are about the same size that they are. Consequently, the units meter and second bear little relation to physically sensible units. You can either define meter to be some arbitrary length and then use the speed of light to work out how long you want a second to be, or you can define a second to be some arbitrary time and use the speed of light to work out how long you want a meter to be. SI uses the latter method.

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. The meter is defined as the length of the path travelled by light in an absolute vacuum during a time interval of 1/299,792,458 of a second.

Neither of these weird choices has any bearing on the speed of light, which in sensible units is always 1. -- Xerxes 16:33, 3 April 2006 (UTC)

By the way, if this statement is correct "the speed of light was infact known more accurately than the length of a meter" then it should actualyl read "the length of a metre prior to its definition in terms of the speed of light". And "current speed of light" also doesn't make sense, it should be the current defintion of the speed of light. 218.102.221.8 10:37, 16 May 2006 (UTC)

Just to confuse those whom don't know much, I like to say c=299,792,458ms^-1.Because ms^-1=m/s.

Can uncharged nuclear matter be accelerated beyond c?

This is a little strange, and I want to know if I am wrong, but it seems to me that no part of relativity theory would be inconsistent with the hypothesis that uncharged nuclear matter (like that comprising neutron stars) could be accelerated to arbitrarily high relative velocities, as by the intense gravitational field of another neutron star or a black hole.

The requirement for relativity theory is that all physical processes must take place with the same value for c. But if the physical processes involved have no electromagnetic interaction with the rest of the universe (as would be the case for neutrons), then there would be no theoretical prohibition against it having a relative velocity greater than c. If this is true (and not the result of a misunderstanding of relativity or some other theory), then I think it would provide better models for the merging of black holes.

I imagine evidence for or against this would be hard to come by. Since particle accelerators only act on charged particles, even if a neutron could be generated, greater-than-light speeds could probably not be achieved due to technical limitations. The only force that could hypothetically accelerate neutrons to velocities beyond c would be intense gravity. Detection of something like a neutron star travelling faster than c would have to take into account gravitational effects on nearby ordinary matter, much the same way that black holes are observed.

Can someone say definitively whether or not this hypothesis is consistent with relativity theory?

Reykjavikingur 01:30, 15 January 2006 (UTC)

My understanding of relativity is that all matter which has a mass (charged or otherwise) increases in mass as its velocity increases. As the velocity tends to the speed of light, the mass tends to infinity - in other words, it would take an infinite amount of energy to accelerate an object to the speed of light. This does not affect photons since they have zero mass to start with. Birkett 17:59, 28 February 2006 (UTC)

Speed of light in space

I have been trying to find out the speed of light in space as opposed to a vacuum. Since a vacuum should have a temperature of 0 K and space has a temperature of 2.7K, the speed of light should be different for both. So what is the speed of light in space and what factors are involved in its calculation.

I believe that the the definition of c as the speed of light in a vacuum refers only to the fact that no matter particles are present in the path of light. Temperature has no part to play here. The speed of light does not depend on temperature.

NOTE: This does not mean light would travel at the same speed in matter in different temperatures. The temperature affects the density of matter, does affecting the speed of light. 59.93.161.76 12:06, 28 Mar 2005 (UTC)

Especially since the 2.7K temperature refers to blackbody radiation - i.e., light.

History of the measurement of the speed of light (vandalism?)

The first two paragraphs of the "History of the measurement of the speed of light" section are

The first quantitative estimate of the speed of light is seen in Indian vedic scholar Sayana's commentary on the Rigveda, one of the main Hindu scriptures. It says sun light travels 2202 Yojanas in a half Nimesa. Yojana is an ancient unit of length. Arthasastra defines it as being equal to 8,000 dhanus, which is equivalent to 9 miles. A nimesa is an ancient unit of time that is equal to 16/75 seconds. Thus 2,202 yojanas in half a nimesa is equal to 185,794 miles per second after conversion. The modern estimate of the speed of light is 186,281.7 miles per second.

It is to be noted that Bhatta Bhaskara (probably in 10th century) made the same statement in his commentary on Taittiriya Brahmana, another Hindu Veda . He says this to be an old tradition.

This sounds pretty suspicous to me. I think that these two paragraphs should be removed (or at least good references should be added). --Jochen 13:46, 15 Mar 2005 (UTC)

Yes, that was also my first reaction. However, there are quite some references to "2202 yojanas" on the internet, including arXiv e-print physics/9804020, a paper by Subhash Kak of Louisiana State University titled "The speed of light and Puranic cosmology". I did not read the paper, but it looks serious to me. -- Jitse Niesen 15:09, 15 Mar 2005 (UTC)

Indeed: I was initially suspicious, but found enough sensible-looking pages via Google to leave it in place. -- ALoan (Talk) 16:08, 15 Mar 2005 (UTC)

Your suspicions were justified. The units in question were not the same for all time periods and places, and experts' opinions differ about how long exactly they were.

[1] says: one day is 8*6*25*10*5*3 nimesas. Since that is 180.000, a nimesa is 12/25s or 6/25s (if "day" is the bright half only, which is improbable since this would mean half a day has 8 dandas, but all the other sources say half a day is 4 dandas. So I'll ignore this.) Farther down the site says that a nimesa is 16/75s.

[2] says that a nimesha is 8/15s, plus, farther down, one "day" (having 12 hours) is either 4*6*15*15*5*3 or 4*7*15*15*5*3 nimeshas.

Now we got the following nimesha/nimesa candidates:

0.213s

0.457s

0.48s

0.533s

[3] says that experts are divided on the size of a yojana - but it is between 6 and 9 miles. If we assume that this is the international mile, which is 1,6 kilometres (there are several others), then that is between 9,6 and 14,4 km.

[4] says it is 10 km.

Just for completeness: [5] has "By rough estimation 100 yojanas can be equivalent to 90 to 130 kms"--Jochen 20:49, 31 Mar 2005 (UTC)

Finally, [6] says that the speed Sayana was talking about was the speed of the sun, not light.

Combining the possibilities, we get for the speed of the sun:

79, 82, 88, 91, 92, 96, 118, 132, 138, 198, 206 oder 297 millions of m/s.

If we assume a longer or shorter mile, the candidates multiply further.

This is a typically pseudoscientific pick-and-choose trick. If you are flexible enough you can find lots of such "coincidences". "Speed of sun? surely he means speed of light. Hmm... let's take this yojana and this nimesha, and we are spot on!"

So Sayana was probably guessing, and he was wrong. Is anybody against deleting the paragraph now? --Hob Gadling 08:25, Mar 17, 2005 (UTC)

Thanks for the research. Perhaps we should mention that Indian scholars wrote about the "speed of the Sun", but the size of the units is uncertain: for certain sizes of the units, the "speed of the Sun" is uncannily close to the modern definition of the speed of light, but other choices for the units produce a "speed of the Sun" which is different from the speed of light by a factor of up to 3 (4?). -- ALoan (Talk) 11:55, 17 Mar 2005 (UTC)

Hob Gadling's research seems convincing to me, including the conclusion "Sayana was probably guessing, and he was wrong". And 'guesses' about the speed of the sun seem irrelevant for the history of 'measurements' of the speed of light. Therefore I will remove the paragraphs in question. --Jochen 01:10, 19 Mar 2005 (UTC)

According to the standard measurement of yojana and nimesa used by Sayana in his books it is spot on. Even if we ignore that, it is still a valid estimate of the speed (which may have been close by a factor of upto 4). We don't know enough of the history of how he came up with the figure - for all we know it could've been a measurement that was undocumented or lost? I may agree that it may not belong in the 'History of Measurements' section but I find "Sayana was guessing and he was wrong" extremely arrogant. Does a figure like 2202 seem like a 'guess'? and even if he was close by a factor of 4, is it still a guess? In a period where Europe was grasping at theories like - light is due to the presence of something, light has infinite/possibly finite speed - is Sayana not even worth mentioning? (Indian astronomy was too advanced to believe Sayana estimated the sun to travel that fast.)

You are misquoting Hob Gadling. --Jochen 19:49, 21 Mar 2005 (UTC)

Indeed. You forget that Sayana was talking about the speed of the sun, not light. Also, what reason do you have for calling the values that lead to the right figure "standard"? On top of that, measurements don't fall from the sky. The first real measurements (Roemer etc.) were widely off, but we know that the method was sound. From Sayana, we don't have any data. No method, no error bars, only a single number given in dubious units and applied to the wrong object.

Also, "Indian astronomy was too advanced to believe Sayana estimated the sun to travel that fast" is not an argument. Our astronomy is even more advanced, but if you ask an arbitrary non-astronomer, even a very wise one, for the speed of the sun (relative to what, by the way? the next fixed star? the core of our galaxy?), you will earn blank stares or guesses. And: Was Indian language so un-advanced that there was no word for "light"?

"Does a figure like 2202 seem like a 'guess'?" Then call it a figure derived from numerological fantasies. People are not automatically right about everything just because they are long dead or from far away, or remembered as wise. Newton, Kepler, and Copernicus wrote a lot of gobbledigook, and there is no reason to assume that Sayana was any better.

Regarding the context of the Sayana quote: There are some people who desperately search the old Indian writings for quotes that allow the interpretation that the writers knew a lot of things Western culture found out only recently. This has yielded lots of fruit. With the type of flexibility I outlined above, this can be done for any other culture, and the same was done for other cultures. According to pseudoscientists, the Parthians had batteries, the Egyptians had lightbulbs (a gift from the aliens), the Greeks had computers, the Africans had everything the Greeks later stole from them, and the Atlanteans invented by Plato had everything else. I guess one of the reasons is that some non-European people want smart ancestors to prove that they are not worse than European people. But they are already not worse. Europe becoming dominant was a coincidence, as Jared Diamond wrote in Guns, Germs, and Steel. Europe was lucky. --Hob Gadling 10:56, Mar 22, 2005 (UTC)

Europe was not just lucky. It also colonized a large part of the world for the last few centuries. In the context of India, the 'declared' intent of it's British colonial rulers was to engineer/perpetrate an 'awe' for European superiority, in order to keep the masses subjugated (not to mention the post-colonial British believed in their superiority). A few narrow-minded missionaries (no offense intended to the religion) also had a vested interest in the same. In addition to impoverishing a wealthy nation, they appropriated and destroyed traditional education systems (which was flourishing by British accounts http://www.swaraj.org/shikshantar/_disc5/00000049.htm) and replaced it with a glorified European version. In other words, they were practising Eurocentrism and had the means to enforce it.

'some non-European people want smart ancestors to prove that they are not worse than European people'

I consider that a biased and simplistic assumption on the intentions of people trying to search for the sciences within their cultures. What non-European cultures are probably seeking is acknowledgment that there are other cultures that have made significant contributions to the sciences. Some Indians atleast are trying to reclaim the science of their ancestors. For an example, see the patent controversy section in Neem.

Some accepted facts:

Around 100 BC (not sure about the 'accepted' dates), India's Taxila University was noted for Science, Medicine (Indian medicine included surgery) and the Arts.

Although the modern study of philosophy is dominated by the Greek Philosophy, Indian philosophy was known to encompass most of that thought and more.

The decimal system, algebra as it's used today originated in India.

- Didn't want to trumpet Indian acheivements but I think I see a condescension in "But they are already not worse."

I performed a random survey on guesses (diff age groups, all college graduates) for the speed of light. I found the following 'guesses' - 15,000 miles/sec, 2800 miles/sec (10 million miles/hour), 36 miles/sec (193,000 ft/sec) and ~2miles/sec (10 times the speed of sound - BTW this was Galileo's guess too). I would encourage you to try it too.

Can you also point me to other 'guesses' from the pre-Roemer days? Gobbledigook is fine too, since what I am attempting to put in this page is alleged to be the 'same'. In the absence of any data, Indians did not make the assumption that he measured it, but you don't hesitate to make the assumption of a numerological fantasy.

Sayana's number has also been validated (units and all) by Subhash Kak, Professor in the Dept of Electrical and Computer Engg in Louisiana State University, if that will lend it any credibility. I believe he mentions that Sayana used the same conversion for yojana & nimesa consistently ('standard') in his book.

And finally, if you read the Wikipedia criticisms on the book you mentioned Guns, Germs and Steel, you will note the charges of racism and also

"It makes little attempt to explain relatively recent geographic transitions in technology, power and wealth; in particular the rise of Europe and the decline of south-west Asia since about 1500 AD." - Pranathi

As an Indian who still lives in India, I will like to add a few words. I believe this discussion has been tainted by the eternal disease of debates, extremism in order to counter-attack. It is not true that aAncient Indians measured things, a guess is far more logical. However, to do this properly you have to understand how most Ancients(of any culture) found their wisdom. When you see a right fact, you tend to assume it was arrived at by following a scientific 'cause-effect' chain. Most of the times this is not true. It is usually a process of trial and error. Ancients did not find the disease-curing capablities of herbs by braking them into their chemical compositin and then studying the effects these chemicals had on cells. They just kept trying herbs at random till they found one that worked. However, dismissing them doesn't do either. I should say that a factor of even 5 in accuracy is remarkable. It has also been mentioned that he talked about the speed of sun. How do we know this? If anyone can give me the original Sanskrit from Rigbed(the first recorded Indian liturature, BTW) then I could check to see what the translation is and if there are any disputes among scholars about the meaning of words. -Molu

Molu, Sayana wrote -

Tathā ca smaryathe yojanānām sahasre dve dve sate dve ca yojane ekena nimishārdhena kramamāna

in his commentary on the 4th verse of the hymn 1.50 of Rgveda on the sun. The verse in the Rgveda does not mention any numbers.

With worst case scenario of #s presented by Hob Gadling, the figure is off by a factor of 3.75. I made it 4 to fend off further arguments and now you made it 5. Let's go back to the 3 :)..

Also, Sayana was from the 14th century - not exactly ancient. If 5th century Aryabhata could give periods of planets wrt the sun, an accurate ratio of earth's rotation to lunar obits etc, is it not possible that him & his successors had gone beyond trial and error? - Pranathi 25th Mar

Two points, Pranathi i)It's just a tiny bit more difficult to measure the speed of light than it is to measure the velocities of massive stellar bodies. Just look at the time europeans found stellar phenomenons and the time when they measured the speed of light with any accuracy. However, it may be possible that Sayana carried out some approximate measurement(such as the gunshot-between-observers-on-hills experiment). ii)Sayana was from the 14th century? Then how possibly did his verse get on the Rigved? Things said by a man as late as the 14th century has no buisness in the Rigvedas. If your time estimation is right, there is the possiblity of Sayanas verses being introduced to the Rigvedas much later by unscruplous priests. Very disturbing! I'll try to resolve whether the verse definitely refers to the sun and to nothing else. 59.93.160.46 17:28, 27 Mar 2005 (UTC)

I am not trying to prove that he measured it, rather I am refusing to make assumptions. I am arguing (to So Sayana was probably guessing, and he was wrong) that a guess not "more" probable than a measurement - not knowing where he got the data from & going by the accuracy of the #. Secondly, Sayana's verse is a commentary on a verse in the Rigveda. It's not a verse in the Rgveda - the original verse has no relation to this topic except that it is in praise of the sun. --Pranathi 21:19, 27 Mar 2005 (UTC)

Oh! I wasn't paying attention, I didn't notice you said commentary. Anyway, without hard facts this discussion is leading nowhere. For all we know, little green aliens from Alpha Centaurio may have come and told the Speed Of Light to Syan!\n59.93.161.76 11:59, 28 Mar 2005 (UTC)

Hello all, sometimes the internet is quite useful: the rig veda is for example at [7], Sayana's commentary is at [8], so we can actually check what this is all about. Should we? Well, let's see. Verse 1.50 is at [9]. It states (relevant verses selected)

4 Swift and all beautiful art thou, O Surya, maker of the light, Illuming all the radiant realm.

5 Thou goest to the hosts of Gods, thou comest hither to mankind, Hither all light to be beheld.

6 With that same eye of thine wherewith thou lookest brilliant Varuna, Upon the busy race of men,

7 Traversing sky and wide mid-air, thou metest with thy beams our days, Sun, seeing all things that have birth.

and so on. This is clearly about the sun ("maker of the light", "thy beams"). An alternative translation together with Sayana's commentary is at [10]:

1.050.04 You, Su_rya, outstrip all in speed; you are visible to all; you are the source of light; you shine throughout the entire firmament. [Smr.ti states that the sun moves 2,202 yojanas in a half a winkle of the eye; jyotis.kr.t = giving light to all things, even to the moon and the planets, by night; for, they are of a watery substance from which the rays of the sun are reflected (like a mirror in a door-way reflecting sun's rays, to light up a chamber); metaphysical explanation: sun is the supreme spirit, who enables all beings to pass over the ocean of existence, who is beheld by all desiring final emancipation, who authors true light, and who illuminates everything through the light of the mind].

Again, this is clearly about the sun ("sun's rays", "authors true light"). Why do these "some" think that this would be about the speed of light? The source really only states that the sun moves 2,202 yojanas in half a winkle of the eye. After this, does anybody still think that the paragraph in question really improves an article about the speed of light and should stay in? --Jochen 21:26, 31 Mar 2005 (UTC)

Yes, I still think it should be mentioned. I think that everybody agrees that verse 1.50.4 of the Rig Veda is about the Sun; the question is what Sayana's comment is about. The translator of the quote that Jochen gave, thinks it is about the speed of the Sun, Kak thinks it is about the speed of light because it would otherwise be inconsistent with what the Indians thought that the distance Earth-Sun is. I do not know the philosophy and physics of that time to gauge the strength of Kak's argument, but he seems a serious scholar and I think his interpretation should be mentioned. -- Jitse Niesen 13:57, 1 Apr 2005 (UTC)

Ok, this is the first time I am editing on Wikipedia so sorry if I do something wrong, cause I don't realy know how these discussions work.. but here goes..

This section is about the measurment of the speed of light through Hiundu scriptures, so I don't know should I make a new section or write it here... Thaaat, another ancient measurment (not so ancient as hindu) of the speed of light is from the muslim religion in the book of Quran, at least I think it is, I've never read it, that is why I would like to verify this. I don't know am I allowed to put links here but, here is a link: http://www.speed-light.info/angels_speed_of_light.htm maybe some of you have stumbled upon it. I am not good at physics and math, so could someone verify are these calculations correct. I hope no one will think that I am changing this discussion into a religious one. Thank you. (unsigned comment by User:Mirton)

I read that page before and I found the science unconvincing. Some details are in the section Relativity in Quran below. -- Jitse Niesen (talk) 15:07, 3 August 2005 (UTC)

I think the facts that a) the correlation between c and the writings of ancient Hindu philosophers is highly specious, at best, and b) such correlation, even if valid, has little or no documented influence on modern understanding of the speed of light, weigh heavily toward a decision to remove such mentions from a scientific article. You might as well throw in "The earth sits on the back of turtles for all values of turtle that equal nothing." Molding religious texts to fit modern science is not our job. --Albatross83 03:56, 13 February 2006 (UTC)

The reason it is ascribed to speed of light and NOT sun, is because it is described at other places that sun is described as stationary in many a places in vedas. I remember reading many mentions of yajur veda as sticking to a heliocentric system. At the same time sun is described as one who rides across space with seven horses... However the preferred description was that sun is static - MUCH before the introduction of the west, and there were many heated debates over it (theory of gravitation was pretty embryonic back then, and wasn't known very well) between indian scholars. Secondly, when the estimates are approximating to speed of light, what else would it be suggesting other than the speed of light, especially when sun is said to be stationary...

As for the "earth sits on back of turtles" crap - that is more in favour of puraanas, whose mythology cannot be trusted by any means, which contradict each other. It is accepted by many hindus that they were written for understanding of a layman.

On the other hand, "molding religious texts to fit modern science" -- this is not over-interpretation, thus it counts. As for the variation of values - just put it in the article, the estimates varied by this range..

Oh yea, and there is indeed no word for 'light'. As said, there are absolutely no words for light in indian languages. There are words for brightness, and for rays, but no word 'light'. We do however say 'sun rays' to mean light at a max, thus most of the skepticism here is flawed. 'surya kirana' means 'sun rays'.

Last but not least, saying "but that is incorrect" would be wrong to adhere to, because the wiki kept the views of an astrologer who measured incorrectly himself. —Preceding unsigned comment added by Leaflord (talk • contribs) 23:56, 24 June 2006

There is a broad consensus here that it is not important enough to warrant more than a sentence, if indeed it should be mentioned at all. See for instance the statements above, and #Pre-modern speed of light nonsense. Therefore, I removed your edits. -- Jitse Niesen (talk) 04:52, 25 June 2006 (UTC)

Faster than c : Improper handling

This question has been raised before, but I still feel that it must be raised again. The writer states with assurance that recent experiments have shown group velocities to exceed c. I beg to differ. These experiments are some of the most debated ever in the history of science and many would go as far to question the moral velues and/or sanity of those advocating speeds greater than c. For example, recently the book "Faster Than The Speed Of Light" brought to light the ridicule faced by the author for propounding his theory. Nature even refused to publish his article for proposing something that challenges the foundations of one of the centurys two greatest theories advanced by a man who is often considered the impersonification of mordern physics, not to mention a Nobel laureate. Whether chnges do occur faster than the speed of light in vaccuam is a highly controversial issue and the author has no right to make the unaware reader believe that his next door neighbour regularly exceeds c. I request permission to make some minor changes to the section so the dispute among the scientific community is more clearly reflected.

Recently, some very strong evidence have emerged supporting the almost supernatural byproduct of Quantum Mechanics known as the Doppleganger Effect, the insteanteneous reflection of changes in one particle in another which implies transfer of information(since the state of a particle is definitely information). A compromise used by the Relativists to counter this is that c can not be exceeded in the 4 ordinary dimensions. However information may travel at c through extra dimensions curled up too small to be noticed(10/11 dimensional supergravity) so that it appears to exceed c.

I am not aware of much controversy surrounding the experiments showing that the group velocity may exceed c. The group velocity is not the velocity of a physical object, hence such experiments do not contradict the relativity theory. The theory mentioned in "Faster than the speed of light" is very different: it does contradict relativity theory and it is not accepted by most scientists. I do not understand which experiments you are referring to in your second paragraph, but I guess it has something to do with quantum entanglement.

You do not need any permission to change the article, you can just go ahead and change it. However, be prepared that your changes may be undone; in fact, this is quite likely if you don't support it by references to the literature. -- Jitse Niesen 20:07, 25 Mar 2005 (UTC)

Sorry, posted in the wrong comment thread

Ancient theory: light is emitted from the eye

From the article: "One of the ancient theories of vision is that light is emitted from the eye, instead of being reflected into the eye from another source.*

How did this theory explain darkness? --Lakefall 18:24, 10 Apr 2005 (UTC)

Dunno, but maybe that's why it's not so popular anymore. Wile E. Heresiarch 00:08, 11 Apr 2005 (UTC)

Obviously it isn't popular, but I just wonder how anyone, no matter how ancient, could have come up with a theory about light, which doesn't explain darkness. So I suppose either it did try to explain darkness somehow or their concept of "light" didn't have anything to do with luminance. In the latter case they weren't even talking about the same light we are, but rather about the speed of vision or something like that. --Lakefall 18:24, 12 Apr 2005 (UTC)

Sounds like something Aristotle would come up with. Now there's a guy who couldn't bring himself to answer "I don't know"!

Actually, the Greek idea was pretty insightful as a theory of visual perception, since it addressed the question of how something can be overlooked for a while but be seen when looked for. An awful lot of seeing happens in the brain -- an active process, not a passive one. Hunter 23:05, 17 Apr 2005 (UTC)

If light came from the eye, then looking into a mirror would make you blind. Ancient "theories" are generally bullshit. --Kvuo 00:44, 4 Jun 2005 (UTC)

Hey, it seemed obvious with the available evidence they had at the time... I suspect they treated colours, brightness and so on as intrinsic properties of items, and considered light as something that went from your eye, sniffed out the state of the colour variable, and then reported back appropiately.--Fangz 15:40, 4 Jun 2005 (UTC)

Somone put on BJAODN that light bulbs do not emit light,they suck dark. Dudtz 8/3/06 8:40 PM EST

"Einstein's constant"

Hello everyone. The intro says that c is called "Einstein's constant". I don't remember ever seeing that usage in any physics book or article. The term seems to be associated with a certain Kenneth Brecher; see for example [11]. Maybe we can move "Einstein's constant" out of the intro and into some section farther down; the term doesn't get enough traction, sfaict, to warrant mention right at the top. Anyone want to weight in here? Regards & happy editing, Wile E. Heresiarch 00:08, 11 Apr 2005 (UTC)

I've also never heard of it. --MarSch 15:25, 6 November 2005 (UTC)

I've heard of "Einstein's Constant" a few times, but only ever as an alternative to cosmological constant - and even then only as "historical" usage. Certainly never for c.

I've never heard of it either. I recommend it be removed. --vex 17:08, 3 February 2006 (UTC)

I've... heard of it never, also. The phrase does very poorly on Google, where it often means "cosmological constant" instead, and Google books is even worse. I'll remove it from the article; for the record, it said:

• The speed of light in a vacuum is exactly 299,792,458 metres per second (or 1,079,252,848.8  km·h^-1, which is approximately 186,282.397 miles per second, or 670,616,629.4 miles per hour). This value is denoted by the letter c, reputedly from the Latin celeritas, "speed", and also known as Einstein's constant.

I'll also make Einstein's constant into an appropriate dab page. Melchoir 03:47, 15 April 2006 (UTC)

Relativity in Quran

I deleted the link Relativity in Quran (Islam) because the explanation on that page is not sound. Specifically (this probably does not make sense unless you read the web page), even if the interpretation of Quran 32.5 that light travels the same distance in one day as the moon in 12000 orbits is correct, the distance travelled by the moon in one period is not computed correctly. The page says that the moon travels 3682.092 km/hr relative to the earth, and finds via an unintelligible reasoning that you have to multiply this number with 0.8915645 to get the speed outside gravitational fields, "relative to the stars". I don't know what exactly they mean with this phrase, but if it's relative to the centre of the Milky Way, then this is clearly incorrect since the sun travels with the much higher speed of 217 km/s (see Sun). -- Jitse Niesen 14:55, 26 Apr 2005 (UTC)

--

Look at the diagram to see why they multiplied by 0.8915645, which is supposed to be cosø and is calculated at footnote 4.

I did read the page. It is incomprehensible, and the diagram does not explain anything. -- Jitse Niesen (talk) 21:33, 8 January 2006 (UTC)

The moon's velocity was broken down into two components: One that's the pull of the sun, and the other is the moon's velocity WITHOUT the sun. I don't see why you say it's incomprehensible. A basic knowledge of vectors would help.

I do have a knowledge of vectors, so that is not the problem. The issue is the physics: why do you want to subtract this component? I maintain that the moon has the same speed, whether there is a sun or not. If the sun would suddenly disappear (which is what the author of the page seems to mean with "isolated system") then the earth and the moon would continue with the same speed, the moon still circling around the earth, but the earth-moon system would shoot of in one direction, instead of circling around the sun. -- Jitse Niesen (talk) 21:16, 12 January 2006 (UTC)

You're saying the sun does not affect the moon? You just said it yourself: The system would go in another direction, in other words the moon would go in another direction. That component is equal to the sun's pull.

I'm saying that the speed wouldn't change. In physics, speed means the magnitude of the velocity, ignoring its direction, see Wikipedia's article on speed. -- Jitse Niesen (talk) 15:26, 13 January 2006 (UTC)

Communications "Beam of Light" Image

A few days ago I removed the animated image in the "Communication" section (See here). It's being animated adds very little information, in fact seems to add nothing, and is also animated on a fast loop, making it very distracting whilst attempting to read text around it. I removed it stating the reason, and suggested that it be replaced by a static version if it must be in there. Perhaps a static step-by-step image (i.e. in the style of a) signal sent out b)signal reflected etc ). Either way, It really doesn't need to be animated.

It was reverted the next day by the images author, for the reason "revert removal" - which suggests to me was purely for vanity reasons. What do people think? Does it add anything to the article being animated and does anyone else think it is a distraction? If there are no objections I'll remove it again in a couple of days with a note to the effect. - Xgkkp 18:58, 3 Jun 2005 (UTC)

The dimensions of the Earth, the Moon and the distance between them are proportional to the real world but on a smaller scale. And the beam of light is animating at approximately the same speed a beam of light travels the Earth to the Moon in the real world. That's why it is crucial this image is animated. That's the whole point. Perhaps this could be made clearer in the image's caption. This is not a vanity issue. —Cantus…☎ 19:37, Jun 3, 2005 (UTC)

On such a tiny scale, I really doubt that relative sizes matters. Also I'd be hesitant to suggest that people can't imagine/understand what the phrase "Houston had to wait nearly 3 seconds" means in the preceeding paragraph. Why couldn't it be a static image with the relative sizes? Xgkkp 19:53, 3 Jun 2005 (UTC)

Yeah, that was pretty obvious I thought. I think it should have a static version in the article that links to the animated version, like a thumbnail links to a large version. - Omegatron 19:53, Jun 3, 2005 (UTC)

I don't have a problem with this. —Cantus…☎ 21:40, Jun 3, 2005 (UTC)

This does seem like a good solution Xgkkp 17:49, 4 Jun 2005 (UTC)

For what it's worth, I find the animated image useless and distracting, and I can't see the point of linking a noninformative static image to a noninformative animation. Why can't we just erase the image in question altogether? Wile E. Heresiarch 02:05, 5 Jun 2005 (UTC)

It may be distracting, but useless is not. It is one of the few ways I can think of you can visualize how fast light travels using real world examples while keeping all proportions true and visible for a computer screen. —Cantus…☎ 23:17, Jun 5, 2005 (UTC)

I turned the "distracting" animation into a link. This should hopefully solve the issue for everybody. —Cantus…☎

3e8 m/s

I understand that the meter is now defined in terms of this constant with regard to the other definition of the second, but am I the only one who usually thinks of this constant in inexact terms as 3 x 10⁸ m/s? --arkuat (talk) 08:07, 13 August 2005 (UTC)

So?... - Centrx 16:08, 26 August 2005 (UTC)

I usually think of it like that as well. That approximation used to be mentioned in the lead section, but one user didn't want it there and it was removed (see the archive for discussion). I would support adding it back. Fredrik | talk 11:28, 25 September 2005 (UTC)

Is it really necessary to explicitly point out that 299,792,458 is close to three hundred million?

Anyone got a calculator ?

The opening couple of lines of this page is definitely in need of checking: "The speed of light in a vacuum is defined to be 4 metres per second (1,079,252,848.8 km/h, which is approximately 3.1 miles per second, or 9miles per hour)."

What?? I can run faster than 9 miles per hour!! If I were to get in my car, I could exceed the stated "speed of light" without breaking any traffic laws!!

Assuming the 1,079,252,848.8 Km/h is the figure to be trusted, light speed is therefore 299792.458 m/sec. I'll leave it to the physicists to do the metric to old conversion. MSkerrett 14:41, 14 October 2005 (UTC)

That was just some vandalism by user:166.109.0.79 It has been fixed now. The figures as they stand now are 299,792,458 metres per second (1,079,252,848.8 km/h, which is approximately 186,282.4 miles per second, or 670,616,629.38 miles per hour) , certainly enough to break traffic laws. Majts 19:45, 14 October 2005 (UTC)

Note:1 minute is 60 seconds, 1 hour is 60 minutes and that 1 mile (statue mile) is 1.609344km.

breaking the speed of light

Let's say you had an extremely long tube in which there is a vaccuum. On one end was an incredibly massive object and on the other an object(let's say a steel ball) was dropped. Let's say that the tube connected right to the pole so that there was no cork-screwing. Now the massive objects mass is about 10 times that its gravity accelerates all objects toward it at the rate of 100m/s/s. At the end of one second it would be going 100 m/s, two it would be 200m/s. At one point wouldn't the steel ball be exceeding the speed of light? K-unit 17:35, 16 October 2005 (UTC)

It is first worth pointing out that Newton's laws of gravitation can't be used to calculate accelerations in this case -- Newtons laws are only an approximation which works at the low-speed and low-mass limit. Secondly, it is true that an object can be massive enough to accelerate spacetime (and its contents) to the speed of light -- it is called a black hole. The mass has to be compressed into a tiny volume in order for sufficient acceleration to be achieved. Hope that's helpful. Rnt20 18:09, 16 October 2005 (UTC)

thanks

Accelerate spacetime? That's weird. --MarSch 15:19, 6 November 2005 (UTC)

speed of light in common materials

The article Speed of light doesn't have the speed of light in common materials!!! as far as i know the speed of light in water is 2.5 x 10⁸ m/s and in glass is 2 x 10⁸ m/s. 59.93.129.144 14:45, 5 November 2005 (UTC)

See the section "Interaction with transparent materials" where those values are given as fractions of the speed in vacuum. Tweak it if you like, realizing that someone might just revert it or change your additions, too (more likely to happen if you edit anonymously; it's quite easy to get a user name and long on first). Be bold! Gene Nygaard 15:05, 5 November 2005 (UTC)

Speed of light in a vacuum

Question moved to reference desk Zhatt 22:40, 20 December 2005 (UTC)

Circular Definition of Speed of Light

It is in my view incorrect to say that the speed of light is *defined* as 299,792,458 m/s when on the other hand the meter is defined as the length of the path travelled by light during a time interval of 1/299,792,458 of a second. The former follows algebraically from the latter and is thus not a definition in its own right.

There is also a discrepancy due to this circumstance regarding the accuracy with which the speed of light is known: the m/s value suggests that this a factor 1/299,792,458 =3.3*10^-9, whereas the km/h value suggests an accuracy 0.1/1,079,252,848.8 =9.3*10^-11. Clearly the speed of light has not been plucked out of thin air but has been measured experimentally, so it would be good to know to which accuracy the speed of light (i.e. the time it takes for a light signal to get from one place to another) is actually known (especially with regard to possible variations depending on wavelength and/or intensity).

Thomas

It is exactly 299792458 m/s - with no experimental inaccuracy (e.g., [NIST]), precisely because the metre is defined in terms of the speed of light. There is no circularity involved because the definition of the second does not depend on the speed of light. That is defined in terms of a certain number of energy-level transitions in a caesium-133 atom; there may be inaccuracies in whatever apparatus and methodology you use to determine the duration of a second. Then the metre is defined as how far light travels in the 1/299792458 part of that duration; and there may be inaccuracies in whatever apparatus and methodology you use to determine that. Either way, the inaccuracy is deemed to lie not in the figure given for the speed of light, but in your calibration of those units. And those, naturally, vary depending on how you do the calibration.

No, the number was not picked out of the air (I can cite a fairly informative Wikipedia article that discusses this), but the redefinition of the metre came later (for the history of which see the cited article). And that, incidentally is why the speed of light is 299,792,458m/s and not the eminently more sensible 300,000,000m/s.

And do you know of "any possible variations depending on wavelength and/or intensity"? None have ever been observed, and there is no theoretical justification I can think of for it.

So what is then the accuracy in determining how far light travels in 1/299792458 of a second? This question should be much more important here than clock stability as the interaction of light with matter always involves a certain statistical uncertainty which typically is of the order of nanoseconds, and this uncertainty should then also hold for the accuracy with which the length of a meter is determined.

And yes, I can think of a possible evidence for a variable speed of light: this is the change in the decay time of supernova lightcurves which is observed to be proportional to the distance. The point is that this change corresponds only to about a fraction of 10^-11 of the total travel time of the light signal, so this effect could well be hidden in other experiments due to the experimental uncertainties addressed above.

Thomas

"So what is then the accuracy in determining how far light travels in 1/299792458 of a second?" It depends entirely on what you're using to measure how long a second is: the definitions of neither the second nor the metre make any mention of what apparatus you're using or what "interactions of light with other matter" may be involved in its use (indeed, the definition of speed of light cites that it is the speed in the absence of any other matter). Since such "statistical uncertainties" would vary from experimental setup to experimental setup, there would be no point in trying to load them into a definition which is supposed to be independent of how what you're using to calibrate your measures.

"And yes, I can think of a possible evidence for a variable speed of light": then that is another matter which, if confirmed, would require changing the definition of the metre to be something other than the distance travelled by light in 1/299792458 of a second; and perhaps a note (with citations) in the article on Speed of Light to the effect that it may not be as constant as the rest of the article claims; and of course a complete rewrite of Special Relativity. Until then....

"Frame of reference" vs "inertial frame of reference"?

"The above equation was derived by Albert Einstein from his theory of special relativity, which takes the principle of relativity as a main premise. This principle (originally proposed by Galileo Galilei) requires physical laws to act in the same way in all reference frames."

In the quoted text above, shouldn't "all reference frames" be "all inertial frames of references"?

yes, special relativity deals with inertial reference frames, but general relativity extends this to all reference frames. -MarSch 11:59, 16 March 2006 (UTC)

Communications section is still bad

The article sez:

For example, given that the equatorial circumference of the Earth is 40,075 km and c, the theoretical shortest amount of time for a piece of information to travel half the globe is 0.067 second.

True? Let's do the math!

Assuming this article is correct about the equatorial circumference of the Earth (it's close enough), 12,756 km would be the average equatorial diameter of Earth. For information to travel "half the globe" it need only pass through the center of the Earth, not travel along an arc on the surface of the planet.

Of course photons don't propagate through opaque solids (phonons do, at the speed of sound within that solid), and sound is slower than light, but there's no physical reason the information couldn't be transmitted with very very low-mass leptons, like neutrinos, which would pass right through the planet (and often do!) at very near the speed of light. So near the speed of light that, given the significant digits we have to work with here (five), the solution is the same. That's good, as it greatly simplifies the rest of the math, and I can just pretend that a signal the speed of light is passing through the Earth!

So, the speed of light is exactly 299,792.458 km/s, and it needs to go 12,756 km. Look mom, I can do basic division, and the solution is 0.04255 seconds! 63.5% the time the article claims! This would also be the solution if you ran a near-zero-latency cable straight down through the Earth to the other side (also not impossible, in principle).

The above solution defines the longest time (using average planetary dimensions) physically necessary to wait anywhere on (or in) Earth. So if by "half the globe" it means a hemisphere, and not the exact opposite side (which would actually end up being "all the globe"), the time is even shorter (70.717%. not 50%, as you F students might now be thinking).

Any signal that has a destination within the same hemisphere need not take as long as above, from the standpoint of physical possibility. For one entire hemisphere of the Earth to receive the same piece of information (transmitted omnidirectionally), the theoretical shortest amount of time would be 0.03009 seconds (this required using the Pythagorean theorem! oh no's!). This is 44.9% Wikipedia's claim (less than half! zomgwtfbbq!). And only the receivers on the very boundary of the hemisphere defined with the transmitter at its center would have to wait that long.

No matter how you interpret it, Wikipedia is wrong. Again. The statement in the article can only be valid if an artificial geometrical constraint is introduced, perhaps "the information must remain parallel to the surface of the Earth". It does not describe any theoretical physical limitation, as it claims to. Introducing an artificial constraint makes it meaningless within the context in which it occurs. 0.067 seconds is not a magic number on Earth. It only arises if you include unwarranted (not justifiable purely through physics) assumptions.

-08:12, 3 February 2006 (UTC)

Be bold and {{sofixit}}. -- 20:04, 4 March 2006 (UTC)

Information generally travels through optical fibers along the floor of the ocean, it doesn't simply pass through earth. Though this may be possible in theory, it's better to use a practical example, so a circumferential measurement is most appropriate in this case. If the information is still wrong taking this into account it should of course be changed. Richard001 06:31, 2 August 2006 (UTC)

FTL

Light travel's at c (299,792,458 m/s) in a vaccuum, but considerably slower in other mediums (i.e. water, air, etc).

Now consider this, if you fire a gun from a vehicle traveling at 100 m/s, wouldn't the bullet travel at the speed it was fired PLUS the 100 m/s?

So why not fire a light from a satellite in deep space (which has never been done, mind you), which travels at hundreds of kilometers per second?

And since the temperature of space is not 0 degrees Kelvin, does that mean that it is not a true vaccuum?

Thanks. --vex 17:04, 3 February 2006 (UTC)

---

The usual Galilean laws for the addition of velocities are a good approximation at the sorts of speeds you and I are used to every day, but just don't apply at relativistic velocities (i.e. anywhere near a sizeable fraction of c). Einstein's equations produce the observed time dilation effects on, for example, atomic clocks orbiting the earth versus those on the surface of the earth, or messages received from the rapidly-receding Voyager probes.

We have sent satellites away at kilometres per second, and they don't emit radio waves that travel faster (or slower) than c.

And no vacuum is perfect :) -- ALoan (Talk) 18:08, 3 February 2006 (UTC)

Quite so, but you haven't answered my question, which was will light travel faster when shot from an accelerated mass in true vaccuum? --vex 18:11, 3 February 2006 (UTC)

No. Light always travels at the same speed. If a gun fires a bullet at 1000 km/h and the gun is fired forward from a car driving 100 km/h, then the bullet goes at 1100 km/h; this is what ALoan refered to as the usual Galilean laws. But they are not valid for light or anything that travels with similar speed. -- Jitse Niesen (talk) 18:33, 3 February 2006 (UTC)

Is this because light has no mass, or for another reason? Also, how much has light been slowed down? .75c? Thanks. --vex 18:36, 3 February 2006 (UTC)

The statement that satellites traveling away at many km/sec don't emit radio waves that travel faster or slower than c is making the assumption that permittivity is the same in outer space as it is in a vacuum on the earths surface. The definition for permittivity gives its value in 'space', but it has never been measured there. That is sloppy science. Currere 21:50, 21 February 2006 (UTC)

No, it's not. We know how far away the satellites are because we know very precisely their trajectories and velocities. We also know very precisely when the radio waves they emit are being sent, because we programmed the satellites. All of this agrees very accurately with relativity and the idea that the permitivity of the vacuum is the same everywhere. Considering all this, there is no reason to think that the vacuum of space is any different than a vacuum on Earth. Indeed, them being the same is a very simple theory and fits extremely well with all measured data, so it would be foolish not to think this. Furthermore, the permitivity of the vacuum in space has been measured, since if it were different in space, that would lead to a difference in the strength of the Coulomb force (among many other things) in space, which would of course cause the space shuttle to either implode or fly apart upon entering space. Grokmoo 22:34, 3 March 2006 (UTC)

Anyone else think it's rather funny that, in the year 2006, we are arguing over the very basic tenets of Einsteinian relativity? This concept is literally decades old and is accepted by every physicist. I can assure the OP that physicists long ago thought about, and resolved, the apparent "paradox" of emitting light from a fastly-moving object. -- 20:03, 4 March 2006 (UTC)

phase/group velocity

I changed references to group velocity exceeding c to phase velocity doing so. This is the standard convention. one may read the phase velocity article or see a proof here [12].

There are certainly experiments in which group velocity exceeds c. Are you absolutely certain that the experiments you changed are phase and not group velocity? Please provide citation. -- Xerxes 13:34, 2 March 2006 (UTC)

Here are links to the experiments the article was discussing [13] [14] [15] which are about the group velocity exceeding c. Howewer, this link (with paper references) suggests that the phase velocity or the group velocity can exceed c [16]. -- ALoan (Talk) 15:19, 2 March 2006 (UTC)

• I've reverted the changes. The phase velocity of light can routinely exceed c. The group velocity can also exceed c in some circumstances (near absorption resonances, for example.) This has been known since the 1910s when Sommerfeld and Brillouin published papers about it. In neither case does this lead to FTL communication. --Bob Mellish 22:39, 4 March 2006 (UTC)

The article as it stands on this issue is a bit confusing. Front-velocity sits awkwardly. Since the group-velocity article states that it is the velocity information travels, then there are inconsistencies across the two articles. It seems this group-velocity>c thing is quite a specialised point. LeBofSportif 01:25, 8 June 2006 (UTC)

Relative to what?

It says in the article that the speed of light is constant, regardless of the velocity of the object it was emitted from. So the light will be travelling at the speed of light from both a staionary car and a moving car. My question is, doest the speed of this light remain constant relative to the car it was emitted from, or relative to an unconnected observer?

Both. The speed of light is constant for all observers. -- Xerxes 18:09, 25 March 2006 (UTC)

Another way to look at it is this: The medium for light (EM waves, in general) is (empty) space itself. So the speed of light relative to space is 3 x 10^8 m/s (1). When we say an object moves at a certain speed, it is with respect to something, and the something is not space, but some object like a building, the earth, solar system, and so on. But all these objects are in space, meaning their relative velocity w.r.t. space itself is zero (2). Combining (1) & (2), it turns out that all objects (including observers) will measure the speed of light to be the same irrespective of their velocities relative to each other. Rohitbd 10:48, 10 April 2006 (UTC)

Speed of light not a constant?

Just came moseying through was wondering if the page should have some kind of reference to http://www.newscientist.com/article.ns?id=dn6092 perhaps this research has been proven wrong since I saw it way back? Its just your page lists light speed as being constant and makes no reference to this paticular line of research. Its all a bit out of my league so Ill leave you all to 'fight' it out to determine if it should be in or not. Hope you find it intresting at any rate. -- Shimirel ^(Talk) 03:27, 27 March 2006 (UTC)

Since the speed of light in sensible units is 1, it is impossible for it to vary over time. What this research actually tries to show is that the electromagnetic coupling constant varies over time. This assertion is not well-received by the physics community, though research into it continues. -- Xerxes 18:18, 27 March 2006 (UTC)

Hi Xerxes thanks for your reply as I said the page was a bit beyond me and I read it a while back and wondered if you editors might find it worth mentioning on the article. If you don't mind, would you please explain what the "electromagnetic coupling" is? Is it electromagnetism inside a photon or something else to which you refer? Confused -- Shimirel ^(Talk) 23:50, 28 March 2006 (UTC)

See fine structure constant - it measures the strength of the interaction between electrons and photons. -- ALoan (Talk) 09:22, 29 March 2006 (UTC)

Thanks ALoan very intresting although if I'm honest pretty deep stuff, going to have to read these articles a few times to see if I can figure it all out. Thanks again. -- Shimirel ^(Talk) 02:18, 1 April 2006 (UTC)

No cloning?

The laws of physics also appear to prevent information from being transferred through more clever ways and this has led to the formulation of rules such as the no-cloning theorem.

Shouldn't that be no-communication theorem? --Tgr 08:09, 20 May 2006 (UTC)

Yes, that should certainly be mentioned. But No cloning theorem also has something to contribute, towards the bottom of the "Consequences" section, so I think both articles should be linked to. Melchoir 08:41, 20 May 2006 (UTC)

You are right, but the phrasing is still inaccurate. I dont think the formulation of no-cloning had anything to do with information transfer - it simply follows from the linearity of QM. --Tgr 16:18, 22 May 2006 (UTC)

Measuerment

Added this to the Fizeau-Foucault apparatus --Arnero 20:56, 25 May 2006 (UTC)

Pre-modern speed of light nonsense

It seems clear from the various edits made to this section of the article that:

1. The quality of these edits are substantially lower than that of the main article.
2. The information contained therein is irrelevant to an understanding of the topic of the article.
3. The information is highly speculative and poorly supported by evidence that anyone before modern times was not just making up random numbers.
4. People are just cheerleading for their favorite cultures.

On these grounds, I suggest that the whole section (up to the point at which experiments were actually tried but failed to determine the speed of light) be deleted. Support? Other opinions? -- Xerxes 19:49, 30 May 2006 (UTC)

I feel uncomfortable removing a section which has good information, but I agree that it attracts edits of a low quality. I'd prefer to keep the section and watch it carefully. I cut out some of the more questionable parts. -- Jitse Niesen (talk) 02:35, 31 May 2006 (UTC)

C for celeritas

Ed cited this article, as saying c does not stand for Latin celeritas. But he may not have read past the first sentence. The article continues, Weber apparently meant c to stand for "constant" in his force law, but there is evidence that physicists such as Lorentz and Einstein were accustomed to a common convention that c could be used as a variable for velocity. This usage can be traced back to the classic Latin texts in which c stood for "celeritas" meaning "speed". The uncommon English word "celerity" is still used when referring to the speed of wave propagation in fluids. Finally, Although the c symbol was adapted from Weber's constant, it was probably thought appropriate for it to represent the velocity of light later on because of this Latin interpretation. So history provides an ambiguous answer to the question "Why is c the symbol for the speed of light?", and it is reasonable to think of c as standing for either "constant" or "celeritas". --Blainster 22:55, 8 June 2006 (UTC)

I read far enough to know that celeritas wasn't the only meaning. I'm glad of a second pair of eyes: constant or celeritas seems about right. Thanks, Blainster. --Uncle Ed 23:46, 8 June 2006 (UTC)