Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Speed of light last value accepted 1983

  1. Sep 26, 2011 #1
    I've been looking into the history of the speed of light values and when the last accepted value was set at. This value we use today was set in 1983. In interest of the Cern experiment and the technology advances today compared to technology that is over 30 years old, Has anyone used todays advanced equipment with lower degrees of error to recalculate/comfirm the speed of light? After all there is a significant difference in the processing power of our electronics etc, seems to me this value may need to be looked at.
    The Cern experiment is 8km/s higher than the accepted speed of light.
    and NO this post is not to be used to discuss the CERN experiment other than the reason for my post so do not consider as such .

    I was surprised at the age of the last update on the value for c, It would place my mind at rest knowing if there have been more recent confirmation tests of that value using equipment with higher precision.
  2. jcsd
  3. Sep 26, 2011 #2


    User Avatar
    Science Advisor
    Gold Member

    The speed of light has a defined value. It is used as a standard instead of a meter stick with scribe marks located in France.
  4. Sep 27, 2011 #3
    In other words, from a metrological point of view, it is no longer possible to make better measurements of the speed of light. Instead, when we attempt to do so, we're actually making more precise measurements of the length of a meter.
  5. Sep 27, 2011 #4
    The Meter

    1983 October 21 – The 17th CGPM defines the metre as equal to the length of the path travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second.[12]
    2002 – The International Committee for Weights and Measures (CIPM) considers the metre to be a unit of proper length and thus recommends this definition be restricted to "lengths ℓ which are sufficiently short for the effects predicted by general relativity to be negligible with respect to the uncertainties of realisation".[13]

    The second

    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.[1]

    During the 1970s it was realized that gravitational time dilation caused the second produced by each atomic clock to differ depending on its altitude. A uniform second was produced by correcting the output of each atomic clock to mean sea level (the rotating geoid), lengthening the second by about 1×10−10. This correction was applied at the beginning of 1977 and formalized in 1980. In relativistic terms, the SI second is defined as the proper time on the rotating geoid.[21]

    The definition of the second was later refined at the 1997 meeting of the BIPM to include the statement

    This definition refers to a caesium atom at rest at a temperature of 0 K.

    . In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1⁄299,792,458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.[4]

    From what I can see the unit the second which is part of the definition of the speed of light has been updated in 1993 however the unit for the value of c has not been recalculated according to the cesium at rest at zero k.

    Also the definition of the meter requires the definition of the light year, but the definition of the speed of light requires the definition of the meter, kind of a chicken before the egg scenario unless I'm missing something here

    Thankfully all 3 were changed in the same year to its final value but no mention is made on why the cesium at rest at zero k changed. Did this affect the value of the second? if so then that also affects the definition of both the speed of light and the definition of the meter.

    My point is that there is a strong need to have a good measure of cesium which is fundamental in our time base to have an accurate measure of the unit metre as well as the speed of light according to these SI definitions.

    Now onto what your reply was they decided to choose the light year as fixed and adjust the definition of the metre instead the following below defines that, however it still does not confirm the validity of the definition of the speed of light

    Because the previous definition was deemed inadequate for the needs of various experiments, the 17th CGPM in 1983 decided to redefine the metre.[132] The new (and current) definition reads: "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second."[79] As a result of this definition, the value of the speed of light in vacuum is exactly 299,792,458 m/s[133][134] and has become a defined constant in the SI system of units.[10] Improved experimental techniques do not affect the value of the speed of light in SI units, but instead result in a more precise realization of the metre

    So I ask you, we have 3 definitions one which depends on cesium the other two depend on each other as well as the cesium, after all if the second is off by a small amount then the other two are effected.

    The definition of the metre requires accuracy in the constant c as well as the the unit second. the value of c is used to define the metre but also requires accuracy of the second.

    You can see the source of my confusion of the twist arounds here lol.
  6. Sep 27, 2011 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Once upon a time, the speed of light was measured. This required the use of a prototype meter, to define distance.

    Having a prototype meter was inconvenient in practice, and when it became technologically possible to create optically pure sources to measure distances using optical interference fringes, the definition of the meter was changed to a certain number of said optical interference fringes, from a defined source.

    THis presumes that the speed of light is constant, but by the time the switchover was made, this fact had been well tested experimentally and was generally accepted.

    http://www.bipm.org/en/scientific/length/former_prototype.html [Broken]

    has some of the history.
    Last edited by a moderator: May 5, 2017
  7. Sep 27, 2011 #6
    good article but again its based on technology measurements in this case krypton in 1983 the measure of the cesium was taken in 1983 and all the subsequent definitions are still based on 1983 measurements. So does that solve my question? would not our current technology have a higher degree of accuracy than those base on 1983?


    according to this the speed of light was set in 1973 and became a fixed convention in 1983 now I understand this is due to its consistencies in measure historically and truthfully I doubt that will change even with our current tech and undertanding, it would still be nice to see if it is still the same using our current accuracies of measures, or at least repeat their methodoligy as more of a just in case scenario. As much as I looked I could find no papers that had more recent confirmations. After all we are requiring in application higher degrees of accuracy in current scientific measurements. Historically from this posted paper the value has changed several times before reaching its final value though those were all techniques and better technology advancements which is also the point of my post .
    Last edited: Sep 27, 2011
  8. Sep 27, 2011 #7
    Curious side note when did they add to the speed of light in a vacuum according to a physics book I have published in 1980 that was not part of its definition. It was simply the speed of light in all circumstances. I also have a 1923 book whose value gives the speed of light at 299,860 km/sec or 186,300 miles/sec
  9. Sep 27, 2011 #8


    User Avatar
    Science Advisor
    Gold Member

    Are you concerned that maybe the speed of light is not a constant value?
  10. Sep 27, 2011 #9
    Mordred. Based upon your reference to a CERN experiment, you question is not really about the absolute speed of light but how it relates to 'c', the proportionality constant between space and time.
  11. Sep 27, 2011 #10


    User Avatar
    Staff Emeritus
    Science Advisor

    They may not have said it but they meant the speed of light "in vacuum". The speed of light can differ sharply passing through various transparent substances and back in 1923 (in fact prior to Newton in the 1600's) it was well known that the speed of light in certain substances was much less than the figure you quote.

    (Well, it may have been the speed of light in air which is close to the speed of light in vacuum.)
  12. Sep 27, 2011 #11
    No I know the speed of light varies in different mediums, side note though there are several not well circulized articles discussing other constants, including light that several scientists have been stating has been changing from past values, one article stated that light is slowing down, another that the fine structure constant is not constant and several that stated that the atomis rate of decay is not constant. Other than the initial hype on those articles I've seen no further news therefore I discount them. Even if one of em was from NASA. Until I hear that the paper has been accepted by the mainstream, I don't trust their validity. Not sure how it would relate to the speed limit or speed of information limit, Those are rather tested in various experiments of GM and LHC applications etc, which would be a decent way to get more up to date values of c. This question I posted I've seen often so its not unusual it would be helpful if wiki etc could post in its history more recent values and experimental confrimation to ease the general public on its accuracy but thats another topic.

    I'm more concerned with how the speed of light reflects our mathematical calculations on the distance to other stellar bodies, such as BB etc, at those distances a small eroor can have a huge impact in our accuracy, as far as Gr and Sr are concerned I doubt the error would be significant should such inaccuracy exist. As far as the CERN reference this merely got me thinking about this problem again I've had this concern well before the published CERN event, again until I hear confirmation on the accuracy of that experiment I withhold any decision on its validity. For that they need more tests and confirmation so it will be a while anyways.

    That part was more a curiousity, I know that the amount of air during the experimentation would have had small consequence, but it could still have affected the degree of accuracy however minute that change would have been. However I wasn't sure they understood the medium properties back then, as their were no reflection of that details in either of the 4 older books on physics I have in my posession.
    Last edited: Sep 27, 2011
  13. Sep 27, 2011 #12


    User Avatar

    Staff: Mentor

    Some discussion and references from article at NIST.

    http://www.nist.gov/pml/div681/museum-length.cfm [Broken]
    Last edited by a moderator: May 5, 2017
  14. Sep 27, 2011 #13


    User Avatar

    Staff: Mentor

    There is no chicken/egg problem here, mordred, scientists have chosen which is which:


    Speed and time are defined, so distance follows them. If time is re-defined, it is still distance that follows.
  15. Sep 27, 2011 #14
    Good link Astronuc its one I'll add to my archive
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Speed of light last value accepted 1983
  1. The speed of light (Replies: 4)

  2. The speed of light (Replies: 7)

  3. Light speed (Replies: 5)

  4. Light speed (Replies: 8)

  5. Speed of light (Replies: 18)