quote: The meter has now been defined so that the speed of light(any electromagnetic wave) in a vacuum has the exact value
c = 299 792 458 m/s
Does anyone know why they chose this number, instead of say, 300 000 000 m/s ? I checked and its only a 0.06% difference.
Posted by King of Men (Member # 6684) on :
That sort of thing is usually done so that old books won't have to be rewritten. It would be a pain to look up some constant and then have to check whether a book was published before or after the New Meter was introduced.
Posted by kaioshin00 (Member # 3740) on :
Ah.
So it is just coincidence that the speed light happens to be 299 792 458 m/s, with however the meter was defined before they knew the speed of light?
Posted by King of Men (Member # 6684) on :
Yep.
Posted by kaioshin00 (Member # 3740) on :
That's pretty cool .
Posted by Dagonee (Member # 5818) on :
I thought the second was defined by how long it takes light to travel 299 792 458 meters.
I'm so confused.
Posted by Bob_Scopatz (Member # 1227) on :
I have a question...
Light's speed, path, whatever, is altered by gravity, right? So, aren't all the measurements we make of the speed of light messed up by the fact that we're measuring under local gravity conditions?
Posted by fugu13 (Member # 2859) on :
Light's speed is not altered, only its path.
The earth does not have a particularly large effect on that.
Posted by Glenn Arnold (Member # 3192) on :
The whole point of each successive definition of "meter" was to make it easier to replicate, without actually changing the length. 0.06% is a huge error when you compare it to the precision that's currently required.
I suspect that the change from a number of wavelengths to a unit of time is simply a matter of solving for a different variable. The result is not arbitrary.
Posted by Glenn Arnold (Member # 3192) on :
quote: So, aren't all the measurements we make of the speed of light messed up by the fact that we're measuring under local gravity conditions?
The whole point of relativity is that it doesn't matter what our frame of reference is, the speed of light is constant. In fact, the reason Einstein came up with the theory was that no matter what direction we measured the speed of light in, it didn't change. Considering that the earth is moving, you ought to be able to add or subtract earth's speed from the speed of light depending which direction you're facing. That doesn't happen though.
Posted by Dagonee (Member # 5818) on :
I just reread the special and general relativity chapters in "The Elegant Universe." It's hard to hold onto the whole thing at once (and by "whole thing," I mean the basic explanation in books like this one and "A Brief History of Time," not even the whole theory). Especially the part about time being slower for each mutually observed observer, and there being no paradox because the clocks can't be brought together without changing one of the observer/observed's inertial frame.
But one part I liked that I hadn't seen before was the idea that everything moves through spacetime at the speed of light. If something is moving through one or more spacial dimensions, then less velocity is available for moving through time, which is why time moves more slowly for things going faster, and why light doesn't experience time.
*head spin*
Dagonee
Posted by kaioshin00 (Member # 3740) on :
quote:I thought the second was defined by how long it takes light to travel 299 792 458 meters.
One second is the time taken by 9 192 631 770 oscillations of the light(of a specified wavelength) emitted by a cesium-133 atom.
[ March 27, 2005, 12:06 AM: Message edited by: kaioshin00 ]
Posted by Tatiana (Member # 6776) on :
The meter was originally defined as some even division of the distance from the equator to the north pole, but because that later turned out to be a really foolish thing to base it on, given that the measurement is difficult to make and not really even very well defined given the oblateness of the earth and the whole mountains-and-ocean-floors deal messing things up, they later changed it.
The inch, though, is now defined as exactly .0254 meters.
The S.I. units are still sort of messed up, even though they did fix most of what was wrong with the English system. For one thing, time and distance should not be defined independently. We should either use as the unit of distance, the distance light travels in 1 second, or else (my preference) use as the unit of time, the time it takes light to travel 1 meter. I think we should do this and call it the Feynman, abbreviation Fy, since Farad already swiped the F.
Then we'd lose those pesky C^2 terms that are sprinkled all over relativity equations. They would just be ones and would drop out, the same way in SI units you don't have silly constants messing up F=mA. (In the English system of units, F does not = mA. It equals mA/g(subC), g(subC) being the constant introduced because the units for force and mass are not defined well relative to each other.)
So that way instead of E = Mc^2, einstein's famous formula would just be E = m.
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