Re: The relationship between meter, speed of light and c

On Feb 10, 8:56 am, "N:dlzc D:aol T:com \(dlzc\)" <d...@xxxxxxx>
wrote:
Dear karandash2000:

<karandash2...@xxxxxxxxx> wrote in message

news:1171123229.212844.130240@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

The meter is defined as the length covered by
light in 1/299792458 seconds (see the NIST website).

How did the metrologists arrive to the 299792458
number? Somehow they must have decided that the
speed of light is exactly 299792458 m/s.

There have been more than 100 very detailed and precise
measurements of c since the late 1600s, and they only served to
highlight problems in metrology. In the 1950s, the time
measurement standard had become good enough that it was evident
that there was a problem with the bar in Paris, in that it had a
secular increase in length of about 1 part in 10^8 per year.

For more see:http://nvl.nist.gov/pub/nistpubs/jres/104/3/html/j43bee.htm
... alloy rod got longer, steel rod got shorter

So they switched in 1960 to a certain number of wavelengths of a
particular light (Krypton I think), which provided a more stable
standard, but some inherent uncertainties. The rest you have
heard from Dirk and Tom.

The meter started out as some fraction of the Earth's equatorial
circumference, so that is likely how we ended up with such an odd
number.

The final NBS (at the time) measurement was 299,792,458.6 m/s.
The value established is close to this.

But the light speed measurements are dependent on the
definition of the meter (and the second). How did they
break this apparent circular dependency?

There was none at the time. c was a fundamental physical
constant, and the meter was either a portion of the equatorial
circumference of the Earth, a rod in Paris, or a number of
characteristic wavelengths. Then in 1983, c was an established
constant, and the meter was derived from it.

It is much simpler, and much more precise, to simply have a very
good clock. That way your transfer standard for length is a
piece of paper with numbers written on it. Nothing to get abused
in shipping, thermally shocked, marred in handling, or resonate
to a building's mechanical systems.

David A. Smith



So let me see if I can get this straight:

1. By 1983 a very stable source for time had been acquired. The
standard "second" was well established, reproducible, etc.
2. Based on the previous speed measurements of light (the ones that
used the old meter standard made out of platinum-rhodium) an average
value of 299,792,458 meters/sec was assigned to the speed of light.
3. Bases on 2, the meter was REDEFINED as being the distance covered
by light in 1/299,792,458 seconds.

Tom,DvM,David , do you agree?

.

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