concept of physics!!?

Question

The metre is defined as the distance travelled by light in 1/299792458 second. Why did'nt people choose some easier number such as 1/300000000 second? Why not 1 second?

Answer 1

The metre was originally defined as 1/40,000,000'th of the polar circumference of the Earth, then as the length of a particular bar of platinum-iridium alloy; then in terms of the wavelength of light emitted by a specified atomic transition; and now is defined as the distance travelled by light in an absolute vacuum during 1/299,792,458 of a second.
In the 300 years from the original metric system, better and better standards have been developed, but they still try to maintain backward compatibility to keep disruptions to a minimum.
For example the second was 1/60'th of 1/60'th of 1/24'th of a solar day. This standard was found to be not precise enough because the Earth's rotation changes (slightly).
The fix was to incorporate "leap seconds" occasionally into the year so that atomic clocks could remain in sync (and you would not have to buy a new set of clocks every time some scientist discovered a better way.)
In a few thousand years when our units prove themselves stable then they can be rationalized.
I am betting we will abandon radix 10 and go to radix 2. There may be many more issues to be dealt with by then as well.

Answer 2

that's not really the definition. You arrive at that answer based on unit analysis and such, solving for metres from, say, the speed of light in a vacuum (2.997 x 10^8 m/s).
And historically, the metre was actually just arbitrarily decided. Scientists wanted a more permanent way to define it and chose something else. Like, a second is defined as the time it takes for some particular isotope to radiate one something or other. The metre is defined as the distance some particle travels ..something something, I dunno.
The kilogramme, however, is still defined by some lump of metal that they take out every few years and re-confirm its mass =p.



To answer the question you didn't want to ask, ..people could have defined it any way they please. In fact, they do, using dimensional analysis (it's used to make life easier when dealing with multi-variable equations and the like. you can ask your teacher about dimensional analysis with regards to Reynold's number and he or she will probably cringe) ..but the long and short of it is, they'd still have to do some tough math at some point to get things right.

It's like ...gravity is 9.81 m/s^2 at most points on the surface of the earth ...it's not a particularly convenient number, ..but it makes the math work out when doing, for example, projectile and range problems

best of luck

Answer 3

Without going into "sciency" details, the meter was originally defined as one ten-millionth of the distance from the north pole to the equator. Over the years, other methods were devised to ensure the precision of the measurement, in particular, defined by the distance between two marks on a metal alloy bar kept under a specific environment at the standards bureau.

Point is, the length was more or less "fixed" as more accurate methods were devised to derive its length from light. To pick a convenient value like 3e-9 seconds, you'd be redefining the length of a meter.

Bad analogy, but it's akin to redefining the length of an inch so there's ten of them in a foot, to make calculations easier.

Answer 4

Because "the distance travelled by light in 1/299792458 second" approximated to what was already arbitrarily defined as a metre. Changing the definition would have meant changing every reference to and use of the metre in the world. (Not just obvious things like road signs, but all objects designed using physics formulae that include the metre)

It wasn't so much a redefinition as a recallibration - the speed of light (which is constant) was used to determine the length of the metre because all previous attempts referred to physical objects which were subject to variation (including the International Prototype Metre - enscribed upon a bar of platinum-irridium which was subject to microscopic variation depending on temperature, gravitation, etc.)

Answer 5

The meter was originally defined as 1/10,000,000 of the distance between the equator and either pole; however, the original survey was inaccurate and the meter was later defined simply as the distance between two scratches on a bar made of a platinum-iridium alloy and kept at Sevres, France, near Paris. More recently, it has been defined as the distance light travels through a vacuum in 1/299,792,458 of a second. The meter is now the legal standard of length for most of the world, other standards, such as the yard, being defined in terms of the meter.

Answer 6

the meter was defined as something before it was designated in light terms. They just recalculated the definition to be in terms of light, because thats a constant value, unlike many other things it could be based on.

same thing with a second. the second has been a division of time for a LONG time, but it is officially designated as some number of oscilations of some atom. they just set it as that so it could be more universally constant, while still having the same traditional value.

Answer 7

should have asked the people who chose. bythe way all i can say is it's physics

Answer 8

coz..its a standard.. on which other calculations are based