/sci/ - Science & Math

[Anonymous No.5844266]

Why is a meter defined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second?

What is the significance of this number(299,792,458)?

Why not define the speed of light as something like 1,000,000,000 meters per second, making the meter the length of the path travelled by light in vacuum during a time interval of 1/1,000,000,000 of a second?

The meter seems more arbitrary than using some guy's foot for a basis of measurement.

[Anonymous No.5844275]

>>5844266
Because it simplifies a metric fuckton of expressions.

[Anonymous No.5844279]

>>5844275

I guess it makes sense that defining it in terms of the speed of light makes things easier, but how is 299,792,458 better than 1,000,000,000?

[Anonymous No.5844287]

because the meter was defined before the speed of light was accurately measured. originally it was semi-arbitrary... water was used as a constant for the relationship between length and mass (one cubic centimeter of water is equal to one gram) but of course if a meter were longer or shorter then a gram would weigh differently

the definition of a meter was changed to be based on the speed of light because the speed of light is a better universal constant than the density of water

[Anonymous No.5844292]

>>5844266
>Why not define the speed of light as something like 1,000,000,000 meters per second
Why not define a second as the time it takes light to travel 1,000,000,000 meters.

[Anonymous No.5844290]

>>5844279
The universe doesn't work in base ten, just because 1,000,000,000 looks pretty to us doesn't mean it's useful for the shit we actually have to do.

[Anonymous No.5844291]

>>5844266
>>5844279

See, the meter's had a long history. The meter was originally defined as 1/10,000,000 of the circumference of the Earth. This is, in fact, a nice round number, and was used for a long time.

But it turns out that the Earth is not, in fact, perfectly spherical, and in any case it's very hard to measure accurately. So the Earth's not a very good standard to base the metric system off of, and in any case it's not practical to measure every time you want to recalibrate something.

So next, the meter was defined as the length of a meter-long platinum-iridium bar kept under vacuum in France. But this still wasn't accurate enough; it changed with thermal expansion and reaction to air.

So they based it on fundamental constants- time, which was measured by the ticks of an atomic clock, and the speed of light.

That precise number- 1/299,792,458 - was chosen because that number was one meter long- remember, the meter already existed and had a definition before it was defined based on light- but it could be measured more precisely because it was based on fundamental constants.

Basically, the meter originally had a rational, round-number definition, but it was too imprecise and variable, so they chose a more precise definition that had the same length.

[Anonymous No.5844301]

>not using plank units

[Anonymous No.5844313]

>>5844290
But people do.

Quick, tell me how many meters light travels in .25 seconds.

If it was my way, I could tell you right now it is 250,000,000.

>>5844291

So basically, europe had the same problem the us does? They got stuck with a shitty arbitrary distance they were unwilling to change?

>>5844292
Because everyone recognizes the second as a standard unit of time. Why change something everyone universally agrees to use, when you can change the unit of distance?

>>5844301

In terms of plank units, how tall are you anon?

[Anonymous No.5844325]

>>5844301
We don't know the planck units accurately enough to make them usable. Most of the calculations for planck units involve G, and G is only known very approximately. As such, the planck units all have imprecisions to the degree of 1/16600, and since they're so very small you need a lot of them, and those imprecisions all really add up.

In other words, the planck units are, although more elegant and fundamental, less accurate for measuring things than basing your calculations off of the length of a climate-controlled bar kept in a bell jar, and far less accurate than modern unit systems.

>>5844313
>shitty arbitrary distance

It's not arbitrary at all- it was chosen for logical reasons. At the time, the circumference of the Earth was thought to basically be a fundamental constant, and one that everybody could easily grasp, and it was a nice round-number multiple-of-ten fraction of that.

It's just that it's now measured in terms of fundamental units, and it's not neatly related to any of those. They could probably have made a "new meter" that's precisely one 300 millionth of a light-second, but that wouldn't have caught on.

[Anonymous No.5844420]

>>5844325
>At the time, the circumference of the Earth was thought to basically be a fundamental constant,

how did they even measure thee circumfereance of the earth

[Anonymous No.5844425]

>>5844420
Good question! In fact, the quest to standardize the meter lead to a major expedition- the French Geodesic Mission- to precisely measure the polar circumference of the Earth, by measuring the meridian arc between two reference points on the same arc of longitude.

http://en.wikipedia.org/wiki/French_Geodesic_Mission

[Anonymous No.5844441]

>>5844266
What you are proposing is changing the length of a metre.

Light will travel the same unit distance per unit time in a perfect vacuum regardless of how we choose to define each unit distance or unit time.
The length of a meter was defined long before we knew about the speed of light or how to measure it.

[Anonymous No.5844445]

What i was trying to say with the above post is that the entire system of measurements is completely arbitrary.

[Anonymous No.5844450]

A great benefit of switching to defining each of out units of measurements using the planck measurements as a base would be that they rely on universal constants.
Some measurements we currently use are defined using physical objects stored in special laboratories as a base for the measurement. The problem with these objects is they change over time. For example the 1kg block International Prototype Kilogram would weight slightly more/less depending on the height above the earths surface, the humidity, the degradation of the material it is made from.

[Anonymous No.5844455]

What you are proposing is changing the length of a metre.
Light will travel the same unit distance per unit time in a perfect vacuum regardless of how we choose to define each unit distance or unit time.
The length of a meter was defined long before we knew about the speed of light or how to measure it.

The entire system of measurements is completely arbitrary.

A great benefit of switching to defining each of out units of measurements using the planck measurements as a base would be that they rely on universal constants.
Some measurements we currently use are defined using physical objects stored in special laboratories as a base for the measurement. The problem with these objects is they change over time. For example the 1kg block International Prototype Kilogram would weight slightly more/less depending on the height above the earths surface, the humidity, the degradation of the material it is made from.

The problem being that measuring planck units is difficult and full of errors.