# Lengths of SI Units

1. Sep 1, 2011

### xeryx35

After the discovery of the speed of light, it may have made a bit of sense to redefine the meter as $\frac{1}{300,000,000}$ the distance light travels in a second. I understand that larger units like the light year may have been distorted a bit, and that conversions would be required for such areas, or others where precise measurement may have been required. But my point is that the current SI system is a bit arbitrary(fortunately less so than the so-called "customary" system) and using the speed of light in a vacuum would base it upon a universal constant, not something that can easily change.

2. Sep 1, 2011

### Staff: Mentor

I don't see how any of the SI units can easily change.

3. Sep 1, 2011

### xeryx35

True. It may be a difficult decision to change old meters into new meters, from a political and social standpoint. But do you think that, if it had been an easier process, that it would make sense?

Maybe the new SI definitions could be:

Second: time taken by light to travel 300,000,000 meters
Meter: Distance trageled by light in $\frac{1}{300,000,000}$second.

The units of weight could be modified, but a gram will still be the mass of water in a cube 1 new centimeter in length, and the Kelvin scale could be used for temperature, with the Celsius scale being used in a non-scientific context.

4. Sep 1, 2011

### bp_psy

"The meter is the length of the path travelled by light in vacuum during a
time interval of 1/299 792 458 of a second."

http://physics.nist.gov/Pubs/SP330/sp330.pdf
page 18

5. Sep 1, 2011

### Staff: Mentor

All of the SI units are very well defined using things that do not easily change, that is what I meant as a response to your statement:

For example, the second is defined as: 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.

The Kilogram is currently the only SI unit that is defined using an actual object instead of fundamental physical properties. However, that may change in the near future. See here: http://en.wikipedia.org/wiki/Kilogram

So other than changing Kilogram, I don't really see what you are getting at.

6. Sep 1, 2011

### xeryx35

Oh, sorry, I did not know that the other units were based off of physical constants. In that event only kilograms need modification.

7. Sep 1, 2011

### Staff: Mentor

Yep, you can click on the units here to get to their articles: http://en.wikipedia.org/wiki/SI_unit