# The exact speed of light

#### Bloodworth

##### Orbinoob
I have a question for the actual physicists in the group. My understanding is, that the statement of the speed of light being 186,000 MPS is a generalization, that the actual speed of light is 186,282 MPS. For most this will seem like picking nits, but for the project I am working on (and especially when dealing with long distances traveling at, or determined by that speed) it can make a HUGE difference.

#### Urwumpe

##### Not funny anymore
Donator
I have a question for the actual physicists in the group. My understanding is, that the statement of the speed of light being 186,000 MPS is a generalization, that the actual speed of light is 186,282 MPS. For most this will seem like picking nits, but for the project I am working on (and especially when dealing with long distances traveling at, or determined by that speed) it can make a HUGE difference.

As official as you can get in the USA

http://physics.nist.gov/cgi-bin/cuu/Value?c

Errors by converting to a unit derived from the width of a horses ass are not my problem of course. :lol:

(And if you wonder why the vacuum speed of light is known exact without uncertainty: The meter is the distance that light travels in vacuum in a fraction of a second....)

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#### Bloodworth

##### Orbinoob
Thank you Urwumpe. Now to convert that into imperial measurements so I can wrap my head around it

#### Urwumpe

##### Not funny anymore
Donator
Thank you Urwumpe. Now to convert that into imperial measurements so I can wrap my head around it

According to Google: 186 282.397 miles per second, with some rounding errors.

186282.3971 when calculating with the 1959 definition of the mile (1609.344 meter)

Or if you like it more imperial:

$1.8026175 \times 10^{12}$ furlongs per fortnight

#### Linguofreak

##### Well-known member
According to Google: 186 282.397 miles per second, with some rounding errors.

186282.3971 when calculating with the 1959 definition of the mile (1609.344 meter)

Or if you like it more imperial:

$1.8026175 \times 10^{12}$ furlongs per fortnight

My preferred way of dealing with the speed of light is to define the foot to equal one nano-lightsecond, making the speed of light exactly 1 billion feet per second by definition. This is about 2% off from the official value of the foot.

#### SolarLiner

##### It's necessary, TARS.
My preferred way of dealing with the speed of light is to define the foot to equal one nano-lightsecond, making the speed of light exactly 1 billion feet per second by definition. This is about 2% off from the official value of the foot.

My preferred way of dealing with it is touching nothing to the units, and say it is 299 792 458 m/s. Or, round it to 3e8 m/s, which leads to a 99.93% accuracy.

#### Cairan

##### Donator
Donator
How about using Plank natural units and just converting the end results?

#### Bloodworth

##### Orbinoob
My idea is to devise a new standard of unit distance measure that does away with imperial and overwrites metric (using the same metric terms) based on our largest NATURAL measuring stick, the speed of light. (as opposed to either a yard or a modern meter both of which are originally based on an arbitrary length). The idea is that if you take 1 light second (time being another natural unit of measure, based upon the cycle of the day), keep dividing that distance by 10, ultimately you should wind up with a unit that is somewhere between 2.5 - 4 feet in length. That would be the new meter. Then divide that into 1000 units and you get decimeters, centimeters, millimeters. The same applies outward as well, Decameters, kilometers etc.

#### Linguofreak

##### Well-known member
The idea is that if you take 1 light second (time being another natural unit of measure, based upon the cycle of the day), keep dividing that distance by 10, ultimately you should wind up with a unit that is somewhere between 2.5 - 4 feet in length. That would be the new meter.

Actually, if you keep dividing the light second by ten, you eventually get a unit that is almost exactly one foot (0.98 feet), as I mentioned above. 1 foot ~= 1 nano-lightsecond. 1 lightsecond ~= 1 gigafoot

#### Thorsten

##### Active member
For most this will seem like picking nits, but for the project I am working on (and especially when dealing with long distances traveling at, or determined by that speed) it can make a HUGE difference.

Dependent on what problem you're working on and what accuracy you need, you might want to note that the usually quoted value is lightspeed in vacuum. In a medium (even a very thin one) light is usually slower than that.

#### Urwumpe

##### Not funny anymore
Donator
as opposed to either a yard or a modern meter both of which are originally based on an arbitrary length

The yard yes, the meter no. The meter was measured. But the quantity that was used for the definition was a bit geocentric.

I think that Planck units are the most unpolitical of them all - even the definition of a second is arbitrary, though we have to go a few thousand years back to find the culture that imposed their numerical system on their neighbours, resulting in us still measuring time like they do.

But even Planck units are not without alternatives, the selection of which constants are used for defining them is again no natural law.

And a Planck length a tiny bit short to be useful.

#### Dantassii

##### HUMONGOUS IMS shipbuilder
Back in 1987, my college buddies and I (all Aerospace Engineers) calculated that the femto-lightyear is about a meter long.. 38.16 inches actually.

This is what happens when a bunch of Rocket Scientists have way too much time on their hands...

Dantassii
HUMONGOUS IMS shipbuilder

#### Quick_Nick

##### Passed the Turing Test
Donator
Back in 1987, my college buddies and I (all Aerospace Engineers) calculated that the femto-lightyear is about a meter long.. 38.16 inches actually.

This is what happens when a bunch of Rocket Scientists have way too much time on their hands...

Dantassii
HUMONGOUS IMS shipbuilder
You may want to recalculate!
http://wolframalpha.com/input/?i=1+femtolightyear

#### Thorsten

##### Active member
I think that Planck units are the most unpolitical of them all

Somehwat simplified - the universe has one scale which you need to pick to assign units to everything. It can be a length scale, a time duration,... it doesn't really matter which, it just has to be a dimensionful quantity.

The rest falls then naturally into powers of that unit if you refuse to add extra units.

Particle physicists like to pick the GeV as basic unit for energy. Then the uncertainty relation says that the conjugate to energy - time - is 1/GeV. Then General Relativity says that space and time are really the same, so length is also 1/GeV. The conjugate of length is momentum, which then gets to be GeV like energy (would also directly follow from GR).

Velocity is then length/ time and comes out dimensionless - if you're moving the same length and time, Special Relativity tells you that you're moving at lightspeed, so c is dimensionless and just gets the number 1, every other velocity is a number <1 and measures velocity as fraction of lightspeed.

Area is then 1/GeV^2 - volume 1/GeV^3, energy density is energy.volume, i.e. GeV^4,... and so on.

You could have picked meters as length units, or lightyears, or parsec, or the mean distance of the hydrogen atoms in a water molecule - say a unit U - then time would also be measured in U, energy and momentum in 1/U, and lightspeed would still be 1.

Which is universal - in natural units, whatever you pick, lightspeed is just unity because space and time is really the same.

#### Xyon

##### Puts the Fun in Dysfunctional
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Actually, if you keep dividing the light second by ten, you eventually get a unit that is almost exactly one foot (), as I mentioned above. ~= 1 nano-lightsecond. 1 lightsecond ~= 1 gigafoot

I very much like the word "Gigafoot".

#### Hielor

##### Defender of Truth
Donator
Beta Tester
My idea is to devise a new standard of unit distance measure that does away with imperial and overwrites metric (using the same metric terms) ...

http://xkcd.com/927/

#### Linguofreak

##### Well-known member
Which is universal - in natural units, whatever you pick, lightspeed is just unity because space and time is really the same.

Which means we can abuse dimensional analysis and say that the LD50 of orally administered caffeine in rats is 60 km/s. :leaving:

EDIT: Also "It's the ship that flew the Kessel Run in less than 40 years."

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#### kamaz

##### Unicorn hunter
Interestingly, a light travel time of 1 nanosecond corresponds to 30cm, which is one foot.

Adm. Grace Hopper was known for carrying around a bunch of 30cm wires which she used to explain satellite link latency to generals:

Also, 30cm corresponds to 1GHz wavelength, which is terribly useful in microwave engineering: to calculate wavelength for a given frequency, you divide 30cm by frequency in GHz. E.g. for 5GHz you have 30/5 = 6cm.

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#### PhantomCruiser

##### Wanderer
Moderator
Tutorial Publisher
I had the priviledge of meeting Adm. Hopper, she did keep a good many strands of "nanoseconds" in her purse. Really smart woman, I never worked for her but I wish I could have.

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