The exact speed of light

Bloodworth

Orbinoob
Joined
Oct 14, 2008
Messages
544
Reaction score
2
Points
16
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
Addon Developer
Donator
Joined
Feb 6, 2008
Messages
37,588
Reaction score
2,312
Points
203
Location
Wolfsburg
Preferred Pronouns
Sire
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....)
 
Last edited:

Bloodworth

Orbinoob
Joined
Oct 14, 2008
Messages
544
Reaction score
2
Points
16
Thank you Urwumpe. Now to convert that into imperial measurements so I can wrap my head around it :)
 

Urwumpe

Not funny anymore
Addon Developer
Donator
Joined
Feb 6, 2008
Messages
37,588
Reaction score
2,312
Points
203
Location
Wolfsburg
Preferred Pronouns
Sire
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:

[math]1.8026175 \times 10^{12}[/math] furlongs per fortnight
 

Linguofreak

Well-known member
Joined
May 10, 2008
Messages
5,017
Reaction score
1,253
Points
188
Location
Dallas, TX
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:

[math]1.8026175 \times 10^{12}[/math] 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.
Addon Developer
Joined
Jun 14, 2010
Messages
1,847
Reaction score
2
Points
0
Location
404 ROAD NOT FOUND
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
Joined
Apr 11, 2008
Messages
601
Reaction score
1
Points
18
Location
Amqui, QC
How about using Plank natural units and just converting the end results? :)
 

Bloodworth

Orbinoob
Joined
Oct 14, 2008
Messages
544
Reaction score
2
Points
16
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
Joined
May 10, 2008
Messages
5,017
Reaction score
1,253
Points
188
Location
Dallas, TX
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
Joined
Dec 7, 2013
Messages
785
Reaction score
56
Points
43
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
Addon Developer
Donator
Joined
Feb 6, 2008
Messages
37,588
Reaction score
2,312
Points
203
Location
Wolfsburg
Preferred Pronouns
Sire
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
Joined
Jul 14, 2012
Messages
508
Reaction score
20
Points
33
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
Joined
Oct 20, 2007
Messages
4,088
Reaction score
204
Points
103
Location
Tucson, AZ

Thorsten

Active member
Joined
Dec 7, 2013
Messages
785
Reaction score
56
Points
43
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
Administrator
Moderator
Orbiter Contributor
Addon Developer
Webmaster
GFX Staff
Beta Tester
Joined
Aug 9, 2009
Messages
6,922
Reaction score
789
Points
203
Location
10.0.0.1
Website
www.orbiter-radio.co.uk
Preferred Pronouns
she/her
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
Joined
May 30, 2008
Messages
5,580
Reaction score
2
Points
0
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/
standards.png
 

Linguofreak

Well-known member
Joined
May 10, 2008
Messages
5,017
Reaction score
1,253
Points
188
Location
Dallas, TX
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."
 
Last edited:

kamaz

Unicorn hunter
Addon Developer
Joined
Mar 31, 2012
Messages
2,298
Reaction score
4
Points
0
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.
 
Last edited:

PhantomCruiser

Wanderer
Moderator
Tutorial Publisher
Joined
Jan 23, 2009
Messages
5,600
Reaction score
165
Points
153
Location
Cleveland
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.
 
Top