# Speed Of Light

• 186,282 miles per second
• impossible to exceed by acceleration

• One foot per nanosecond - about 10^6 times the speed of sound, which is one foot per millisecond.

• 299792458 m/s in vacuum.
• The meter is defined as 1/299792458th the distance light travels in vacuum in a second nowadays. This only makes sense, because light wavelengths are the most accurate form of measurement currently at our disposal. It also makes for fewer decimal places to remember.

More flippantly, perhaps less usefully, it's:
• the square root of (energy divided by mass).
• the square root of the reciprocal of the product of the permittivity and permeability constants.
• somewhat slower than Hammy the Squirrel in the denoument of "Over the Hedge"

So there you are.

"What do you mean? A European speed of light or a Bose-Einstein condensate speed of light?" (with apologies to Monty Python)

:-) But since you bring it up: there's only one speed of light. When people talk about the speed of light varying in different media, that's a shorthand/euphemism for rather complex phenomenon, and a reasonably descriptive euphemism -- but not literally true.

Why does this page exist? Can we have c in units people are likely to understand please?

• 1m in pi nanoseconds?
• 1km in pi micro-seconds?
• North Pole to Equator in 33 thousandths of a second?
• Eight times round the world in one second?
• Sun to Earth in 8 minutes?

What units would you like? If you're a programmer, or a scientist, how is it that you are not capable of converting units for yourself?

as a physicist I'd usually use 3*10^8 m/s (or 1 and convert other quantities e.g. energy in eV )

Well the problem is that most of the world does metric and a small portion of the world does more archaic units. Therefore it wouldn't be fair to enumerate the speed in units "we" would understand because some people will not understand it. I therefore submit that the speed of light is 1799884800000 furlongs per fortnight.

Oh fine. 299337984 meters per second

-- BruceIde

Do people really find 300,000,000 metres/sec easier to grasp than 1 foot/nanosecond? Just how far is 3*10^8 metres? Can you truly grasp that?
• Well, while 3 hundred million metres/sec is useful most of time, the guys that design our circuit boards find that a little clumsy (except for our earlier boards, which were several billion metres across).

Yes, but only because we want to calculate another quantity in SI units. approx 9*10^8 feet. Just how quick is a nanosecond, can you truly grasp that? The speed of light doesn't really make sense in a human way, SI units are a nice consistent way of dealing with dimensions when plugging constants like this into equations, which is the only time numbers like the speed of light are useful, it just isn't a number that is useful on a human scale. -- JamesKeogh

Sampling at 50 MHz, the speed of light corresponds to a 3 metres intersample distance for radar. And no, there isn't a factor of two error here, that's left as an exercise for TheInterestedReader.

• If the radar is being used within the atmosphere, the speed of light is different from the above constant. It's also different in water and glass -- hence refraction. By the way, I always used 1 ft/nsec during my hardware days in the seventies. Even then, computers were fast enough that backplanes and external buses had significant transmission-line effects.-- TomStambaugh
• GraceHopper was also known for carrying around a "nanosecond". It was a one-foot length of copper wire, which, of course, is that distance that electricity will travel in that length of time. (See WhimsicalUnitsOfMeasurement) Remember, this was back in the '70s. Her point was if we were going to build computers with cycle speeds faster than 1MHz we would have to put the logic boards closer to each other than the one foot they were in the machines of the late '60s and '70s. Basically, she envisioned the advent of microprocessors.
• My father attended a lecture by Commodore Hopper where she distributed some nanoseconds, followed by packets of pepper - picoseconds.
• Not exactly. Supercomputers in the 1960s already had to take distance vs the speed of light into account (actually, the speed of signal propagation, which is a large percentage of the speed of light, not c itself). Same issues arose in the analog domain with high frequence (especially radar).

If you're dealing with the speed of light on a regular basis, you probably want to use PlanckUnits?, where c = 1. This is the appeal of 1 foot/nanosecond; it's only a power of 10 from the appropriate Planck unit, so you don't need to carry around numeric constants. SI is just as arbitrary as any other unit system when you're using ScientificNotation?; PlanckUnits? are sometimes described as "God's own units" because they let you drop the constants from most of the laws of physics. -- JonathanTang
• Frequently called "natural units".
Yes - all unit systems are arbitrary, hence 1 foot/nanosecond is not universally easier to understand than 3*10^8 m/s. I don't consider having a numeric constant that is close to a power of ten an advantage over one which is close to 3 times a power of ten, but then I've been brought up with the metric system alongside imperial units. I would use PlanckUnits? normally but that is because I normally don't mind getting an energy in MeV, it's all about the units you want to end up in (which is an arbitrary choice until a 3rd party supplies one of the numbers in particular units, if you don't use their units you will need a numerical constant to convert units anyway). -- JamesKeogh
• Natural units/Planck units are arguably not arbitrary, the only such.

JulyZeroFive

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