please empty your brain below
I also don't understand why people talk of the fundamental units as 'distance, time, weight'. Apart from mass not being the same as weight, we have moved on to a situation where the fundamental units are speed (of light), time, mass (plus four others - less obvious). Distance is now a derived unit (from speed and time).
And to be really pedantic we are taking about the velocity of light not the speed of light.
When was this year's open day first advertised, and would a three hour visit suffice (I am a physicist by the way)?
dg writes: I spent three and a half.
As a physicist I really should have known about this event coming up and gone along.
One day I'll get a Caesium fountain clock on ebay, but until then I'll stick with my 3 Rubidium clocks at home - all of which give a fractionally different time...
dg writes: It could, and an alternative plan was to use an Avogadro Sphere made from a precise number of silicon atoms.
Several have been made, but for the official definition the Kibble Balance won out.
Originally. Just like the metre was originally 1/10,000 of the distance between the North Pole and the Equator *through Paris*. But things are more complicated than that these days.
I think now a litre is defined as a decimetre cube of water, or one thousandth of a cubic metre.
In terms of school physics (in my day, at least) velocity was a vector, that is speed in a particular direction. Light always travels at the same speed (in a vacuum), but may have varying velocity.
But nowadays really the "speed of light" is a thing. It is part of the structure of spacetime. You do not measure it, you define it. So it's not really a speed or a velocity, it's just the speed-of-light.
It was a rather odd thing to go with (I believe the initial definition was 1cm = a trillionth of the distance) as it was neither measurable nor the size able to be estimated from the concept alone (in the highly unlikely event you can imagine how far it is to do a quadrant of earth, the divisor is an unpicturably huge - even 10,000 for the kilogram): its merely a calculable distance.
It makes units initially defined by royal body parts seem not absurd - obviously both need some sort of standard stick/rule (or definition involving em-waves) to replicate the distance - but the latter of the sort 'from the king's nose to his pointing finger' isn't deliberately trying to be as arbitrary and aloof from the mundane and human as possible.
What's the difference between a copy and a replica?
https://en.wikipedia.org/wiki/Estimated_sign
French surveyors Méchain and Delambre spent many years crossing France with surveying equipment to make the necessary measurements that produced the metre. A decimetre cube of water gave the litre, the mass of which was a kilogram. The rational French also proposed a decimal time system, but that never caught on.
We may even have chatted, if you visited the Earth Observation exhibit - I was wearing a blue NPL polo shirt... but then so was everyone else (there's not usually a dress code). Glad that you enjoyed the day!
I wonder if NPL have Imperial standards too?
More nerds per square foot (sorry metre) than I have ever seen. It was great. Spent 4 hours there.
Sorry to those who missed being there. I doubt they will repeat it any time soon as it must have been a huge effort, cost and committment for NPL to put on. Just glad I spotted it several months ago. One can sign-up to receive information from them on their website so if they do, you will find out.
I feel uneasy with the definition of a metre in terms of the radiation emitted by anything, or the distance travelled by light in a certain time.
(a) How can anybody measure those things? Anybody could go to Trafalgar Square and measure their ruler against the standard yard.
(b) Is the wavelength of the radiation constant for ever?
(c) Is the speed of light in a vacuum constant for ever? Or does light speed up or slow down over time? (But what is time, anyway?)
(d) Light of different wavelengths travels at different speeds (or velocities) through material (that's how we get diffractions to make a rainbow); is the speed of all wavelengths of light the same in a vacuum? (I can't remember from A Level Physics nearly 40 years ago.)
If the definition has to refer to light OF A CERTAIN WAVELENGTH, then don't we end up defining a metre in terms of itself we go round in circles ... because wavelength (like any other length) is defined in terms of how far light travels ...
dg writes:
a) Scientists can
b) Yes
c) Yes
d) No
"a litre is defined as a kilogram of water"
Other way round - 1kg was originally defined as the mass of 1 litre of water. (It is also, in round numbers, the mass of 1 cubic metre of air)
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