An exact second...

godnroh1

Say I wanted to make a clock and I have nothing to help me.

How would I go by defining an exact second? What is it? How do I time it?

It's got to be something with the way the Earth is divided into minutes and seconds but I just can't work out how it would be done...

It's doing my head in.

Help!

bonkey

The unit of duration the Systeme International (SI) second defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133.

Its very definition should tell you the only way to exactly measure a, exact second. Anything else is an approximation

jc

Kappar

Originally posted by bonkey
The unit of duration the Systeme International (SI) second defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133.

Its very definition should tell you the only way to exactly measure a, exact second. Anything else is an approximation

jc

Is there not something like the duration it takes light in a vacum to travel over some distance aswell?

Gordon

But when our time was invented or thought up - what was a second? It wasn't surely what bonkey suggests originally was it?

Capt'n Midnight

is 1,659,763.73 wavelenths of a certain orange radation emitted from krypton 86 ....

you can work out the speed of light from maxwells equations - you could measure it too and work out the meter that way...

but for your average punter the best bet is GPS - they have atomic clocks...

Back in the days of the romans midnight and miday were both 12 oclock - and 6 and 6 were sunset/sunrise - so the hours were not variable

base 6 came from way way back from the iraqi's no less

back in the middle ages a moment was about three of our modern minutes,,,

Sangre

So when did the second come about?

Fabritzo

The second was originally defined as 1/86,400 of the mean solar day. However, since the Earth's rotation is slowing down by almost 1 second every 10 year.

the second was redifined as 9,192,631,770 periods of the electromagnetic radiation emitted by a particular transition of cesium atom.

My 2 cent...now it's up to someone else to go another step backward and explain my definition.

Got it from a book by Boylestad btw.

Capt'n Midnight

It was based on 1/(365*24*3600) - the year used was 1900
Without the use of a time machine this is not very reproducable...

Quasers make nice timepieces but they are slowing down too,

From looking at them yokes in shark bay they can tell how many days there were - 400 per year in the dinosaur days

moon drifting away - and will one day drop back down into again...


BTW - Ireland is in a different time zone to the UK BTW

Turner

Everbody knows its the time it takes you to say "one elephant"

Sangre

A more exact definition is...

The time it takes to say 'one missisippi'

Capt'n Midnight

How long would it take Zag to say effelump correctly ?
And as for mississippi try that with a lisp

The shortest second is a "New York Second" - the time between when the lights go green and the taxi behind you starts blowing it's horn...

Swinging a meter stick is also fairly accurate, or measuring your heat beat at rest.

"how long did that take then ?"
"it took 0 seconds - oh actually my pulse has gone - goodbye world" THUD !

Syth

The name 'second' came about when they were able to make accurate clocks like pendulum clocks and were able to measure seconds easily. Since there was 60 minutes in a hour, they said there would be 60 seconds in a minute. Since they were dividing by 60 a second time, they called it a second. Smart, eh?

And I always thought it was the time to say "one thousand", eg
"one onethousand, two one thousand, three one thousand" etc.

Sangre

but who decide how many minutes there would be? And whats the exact defintion of a first, i mean a minute?

Syth

...oh and BTW all the other fundamental SI units are defined like that, except for the kilogramme. They havent' been able to derive from any where and so they're stuck with just defining it as the mass of the lump of metal in Paris. Funny thing is, is that it's actually losing about 40mg a year or so, so you everyone's getting heavier coz the kilogramme is getting lighter. hehe. I'd like to sneek in there and take a chip off that lump of metal. They'd have to change all the scales.. hehe

Sangre

Isn't it the mass that 1N can push 1M in space or something?

mehhhh....

Capt'n Midnight

Funny that a litre is not the same as 1,000cc...
also funny that a kilogram weighs more at the poles than at the equator and also varies between nepal and the deccan plateau not so far away....

Having centigrade as a scale based on water makes a lot of sense - freezing / boiling - fairly obvious stuff and kinda important when liquid water is essential for know lifeforms (ok lets not go into effect of pressure or solutes etc.) - So we could redefine the Kilogram to be the weight of 3.34E25 molocules of water (excuse the exponent I'm lazy) but water = H20 but based on C14 weights or natural distributions in the ocean ??

The Amp is also based on the kilogram too.... - maybe we could do the other way around - the Newton being the force generated per unit length by (a big number of) electrons moving in two parellel wires (of infinite lenght) ref. current balance

PS. the only thing Newton can push is daisies

Syth

I reckon 60 minutes=1 hour is a hang over from the Babylonian base-60 system

A kilogramme can't be defined as 6x10^26 atoms of C12 because it is currently not possible to measure accuratly enough how many atoms are needed to make a kilogramme (if we were to convert from one definition to another).

For more info see http://www.ex.ac.uk/cimt/dictunit/dictunit.htm

New Scientist or Scientific American had an article on this not long ago, but I can't find it... damn...


BTW Cap'n Midnight, a kilogramme is the unit of mass not weight. A kilogramme of something can have any weight. It's weight is measured in Newtons, and depends on the mass and the gravity at a place. A kilogamme weighs less at the poles because the earth is squashed and the poles are nearer to the earths centre of mass. So you wanna loss weight, go south!

Capt'n Midnight

Actually most of the weight loss is due to - shall we call it "centrifugal force"
The reason the poles are closer to the centre of the earth is that are heavier - they are not heavier because they are closer...

I was not suggesting it was easy to count atoms (hint - start off with a mass spectrometer and a photomultiplier) but you could base more of the basic units on reproducable stuff. - you can't exactly tell the aliens - well there's a Kilogramme in Paris suggest you drop in the next time you are over there....

Back on topic -
the length of a second is of course relative.

disco_rob_funk

I was told in primary school that a gramme was based on the weight of a standard housefly.

Obviously that varies, but I'm sure it was good enough for most of the history of the metric system; up until the standard lump of metal in Paris was adopted

RC

Capt'n Midnight

The metre was originally defined as one ten millionth of the distance between the equator and the north pole on a line running through Paris. - one of the more interesting things about this definition was that no one had yet been to the pole...

Also I'm sure someone can pull up details about the other units proposed around the same time - including Gradians.. and of course the new decimal units of time.

And I think it was a mix up of temperature that resulted in wrong volume of water being weighed to produce the standard kilogramme. 0 deg Vs. 4deg.

Saruman

I thought it was what Fabritzo said! I have never heard of bonkeys suggestion before.. then again i dont do science.

However is it based on the mean solar day or the Sidereal day?

Capt'n Midnight

'Cos that way you don't have to take into account the extra day caused by rotation around the sun (it's a bit like the hours and intenational data line)

Other ways of getting a second are
penduilum of the right length
dropping things
for musical people there should be some way of working back from the tone of a note (though this is better for finding lenght)

so using a hollow tube - cut it's length to make it resonate at middle C (or whatever) - you can then use it as a pendilium to make the appropiate swing.

nesf

From a physics point of few you're looking at it in completely the wrong way. It doesn't matter what a second is exactly, so long as as we settle on a single definiton, and then use that to measure.


All the different definitons are something just to keep the engineers happy.

Sleipnir

a second is just a way human's have created tomeasure the passage of time. Even

The unit of duration the Systeme International (SI) second defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133.

is just our perception of a unit of time.
We just had to make a definition to make sense of time. It has been redefined before and could be redefined as "how long it takes for a dwarf to walk to the shops through ice-cream, the actual length of a second is less important than everybody agreeing on the definition..
Time is curved and there is no real way to measure this passage of time.
Think of the first two seconds of time when the universe began; 1 & 2.
The first second started running and when like this 1, 1.1, 1.2, 1.3, 1.4, 1.5, .....1.99, 1.991, 1.992..........1.999999999999999991
Bear in mind that this goes on into infinity. All the above are simply points or tick-marks that humans make to mark the passage of time. At what point (bearing on mind 1.99999999999999999999999999999999991, 1.99999......2) does the first second stop and the 2nd second begin. It can't due to the infinate number of tenths, hundreths, millionths, billionths of a second between the two. The first second doesn't end because, really there's no such thing. we just made it up because we can't comprehend time as just moving like a curve.
or did I dream it?

Capt'n Midnight

it can be broken down only so far (10E -43 of a second ?)

eg: speed of light across planck distance (or whatever the smallest physical distance that can exist)

Sleipnir

nothing that has a finite value (like the time between 1 second and the next one) can be applied to measure the passage of time really. A planck distance is still something that humans have invented to measure something.

Capt'n Midnight

Tying together the theory of gravitons with the shortest possible measurements of time, quantum theory says time would move in miniscule, Planck time-sized bits - like grains of sand passing chaotically through an hourglass, or a sequence of jittery freeze frames that on average last one Planck time rather than a continuous, seamless flow.



http://www.ebtx.com/ntx/ntx27.htm - measurement system based on Planck

Also
http://www.planck.com/intuitiveplanck.htm
"The Planck quantities are a system of natural units which arise purely from gravity and light. They don't depend on the mass of any one particular thing, like an electron or the universe."

disco_rob_funk

Discrete distance? and therefore discrete time?

Tough call. This (or last/week before) week's New Scientist featured a chap (Fredkin) who reckoned all reality was in the form of a cellular automaton with only very basic rules generating all of existance. In that model of reality, both were certainly discrete. Quantum theorists would probably agree (Planck distance etc.).

However, I'm sure there exist mathematical models of the universe where space/time has to be treated as continuous (gen. rel? haven't done it yet) for certain rules to hold.

RC

Victor

Originally posted by Capt'n Midnight
Funny that a litre is not the same as 1,000cc...

Really?

Originally posted by Capt'n Midnight
also funny that a kilogram weighs more at the poles than at the equator and also varies between nepal and the deccan plateau not so far away....

As (not quite elegantly) said above, the mass the constant, the weight will vary dependant on the distance to the centre of gravity of the Earth. As the mass of the earth suffers centrifugal forces as it spins on it's axis, the circumference (and there radius and didstance to centre) around the equator is great than one through the poles.

Capt'n Midnight

Our universe is infinte in the sense that all the energy in it is not enough to carry anyone outside it.

RE: Litre Vs. cc's - In the USA they have used imperial feet, metric feet and US gelogical feet which are all different....

Funny thing is according to Mr Hawking at the big bang you get complex time ( square root of -1 stuff ) or rather the equations work out if you use funny values. This stuff is much wierder than negative time....

Capt'n Midnight

3600
The number of seconds in an hour (60×60), the number of fingers in a cord (units of length), the number of shekels in a talent (units of weight), the number of years in the (Babylonian) long "saros", and the product of squares of the simplest Pythagorean triangle (32×42×52). All of these were significant to the Babylonians.
The first of these, the seconds per hour number, was probably picked as a compromise of convenience. The hour had been long established as 1/12 part of the daylight period (to such an extent that, in many cultures, the length of the hour increased and decreased wIth the seasons!). The second arose as a natural unit of time because it is close to the frequency of a human heartbeat. Since hours and heartbeats were already pretty well established, it was necessary to choose a number that was pretty close to the right ratio but also was convenient to work with. 3600 was by far the best choice... base of the Babylonian number syste= 60.