The metre was originally defined as "one ten-millionth of the length of the Earth's meridian along a quadrant", that is one ten-millionth of the distance from the equator to the North pole. The problem was the above definition brought about a degree of uncertainty and some other issues, so the metre was redefined to the current definition. As such, the number used isn't an arbitrary number but is the number that allowed the new definition to be the same length as the old definition.

To be clear the international prototype bars themselves are not that very expensive for so much security. It's that once these metal bars were made and length defined according to that, if they were accidentally manhandled or damaged it would be impossible to make exact same replica and hence one would have to redefine the metre. So they ensured (and still do) high security for many instruments were calibrated first by using these as reference.

I want to see how they actually come up with the meter. Show us how you actually did it NPL.

Where do I buy my new tape measure that works by the speed of light.

Sir a doubt without knowing a metre how did you told speed of light is 299 792 482 metres per seconf.

huh... okay so the speed of light is defined by meters per second a meter is defined as the distance light travels in a fraction of a second and a second is defined by amount of time it takes a cesium 133 atom to complete 9,192,631,770 cycles ... so how much time does it take for a cesium 133 atom to complete one cycle? the answer is 1 / 9,192,631,770ths of a second! is it just me or is this some kind of paradox?? each measurement unit defines the other

Any measurement unit can be translated into a speed of light of course. Maybe you could also translate feet and yard and cubit in speed of light in another vid and see which numbers you get and maybe find patterns. The metric system is just 1 of systems to look into.

Can I, as a citizen, replicate the meter with something cheap? Or do I need extremely expensive equipment to recreate it at home? I would love to do an experiment myself

How do we establish that the wavelength is in vacuum? And does quantum theory enter into defining what a vacuum is and how to realize it?

Such a big lab as NPL could have done a way better video on the topic. Video starts with a boring 1mn introduction; spends 2mn opening the vault where a local meter is held, and does not even show is clearly (box says "meter with graduations"; but video does not zoom on any). No close view of the bar. No mention of where to find the historical international one (BIPM in France). Jeremy's message is so important; can't udnerstand why this was omitted.

A meter is defined in terms of second, one of the basic SI units. Doesn't that make it a derived unit?

Thanks Dr.Andrew Lewis Robert lambourne Ewan Macfarlan George Butcher

Some forget that the speed of light is defined by using the two fundamental constants of permeability (magnetic) and permittivity (electrical) of a vacuum - it has nothing to do with distance, temperature, gravity

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

The fraction of time used to define meter is a number indicating speed of light in (meters/second) itself. Isn't it mind bending in a way because to get that number (2972..) one needs to measure distance traveled by light in meters-- the same quantity which we are trying to define

Class 11 physics meter concept is finally clear as compared to class 9 😎

Since the rate of time changes based on the gravitational field, does this mean that the length of a meter here on Earth is different than the Moon?

ok but why 299,792,458?

Would the length of metre change if the wavelength of light is changed??

this is not he one way speed of light instead its the two way speed of light.. nobody has ever measured the one way speed of light. so we actually dono how long is one meter ACTUALLY is

This is difficult for me to actually see a visual demonstration of this measurement unit , so far everything is leading me to this unit of time that you have to construct . I will only accept some thing that is physically tangible or visible . How and where can I see this done, I'm not quite grasping how I can see this here

The candela is the most enlightening.

Not a good definition. Why would you define the speed of light to be the distance travelled in 1/(24*60^2) of an earth day? Our current time system sucks. Since metric uses base ten, shouldn't we define it using decimal time, 1/(10^5) of an Earth day? And should it even be defined according to an Earth day? If you change to dozenal time (as I think we should), a second would be 1/248832nd of an Earth day, or 1/10^5 in dozenal. But it would still be defined on an Earth day, which is still not good.

so how do we define speed of light .what defined first and how do we define something without know something .we used speed light to define meter what's the first

Thanks

gasdive: Yas and no. c is considered as a universal constant (like Avogrado's number, and Plank constant which are deeply involved in redefining the kg). The meter needs the second; the definition of second could affect the definition of m. That's why many people consider that the real base unit is c (what is ... a speed). For practical reasons, we had to re-define an existing unit. We can not change the world and say "hey, you will stop using the meter, and start using c*t". But from scientific point of view, yes, the basic unit is now c, and m is derived from c. And there is a difference between the technical, and the legal definition. The natural fundamental entity is c; but the legal term will stay m ... even if it technically depends on an other unit. Same mess is about to happen soon for the poor old kg. Wikipedia ... SI_base_unit ... The pciture top right shows that few units are really fundamental: only K, and s at the moment. kg also is, for now, but this will (has to) change very soon. Not speaking about meter (which is your question), A, mol, and cd are already derived from other sources. The relationship between kg and mol and A will probably be reversed; maybe making the mol a new independant root unit, like K and s. You have to understand that the meter is not a subset of the second, because c introduces new concepts that were compeltely absent in the second. m or c can not be replaced by s. They are bonded by the definition of speed formular (V=D/t). Defining only one does not work. You have to define two elements. They are related, by essence. We could as well state about the meter, and c, and then derive time from speed and length (this would be unpractical, and unreliable ... but it could be feasable). Amongst other things, I had, personally, the idea that, if we can define the new kg by counting carbon atoms in a cube, or silicon atoms in a sphere ... we could technically define the meter as being the multiple of the size of a given atom. Such a definition would be a pure length, but would probably be less practical (and less precise), otherwise, the BIPM would have thought about it before me :) We have defined the time according to atoms; and think hard about doing the same for weight; why not length ? (the technical issues would be about the cristal regularity, and variation with temperature). Then c would be a consequence of s and m. Its just better pratical, and more precise (and cheaper) to use s and c. The BIPM does not like the watt-balance ... because it's too expensive, and too hard to duplicate. Takes 10y to build one. By comparison, you can build a good atomic clock within a day, by reusing spare parts from other projects.

Come on man, how the hell am I supposed to find the length of my house with the speed of light?

Since the metre is defined in terms of a period of time in seconds times c, wouldn't that make it a derived unit rather than a base unit?

3:44 how the hell is the speed of light "fixed" when it literally slows down in water?

How can you use 299,792,458 m/s as a calculation of a metre when it literally uses the metre in of itself???

Uh……what?

Sir, could we have a practical plz!!!!

Woah😶

Second

Very disappointed with this grossly incomplete presentation.