# A present for the admin

## Main Question or Discussion Point

I didn't think it was right having having a Physics Forum without a proper h-bar symbol so I made another smilie for you. I should add that it isn't a transparent gif so if you have a proggy to make it one you may want to do that.

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It didn't show up

maybe this time.

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drag

Ah... Someone told me this once but I forgot,
what's h-bar again ?

ahrkron
Staff Emeritus
Gold Member
Planck's constant (usually denoted by h), divided by 2pi (=2 x 3.14159265...).

drag
Oh, right !
Why's it called h-bar then ?

Because it loks like the h for planck's constant, but it has a bar going through it.

marcus
Gold Member
Dearly Missed
Originally posted by drag
Oh, right !
Why's it called h-bar then ?
some strange trivia: Planck derived the value of hbar, which he then denoted by the letter "a", in 1899.
This was BEFORE the appearance of his "h" constant (larger by a factor of 2 pi) in his famous 1900 paper on the black body spectrum.

The 1899 paper contains values of the Planck units (which are based on hbar) approximately equal to those we use now, including the Planck temperature unit.

In other words, in his 1899 paper "On irreversible radiation-processes" (which he presented before the Prussian Academy) he recognized this constant "a" as a fundamental constant of nature and proceeded to base a system of natural units on it---together with the speed of light c, the gravitation constant G, and the temperature coefficient k.

But you will ordinarily find that people date the first appearance of "Planck's constant" 1900 and think of the original constant as "h" which is actually 2 pi times the original "a" or hbar.

hbar is also called the "quantum of angular momentum" or the "quantum of action"

It is a product of energy with time. In metric terms 1.05457 E-34 joule second.

If you think of a photon of light as carrying a certain energy E and having a certain (angular) frequency &omega; then the photon's two key properties are related by E = hbar &omega;

Or if you think of the small characteristic interval of time it takes the photon's vibration to go thru one radian of phase---then this small interval multiplied by the photon's energy is equal to Planck's hbar constant.

It is present as a product of energy and time in every bit of light in the universe as far as we know----but is not confined to light and pervades the rest as well.

How do we make use of Tyger's hbar symbol in messages?

To make use of it

it should be converted to a transparent gif if possible, and made into an smilie. I made it the same height as the other smmilies.