# Planck units

1. Nov 20, 2004

### StatusX

I was reading about the planck units, and they said that Planck came up with them before his quantum theory of blackbady radiation, and before the constant h had been discovered. They said it's unclear how he found these units. This seems pretty wierd to me. How did he come up with these numbers? Did he know something we still don't?

2. Nov 20, 2004

### Chronos

Plancks determination of the Planck constant was the result of his work on blackbody spectrum. It is empirical, which is to say it is directly measured. All planck units are derived from the Planck constant. See here for a more:
http://physicsweb.org/articles/world/13/12/8

3. Nov 20, 2004

### StatusX

http://en.wikipedia.org/wiki/Natural_units

The above site says:

"At the time he presented the units, quantum mechanics had not been invented. He himself had not yet discovered the theory of black-body radiation (first published December 1900) in which the Planck's Constant made its first appearance and for which Planck was later awarded the Nobel prize. The relevant parts of Planck's 1899 paper leave some confusion as to how he managed to come up with the units of time, length, mass, temperature etc. which today we define using Dirac's Constant and motivate by references to quantum physics before things like $$\hbar$$ and quantum physics were known. "

4. Nov 20, 2004

### Integral

Staff Emeritus
To say that Planck units preceded Planck's constant is to say that the car preceded the wheel. What are your sources for any other ideas about this?

5. Nov 20, 2004

### Chronos

In the 1899 paper, Planck introduced two physical constants, a and b, which he had calculated from experimental data. In 1900, he replaced b with the now better known letter h. He also replaced the 'a', with the letter b [Boltzmann constant]. To correct my earler statement, there are a few planck units that can be derived without knowing the Planck constant.

6. Nov 21, 2004

### StatusX

ok thanks. I don't know why they acted like it was some unsolved mystery.

7. Nov 22, 2004

### Chronos

Errata: the constant 'a' was replaced by the letter 'k' [Boltzmann's constant]. That was bugging me and it was too late to edit the typo.

8. Nov 22, 2004

### tfleming

is there an online version of his original 1899 paper?? i know he (and others) thought his quantum 'leap' was nothing more than a mathematical fudge to solve the analytics and numerics