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M.~Planck, Verhandl. Dtsch. phys. Ges., {\bf 2,} 237 \hfill {\large \bf 1900}\\
\vspace{2cm}
\begin{center}
{\bf On the Theory of the Energy Distribution Law of the
Normal Spectrum}\\
\end{center}
\begin{center}
M.~Planck\\
Berlin\\
(Received 1900)
\end{center}
\centerline{--- ---~~~$\diamond~\diamondsuit~\diamond$~~~--- ---}
\noindent
English translation from ``The Old Quantum Theory,''
ed. by D. ter Haar, Pergamon Press, 1967, p. 82.
\centerline{--- ---~~~$\diamond~\diamondsuit~\diamond$~~~--- ---}
\vspace{1cm}
GENTLEMEN: when some weeks ago I had the honour to draw your attention to a new
formula which seemed to me to be suited to express the law of the distribution
of radiation energy over the whole range of the normal
spectrum \cite{Planck1900}, I mentioned
already then that in my opinion the usefulness of this equation was not based
only on the apparently close agreement of the few numbers, which I could then
communicate, with the available experimental data,\footnote{Verh. Dtsch. Phys.
Ges. Berlin 2, 237 (1900)} but mainly on the simple structure of the formula
and especially on the fact that it gave a very simple logarithmic expression for
the dependence of the entropy of an irradiated monochromatic vibrating resonator
on its vibrational energy. This formula seemed to promise in any case the
possibility of a general interpretation much rather than other equations which
have been proposed, apart from Wien's formula which, however, was not confirmed
by experiment.
Entropy means disorder, and I thought that one should find this disorder in the
irregularity with which even in a completely stationary radiation field the
vibrations of the resonator change their amplitude and phase, as long as
considers time intervals long compared to the period of one vibration, but short
compared to the duration of a measurement. The constant energy of the stationary
vibrating resonator can thus only be considered to be a time average, or, put
differently, to be an instantaneous average of the energies of a large number of
identical resonators which are in the same stationary radiation field, but far
enough from one another not to influence each other. Since the entropy of a
resonator is thus determined by the way in which the energy is distributed at
one time over many resonators, I suspected that one should evaluate this
quantity in the electromagnetic radiation theory by introducing probability
considerations, the importance of which for the second law of thermodynamics was
first of all discovered by
Mr. Boltzmann\cite{Boltzmann77}.
This suspicion has been confirmed;
I have been able to derive deductively an expression for the entropy of a
monochromatically vibrating resonator and thus for the energy distribution in a
stationary radiation state, that is, in the normal spectrum. To do this it was
only necessary to extend somewhat the interpretation of the
hypothesis of ``natural radiation'' which is introduced
in electromagnetic theory. Apart from
this I have obtained other relations which seem to me to be
of considerable
importance for other branches of physics and also of chemistry.
I do not wish to give today this \mbox{deduction~--~which} is based on the laws of
electromagnetic radiation, thermodynamics and probability
\mbox{calculus~--~systematically} in all details, but rather to explain as clearly as possible the
real core of the theory. This can be done most easily by describing to you a
new, completely elementary treatment through which one can \mbox{evaluate~--~without}
knowing anything about a spectral formula or about any \mbox{theory~--~the}
distribution of a given amount of energy over the different colours of the
normal spectrum using one constant of nature only and after that the value of
the temperature of this energy radiation using a second constant of nature. You
will find many points in the treatment to be presented arbitrary and
complicated, but as I said a moment ago I do not want to pay attention to a
proof of the necessity and the simple, practical details, but to the clarity and
uniqueness of the given prescriptions for the solution of the problem.
Let us consider a large number of monochromatically vibrating
\mbox{resonator -- $N$} of
frequency $\nu$ (per second), $N^{\prime}$ of frequency $\nu^{\prime}$,
$N^{\prime \prime}$ of frequency \mbox{$\nu^{\prime \prime}$, ...,} with all $N$ large
number -- which are at large distances apart and are enclosed in a diathermic
medium with light velocity c and bounded by reflecting walls. Let the system
contain a certain amount of energy, the total energy $E_t$ (erg) which is
present partly in the medium as travelling radiation and partly in the
resonators as vibrational energy. The question is how in a stationary state this
energy is distributed over the vibrations of the resonator and over the various
of the radiation present in the medium, and what will be the temperature of the
total system.
To answer this question we first of all consider the vebrations of the
resonators and assign to them arbitrary definite energies, for instance, an
energy $E$ to the $N$ resonators $\nu$, $E^{\prime}$ to the
$N^{\prime}$ resonators $\nu^{\prime}, \ldots$ . The sum
$$
E + E^{\prime} + E^{\prime \prime} + \ldots = E_0
$$
must, of course, be less than $E_t$. The remainer $E_t - E_0$ pertains then to
the radiation present in the medium. We must now give the distribution of the
energy over the separate resonators of each group, first of all the distribution
of the energy $E$ over the $N$ resonators of frequency $\nu$.
If $E$ considered to be
continuously divisible quantity, this distribution is possible in infinitely
many ways. We consider, however -- this is the most essential point of the whole
calculation -- $E$ to be composed of a very definite number of equal parts and
use thereto the constant of nature $h = 6.55 \times 10^{-27}$
erg $\cdot$ sec.
This constant multiplied by the common frequency $\nu$ of the resonators gives
us the energy element $\varepsilon$ in erg, and dividing $E$
by $\varepsilon$ we get the number $P$ of energy elements which must be
divided over the $N$ resonators.
If the ratio is not an integer,
we take for $P$ an integer in the neighbourhood.
It is clear that the distribution of $P$ energy elements over $N$ resonators can
only take place in a finite, well--defined number of ways. Each of these ways of
distribution we call a ''complexion'', using an expression introduced by Mr.
Boltzmann for a similar quantity. If we denote the resonators by the numbers 1,
2, 3, $\ldots$, $N$, and write these in a row, and if we
under each resonator put the
number of its energy elements, we get for each complexion a symbol of the
following form
\vspace{0.1cm}
\begin{center}
\begin{tabular}{rrrrrrrrrr}
1~~~&2~~~&3~~~&4~~~&5~~~&6~~~&7~~~&8~~~&9~~~&10\\
\hline
7~~~&38~~~&11~~~&0~~~&9~~~&2~~~&20~~~&4~~~&4~~~&5\\
\end{tabular}
\end{center}
\vspace{0.2cm}
We have taken here $N = 10$, ~~ $P = 100$. The number of all possible
complexions is
clearly equal to the number of all possible sets of number which one can obtain
for lower sequence for given $N$ and $P$. To exclude all misunderstandings, we
remark that two complexions must be considered to be different if the
corresponding sequences contain the same numbers, but in different order. From
the theory of permutations we get for the number of all possible complexions
$$
\frac{N(N + 1) \cdot (N + 2) \ldots (N + P - 1)}{1 \cdot 2 \cdot 3 \ldots P}
= \frac{(N + P - 1) !}{(N - 1) ! P !}
$$
or to a sufficient approximations,
$$
= \frac{(N + P)^{N + P}}{N^N P^P}.
$$
We perform the same calculation for the resonators of the other groups, by
determining for each group of resonators the number of possible complexions for
the energy given to the group. The multiplication of all numbers obtained in
this way gives us then the total number $R$ of all possible complexions for the
arbitrary assigned energy distribution over all resonators.
In the same way any other arbitrarily chosen energy distribution\\
$E, E', E'' \ldots$ will correspond to a definite
number $R$ of all possible
complexions which is evaluated in the above manner. Among all energy
distributions which are possible for a constant
$E_0 = E + E^{\prime} + E^{\prime \prime} + \ldots $ there is one
well--defined one
for which the number of possible complexions $R_0$ is larger than for any other
distribution. We look for this distribution, if necessary by trial, since this
will just be the distribution taken up by the resonators in the stationary
radiation field, if they together possess the energy $E_0$. This quantities $E,
E^{\prime}, E^{\prime \prime}, \ldots $ can then be expressed in terms of $E_0$.
Dividing $E$ by $N, E^{\prime}$ by $N^{\prime}, \ldots$ we obtain the stationary
value of the energy
$U_{\nu}, U^{\prime}_{\nu^{\prime}}, U^{\prime \prime}_{\nu^{\prime \prime}}
\ldots$ of
a single resonator of each group, and thus also the
spatial density of the corresponding radiation energy in a diathermic medium in
the spectral range $\nu$ to $\nu + d\nu$,
$$
u_{\nu} d\nu = \frac{8 \pi \nu^2}{c^3} \cdot U_{\nu} d \nu,
$$
so that the energy of the medium is also determined.
Of all quantities which occur only $E_0$ seems now still to be arbitrary. One
sees easily, however, how one can finally evaluate $E_0$ from the total energy
$E_t$, since if the chosen value of $E_0$ leads, for instance, to too large a
value of $E_t$, we must decrease it, and the other way round.
After the stationary energy distribution is thus determined using
a constant $h$,
we can find the corresponding temperature $\vartheta$ in degrees
absolute\footnote{ The original states
``degrees centigrade'' which is clearly a slip [D.
t. H.]} using a second constant of nature
$k = 1.346 \times 10^{-6}$
erg $\cdot$ degree$^{-1}$ through the equation
$$
\frac{1}{\vartheta} = k~ \frac{d~ ln ~R_0}{dE_0}.
$$
The product $k ~\mbox{ln}\cdot R_0$ is the entropy of the system of
resonators; it is the sum
of the entropy of all separate resonators.
It would, to be sure, be very complicated to perform explicity the above--
mentioned calculations, although it would not be without some interest to test
the truth of the attainable degree of approximation in a simple case. A more
general calculation which is performed very simply, using the above
prescriptions shows much more directly that the normal energy distribution
determined in this way for a medium containing radiation is given by expression
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
$$
u_{\nu} d \nu = \frac{8 \pi \nu^3}{c^3} \frac{d \nu}{e^{h \nu / k \vartheta} -
1}
$$
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
which corresponds exactly to the spectral formula which I give earlier
$$
E_{\lambda} d \lambda = \frac{c_1 \lambda^{-5}}
{e^{c_2/\lambda \vartheta} -1} d \lambda.
$$
The formal differences are due to the differences in the definitions of
$u_{\nu}$ and $E_{\lambda}$. The first equation is somewhat more general inasfar
as it is valid for arbitrary diathermic medium with light velosity $c$. The
numerical values of $h$ and $k$ which I mentioned were calculated
from that equation
using the measurements by $F$. Kurlbaum and by O. Lummer and
E. Pringsheim.\footnote{F. Kurlbaum \cite{Kurlbaum1898} gives
$S_{100} - S_0 = 0.0731$ Watt cm$^{-2}$,
while O.
Lummer and E. Pringsheim \cite{Lummer1900}
give $\lambda_m \vartheta = 2940 \mu \cdot \mbox{degree}$.}
I shall now make a few short remarks about the question of the necessity of the
above given deduction. The fact that the chosen energy element $\varepsilon$ for
a given group of resonators must be proportional to the frequency $\nu$ follows
immediately from the extremely important Wien displacement law. The relation
between $u$ and $U$ is one of the basic
equations of the electromagnetic theory of
radiation. Apart from that, the whole deduction is based upon the theorem that
the entropy of a system of resonators with given energy is proportional to the
logarithm of the total number of possible complexions for the given energy. This
theorem can be split into two other theorems: (1) The entropy of the system in a
given state is proportional to the logarithm of the probability of that state,
and (2) The probability of any state is proportional to the number of
corresponding complexions, or, in other words, any definite complexion is
equally probable as any other complexion. The first theorem is, as for as
radiative phenomena are concerned, just a definition of the probability of the
state, insofar as we have for energy radiation no other a priori way to define
the probability that the definition of its entropy. We have here a distinction
from the corresponding situation in the kinetic theory of gases. The second
theorem is the core of the whole of the theory presented here: in the last
resort its proof can only be given empirically. It can also be understood as a
more detailed definition of the hypothesis of natural radiation which I have
introduced. This hypothesis I have expressed
before \cite{Planck00} only in the form that
the energy of the radiation is completely ''randomly'' distributed over the
various partial vibrations present in the
radiation.\footnote{When Mr. Wien in
his Paris report about the theoretical radiation laws did not
find my theory on
the irreversible radiation phenomena satisfactory since
it did not give the
proof that the hypothesis of natural radiation is
the only one which leads to
irreversibility, he surely demanded, in my
opinion, too much of this hypothesis.
If one could prove the hypothesis, it would no longer be a hypothesis, and one
did not have to formulate it. However, one could then not derive anything new
from it. From the same point of view one should also declare the kinetic theory
of gases to be unsatisfactory since nobody has yet proved that the atomistic
hypothesis is the only which explains irreversibility. A similar objection could
with more or less justice be raised against all inductively
obtained theories.} I
plan to communicate elsewhere in detail the considerations, which have only been
sketched here, with all calculations and with a survey of the development of the
theory up to the present.
To conclude I may point to an important consequence of this theory which at the
same time makes possible a further test of its reliability.
Mr. Boltzmann \cite{Boltzmann1877}
has shown that the entropy of a monatomic gas in equilibrium is equal
to
$\omega R \ln P_0$, where $P_0$ is the number of possible
complexions (the ``permutability'') corresponding to the most
probable velocity distribution, $R$
being the well known gas constant
($8.31 \times 10^7$ for O = 16), $\omega$ the ratio of the mass of a real
molecule to the mass of a mole, which is
the same for all substances. If there are any radiating resonators present in
the gas, the entropy of the total system must according to the theory developed
here be proportional to the logarithm of the number of all possible complexions,
including both velocities and radiation. Since according to the electromagnetic
theory of the radiation the velocities of the atoms are completely independent
of the distribution of the radiation energy, the total number of complexions is
simply equal to the product of the number relating to the velocities and the
number relating to the radiation. For the total entropy we have thus
$$
f ~\mbox{ln}~(P_0 R_0) = f ~\mbox{ln}~ P_0 + f~ \mbox{ln}~ R_0,
$$
where $f$ is a factor of proportionality. Comparing this with the earlier
expressions we find
$$
f = \omega R = k,
$$
or
$$
\omega = \frac{k}{R} = 1.62 \times 10^{-24},
$$
that is, a real molecule is $1.62 \times 10^{-24}$ of a mole or, a
hydrogen atom weighs $1.64 \times 10^{-24}$ g, since H = 1.01, or,
in a mole of any substance there are $1/ \omega = 6.175 \times 10^{23}$
real molecules. Mr. O.E Mayer \cite{Mayer1899}
gives for this number $640 \times 10^{21}$ which agrees closely.
Loschmidt's number $L$, that is, the number of gas molecules in 1 cm$^3$ at
$0^{\circ}$ C and 1 atm is
$$
L = \frac{1~ 013~ 200}{R \cdot 273 \cdot \omega} = 2.76 \times 10^{19}.
$$
Mr. Drude \cite{Drude1900} finds $L = 2.1 \times 10^{19}.$
The Boltzmann-Drude constant $\alpha$, that is, the average kinetic energy of
an atom at the absolute temperature 1 is
$$
\alpha = \frac{3}{2}~ \omega R = \frac{3}{2}~ k = 2.02 \times 10^{-16}.
$$
Mr. Drude \cite{Drude1900} finds $\alpha = 2.65 \times 10^{-16}$.
The elementary quantum of electricity $e$, that is, the electrical charge of a
positive monovalent ion or of an electron is, if $\varepsilon$ is the known
charge of a monovalent mole,
$$
e = \varepsilon \omega = 4.69 \times 10^{-10} \mbox{c.s.u.}
$$
Mr. F. Richarz \cite{Richarz1894} finds $1.29 \times 10^{-10}$
and Mr. Thomson \cite{Thomson1898}
recently $6.5 \times 10^{-10}$.
If the theory is at all correct, all these relations should be not
approximately, but absolutely, valid. The accuracy of the calculated number is
thus essentially the same as that of the relatively worst known, the radiation
constant k, and is thus much better than all determinations up to now. To test
it by more direct methods should be both an important and a necessary task for
further research.
\vspace{0.5cm}
\begin{thebibliography}{99}
\bibitem{Planck1900}
M. Planck, Verh. D. Physik. Ges. Berlin 2, 202 (1900)
(reprintedas Paper 1 on p. 79 in the present volume).
\bibitem{Rubens1900}
H. Rubens and F. Kurlbaum, S.B. Preuss. Akad. Wiss. p. 929 (
1900);
\bibitem{Boltzmann77}
L. Boltzmann, S.B. Akad. Wess. Wien 76, 373 (1877);
\bibitem{Kurlbaum1898}
F. Kurlbaum, Ann. Physik 65, 759 (1898);
\bibitem{Lummer1900}
O. Lummer and E. Pringsheim, Verh. D. Physik. Ges. Berlin
2, 176 (1900);
\bibitem{Planck00}
M. Planck, Ann. Physik 1, 73 (1900);
\bibitem{Boltzmann1877}
L. Boltzmann, S.B. Akad. Wiss. Wien 76, 428 (1877);
\bibitem{Mayer1899}
O.E. Mayer, Die Kinetische Theotie der Gase, 2nd ed., p. 337 (
1899);
\bibitem{Drude1900}
P. Drude, Ann. Physik 1, 578 (1900);
\bibitem{Richarz1894}
F. Richarz, Ann. Physik 52, 397 (1894);
\bibitem{Thomson1898}
J.J. Thomson, Phil. Mag. 46, 528 (1898).
\end{thebibliography}
\end{document}
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