Ultraviolet catastrophe

The ultraviolet catastrophe, also called the Rayleigh-Jeans catastrophe, was a prediction of late 19th century/early 20th century classical physics that an ideal black body at thermal equilibrium will emit radiation with infinite power.

Solution

Einstein pointed out that the difficulty could be avoided by making use of a hypothesis put forward five years earlier by Max Planck. Planck postulated that electromagnetic energy did not follow the classical description, but could only oscillate or be emitted in discrete packets of energy proportional to the frequency, as given by Planck’s law. This has the effect of reducing the number of possible modes with a given energy at high frequencies in the cavity described above, and thus the average energy at those frequencies by application of the equipartition theorem.

Historical inaccuracies

Many popular histories of physics, as well as a number of physics textbooks, present an incorrect version of the history of the ultraviolet catastrophe. In this version, the “catastrophe” was first noticed by Planck, who developed his formula in response. In fact Planck never concerned himself with this aspect of the problem, because he did not believe that the equipartition theorem was fundamental – his motivation for introducing “quanta” was entirely different. That Planck’s proposal happened to provide a solution for it was realized much later, as stated above.

Though this has been known by historians for many decades, the historically incorrect version persists, in part because Planck’s actual motivations for the proposal of the quantum are complicated and difficult to summarize to a modern audience.

— Wikipedia on Ultraviolet catastrophe

2010.08.14 Saturday ACHK

電磁波

這段改篇自 2010 年 3 月 18 日的對話。

光是電磁波。那樣,什麼是「電磁波」呢? 

很久以前的人以為「電力」和「磁力」是兩樣沒有關係的東西。後來,物理學家發現,它們原來是背後同一樣東西的兩個化身。

然後呢,你要留意,「靜電無磁」。意思是,如果我放一粒電荷在這裡,而它是靜止的話,它只會有電場,而不會產生磁場。另一方面,「靜磁無電」。意思是,如果我放一塊磁石在這裡,而它是靜止的話,它只會有磁埸,而不會產生電場。

暫時的結論是,「靜電無磁;靜磁無電」。

那樣,「電」和「磁」有什麼關係呢?

如果那粒電荷在移動的話,它除了會有原本的電場外,亦會額外產生一個磁場。詳細一點的講法是,如果那粒電荷在移動的話,它原本的電場就會有變化。而變化的電場會產生一個磁場。整個過程簡稱為「電動生磁」。

另一方面,如果那塊磁石在移動的話,它除了會有原本的磁場外,亦會額外產生一個電場。詳細一點的講法是,如果那塊磁石在移動的話,它原本的磁場就會有變化。而變化的磁場會產生一個電場。整個過程簡稱為「磁動生電」。

簡言之,「電動生磁;磁動生電」。

— Me@2010.08.14

2010.08.14 Saturday (c) All rights reserved by ACHK