What is the blackbody problem and the ultraviolet catastrophe?by kweba Tags: blackbody, catastrophe, ultraviolet 

#1
Jan2814, 10:02 PM

P: 43

Hello! I'm trying to research about the history and development of Quantum theory, especially with regards to Max Planck's energy quanta.
I don't understand what was the problem in classical physics that required Planck to formulate quantum theory/energy quanta, which seemed to have revolutionized physics at the time. I read about the blackbody radiation problem and the ultraviolet catastrophe that Planck had tackled on, but I'm having a hard time understanding it. All I could pick up is about some infinite energy in the body's system when it absorbs all incoming radiation. I don't know if that's even right. Anyway, if somebody could explain it to in layman's terms (if possible, with analogies), as I don't really have a working technical knowledge of physics and mathematics. Thanks you very much! 



#2
Jan2814, 11:35 PM

P: 1,271

According to Wiki
http://en.wikipedia.org/wiki/Ultraviolet_catastrophe Plank did not develop his theory in response. 



#4
Jan2814, 11:52 PM

P: 43

What is the blackbody problem and the ultraviolet catastrophe?So the blackbody problem/UV catastrophe had nothing to do with Planck and the development of "quanta"? But still, my question remains, what was the motivation of Planck behind introducing the concept of "quanta"? Did it stil have someting to do with rising problems in classical physics? I know you said in your quote that Planck's actual motivations are difficult to explain to a lay audience, but maybe you (or somebody out there :D) could give a general idea? Thanks so much, sorry for my persistence. :) 



#5
Jan2914, 02:23 AM

P: 43

Why was he called the "reluctant revolutionary?", just to be sure? 



#6
Jan2914, 02:35 AM

P: 740





#7
Jan2914, 02:54 AM

P: 43

Okay, I think I just found the answer to my own question from this website/blog:
http://www.daviddarling.info/encyclo...y_origins.html As it turned out, Max Planck was really "into" the concept of Entropy and the 2nd law of Thermodynamics, and the Time's arrow (the Universe only going forward in one direction); and that he doubted the existence of atoms (which is pretty ironic considering the concept of "quanta"). To quote the site/blog post: Finally, Planck's "quanta" was not in fact a direct response to the Ultraviolet Catastrophe: 



#8
Jan2914, 03:05 AM

P: 1,271

That is a Wiki quote  their words, not mine.
This should give you an insight and make some sense out of it, contrary to what wiki says. http://www.daviddarling.info/encyclo...y_origins.html a few paragraphs down, EDIT: I see you found the same reference. 



#9
Jan2914, 03:14 AM

P: 43

Because he "really did not give it much thought" except to bring a positive result, and that he didn't see his work at the time as representing any kind of threat to classical mechanics or electrodynamics. That's why some historians question whether Planck really ought to be considered the founder of quantum theory, that it wasn't until years later when the "physical"concept of energy quanta was fully realized, when Albert Einstein published his Nobelwinning work on the Photoelectric effect (where he quantized light as packets called photons). To quote: 



#10
Jan2914, 03:53 AM

P: 43

Planck believed that such a formula (blackbody) might provide the link between irreversibility and the absolute nature of entropy: his scientific holy grail. But what was the "problem" or "idea" about Entropy/2nd law of Thermo, that he intended to "solve" or "achieve"? Our source also explained it in detail: (Note the bold's) But anyway, I still don't understand the problem behind the UV Issue, and how his quantum energy had resolved it. So maybe, if it's okay with you to explain it to me briefly and simply? I'd appreciate it. Thanks very much for posting and your help! 



#11
Jan2914, 05:03 AM

P: 1,271

Loosely described, and baring the actual physics, which I think you can look up Wein, Raleigh, and some of the other terms that you have seen, it became apparent that a radiating body due to the classical investigation of radiation, that a blackbody should emit energy at all wavelengths, and more so at the shorter wavelengths on into an emission of energy on into infinity. Of course, if you take a blackbody, the emission spectrum does not behave that way, but has a peak at a certain frequency, and falls lower at higher and lower wavelengths.
Wiki uses an analogy as a vibrating string, onto of which countless standing waves could be imposed, with each standing wave wavelength, or frequency, if you prefer, as having equal energies. The string would then have limitless energy as the number of standing waves would also be limitless. You could relate it to sound waves also. A musical instrument giving a note, emits that note, and harmonics, the strength of which is less than the previous harmonic (or note). The sound level of the harmonic(s) decays the farther it is from the basic note. That is the reality. For an "ultraviolet catastrophe" of the musical instrument, each harmonic would not be at a lessor strength but of the same level as the basic note. Since there are many harmonics, a musician playing an "ultraviolet catastrophe" musical instrument, would have the sound levels of each harmonic add ( which is not additive in the usual sense but of some formula ), and you and your fellow audience members would not enjoy having your eardrums burst with a high decibel output. For sure there are some analogical errors there, but I hope you get the sense of it. PS. Rereading that, it sounds like someone writing from the 1880's 



#13
Jan2914, 07:09 AM

P: 124

Hehe, I think we once derived both results in a statistical mechanics class, the RayleighJean and Plancks law. I think deriving them will let you understand why one is wrong and the other isn't  or just finding the deriviation in a book with some good explanations, of why they did as they did, and which assumptions they made (and thus which were wrong).




#14
Jan2914, 12:05 PM

P: 1,271

( yes. I looked up wein  grape, wine in german and Yidish ) 


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