6 ms·

My black body story (it's physics)

133 points by winkywooster 4y ago

deleted 4y ago
[deleted]
awinter-py 4y ago
history of 19th century physics is fascinating; big questions that took a really long time to resolve, often because the right answer defied common sense the guy who in 1901 developed the double-slit experiment, actually a single paper card back then, created a mystery that lasted at least a century, if not until today. (it was the same guy who decoded the rosetta stone, weirdly). In a large way the project of the century was understanding the elements, separating them and measuring their weights. I think constantly about this 1870s essay by an important geochemist asking 'are the elements elementary'[1]. It is nearly poetry (to me at least). the key observations coevolved with the math, so sometimes experiment led (someone put a wire near a compass and the compass moved), other times the math led. (As in the 20th century, when lasers were a numeric phenomenon before anyone thought to build one). Maxwell's math feels to me like it's at the center of it all. A 20th century physicist, Wigner, has an essay about the 'unreasonable effectiveness of math in science'. maxwell's equations predict a speed limit for light, and were largely intact in einstein. (the speed of light was known experimentally way before this from eclipse observations, I think). you could say that maxwell + electron research + planck led naturally to einstein, and there's no reason someone's annus mirabilis couldn't have been 50 years earlier other than that it took this long to think of these ideas. if there's any takeaway from this history it's that big questions, if you sink your teeth into them, lead to big answers 1. https://books.google.com/books?id=CSIDAAAAMBAJ&pg=PA463 https://books.google.com/books?id=CSIDAAAAMBAJ&pg=PA463
- gjm11 4y ago
  <pedant>Thomas Young was one of the people involved in figuring out the Rosetta Stone, but it was J-F Champollion who actually cracked it.</pedant>
  - hilbert42 4y ago
    pedant#2 here. Typo, Thomas Young's work on the 'double slit' was in 1801.
- devoutsalsa 4y ago
  Starting quantum mechanics & decoding hieroglyphics is quite a résumé.
capn_duck 4y ago
Wow, I feel like this article was written for me. I too just recently stumbled upon the significance of Planck's work while reading David Bohm's "Quantum Theory". It was such a pleasing thing and wasn't too difficult to follow at all. It uses classical electrodynamics to get an expression for the energy contained in the box with respect to frequency, which of course disagrees with experiment. Then he supposes that energy could only be transferred in certain chunks, and uses statistical mechanics to show what the new distribution looks like. As you say, the existence of photons wasn't immediately jumped to. All that he deduces is that energy transfer must be quantized, and this is the real insight of quantum mechanics, but it gets lost in the woo-ey clickbaity world of modern pop sci. It was so nice to read the derivation from the point of view of a contemporary 20th century physicist, made me want to start reading a bunch of other 20th century physics papers in their original form.
- hilbert42 4y ago
  David Bohm is an interesting physicist, he often gets 'eggs' chucked at him for his Pilot Wave interpretation of QM but that shows he was a lateral thinker trying to find an explanation for one of physics' most intractable problems. But he was no lightweight either as we've seen with his work on the Aharonov–Bohm effect. That said, I'm ashamed to say I've not had a copy of his book Quantum Theory in my hands. You're one of many I've seen mention it in recent times so I'll have to remedy that. :-) Incidentally, I have read the original Aharonov–Bohm paper. Understanding that physics I reckon is crucial to getting a good handle on EM, potentials, etc.
  - capn_duck 4y ago
    I'd strongly recommend it! I think it's worth the price of admission for the first few chapters alone, although I admit that at a certain point, I was struggling to keep up with all of the mathematical arguments as I am not a trained physicist. But I was able to keep up by hand through all of the thermodynamic, electromagnetic, and statistical mechanical arguments of the black box portion. But the first few chapters are so conversational and grounded in historical context that it's a real joy to read, even if you're not following along on pen and paper through all of the steps. If you know of any other "must reads" of the 20th century like this, please let me know! I love to hear the arguments presented as they were actually discussed at the time, not just what was settled on as the end result a century later.
antognini 4y ago
Physics in the early 20th century is fascinating because by this point physicists had pushed classical mechanics right to the limit of where classical physics could go and were starting to grapple with the contradictions that popped up when they went any further. One of the big problems at the time was the problem of the self-force of electric fields and the structure of the electron. In classical mechanics we approximate an electron as a point charge --- an infinitesimally small source of electric charge. This works for a lot of practical problems, but by the 20th century it was known that this couldn't be the real story because if you integrate up the energy in the electric field around a point source you find that it's infinite! And with Einstein's discovery of relativity, that meant that it would be infinitely massive. To avoid this infinity physicists generally assumed that the electron had to have some finite radius. (Estimates of this size are how we get the notion of the "classical radius of the electron.") But now there was a new problem. If you assume that the electron is a little sphere packed with charge, what keeps it together? The self-repulsion from the electric field will try to make it explode apart. That means there has to be a new force acting like rubber bands to hold the thing together. To make matters worse, if you accelerate an electron, the electromagnetic waves generated by one part of the electron will act on the other part. This will introduce new forces. Moreover, when the electron radiates an electromagnetic wave, that wave will in turn impart a force to the electron, resulting in a "self-force." Calculating these self-forces and internal stresses occupied many of the brightest physicists of the early 1900s, including Poincaré. But the results they got were paradoxical. They imply that due to its self-force an electron can spontaneously accelerate exponentially --- or, even worse, act with retrocausality and start moving before you even touch it! In the end these problems were "resolved" because the classical picture doesn't hold on these small length scales --- you need a quantum mechanical description. But interestingly, the fundamental problems that those physicists were grappling with didn't exactly disappear in the quantum picture, they just got transmuted into new problems in quantum electrodynamics. The problem of the diverging energy of an electric field around a point source has a mirror in the issue around renormalization in QFT.
- hilbert42 4y ago
  That's a nice understandable summary. More often than not it's marred in gobbledygook that's not immediately clarifying.
dsabanin 4y ago
Captivating writing!
ggm 4y ago
I believe it is not uncommon for the pedagogical path to some aspect of physics or maths to follow a route teachers like even if it's a-historical. Maxwell's daemon comes to mind. Fermi is noted in Richard Rhodes abomb history having a huge unconscious drive to test a ludicrous substance, against his plans, uncovering how radioactivity behaved.
qazpot 4y ago
One of the many reason's I like Apostol's Calculus book is that it starts with integration first and then diffrentiation which is how the field developed historically.
- selimthegrim 4y ago
  Apostol has some nice innovations - check out the article “The proof Euclid missed” but volume II shows its age - Putzer’s method is bit old for example.
the-mitr 4y ago
Thomas Kuhn's Black-body theory and the quantum discontinuity, (1894-1912) explores the historical development of this topic in depth. https://press.uchicago.edu/ucp/books/book/chicago/B/bo3622390.html https://press.uchicago.edu/ucp/books/book/chicago/B/bo362239...
francasso 4y ago
If you are interested in the old (and new) quantum theory, with all reasoning and formulas derived in detail, I cannot recommend highly enough M. Longair's two books: "Theorerical concepts in physics" and "Quantum concepts in physics". They are historically based derivations of the main physics theories from Galileo to the Solvay conference of 1927. If you don't know the history of electrodynamics and thermodynamics already I wouldn't skip the first book (which also contains the work of Plank on black body radiation)
Koshkin 4y ago
> as the wavelength increased, the energy of the required to fill the quantized slots (photons) was no longer available Is it wavelength or frequency?
dheerajvs 4y ago
> I was being taught a way to understand the physics, which is fine, but not how the ideas came to life. I've always felt that science should be taught in schools by a process of discovery or historical discoveries.
quickthrower2 4y ago
This is the most interesting thing I have read for a while. Beautifully written. Accessible for lay person. tl;dr: just read it!
phkahler 4y ago
Similarly, when it comes to the "galactic rotation problem" I wonder if people modeling galactic dynamics need to remember that matter is quantized. A galaxy is not a disk. There are other possible mistakes too, but the worst seem to be treating them as a continuous distribution and misapplication of the divergence theorem.
- nh23423fefe 4y ago
  Why would you wonder if experts are making basic mistakes?
lordnacho 4y ago
This is actually how math and science should be taught, with history. Without history explaining what people were thinking, you get a mess of formulas and Eureka moments, conclusions coming out of seemingly nowhere, and very hard to follow.

133 points by winkywooster 4y ago

deleted 4y ago

[deleted]

history of 19th century physics is fascinating; big questions that took a really long time to resolve, often because the right answer defied common sense the guy who in 1901 developed the double-slit experiment, actually a single paper card back then, created a mystery that lasted at least a century, if not until today. (it was the same guy who decoded the rosetta stone, weirdly). In a large way the project of the century was understanding the elements, separating them and measuring their weights. I think constantly about this 1870s essay by an important geochemist asking 'are the elements elementary'[1]. It is nearly poetry (to me at least). the key observations coevolved with the math, so sometimes experiment led (someone put a wire near a compass and the compass moved), other times the math led. (As in the 20th century, when lasers were a numeric phenomenon before anyone thought to build one). Maxwell's math feels to me like it's at the center of it all. A 20th century physicist, Wigner, has an essay about the 'unreasonable effectiveness of math in science'. maxwell's equations predict a speed limit for light, and were largely intact in einstein. (the speed of light was known experimentally way before this from eclipse observations, I think). you could say that maxwell + electron research + planck led naturally to einstein, and there's no reason someone's annus mirabilis couldn't have been 50 years earlier other than that it took this long to think of these ideas. if there's any takeaway from this history it's that big questions, if you sink your teeth into them, lead to big answers 1. https://books.google.com/books?id=CSIDAAAAMBAJ&pg=PA463 https://books.google.com/books?id=CSIDAAAAMBAJ&pg=PA463

gjm11 4y ago
<pedant>Thomas Young was one of the people involved in figuring out the Rosetta Stone, but it was J-F Champollion who actually cracked it.</pedant>
- hilbert42 4y ago
  pedant#2 here. Typo, Thomas Young's work on the 'double slit' was in 1801.
devoutsalsa 4y ago
Starting quantum mechanics & decoding hieroglyphics is quite a résumé.

capn_duck 4y ago

Wow, I feel like this article was written for me. I too just recently stumbled upon the significance of Planck's work while reading David Bohm's "Quantum Theory". It was such a pleasing thing and wasn't too difficult to follow at all. It uses classical electrodynamics to get an expression for the energy contained in the box with respect to frequency, which of course disagrees with experiment. Then he supposes that energy could only be transferred in certain chunks, and uses statistical mechanics to show what the new distribution looks like. As you say, the existence of photons wasn't immediately jumped to. All that he deduces is that energy transfer must be quantized, and this is the real insight of quantum mechanics, but it gets lost in the woo-ey clickbaity world of modern pop sci. It was so nice to read the derivation from the point of view of a contemporary 20th century physicist, made me want to start reading a bunch of other 20th century physics papers in their original form.

hilbert42 4y ago
David Bohm is an interesting physicist, he often gets 'eggs' chucked at him for his Pilot Wave interpretation of QM but that shows he was a lateral thinker trying to find an explanation for one of physics' most intractable problems. But he was no lightweight either as we've seen with his work on the Aharonov–Bohm effect. That said, I'm ashamed to say I've not had a copy of his book Quantum Theory in my hands. You're one of many I've seen mention it in recent times so I'll have to remedy that. :-) Incidentally, I have read the original Aharonov–Bohm paper. Understanding that physics I reckon is crucial to getting a good handle on EM, potentials, etc.
- capn_duck 4y ago
  I'd strongly recommend it! I think it's worth the price of admission for the first few chapters alone, although I admit that at a certain point, I was struggling to keep up with all of the mathematical arguments as I am not a trained physicist. But I was able to keep up by hand through all of the thermodynamic, electromagnetic, and statistical mechanical arguments of the black box portion. But the first few chapters are so conversational and grounded in historical context that it's a real joy to read, even if you're not following along on pen and paper through all of the steps. If you know of any other "must reads" of the 20th century like this, please let me know! I love to hear the arguments presented as they were actually discussed at the time, not just what was settled on as the end result a century later.

antognini 4y ago

Physics in the early 20th century is fascinating because by this point physicists had pushed classical mechanics right to the limit of where classical physics could go and were starting to grapple with the contradictions that popped up when they went any further. One of the big problems at the time was the problem of the self-force of electric fields and the structure of the electron. In classical mechanics we approximate an electron as a point charge --- an infinitesimally small source of electric charge. This works for a lot of practical problems, but by the 20th century it was known that this couldn't be the real story because if you integrate up the energy in the electric field around a point source you find that it's infinite! And with Einstein's discovery of relativity, that meant that it would be infinitely massive. To avoid this infinity physicists generally assumed that the electron had to have some finite radius. (Estimates of this size are how we get the notion of the "classical radius of the electron.") But now there was a new problem. If you assume that the electron is a little sphere packed with charge, what keeps it together? The self-repulsion from the electric field will try to make it explode apart. That means there has to be a new force acting like rubber bands to hold the thing together. To make matters worse, if you accelerate an electron, the electromagnetic waves generated by one part of the electron will act on the other part. This will introduce new forces. Moreover, when the electron radiates an electromagnetic wave, that wave will in turn impart a force to the electron, resulting in a "self-force." Calculating these self-forces and internal stresses occupied many of the brightest physicists of the early 1900s, including Poincaré. But the results they got were paradoxical. They imply that due to its self-force an electron can spontaneously accelerate exponentially --- or, even worse, act with retrocausality and start moving before you even touch it! In the end these problems were "resolved" because the classical picture doesn't hold on these small length scales --- you need a quantum mechanical description. But interestingly, the fundamental problems that those physicists were grappling with didn't exactly disappear in the quantum picture, they just got transmuted into new problems in quantum electrodynamics. The problem of the diverging energy of an electric field around a point source has a mirror in the issue around renormalization in QFT.

hilbert42 4y ago
That's a nice understandable summary. More often than not it's marred in gobbledygook that's not immediately clarifying.

dsabanin 4y ago

Captivating writing!

ggm 4y ago

I believe it is not uncommon for the pedagogical path to some aspect of physics or maths to follow a route teachers like even if it's a-historical. Maxwell's daemon comes to mind. Fermi is noted in Richard Rhodes abomb history having a huge unconscious drive to test a ludicrous substance, against his plans, uncovering how radioactivity behaved.

qazpot 4y ago

One of the many reason's I like Apostol's Calculus book is that it starts with integration first and then diffrentiation which is how the field developed historically.

selimthegrim 4y ago
Apostol has some nice innovations - check out the article “The proof Euclid missed” but volume II shows its age - Putzer’s method is bit old for example.

the-mitr 4y ago

Thomas Kuhn's Black-body theory and the quantum discontinuity, (1894-1912) explores the historical development of this topic in depth. https://press.uchicago.edu/ucp/books/book/chicago/B/bo3622390.html https://press.uchicago.edu/ucp/books/book/chicago/B/bo362239...

francasso 4y ago

If you are interested in the old (and new) quantum theory, with all reasoning and formulas derived in detail, I cannot recommend highly enough M. Longair's two books: "Theorerical concepts in physics" and "Quantum concepts in physics". They are historically based derivations of the main physics theories from Galileo to the Solvay conference of 1927. If you don't know the history of electrodynamics and thermodynamics already I wouldn't skip the first book (which also contains the work of Plank on black body radiation)

Koshkin 4y ago

> as the wavelength increased, the energy of the required to fill the quantized slots (photons) was no longer available Is it wavelength or frequency?

dheerajvs 4y ago

> I was being taught a way to understand the physics, which is fine, but not how the ideas came to life. I've always felt that science should be taught in schools by a process of discovery or historical discoveries.

quickthrower2 4y ago

This is the most interesting thing I have read for a while. Beautifully written. Accessible for lay person. tl;dr: just read it!

phkahler 4y ago

Similarly, when it comes to the "galactic rotation problem" I wonder if people modeling galactic dynamics need to remember that matter is quantized. A galaxy is not a disk. There are other possible mistakes too, but the worst seem to be treating them as a continuous distribution and misapplication of the divergence theorem.

nh23423fefe 4y ago
Why would you wonder if experts are making basic mistakes?

lordnacho 4y ago

This is actually how math and science should be taught, with history. Without history explaining what people were thinking, you get a mess of formulas and Eureka moments, conclusions coming out of seemingly nowhere, and very hard to follow.