Robert McNees
Happy #QuantumDay to all who celebrate!
Max Planck presented work on blackbody radiation to the German Physical Society #OTD in 1900. His novel “quantum hypothesis” suggested that matter emits and absorbs light with frequency f only in discrete chunks of energy E=hf.
Image: AIP
Robert McNees
Planck’s quantum hypothesis would revolutionize physics, but he initially thought it wasn’t real. He suspected that the interaction of matter and radiation was tremendously complicated but still governed by the physics known at the time — what we now call “classical physics.”
Invoking quanta of radiation to derive the blackbody emission spectrum was, it seemed to Planck, just a mathematical trick that somehow encapsulated all that complication.
Robert McNees
@GuyDudeman Quanta are real! There isn’t some complicated classical process underlying it all. Excitations of the electromagnetic field (photons) carry energy only in discrete chunks.
Guy Dudeman
@mcnees Forgive my ignorance, I am but an art major…. What makes it not a mathematical trick?
Robert McNees
If so, it was a trick that *worked*.
Previous efforts to derive the blackbody emission spectrum using classical physics gave nonsensical results at high frequencies. Replace continuous emission with discrete quanta and things work out perfectly.
Evidence for the reality of quanta began with Einstein’s explanation of the photoelectric effect in 1905. The proposal that radiation of frequency f exists only in discrete chunks of energy E=hf was the first result of Einstein’s “Miracle Year.”
Robert McNees
Many physicists remained skeptical. But Arthur Compton’s 1923 explanation of X-rays scattering off electrons was near-incontrovertible evidence that quanta were real.
Robert McNees
Here's a great APS Landmarks entry where you can read more about Compton Scattering and the reality of photons:
Tim Richardt
@mcnees Is your quantum day every 14th? :) The https://worldquantumday.org/ is Apr 14.
Robert McNees
@tim this is the reduced quantum day
Robert McNees
@tim quantum day-bar
Robert McNees
This sequence of events was a not uncommon narrative in the transformative years of late 19th and early 20th century physics.
An experimental result wouldn't make sense, and a physicist would propose a radical solution. The solution would seem too weird to be true so they'd write it off as a convenient "trick.” Eventually, everyone would realize that it wasn't a trick at all and reality was in fact weirder than they'd expected.
Robert McNees
Planck’s hypothesis was in fact a deep statement about the nature of Nature. Electromagnetic adiation of frequency f comes in discrete chunks of energy E=hf. It's not some artifact of a complicated interaction with matter.
Nowadays we refer to the proportionality constant h as "Planck's constant." In the SI system it takes the value h=6.626 x 10⁻³⁴ kg*m²/s, or 4.136 x 10⁻¹⁵ eV*s.
As far as we can tell, h is a fundamental constant that is hardwired into our Universe.
zheng3_jim
@mcnees Aw yeah, it’s PLANCK TIME, baby!
…and now it’s over. #dadJokes
Robert McNees
The results of Planck’s December 14th lecture were submitted to Annalen der Physik a few weeks later, on January 7th, and published not long after that.
You can see the original (in German) here:
https://onlinelibrary.wiley.com/doi/epdf/10.1002/andp.19013090310
An English translation of Planck’s paper, taken from Dirk ter Haar’s “The Old Quantum Theory,” is available here:
http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Planck%20(1900),%20Distribution%20Law.pdf
Robert McNees
Something that isn’t as widely known is that “h” is not the only fundamental constant that was introduced during Planck’s presentation to the German Physical Society #OTD in 1900!
Planck was investigating emission and absorption by blackbodies, so he needed a way to relate energies and temperatures. Thus, he had to introduce what we now refer to as “Boltzmann’s constant.”
Robert McNees
That’s right, the factor of “k” that appears in the formula “S = k log W” — the formula engraved on Boltzmann’s memorial! — was actually introduced by Max Planck. Boltzmann expressed his results in terms of the ideal gas constant R.
Image: user Daderot on Wikipedia
Robert McNees
The transition from classical to quantum physics went about as smoothly as you would expect. Happy #QuantumDay.
(Sorry, I don’t know who made this classic physics meme. If you know, please tell me!)
Piggleston Pecanpants
@mcnees Didn't this dude die of radiation poisoning?
John Carlos Baez
@mcnees - and I believe "Boltzmann's constant" k was introduced in the same paper by Planck where he introduced what we now call "Planck's constant" - a nice confirmation of the deep unity between statistical mechanics and quantum mechanics.
AK
@mcnees I love this representation of Boltzmann. Please tell me he had anime boss style shoulders in real life.
Robert McNees
@ak Classic Street Fighter villain M. Boltzmann
TheSauce
@mcnees Some Bill the Cat wavicles going on in 1901 Planck.
Robert McNees
@TheSauce Ack! Thppt!
SnoopJ
@mcnees my favorite example of a scientific discovery that required some humility:
"When I began my physical studies and sought advice from my venerable teacher Philipp von Jolly...he portrayed to me physics as a highly developed, almost fully matured science...fairly secured, and theoretical physics approached visibly that degree of perfection which, for example, geometry has had already for centuries."
Lucky thing that he wasn't dissuaded by von Jolly's picture of "dead end" physics!
Joel VanderWerf
@mcnees Planck's constant? Not so much.
David Wakeham
@mcnees great thread, thank you for writing it.
Robert McNees
@joelvanderwerf loll
Robert McNees
Björn Heile
@mcnees Odd title for a novel. (Sorry, I get my coat...)
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