well jfred, if you are interested in this subject, historical developement which led to quantization of em radiation, i can tell you more, so long as you promise me not to bug me with spelling and in the case the date that i give differs by less than 0.1% of the true date. i'll be very brief, but ready to tell more, if interested.
the greeks (many of them) believed that light is corpuscules emitted by eyes, and when reflected by obgects, these corpuscules created visible images. the next set of major advances came in XVII and XVIII centuries, when phenomena of refraction and dispersion were discovered and "explained". huygens believed that light is waves in the ether, which was introdused specifically to support lite waves. his theories explained everithing. on the other hand, newton, who is often credited with the discovery of dispersion, believed that light consisted of corpuscules, and he had his own theories that also explained everything. [just like mory-g-] than in the early XIX century phenomena of diffraction and interference were discovered and were naturally explained within the ether wave theories [there were several variations of them] of light, and the naive corpuscular theories of newton were dead. the further progress (skipping some stuff again) was made by maxwell (and also heaviside and j.j. thompson) who synthesized Maxwell equations (~1865, which include faraday's eqn, ampere's law and poison eq.) and predicted that light is electromagnetic waves, which still propagated in ether. hertz et al verifyed maxwell's eqs. and discovered radio waves. also at that time (1870-80's) infrared and yltraviolet lights were discovered and everything seemed super. however, further experimental studies (in particular by kirchhoff and wien) on blackbody radiation showed a misterious discrepancy. you know what blackbody radiation spectrum loks like (it's a kind of bell-like curve when spectral density of radiation energy is plotted vs. wavelength), so the long wavelength part of the curve, off the peak, was well described by the wien formula, derived using maxwell eqs, thus treating light (light = em radiation) as waves. however, the wien formular diverges (goes to infinity) as wavelength goes to zero, and it predicts an infinite total power emmited integrated over the wavelength, bith in an obvious contradiction with experiment. lord rayleigh (who used to be tompson) derived a different formula based on a statistical mechanics, thus treating light as consisting of particles, and his formula (which had an unknown constant in it) described well the low wavelength part of the experimental spectrum, but diverged at long wavelenghts. plus, everybody, mory included, knew that light IS em waves, so that result could not be taken very seriously. now plank, who in 1887 became THE physics professor in berlin u, inheriting kircchoff's chair, was involved with the german experimentalists who kept measuring the radiation curve, absolute power levels etc better and better, and he tried for some 10 years (mory, he did not work hard, was just having fun-g-) to reconcile the theory and experiment of blackbody radiation. in general, he worked in thermodynamics and stat mech. and, finally, in 1900 had an "idea" of a trick. namely, assuming em waves vere emmited discontiniously by the atoms of the "black body", in particular, in quantities proportional to frequency of radiation (inversly proportional to wavelength), he could derive a formular, based on maxwell's eqs, which described a bell-like curve similar to the experimental ones. he also had a new, unknown constant, which was the constant of proportionality of the amount of energy emitted wave to the frequency of the em wave emitted, E=hf, h is called plank's constant. the experimental curves were actually found to deviate slightly from the plank's formula in the hi-f regin, but subsequent better experiments came outcloser to the formula.
now, there was another concurrent mistery. in 1880's hertz also discovered that when uv light hit a metal, electrons were emitted, the photoelectric effect. subsequently it (the effect) was studied in detail by lenard (who got a nobel prize for that) and others, who found that there were three major misteries, incinsistent with the wave theory of light: (i) the effect had a low frequency cutoff, i.e., light had to have wavelength (L = 1/f) shorter than so much (depending on metal and some other details) for the effect to exist; (ii) the number flux of electrons emitted was proportional to the intensity of light, but (iii) the kinetic energy of emitted electons was proportional to light frequence in excess of that cutoff freq. all these can't be understood, and contradict within wave theory of light. einstein, who was 8 years old when plank became THE berlin professor, came up with the explanation of photoelectric effect in 1905, where he assumed that light was composed of electromagnetic waves quantized in free space according to the plank's idea E = hf. his theory explained the observed phenomena of the photoelectric effect. he also rederived the plank's formula in a better way. he went on to apply quamtum ideas to specific heat of solids (where there was also disagreement with experiment), later elaborated by debye, to stimulated emission of light (in 1917), which is the way lasers work, to quantum statistical mechanics (bose-enstein statistics), and to encourage bohr, born, sommerfeld, de broglie and others to work on quantum phenomena.
there is a conseptual difference between plank's and einstein's view of quantization of em waves. plank denied it, eistein was the one who proposed it. plank's view was that what was quantized was the process of emission or absorbtion of continuous light waves by atoms in solids, that is, that the apparent relation E = hf was a concequence of interaction of light with matter, not intrinsic property of light. even as late as 1920, nominating einstein for nobel prize for photoelectric effect, plank wrote, paraphrasing, "einstein has made great contributions to physics ...blah blah blah... his speculative hypothesis on the quantum nature of light should not be held against him (!) because of his significant solid work elsewhere...blah blah blah..." einstein was the one who "invented" photons.
since einstein has done so much, it is not often appreciated that he was the most important figure in the "old" quantum physics, prior to development of modern (nonrelativistic) quantum mechanics in 1925-1927. that was the reason why all quantum mechanics, like heisenberg, pauli, bohr, born,sommerfeld, jordan, schroedinger, dirac, sought out his approval.
philosophy, i'm not interested in. |
