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parallaxicality
2015-Nov-18, 03:07 AM
What was particle physics's equivalent of the Michelson/Morley experiment, the one that left that dangling thread that, when tugged, unravelled the whole ancient understanding and paved the way for the new way of thinking, ie quantum mechanics?

cjameshuff
2015-Nov-18, 04:19 AM
https://en.wikipedia.org/wiki/Robert_Andrews_Millikan#Photoelectric_effect

Jens
2015-Nov-18, 05:46 AM
I think the main "experiment" was not really an experiment but rather the discovery that the atom was composed of a nucleus with electrons orbiting around it. If you had classical physics, then logically the electrons should spiral into the nucleus and get destroyed. But quantum theory came to the rescue by showing that they could only occupy certain states, and thus could not spiral in.

ShinAce
2015-Nov-18, 06:44 AM
Technically, Planck's radiation law(1900) came before Eintein's paper on the photoelectric effect(1905) and Rutherford's atomic model(1910).

The only problem is that no one really took him seriously. That's why it's hard to compare the birth of quantum mechanics with the birth of special relativity. They just aren't comparable.

But the answer lies in the same experiment performed repeatedly over several thousands of years. Every time someone heated up a material until it was glowing, they were staring at quantum mechanics but never realized it.

Cougar
2015-Nov-18, 12:48 PM
What was particle physics's equivalent of the Michelson/Morley experiment....?

Planck's solution to the ultraviolet catastrophe (https://en.wikipedia.org/wiki/Ultraviolet_catastrophe)?

Ken G
2015-Nov-18, 03:10 PM
I think the main "experiment" was not really an experiment but rather the discovery that the atom was composed of a nucleus with electrons orbiting around it. If you had classical physics, then logically the electrons should spiral into the nucleus and get destroyed. But quantum theory came to the rescue by showing that they could only occupy certain states, and thus could not spiral in.Yes, I think that's true. There was long a debate about whether light was more a particle or more a wave, so experiments on light (such as the Young two-slit experiment) weren't really enough to usher in quantum mechanics (though it's true that Planck did not originally think it was the light that was discrete, rather the system that emitted the light, so he had arrived first at quantum mechanics, but it didn't catch on until atoms got into the act). So the really big idea is that all particles exhibit that duality. There was no "particle" version of the Young experiment until the 1960s, believe it or not, but by then the outcome of the experiment was already well anticipated by quantum mechanics. It had to fit what was already known about the behavior of electrons in atoms, things like Rydberg's formula (in 1888) for the transitions in hydrogen.

So I'd agree that was a different but also common situation-- an experimental result that was already known and thought to be mysterious, later explained by theory. That contrasts to the Michelson/Morely situation of having a theory that gave a clear expectation for an experiment, but the experiment didn't come out as expected. I don't know which is more common, but on reflection, it seems there are examples of both types of discovery.

George
2015-Nov-18, 09:09 PM
I would have guessed the experiment that brought us Einstein's Photo Electric Effect could be on top the list.

Reality Check
2015-Nov-18, 11:48 PM
OMO: The paradigm shifting experiment in particle physics is the Geiger–Marsden experiment (https://en.wikipedia.org/wiki/Geiger–Marsden_experiment) that discovered atomic structure and the need for physics beyond classical physics to explain that structure. As far as quantum mechanics is concerned, I would say there is no one paradigm shifting experiment, rather a series of experiments. Emission lines in hydrogen could be fitted by the Rydberg formula with "quantum" numbers. Thomas Young's double-slit experiment. Discovery of radioactivity (not easily explained using classical physics). Observed black body spectrum could be explained by Planck's law with quantization of energy. Einstein similarly explaining the photoelectric effect. The above Geiger–Marsden experiment.

StupendousMan
2015-Nov-19, 12:02 AM
There was no "particle" version of the Young experiment until the 1960s, believe it or not,

Does the work of Davisson and Germer in 1927 count?

http://spiff.rit.edu/classes/phys314/lectures/e_diffr/e_diffr.html

Ken G
2015-Nov-19, 05:04 AM
Yes, it seems like it should. Perhaps we should just say there wasn't a conventional "two slit" experiment on particles, but a diffraction experiment like you cite, far earlier, is pretty much the same thing-- the wave nature of particle motion.

George
2015-Nov-19, 07:45 PM
Einstein's 1905 paper confirming Planck's quantum ideas was an explanation of Hertz's 1887 experiment that showed a reduction in a the size of a spark across a gap when it was placed inside a glass-paneled box. Although Hertz thought that his experiment was useless, Einstein got the Nobel Price for explaining it based on the quantum ideas of Planck. Milikan determined Planck's constant from his work published in 1914, though he did not think Einstein right even though his work supported Einstein. [I think I have this right.]

[Added: Humble beginnings are often the best. :)]