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Questions tagged [quantum-mechanics]

The branch of physics that relates to the behavior of objects, typically particles, on small scales. Probability is very important in quantum mechanics.

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How did group theory enter quantum mechanics?

How did the physicists in the 1920s become aware of the importance of group theory in quantum mechanics? Was group theory already part of the physics curriculum at that time, perhaps in connection to ...
Felix's user avatar
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Why were 20th Century German scientists so impressive?

German (and Austrian) scientists of the late 19th - early 20th centuries seem to have been the backbone of most of modern physics - namely quantum theory/mechanics. The following are a few predominant ...
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I want to know the tricks to search for and find old academic journals for free

In the research of scientific and mathematical history, efficient skills in searching for and accessing old academic journals (preferably for free) are essential. However, even when using platforms ...
enjin2000's user avatar
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What was different about Planck's quantization of light compared to Einstein's?

In describing black body radiation Planck assumed that the energy that can be absorbed or emitted by charges is quantized, i.e., they can only absorb or emit certain quantities of energy. But it was ...
Quantum spaghettification's user avatar
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What did Schroedinger try to say with the cat thought experiment?

In many books one finds different explanations. Specifically popular seems to be that he "argued against the Copenhagen interpretation". But what did he really intend to communicate? I for ...
manuel459's user avatar
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Why isn't Feynman's path integral taught more widely and earlier in today's academic physics curricula?

Anyone who has studied Feynman's path integral will know that it makes quantum mechanics more like classical mechanics. A student who has learned about the Lagrangian will easily understand the ...
Ooker's user avatar
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17 votes
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Did light bulb companies commission Planck to study black body radiation?

Background When introducing Planck's switch to looking at black-body radiation, a number of sources -- like MinutePhysics, the Economist, random online encyclopaedias and even here on HSM.SE (plus ...
Artem Kaznatcheev's user avatar
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5 answers
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Why did Einstein help in the development of the Quantum theory if he didn't agree with it?

I read the book "A Brief History of Time" By Stephan Hawking. It states that Einstein helped scientists like Pauli etc. in the development of the quantum theory and even shared the Nobel Prize with ...
user3459110's user avatar
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Was Dirac really trying to take the square root of the Klein-Gordon operator?

As a student of physics one will, on several occasions, indubitably hear professors or other physicists (here is an example, from Physics.SE's highest-rep user John Rennie) tell the famous story that ...
Danu's user avatar
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14 votes
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How did quantum mechanics operators come into being?

Now I am starting to learn Quantum Mechanics. In the class I am taught about operators, postulates and all other basic stuff. I understand operators to be +, -, /, etc; but quantum mechanical ...
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How did Planck derive the black body radiation formula without using the Bose statistics?

It is so funny that science never develops as in the textbooks. Bose only introduced his statistics in 1924, so Planck could not possibly have used it to derive the radiation formula in 1900. So how ...
John's user avatar
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Why was Alain Aspect discouraged from doing his now Nobel-winning work?

In an interview following Aspect's winning of the Nobel Prize in Physics, he claims that John Stuart Bell discouraged him from pursuing his now-famous 1982 experiment on quantum entanglement. The ...
Tfovid's user avatar
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1 answer
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Einstein's confusion about Stern-Gerlach

In a historical description of the Stern-Gerlach experiment, [Friedrich 2003] says: Einstein and Paul Ehrenfest, among others, struggled to understand how the atomic magnets could take up definite, ...
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2 answers
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Who coined the term ''Born's rule''?

Who assigned the term ''Born's rule'' to the statement that the measurement of a quantum observable is one of its eigenvalues, with a probability given by the square of the coefficient in the spectral ...
Arnold Neumaier's user avatar
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1 answer
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How did gyromagnetic ratio come up before quantum mechanics, and who introduced it?

I am really curious about who was the first one to define the gyromagnetic ratio $\gamma$ of a body, i.e. the ratio of its magnetic dipole momento to its angular momentum. It is a very important ...
AccidentalFourierTransform's user avatar
11 votes
2 answers
790 views

Did wave optics anticipate quantum mechanics?

I heard in wave optics and electromagnetism that Hamilton could have discovered the Schrödinger equation, or that he was the first man who used the expression $$ \Psi(x)= \exp(i S(x)/\hbar)\,. $$ I ...
Jose Javier Garcia's user avatar
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2 answers
632 views

Who popularized the atom icon (atomic whirl or planetary model)

There is a symbol or icon for an atom that is instantly recognizable and is associated with nuclear physics and with chemistry. A search for "nuclear atom symbol" (on 3/23/2023) shows what ...
Karsten Theis's user avatar
11 votes
1 answer
560 views

Were there serious attempts to model the photoelectric effect classically?

Today we see the photoelectric effect as one of the simplest pieces of empirical evidence that leads to quantum physics. The historical development of the subject, however, seems to have involved much ...
user avatar
11 votes
1 answer
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What new physics was discovered or needed as a result of the Manhattan Project?

I originally asked this question on the Physics StackExchange and was told to migrate it here. I've tightened up the question a bit. I recently got into a discussion with colleagues regarding ...
irritable_phd_syndrome's user avatar
10 votes
4 answers
13k views

Who was the first to say "Shut up and calculate!"?

The best thing I could find on the internet was this apparently forgotten article from 2004: N. David Mermin, Could Feynman have said this?, Physics Today 57 (5), 2004.
user 85795's user avatar
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1 answer
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What equation is Stephen Hawking most noted for?

I am trying to equate the famous Stephen Hawking to some of our other famous scientists and noted that the vast majority have an associated equation with their name. As for example Einstein was the ...
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Who introduced the "dagger"symbol as conjugate transpose in quantum mechanics?

The $\dagger$ symbol is often used in quantum mechanics,and also often in general mathematics to represent the conjugate transpose operation.For Hermitian matrices we can write $$A^\dagger=A$$Who ...
Manas Dogra's user avatar
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1 answer
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Does anyone know the physics student who passed a quantum mechanics oral exam without taking quantum mechanics?

Many years ago I read an interview with a physics professor, where he recounted a funny situation when he was a graduate student at Harvard. When a first year, he was supposed to take the quantum ...
FurryTheorem's user avatar
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1 answer
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What is the origin of the "virtual particle pair" metaphor for vacuum fluctuations?

In any layman level description of vacuum fluctuations in quantum field theory the fluctuations are described as a pair of virtual particles spontaneously appearing then disappearing within some short ...
John Rennie's user avatar
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2 answers
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Why did Einstein oppose quantum uncertainity?

Einstein always believed that everything is certain, and we can calculate everything. That's why he rejected quantum mechanics, due to its factor of uncertainty. But still quantum physics was right. ...
Creepy Creature's user avatar
9 votes
1 answer
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What theories preceded the wave, particles and duality models of light?

Currently, the wave-particle duality model for light is the accepted model. From HyperPhysics: The evidence for the description of light as waves was well established at the turn of the century ...
user avatar
9 votes
1 answer
1k views

What actually led Feynman to the path integral formulation of quantum mechanics?

It is commonly known that Feynman's path integral was inspired by Dirac's observation that the kernel is proportional to $\exp{iS/\hbar}$. It was Feynman, however, who had the idea of expressing the ...
seeker_after_truth's user avatar
9 votes
2 answers
2k views

When was the measurement problem solved?

I have been looking into the measurement problem that arises when considering different interpretations of quantum mechanics. Nowadays it seems to be considered a solved problem (in fact some people ...
Wolpertinger's user avatar
8 votes
1 answer
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Why do many names of technical and scientific subjects end with "ics"?

The names of many technical and scientific subjects, like mathematics, physics, statistics, etc., etc., end with letters "ics". What is meant by this, if anything? Was there any logic behind it or is ...
FAHDI GORSY's user avatar
8 votes
3 answers
1k views

Origin of operators in quantum mechanics

Historically, where did the concept of operators in quantum mechanics come from? How did people first understand that momentum operator should be of the form of $i \hbar \frac{{\rm d}}{{\rm d}x}$? ...
user157588's user avatar
8 votes
2 answers
433 views

Heisenberg's last work on a non-linear generalization of quantum mechanics?

It is claimed here that toward the end of his life Werner Heisenberg worked on a non-linear broadening or generalization of quantum mechanics. What work was that? Was it published? Is it listed in ...
Geremia's user avatar
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8 votes
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Why is the azimuthal quantum number so named?

The name "azimuthal quantum number" is often used for the total orbital angular momentum quantum number $\ell$ in an atom. What is the origin of this name? It makes no sense to me, since the usual ...
Brian's user avatar
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8 votes
2 answers
478 views

Historically how it was discovered that we need fields to describe matter?

This question is from one historical perspective. The question is: how physicists historically found out that one needs quantum fields to describe matter? Being more detailed. Let us consider the ...
user1620696's user avatar
8 votes
1 answer
629 views

Who solved the particle-in-a-box model first?

I got curious who invented the particle-in-a-box model first. It is really simple and intuitive. I was googling to find the original author who suggested it but I only get textbook or webpages as ...
Suzanka's user avatar
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7 votes
5 answers
885 views

Were matrix theory and functional analysis well-known to physicists before the invention of matrix mechanics?

Were matrix theory and analysis well-known to physicists circa 1920-1925? Did physicists make extended use of this theory in that period? The question is related to the discussion in How did ...
Frobenius's user avatar
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7 votes
2 answers
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Which physicist is this quote attributed to?

There is a quote from a 19-20th century scientist that goes (and I am paraphrasing): New scientific theories are never accepted until old scientist die. Who is this cynical quote attributed to, ...
cms's user avatar
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7 votes
1 answer
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What is the original source for Einstein's description of entanglement as "Spooky Action at a Distance"?

Try as I may, I cannot find the original source where Einstein described entanglement as "Spooky Action at the Distance." The work "spooky" is not in the 1935 EPR paper, which was written in English ...
vy32's user avatar
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7 votes
1 answer
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How did Planck calculate the Planck constant?

Having started to learn about quantum behavior, this formula came up: $$E = hf $$ Where $E$ is energy, $h$ is the Planck constant and $f$ is the frequency. My physics teacher suggested an ...
Darth Vader's user avatar
7 votes
1 answer
357 views

What is the origin of the terminology 'spin up/down'?

In my research area one seminal reference is H. Bethe, ''Zur Theorie der Metalle'', Z. Phys. 71 205 (1931), see also the English translation by T. C. Dorlas (2009). On page 206 of the original ...
Jules Lamers's user avatar
7 votes
2 answers
249 views

Early helium spectrum measurements and their challenge for Bohr's quantum mechanics

My understanding is that explaining ortho- and para- helium spectral lines was a key motivation for Heisenberg's new quantum theory. For example, Birthwistle's 1928 "The New Quantum Mechanics&...
David Bailey's user avatar
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7 votes
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What was the first journal to have Feynman Diagrams?

I'm writing a paper on the history of Feynman diagrams and am having trouble finding the first paper published by Feynman which features a calculation done with Feynman Diagrams.
TheStrangeQuark's user avatar
7 votes
1 answer
206 views

Did classical physicists feel ill-at-ease about point charges?

The point charge concept is clearly a very useful mathematical fiction, but it is also problematic from the point of view of "physical intuition". Even a layperson would feel that an explanation is ...
David Holden's user avatar
6 votes
2 answers
546 views

Are there any records that show how Hilbert came to "invent" or "discover" Hilbert spaces?

I think it's fuzzy as to whether or not this question is appropriate to ask on this site. The reason I ask it that the characteristics of Hilbert spaces are very much used in expressing quantum ...
Sedumjoy's user avatar
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6 votes
2 answers
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Did Einstein propose a perpetual motion machine to try to disprove quantum mechanics?

In response to quantum mechanics, so the story goes, Einstein proposed a machine, that, based on the uncertainty principle, was a perpetual motion. This showed that quantum mechanics was at odds with ...
Christopher King's user avatar
6 votes
1 answer
394 views

Where did Einstein propose interpreting the square of amplitude as probability density (Born's rule)?

In Max Born's Nobel lecture, he alludes to Einstein's proposed interpretation of EM wave amplitude (squared) as being the probability density of detecting a photon: Again an idea of Einstein’s gave ...
Dave's user avatar
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6 votes
1 answer
262 views

Does the prototype BB84 quantum cryptography machine still exist?

According to Wikipedia's timelines, quantum computing may have had some inceptions as early as the late 60's, when Stephen Wiesner invented conjugate coding. Around the early 80's, Wiesner's ideas ...
Mark S's user avatar
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6 votes
1 answer
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Was Von Neumann and Birkhoff's original formulation of Quantum Logic related with projective geometry?

I was looking at how did von Neumann and Birkhoff formulate their Quantum Logic formalism back in 1936. To solve some questions, I contacted via email a philosopher who studied this topic. I thought ...
vengaq's user avatar
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6 votes
0 answers
136 views

What technology was used to determine the shape of the blackbody spectrum at the 19th century?

The shape of the blackbody radiation spectrum was known in the 19th century from experimental measurements, and before the theoretical discovery of Planck's law. At those times, how did people manage ...
Solidification's user avatar
5 votes
3 answers
398 views

How did quantum mechanics become well known by (non)-scientific community.

Today it is well known even by the non-scientific community that there is a part of physics that deals with the very small called quantum physics. My question is how this became "common knowledge". I ...
AngusTheMan's user avatar
5 votes
2 answers
1k views

How did Stern or Gerlach, of Stern-Gerlach experiment, create individual silver atoms? How were they accelerated?

How, a century ago, could Stern and/or Gerlach KNOW that they had created single silver atoms? How were they moved, or accelerated?
Kurt Hikes's user avatar