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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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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
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2k views

Why Doesn't Einstein Get More Credit for Being the Father of Quantum Mechanics?

I'm not simply referring to the notion that Einstein treated the discrete emission and transference of energy (and matter) as "real" physical phenomena, but rather his major continuous role in the ...
Albert Heisenberg's user avatar
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Einstein's overdetermination theory

In 1923 [1], Einstein proposed an idea for a classical theory that would explain some features of quantum mechanics, via the overdetermination of the EoM, so that only certain configurations would be ...
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Origin of the recurrence relation for Clebsch-Gordan coefficients

The Clebsch-Gordan coefficients $C_{\pm }(J,M)$ arise in quantum mechanics in the problem of addition of angular momentum. They also arise in mathematics in the more theoretical (but related) problem ...
roymend's user avatar
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During the development of QFT was this theory actually used to build any invention like the MRI?

I have always wondered if the equations of quantum field theory were actually ever used in the production of some invention or device other than needed to make predictions about the Standard Model of ...
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What's the early history of the "inner quantum number"?

Pais, in his "Inward Bound", describes the early history of spin. He tells us that Goudsmit and Uhlenbeck interpreted Pauli's "doubled valuedness" as spin, while in turn Pauli re-...
David Schrittesser's user avatar
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148 views

Understanding Bohr's response to a 1930 Einstein thought experiment that challenged the uncertainty principle

Structure of this post I want to better understand how Bohr responded to Einstein's 1930 attempt to demonstrate that quantum mechanics was inherently contradictory under the Copenhagen interpretation....
a_former_scientist's user avatar
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Etymology of 'qubit'; is there any relation to cubits?

Whilst several not-very-authoritative sources e.g. Wikipedia state that the word qubit was derived, partially, as a play on the word cubit (obviously it also stands for 'quantum bit'), is there any ...
Toby Hawkins's user avatar
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162 views

What was Einstein's contribution to the Einstein-Podolsky-Rosen (EPR) paper?

Do we have any insight or document into theoretical development of the EPR papers both in terms of math, ideas and anything related to actual physical problem. I know that Einstein didn't write the ...
user avatar
3 votes
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77 views

What is the first effect demonstrating the importance (or existence) of quantum coherence?

As far as I know, in the old quantum theory, a system can only reside in a stationary state. There was no Hilbert space, and no linear superposition. Or, a system cannot be simultaneously in two ...
wdlang's user avatar
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Did the concept of the Dirac Sea influence the development of semiconductors like the transistor?

Dirac (1928), (http://rspa.royalsocietypublishing.org/content/royprsa/126/801/360.full.pdf) explained the negative-energy solutions of his equations as holes in a sea of electrons. The concept seems ...
highsciguy's user avatar
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Whom did Sommerfeld prefer as student?

On the occasion of Heisenberg's death, Wigner wrote an obituary in April 1976 issue of Physics Today (pp. 86-87): Sommerfeld, an excellent teacher with a wonderful overall knowledge of his discipline,...
hyportnex's user avatar
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Understanding how Stern-Gerlach tests Bohr-Sommerfield Hypothesis

I am trying to get to the bottom of a few things in the Stern-Gerlach experiment. First, on wikipedia, it says The Sommerfeld model predicted that the magnetic moment of an atom measured along an ...
Relativisticcucumber's user avatar
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583 views

What did Einstein say about the Dirac equation?

The wiki article on Dirac quotes Einstein as saying of Dirac I have trouble with Dirac. This balancing on the dizzying path between genius and madness is awful. and I don't understand Dirac at all. ...
Adam Herbst's user avatar
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What are the favorite interpretations of quantum mechanics by the recent Nobel laureates in physics?

The new Nobel laureates in physics have been given the prize because of their contributions to quantum mechanics (QM). Of course, the Nobel prize focusses more on groundbreaking work that has links to ...
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Did John von Neumann make any comments about the Many Worlds Interpretation of Hugh Everett?

I was having a discussion with a physicist about Many Worlds Interpretation and he told me that von Neumann like the idea of having multiple worlds in quantum mechanics. When I asked him about more ...
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The original Dirac equation

In the original 1928 paper (pdf) the Dirac equation appears on page 615 in equation (9) as $$ [p_0+\rho_1\left(\boldsymbol{\sigma},\boldsymbol{p}\right)+\rho_3mc]\psi=0\qquad(1) $$ Using the ...
Trond Saue's user avatar
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On the origin of "sandwiches" in quantum mechanics

The term "sandwich" and the verb "to sandwich" appear pretty common but informally in quantum mechanics. Generally when describing some kind of inner product of the form: $$\langle ...
Mauricio's user avatar
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Background on the Stone-von Neumann theorem

I'm a mathematician. I'm required to give a lecture on the Stone-von Neumann theorem. I already have all the mathematical details figured out, but I wish to make the lecture more interesting by giving ...
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Did Rydberg ever learn of Bohr's quantum-mechanical explanation of his formula?

The Rydberg formula on the wavelengths of a spectral line in chemical elements was first stated empirically in 1888 by Johannes Rydberg. A theoretical explanation of the formula wouldn't arive until ...
Mr. Palomar's user avatar
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Who first noted the connection between Heisenberg's uncertainty principle and the Fourier transform?

Who first noted the connection between Heisenberg's uncertainty principle and the Fourier transform?
Marc's user avatar
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Who introduced the fermionic creation and annihilation operators?

It is strange that this is never mentioned when textbooks introduce the 2nd quantization formalism.
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Is the first paper attempting to apply Hilbert space theory to quantum mechanics in 1926 Germany available for free online?

I would like to view the quantum mechanics paper "Über die Jacobischen Transformationen der Quantenmechanik," published in Germany in 1926 by Fritz Wolfgang London. The reference is Z. Phys. ...
enjin2000's user avatar
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Was Bell’s theorem historically seen as evidence against realism and determinism?

As Tim Maudlin points out in his paper: Early on, Bell’s result was often reported as ruling out determinism, or hidden variables. Nowadays, it is sometimes reported as ruling out, or at least ...
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Origin of Einstein quote "Quantum mechanics: Real Black Magic Calculus"

The description of the 1993 English translation of The Quantum Dice, by Ponomarev and Kurchatov, as well as one of the quotes for chapter 2 of Quantum Computation and Quantum Information, by Nielsen ...
elutionary's user avatar
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73 views

What paper or papers about molecules did Heisenberg "like", and what has Heisenberg published or otherwise commented about it?

In the 2023 film Oppenheimer based on the 2005 biography American Prometheus by Kai Bird and Martin J. Sherwin, there seems to be two references to work by Oppenheimer on molecules: RABI: I caught ...
uhoh's user avatar
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Who first stated the three polarizers experiment in quantum mechanics?

In my experience, Dirac is most often cited as the origin of this thought experiment. However, from what I've read in his Principles of Quantum Mechanics, he never actually introduces the idea with ...
Sam Gallagher's user avatar
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116 views

Why is 'total angular momentum' denoted by the letter $J$ in quantum mechanics?

In quantum mechanics, we say $J$ ('total angular momentum') = $L$ ('orbital angular momentum') + $S$ ('spin angular momentum'). Apparently $S$ is from 'Spin', but why $J$ for the total angular ...
edpidufd's user avatar
1 vote
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163 views

Did Felix Bloch really discover Bloch oscillations?

A 1929 paper by Felix Bloch, Über die Quantenmechanik der Elektronen in Kristallgittern, is widely cited as having predicted the phenomenon of Bloch oscillations: The oscillatory motion of an electron ...
Carlo Beenakker's user avatar
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65 views

Who was the first scientist to give a formula for the probability density function of the position of a photon in the double slit experiment?

The double-slit experiment shows the fundamentally probabilistic nature of quantum mechanical phenomena. On Wikipedia one can read: This type of experiment was first performed, using light, by Thomas ...
AdVen's user avatar
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Why are pilot waves in quantum mechanics said to be Faraday waves?

De Broglie proposed a pilot wave in connection with quantum mechanics. He was more or less forced to abandon this wave in favor of the Copenhagen view. Bohm furthered his approach in the fifties but ...
Deschele Schilder's user avatar
1 vote
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56 views

What is the origin of the probability interpretation of quantum mechanics?

Born came up with his probability interpretation of quantum mechanics. But why did he think about such an interoretation? Why not thinking that a deterministic process has to lay at its foundation? ...
Deschele Schilder's user avatar
1 vote
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32 views

Dirac's non-neglibile disturbances argument and epistemic quantum mechanics

Here is Dirac's argument about one reason to motivate quantum mechanics: "It is usually assumed that, by being careful, we may cut down the disturbance accompanying our observation to any desired ...
J Kusin's user avatar
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129 views

How did Bohr guess the quantization rule?

We all know Bohr's quantization rule as $L = \frac{nh}{2\pi}$. How did Bohr derive this? If it is a guess, what is the reason behind choosing $\frac{h}{2\pi}$ and not some other constant? Why did he ...
J Arun Mani's user avatar
1 vote
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185 views

Did Werner Heisenberg ever agree or propose the existence of some kind of multiverse?

I was watching a video (https://www.youtube.com/watch?v=muJYTeQlvC4) where the director of a videogame company speaks about one of its most successful games. This game is set in a floating city which ...
Maribel's user avatar
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What is the history and motivation for the (d-1,1) notation used to describe a field theory?

Very often in the literature of research papers and other articles, and maybe text books, on topics of quantum field theory, a theory may be described as a 3+1 or 0+1, or maybe even 1+1 theory. I ...
K7PEH's user avatar
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A brief history of "delocalization" of electrons

I have been studying the concepts of "resonance" and "mesomerism" recently and a common principle of these concepts is the "delocalization" (of electrons, molecular ...
Bhavya Jain's user avatar
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41 views

Who was the first to make a Stern-Gerlach experiment with two magnets?

I'm trying to fill a conceptual gap I have in the history of physics In 1922 Stern and Gerlach make their experiment, proving that electrons have intrinsic angular momentum, however it takes a while ...
HighlyEntropicMind's user avatar
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142 views

Did Heisenberg say free will could arise from quantum probabilistic mechanics?

I see this view attributed to him a lot during Twitter debates but I never found the source for it does anyone know if Heisenberg actually held this view/suggested it?
Hisham's user avatar
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question regarding Stern-Gerlach experiment

When Stern-Gerlach experiment was done with quantum electron , two peaks were observed rather than a continuous distribution as in case of small magnets , but how did those peaks confirm that angular ...
Astitva Roy's user avatar
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99 views

According to Heisenberg, is quantum theory merely a calculation procedure for predictions?

Is quantum theory just a mathematical tool for correct predictions, according to Heisenberg's views? What are his personal ontological commitments in relation to quantum theory (if he had any ...
Pavel Borisov's user avatar
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70 views

On implications of Schrodingers Cat regarding macroscopic quantum states and decoherence

How exactly did Schrodingers Cat lead to development regarding macroscopic quantum states and decoherence? One often hears that the thought experiment was the initiator to the question, whether ...
manuel459's user avatar
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Was Landau interested in the interpretations of quantum mechanics?

I am interested in Lev Landau. I was trying to find what was his approach to the interpretation of quantum mechanics, I am unable to find any particular source were this is discussed. Was he an ...
Mauricio's user avatar
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Did the mathematician Garrett Birkhoff ever work with or mention Feynman's path integrals?

Did Garrett Birkhoff ever work with Feynman's path integral? Did he ever work in his Many-Histories interpretation? Or at least, did he mention it in any of his articles?
vengaq's user avatar
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Did Einstein or the scientific community know about quantum tunneling during the EPR debates?

It is my understanding that Einstein produced the EPR paper to necessarily elucidate the existence of physical properties that could not exist prior to measurement. That was his assumption which was ...
Weezy's user avatar
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0 answers
305 views

What did Hans Bethe think of von Neumann's quantum logic?

Nobel laureate Hans Bethe was a friend of mathematician-physicist John von Neumann, and he once said: "I have sometimes wondered whether a brain like von Neumann's does not indicate a species ...
stifenahokinga's user avatar
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71 views

Which is the physical interpretation of the "strange" constant $e \cdot c/ 4 \pi$?

This constant is assignable to charged leptons $(e, \mu, \tau)$ representing the fraction of their individual magneton $\hat{m}= e \cdot \hbar/ 2 m$ to their Compton-wavelength $\lambda_{C} = h / m ...
Realist753's user avatar
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0 answers
111 views

Max Planck and energy quantization idea

Did Planck have an intuition behind the idea of energy quantization of atomic oscillators and radiation, or was it just a mathematical trick to derive his distribution law?
Karim mohie's user avatar
0 votes
0 answers
106 views

Why is the angular momentum written as JJ in quantum mechanics?

Why is $\textbf{J}$ called angular momentum operator? Can anyone explain why the expectation value of J is angular momentum? Here is how $J$ is defined: The rotation operator $$ U(\alpha)=\exp(-i {\...
user avatar
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Planck's quantization idea

First of All, I doubt there has ever been any new idea that did not involve intuition. However, most textbooks suggest that the quantization idea was just a mathematical trick, that Planck introduced ...
Karim mohie's user avatar