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Results tagged with quantum-mechanics
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user 7924
Quantum mechanics describes the microscopic properties of nature in a regime where classical mechanics no longer applies. It explains phenomena such as the wave-particle duality, quantization of energy, and the uncertainty principle and is generally used in single-body systems. Use the quantum-field-theory tag for the theory of many-body quantum-mechanical systems.
50
votes
Can the photoelectric effect be explained without photons?
Yes, the photoelectric effect can be explained without photons!
One can read it in
L. Mandel and E. Wolf,
Optical Coherence and Quantum Optics,
Cambridge University Press, 1995,
a standard reference f …
- 45.7k
27
votes
Accepted
Difficulties with bra-ket notation
The wording used in your textbook was sloppy.
$A$ acts as $A^*$ on a bra, as $\langle u\rvert A\lvert v\rangle:=\langle u\lvert Av\rangle~$ is the same as $\langle u\rvert A\lvert v\rangle=\langle A …
- 45.7k
26
votes
Accepted
Is the statistical interpretation of Quantum Mechanics dead?
The statistical interpretation of quantum mechanics is alive, healthy, and very robust against attacks.
The statistical interpretation is precisely that part of the foundations of quantum mechanics wh …
- 45.7k
23
votes
Accepted
Is there any theorem that suggests that QM+SR has to be an operator theory?
What you call an operator theory is usually called the Heisenberg picture of quantum mechanics. What you call a wave function theory is usually called the Schrödinger picture of quantum mechanics.
It …
- 45.7k
21
votes
In what sense (if any) is Action a physical observable?
The book propagates a myth.
Experiments measure angular momentum, not action - even though these have the same units. One finds empirically that angular momentum in any particular (unit length) direct …
- 45.7k
21
votes
Intuitive explanation of why momentum is the Fourier transform variable of position?
Momentum is not the Fourier transform of position.
In the position representation, position is the operator of multiplication by $x$, whereas momentum is a multiple of differentiation with respect t …
- 45.7k
20
votes
Path integral vs. measure on infinite dimensional space
In 2-dimensional space-time, Feynman path integrals are perfectly well-defined, though understanding how this is done rigorously is somewhat heavy-going. But everything is spelled out in the book ''Qu …
- 45.7k
19
votes
Does measurement, quantum in particular, always increase the total entropy?
Quantum statistical irreversibility ("the second law") and quantum measurement irreversibility are almost the same thing. Indeed,the latter is the special case of the former where one assumes a more s …
- 45.7k
18
votes
Accepted
Does the canonical commutation relation fix the form of the momentum operator?
No. You can add an arbitrary constant shift (or an arbitrary operator commuting with $x$) without affecting the CCR.
For 1-dimensional QM, the general solution of the CCR with $\hat x$ represented as …
- 45.7k
18
votes
Is the density operator a mathematical convenience or a 'fundamental' aspect of quantum mech...
No macroscopic quantum system is described by a pure state. For example, notions like temperature or pressure, which apply to macroscopic systems do not even exist for systems described by a pure stat …
- 45.7k
18
votes
Accepted
Is it theoretically possible to reach $0$ Kelvin?
By the third law of thermodynamics, a quantum system has temperature absolute zero if and only if its entropy is zero, i.e., if it is in a pure state.
Because of the unavoidable interaction with the …
- 45.7k
17
votes
Why does spin have a discrete spectrum?
The deeper reason is that the components of the spin (angular momentum) vector generate the group of rotations. This group is compact, which means that a rotation perpendicular to an arbitrary directi …
- 45.7k
17
votes
Why would Klein-Gordon describe spin-0 scalar field while Dirac describe spin-1/2?
Spin is a property of the representation of the rotation group $SO(3)$ that describes how a field transforms under a rotation. This can be worked out for each kind of field or field equation.
The Kl …
- 45.7k
17
votes
Accepted
What is the Copenhagen interpretation of quantum field theory?
The Born rule (and hence any discussion of collapse in the sense of the Copenhagen interpretation) is relevant only when an observer has made a distinction between a (tiny, observed) system and its (h …
- 45.7k
17
votes
Is the Mendeleev table explained in quantum mechanics?
While quantum mechanics explains the gross features of the periodic system, many fine details of the periodic table of elements are computable numerically from various approximations to QED, but are c …
- 45.7k