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Results tagged with quantum-mechanics
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user 386
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.
3
votes
Is a permutation of coordinates or labels really equivalent?
I suspect that the root of this (quite standard) confusion is that one thinks that wavefunctions are the basic notion of quantum mechanics. While you have to think about Hilbert spaces, vectors, scala …
- 20.2k
8
votes
Accepted
Entangled electron-positron pair
I've already quite a long time ago noticed that in particle physics we usually do stuff that quantum-computing people will call an "entaglement". We just don't phrase it like that, because we are used …
- 20.2k
1
vote
Photon absorption probability for a given molecule in gas phase
Well my first guess that you shouldn't consider naive "cross-sectional area" of your molecule. It would be better If you calculated the cross section of absorption/scattering of photons on your molecu …
- 20.2k
15
votes
What physics does occur at short distances in QED?
Let us take standard fluid dynamics as a model of continuum media physics. It is obvious what is its "short-distance" physics isn't it? These are atoms and molecules. Let's apply your questions to thi …
- 20.2k
4
votes
Unstable energy levels
Is this right? An electron could stay a little time in a unstable energy level?
Short answer is -- yes, this is a correct standard description of the process.
Let me elaborate. In quantum mechan …
- 20.2k
1
vote
When is the "minus sign problem" in quantum simulations an obstacle?
I think that there is a slight misrepresentation of a problem in your question. The fact that quantum-mechanical amplitudes are generally complex numbers is not the root of the problem. It is usually …
- 20.2k
7
votes
Why position is not quantized in quantum mechanics?
So, suppose you have an eigenstate of $\hat{x}$:
$$\hat{x}|\psi\rangle = x|\psi\rangle$$
Now let us act with $\hat{x}$ on $e^{i\hat{p}\delta}|\psi\rangle$, and use this formula (I have $\hbar=1$):
$$\ …
- 20.2k
1
vote
It appears that stationary states aren't so stationary
Imagine a ball, laying still on the ground. While you are looking at it, slowly walking around. The state of the system is staying the same, while the things that you see depends on your aim.
State …
- 20.2k
0
votes
Is the density operator a mathematical convenience or a 'fundamental' aspect of quantum mech...
I'll throw in two "classical" citations from Landau-Lifshitz, book 5, chapter 5:
The averaging by means of the statisitcal matrix ... has a twofold nature.
It comprises both the averaging due …
- 20.2k
7
votes
Calculation of the cross section
We do all the "cross section business" because we want to predict results of experiments.
Let's take for example some particle with two polarizations states: "+" and "-". You know that experimentalist …
- 20.2k
3
votes
Pauli exclusion principle and Entangled pairs
Let's start from the definition of entangled state.
Briefly -- if the state of your system can be described by separately defining the states of its components, then we call the state of this system …
- 20.2k
8
votes
Are these two quantum systems distinguishable?
Let me give some reference that might be useful to make things clear.
It's Landau-Lifshitz, book 5, chapter 5:
The averaging by means of the statisitcal matrix ... has a twofold nature.
It comp …
- 20.2k
10
votes
Accepted
What is the difference between a bit and a qubit?
After several discussions on that topic, I came to think that it is more appropriate to separate those different types of "bits" into four instead of two classes:
A deterministic classical bit is an …
- 20.2k
1
vote
Terminology for "measurable" and "hidden" realms in quantum physics
There are classical and quantum descriptions of the world. One of the differences of quantum description is paying attention to the process of measurement and how it affects the measured system.
Desc …
- 20.2k
5
votes
Accepted
What is the range of validity of Fermi's Golden Rule?
Here:
...the assumption that the time of the measurement is much larger than the time needed for the transition...
- 20.2k