All Questions
42
questions
-3
votes
1
answer
79
views
Probabilistic behavior of quantum mechanics [closed]
In a hypothetical scenario, if I were to measure the quantum spin of an electron and it showed "up," and then I traveled back in time without changing the initial conditions, would measuring ...
- 15
3
votes
3
answers
337
views
Understanding time translations
I suppose this question ultimately boils down to: when we speak of a time translation (in nonrelativistic mechanics, so that the Galilean group is the apporpiate symmetry group under which the physics ...
- 1,095
1
vote
1
answer
380
views
What is the topology of a de Sitter spacetime with multiple timelike dimensions?
(I believe that) de Sitter space is the only maximally symmetric Lorentzian spacetime, and that for $n$ spacetime dimensions, it has the hypercylindrical topology $\mathbb{R} \times S^{n-1}$.
This is ...
- 48.4k
0
votes
1
answer
92
views
Time dependent Quantum Generators
We know a space-translation generator can be written as:
\begin{equation}
T(\textbf{r}_{0})|\alpha\rangle=e^{-i\frac{\textbf{p}\cdot\textbf{r} _{0}}{\hbar}}|\alpha\rangle=|\alpha'\rangle.
\end{...
- 77
1
vote
1
answer
81
views
Quantum Time Crystals
I am not sure I appreciate the implication made by Wilczek here:
I definitely see how the expectation value for $\dot\phi$ becomes zero for an energy eigenstate $\Psi_E$ but I do not see what he is ...
- 713
18
votes
7
answers
2k
views
What makes energy "the" conserved quantity associated with temporal translation symmetry?
This kind of relates to my prior question about the non-triviality of temporal translation symmetry and will use some of the same concepts:
How is energy conservation & Noether's theorem a non-...
- 20.6k
7
votes
0
answers
187
views
Correct statement of Birkhoff's theorem (spherically symmetric does not imply static?)
If I understand correctly, the appropriate statement of Birkhoff's theorem in general relativity is that
The Schwarzschild metric is the unique spherically symmetric vacuum
solution.
(Or we might ...
- 684
0
votes
2
answers
108
views
Intuition for the time/energy connection?
I find the following analogy reasonably intuitive:
translational symmetry : conservation of linear momentum :: rotational symmetry : conservation of angular momentum
In contrast, I find the analogy ...
- 177
1
vote
1
answer
122
views
How do I prove the Euler-Lagrange equations of an isolated system do not depend explicitly on time?
Disclaimer. This is my first question, apologies for any potential standard of the community that I broke or if this question was already asked.
How do I prove the Euler-Lagrange equations of an ...
- 43
4
votes
0
answers
83
views
Time-independent source and quantum field theory
Can anyone explain the fundamental reason of why time-independent sources cannot emit or absorb energy. Does it have to do with time-translation symmetry and Noether's theorem?
I was studying the ...
- 369
1
vote
1
answer
403
views
Time Reparametrization Invariance in GR
I've heard that the time reparametrization invariance in GR can be thought of as a gauge symmetry. Could someone specifically explain how, while also explaining if there is any connection with the ...
- 814
0
votes
1
answer
31
views
Is the EDM of a fundamental particle T-odd?
Is the EDM of a fundamental (or almost) particle (e.g. electron, neutron) T-odd? On wikipedia it says that it is not (check the diagram). But I found some papers (e.g. this review and the references ...
- 643
3
votes
2
answers
644
views
Does energy conservation imply time invariance?
This is similar to this question: Is the converse of Noether's first theorem true: Every conservation law has a symmetry?. However, the answer given there is very technical and general. I am only ...
- 510
2
votes
2
answers
813
views
How to prove time translation invariance of Lagrangian for a free particle?
In my textbook, the author deduce the expression of the lagrangian $L(q_i(t), \dot q_i(t), t )$ of a free particle only using classical physical symmetries where the $q_i(t)$ are independent ...
- 33
1
vote
2
answers
601
views
Expanding universe and Time-translation invariance
My understanding is that there are 5 implicit assumptions when introducing the Lagrangian formulation.
Assumptions:
We appear to know that the principle of stationary action is true for the universe
...
- 143