Questions tagged [time]
Time is defined operationally to be that which is measured by clocks. The SI unit of time is the second, which is defined to be
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Time diffeomorphisms breaking in inflation
I am currently working on the topic of inflation.
It seems that at the stage of inflation, the universe can be described as a de Sitter space. In such a space, all spacetime diffeomorphisms are ...
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Why is my approach to the equation of time off by a constant?
I'm trying to better understand the causes for the equation of time by deriving an approximation from first principles.
My naive approach, $EOT_{NAIVE}$, is to take the difference between the right ...
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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 ...
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Why are the propagators in old-fashioned QED oblique, while in modern QED they are horizontal (or vertical)?
In old-fashioned Quantum Electrodynamics, one can find diagrams such as these (probably Stückelberg was the first to use this notation, a kind of predecessor of Feynman diagrams):
In modern QED this ...
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In the context of condensed matter physics, what does it mean for time to have two dimensions?
In an online article that describes condensed matter physics for laypersons, the author describes various so-called "designer materials" that have exotic properties, including one in which ...
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Does electron have some intrinsic ~$10^{21}$ Hz oscillations (de Broglie's clock/Zitterbewegung)?
Louis De Broglie has postulated in 1924 that with electron's mass there comes some $\approx 10^{21}$Hz inner oscillation: $E=mc^2=h f=\hbar \omega$.
We would get such oscillation e.g. if using $E=mc^...
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What does the time reversibility of the laws of physics mean for causality?
Does the fact that the fundamental laws are symmetric with respect to direction of time show that causation does not exist? Since causality always requires the cause to precede the effect, but laws of ...
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Imaginary Hamiltonian
The Hamiltonian for nuclear spin independent parity violation in atoms is given by: $$H_{PV} = Q_w\frac{G_F}{\sqrt{8}}\gamma_5\rho(r)$$ Here $Q_w$ is the weak charge of the nucleus (which is a scalar),...
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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 ...
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What does the operator's explicit dependence or independence on time actually mean in Quantum mechanics?
Consider the equation of motion for the expectation value of an operator $A$
$$\frac{d\langle A\rangle}{dt} = \frac{1}{i\hbar}\langle [A,H]\rangle + \left \langle \frac{\partial A}{\partial t} \right \...
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Cases of various time symmetries
Is it possible to cook up three physically relevant examples where the Lagrangian has explicit time dependence but the system still has one of the following?
time-reversal invariance,
time ...
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How is time "homogeneous"?
My book$^1$ states:
Let's consider a clock moving freely over a curve such as:
\begin{equation}
\frac{dx^i}{dt}=\text{const} \tag{1.20}
\end{equation}
We define the proper time $\tau$ as the ...
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Equivalence of $d$ dimensional quantum system to $d+1$ dimension stats system
" There are close analogies between quantum field
theories in d dimensions and classical statistical mechanics in d + 1."
What does this statement imply and from where does this extra dimension ...
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Should we consider space and time as separate entity?
In general relativity, we think of space and time in spacetime framework. As some people say, metric tensor sign difference, along with our inability to go backward in time suggests that space and ...
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How to express Allan variance without neglecting clock drift
Allan variance, $\sigma^2[ \tau ]$, or its square root (Allan deviation, $\sigma[ \tau ]$) is a quantity (as function of parameter $\tau$) which is said to be a measure of (or related to) "stability ...
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