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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598 views

Does time emerge from entanglement?

I stumbled upon the hypothesis that time emerges from quantum mechanics as long as the observer is part of the system. That is, the evolution of an entangled state can serve as a clock for an observer ...
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437 views

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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162 views

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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1answer
360 views

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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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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704 views

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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175 views

Is time ordering defined for a single operator depending of two time variables?

The time ordering for the purpose of quantum mechanics is e.g. given by $${\mathcal T} \left[A(x) B(y)\right] := \begin{matrix} A(x) B(y) & \textrm{ if } & x_0 > y_0 \\ \pm B(y)A(x) & \...
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Is the Universe 11 billion years old?

This clip claims scientists found a star 200M years older than the Universe. However, I took another assertion more seriously: scientists estimated a faster expansion rate of the Universe, driving age ...
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76 views

Could two atomic clocks really be used to detect gravitational waves from a distant source? If so, how?

Three articles report on the recent paper in Phys Rev. D: Flanagan, Éanna É. et al. 2019 Persistent gravitational wave observables: General framework (also ArXiv): Phys.org: Gravitational waves leave ...
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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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42 views

Is there any feature which distinguishes the Hamiltonian in the Poincare algebra?

The Poincare algebra is defined as \begin{align*} i[J^{\mu\nu},J^{\rho\sigma}]&=\eta^{\nu\rho}J^{\mu\sigma}-\eta^{\mu\rho}J^{\nu\sigma}+\eta^{\sigma\nu}J^{\rho\mu}-\eta^{\sigma\mu}J^{\rho\nu}\\ ...
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166 views

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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56 views

Is there a scheme for secure physical time-stamping?

Is there a physical phenomenon that could be used to record digital information in such a way that it has the following properties: it relies on some immutable physical law and does not rely on any ...
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123 views

Why does the law of conservation of energy not hold true when the work-function $U$ depends explicitly on $t$?

[...] the infinitesimal work $\overline{\mathrm dw}$ comes out as a linear differential form of the variables $q_i$: $$\overline{\mathrm dw}= F_1~\mathrm dq_1 +F_2~\mathrm dq_2+ \ldots + F_n~\mathrm ...
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146 views

What is the shortest controllable time?

I was reading http://en.wikipedia.org/wiki/Planck_time and noticed a reference to: http://phys.org/news192909576.html where it is stated that: 12 attoseconds is the world record for shortest ...
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61 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 ...
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1answer
39 views

Time dilation in 2+ dimensions. Relativity problem

I am a student and due to school closures, I am reading ahead in physics. I have been learning about special relativity and I made up a problem for myself. I've drawn a diagram below: The diagram ...
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Nicholas Gisin's papers about Time in Physics

I recently read the article from QuantaMagazine which says Over the past year, the Swiss physicist Nicolas Gisin has published four papers that attempt to dispel the fog surrounding time in ...
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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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What was the rationale behind Stephen Hawking's “biggest mistake”?

Albert Einstein said he made his biggest blunder, while Stephen Hawking spoke of his "biggest mistake". One can read this in this interesting book review (The Nature of Space and Time by Stephen ...
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1answer
60 views

Age of universe

It's said that at the moment of the Big Bang, space expanded so that to an objective observer matter moved at thousands of times the speed of light. If so, how can we say the universe is 10-30 billion ...
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Why is there a Moore's Law for atomic clocks?

From the NIST website: If you add the more recent data points, like this one in 2019, which achieved $~10^{-17}$ fractional instability, the trendline continues. Why should there be a Moore's law ...
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1answer
136 views

How to define time in a time-dependent solution?

If a spacetime has no timelike killing vector, how can we define "time" in such spacetime, in order to calculate the time evolution behaivor of some quantities in it?
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Time derivative in rotating frame

In Goldstein (2ed) sec 4.9 - Rate of change of a vector, why does he say that the instantaneous angular velocity $\omega$ is not a derivative of any vector? $$ (d\textbf{G})_{space} = (d\textbf{G}...
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Time and Quantum Mechanics

my question is about the nature of time across classical/macro and quantum scales. I understand that the 2nd law of thermodynamics and entropy increase has a lot to do with our understanding of time ...
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139 views

Does our universe admit of a global time function?

My understanding of a global time function is this: a function whose value always increases as a body moves into its local future. My confusion is this: I gather that the Gödel Universe is bizarre ...
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Sakurai"s Comment Anti-particle in Klein-Gordon equation: Reversing momentum and Moving backward in time

In JJ Sakurai, p.492-493, he made Comments on Anti-particle in Klein-Gordon equation. By analyzing the current EOM, he said that charge current density is the same for all particles, (regardless ...
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352 views

What is the difference between an ideal clock and a physical clock in relativity

A letter from Einstein's friend Besso mentioned what Einstein called his "original sin." Einstein's sin was that he defined a clock to be both a physical thing (an actual clock) and an ideal entity ...
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Gravitational time dilation and the pace of time

If we are in empty space far from a black hole, at rest relative to the hole, we would look at a clock and a light source inside the gravitational field of the hole, then we would, according to the ...
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1answer
174 views

How much is an incandescent lamp's filament stressed by an on/off cycle?

The coiled tungsten alloy filaments of incandescent heat up greatly in use. Eventually, they fatigue and fail. Lightbulbs are rated by an number of hours of use, but along with the lit hours and any ...
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Does the intensity of LED lights decrease after turning on?

LED lights come on immediately the power is supplied. This is different to the older compact fluorescent-tube lights. I am always impressed by the brightness of the LED lights when they come on. Later ...
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238 views

Causality in CPT symmetry analogue of free electron laser (stimulated absorbtion)?

While 2nd law of thermodynamics emphasizes past->future time direction, CPT theorem says that at least microscopic physics has some symmetry between past and future. For example the Feynman-...
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161 views

ADM decomposition and hyperbolicity

Given a spacetime $\mathcal M$ I am interested in the ADM decomposition of a metric $g_{\mu\nu}$ $$ ds^2 = - N_t^2 dt^2 + \gamma_{ij} (N^i dt + dx^i) (N^j dt + dx^j). $$ By reading the literature I ...
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If microscopic dimensions were found in particle experiments, how do we determine whether it is spatial or temporal?

This is not a question asking why our universe is 1T+3D dimensional, and hence not about how the various models such as Itzhak Bars and F theory can incoporate multiple time into a model to describe ...
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1answer
183 views

Fixing time in Feynman phase space path integral

The phase space version of Feynman's path integral expression for the free particle propagator involves a (formal) sum over paths in phase space with fixed $q$ endpoints and (as far as I'm aware) ...
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In KK theory, is proper time defined using the 5 dimensional or the 4 dimensional line element?

Let's consider five dimensional KK theory. This is Klein's metric $\hat{g}_{AB}= \begin{pmatrix} g_{00}+A_{0}A_{0}&g_{01}+A_{0}A_{1}&g_{02}+A_{0}A_{2}&g_{03}+A_{0}A_{3}&A_ 0\\ g_{10}+...
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108 views

Expectation value for the time of a photon reflection

A photon is reflected by matter (by an electron in empty space). How long does the reflection take? (i.e. is there any infinitesimal time elapsing during the reflection process?), or more precisely, ...
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How to prove that a time-oriented spacetime possesses a nowhere vanishing timelike vector field?

Penrose gave a very brief proof to this question. Since the spacetime is paracompact, there exists a positive definite metric called $h_{ab}$. Then, the nowhere vanishing time-like vector field $V^a$ ...
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What's the connection between being homogeneous in energy and having a proper time scale?

Towards the end of page 8 of this scientific paper, I have found the following sentence: ...given the fact that the system is homogeneous in energy, or equivalently, that it has no proper time ...
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Can we derive the relation between proper time and the spacetime interval?

In GR, it's usually taken for granted - or as a definition - that the time measured by an observer's clock is related to the geometry in a very simple way, $d\tau^2 = |ds^2|$. This is easy enough to ...
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548 views

Time ordering and Fermions

Having time ordering operator for fermions, should it reverse sign if it swaps operators with opposite spin variable? In other words should $T[c_{t_1,\uparrow}c_{t_2,\downarrow}^\dagger]$ return $T[...
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How to make timelike entanglement in the laboratory?

http://io9.com/5744143/particles-can-be-quantum-entangled-through-time-as-well-as-space http://arxiv.org/abs/1101.2565 How to make timelike entanglement in the laboratory? How to test whether mixed ...
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797 views

Is large-scale “time reversal” (Poincaré recurrence) possible given infinite time?

The following are some assumptions I'm basing my question on, from what (little) I understand of physics. I list them so an expert can (kindly) tell me where I'm going wrong. There is a probability ...
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Free falling into a black hole: time to the singularity doubt!

I have trying to get a more precise insight into the calculation of the time to the singularity of a test mass into a non-rotatic uncharged black hole. Prelude: the lagrangian for a massive particle ...
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2answers
57 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 ...
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30 views

How does one get ticks of a clock from an atomic clock's microwave signal?

I understand that in an atomic clock there is a microwave signal that is locked to the frequency of the atomic rotation. However, if one wants to extract time ticks from the microwave signal: $v(t) =...
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2answers
56 views

Water electric dipole and time reversal

Why some objects, such as water molecule, can have a (large) electric dipole moment without violating time reversal symmetry, while the existence of an electron or neutron electric dipole moment would ...
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1answer
54 views

Is there an ambiguity in time in the absence of clocks?

Imagine 2 clocks space 2 meters apart with a vacuum in between. These are macroscopic objects with which we can measure time. But in between these, according to general relativity, we can foliate ...
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Time ordering identities in integration over gluon fields

My question arised when trying to compute the Wilson Loop of a hybrid meson. When calculating the loop one has to keep in mind the path ordering and time ordering respectively. I have the following ...