# 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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### Expectation value of time derivative of operator vs. time derivative after operator

Problem 3.18 in Griffiths's Introduction to Quantum Mechanics (3rd ed.) asks to apply the generalised Ehrenfest theorem to operators like the Hamiltonian and momentum operator. The purpose of the ...
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### Is temporal causality broken by quantum entanglement?

When one particle of an entangled pair is measured, it forces the state of the other particle. This effect propagates faster than we can measure, certainly faster than light and possibly ...
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### Is there any method to know the real age of the universe? [duplicate]

Well, I was wondering about the real age of our universe, I found that it's estimated to be $13.8\times 10^9$ years. Is it an approximation, or laws behind this age?
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### Applying force in zero seconds? How is this possible?

$F\,\Delta t=m\,\Delta v$, is the equation of impulse. Let's say that I push a wall with my hands, we can have $0$ change in velocity; therefore, $F\,\Delta t=0$, since I'm pushing the wall there is ...
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### Particles reaching a singularity [closed]

If the gravity well steepens to infinity at the point of a singularity, then so does the "time well". Understandingly, the inward falling energy/matter is not aware of this change in time relative to ...
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### What is the preferred clock, does choice of clock define the laws of physics?

I was wondering about the idea of time and its measurement, i.e. clocks. As I understand, we define time by the count of a recurring event. Let's assume that we use two atomic clocks A and B to ...
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### Does a time-independent system have a time-independent Lagrangian?

Consider a Lagrangian $L:\mathbb{R}^3\times\mathbb{R}^3\times\mathbb{R}\to\mathbb{R}$. Suppose it has the property that if $\gamma:[t_1,t_2]\to\mathbb{R}^3$ solves the Euler-Lagrange equation, then (...
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### Is simultaneity independent to reference frame?

Imagine two events happening simultaneously with respect to my frame of reference. Is this possible to have a reference frame to which the events are observed NOT to be simultaneous?
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### liquid flowing through tiny holes super slowly [closed]

How could I make a liquid in a container to flow out of several tiny holes super slowly, let's say that the whole container would empty in a month duration. Is it possible? does it only depend of ...
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### How could a relation of the form $t=-i\tau$ hold with both $t$ and $\tau$ being real?

The physical time is a real quantity. But in quantum field theory, whenever we find oscillatory exponentials in time and we cannot literally take the limit $t\to \infty$, we make a change of variable, ...