# 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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### Chaining together two successive time dilations through two Lorentz transformations [duplicate]

The three clocks $A, B, C$ at the same position in spacetime at the beginning of the measurement. $A$ starts moving to the right with the velocity $v$ with respect to $B$ and $B$ starts tracing $A$ ...
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### Checking the Lorentz Invariance

This seemed at first glance very easy. But there appeared some confusion. $A$ is moving to the right with velocity $v$ with respect to $B$. The proper time for $A$ is $$t_a=t_b\sqrt{1-v^2/c^2}$$ ...
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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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### Example of Heisenberg Uncertainty Principle (energy & time)

Beside momentum and position I read that Heisenberg Uncertainty can also apply to energy and time so for instance virtual particle (energy) could pop into existence but for a very brief moment (time) ...
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### If a state is an energy eigenstate, then it cannot be localized in time

In Weinberg books The quantum field Theory he says that if a state is an energy eigenstate, then it cannot be localized in time. I am not understanding this statement. For example if $\Psi$ is energy ...
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### Solar panel and Equinox

At Equinox the number of hours of daylight and night is equal so 12 hours of each... is this from 6am to 6pm? how can we know which hours this daylight happens for. The question is: Over which ...
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### Can anitmatter be scattered in other parts of time?

Is it possible that during the Big Bang, the matter and antimatter was scattered in the spacetime, along the time axis, as well, not just in space? (I am not sure, how to ask this properly, I hope ...
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### Do physicists talk about instants and infinitely small moments in time? [duplicate]

Do physicists talk about instants and infinitely small moments in time? If so, how do they measure something like that? If they don't measure it, why do they think it exists?
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### Are we really moving at the speed of light in the time dimension? [duplicate]

I have seen a vector explanation of time dilation that our net velocity in space-time is the speed of light. Most of that velocity for slow spatial speeds is in the time dimension. As you go faster ...
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### The beginning of a movement and the continuity of time? ( How can movement begin without a first beginning instant ?) [closed]

The question I am asking is not original , it is a very old problem dating back from antiquity. I would like to know how modern science has solved it; or, maybe, in which fallacious reasoning the ...
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### How is time notated for the overlapping hour when switching to daylight savings? [closed]

How is time notated for the overlapping hour when switching to daylight savings? At 1:59:59 AM (#1) -- daylight saving time in effect. Then clocks get turned back to: 1:00:00 -- standard time. ...
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### Graphing time as a function of another variable [closed]

I'm sorry if this is not the correct place to post this, but it seems most applicable. I'm graphing the time for a satellite to decay, which depends on the altitude it orbits at, making time a ...
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### How is it possible that a Hamiltonian that commutes with the time reversal operator $\Theta$ has eigenstates that change sign when $\Theta$ s applied?

$\newcommand{\Ket}{\left|#1\right>}$ For example, $H=AS_z^2+B(S_x^2-S_y^2)$ where $s=1$ is time-reversal invariant, but one of its eigenvalues behaves like $\Theta\Ket{\alpha}=-\Ket{\alpha}$, ...
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### Pegging a number to the when the measurement happens?

Motivation Usually the measurement is denoted by for example: $$\alpha |0\rangle+\beta |1\rangle \to |1\rangle$$ Note that there is no mention of when this measurement happened. I was wondering ...
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### What is the experimental evidence for gravitational time dilation?

There are many experiments done to prove time can be changed. I don't know lots of them, but I think the most popular (most cited) is to put a clock on a fast moving object (airplane), fly around and ...
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### Can a wormhole change the wavelength of a light beam?

If a light beam went into one mouth of a wormhole, and came out the other, are their factors that would result in it coming out red or blue shifted? Examples, if it came out of a mouth that was in ...
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### How is the global time coordinate $t$ (“observer at infinity” time) defined operationally e.g. in the Schwarzschild metric?

This is a question about coordinate time versus clock time / observed time, which I want to understand because I am teaching a GR course. Consider the Schwarzschild metric for specificity. I ...
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### Why does nature prefer simultaneous events? [closed]

Nature mostly prefers simultaneous events: Acceleration is produced without any delay on applying force. Angular acceleration is produced without any delay on applying torque. A bulb glows ...
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### Are all processes time/CPT-reversible, e.g. measurement, stimulated emission, state preparation, Big Bang?

"The CPT theorem says that CPT symmetry holds for all physical phenomena, or more precisely, that any Lorentz invariant local quantum field theory with a Hermitian Hamiltonian must have CPT symmetry." ...
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### Clock postulate validity

As far as I understood, the generalized clock postulate says that there is no difference between the measurements of a non-inertial observer (NIO) and those of his co-moving inertial observer (CMIO) ...
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### Can a region of space-time be viewed as a particle with properties of it's own? [duplicate]

Can a piece of space-time having 3 spacial dimensions and 1 dimension of time be viewed as a particle? Could this piece of space-time have properties of its own (mass, spin, charge) independent of ...
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### Moving clocks observed with different observers

If a clock is moving near to speed of light with constant velocity it'll slow down, but if I were to observe from the clock will the observer on Earth slow down?
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### Are these Stephen Hawking's statements legitimate? [closed]

I am reading 'Brief Answers to the Big Questions' by Stephen Hawking and it seems to me that some of his statements are just hypotheses, but they are written in such a way that they 'sound' like they ...
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### Analytic Elements Haag-Araki theory

Why the elements of a local von Neumann algebra cannot be analytic elements of the timelike translation?
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### Is it possible to measure the speed of Time? [duplicate]

Is it possible to somehow measure the rate at which time is passing?
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### Calculating mean velocity: using time or velocity? [closed]

Background I have been running tests which involve timing an object moving a certain (fixed) distance, $s$. Each test has been repeated 3 times, and the 3 times ($t{_{1}}, t{_{2}}, t{_{3}}$) for the ...
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### Is Young Earth universe distinguishable from non-creationist one? [closed]

While talking today about some religious beliefs I was struck by the next question: So what if some super-natural being forks our universe 6,000 years ago, i.e. creates the exact same copy of ...
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### Is vacuum energy transient or permanent?

Vacuum energy is based on Heisenberg's energy–time uncertainty principle which says that $\Delta E$ can exist for $\Delta t$. This $\Delta E$ can produce electron positron pair for $\Delta t$ by ...
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### Highly isolated two different reservoir

Lets say we have highly isolated two different reservoir.One is cold the other one is hot.We just connected them with a tube.(Which is also isolated) Tube has "0" thermal conductivity. If we let ...