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?

$\newcommand{\Ket}[1]{\left|#1\right>}$ 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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Time derivative of the Lagrangian

I have the time derivative of the lagrangian: $$\frac{\mathrm d \mathcal L}{\mathrm d t}=\sum_i\left(\frac{\partial \mathcal L}{\partial q_i}\frac{\mathrm d q_i}{\mathrm d t}+\frac{\partial \mathcal ...
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Is there any experimental evidence of CTC?

One consequence of general relativity is a solution called the closed time like curve or CTC...which could allow you to view a place at an earlier point in time or possibly even travel back in time. ...
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Is Time a container of Space, or separate from Space? [duplicate]

Those of you more learned than me in this topic, please bear with me here. Hopefully it presents an interesting and enlightening discussion for those of us who are just readers of science and may ...
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Do all photons have the same proper time in different environment?

All photons have the same speed. In "space" (I mean when there is practically no matter) a photon will travel a distance faster than a photon in matter. But the speed is still the same. Do 2 photons ...
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8answers
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Are time and motion the same thing?

My question might be completely irrelevant or stupid - sorry in advance if it's the case. I'm kinda lost with the time and space notions, and even formulating adequate questions is hard at that point. ...
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Equivalence of two definitions of proper time in special relativity

According to Wikipedia, proper time along a timelike world line is defined as the time as measured by a clock following that line. This makes sense to me, but my book defines proper time in the ...
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Is there a notation alternative to Hertz?

Hertz, as we all know, is a unit of frequency where 1 Hz equals one cycle per second. Conversely ...
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Problem of now. How physics deals with it? [duplicate]

i know this is kind of off topic. from my understanding in relativity spacetime is frozen, nothing changes, there's no "time" as we understand it in everyday life. is there anything in physics that ...
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Does the age of the universe depend on the speed of light?

In the Big Bang theory of the universe, does the value of the speed of light enter into the calculation that determines the age of the universe, and if yes, in which way?
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Is mass persistent in spacetime?

As an object moves through spacetime is its mass persistent? What I mean is, if we find a way to look backwards into time, will a pen sitting on a desk when I was in college still be sitting on that ...
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Notion of Present

Can't I sync all watches in spacetime and call this time slice the present? In Carlo Rovelli's book he tried to explain that the notion of the present is local only, which I could not follow.
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Find velocity from acceleration equation [closed]

Suppose the acceleration of a particle is a function of $x$, where $$a(x) = (2.2 s^{-2})x$$ (a) If the velocity is zero when x = 1.0 m, what is the speed when x = 3.4 m? (b) How long does it take ...
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Why don’t I see proper time?

Say we have my friend blasting off in a spaceship to Planet Physics and I’m sitting here on earth. I think it’s quite obvious that I will measure the proper length between the planets (because I could ...
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Time in the second law of thermodynamics versus relativity [duplicate]

I recently had a debate with someone about time and argued that the most concrete definition of time would be an increase in entropy. The entropy of a localized area of space might reduce, but that of ...
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Why is the age of the universe important?

Age may only be a number, but when it comes to the age of the universe, it's a pretty important one. Why?
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Expectation of partial time derivatives of $x$ in QM

In Ehrenfest theorem we know that $$m\frac{d\left< x\right>}{dt}=\left< p\right>+m\left<\frac{\partial x}{\partial t}\right>.$$ So how can I exactly calculate a specific $\left<\...
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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 ...
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Validity of the derivation of time-energy uncertainty principle?

Background The top voted answer to this question seems to make some assumptions I'm uncertain about (to say the least): What is $\Delta t$ in the time-energy uncertainty principle? Assumptions The ...
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Time-dependent pertubation theory assigning the order of expansion to squares of the solution

What is the square of a solution from time dependent pertubation theory? Assume we have found the corrections up to second order such that $$ |\psi(t)\rangle \approx |\psi^0(t)\rangle + |\psi^1(t)\...
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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 ...
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Does the age of the universe depend on the way it is defined?

I was reading some earlier posts on this question in stack exchange. These posts give me the impression that the age of the universe is defined in a certain way following prescriptions such as that: ...
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1answer
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Are there any experimental data about potential time dilation caused by cyclical movement of heavy objects?

I am dealing with a completely fabricated scenarios (while trying to write a sci-fi book), so my thougts are completely hyphotetical. But I would like to ask you to help me find out if following ...

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