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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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Work-kinetic energy theorem for time-dependent force

Does the work-kinetic energy theorem hold in all the cases? Specifically is true for a time-dependent force?
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53 views

New S.I. and the second definition standard

The new SI defines the second as the hyperfine splitting transition time associated to the frequency of that transition of caesium-133, 9192631770 Hz, that is 12 decimals precision, but...I wonder two ...
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Why do the directions of two moving objects affect the time they share?

Before starting, I'm not a native speaker of English, so please pardon my poor command of the language. So, I watched a video that says the future is predetermined. The image below is what I found ...
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Lagrangian and equations of motion in a time-varying coordinate system

I am assuming a very simple case, where there is only a mass $m$ with position $x$ under an external force $F$. we know that the Lagrangian takes the form $L = (1/2) m \dot{x}^2$ from which equations ...
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How to compute elliptic integral of the first kind using Fortran? [closed]

I am trying to solve the computational problem using fortran to get the value of Time period based on amplitude. The formula is where value of α ranges from 0 to pi/2 which I can convert degree into ...
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Special Relativity - Clock Synchronization

In Special Relativity - Is their a definition of "synchronizing" clocks which are moving relative to each other? I suspect such a definition makes no sense - as moving clocks belong to separate ...
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Another Free Energy Question :P :)

Hello StackExchange users. Before you read my question I would like to thank you for taking the time to help me out :). So anyways, my question is about the formation of coal seeming to break the laws ...
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2answers
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On the derivation of Ward-Takahashi identity

I am reading Weinberg's QFT book and in 10.4 he introduced a derivation of Ward-Takahashi identity (where $T$ is the time ordering): $$\begin{align} \frac{\partial}{\partial x^\mu}T{\{J^\mu(x)\Psi_n(y)...
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Where does the 379,000 year recombination prediction of the Big Bang theory come from?

The Big Bang theory postulates that recombination happened 379,000 years after the Big Bang. However, I have never seen a single statement as to how this number came about. I don't imagine they ...
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If I place a live video camera on another planet where time moves with a different speed

remember that movie "Interstellar"? Where the astronauts came to a planet where one hour here ment 7 years back on earth? Well, if you don't: Imagine a planet, far, far away where time moves in a ...
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1answer
42 views

How to know is four-velocity is directed towards future or towards past?

Consider a four-velocity vector $\mathrm{u}=(u^0,u^1,u^2,u^3)$. The zeroth component of position is $x^0=ct$. How do I know if $\mathrm{u}$ is directed towards the future or towards the past? Is it ...
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1answer
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Could be experience Time without any changes in our enviroment?

Being a beginner in Physics, I found this question to be interesting which came in mind. Time has been expressed as an dimension which is scalar in nature. Also the GTR also explains Time Dilation. ...
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Time dilation in a geosynchronous orbit

since gravity slows time, and velocity slows time, would observer A on the equator directly under observer B in a geosynchronous orbit experience time at the same rate? Time would slow on the surface ...
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1answer
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Black Holes and Time [duplicate]

My understanding is that if I am outside a black hole and watch something fall in, the object will appear to slow down and will never actually get sucked in. So how can we ever get a black hole if ...
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2answers
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Difference continous - discrete symmetry

I am trying to understand the difference between the two types of symmetries.Wiki Wikipedia says that Translation in time : $t \rightarrow t + a$ is a $\textbf{continuous}$ symmetry, for any real $...
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1answer
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Is there an intrinsic speed of time? [closed]

If time and speed is relative, then we could double the speed of every particle in the universe, as this increases the clocks rate we shouldnt be able to tell any difference. But my question is, in ...
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2answers
56 views

If an object is travelling near light speed, would it's actions seem to be in slow motion?

Hypothetically if we were observing a clock travelling near light speed relative to us, we would see the clock ticking at a much slower speed than us. If that is true, then would all actions that are ...
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2answers
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How can time go in different directions in the Universe?

This is not a duplicate, I am not asking about any kind of time dilation caused by a BH. My question is about the direction of time (and if it is possible to have different directions for time, other ...
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2answers
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Perception of time [duplicate]

A video I came across in youtube recently had mentioned how Einstein perceived time. It said he thought of all moments, the present, past and future to exist together and how it is not like one ...
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Why is time 'negative' in a proper distance in Relativity? [duplicate]

so my question is as follows: the proper distance between two events $a$ and $b$, $S_{ab}$ is given by $$S_{ab}^2 = -[x_0(b)-x_0(a)]^2 +[x_1(b)-x_1(a)]^2 +[x_2(b)-x_2(a)]^2 +[x_3(b)-x_3(a)]^2.$$ ...
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781 views

Why do we perceive time?

Since everything (at least what we observe) is travelling through the time in the same direction and speed why do we even perceive the time? Of course, there is the time dilatation, but it is ...
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Timing in double slits experiment

In this setup the source, the slits and the screen are a lightsecond away from each other. I would like to know if there is any difference if I tempered with the slits before and after the photon is ...
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Antiparticles travelling backward through time [duplicate]

Could someone please explain to me Feynman and Wheeler's theory of waves acting forward and backward in time? I have read about their idea of an antiparticle behaving as a particle travelling ...
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1answer
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Do position operators commute at different time?

I had seen a general case that $\hat{q}(t)$ and $\hat{q}(t')$ doesn't commute at different time $t$ and $t'$, where $\hat{q}(t)$ and $\hat{q}(t')$ are Operators in Heisenberg's view. I tried to ...
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Is this a good clock for quantum cosmology?

I was thinking about Quantum Cosmology and the wave function of the universe $\Psi[g,\phi]$ in the no boundary proposal. I was thinking about what would make a good clock. It would be some function ...
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Why can't we rule out other synchronization procedures?

According to what I've read, it is generally regarded that Einstein's choice to have the speed of light equal in every direction is only a convention, and since there is no way to measure the one-way ...
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1answer
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The relationship between time, relativity and entropy

I came across a discussion about the nature of time and whether or not time is an illusion on a physics forum. I'm not so much interested in the philosophical issues regarding time, but the following, ...
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1answer
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What is the true way to determine time parameters from semilog graph?

I have experimental nuclear magnetic resonance data that describe T2-relaxation of the nuclei in the sample of interest. The experimental points obey bi-exponential dependence: I = I1*exp(-t/T2_1) + ...
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1answer
136 views

Why do we assume the potential is independent of time in the Schrödinger equation?

In just about every text I read (online or in paper), when they handle the time-dependent Schrödinger Equation, I see something along the lines of "we always assume the potential is independent of ...
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2answers
54 views

Is time a mathematical entity or there is any demostration of its existence? [duplicate]

I an having trouble trying to understand time as a physical entity. We can demonstrate the existence of air, the source of infection, electromagnetism, voltage, amperage, among many other physical ...
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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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Why don't we normalize the wavefunction in time? [duplicate]

I am aware that because a particle whose wavefunction we are dealing with must be found somewhere, we normalize the probability density in position. Why do we not normalize the wavefunction in time? ...
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4answers
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Understanding intuition behind time translation in classical mechanics

In V.Arnold book "Mathematical Methods of Classical Mechanics" he says that invariance with respect to the time for isolated systems means that "the laws of nature remain constant", i.e., if $\phi(t)$ ...
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1answer
103 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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Why does entropy increase along the relativistic time dimension?

I've repeatedly read that time flows / is asymmetric because entropy increases as you move away from the big bang and towards the future. This makes sense. But why is this direction in which entropy ...
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7answers
187 views

What happens to velocity when Time equals zero?

I am not formally educated in Science but natural questions have always intrigued me.The way I put it is that I am married to Commerce but Science has been a childhood love. Now I have this very basic ...
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Effect of variation in day length on timekeeping

I was reading about atomic clocks when I came to know that precise measurements of time have shown that the time for earth's rotation is slightly more that 24 hrs (on the order of 10^-3). I was ...
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1answer
54 views

Convective derivative vs total derivative

I was wondering what is the difference between the convective/material derivative and the total derivative. We were introduced to the notion of material derivative $$ \frac{D\vec{u}}{Dt}=\frac{\...
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1answer
92 views

What is the physical interpretation of the Wick rotation?

What is the physical interpretation of the Wick rotation? How is it that we can just propose there's a new time coordinate tau? Are physicists saying time is modeled by an imaginary number? Isn't ...
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1answer
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Would not the oscillation of light slow down the wave? [duplicate]

Would not the oscillation of the light wave slow down the wave? Does light travel at "c" of its net position over time, or does it travel at "c" on the wave?
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1answer
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How do you calculate conformal time?

I've got several references that indicate that conformal time is the integral of the scale factor: $$\eta=\int_0^t \frac{1}{a(t)}\mathrm dt.$$ So I tried to calculating this over a range of {0, ...
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1answer
175 views

Is the M87 black hole really a black hole? [duplicate]

I apologize if this is a silly question, but... From what I've been told, new black holes cannot form, because time slows to nearly stopped as matter approaches the event horizon. Instead, a new ...
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2answers
172 views

Why the time period of pendulum with infinite length is $84.6$ minutes? [closed]

In a book I was reading about SHM it stated: If the length of a simple pendulum is increased to such an extent that $\ell\to\infty$, then its time period is given by, $$T=2\pi\sqrt\frac{R}{g}\...
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1answer
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Aging while traveling faster than speed of light? [closed]

theoretically, if organic matter was able to travel faster than the speed of light, without becoming "pure energy", and since theoretically traveling faster then the speed of light would travel ...
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4answers
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How can a day be exactly 24 hours long? [closed]

The longest solar day of year is approximately 24 hours 0 min 30 seconds (occurs at mid to late December) while the shortest solar day of year is approximately 23 hour 59min 38 seconds. If I average ...
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3answers
565 views

Why are time dilation and length contraction needed to fix time? [duplicate]

In many textbooks and online answers I have heard people go on about how time dilation accounts for the gap of time experienced between two observers. However, they continue that in reality this is ...
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2answers
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Clocks only measure repetitions and not time [closed]

To measure time, we use clocks. From sophisticated atomic clocks to everyday wall clocks. When we measure the length of a time interval using a clock, we are really measuring the number of times the ...
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0answers
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Does time flow or do we move through time? [duplicate]

I hear time spoken of as something that flows like a river. Something that stops. I've also heard of time called a dimension. Something through which we move. These two ideas seem incompatible. Does ...
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3answers
103 views

What is the best way to conceptualise a 'division' [closed]

This may seem like a strange question, but my question is more along the lines of trying to figure out how people were able to 'discover' the formulas which have shaped our understanding of the ...
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3answers
52 views

Inserting an arbitrary phase in the equation for driven damped oscillations

In Classical Mechanics by Taylor, we find the solution to the differential equation of a damped oscillator with a sinusoidal driving force: $$\ddot{x} + 2\beta\dot{x} + \omega_0^2x = f_0\cos\left(\...