# 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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### Does time exist in a universe with no movement?

If everything just stays in place, is it meaningful to say that everything stays in place as time passes? Or does time not exist at all?
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### What is the physical interpretation of four-force in SR?

Four-force in SR is the ratio of the relativistic momentum to the proper time $\tau$, however, the three-force has a different definition so that the coordinate time of $t$ replaces the proper one. ...
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### Does the four-force, in some ways, invalidate the results of the three-force in SRT? [closed]

I have recently submitted an article to PNAS, however, the editor, oddly enough, claimed that he has issues with the traditional Lorentz transformation for force, he stated: The argument of this ...
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### Total time derivatives and partial coordinate derivatives in classical mechanics

This may be more of a math question, but I am trying to prove that for a function $f(q,\dot{q},t)$ $$\frac{d}{dt}\frac{∂f}{∂\dot{q}}=\frac{∂}{∂\dot{q}}\frac{df}{dt}−\frac{∂f}{∂q}.\tag{1}$$ As part of ...
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### Time and the incompatibility of General Relativity and Quantum Mechanics [duplicate]

Is GR is incompatible with QM because GR has time as an essential dimension whereas QM does not have time? Is time the key problem to reconciling the two theories?
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### How does the derivative of time with respect to proper time equals gamma?

So, I want to really get a deep understanding of everything that goes beneath special relativity. Since I teach myself with books, I have no teacher to ask to which makes things a bit harder most of ...
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### Least count of cesium clock and maximum possible significant figures for time

We know that a second is defined as being equal to the time duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental ...
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### How long does it take an electron to emit (or absorb) a photon?

A photon is emitted (or absorbed) by a transitioning electron. How fast is this process?
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### What is the role of the laws of physics in a block universe?

Definition of a block universe - The idea that the whole universe exists simultaneously and time doesn’t flow. For those who favor this kind of theory (the few of you), what is the role of the laws of ...
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### Correlation function of time derivative of fields

Suppose you have calculated a two point function for a field $\phi$, and the result is some function of the positions (it can be a generic function, not necessary a function of the distance $x_1-x_2$):...
932 views

### Is velocity real?

This sounds like a stupid question but I am do not grasp physics concepts easily. "Velocity" is just the change in displacement over the change in time. I can see displacement and time as ...
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### Is it even theoretically possible for a perfect clock to exist?

I have heard that even atomic clocks lose a second every billion years or so. That raises the question, is it even theoretically possible for a perfect clock to exist, one that never gains or loses ...
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### Age of a person going to space [closed]

When a person goes into space and stays there for a specific time without moving the spacecraft, will he age faster than those on earth?
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### If all motion stops, does time cease to exist? Is awareness then still possible in this stae? [closed]

I mean on an entire universe level. We rotate the sun, the sun some black hole, the galaxy speeding off from some explosions in an unknown central location... On it goes. Everything moving at ...
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### How to calculate the components of the displacement of unit magnitude of an accelerating particle in 2D? [closed]

A particle is moving in 2D having a constant acceleration $\vec a$. Given initial velocity $\vec u$, after time $t$ the magnitude of its displacement $\vec S$ is 1. I need to calculate $S_x$...
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### Time-dependent Schrieffer-Wolff Transformation

The time-independent Schrieffer-Wolff Transformation has the form: $H' = e^{S}He^{-S}$. I see that the time-dependent form is: $H' = e^{S}He^{-S} +i\hbar\frac{\delta}{\delta t}(e^{S})e^{-S}$. Why ...
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### Infinite time period vs zero time period

Lets suppose a pendulum of infinite length is given a small disturbance. It should take an infinite amount of time to move. Can this be considered equivalent to the case where the pendulum doesnt move ...
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### Why should time slow down for light?

So I just learnt about relativity and time dilation. One of the visualisations used to explain time dilation is an apparatus contaning a beam of light moving between two mirrors. When the apparatus is ...
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### Calculation of time for electronic transitions

How do we determine the time for electronic transitions in atoms or in semiconductor devices, from one energy level to another?
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### What is the difference between the two time derivatives $\frac{\text{dL}}{\text{dt}}$ and $\frac{\partial \text{L}}{\partial \text{t}}$? [duplicate]

I am taking a course in classical mechanics, and I was wondering what the difference between the two time derivatives $\frac{\text{dL}}{\text{dt}}$ and $\frac{\partial \text{L}}{\partial \text{t}}$ in ...
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### Special Relativity can my past be in someone's future

My question is the following: Is it possible that an event which is in my past light cone is currently in another observer's future light cone? (i.e. for an observer sitting at a point in the set of ...
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### Is time a measurement of loss of energy? [closed]

I know it sounds dumb . I'm not entirely sure whether this question is indeed suitable to ask here or the philosophy stack exchange . Anyways just hear me out alright ? People age . objects age . But ...
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### Carlo Rovelli's analogy of gravity as running down the beach into the sea where the resistance of water makes us fall down into the waves

In "The Order of Time" (page 12), Carlo Rovelli explains how things fall due to the slowing down of time and uses the following metaphor: "Where time passes uniformly, in ...
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### Does time expand?

If the time component of spacetime does not "expand" in the sense that space expands, doesn't that imply that time must already exist in its entirety, past and future? I believe I have seen ...
366 views

### Can we explain velocity without time? [closed]

I'm trying to wrap my head around inertia/velocity, or just change in general, I guess. I'm unsure exactly sure were my gap is, or what question to ask, so I'll explain with a scenario. Lets say I'm ...
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### How would a motionless person/object with no gravitational forces acting upon them experience time relative to a person on earth?

Regarding time dilation, how slowly would a theoretically completely stationary person experience time in comparison to those on earth, with that person being infinitely far away from any other object ...
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### Can there exist a motion which cannot be described as a function of time?

I know there exist systems for which we cannot solve its differential equation, But I was wondering if there could be a motion that cannot be represented as a differential equation with respect to ...
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### Evidence that Time is a Natural Phenomenon? [duplicate]

Time is a tool we've invented to measure changes of state. What evidence do we have that it is a natural phenomenon?
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### A vector represents a force with its magnitude and direction but could it also represent the time it was applied for?

I'm an A2 igsce math student and I'm taking mechanics for the first time in math this session. However, I was adviced to ask this question in the physics forum instead. I know that what I'm going to ...
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### Why is there a “problem of time” in Quantum Gravity?

It’s often said that the problem of time exists because time is treated as absolute in quantum mechanics but not so in General Relativity, see e.g. A list of inconveniences between quantum mechanics ...
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### What exactly is dark matter? [duplicate]

Astronomers have shown that the objects in the universe from galaxies a million times smaller than ours to the largest clusters of galaxies are held together by a form of matter that is not what we ...
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### Is there any way to create the distinction between time slowing down and clocks slowing down?

As Einstein once said, "Time is what you measure with a clock". So my question is: is there any way to create the distinction between time slowing down and clocks slowing down because of ...
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### One-dimensional harmonic move - input parameter conversion

I have one-dimensional harmonic move, but velocity is defined as function of distance from the beginning V(x) = (Vmax + Vmin)/2 + (Vmax - Vmin)/2 * cos((2 * PI / K ) * x ) Where Vmax > Vmin, Vmin &...
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### If the wave function is a single energy eigenstate, why is the probability distribution independent on time?

I thought wavefunctions always depend on space and time. Even if it is just one energy eigenstate I do not see why there would be no time dependence. That exponent $e^{iEt/\hbar}$ should still be ...
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### Explicit independence of Hamiltonian phase-space variables from the time parameter

In general, we have for a Hamiltonian flow $H$ of some "time" parameter $t$, the following relation for any function $f=f(q,p;t)$ of the phase-space generalized position ($q$) and conjugate ...
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### What do you think of the idea of measuring macroscopic Special Relativistic-kinematic time dilation by placing atomic clocks onto moving dollies?

We first attempt to synchronize two stationary clocks placed apart, and we assume we succeed using the Einstein synchronization convention. Then, we run a third clock along a dolly at a constant speed....
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### Is the Big Bang occurring or did it already occur (a question about time) [duplicate]

I understand how time (space/time) is perceived according to the theories of relativity, but I still don’t understand: Did the Big Bang occur 13.8 billion years ago or is it occurring relative to us ...
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### Number of variables in the Hamilton-Jacobi equation

In Goldstein's Classical Mechanics, while introducing the Hamilton-Jacobi equation, he argues that the equation H(q_1, ... , q_n; \frac{\partial S}{\partial q_1}, ..., \frac{\partial S}{\partial q_n}...
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### Subsequent measurement of energy [duplicate]

I am interested in the subsequent measurement of energy, like assume the energy of the electron was measured to be $E_1$ and the wavefunction of the electron is described by a finite superposition of ...
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### What is meant by coordinate time? Isn't it time measured on a clock? If not then what does it measure?

In this question- Is proper time equal to the Invariant Interval or the time elapsed in the Rest Frame? @Dale in the comments says- no $dt$ is never physical time. It is always coordinate time. The ...
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### Is proper time equal to the Invariant Interval or the time elapsed in the Rest Frame?

In SR consider two time like separated events - In some frame $$ds^2= dt^2 - dx^2$$ In a frame where the events occur at the same place ( rest frame; $dx' =0$) then according ...
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### How does a capacitor get charged instantly in AC whereas it takes infinite time in DC? [closed]

I have seen when a capacitor is connected to a dc source it takes infinte time to charge, but when connected to ac it takes the potential of the source instantly, probably the approach in the books is ...
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### Why is time explained differently?

In quantum mechanics time is a classical background parameter and the flow of time is universal and absolute. In general relativity time is one component of four-dimensional spacetime, and the flow of ...
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### Reconciling two types of time in QFT

When I took field theory 25 years ago, I learned to do the mathematical manipulations very fluently but had no idea whatsoever what it all meant. Now, with a more mature understanding of quantum ...
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### Real life applications to the simple pendulum experiment

I have been wondering quite lately what were the real life applications used in a simple pendulum experiment. Time measurement is one of the applications that could be used for simple pendulum ...
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### How does matter and energy exist in a block universe?

If we suppose that the block universe is indeed true, and that the past, present and future exist simultaneously, then what happens to matter and energy? If matter and energy cannot be created nor ...
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### How do I prove the Euler-Lagrange equations of an isolated system do not depend explicitly on time?

Disclaimer. This is my first question, apologies for any potential standard of the community that I broke or if this question was already asked. How do I prove the Euler-Lagrange equations of an ...
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### How is this possible? [duplicate]

Imagine three observers(top, right and left). From the top observer's point of view left and right's speed is the same, so they would go equally slower. From the right observer's point of view, the ...
Two point charges in the vacuum (so ignore all frictions). They have opposite signs so they will move towards each other under Coulomb's force. The magnitude of their charges are $q_1$ and $q_2$ ...