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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86 views

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, ...
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How sun gravitational field changes the rate of flow of time on earth [duplicate]

According to the Theory of relativity More the gravitational field slower the time because of more bending in spacetime fabric. Earth orbit is not completely elliptical it means distance from sun is ...
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A hypothesis for obtaining energy from gravity in conjunction with time [closed]

Have a scenario which appears to be a contradiction to the established laws with regard to conservation of energy. It goes as follows - If a mass of 2 kg is dropped from a ceiling (height) of 100 ...
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In the twin paradox, can light signals be used between twins to figure out what their proper times were when the light signals were sent?

Einstein used light signals to synchronize clocks in the same reference frame to the same time no matter the distance separation between those clocks to come up with some kind of perspective "present"....
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If time is relative, then how is the age of the universe calculated? [duplicate]

I am very new to the world of relativity, so the answer may be trivial, but to me measuring the age of the universe seems an impossibility due to there being no fixed frame of reference from which to ...
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Energy-Time Uncertainty Relation and Virtual Particles [duplicate]

I've come across a hole in my understanding. Heisenberg's Uncertainty Principle can be expressed in terms of energy and time as $$ \Delta E \, \Delta t \geq \frac{\hbar}{2} $$ where $\Delta E$ is ...
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How to assign a temperature and a time to cosmic events in the universe?

Sorry if this question to too silly for the experts here. But how do cosmologists assign a temperature and a time to each important cosmic event in the history of evolution of the universe such as ...
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HD 140283 and the age of universe

I am watching a video on a star that is supposedly older than the universe. HD140283 (Methuselah). According to the first 20 years of research on the star, scientists believed the star is just over ...
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Infinity at event horizon boundary [duplicate]

It has been repeatedly explained to me that from the perspective of an outside observer a particle takes "infinite time" to cross the event horizon of a black hole. Also, according to these ...
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Can the Loedel reference frame be used as the basis for establishing “Loedel age difference” as a viable term? [closed]

When I first posted on the PSX, I kept using the term "age difference" as meaning something other than the term "reciprocal time dilation". It turns out that age difference here means the coordinate ...
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Asynchronous clocks in motion special relativity

I was watching this derivation of 'asynchronous' nature of clocks from which I got slightly confused. Consider two reference frames moving relative to each other call them 'Red' and 'Blue' Frame. ...
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Is the universe expanding at an increasing rate or is time slowing down or is $c$ changing? [closed]

If time slows down for an observer as velocity increases, then the earth is moving as fast as the speed of light away from objects at the edge of the observable universe. However when those objects ...
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What is the age of planet Earth, if we are not looking for the age at sea level?

I remember to learn at school that our planet is approximately 4.543 billion years old. But since the time is affected by mass of an object, this number is actually the age on the sea level, where ...
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Is general relativity compatible with an arrow of time?

The spacetime of Special Relativity is Minkowski's "Block Universe" where time is treated as a dimension. Does General Relativity allow for an arrow of time or "flow" of time? If yes, can this be ...
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Experience of Time by observers in General Relativity

I want to know how to obtain the "experience of time" of an observer moving on an arbitary timelike worldline. By "experience of time" I mean the time expereinced by the observer as a function on ...
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The time-energy uncertainty relation on a compact space

This post follows this answer of Ben Crowell. The problem with the time-energy uncertainty relation $$[H,T]=i\hbar$$ is that it implies that the spectrum of the energy operator $H$ cannot be ...
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Explain Imaginary Time and Temperature [duplicate]

I was amazed to learn that we can use Imaginary unit iota into physical quantities like time and Temperature but how exactly? The explanation was not something I would say stellar so I am hoping can ...
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A quantum gravity replacing the time variable by an operator

In this video (from 57:38 to 58:31) Aurélien Barrau talked about a simple obstruction for the existence of quantum gravity, stated in an unusual way (to me): Original in French: Ce qui m'intéresse ...
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In what frame of reference is time measured, just after the big bang?

We often read about the expansion of the universe in the first seconds, or milliseconds after the big bang. What frame of reference is used for these estimates of time? If the entire universe was ...
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Why is the symmetry of measurements in SR no longer valid in GR?

In special relativity, we know that if an inertial observer $A$, having a relative motion WRT an inertial observer $B$, attributes, say, time dilation and length contraction to respectively $B$'s ...
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Is the Universe 11 billion years old?

This clip claims scientists found a star 200M years older than the Universe. However, I took another assertion more seriously: scientists estimated a faster expansion rate of the Universe, driving age ...
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If force depends on both position and time, how to find potential energy?

Suppose force on a particle depends on both position of the particle and time. In this case, how to find potential energy of the particle? To be specific, force is $$ F(r,t) = (k/r^2) \exp(-\alpha t)...
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Are the situation of a man in an obscured freely falling ship in a uniform gravity field and a man in such a ship in deep space equivalent?

Suppose a man finds himself in a spaceship in which he can't see anything that happens outside. And imagine his ship is falling in a uniform gravitational field. For a truly uniform gravitational ...
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Confusing definition of proper time – which is correct?

I have googled for „definition of proper time“ This source https://www.collinsdictionary.com/dictionary/english/proper-time gives the following definition: proper time ... measured by a clock that ...
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How we can measure speed with time, when the rate at which time passes depends on speed?

I was thinking about relativity, specifically about how we travel at the speed of light in 4 dimensions. The higher one’s velocity in space, the lower one’s velocity in time. Inversely, the lower one’...
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As space expands, does this not imply that time also expands? [duplicate]

According to General Relativity, space and time are aspects / dimensions of a unified whole. If the speed at which wave energy travels through spacetime is constant, $c$, then as distance expands, ...
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Why the Lagrangian doesn't have an explicit time dependence?

I have a simple question regarding an example presented by Leonard Susskind and George Hrabovsky in their book on Classical Mechanics The Theoretical Minimum. In page 151, they state: "If there is ...
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If there is no absolute time, how can we say the Big Bang was 13.8 billion years ago? [duplicate]

The Big Bang is supposed to have happened 13.8 billion years ago. Question: Given that there is no absolute time and different observers do not agree on the amount of time that has elapsed, how can ...
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Finding how much time it takes for a complete Earth revolution around the Sun

Long story short, my brother made a joke about how stupid it is to celebrate the Earth making one "trip" around the Sun: New Year's Eve. So I got curious and was wondering: how could the first ...
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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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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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