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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How does the scale of a cataclysm determine if we can look beyond it?

The textbook I am referring to (Physics for Scientists and Engineers ($9e$), page $1 469$, section $46.11$), has the following paragraph regading the Big Bang: ... This theory of cosmology states ...
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How do clocks, at rest wrt to us, in a saddle-shaped 3D space tick relative to one another?

Clocks placed in a 3D flat space all tick at the same rate, if we look at them when we are at rest wrt to them. On a positively curved space around a mass, the clocks show different paces if we are at ...
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How to convert volt-second data into power-frequency domain?

guys, I am new to physics. Due to my project, I have to utilize my data in the frequency domain. However, my data is in the time domain. So I wanted to ask how I can transform my Volt-time domain data ...
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How do I numerically calculate time it takes a car to travel along a predefined path?

How do I numerically calculate the time it takes a car to travel along a predefined path? Given these inputs: path as a set of points (x, y, plus curve radius at each point) car mass car max absolute ...
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Are time and length independent or can one be derived from the other?

In particular, the time coordinate $t$ can be choosen so that the mathematical expression of the physical laws reflects their inherent symmetries. Already Newton's first law then fixes the time rate ...
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How fast would a clock falling into a blackhole tick relative to the reflection of a clock stationed far away?

Lets say you have two clocks, you attach one to a mirror and drop in into a black hole, while you keep the other next to you, point it towards the black hole and view it's reflection on the mirror. ...
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What is the speed of time/gravity? [duplicate]

Is there a speed of time, or gravity? (Sonic boom > optic boom > time boom)
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The speed of time [closed]

If the speed of light is 14,545.075 x the speed of sound, could the speed of time (and/or gravity) be 'something' like that times the speed of light, say 2,705,384,023.877 m.p.s? (Sonic boom > ...
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How to find time taken for a faster object to cross a slower object of same length, both moving parallel to each other in the same direction? [closed]

Can you explain me how to determine the time taken for a faster object to cross a slower object when they're both of same length and are moving parallel to each other in the same direction?
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How can a time-dependent gravitational field be conservative?

Let's consider 2 point particle graviting the one around the other. Can that gravitational field be considered conservative? I can go from A to B and then, after a time $\Delta t$ come back to A with ...
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The space of the universe is expanding. Is there any corresponding universal phenomenon for time (contraction/dilation)? [duplicate]

Just like how electric and magnetic fields are complimentary, space and time are complimentary as they share the same "fabric". We observe that space in the universe is expanding at about a ...
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What are Time Capsules in Julian Barbour's theory of a Timeless Universe?

Barbour (The End of Time, The Janus Point) describes Time Capsules as Nows (instants of the universe) with awareness of other instants. That we experience time because we 'live in a Time Capsule'. I ...
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Is this line of thought compatible with current physics thinking?

The closer we get to the max speed of light, the faster time is moving from the perspective of the people "stationary" on Earth. This time speeding value is henceforth 'A'. The closer we get ...
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Time dilation - do all moving clocks run slower? [duplicate]

I have some doubts about getting the time-dilation concept right, so I would like to suggest a scenario and see if my conclusions are right or wrong. Person A is on Earth, Person B is on a spaceship. ...
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Confusion with the variational operator $\delta$ and finding variations

I have recently started studying String Theory and this notion of variations has come up. Suppose that we have a Lagrangian $L$ such that the action of this Lagrangian is just $$S=\int dt L.$$ The ...
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Deriving the age of the universe

I am trying to work out the solution to exercise 8.4 from An Introduction to Modern Cosmology by Andrew Liddle. I could derive the Friedmann equation as below, $$\dot{a}^2 = H_0^2 \left[\Omega_0a^{-1} ...
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Quantum Time Crystals

I am not sure I appreciate the implication made by Wilczek here: I definitely see how the expectation value for $\dot\phi$ becomes zero for an energy eigenstate $\Psi_E$ but I do not see what he is ...
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What makes energy "the" conserved quantity associated with temporal translation symmetry?

This kind of relates to my prior question about the non-triviality of temporal translation symmetry and will use some of the same concepts: How is energy conservation & Noether's theorem a non-...
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The definition of Time and second

From the book "University of Physics 15th edition", in chapter 1, they talk about the fundamental units. They stated that the definition of unit of time is based on an atomic clock, where 1s ...
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Gravitational collision [duplicate]

Two point particles of exactly the same m = mass 1kg is separated by a distance of d = 4m. After how much time, they will collide with each other due to the force of gravity? Assume that, the ...
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What is entropy in the concept of arrow of time? [duplicate]

When I first started to study arrow of time, I first heard the word Entropy and after watching and reading internet resources I can't get what is this in the concept of arrow of time. So, please tell ...
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Negative time derivative of state function

I have been working on a relatively simple problem. Just take a quantum wave function for which a physical requirement is that an arbitrary displacement of x or an arbitrary shift of t should not ...
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Perception of light speed when traveling between two light sources

I'm struggling with a fundamental understanding of time dilation and special relativity (if I'm correct). Many online sources explain it as the following: With the ship moving away from the lightbeam ...
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Is the proper time invariant while going from stationery frame to freely falling frame in Genral Relativity?

While reading GTR, I found the following calculation: Consider an arbitrary gravitational field and let us take $x^{\mu}$ as the stationery/lab frame and $\xi^{\mu}$ as the freely falling frame, where ...
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Are Coherent Superpositions of Energy Forbidden in Page-Wootters Construction?

Let me start with a summary of the Page--Wootters (PW) construction to set up convention. My description follows that of Section II.A in this paper. In the PW construction, one considers a Hilbert ...
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Can light exist if it never interacts with anything? [closed]

I've heard some physicists mention that photons do not experience time. From the perspective of the photon, it begins and ends instantaneously. With all the photons shooting out into empty space, it ...
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How does the clock postulate apply in non-inertial frames?

I've read countless answers and other sources on the question of whether time dilation is caused both by velocity and acceleration or only by velocity, but they all look at things only from an ...
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Is time dilation due to relative velocity equivalent in principle to time dilation due to relative gravitational strength?

Is time dilation due to relative velocity and relative gravitational strength equivalent? That is, similar to Einstein's thought experiment where an observer in an enclosed capsule with no windows ...
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Does it make sense to discuss entropy change at different time frames?

The classic cup of tea cooling problem, why does it not violate the 2nd law of thermodynamics. The classic answer would be that the increase in the entropy of the surrounding is greater than the ...
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Can the age of the universe be much bigger than 13.8 billion

If observable universe is only a small fraction of the existing universe, does it imply that the age of the universe is much more than 13.8 billion years or the expansion of the universe is much ...
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How fast can clocks go?

We know that clocks can move slower when near a massive object, however, how FAST can clocks go? Can clocks go as fast as they want?
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Prove for chronological future of open subset of a spacetime $(M, g)$

I'm currently reading $\textit{Semi-Riemannian Geometry}$ of B. O' Neill and after giving the definition of chronological (causal) future (past), he says that: Given a spacetime $(M, g)$ and a open ...
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How to get Lorentz metric from a given light cone structure?

Given a Lorentz metric it's usually easy to figure out how the the light cone structure looks like (e.g. by calculating the null geodesics). But is there a straightforward way to determine the ...
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Reversal of the psychological arrow of time on the contraction of the universe [closed]

In Brief History of Time, it says that the people living in a contracting phase (although unsuitable for living or even existence itself, but lets assume) would remember the future but not the past. ...
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Free Fall time in General Relativity

Suppose you are working with the Schwarzschild metric in GR. $$ -c^2 \mathrm d \tau ^2 = -c^2 \left (1 - \frac{2 G M}{c^2 r} \right) \mathrm d t^2 + \left (1 - \frac{2 G M}{c^2 r} \right)^{-1} \...
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Does time exist in vacuum? [duplicate]

The universe is known as 4D universe (3 space dims, 1 time dim). Does time exist in vacuum? coz no material there to experience time. And if not existing, the universe is just 3D in vacuum?
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Searching for an "intuitive" explanation about how gravitational waves can be detected by a laser interferometer like LIGO

Since LIGO's first detection of gravitational waves, I have been searching for an intuitive way -as long as intuition can be useful in relativity, which often it isn't- to understand how the detector ...
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Will a clock that is isolated and stationary with respect to the CMB report the highest possible value for the age of the universe?

We have a very special clock that has existed since the dawn of time. Its purpose is to measure the age of the universe. It is always very far from any massive body or gravitational field and it is ...
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What if 2 identical pebbles were carbon dated, 1 exposed to time dilation through an extreme force of gravity, and then both carbon dated again? [duplicate]

Essentially the question asks if time dilation has any effect on carbon dating
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When one speaks of spacetime does space mean a location or a distance?

I am a layman who asks ... When one speaks of spacetime does the space part mean a distance as opposed to a location? Is it fair to think of time and space as two sides of a coin in that the ...
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Why are people using 'lightyear' as a measurement of time? [closed]

I came across 2 people in an open forum, who are college students, who that agree using the term 'lightyear' as a measurement of time is correct. Their context was "This country's technology is ...
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Impossibility of closed timelike curves

Hawking's Chronology Protection Conjecture says that the laws of nature must always conspire to prevent a CTC from forming. Why can't we conclude that this is proven? An inconsistent CTC is s ...
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Which parameter to use in relativistic Lagrangian mechanics?

According to Wikipedia the formulation of the Relativistic Lagrangian is: $$L = -mc\sqrt{g_{\alpha\beta}\dot{x}^{\alpha}\dot{x}^{\beta}}+L_I(x,\dot{x}).$$ However, I have read that using both ...
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1 vote
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Energy conservation and time uniformity

Reading Peter Atkin's book Four laws that drive the universe, I cannot see the link between time uniformity and conservation of energy the way it is tackled in this excerpt: " Noether’s theorem, ...
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How "accurately" does the CMB tell the age of the universe?

Just to clarify, I am not asking about clock accuracy per se at all. The reason I am asking this question stems from the fact that on this site, most questions about the age of the universe answer ...
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2 votes
4 answers
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Time's Arrow and QM

Something puzzles me. It is sometimes said that the fundamental equations of physics are time-reversible, creating the problem of time's arrow. But... isn't Schrodinger's wave equation time-dependent? ...
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IF heat was the absence of cold, how would that change physics [closed]

Back in high school, I had an argument with my physical science teacher. She said cold is the absence of heat. I asked why it couldn't be the other way around. She simply stated that was not possible. ...
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Is there evidence for the existence of time? And what's eternalism (block universe)? [closed]

I have been told that: "time is an illusion. Rather than a chronological progression of events, all events occur simultaneously in space, and so right now, dinosaurs exist, WW2 just ended, you're ...
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Why does light move? [closed]

According to Einstein's theory of special relativity, the phenomenon known as time dilation occurs when an object is moving faster than another. The closer to the speed of light this object gets, the ...
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Is this idea/concept very useful to alternatively tackle a brachistochrone problem?

As we know initially we would be given two points on a 2D surface (consider a plane perpendicular to ground ) we would like to run a small ball from higher level point to lower level point from a ...
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