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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28 views

Are the laws of physics eternal? [duplicate]

It is scientifically assumed that objects on a planet or heavy bodies fall towards it, when not supported by some ground; irrespective of time. Over a million years this does not change. Its the same ...
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What are timelike singularities? [duplicate]

What are timelike singularities? I'm trying to learn about the properties of timelike singularities.
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What is the remarkable difference between dream and life - as we traverse or jump from an event to another event? [closed]

Is it not possible, to know more about how our consciousness jumps from an event to another event in real life, if we studied how we jump from event to another event in a dream. To start with, what ...
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Using Maxwell and electromagnetism to solve for $T$ [closed]

$$\textbf{F} \cdot d\textbf{l}=q(\textbf{E}+\textbf{v} \times \textbf{B})\cdot \textbf{v} dt$$ If we denote $q=\rho d \tau$ and $\rho \textbf{v}=\textbf{J}$ $$\frac{dW}{dt}=\int_{V} (\textbf{E} \cdot ...
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What does the plot of the universe's Entropy versus Time starting from the big bang look like?

What does the plot of the entire universe's Entropy versus Time starting from the big bang look like? Is it even possible to create such a figure?
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Why do scientists need to measure extremely small intervals of time?

Why do scientists need to measure extremely small intervals of time? Why is it necessary?
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What are the accepted abbreviations for Million Years and Billion Years?

I'm putting together some charts and the time axis in one shows billions of years, the time axis in another shows millions of years. I've seen in various publications $my$ and $myr$ (also $ma$) for ...
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How to prove time translation invariance of Lagrangian for a free particle?

In my textbook, the author deduce the expression of the lagrangian $L(q_i(t), \dot q_i(t), t )$ of a free particle only using classical physical symmetries where the $q_i(t)$ are independent ...
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Time dilation in 2+ dimensions. Relativity problem

I am a student and due to school closures, I am reading ahead in physics. I have been learning about special relativity and I made up a problem for myself. I've drawn a diagram below: The diagram ...
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When it's noon on the Moon, then what time is it here?

Is there a way to calculate the time it has to be on Earth at any moment in time when a person on the Moon would say their clock reads noon (i.e. a time at which a sundial on the Moon would cast no ...
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Why don't these conversions of time between reference frames seem consistent? [closed]

I am a student and due to school closures I am reading ahead in physics. I have been learning about relativity and I made up a problem for myself. You can see a diagram that I drew below: The diagram ...
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Investigating colour of a star when in motion [closed]

The star Alpha Centauri is 4 light years from Earth. For an astronaut travelling at $c/2$ from Earth to Alpha Centauri. So, this apparently means The colour of a distant star perpendicular to the ...
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Does Time Require Matter to Exist?

I've heard that General Relativity entails matter (or mass) is necessary for time to exist. However, there are vacuum solutions where the universe is empty of matter but still has spacetime. P.s. I'...
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Does time travel faster than the speed of light? [closed]

I don't know enough about the subject to know that I am wrong.
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Time Dilation and “existence” of time [closed]

I came up with what might be considered a strange conclusion when thinking about time dilation, and more specifically the Hafele and Keating experiment from 1971. It was shown that time either went ...
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Consistency in the laws of physics [duplicate]

When I read about past events in the universe (i.e. big bang, formation of the sun and planets, etc) future events (i.e. sun becoming a red giant) and distant objects (black holes, nebulas, galaxies, ...
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Difference between Gullstrand-Painlevé (GP) time coordinate and proper time $\tau$

The GP coordinates version of Sch. metric uses a new time coordinate, t', which is the time read from a free-falling clock that was dropped from rest at large r and falls past the event we're ...
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If photon experiences time in an instant, does it mean that it's predestined to be consumed? [duplicate]

That one bothers me a lot. If time is collapsed to an instant for the photon, and it experiences it's own end of lifetime, what should happen if photon is never going to be absorbed? Especially ...
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How is time defined in time dilation?

It appears that there are two different concepts of time dilation (in my amateur point of view): Time delation due to relative velocity. Let‘s say a second is measured by the time it takes for light ...
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What does a function of proper time mean and how do you transform from proper time to real time?

A covariant formulation of electromagnetism says the equation of motion of a charged particle in an electromagnetic field is $$m\frac{du_\mu}{d\tau} = qF_{\mu\nu}u^\nu.$$ But this equation is in terms ...
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Can we provide a universal clock with atomic clock?

If we use normal clocks then due to the relativiy there will be misleading time, but what will happen with certain atomic clocks with less mistake and relative? If we can measure the time, what ...
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Meaning of “time existing locally relative to a reference frame”

I'm reading a book in which it's written Relativity shows that time exists locally, relative to a given reference frame. What does this mean? I'm a bit confused because this seems like a ...
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How does entropy increase over time?

According to 2nd law of thermodynamics, In an isolated system entropy increases over time. In an open system entropy might seems decreases but combined result of all related events results in ...
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What is the relationship between Cosmic Time and Scale Factor (or Redshift)?

I was looking at the answer to this question: The difference between comoving and proper distances in defining the observable universe and wanted to reproduce the chart in order to better understand ...
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Expanding universe and Time-translation invariance

My understanding is that there are 5 implicit assumptions when introducing the Lagrangian formulation. Assumptions: We appear to know that the principle of stationary action is true for the universe ...
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Trouble with conflicting results for time dilation for two sets of events [closed]

I was just trying to calculate the time dilation for two sets of events using the Lorentz-Transformation. (I'm trying to understad the special theory of relativity at the moment - I mean what better ...
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Time dilation to speed an experiment

I don't know a lot about special or general relativity, but I was always intriguing about the time dilation effects that it provokes and I always tought, it is possible to use this effect to increase ...
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What does $\frac{Dv}{Dt}$ and $\frac{D\rho}{Dt}$ notion mean here?

Momentum conservation: $$\rho\frac{Dv}{Dt}=\nabla\cdot\sigma+\rho g$$ Mass conservation: $$\frac{D\rho}{Dt}+\rho\nabla\cdot v=0$$ What does $\frac{Dv}{Dt}$ and $\frac{D\rho}{Dt}$ notion mean here ...
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What Ron Mallett doing for time machine here? [duplicate]

Here Ron Mallett pouring liquid chemical on array of lasers for progress of time machine. I didn't understand it. Source What exactly Ron Mallett doing for time machine here?
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Nicholas Gisin's papers about Time in Physics

I recently read the article from QuantaMagazine which says Over the past year, the Swiss physicist Nicolas Gisin has published four papers that attempt to dispel the fog surrounding time in ...
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What is the $d/dt$ in the Schrödinger equation? [duplicate]

I'm wondering, in the Schrödinger equation, $$ {\displaystyle i\hbar {\frac {d}{dt}}\vert \Psi\rangle ={\hat {H}}\vert \Psi } $$ what is the $ {\displaystyle {\frac {d}{dt}}} $ ? I understand it has ...
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How does one get ticks of a clock from an atomic clock's microwave signal?

I understand that in an atomic clock there is a microwave signal that is locked to the frequency of the atomic rotation. However, if one wants to extract time ticks from the microwave signal: $v(t) =...
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Expectation value of time derivative of operator vs. time derivative after operator

Problem 3.18 in Griffiths's Introduction to Quantum Mechanics (3rd ed.) asks to apply the generalised Ehrenfest theorem to operators like the Hamiltonian and momentum operator. The purpose of the ...
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Is temporal causality broken by quantum entanglement?

When one particle of an entangled pair is measured, it forces the state of the other particle. This effect propagates faster than we can measure, certainly faster than light and possibly ...
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Is there any method to know the real age of the universe? [duplicate]

Well, I was wondering about the real age of our universe, I found that it's estimated to be $13.8\times 10^9$ years. Is it an approximation, or laws behind this age?
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Applying force in zero seconds? How is this possible?

$F\,\Delta t=m\,\Delta v$, is the equation of impulse. Let's say that I push a wall with my hands, we can have $0$ change in velocity; therefore, $F\,\Delta t=0$, since I'm pushing the wall there is ...
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1answer
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Particles reaching a singularity [closed]

If the gravity well steepens to infinity at the point of a singularity, then so does the "time well". Understandingly, the inward falling energy/matter is not aware of this change in time relative to ...
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What is the preferred clock, does choice of clock define the laws of physics?

I was wondering about the idea of time and its measurement, i.e. clocks. As I understand, we define time by the count of a recurring event. Let's assume that we use two atomic clocks A and B to ...
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Does a time-independent system have a time-independent Lagrangian?

Consider a Lagrangian $L:\mathbb{R}^3\times\mathbb{R}^3\times\mathbb{R}\to\mathbb{R}$. Suppose it has the property that if $\gamma:[t_1,t_2]\to\mathbb{R}^3$ solves the Euler-Lagrange equation, then (...
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What does the operator's explicit dependence or independence on time actually mean in Quantum mechanics?

Consider the equation of motion for the expectation value of an operator $A$ $$\frac{d\langle A\rangle}{dt} = \frac{1}{i\hbar}\langle [A,H]\rangle + \left \langle \frac{\partial A}{\partial t} \right \...
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140 views

About the definition of “common time”

In a translation of Einstein's article about special relativity, we can read, If at the point A of space there is a clock, an observer at A can determine the time values of events in the ...
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Second dominate phenomenon to effect the speed of time? [closed]

Part of Einstein's Theory Of Relativity is that the faster an object and/or person moves through space, the slower they move through time. What other phenomenon could potentially effect the passage ...
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Does time dilation mean an object moving faster advances faster in time? [duplicate]

I tried to read a bit about relativistic time dilation, but the maths and complex physics overwhelmed me, confused me -I'm not trained in this field beyond common divulgation and basic education, and ...
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Travelling to the future while seemingly standing still?

Some time ago I thought about this possibility, knowing about the time dilation from general relativity. What would theoretically happen if I travelled at near the speed of light in a circle that has ...
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The origin and current status of the equation $\boldsymbol {Et}-\boldsymbol {tE}=\frac{h}{2\pi i}$ [closed]

Heisenberg already in 1927 quotes this equation as a "known equation".[1] I would like to know that what was the original conception behind this equation, and what is its current status. According to ...
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Does time exist without Gravity?

If you travel the universe and move into an area where there is negligible gravitational influence, does time stop? As an observer, would this be like watching someone cross the event horizon of a ...
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Water electric dipole and time reversal

Why some objects, such as water molecule, can have a (large) electric dipole moment without violating time reversal symmetry, while the existence of an electron or neutron electric dipole moment would ...
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Cases of various time symmetries

Is it possible to cook up three physically relevant examples where the Lagrangian has explicit time dependence but the system still has one of the following? time-reversal invariance, time ...
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What was the rationale behind Stephen Hawking's “biggest mistake”?

Albert Einstein said he made his biggest blunder, while Stephen Hawking spoke of his "biggest mistake". One can read this in this interesting book review (The Nature of Space and Time by Stephen ...
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Smaller elapsed time with higher velocity, but free fall maximize elapsed time, who clearly have a velocity compare to a stationary object

I am a little confused because an object with velocity would experience smaller elapsed time compared to an object that is not. But in GR elapsed (proper) time is maximized by free fall who have ...

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