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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67 views

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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64 views

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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129 views

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
85 views

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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1answer
61 views

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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149 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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30 views

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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78 views

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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77 views

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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219 views

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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100 views

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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Observing from a true stationary point in space is there a “True time” we can observe?

Aren't all points on planets, in solar systems, in galaxies moving through space subject to different amounts of time dilation due to gravity and velocity when observed from a stationary point in ...
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Definition of 1 second [closed]

"One second is the time that elapses during 9,192,631,770 cycles of the radiation produced by the transition between two levels of the cesium 133 atom." Can we alternatively define it as, Frequency ...
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155 views

Is the magnitude of the four-velocity vector in spacetime arbitrary?

I know that the squared magnitude of the four-velocity vector is plus or minus $c^2$, but I’m a little confused on whether or not this vector has been normalised arbitrarily, since it is often claimed ...
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Hubble parameter

When we observe a distant galaxy we must see that as it is moving from us its speed regarding Earth increases according hubble constant.But as seems it is not a constant but an increasing function of ...
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41 views

Synchronization of clock in general relativity

I was reading up on Synchronous Frame on Wikipedia, where this term arises, $$ y^0 = x^0 - \frac{g_{0\alpha}}{g_{00}}dx^\alpha $$ I'm a bit confused as of how this equation can hold? The reason ...
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What conditions would need to exist for another earth-like planet to experience time 4 times faster than earth?

I'm trying to write a story and I want it to be as scientifically plausible as possible. By saying experiencing time 4 times faster, I mean 20 years on that planet would be 5 years on this earth. ...
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2answers
84 views

Can the Hilbert space of a system depend on time?

It's postulated that the state of a quantum mechanical system is represented by a ray in a Hilbert space. I'm aware that the ray may depend on time, but can the Hilbert space itself depend on time? ...
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1answer
50 views

If a device's uncertainty for measuring time is $ 10^{-4} $, what happens to the uncertainty when the experimental value is squared?

In a case where the device's uncertainty is found to be $ 10^{-4} $. As it is a device that measures a time interval; when squaring the experimentally obtained value, what should one do with the ...
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Can we revert back a broken egg into the original one? Given that we are allowed to increase entropy in some other part of the system

Background (and much of the argument for the question) The second law of thermodynamics says(as my book states it): If a process occurs in a closed system, the entropy of the system increases for ...
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1answer
60 views

Age of universe

It's said that at the moment of the Big Bang, space expanded so that to an objective observer matter moved at thousands of times the speed of light. If so, how can we say the universe is 10-30 billion ...
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1answer
108 views

Time between Big Bang and 'now'

To point at first my question has a part of trying to make a concept and is not about was there a Big Bang or not. We accept expansion so it is right to think about a starting point of expansion and I ...
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1answer
54 views

Is there an ambiguity in time in the absence of clocks?

Imagine 2 clocks space 2 meters apart with a vacuum in between. These are macroscopic objects with which we can measure time. But in between these, according to general relativity, we can foliate ...
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121 views

Misconeption about age of universe [duplicate]

If we take the most distant quasars and galaxies from as in all directions we get a sphere of radius 13,7 billion light years. As we know they were there 13,7 billion years ago.This suggests the ...
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80 views

In what circumstances would a 1000 years on Earth be a single day

I am not sure if this is the right place to ask this question. If if isn't then please move it accordingly. I was googling about time dilation and age of the earth and I ran into some argument by ...
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163 views

If photons never experienced time, how do they exist?

If photons are said to always travel in a vacuum at speed of light, then according to the time dilation equation, photons shouldn't "experience" time. If they don't experience time, doesn't that mean ...
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140 views

What is the definition of time in General Relativity?

In physics, we have two categories of quantities: Fundamental: These quantities are defined by a rule how to measure them. Non-fundamental: These quantities are (directly or indirectly) defined in ...
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1answer
114 views

Does matter take infinite time to leave a black hole?

To this question: Can a sufficiently large black hole be singularity-free? about the possibility of a sufficiently large black hole not containing a singularity John Rennie posted an answer ...
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2answers
127 views

What is meant by 'now'? [duplicate]

I'm sorry for my naivety! What is 'now'? How is it different from 'now' that was dt ago, or the 'now' that is dt in the future? That is, what quality might the 'now' possess which the set of all ...
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1answer
83 views

Connection between velocity and time

I have some understanding that a relationship exists between time and velocity. As velocity increases, time passes slower for the person in motion at any significant velocity more than those at rest ...
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2answers
123 views

Problem with the proof of Ehrenfest theorem

My quantum mechanics book has this proof of Ehrenfest theorem: let $A$ be an observable and $\hat{A}$ the operator that represents it. Then we have $$\frac{d}{dt}\langle \hat{A}\rangle=\frac{i}{\hbar}...
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36 views

Is simultaneity independent to reference frame?

Imagine two events happening simultaneously with respect to my frame of reference. Is this possible to have a reference frame to which the events are observed NOT to be simultaneous?
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1answer
49 views

Is there a physical interpretation of the wave equation with a third (or higher) order time derivative?

In physics we learn about the heat equation, $$ \frac{\partial u}{\partial t} = k \Delta u,$$ which describes how heat propagates in a material. We also learn about the wave equation, $$ \frac{\...
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29 views

The passing of time [duplicate]

Did the passing of time come before everything else? As in, how long was there "nothing"? And if we can put a time on that, wouldn't time itself be something? And if so, is time the thing that "always ...
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3answers
103 views

First Law of Thermodynamics as a rate equation

In a standard engineering thermodynamics textbook (Fundamentals of Thermodynamics, Sonntag, et. al, 6th ed) the First Law of Thermodynamics is differentiated with respect to time, and written as a ...
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1answer
59 views

Measuring the solar system's speed through ether with the help of an eclipse

In A.P. French's Special Relativity, the author said, Jupiter has a period of $12$ terrestrial years, and so in half a terrestrial year, while the earth moves from $A$ to $B$ (Fig. $2$-$7$), ...
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51 views

How fast does an electric shock pass through the human body?

So essentially I want to make a 1,000,000 volt Van de Graaff machine and I'm trying to calculate the energy that would pass through the human body if I charged it completely and let it arc to my hand. ...
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2answers
96 views

Connection between affine parameter and time in general relativity

Setup Consider the metric for a spherically symmetric and isotropic spacetime: $$ds^2 = -B(r)c^2dt^2 + A(r)dr^2 + r^2d\Omega^2.\tag{$\Delta$}$$ With this metric we can find (with the geodesic ...
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2answers
97 views

Time in Lorentz transformation

While doing Lorentz transformation for position $$x'=\gamma(x-ut)$$ Here I understand that instead of just length contraction we also use the term $$ut$$ To include the distances between the ...
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1answer
57 views

liquid flowing through tiny holes super slowly [closed]

How could I make a liquid in a container to flow out of several tiny holes super slowly, let's say that the whole container would empty in a month duration. Is it possible? does it only depend of ...
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1answer
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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23 views

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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2answers
86 views

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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