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# 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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### A question about the definition 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 ...
8k views

### Can black holes form in a finite amount of time?

One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an ...
10k views

### What is the proper way to explain the twin paradox?

The paradox in the twin paradox is that the situation appears symmetrical so each twin should think the other has aged less, which is of course impossible. There are a thousand explanations out there ...
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### Who change is constant? [closed]

How change is constant? As we know when universe created n till today every thing is changing then who would we say change is constant? And also when a baby is born and till his or her death they ...
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### What is the relationship between time and causality? [closed]

In one equation, could someone theoretically relate time and causality? What would that equation look like? This is my first question :)
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### Calculating Acceleration To a Predetermined Point [closed]

I've been struggling to find a solid solution for a problem I'm having. I have an object in 2D space moving at some initial velocity, from some initial point to some final point, over a determined ...
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### How should be 'time' understood? [duplicate]

Is time an invention or discovery? How to relate time with the gravity via general relativity theory?
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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 ...
8k views

### What is time, does it flow, and if so what defines its direction?

This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address ...
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### Problem of now. How physics deals with it? [duplicate]

i know this is kind of off topic. from my understanding in relativity spacetime is frozen, nothing changes, there's no "time" as we understand it in everyday life. is there anything in physics that ...
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### Notion of Present

Can't I sync all watches in spacetime and call this time slice the present? In Carlo Rovelli's book he tried to explain that the notion of the present is local only, which I could not follow.
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### Is mass persistent in spacetime?

As an object moves through spacetime is its mass persistent? What I mean is, if we find a way to look backwards into time, will a pen sitting on a desk when I was in college still be sitting on that ...
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### Does the age of the universe depend on the speed of light?

In the Big Bang theory of the universe, does the value of the speed of light enter into the calculation that determines the age of the universe, and if yes, in which way?
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### What does one second after big bang mean?

Consider the following statement: Hadron Epoch, from $10^{-6}$ seconds to $1$ second: The temperature of the universe cools to about a trillion degrees, cool enough to allow quarks to combine ...
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### Does time emerge from entanglement?

I stumbled upon the hypothesis that time emerges from quantum mechanics as long as the observer is part of the system. That is, the evolution of an entangled state can serve as a clock for an observer ...
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### Clock postulate validity

As far as I understood, the generalized clock postulate says that there is no difference between the measurements of a non-inertial observer (NIO) and those of his co-moving inertial observer (CMIO) ...
843 views

### How to explain (pedagogically) why there is 4 spacetime dimensions while we see only the 3 spatial dimesions?

I have been asked this question by a student, but I was able and in the same time incapable to give a good answer for this without equations, so do you have ideas how one can explain this in a simple ...
68 views

### Find velocity from acceleration equation [closed]

Suppose the acceleration of a particle is a function of $x$, where $$a(x) = (2.2 s^{-2})x$$ (a) If the velocity is zero when x = 1.0 m, what is the speed when x = 3.4 m? (b) How long does it take ...
785 views

### The Big Crunch and perceived entropy

I'm aware of the Big Crunch theory, that once at capacity, the universe may collapse in on itself. Hawking once theorized that time may go backwards during this crunch. So, that got me thinking: how ...
213 views

### Cellular automata and time T

Reading around about various CA models I saw that there are even those who are following the track to provide a single mechanicist model of the universe. For example Gerard van 't Hooft claims that ...
145 views

### Why don’t I see proper time?

Say we have my friend blasting off in a spaceship to Planet Physics and I’m sitting here on earth. I think it’s quite obvious that I will measure the proper length between the planets (because I could ...
193 views

### How can we understand acceleration direction (positive or negative)?

I am studying motion in one dimension. I am having hard time to determine acceleration either positive or negative. How can I understand this? For example in this question I thought accelaration was ...
21 views

### Time in the second law of thermodynamics versus relativity [duplicate]

I recently had a debate with someone about time and argued that the most concrete definition of time would be an increase in entropy. The entropy of a localized area of space might reduce, but that of ...
90 views

### Why is the age of the universe important?

Age may only be a number, but when it comes to the age of the universe, it's a pretty important one. Why?
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### Why does nature prefer simultaneous events? [closed]

Nature mostly prefers simultaneous events: Acceleration is produced without any delay on applying force. Angular acceleration is produced without any delay on applying torque. A bulb glows ...
10k views

### What is the best way to calculate impact time with collisions?

I've been teaching myself physics and I've been wondering about the impact time in collision calculations. The scenario I've been using to learn is an object with a mass of 4000 kilograms colliding ...
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### Time ordering identities in integration over gluon fields

My question arised when trying to compute the Wilson Loop of a hybrid meson. When calculating the loop one has to keep in mind the path ordering and time ordering respectively. I have the following ...
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What is the square of a solution from time dependent pertubation theory? Assume we have found the corrections up to second order such that $$|\psi(t)\rangle \approx |\psi^0(t)\rangle + |\psi^1(t)\... 1answer 314 views ### Does electron have some intrinsic ~10^{21} Hz oscillations (de Broglie's clock/Zitterbewegung)? Louis De Broglie has postulated in 1924 that with electron's mass there comes some \approx 10^{21}Hz inner oscillation: E=mc^2=h f=\hbar \omega. We would get such oscillation e.g. if using E=mc^... 2answers 148 views ### Validity of the derivation of time-energy uncertainty principle? Background The top voted answer to this question seems to make some assumptions I'm uncertain about (to say the least): What is \Delta t in the time-energy uncertainty principle? Assumptions The ... 1answer 328 views ### Correlation Amplitude in QM The following is a section "Correlation Amplitude and the Energy-Time Uncertainty Relation" from Sakurai's Modern Quantum Mechanics book page 79: Question: Why does it state that the oscillations ... 2answers 98 views ### What is the experimental evidence for gravitational time dilation? There are many experiments done to prove time can be changed. I don't know lots of them, but I think the most popular (most cited) is to put a clock on a fast moving object (airplane), fly around and ... 3answers 175 views ### Are all processes time/CPT-reversible, e.g. measurement, stimulated emission, state preparation, Big Bang? "The CPT theorem says that CPT symmetry holds for all physical phenomena, or more precisely, that any Lorentz invariant local quantum field theory with a Hermitian Hamiltonian must have CPT symmetry." ... 1answer 579 views ### When can one omit a total time derivative in the Lagrangian formulation? I am studying Lagrangian and Hamiltonian mechanics and i am using Landau & Lifshitz and Goldstein books. Both of them state that a modified lagrangian$$L'=L+\frac{df}{dt}$$gives the same ... 1answer 4k views ### How to determine day/night based on latitude, longitude and a date/time? Is there a simple method of determining, given a UTC date/time, whether it is day or night at a given lat/long coordinate? I am currently using a formula based on a Sunrise/Sunset Algorithm from the ... 2answers 179 views ### How is the global time coordinate t (“observer at infinity” time) defined operationally e.g. in the Schwarzschild metric? This is a question about coordinate time versus clock time / observed time, which I want to understand because I am teaching a GR course. Consider the Schwarzschild metric for specificity. I ... 0answers 24 views ### Can a wormhole change the wavelength of a light beam? If a light beam went into one mouth of a wormhole, and came out the other, are their factors that would result in it coming out red or blue shifted? Examples, if it came out of a mouth that was in ... 0answers 23 views ### Equations of motion due to gravity [duplicate] This question has probably been asked before, but I couldn't immediately find anything with a search. Two masses of m_1 and m_2 are in space separated by a distance of x. They now accelerate ... 14answers 32k views ### Is time continuous or discrete? I was coding a physics simulation, and noticed that I was using discrete time. That is, there was an update mechanism advancing the simulation for a fixed amount of time repeatedly, emulating a ... 0answers 33 views ### Can a region of space-time be viewed as a particle with properties of it's own? [duplicate] Can a piece of space-time having 3 spacial dimensions and 1 dimension of time be viewed as a particle? Could this piece of space-time have properties of its own (mass, spin, charge) independent of ... 3answers 5k views ### What is the length of 1 second in meters If time is treated as a fourth dimension of spacetime, what is relation between length and time units? Or in other words, how can I convert time units to length units, for instance seconds to meters? 1answer 41 views ### Moving clocks observed with different observers If a clock is moving near to speed of light with constant velocity it'll slow down, but if I were to observe from the clock will the observer on Earth slow down? 2answers 115 views ### Are these Stephen Hawking's statements legitimate? [closed] I am reading 'Brief Answers to the Big Questions' by Stephen Hawking and it seems to me that some of his statements are just hypotheses, but they are written in such a way that they 'sound' like they ... 0answers 40 views ### Analytic Elements Haag-Araki theory Why the elements of a local von Neumann algebra cannot be analytic elements of the timelike translation? 2answers 127 views ### Eulerian mass conservation on a stream line to Lagrangian mass conservation if the density of a fluid particle is conserved on a streamline,$$\frac{d\rho}{dt}=0.$$Why does this mean$$\frac{\partial \rho}{\partial t}+(\mathbf{v}\cdot\nabla)\rho=0 is true everywhere? Why ...
Background I have been running tests which involve timing an object moving a certain (fixed) distance, $s$. Each test has been repeated 3 times, and the 3 times ($t{_{1}}, t{_{2}}, t{_{3}}$) for the ...