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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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1answer
44 views

Is Time a container of Space, or separate from Space?

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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2answers
72 views

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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8answers
95 views

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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2answers
170 views

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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0answers
25 views

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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0answers
74 views

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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22 views

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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2answers
81 views

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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4answers
108 views

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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3answers
125 views

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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3answers
71 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 ...
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4answers
146 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 ...
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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 ...
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1answer
91 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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1answer
41 views

Expectation of partial time derivatives of $x$ in QM

In Ehrenfest theorem we know that $$m\frac{d\left< x\right>}{dt}=\left< p\right>+m\left<\frac{\partial x}{\partial t}\right>.$$ So how can I exactly calculate a specific $\left<\...
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0answers
15 views

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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2answers
149 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 ...
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1answer
33 views

Time-dependent pertubation theory assigning the order of expansion to squares of the solution

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)\...
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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 ...
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0answers
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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 ...
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2answers
180 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 ...
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4answers
3k views

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 ...
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3answers
176 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." ...
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1answer
63 views

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) ...
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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 ...
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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?
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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 ...
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0answers
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Analytic Elements Haag-Araki theory

Why the elements of a local von Neumann algebra cannot be analytic elements of the timelike translation?
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1answer
68 views

Is it possible to measure the speed of Time? [duplicate]

Is it possible to somehow measure the rate at which time is passing?
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5answers
136 views

Calculating mean velocity: using time or velocity? [closed]

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 ...
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2answers
119 views

Is Young Earth universe distinguishable from non-creationist one? [closed]

While talking today about some religious beliefs I was struck by the next question: So what if some super-natural being forks our universe 6,000 years ago, i.e. creates the exact same copy of ...
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2answers
110 views

Is vacuum energy transient or permanent?

Vacuum energy is based on Heisenberg's energy–time uncertainty principle which says that $\Delta E$ can exist for $\Delta t$. This $\Delta E$ can produce electron positron pair for $\Delta t$ by ...
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1answer
39 views

Highly isolated two different reservoir

Lets say we have highly isolated two different reservoir.One is cold the other one is hot.We just connected them with a tube.(Which is also isolated) Tube has "0" thermal conductivity. If we let ...
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4answers
181 views

Does the age of the universe depend on the way it is defined?

I was reading some earlier posts on this question in stack exchange. These posts give me the impression that the age of the universe is defined in a certain way following prescriptions such as that: ...
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1answer
52 views

Are there any experimental data about potential time dilation caused by cyclical movement of heavy objects?

I am dealing with a completely fabricated scenarios (while trying to write a sci-fi book), so my thougts are completely hyphotetical. But I would like to ask you to help me find out if following ...
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3answers
255 views

Why does everyone seem to think that moving clock appear to tick slower? [duplicate]

I'm getting interested in special relativity and almost all people and explanations about the subject I found on the internet seem to think that when A and B would be in relative motion, they would ...
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2answers
36 views

Does density vary with time in a steady flow?

Steady flow is defined as the type of flow where density, pressure and velocity don't change with time, as far as my understanding goes. However, in the video below, they say that the density in a ...
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4answers
486 views

What is time? Does it flow linearly? If so, how are we sure? [duplicate]

Distances in our world (classically at least) can be defined by considering our world to be a model for the Euclidean normed three dimensional space. But how can we set the notion of time on a firm ...
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7answers
628 views

What is the difference between position, displacement, and distance traveled?

Suppose the question is somewhat like this: If $v=8-4t$ and the position at time $t= 0\ \rm s$ is $2\ \rm m$, find the distance traveled, displacement, and final position at $t=3\...
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1answer
65 views

No explicit time-dependence in Lagrangian means constraints are explicitly time-independent?

Suppose a Lagrangian is not explicitly time-dependent. Does it mean that the constraint equations are also explicitly time-independent, and (as a result) the kinetic energy is necessarily a ...
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1answer
28 views

Why and in what cases we use $E$ instead of $\Delta E$ in energy-time uncertainity relation?

sometimes in problems of energy or time uncertainity we use the value of energy of the particle/system to calculate the delta t. Now what are those cases specific examples, where we can apply this?
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2answers
76 views

Is our physics and system of units based on time (second) or the speed of light?

The speed of light in vacuum is stated in our physics as a universal physical constant, c, when measured locally, in vacuum. The speed of light in vacuum, commonly denoted c, is a universal ...
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3answers
129 views

What's the advantage of Conformal Time?

I'm trying to follow the analysis of CMB Acoustics and several charts are done using conformal time instead of chronological time. Conformal time corresponds to the quotient of the particle horizon ...
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1answer
97 views

How quickly does an atomic electron decay?

I've looked in lots of books on atomic physics, but I've not been able to find an answer to: Question: Take a hydrogen in empty space whose electron is excited. How long will it take for the ...
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1answer
53 views

Different statements of the Heisenberg uncertainty principle [duplicate]

I know that from the Heisenberg uncertainty principle, ∆x∆p=ℏ/2 . And I know that this equation can be rewritten as ∆t∆E=ℏ/2. From QED I also know that the equation ∆t∆E=ℏ/2 claims that some energy ...
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1answer
117 views

How to define the proper time of a photon?

I'm writing a paper about the motion of photons near a Schwarzschild black hole. At some point there's a derivative of the Hamiltonian of the system with respect to time $\tau$. I need to explain what ...
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1answer
57 views

What does this mean? World consume 14 trillion watts of power [closed]

I am not very clear about this statement. Does that mean in every second we are consuming that much energy?
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2answers
53 views

Time to move along a path?

Let's say I have an arbitrary path from point $A$ to point $B$. A particle moves from $A$ to $B$ under the influence of gravity but also impeded by a frictional force $\vec{F_\mu}$. The gravitational ...
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WHY did physicist defined velocity as displacement divided by time, why not displacement * time? [closed]

V=S/T. As per my knowledge i think ratio as division and it don't give any meaning like this much displacement in this much time. So i think physicists only used division as notion for velocity. But ...
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0answers
17 views

Do orbiting masses accelerate or decelerate (relative to us) near black holes? [duplicate]

Apologies if this question seems stupid; I was just curious after watching a documentary, and I am not a trained astrophysicist. Simply put, I cannot reconcile the (perhaps misunderstood) facts that ...