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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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Can time dilation show who's moving?

Imagine a person going in a spacecraft moving away from the Earth, when he return from his journey he notice the time has passed slowly than for the habitants on the Earth. Noting this, we can ...
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1answer
45 views

Special relativity and time dilation

Im studying Relativity and there is something that my mind really cant proccess right now. That would be the Deformation of time on heavily gravitational bodies. For an example lets take the movie ...
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1answer
31 views

Could universe expansion continue after the universe reached thermodynamic equilibrium

Stimulated by reading this question Would time still exist after Heat Death of the Unverse? it seems that a literally eternal expansion is not precluded. This would be in accordance to a real ...
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Interpretation of the energy-time uncertainty [duplicate]

From the uncertainty relation it follows that: $$\Delta E \ \Delta t = \hbar$$ $\Delta E$ is the energy uncertainty of a state, $\Delta t$ should be the uncertainty of the lifetime $\tau_b$ of the ...
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The speed of light and friction [on hold]

In a friction free universe, light and really any object should be able to cross the universe in an instant. But without friction this wouldn't happen and time wouldn't exist. Friction=Time
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1answer
39 views

Can position be function of a variable except time? [on hold]

Can position be function of an independent variable other than time ? or it doesn't have a physical meaning (e.g. x(P)=3P-7 where P is pressure) ? from mathematical point of view such a restriction ...
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What is the best adopted theory of time? [closed]

I have read lots of theories in physics, but I haven't come across any about my favorite topic. Time. My question is, what is the best theory that explains the concept of time.
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Is there a correlation between aging and the increase of organisms entropy?

This question is a restatement of Death by entropy. Question: Does aging, as we understand it nowadays (shortening of telomeres etc.) goes hand in hand with the increase of entropy? If so and an ...
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Horizontal Projectile motion question [closed]

The question is that: An electron in a cathode-ray tube is traveling horizontally at 2.60×10^9 cm/s when deflection plates give it an upward acceleration of 5.60×10^17 cm/s2 . How long does it take ...
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Time translation invariance of Hamiltonian

I am learning about the time translation invariance of the Hamiltonian. I read that the time translation invariance is already manifest in the fact that our Hamiltonian is chosen an ...
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1answer
39 views

How does time-translation symmetry morph into evolution in time?

I am reading Ballentine's textbook "Quantum Mechanics: A Modern Development". In it he transitions from discussing time-symmetry to discussing evolution (of the state) in time. I'm finding it ...
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Meaning of time derivative of an operator

Today when my professor was deriving this equation: $$\frac{\mathrm d\langle A\rangle}{\mathrm dt}=\frac{i}{\hbar}\langle\left[H,\,A\right]\rangle+\left\langle\frac{\partial A}{\partial t}\right\...
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1answer
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Hamiltonian time-independent, partial derivative always zero?

For conceptual simplicity, let's restrict the discussion to systems with a two-dimensional phase space $\mathcal P$ with generalized coordinates $(q,p)$. Hamiltonian is a function that maps a pair ...
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1answer
45 views

Consistency of time-dependent and time-independent perturbation theory

I am confused as to how time-independent and time-dependent perturbation theories in QM give consistent results at the instant the perturbation is switched on. Suppose I have a two-level system which ...
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2answers
35 views

Is there experimental evidence for the irreversibility of the collapse of the state of a system due to a measurement?

It has been pointed out by one of my professors that, in quantum mechanics, the collapse of the state of the system by measurement is not time-reversible, i.e once the system collapses due to a ...
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1answer
45 views

Biological aging interval equals Proper Time interval?

In special relativity does proper time interval equal biological aging interval in every inertial reference frame?
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106 views

Time's ticking slower vs. fewer events' occurring [closed]

In modern physics, or at least its non-mathematical version for laymen, we hear of time's "going slower" or "faster." In the rocket experiment for the Twin Paradox, for example, time "went slower" ...
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Why is the motion described by $F = -kx^{2n+1}$ periodic?

I understand the motion when: $$ F=-kx $$ which leads to the an ODE that then has solutions expressed as: $$ x(t)=A\cos(\omega t)+B\sin(\omega t) $$ This is periodic since a cosine function is ...
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1answer
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Swing - time taken [duplicate]

I was thinking about how I would go about calculating the time taken for a swing to swing from one side to the other, assuming that there only exists a gravitational force and discarding all other ...
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1answer
61 views

How is four-velocity automatically normalized?

This is a page from Sean Carroll's Spacetime and Geometry.There is a line in this page which says that the four velocity is automatically normalized.This absolute normalization is a reflection of the ...
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2answers
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How does calculating Hubble time work?

For calculating we take $t=\frac dV$, where $V=Hd$ . But velocity isn't uniform for an object as distance also increases with time. And if velocity isn't constant the equation $t=\frac H d$ also isn't ...
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2answers
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What would the position vs. time graph look like if the velocity vs. time graph was a curve?

In my physics class we are learning about velocity vs. time graphs and position vs. time graphs for an object. We have learned that if the object is moving with constant velocity (this would be a ...
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1answer
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What do you think of this conceptualization of the universe and time dilation? [closed]

So the universe is ever-expanding. It's been expanding in all directions since the Big Bang, like how a sponge expands when you give it water. The universe is made of a fabric known as space-time. The ...
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Why is the MKS unit of time the same as the CGS unit? [closed]

There are many system of units used in physics. In the CGS, the units are, length : centimetre mass : gram time : second And in the MKS system the units are, length : metre mass : kilogram time : ...
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What formulas do I need to calculate the time it takes for a room with opened window to become the same temperature as outside?

I want to calculate how long I have to open my window to become the same temperature as outside. To make things more easy let’s assume there’s almost no wind helping the exchange of temperature.
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2answers
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Calculation of Gravitational Collapse time

The answers to Gravitational collapse and free fall time (spherical, pressure-free) describe the calculation of collapse time for a gas. For the case of neutral hydrogen gas with number density $10^4$/...
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Is it possible that D-T graph could be linear , but V-T graph will be log?

So , I have plotted the V-T & D-T plots for solar sail using the parameters (Using euler's method for solving the ODE based on Solar sail, estimate force & Here's my program :- https://github....
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1answer
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Average Velocity Question (isolation?)

So there is this question i've been having trouble with for the past hour. It involved a child going down a ramp on a sled at and angle theta with a total system of mass. accelerates down a hill of ...
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Why does a sign difference between space and time lead to time that only flows forward?

Ever since special relativity we've had this equation that puts time and space on an equal footing: $ds^2 = -dt^2 + dx^2 + dy^2 + dz^2$ But they're obviously not equivalent, because there's a sign ...
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1answer
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Does light know the future? [duplicate]

As I understand it, if light is traveling at the speed of light, then from it's point of view space is fully compressed in its direction of travel. Does that mean that from it's point of view, light ...
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1answer
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Interdependence of time, the speed of light and distance in General Relativity

In the relation $$ \text{time}\cdot\text{speed of light} = \text{distance}\,,$$ by definition, the speed of light is constant. When bringing a clock, like in a GPS satellite, up into orbit, i.e. ...
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2answers
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Why did Einstein pick his definition of synchronous clocks?

In his paper "On the Electrodynamics of Moving Bodies", Einstein defines synchronous clocks as clocks which satisfy $t_B-t_A=t'_A-t_B$. $A$ and $B$ are two points separated by empty space, and ...
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If things travel at different time rates due to relativity wouldn't one of them disappear from my perspective? [closed]

If I travel to a 3d location where Einstein was known to have existed at some other time, I do not expect to see him, because his 4d location is different! Why then can I send a clock around a black ...
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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 $m_2$ kgs are in space separated by a distance of $x$ meters. They now ...
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3answers
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Does time speed up or slow down near a black hole?

The Schwarzchild geometry is defined as $$ds^2=-\left(1-\frac{2GM}{r} \right)dt^2+\left(1-\frac{2GM}{r} \right)^{-1}dr^2+r^2(d\theta^2+\sin^2(\theta) d\phi^2)$$ Lets examine what happens close to ...
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1answer
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Can we always choose a gauge in GR in which time is constant?

In General relativity the metric describes the curvature of 4D space-time. But due to diffeomorphism invariance, many metrics describe the same physics. Can we always choose a metric such that we can ...
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3answers
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If I could halt all motion would I stop time? [duplicate]

I've never really understood time. Time is not a force. It isn't energy or matter. Time doesn't MAKE anything do anything. Time doesn't make a clock tick (motion or batteries do). Time doesn't make ...
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1answer
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How do we write free body diagram for an object that is rebounding?

How do we write free body diagram for an object that is rebounding? Here's an illustrated example Let us assume that the object has an initial force. So, I want to draw free body diagram for each ...
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What does it actually mean to say that time “speeds up” or “slows down”?

I was thinking lately about relativistic time dilation and time passing differently on other planets (which both, eventually turned out to be caused by the same principle). Now, quite naturally (as ...
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1answer
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Does a black body in zero gravity emit EM radiation? [closed]

I have just recently thought of this query. However, I can find no evidence of any experiments ever being carried out on the effect of zero gravity on black body radiation.Perhaps someone here has ...
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1answer
60 views

Can body of one temperature ever cause higher temperature in another body?

If you have one body that radiates heat at a constant rate and, forgive my simple assumption, thus constant temperature, and a body that may absorb this heat, may the temperature of second body ever ...
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1answer
58 views

Time Travel, CTC and Causal Loop

What is the difference between a closed timelike curve and a causal loop? If someone is travelling in a closed timelike curve, are they also in a causal loop?
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1answer
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Is speed of light is the only way to measure time [closed]

After reading up on special relativity, I understood that because of Einstein's postulates -the speed of light in free space is a universal constant Everytime we measure time for an event, directly ...
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1answer
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What are the relativistic arguments for existence of space? [duplicate]

One of the interpretation of special relativity states that the time is an illusion and does not really exist. This interpretation promoted Einstein himself in his condolence letter after the death of ...
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3answers
117 views

Concept of time in General Relativity

I am starting to read General Relativity, and from what i have understood, in this theory, time is a derived notion. An event is not associated with a particular time. But then what is the physical ...
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1answer
27 views

What does “Real-time” mean?

In the context of describing Real-time dynamics of Lattice gauge theories, have they specifically mentioned real-time in order to differentiate it from imaginary-time. Or does it have any other ...
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1answer
55 views

Are there other processes theoretically out there that can cause time dilation besides gravity and velocity?

I am just wondering if there is anything else besides gravity or velocity that causes time dilation in regards to special relativity or other theoretical models of our universe.
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If time was non-existent would we be able to move? [closed]

If time would stop, would we be able to move, or would we be "frozen"? also if we could move, would we be able to move at a infinite speed ?
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How to get the UT1 directly? or How to get the UT1-UTC

I have a question to consult you. Even though I know the relationship between time scales like UT1, UTC, TAI, and TT, I do not know which one is the key point for the computation. That is, from which ...
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How time dilated is the average particle relative to the big bang? [duplicate]

Particles created during the being bang have experienced time dilation by means of gravitation and general acceleration for the past 13.8 billion years. On average, how time dilated is the average ...