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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 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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1answer
31 views

Absolute zero with time measurement as an observer

When referring to absolute zero which I understand as o E(kinetic) are Electrons still in motion. There are explanations as to matter still having some sort of E such as Heisenberg laid out. So when ...
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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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23 views

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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2answers
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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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Time for writing fiction [closed]

I'm going to apologize early if I'm in the wrong section, or that the title is too vague, or that this is just something that shouldn't really be here. I'm writing a fictional character that deals ...
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1answer
61 views

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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51 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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Is Time a relative quantity if yes, How? [duplicate]

According to the theory of relativity, time dilation is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other, or by being differently ...
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46 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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Time dialation in General realtivity and Special relativity are same? [duplicate]

I came across a thought experiment .Consider two persons A and B both have a very accurate and synchronized clocks now if A moves with a velocity compareable to c then its his clock will run slower ...
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1answer
70 views

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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68 views

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
109 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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42 views

If I walk and you stand still, do you get older than me faster?

I was watching a video from Aurelien Barrau about relativity and time. As he was talking to the audience, he said that since he was walking on stage, he was ageing slower than people sitting in the ...
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1answer
26 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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51 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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How does physics explain why time flows? [duplicate]

All the math I've seen explains how things flow in time, as mapped in 4+ dimensional space, but I haven't seen a single model/formula that explains why time flows at all. People ask why it flows ...
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1answer
34 views

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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25 views

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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0answers
17 views

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 ...
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1answer
77 views

What is the meaning of the negative sign in $\Delta s^2 = \Delta x^2 + \Delta y^2 + \Delta z^2 - (c\Delta t)^2$?

In the equation of the spacetime interval formula $\Delta s^2 = \Delta x^2 + \Delta y^2 + \Delta z^2 - (c\Delta t)^2$ is there meaning for the minus sign before the $(c\Delta t)^2$ or is it just a ...
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1answer
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Is time passing really differs on other planets/in interstellar? [duplicate]

I am aware of the idea of time dilation (which I, by the way, asked another question about) on speeds, that get close to the speed of light. Though, I wonder what other (if any) forces and laws affect ...
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How to calculate time of two objects collide? [duplicate]

There are two objects in the space. The first object has mass m1 of 10kg, and the second has mass m2 of 20kg. These objects (centers of mass) are 5 meters away (d = 5m). But how long does it take to ...
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1answer
40 views

Gravitational attraction between two kilogram weights [duplicate]

Imagine two weights, each of mass one kilogram, floating in outer space. Starting one meter apart, and under no forces other than their own gravity, how long would it take before these two weights ...
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1answer
61 views

Why are pendulums' time periods amplitude-invariant? [closed]

Whether the angle is low or high, it returns at the same time. How come this is true?
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4answers
801 views

Twin paradox in curved space time [duplicate]

In a flat space, where special relativity works, a travelling body can only return to the same point if we apply some kind of acceleration to the body. So twin paradox is not a paradox because a ...
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0answers
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Event Horizons and Events [duplicate]

How can two black holes with event horizons merge within the lifetime of the universe? Basically events are happening where events stop (to the outside observer) so does an event horizon actually ...
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2answers
62 views

Conversion from Planck unit to SI [closed]

Good evening, I'm reading the paper Prehawking radiation by William G. Unruh where it says: ...a time scale of order of $m^{3}$ in Planck units, or $10^{53}$ ages of the current universe for a ...
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0answers
62 views

Testing if antimatter is backwards-travelling matter?

I've read about how antimatter can be modeled as matter traveling backwards in time before but always viewed this as a sort of mathematical curiosity. With this question I'm not so much asking if ...
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2answers
68 views

What is the “rate” of the “passage of time”? How does a physicist define and measure time? [duplicate]

I'm trying to separate common concepts of time from the use of time as a standard measurement in physics. One thing I'm struggling to define is the "rate of the passage of time". For example, ...
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3answers
58 views

What are the units of time when planck's constant is equal to 1?

If I express a Hamiltonian $H$ in units of Hz by dividing the energy terms in the Hamiltonian by hbar $\tilde{H}=\dfrac{H}{\hbar}$ which means you set $\hbar =1$. Then what are the units of time? Also ...
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1answer
54 views

Is the violation of time-reversal always associated with the violation of time-translation and vice-versa?

Is the violation of time-reversal symmetry always associated with the violation of time-translation symmetry? What about the converse? Is it possible for one to be violated while the other remaining ...
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If photons don't experience time, does that mean time is a consequence of mass? [closed]

If photons don't experience the passage of time because they have no mass, does that mean time is a consequence of mass? To me that's a profound conclusion yet not one I see printed. Is it right?
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2answers
95 views

Effect of time dilation in Cs atomic clocks?

Let's say, Observer A is at rest and Observer B is in a spaceship moving at a significant fraction of speed of light. B synchronized his atomic clock with A's when his spaceship just passed A. Now, ...
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4answers
6k views

With what velocity are we moving along the time dimension?

Does the question make sense? Velocity along time axis means $v_t=\mathrm dt/\mathrm dt$? If it doesn't, please explain where the flaw is. Taking time as measure like length? Or do we need to ...
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32 views

Why is the dimension of time unique? [duplicate]

Why is the dimension of time seem unique for physicists when any dimension could be perceptually temporal from a different point of relative reference?
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35 views

What is the conformal time today? And at Planck epoch?

We can define conformal time, $\eta$, in terms of physical time, $t$, through $$ dt = ad\eta $$ where $a$ is the scale factor, which is a function of $t$ or $\eta$. I'm trying to compute conformal ...
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Can time flow backwards? [duplicate]

Is there any equations in physics or laws that say time can flow backwards instead of forward?
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1answer
43 views

Is the formula for time dilation applicable only in 1D?

According to time dilation formula $$ \Delta t_{observer} = \frac{\Delta t_{proper}}{\sqrt{1-(v/c)^2}} $$ I have basically two questions: (1) Is this formula applicable only for 1D motion, i.e. ...
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2answers
93 views

How to deal with explicit time dependence of the Lagrangian?

Clearly, if the Lagrangian in explicitly time dependent, the Euler-Lagrange equations being satisfied does not extremise the action. I am unclear as to how to deal with systems with an explicitly time-...
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2answers
148 views

Is spacetime really just time, with no aspect of space? [closed]

In my questions on gravity, I've come to the understanding that spacetime is really just time, and there is no need for space. Anything that requires a distance (for example the distance between two ...
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1answer
62 views

Time and acceleration [duplicate]

I am confused by the explanations of time dilation and gravity vs. acceleration. The consensus (and apparently experiments) is that time dilation in a gravitational field (not free-falling) is "as if" ...
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0answers
85 views

Time and Quantum Mechanics

my question is about the nature of time across classical/macro and quantum scales. I understand that the 2nd law of thermodynamics and entropy increase has a lot to do with our understanding of time ...
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1answer
61 views

I need to find the drag coefficient of a pendulum bob [closed]

Is it possible to find the drag coefficient of a pendulum bob from the damping caused on it during swinging. I will be able to measure its displacement from the point of origin and plot it against ...
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1answer
59 views

Chronology protection: current status

I am looking for some fresh references on the Chronology Protection Conjecture. I am aware of this question, but the answer there seems to resort to energy conditions. But, weren't they shown violated ...
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2answers
80 views

Do we know, or at least have a strong argument for the fact that for a given time interval, we can always find a smaller time interval? [duplicate]

Motivation: In Biology, when, for example, biologists try to model the population dynamics of a population, they say: Let $N: \mathbb{R}^{nn} \to \mathbb{R}^{nn}$ be a function that represents ...
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3answers
85 views

Does the Einsteins theory of a clock tower observed from a tram really explain time dilation?

We were shown a video in our class about time dilation which explained a Einstein's theory - the one where he was travelling in a tram and realized that if he travels at light speed, time will flow ...
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1answer
53 views

Cosmic scale factor $a(t)$ in a general frame of reference

In considering an FLRW type universe, the scale factor $a$ is generally indicated as being strictly a function of time $a(t)$. Isn't this only true for a comoving frame? In some other reference ...
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1answer
101 views

How do you write the Wightman function $\langle\phi(t_1)\phi(t_2)\rangle$ for a massive scalar field in position space?

For a free real scalar field $\phi(t,\mathbf{x})$, we define the Wightman function as: $$ W(t_1,t_2) \equiv \langle 0 | \phi(t_1,\mathbf{x}_1) \phi(t_2,\mathbf{x}_2) | 0 \rangle $$ I'm suppressing the ...