# 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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### 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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### 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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### 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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### 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 ...
128 views

### 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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### 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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### 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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### 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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### 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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### 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 ...
142 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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### 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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### 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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### 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 ...
480 views

### 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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### 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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### 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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### 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 ...
28 views

### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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 ...
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### What causes clock drift in quartz oscillators?

Usually, computer seem to use quartz oscillators. In contrast to atomic caesium clocks they seem to have a relatively big drift and thus we need protocols like NTP to correct them. What causes this ...
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### QM: Time evolution with $H = H(t)$

In order to calculate time evolution in QM we use Schrödinger equation \begin{align*} i \partial_t |\psi\rangle_t = H(t) | \psi\rangle_t. \end{align*} If $H\neq H(t)$ then \begin{align*} i \partial_t ...
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### Time dilation and The Big Crunch [closed]

When/if the universe collapses in onto itself as described by the big crunch theory, would time slow down due to time dilation (relative to our experience) as all matter gets pushed together, or do I ...
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### Can time be described as a distance?

Can we do away with time in the coordinate system? When I picture the x,y and z planes and plot positions, the idea of adding a time element seems redundant. The time element adds a reference to an ...
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### Where in the solar system could we place a clock, such that it would run the fastest (relative to a clock on Earth)?

I know that time dilation occurs between two reference frames with differing velocity, or differing positions in a gravitational field. Many examples that illustrate this point involve things like ...
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### What is the relationship between how time is viewed in thermodynamics and how time is viewed in general relativity?

From my limited understanding of physics, it seems that the second law of thermodynamics, in which entropy never decreases over time in a closed system, relates to how time can only go forward i.e. ...
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### How does gravitational time dilation affect how we age?

Suppose you could live on Jupiter without dying from anything except age, how much longer would you be able to live there compared to how long you could live on Earth?