# 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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### The time derivative term in the continuity equation

The integral form of the continuity equation is written as: $$\frac{dq}{dt} + \oint_{S} \textbf{j} . d\textbf{S} = \Sigma$$ where $q$ is the amount of quantity in a certain volume $V$, $t$ is the time,...
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### What about just "postulating" that time runs only forward?

I understand that $v \cdot t$ is a spatial dimension. And I understand that mathematically, the processes are the same if I move it in time or in space direction. However, time is different to space: ...
1 vote
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### Questions about E. Minguzzi's article on Synchronization (arXiv:1009.3005)

Only recently I learned of E. Minguzzi's article "Clocks' synchronization without round-trip conditions", [gr-qc: arXiv:1009.3005] ... (Notably, the article available for download is dated ...
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### Since we observe stars, galaxies, etc in their past - is it correct to say that our present is only present for us but is in the past from afar? [closed]

So - I know that when we observe galaxies, we are observing their past. And that if the same past-them were to be looking at us, they would see our past. But when we look at their past - that is a ...
23 views

### Calculate mass from time dilation

Hi not a physicist in any way so was wondering if I know the age (time passed) and mass of an one object and the age (time passed) of another object, can I calculate its mass? So Object 1 is Earth: ...
44 views

### Relationship between clock rate and speed

Let's consider example: The propagation of light from the middle of a moving train to its left and right ends. From the point of view of a train passenger, the light will reach the right and left ends ...
1 vote
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### Equality of variables for small values of time, when the time derivative of the variables are equal to one another

It is given that $\frac{ds}{dt} = \frac{d\theta}{dt}$, i.e. the time derivative of s and $\theta$ are equal to each other. Does it follow that for small values of $t$, $\Delta s ≈ \Delta \theta$? My ...
1 vote
46 views

### Intuition behind energy not being conserved in Rheonomous mechanical system [closed]

firstly, this is what Rheonomous System means. So, in such a system, the kinetic energy is not exactly just a quadratic function of generalized velocities because one of the generalized coordinates ...
68 views

### Reality of speed and time [closed]

I have a very simple question. How does a faster object takes less time for the same distance? A car with a speed of 100km per hour takes one hour to travel 100km while a car with the speed of 50km ...
1 vote
448 views

### How to explain the time dilation effect non-mathematically (without Lorentz transformation)?

I read many good and rigorous mathematical explanations of the time dilation effect, but I didn't see any non-mathematical explanations. Is it possible to explain the time dilation effect non-...
64 views

### How to measure time in reference frame with clock?

I considered example of time dilation with light clock. I have a question about measuring time in reference frame with clock. If we know that clock move from A to B in the reference frame with clock ...
56 views

### Time dilation query [closed]

In the light clocks, time ticks via the motion of light and since speed of light is constant therefore when the clock is in motion ,the photon has to cover a greater distance by the perspective of an ...
1 vote
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### Why is the Accuracy in the measurement of Time so important in GPS?

I read in a physics book that Without clocks of extreme accuracy, the Global Positioning System (GPS) that is now vital to worldwide navigation would be useless. What would be effects on navigation ...
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### What exists in the world according to the special relativity? [closed]

Before I learned about special relativity, I thought that only one 3-dimensional state of the world exists. Then, like in game of chess, in one "turn" previous state is destroyed - and the ...
470 views

### Has the age of the universe changed in 2023?

I teach high school physics and physical science. I was going through the definitions of theory and law when a couple of my students (of different periods) asked about some recent development that ...
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### Does special relativity imply that there's such a thing as absolute time, or base time?

If time measured by one observer moving at a greater velocity than another observer is observed to be passing more slowly, does this imply that there's such a thing as "absolute time" or &...
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### Theoretical: what is the meaning of nothing? [duplicate]

Before the big bang, there was a point surrounded by nothing (no space or anything). Then the big bang happened and the universe expanded. so beyond the universe's limits, there is nothing? I don't ...
55 views

### What is the reason of different clock's measurements in different frames? [closed]

I tried to understand what is the reason of different clock's measurements in different frames. For this I looked at easy example with distance S traveled by man with moving clock (velocity V1=2m/s) ...
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### I need a ship at rest to accelerate under preferably constant acceleration/deceleration to arrive at rest at an object 55 AU away [duplicate]

I'm working on the story and I need help with the plot point. Assume that the energy needed for constant acceleration is not a problem. And there's no need to complicate this with outside forces. I ...
1 vote
### A unit of time $\frac{GM}{c^3}$ [closed]
I have come across a time unit $M$ that should be equal to $\frac{GM}{c^3}$. I cannot find anything at all about this anywhere. I came across it while doing flux analysis on the flux from black hole ...