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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What is time, does it flow, and if so what defines its direction?
This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address ...
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What is $\Delta t$ in the time-energy uncertainty principle?
In non-relativistic QM, the $\Delta E$ in the time-energy uncertainty principle is the limiting standard deviation of the set of energy measurements of $n$ identically prepared systems as $n$ goes to ...
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What is the proper way to explain the twin paradox?
The paradox in the twin paradox is that the situation appears symmetrical so each twin should think the other has aged less, which is of course impossible.
There are a thousand explanations out there ...
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Can black holes form in a finite amount of time?
One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an ...
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Is time continuous or discrete?
I was coding a physics simulation, and noticed that I was using discrete time. That is, there was an update mechanism advancing the simulation for a fixed amount of time repeatedly, emulating a ...
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The Time That 2 Masses Will Collide Due To Newtonian Gravity [closed]
My friend and I have been wracking our heads with this one for the past 3 hours...
We have 2 point masses, $m$ and $M$ in a perfect world separated by radius r. Starting from rest, they both begin to ...
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Is anti-matter matter going backwards in time?
Some sources describe antimatter as just like normal matter, but "going backwards in time". What does that really mean? Is that a good analogy in general, and can it be made mathematically precise? ...
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Is time travel possible? Is it possible to go back in time?
I read somewhere that according to relativity, it is possible - involving black holes and other stuff - to jump into the past. Is it possible for anything to go back in time either continuously or by ...
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Intuition for multiple temporal dimensions
It’s easy, relatively speaking, to develop an intuition for higher spatial dimensions, usually by induction on familiar lower-dimensional spaces. But I’m having difficulty envisioning a universe with ...
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Why are objects at rest in motion through spacetime at the speed of light? [closed]
I read that an object at rest has such a stupendous amount of energy, $E=mc^2$ because it's effectively in motion through space-time at the speed of light and it's traveling through the time dimension ...
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What is the difference between implicit, explicit, and total time dependence, e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$?
What is the difference between implicit, explicit, and total time dependence, e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$?
I know one is a partial derivative and the other is a ...
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More than one time dimension
We know that space-time dimensions are 3+1 macroscopically, but what if 2+2?
Obviously it is tough to imagine two time dimensions, but mathematically we can always imagine as either having two ...
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understanding time: Is time simply the rate change?
Is time simply the rate of change?
If this is the case and time was created during the big bang would it be the case that the closer you get to the start of the big bang the "slower" things change ...
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Symmetrical twin paradox in a closed universe
Take the following gedankenexperiment in which two astronauts meet each other again and again in a perfectly symmetrical setting - a hyperspherical (3-manifold) universe in which the 3 dimensions are ...
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Time as a Hermitian operator in quantum mechanics
In non-relativistic QM, on one hand we have the following relations:
$$\langle x | P | \psi \rangle ~=~ -i \hbar \frac{\partial}{\partial x} \psi(x),$$
$$\langle p | X | \psi \rangle ~=~ i \hbar \...
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