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
1,479
questions
0
votes
0answers
37 views
Justification for “running” cosmological constant
I am looking for papers which seek out to justify that the cosmological "constant" could be time-dependent. I am aware that within the framework of GR the cosmological constants really has to be a ...
4
votes
0answers
117 views
What does the time reversibility of the laws of physics mean for causality?
Does the fact that the fundamental laws are symmetric with respect to direction of time show that causation does not exist? Since causality always requires the cause to precede the effect, but laws of ...
7
votes
1answer
131 views
Why are massive particles clocks? Or are they not?
I recently came across a public lecture "Dark matter decay?" by Sir Roger Penrose.
In his lecture he states that the two equations
$E=mc^2$ and $E=h\nu$ can be combined to form a formula for the ...
1
vote
0answers
34 views
Co-moving distance and universal time
I am reading about Cosmology, in particular, the concept of co-moving distance. I am trying to derive the equation for calculating the co-moving distance with the following argument.
If the source ...
2
votes
2answers
165 views
How long does it take to cool down a drink with ice?
If I have a bottle or container with X fluid ounces of water at temperature Y (say, fridge temperature or room temperature), and I put ice or ice sticks (composed of water) in it that make up Z fluid ...
0
votes
0answers
30 views
Why the Chipping Rate of 1.023 MHz for the GPS L1 C/A Signal?
Why was the chipping rate for the GPS L1 C/A signal chosen to be 1.023 MHz (1023 chips per symbol, one symbol per ms)? It seems to me like you'd want to get one chip as close as possible to an ...
2
votes
1answer
30 views
Time dilation : Hourclock on Jupiter
Time slows down on Jupiter because of higher gravity, but Hourclock(sand clock) run faster due to higher g. If I am correct than how?
0
votes
0answers
67 views
Is time an invariant of Galilean transformation?
Is time an invariant of Galilean transformation? By saying that I mean if there is a quantity analogous to spacetime interval in Lorentz transformation. What is the geometry of "Galilean spacetime"?
0
votes
1answer
34 views
Time taken to reach common velocity - collision
Say for instance I have two objects with different masses that collide in-elastically the resulting velocity will be $ v =\frac{m_1 v_1 + m_2 v_2}{m_1 + m_2}$.
What I don't fully grasp is how to ...
0
votes
2answers
41 views
Work-kinetic energy theorem for time-dependent force
Does the work-kinetic energy theorem hold in all the cases? Specifically is true for a time-dependent force?
1
vote
1answer
76 views
New S.I. and the second definition standard
The new SI defines the second as the hyperfine splitting transition time associated to the frequency of that transition of caesium-133, 9192631770 Hz, that is 12 decimals precision, but...I wonder two ...
3
votes
1answer
132 views
Lagrangian and equations of motion in a time-varying coordinate system
I am assuming a very simple case, where there is only a mass $m$ with position $x$ under an external force $F$.
we know that the Lagrangian takes the form $L = (1/2) m \dot{x}^2$ from which equations ...
1
vote
3answers
87 views
Special Relativity - Clock Synchronization
In Special Relativity - Is their a definition of "synchronizing" clocks which are moving relative to each other?
I suspect such a definition makes no sense - as moving clocks belong to separate ...
1
vote
2answers
72 views
On the derivation of Ward-Takahashi identity
I am reading Weinberg's QFT book and in 10.4 he introduced a derivation of Ward-Takahashi identity (where $T$ is the time ordering):
$$\begin{align}
\frac{\partial}{\partial x^\mu}T{\{J^\mu(x)\Psi_n(y)...
1
vote
2answers
134 views
Where does the 379,000 year recombination prediction of the Big Bang theory come from?
The Big Bang theory postulates that recombination happened 379,000 years after the Big Bang.
However, I have never seen a single statement as to how this number came about.
I don't imagine they ...
-1
votes
2answers
64 views
If I place a live video camera on another planet where time moves with a different speed
remember that movie "Interstellar"? Where the astronauts came to a planet where one hour here ment 7 years back on earth?
Well, if you don't:
Imagine a planet, far, far away where time moves in a ...
1
vote
1answer
51 views
How to know is four-velocity is directed towards future or towards past?
Consider a four-velocity vector $\mathrm{u}=(u^0,u^1,u^2,u^3)$. The zeroth component of position is $x^0=ct$.
How do I know if $\mathrm{u}$ is directed towards the future or towards the past? Is it ...
0
votes
1answer
42 views
Could be experience Time without any changes in our enviroment?
Being a beginner in Physics, I found this question to be interesting which came in mind.
Time has been expressed as an dimension which is scalar in nature. Also the GTR also explains Time Dilation.
...
0
votes
0answers
198 views
Time dilation in a geosynchronous orbit
since gravity slows time, and velocity slows time, would observer A on the equator directly under observer B in a geosynchronous orbit experience time at the same rate? Time would slow on the surface ...
0
votes
1answer
53 views
Black Holes and Time [duplicate]
My understanding is that if I am outside a black hole and watch something fall in, the object will appear to slow down and will never actually get sucked in.
So how can we ever get a black hole if ...
1
vote
2answers
53 views
Difference continous - discrete symmetry
I am trying to understand the difference between the two types of symmetries.Wiki
Wikipedia says that
Translation in time : $t \rightarrow t + a$
is a $\textbf{continuous}$ symmetry, for any real $...
-5
votes
1answer
70 views
Is there an intrinsic speed of time? [closed]
If time and speed is relative, then we could double the speed of every particle in the universe, as this increases the clocks rate we shouldnt be able to tell any difference.
But my question is, in ...
2
votes
2answers
61 views
If an object is travelling near light speed, would it's actions seem to be in slow motion?
Hypothetically if we were observing a clock travelling near light speed relative to us, we would see the clock ticking at a much slower speed than us. If that is true, then would all actions that are ...
2
votes
2answers
84 views
How can time go in different directions in the Universe?
This is not a duplicate, I am not asking about any kind of time dilation caused by a BH. My question is about the direction of time (and if it is possible to have different directions for time, other ...
0
votes
2answers
54 views
Perception of time [duplicate]
A video I came across in youtube recently had mentioned how Einstein perceived time. It said he thought of all moments, the present, past and future to exist together and how it is not like one ...
0
votes
0answers
24 views
Why is time 'negative' in a proper distance in Relativity? [duplicate]
so my question is as follows:
the proper distance between two events $a$ and $b$, $S_{ab}$ is given by
$$S_{ab}^2 =
-[x_0(b)-x_0(a)]^2
+[x_1(b)-x_1(a)]^2
+[x_2(b)-x_2(a)]^2
+[x_3(b)-x_3(a)]^2.$$
...
3
votes
3answers
804 views
Why do we perceive time?
Since everything (at least what we observe) is travelling through the time in the same direction and speed why do we even perceive the time? Of course, there is the time dilatation, but it is ...
0
votes
2answers
61 views
Timing in double slits experiment
In this setup the source, the slits and the screen are a lightsecond away from each other. I would like to know if there is any difference if I tempered with the slits before and after the photon is ...
0
votes
0answers
40 views
Antiparticles travelling backward through time [duplicate]
Could someone please explain to me Feynman and Wheeler's theory of waves acting forward and backward in time?
I have read about their idea of an antiparticle behaving as a particle travelling ...
3
votes
1answer
104 views
Do position operators commute at different time?
I had seen a general case that $\hat{q}(t)$ and $\hat{q}(t')$ doesn't commute at different time $t$ and $t'$, where $\hat{q}(t)$ and $\hat{q}(t')$ are Operators in Heisenberg's view. I tried to ...
0
votes
0answers
31 views
Is this a good clock for quantum cosmology?
I was thinking about Quantum Cosmology and the wave function of the universe $\Psi[g,\phi]$ in the no boundary proposal.
I was thinking about what would make a good clock. It would be some function ...
0
votes
0answers
34 views
Why can't we rule out other synchronization procedures?
According to what I've read, it is generally regarded that Einstein's choice to have the speed of light equal in every direction is only a convention, and since there is no way to measure the one-way ...
2
votes
1answer
83 views
The relationship between time, relativity and entropy
I came across a discussion about the nature of time and whether or not time is an illusion on a physics forum. I'm not so much interested in the philosophical issues regarding time, but the following, ...
0
votes
1answer
42 views
What is the true way to determine time parameters from semilog graph?
I have experimental nuclear magnetic resonance data that describe T2-relaxation of the nuclei in the sample of interest. The experimental points obey bi-exponential dependence: I = I1*exp(-t/T2_1) + ...
4
votes
1answer
161 views
Why do we assume the potential is independent of time in the Schrödinger equation?
In just about every text I read (online or in paper), when they handle the time-dependent Schrödinger Equation, I see something along the lines of "we always assume the potential is independent of ...
0
votes
2answers
54 views
Is time a mathematical entity or there is any demostration of its existence? [duplicate]
I an having trouble trying to understand time as a physical entity. We can demonstrate the existence of air, the source of infection, electromagnetism, voltage, amperage, among many other physical ...
3
votes
0answers
73 views
Could two atomic clocks really be used to detect gravitational waves from a distant source? If so, how?
Three articles report on the recent paper in Phys Rev. D: Flanagan, Éanna É. et al. 2019 Persistent gravitational wave observables: General framework (also ArXiv):
Phys.org: Gravitational waves leave ...
0
votes
0answers
40 views
Why don't we normalize the wavefunction in time? [duplicate]
I am aware that because a particle whose wavefunction we are dealing with must be found somewhere, we normalize the probability density in position.
Why do we not normalize the wavefunction in time? ...
3
votes
4answers
142 views
Understanding intuition behind time translation in classical mechanics
In V.Arnold book "Mathematical Methods of Classical Mechanics" he says that invariance with respect to the time for isolated systems means that "the laws of nature remain constant", i.e., if $\phi(t)$ ...
2
votes
1answer
108 views
How to define time in a time-dependent solution?
If a spacetime has no timelike killing vector, how can we define "time" in such spacetime, in order to calculate the time evolution behaivor of some quantities in it?
0
votes
0answers
24 views
Why does entropy increase along the relativistic time dimension?
I've repeatedly read that time flows / is asymmetric because entropy increases as you move away from the big bang and towards the future. This makes sense.
But why is this direction in which entropy ...
0
votes
7answers
344 views
What happens to velocity when Time equals zero?
I am not formally educated in Science but natural questions have always intrigued me.The way I put it is that I am married to Commerce but Science has been a childhood love. Now I have this very basic ...
0
votes
0answers
16 views
Effect of variation in day length on timekeeping
I was reading about atomic clocks when I came to know that precise measurements of time have shown that the time for earth's rotation is slightly more that 24 hrs (on the order of 10^-3). I was ...
1
vote
1answer
97 views
Convective derivative vs total derivative
I was wondering what is the difference between the convective/material derivative and the total derivative. We were introduced to the notion of material derivative
$$ \frac{D\vec{u}}{Dt}=\frac{\...
3
votes
1answer
162 views
What is the physical interpretation of the Wick rotation?
What is the physical interpretation of the Wick rotation?
How is it that we can just propose there's a new time coordinate tau? Are physicists saying time is modeled by an imaginary number? Isn't ...
0
votes
1answer
28 views
Would not the oscillation of light slow down the wave? [duplicate]
Would not the oscillation of the light wave slow down the wave? Does light travel at "c" of its net position over time, or does it travel at "c" on the wave?
1
vote
1answer
49 views
How do you calculate conformal time?
I've got several references that indicate that conformal time is the integral of the scale factor:
$$\eta=\int_0^t \frac{1}{a(t)}\mathrm dt.$$
So I tried to calculating this over a range of {0, ...
1
vote
1answer
181 views
Is the M87 black hole really a black hole? [duplicate]
I apologize if this is a silly question, but...
From what I've been told, new black holes cannot form, because time slows to nearly stopped as matter approaches the event horizon. Instead, a new ...
4
votes
2answers
203 views
Why the time period of pendulum with infinite length is $84.6$ minutes? [closed]
In a book I was reading about SHM it stated:
If the length of a simple pendulum is increased to such an extent that $\ell\to\infty$, then its time period is given by,
$$T=2\pi\sqrt\frac{R}{g}\...
-3
votes
1answer
55 views
Aging while traveling faster than speed of light? [closed]
theoretically, if organic matter was able to travel faster than the speed of light, without becoming "pure energy", and since theoretically traveling faster then the speed of light would travel ...
