# Tag Info

### Intuition for multiple temporal dimensions

I have been a self-guided student of theoretical physics for over 30 years. I don't have any formal training. I have been working on a model of our universe ever since I purchased and read Dr dr ...
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Accepted

### What does $\delta/\delta t$-derivative represent in tensor calculus?

If $t$ is parameter which varies along some time-like or null geodesic, and say the tangent vector at each point on the geodesic is given by $V^a=\frac{dx^a}{dt}$, then the acceleration vector is ...
• 778
Accepted

### How does the scale of a cataclysm determine if we can look beyond it?

Cataclysmicity is not a physically measurable observable. That part of the quote is not physics. The first sentence should refer to the theories of cosmology, plural possessive. Cosmology is a branch ...
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1 vote

### Are time and length independent or can one be derived from the other?

"Time" is not a quantity at all, it is a coordinate similar to "north", "east", or "up" (or "x", "y", "z"). What Rindler is saying ...
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### How fast would a clock falling into a blackhole tick relative to the reflection of a clock stationed far away?

If you drop it from rest at radial coordinate $\rm r=r_0$ the free fall velocity at $\rm r=r_1$ is $$\rm v=c \ \sqrt{\frac{r_s \ (r_0-r_1)}{r_1 \ (r_0-r_s)}}$$ which in the limit of $\rm r_0=\infty$ ...
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### The speed of time

Are time and gravity the same thing? No, they are not. Could there be a speed of gravity (cause and effect)? Gravitational waves travel at the same speed as light. In quantum theory both as ...
• 7,265
1 vote
Accepted

### How to find time taken for a faster object to cross a slower object of same length, both moving parallel to each other in the same direction?

When solving physical problems it is essential to draw diagrams. This helps to develop a concept in your mind (especially when having no idea yet about the solution). Do this before writing down any ...
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### How to find time taken for a faster object to cross a slower object of same length, both moving parallel to each other in the same direction?

Here you're only trying to find the time taken by this object to cover a distance equal to it's length. So, time=length/speed. Have you been given the speed and length?
1 vote

### How can a time-dependent gravitational field be conservative?

Conservativeness of a field is defined based on its state at single point of time. The closed path trip of the particle is only imagined, it is not supposed to take non-zero time. So it makes no sense ...
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### How can a time-dependent gravitational field be conservative?

Let't consider 2 point particle graviting the one around the other. Can that gravitational field be considered conservative? I can go from A to B and then, after a time $\Delta t$ come back to A with ...
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### Recordings of journey traveling near speed of light

Each recorder shares proper time with its corresponding clock, so both sets record and show the same amount of time during playback. The clocks themselves though, after luminal travel, would show ...

### How can time go in different directions in the Universe?

If time represents all change in the current state of the universe, then any change in the relationship of variables in spacetime could be interpreted as a change in the direction of time. Example - ...
1 vote

### Is this line of thought compatible with current physics thinking?

Statement: "The closer we get to the max speed of light, the faster time is moving from the perspective of the people "stationary" on Earth." Not correct, Time does not speed up, ...

### Time dilation - do all moving clocks run slower?

So $A$'s clock is going to tick off a time of $$t_A=t_0$$ which is the proper time between the two relevant events because $A$ is at rest. As $B$ approaches the planet, he has been moving at $\beta$ ...
• 23k
1 vote

### Time dilation - do all moving clocks run slower?

If person A on Earth looks at his own clock A', he sees his proper time, because he is at rest with respect to his own clock. Similarly, if person B on the rocket looks at his own clock B', he sees ...
• 4,480
Accepted

### Deriving the age of the universe

While doing these types of calculations always use integral calculator site (https://www.integral-calculator.com/) There’s a trick that always works while taking these kind of integrals. Write down ...
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### What makes energy "the" conserved quantity associated with temporal translation symmetry?

The answer is that the statement that there is such a thing as "the" conserved quantity associated to temporal translation - or to any symmetry - of a dynamical system, is simply wrong: ...
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### What makes energy "the" conserved quantity associated with temporal translation symmetry?

I think this is an interesting question. One should keep in mind that the symmetries are in the action S, which is in many cases composed of dual Fourier pairs (position and momentum, or energy and ...
1 vote

### Confusion with the variational operator $\delta$ and finding variations

Yes, that happened. I guess you meant $$\delta f = \sum_i \frac{\partial f}{\partial x_i} \delta x_i$$ on your third equation. Also you've implicity fixed inital $t_0$ and final $t_1$, so that your ...
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### Confusion with the variational operator $\delta$ and finding variations

If the Lagrangian only depends on time through $X$ or $\dot{X}$, then we say that the Lagrangian has implicit but not explicit time dependence. So in your example, we would write L(X, ...
• 33.3k
1 vote
Accepted

### Quantum Time Crystals

He is referring to a more precise definition of order parameter. The idea is to capture spontaneous symmetry breaking: the ground states of the system are not individually invariant under the ...
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### What makes energy "the" conserved quantity associated with temporal translation symmetry?

If I understand the question correctly, then a flow $\Phi^t$ uniquely determines a Hamiltonian vector field $\mathbf X := \mathrm d\Phi^t\big|_{t=0}$. Such a vector field can be written as \mathbf ...
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### How does time work if the universe is the surface of a hypersphere

There is a theory in which our 4 dimensional universe is the boundary of a 5 dimensional hyper-sphere. One needs to send the radius of this sphere to infinity. This theory predicts the emergence of a ...
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What is special about the conserved quantity $Q(x, p) := \frac{1}{2} (x^2 + p^2)$, when also the quantity $Q_2(x, p) = \sin(x^2 + p^2)$ is conserved too, by the same temporal translational symmetry? ...