Time Definition and 1000 Threads

General relativity permits some exact solutions that allow for time travel. Some of these exact solutions describe universes that contain closed timlike curves, or world lines that lead back to the same point in spacetime. I wondered if these solutions also permits Causal loops? Such as the one...

The thread "twin paradox without math" inspired me try to find fully graphical solution of length contraction. Here is the result: Graph is 3D as I think that is ilustrative to make all in one graph, but I am sure that it is possible to do it in 2 or 3 2D slices. X and Y are spatial axes, T is...

This graph shows ##H## as a function of time related to the L-CDM model. Do we (@Jorrie) have similar graphs e.g. for ##\Lambda=0##; ##k=-1## critical, ##\Lambda=0##; ##k=0## open, ##\Lambda=0##; ##k=+1## closed? That would be great, thanks in advance.

I'm wondering is whether it is the gravitational potential (in J/kg) at a point in space that determines the rate of passage of time, or whether it is the gravitational field strength (in m/s2). To clarify, suppose you had a very heavy hollow spherical shell. The gravitational potential would...

I want to make a few of these if it will not cost a lot.

We know that if a projectile is thrown vertically, there will be a brief time when it momentarily stops. Can we calculate this brief moment?

So, I have a question. The time dilation formula is: t = t₀ • 1 / √(1 - v²/c²) Let's take a photon that travels at c. In my opinion, for a photon "clock doesn't tick" and its life is just a moment. But when we calculate time dilation by this formula, then c over c is 1 and the root of 1 minus...

Hi I posted this differential equation to WolframAlpha https://www.wolframalpha.com/input?i2d=true&i=Power[\(40)Divide[a,1+b*cos\(40)y\(40)x\(41)\(41)]\(41),2]*y'\(40)x\(41)=c but no solution , " Standard computation time exceeded... Try again with Pro computation time " Should I ( buy and )...

I have been thinking about the classic example of two twins traveling at different speeds and the effect of time passage. One twin that goes off to travel near the speed of light while the other twin is left on earth. We theorize that the traveling twin will pass through less time and be...

In special relativity we've the invariant ##\begin{aligned} d s^2=&-d t^2 \\ &+d x^2 \\+d y^2+d z \end{aligned}##. For a clock moving along a worldline the above equation reduces to ##\begin{aligned} d s^2=&-d t^2\end{aligned}## , hence we can say that the time measured by the clock moving...

This is a question on Einstein's 1907 paper first discussing equivalence principle and uniform acceleration. Picture a rigid accelerating object of length £ with a clock at each end. The rear accelerates for time τ (measured by the clock there) at a proper acceleration γ. The clock at the front...

Let's say that we're sending a collimated beam of EM radiation through a slit that's 3 cm wide, and it diffracts and forms a clear interference pattern on a screen, with the bands of brightness and darkness, or of high and low intensities. And next, we reduce the intensity of the beam until it...

Interested in Banks' HST these days, for discussion,here are some abstract and an email from L.Smolin. A recent conclusive paper as well https://arxiv.org/abs/2201.06988 The formalism of Holographic Space-time (HST) is a translation of the principles of Lorentzian geometry into the language...

U=U(x,t) Ut=DUxx; 0<=x<=L, t>0 U(x,0)=0 0<x<=L U(0,t)=a(t); t>0 *a(t) is known function* (dU/dx)=0 for x=L I have looked into many ways but not one is usable for diffusion equation with this boundary conditions.

Hi. If I turn on an antenna, it starts sending out radiation. If I turn it off again, the radiation doesn't instantly disappear but dies out smoothly (exponentially?). But this also means the radiation is never completely gone. This looks time-asymmetric, which is weird for electrodynamics. It...

I'm having a hard time understanding some concepts and would really appreciate some help(not super smart so I need some things basically dumbed down). In my physics lab we're going over Newton's Second Law. There's a statement in the lab papers I don't understand. It states "As you should know...

The question is to extremize the proper time : ##\begin{aligned}\int d\tau=\int ( dt^{2}-dx^{2}\\ -dy^{2} \\ -dz^2)^{1/2} \end{aligned}## I've studied calculus of variations somewhat and a can solve a similar problem which I found in the mathematical methods book the author Mary Boas. The way...

In describing the Galelian or Lorentz transformations, All books I've read keep talking about clocks and meter sticks, but I don't see how an event happening away from the observer could be instantaneously described by a set of coordinates and a point in time; information conveying the event...

Hi, It is easy to find discussions about time dilation and muon Half-Life. Is it meaningful to discuss whether bosons capable of pair production can have their decay rate changed if they pass through material?

Could kinetic energy possibly depend explicitly on time in the lagrangian for some arbitrary set of generalized coordinates?

The goal is to calculate the difference in the passage of time between two ends of a cabin which is accelerating upwards due to length contraction. To help in the calculations we can consult: https://arxiv.org/pdf/1807.05338.pdf There is an old problem called the 4/3 problem which has been...

My understanding is that a sub atomic particle has no arrow of time. Clearly the Arrow of time as understood in our macroscopic world has one direction. Entropy being an arrow in the direction toward uniform distribution of the individual and or sums of all the forms of energy in a system and ...

Wikipedia article on proper time "Given this differential expression for ##\tau##, the proper time interval is defined as ## \Delta \tau=\int_P d \tau=\int \frac{d s}{c} . ## Here ##P## is the worldline from some initial event to some final event with the ordering of the events fixed by the...

I agree with Doc. Al that, "For the simplified case of a uniform density spherical planet, the gravitational field varies linearly from 0 at the center to its full value at the surface." But, what is the effect, if any, on the shape and density distribution of such a sphere over time (e.g...

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I'm stuck on this problem, I've tried to follow techniques for similar questions, namely I seem to be struggling with these questions where I have to use an equation inside an equation. I've attached photos of my process so far, but obviously, I'm not getting the right answer because what I'm...

The parts in bold are the questions and also highlights of my attempts at the solutions for each number and letter portion of the question. I attached a word doc to show the diagram and it also has the same questions ;) 4) A launcher is setup at the edge of a table as shown below: (I attached...

Summary: Which are the best engineering topics to really focus on during my spare time to prepare for the real world. Im on a physics and engineering degree, but will hopefully be doing a masters in engineering, my engineering modules are all the standard broad ones that most engineering...

Hartle, gravity pg 34 " An observer in a inertial frame can construct a clock that measures the time t"*)Is time something else that exists irrespective of clocks? What we do is just that we measure it? Pg 35 " ...It's a central assumption of Newtonian mechanics that there is a single notion of...

I am currently in a college. And the college that I am currently at doesn't offer the class online. Can I go to a different college to take the classes online, and still be at the same college I'm at? I need the class online, it makes my school life and work life function more smoothly.

In texts on General Relativity, the proper time ##d\tau^2 = -ds^2## (with an appropriate choice of metric signature) is commonly said that the time measured by a timelike observer traveling along a path is given by the integral of ##d\tau## along this path. Of course it's possible to construct a...

$$B(t) = B_{0} \frac{t^2}{T^2}$$ for ##0 \leq t \leq T## The issue here is more conceptual, because once I find the flux of B I know how to proceed to find the current. I got velocity (but it seems to me that it is the initial velocity), I could use it to find the time in function of space...

Spearmint extract and lemon balm have an effect of making you faster in ball games and martial arts, not only improving mental cognition. https://www.nutritionaloutlook.com/view/nootropic-spearmint-extract-may-improve-reactive-agility-according-new-study The active compound in it seems to be...

Hartle, Gravity "An observer in an inertial frame can discover a parameter ##t##with respect to which the positions of all free particles are changing at constant rates. This is time" Then goes on to say "Indeed, inertial frames could be defined as Cartesian reference frames for which Newton’s...

This is really a conceptual question about time dimensions bit it came up in the context of astronomy so I'll put it here. I was gazing up at the sunset sky the other night and saw a very bright (twinkling) star. I was about to make a note about its location so I could look it up later. I...

hello I'm korean high school student and sorry for my poor English. I saw ## t_0=t_f\sqrt{1 -\frac{ 2GM}{rc^2}} ## in wikipedia. does ## \sqrt{1 -\frac{ 2GM}{rc^2}} ## of this equation have name like lorentz factor ## \frac{1}{\sqrt{1 -\frac{v^2}{c^2}}} ##of ## t=\frac{t_0}{\sqrt{1...

Another noob relativity / cosmology question (although at least this time won't turn out to be a coding bug, as no code is involved...) AIUI, according to relativity, there is no privileged reference frame, and any inertial reference frame is as "correct" as any other. But... In practice, in...

how can l prove Newton's law is time invariant? if x (t) is a solution of dd/ddx x(t) = f(x(t)) then if l put y(-t) dd/ddt y(t)=dd/ddt x(-t). Now how dd/ddt x(-t) is equal to f(x(-t))?dd/ddt is second derivative with respect to time

Hello. Would you please explain the factors that determines the opening time of a program or a big file. For example if you have a large excel file, why would you have a difficulty to open it, if it is very large? If it is in your PC's main storage, that is either HDD or SDD, is it reading...

Vvertical=15sin30 = 7.5m/s d=Vvertical*t+0.5atsquared 0=(7.5m/s)t+0.5(-9.81m/s)t squared 0=t(7.5m/s-4.9t) t= 1.53s t=1.53/2 t= 0.76s to reach the maximum height Is this correct?

How is the real understanding, when an external constant E potential (voltage) is imposed/applied on a LR circuit, that is being charged as the characteristic L/R inductor time constant: the greater R the shorter time inductor get (full) charged This absolutely independent to the E; it could...

Our current model (FLRW) is clear that the universe has a continuous temporal asymmetry. This is seen as the expansion factor grows with time, and thermodynamically with entropy. A continuous transformation in the current model ##t \rightarrow t + dt## is not the same as ##t \rightarrow t - dt...

Hi, I have been thinking about pendulums a bit and discovered that a HO(harmonic Oscillator) will take the same time to complete one period T no matter which amplitude A/length l it has, if stiffness k and mass m are the same. But moving on to a simple pendulum suddenly the time period for one...

in special relativity we have moving reference frames resulting in a different flow of time in each reference frame. This we can explain because we use the fact that the speed of light is the same in all reference frames, leading to the Lorenz transformation giving the amunt of slowdown of time...

https://i.stack.imgur.com/hSOU2.png https://i.stack.imgur.com/wE2Iy.png https://i.stack.imgur.com/R7FKK.png cannot understand this deadtime compensation method. So I have provided the whole cut out of the section of this book. BOOK: Control of Electric Machine Drive Systems Book by Seung-Ki...

Some BSM theories, often but not exclusively string theory inspired, have an extra time dimension, as well as extra spatial dimensions. I'm trying to make sense of what it even means to have more than one time dimension, ideally, with a concrete example that illustrates how the second time...

I put the net charge density ##\rho_q = e\delta## so that ##\nabla \cdot \mathbf{E} = e\delta / \epsilon_0##, then I tried Maxwell IV:\begin{align*} \dot{\mathbf{E}} + c^2 \mu_0 \mathbf{J} &= 0 \overset{\mathrm{div}}{\implies} e\dot{\delta} + \nabla \cdot \mathbf{J} = 0 \end{align*}but this...

Using the above formula I get that the time goes 6.5∗10−86.5∗10−8 percent faster in ISS. Thus, this is approximately 2 seconds in a year. But the answer is much lower. Where am I making a mistake?

Hi everyone, After completing high school, I will enroll this fall at a 3 year technical school in China studying industrial robotics, but I still plan on getting into an undergrad physics program at a later time. (Some colleges can allow a transfer to their undergrad program after technical...

I tweeted this on a Sabine Hossenfelder’s Twitter thread: Is there an example, definition, explanation of time that is not derivative of a physical system like a clock, e.g., that's based on the rotation of the earth? If not. If time is not a stand-alone definable entity how can it qualify as a...