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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18 views

Time in the second law of thermodynamics versus relativity [duplicate]

I recently had a debate with someone about time and argued that the most concrete definition of time would be an increase in entropy. The entropy of a localized area of space might reduce, but that of ...
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Why is the age of the universe important?

Age may only be a number, but when it comes to the age of the universe, it's a pretty important one. Why?
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Is the age of the universe 13.799±0.021 billion years OR 2.935364e+18 second(s)? [duplicate]

According to wikipedia (https://en.wikipedia.org/wiki/Observable_universe), The observable universe is thus a sphere with a diameter of about 28.5 gigaparsecs (93 billion light-years or 8.8×10^26 ...
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What if the moon was destoryed? [on hold]

Speculating about how would be the situation in the Earth if there was no moon. I made some research and this are some of the consequences I could find (though im unsure if they're accurate): Earth ...
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Expectation of partial time derivatives of $x$ in QM

In Ehrenfest theorem we know that $$m\frac{d\left< x\right>}{dt}=\left< p\right>+m\left<\frac{\partial x}{\partial t}\right>.$$ So how can I exactly calculate a specific $\left<\...
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Time ordering identities in integration over gluon fields

My question arised when trying to compute the Wilson Loop of a hybrid meson. When calculating the loop one has to keep in mind the path ordering and time ordering respectively. I have the following ...
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Validity of the derivation of time-energy uncertainty principle?

Background The top voted answer to this question seems to make some assumptions I'm uncertain about (to say the least): What is $\Delta t$ in the time-energy uncertainty principle? Assumptions The ...
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1answer
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Time-dependent pertubation theory assigning the order of expansion to squares of the solution

What is the square of a solution from time dependent pertubation theory? Assume we have found the corrections up to second order such that $$ |\psi(t)\rangle \approx |\psi^0(t)\rangle + |\psi^1(t)\...
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What is the experimental evidence for gravitational time dilation?

There are many experiments done to prove time can be changed. I don't know lots of them, but I think the most popular (most cited) is to put a clock on a fast moving object (airplane), fly around and ...
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Can a wormhole change the wavelength of a light beam?

If a light beam went into one mouth of a wormhole, and came out the other, are their factors that would result in it coming out red or blue shifted? Examples, if it came out of a mouth that was in ...
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How is the global time coordinate $t$ (“observer at infinity” time) defined operationally e.g. in the Schwarzschild metric?

This is a question about coordinate time versus clock time / observed time, which I want to understand because I am teaching a GR course. Consider the Schwarzschild metric for specificity. I ...
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Why does nature prefer simultaneous events? [on hold]

Nature mostly prefers simultaneous events: Acceleration is produced without any delay on applying force. Angular acceleration is produced without any delay on applying torque. A bulb glows ...
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Are all processes time/CPT-reversible, e.g. measurement, stimulated emission, state preparation, Big Bang?

"The CPT theorem says that CPT symmetry holds for all physical phenomena, or more precisely, that any Lorentz invariant local quantum field theory with a Hermitian Hamiltonian must have CPT symmetry." ...
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Clock postulate validity

As far as I understood, the generalized clock postulate says that there is no difference between the measurements of a non-inertial observer (NIO) and those of his co-moving inertial observer (CMIO) ...
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Can a region of space-time be viewed as a particle with properties of it's own? [duplicate]

Can a piece of space-time having 3 spacial dimensions and 1 dimension of time be viewed as a particle? Could this piece of space-time have properties of its own (mass, spin, charge) independent of ...
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Moving clocks observed with different observers

If a clock is moving near to speed of light with constant velocity it'll slow down, but if I were to observe from the clock will the observer on Earth slow down?
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Are these Stephen Hawking's statements legitimate? [closed]

I am reading 'Brief Answers to the Big Questions' by Stephen Hawking and it seems to me that some of his statements are just hypotheses, but they are written in such a way that they 'sound' like they ...
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Analytic Elements Haag-Araki theory

Why the elements of a local von Neumann algebra cannot be analytic elements of the timelike translation?
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Is it possible to measure the speed of Time? [duplicate]

Is it possible to somehow measure the rate at which time is passing?
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Calculating mean velocity: using time or velocity? [closed]

Background I have been running tests which involve timing an object moving a certain (fixed) distance, $s$. Each test has been repeated 3 times, and the 3 times ($t{_{1}}, t{_{2}}, t{_{3}}$) for the ...
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Is Young Earth universe distinguishable from non-creationist one? [closed]

While talking today about some religious beliefs I was struck by the next question: So what if some super-natural being forks our universe 6,000 years ago, i.e. creates the exact same copy of ...
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Is vacuum energy transient or permanent?

Vacuum energy is based on Heisenberg's energy–time uncertainty principle which says that $\Delta E$ can exist for $\Delta t$. This $\Delta E$ can produce electron positron pair for $\Delta t$ by ...
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Highly isolated two different reservoir

Lets say we have highly isolated two different reservoir.One is cold the other one is hot.We just connected them with a tube.(Which is also isolated) Tube has "0" thermal conductivity. If we let ...
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4answers
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Does the age of the universe depend on the way it is defined?

I was reading some earlier posts on this question in stack exchange. These posts give me the impression that the age of the universe is defined in a certain way following prescriptions such as that: ...
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1answer
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Are there any experimental data about potential time dilation caused by cyclical movement of heavy objects?

I am dealing with a completely fabricated scenarios (while trying to write a sci-fi book), so my thougts are completely hyphotetical. But I would like to ask you to help me find out if following ...
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Why does everyone seem to think that moving clock appear to tick slower? [duplicate]

I'm getting interested in special relativity and almost all people and explanations about the subject I found on the internet seem to think that when A and B would be in relative motion, they would ...
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Does density vary with time in a steady flow?

Steady flow is defined as the type of flow where density, pressure and velocity don't change with time, as far as my understanding goes. However, in the video below, they say that the density in a ...
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What is time? Does it flow linearly? If so, how are we sure? [duplicate]

Distances in our world (classically at least) can be defined by considering our world to be a model for the Euclidean normed three dimensional space. But how can we set the notion of time on a firm ...
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What is the difference between position, displacement, and distance traveled?

Suppose the question is somewhat like this: If $v=8-4t$ and the position at time $t= 0\ \rm s$ is $2\ \rm m$, find the distance traveled, displacement, and final position at $t=3\...
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1answer
61 views

No explicit time-dependence in Lagrangian means constraints are explicitly time-independent?

Suppose a Lagrangian is not explicitly time-dependent. Does it mean that the constraint equations are also explicitly time-independent, and (as a result) the kinetic energy is necessarily a ...
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Why and in what cases we use $E$ instead of $\Delta E$ in energy-time uncertainity relation?

sometimes in problems of energy or time uncertainity we use the value of energy of the particle/system to calculate the delta t. Now what are those cases specific examples, where we can apply this?
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Is our physics and system of units based on time (second) or the speed of light?

The speed of light in vacuum is stated in our physics as a universal physical constant, c, when measured locally, in vacuum. The speed of light in vacuum, commonly denoted c, is a universal ...
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What's the advantage of Conformal Time?

I'm trying to follow the analysis of CMB Acoustics and several charts are done using conformal time instead of chronological time. Conformal time corresponds to the quotient of the particle horizon ...
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How quickly does an atomic electron decay?

I've looked in lots of books on atomic physics, but I've not been able to find an answer to: Question: Take a hydrogen in empty space whose electron is excited. How long will it take for the ...
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1answer
51 views

Different statements of the Heisenberg uncertainty principle [duplicate]

I know that from the Heisenberg uncertainty principle, ∆x∆p=ℏ/2 . And I know that this equation can be rewritten as ∆t∆E=ℏ/2. From QED I also know that the equation ∆t∆E=ℏ/2 claims that some energy ...
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How to define the proper time of a photon?

I'm writing a paper about the motion of photons near a Schwarzschild black hole. At some point there's a derivative of the Hamiltonian of the system with respect to time $\tau$. I need to explain what ...
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What does this mean? World consume 14 trillion watts of power [closed]

I am not very clear about this statement. Does that mean in every second we are consuming that much energy?
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Time to move along a path?

Let's say I have an arbitrary path from point $A$ to point $B$. A particle moves from $A$ to $B$ under the influence of gravity but also impeded by a frictional force $\vec{F_\mu}$. The gravitational ...
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WHY did physicist defined velocity as displacement divided by time, why not displacement * time? [closed]

V=S/T. As per my knowledge i think ratio as division and it don't give any meaning like this much displacement in this much time. So i think physicists only used division as notion for velocity. But ...
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Do orbiting masses accelerate or decelerate (relative to us) near black holes? [duplicate]

Apologies if this question seems stupid; I was just curious after watching a documentary, and I am not a trained astrophysicist. Simply put, I cannot reconcile the (perhaps misunderstood) facts that ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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1answer
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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?
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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"?
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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 ...
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2answers
39 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?