# 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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### Can physics be done without the use of time?

Carlo Rovelli is interviewed in this article (The illusion of time): Alongside and inspired by his work in quantum gravity, Rovelli puts forward the idea of 'physics without time' This stems from the ...
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### Does the Hubble constant measured for huge distances mean it is not the actual Hubble constant but a very old one?

I am citing Wikipedia about Hubble constant.... "Hubble constant is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec (3.09×1019 km) away, and its value ...
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### Time Constant of a Capacitor in DC

I've just started studying Physics and I'm not sure how you're supposed to calculate the time constant of a capacitor when a direct current is applied. DC means that frequency equals = so the ...
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### What is the role of time (time interval) in principle of least action?

Action is represented by $S[Q(t)]$ where $Q(t)$ is the name of a single complete path in the configuration space of a system. The path starts at the point $q_i$ and ends at the point $q_f$. Suppose ...
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### Time-independent source and quantum field theory

Can anyone explain the fundamental reason of why time-independent sources cannot emit or absorb energy. Does it have to do with time-translation symmetry and Noether's theorem? I was studying the ...
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### Why doesn't delayed choice quantum eraser experiment imply retrocausality?

WRT: https://www.youtube.com/watch?v=0ui9ovrQuKE&ab_channel=ArvinAsh Now I found a paper "Why Delayed Choice Experiments do NOT imply Retrocausality by David Ellerman" on the internet ...
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### Is there a physical process that - ignoring measurement uncertainties - has a fixed duration?

I'm looking for a process that has an absolutely fixed duration, where all the variance obtained by measurement is just due to measurement uncertainty, and the obtained mean is the actual duration of ...
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### Why is the period of a geostationary satellite not exactly 1440 minutes?

When reading about Astra satellites on Wikipedia (https://en.wikipedia.org/wiki/Astra_1KR), I saw that the period of the Astra 1KR satellite, positioned at 19.2° E, is 1,436.1 minutes (source: NORAD ...
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### Is local time (age) and quantum information required for an object to exist? [closed]

Is quantum information coherent while age is decoherent? Is the information paradox suggesting that coherence can't be deleted? Is Time only cyclical when it is decoherent? Linear when coherent? Is it ...
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### Do coherent waves USE Time while decoherent/localized waves HAVE Time? [closed]

Could Time have States? Is there universal Time for coherent waves to use? When an object HAS Time, does it age? Don't physical objects require Time to exist? Is time swapping from being coherent, ...
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### Are there particles that carry time? [closed]

As every event in the universe can be reduced to interaction between particles via the 4 fundamental forces, can the same be applied to time? Could the flow of time be reduced to a particle that ...
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### How can spacetime change with time?

As far as I understand, spacetime fuses the 3 dimensions of space with the dimension of time into a single entity. If there's a big curvature, things will experience a change in how they experience ...
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### Acceleration function of position and time

I have an acceleration function in python with position and time parameters and returns the acceleration value. I need the end velocity at a position ,start velocity is zero. how to calculate this ...
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### Is our perception of time linked to the frequency of quantum waves? [closed]

Is it correct to say that a change in the amplitude of a wave is proportional to the time measured by an observer: $$d\Psi=c*dt$$
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### Finding displacement from acceleration and time

Can someone open my eyes my disproving the formula that I arrived. I understand it is wrong but where exactly is the mistake?
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### Why are the propagators in old-fashioned QED oblique, while in modern QED they are horizontal (or vertical)?

In old-fashioned Quantum Electrodynamics, one can find diagrams such as these (probably Stückelberg was the first to use this notation, a kind of predecessor of Feynman diagrams): In modern QED this ...
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ABCD is a track with a horizontal section BC of length 5 m. A 0.3 kg steel sphere is released from a point on the slope AB and it rolls towards the end D of the track. A stop watch was started shortly ...
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### Is our metrics and/or constants geocentric?

Considering that all our metrics is now based on physical constants as measured here on earth, With earth having both gravity and velocity, as well as the whole solar system having too, and special ...
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### Suspecting that the universe is even more bigger

Is it plausible that a possible extraterestrial scientist from a galaxy 13 bilion light years from us sees this part of the universe nothing else than just background cosmic radiation... and even more ...
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### Many-world interpretation and absolute time

Consider a quantum system $S$ in superposition of two state,with an observer $A$, forming a bigger quantum system $S'$, with observer $B$. At $t=0$, observer $A$ measure $S$ and according to MWI, the ...
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### How much anti-matter (combining with matter) is needed to give a rocket a velocity that approaches the speed of light (say 95 % of c)? [closed]

A question (very hot presently) about the propulsion of a rocket with compressed air prompted me to ask a somewhat related question. Maybe one sunny day it will be possible to construct an anti-matter ...
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### Can time be just another spatial dimension which the universe travels through with a particular direction and constant velocity?

Is it possible that time is not a 'time-like' dimension, but a proper spatial dimension just like the others but we only have access to a 3D cross-section of it? I did some research about the internet ...
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### Novikov self-consistency principle and probabilities?

According to the Novikov's self consistency principle (also proposed by other authors such as Kip S Thorne) 1 2, if an event exists that would cause a paradox or any "change" to the past, ...
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### What are the units of time in the Quantum Adiabatic Theorem?

To preface my question, I'm coming to adiabatic quantum computing from a background in classical computer science with little knowledge of quantum physics, so simple, step-by-step explanations or ...
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### Why should the particles meet at a common point? [closed]

I saw a question in my physics book asking for the time when all the three particles (each at the corner of an equilateral triangle and each having constant velocity v along the sides of the triangle) ...
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### How long will it take for a projectile to be knocked off a wall? (See details)

I am trying to calculate the impulse of an object that is thrown against a wall. Problem is, to calculate the impulse one needs to know not just the force, but also the time it takes for the momentum ...
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### How robust is the international atomic time standard against malicious actors?

This answer explains the international atomic time standard (TAI). The fact that the Russian GLONASS and American GPS systems are referenced to their national clock systems, which are then ultimately ...
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### What reference clock is an atomic clock measured against?

I looked at a few of the other posts regarding the accuracy of atomic clocks, but I was not able to derive the answer to my question myself. I've seen it stated that atomic clocks are accurate on the ...
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### Interpretation of $t$ coordinate for charged black hole in Kerr-Schild form

In GP coordinates, the $t$ coordinate matches the proper time of a free-falling observer starting at infinity with zero velocity. Now consider the Reissner–Nordström metric in Kerr–Schild form:  g_\...
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### How big (in meters) could the difference be between ECI and ECEF coordinates at midnight UTC?

Is the only difference due to leap seconds, etc. or other differences between UTC and updated forms of universal time, such as UT1? In other words, are all earth-centered inertial (ECI) coordinate ...
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### Does the integral of voltage with time (as in AC circuits) has any physical significance in resistive circuits?

I tried to study the relationship between voltage and charge flow through a resistor (Whose rate of change with time indeed gives the current). I figured out that rate of integral of voltage with time ...
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### Is this formula for the time-temperature relation in this universe correct, or is there a more appropriate one?

I'm trying to simplify an equation into a form that uses the ratio of the number density of sterile to active neutrinos. For this, I need a relation between the time and temperature of the universe, ...
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### When Matter condensed — the condenser

Did gluons, quarks, and electrons first appear at the same age of the universe or was one first?
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### Can gravity act through time? [closed]

I know, that gravity acts through time and space, as it propagates at the speed of light, through space. I mean to discuss whether gravity has an effect in spacetime, along the time axis. Effects of ...
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### Why would time behave differently in different parts of the galaxy? [closed]

I watched Interstellar and I got really confused around different time thing. How can it be possible at all that time is different at different places in the universe? Please explain.
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### Is time constant the same for linear system for all time?

I have heard that if you take a measurement $T_1$ and wait, then take another measurement $T_2$ and then find $\Delta T = T_2 - T_1$. Then 63% of $\Delta T$ + $T_1$ will represent the measurement when ...
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### Time quantization [closed]

There is no evidence to support that time is quantized. So wouldn't the use of discrete values like $dt$ in calculus suggest time is quantized and comes in discrete durations of $dt$?
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### Why does time flow? [duplicate]

People have explained how time flows but not why time flows or how time flow is possible at all. How does science explain time flow?
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### Why is the photon clock equivalent to all clocks?

I can understand why, if the speed of light is invariant, a photon clock would tick slower. I find this explanation very useful in terms of introducing the idea of time dilation (also because it ...
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### What would be the increase of gravity if we create a new world slightly bigger than the earth? [closed]

I need to know what would happen if I increased the earth's size by a small percentage to make a fictional world similar to earth. So my thought process is this: My thinking started with increasing ...
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### How to determine the correct time using quadratic functions if there are multiple “correct” roots exist in a 3D system? [closed]

Time can be calculated from a modified kinematics quadratic formula using the initial velocity, displacement, and acceleration. However, the acceleration vector isn't limited to 1 axis. When applying ...
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### Time dilation in Einstein's train

Wikipedia states that a clock that is moving relative to [observer] will be measured to tick slower than a clock that is at rest in their frame of reference and they explain it using two pictures: ...
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### Time (or rather, $dt$) as a Vector? [duplicate]

I want to preface this by saying that I'm by no means experienced with the field of physics, but while on my commute this morning, I started thinking: Basically, I thought about time being a fourth ...
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### What is the smallest time that the current technology can measure?

Many physical phenomena are associated with unbelievably small time scales. Let me give a few examples. The time scale associated with visible light is $\sim$ a femtosecond ($10^{-15}$ sec), The ...
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### Difference between coordinate time and proper time in general relativity

I was watching a video on relativity on YouTube that talked about the difference between coordinate time $t$ and proper time $\tau$ and I have a couple of questions. As I understand it, the video said ...
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### How to get radius of trail with velocity and angular velocity?

If I know the velocity of something and the angular velocity of something, how could I figure out the distance between the object and the center it is rotating at? I just need an equation for the ...
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### How can I calculate the UV LED dose having few parameters such as wavelength and output power of LED?

i have UVC led with 270nm wavelength and output power of 70mW. The total length of the passage for the de-contamination is 40cm and the total time is 60 seconds. Is it possible to calculate the UV ...
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### Looking for a simplified explanation for why you cannot measure velocity and pin point location

Because of uncertainty, you cannot measure both velocity and exact position. Is this because when you measure the position of a particle, it is freezing it in its frame of reference? When measuring ...
In mechanics, the conjugate momentum is defined to be $\pi=\dfrac{\partial L}{\partial \left(\frac{dx}{dt}\right)}.$ This can be regarded as a partial derivative with respect to a derivative of a ...