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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When are leap seconds added in various time zones?

I understand that technically when a leap second is added, it is added after midnight UTC, but I'm unclear how the addition is handled in other timezones. For precise reckoning of course (e.g. ...
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3k views

When moving fast Time slows down Vs speeds up

I was watching an old cartoon movie where a scientist makes a gadget, which when bound on the wrist, freezes the movement of the whole world. So, that one may do 100s of things in a single second. ...
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95 views

A clock devoid of motion

My understanding is that every clock mechanism we have depends on motion of something w.r.t the observer. From atomic transitions to clockwork gears. So, does this property/constraint makes every ...
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212 views

How can an action be dependent on both its past and future?

Is it true that whenever an action takes place it is dependent on both its past and future? I mean if we already know that whatever we are doing is dependent on future as much as it is dependent on ...
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576 views

How precisely can we date the recombination?

The early universe was hot and opaque. Once it cooled enough, protons and electrons were able to form hydrogen atoms. This made the universe transparent, and was known as recombination. We can see the ...
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How to describe a well defined “zero moment” in time

Suppose you have to specify the moment in time when a given event occurred, a "zero time". The record must be accurate to the minute, and be obtainable even after thousands of years. All the measures ...
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78 views

Atomic Clocks: How is time measured?

I am trying to understand atomic clocks better. I am not getting HOW the cesium oscillation is actually being counted. So from my understanding of an older atomic clock: cesium gets heated-> ...
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1answer
36 views

Scientific definitions of “moment (of)” and “instant”? [closed]

What are the scientific definitions of "moment (of time)" and "instant"? Are they different with their definitions in everyday language? I also don't know the definitions in everyday language, of ...
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96 views

Three questions about time [closed]

Since I don't want to abuse to the ASK QUESTION form, and since those questions are all about time, I decided to write them all here. Hope it's ok. First Question Could it be possible (how?) to ...
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102 views

Is the cosmological time grosso modo isochrone?

Is the cosmological time grosso modo isochrone? by analogy with space isotropy. Or else do we have possibly great differences by analogy with great voids in the space. We know that it's not strictly ...
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30 views

Does matter stacks up as it approaches Black hole?

When something approaches Black hole it'll experience time dilation with respect to a frame away from the black hole. So to an observer away from the hole the object would seem to slow down until ...
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93 views

Will physics ever be able to answer the question: “What caused the universe to come into existance”? [closed]

Or is this just the flip side of a metaphysical question "why does the universe not exist", if the opposite was the case and nothing existed i.e. a universe devoid of all energy, matter forces, ...
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50 views

Question regarding gravity and time

I and my friend (Age 15) were discussing about light and speed of light when we thought of a question. Imagine you are travelling in a spaceship at the speed of 2.9*10^8 m/s circling the earth. ...
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117 views

What if a light clock travels perpendicular to mirrors that make up the clock?

I'm guessing you're all familiar with the classic intuitive way of explaining time dilation: with a light clock traveling at velocity v directed at a parallel direction to the mirrors that make up the ...
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105 views

Full time-derivative, Poisson brackets and Hamilton's equations (classical mechanics)

While studying Poisson brackets in classical mechanics and the derivation of $\dot{q_j}=\{q_j,H\}$ and $\dot{p_j}=\{p_j,H\}$ form of Hamilton's equations I encountered a surpsing identity, which led ...
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861 views

Significance of Time constants in LR and RC circuits [closed]

What is the significance of a time constant in a circuit? Is there any use of knowing the time at which the charge in a capacitor is 1-1/e times its total charge? What is special about the capacitor ...
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65 views

Gravity vs. Velocity time-dilation [duplicate]

Is there any difference in effects from time dilation of that due to gravity vs. that due to velocity?
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242 views

What is meant by 'probability of transition per unit time'?

Today I came across a term used by Feynman in his thirteenth lecture: 'probability per unit time' to go from $| 1\rangle$ to $|2\rangle$ while initially being at $|1\rangle$. This is the excerpt fom ...
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93 views

Would a giant see our year as a second? [closed]

If there were an enormous being whose arm span is one light year across, how would that being perceive time? Wouldn't what we perceive as a year be virtually nothing to that being? Explanation: ...
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97 views

Can we observe a star collapse into a black hole? [duplicate]

Let's say that I happen by a star that has just burned out and is collasping into a black hole. I seat at infinity to watch it as a Schwarzschild observer. Will I ever see the black hole collapse into ...
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91 views

Standardising shadow length on sundials

The sundial is fundamentally flawed in that the length of each hourly shadow changes with the seasons. If the base of the sundial was engineered to move cyclically on an anual basis however, the ...
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3answers
29 views

How can we be sure that a new frequency standard is better than the old one?

Lets assume for this question that at one point in scientific history everyone was running along well with their cesium frequency standards, and someone has a brilliant idea and builds an active ...
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75 views

Understand the relation of time and the space dimensions of spacetime?

I assume the relation of the three spacial dimensions and the time dimension is handled purely in the mathematical domain, usually. But is there any intuitive description of this relationship, how ...
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140 views

Speed of light and perception

So I'm reading a book called "The Elegant Universe" and here is a part of it Imagine two countries that have been at war are sitting down to sign a treaty ending hostilities while traveling aboard a ...
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201 views

Is time dilation based on the formula for period of a pendulum?

The theory Albert Einstein put forward about special relativity mentions a possibility for time dilation, in which he states gravity has a considerable effect on time. And in high school physics we ...
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247 views

What is the difference between a parameter, a variable, and an operator in QM?

On the question why time isn't an operator, people will usually say that time is a parameter in QM (Time as a Hermitian operator in QM?) and not a variable. Can someone please distinguish between a ...
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188 views

Modeling the motion of a bouncing ball

I'm writing a program that displays a line of text, and animates a ball that bounces from syllable to syllable (like a sing-along). The program knows the location of each syllable, and it knows at ...
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2answers
96 views

Does information paradox in the Many Worlds interpretation cause a problem? [closed]

I'm taking a philosophy of time travel class. In one of the lectures, the teacher was discussing problems with the Many Worlds interpretation. He talked about how since anything that can possibly ...
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125 views

Time dilation: faster or slower?

I was wondering about Time-dilation in Special Relativity. I am still a middle school student who wonders so please excuse me if I missed any important aspects. Let us assume we have a system of ...
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60 views

Rate of time progression

We know time can dilate, but what about the base "rate" of how "fast" time progresses? Is it even possible to conceptually pinpoint what time would be progressing relative to? Some relevant points ...
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86 views

can one measure energy to a finite accuracy?

Can one measure energy to a finite accuracy in bounded amount of time? I don't know much about QM, but someone told me that the energy-time uncertainty principle says that it would take infinite ...
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454 views

Magnetism, spin and time reversal

I am somewhat confused about this topic. It is usually explained how magnetic fields avoid break*ing* time reversal symmetry by the example of a field produced by a circulating charge current - run ...
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2answers
138 views

Definite energy states for a single non-relativistic particle with a time dependent potential

Do definite energy states exist for a single particle when its potential itself changes with time? I tried solving it and the equations seem to show that they do not exist. But then i am confused as ...
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55 views

Time, distance perception [duplicate]

If photons going c have no local time or distance 'perception' why isn't the same true for us as we're going c relative to them?
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242 views

Displacement of an object due to opposite and equal forces acting at different times?

This question is about mechanical waves in solids, the speed at which forces propagate through solids, and the displacement that might occur due to unequal starting times of equally sized forces. ...
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112 views

How do we know that clocks slow down relative to each other? [duplicate]

For example if a body in motion experiences time dilation, why does it see a body at rest slow down relative to it? wouldn't it make more sense if the body at rest has a faster clock relative to it?
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145 views

Beginner question: timelessness of massless particles [duplicate]

I am not very familiar with the quirkiness of relativity, and I was wondering how to explain this situation. If a beam of light is shining at some object at some distance from the origin of the beam, ...
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84 views

About time measurements

What we regard as TIME is just a way of measuring duration for various phenomena. Like a ruler is a measuring device for measuring length ( or breadth or width). Saying Time is an illusion is like ...
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56 views

Is this “cloaking in time” serious and what is really meant?

I just found this news article: http://www.nature.com/nature/journal/v481/n7379/full/nature10695.html What did those researchers actually do? The article itself doesn't sound to me like it can be ...
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94 views

How has the age of the Universe been derived from the observations made by the Planck mission?

The parameters of $\rm\Lambda CDM$ model have been determined to an amazing high precision from the measurements made by the Planck mission. In particular, the Hubble "constant" (the value of Hubble ...
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1answer
4k views

If gravity bends space time, could gravity be manipulated to freeze time? [closed]

You age at a different rate depending on the force of gravity. Astronauts age fractions of fractions of fractions of a second less than earthlings. If you took a sphere of equal masses, separated by ...
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273 views

Confused about the concept of time and time dilation [duplicate]

I am having a hard time understanding what is time. If scientists define time as a multiple of caesium frequency, then time itself is dependent on motion, so what if I have a number of particles that ...
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83 views

Increase thinking capability through relativity

As Einsteins relativity suggests that everything slows down along with the speed of light. So, if we were to live in space would our thinking capability (essentially speed) increase or decrease (due ...
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364 views

Special Relativity Time Dilation Misconception

Imagine a boxcar with a lamp on the left end and a mirror on the right end, so that a light signal can be sent down and back. The boxcar is moving to the right with velocity v. They find the length of ...
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897 views

How does time relate to mass and velocity

I understand that the larger the mass the greater gravity is and the slower time is, as well the faster an object is traveling the slower time passes. My question is that since the faster an object ...
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390 views

Why does the guy moving on spaceship look younger in twin paradox?

If there is no particular absolute choice of frame of reference, the guy who sits on Earth is also moving away from the guy on spaceship perspective and hence time on Earth should also dilate when ...
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1D Kinematics - Relative Motion involving non-inertial frames of reference

I recently came across a question involving non-inertial frames of reference. I didn't quite understand the way it had been solved due to some conceptual confusion regarding certain deductions made to ...
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2k views

What is the common difference between partial time derivative and ordinary time derivative? [duplicate]

What is difference between partial and ordinary time derivative? for example: what is difference between $\frac {\partial v}{\partial t}$ and $\frac {dv}{dt}$? where the $v$ is velocity.
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330 views

Will observers moving on a sphere experience time dilation?

A single source of light exists at a fixed point in space relative to two observers. The two observers move on the surface of a shell with a fixed radius with the light source at its centre. They move ...