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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Atomic clocks and how to synchronize them

During teaching measurement section in the class, Our teacher told us about atomic clocks. I have two questions: What is exactly an atomic clock? and how do we synchronize two atomic clocks far ...
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How to determine day/night based on latitude, longitude and a date/time?

Is there a simple method of determining, given a UTC date/time, whether it is day or night at a given lat/long coordinate? I am currently using a formula based on a Sunrise/Sunset Algorithm from the ...
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Nonlinear combination of velocities implies no absolute time?

Landau 1961 begins with a brief presentation of special relativity. This question is about the validity of a certain argument that they use in building up the foundations of the subject from scratch, ...
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Imaginary time in QFT

I'm reading chapter 4 of Introduction to Quantum Field Theory by Peskin & Schroeder. In the $\phi^4$ theory, the authors state that the ground state of the interaction theory $|\Omega\rangle$ can ...
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754 views

Finding radius of Earth through observation of Sun's motion

The question I'm about to pose is from a physics book I had recently bought. Since I am very interested in physics I am quite keen in understanding how this question can be solved. Before I present ...
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There are plans to develop a better definition of a “second”. How does the current definition fall short?

The current definition of a second is stated here and I found a presentation on the BIPM site which discusses plans to change to a "better" definition of a second. You can find the presentation here. ...
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How long would it take to travel through a wormhole?

Assuming wormholes exist and you put some matter into one, how long would it take to reach the other end versus how far apart the two ends are? Basically, by how much does a wormhole stretch ...
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110 views

Would time dilation be too great for the early universe to expand?

I read that one second after the big bang the universe was composed of photons electrons and neutrinos. Wouldn't the density of energy/matter have caused such extreme time dilation that the universe ...
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Time Slowing Down [duplicate]

I'd like to ask a question if I may about the relativity. When travelling at the speed of light(which is impossible for particles with masses) or very close to it, would it be me, myself, seeing my ...
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The heat equation in relation to smoothing algorithm

Recently I learned about a technique in image processing, which has its roots in something called the 'heat equation' from physics. The original creators of this technique were inspired by the physics ...
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614 views

Is there an observable of time? [duplicate]

In Quantum Mechanics, position is an observable, but time may be not. I think that time is simply a classical parameter associated with the act of measurement, but is there an observable of time? And ...
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150 views

Why isn't sunset time in sync with solstice?

The winter solstice in the Northern Hemisphere in 2011 is on December 22. But if I look at the sunset times for a location such as Washington, DC on the USNO site, the sunset time starts reversing ...
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Is it time or duration?

Taken this post: Is there a proof of existence of time?, as a starting point. There was mentioned that there is confusion between: time and flow of time. There was a comment (of mine) that the ...
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105 views

How is UT1 being computed?

I've recently read up about time standards and now understand that UTC is a second-corrected version of international atomic time so that it is kept within 0.9s from UT1. And then UT1 is defined as ...
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183 views

Calculating the length of day at any time of the year? [duplicate]

Is it possible to calculate the length of the day at various location (distances) from the sun? I hear agricultural scientists are able to do that. To make calculations easier assume there is no axis ...
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662 views

When is the right ascension of the mean sun 0?

I understand that the right ascension of the mean sun changes (at least over a specified period) by a constant rate, but where is it zero? I had naively assumed that it would be zero at the most ...
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1answer
110 views

Is GPS time measuring the proper time on the mean sea level or the GPS station itself?

LeapSecond.com states: Global Positioning System time is the atomic time scale implemented by the atomic clocks in the GPS ground control stations and the GPS satellites themselves. Does GPS ...
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Time in special relativity and quantum mechanics

The time is treated differently in special relativity and quantum mechanics. What is the exact difference and why relativistic quantum mechanics (Dirac equation etc.) works?
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How can we measure time?

If we cannot define a proper time (or synchronize clocks in different positions) in an inertial frame (independent with the theory of relativity), there seems to be no direct way to confirm the 2 ...
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Is time quantized? Is there a fundamental time unit that cannot be divided? [duplicate]

Is the present just a sharp line between the past and the future with no time at all, or is the present a short frozen unit of time? Could time be quantized into a fundamental units? Like Planck ...
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How is the physical Lagrangian related to the constrained minimization Lagrangian?

If we're minimizing an energy $V(q)$ subject to constraints $C(q) = 0$, the Lagrangian is $$L = V(q) + \lambda C(q).$$ I have fairly solid intuition for this Lagrangian, namely that the energy ...
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How would an observer perceive movement on a train that's travelling near the speed of light?

Person A is on Earth and a train (or whatever you want to imagine) travels past him at near the speed of light. How would person A perceive movement on the ship? If time is slowed on the ship from the ...
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392 views

Time to establish saturated vapour pressure above liquid

Thought experiment - a liquid is in a closed container in equilibrium with its vapour, and then suddenly all the vapour is pumped away. Switch off the pump so that instantaneosuly there is no vapour ...
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How did Cook and other astronomers time the 1769 Venus transit?

The 1769 transit of Venus was observed and coordinated by over one hundred astronomers around the world. How did they measure time so accurately, key to the observations having any scientific value? I ...
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What is a clock?

Relationalists love to define time operationally as what clocks measure, but this begs the question of what counts as a clock. Clearly, it's a measuring instrument and what it measures is supposedly ...
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242 views

Special Relativity and time [duplicate]

Possible Duplicate: Question about Time Dilation.. I have a question about special relativity which was bothering me for a while now. I know that as one approaches the speed of light, time ...
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Does the definition of the SI unit “second” require that possible perturbation of primary frequency standards should be measured?

The definition of the SI unit "second" is stated as The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground ...
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How is time not a constant?

I am a visual person, so it's hard to imagine the information I keep getting, but shouldn't time be a constant? If you were traveling at the speed of light and your able to cover $299{,}792{,}458$ ...
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How to interpret $t^2$? [closed]

I can't think of the meaning of squaring the Time (multiplying it by itself). It makes sense in Mathematics. But how can you figure it out in nature (or physics). As an example, the formula ...
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What is the shortest controllable time?

I was reading http://en.wikipedia.org/wiki/Planck_time and noticed a reference to: http://phys.org/news192909576.html where it is stated that: 12 attoseconds is the world record for shortest ...
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Does anybody actually do astrometry in the 21st century?

I mean the nitty-gritty measurements of positions of stars and planets using transit circles. The transit instruments I've seen are mostly in national observatories and are essentially museum pieces: ...
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217 views

Accuracy and Error of Atomic Clocks

I'm quoting a passage from my notes: The development of clocks based on atomic oscillations allowed measures of timing with accuracy on the order of $1$ part in $10^{14}$, corresponding to errors ...
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730 views

Can Flow of time become still?

According to theory of time dilation, flow of time slows down significantly at the speed of light.Is there any conditions practically or theoretically when flow of time is reduced to zero means it ...
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Scale factor and age of the universe

Why is the cosmological scale factor (expansion rate of the universe) not simply the time $t$, i.e. the age of the universe?
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In general relativity (GR), does time stop at the event horizon or in the central singularity of a black hole?

I was reading through this question on time and big bang, and @John Rennie's answer surprised me. In the immediate environment of a black hole, where does time stop ticking if one were to follow a ...
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Are circularly defined {velocity, distance, and time} a problem in physics?

In order to measure velocity, one needs a calibrated measuring stick and clock. But in order to calibrate a measuring stick you need a calibrated clock and velocity. And in order to calibrate a clock ...
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How much could one slow down aging using existing spacecraft?

Imagine that I board a spacecraft and head away from the Earth at top speed until I've aged twenty years, then I turn around and come back. How much younger will I be than the unexciting folk who hung ...
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Do algorithms have an intrinsic time direction?

This article says There is no intrinsic time direction in Newton's mechanics nor in the differential equations of the new physics. My question is, do other types of mathematics, say a cellular ...
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does light experience time? [closed]

If we positioned a mirror 1 light year away from earth and shot a particle of light at the mirror so that it would reflect and come back to earth, how long would it take for us to receive that ...
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What are the Time Operators in Quantum Mechanics? [duplicate]

I don't understand at all what the time operators are in quantum mechanics. I thought that given a wave function, because it's a function of time, we could simple put in any time in the future to find ...
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375 views

Does time expand with space? (or contract)

Einstein's big revelation was that time and space are inseparable components of the same fabric. Physical observation tells us that distant galaxies are moving away from us at an accelerated rate, and ...
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Proper Time for Deceleration in Special Relativity

As stated here: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html A rocket which accelerates to some point, then decelerates back to 0 over the same distance will measure a proper time ...
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Area under a displacement graph

If the area under an acceleration-time graph denotes velocity and the area under a velocity-time graph denotes displacement, what exactly does the area under a displacement-time graph denote?
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Anti-matter black hole and time

I have recently read some hard science-fiction story based on an assumption that if time stops (from external observer's perspective) on the event horizon of black hole, then in an anti-matter black ...
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Is it possible to hear the past?

From this Stack Exchange Physics Post, I am certain that it is possible to view the past. But then this interesting question came to me. Is it possible to hear the past? Ok, you might say, "Well, ...
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1answer
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What did Hawking mean? 'Time started at the big bang'. Book suggestions please [closed]

After writing down this question, I have come to realize that. What I really want is reading materials on the questions below. Before the big bang there was no such thing as 'time' (Steven Hawking on ...
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Question about Time Dilation

I just have a quick question about time dilation/proper time because my physics book makes it a little confusing. Let's say we have an observer on Earth, and then an observer on a space ship. The ...
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3answers
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Can we measure rates in real time?

I know what it means to say that my position is "X" at a particular moment in time. I can easily take a picture of my motion and observe my exact location at the instant the picture was taken. That is ...
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2answers
114 views

How is time distributed through space?

I was greatly confused when I saw video saying that if two wormholes were separated by time instead of distance, it would allow time travel. My question is what did they mean by this statement? What ...
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Can we live forever by taking advantage of the time dilation effect?

I have been reading about Time dilation and twin paradox in Relativity. So,I thought that if It can be the difference of 10 yrs between the Girl in space and on Earth, then why can't anyone not age ...