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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Time Contraction

This is my first time posting on this site. I am a computer programmer that stumbled across a physics text book and have a question on special relativity. So firstly, I understand that there is no ...
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How does light travel create time travel violating causality?

Saw a question about faster than light travel... I still have the same question though none of the answers offered any resolution for me. It is so summarily assumed by all physicists and commentaries ...
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How to make timelike entanglement in the laboratory?

http://io9.com/5744143/particles-can-be-quantum-entangled-through-time-as-well-as-space http://arxiv.org/abs/1101.2565 How to make timelike entanglement in the laboratory? How to test whether mixed ...
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The analogy between temperature and imaginary time

There are many statements about the relation between time and temperature in statistical physics and quantum field theory, the basic idea is to interpret (inverse) temperature in statistics as "time" ...
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237 views

Is there a “present state” of distant stars if simultaneity is relative?

Special relativity theory says simultaneity is relative, meaning that different observers will not agree on what happened first and what second. Does it then make sense to say that looking at distant ...
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440 views

What accounts for the discrepancies in my calculations of year lengths?

A common exercise in many introductory astronomy texts is to use the lengths of various kinds days to calculate the approximate length of the corresponding year. For example, ratio $k$ of the length ...
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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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862 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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More than one time dimension

We know that space-time dimensions are 3+1 macroscopically, but what if 2+2? Obviously it is tough to imagine two time dimensions, but mathematically we can always imagine as either having two ...
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Solving time dependent Schrodinger equation in matrix form

If we have a Hilbert space of $\mathbb{C}^3$ so that a wave function is a 3-component column vector $$\psi_t=(\psi_1(t),\psi_2(t),\psi_3(t))$$ With Hamiltonian $H$ given by $$H=\hbar\omega ...
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How to explain (pedagogically) why there is 4 spacetime dimensions while we see only the 3 spatial dimesions?

I have been asked this question by a student, but I was able and in the same time incapable to give a good answer for this without equations, so do you have ideas how one can explain this in a simple ...
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Intuition for multiple temporal dimensions

It’s easy, relatively speaking, to develop an intuition for higher spatial dimensions, usually by induction on familiar lower-dimensional spaces. But I’m having difficulty envisioning a universe with ...
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Speed of Entropy change

If time in systems moving with different speed goes differently, does speed of entropy change differ in these systems? (is "speed of entropy change" a valid term? can we compare them?)
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Energy-time uncertainty and pair creation

Usually, the energy-time analogue of the position-momentum uncertainty relation is quoted as $\Delta E \Delta t \geq \frac{h}{4 \pi}$. This has interpretational issues and such. But, with a suitable ...
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What is the length of 1 second in meters

If time is treated as a fourth dimension of spacetime, what is relation between length and time units? Or in other words, how can I convert time units to length units, for instance seconds to meters? ...
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224 views

Is there a device that could measure the speed of time?

Is there (or can there be) a device that could measure the speed and acceleration of time?
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174 views

Is time the rate at which one moves through space

I'll start out with the cliche attempt in a protective shield of my dignity. I am a young highschool kid just eager to learn and understand. If I'm way off or this is already a known idea, or maybe ...
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Time crystals : fake or revolution?

This article about "crystals of time" just appeared on the PRL website. Viewpoint: Crystals of Time (http://physics.aps.org/articles/v5/116) The authors (including famous Frank Wilczek) claim that ...
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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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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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In a very small static universe with only a particle, does it make sense to talk about time?

I am sorry if this question is silly; it′s just one of those things I wished I asked before leaving university. If there were a static universe only as big as the size of two particles, say ...
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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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187 views

Why do we treat time as parameter in non relativistic QM instead of treating position as parameter?

Why do we treat time as parameter in non relativistic QM instead of treating position as parameter? What is actually a parameter? I have read this Wikipedia page, but could not understand why should ...
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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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Does the Earth's revolution around the Sun affect radioactive decay?

Premises: The radioactivity is either hastened or slowed inside a fast moving aircraft. Speed of fastest aircraft: 3,529.6 km/h. The earth's revolution is: 107278.87 km/h. The earth's ...
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Calculate time to go from one point to another point [closed]

in a 2D football game I want to calculate cycle(time) until ball go from one point to another point. in this link exist all fomulas that we need for my problem but anyone does not have any parameter ...
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545 views

Is it true looking at an object from a great distance with a telescope will show the past version of the object?

Once again from my son's workbook. It discussed standing on a planet 65 million light years away from Earth, with an extremely powerful telescope pointed at Earth. It claimed that then you could see ...
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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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Is time continuous?

While working on physics simulation software, I noticed that I had implemented discrete time (the only type possible on computers). By that I mean that I had an update mechanism that advanced the ...
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Is a 1D vector also a scalar?

A vector in one dimension has only one component. Can we consider it as a scalar at the same time? Why time is not a vector, although it can be negative and positive (when solving for time the ...
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Time slowing down problem

When someone moves, time slows down for him. Let, a man standing still and another moving very very very fast, this happens for an hour (as measured by the standing man). Time has moved slower for the ...
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What is the difference between “at all times” and “at any particular time”?

Morrison writes in "Morrison, Michael A. : Understanding quantum physics : a user's manual" $ |\Psi(x,t)|^2 \xrightarrow[x\rightarrow\pm \infty ]{} 0$ at all times t [bound state] $ ...
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Is there really time reversibility in physics?

First, how i got to the question. I was randomly looking at this page: http://en.wikipedia.org/wiki/Langton%27s_ant describing some sort of cellular automaton. Here is the path of this automaton ...
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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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How can we be sure about the constancy of atomic clocks as in the Hafele and Keating time dilation test?

Atomic clocks were used in Hafele and Keatings experiment which supposedly helped to prove time dilation. Time Dilation Proof - Hafele and Keating How can we be sure other forces didn't act upon the ...
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Why are objects at rest in motion through spacetime at the speed of light?

I read that an object at rest has such a stupendous amount of energy, $E=mc^2$ because it's effectively in motion through space-time at the speed of light and it's traveling through the time dimension ...
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Is large-scale “time reversal” (Poincaré recurrence) possible given infinite time?

The following are some assumptions I'm basing my question on, from what (little) I understand of physics. I list them so an expert can (kindly) tell me where I'm going wrong. There is a probability ...
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What's the relationship between quantum entanglement and the relativity of time?

Apologies in advance for what may be a stupid question from a layman. In reading recently about quantum entanglement, I understood there to be a direct link between entangled particles, even at ...
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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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“Time” by epistemic subdivision of a closed system [closed]

There is the idea that there is no time in a completely closed (thus unobservable) system. Within such a system, a subsystem may be imagined to be split off by some virtual boundary. However, one ...
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Does String theory say that spacetime is not fundamental but should be considered an emergent phenomenon?

Does String theory say that spacetime is not fundamental but should be considered an emergent phenomenon? If so, can quantum mechanics describe the universe at high energies where there is no ...
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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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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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What isotope has the shortest half life?

Question: What isotope has the shortest half life?
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Relation between comoving distance and conformal time?

In cosmology, we have two quantities and I want to understand the physical relation between these two : $\chi = \int_{t_e}^{t_0}c\frac{dt'}{a(t')}$ : the comoving distance with $t_e$ the time at ...
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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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Finding deceleration and velocity using distance and time

A car is moving down a street with no brakes or gas. The car is slowing due to wind resistance and the effect of friction. The road is flat and straight. The only data I have are timings taken at 100m ...
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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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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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How to calculate time dilation in approaching speed of light

If a spaceship travels close to the speed of light (say, at 0.9c), how do I calculate the time as the spaceship pilot experience it? I thought the formula was $$t = \frac{t_0}{\sqrt{1-v^2/c^2}}$$ ...