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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3answers
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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: ...
8
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7answers
883 views

Time, what is it? [closed]

If you ask any person about time, she/he will give you some answer. I suspect that it is extremely difficult, (if not impossible) to define time. Is there a definition of what it is in physics? Is it ...
0
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1answer
176 views

Red shift and time distortion

Superman throws a light emitting object away from himself fast enough to notice a red-shift. The object passes through a region in which time runs more slowly. From Superman's perspective, does the ...
0
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2answers
86 views

Resources for current thought on time/spacetime?

Are any of the big-name physicists associated with the time in the same way that Hawking and Penrose are associated with black holes? I'm interested in some good books that focus on the topic.
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4answers
949 views

Energy is actually the momentum in the direction of time?

By comparatively examining the operators a student concludes that `Energy is actually the momentum in the direction of time.' Is this student right? Could he be wrong?
4
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7answers
426 views

Is 'now' smeared over time?

Conventional physics as is usually presented in textbooks deals with the evolution of states in phase space parameterized by sharp instances in time, a real parameter. However, quantum fluctuations ...
3
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1answer
151 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 ...
5
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3answers
4k views

Convert acceleration as a function of position to acceleration as a function of time?

Suppose I have acceleration defined as a function of position, $a(x)$. How to convert it into a function of time $a(t)$? Please give an example for the case $a(x)= \frac{x}{s^2}$.
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15answers
2k views

What are the mechanics by which Time Dilation and Length Contraction occur?

What are the mechanics of time dilation and length contraction? Going beyond the mathematical equations involving light and the "speed limit of the universe", what is observed is merely a phenomenon ...
4
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9answers
1k views

understanding time: Is time simply the rate change?

Is time simply the rate of change? If this is the case and time was created during the big bang would it be the case that the closer you get to the start of the big bang the "slower" things change ...
5
votes
2answers
80 views

Tropical year is 27s/year off from Gregorian year?

http://hpiers.obspm.fr/eop-pc/models/constants.html states the tropical year is $365.242190402$ days. The Gregorian calendar's average year is $365.2425$ days (every 4th year a leap year, except ...
1
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1answer
236 views

Is time the property of an object?

I don't know if the title makes much sense, but hopefully it will become clear with the text. Temperature is not a property of a point in the three dimensions, but actually of the object occupying ...
5
votes
1answer
688 views

How to calculate how many degrees the Sun is from the horizon?

Given a position on earth (latitude/longitude, and maybe also height relative to sea-level) and a time, what's the algorithm to find how many degrees the Sun is from the horizon? And the same for ...
8
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5answers
4k views

The Time That 2 Masses Will Collide Due To Newtonian Gravity

My friend and I have been wracking our heads with this one for the past 3 hours... We have 2 point masses, $m$ and $M$ in a perfect world separated by radius r. Starting from rest, they both begin to ...
1
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0answers
109 views

Confusion with infinity and time [closed]

I have some confusion between with resolving the following situation. I know that no measurable quantity can have a value of infinity. For example, I just wrote something out, but clearly this ...
5
votes
2answers
614 views

Why is time special?

In Special Relativity, the spacetime interval between two events is $s^2 = -(c{\Delta}t)^2+({\Delta}x)^2+({\Delta}y)^2+({\Delta}z)^2$ giving the Minkowski metric $\eta_{\mu\nu}=\text{diag}(-1, 1, 1, ...
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3answers
1k views

How accurate is the time on a digital clock versus the time on internet clocks? [closed]

How accurate is the time on a digital clock versus the time on internet clocks? The internet clocks seem slower than internet clocks. So which is the correct time? What are the most accurate clocks?
7
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3answers
5k views

How fast will the sun become a red giant?

I've read many accounts of our sun's distant fate, but what I've never heard is on what time scale these events occur. For instance, when the sun runs out of hydrogen, I presume it doesn't just WHAM! ...
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4answers
1k views

Relativity in Real Life

I have no formal training at all in relativity. But I was just wondering if our perception of time is altered by different events. Here are some examples I had in mind: If I touch a stove for 10 ...
3
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3answers
722 views

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?
9
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3answers
907 views

Can black holes actually merge?

If time stops at the event horizon, can we ever detect two black holes merging? In other words, if you are a short distance away, would you encounter a spherically symmetric gravitational field, or a ...
13
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6answers
18k views

How exactly does time slow down near a black hole?

How exactly does time slow down near a black hole? I have heard this as a possible way of time traveling, and I do understand that it is due in some way to the massive gravity around a black hole, but ...
14
votes
2answers
1k views

Has everything we see happened in the past?

Whatever we see is basically based on the light that hits our eyes, right? When we look at the Moon we are looking at the Moon as it was couple of seconds ago, as the light takes some seconds (~2 ...
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2answers
214 views

length, width, and time

is it possible to have length, width and time. A 2D or 1D object with the 4th dimension (although then there would not be 4 dimensions so would it b 3D?) .
7
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2answers
188 views

How to create unusual sundial?

I am considering small "artsy" project. I would like to create sundial by placing gnomon on the window and painting hour lines on the window facing wall. Since this is to be placed in bedroom I am ...
9
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3answers
132 views

Seeing cosmic activity now, really means it happens millions/billions of years ago?

A Recent report about a cosmic burst 3.8 billion light years away. It is written as though it is happening now. However, my question is, if the event is 3.8 billion light years away, doesn't that mean ...
0
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1answer
191 views

If you removed every particle from space…? [closed]

I'm trying to find something Einstein (I think) said about time...It was something like.. "If you removed every particle from space and were left with only one pocket watch (clock, timepiece?), time ...
3
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1answer
610 views

Universe Expansion as an absolute time reference

Why we call "constant" to the Hubble constant?, if the universe were really expanding then the Hubble "constant" should change, being variable, smaller and smaller..with "time". Other example/view ...
3
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2answers
244 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 ...
0
votes
1answer
198 views

Is relativistic motion equivalent to fluctuating gravitational fields?

The theory of relativity makes very precise predictions about how an object's motion through space-time affects the passage of time for both the object and observers in other frames of reference. I ...
8
votes
2answers
555 views

What is the fastest process or shortest time in nature?

We know about some events that happen very quickly. For example, the dielectric relaxation time is about $10^{-14}\, \mathrm{seconds}$. I'm interested in other processes that switch extremely fast ...
2
votes
1answer
976 views

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 ...
20
votes
3answers
3k views

What is the difference between implicit and explicit time dependence e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$?

What is the difference between implicit and explicit time dependence e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$? I know one is a partial derivative and the other is a total ...
7
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4answers
4k views

How can gravitational forces influence time?

How does it work that gravitational forces can affect time and what usable applications could arise from this?
0
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1answer
407 views

When are we now? Time coordinate system [closed]

The Second and the metre are well defined. We have those units available. Of course units are meant to express measurement, tools to compare with precision, and interchange that information ...
1
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2answers
543 views

what about there is no time, only space [closed]

what about a theory where no time exists, only discrete space, steady states and occasion chains. what would speak against this? ;-)
5
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2answers
1k views

on causality and The Big Bang Theory

With the notion of causality, firmly fixed by GR, we derived the concept of a singular point from where space-time begun. Causality alone gives us the possibility to talk about a known past (i.e. ...
8
votes
2answers
457 views

What is the closest general-relativistic equivalent of a “time slice”?

In a newtonian universe, one can talk of a "time slice", that is, the state of the universe at a given point in (global) time. In a "typical" classical universe, a time slice would contain enough ...
14
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2answers
2k views

Time as a Hermitian operator in QM?

In non-relativistic QM, on one hand we have the following relations: $$\langle x | P | \psi \rangle ~=~ -i \hbar \frac{\partial}{\partial x} \psi(x),$$ $$\langle p | X | \psi \rangle ~=~ i \hbar ...
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7answers
6k views

I need help with finding distance traveled

How do I find the distance traveled of an object if the speed is not constant?
9
votes
4answers
783 views

Special Relativistic Time Dilation — A computer in a very fast centrifuge

Ok, I've stumbled onto what I think is a bit of a paradox. First off, say you had some computer in a very fast(near light speed) centrifuge. You provide power to this computer via a metal plate on ...
13
votes
4answers
1k views

Can black holes form in a finite amount of time?

One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an ...
4
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1answer
2k views

time difference with airplane travel direction

a fight from east to west, (e.g. Australia melbourne to perth) takes longer than travel backwards. (west to east). What's the reason?
6
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2answers
449 views

Is Time Significant in the Double Slit Experiment

When doing the classic double slit experiment is the time between emitting photons significant at all? Say, a single photon is emitted, the scientist waits T seconds, then emits another photon. Are ...
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votes
5answers
1k views

2 balls falling hit the ground at the same time

There are 2 balls in a vacuum, next to each other but not touch. They are on the edge of a surface they will both leave the table at exactly the same time. One gets pushed harder than the other. The ...
1
vote
4answers
783 views

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 ...
18
votes
5answers
3k views

How is the classical twin paradox resolved?

I read a lot about the classical twin paradox recently. What confuses me is that some authors claim that it can be resolved within SRT, others say that you need GRT. Now, what is true (and why)?
11
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4answers
4k views

Did time exist before the creation of matter in the universe?

Does time stretch all the way back for infinity or was there a point when time appears to start in the universe? I remember reading long ago somewhere that according to one theory time began shortly ...
2
votes
0answers
4k views

What's the relationship between mass and time? [closed]

This question has arisen from a wish to understand an end-of-universe scenario: heat death. Are time and mass intrinsically linked? If so, does time "run slower" (whatever that may mean) in a ...
13
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7answers
3k views

Is time travel possible? Is it possible to go back in time?

I read somewhere that according to relativity, black holes and other space related stuff it is possible to jump into past. Is it possible for anything to go back in time either continuously or by ...