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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Does this Zeilinger group result provide experimental proof of backward-in-time causation?

Does this recent Zeilinger group delayed choice entanglement experiment imply backward-in-time influences? http://arxiv.org/abs/1203.4834 From the abstract: "This can also be viewed as “quantum ...
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2answers
151 views

Is there any scientific evidence that demonstrates why time passes?

Is there any scientific evidence that demonstrates why time passes? Or is it just an opened question?
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1answer
572 views

The nature of time, according to quantum field theory

I will try my best to ask the question that best fits something I have been pondering on for a few days. Are virtual particles really constantly popping in and out of existence? Or are they ...
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1answer
190 views

Accuracy of atomic clocks

When you hear about atomic clocks, it's accuracy is described by saying something like, " it neither gains or loses $x/y$th of a second in $z$ years." How is this error calculated? Does an error imply ...
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1answer
198 views

Approximate Time Dilation at Rocket Speeds

How do you calculate the time dilation effect experienced by a traveler traveling at a relatively low speed? Specifically, how much time dilation would a traveller moving at $v=0.0007 c$ (speed of ...
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4answers
691 views

Is time fundamentally different from space?

Note: This is a rewrite of the original question, which was titled What would time be for 2D beings? In my current, non-physicist's understanding, every instant of our three‑dimensional world ...
2
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1answer
96 views

Synchronising the Earth's rotation via mass redistribution

How much material would have to be moved per year from mountain-tops to valleys in order to keep the Earth's rotation synchronised with UTC, thus removing the need for leap seconds to be periodically ...
2
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1answer
2k views

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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2answers
736 views

Time dilation as an observer in special relativity

I've been having a discussion regarding time dilation relating to special relativity and how it should be observed from the FoR (Frame of Reference) of "the person moving" : I assert- If we have a ...
2
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1answer
404 views

Einstein Relative Motion and Time Order of Events

According to Einstein, do observers in relative motion agree on the time order of all events? I don't think they would agree on the timing of events, but I am having trouble figuring out why they ...
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4answers
1k views

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 ...
3
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5answers
3k views

How many colors exist?

How many "colors" do exist? Our perception: As far as I know, colors are just different frequencies of light. According to wikipedia, we can see wavelengths from about 380 nm und 740 nm. This means ...
1
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1answer
326 views

Is it a total or an explicite time derivative in the Schrödinger equation?

I am always dubious when I need write Schrödinger equation: do I write $\partial / \partial t$ or $d/dt$ ? I suppose it depends on the space in which it is considered. How?
1
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1answer
112 views

How does the winter solstice relate to the “longest night”

The winter solstice just gone was on the 22nd at 11:12 UTC. Does that mean the longest night was the night before or the one after? Or more generally, given the time of the winter solstice of any ...
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2answers
66 views

When do natural leap years occur, on years based on the northern winter solstice?

I'm designing a new calendar and I want the first day of the year to be the first day after the longest night of year in the northern hemisphere. Under this plan, "natural" leap years, where the first ...
4
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5answers
151 views

Could there be more universes?

In the documentary: "Curiosity - Did God Create the Universe (on YouTube)", theoretical physicist and cosmologist Stephen Hawking states that time did not exist before the big bang. The first ...
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1answer
91 views

Time passing near a black hole

Given distance $d$ from a normal black hole with mass $m$, how much seconds will pass at that distance while in infinite distance from the black hole $t$ seconds has passed?
3
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2answers
419 views

What is a single word that describes the idea of the second time derivative of energy?

I think about position, its time derivative speed, and its second time derivative, acceleration. I would like to identify a single word that can be used as a handle for the second time derivative of ...
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0answers
265 views

Does time actually exist or is there just a single stateful system being updated over time? [closed]

What I mean is, is there a past, future and present? or just a NOW which changes? I'm a computer programmer so the second alternative is from my programming experience. In this second alternative, I ...
6
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2answers
4k views

How might clock synchronization work with RSA SecurID tokens?

My workplace uses these things to generate one-time passwords which only work within a short time period. I have always been curious about how the clock synchronisation between the authentication ...
5
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3answers
333 views

How can we know, today, that there's something from 100 light-years from here?

In my understanding, to take a picture of something that is 100 light-years from here, our "camera" would have to travel 100 years at light speed, take the picture, send to us, and 100 years later we ...
4
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4answers
2k views

That 10km/day error predicted if GPS satellite clocks not corrected for relativity

Some authorities have stated publicly and without explanation that if the theories of Special and General Relativity were not taken into account in the design of the GPS (by building the satellite ...
6
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3answers
307 views

If the entire universe is the same age, can one assume that any potential life elsewhere is at the same stage of development as us?

There was some discussion in another question regarding whether you can consider all parts of the universe to have existed for the same length of time, and the conclusion seemed to be that, because of ...
2
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4answers
280 views

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 ...
2
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3answers
19 views

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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7answers
866 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 ...
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1answer
166 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
902 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
417 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
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 ...
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 $a(x)= \frac{x}{s^2}$.
14
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14answers
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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
74 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
219 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
652 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 ...
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0answers
108 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
594 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
4k 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
713 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
890 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
17k 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
209 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
183 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
126 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 ...