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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1answer
77 views
US time zones and Daylight saving time, energy efficient? [closed]
As a foreigner, United States has a very complex time system for me. Central Time Zone, North American Eastern Time Zone, ...
1
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0answers
76 views
Do particles travel backward and forward in time? [duplicate]
All these classical ideas are pointless and obsolete today, because in quantum mechanics, the particles are completely different objects, defined by quantum motion of fields, not by the location of ...
0
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1answer
87 views
Does our local time speed up as the Universe expands?
Starting from a simplified radial Freidman Walker metric we have
$$ds^2 = -c^2 dt^2 + a(t)^2 dr^2 $$
How does one measure one's proper time operationally?
One times a light beam along an element of ...
0
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1answer
54 views
Time Dilation in relation to Acceleration
What I am looking for is a layman's explanation on the equations required to work out Time Dilation at high speeds including acceleration and deceleration of velocity. Or I would greatly appreciate it ...
2
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3answers
173 views
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 ...
1
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1answer
86 views
How much time has passed for Voyager I since it left the Earth, 34 years ago?
34 years have passed since Voyager I took off and it's just crossing the solar system, being approximately at 16.4 light-hours away. How much time have passed for itself, though?
1
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2answers
183 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 ...
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votes
3answers
411 views
How does Time traveling work in practice?
There are multiple theories about time traveling. One is "proven": The time slows down according to your speed. The satellites in space are traveling faster than us, thus their clocks slows down a ...
0
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0answers
27 views
What is the formula for calculating the length of any given day (sunrise to sunset)? [duplicate]
In a specific date what law gives us perfect measurements and how will we measure if latitude is given?
4
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1answer
93 views
Understanding how the rate of time changes
The rate at which time passes is relative depending on speed and the gravity as predicted in general relativity. This theory has been tested by scientists by comparing two identical atomic clocks, one ...
1
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1answer
92 views
How can we know the time frames for events in the early universe?
I just finished watching Into the Universe with Stephen Hawking (2010). Specifically the third episode titled 'The Story of Everything.'
In the episode Hawking is explaining the mainstream theories ...
6
votes
8answers
485 views
Can you completely explain acceleration to me?
I understand what acceleration is, and I know the formula, and I understand it's a vector.
I just don't understand how the equation works exactly. I'm kind of picky, I know, but bear with me.
...
3
votes
2answers
289 views
Is there an observable of time?
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 ...
0
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0answers
34 views
Why there is no operator for time in QM? [duplicate]
Is there one central reason why there is no "Time" operator in QM?
I know this question has been asked before, but I thought I would try to stimulate some fresh thinking.
2
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2answers
943 views
Is time speeding up due to the expansion of space?
If we just look at our local galactic cluster, if all of the galaxies that are a part of it are moving away from each other, and so the overall 'density' of the strength of gravity in the cluster is ...
0
votes
2answers
160 views
Imaginary time and string theory
Is imaginary time an extra dimension?
In other words, are time and imaginary time considered two separate dimensions?
If so, does imaginary time appear (as a separate dimension) in string theory (thus ...
3
votes
1answer
91 views
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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1answer
106 views
How do photons experience time? [duplicate]
I know that as velocity approaches the speed of light the time dilation shoots to infinity as shown below.
1)So I want to know how time is perceived from the point of view of the photon?
2)Since ...
0
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1answer
93 views
Deriving infinitesimal time dilation for arbitrary motion from Lorentz transformations
I'm trying to derive the infinitesimal time dilation relation $dt = \gamma d\tau$, where $\tau$ is the proper time, $t$ the coordinate time, and $\gamma = (1-v(t)^2/c^2)^{-1/2}$ the time dependent ...
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votes
1answer
154 views
What is the difference between UT0, UT1 and GMT time?
Every reference I find says that they are "essentially" the same, which we all know really means that they are not the same, but different only by a some small amount that someone else other than me ...
0
votes
4answers
202 views
The bigger the mass, the more time slows down. Why is this?
If I were to stand by a pyramid, which weighs about 20 million tons, I would slow down by a trillion million million million of second. Don't know if that's exactly right, but you get the point. Also, ...
2
votes
4answers
316 views
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 ...
8
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2answers
249 views
Question about proper time in general relativity
I think I may have some fundamental misunderstanding about what $dt, dx$ are in general relativity.
As I understand it, in special relativity, $ds^2=dt^2-dx^2$, we call this the length because it is ...
3
votes
1answer
113 views
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. ...
2
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1answer
115 views
Life of a photon [duplicate]
I am a student of class 12th and as far as i know when anything reaches about 99.99% of the speed of light it starts traveling in time or time for it slows down so that it don't breaks the speed ...
1
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1answer
84 views
parallel=time, perpendicular=space? on multiple time dimensions
There are some discussions on more than one time dimensions, e.g., Intuition for multiple temporal dimensions and More than one time dimension.
If we define that the parallel direction is time, of ...
1
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1answer
56 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 ...
4
votes
1answer
102 views
How is the direction of time determined in general relativity?
In special relativity every frame has its own unique time axis, represented in Minkowski diagrams by a fan-out of time vectors that grows infinitely dense as you approach the surface of the light cone ...
5
votes
3answers
140 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 ...
1
vote
1answer
139 views
Do residents of the Hudson Bay area have more time?
Apparently there is a gravity anomaly in the Hudson Bay Area in Canada: gravity is "missing" or it is slightly less than it is in the rest of the world.
Does that mean that things in the Hudson Bay ...
4
votes
5answers
302 views
What is $\Delta t$ in the time-energy uncertainty principle?
In non-relativistic QM, the $\Delta E$ in the time-energy uncertainty principle is the limiting standard deviation of the set of energy measurements of $n$ identically prepared systems as $n$ goes to ...
1
vote
4answers
484 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 ...
17
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7answers
739 views
What grounds the difference between space and time?
We experience space and time very differently. From the point of view of physics, what fundamentally grounds this difference?
Dimensionality (the fact that there are three spatial dimensions but only ...
12
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9answers
444 views
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 ...
2
votes
1answer
80 views
What was ticking just after the Big Bang?
When reading about the Big Bang, I see phrases like 3 trillionths of a second after... So, what was ticking to give a time scale like this? We define time now in terms of atomic oscillations, but ...
3
votes
3answers
310 views
Why do clocks measure arc-length?
Apologies in advance for the long question.
My understanding is that in GR, massive observers move along timelike curves $x^\mu(\lambda)$, and if an observer moves from point $x^\mu(\lambda_a)$ to ...
0
votes
1answer
156 views
The real meaning of time dilation
Is this true or false: If A and B have clocks and are traveling at relative velocity to each other, then to B it APPEARS that A's clock moving slower, but A sees his own clock moving at normal speed.
...
1
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1answer
100 views
Why does only one twin travel in the twin paradox?
The wikipedia page repeatedly says that the twin travelling in space is the only one which travels, and also is the only one which faces acceleration and deceleration. So it does not age, while the ...
2
votes
3answers
270 views
Is time dilation an illusion?
It is said that we can verify time dilation by flying a very accurate clock on a fast jet or spaceship and prove that it registers less time than the clocks on earth. However, the clocks on earth ...
2
votes
1answer
74 views
Degree of Time Dilation At a Distance From the Sun where acceleration = g?
At higher altitudes above a body, clocks tick more slowly, and gravitational field is weaker. But what is the relationship? It is tempting for a GR newbie such as myself to think that anywhere that ...
3
votes
3answers
2k 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! ...
3
votes
2answers
88 views
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 ...
4
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5answers
494 views
Why isn't the symmetric twin paradox a paradox?
Two twin sisters synchronize their watches and simultaneously (from the earth frame) depart earth in different directions. Following a predetermined flight plan, each sister accelerates identically to ...
0
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1answer
106 views
What is the maximum time dilation between two objects, if one is standing still and the other is moving at $c$?
What is the maximum ratio in the rate of change in time in reference to object $A$ which is standing still and object $B$ which is moving at the speed of light?
0
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1answer
97 views
Confusion about time shift in special relativity
I have never really found a way to comfortably comprehend the idea of time shift even though I know its not the hard part of relativity theory. In that light, can someone point out what is wrong or ...
5
votes
1answer
225 views
What is the speed of time
When we measure the speed of a moving element we do it with the help of a reference frame. Now if we need to measure the speed of time, is it possible? Is time really has speed?
Thanks in advance.
5
votes
3answers
481 views
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 ...
0
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2answers
171 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?
3
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1answer
112 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 ...
1
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1answer
70 views
Looking backwards in time at yourself
If a person on Earth today is looking at a star, say, 10 billion light years away, is it possible that some of the atoms he is looking at will eventually go on to make him?
