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
0
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1answer
153 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
vote
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 ...
17
votes
7answers
737 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 ...
2
votes
3answers
268 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
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 ...
0
votes
1answer
104 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
votes
1answer
96 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
224 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.
0
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4answers
201 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, ...
3
votes
1answer
111 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
vote
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?
8
votes
4answers
559 views
Does the future already exist? If so, which one?
In the NOVA Fabric of the Cosmos program, Brian Greene explains a theory in which there is no "now", or more specifically, now is relative. He describes an alien riding a bicycle on a far off planet ...
0
votes
1answer
134 views
Does photon possesses no time to cover any arbitrary distance?
Photon travel 8 minutes (with speed $c$) from the sun to reach the earth. Any particle (or space-ship) with velocity $0.99 c$ covers the same distance (93 millions km) within less than 2 minutes ...
6
votes
8answers
482 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.
...
2
votes
2answers
931 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 ...
-1
votes
2answers
114 views
Can time dilation be explained by limitations on computing power?
Are there any ideas of explaining the time dilatation as limits in "computing power"? What I mean is basically that the greater is a concentrated mass, the harder is to "compute" what happens in such ...
1
vote
1answer
59 views
When are leap seconds added in various time zones?
I understand that technically when a leap second is added, it is added after midnight UTC, but I'm unclear how the addition is handled in other timezones. For precise reckoning of course (e.g. ...
0
votes
0answers
36 views
Can a black hole actually grow, from the point of view of a distant observer? [duplicate]
Possible Duplicate:
Black hole formation as seen by a distant observer
I've read in several places that from the PoV of a distant observer it will take an infinite amount of time for new ...
1
vote
1answer
213 views
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 ...
-1
votes
1answer
96 views
Existence of time in some other universe [closed]
Is it necessary that time should exist in another universe if it (universe) is there? How do we perceive timelessness?
4
votes
3answers
181 views
Black hole formation as seen by a distant observer [duplicate]
Possible Duplicate:
How can anything ever fall into a black hole as seen from an outside observer?
Is black hole formation observable for a distant observer in finite amount of time? ...
4
votes
1answer
68 views
Could a bipolar nebula be produced by a time gradient?
M2-9 is an example of a bipolar nebula that resembles two back-to-back rocket nozzles. Is it possible that this shape (somewhat unusual for an explosion) is the result of a time gradient? A rotating ...
5
votes
0answers
211 views
Why is my approach to the equation of time off by a constant?
I'm trying to better understand the causes for the equation of time by deriving an approximation from first principles.
My naive approach, $EOT_{NAIVE}$, is to take the difference between the right ...
-4
votes
1answer
152 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 ...
1
vote
1answer
131 views
When calculating the local apparent sidereal time, which time scale should I use?
UT1, UTC, TAI, TDB, or what?
I need to determine the time difference between a given observation and the epoch from which certain constants apply. I typically work with the J2000.0 epoch. This is to ...
9
votes
3answers
436 views
The meaning of imaginary time
What is imaginary (or complex) time? I was reading about Hawking's wave function of the universe and this topic came up. If imaginary mass and similar imaginary quantities do not make sense in ...
3
votes
2answers
259 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 ...
9
votes
0answers
224 views
How can two time theories be compactified to 3+1 without any Kaluza-Klein remnants
I have recently been looking into the two-time theories and the implied concepts.
For me this seems slightly hard to grasp.
How can I see the basic concept in this theory in a fundamental way based ...
3
votes
2answers
236 views
How would we perceive time going backwards?
I haven't taken Physics in University. Lately, I've been reading about some of the branches of physics through Wikipedia. I read several times that many of the theoretical models do not explain why ...
5
votes
2answers
377 views
What's the difference between space and time?
I'm having a hard time understanding how changing space means changing time. In books I've read people are saying "space and time" or "space-time" but never explain what the difference is between the ...
2
votes
1answer
124 views
How do physicists and astronomers handle leap seconds?
I'm confused by the many contradictory descriptions I see about how UTC leap seconds are accounted for. I understand that there are various ways to handle them in common practice, and I've seen a ...
1
vote
1answer
289 views
Laser Coherence Length/Time
Scenario:
Imagine a collimated beam of white light falling on one refracting face of a prism. Let the light emerging from the second face be focused by a lens onto a screen. Suppose there is linear ...
4
votes
1answer
135 views
What time scale is used by the JPL HORIZONS system?
I'm confused by the ust of the term "UT" in the description of time scales used by the JPL HORIZONS system.
Their manual states that
UT is Universal Time This can mean one of two non-uniform ...
4
votes
1answer
75 views
The Effects of Moving Matter Across Light-Year distances
If I were to stand at one end of a light-year long metal pole, and another person were to stand one light-year away at the other end, and then I were to push on my end of the pole. How long would it ...
0
votes
1answer
114 views
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 ...
2
votes
1answer
87 views
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 ...
2
votes
0answers
57 views
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 ...
5
votes
2answers
311 views
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" ...
1
vote
2answers
143 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 ...
10
votes
1answer
366 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 ...
3
votes
0answers
77 views
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 ...
1
vote
2answers
161 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. ...
6
votes
5answers
358 views
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 ...
2
votes
1answer
305 views
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
...
12
votes
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 ...
5
votes
1answer
329 views
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 ...
2
votes
2answers
120 views
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?)
1
vote
3answers
271 views
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 ...
15
votes
3answers
740 views
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?
...
