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

learn more… | top users | synonyms (2)

3
votes
1answer
119 views

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 ...
3
votes
3answers
798 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?
3
votes
1answer
79 views

Time Energy symmetry in General Relativity (not asking about energy conservation)

In General Relativity is there a TE symmetry similar to CPT symmetry in the Standard Model ? It's pretty easy to understand that by flipping charge and parity you merely get a time reversed equivalent ...
3
votes
2answers
151 views

How can we measure time?

If we cannot define a proper time (or synchronize clocks in different positions) in an inertial frame (independent with the theory of relativity), there seems to be no direct way to confirm the 2 ...
3
votes
1answer
138 views

Regarding the possibility of Closed Timelike Curves

I've been looking a lot at Closed Timelike Curves, and how if a theory allows for these curves it doesn't respect causality. I understand that about the curves themselves (Grandfather Paradox), but ...
3
votes
2answers
166 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 ...
3
votes
1answer
274 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 ...
3
votes
2answers
468 views

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 ...
3
votes
1answer
117 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 ...
3
votes
3answers
278 views

Slow thermal equilibrium

I have a question which is inspired by considering the light field coming off an incandescent lightbulb. As a blackbody radiation field, the light is in thermal equilibrium at temperature $T$, which ...
3
votes
2answers
260 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 ...
3
votes
3answers
114 views

How can we justify dropping the absolute time hypothesis?

In some approaches to Special Relativity the theory is motivated talking about the Michelson-Morley experiment and how this relates to the postulate that the speed of light is the same in every ...
3
votes
0answers
106 views

Do clocks really measure time?

Each time I listen to this quote from The man from Earth, I'm absolutely intrigued: Dan: Time... you can't see it, you can hear it, you can't weigh it, you can't... measure it in a laboratory. It ...
3
votes
0answers
66 views

When did people start to regard “time” as a physical quantity? [closed]

I was trying to figure out how people came to know about time then I realized that people started keeping track of time to know about sunset and sunrise. But I can't figure out how did time came into ...
3
votes
1answer
141 views

Does time invariance conclude conservation of energy? [closed]

I find it hard to understand that time-translation invariance necessarily implies conservation of energy. As I understand it, Noether's theorem says that there is an energy conservation because the ...
3
votes
2answers
87 views

Can time be interacted with? [duplicate]

Astronauts come back to Earth younger than they would have been had they stayed on Earth for that same period of time. They are traveling so fast relative to the Earth that time slows down for them. ...
3
votes
2answers
223 views

How is time not a constant?

I am a visual person, so it's hard to imagine the information I keep getting, but shouldn't time be a constant? If you were traveling at the speed of light and your able to cover $299{,}792{,}458$ ...
3
votes
1answer
120 views

How to interpret $t^2$? [closed]

I can't think of the meaning of squaring the Time (multiplying it by itself). It makes sense in Mathematics. But how can you figure it out in nature (or physics). As an example, the formula ...
3
votes
0answers
55 views

What is the shortest controllable time?

I was reading http://en.wikipedia.org/wiki/Planck_time and noticed a reference to: http://phys.org/news192909576.html where it is stated that: 12 attoseconds is the world record for shortest ...
3
votes
5answers
249 views

Two clocks along different worldlines

I have been reading and watching videos about this subject for a while now. I just can not seem to grasp the idea. Let's say we have two clocks. I leave one at home and keep one in my pocket. Then, ...
2
votes
2answers
511 views

Approximation to the number of seconds in a year?

Is there any mathematical formula which shows that there are approximately $\pi \times 10^7$ seconds in one year. I understand that the pi is probably due to the earth's circular orbit, but am not ...
2
votes
4answers
1k views

Which Schrödinger equation is correct?

In the coordinate representation, in 1D, the wave function depends on space and time, $\Psi(x,t)$, accordingly the time dependent Schrödinger equation is $$H\Psi(x,t) = ...
2
votes
3answers
21 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: ...
2
votes
5answers
341 views

What is the exact mechanism by which time dilates?

What is the exact mechanism by which time dilates for a fast moving object? Can the time dilation be explained by any theory other than relativity?
2
votes
3answers
185 views

Expansion of the Universe regarding Time and lightspeed

According to our current understanding, the universe is expanding... it is often said that every galaxy is moving away from us (I'm assuming that isn't true for galaxies in our local group), and it's ...
2
votes
2answers
363 views

Accuracy and Error of Atomic Clocks

I'm quoting a passage from my notes: The development of clocks based on atomic oscillations allowed measures of timing with accuracy on the order of $1$ part in $10^{14}$, corresponding to errors ...
2
votes
2answers
753 views

Can Flow of time become still?

According to theory of time dilation, flow of time slows down significantly at the speed of light.Is there any conditions practically or theoretically when flow of time is reduced to zero means it ...
2
votes
1answer
285 views

What does v=c in the Lorentz transformation for time tell us?

For the simpler cases as boost in the x-direction, the time dilation formula following the Lorentz transformation for time is $$\Delta t'=\gamma(\Delta t-v\frac{\Delta x}{c^2})$$Now, we observe that ...
2
votes
4answers
1k 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. ...
2
votes
1answer
372 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?
2
votes
4answers
306 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
votes
4answers
133 views

How much could one slow down aging using existing spacecraft?

Imagine that I board a spacecraft and head away from the Earth at top speed until I've aged twenty years, then I turn around and come back. How much younger will I be than the unexciting folk who hung ...
2
votes
3answers
160 views

Do algorithms have an intrinsic time direction?

This article says There is no intrinsic time direction in Newton's mechanics nor in the differential equations of the new physics. My question is, do other types of mathematics, say a cellular ...
2
votes
1answer
207 views

What are the Time Operators in Quantum Mechanics? [duplicate]

I don't understand at all what the time operators are in quantum mechanics. I thought that given a wave function, because it's a function of time, we could simple put in any time in the future to find ...
2
votes
1answer
114 views

How many hours will there be in a day 5,000,000,000 years from now?

It is known that the moon is moving away from Earth 2cm a year, and in doing so makes the days longer. I want to know how many hours will have one day, when our planet is near its end.
2
votes
2answers
206 views

does light experience time? [closed]

If we positioned a mirror 1 light year away from earth and shot a particle of light at the mirror so that it would reflect and come back to earth, how long would it take for us to receive that ...
2
votes
2answers
347 views

Proper time for an accelerating object

As far as I have read so far, proper time is the time measured on the clock of an inertial frame moving uniformly with respect to another inertial frame. The concept and the mathematical expression ...
2
votes
3answers
237 views

Proper Time for Deceleration in Special Relativity

As stated here: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html A rocket which accelerates to some point, then decelerates back to 0 over the same distance will measure a proper time ...
2
votes
2answers
6k views

Area under a displacement graph

If the area under an acceleration-time graph denotes velocity and the area under a velocity-time graph denotes displacement, what exactly does the area under a displacement-time graph denote?
2
votes
1answer
1k views

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 ...
2
votes
2answers
306 views

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, ...
2
votes
1answer
1k 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 ...
2
votes
1answer
58 views

Does time really differs drastically?

As I heard/read time moves slower and slower for me as I speed up, but does it change drastically? Our current formulas say, If I go at a speed very close to light's, time almost stops. I've seen ...
2
votes
1answer
105 views

How do sharp time intervals arise in a mesoscopic/macroscopic system?

$\newcommand{\ket}[1]{\left|#1 \right\rangle}$ $\newcommand{\bra}[1]{\left\langle #1 \right|}$ For a physical process in a mesoscopic/macroscopic system, how exactly can one deduce the time that ...
2
votes
1answer
116 views

Death by entropy

An idea struck me as I was walking to class today. According to Wikipedia, entropy is defined as the number of specific ways in which a thermodynamic system may be arranged, commonly understood as a ...
2
votes
1answer
77 views

What was a second in the early universe?

I have read some popsci articles and documentaries about the early universe and they often explain how various features of the universe came about and at what time. For example hydrogen atoms came ...
2
votes
3answers
65 views

Can we measure rates in real time?

I know what it means to say that my position is "X" at a particular moment in time. I can easily take a picture of my motion and observe my exact location at the instant the picture was taken. That is ...
2
votes
2answers
141 views

How is time distributed through space?

I was greatly confused when I saw video saying that if two wormholes were separated by time instead of distance, it would allow time travel. My question is what did they mean by this statement? What ...
2
votes
1answer
599 views

Can we live forever by taking advantage of the time dilation effect?

I have been reading about Time dilation and twin paradox in Relativity. So,I thought that if It can be the difference of 10 yrs between the Girl in space and on Earth, then why can't anyone not age ...
2
votes
1answer
822 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 ...