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Questions tagged [time]

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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Traveling almost speed of light [duplicate]

I am in a huge rocket which is big as the moon. I will accelerate to 99% of the speed of light in 1 nanosecond, so I will travel 300 000 km distance. Then immediately turn 180 degrees and came back to ...
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Covered distance by 2 cars starting from the same point [on hold]

Let's be two cars: $A$ and $B$. The $B$ will start to move after $2s$ after $A$ started to move. These 2 cars started to move from the same point and have the same direction. The initial velocities ...
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1answer
56 views

The absoluteness of time intervals in Newtonian mechanics: how is this input used?

One of the assumptions of Newtonian mechanics is that "time is absolute". Absolute, as I understand, implies that it is the same for all observers. But it's not quite true because if Tom's watch is ...
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1answer
94 views

If I could stop ALL motion would I stop time? [duplicate]

I've never really understood time. Time is not a force. It isn't energy or matter. Time doesn't MAKE anything do anything. Time doesn't make a clock tick (motion or batteries do). Time doesn't make ...
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How does an imaginary coefficient in the heat equation ensure no special direction in time?

In Pauli(1973): Wave Mechanics (Pauli lectures on physics Volume 5) he derives on page 4 the Schrödinger equation (similar to Tong: Lectures on QFT, p. 42) $$ \nabla^2 \psi' + i \frac{2m}{h}\frac{\...
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60 views

Time-independent canonical transformations

Lie's criterion tells us that $(q,p) \to (Q,P)$ is a canonical transformation, for a system with Hamiltonian $H$ and "Kamiltonian" $K$, if and only if the identity $$\sum_k p_k dq_k -Hdt = \sum_k P_k ...
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1answer
31 views

Long lifetime pair creation in nonrelativistic $v\ll c$ regime?

At the Bottom of page 12 of the notes Quantum Field Theory I by Michael G. Schmidt (U. Heidelberg, 2007), I have seen that also for very slow massive particles there are quantum field theoretical ...
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If Time and Space are intrinsically linked and space is ever expanding then surely time is slowing down [duplicate]

When we try to imagine the initial expansion after the big bang we see with the CMB that the universe went from initially smaller than a hydrogen atom to what we see today. However, when we try to ...
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2answers
78 views

Given Max Speed, Max Acceleration and Jerk - How do I calculate time?

I'm developing some python code that controls an end effector moving in a two-dimensional plane (XY). It moves through a list of points sequentially and I'd like to estimate the time it will take as ...
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1answer
62 views

Time and “fundamenta”l equations

I have been reading articles by Carlo Rovelli. About quantum mechanics, he says: “The Heisenberg and Schrödinger pictures are equivalent if there is a normal time evolution in the history. In the ...
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3answers
50 views

Thermal time hypothesis vs. cosmological time

How can Rovelli's thermal time hypothesis be consistent with the apparent existence of a privileged time in the sense of cosmology?
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138 views

Is Time emergent from quantum entanglement?

This paper shows that 2 photons in a toy universe (isolated box) are seen as static to an outside observer unless interacted with/entangled with. It states that it shows that time emerges from ...
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Waves Introduction Course Doubts

Here I have tow basic problems on wave theory, but I'm having a struggle to nail what the problems are asking for Problem 1. Information is transmitted through a wire with electric pulses at 100, 000 ...
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6answers
213 views

Noether's theorem, energy and time invariance

I have studied high school physics but only have a pop-sci kind of understanding of ideas like Noether's theorem so go easy. As far as I know, Noether's theorem simply states that any kind symmetry ...
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1answer
36 views

Explanation of the time quantities in the travelling wave equation

I know that $y=A\sin(\omega t-kx)$, but this can also be written as $y=A\sin \omega (t-x/v)$. What I don't understand is what the quantity $(t-x/v)$ represents. Both quantities have the units of time, ...
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1answer
57 views

How close are we to measure Planck time

I know that Plack time is considered a fundamental time unit by some of the physicists. To test this we need to try to falsify it. I want to learn what the smallest time interval we can measure is.
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Is time moving or we? [duplicate]

We know that time moves in one direction and causes entropy,but what if we are in an enclosed system and instead of time moving through it,we ,a universe is moving through a stationary substance time ...
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2answers
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What happens the present when it has passed? (If there is a physical explanation, I would really appreciate it) [duplicate]

It may seem more like a philosophical question but I wonder if there is a physical explanation for that. Can someone tell me what happens the present (imagine I put the present in my reference frame ...
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52 views

Is Time a reification? [duplicate]

"Time" seems to fit the definition of a Reification "when an abstraction (abstract belief or hypothetical construct) is treated as if it were a concrete real event or physical entity." So how then is ...
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1answer
48 views

Hamiltonian operating on a function of time

I've seen a few people claiming: $$\hat{H(t)}[\psi(x)T(t)] = \hat{H(t)}[\psi(x)]T(t)\tag{1}$$ i.e. an explicit function of t is not acted upon by H, even if H itself may be dependent on t. A more ...
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1answer
49 views

Looking at Earth's history using a cosmic mirror [duplicate]

Assuming we have a very powerful telescope, and a huge mirror 1000 light years away from earth, is it possible to look at this mirror to see what people did on earth 2000 years ago? Is it even ...
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2answers
78 views

Does object from which the universe is created still exist in its original form? [closed]

If time is passing more slowly near objects with bigger gravitational mass, could it be that the time at the object from which the universe is created has not started yet (or very little of time has ...
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(2+2)-dimensional flat spacetime with neutral signature [duplicate]

Is there an example of (2+2)-dimensional flat spacetime with neutral signature (i.e. (+,+,-,-))? ...for example the metric $ds^2=(dt_1)^2+(dt_2)^2-(dx)^2-(dy)^2$, has been studied? Are there any ...
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49 views

Magnetism and Space/Time

I have a permanent magnet 1/2" from a steel plate and it requires a considerable force to keep them apart. How can I visualize this being related to reshaping Space/Time?
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2answers
49 views

Events occuring acording to Relativity of simultaneity

I have seen a picture on wikipedia, which states that Events A,B and C can occur - depending on motion - in the order A,B,C; C,B,A or at the same time. Let Event A be the birth Odilo, Event B is ...
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2answers
102 views

Why do physical processes… take time? [closed]

A spoon is dropped in a vat of acid. The outside of it melts away... over time. A pill is swallowed and dissolves in a stomach... over time. A flammable gas is lit, and the flame travels from the ...
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34 views

Showing time-dependent transformations are canonical

In Goldstein's mechanics book he defines a canonical transformation as a transformation from $q, p$ to $Q, P$ with $$ Q = Q(q,p,t) \tag{9.4a} $$ and $$ P = P(q,p,t) \tag{9.4b} $$ such that if $H$ ...
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81 views

A little help on Gravitational time dilation

Could somebody explain Gravitational time dilation in layman's term for me please!? And in the formula of Definition, it mentioned a capital H, but it didn't ...
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2answers
105 views

Time and the Multiverse [closed]

Is a clock in one universe of any use to someone in a different universe? Supposing the multiverse model is correct, it posits the existence of several distinct universes that exist independently of ...
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4answers
161 views

The flow of time and the Light Clock / fundamental phenomena wrt time?

Two statements from Special Relativity are: The Light Clock measures time. Time is what a clock measures. This seems somewhat circular, but I wonder if it is possible to use the light clock as a ...
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1answer
101 views

How do photons travel in time? [duplicate]

How can particles such as photons travel in time if they do not have any mass? They are inseparable, so how can you have one but not the other?
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1answer
52 views

How does going faster than light cause going backward in time? [duplicate]

I have read in many places that some virtual particles can "travel faster than light (1)" and thereby "go backwards in time (2)". My question is about the connection between these two. I have always ...
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Expansion of the time dimension [duplicate]

Given that the universe is expanding in the three spatial dimensions, is it possible that the time dimension is also changing over time? If the rate of the passage of time is changing over time, ...
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38 views

Is there an end to the know universe?

The second law of thermodynamics states that the entropy of the system, in this case the entire universe (everything), would always increase. It is also true that nature sometimes brings order in the ...
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2answers
508 views

Are the creation and annihilation operators time-dependent?

Something that always confused me when first hearing about second quantization were the dependencies of the creation and annihilation operators. On the one hand I have seen expressions such as $$ \...
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1answer
122 views

How to understand Carlo Rovelli's notion that time “does not exist”?

Media coverage of Carlo Rovelli's book The Order of Time has had headlines like "There is no such thing as past or future", or "Carlo Rovelli: 'Time does not exist'." Is there a way to explain what he ...
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3answers
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Since astronomers are adding 'leap seconds' to our years nowadays, does that mean Earth's orbital period is getting longer?

How long was an Earth year several billion years ago? (I'm assuming constant days, even though I know days were much shorter back then).
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1answer
31 views

Calculate work from Force and time

Everywhere it states, that time does not matter when calculating work, but can't you do this: $$F=ma$$ so $a=F/m$ taking the second integral (dt): $$s=\frac{1}{2}\frac{F}{m}t^2$$ since Force is ...
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Is simulating quantum field theoretical equations in the continuous limit the same as simulating really quantum continuous processes?

At the end of this article: https://hal.archives-ouvertes.fr/hal-00542373/document it is said that: "For example, several works simulate quantum field theoretical equations in the continuous limit of ...
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40 views

Formulaic representation of time differential

It is a known fact that time passes at a different rate on earth than it does elsewhere in the universe. Where a second on earth passes uniformly, at an altitude of 11000 or also passes uniformly but ...
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41 views

With relativity, what does the age of the universe *mean*? [duplicate]

A friend, after attending a talk recently discussing relativistic and other time-like notions, pondered on this question: "Given the fact that much of the talks I attended discussed at length how ...
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1answer
54 views

Is time really a 1-dimensional object that we only traverse through? [duplicate]

Is time really a 1-dimensional object that we traverse || is it possible that time can also be traversed horizontally. I think the later would also put it on a 2 dimensional plane.
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3answers
118 views

Why does time dilation require events to occur at same point in space?

I was reading through "University Physics with Modern Physics ~ Sears and Zemansky" and they mentioned the following paragraph in time dilation : We may generalize this important result. In a ...
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31 views

What distinguishes time directions from spacial directions?

Other than the fact that one may move in only one direction in time but can move in two directions in each spacial dimension (e.g. arrow of time), are there any other physical characteristics that ...
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54 views

Is the Many-Worlds Interpretation consistent with our best understanding of time? [closed]

Einstein's theory of relativity implies a B-theory of time. Deterministic interpretations of quantum mechanics such as the Many-Worlds would seem to be consistent with the B-theory. Regardless of ...
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68 views

Is the universe a lattice?

My understanding is that cause and effect create a partial order of events in the universe (and that relatively prevents anything stronger). My understanding is also that it’s generally useless to ...
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1answer
87 views

Does one age quicker at higher altitudes?

See title: Does one age quicker at higher altitudes? A few years ago I heard that you would age slightly faster (or slower) at higher altitudes. Is this true? What is the theory or evidence for ...
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1answer
65 views

Does the big bang model describe a first momment in time for the entire universe or just the observable universe

The big bang model describes the universe as contracting as we wind time backwards. Since the observable universe is of a finite size this ultimately sets the first moment of time at around 13.8 ...
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#36, Problem by Walter Lewin [duplicate]

I was solving a problem by Prof. Walter Lewin. The problem states that there are two masses M,m in space at a distance R(centre to centre) from each other, where M is Earth and $ m\lt\lt M $. We ...
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Can someone review this mathematical derivation that in Special Relativity FTL implies Time Travel

I am looking to show mathematically (i.e. with derivations and algebraic relationships) that FTL implies time travel. Here's what I got: Suppose we have 2 ships $O_1, O_2$ traveling at $0.9c$ in ...