# 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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### What does the observable 'time' exactly mean in quantum mechanics? [duplicate]

The uncertainty principle for time and energy is $$\Delta t \Delta H >\frac{|[H,t]|}{2}.$$ What are the eigenstates of the time operator? How can anything be in a superposition of time states? What ...
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### Theory on the Role of Zero-Point Energy in the Big Bang and the Nature of Time [closed]

In this hypothesis, I propose that the universe began from an initial state where zero-point energy (ZPE) was zero, resulting in static quantum fields with no fluctuations or excitations. This static ...
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### Is any planets angular speed roughly constant?

The duration of a sidereal day stays pretty much constant on Earth as far as I know. But I was wondering if that is typical or even always the case. For example, can moons influence it? Or can ...
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### How are rotations invariant by time reversal?

The title summarizes it. I'm confused as to how the anti unitary operators work. If it inverts the signal of the angular momentum and conjugates the rotation operator, I can see why the rotation ...
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### Relation between energy and time

I would like help in understanding something that has been causing me a lot of trouble recently: Why is energy always related to time in physics? Examples include the 4-momentum, the energy-time ...
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### Continued calibration of atomic clocks

First off I am not well versed in physics, but as I understand things the second is defined by the ceasium fountain clock which is calibrated as follows How was the first atomic clock calibrated?. Now ...
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1 vote
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### Do clocks tick faster when gravitational forces are weaker?

A professor last year taught us that "gravity slows clocks," when teaching about the relationship between gravity and time. This led me to think about places, such as intergalactic space, ...
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### Probabilistic behavior of quantum mechanics [closed]

In a hypothetical scenario, if I were to measure the quantum spin of an electron and it showed "up," and then I traveled back in time without changing the initial conditions, would measuring ...
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### Does Hamilton's principle allow a path to have both a process of time forward evolution and a process of time backward evolution?

This is from Analytical Mechanics by Louis Hand et al. The proof is about Maupertuis' principle. The author seems to say that Hamilton's principle allow a path to have both a process of time forward ...
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### How exactly did Harrison's chronometer circumvent the impulse problem of time-keeping on a moving ship?

According to folklore, around the time of the exploration of the New World, there was a quandary regarding how to measure time on the open sea. Time keeping then was based on the pendulum clock, which ...
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### Multiple time dimensions in the eternal inflation model

From a lecture by Prof. Kaiser, I reckoned that according to the Eternal Inflation model, it is possible that all of the 10500 topologies posited by string theory could exist somewhere in the region ...
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### Extrapolating time to infinity for a particle moving spirally outward

If I have a particle moving radially away from me in a straight line and I extrapolate time to infinity, I could see the particle in a particular direction (given enough time for the light to reach me)...
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### Could chaos theory and butterfly theory prove that time only exists in the present? [closed]

I am sorry if this were to sound dumb but, chaos theory is about reconizing patterns and the universe itself can only repeat it self , would that mean that time only exist in the present cause if time ...
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### If time reversal symmetry was preserved would past and future notions be defined

If time reversal symmetry was preserved would someone be able to define future as a time interval from a given time moment as measured by a clock (assuming they would still function as normal)?
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### Gravitational time dilation near the Earth [closed]

I recently read the statement that near the Earth, in the Newtonian weak gravitational field, gravity is 99.9999% mainly due to "curvature of time" (ie gravitational time dilation), and only ...
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### Does it make sense to talk about time in absence of matter? [duplicate]

The equations of general relativity should predict (although I might be wrong) that in absence of mass the spacetime is everywhere flat. That is, time is the same everywhere. However, I'm not sure ...
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### Does quantum entanglement arise from perpendicular time vectors? [closed]

From what I understand, "quantum entanglement" is a phenomenon where certain information travels instantly between entangled particles, regardless of distance in space. When thinking of ...
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### Does time arising from entropy agree with GR?

There's a theory that the direction of time arises from entropy and the correlations (interactions) between bodies. However, I don't see how this would incorporate the effects of General Relativity, ...
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### Speed is equal to distance divided by time but is this correct?

In this study https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9784821/, the distance the punch travelled from start to impact is 0.49 meters and the time taken from start of punch (that's it, they define ...
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### Is the meter relative to time?

Is the meter relative when we are near the speed of light? I was reading a physics book and I found that the meter is the length that light travels for an amount of time, so since time is relative ...
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### Intuitive explanation of COSMIC TIME?

I came across the following statement, while studying a Newtonian model for cosmic expansion: "If $R(t)$ is the scaling factor, we can define the Hubble parameter as \$H(t)=\frac{\dot{R(t)}}{R(t)}...
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### Gravitational effects on a celestial body and the difference between a pendulum and a regular clock

Imagine a planet with the same properties as Earth, this time moving in an elliptical orbit around a heavy star of a large number of solar masses. Also imagine that the surface of this planet is as ...
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