# 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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### Time in the second law of thermodynamics versus relativity [duplicate]

I recently had a debate with someone about time and argued that the most concrete definition of time would be an increase in entropy. The entropy of a localized area of space might reduce, but that of ...
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### Why is the age of the universe important?

Age may only be a number, but when it comes to the age of the universe, it's a pretty important one. Why?
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### Is the age of the universe 13.799±0.021 billion years OR 2.935364e+18 second(s)? [duplicate]

According to wikipedia (https://en.wikipedia.org/wiki/Observable_universe), The observable universe is thus a sphere with a diameter of about 28.5 gigaparsecs (93 billion light-years or 8.8×10^26 ...
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### What if the moon was destoryed? [on hold]

Speculating about how would be the situation in the Earth if there was no moon. I made some research and this are some of the consequences I could find (though im unsure if they're accurate): Earth ...
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### No explicit time-dependence in Lagrangian means constraints are explicitly time-independent?

Suppose a Lagrangian is not explicitly time-dependent. Does it mean that the constraint equations are also explicitly time-independent, and (as a result) the kinetic energy is necessarily a ...
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### Why and in what cases we use $E$ instead of $\Delta E$ in energy-time uncertainity relation?

sometimes in problems of energy or time uncertainity we use the value of energy of the particle/system to calculate the delta t. Now what are those cases specific examples, where we can apply this?
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### Is our physics and system of units based on time (second) or the speed of light?

The speed of light in vacuum is stated in our physics as a universal physical constant, c, when measured locally, in vacuum. The speed of light in vacuum, commonly denoted c, is a universal ...
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### What's the advantage of Conformal Time?

I'm trying to follow the analysis of CMB Acoustics and several charts are done using conformal time instead of chronological time. Conformal time corresponds to the quotient of the particle horizon ...
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### How quickly does an atomic electron decay?

I've looked in lots of books on atomic physics, but I've not been able to find an answer to: Question: Take a hydrogen in empty space whose electron is excited. How long will it take for the ...
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### Different statements of the Heisenberg uncertainty principle [duplicate]

I know that from the Heisenberg uncertainty principle, ∆x∆p=ℏ/2 . And I know that this equation can be rewritten as ∆t∆E=ℏ/2. From QED I also know that the equation ∆t∆E=ℏ/2 claims that some energy ...
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### How to define the proper time of a photon?

I'm writing a paper about the motion of photons near a Schwarzschild black hole. At some point there's a derivative of the Hamiltonian of the system with respect to time $\tau$. I need to explain what ...
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### What does this mean? World consume 14 trillion watts of power [closed]

I am not very clear about this statement. Does that mean in every second we are consuming that much energy?
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### Time to move along a path?

Let's say I have an arbitrary path from point $A$ to point $B$. A particle moves from $A$ to $B$ under the influence of gravity but also impeded by a frictional force $\vec{F_\mu}$. The gravitational ...
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### WHY did physicist defined velocity as displacement divided by time, why not displacement * time? [closed]

V=S/T. As per my knowledge i think ratio as division and it don't give any meaning like this much displacement in this much time. So i think physicists only used division as notion for velocity. But ...
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### Do orbiting masses accelerate or decelerate (relative to us) near black holes? [duplicate]

Apologies if this question seems stupid; I was just curious after watching a documentary, and I am not a trained astrophysicist. Simply put, I cannot reconcile the (perhaps misunderstood) facts that ...
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### Justification for “running” cosmological constant

I am looking for papers which seek out to justify that the cosmological "constant" could be time-dependent. I am aware that within the framework of GR the cosmological constants really has to be a ...
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### What does the time reversibility of the laws of physics mean for causality?

Does the fact that the fundamental laws are symmetric with respect to direction of time show that causation does not exist? Since causality always requires the cause to precede the effect, but laws of ...
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### Why are massive particles clocks? Or are they not?

I recently came across a public lecture "Dark matter decay?" by Sir Roger Penrose. In his lecture he states that the two equations $E=mc^2$ and $E=h\nu$ can be combined to form a formula for the ...
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### Co-moving distance and universal time

I am reading about Cosmology, in particular, the concept of co-moving distance. I am trying to derive the equation for calculating the co-moving distance with the following argument. If the source ...
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### How long does it take to cool down a drink with ice?

If I have a bottle or container with X fluid ounces of water at temperature Y (say, fridge temperature or room temperature), and I put ice or ice sticks (composed of water) in it that make up Z fluid ...
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### Why the Chipping Rate of 1.023 MHz for the GPS L1 C/A Signal?

Why was the chipping rate for the GPS L1 C/A signal chosen to be 1.023 MHz (1023 chips per symbol, one symbol per ms)? It seems to me like you'd want to get one chip as close as possible to an ...
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### Time dilation : Hourclock on Jupiter

Time slows down on Jupiter because of higher gravity, but Hourclock(sand clock) run faster due to higher g. If I am correct than how?
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### Is time an invariant of Galilean transformation?

Is time an invariant of Galilean transformation? By saying that I mean if there is a quantity analogous to spacetime interval in Lorentz transformation. What is the geometry of "Galilean spacetime"?
Say for instance I have two objects with different masses that collide in-elastically the resulting velocity will be $v =\frac{m_1 v_1 + m_2 v_2}{m_1 + m_2}$. What I don't fully grasp is how to ...