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Perhaps the question is better answered by asking, "what makes for a BAD clock". Most people know, at least intuitively, the answer to this question: a bad clock is one that shows (1) no direct correspondence to other clocks and (2) variations in the period after many cycles compared to another clock of the same type that has undergone 1 period.


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Entanglement experiment performed in one frame of reference guarantees that the two measured results are synchronized in this frame of reference. If we try to perform the same entangled experiment while the two ends of the same length fiber optic lines are attached to two frames of references moving with a constant relative velocity, the two measured results ...


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Time is defined as the change we experience. Which is things being different in two different places in time. Which is annoyingly circular. Why can't we freely move back and forth in time like spatial dimensions? Because then it wouldn't conform to our idea of time. It's perfectly plausible though, to have another kind of being who perceives one of our ...


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Time such a pesky pest To leave it alone will be the best Cause its sure not to let you rest Until your thoughts explode in chest. What you say cannot be refuted. But speed of light alone cannot define it But you are accepting the notion of space , then of course space and motion will together be able to define time. Similarly c can be thought of as a ...


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If you are abit more careful about making the statements, then "both perspectives" are actually correct. Let me be more concrete and explain. Let $E$ be the one who stays on Earth and $S$ be the one who is on the spaceship. The first issue you have is you said "let's say seven years passes on Earth" - this is an ambiguous statement: from whose perspective? ...


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Newtonian mechanics, with no upper limit on velocities, is perfectly consistent and has no problem talking about time.


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Why isn't time just a function of the speed of light being finite In a way it is. If the speed of light was infinite, everything would happen at once. And it doesn't. But more generally I think it's better to say time is a function of motion. The mechanism of a clock is called a movement. A clock doesn't literally measure the flow of time like it's some ...


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Time is a physical quantity which cannot be defined, just like mass and length. They happen to be the fundamentals of our knowledge regarding understanding of nature. Nobody in this world can define time. Moreover, c ,that is, the speed of light in vacuum is used to define the unit of time second. This is because speed of light in vacuum has been observed ...


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When you are given acceleration, density, area and time, you can indeed find an expression for mass in terms of these. Here is how you go about it: Make a table of the units that occur in each, and their exponents: L M T a 1 -2 D -3 1 A 2 t 1 As you can see, you need to use D (density) as the only one that contains mass. But ...


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regarding the first half: Density is Mass/Volume (not the other way around), so you need a Volume.. You could achieve that by a sufficient power of the area or find yourself a length-scale from acceleration and time regarding the second half: that's the whole point of dimensional analysis, a mass-like quantity is a mass-like quantity and must come from ...


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It should be clarified that the Higgs boson does not carry mass. The correct statement is that the Higgs field (not boson) is giving mass to some (not all) particles. In fact most of your mass is not given by the Higgs field. Most of the mass of atomic nucleus (protons and neutrons) is due to the binding energy of strong interaction. The Higgs field is ...


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To answer question 2: If it fires a photon orthogonal to its heading, it will travel orthogonal to the source's heading from the POV of the source. From the POV of an outside observer, it will travel not orthogonal to the motion of the source, but slightly along its direction of motion. This can be easily intuited because in its own frame, the source is at ...


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Do photons experience every moment in time and position in space simultaneously? No. LIke WillO said, photons don't experience anything. Would it be more correct to say that a photon, traveling at the speed of light, would experience all points in time simultaneously, and therefore be everywhere at once? No it wouldn't. A photon is emitted from ...


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A photon traveling at speed of light has a lightlike worldline. It has one place of emission and one place of absorption. The spacetime interval between both points is empty (=0), that means that no spacetime is between them. That means, if a photon would experience something, it would experience both points as simultaneous. But there is no reference frame ...


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If you don't want to violate causality then you can only have a few types of faster than light travel. One is to not allow it to slow down. Another is to only allow it to go in one direction and hope the universe is infinite. Otherwise it is rather trivial to use regular slower than light motion between some FTL trips to generate a time machine. As for ...


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Quantum mechanics is not silent about time, instead it specifically says exactly how states evolve in time. There are some interactions that change things to a fixed set of final states. So in general they do change an arbitrary state. But some of those interactions have a consistency that if something was already in one those fixed states that can be ...


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Let $\mathcal{H}$ be the space of states of our theory. Then, time evolution is given by a unitary operator $U(t_2,t_1) : \mathcal{H}\to\mathcal{H}$ that evolves "stuff" from time $t_1$ to time $t_2$. For time-independent Hamiltonians it is just $\mathrm{e}^{\mathrm{i}H(t_2 - t_1)}$. If we are in the Schrödinger picture, we say states "carry the time ...


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To address the question of whether the age estimates are in years on Earth or years for a comoving observer in deep space, I could tell you the same thing John Rennie said. But that would make this redundant, unproductive, and redundant. Instead, let me show you that it doesn't really matter. The equation for time dilation due to gravity is as follows: ...


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The geometry of the expanding universe is (approximately) described by the FLRW metric, and to express this there is a natural space/time split called comoving coordinates. The comoving time coordinate is roughly speaking the time measured by an observer who is at rest with respect to the cosmic microwave background. Since most galaxies have peculiar ...


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First, let us suppose you have the energy to travel at any speed you like. The only restriction you have is that you cannot travel at the speed of light, because you would need infinite energy, and even though you have infinite energy, you cannot apply infinite energy if you don't have infinite time to apply it. So you accelerate in a ship let us say at ...


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If you're asking whether there can be more than one time dimension, that's hard to answer. It's unclear how to reformulate physics to accommodate more than one time dimension in any field, except for maybe special relativity. In special relativity, time is "just like" a space dimension, except it has a minus sign in the metric. As a result, you can easily ...


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It's quite common to use time to parameterise an equation. For example suppose you have a particle moving in a circle (or radius $r$). One way of describing its motion would be to say the trajectory describing its motion is: $$ x^2 + y^2 = r^2 \tag{1} $$ but an alternative description would be to use the pair of equations: $$\begin{align} x &= ...


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Quantum mechanics has an absolute time concept, and general relativity has the dynamic time concept of spacetime. The incompatibility of both concepts is called the problem of time in quantum gravity. Yes, but that Wikipedia article needs attention from an expert. It says "therefore, we arrive at the conclusion that 'nothing moves' ('there is no time') ...


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A physical quantity is introduced by its operational definition. Yes. Excellent. And physical quantities would include facts like whether an observer receives one signal between receiving two other signals or sees one mark between two other marks (clocks and rulers are designed on these principles) In general relativity we use a differential ...


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There isn't a simple answer to your question because, well, it depends. In the lab the only time I can think of that this issue arises is with gravitational time dilation e.g. experiments with atomic clocks or GPS satellites. In that case I would guess we'd use EM signals (light or radio) and correct for the known travel time. In astronomical contexts the ...


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You may be speaking of the subjective experience of time, which is not a physics question, but rather a psychological one. Otherwise, the last paragraph of your question doesn't seem to make any physical sense. Time is an arbitrary standard by which a motion or a process is measured relative to other motions and processes. For example, you can synchronize ...


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The truth of the Lorentz transformation as an accurate description of the co-ordinate transformation between relatively uniformly moving observers needfully implies relativity of simultaneity. Contrapositively, the Lorentz transformation cannot be sound if simulteneity is not relative. So, in the sense that the soundness of the Lorentz transformation has a ...


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Comments to the question (v2): In (non-relativistic) quantum mechanics, time is a parameter (as opposed to a selfadjoint operator), cf. e.g. this Phys.SE post and links therein. In the phase space path integral $$ K(q_f,t_f;q_i,t_i) ~\equiv~\langle q_f,t_f \mid q_i,t_i\rangle ~=~\int_{q(t_i)=q_i}^{q(t_f)=q_f} \!{\cal D}q~ {\cal D}p~ ...


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It's important to state exactly what one means by "aether" when saying that aether theories are discredited. Specifically, the notion of medium that one can in principle detect one's motion relative to is what has been ruled out by experiment. Mediums such as water for acoustic waves fall into this category: the acoustic wave equation changes its form when ...


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From Wikipedia: James Clerk Maxwell began working on Faraday's lines of force. In his 1861 paper On Physical Lines of Force he modelled these magnetic lines of force using a sea of molecular vortices that he considered to be partly made of aether and partly made of ordinary matter - light and monopoles. He derived expressions for the dielectric constant and ...


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Einstein didn't actually get rid of the aether. He said the luminiferous aether was redundant when he was doing special relativity in 1905. But later when he was doing general relativity, he described space as an aether. See his 1920 Leyden Address. He said this: "Recapitulating, we may say that according to the general theory of relativity space is endowed ...



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