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79

"Because that is how the second is defined" is nice - but that immediately leads us to the question "why did Cesium become the standard"? To answer that we have to look at the principle of an atomic clock: you look at the frequency of the hyperfine transition - a splitting of energy levels caused by the magnetic field of the nucleus. For this to work you ...


20

The choice of cesium is due to various factors. It's worth noting that your statement "Modern atomic clocks only use caesium atoms" is simply untrue. At the very least, rubidium and hydrogen clocks are common, and you can get rubidium standards on eBay for well under $200. But the best performance comes from using cesium. In part this is because it was ...


4

As mentioned by WhatRoughBeast, caesium offers several advantage over other microwave standards. Its most important feature is the presence of an atomic transition with a very small linewidth. This allows the energy of this transition to be established very accurately (see the uncertainty principle). However, caesium is not the only atom with a narrow ...


4

Light travels at the speed $c$ this speed is finite and with out using any relativity we can calculate the time it takes for something travelling at this speed to reach us: $\text{time} = \frac{\text{Distance}}{\text{speed}}$ or $ t= \frac{d}{c} = \text{8 minutes}$ in this case. For a person travelling very close to the speed of light with velocity $v$ from ...


4

Because one second is defined as (from the SI brochure): the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom, ${}^{133}\mathrm{Cs}$. Thus, using any other atom is irrelevant (even if calculate some correction time factor).


2

Antimatter increase in entropy over time. We can verify this with a thought experiment. Take ten positrons. Put five in one side of a chamber with a barrier and then the other 5 on the other side of the barrier in the same chamber. The chamber and barrier are also made of antimatter. The positrons repel each other and so each have a certain amount of kinetic ...


2

If the raindrop's vertical velocity is constant as the train is both stationary and moving, the time taken for the raindrop to travel down the window would be: $$t = \frac{1\ \text{m}}{5\ \text{m}/\text{s}} = 0.2\ \text{s}$$ Remember, the time $t$ would not depend on the speed of the train. The exercise also specifically states that the raindrop's vertical ...


2

Is there an absolute pace of time, no. Is your clock , in a region without gravity, (and at "rest" relative to other objects) "ticking" faster than your alarm clock on the Earth's surface, yes. But obviously you physically cannot escape the effect of gravity, no matter how far away the mass-energy sources are, so this will vary from observer to observer, ...


1

There isn't a such thing as "absolute time." Some events – they are called space-like events – can't even be agreed to happen in an "objective order." Only time-like events can be universally agreed to happen in a particular order, but there's no such thing as "universal time." For you, time will always tick per one second by second, and that will apply to ...


1

This is one of those questions that can drive you crazy, since there is a great deal assumed and not stated. Let me try an alternate possibility. Conceivably, the problem wants you to assume that, when moving, the overall speed of the raindrop remains fixed at 5 m/sec, but it travels in a straight line at an angle due to horizontal wind forces, and this may ...


1

If there were an enormous being whose arm span is one light year across, how would that being perceive time? Wouldn't what we perceive as a year be virtually nothing to that being? It would probably have to have decenteralized brains spread throughout its volume here and there. Otherwise, yes, there is a distinct problem that a being with a single large ...


1

If the Universe has such a time interval, we have not observed it yet. Moreover: if the Universe's "refresh time" is less than a Planck time, roughly 10-43 seconds, we have no direct way of detecting it, as any wave which we created which had that frequency would have a Schwarzschild black-hole radius bigger than its own wavelength, making it very hard to ...


1

Sitting at infinity, you will see something more and more red-shifted - but never actually stop radiating as you would expect it from a black hole. The reason being that your coordinates (the asymptotically flat ones) diverge at the radius of the event horizon. Specifically, in your coordinates the metric of a Schwarzschild black hole (which is not entirely ...


1

Time might be relative, aging (time passing) is absolute. Run around, jump into a rocket, speed up and circle a few times around a black hole, and do whatever else you fancy, all observers will agree how much you have aged in the process. Here, for 'aging' you can read 'proper time': the time that has passed according to your wristwatch.


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The thing is, in relativity you cannot have a reference frame "chasing" a photon. You'll get singularities if you try to view the world from a photon's perspective. A photon cannot move like you and you cannot move like a photon. As a photon, travelling along a light-like world line, experiences no proper time it's proper velocity is simply undefined. ...



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