Tag Info

Hot answers tagged

72

"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

To know what a closed timelike curve looks like, you just do like every spacetime metric. You compute geodesics and field equations and all of that. Unfortunately, things start getting complicated. Closed timelike curves have a lot of weird behaviours, especially when it comes to matter fields upon them. They may not have a properly defined Cauchy problem, ...


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).


3

Proper time of an observer is time as measured by the observer's own clocks. So it's obviously frame-independent because calculating proper time of a given observer requires to use his own frame of reference.


3

I once read of a science fiction scenario where on a planet the distance from a specific center was time. Life in that format progressed in height from the center, grew contours of certain height and became flat at death. The consciousness of those entities had time defined by their changes but humans just saw a completed contoured landscape unchanging in ...


2

Contrary to popular misconception, below a specific temperature, glasses do not flow. At all. A glass by definition is a solid sans repeating crystalline structure. Anything which flows (see "pitch-drop experiment which drops every 80 (or something) years") is a liquid, however viscous. Liquid glasses tend to have reasonably high viscosity, but once ...


2

You can easily get that answer by noticing that in when you have constant acceleration the average velocity after a time $t$ is: \begin{equation} \langle v\rangle_{acc.=w}=\frac{v_{start}+(v_{start}+w t)}{2} \end{equation} for your case $v_{start}=0$ and then $\langle v\rangle_{acc.=w}=\frac{wt}{2}$. For the deceleration we get the same result. Now you ...


2

I think you are mixing up two different concepts, which is muddying the waters. Firstly, relativity (both special and general) is a geometrical theory and the proper time for an observer has a precise definition as the length of a world line along which the observer travels (give or take a factor of $c$). This length is calculated using the metric. As ...


2

You are mixing up time with the flow of time. Search this site for more on this topic. Although we're all used to the fact that for us humans time flows, the flow of time does not exist in relativity. Time is just a coordinate like the spatial coordinates $x$, $y$ and $z$. We identify points in spacetime by the four coordinates $(t, x, y, z)$, so time is ...


2

You ask: If the clock is running slowly compared to a distant clock is this equivalent to the clock having a lower energy compared to a distant clock? but you have to very careful what you mean by energy in general relativity. As it stands your question too vague to be usefully answered. However in the weak field limit there is a sense in which time ...


2

Is it possible for any two thing to occur at the exact same time This is a physics question and answer site. In physics our examination of nature has shown that there exist many frameworks for defining "events" , as in your title, or "things" as in your question. The main frameworks where "simultaneity" and "event" have to be defined so as to make the ...


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, ...


2

As the equations of motion are of second order, the higher derivatives give no new information (but follow uniquely from the initial conditions of position and velocity), therefore they usually are not discussed. (Note: As Timaeus pointed out there are specific scenarios, e.g. Norton's dome where intial values for the higher order derivates will change the ...


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

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 ...


1

Is it possible for two events happen at the exact same time? No. Even at any one event itself there can be several (or in though-experimental principle even arbitrarily many) distinct participants (encountering and passing each other, momentarily). All their individual distinct times (indications) are attributable to this one event of their meeting. ...


1

Your problem is that $E=mc^2$ is false here. That's only rest energy: the complete formula would be $E=mc^2 + \frac12 mv^2 + mgh$ (in the non relativistic limit).


1

Actually we can't reach to a thing that is more speeder than light. Because as we read in the Einstein's theory, the speed of light is constant. When you reach the speed near the speed of light, the time will be stretched, and more you get close to the speed of light, more the time will be stretched and YOU WILL NEVER GET TO THE SPEED OF LIGHT. So, due to ...


1

A vector is a scalar with direction. So Time can be a vector, but what it means depends on the context. In 1D it has only 2 directions, positive and negative with zero being positive. In 2D it can be an angle between ÷/-Pi radians. And so on. Time can be a single dimension attached to the familiar 3 Euclidian spacial dimensions and in this case it is ...


1

John, have a look at the simple inference of time dilation due to relative velocity. If you and I are identical twins, and you take a fast out and back trip, when you come back we agree that you've experienced less time than me. As you pointed out, we can relate this to the Lorentz factor and write: $$\Delta t' = \frac{\Delta t}{\sqrt{1-\frac{v^2}{c^2}}}$$ ...


1

If time was reversed we would remember only things we hadn't done yet and nothing that had happened. The laws of physics work equally well forwards or backwards, yet our everyday experiences of cause preceding effect, not the other way around makes this seem counter-intuitive. Just imagine some kids playing cricket in a universe where time is reversed. In a ...



Only top voted, non community-wiki answers of a minimum length are eligible