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My 1st thought was the Solar Cycle--which is slow, but certainly observable Earth-wide. After reading your comment, I am going with pulsars. They are not just Earth wide, they are solar system (and be …

In quantum mechanics, time is not a dimension, it's a universal parameter (involved in the evolution of states/operators depending upon your so-called picture). … Quantum fields permeate space-time, and an electron is quanta of that field. Since computations are so difficult, we usually think in approximations. …

The laws of physics being time reversal symmetric means the equations are still valid if you reverse time. … So that is what time reversal invariance is all about. …

So $A$'s clock is going to tick off a time of $$t_A=t_0$$ which is the proper time between the two relevant events because $A$ is at rest. …

One way is: "...The unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s$^{−1}$" which measures oscillat …

So the observer on the train ($\gamma=10$) has a clock. He looks out the window and sees a single clock (A). Let's say they are momentarily synchronized, so they both read 12:00. The instantaneous spe …

You need to find a function of time that works. … So the cube of the radius goes like time-squared? Sounds like Kepler's Law. Maybe you could have solved the problem with dimensional analysis. …

that directly depends on that time period. … The lifetime is around 5 ys, or roughly the time it takes light to cross a proton. …

If you are talking about photons from galaxies and other distant sources, they are red shifted relative to the rest frame of their source because we are moving away from the source rapidly. The photon …

Since, based on the equation, you are comparing 2 time differences among 3 events. Let's say the 1st even is the start. … If you just use abstract $\Delta$'s, confusion will result, as you'll forget which hyperplane of simultaneity (defined by equal coordinate time in a reference frame) by which you are comparing. …

It represents the solution running the experiment backwards in time (or at least continuing it to earlier times, with the same kinematics). …

A thousand years on earth would be a single (solar) day if the earthed had a sidereal rotation period of 0.999001 years.

The problem with this question is it imbues special meaning to the light's source. That is important when doing astronomy and spectroscopy, but this is not that. The problem is that it implies there i …

You, referring to yourself, always have a 4-velocity: $$ u_{\mu} = (c, \vec 0) $$ which has: $$ u_0 = c $$ Hence, moving through the time-direction at the speed of light. …

Regarding time, there is no anti-time; however, time-reversal was once thought to be a symmetry of space-time (the Weak interaction broke that). … Velocity is "time-odd", while position is time even ($T:\vec x \rightarrow +\vec x$). …