How do we presently understand time?

Can a second be related directly to physical phenomena?

What is a second really?

Why are we sure that time is linear?

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    $\begingroup$ In re to "What is a second really?": From wikipedia, a second is defined as "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." $\endgroup$ – Jim Jan 7 '15 at 19:50
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    $\begingroup$ that also answers the second question I guess. $\endgroup$ – Jim Jan 7 '15 at 19:51
  • $\begingroup$ About the fourth question (v1 of your post), what do you mean linear? Always moving forward? Or do you mean a linear function of something else? Can it be cubic? $\endgroup$ – Jim Jan 7 '15 at 19:52
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/15371/2451 and links therein. $\endgroup$ – Qmechanic Jan 7 '15 at 19:56
  • $\begingroup$ @jim - yes always moving forward, isnt that the assumption? that things are moving in a particular direction? $\endgroup$ – ruben_KAI Jan 7 '15 at 20:12

We, the human beings, introduced the concept of time, because of irreversible processes going on us - e.g. we grow older. To introduce a fixed unit of time, as we introduce a fix unit of length (the meter) or a fixed unit of mass (Kg), we used those atomic clocks.

Now, besides the biological irreversible processes, there are others, e.g. the decay of nuclei in which we know the exponential law $N(t) = N(t_0) e^{-i(t - t_0)}$.

In short the idea that "time passes" (in one direction) comes from irreversible processes, and first of all was introduced because of our biological process.

  • $\begingroup$ How do we know that there are processes that are truly irreversible? $\endgroup$ – ruben_KAI Jan 8 '15 at 0:23
  • $\begingroup$ Well, did someone get younger? Did someone see emitted articles return to the emitted nucleus? (By the way, bombarding the daughter nucleus with a flux of particles of the emitted type, is not the inverse of the decay). Nobody saw such things. All that we know is from the experience of humankind. Maybe if one would live a life of the order of magnitude of the age of the universe, one would see other things. But, we rely our conclusions on what the humankind experience tells us. $\endgroup$ – Sofia Jan 8 '15 at 1:08

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