# How do we presently understand time? [duplicate]

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?

• 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." – Jim Jan 7 '15 at 19:50
• that also answers the second question I guess. – Jim Jan 7 '15 at 19:51
• 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? – Jim Jan 7 '15 at 19:52
• Possible duplicates: physics.stackexchange.com/q/15371/2451 and links therein. – Qmechanic Jan 7 '15 at 19:56
• @jim - yes always moving forward, isnt that the assumption? that things are moving in a particular direction? – ruben_KAI Jan 7 '15 at 20:12

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