## BIG IDEAS - Time is a fundamental [[Physical quantity]] used to describe [[Rest and Motion|motion]]. - Time is the reference used to describe speed, velocity, and acceleration. - Time (second) is the “reading of a clock.” - The **time interval** is a difference between two time readings. Time is a basic **==[[SI units]]==**, symbolized with a lowercase _t_ and measured in seconds. Seconds is abbreviated an italicized lowercase _s_ or _sec_. We add subscripts to add specificity to the start and stop of the measurement of time. For example, $t_0$ is the initial time. The second (s) is defined by taking the fixed numerical value of the cesium frequency ∆νCs, the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to $s^{-1}$ . ([NIST](https://www.nist.gov/pml/weights-and-measures/si-units-time)) --- In Newtonian mechanics, time and time interval are ==**[[Scalars|scalar]]**== quantities. They solely have magnitude (a number and a unit) and not direction. However, in modern physics, time is a coordinate. - Example: 5 seconds is written 5 _s_. --- Typically physicists agree that time is a continuum; that is, time is infinitely divisible. For this course, time is ==**absolute time**==, also called ==**Newtonian time**==. In general, time “progresses at a consistent pace for everyone everywhere throughout the universe, and is essentially imperceptible and mathematical in nature.” Newtonian time is different from **==relativistic time==**, which is used to explain the modern concepts of time travel, time dilation, and special relativity. Time is also considered to have an origin, starting at the moment of the expansion of the Universe (Big Bang Theory). This is called an “_arrow of time_.” What experimental evidence exists to support the idea that time is not constant? --- Return [[Home|Home]] | [[SI units]] | [[Base Units]] | [[Mass]] | [[Length]]