<P> An order of magnitude of time is (usually) a decimal prefix or decimal order - of - magnitude quantity together with a base unit of time, like a microsecond or a million years . In some cases, the order of magnitude may be implied (usually 1), like a "second" or "year". In other cases, the quantity name implies the base unit, like "century". In most cases, the base unit is seconds or years . Prefixes are not usually used with a base unit of years, so we say "a million years", not "a megayear". Clock time and calendar time have duodecimal or sexagesimal orders of magnitude rather than decimal, i.e. a year is 12 months, and a minute is 60 seconds . </P> <P> The smallest meaningful increment of time is the Planck time, the time light takes to traverse the Planck distance many decimal orders of magnitude smaller than a second . The largest realized amount of time, given known scientific data, is the age of the universe, about 13.8 billion years - the time since the Big Bang as measured in the cosmic microwave background rest frame . Those amounts of time together span 60 decimal orders of magnitude . Metric prefixes are defined spanning 10 to 10, 48 decimal orders of magnitude which may be used in conjunction with the metric base unit of second . Metric units of time larger than the second are most commonly seen only in a few scientific contexts such as observational astronomy and materials science although this depends on author; for everyday usage and most other scientific contexts the common units of minutes (60 s), hours (3600 s or 3.6 ks), days (86 400 s), weeks, months, and years (of which there are a number of variations) are commonly used . Weeks, months and years are significantly variable units whose length crucially depends on the choice of calendar and is often not regular even with a calendar, e.g. leap years versus regular years in the Gregorian calendar . This makes them problematic for use against a linear and regular time scale such as that defined by the SI since it is not clear as to which version of these units we are to be using . Because of this, in the table below we will not use weeks and months and the year we will use is the Julian year of astronomy, or 365.25 days of 86 400 s exactly, also called an annum and denoted with the symbol a, whose definition is based on the average length of a year of the Julian calendar which had one leap year every and always every 4 years against common years of 365 days each . This unit is used, following the convention of geological science, to form larger units of time by the application of SI prefixes to it at least up to giga - annum, or Ga, equal to 1 000 000 000 a (short scale: one billion years, long scale: one milliard years). </P> <Table> <Tr> <Th> Unit (s) </Th> <Th> Multiple </Th> <Th> Symbol </Th> <Th> Definition </Th> <Th> Comparative examples & common units </Th> </Tr> <Tr> <Td> 10 </Td> <Td> 1 Planck time </Td> <Td> </Td> <Td> Presumed to be the shortest theoretically measurable time interval (but not necessarily the shortest increment of time - see quantum gravity) </Td> <Td> 6956099999999999999 ♠ 10 ys = 6956099999999999999 ♠ 10 s: One Planck time t = ħ G / c 5 (\ displaystyle (\ sqrt (\ hbar G / c ^ (5)))) ≈ 6956538999999999999 ♠ 5.39 × 10 s is the briefest physically meaningful span of time . It is the unit of time in the natural units system known as Planck units . </Td> </Tr> <Tr> <Td> 10 </Td> <Td> 1 yoctosecond </Td> <Td> ys </Td> <Td> Yoctosecond, (yocto - + second), is one septillionth of a second </Td> <Td> 156 ys: mean lifetime for the decay of a Higgs Boson, the quantum of energy in the field which gives elementary particles their masses </Td> </Tr> <Tr> <Td> 10 </Td> <Td> 1 zeptosecond </Td> <Td> zs </Td> <Td> Zeptosecond, (zepto - + second), is one sextillionth of one second </Td> <Td> 2 zs: representative cycle time of gamma ray radiation released in the decay of a radioactive atomic nucleus (here as 2 MeV per emitted photon) </Td> </Tr> <Tr> <Td> 10 </Td> <Td> 1 attosecond </Td> <Td> as </Td> <Td> One quintillionth of one second </Td> <Td> 12 attoseconds: best timing control of laser pulses . </Td> </Tr> <Tr> <Td> 10 </Td> <Td> 1 femtosecond </Td> <Td> fs </Td> <Td> One quadrillionth of one second </Td> <Td> 1 fs: Cycle time for 300 nanometre light; ultraviolet light; light travels 0.3 micrometres (μm). 140 fs: Electrons have localized onto individual bromine atoms 6Å apart after laser dissociation of Br . </Td> </Tr> <Tr> <Td> 10 </Td> <Td> 1 picosecond </Td> <Td> ps </Td> <Td> One trillionth of one second </Td> <Td> 1 ps: mean lifetime of a bottom quark; light travels 0.3 millimeters (mm) 1 ps: lifetime of a transition state 4 ps: Time to execute one machine cycle by an IBM Silicon - Germanium transistor </Td> </Tr> <Tr> <Td> 10 </Td> <Td> 1 nanosecond </Td> <Td> ns </Td> <Td> One billionth of one second </Td> <Td> 1 ns: Time to execute one machine cycle by a 1 GHz microprocessor 1 ns: Light travels 30 centimetres (12 in) </Td> </Tr> <Tr> <Td> 10 </Td> <Td> 1 microsecond </Td> <Td> μs </Td> <Td> One millionth of one second </Td> <Td> 1 μs: Time to execute one machine cycle by an Intel 80186 microprocessor 4--16 μs: Time to execute one machine cycle by a 1960s minicomputer </Td> </Tr> <Tr> <Td> 10 </Td> <Td> 1 millisecond </Td> <Td> ms </Td> <Td> One thousandth of one second </Td> <Td> 1 ms: time for a neuron in human brain to fire one impulse and return to rest 4--8 ms: typical seek time for a computer hard disk </Td> </Tr> <Tr> <Td> 10 </Td> <Td> <P> centisecond </P> </Td> <Td> cs </Td> <Td> One hundredth of one second </Td> <Td> 18--300 ms (= 0.02--0.3 s): Human reflex response to visual stimuli <P> 16.667 ms period of a frame at a frame rate of 60 Hz . 20 ms: cycle time for European 50 Hz AC electricity </P> </Td> </Tr> <Tr> <Td> 10 </Td> <Td> <P> decisecond </P> </Td> <Td> ds </Td> <Td> One tenth of a second </Td> <Td> 100--400 ms (= 0.1--0.4 s): Blink of an eye </Td> </Tr> </Table> <Tr> <Th> Unit (s) </Th> <Th> Multiple </Th> <Th> Symbol </Th> <Th> Definition </Th> <Th> Comparative examples & common units </Th> </Tr>

What measurement of time is smaller than a second
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