Unit of Time - SI Unit, CGS Unit & Other Time Units, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:59 PM IST

We all have experienced time and have seen its amazing and phenomenon results.
So, in this article we will study and will try to find out what is time, what is unit, what is unit of time, what is the SI unit of time, what is basic unit of time, how much one second is equal to and what is standard unit of time.

Unit of Time - SI Unit, CGS Unit & Other Time Units, FAQs

time

First let’s discuss about,

What is time? Definition of time in simple words.

As soon as we hear the word, we think of something which is continuous in nature and makes the life easier by categorizing it in seconds, minutes, hours, days, weeks, months and years. Hence, order of life is being categorized from past events to present and then to the future events.

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What is unit?

There are many types of units in physics, because of a large number of independent quantities existing for measuring different physical or non - physical quantities. Length, mass and time three basic quantities which are unique and all time necessary. Therefore, they have three separate units for their system of measurement hence it is required to define system of units a system of unit is a collection of unit in which certain units are chosen as fundamental and all others are derived from them this system is called absolute system of unit in most system mass length and time are connected considered to be fundamental quantities and their units are called fundamental units the following are some system of units which are used commonly

1 . cgs system of units the unit of length is centimeter ( cm ), the unit of mass is gram ( g ) and the unit of time is second (s) .

2 . mks system of units the unit of length is meter (m) , the unit of mass is kilogram (kg) , the unit of time here also is second (s) .

3 . fps system of units here unit of length is measured in foot (ft) , the unit of mass is calculated in pound (lb) and the unit of time is seconds (s) . but this system of unit is no more in use widely used system of unit is metric system of units .

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Unit of time

There are various units of time.

As, we have already discussed about that Second, minute, hour, day, week, month and year are the terms or we can say the basic unit of time that we consider. These are time measurement units.

All these units of time can be in a relation as under:

60 seconds is equal to 1 minute and 1 minute is equal to 60 seconds .

Also,
60 minutes is equal to 1 hour and 1 hour is equal to 60 minutes .

24 hours is equal to 1 day (day and night) , 7 days is equal to 1 week

12 months is equal to 1 year , 52 weeks is equal to 1 year .

365 days or 366 days makes a year and 10 years make a decade .

50 years make half century and 100 years make a century .

These are some general units of time that we use in our daily life for making our life sort.

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What is SI (standard unit of time) unit of time?

The basic or SI (standard unit of time) unit of time is second (SI units of other physical quantities are: metre for length, kilogram for mass, ampere for current, kelvin for calculating temperature, candela for measuring luminous intensity, and mole is considered for the amount of substance). The second can be shown as a symbol by s or sec.

As soon as this system came into existence the SI system, a second is physically in terms of science, we define time in physics in more accurate and absolute atomic concept as “the time of 9,192,631,770 periods of the radiation be consistent with the transitions between 2 levels of the ground (zero) state of the caesium 133 atom”, this definition was made even more exact with the stipulation that this means to a caesium atom at rest 0° Kelvin temperature. This is known as 1 second of time.

Some basic SI units of time chart are

For less than a second,

Deci second is equal to 1/10 second cent second is equal to 1/100 second millisecond is equal to 1/1,000 second microsecond is equal to 1/1,000,000 second

nanosecond is equal to 10-9 second

picosecond is equal to 10-12 second

femtosecond is equal to 10-15 second

attosecond is equal to 10-18 second

zeptosecond is equal to 10-21 second

yoctosecond is equal to 10-24 second

For more than a second

Deca second is equal to 10 seconds hector second is equal to 100 seconds kilo second is equal to 1,000 seconds (about 16.7 minutes) mega second = 1,000,000 seconds (about 11.6 days)

Giga second = 109 seconds (about 31.7 years)

Tera second = 1012 seconds (about 31,700 years)

Peta second = 115 seconds (about 31.7 million years)

Exa second = 1018 seconds (about 31.7 billion years)

Zetta second = 1021 seconds (about 31.7 trillion years)

Yotta second = 1024 seconds (about 31.7 quadrillion years)

Now, in this article we have understood the concept of what is time, what is unit, what is unit of time, what is the SI unit of time, what is basic unit of time, how much one second is equal to and what is standard unit of time.

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NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. What is the largest unit of time?

Largest unit of time is eon. Earth’s 4.6 billion year history is divided into four eons three out of the four eons are grouped together to form the pre Cambrian time.

2. What is SI unit system?

International system of units is the best and today’s form of a metric system it is the only system of measurement with considered status in every country around the globe, there are 7 basic SI units: length, time, amount of substance, electric current, temperature luminous intensity and mass.

3. What is the smallest unit of time?

Zeptoseconds is the smallest unit of time. While shake is also considered as smallest unit of time but not in official status.

4. Calculate 7 days in seconds and three days in seconds.

7 days in seconds are 604,800 and 3 days in seconds are 259200.

5. Five standard unit for measuring length.

Five standard unit for measuring length are inches, feets, yarns, and mile in US system. millimetre, centimetres, metres, kilometres in metric unit system.

6. What is the SI unit of time?

The SI unit of time is the second (s). It is one of the seven base units in the International System of Units (SI) and is used as the fundamental unit for measuring time in scientific and everyday contexts.

7. How is the second defined in modern physics?

The modern definition of a second is based on atomic physics. It is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom at rest at a temperature of 0 K.

8. What is the CGS unit of time?

The CGS (centimeter-gram-second) unit of time is also the second (s). Although the CGS system uses different units for length and mass compared to SI, it shares the same unit for time.

9. Why do we need different time units besides seconds?

Different time units are needed to conveniently express various durations. Seconds are suitable for short intervals, while minutes, hours, days, and years are more practical for longer periods. This allows for easier communication and understanding of time in different contexts.

10. How does a leap second affect our timekeeping?

A leap second is occasionally added to Coordinated Universal Time (UTC) to account for the slowing of Earth's rotation. It ensures that atomic time (extremely precise) stays synchronized with solar time (based on Earth's rotation). This adjustment is crucial for maintaining accurate global timekeeping systems.

11. How does time dilation in special relativity challenge our understanding of time as absolute?

Time dilation in special relativity shows that time is not absolute but relative to the observer's frame of reference. It predicts that time passes more slowly for objects moving at high speeds relative to an observer, contradicting our everyday experience of time as a constant, universal quantity.

12. What is the concept of "now" in physics, and why is it problematic?

The concept of "now" or simultaneity is problematic in physics, especially in special relativity. Events that appear simultaneous to one observer may not be simultaneous to another observer in a different reference frame. This challenges our intuitive understanding of a universal "present moment" across the universe.

13. What is Planck time and why is it significant?

Planck time is the smallest measurable unit of time, approximately 5.39 × 10^-44 seconds. It's significant because it represents the time scale at which quantum effects of gravity become relevant. Below this scale, our current understanding of physics breaks down, and a theory of quantum gravity would be needed.

14. What is the difference between proper time and coordinate time in relativity?

Proper time is the time measured by a clock carried by an observer in their own reference frame. Coordinate time is the time measured in a specific coordinate system, often by a distant observer. In relativity, proper time can differ from coordinate time due to effects like time dilation.

15. How does gravity affect the passage of time according to general relativity?

According to general relativity, gravity causes time dilation. Time passes more slowly in stronger gravitational fields. This means that clocks on Earth's surface tick slightly slower than clocks at higher altitudes where the gravitational field is weaker. This effect, though small on Earth, becomes significant near massive objects like black holes.

16. How do physicists reconcile the apparent irreversibility of time in thermodynamics with the time-reversal symmetry in fundamental physics equations?

This apparent contradiction, known as Loschmidt's paradox, arises because fundamental physics equations are time-symmetric, while macroscopic phenomena show a clear time direction. The reconciliation involves understanding that macroscopic irreversibility emerges from the statistical behavior of large numbers of particles, even though individual particle interactions are reversible.

17. What is the "twin paradox" in special relativity, and how is it resolved?

The twin paradox is a thought experiment where one twin travels at high speed and returns to find the stay-at-home twin has aged more. It seems paradoxical because each twin could view the other as moving. The resolution lies in understanding that the traveling twin experiences acceleration and changes in reference frame, breaking the symmetry and causing real time dilation.

18. What is the significance of the Planck epoch in cosmology?

The Planck epoch refers to the earliest period of the universe, from the instant of the Big Bang to about 10^-43 seconds later. It's significant because it represents the time when quantum effects of gravity were dominant. Our current physics theories break down at this scale, making it a frontier for theoretical physics and our understanding of the universe's origins.

19. How does time dilation affect GPS satellites?

GPS satellites experience time dilation due to both their high speed (special relativity) and their position in a weaker gravitational field (general relativity). The combined effect causes the atomic clocks on GPS satellites to tick faster than ground-based clocks by about 38 microseconds per day. This difference must be accounted for to maintain the accuracy of GPS systems.

20. How does time reversal symmetry work in physics equations?

Time reversal symmetry refers to the invariance of physical laws under the reversal of the time coordinate (t → -t). Many fundamental equations in physics, such as Newton's laws of motion, exhibit this symmetry. However, certain processes, particularly those involving entropy increase, do not appear time-reversible, leading to the arrow of time in macroscopic systems.

21. How does time dilation affect particle decay in high-energy physics?

Time dilation affects particle decay rates in high-energy physics experiments. Particles moving at very high speeds experience time more slowly in the laboratory frame, leading to longer observed lifetimes. This effect allows short-lived particles produced in accelerators or cosmic rays to travel much further than they would at rest, enabling their detection and study.

22. What is the significance of the Hubble time in cosmology?

The Hubble time is an estimate of the age of the universe based on the current expansion rate. It's the inverse of the Hubble constant and represents the time it would take for the universe to expand to its current size if the expansion rate had been constant. While not the actual age of the universe (due to changing expansion rates), it provides a useful scale for cosmic time.

23. What is the relationship between frequency and time period?

Frequency and time period are inversely related. Frequency (f) is the number of occurrences of a repeating event per unit time, while the time period (T) is the duration of one cycle. Their relationship is expressed as: f = 1/T or T = 1/f.

24. How do atomic clocks work, and why are they so precise?

Atomic clocks work by measuring the electromagnetic waves emitted by electrons in atoms as they change energy levels. They use cesium atoms and microwave radiation to measure time with extreme precision. Their accuracy comes from the consistency of atomic transitions, which are far more stable than mechanical oscillations used in traditional clocks.

25. How do we measure extremely short time intervals in physics experiments?

Extremely short time intervals are measured using techniques like pump-probe spectroscopy, streak cameras, or attosecond lasers. These methods can measure events occurring on femtosecond (10^-15 s) or even attosecond (10^-18 s) timescales, allowing scientists to study ultrafast phenomena in atoms and molecules.

26. What is the relationship between time and entropy?

Time and entropy are closely related through the second law of thermodynamics. As time progresses, the entropy (disorder) of an isolated system tends to increase. This gives time a direction, known as the "arrow of time," explaining why we remember the past but not the future and why certain processes are irreversible.

27. How does time factor into the uncertainty principle?

The time-energy uncertainty principle states that the uncertainty in energy (ΔE) multiplied by the uncertainty in time (Δt) must be greater than or equal to ħ/2 (where ħ is the reduced Planck constant). This means that very precise energy measurements require longer observation times, and very short time intervals lead to greater uncertainty in energy.

28. What is the difference between UTC and GMT?

UTC (Coordinated Universal Time) and GMT (Greenwich Mean Time) are very similar but not identical. GMT is based on the Earth's rotation and the position of the sun, while UTC is based on atomic clocks. UTC includes leap seconds to stay synchronized with the Earth's slowing rotation, making it more precise for scientific and technological applications.

29. How does the concept of time differ between classical and quantum mechanics?

In classical mechanics, time is a continuous parameter that flows uniformly. In quantum mechanics, time is treated differently. The time evolution of quantum states is described by the Schrödinger equation, but time itself is not an observable quantity. This leads to challenges in defining time on very small scales and in understanding the nature of time in quantum systems.

30. What is the role of time in wave-particle duality?

Time plays a crucial role in wave-particle duality. The time-dependent Schrödinger equation describes how quantum waves evolve over time. The wave nature of particles becomes evident in phenomena like interference patterns, where the time of arrival of particles at a detector is crucial. The particle nature is observed in instantaneous events like detection or measurement.

31. How does the concept of time apply to the early universe in cosmology?

In cosmology, the concept of time becomes complex when considering the early universe. Near the Big Bang, extreme conditions make our usual understanding of time break down. Some theories suggest that time itself may have emerged as the universe expanded and cooled, challenging the idea of time existing "before" the Big Bang.

32. What is meant by the "arrow of time" in physics?

The "arrow of time" refers to the observed asymmetry of time in many physical processes. It's most evident in the second law of thermodynamics, where entropy increases over time in closed systems. This directionality of time explains why we can remember the past but not the future, and why many processes (like mixing or breaking) are irreversible.

33. How do time zones relate to the rotation of the Earth?

Time zones are based on the Earth's rotation. The Earth rotates 360° in about 24 hours, or 15° per hour. Each time zone ideally covers 15° of longitude, with local time increasing by one hour for each zone east of the Prime Meridian (0° longitude) and decreasing by one hour for each zone to the west.

34. What is the relationship between time and causality in physics?

Causality in physics requires that causes precede their effects in time. This principle is fundamental to our understanding of physical laws and is preserved in special relativity through the concept of light cones. Events outside an observer's past light cone cannot influence their present, maintaining causality and preventing paradoxes like time travel to the past.

35. How does the concept of time apply to black holes?

Time behaves strangely near black holes due to extreme gravitational effects. As an object approaches the event horizon, time appears to slow down from an outside observer's perspective, eventually appearing to stop at the horizon. For an object falling into a black hole, time would seem to pass normally, but it would be unable to communicate with the outside universe.

36. What is the difference between proper time and improper time in physics?

Proper time is the time measured by a clock moving with an object or observer, experiencing the same accelerations and gravitational fields. Improper time, or coordinate time, is the time measured in a specific reference frame, often by a distant observer. The difference between these becomes significant in relativistic scenarios, leading to phenomena like time dilation.

37. How do we measure the age of the universe?

The age of the universe is primarily determined by measuring the cosmic microwave background radiation and the rate of expansion of the universe. By analyzing the properties of this radiation and using models of cosmic evolution, scientists can trace back to the Big Bang. Current estimates put the age of the universe at about 13.8 billion years.

38. What is the role of time in quantum entanglement?

Time plays a complex role in quantum entanglement. Entangled particles show correlated properties instantaneously, regardless of their separation in space, which seems to challenge our understanding of causality and time. This "spooky action at a distance," as Einstein called it, doesn't allow for faster-than-light communication but raises questions about the nature of time and information in quantum systems.

39. How does time feature in Feynman diagrams?

In Feynman diagrams, which are used to visualize particle interactions in quantum field theory, time is typically represented as progressing upward on the vertical axis. These diagrams allow physicists to represent complex quantum processes, including the creation and annihilation of particles, in a time-ordered manner, helping to calculate probabilities of different outcomes.

40. What is the concept of imaginary time in theoretical physics?

Imaginary time is a mathematical concept used in quantum mechanics and cosmology. It involves treating time as an imaginary number (multiplied by i, the square root of -1). This approach can simplify certain calculations and has been used in theories about the early universe, potentially removing the singularity at the Big Bang.

41. How does time factor into the measurement problem in quantum mechanics?

The measurement problem in quantum mechanics involves the apparent collapse of a quantum state upon measurement. Time plays a crucial role here, as the act of measurement is considered an instantaneous event that causes the wavefunction to collapse. This raises questions about the nature of time in quantum processes and the role of the observer in quantum mechanics.

42. What is the significance of the Chronon in theoretical physics?

The Chronon is a hypothetical unit of time proposed as the smallest possible measurable interval. While not widely accepted, the concept explores the idea that time might be quantized at extremely small scales, similar to how energy is quantized in quantum mechanics. This could have implications for our understanding of time at the most fundamental level.

43. What is the relationship between time and the speed of light?

The speed of light (c) is a fundamental constant in physics and plays a crucial role in our understanding of time. According to special relativity, c is the maximum speed at which information or matter can travel. It sets a limit on causality and defines the structure of spacetime. The constancy of c leads to effects like time dilation and length contraction at high speeds.

44. What is the concept of proper time in general relativity?

Proper time in general relativity is the time experienced by an object along its worldline (path through spacetime). It's the time measured by a clock carried by the object itself. In curved spacetime, proper time can differ significantly from coordinate time measured by distant observers, leading to effects like gravitational time dilation near massive objects.

45. How does the concept of time apply to virtual particles in quantum field theory?

In quantum field theory, virtual particles can briefly exist, seemingly violating energy conservation. The time-energy uncertainty principle allows for these short-lived fluctuations. The "lifetime" of virtual particles is extremely short, related inversely to their energy. This concept challenges our classical notion of time and particles in the quantum realm.

46. How does time feature in the formulation of Lagrangian mechanics?

In Lagrangian mechanics, time is a fundamental parameter. The Lagrangian (L) is typically expressed as the difference between kinetic and potential energy, both of which can be time-dependent. The principle of least action, central to Lagrangian mechanics, involves finding the path that minimizes the action integral over time, connecting initial and final configurations of a system.

47. What is the concept of proper distance in relation to time in cosmology?

Proper distance in cosmology is the distance between two points measured at a specific cosmic time. Unlike comoving distance, which remains constant as the universe expands, proper distance changes with time due to cosmic expansion. This concept is crucial for understanding how distances in the universe evolve and how we observe distant objects.

48. How does time factor into the concept of half-life in radioactive decay?

Half-life is the time required for half of a given quantity of a radioactive isotope to decay. It's a probabilistic concept, representing the average time for half the atoms to decay. The exponential nature of radioactive decay means that the half-life is constant, regardless of the amount of material, making it a useful measure of time in fields like archaeology and nuclear physics.

49. What is the role of time in the formulation of Maxwell's equations?

Time plays a crucial role in Maxwell's equations, which describe the behavior of electric and magnetic fields. These equations include time derivatives, showing how changing electric fields create magnetic fields and vice versa. This time-dependence led to the prediction of electromagnetic waves, traveling at the speed of light, fundamentally linking time, space, and electromagnetism.

50. How does the concept of time apply to the evolution of quantum states?

The evolution of quantum states over time is described by the time-dependent Schrödinger equation. This equation shows how the wavefunction of a quantum system changes with time, allowing predictions about future states. In quantum mechanics, time is treated as a parameter, not an observable, leading to interesting consequences for our understanding of time at the quantum level.

51. What is the significance of the Planck time in quantum gravity theories?

Planck time (approximately 10^-43 seconds) is theoretically the smallest meaningful unit of time. At this scale, quantum effects of gravity become