# What is Time: Definition and 998 Discussions

Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.Time has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars.
Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.Time in physics is operationally defined as "what a clock reads".The physical nature of time is addressed by general relativity with respect to events in space-time. Examples of events are the collision of two particles, the explosion of a supernova, or the arrival of a rocket ship. Every event can be assigned four numbers representing its time and position (the event's coordinates). However, the numerical values are different for different observers. In general relativity, the question of what time it is now only has meaning relative to a particular observer. Distance and time are intimately related and the time required for light to travel a specific distance is the same for all observers, as first publicly demonstrated by Michelson and Morley. General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. At this time, there is no generally accepted theory of quantum general relativity. Time is one of the seven fundamental physical quantities in both the International System of Units (SI) and International System of Quantities. The SI base unit of time is the second. Time is used to define other quantities – such as velocity – so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. To describe observations of an event, a location (position in space) and time are typically noted.
The operational definition of time does not address what the fundamental nature of it is. It does not address why events can happen forward and backward in space, whereas events only happen in the forward progress of time. Investigations into the relationship between space and time led physicists to define the spacetime continuum. General relativity is the primary framework for understanding how spacetime works. Through advances in both theoretical and experimental investigations of space-time, it has been shown that time can be distorted and dilated, particularly at the edges of black holes.
Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms (see below). Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.
There are many systems for determining what time it is, including the Global Positioning System, other satellite systems, Coordinated Universal Time and mean solar time. In general, the numbers obtained from different time systems differ from one another.

View More On Wikipedia.org
1. ### B How do space and time fuse together to form “spacetime?”

Here is the definition of spacetime? “In physics, spacetime is any mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum.” But if space is literally the absence of matter or physical properties, and time has no...
2. ### A Inquiry Regarding Concept in Time and Celestial Motion

I am writing to investigate a unique theoretical perspective that explores the relationship between time and the motion of celestial bodies, particularly focusing on Earth's trajectory through space. I posit that time, particularly in the past, is intricately connected to the precise path that a...
3. ### Will The Cars Stop In Time? - Check My Work Please :)

Model attached below. Changed Sarah's 65 mph to m/s and got 29 m/s. Sarah's initial position is positive. Her initial velocity is negative. Her acceleration is positive; Andrew's initial position is negative. His initial velocity is positive. His acceleration is negative. Graph is attached...
4. ### B Is velocity the reason for the time dilation effect?

Hello! I try to understand how in different frames clocks tick and stop simultaneously but show different time? I suppose that velocity is reason of time dilation effect but I'm not sure. Thanks.
5. ### B How to measure time in reference frame with clock?

I considered example of time dilation with light clock. I have a question about measuring time in reference frame with clock. If we know that clock move from A to B in the reference frame with clock then what time of motion is measured in this reference frame? (In non-moving reference frame...
6. ### Verifying the acceleration of gravity in our lab (help with error please)

Data and graphs here (the time is measured every 1/30 of a second, but for some reason, Google Sheets thought 1/30 wasn't a number so its in decimals!!! very confusing!!! so sorry about that!). why is the slope only 4.68, should it not be 9.81m/s^2? is the slope of m/s^2 not supposed to be...
7. ### I "Quasar clocks" and time running slower long ago

Regarding the new "quasar clock" data showing that time ran more slowly (~5-fold) shortly after the Big Bang: is it all based on the periodic variation in luminosity at various wavelengths of some quasars (and presumably explained by the special relativistic time dilation effect from "spatial...
8. ### Running princess and spider - time problem

If the veil stretches prefectly, then when the princess moves by ##\Delta x##, then if the spider was not moving, he would also move by ##\Delta x## from the door, so he would be in the same distance from the princess. So assuming it, the time would be ##t_f=\frac {l_0}{u}## but that is no...
9. ### B Is Time the Fifth Force?

We all know the four fundamental forces in physics: gravity, electromagnetism, and the strong and weak nuclear forces; and there is now some discussion about a fifth, more obscure force involving muons. But I have a question; could the fifth force be something we deal with every day? Could it...
10. ### Relating acceleration to distance and time

I’m an absolute beginner and I need someone to show me where I’m wrong. Knowing the formula of acceleration ∆v (change in velocity) / ∆t (change in time) where ∆v = ∆x (distance) / ∆t, a common way of relating acceleration to distance is to say a (acceleration) = (distance/time)/time =...
11. ### B Trying to understand what "time" is (layperson)

Hi Everyone, Just a disclaimer, I'm not a physicist or a physics student, just a layperson with an interest in physics. So please forgive any lack of understanding on my part :). Anyway, I recently started trying to gain a better understand of what time is in the real world, because some...
12. ### I Integrity of matter / space and time

I have probably a basic question from Space, Time and Matter area. My 11 years old daughter asked me once why we exist physically in a stable form if everything is infinite. We had a conversation about it but then it got me thinking about this and it seems I can't find the answer. There is...
13. ### I Entropy Real? End of Time? - Veritasium Video

Is entropy real? It seems like it's not real because it depends on how you group microstates together into a macrostate, and the way you group them can be arbitrary. For example (at 13:04 of the video below), there are 91,520 microstates in the macrostate “9 in left; 1 in right” but 627,264...
14. ### I Does the past of an observer still actually exist in their "now"?

This is a tough question to phrase but I haven’t been able to find an answer yet, although I’m sure one exists. I’ll try expand what it is I’m asking. I’m no physicist but I think I have a general understanding of what they are saying when they describe a block space-time universe where the past...
15. ### How much time do I have to catch a coin?

I am a bit confused with velocities in this problem. From Philipp's view, the coin's initial velocity is zero, so its transfer kinetic energy is also zero. When I am standing on a non-moving ground, is the coin's initial velocity ##v## in direction the walkway is moving? But won't I get then...
16. ### Distance as a function of time for two falling stones

I am aware that this question is very simple and basic. Using ##y(t)=y_0+v_{0,y}t-\frac {1}{2}gt^2## we can find distance as a function of time: ##|y_1-y_2|=|y_0+v_{0,y}t|=-y_0- v_{0,y}t## I assumed the downward direction to be negative. So as I wrote ##D(t)=-y_0- v_{0,y}t##. It tells that the...
17. ### B Examples of less popular pairs of physical properties with uncertainty (HUP)

Position and momentum are the popular pairs of properties with uncertainty we often hear about, for example that we cannot know with precision where an electron is and its momentum at the same time. What are others? Such as an example of an energy and a time that we cannot know both...
18. ### Impulse integration for a Tennis Racket hitting a Tennis Ball

For this, Can someone please tell me why they integrate the impulse over from ##t_i## to ##t_f##? Why not from ##j_i## to ##j_f##? It seems strange integrating impulse with respect to time. Many thanks!

49. ### I Time for a cold volume of air to reach a higher air temperature

I have a cube with a volume of 1000m3 at an initial temp of 290K. The bottom side (10m by 10m) is open to the ambient air. I put this cube into a huge fridge and cool the whole volume by 5K. I close the open side by placing a cover on it. This cube has now got a volume of air at a temperature of...
50. ### Should a central heating boiler be kept on all the time or on a timer?

Yet no-one has been able to provide any formulae etc to expand the topic further. A lot is said about house construction (high mass vs low mass) but that doesn't take the discussion any further. I think the key question is if your house is badly insulated then it's losing heat all the time...