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.

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  1. Wiredcerebellum

    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. R

    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. V

    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. M

    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. M

    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. feynmansorange

    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. H

    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. Lotto

    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. Jim Robison

    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. golya

    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. Josh S

    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. T

    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. H

    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. oxbaker

    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. Lotto

    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. M

    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. S

    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. C

    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!
  19. George Wu

    A Do Time-ordering and Time Integrals commute? Peskin(4.22)(4.31)(4.44)

    In Peskin P85: It says the Time-ordered exponential is just a notation,in my understanding, it means $$\begin{aligned} &T\left\{ \exp \left[ -i\int_{t_0}^t{d}t^{\prime}H_I\left( t^{\prime} \right) \right] \right\}\\ &\ne T\left\{ 1+(-i)\int_{t_0}^t{d}t_1H_I\left( t_1 \right)...
  20. L

    Discharging a capacitor -- Calculate the current as a function of time

    Hi, I am not sure if I have calculated the task b correctly. I always interpret an open switch as an infinitely large resistor, which is why no current is flowing through this "resistor". So there is no current in the red circle, as it was the case in task part a, but only in the blue circle...
  21. W

    B Seeing both B field lines and E field lines at the same time

    After watching this clip Electric Field Lines Lab I wonder if it is possible to see both electric field lines and magnetic field lines at the same time by swapping the two nails in the video with two bar magnets, as the conductors as we understand bar magnets are metals and metals are good...
  22. M

    Electron in a time variable magnetic field

    TL;DR Summary: Find acceleration of electron in dB/dt >0 Hello. Here is a problem that i'm not so sure about: Inside a solenoid there is a time-dipendent magnetic field B, so we have dB/dt = b (constant). We want to know the acceleration of an electron: a) placed in the center of the solenoid...
  23. jedishrfu

    B Time Travel: Is it Physicsly Possible? | Scientific American

  24. E

    I Sabine Hossenfelder says time dilation is due to acceleration

    Sabine Hossenfelder says time dilation is due to acceleration in the twin's paradox. Is this true? At 12 minutes into this video , Hossenfelder states, "This is the real time dilation. It comes from acceleration." Looking at the equations for time dilation, time dilation comes from...
  25. G

    B Time Dilation: Is This a Reasonable Explanation?

    I thought of this description recently and I think it's pretty intuitive, but I've gotten some side eye telling it to friends and family (maybe because relativity is screwy, maybe because I'm confused, maybe both?) so I want to get some confirmation that it's reasonable. Here goes: If you're in...
  26. ejacques

    Calculating Train Deceleration: Kid's Free Fall Time

    I think, since the train is decelerate the kid will fall off at free falling. the time for that is: y=0.5*9.8*t2 ⇒ t=√(2*y/g)=√(2*3/9.8)=0.8sec. Now i think i need to find what is the distance the train is doing in this time, but i can't figure this out.
  27. A

    Programs My Story: Physics Undergraduate Struggling to Graduate on Time

    I'm a physics undergraduate student. Im gonna try to tell you the short version of my story. I used to be in a different science field, but always wanted to pursue physics. The reason I didn't do so at the start is because my parents were opposed to the idea (i was 17 and didn't argue much at...
  28. E

    Ensemble vs. time averages and Ashcroft and Mermin Problem 1.1

    The question is as seen below: My attempt (note that my questions are in bold below) is below. Please note that I am self-studying AM: (a) By the independence of any interval ##dt## of time and time symmetry, we expect these two answers are the same (Is there any way to make this rigorous?)...
  29. sbrothy

    I Classic 'Double-Slit' Experiment Using Time Instead of Space

    (Not sure about the prefix. Choose intermediate to be sure.) This article Physicists Recreate Classic 'Double-Slit' Experiment Using Time Instead of Space has it's basis in this paper: Double-slit time diffraction at optical frequencies It certainly sounds intriguing but is there actually...
  30. snoopies622

    I How to add time variation to a Schrodinger operator?

    I'm looking at Dirac's "Lectures on Quantum Field Theory" and I have a question about the basic mathematics of something that's part of ordinary quantum mechanics. On page 3, he says, The two pictures are connected in this way: any Schrodinger dynamical variable is connected with the...
  31. Onyx

    B Four Velocity Sign of Time: \dot t>0?

    Is it generally the case even with light like paths that ##\dot t>0##?
  32. AronYstad

    Special Relativity - Which reference frame experiences which time?

    This was a practice question, so it had the answer with it, which is 31 minutes. However, I'm confused as to why Lisa experiences T0. It isn't exactly an event happening in Lisa's rocket, but rather her just moving through space. From her perspective, it should look like Earth is moving at the...
  33. jselms99

    Time Travel: Calculating Velocity for 10 Year Trip

    So at first I thought that the time would be 10 years, and that I’d have to consider the outbound motion as v = .87c and inbound motion as v = -.87c but I’m struggling with addition of the velocities and whether or not this is even the right approach?
  34. G

    Time dilation for a clock thrown vertically

    The non-moving clock will see the other one move upwards and land as predicted by Newton's laws, so using the equation ##z=v_0t-\frac{1}{2}gt^2##, and assuming the moving clock starts at ##t=0##, it will land at ##t=\frac{2v_0}{g}##. Now, using SR only, and the Minkowski metric (with signature...
  35. T

    I Pickleball Physics -- Player reaction time vs aging

    Out of curiosity, how fast are the hands of the pros? I have timed an exchange between BJ/ALW and JD/JI that occurred at the Baird PPA in September 2022. The ball made the (about) 14 foot trip 11 times over 4.0 seconds, for an average trip time of 363mS (milliseconds). (Average speed of 26.3...
  36. doudou

    B Explaining Distant Time Dilation & Enhanced Gravitation

    When we observe distant time dilation effects, usually indicated by redshift, there are three possible explanations: 1) The speed of light, is slower there and then; 2) Space, is contracted for light there and then; 3) The frequency, of specific light is slower there and then. However, it is...
  37. brotherbobby

    The gravitating of a small mass towards a big mass

    Diagram : I draw a picture of the problem situation and paste it to the right. Attempt : Let me assume that the position of the smaller mass ##m## at a given instant of time ##t## is ##x(t)##. (1) Gravitational potential energy ##\frac{GMm}{x} = \frac{1}{2}mv^2(x)##, where ##v(x)## is the...
  38. A

    I Ice in space, sublimation time

    How long does it take water ice H20 in space in our solar system to sublimate, say a basic ice cube? It starts as a solid cube at the temperature of whatever space is above Earth and then completely turns to vapor. Just looking for ballpark situation here. Does anyone know of a table or place...
  39. Asem

    Find Initial Velocity of a projectile given flight time and angle

    Vix=Vcosθ Vix=0.76604*V Xf=Vixt Xf=0.77604V*2 V=Xf/1.55208
  40. A

    Calculating time to reduce alcohol in wine using heating method

    Hi, This is my first post on PF. I am engaged in a project seeking to find a simple way to reduce ethanol content in wine for those wishing to minimize alcohol intake for health or personal reasons. The alcohol-reduced wine will be used for cooking. Seems heating would do the trick however, the...
  41. T

    B Time dilation interpretation

    Hi, i am not a physicist but i have the intuition that time dilation is just slow in the movement of particle's and causality instead of slow in time itself and that this does not affect photons. I understand that there is no way to distinguish between a slow in time and a slow in movement and...
  42. L

    B Exploring Space-Time: Is Einstein's Theory Overfitted?

    I understand people collapse multi dimensional functions to make simpler visualisations, eg if you have a 500 dimension objective function in machine learning you can collapse it to 2D or 3D to get a visual idea of the objective-space.is this why Einstein did it as well? to make simpler...
  43. S

    I What Milky Way stars could go supernova to < +3 at any time?

    (I use +3 as the cutoff as that seems to be the limit of easily visible stars.) Yes, I know that Betelgeuse could go up in a < -10 blaze of glory, but I wonder what other ones are out there. On a side note, how fast could the big observatories move to it to observe it? And how quickly would...
  44. haziq

    Problem 2 in "Quantum Theory for Mathematicians", Solving for the travel time of a particle in a potential

    I’ve been trying to solve this for ages. Would really appreciate some hints. Thanks
  45. A

    B Phase Change in Light: A & B Signal Reflections

    I would appreciate knowing if there is any error in the mechanics illustrated in this animation with respect to phase change in the signal reflections shown at A and B, or in the description accompanying it. I have embedded the description with the animation and included it in this post for...
  46. AtoMick-u235

    B Mick's Spooky Time Dilation Puzzle

    Hmmm, , Does gravitational time dilation (speed up) cancel out earth orbit time dilation (slow down) for astronauts, , , it must do, to a certain extent Hmmm, , ,Mick's been thinking = the present is a continuous but fleeting moment, that allows the future to flow into the past, , ,so does the...
  47. L

    B If gravity was a force wouldn't going back in time cause us to float?

    This might sound as a dumb and silly question but if you think about it, it makes sense. If we wrongly assume that gravity is a force just like any other, and given the fact that time is closely related to gravity and that gravitational time dilation is a thing, wouldn't reverse time travel...
  48. Leo Liu

    How to take the double integral of a data set with respect to time

    Question: Suppose I have a data file for the acceleration of an object after every ## \Delta t_i##, how do I obtain the displacement of it? Context: Integral in a PID loop, although not exactly what I am asking as one is sum of error: $$\int_0^T \int_0^T \ddot {\vec \theta(t)}dtdt$$ the other...
  49. B

    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. C

    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...