Time Definition and 277 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. ohwilleke

    I Trying To Conceptualize Two Dimensions Of Time

    Some BSM theories, often but not exclusively string theory inspired, have an extra time dimension, as well as extra spatial dimensions. I'm trying to make sense of what it even means to have more than one time dimension, ideally, with a concrete example that illustrates how the second time...
  2. S

    I It's About Time

    I tweeted this on a Sabine Hossenfelder’s Twitter thread: Is there an example, definition, explanation of time that is not derivative of a physical system like a clock, e.g., that's based on the rotation of the earth? If not. If time is not a stand-alone definable entity how can it qualify as a...
  3. trryan5

    Find Projectile Flight Time Given Only Maximum Height

    I've been trying some online projectile problems. Specifically, I was using this one on master difficulty, looking at row e. It uses a random number generator; I shared the data I received in the Homework Statement above. According to the help section, you can solve this with the formula...
  4. Spockishere

    B How long would a 7ly trip feel like to you if you travel at 50000km/s

    let's say i would like to drop by one of my pals on a certain planet, 7ly away. I got to 42 years but it doesn't really sound correct.
  5. V

    I Trouble understanding the SI definition of 1 second

    Homework Statement:: The SI definition of unit of time says the following. "The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." Relevant Equations:: None I know an...
  6. Anmol Dubey

    Making a Velocity vs Time graph for a bullet

    Here is my working in case it helps :)Ignore the v = 0.6/0.0001 = 6000m/s
  7. entropy1

    I Quantum Mechanics without time?

    Is there a view in quantummechanics, of quantummechanics, without time as a concept?
  8. AntSC

    Acceleration-Time Graph and Velocity Multiple Choice Question

    The variation with time t of the acceleration a of an object is shown What is the change in velocity of the object from ##t=0## to ##t=6##? A. ##6ms^{-1}## B. ##8ms^{-1}## C. ##10ms^{-1}## D. ##14ms^{-1}## So apparently the answer is B, which I am having trouble reconciling. Using methods...
  9. R0dr1go

    B Acceleration of time due to entropy

    Hello, I am a 15 year old who has done research around the topic of why the universe is expanding at an accelerating rate, and how it will come to an end. After years of thought (since I was 11) I have come up with my hypothesis that time itself is accelerating and slowing down, and has been...
  10. Ebi Rogha

    A Block universe and entropy increase

    I wonder how physicists who support this theory explain the increase of entropy?
  11. Ebi Rogha

    B Is time a consequence of 2nd law of thermodynamics?

    I have heard from a knowledgable physics proffessor, time exists independently and it is not a consequence of arrow of time. Could some body explain this?
  12. tworitdash

    MATLAB Creating and recovering a frequency shift in time domain in MATLAB

    I am trying to simulate and process the Doppler signals. My main problem is a little more complex so I am only posting a simple version of it. Task1: I have a time-domain signal with the velocity of the target as mu. I need to change the velocity to mu cos(theta) where theta is a vector from 0...
  13. N

    Position vs Time graphs

    The graph: 1) $$v_1 = \frac{\Delta{x}}{t} = \frac{5 - 3}{3} = \frac{2}{3}$$ $$2 = \frac{\Delta{x}}{t} = \frac{4 - 0}{5 - 1} = 1$$ $$\frac{v_2}{v_1} = \frac{1}{2/3} = \frac{3}{2} = 1.5$$ 2)Points of intersection of the lines with the x-axis: ##I## (0; 3) and ##II## (0; -1), thus $$\frac{2}{3}t...
  14. AlfSalte

    B What happens to time as space is expanding?

    I have one question I hope someone here can answer for me. Relativity theory tells us that space and time are sort of the same thing, as a spacetime. So when space is expanding, what happens to time? I find it hard to believe that time is somehow unaffected by the expansion of space, so while...
  15. M

    Please help me understand time dilation

    Good day. I have read many books, forums, and articles, and watched many youtube videos, all in an effort to understand special relativity and time dilation. Or rather, more precisely, trying to find a valid explanation that does not result in contradictions and paradoxes, and if it does, one...
  16. adafus

    B Are time and light at the same speed?

    I'm probably am wrong in this but i guess you guys could light me in this one
  17. thegroundhog

    B “Direction” of time

    I've seen a lot of Youtube clips and listened to podcasts recently on cosmology and quantum physics and something that has come up frequently that I don't understand is descriptions of time. Specifically Sean Carroll on various podcasts and Carlos Ravelli (when he finally got to the point). 1...
  18. VarnaiKristof

    B Can our movement in time be decelerating?

    Is it theoretically possible that the whole universe is decelerating in time? My question originates from the discovery, that objects further away accelerates away faster. Is that acceleration measured from us or from the center of the big bang?
  19. M

    What’s a good name for a phenomenon where time moves differently?

    I’m trying to come up with a term to describe how time moves slower or faster in different places of a galaxy. I’m trying to explain how in Star Wars, things seem to move differently for different characters in separate locations. I’m trying to argue that Han limping to Bespin took “longer” than...
  20. E

    Question about variations of the Sun's path over long periods of time

    Hello. I am a filmmaker writing a screenplay and I am in need of technical support to validate a method being used in the film. I am going to post this in the Earth Science forum but I'll copy it here, in case anyone reading this might know. Thank you all! "I am wondering if shadows cast...
  21. L

    Which way is correct and why?

    I get two different answers even the area of the graph. I think there is something wrong with the equation I constructed for average speed part. I know the method 1 is correct. But I'd like to know why the average speed cannot be used. And why I get two different areas for the graph Thank you
  22. TammyTsang

    Engineering Scaling of units for equations of motions

    In the picture, there is a problem where the t is in units of square root(l/g), and V in square root(gl) I am wondering 1. What it means when time is in units other than time? Does it mean that when solving I have to take time/squareroot(l/g) 2. How did they get square root(l/g). Thank you...
  23. M

    A Vast distances=Time travel?

    If you could instantaneously teleport from Earth to a planet in the Andromeda galaxy; when you arrive, would you be in what to Earth would be the distant past? Meaning that when you looked at Earth, you would be seeing what it looked like millions of years ago?
  24. D

    Getting Force with distance, mass and time

    I'm sorry but I can't even attempt this question since I'm pretty much clueless, I got the notes but I don't understand them.
  25. velocityoverrtime2

    Pendulum projectile problem

    Tried to find time in seconds in order to use the formula d=vt and find the distance in the x plane in which the target must be placed, to no avail.
  26. S

    Battery Charge and Discharge Time Calculations

    I am performing some calculations but struck with confusion whether I am doing it correct or wrong due to contradictions of these calculations on different calculators websites. I hope here someone will help me with the validation of my calculations. Battery Capacity = 150 Ah Battery...
  27. entropy1

    Is the past in MWI unambiguous?

    I couldn't match this question with another topic, so I made a new topic. Suppose we have past H and present P at a certain moment. In present P a measurement is made. For matter of speaking we adopt MWI, so we have the two measurement outcomes A and B diverging in two realities A and B in...
  28. Prabs3257

    Kinetic energy as a function of time

    I got acceleration by dividing force by m then replaced a by dv/dt and then integrated it to get velocity as a fxn of time and hence got kinetic energy but problem is my ans does not match with any option can someone please compare their ans
  29. W

    I Entanglement/correlations in time

    From what I know about (bi-partite) entanglement, we write the combined Hilbert space as a tensor-product of Hilbert spaces for a particle at ##A## and a particle at ##B##, ie ##\mathcal{H} = \mathcal{H} ^{A} \otimes \mathcal{H} ^{B}##. If the particles share a non-separable state, they are...
  30. menniandscience

    Retarded and advanced potentials

    "of the two types of solutions which the Maxwell equations yield for the wave equation, the retarded and advanced potentials, only the retarded field seems to have a physical meaning," let's start please with basic (and detailed as possible for the knowledgable layman! p.s-which equation is...
  31. W

    I Time-reversal symmetry in QM

    I've bumped into a few interesting papers talking about time-reversal symmetry in QM (eg: https://arxiv.org/abs/1507.07745) but I can't seem to wrap my head around the concept. 1) What does it mean for one to say that standard QM isn't time-reversal symmetric? Does this have to do with the...
  32. A

    Time it takes to swim across a river

  33. R

    Frequency to temporal period conversion

    I am working with signals that look like the following. The curve has the same form in both frequency and time domain. I am trying to calculate the ratio of two distances: The distance between two adjacent red "x"s (known as the free spectral range (FSR)). The distance between two adjacent...
  34. Samwell

    Force as a function of time

    Hello, I have the force defined as a function of time, where F=A-Bt and A=100N, B=100Ns-1. I have to determine, how long it will take for object to stop, if t0=0s and v0=0,2ms-1 and mass of the object is m=10kg. Can somebody please help me with this, because I'm having hard time with this task.
  35. J

    B How physics equations transform under inverse of parameters

    It is obvious that there is a one-to-one relationship between real numbers (defined to include infinity) and their multiplicative inverses (assuming we map the inverse of zero to infinity and vice versa). Thus, one should be able to replace the distance between two points in space with it's...
  36. mousheng xu

    I Is time bidirectional in a black hole?

    I just watched a youtube video saying that somewhere in a blackhole, time is bidirectional (can go to the future and can go to the passed), but space is one-directional. Any introduction material on this subject? Thanks in advance!
  37. Abhishek11235

    Calculating elapsed time in different frames

    The attempt is above
  38. Ryu

    Theoretical Higher spatial dimensions question

    So I had a topic which I would like to fact check from an informed scientific source. Basically there is an argument about whether or not an object that naturally exists in a fourth dimensional space, would by default have more than countably infinite times the energy of a 3 dimensional Object...
  39. B

    Relative velocity between a Bus and a Car

    for part 1: t= d/s = 9000/13 = 692.31s for part 2: What i am unsure about is wether or not this is after the initial observation or exactly what they are asking honestly. so i found the relative velocity of the bus to the car and vice vera and came up with: t=d/s = 9000/4 = 2250s however i dont...
  40. A

    What do you think of this FTL/time travel engine?

    So, I have this idea for the FTL engine for artistic purposes and I wanted to ask if anybody could point out it's flaws. I would greatly appreciate if you were willing to discuss them further. It is just a rough summary, so if you want me to disclose more details, ask away. The idea is as...
  41. W

    Retarded time calculations: Electrodynamics

    Homework Statement A positive charge ##q## is fired head-on at a distant positive charge ##Q## that is held stationary. It comes in at speed ##v_0## and comes to an instantaneous halt at distance ##r_f## away from Q. What is the amount of energy radiated due to acceleration in this time...
  42. L

    Loading/charge time of an electromagnet

    Hej Guys, Did anyone see anywhere an equation to this? If nothing else then a loading time formula for solenoid would do. Thank you!
  43. U

    Are all arrows of time special cases of the thermodynamic one?

    Are all arrows of time special cases of the thermodynamic one? The arrows of time I am referring to are the psychological arrow of time and the cosmological arrow of time. Thanks.
  44. J

    I Relativistic Reference Frames and the Big Bang?

    Peeling this out into its own thread for clarity: How is time dilation of extreme reference frames (photons, black holes, intergalactic space-time) taken into account in Big Bang cosmology? Since from the POV of a singularity or a photon, their clocks have effectively stopped and any lower...
  45. Antony Death

    B Is the current definition of gravity accurate?

    The current definition of Gravity is: The force of attraction between bodies as a result of their mass. Gravity affects both the space and time of the area surrounding a mass, diminishing with distance, so is the current accepted definition truly accurate? Do I have the correct definition and if...
  46. A

    I Back to Muller's 'Now and the Flow of Time'

    Folks, I'm back to reading Muller's paper (https://arxiv.org/pdf/1606.07975.pdf) about the flow of time. He postulates that time is expanding similar the way space is expanding (ok... so if you can swallow that). He asks: "why are the new nows created at the end of time, rather than uniformly...
  47. L

    I Do we have to accept the physical existence of time?

    So, I am confused about the following.I learned in high school and in my first year of university that there is something called time dilation that observers observe that time is slower for objects that are moving faster. As in if there were two initially synchronized clocks that you could...
  48. C

    B Time Dilation Theory Flaw

    The theory regarding two persons experiencing different amounts of time dilation due to the two persons being influenced by different amounts of gravity states that while one person stays on earth under its gravity and another person travels through space under less influence of gravity (less...
  49. T

    Time of Fall in triangular and rectangular frame

    Homework Statement This question is actually two question. We have two hollow frames - one is rectangular and another is triangular. the rectangle is rotated and fixated such that the angles in shape are ##\alpha , \beta = 90 - \alpha## and the angle of triangle is ##\alpha##. We have two balls...
  50. entropy1

    B Entropy, irreversability, and the arrow of time

    It is said that entropy causes an arrow of time. However, how about the irreversability of a measurement like electron spin. When measured a certain spin, the previous value gets lost. So does that also require an arrow of time?