Time dilation Definition and 999 Threads

In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them (special relativistic "kinetic" time dilation) or to a difference in gravitational potential between their locations (general relativistic gravitational time dilation). When unspecified, "time dilation" usually refers to the effect due to velocity.
After compensating for varying signal delays due to the changing distance between an observer and a moving clock (i.e. Doppler effect), the observer will measure the moving clock as ticking slower than a clock that is at rest in the observer's own reference frame. In addition, a clock that is close to a massive body (and which therefore is at lower gravitational potential) will record less elapsed time than a clock situated further from the said massive body (and which is at a higher gravitational potential).
These predictions of the theory of relativity have been repeatedly confirmed by experiment, and they are of practical concern, for instance in the operation of satellite navigation systems such as GPS and Galileo. Time dilation has also been the subject of science fiction works.

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  1. newjerseyrunner

    Can time dilation be explained by changes in the rate of entropy?

    I was watching a Youtube video and the narrator mentioned that sometimes antimatter can be thought of matter traveling backwards in time and that confused me. I started thinking about what time is and it doesn't make any sense that antimatter acts in any way like it's traveling backwards in...
  2. N

    First Year Special Relativity Question

    Homework Statement Planets A and B are 10 light years apart in the reference frame of planet A. A deep-space probe is launched from A, and 5 years later (in reference frame A) a similar probe is launched from B. Does a reference frame exist in which these two events (a) are simultaneous and (b)...
  3. W

    Time Dilation & Acceleration: Leaving Earth Effects

    Leaving Earth at constant acceleration will make time on board go slower. Acceleration will take the ship closer and closer to c Question is: How long will acceleration act on the ship? The time as measured on Earth or on the ship? The final v will be greatly different. Of course, given enough...
  4. T

    I Black Holes - Spaghetification countered by time dilation?

    Here is an oddball that I am wondering - could the spaghetification be countered by time dilation? Because as you approach a black hole (assuming you go in legs first) not only do your legs experience higher gravity than your torso, but they are also subjected to more time dilation - as they...
  5. E

    B Is time dilation just a problem with our clocks?

    [Mentor's note: This thread was split off from https://www.physicsforums.com/threads/time-dilation-and-redshift-for-a-static-black-hole-comments.835277/ as clarifying misunderstandings about time in general is off-topic in a thread about the specifics of time dilation around a black hole]...
  6. B

    Time dilation - Is the time between events longer or shorter

    So let's say there's a train and according to the passengers on the train the trip takes a time t to go from the station and back. According to an observer at the station, would they measure a time interval of gamma *t or t/gamma? I know they both should see the others clock ticking at a...
  7. stevebd1

    Insights Time Dilation and Redshift for a Static Black Hole - Comments

    stevebd1 submitted a new PF Insights post Time Dilation and Redshift for a Static Black Hole Continue reading the Original PF Insights Post.
  8. RealTwistedTwin

    What happens to matter when it enters a black hole?

    There are many videos and articles about this topic (what it looks like if you fall into a black hole). I remember hearing that, inside the event horizon of a black hole, time has essentially stopped for an outside observer. However, if you fell into one that would mean that any amount of time...
  9. H

    I Exploring Time Dilation: The Effects of Destroying an Atomic Clock

    I will not bother saying to much because the post will just get closed if I do not accept the forced discipline. If I shot the aeroplane out of the sky with the Keating experiment on board, destroying the atomic clock in essence stopping the beats per second of the clock, does this mean time...
  10. Keermalec

    Time dilation in an elliptical orbit

    The formula for time dilation in a circular orbit is readily available but the literature seems to indicate it would not be so simple in the case of an elliptical orbit, and no simple formula seems to be available. Given that time dilation in a circular orbit adds the velocity effect (GM/r) to...
  11. A

    Time dilation on the surface of the sun

    Hi, I am wondering. Is it possible to predict the amount of time dilation on the surface of the sun compared to the surface of the earth? Is this a correct question to ask, or have I made a bad assumption? Thanks in advance. Ayjay
  12. Dynamotime

    Time Dilation: Twins Separated by Black Hole's Gravity

    is This example correct? I made it up Twin Z get close to a black hole to observe it (theoretically he is Not in orbit, Not moving but close in off to an black hole to be affect by the gravity of it, and the black is not moving in space or rotating) And Twin X stay behind on the mothership...
  13. Dynamotime

    Time Dilation due to Gravity: Formula Explained

    If possible can someone tell me what is the formula of time dilation for an object Which theoretically is Not in orbit, Not moving but close in off to an black hole to be affect by the gravity of it. It will be greatly appreciated.
  14. Gaz

    Determine Absolute Speed Using Time Dilation

    So if time dilation is what's left after correcting for the optical effect and we want to know our motion through the universe. We can't use light because we have no reference point to what not being in motion is and there's lots of attempts using light which doesn't work. Why don't we just take...
  15. Tomvader1988

    Does time dilation affect only the measurement of time?

    As I understand, 'Einstein's mirrored light clock' shows us that a vessel (containing said clock) traveling at any speed greater than that of a duplicate, at rest will observe a slower passage of time. I understand that this is because of simple Newtonian physics that shows us that in the case...
  16. D

    Time dilation and speed of light

    Hello Physics Forum. Given time runs slow as seen by a moving observer, why doesn't light travel further in the slowed time? Thereby negating the greater distance light has to travel, mirror to mirror in the light clock on the train thought experiment?
  17. Gaz

    Time Dilation & Doppler Effect: Conclusions?

    [Mentor's note - split off from https://www.physicsforums.com/threads/are-time-dilation-experiments-conclusive.826952/ as this is a different question] This makes sense. And fact they appear to slow down is due to red shift of light created between the two frames?
  18. Boro Petrovic

    Are time dilation experiments conclusive?

    Before posting reply, please think about time as a concept that is used to describe a change relative to some other change. For example, a traveler will arrive on a destination after Earth completes x rotations. At the beginning, people used Earth rotation as a reference change and called it a...
  19. P

    B Do Gravitational Time Dilation and Redshift Contradict Each Other?

    Timedilation seems clear when you study special relativity and read about the Hafele-Keating experiment. Gravitational redshift seems logical when you assume that light should lose energy when it is leaving a gravitational field. But the two seem to be contradictory to each other. Question 1...
  20. H

    Questions about black holes and wormholes

    Questions about black holes: Various articles mention that it takes infinite amount of time to observe something pass through the event horizon. Does this imply that the redshift observed from afar would carry on forever, that the infalling object would just become dimmer and dimmer, but never...
  21. Raman Choudhary

    How does this proof demonstrate the concept of time dilation?

    Suppose there is person A in a frame S with respect to which a light beam clock is at rest , now time taken by a photon to go from bottom to top is L/c according to A. now the same clock is looked at by a person B traveling with velocity v w.r.t S and according to him time taken by photon to...
  22. Stephanus

    Understanding the Doppler Effect: Formula and Examples for Light Waves

    Dear PF Forum, I wonder about this Doppler formula. https://en.wikipedia.org/wiki/Doppler_effect#General ##f=(\frac{c+V_r}{c+V_s})f_0## The speed of sound is 343m/s Supposed S (Source) moves to the east, toward R(Receiver) at 70 m/s and R moves to the east at 40 m/s. So the formula is...
  23. M

    How to determine which clock runs slower in relativity?

    So I was recently reading Stephen Hawkings' "The Universe in a Nutshell" and came across the famous Twins Paradox thought experiment. My question is, since motion is relative, couldn't we extrapolate that either the observer on Earth is stationary and the rocket is traveling near light-speed...
  24. G

    What is the cause of time dilation in Special Relativity?

    One of the basic foundation of special relativity is that the speed of light is a constant, hence time is relative to the inertial framework reference. Usually GPS is also mentioned as a modern experimental evidence. My questions: if Maxwell's equations describe EM fields, how can we derive a...
  25. godzenon

    Time Dilation of Fast-Moving Objects: Exploring Forces

    I meant to say fast moving "objects" in the title, my bad... my title kind of sucks https://en.wikipedia.org/wiki/Relativistic_electromagnetism#The_field_of_a_moving_point_charge That section is basically what my questions are about. I want to know generally about the overall forces. Two...
  26. S

    Time Dilation with a Tennis Ball

    So I just finished reading my textbook on special relativity and am a bit confused by the discussion of time dilation. We equip an observer with a light source, a mirror, and a clock and herd him onto a train moving at constant velocity relative to a second observer located at the station. If...
  27. K

    Time Dilation & Mass: Does 2x Mass Mean 2x Slower Time?

    I was wondering if time on a planet which is exactly twice the mass of the Earth would pass exactly twice as slow relative to Earth time? So after a year on this big earth, two years would have passed on earth? Thanks
  28. SpiderET

    Lower gravity and higher time dilation?

    When I started to learn about GR, I have been thinking, that there is simple relation to gravity and time dilation, that higher gravity means bigger time dilation. But later thanks to PF discussions I learned time dilation is related to gravitational potential and not directly to gravity. But I...
  29. L

    Could extreme time dilation ever be directly observed?

    Hi Folks-I am interested in knowing whether, in actual practice, people on Earth would see their 99% of c colleagues moving around in fast forward motion and if the reverse would be true from the vantage point of the relativistic astronauts who are moving away from earth. I suspect that since...
  30. J

    Relationship between gravitational time dilation and energy?

    The rate that a stationary clock slows down near a massive object, relative to one far away, can be read off from the Schwartzschild metric: $$c^2d\tau^2=\left(1-\frac{r_s}{r}\right)c^2dt^2-\left(1-\frac{r_s}{r}\right)^{-1}dr^2-r^2\left(d\theta^2+\sin^2\theta d\phi^2\right)$$ by setting...
  31. Invutil

    Time Travel & Gravitational Time Dilation Function

    If you square both sides of the gravitational time dilation function for non-rotating spherical bodies, do you not get a "time travel function" that allows you to travel back in time with a massive enough body like a black hole?
  32. kweagle

    Do different frames of reference experience time dilation at the same rate?

    Hi Everyone, first I want to say I have no formal education or background on these topics, but find them very interesting and research and learn as much as I can on my own. With that in mind, I am hoping some of you will have the patience to explain what I don't seem to understand. What I am...
  33. Kairos

    Time dilation, length contraction, but velocity invariant

    If a frame is moving at constant velocity relative to an observer, this observer perceives a time dilation and a length contraction. But in this case how the velocity (length/time) can appear constant ? It is expected to be contracted.. Thank you in advance for the explanation
  34. A

    Calculate Time Dilation b/w t1 & t2 Using Average Mean Density

    I thinking how to calculate time dilation in different time with usage of average mean density. Lets set that Universe have average mean density ##\Omega_{t1}## at time t1 and ##\Omega_{t2}## at time t2. How to get time dilation, for comoving observers, between time t1 and t2?
  35. Muhammad Zubair

    Time dilation in relativity theory

    Is time dilation an actual phenomenon or it is just an apparent change relative to some other frame of reference? If it is so then why the age of a person slows down and elongates actually?
  36. D

    Time Dilation Confusion: Need Help Understanding

    Hello. I need some help understanding time dilation. So the idea is that moving clocks tick slowly. Say there was an observer A who, by his frame of reference, is stationary, and he sees an object B moving past it at high velocity. From what I understand, to A it would appear as if B's clocks...
  37. T

    Is Time Measured Differently Due to Time Dilation Since the Big Bang?

    When we talk about the BB occurring 13.8Billion years ago (see time chart below) http://en.wikipedia.org/wiki/Big_Bang#/media/File:History_of_the_Universe.svg Does a 1ns of interval time at each of those points in time mean the same thing as a 1ns interval of time now, or is the measurement of...
  38. R

    Time dilation in accelerating reference frames

    Sorry for the lame question, but I was wondering if someone could help answer the following. I have two synchronised clocks which I place on two different space ships which then accelerate away from me at the same rate and time until they reach a given speed. Ship A then slows down to be at...
  39. Mitchell Swan

    Time Dilation: 1 Hour=77 Hours - Can I Phone?

    I was talking to my dad not long ago and he asked me an extremely valid question, so basically, the question is. If I'm next to a supermassive black hole, and 1 hour for me is 77 hours on earth, and my dad rings me, bearing in mind you would an extremely good signal, but let's just say i...
  40. M

    Gravitational Time Dilation at Event Horizon

    According to Wikipedia, the gravitational time dilation formula is given by t_0 = t_f \sqrt{1 - \frac{2GM}{rc^2}} = t_f \sqrt{1 - \frac{r_0}{r}} where t0 is the proper time between events A and B for a slow-ticking observer within the gravitational field, tf is the coordinate time between...
  41. C

    Time Dilation happens on moving frame - but which one?

    As per theory of special relativity the time in moving frame R' is supposed to run slowly compared to the stationary frame R. But isn't the stationary frame R moving wrt frame R' in the opposite direction. For an observer in stationary frame R', R would be moving with the same velocity wrt to it...
  42. O

    Calculating Time Dilation for Earth Signals at 0.600c

    A person on Earth signals with a laser beam at 6 minute intervals. Another person on a rocket moving away from Earth at 0.600c detects the signals. At what time intervals does the person on the rocket receive the signals from the Earth. The formula we use is: Δt=Δto/sqrt((1-u2/c2)) u = 0.600c...
  43. T

    Exploring Time Dilation: Understanding Aging in Space and the Effects of Speed

    Hi all, First post here. I have been debating with a colleague today about time dialation. I am certain that an astornaut in space will experience more time or age faster than people on Earth due to time dilation of a massive object. He is saying that he will come back younger. Whos right...
  44. T S Bailey

    Lifespan of a black hole as measured by different observers

    I've read that a stellar-mass black hole has a lifespan on the order of 10^67 years. Does this mean that a clock which is at rest with respect to (and sufficiently far away from) a stellar-mass black hole will tick off 10^67 years before the black hole evaporates? Also, will shell observers...
  45. J

    B Is time dilation accounted for in calculating expantion

    Is time dilation a factor in the calculation of the expansion speed of the universe?
  46. Noctisdark

    Observing Objects Near Light Speed: Time Dilation or Reality?

    Hello PF, A question on special relativity I've not found an answer to,I tried to google it but no luck, so here is it : If you were to observe a moving body whose speed is very near to the speed of light, will you see it slowly due to time dilation, or you'll observe it as it really is, very...
  47. Stephanus

    Twins Paradox: Exploring Clocks in a Closed Room

    Dear PF Forum, I have a tought experiment here. I'm asking about twins paradox, but instead of using twins, I'm using clocks to lock them up in a closed room. Sort of Einstein elevator. (unlike Schrödinger, even in tought experiment, I can't imagine locking human being -- or cat -- in a closed...
  48. P

    Schwarzschild vs vs Alcubierre & Time Dilation

    Let me begin with the fact I am a rube in the field of Quantum Physics. I seem to have an innate grasp of certain concepts but if it comes to proving theory with math, I’m out. That being said, I am completely fascinating with Miguel Alcubierre’s theory on collapsing, or “warping”, space...
  49. RisingSun361

    Equivalence principle and time dilation

    I'm currently reading https://www.amazon.com/dp/0199236224/?tag=pfamazon01-20. The book claims that, according to the equivalence principle, acceleration and gravity have the same effects. So if gravity slows down time, shouldn't acceleration also slow down time? The book seems to state this as...
  50. SpiderET

    What is causing time dilation: Speed or acceleration?

    In case when we have accelerating spaceship without any influence of gravity, what is causing the time dilation? Is it primary caused by the speed of the spaceship or it is caused by acceleration leading to higher speed? What is the primary cause of time dilation?
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