# What is Time dilation: Definition and 999 Discussions

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. ### B Entering a black hole -- I know I must be wrong (Help me understand why)

From what I understand about time dilation and the relativity of simultaneity; if we imagine two people near a black hole and one of them begins to approach the black hole on a trajectory that crosses the event horizon. The stationary observer will never see the moving observer enter the black...
2. ### I A question about Relativity of Time using Time dilation experiment

The time dilation experiment involves two frames in relative motion, let one at ground and other at train with velocity V. The light clock runs faster in rest frame, as seen by an observer A at rest in train ( just beside clock ) than that observed by an observer B in ground frame which observes...
3. ### Question about time dilation

My try which was failed : Observer at rest measures time ##\Delta {t'}= 2.2 \mu s## In the frame of reference of the Earth observer measures time ##\Delta t=\dfrac {\Delta {t'}}{\sqrt {1-u^2/c^2}}## I have two unknowns ##u## and ##\Delta t## so I cannot find ##u##. Is there another equation...
4. ### B Question about "odometer" analogy and time dilation

Hey again physics forums! I'm just a lug who likes watching science videos and appreciates the occasional answer to questions that pop up as I watch them. This morning I came across a video that explained time dilation really well by using an odometer analogy, that a clock is like an odometer...
5. ### I What causes time dilation and length contraction

This Video [Moderator's note: Unacceptable Reference removed] right here is describing a way of understanding SRs time dilation and length contraction. He tries to explain the phenomena with aether theory and argues that light does not have/is in a preferred frame of reference. So some context...
6. ### B Would We See the Same Light After Travelling Back to Earth at Double the Speed of Light?

Just joined because I have been unable to find an explanation: please avoid comments about FTL speed since they don't pertain to the question. The question: A planet is 30 light years away, so when viewing it from earth we are seeing it from "30" years previous. Now, we travel 30 years at light...
7. ### Calculating Time Dilation for Space Travelers

If we took the perspective of the space traveller themselves, they are stationary and the whole universe goes past them at 0.7c. THen th elapsed time of 6.5 yr looking outside is \Delta t =6.5 \frac{1}{\sqrt{1-0.7^2}} = 9.11 yrs THen, when the traveller looks at the person travelling at 0.9c...
8. ### Time slows down at lower gravitational potential

Common interpretation is that time slows down at lower potential. I wonder if people are simply saying for the time interval between two events at lower potential, it's smaller than what would be measured at greater potential ##d \tau < d t##. i.e. Clock at lower potential shows a time interval...
9. ### I Time Dilation Near a Black Hole: Accelerating Objects Approaching the Event Horizon

First of all, I wish everyone a Happy New Year. I am interested in your expertise on a special constellation, which I will first briefly describe. If you observe an object that is approaching the event horizon of a black hole, it is said that at some point the distant observer will have the...
10. ### B Question about choice of reference frame

Hello everyone, I might be highlighting my ignorance in this post (it might be ignorance of the particulars of simultaneity) but that's a good thing because then I might be able to figure out what I don't know. If the Lorentz factor is 2 for a spaceship that is launched from Earth then...
11. ### B Help understanding the Relativity of Simultaneity

Hello everyone, So I think maybe what confused me with the symmetry of time dilation was not understanding relativity of simultaneity. After looking into it if you have two clocks P and Q that synchronize when they meet and if ##\gamma## is 2 then from P's reference frame P equaling 10 and Q...
12. ### B What does symmetry of time dilation really mean?

Hello everyone, I've been studying Morin's book and the case for time dilation makes sense, a clock in the rest frame of the moving body counts to ##T_A##, and a clock in the lab frame counts to ##T_B## and we find ##T_B = {\gamma}{T_A}## What I might be failing to do is understand what the...
13. ### I Question about time dilation symmetry

Hello everyone, Suppose there is an airplane that is taking off from an airport but before it takes off it synchronizes it's clock to zero with the clock at the airport. In the rest frame of the plane the airport is moving, so you could argue ##T_{plane} = \gamma*T_{airport}## In the rest...
14. ### B Question about Morin's time dilation explanation

Hello everyone, I'm reading Morins book which I like, and I feel I kind of understand the part on time dilation, however I'm a little confused by the geometry of the Pythagorian theorem when applied to velocities. On the moving clock he shows the velocity of light on the diagonal it traces...
15. ### B Intuition for time dilation in a cesium clock

A common way to introduce time dilation is to show the example of a "light clock" which bounces photons back/forth and ticks each time a photon passes a certain point. Wikipedia does it this way, for example. From such a clock, it's easy to see why the constancy of the speed of light would...
16. ### B A well defined thought experiment

Bob lives on top of a skyscraper which is 1000 meters high. There are exactly 10 meters between 2 (glass) walls in his penthouse apartment. Between these walls, - a photon moves back and forth (is reflected) for exactly 100 billion years. Bob can measure the elapsed time to be...

Consider a distant observer traveling at .867 c ( 𝛾=2 ) relative to the solar system along the line that is collinear with the sun's axis of rotation. As the clockwork solar system spins beneath him, the distant observer peers through his powerful telescope at Big Ben in London. After taking...
18. ### I Question about the reciprocity of time dilation

Hello everyone, After studying time dilation I believe we pick a rest frame as a place we situate a clock and because of time dilation when we have a "moving" clock it will have a longer period when compared to the rest clock. I've heard this is reciprocal and if we situate a clock at either...
19. ### I Question about length contraction

Hello everyone! I was researching about time dilation and I feel I have a pretty good understanding of it its very cool and fascinating, but my understanding of length contraction is fuzzy. It's not so clear why to me length contraction only happens in the direction of motion. If there is a...
20. ### B Does Time Move Slower for Travelers in Special Relativity Experiments?

In the mental experiment of special relativity (which has been experimentally proven anyway), in which it is shown that for the traveler on the spaceship, time passes more slowly because the clock (the tick-tock of light beams) goes slower... What is the correlation between the traveler's...
21. ### B Difference between special relativity and redshift?

For quite a while I thought that time dilation was the effect of bodies moving close to the speed of light dilates the passage of time. I also have read about red shift, which seems to be the same thing, though I’m sure they are totally different. I learned in special relativity (the time...
22. ### 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.
23. ### I What is the Connection Between Black Holes and the Big Bang?

*********Disclaimer******** Hi, I want to ask this community about some black hole shinanigans. I've spent some time searching for this topic here because I don't want to be the guy who spams a forum with a question already answered a hundred times over. Since none of the threads I found...
24. ### I Twin paradox, virtual clock on ship with Earth time, discontinuity

Conventional Twin Paradox. A ship with speed v = 0.8c makes a round trip from Earth and back. It lasts 6 years and on earth 10 years have passed. The ship carries its own clock and also a computerized clock that always shows the time on earth at that moment. It is about knowing the T - T'...
25. ### 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...
26. ### Measuring Time Dilation from 25Kg Lead Brick w/3 Cesium Oscillators

I have access to 3 Cesium 10MHz Oscillators. If I place one on top, one under and one 10' from a 25Kg lead brick what would to the ns/time rate of change? I am thinking this would be a grate HS Science Fair project.
27. ### I Is anything not spinning in the Universe?

so I've started doing a few equations involving time dilation due to gravity and have been told it changes depending on if its spinning or not (like with a black hole), how different is it, and whether there is any time something won't be spinning and it needing the altered calculations
28. ### B Time dilation in a planet-moon system

In Wikipedia time dilation is considered: As far as I know, in a planet-moon system, the difference in elapsed time between a clock on the planet and a clock on its moon is calculated using GR/proper time, so it's not very obvious if/how the movement/velocity of the moon around the planet...
29. ### B Transfer speeds of forces affected by time dilation?

Are forces subject to the Lorentz transformation? Not force carriers; I already got that question answered, thanks to @PeterDonis. But forces. The different forms of them, such as the contact forces etc.: https://en.wikipedia.org/wiki/Contact_force With time dilation, does the rate of force...
30. ### B Time Dilation in Non-Stationary Reference Frame: A, B, C

Let's consider three observers A, B and C. The experiment starts at t = 0. A is 'absolutely' stationary. B immediately (please imagine it) starts moving at speed v1 with respect to A. C immediately starts moving at speed v2 with respect to A in the same direction as B. Let's say A measures a...
31. ### B Understanding Time Dilation: How Passing Photons Affect Time Measurement

Now if two different time durations are measured for one and the same event by two different observers, for example T+1 and T-1 seconds. Is the speed of passage then (T x c)/(T+1) and (T x c)/(T-1) respectively? So not c? You may be wondering…, and yes, there is an example of it!
32. ### B Triplets Paradox: Is There a Solution?

One person (A) stays on Earth, while another (B) goes on a long journey and returns later. We can calculate the time interval for A and the proper time interval for B. Let's say we get Δt=100y and Δτ'=50y. We then consider a second traveler (C) going on a different journey, departing and...
33. V

### Does Time Dilation Differ Between Earth's Day Side and Night Side?

I’m physics curious and a middle school science teacher. I was wondering about time dilation on day side of Earth vs. night side. Pervect gave an interesting response on the subject in 2005, complete with link. The link doesn’t work and I cannot find the paper on OpenSky. I’m hoping someone can...
34. ### 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...
35. ### 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...
36. ### 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...
37. ### 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...
38. ### 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...
39. ### 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...
40. ### Effect of time dilation on Earth/space communication?

Let's say there is an audio oscillator on earth sending a 440 Hz audio tone over radio to a spacecraft. There is also an audio oscillator on the spacecraft sending a 440 Hz audio tone over radio to earth. Time is slower in the high gravity of earth than the weak gravity in space. Is the 440...
41. ### I Time Dilation Paradox: Geosynchronous Satellites

Time moves faster when higher above the earth. Time move slower the faster you are moving. A geosynchronous satellite has to be very high to be geosynchronous so it's time should be fast, but it also has to be orbiting the earth extremely fast to keep up with a geosynchronous position, so it's...
42. ### I Muon Decay Example from Morin's Book: Issues Explained

From the Lorentz transformation equations we know that $$t = \gamma(t^{'} - x^{'} v/c^2)$$ but for the Muon decay example where the setup is as follows : "Assume for simplicity that a certain muon is created at a height of 50 km, moves straight downward, has a speed v = .99998 c, decays in...
43. ### I Effects of time dilation for near-speed-of-light travel

Hello everyone, I've been learning about special relativity, and so far I believe based on what I read that if you are traveling at a velocity of .6c, you will experience time 20 percent slower than people on earth. Each second in the spaceship will be 1.25 earth seconds. Each second on earth...
44. ### I Time dilation from galactic gravitational mass

I've been thinking about how rotational speeds don't fall off high distances from galactic centers, for which dark matter is generally an explanation for the increase in acceleration Speed = distance / time But time is relative What "time" is used in these calculations? Wouldn't time be...
45. ### B Time dilation - what is the time on the Earth?

If I understand it well, 10 s did really passed in the rocket, it is according to the observer on the Earth, but if the man in the rocket measured the time, he would measure 10 s. But when we say that the man in the rocket is in an inertial frame of reference as well, he can claim that because...
46. ### I Time Dilation & Length Contraction: Graphically Explained

The thread "twin paradox without math" inspired me try to find fully graphical solution of length contraction. Here is the result: Graph is 3D as I think that is ilustrative to make all in one graph, but I am sure that it is possible to do it in 2 or 3 2D slices. X and Y are spatial axes, T is...
47. ### I Time Dilation in Gravitational Field: Potential vs Field Strength

I'm wondering is whether it is the gravitational potential (in J/kg) at a point in space that determines the rate of passage of time, or whether it is the gravitational field strength (in m/s2). To clarify, suppose you had a very heavy hollow spherical shell. The gravitational potential would...
48. ### I Time Dilation for Photons: Explained

So, I have a question. The time dilation formula is: t = t₀ • 1 / √(1 - v²/c²) Let's take a photon that travels at c. In my opinion, for a photon "clock doesn't tick" and its life is just a moment. But when we calculate time dilation by this formula, then c over c is 1 and the root of 1 minus...
49. ### B Time Dilation & Bosons: Decay Rate Changes?

Hi, It is easy to find discussions about time dilation and muon Half-Life. Is it meaningful to discuss whether bosons capable of pair production can have their decay rate changed if they pass through material?
50. ### I Time difference caused by length contraction

The goal is to calculate the difference in the passage of time between two ends of a cabin which is accelerating upwards due to length contraction. To help in the calculations we can consult: https://arxiv.org/pdf/1807.05338.pdf There is an old problem called the 4/3 problem which has been...