<p>I don't like analogies. They are always wrong (if they weren't wrong, they wouldn't be analogies) but they often look like explanations. Instead of an analogy, I will give you a thought experiment. It has no math, but you still have to concentrate to follow it:</p>

<p>1) Imagine that you are in a tower and you drop a cannon ball. The cannon ball speeds up because gravitational potential energy is converted into kinetic.</p>

<p>2) Now instead imagine that you shoot a cannon ball straight up. The cannon ball slows down because it's consuming energy to climb Earth's gravitational well.</p>

<p>3) Now imagine that instead of a cannon ball, you shoot a photon. The photon is climbing up the gravitational well too, so it is not unreasonable to imagine that it must also be losing kinetic energy.</p>

<p>But here is the thing: a photon cannot slowdown. We know that from experiments. The speed of light is a constant of the universe. So if the photon cannot slow down, how can it lose energy? Well, to lose energy the photon needs to change wavelength. Longer wavelength means less energy. So as the photon climbs up the gravity well, the wavelength gets longer. This is known as gravitational redshift.</p>

<p>Now, to finish the thought experiment, imagine that you have a friend at the International Space Station, looking at your photon. Imagine also that on the ground you have a clock, that is synchronized with the photon. For every crest of the wave, your clock does one tick. You would imagine that if you see that your clock ticks always happen together with the crest of each wave, your friend at the space station must agree with that fact too. But here is the thing: He is looking at the photon when it has a longer wavelength. If he has to agree that your clock ticks coincided with the crest of each wave, then he has to see your clock ticks happening farther apart than what you saw. In other words, he will see your clock ticking more slowly. This is time dilation.</p>

<p>To conclude: What I wrote here is not a proof, but it is a thought experiment that hopefully conveys that time dilation is reasonable.</p>

CC BY-SA 3.0
2017-08-05T08:38:08.903Z
2017-08-05T08:38:08.903Z
DanielC
<p>It's a bit odd, but I hope this helps. It's like a coordinate system with time on the x axis and space on the y. For example, if you move at a speed of 5 mph, that can be split between the x and y axes. If you're going mostly in the x direction, you're not moving that fast in the y. With time, the idea is that if you are moving extremely fast through space (our y axis in this case), there's not as much left over to travel through time (the x axis).</p>

<p>So if we have a fixed "speed" through space-time, the faster we move through space, the slower we move through time (It's like placing a ladder, the length stays the same, but depending on how we angle it, it can go very high, or just barely off the ground)</p>

<p>Obviously not a perfect explanation, but it at least gets the right idea for what you're looking at.</p>

CC BY-SA 3.0
2017-08-07T16:36:50.387Z
Yes, I understand that, but what I don't understand is  as follows. Let's say I moved through space so much faster than light, that I reached at the edge of the observable universe in an hour. And again, I travelled back to Earth in an hour. Time I spent travelling was 2 hours. Time on Earth should also change 2 hours according to my watch. Right  or wrong ?
GypsyCosmonaut
CC BY-SA 3.0
2017-08-07T16:41:32.463Z
Not quite. First, traveling faster than light is impossible for anything with mass, but ignoring that, if it took 2 hours on Earth, let's call that 2 hours on the "outside," you wouldn't feel 2 hours go by, it would feel and be instantaneous for you. Your watch would show the same time and nothing about you would have aged (other than the fraction of a second it felt like for you). The idea is that time isn't a fixed thing. It moves forward, but not the same for everything. You actually experience different time.
Nick van alfen
CC BY-SA 3.0
2017-08-07T16:21:06.873Z
2017-08-07T16:21:06.873Z
Nick van alfen
<p>I could never understand how in Interstellar, they say our every hour spent on the planet is equal to 7 years on Earth and when he returns back to her daughter, the father is older than her daughter. How is this possible when both the planet over there and Earth and for the fact everything in the universe is moving with the same time reference point. The age of daughter and father should have been the same.</p>

<p>I don't know but is this termed as Time Dilation ?</p>

<p>How does this work ?</p>

<p>Please avoid any formulae in explanation.</p>

CC BY-SA 3.0
2017-08-04T12:00:50.497Z
There's no preferred frame of reference in the universe. Although a bit of math is almost unavoidable, you might still just like reading the wiki articles on [Time dilation](https://en.wikipedia.org/wiki/Time_dilation) and [Gravitational Time Dilation](https://en.wikipedia.org/wiki/Gravitational_time_dilation).
Dhruv Saxena
CC BY-SA 3.0
2017-08-04T14:17:35.517Z
@DhruvSaxena There must be an analogy for explaining this, not every person understands concepts with maths
GypsyCosmonaut
CC BY-SA 3.0
2017-08-04T17:09:01.293Z
Why do you think there has to be an analogy? The analogy is just that time moves at different rates for different observers due to the geometry of spacetime.
A. C. A. C.
CC BY-SA 3.0
2017-08-04T19:45:49.667Z
If you are so sure that you cannot understand concepts with maths, why can you be so sure that there has to be an explanation so that you can understand concepts with physics? Targeting your question ”How does this work ?“ from a philosophical perspective: We do not know how anything works. All physical laws are deductions from empirical observations. We are very certain that these deducted laws work under some preconditions, but we never know why they work. So your question is in fact unanswerable.
Xaver
CC BY-SA 3.0
2017-08-04T20:29:35.420Z
Possible duplicate of [Time Dilation in the movie Interstellar](https://physics.stackexchange.com/questions/220542/time-dilation-in-the-movie-interstellar) on Physics SE.
StephenG - Help Ukraine
CC BY-SA 3.0
2017-08-04T20:34:58.720Z
@GypsyCosmonaut A completely non mathematical answer to a question that asks _how_ time dilation works would barely manage to put any point across. I only meant to suggest looking at some basic equations used in the calculations (and not their entire derivation). The experience of passage of time is _not_ the same for all the observers in the universe. At the heart of the issue is the [constancy of speed of light - $c$](https://en.wikipedia.org/wiki/Speed_of_light). It remains the same for all the observers in the universe, as derived using Maxwell's equations.
Dhruv Saxena
CC BY-SA 3.0
2017-08-04T20:35:09.203Z
Now since $c$ is constant (where $speed = distance / time$), Special Relativity asserts that for an object in motion, distances should shrink and time should dilate to keep the speed of light invariant for all the observers (including that for the moving object). Further, because masses accelerate under the influence of gravity, General Relativity shows that time moves slower for objects closer to the gravitational potential than for those farther away from it. For a spaceship closer to a supermassive black hole, time would slow down significantly with respect to an external observer on earth.
Dhruv Saxena
CC BY-SA 3.0
2017-08-06T09:31:50.660Z
"the fact everything in the universe is moving with the same time reference point". Why do you say that's a fact? Sure, it's a reasonable _assumption_; however, for over a century we've known that it's _not_ strictly true. Numerous experiments have demonstrated that the time in one reference frame can differ from that in another reference frame. In fact, the GPS system would produce incorrect location information if it didn't incorporate the time changes predicted by relativity.
PM 2Ring
<time-dilation>
How do I imagine time difference?
GypsyCosmonaut
# How do I imagine time difference?

(Asked by: username_0 on 2017-08-04)

I could never understand how in Interstellar, they say our every hour spent on the planet is equal to 7 years on Earth and when he returns back to her daughter, the father is older than her daughter. How is this possible when both the planet over there and Earth and for the fact everything in the universe is moving with the same time reference point. The age of daughter and father should have been the same.

I don't know but is this termed as Time Dilation ?

How does this work ?

Please avoid any formulae in explanation.

**Tags:** time-dilation

> **Comments:**
> **username_1**: There's no preferred frame of reference in the universe. Although a bit of math is almost unavoidable, you might still just like reading the wiki articles on [Time dilation](https://en.wikipedia.org/wiki/Time_dilation) and [Gravitational Time Dilation](https://en.wikipedia.org/wiki/Gravitational_time_dilation).  
> **username_0**: @username_1 There must be an analogy for explaining this, not every person understands concepts with maths  
> **username_2**: Why do you think there has to be an analogy? The analogy is just that time moves at different rates for different observers due to the geometry of spacetime.  
> **username_3**: If you are so sure that you cannot understand concepts with maths, why can you be so sure that there has to be an explanation so that you can understand concepts with physics? Targeting your question ”How does this work ?“ from a philosophical perspective: We do not know how anything works. All physical laws are deductions from empirical observations. We are very certain that these deducted laws work under some preconditions, but we never know why they work. So your question is in fact unanswerable.  
> **username_4**: Possible duplicate of [Time Dilation in the movie Interstellar](https://physics.stackexchange.com/questions/220542/time-dilation-in-the-movie-interstellar) on Physics SE.  
> **username_1**: @username_0 A completely non mathematical answer to a question that asks _how_ time dilation works would barely manage to put any point across. I only meant to suggest looking at some basic equations used in the calculations (and not their entire derivation). The experience of passage of time is _not_ the same for all the observers in the universe. At the heart of the issue is the [constancy of speed of light - $c$](https://en.wikipedia.org/wiki/Speed_of_light). It remains the same for all the observers in the universe, as derived using Maxwell's equations.  
> **username_1**: Now since $c$ is constant (where $speed = distance / time$), Special Relativity asserts that for an object in motion, distances should shrink and time should dilate to keep the speed of light invariant for all the observers (including that for the moving object). Further, because masses accelerate under the influence of gravity, General Relativity shows that time moves slower for objects closer to the gravitational potential than for those farther away from it. For a spaceship closer to a supermassive black hole, time would slow down significantly with respect to an external observer on earth.  
> **username_5**: "the fact everything in the universe is moving with the same time reference point". Why do you say that's a fact? Sure, it's a reasonable _assumption_; however, for over a century we've known that it's _not_ strictly true. Numerous experiments have demonstrated that the time in one reference frame can differ from that in another reference frame. In fact, the GPS system would produce incorrect location information if it didn't incorporate the time changes predicted by relativity.  

---

## 2 answers

### Answer by username_6 on 2017-08-05. Score: 2

I don't like analogies. They are always wrong (if they weren't wrong, they wouldn't be analogies) but they often look like explanations. Instead of an analogy, I will give you a thought experiment. It has no math, but you still have to concentrate to follow it:

1) Imagine that you are in a tower and you drop a cannon ball. The cannon ball speeds up because gravitational potential energy is converted into kinetic.

2) Now instead imagine that you shoot a cannon ball straight up. The cannon ball slows down because it's consuming energy to climb Earth's gravitational well.

3) Now imagine that instead of a cannon ball, you shoot a photon. The photon is climbing up the gravitational well too, so it is not unreasonable to imagine that it must also be losing kinetic energy.

But here is the thing: a photon cannot slowdown. We know that from experiments. The speed of light is a constant of the universe. So if the photon cannot slow down, how can it lose energy? Well, to lose energy the photon needs to change wavelength. Longer wavelength means less energy. So as the photon climbs up the gravity well, the wavelength gets longer. This is known as gravitational redshift.

Now, to finish the thought experiment, imagine that you have a friend at the International Space Station, looking at your photon. Imagine also that on the ground you have a clock, that is synchronized with the photon. For every crest of the wave, your clock does one tick. You would imagine that if you see that your clock ticks always happen together with the crest of each wave, your friend at the space station must agree with that fact too. But here is the thing: He is looking at the photon when it has a longer wavelength. If he has to agree that your clock ticks coincided with the crest of each wave, then he has to see your clock ticks happening farther apart than what you saw. In other words, he will see your clock ticking more slowly. This is time dilation.

To conclude: What I wrote here is not a proof, but it is a thought experiment that hopefully conveys that time dilation is reasonable.


---

### Answer by username_7 on 2017-08-07. Score: 0

It's a bit odd, but I hope this helps. It's like a coordinate system with time on the x axis and space on the y. For example, if you move at a speed of 5 mph, that can be split between the x and y axes. If you're going mostly in the x direction, you're not moving that fast in the y. With time, the idea is that if you are moving extremely fast through space (our y axis in this case), there's not as much left over to travel through time (the x axis).

So if we have a fixed "speed" through space-time, the faster we move through space, the slower we move through time (It's like placing a ladder, the length stays the same, but depending on how we angle it, it can go very high, or just barely off the ground)

Obviously not a perfect explanation, but it at least gets the right idea for what you're looking at.

> **Comments:**
> **username_0**: Yes, I understand that, but what I don't understand is  as follows. Let's say I moved through space so much faster than light, that I reached at the edge of the observable universe in an hour. And again, I travelled back to Earth in an hour. Time I spent travelling was 2 hours. Time on Earth should also change 2 hours according to my watch. Right  or wrong ?  
> **username_7**: Not quite. First, traveling faster than light is impossible for anything with mass, but ignoring that, if it took 2 hours on Earth, let's call that 2 hours on the "outside," you wouldn't feel 2 hours go by, it would feel and be instantaneous for you. Your watch would show the same time and nothing about you would have aged (other than the fraction of a second it felt like for you). The idea is that time isn't a fixed thing. It moves forward, but not the same for everything. You actually experience different time.  

---

