This article will address and answer the following:

What is Time Dilation?
How does speed affect time?
How does gravity affect time?

Yes, time does slow down as you move. Absolutely. Not just hypothetically. It has been verified beyond any doubt by experiments. Speed and Gravity affect how fast your time ticks. It is not just ostensibly.

The measurement is not connected to any perception of light, which is an incorrect explanation I often meet. All observers agree that a moving frame or a frame nearer gravity shows slower ticking time. The higher the speed, the slower the referenced time. The stronger the gravity, the slower the referenced time. And the formula is all Einstein. In fact, we have two.

What is Time Dilation?

“Special Theory of Relativity” (1905) taught us that time is a function of speed. We call the phenomenon: “Velocity Time Dilation”, and it has been verified beyond any doubt.

Anyone speeding relatively to someone not speeding will experience time dilation (aka a different flow of time). And this goes for any speed - be it a stroll in the park or a spaceship whooshing away. Wave your hand, and it experiences time slightly slower than your shoulder. Your circulating blood cells age slower than your stationary heart. Etc. Etc. Everything is relative.

Popular media often mentions that this happens only at near the speed of light. This is false. Naturally, the difference is more noticeable the closer to the speed of light one travels, but the effect appears at any speed.

Would you move at the speed of light, your time would have slowed so much down that it would have stopped entirely. Of course, Einstein also tells us that we can never achieve this speed, but could we, and our time would stop. Meaning that we could factually travel to any part in the universe, instantly - from our perspective, that is. If time does not tick, you will not experience anything except the beginning and the end of your voyage.

On top of this, we also have the “General Theory of Relativity” (1915). It taught us time too is a function of how close something is to a source of gravity. We call it “Gravitational Time Dilation” or sometimes “Acceleration Time Dilation”. So just by being on Earth, our time ticks a bit slower than, for example, if we were floating in space far away. It also says that since your feet are closer to Earth than your head is, your feet are a tiny bit younger than the rest of you. Crazy, indeed, but very true.

Anyway, all objects will have their time ticking quicker or slower, depending on how fast they relatively move or how close to a source of gravity they are.

The speed you experience on earth (like when you are taking the bus, or flying in an airplane, or running down the stairs) affect your time a tiny bit, but the effect is so tiny it really does not matter for any of us - even that the difference is real. Stay near a mountain, and it too affects your time because of its gravitational pull. But still, very little.

Timedilation 3.JPG — GPS Satellites

Ticking 38 microseconds faster than us, all the time.

But look at our GPS satellites. They tick faster than our clocks on Earth. They tick precisely 38 microseconds faster.

The thing is: According to “Gravitational Time Dilation,” they should tick 45 microseconds faster because they are much further away from the gravitational force of Earth than we are.

So why is it not 38?

Well, “Velocity Time Dilation” tells us that the satellite’s internal time should slow down because their relative speed to us is faster. It should slow down precisely 7 microseconds.

In other words; those two Time Dilation phenomena explain together how the clocks onboard the GPS satellites tick:

+45 -7 = +38

In order for us to use them, we need to compensate for this different flow of time. Otherwise, the GPS systems would be utterly useless.

Timedilation 4.JPG — Thank you, Einstein

All this begs the question, HOW? Fancy words and postulates is worth little without understanding.

Before I begin to explain the “how” you need to know two things:

The speed of light is always constant no matter how you approach it - if you run towards a beam of light, its speed will not increase relative to you. It remain 300 000 km/s. If you run with a beam of light, it will not decrease relative to your speed. It remain 300 000 km/s. Unlike speeds for anything else, the speed of light is invariant and not a relative phenomenon. The highest-energy photon and the lowest-energy photon ever observed, travel at precisely the same speed. Always. That the speed of light is the same for all observers is known as Maxwell’s equations. And it means that if something seems to indicate the speed of light varies under certain conditions, the explanation is likely found elsewhere. You can read more on the speed of light here. This scientific axiom or law is pivotal to understanding how and why time dilation occurs.
Any measurement in one frame, depends on another relative frame. Consider two frames of reference. If they move together, there is no measured time dilation between them - they each experience time the same. In fact, they are in the same frame of reference. If they move at different speeds, then there is a measured time dilation between them - the one moving faster experience time slower. In this case, they are in different frames of reference.
It means time dilation is a function of relative speed or relative gravity - hence “theories of relativity”. The theories of relativity are tied up to this; “frames of reference”. If there is no relative frame, there is no relative measurement. So to measure the effect, you must always have relative frames - two points of view. The effect is still factual inside a singular speeding frame, but to measure it or realize that it occurs, you need an external point of reference.
No matter how fast or slow you move, you - the mover - will never experience any difference yourself. It is only when you will compare it to another frame of reference that this relativity difference becomes apparent.

How does speed affect time?

I will start with “Velocity Time Dilation”. Take a look at this illustration:

Look at the boxes and the dotted lines.

The left side of the illustration and the right side of the illustration is the same box.
The first box is at rest - standing still on the ground together with us.
The second box (illustrated as the three boxes) is this same box, only moving relative to us. This is why it looks like three.
The dotted lines go up and down at a fixed distance of L.
Move the box, and for someone moving with it, its dotted lines will still move up and down at a fixed distance of L.

HOWEVER, when you are looking at the moving box from outside, you can see these dotted lines are now longer (they are dilated) than when it was standing still right next to you.

We must, therefore, conclude that just by moving this box, the dotted lines (or gray line in the above GIF) have a longer distance to cross, than if it was standing still. And YET, were you moving with the boxes they would still only move the distance of L - so it depends from where you look.

Move with the box, and the dotted lines do not change (you are with it - the box on the left).
Stand outside the moving box, and the dotted lines become dilated (longer, as you stand outside looking at it move).

In other words; Moving makes the dotted line longer for an outside observer. And yet makes no difference for an inside observer. This dotted line represents time. It is a beam of light.

The difference in the lengths of this dotted-line/beam of light can be explained by one of two things:

Light varies in speed, depending on where you observe it (inside the moving box vs. outside the moving box).
Time ticks differently when moving.

Since the speed of light in a vacuum is the same for all observers, regardless of their motion relative to the source, it cannot be the first explanation. Light speed is constant. Ergo, time must tick at different rates when you move.

Let us say L equals a second. So, since the dotted lines become longer (dilated) when viewed from outside the moving box, it means the second takes longer. It means the second inside the moving box is slower, when compared to a non-moving box.

Ergo, the rate of time is a function of relative movement — time dilates (slows) when you move.

Take a look at this little clip (1 minute) - same as the illustrations above, only animated:

How does gravity affect time?

As we learned above, time is also affected by gravity. But, how does “Gravitational Time Dilation” actually do it?

To understand this strange and counter-intuitive phenomenon, it helps to realize that constant acceleration is effectively the same as gravity (aka “the principle of equivalence”). Also why “Acceleration Time Dilation” and “Gravitational Time Dilation” are two labels for the same.

Timedilation 111.JPG — From 2001: A Space Odyssey

(1968)

In the pictures above (from the brilliant movie; "2001: A Space Odyssey" (1968)), gravity is achieved by spinning, which scientifically equals a constant uniform acceleration. In science, constant acceleration and gravity are equivalent. It helps, when we need to understand how gravity may affect the flow of time.

Imagine that you are standing inside an elevator in deep space, and it is being pulled at a fixed rate of increased acceleration. In this scenario, there would be no way for you to figure out if you were on a planet with gravity or if you were being pulled in space.

This fact helps to visualize Gravitational Time Dilation.

Imagine a very long elevator in space.
Imagine the elevator has someone standing on the floor, with a regular ticking watch. Let us call him Niels.
Imagine someone about to pull the elevator (he is faraway outside, and pulling in the direction of the ceiling). Let us call him Albert.
Imagine the elevator has a light-pulse-watch on the ceiling. It sends a light pulse every second - 60 a minute - straight down to Niels on the floor.
Niels receives 60 light pulses every minute from this ceiling light-pulse-watch.
Now, imagine Albert pulls the elevator, and thereby create acceleration in the direction of the ceiling (and so also gravitation inside the elevator - from the bottom up).

Niels will experience an increase in gravity, because Niels is now being accelerated.

Niels will also record more than 60 pulses a minute because he is being pulled in the direction of the pulses that are sent to him from the ceiling. You can say that he catches up to them (let us say he now receives 120 pulses in one of his minutes). However, the ceiling light-pulse-watch is still only sending 60 per minute.

There can only be one of two explanations:

Either the speed of these light photons/pulses are variant, and changes as Niels is being pulled into them. From Niels’ perspective, the photons increase in speed. In other words; relative to Niels, the light-pulses become faster than light.
The time on the floor becomes slower than on the ceiling.

Since the speed of light in a vacuum is the same for all observers, regardless of their motion relative to the source, it cannot be the first explanation. There is no way that light can increase its speed, even relative to Niels. The speed of light is always c, even when Niels is being pulled towards it. Ergo, it must be the second option: Time is now different.

Niels must conclude that when just 60 seconds passed at the floor where he is, 120 seconds must have passed at the ceiling where this light-pulse-watch is. Niels’s clock ticks slower now on the elevator floor, when the acceleration/the gravity increases.

The stronger the gravity, the slower the time.

Does Time Slow Down When You Move?

This article will address and answer the following: