Saying that the way mass-energy affects space and time are complicated is an understatement. Time dilation and length contraction are fundamental underlying consequences of relativity. Your help fleshing out this article and making it simple, awesome, and accurate is appreciated. Comment below.
Is Time Relative? Is Space Relative?
Time is relative to speed and gravity (time dilation), and so is space (length contraction). Light speed is constant for all observers, so time and space can’t be.
TIP: Time dilation and length contraction are a consequence of relativity, this means we’ll be pulling out everything in the physics 101 toolkit to explain them. With this in mind this page is written for laymen, and thus, we don’t use any mathematics in our explanations.
A Simple Overview of Time Dilation and Length Contraction
There are only a few universal constants in the physical universe (including the constant speed of light in a vacuum and the constant effect of gravitational mass). Everything else, (including time, space, and motion), is relative to a given frame of reference (coordinates in spacetime accounting for speed and gravity).
All observers will always record light speed in a vacuum (c) as 299,792,458 m/s regardless of their own speed (velocity) or gravity (proximity to mass).
When an object speeds up, or becomes more massive (if for example it approaches a massive planet or blackhole and is effected by its gravity), its clock runs more slowly relative to a slower moving and/or less massive observer, and it also appears to shrink relative to that observer.
The opposite is true for the faster moving and/or more massive observer, the clock of the slower and less massive objet moves more quickly relative to that observer’s frame of reference, and, in terms of size, the object appears to enlarge relative to the observer.
This is true for both speed and gravity, because in both cases a type of mass is being gained. The quicker object is getting closer to light speed, and thus it gains a type of mass called “relativistic mass“. The more massive object has increased its proximity to mass, and thus its gravitational mass has increased. Likewise, slowing down or moving away from the mass has the opposite effect, it sheds some relativistic mass.
Mass by any name is always the same general thing, the curving of spacetime around an object (so velocity and proximity to mass have the same effect, and generally any type of mass added or subtracted from a system will have the same effect).
Here we can note that direction and other types of velocity have unique, but related effects (which are illustrated in the more complex triples example below).
No matter how close to light speed an object is, it will always record light speed as the same, so time and space (not the speed of light) have to change from an observer’s frame of reference to account for this. This is what relativity means, it describes how things are relative to a frame instead of constant in all frames (like light speed).
This doesn’t imply that time and space aren’t real, or that motion doesn’t happen, it just implies that we can’t determine exact time, space, or motion from our frame of reference without accounting for relativity. Even though we can’t get accurate measurements of time or space that work for all observers from a single frame without accounting for relativity, we can find fundamental truths like the order of causality by measuring light and by applying a little relativity related math and physics (so, for example, the end result is we can build satellites that work despite the relative differences).
We explain this all in simple terms below, but first lets go over the long list of terms related to time dilation and length contraction. There are still a few more key bits of information that will help us to understand this all, so keep reading and watching the videos if you feel like you are still missing something.Einstein’s Theory Of Relativity Made “Easy”.
TIP: Understanding time dilation is a little like trying to understand the movement of the planets from the “frame of reference” of a telescope on Earth (the planets appear to move in odd patterns, but this is due to our perspective, what is actually happening is less complex… conceptually at least).
FACT: They set clocks fast on GPS satellites to offset time dilation. Speed makes time run slower, but this is offset by gravity which makes time run faster compared to Earth. If we didn’t account for relativity and time dilation, GPS would be off by about 5 kilometers a day.
Time Dilation and Length Contraction Cheat Sheet
The bullet-pointed list below contains an overview of all terms one needs to know to understand time dilation and the related length contraction.
- Time dilation is when time speeds up or slows down relative to an observer due to speed or gravity.
- Length contraction is when space shrinks or grows relative to an observer.
- General relativity is Einstein’s theory that gravity is mass curving spacetime.
- Special relativity is Einstein’s theory that speed increases relativistic mass (energy acting as mass and curving spacetime).
- Mass-Energy mass and energy can be considered as mass-energy. They don’t convert into each other, but they are conserved as each other. Mass-energy is a measurable property of the four forces and elementary particles. All matter is made from mass energy including light and everything in the universe.
- Light Speed is the constant speed of light (pure energy) in a true vacuum. Since the speed of light is constant, and both we and light are made of mass-energy, most everything else ends up being relative.
- The Constants are the only things in the universe that aren’t constant. There are a lot of constants, but only a few underlying constants. For the 101 version we can consider the important constants light speed (max speed of light) and the gravitational constant (the constant force between two bodies).
- Spacetime is 3 dimensions of space and 1 dimension of time. Together they are a constant.
- A Frame of Reference is coordinates in spacetime, this accounts for velocity and speed.
- Gravitational time dilation, is when time speeds up or slows down due to gravity. The more massive an object is relative to the observer, the slower the observer sees the object’s clock run.
- Gravitational length contraction, is when length shrinks or expands due to gravity.The more massive an object is relative to the observer, the smaller the observer sees the object get.
- Velocity time dilation is when time speeds up or slows down due to speed. The faster an object is moving relative to the observer, the slower the observer sees the objects clock run.
- Velocity length contraction is length contracting due to velocity. The faster an object goes relative to an observer, the more length appears to shrink. The slower an object goes, the more length appears to grow.Your twin flying in a rocket ship away from you through space is younger.
TIP: If there is a change in speed, direction, or both, then the object has a changing velocity and is said to be undergoing an acceleration. Acceleration is key in physics, but we are focusing on general concepts so we won’t harp on specifics here. The twin example at the bottom of the page goes into specifics on how direction affects dilation and contraction.
Time Dilation and Length Contraction Explained in More Detail
Time, space, and motion are relative to frame of reference (point of view), while light speed (the maximum, and only, speed of pure energy in a true vacuum) and the gravitational constant (the constant gravitational force between two bodies with mass, not earth’s local gravity) aren’t. This is because everything that exists in spacetime is made out of mass-energy.
Mass-energy is, in simple terms, light speed particles either moving in a single direction at light speed (energy), or not (mass). Given that everything is made out of the same star stuff, and that stuff is light moving at light speed (laymen for electromagnetic energy and the related forces), the rest of the universe has to be relative to that.
As Einstein noticed, “light speed is constant, so either time and space can’t be”. As it turns out, neither time and space are constant. The only real constant is the nature of light itself. The effects of this are time dilation and length contraction.
From the observer’s frame of reference (AKA point of view), objects that are moving faster, or are closer to mass, have time move slower and space contract. Meanwhile, to the observer, objects that are moving slower, or are further away from mass, have time move faster and space expand. (see a visual here).
This may seem strange, but remember, the only thing “actually” happening in the universe is light moving at light speed or not moving at light speed, relativity just explains the physics of light not moving at light speed and instead acting as mass. To what extent light is moving in a single direction at max velocity, or is not (mass curbing spacetime and acting as gravity), determines how other observers see time dilate and length contract relative to each other, from their point of view.
In other words, if we put together every single one of the theories related to relativity (including Maxwell, Einstein, Lorentz, and more), then the obvious conclusion is time and space are relative.
Gravitational and Velocity Time Dilation and Length Contraction in More Detail
When time slows or speeds up it’s called “time dilation”. There is gravitational time dilation (where time slows due to gravity) and velocity time dilation (where time slows down due to speed). At light speed or in a black hole, time appears to stop (at least from our frame of reference), while light speed is the maximum and only speed of the universe (so no information ever travels faster than light, and light can’t slow down unless impeded). 
The same thing happens with length, space contracts and expands due to gravity (gravitational length contraction) and speed (velocity length contraction). Space also ceases to exist in black holes and beyond the speed of light (at least from our frame of reference).This video explains time dilation AKA “why time is relative”.
The Speed of Causality and The Universal Constants: Mother Nature’s Measuring Stick
Even though time is relative to the observer based on things like velocity and mass, we can still (sort of) measure time as a constant by measuring the distance light travels (at its constant speed in a vacuum), the minimum distance light can travel (Planck Units), and the constant force between bodies (big G gravity, not earth’s local gravity little g).
These universal constants can be used to create “natural units” and “standard units (SI)”, like the light years or light seconds, based on the nature of light (AKA mass-energy, AKA electromagnetic energy). These constants can act as a sort of universal agreed on clock (although they technically measure length, so it’s more a natural ruler). Despite this universal measuring stick, time and space are truly relative to the observer’s frame of reference (not the world’s simplest concept).
More Concepts Related to Relativity, Time Dilation, and Length Contraction
Before we get to examples, here are some more explainer videos, facts, and clarifications regarding relativity, time dilation, length contraction, and the nature of mass-energy.One way to look at time dilation and light speed.
- FACT: All objects observe light traveling at a constant speed of 299,792,458 m/s regardless of the speed it is traveling or its proximity to mass. The effects of this are things like time dilation and length contraction (space and time, not light speed are relative).
- TIP: The speed of light is a measure of distance, not time. On the other side of the “ruler” is “Planck length”. That is the smallest distance between two “frames of reference”. When motion happens, particles jump from Planck frame to Planck frame in relative time (or exist in-between frames in a state of quantum super position). We can measure the distance of those jumps at “light speed”. Thus, the Planck length and light speed give us a measurement system for causality. That is as close as we get to a universal “clock”, and it’s really just a universal measuring stick (keep in mind that length contracts, so a ruler gets wonky just like a clock when considering relativity).
- FACT: Time, from our perspective, stands still at light speed and time stands still in black holes. So time, relative to a frame of reference in spacetime, can slow down, speed up, or stop altogether.
- How Do Gravity and Velocity Affect Space and Time? Gravitational mass and velocity both cause time to slow down (time dilation) and space to contract (length contraction). This allows all objects moving through spacetime to experience time and space normally for themselves in relation to the speed of light but causes all objects to experience time and space differently from each other.
- E(energy) =(is equivalent to) m(mass) times c2 (the constant speed of light). Energy can be measured as mass, and mass can be measured as energy. In simple terms, if an object is “still” it has potential energy; when it moves it trades potential energy for kinetic energy. Mass is a property of massless energy interacting with other massless energy. Mass-energy distorts spacetime.
- FACT: On some fundamental level, there is no actual thing called “mass”, mass is a measure of energy interacting with other energy (both mass and energy are simply “mass-energy”). This helps explain why mass and velocity both have similar effects.
- FACT: Like the concept of mass, gravity isn’t something you can hold in your hands. General relativity says “gravity a consequence of the curvature of spacetime caused by the uneven distribution of mass/energy”.
- What does it mean that the speed of light is constant? The speed of light is the maximum speed in the universe. No matter how fast an object is going or how much mass an object has, it will always record the speed of light as C (approximately 3.00×108 m/s). This can be thought of as the speed of causality, a sort of absolute time, in which everything else exists. We can measure this in light years or light seconds. From our perspective things lose mass at light speed and appear to stop, but from an outside frame of reference light moves at a constant speed (the rate at which things actually happen in spacetime).
- FACT: The closer you get to light speed, the more distance it seems as though there is between you and light speed. Time is relative; to move at light speed, you must trade all potential energy for kinetic energy. Don’t try this at home.
- What does it mean that spacetime is constant? Spacetime is a mathematical concept combining space and time, which all objects move through. Mass curves spacetime and “gravity” dilates time and contracts space. Meanwhile, acceleration causes time to slow down, and deceleration causes time to speed up. Spacetime is used to understand all the more complex concepts related to special relativity, time dilation, and length contraction. A good explanation of this concept may be found in the PBS series videos on this page, specifically the ones on spacetime and the illusion of gravity (PBS Space Time).
Examples of Time Dilation
Since time dilation and the concept that time is relative can be hard to grasp we have a few examples below to help you visualize time dilation (and length contraction).
Example 1 (Time Dilation Due to Speed)
Consider a 2-dimensional graph. On the Y-axis, we have space, on the X-axis time. An object can move in a straight line anywhere on the graph. If an object moves through space, it must sacrifice time and vice versa. Thus, objects moving through the graph measure time differently.
Spacetime is an absolute that all objects exist in and must move through. In order to move through spacetime, an object must either stand still (thus moving at the maximum speed of time) or sacrifice time to move through space more quickly. To someone observing an object moving through spacetime time will “dilate”. The observer standing still will experience time as passing more quickly and the object will experience time as passing more slowly.
Example 2 (Time Dilation Due to Speed)
We have 3 people, each with a clock that will tell time accurately. One stands on the ground and observes the other two who take off in jet planes: one fast jet plane and one slow jet plane. The jet planes fly off for a day (earth time) and then come back. While they are on their respective journeys each measures the speed of light relative to their speed and each finds that it is exactly the same (about 300,000,000 m/s). When they get back they compare clocks, 24 hours has passed for the person standing still, slightly less time has passed for the slower plane, and even less time has passed for the fastest plane. This is because time is relative to an object and the speed it is traveling through spacetime.
Example 3 (Gravitational Time Dilation)
We have 3 people, each with a clock that can tell time accurately. One person sits on earth, another person magically becomes a celestial body with a high gravitational mass, and another person warps out into space far from the gravitational mass. A day later (earth time) everyone comes back and compares clocks. The person on earth’s clock shows 24 hours have passed, the person with no gravity shows much more time has passed, the person with high gravity shows less time has passed.Here is a simple explainer of special relativity and the twin paradox. This will help you understand the twin paradox example below. We apply to the twin paradox to gravity as a triple paradox, but it’s the same gist.
Example 4 – The Twin Paradoxes: Example of Special Relativity, Length Contraction, and Time Dilation
You have two identical twins (making the three of you identical triplets). One on a planet with high gravitational mass, and one on a spaceship traveling away from you at a high speed. From any single twin’s point of reference, all other twins appear to age at different rates (time dilation) and appear to be of different sizes (length contraction). Despite this, all experience time and space normally in their own frame of reference.
The twin on the gravity-heavy planet sees time moving very fast when looking at you from their frame and you see them as moving slowly. By the time they get back to earth they are younger than you.
Speed is much trickier than gravity. The twin on the ship sees you moving slow and you see them moving slow as they fly away. When twin turns around and starts heading back to earth, his or her frame of reference changes and they see you appearing to speed up (but you seem them still slow as your frame didn’t change). This results in the moving twin being younger than you when they get back to earth.
In both cases, mass-energy (velocity and gravitational mass) slowed time and thus slowed down the twins aging process relative to you. This is true even though everyone involved personally experienced time normally. Length contraction also happened for each twin the same way time dilation did.
You and the twins didn’t experience events happening at the same time (relativity of simultaneity), you didn’t agree on who was moving (relative motion), you didn’t agree on the amount of time that passed (time dilation), and you didn’t agree on sizes (length contraction). Despite the differences, you all observed light traveling at the same speed at all times. This is because time and space are relative, but light speed and spacetime are not.
FACT: It’s not only theoretically possible to time travel forward, but you actually do it in small amounts every time you accelerate.
Experience of Time Dilation For the Non-Observer
For the observer in a time dilation experiment, the other object appears to have had time pass slower or faster. However, for the object experiencing time dilation, time acts normally. Time appears constant to the object experiencing it, but it is observed as relative when compared to the time that other objects are experiencing.
So Time is Not Absolute then?
Despite Isaac Newton’s theory of “absolute time” and our everyday experience, Einstein showed time moves more slowly for objects moving quickly and moves faster for objects near a concentration of mass. This makes time relative to an object’s speed and proximity to mass.
A number of experiments prove Einstein’s theory, or rather they prove that the only way all the experiments could work is if time and space, and not the speed of light or spacetime, are relative.
Time can be shown to be relative using Albert Einstein’s theories of special relativity and general relativity as well as corresponding experiments. If we zoom out of our perspective we can see that the speed of light is the speed of the universe and we realize time is better quantified in standard units based on the constant speed of light in a vacuum.
There is no one simple way to grasp the concept of time dilation or length contraction, but understanding that the speed of light and spacetime are absolute allows us to infer that time is relative to velocity and mass. Once we understand how relative motion, time, and space all work we can start to grasp why it all acts so strange from our perspective.
To sum this all up:
Everything is made of “massless energy particles”, massless energy has a property called “mass” when it interacts with other massless energy, light speed (in a vacuum) is the max speed of the universe, and all objects will always measure light speed the same, time dilation is a byproduct of these truths. Time dilation is really happening, but it only seems strange from our perspective. When we measure causal relationships in the universe with standard light units, and see objects experiencing time dilation from that perspective, everything becomes much clearer (sort of).