the space time continuum as used in physics and/or science fiction.
For context, I have no background whatsoever in science
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Not sure I’ve ever heard the “continuum” part in any real science context, but spacetime is the word that refers to both space and time.
For most purposes, space and time are two different things. I can’t, for instance, rotate a stick so that it points forward in time. That just doesn’t make sense.
Normally, everything is moving forwards in time together. Myself, my cat, and my car are all marching forward through time side by side – my watch and my car’s clock are in agreement. *However*, by moving quickly through space or by sitting in a deep gravity well, our movement through time is partially replaced by our movement through space. The astronauts on the ISS experience time a little bit slower, because they are moving through space a little bit faster than I am. In this sense, time and space are somewhat interchangeable, but still not exactly the same thing. This is spacetime.
A “continuum” is a thing that is “continuous”, meaning that each part is right next to another part, and broadly similar to it.
The normal numbers are a continuum. Give me any number and I can find another number as close to it as I like. And those numbers behave in similar ways.
In some physics models “space” is a continuum – each bit of space is right next to more space, and space generally looks the same wherever you are. Since the early 20th century physics has tended to combine space and time into a single continuum, modelling time as something similar to space, and overlapping it in some situations (in General Relativity one of the ways of understanding why things “fall” because their local “forwards through time” direction overlaps the wider “down” direction).
The term “continuum” isn’t used that much in physics to refer to spacetime (although it does get used in some other areas, like continuum mechanics, which is the study of how objects or systems deform – when you treat them as a single continuous thing rather than being made up of individual particles); generally rather than talking about the “spacetime continuum” we’d just talk about “spacetime.” It is one of those terms that gets used a lot in science fiction, though, as it sounds fancy.
Basically it means everything. All of space and all of time.
Spacetime is a mathematical construct. It is used in the theory of General Relativity to unify space and time into a single object, because how an object is moving affects how another observer will measure its clock, and how it will measure distances.
The mathematical object is related to the mathematics of geometry and curved surfaces.
It is not known if it is a real thing.
Imagine a teeter-totter.
On one end of the teeter-totter is a ruler, representing the three dimensions of space (forward / backward, up / down, side / side). On the other end of the teeter-totter is a clock, representing the one dimension of time (past to future). The clock and the ruler share something in common with each other – motion. It should be obvious how space involves motion, you experience it every single day. But you also experience motion with time every single day too. Right now, as you read this, even if you’re sitting in your chair you’re still moving from past to future, moving away from yesterday and headed towards tomorrow. In this analogy our teeter-totter represents motion – the thing that both our clock and our ruler share in common.
Now, like any teeter-totter, the more motion we put on one side the less balanced it becomes. The more motion we give to space, like driving in our car, the less motion we give to time. Conversely the more motion we give to time the less motion we give to space, and so the teeter totter becomes imbalanced in the opposite direction.
Makes sense so far? Ok, great. Let’s keep going.
Way back when this really smart dude that went by the name of Albert Einstein had a realization. Recent experiments had shown evidence that the speed of light is invariant for all inertial observers, which is fancy physics talk to say that everyone always observes the speed of light moving exactly [this fast,](https://en.m.wikipedia.org/wiki/Speed_of_light) no matter how fast they themselves are moving. Take a second to appreciate what a true mind fuck this really is. No matter how fast you’re moving you will always measure the speed of light to be exactly the same as if you were standing still. If you were in a car and turned on your headlights the speed of the head beam would rush away from you the exact same no matter how fast the car was moving – if the only thing you could see from your car was the headlight it would be impossible for you to know whether you were racing down the highway or sitting in your driveway. Einstein realized the way you’d see the beam of light move from both perspectives would be **identical.**
In teeter-totter speak what this means is that no matter how fast you’re moving you never see the teeter-totter shift. Think about it. If the teeter-totter starts out level when there isn’t any motion, and you measure the speed of light to be [this speed](https://en.m.wikipedia.org/wiki/Speed_of_light) then the teeter-totter should remain level so long as you continue to measure the speed of light going [this speed](https://en.m.wikipedia.org/wiki/Speed_of_light). So no matter how fast you’re moving you never see the teeter-totter shift.
This presented a bit of a pickle for ol’Einstein. You see… if you measure the speed of light to be [this number](https://en.m.wikipedia.org/wiki/Speed_of_light) and I measure the speed of light to be [this number](https://en.m.wikipedia.org/wiki/Speed_of_light) but one of us is moving and the other isn’t then the teeter-totter *must* shift. If speed is distance (space) over time and you can’t change the value for speed then you have to be able to change the value for distance (space) or time. In other words for the speed of light to be constant the teeter’s gonna have to totter.
Here is where Einstein had his major revelation that completely changed the way we understand our universe. What he realized is that **there isn’t just one teeter-totter**. He realized that **everyone and everything** has a teeter-totter. What he realized is that *from your perspective* your teeter-totter always remains in the same position, because remember you always measure the speed of light going [this fast](https://en.m.wikipedia.org/wiki/Speed_of_light). What changes when we add motion is how you observe everyone else’s teeter-totters. From your perspective the more motion between you and everyone else the more their teeter-totters become increasingly unbalanced. What this means is from your perspective their clock measures a different time and their ruler measures a different distance because their clock and their ruler is in a different state of balance then your clock and your ruler. Remember though, from their perspective their teeter-totter is the one that is always balanced and so it’s your teeter-totter that is imbalanced *to them*.
That realization led Einstein to the understanding that there is no such thing as absolute time or even absolute space. If you have a banana and your ruler measures that banana to be 7 inches long and your clock measures that it takes you 1 minute to eat that banana then those measurements are according to your ruler and your clock from your teeter-totter. The ruler on my teeter-totter might measure that same banana to be 5 inches long and the time it takes you to eat it to be 2 minutes. From my perspective my teeter-totter is the one that is balanced (because remember I measure the speed of light going [this fast](https://en.m.wikipedia.org/wiki/Speed_of_light)) so my measurements are equally as valid and correct as yours. There is no such thing as a universal truth to how big the banana is or how long it takes you eat it, all that matters is whose teeter totter we’re talking about when we ask the question.
It’s the fact that you can have two different perspective and both be equally correct that forms the basis of relativity and what Einstein realized is that time and space themselves are relative.
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