eli5 : can somebody explain the idea of “spacetime” for me?


I’ve been studying pharmacy/medical sciences for all my adult life when i stumbled upon “a brief history of time” by stephen hawking
Anyway,for a person who hasn’t studied physics much..how does “spacetime” exist? I mean einstein abandoned the idea of absolute time
But what is spacetime? And how does time exist as a physical dimension? I mean it’s just something we measure as it passes
How does all this exist?

In: 3

Time is “just something we measure as it passes” in the same way that distance is “just something that we measure as we pass it”.

If you want to uniquely identify an event in the universe you need to say where it happened (space) and when it happened (time). You need four values…three spatial dimensions and one time. Without all of that, you can’t tell if two events are the same thing (same time and position) or different things (same time/different positions or same position/different times). Our universe is four-dimensional in the sense that you need (at least) four unique values to specify a unique event.

It turns out that a *lot* of physics gets a lot cleaner, including some things we observe experimentally but couldn’t explain before, if you treat the universe as a four dimensional thing where three dimensions are space-like (we can move freely in either direction along them) and one as time-like (we can only move in one direction along it).

Have you heard before that, the faster something moves through space, the less time passes for it? Because of this, Einstein figured into special relativity that space and time are joined together in what is called a continuum. Without time, nothing in space would be able to move, as something other than the three spatial dimensions needs to progress for that to be possible. A position in three dimensional space is referred to as a position. When time is added, it is referred to as an event. Moving something is a series of events.

Think of the movement of a ball, bouncing all around a closed box. How many dimensions do you need to be able to fully describe its motion inside the box? And what even *is* a dimension?

We can think of a dimension as an ‘index’ of a quantity. For example; let’s say something like ‘I have ten’. Just ‘ten’. The question immediately then becomes ‘okay… ten *what?* That *what* can be considered the dimension of the quantity. Without dimensions, quantities are just disembodied numbers with no meaning.

Okay, so for normal Euclidean space inside this box, I need a dimension that quantifies up and down position, I need a dimension for left and right position, and I need a dimension for forward and back position. With those three dimensions, I can fully describe the ball’s location inside the box. But remember; the ball is *moving.*

An stationary object’s position is completely time-independent. A static position doesn’t ‘occur over ten seconds.’ It just *is* a position. You don’t need to measure the passage of time to describe a thing’s location. But you *do* need to track time for **movement**. Movement is a *transient* phenomenon – that is to say, it occurs over an amount of time. The balls position inside the box is dependent on time, because it is moving; one second, it’s somewhere, another second, it’s somewhere else. How is a ball meant to bounce without the passage of time?

In this manner, time is the fourth dimension we use to fully quantify the behaviour of the ball. We have three *spatial* dimensions that fully describe its position, and we have one *temporal* dimension that describes how that position changes with the flow of time.

In the universe, all things move, and time never stops flowing forward. All things are moving through space, and all things are moving through time as well. For this reason, we consider space and time to be an inseparable entity; four-dimensional spacetime. We need these four dimensions because they are the number required to fully describe an object’s position, at any point in time.

I picture a sort of subway tunnel, with all the universe as a traincar. We’re all doing our thing in the car, and the car is traveling through the subway at a consistent rate (time). So we have the usual 3 dimensions we know about, and we have the fourth which, so far as we know or can discern, only goes one speed and one direction

We’ve confirmed via experiment a couple different phenomena with time. Gravity causes clocks to run slower. If you could stand on the suface of Jupiter your watch would tick slower than standing on the surface of Earth. We also know that no matter how fast you are currently moving light always appears to move at the speed of light. Meaning if you were flying through space at like .25x the speed of light and a light beam passes you it would _appear to you_ as if the light were moving at the speed of light and not .75% the speed of light. In turn this means that for you moving at that speed your experience of time slows and your clock would be running slower than someone standing still.

This tells you a couple things. Mass and energy are fundamentally related and space and time are fundamentally related because mass collected into a defined space distorts the space AND time around it and mass moving through space with high energy slows time. So spacetime is what appears to be the same _thing_ affected by mass and energy.