Are Space and Time An Illusion?

0:46

Spacetime refers to whichever external reality underlies our collective experiences of the space between things and the time between events.

0:55

Why can't space and time just be a reality?
Why add spacetime as an extra concept?
Here's why.
Suppose two observers are moving relative to each other, and particles count as observers.
Fact-- those observers don't agree about how much time passes between events.
Fact-- they don't fully agree on how much space there is between things at any given moment.
Fact-- they don't even agree on the chronological order of all events.
And yet, each observer measures things properly and is entirely consistent, which means neither of them is wrong.

2:35

A good starting point for objective reality is universal agreement. And as luck would have it, all observers do agree about this thing.
It's called the spacetime interval, or spacetime separation between two events:

s.i. = (Δx)²-(cΔt)²

3:34

It appears that the spacetime interval between events A and B tells you whether A can influence B.
In other words, even though we can't agree about past, present, future, time, or distance, we all appear to agree about causality.

3:53

Normally we think that time is responsible for causality. But actually it's the other way around.