I'm at a wedding at the moment
and I met the groom's grandfather
who used to be a nuclear engineer
and who gave me an amazing explanation
of what time is and how to picture it.
He agreed to tell it again on camera,
let me introduce him to you.
The most gripping experiment I know when it comes to time
is Saint Augustine's.
He said, "If you ask me whether I know what time is,
I do.
But if you ask me to explain what time is,
I don't know anymore."
And indeed, just try to explain what time is.
Nobody can.
We can't define it, we can't give an explanation
of what time is.
Time is just there.
On the other hand,
you can also wonder whether time actually exists.
We can't be so sure.
All you need to do
is consider it as a dimension
and that's when you don't need it anymore.
So time is just a trick we found
to explain movement.
We live in a three-dimensional world.
There's length, width and height.
If I move my pencil from right to left,
I need to use time in order to explain that movement.
My pencil is over here on instant t,
then on instant t+ 1s it's over there,
and on instant t+ 2s it's further away.
Why do I need time to explain this?
Because I don't know of a fourth dimension.
In fact, a four-dimensional universe isn't something we can even imagine.
Nobody can picture it,
it surely goes beyond the capacities of the human brain.
We can't even imagine
how to go from a three-dimensional universe to a four-dimensional universe.
We're not able to picture a way to not need time.
And yet we may not.
Just imagine going from a two-dimensional universe
to a three-dimensional universe.
So let's imagine some kind of animal
which lives in a two-dimensional universe.
His universe is merely a flat surface.
Take this pencil.
Thanks to our three dimensions, we can see the object for all that it is.
But the two-dimensional animal can't.
He misses one dimension.
So what does he do?
That third dimension he misses
he replaces with time.
What is this pencil for that being?
It's the amount of time it'll take him to explore it.
So how does it work?
Imagine the flat surface universe.
What will the animal see?
First, he will see a metallic cone appear in his universe.
The cone grows bigger,
then suddenly it'll turn black and hexagonal
or maybe octogonal.
That's going to last for some time
until it stops,
until it turns into a metallic part again
and then disappears.
In other words,
birth,
evolution or life,
and death.
Just like we see human beings
that are born,
then grow up, evolve,
and then die.
So we can imagine that a being that would live in a four-dimensional world
could see all of what we are
all at once.
Just like we can see all that this pencil is
which the two-dimensional animal cannot perceive
and has to use time
to explore the entirety of the object.
That's it.
Many thanks to Bernard Jampsin (and congratulations to the newlyweds!)