Again?
I feel like we've done this before.
Does anybody else notice this?
How do I break this time loop?
You know that feeling of deja vu,
when you just know
you've seen this happen before?
Now imagine if your entire day
is a big deja vu moment.
And tomorrow.
And then the day after tomorrow.
And nobody seems to notice it but you.
I understand you've got a lot questions.
But before we start,
I'd like to introduce you
to some mind-blowing science.
As weird science theories often do,
this one starts with Albert Einstein's
theory of general relativity.
Basically, the idea is that the three dimensions of space,
and a dimension of time,
are joined together in a continuum called space-time.
Gravity can bend this space-time,
which, theoretically, makes time travel possible.
That is, if you have access to a big enough black hole,
or to a time machine like a Tipler cylinder.
We included the instructions on how
to build one in another video,
you might want to check that out.
Without getting into too much detail,
a time machine like a Tipler cylinder
could create a closed timelike curve.
In this system, a particle
could go back in time to where it started.
We don't understand quantum physics enough
to know if these closed timelike curves
could or couldn't exist.
But for the sake of this scenario,
we'll assume they are possible,
and are a result of space-time folding upon itself.
For one thing, you could
travel back into yesterday,
and start your day all over again.
In that case, reliving the same day again
wouldn't be surprising to you.
You'd be in control of when to go back to the future.
And if you die during one of those travels,
you wouldn't wake up in your bed the morning before.
That would be the real end for you.
But you wouldn't be restricted
to only going back to yesterday.
You could travel back
to the time when you were born.
Take a heck of a trip down memory lane.
No matter how far in the past you went,
you wouldn't change it in any way.
You wouldn't create any paradoxes,
like killing your yesterday's self,
or whatever else you might come up with.
Ironically, what's preventing you
from creating any time paradoxes
is called a Predestination Paradox.
In a closed timelike curve,
when you travel to the past,
you've always been a part of that past.
Your presence wouldn't change a thing.
At the same time, if you find yourself reliving yesterday,
that could mean that somebody put you in a time machine,
and sent you one day back.
In that case, you should
just go on with your day till it's over,
and the next day you'd be living in tomorrow.
If you want to geek out on time traveling,
here's some food for thought.
Maybe closed timelike curves aren't closed at all?
What if, instead, an alternate timeline would appear
appear every time you traveled to the past?
Then, you'd continue living in an alternate universe,
and you wouldn't be able to go back
to your original one,
unless you open some kind of
portal to a parallel world.
But that's a story for another WHAT IF.
And if you want to learn more about time loops
and closed timelike curves,
check out What If Discussed episode,
where we welcomed Danny Rubin,
the screenwriter of the ultimate
time loop movie, "Groundhog Day."