We all know, if Marty travels back into the
past, and messes with it, he makes the Biff
timeline.
Casual.
We all get it.
Moral of the story: If you time travel and
mess with anything in the past, you automatically
mess with your future, but if Marty was a
quantum particle?
Maybe not so much.
From what we commonly understand, time has
One Direction and that direction is forward;
past, present, future.
But physicists are playing with the theory
of “retrocausality”, which if real, is
the idea that the future can influence the
past.
That’s like if I spill coffee on my shirt
TOMORROW, but I will see the coffee stain
on my shirt today.
It’s kooky and is an idea that has been
around for a long time, but it never caught
on, because why would it??
We live on a cause-and-effect timeline and
the theory of retrocausality didn’t really
fit, until recently.
Some physicists think accepting retrocausality
can answer a lot of unanswered questions in
quantum mechanics, and deepen our understanding
of “spooky action at a distance” .
“Spooky action” is what Einstein called
quantum entanglement back in the 1930’s.
He thought it was crazy that when two particles
are created at the same time, they can be
linked together, or entangled, even if they
are then separated light-years apart.
And according to quantum theory, elementary
particles like photons that are entangled,
have no definite state until they are measured
by us.
By measuring the spin of, say Particle A,
you automatically know the value of its entangled
partner, Particle B.
This is because Particle B is the opposite
of whatever Particle A was, even though we
didn’t actually know what Particle B was
before...and didn’t observe the two particles
exchange any signals when they were measured.
It’s spooky.
But some theorists are thinking that instead
of this just being a spooky occurrence, the
exchange of A and B’s spins are caused by
retrocausality.
The way some physicists see it, if time-symmetry
exists….
(the idea that time looks the same going backward
or forward without any issue in physics)
… then retrocausality would allow Particle
B to go backwards in time.
When it does so, it takes the spin to the
moment it was created, and therefore affected
its entangled partner.
Right!?
I know.
Buuut how time flows is still up for debate.
The Second Law of Thermodynamics is thought
to drive time as we experience it -- the law
says says entropy must increase, making time
A-symmetrical.
And retrocausality would break that Law.
Buuut no one has proved time must be symmetrical,
or able to move either forward or backward,
and until that day happens retrocausality
isn’t going
So, we’re not time-traveling anytime soon,
the Laws of Thermodynamics are against us
and retrocausality is difficult to test because
we can’t witness it in our everyday lives.
People are working on theories around quantum
retrocausality, but it’s on the bleeding
edge of the discipline... it’s pretty out
there.
They believe if we can find evidence of retrocausality
in quantum states that would be huge.
If it does exist, we might be able to move
particles through a dimension of time…
Which would be like time travel.
And the first step to figuring it out, is
determining if it even exists in the first
place.
If physicists do figure it out, you know it’s
going to be a mind-melting explanation that
I am 100% ready to surrender my brain to.
Future me will hopefully come back to this
exact moment and tell us 
all about it.
Future you chimed in and wants to let you
know you should subscribe for more videos
because you like them.
One last thing: Some cosmologists say there
are an infinite number of universes, with
an infinite number of yous, and us….But
only one where we beat Thanos.
Thanks for watching
