we all know if Marty travels back into
the past and messes with it he makes the
Biff timeline it's 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
forward into the future from the past
through the president but physicists are
playing with the theory of retro
causality which if real is the idea that
the future can influence the past it's
like if I spill coffee on my shirt
tomorrow but I see the coffee stain on
my shirt today
it's an idea that's 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 retro causality didn't really fit
until recently some physicists think
accepting retro causality can answer a
lot of unanswered questions about
quantum mechanics and deepen our
understanding of spooky action at a
distance spooky action is what Einstein
called quantum entanglement back in the
1930s 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're measured by us I mean literally
that's like yeah physicists yeah 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 and even though
we didn't actually know what particle B
was before and we didn't observe the two
particles exchanged any signals when
they were measured and 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 retro causality the way some
physicists see it if time symmetry
exists that's the idea that time looks
the same going forwards and backwards
without any issues in physics then retro
causality would allow particle B to go
backwards in time when it does so it
takes this spin to the moment it was
created and therefore affected its
entangled partner but 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 that entropy must increase with
time making time asymmetrical and retro
causality well it would break that law
yeah but no one has proved that time
must be symmetrical and until that day
happens retrocausality isn't going
anywhere so we're not time-traveling
anytime soon the laws of thermodynamics
are against us and retro causality is
difficult to test because we can't
witness it in our everyday lives people
are working on theories around quantum
retro causality but it's on the bleeding
edge of the discipline it's a bit out
there they believe if we can find
evidence of retro causality 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 gonna be a
mind-melting explanation that I'm 100%
ready just to render my brain - totally
absolutely future me would come back to
this exact moment and tell us all about
it everyone you probably already know
this but this is Diana from physics girl
on YouTube
hi Diana joined us for this episode
because she knows a lot more about
quantum mechanics than I do okay just a
slight allotment that's like a bit a lot
so where can people find you if they
want to learn more stuff about physics
Oh youtube.com slash physics girl or
just google physics girl it pops up
definitely also Twitter and wherever
else yeah mm-hmm thanks for coming in
yeah thanks I mean 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 that there are an
infinite number of universes with an
infinite number of use and Isis but only
one where we beat Thanos thanks for
watching
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