the number of shows and movies featuring branching  timelines and multiverses have been multiplying,  

and you may be low-key wondering just how  much of the fiction resembles reality. 

Scientists have been asking these same questions  for decades because from what we've seen,  

the future is far from certain. The idea of  multiple worlds is compelling because it solves a  

thorny issue that arises from quantum mechanics. Classical physics is typically thought to be  

deterministic: if you know the speed and position  of something like a ball flying through the air,  

then you can predict where  the ball is going to land. 

But in the quantum world,  things don't work like that.  

It is impossible to know the exact speed  and position of a particle like an electron,  

and so it is impossible to say for certain  what it will do, only the probabilities. 

The mathematical expression of all known  possibilities of a particle's location  

and characteristics is called its wave  function. Bizarrely, in the quantum  

realm particles appear to exist in multiple  states simultaneously, until something causes  

only one outcome to result, what's known in  science-y talk as “collapsing the wave function”.

The mechanism that causes wave functions  to collapse is still debated, but one of  

the most widely accepted is the Copenhagen  Interpretation. It states wave functions  

collapse when they are observed, measured, or  interact with the classical world in some way. 

It sounds bizarre but that's a conclusion the  evidence points to. It's beyond the scope of this  

video but if you want to know more about it, check  out this video on the double slit experiment here.

The Copenhagen interpretation comes with its own  issues, which physicist Edwin Shrodinger tried  

to point out when he devised a thought experiment  where a cat in a box could be both alive and dead  

at the same time so long as no one looked inside. But what if we're looking at this the wrong way,  

and the wave functions don't actually collapse? What if instead all possibilities continue  

to exist and our universe simply branches  off into separate worlds for each outcome?  

A mind-boggling number of worlds  could be constantly branching off for every  

single possible outcome in this vast universe. That was the idea put forward by Hugh Everett  

in 1957. It's an intriguing thought  but if the worlds are truly split,  

then there's no way to ever directly observe  them, let alone open a doorway between them. 

On the one hand, that's great news if you're  worried about some some  

parallel-world- hopping conqueror 

suddenly showing up. On the other hand, it's  frustrating if you're a scientist or if you have  

aspirations of inter-universe conquest… or both.

Of course, the many-worlds interpretation just  

creates its own set of questions, fittingly.  Like, what actually causes worlds to split?  

One possible mechanism is decoherence where  quantum states gradually unravel as a result  

of their interactions with their environment.  If that's the case, then worlds don't have hard  

splits, more like gradual separations until  they reach a point when they can no longer  

affect each other and are truly distinct. There are more issues with the many-worlds  

interpretation, some that are more in the  realm of philosophy than physics. Like,  

if worlds are constantly branching off creating  countless slightly different versions of you,  

then what does it mean to be you? Some versions  of you should be incredibly similar while a huge  

number of others are profoundly different. Which one is the real one? Some physicists  

might say that no single one is, but the real  you is the sum total of all versions of yourself.  

And other physicists might say that the  whole idea of many worlds is hogwash.  

The debate rages on and because there's no proof  

yet for one argument or the other, it could  very well continue for a long time to come. 

One thing I will confidently predict is that due  to its intriguing nature and the possibilities  

it opens up, the many-worlds interpretation of  quantum mechanics is definitely going to be a  

staple of science fiction until the end of time.

While we're still searching for branching worlds,

another universe may have bumped into ours.

Check out this video on that right here.

So, do you buy into the many worlds idea, and if so,

what do you think about all the other you's that could be running around

out there. Let us know in the comments,  

be sure to subscribe, and I or some version  of myself will see you next time on Seeker.