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Consider throwing a ball
straight into the air.

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Can you predict the motion
of the ball after it leaves your hand?

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Sure, that's easy.

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The ball will move upward
until it gets to some highest point,

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then it will come back down
and land in your hand again.

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Of course that's what happens,


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and you know this because you have
witnessed events like this countless times.

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You've been observing the physics
of everyday phenomena your entire life.

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But suppose we explore a question
about the physics of atoms

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like, what does the motion of an electron

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around the nucleus of a 
hydrogen atom look like?

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Could we answer that question based on
our experience with everyday physics?

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Definietly not. Why?

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Because the physics that governs the
behavior of systems at such small scales

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is much different than the physics
of the macroscopic objects

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you see around you all the time.

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The everyday world you know and love

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behaves according to the laws 
of classical mechanics.

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But systems on the scale of atoms

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behave according to the laws 
of quantum mechanics.

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This quantum world turns out to be
a very strange place.

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An illustration of quantum strangeness
is given by a famous thought experiment:

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Shrodinger's Cat.

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A physicist, who doesn't particularly 
like cats, puts a cat in a box,

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along with a bomb that has a 50% chance
of blowing up after the lid is closed.

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Until we reopen the lid,
there is no way of knowing

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whether the bomb exploded or not,

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and thus, no way of knowing
if the cat is alive or dead.

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In quantum physics,
we could say that before our observation

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the cat was in a superposition state.

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It was neither alive nor dead but
rather in a mixture of both possibilities,

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with a 50% chance for each.

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The same sort of thing happens
to physical systems at quantum scales,

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like an electron orbiting
in a hydrogen atom.

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The electron isn't really orbiting at all.

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It's sort of everywhere in space,
all at once,

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with more of a probability of being
at some places than others,

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and it's only after 
we measure its position

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that we can pinpoint where it is
at that moment.

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A lot like how we didn't know
whether that cat was alive or dead

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until we opened the box.

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This brings us to the strange
and beautiful phenomenon

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of quantum entanglement.

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Suppose that instead of one cat in a box,
we have two cats in two different boxes.

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If we repeat the Schrodinger's Cat
experiment with this pair of cats,

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the outcome of the experiment
can be one of four possibilites.

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Either both cats will be alive,
or both will be dead,

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or one will be alive 
and the other dead, or vic versa.

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The system of both cats
is again in a superposition state,

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with each outcome having a 25% chance
rather than 50%.

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But here's the cool thing.

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Quantum mechanics tells us
it's possible to erase

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the both cats alive and both cats dead
outcomes from the superposition state.

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In other words,
there can be a two cat system

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such that the outcome will always be
one cat alive and the other cat dead.

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The technical term for this is that the
states of the cats are entangled.

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But there's something truly mindblowing
about quantum entanglement.

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If you prepare the system of two cats
in boxes in this entangled state,

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then move the boxes to opposite 
ends of the universe,

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the outcome of the experiment
will still always be the same.

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One cat will always come out alive,
and the other cat will always end up dead,

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even though which particular cat
lives or dies is completely undetermined

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before we measure the outcome.

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How is this possible?

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How is it that the states of cats
on opposite sides of the universe

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can be entangled in this way?

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They're too far away to communicate
with each other in time,

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so how do the two bombs always 
conspire such that

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one blows up and the other doesn't?

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You might be thinking,

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"This is just some theoretical
mumbo jumbo.

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This sort of thing can't happen
in the real world".

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But it turns out that quantum entanglement


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has been confirmed in 
real world lab experiments.

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Two subatomic particles entangled
in a superposition state,

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where if one spins one way
then the other must spin the other way,

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will do just that,
even when there's no way


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for information to pass
from one particle to the other

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indicating which way to spin
to obey the rules of entanglement.

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It's not surprising then that
entanglement is at the core

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of quantum information science,

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a growing field studying how to use
the laws of the strange quantum world

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in our macroscopic world,

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like in quantum cryptography, so spies
can send secure messages to each other,

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or quantum computing,
for cracking secret codes.

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Everyday physics may start to look
a bit more like the strange quantum world.

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Quantum teleportation 
may even progress so far,

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that one day your cat will 
escape to a safer galaxy,

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where there are no physicists,
and no boxes.