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The Heisenberg Uncertainty Principle
is one of a handful of ideas

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from quantum physics to 
expand into general pop culture.

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It says that you can never simultaneously
know the exact position

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and the exact speed of an object
and shows up as a metaphor in everything

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from literary criticism
to sports commentary.

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Uncertainty is often explained as a result
of measurement,

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that the act of measuring an object's
position changes its speed, or vice versa.

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The real origin is much deeper
and more amazing.

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The Uncertainty Principle exists
because everything in the universe

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behaves like both a particle and a wave
at the same time.

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In quantum mechanics, the exact position
and exact speed of an object

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have no meaning.

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To understand this,

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we need to think about what it means
to behave like a particle or a wave.

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Particles, by definition, exist in 
a single place at any instant in time.

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We can represent this by a graph
showing the probability of finding

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the object at a particular place,
which looks like a spike,

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100% at one specific position,
and zero everywhere else.

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Waves, on the other hand,
are disturbances spread out in space,

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like ripples covering 
the surface of a pond.

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We can clearly identify features
of the wave pattern as a whole,

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most importantly, its wavelength,

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which is the distance between two 
neighboring peaks,

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or two neighboring valleys.

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But we can't assign it a single position.

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It has a good probability of 
being in lots of different places.

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Wavelength is essential for
quantum physics

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because an object's wavelength
is related to its momentum,

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mass times velocity.

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A fast-moving object has lots of momentum,

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which corresponds to 
a very short wavelength.

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A heavy object has lots of momentum
even if it's not moving very fast,

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which again means a very short wavelength.

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This is why we don't notice
the wave nature of everyday objects.

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If you toss a baseball up in the air,

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its wavelength is a billionth of a 
trillionth of a trillionth of a meter,

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far too tiny to ever detect.

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Small things, 
like atoms or electrons though,

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can have wavelengths big enough
to measure in physics experiments.

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So, if we have a pure wave, 
we can measure its wavelength,

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and thus its momentum,
but it has no position.

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We can know a particles position
very well,

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but it doesn't have a wavelength,
so we don't know its momentum.

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To get a particle with both position
and momentum,

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we need to mix the two pictures

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to make a graph that has waves,
but only in a small area.

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How can we do this?


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By combining waves 
with different wavelengths,

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which means giving our quantum object some
possibility of having different momenta.

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When we add two waves, 
we find that there are places

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where the peaks line up,
making a bigger wave,

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and other places where the peaks of one
fill in the valleys of the other.

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The result has regions where
we see waves

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separated by regions of nothing at all.

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If we add a third wave,

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the regions where the waves cancel out
get bigger,

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a fourth and they get bigger still,
with the wavier regions becoming narrower.

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If we keep adding waves,
we can make a wave packet

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with a clear wavelength
in one small region.

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That's a quantum object with both
wave and particle nature,

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but to accomplish this,
we had to lose certainty

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about both position and momentum.

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The positions isn't restricted 
to a single point.

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There's a good probability
of finding it within some range

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of the center of the wave packet,

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and we made the wave packet
by adding lots of waves,

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which means there's 
some probability of finding it

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with the momentum corresponding
to any one of those.

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Both position and momentum
are now uncertain,

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and the uncertainties are connected.

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If you want to reduce 
the position uncertainty

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by making a smaller wave packet,
you need to add more waves,

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which means a bigger momentum uncertainty.

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If you want to know the momentum better,
you need a bigger wave packet,

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which means a bigger position uncertainty.

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That's the Heisenberg Uncertainty Principle,

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first stated by German physicist
Werner Heisenberg back in 1927.

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This uncertainty isn't a matter
of measuring well or badly,

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but an inevitable result
of combining particle and wave nature.

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The Uncertainty Principle isn't just 
a practical limit on measurment.

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It's a limit on what properties 
an object can have,

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built into the fundamental structure
of the universe itself.