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The Heisenberg Uncertainty Principle[br]is one of a handful of ideas

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from quantum physics to [br]expand into general pop culture.

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It says that you can never simultaneously[br]know the exact position

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

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from literary criticism[br]to sports commentary.

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Uncertainty is often explained as a result[br]of measurement,

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

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The real origin is much deeper[br]and more amazing.

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The Uncertainty Principle exists[br]because everything in the universe

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

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In quantum mechanics, the exact position[br]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[br]to behave like a particle or a wave.

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

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

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the object at a particular place,[br]which looks like a spike,

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

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

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like ripples covering [br]the surface of a pond.

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We can clearly identify features[br]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 [br]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 [br]being in lots of different places.

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Wavelength is essential for[br]quantum physics

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because an object's wavelength[br]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 [br]a very short wavelength.

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A heavy object has lots of momentum[br]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[br]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 [br]trillionth of a trillionth of a meter,

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far too tiny to ever detect.

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Small things, [br]like atoms or electrons though,

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can have wavelengths big enough[br]to measure in physics experiments.

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

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and thus its momentum,[br]but it has no position.

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We can know a particles position[br]very well,

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

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To get a particle with both position[br]and momentum,

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we need to mix the two pictures

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

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How can we do this?[br]

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By combining waves [br]with different wavelengths,

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

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When we add two waves, [br]we find that there are places

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where the peaks line up,[br]making a bigger wave,

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

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The result has regions where[br]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[br]get bigger,

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

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If we keep adding waves,[br]we can make a wave packet

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with a clear wavelength[br]in one small region.

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

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but to accomplish this,[br]we had to lose certainty

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about both position and momentum.

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The positions isn't restricted [br]to a single point.

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There's a good probability[br]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[br]by adding lots of waves,

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which means there's [br]some probability of finding it

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with the momentum corresponding[br]to any one of those.

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Both position and momentum[br]are now uncertain,

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and the uncertainties are connected.

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If you want to reduce [br]the position uncertainty

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by making a smaller wave packet,[br]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,[br]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[br]Werner Heisenberg back in 1927.

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

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but an inevitable result[br]of combining particle and wave nature.

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

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

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built into the fundamental structure[br]of the universe itself.