What is the Heisenberg Uncertainty Principle? - Chad Orzel

5,002,187 views ใƒป 2014-09-16

TED-Ed


Please double-click on the English subtitles below to play the video.

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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.
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