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  • The Heisenberg Uncertainty Principle is one of a handful of ideas

  • from quantum physics to expand into general pop culture.

  • It says that you can never simultaneously know the exact position

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

  • from literary criticism to sports commentary.

  • Uncertainty is often explained as a result of measurement,

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

  • The real origin is much deeper and more amazing.

  • The Uncertainty Principle exists because everything in the universe

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

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

  • have no meaning.

  • To understand this,

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

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

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

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

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

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

  • like ripples covering the surface of a pond.

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

  • most importantly, its wavelength,

  • which is the distance between two neighboring peaks,

  • or two neighboring valleys.

  • But we can't assign it a single position.

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

  • Wavelength is essential for quantum physics

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

  • mass times velocity.

  • A fast-moving object has lots of momentum,

  • which corresponds to a very short wavelength.

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

  • which again means a very short wavelength.

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

  • If you toss a baseball up in the air,

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

  • far too tiny to ever detect.

  • Small things, like atoms or electrons though,

  • can have wavelengths big enough to measure in physics experiments.

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

  • and thus its momentum, but it has no position.

  • We can know a particles position very well,

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

  • To get a particle with both position and momentum,

  • we need to mix the two pictures

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

  • How can we do this?

  • By combining waves with different wavelengths,

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

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

  • where the peaks line up, making a bigger wave,

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

  • The result has regions where we see waves

  • separated by regions of nothing at all.

  • If we add a third wave,

  • the regions where the waves cancel out get bigger,

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

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

  • with a clear wavelength in one small region.

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

  • but to accomplish this, we had to lose certainty

  • about both position and momentum.

  • The positions isn't restricted to a single point.

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

  • of the center of the wave packet,

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

  • which means there's some probability of finding it

  • with the momentum corresponding to any one of those.

  • Both position and momentum are now uncertain,

  • and the uncertainties are connected.

  • If you want to reduce the position uncertainty

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

  • which means a bigger momentum uncertainty.

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

  • which means a bigger position uncertainty.

  • That's the Heisenberg Uncertainty Principle,

  • first stated by German physicist Werner Heisenberg back in 1927.

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

  • but an inevitable result of combining particle and wave nature.

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

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

  • built into the fundamental structure of the universe itself.

The Heisenberg Uncertainty Principle is one of a handful of ideas

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B2 US TED-Ed wave momentum uncertainty wavelength object

【TED-Ed】What is the Heisenberg Uncertainty Principle? - Chad Orzel

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