This Quantum World/Implications and applications/How fuzzy positions get fuzzier

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How fuzzy positions get fuzzier[edit | edit source]

We will calculate the rate at which the fuzziness of a position probability distribution increases, in consequence of the fuzziness of the corresponding momentum, when there is no counterbalancing attraction (like that between the nucleus and the electron in atomic hydrogen).

Because it is easy to handle, we choose a Gaussian function

which has a bell-shaped graph. It defines a position probability distribution

If we normalize this distribution so that then and

We also have that

  • the Fourier transform of is
  • this defines the momentum probability distribution
  • and

The fuzziness of the position and of the momentum of a particle associated with is therefore the minimum allowed by the "uncertainty" relation:

Now recall that

where This has the Fourier transform

and this defines the position probability distribution

Comparison with reveals that Therefore,

The graphs below illustrate how rapidly the fuzziness of a particle the mass of an electron grows, when compared to an object the mass of a molecule or a peanut. Here we see one reason, though by no means the only one, why for all intents and purposes "once sharp, always sharp" is true of the positions of macroscopic objects.

Spreading wave packet electron1.gif

Above: an electron with nanometer. In a second, grows to nearly 60 km.

Below: an electron with centimeter. grows only 16% in a second.

Spreading-wavepacket electron2.gif

Next, a molecule with nanometer. In a second, grows to 4.4 centimeters.

Spreading wavepacket C60.gif

Finally, a peanut (2.8 g) with nanometer. takes the present age of the universe to grow to 7.5 micrometers.

Spreading wavepacket peanut.gif