# Quantum particle spead to macroscopic size

Is there experimental evidence of a single, free massive particle wavefunction can spread in space to macroscopic size?

• What is your definition of "macroscopic"? What constitutes a "particle" at that size? Oct 21 '20 at 14:55
• 1.: Defined by the experiment 2.:Please, read the description Oct 21 '20 at 15:00

Theoretically, this is possible (as are a lot of things, as it turns out). However, there are a few issues with determining something like this experimentally.

1. Free particles are hard to come by. Although particles can be close to free (i.e. relatively little force is acting on them), truly free particles are rare (if they exist).
2. We can't measure a wavefunction. Wavefunctions are a mathematical tool. We can only calculate and work with them in the theoretical realm. By attempting to measure one, it collapses.

Although I can't be certain, due to the reasons listed above, I doubt there is experimental evidence that a free particle wavefunction can be macroscopic.

• I didn't write about measuring a wavefunction. I meant in case of a Gaussian wavefunction the deviance grows so high that the observation positions are scattering in a macroscopic domain. Oct 21 '20 at 17:29
• Right. But you said experimental evidence, which means (or at least implies) some sort of measurement. Oct 21 '20 at 17:30
• Yes, like a scattering experiment or using a sensitive screen in a double slit experiment. But the point is : measuring a particle's extreme large position uncertainty. Oct 21 '20 at 18:23
• Uncertainty can only be measured theoretically. If you have a large enough sample size, you can approximate it Oct 21 '20 at 18:27
• Although there are certainly caveats, such an experiment would be theoretically possible (I think). I’m not sure if it’s been done Oct 22 '20 at 11:20

Suppose you prepare identical particles in an identical free state. That is, they have the same spread in space. You send them in opposite directions.

If you make position measurements on the particles on one side after a time that is much shorter than the measurements that you make on the particles on the opposite side, you'll see that the spreading in space of the first particles will be much smaller than that on the opposite space.

The longer you wait to measure the second part of the particles (i.e. you measure their position after a sufficiently long time), the bigger the spread, which can reach macroscopic measures.