Every finite width beam will have some divergence.

Is that a law or something?

I was thinking of a hypothetical case.

Yes, the so called diffraction limit. Has a lot of applications in optics.

>"Diffraction refers to various phenomena that occur when a wave encounters an obstacle or opening."

>In a vacuum.

Read more.

May I refer you to https://www.edmundoptics.com/knowledge-center/application-no... ? Pay particular attention to the last paragraph:

> Achieving a truly collimated beam where the divergence is 0 is not possible, but achieving an approximately collimated beam by either minimizing the divergence or maximizing the distance between the point of observation and the nearest beam waist is possible.

You confuse theory w/ practice.

The diffraction limit is a well supported theoretical limit that is caused by the wave nature of light. There is ways to get around it in the near field (up to a few ten wavelength away from the source). But in the far field you can not get a perfectly divergence-free finite-width beam. [1] Nor can you focus a beam to a beam waist smaller than about the wavelength.

In practice the situation is even worse and you can't even get the performance that a perfect Gaussian beam would allow. We often express the performance of real beams by a thing called M^2 or beam quality parameter [2]. In some sense is measures how much wider the beam is than it needs to be, and this number is never less than one.

[1] Btw such a beam would have infinite brilliance. [2] https://en.wikipedia.org/wiki/M_squared

>The diffraction limit is a well supported theoretical limit that is caused by the wave nature of light.

That's not written in stone yet, it's a good approximation but still falls down under specific conditions.