What happens when you have infinite energy?

[gomipile]

Unless I'm really missing something, the energy density of a laser still decreases with distance, because the photons spread apart as a function of the wavelength.

I think what was meant is that with a laser, one can set up a situation where the intensity of a beam increases with distance up to a chosen focal point, after which it starts spreading normally. Yes, the maximum possible intensity at the focal point will be less the further away the focal point is set up to be. That's restricted by the spread you're thinking about, of the beam produced by the laser hardware itself.

However, in practice the intensity of the entire beam will still be increasing up until that point.


What one would do is take a collimated beam, put it through diverging optics, collimate it again at the desired minimum initial intensity for a short distance, then pass it through focusing optics with the desired focal point. The maximum intensity at the focal point will be limited by the laser hardware and the total distance traveled, including within the mentioned optics.