Quote Originally Posted by warty goblin View Post
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.
Fun fact: lasers are weird. The field intensity of a laser looks the same whether the laser is focusing (getting smaller) or diffusing (getting bigger). How tight the laser can be focused is a function of both frequency and how quickly it is being focused (with farther focal lengths having tighter focusing). If I have a laser that at its output has an energy density of 5 W/m^2 and a 1 m^2 aperture, I can focus it down to a much smaller area, still have almost 5 watts, but an energy density in the hundreds or thousands of watts per square meter. If I align several lasers so their focuses are in the same spot, I can add together their energy densities (because electromagnetism is, to the best of our knowledge, linear in a vacuum). I don't remember the theoretical amount of energy density you need to make a black-hole with UV light, but I do remember that it was something absurd.