h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Next to the 'small' black holes created from just one star, astronomers believe that in the centre of galaxies giant black holes exist, containing the mass of thousands upon thousands of stars.

Theoretically an even more weird version is possible, containing no normal mass at all, and consisting just of photons.
Each photons has a bit of kinetic energy, which can be seen as mass, using E=mc2. This means a photon is generating a very small gravity.
Take enough fotons in a very small space (being bosons this must be possible) and the combined gravity may generate a black hole.

From the outside there would be no difference and it will collect normal matter over time anyhow.

how do the photons come to be at the massive density required for gravitational collapse? the only thing I can think of with enough gravity is a black hole, in which case the photons would be lost inside the event horizon.

Daneel,
as I said, it is a theoretical possibility, and I really don't know how to achieve it. A immense number of laser-cannon all aimed at the same point and at the same moment might do the trick, but where to get the energy?
The concept was used by Frederik Pohl in his Heechee saga. I cannot remember any story by your good old Isaac mentioning anything like this.

you could make a massive optical cavity and put it near a bright star and wait, would get a bit hot though so you would also need unbelievably thick oven gloves to try and move it. wait too long and it will collapse and eat your star too.

Isaac liked his robots and his empires. it is a rare and often worrying breed that finds all of this conceptual physics interesting.

Isaac liked deep physics also, and was one of those rare and worrying breed. Maybe we humans want to know more than there is to know.

By the way, can you define a 'human being' to me? The robotic laws cannot be implemented if you cannot tell the difference between 'a human being' and 'not a human being'.

Human being = any being which can be classified under homo sapiens. Bipedal, brain is disproportionate to body mass based on all other mammals, lacks fur for the most part, and also is the only known sentient lifeform. (Yeah, yeah, dolphins recognize their reflections, big woop.)

So if we combine Asimov with DNA, and Marvin has been programmed with the three rules of robotics, he may snuff out most of the characters from the H2G2 series, as they are not human beings.

Especially the Vogons, who are hurting a lot of human beings by dismantling the earth

Yep. They can.