[Vermont Lt]

I like the vacuums that are empty. What other kinds are there?

[Boogieman]

The ones where you forgot to empty the bag?

[CharlesWayneCT]

In this case, the vacuum is actually cluttered with particles and anti-particles, which come into existence and then obliterate each other, but the light is being absorbed by these particles and then being re-transmitted, but with a slight delay.

At least, that is the claim in this paper.