system
Given the following post, respond as if you're commenting on Reddit:
human
When physicists refer to "nothing",  what does that really mean?

Arguments have been made that Hawking's statements that the universe can create itself from nothing are self-contradictory, because it breaks causailty.  My hunch is that those making such arguments are just not familiar with the more complex aspects of physics, but then again, neither am I.  I've heard of particles "popping in and out of existence", but I'm not sure what that really means.  Can anyone explain to the layman what physicists mean when they say something comes from nothing?
gpt
You're probably talking about Lawrence Krauss' lecture "A Universe from Nothing."

What he means is that if you count the total energy density of the universe as positive, and the gravitational potential energy as negative, it cancels out to zero.