Science & Technology Q&A for Kids (and others) [November 18, 2022]

Stephen Wolfram answers general questions from his viewers about science and technology as part of an unscripted livestream series, also available on YouTube here: https://wolfr.am/youtube-sw-qa Questions include: Does gravity's strength cause a fundamental limit for the size a planet? What about a star? What about a black hole? What about a galaxy? What about the universe? - Internal gas pressure and gravity are two main forces for star formation from nebulas. - What was the pressure of the early universe vs. today? Just as a thought experiment. - Can one stretch a vacuum beyond a "breaking point" similar to how matter can be compressed beyond a "breaking point" that leads to black holes? - In both quantum field theory and general relativity, the zero-point energy seems to be arbitrary: you can add a constant to the equations and it will still be a valid solution. But in general relativity there seems to be a notion of absolute energy because of its gravitational effects. This zero point seems to be associated with flat space. Why is flat space non-gravitational, i.e. why is flat space the lowest possible energy state? - Any ideas about "hacking nature" to gain powers (get infinite energy, travel faster than light, etc.). Do you think all these are possible at all? Can we really "hack" or "alter" the rules of nature? - You can travel faster than light if the space between you and your destination changes; this happens quite frequently as the universe expands, and it's why we get measurements faster than the speed of light in space. It's just a fabrication. - This brings up a related question. You cannot distinguish the geometry of empty space from that which has matter that is uniformly distributed. So it is perhaps uniformity that determines the geometry (without dark energy). But this assumes matter can be spread out like a fluid, instead of being discrete. So perhaps flat space is indeed the lowest-energy state. Uniform matter cannot exist because of the discreteness of matter, which leads inevitably to inhomogeneities. - It's almost like you need to solve the puzzle of constructing the space you want to travel through before you can travel through it. - Why is the refractive index for x-rays into matter smaller than 1? Does that mean that the speed of light for x-rays is faster in matter than in a vacuum?