Live Q&A with Brian Greene | World Science Festival

The Artemis missions are not designed to test fundamental physics theories like quantum gravity or general relativity deviations. Quantum gravity effects emerge in extremely energetic and small realms, far beyond the gravitational influences relevant for lunar missions. Newtonian gravity is sufficient for most calculations related to Artemis.

Brian Greene states, 'The issues with quantum gravity emerge in extreme realms, realms that are extraordinarily energetic, extraordinarily small. The kind of gravitational influence that's relevant for Artemis, Isaac Newton could have done the calculations.'

It is theoretically possible to create a black hole by concentrating enough energy, such as light (photons), into a sufficiently small region. Einstein's theories emphasize that energy, not just matter, sources gravity. The same formula used for matter-based black holes applies, with E=mc² converting mass to energy.

Greene confirms, 'all you need is energy to create a black hole. Energy is what sources gravity. And so photons, radiation more generally, they have energy. And so yeah, if you get enough of that energy in a small enough region, you can create a black hole.'

Fundamentally, the nature of time is not fully understood, but from a practical physics perspective, time is what allows for change. In a hypothetical universe of absolute stasis, where no change occurs, the role and meaning of time would effectively disappear, as even thought requires change.

Greene explains, 'from a practical sense, I would say absolutely because time is what allows for change... if it's total stasis, from any practical standpoint, a notion of time completely drops away.'

The expansion of space, driven by gravity (including repulsive gravity from uniform energy like dark energy), does not cause objects like our bodies or solar systems to expand. This is because these structures are held together by much stronger non-gravitational forces (electromagnetic, strong, and weak nuclear forces) that counteract the expansive gravitational push.

Greene clarifies, 'Our bodies are held together by non-gravitational forces that are stronger than the gravitational forces... because those forces are so much more powerful than the force of gravity, our bodies and the world around us is able to hold together and not succumb to the expansion of space.'

If multiple universes exist, the concept of time as we typically understand it (e.g., 'time began with the Big Bang') becomes a local description specific to our universe. Each universe would have its own 'Big Bang' and associated timeline. A universal notion of time applying to the entire multiverse is difficult to define because the rate of time's passage is influenced by local matter and energy conditions, which would vary greatly across different universes.

Greene states, 'If there multiple big bangs, say giving rise to universes at distinct locations in some larger multiverse reality, then time in each of those universes would have come into existence in the same sense with the big bang that drove the expansion of space in that universe.'

It is conventionally understood that entangled particles can exist with one inside and one outside a black hole's event horizon. Measuring the outside particle would reveal information about the inside one without violating the black hole's escape boundary. However, the 'firewall paradox' (inspired by Hawking radiation) suggests that the event horizon might sever entanglement, creating an intensely energetic 'firewall' that incinerates anything attempting to cross it, challenging the traditional view of smooth black hole entry.

Greene explains, 'the most conventional answer from general relativity is yes. Right? So if you measure that entangled member on the outside and it's spinning up, then you will have learned that the one on the inside is spinning down.' He then introduces the 'firewall' concept: 'some suggest that maybe the event horizon of a black hole severs the entangled relationship between those two particles... which means that the surrounding region of a black hole, the event horizon would be highly energetic.'

The fundamental constants of nature (e.g., speed of light, electron mass) could theoretically differ in other universes within a multiverse. The anthropic principle suggests that we observe our specific set of constants because these are the only values that allow for the formation of stable structures and life as we know it. While some find this explanation 'repellent' for placing humanity at the center, it offers a potential answer to why our universe has its particular physical laws.

Greene notes, 'Maybe if the numbers were significantly different, we couldn't live. We couldn't exist. Life as we know it wouldn't form. So why are we in a universe with the particular values of the constants that we observe? Because this is the only kind of universe in which we could exist.'

Einstein's general relativity posits that gravity is fundamentally the warping of time, not just space. Objects fall because they 'want' to move towards regions where time elapses more slowly, which occurs closer to massive bodies. This perspective offers a more accurate understanding than the common analogy of objects sliding down indentations in warped space.

Greene clarifies, 'what Einstein really showed with general relativity is that you can think about gravity at least in the familiar context like the earth or even a black hole. You can think of gravity as that which slows the passage of time... it's time that's warped.'

AI is rapidly changing physics research by providing 'an army of a hundred or a thousand of the best graduate students' to physicists. AI systems can quickly learn and synthesize vast amounts of complex mathematical and scientific knowledge, accelerating problem-solving. While currently lacking the 'creative leap' of human inspiration, AI excels at novel combinations of existing knowledge, suggesting a future where human and artificial intelligence hybridize to push scientific frontiers.

Greene recounts an experiment where an AI solved a complex, esoteric physics problem in half an hour that would take a human graduate student months to learn and solve. He adds, 'a leap of insight can be putting together novel qualities or I should say putting together existing qualities in a novel pattern a novel way and that's the kind of thing that these systems you know are are pretty good at.'