10/4/2023 0 Comments Gravity as emergent phenomena![]() ![]() If two theories are really dual, then no observation can distinguish them: they make exactly the same predictions. The skeptics, though, bring up an important point. ( I’ve made arguments like that in the past too.) Using those theories, they argue that space and time might “break down”, and not be really fundamental. As evidence, they like to bring up dualities where the dual theories have different concepts of gravity, number of dimensions, or space-time. They expect that distances and times aren’t really fundamental, but a result of relationships that will turn out to be more fundamental, like entanglement between different parts of quantum fields. The “spacetime is doomed” crowd think that not just gravity, but space-time itself is emergent. (For the experts: I, like most physicists, am talking about “weak emergence” here, not “strong emergence”.) For example, hydrodynamics, the theory of fluids like water, emerges from more fundamental theories that describe the motion of atoms and molecules. An emergent theory is one that isn’t really fundamental, but instead a result of the interaction of more fundamental parts. ![]() If theories with gravity can be described by theories without gravity, does that mean gravity doesn’t really exist? If you’re asking that question, you’re asking whether gravity is emergent. No matter what you observe, neither description will fail. For any question you can ask about the gravitational “bulk” space, there is a matching question on the “boundary”. In these situations, a theory of quantum gravity in some space-time is dual to a theory without gravity describing the edges of that space-time, sort of like how a hologram is a 2D image that looks 3D when you move it. Some of the more popular examples are what we call holographic theories. In physics, sometimes we find that two theories are actually dual: despite seeming different, the patterns of observations they predict are the same. At the heart of the argument is the distinction between two related concepts: duality and emergence. Joined by a few philosophers, they think the “spacetime is doomed” crowd are over-excited and exaggerating the implications of their discoveries. Other, grumpier physicists are skeptical. These physicists believe that what we think of as space and time aren’t the full story, but that they emerge from something more fundamental, so that an ultimate theory of nature might not use space or time at all. They don’t mean this as a warning, like some comic-book universe-destroying disaster, but rather as a research plan. Spacetime is doomed! At least, so say some physicists. ![]()
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