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Yes, we do need a "theory of everything," or at a minimum we need a model of quantum gravity. Consider the color of gold, as a famous example; gold has its uniq
by Kednicma 6y ago
Yes, we do need a "theory of everything," or at a minimum we need a model of quantum gravity. Consider the color of gold, as a famous example; gold has its unique and curious color because of a combination of relativistic and quantum effects at the subatomic level [0].
Edit: Wow, fast downvotes! Sorry, I'll add another paragraph: The reason why we cannot use quantum mechanics (QM) or general relativity (GR) alone to explain the color of gold is because we know that the color comes from photons being emitted by electrons, as in QM, but the electron orbitals are distorted as if the electrons were moving at a serious fraction of the speed of light, as in GR. I think that this flatly contradicts her when she says:
> So far, this is a purely theoretical problem because with the experiments that we can currently do, we do not need to use quantum gravity. In all presently possible experiments, we either measure quantum effects, but then the particle masses are so small that we cannot measure their gravitational pull. Or we can observe the gravitational pull of some objects, but then they do not have quantum behavior. So, at the moment we do not need quantum gravity to actually describe any observation.
The color of gold, the liquidity of mercury, and even the ability of lead-acid batteries to turn over car starters are all everyday observations which need both QM and GR to explain. But, as she correctly notes, we can't put QM and GR together yet! So we do need some sort of quantum gravity.
[0] https://en.wikipedia.org/wiki/Relativistic_quantum_chemistry https://en.wikipedia.org/wiki/Relativistic_quantum_chemistry
- Strilanc 6y agoDownvotes might be because the article distinguishes between a theory of quantum gravity and a theory of everything: > Physicists would much rather have all these forces unified to one, which means that they would all come from the same mathematical structure. [...] At the energies that we have tested it so far, the symmetry would have to be broken, which gives rise to the standard model. This unification of the forces of the standard model is called a “grand unification” or a “grand unified theory,” GUT for short. > What physicists mean by a theory of everything is then a theory from which all the four fundamental interactions derive. This means it is both a grand unified theory and a theory of quantum gravity.
- Kednicma 6y agoAs she says, that's specifically "grand" unified theories (GUTs). A theory of everything might be imagined as similar to a GUT, in that they should both have all of the same stuff on the blackboard and arranged into roughly the same sorts of equations, but a theory of everything might not be able to express all four forces in terms of just one force. Indeed, what we mean by "force" is two different things entirely: in GR, "force" is the shape of the fabric of space, while in QM, "force" is purses of coins (I don't want to say "packets of data") traveling through that space. I don't quite agree with Wikipedia [0] on this; they say: > Unifying gravity with the electronuclear interaction would provide a theory of everything (TOE) rather than a GUT. GUTs are often seen as an intermediate step towards a TOE. We all have different opinions on this terminology, it seems! [0] https://en.wikipedia.org/wiki/Grand_Unified_Theory https://en.wikipedia.org/wiki/Grand_Unified_Theory
- Strilanc 6y agoYes, I was also surprised about that particular definition of "theory of everything", but it is the one used by the article.
- BoiledCabbage 6y agoHer terminology and Wikipedia's terminology match 100%. Are you certain you're using the correct terminology?
- sumanthvepa 6y agoNot a physicist. But perhaps this is referring to combining quantum mechanics with special relativity, not GR. So these examples are not examples of problems where you need a theory of quantum gravity.
- Iolaum 6y agoWrong. Special relativity ignores gravity interactions. This article is explicitly mentioning gravity. Describing gravity is why Einstein researched/invented GR.
- garmaine 6y agoI’m a physicist and I downvoted. Relativistic quantum field theory (which explains gold’s color) and quantum gravity are two entirely different things. We already have a relativistic quantum theories which combine quantum mechanics with Einstein’s special relativity, and explains everything we know about chemistry. Quantum gravity would combine the standard model with general relativity, which is only really needed for non-human scale things like black holes and the origin of the universe. Special relativity and general relativity are two very different things. Don’t conflate them.
- Kednicma 6y agoThanks. There's a recent video [0] out by Sean Carroll that delves more into the difference between SR and GR. [0] https://www.youtube.com/watch?v=353lEB63iyg https://www.youtube.com/watch?v=353lEB63iyg