I wanted to make a thread to discuss the possible physical implications of a 2D universe as opposed to our 3D one. Note this is meant to be purely to be a thought experiment rather than a true analysis of something real. Also note that when I say 2D and 3D, I am referring to the number of spatial dimensions. Both are assumed to include time. I'll start off, but please add anything else that you don't think I've considered. I'll mostly be discussing 2-Dimensional implications on classical physics, as it it what I know best, but feel free to discuss any field you want. First Off, I'm pretty sure almost all of Newtonian Mechanics would be unaffected. Collisions and other interactions between macroscopic objects, to my knowledge, work just as well in 2D as 3D. Gravity, at least in the classical sense, would still function in the same way, as would the other 3 fundamental forces I believe. Rather than pulling itself into spheres, matter would pull itself into circles. This means planets and stars that are circular rather than spherical. Atoms and their constituents would also be circular rather than spherical. Instead of atoms having a 3D electron cloud, they would simply have a 2D one. Interestingly, any quantities that were related to volume would have to use area instead, and some uses of area would have to just use length. For example: Density units: kg/m2 (Instead of kg/m3) Pressure: P=F/x (Instead of P=F/A) One of the most prominent problems one might encounter when dealing with physics in a 2D universe would be the fact that as there is no third spatial dimension, there would be no cross products. This would mean no torque (or at least no 3D torque) and also, very sadly, no magnetism or magnetic fields Many compounds would not be able to exist, or at least not exist as we know them. Compounds that were already flat, such as water or anything with only 2 atoms, would translate perfectly into just 2 dimensions. Other compounds, like ammonia, would have to be flattened, but would still be structurally viable I think. However, most remotely complex compounds such as glucose, propane, and perchloric acid, which pretty much need 3 dimensions to be constructed correctly, would probably not work, and I only named a few of the more simple ones. Life, as we know it, would probably be impossible. As mentioned above, complex chemicals would not work quite the same way. Because of this, proteins, DNA, and other such molecules needed for life could probably not exist. However, I'm sure that some simplified version of life, using flat variations of different compounds could be conceived of. I do not know enough about Quantum Mechanics to make any assumptions relating to it, so I will skip it entirely. From what I do know of Special Relativity, it would be unaffected, and I believe that GR would be as well. That's all I can think of for now. Please tell me what you think of this and add any other ideas that you can think of.