A universe without light?

Hmm, this is a tricky question because there are so many factors that go into making life possible that it's hard to really be sure about what changes we could make to physics that would still allow life. Hell, it's pretty difficult to even come up with an airtight definition of what constitutes 'life' in the first place.

But even more pertinently, there's the problem that if you tweak one part of physics, there's no assurance any of it is gonna work the same way. Reality doesn't politely follow a set of rules that we lay out, it just kind of does what it does-- scientific theories are just ad-hoc models we've found that seem to do a good job at predicting what that's gonna be. So if I change one model that describes how an aspect of nature works, it's not always clear how a different model describing a different aspect of nature will change, because scientific theories don't explain nature, they model it. The only way to definitively answer the question of how the universe would look if we tweaked our physics is if we already had a theory that perfectly modeled nature, which unfortunately we don't have.

With all that being said, I'll take a stab from the framework of classical electromagnetism. My idea here would be to set the permeability of free space, $\mu_0$, to $0$. Maxwell's equations governing magnetism read $$\nabla \times \mathbf{B} = \mu_0 \big( \mathbf{J} + \epsilon_0 \frac{\partial \mathbf{E}}{\partial t}\big)$$ $$\nabla \cdot \mathbf{B} = \mathbf{0}$$

We can see from this that if $\mu_0=0$, the magnetic field is zero everywhere (assuming the standard vanishing boundary conditions). But if the magnetic field is always zero, you can't have propagating electromagnetic waves, since those are brought about by the coupling between the electric and magnetic fields. Thus, there's no light in this universe! Now, it's still possible to transmit long distance information by moving charged particles and measuring how the electric field changes, but this is impractical since most matter is very close to electrically neutral (indeed, this is also possible to do in our own universe, but it is so impractical that no lifeforms use it to 'see').

Like I've said before, it's hard to know exactly how this will affect physics, but for the most part it shouldn't change things too much since the electric force tends to dominate magnetic forces in most problems. A few notable exceptions:

• Perhaps most profoundly, this completely screws up relativity because forces via the electric field can now propagate instantaneously. This basically has the effect of uncoupling spacetime and turning time into an absolute coordinate, making newtonian mechanics much more accurate.
• Plasma physics, especially within astrophysics, will look very different. The movements of most astrophysical plasmas are governed by magnetic forces, because Debye shielding makes them more or less electrically neutral.
• It will be impossible to generate electricity via induction-- the only ways would be electrostatic methods like batteries and Van de Graaff generators.

• Within the framework of quantum mechanics, no relativity means no spin, which would have some pretty significant effects on atomic structure. The Pauli exclusion principle doesn't require relativity to derive, so electrons would still stack in orbitals and so there would still be different elements, which is good. However, they would stack more quickly since they can no longer pair up with electrons of opposite spin. So this world would still have a chemistry capable of building complex elements, but those elements would act very different than our own*.

• Also related to the lack of spin in quantum mechanics: their alien scientists won't observe fine or hyperfine structure of atoms, and superconductors won't be a thing.

I'm ignoring problems the lack of relativity produces when it comes to describing creation and annihilation of particles, ie when QFT and the other fundamental forces start getting into the mix. This is mainly because I don't know enough of the math behind QFT to give a confident answer, but also because of my previous conversation about how it can be unclear how changes in physical models would affect other models within that universe.

*EDIT-- I was actually a bit sloppy here-- you don't need relativity to show that parity of a wavefunction is a conserved value. However, you do need it to prove the spin statistics theorem, which says that states of mixed parity do not occur for particles, and that parity is linked to the spin of the particle. to ensure that your chemistry works, I would just make it a postulate that particles can only have symmetric or antisymmetric wave functions.

posted almost 5 years ago

el duderino‭

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