# A universe without light?

In the story Wang's Carpets (and part of the novel Diaspora), Greg Egan sketchily describes a high-dimensional universe which contains no analog for light, such that the aliens who inhabit this universe can only gain information about their surroundings through touch. (One could also imagine gaining information through sound, but the brief description of this universe also implies weird things about motion, so maybe sound doesn't exist there, either.)

If you just delete electromagnetism from our universe, things do not turn out well. And fleshing out a completely alien universe, where even the basic laws of mechanical motion are weird, but which nevertheless permits intelligent life to evolve, is a bit of a tall order! (Part of the point of the Wang's Carpet universe is that it is, intrafictionally, weirder than anything Earthling civilization has ever been able to imagine in simulation--so the lack of detailed description can be excused, as describing it in detail would, by construction, invalidate the narrative point it was intended to make!)

So, here's the question: how can we tweak our physics (rather than just starting from scratch) to produce a universe which contains complex biochemistry-analogous structures, but does not have any perceptual equivalent to light? I.e., which lacks a stable, strongly-interacting, low-mass particle that can transmit coherent images over long distances?

• Comments are not for extended discussion; this conversation has been moved to chat.
– L.Dutch
Jul 17 '19 at 3:04

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.

• Well, dang! That's a heck of a lot simpler than I was expecting! Turning spacetime Newtonian has some rather drastic side effects--like requiring a completely reworked definition of mass, energy, and time...--but those are issues that have been discussed here before, so might be solvable.... Jul 17 '19 at 3:54
• This is probably the easiest fix. Historically, quantum mechanics were the result of trying to understand light - the simplest way to erase one is to erase the other. And classical mechanics is so self-consistent (light-related issues aside) that up until the quantum revolution, many believed that it explained everything. Jul 17 '19 at 8:50
• How would life even develop under these conditions? Don't all basic metabolic processes sort of depend on the kinds of energy interactions that this renders nonexistent? Jul 17 '19 at 17:34
• @MorrisTheCat the main thing I was concerned about was keeping some form of chemistry intact. As I explained in my answer, there will still be different elements, and since the electric force still exists chemical bonds will also be able to develop. The main problem is that without light, it's much more difficult to transfer energy from stars to planets. However, this is a fundamental problem with the question itself, and my rationalization is that the life in this universe may operate in a very alien way that requires little energy input-- perhaps they only move on geological time scales. Jul 18 '19 at 5:39

There are basically three options to work with:

1. Delete the electromagnetic field, and replace it with something completely different.
2. Delete the electromagnetic field, and try to adjust the other forces to make up for it in a way that doesn't re-introduce another kind of light (e.g., massless pions).
3. Alter the electromagnetic field so free photons can't propagate.

I have spent some time contemplating options 1 and 2, and they always seem to go off the rails rather quickly, becoming sufficiently complicated that you might as well just build a whole new universe from scratch. El duderino's answer is in the realm of option 3, and so is my next alternative:

In order to restrict photon propagation, rather than altering the properties of space as in el duderino's answer, we can instead make the photon decay over relatively short distances. In order for photons to decay, they will need to experience time, which means they need mass--but not necessarily very much mass. Giving photons mass restricts the range of the static EM force as well, replacing the usual Coulomb potential with a generalized Yukawa potential with exponential decay, just like the residual nuclear force carried by massive pions

$$U(r) = -g^2\frac{e^{-\alpha m r}}{r}$$

where $$g$$ is the force coupling constant, $$\alpha$$ is a scaling factor to get the units right, and $$m$$ is the mass of the gauge boson and determines the field decay rate. As $$m$$ approaches zero, this simplifies to the normal Coulomb potential, so if $$m$$ merely remains very small, we can retain atomic and chemical structure with only minor perturbations, and possibly even retain some macroscopic EM effects (or not, as we choose).

However, giving photons mass is only a necessary, not sufficient, condition for short-range decay of free photons. We also need something for them to decay into. The obvious first option to try is just having photons decay into electron-positron pairs, so the universe ends up filled with a positronium gas as the dominant form of matter, rather than hydrogen. That might be a cool avenue to explore independently, but it means that the photon mass must be at least 1.022MeV--which in turn limits its range to nuclear scales. That would definitely screw up chemistry! Next we might be tempted to ask if photons could decay into neutrinos--and unfortunately, the answer is "no". The reason is that neutrinos have no electric charge--only weak isospin--and so do not couple to the electromagnetic field, and cannot participate in production, decay, or absorption of the electromagnetic boson.

So, we're going to have to add a whole new fermion field to permit a route for photon decay. Fortunately, considered and rejected both electron decays and neutrino decays as suitable candidates, there is a glaring whole in our universe's particle inventory which nicely solves the problem!

Note that electrons have both electric charge and weak isospin, which is why they interact both electromagnetically and weakly. Neutrinos have only weak isospin, which is why they only interact weakly. But where are the particles with only charge, and no weak isospin?

If we add such particles and give them very low mass, that solves most of our problems. Being low mass, they will not bind to atoms, so electrons are left unmolested to continue supporting chemistry. Having charge, they will support photon decay...

...but, we may have a new problem: we may have just destabilized the electron. If there is a low-mass weak isospin particle (the neutrino), and a low-mass charged particle (the new thing--let's call it an electrino), then perhaps electrons can decay into a pair of electron-neutrino and electrino, splitting up its charges between them.

Fortunately, though, two other conservation laws come to the rescue: conservation of spin (i.e., angular momentum), and conservation of lepton number! If "electronness" is also conserved (as it appears to be in the low-energy regime, modulo whatever process ends up explaining the matter/antimatter asymmetry), then an electron could produce either an electron-neutrino, or an electrino, but not both. And giving our new particles half-integer spin (i.e., making them fermions) just like other leptons means that a parent particle must be either spin 0 or spin 1--which an electron isn't.

The introduction of a new low-mass charged field will change how vacuum polarization works, and introduce an additional weakening of the electromagnetic field over very large distances--but that will be difficult to disentangle from the exponential decay induced by the photon mass, anyway.

At this point, ensuring that photons are not useful for vision is just a matter of tweaking the mass ratios of the photon and electrino to keep the photon half-life really low, thus limiting its range. Unfortunately, the mathematics governing boson decay rates of this type (the closest analogues would be the decays of W and Z bosons) are way over my head, so I have to get a little bit handwavy with the exact values--but thus far, it seems like this approach of simply adding a decay path for photons should work.

• Giving photons a mass is one way to make the electromagnetic force operate only at very short ranges; as you note, this parallels the weak nuclear force where the force carrying particles are the W and Z bosons which have mass. But I don't think it's a necessary condition as you suggested--you might also give photons charge, which would parallel the situation with massless gluons that carry the strong nuclear force, their charge explains why the force only works at short range as explained here. Jul 17 '19 at 17:50
• @Hypnosifl It's not a necessary condition for restricting range, but for permitting decay. Altering EM to be a charged boson field is an intriguing alternative, but I fear it might make things too weird to figure out. If you want to expand that into an answer, though, I'd love to see it! Jul 17 '19 at 17:53