I was thinking of a universe that would have similar physics to ours but in which there would be no quantum physics at all. So this universe would be described entirely through classical physics with no quantum effects. In this universe special relativity would apply meaning that there would be the effects of time dilation and length contraction for objects that are moving near speed of light relative to each other but there would be no quantum mechanics. Also general relativity would apply in this universe so there would also be the effect of time dilation from gravitational fields.

How would this universe evolve differently from our own? Could complex systems develop in this universe?

  • $\begingroup$ So... Basically, you want a universe that functions the way we thought it did a few centuries ago, but with less anthropomorphic principle? $\endgroup$
    – user867
    Feb 29, 2016 at 6:40
  • 3
    $\begingroup$ OMG! the damned ultraviolet catastrophe is back... $\endgroup$
    – user6760
    Feb 29, 2016 at 9:00
  • $\begingroup$ This is an unanswerable question. Or rather, the only answer is "we don't know". $\endgroup$ Feb 29, 2016 at 10:54
  • $\begingroup$ "How would this universe evolve differently from our own?" is inherently too broad because any answer would be drawing on a huge variety of disciplines and would need to go into some depth on each. You should focus on some aspect such as, for example, stellar formation. $\endgroup$
    – user
    Feb 29, 2016 at 12:13
  • $\begingroup$ Is there a particular effect you're trying to go for? Like, you don't want to deal with the possibility of non-deterministic futures? For something like that, I'd say handwave it as your scientists discovered sub-layers to quantum mechanics that are purely deterministic. $\endgroup$
    – MichaelS
    Feb 29, 2016 at 12:36

3 Answers 3


Maybe the shortest answer possible: No quantum physics, no stars, no life. As we know it.

At least that's what the actual knowledge about stellar nucleosynthesis says: inside our sun its too cold (!) and/or the density of the fusion medium is way too thin (!) for fusion to happen in a manner that could uphold the star in its current form. It just works thanks to quantum tunneling effects — in pretty rare occasions the atom cores needed for fusion (quantum)tunnel their way past the magnetic forces that repels them on a atom scale. These effect might be pretty rare, but there are pretty much atom cores inside a stars core, so after all it happen often enough to keep this... equilibrium in place (that "even out" between gravity and ray pressure) due to fusion reactions.

And... no quantum effects, no tunneling. Well maybe really big stars will be able to exist, but usual they go boom (bada boom) in a time to short to develop complex forms of life. Well, lets just say: no stars as steady as we would need them would be able to persist for a time long enough to random generate intelligent life. Would be a pretty boring universe, because no one would be there to watch all the giant stars rip themselves apart.

EDIT: Something more to add... if you are going to break down where Energy from fusion does come from, you'll end up with stuff like quantum-fluctuation. While I do not understand this topic as good as the nucleosynthesis, I'm afraid that even light would be something you'll miss in such an universe. Protons... pop up if electrons switch their orbit (most easy explanation :); the reason they do appear after all is a quantum effect too.

Even more: no ionic radiation (alpha, beta and gamma as we know them from radioactive stuff)... well, no radiation at all. No heat too. No sir, that universe of yours would be a place with Quarks doing brownian motion until infinity, because they can't transmit any form of energy.... or something like this.

EDIT2: As suggested: if you want just 1899 style universal physics without needing to thing about quantums, maybe this https://en.wikipedia.org/wiki/Space:_1889 can provide some impressions. There are more of them out there, but it did come to mind first.

  • $\begingroup$ So no "tunneling" but still have weak interaction turning protons+electrons into neutrons? $\endgroup$
    – JDługosz
    Feb 29, 2016 at 11:18
  • $\begingroup$ @JDługosz OK, so there are more reasons stars (and thus life) as we know it couldn't exist. But OP's argument is sufficient to prove his point, fact that other arguments exists doesn't change that much. Maybe some things no one would observe anyway, but for a story world, what does it matter? $\endgroup$
    – Mołot
    Feb 29, 2016 at 11:25
  • $\begingroup$ after a while, I start doubting that even basic atom parts (electrons, protons and neutrons) could form without quantum physics... hell, even the energy-release due to fusion is some kind of quantum effect, one of my more lazy brain-areas did mention just a second ago. But that's something I cannot explain properly, just ... wild guess. $\endgroup$ Feb 29, 2016 at 11:43
  • $\begingroup$ Yea, see my answer. It took a while to enter, and I could keep going :) $\endgroup$
    – JDługosz
    Feb 29, 2016 at 11:43
  • $\begingroup$ Re your edit: if you cut off the universe at the ankles, all physics that emerges from the most fundamental rules would be gone. I'm supposing that he defines stuff at a 19th century level instead. Newton's laws and Maxwell's equations are the easy things to specify, so light is ok. You'll have to fix the bleeding edges of 1899 physics in other ways so it holds together. $\endgroup$
    – JDługosz
    Feb 29, 2016 at 11:59

Without quantum physics, everything in the universe is the same temperature, as anything that gets hotter than the background instantly radiates away all the excess heat as black-body radiation. It's only the quantisation of the energy spectrum that keeps the rate of radiation finite.


Does your lack of quantum effects include chemistry?

While Newtons Laws of motion are a low-energy approximation that's in fact the limit as energy approaches zero, that makes it a very good approximation. Chemistry as a set of rules of thumb is a very rough and crude guide that doesn't give exact results and falls down for lots of organic chemistry.

So, if made a simple set of rules, it would not have the richness and subtlety needed to support life as we know it. If you tried to codify the real results of chemistry to better approximate what really happens, you need a special rule for every distinct interaction. It doesn't reduce to an underlying simpler model.

What about the very existence of stuff in general? Electrons don't fall in to the nucleus because of quantum effects. Would atoms have a substructure? Is every atom a distinct kind of thing, rather than reducing to simpler basics? How complex do you want to make the underlying rules, and would having deeper rules violate your premise?

Matter has essential matter-like qualities (hard and solid) because of spin. Spin just shows up because of quantization and Minkowski spacetime. So instead you need rules to make atoms hard and solid as a direct fiat. Will that cover all the subtleties or will your rules be simple? I guess you'll need special rules to make metal behave the way it does such as conducting electricity, on top of all the other special rules already discussed.

Ok you programmed in metal wires and classical electrodynamics, but what about semiconductors? Can you come up with rules for that which are not in your forbidden territory?

In short, it will be as much as you invent using explicit rules for all the effects you want to support. Because all that just pops out for free thanks to the more fundamental rules of QM.

Actually, the basic ideas of QM exist because the universe has a smallest scale. If you don't have QM does that mean your universe is continuous and infinitely detailed? Or does it mean that you have a system that's so utterly different and alien that the question is meaningless?


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