If humans had the ability to physically travel to a universe similar to our own, how different could its fundamental constants be and still allow humans and other terran life to survive there long-term?

This would obviously require that matter, stars and planets are still able to exist, and that Human biochemistry still functions sufficiently to allow survival long-term.

What effect would these changes to the physical constants of the universe have upon humans placed in such an environment? Would there be any additional limitations or advantages?

Additionally, is there any combination of changes to the fundamental constants that would allow a long-term human-survivable universe to have the following characteristics:

  • Planets have the same orbits as in our own universe.

  • Stars are larger and cooler (I.e. redder) but do not significantly change the Goldilocks zone, save perhaps to move it outward such that Earth would be hotter but still survivable.

  • Brighter night sky due to more/closer/larger/brighter nearby stars, and/or a hydrogen-fusing Jupiter

  • An earth-sized world in the same position as Earth with respect to its primary with lower surface gravity and an atmosphere with equivalent pressure.

Or even:

  • Clusters of separated solid and liquid matter suspended in an atmosphere with equivalent pressure to that of earth at the equivalent distance from the centre of gravity as earth's sea level. Very low gravity. For this case, gravity bleedthrough from a very close parallel/alternate universe (our own) would be permitted in order to allow such a world to be human-viable.


I am well aware that any significant change to any one of the universal constants will result in a universe uninhabitable by humans, for reasons than our biochemistry just won't function, to stars not forming or burning out too quickly, or even to atoms not being possible... and an insignificant change won't likely result in anything other than human biochemistry just not quite working as well as it could, and nothing else noticeable.

So, this question is more about combinations of changes... can we change several of the constants in such a way that changing one would push the universe toward unsurvivability in one direction, and changing another would push it the other way, just happening to make it survivable.

Is there a combination of factors - including different stellar masses - that could allow a planet orbiting its star in the same orbit as Earth, but the star might be fusing helium without it engulfing or roasting the planet.

It doesn't actually matter if life as we know it could never spontaneously occur or evolve, since my intention is to transplant life from our universe to the altered universe. In a way, it could even improve matters for purposes of my setting if any human scientists over there could conclude that their universe isn't viable long-term (more than a few tens of thousands to a million years into their future). If they can predict that their version of the sun will become a supernova in no more than a thousand years from now, all the better.

So, for my purposes, I only need the altered universe's solar-system analogue to be human-survivable, or even to be able to exist for an internal duration of 10 million years at a minimum.

So, all I really need in this new, altered universe is:

  • An Earth-sized world (not necessarily earth-mass or composition) with an atmosphere (or capable of retaining a transplanted atmosphere) also similar to that of Earth's in pressure, orbiting at the same distance and with the same periodicity from its solar system's centre of mass as Earth in our own universe.

  • Human survivability with a transplanted biome for one million years. It doesn't have to be pleasant or easy.

  • A system primary star that puts the earth-analogue planet near the inner edge of its goldilocks zone, and appears optically to be bigger, redder, and possibly dimmer. I don't care if it has to burn helium to meet the requirements or if it can do so with hydrogen, and other things must change so that it doesn't engulf the inner planets... or if it must have a different mass to our own sun.

This is a companion question to When universes collide, and I'll be having lifeforms from Earth seeded repeatedly onto this other universe's planet, which can take care of atmosphere too, if necessary. Survival of any of it greatly beyond the scope of the story is irrelevant.

Time need not pass at the same rate in this altered universe as in our own, as each will have its own seperate time dimension.

Finally, if no other option is viable, I may be open to a little handwavium in the form of introducing a factor or two that we either don't have or aren't currently aware of also being tweaked to bring things back to being human-viable.

  • $\begingroup$ This is referring to tooling around with the fundamental forces - gravity, electromagnetism, weak force, and strong force, correct? $\endgroup$
    – Halfthawed
    Commented Jun 1, 2020 at 15:17
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    $\begingroup$ @RandySavage Variable speed of light is not mainstream physics. Here's a link to just some of the criticisms of these ideas. Note headlines saying things like "scientists say" should really say e.g. "just these scientists say ...". $\endgroup$ Commented Jun 1, 2020 at 17:16
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    $\begingroup$ I find that closing questions for being opinion based too early on is a lot like accepting answers too quickly. Until you see what answers are out there, you can't really assume there isn't a good one. I've seen a lot of questions nearly get closed as opinion based pretty quickly only for someone to come along with a very soild and indisputable best answer that other people just did not think of first. $\endgroup$
    – Nosajimiki
    Commented Jun 1, 2020 at 21:56
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    $\begingroup$ The main issue with POB in this particular forum is that, if we really want to follow Stack Exchange's rules anything like close to the letter of the law, then we'd have to close something like 90% of all questions we get here. Simply because, even though there may be some kind of "objective basis", the reality is that those 90% all deal with irrealia -- places that don't exist, natural systems that are at odds with our world. And the 10% that can remain open are the real world questions, the maths questions, the history questions that we've never bothered weeding out. Long and short of it:... $\endgroup$
    – elemtilas
    Commented Jun 1, 2020 at 23:40
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    $\begingroup$ I don't see how this can be opinion-based when it can clearly be shown that anything more than an insignificant change to any one of the constants will result in a human-uninhabitable universe. $\endgroup$
    – Monty Wild
    Commented Jun 2, 2020 at 0:57

2 Answers 2


It sounds like you are asking if we can play around with the fundamental constants of the universe and still have it capable of playing host not just to generic life, but human life.

Based on the observed "coincidences" around the anthropic principle I am afraid we don't have that much room to maneuver.

The observed values of the dimensionless physical constants (such as the fine-structure constant) governing the four fundamental interactions are balanced as if fine-tuned to permit the formation of commonly found matter and subsequently the emergence of life. A slight increase in the strong interaction would bind the dineutron and the diproton and convert all hydrogen in the early universe to helium; likewise, an increase in the weak interaction also would convert all hydrogen to helium. Water, as well as sufficiently long-lived stable stars, both essential for the emergence of life as we know it, would not exist. More generally, small changes in the relative strengths of the four fundamental interactions can greatly affect the universe's age, structure, and capacity for life.

What would happen to humans traveling to these exotic universe?

Their atoms would probably, due to the new constants they would have to adhere, become unstable and decay.

"Houston, I am seeing the light" would probably be the only sentence we would hear back from them.

An interesting exploration of the Anthropic Principle which I think pertains nicely to this question.

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    $\begingroup$ I know this is asking a lot, but do you suppose it'd be survivable if the constants were only adjusted slightly - say by a factor of less than 0.1? $\endgroup$
    – Halfthawed
    Commented Jun 1, 2020 at 15:48
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    $\begingroup$ Exactly as Halfthawed said... I'm talking about very small or balanced changes that change a few things about the nature of stars and planets, while leaving human biochemistry functional... not necessarily optimal, but working well enough for people to be able to colonise the place. $\endgroup$
    – Monty Wild
    Commented Jun 1, 2020 at 15:52
  • $\begingroup$ @MontyWild since we have different strengths of gravity at different astronomical objects, could that be changed without a cascade of other aspects breaking down? $\endgroup$
    – user69935
    Commented Jun 1, 2020 at 15:56
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    $\begingroup$ @RandySavage I don't really know enough to say... it seems that if you reduce the gravitational constant enough, stars don't ignite, and if you increase it enough, more gas-giants would be stars instead... but the stars would burn hotter and faster, and in a way, I'm after bigger and cooler, but still fusing hydrogen, so the gravitational constant isn't the only thing to tweak. $\endgroup$
    – Monty Wild
    Commented Jun 1, 2020 at 16:02
  • $\begingroup$ @MontyWild - I think the basic point, as far as I can understand the explanations of what's going on regarding the principle, is that even changes in the range you're talking about screw things up for life as we know it! In other words, and these are strong implications for most of us worldbuilders, we can't really play around too much with those physical constants lest we end up with a universe that is entirely hostile to the life we want to put in it. $\endgroup$
    – elemtilas
    Commented Jun 1, 2020 at 16:19

Electromagnetism cannot even be modified by even a fraction of a %. Molecular structures found in biology are formed under very tight controls. Minor alterations would cause the rules of chemistry to change which would make your DNA restructure in ways that would for all intents and purposes turn your body into solid cancer. It could also cause materials in your body that do not react to spontaneously combust as things resettle into thier new chemical bonds. Between the two of these issues, jumping into a universe with even slightly different rules of electromagnetism would probably be a very unpleasant way to go. Such a universe may still have life, it just would not be compatible with our version of life.

Because the speed of light is so tied up into electromagnetism, it is reasonable to assume that manipulating the speed of light would have unintended consequences that would affect how electromagnetism works; thus, also off limits as far as safety is concerned.

The strong force, weak force, and gravity would give you a bit more wiggle room as long as you don't need your timelines to match up between over-layed universes. Messing with these you could mess around a decent amount with the way stars and planetary cores form, without making the universe total inhospitable.

Planets have the same orbits as in our own universe.

If gravity were greater or less than it is, you would have to speed up or slow down the orbits of the planets to maintain the same orbital distances. This mean your other Earth with lower gravity would have a longer year and vise versa.

If you mess with the strong force you can change at what points stars fuse and explode. Messing with this could mess a lot with the composition and behaviors of the planets and stars without changing the orbital distance or period.

Messing with the weak force will not have a lot to do with this.

Stars are larger and cooler (I.e. redder) but do not significantly change the Goldilocks zone, save perhaps to move it outward such that Earth would be hotter but still survivable.

Higher gravity would cause a smaller bluer hotter sun that burns out more quickly, and lower gravity would cause a larger redder cooler sun that lasts longer. Despite being bigger the red star would make Earth colder because there were be less fusion happening; so, less energy would be radiated by it. The physics of a helium burning red star, and a hydrogen burning red star are not the same; so, to make predictions about your stars, you should think of your sun as a large red dwarf and not as a small red giant.

Changes to the strong force could also red/blue shift a star since it would make fusion easier or harder under the same gravity. But doing this would make your red shifted stars smaller and your blue shifted stars bigger which is the opposite of what changing gravity would do.

Changes to the weak force could push radioactive decay to make the Earth's core hotter or cooler. Making the weak force stronger would make the Earth's core hotter in its infancy, but would expend the Earth's radioactive fuels that keep the core molten faster. So, there are a few ways to play with this to heat or cool the world a bit differently, to compensate for the heat of the sun a little bit.

Brighter night sky due to more/closer/larger/brighter nearby stars, and/or a hydrogen-fusing Jupiter

Jupiter fusing enough hydrogen to light up the Earth's night sky requires significantly more gravity or weaker strong force. This means you would not have this phenomenon with a red sun, but rather a blue one. This creates a bit of a paradox though because blue stars burn out so much faster than yellow ones that changing gravity enough for Jupiter to become a red dwarf like this would not give life on Earth enough time to evolve before being consumed by the sun's Red Giant phase. If the Earth somehow survived in this universe, the sun would merely be a solar remnant by now like a white dwarf or a neutron star. I would not put money on Earth having any life on it at this point... also, there is a good chance that the Earth and Jupiter alike would have been destroyed by the red giant and supernova phases of the sun's life cycle.

A better approach here would be to bring the moon closer to the Earth. The relative brightness of a full moon is 11 orders of magnitude greater than that of any star; so, a small change here will have a much greater effect than on the stars themselves. The moon is believed to have been created by a massive asteroid impact long ago. Even a very tiny change in the constant of gravity could have caused this massive impactor to miss the Earth, or put the Earth in the path of other impactors, or just make the massive impactor have to make a few million more laps around the sun before syncing up with the Earth's orbit. So, a slight nudge in the constant of gravity could make the moon hundreds of millions of years younger (and therefore closer), it could make multiple moons, or it could make no moons at all.

Changing the weak force should not have a huge impact here.

An earth-sized world in the same position as Earth with respect to its primary with lower surface gravity and an atmosphere with equivalent pressure.

A noticeably lower gravity would not have the same atmospheric pressure. Less gravity would make the Earth's core less active which means the Earth would be slower to release new gases into the atmosphere. Lower gravity also means a shallower gradient to the atmosphere that we do have so less of the air would be down low for us to breath. Putting more of the air up higher and less magnetism from our active core also mean that solar radiation would whisk it away faster. Some of this may be offset by a less radiation from the weaker sun though. (Higher gravity would reverse all these variables and probably give the Earth a runaway greenhouse effect leaving us looking a lot like Venus.)

A stronger weak force might make the Earth's core warm enough from radioactive decay to compensate for less gravity, but again, you have to worry about having enough fissile fuel in the core to maintain that heat.

Messing with the strong force too might produce an Earth with enough fissile fuel to maintain this, but then you are looking at a complex balancing act of these three forces which will have all sorts of consequences for the universe as a whole which would probably prevent our entire solar system from actually forming.

Clusters of separated solid and liquid matter suspended in an atmosphere with equivalent pressure to that of earth at the equivalent distance from the centre of gravity as earth's sea level. Very low gravity. For this case, gravity bleedthrough from a very close parallel/alternate universe (our own) would be permitted in order to allow such a world to be human-viable.

This would require a re-writing of physics on a much bigger level than some basic universal constants, but my guess is that whatever makes this happen would probably be so alien to our physiology as to result in a very unpleasant demise.

Making Your Hellscape:

To get a darker world with red skies and increased temperature you want to start by increasing the weak force. This will hasten radioactive decay making the core of the Earth much hotter and increase volcanic activity. The increased number of volcanoes will spit out more CO2 and water vapor into the atmosphere which scatter lower frequency lights than O2 and N2 which will both create a greenhouse effect and shift the appearance of the sky toward white instead of blue. As an added bonus, nothing says hellscape like a planet with lots of volcanoes!

Alone, this would give you a lethal runaway greenhouse effect; so, you also need to drop the intensity of the sun by either decreasing gravity to reduce the amount of fusion causing pressure inside the sun or by increasing the strong force which would stabilize nuclear binding strengths making fusion harder to achieve. Either way you get a redder darker sun.

With a red sun shining through a "white" atmosphere, the sky should then appear red at all times.

This said, small changes have big outcomes, and the more you you go back in time the more significantly changes impact the present. If you make these changes at the beginning of time, then Earth will never form the way we known it. If you want planets that are in the same places between universes you should introduce a divergent point in the timeline. Basically, you say that instead of two universes that have always existed, you say that the universes split X number of years ago and thier constants drifted apart after the split. This way you can place the stars and planets along with what they are made out of all in the same places, but then go back a few million years ago and then allow them to evolve separately past that point. This way the Earth will still have continents in about the same places, and still have all the fissile fuel it needs to keep the core molten, etc.

The only significant hazard this might have on human physiology could be a greater amount of ambient radiation. Minerals that are not radioactive on Earth would be radioactive on Hell; so, your colonists will need to be a lot more careful about heavy metal poisoning. A lot of your early colonists will probably die of cancer, but by eliminating things like tungsten, lead, and gold from consumer products and designing architecture to insulate against ground radiation should solve these issues. If the humans on your other world natural evolved there (not a colony from this world), then this can just be handwaved away by saying these humans have evolved to live in higher radiation levels. On Earth, there are creatures than can survive in 100 times the radiation a human can; so, we know biology allows for some variance here.

  • $\begingroup$ Unfortunately, a containment system isn't going to work for me, as I need the alternate universe's planet populated by humans. $\endgroup$
    – Monty Wild
    Commented Jun 1, 2020 at 22:14
  • $\begingroup$ Is there any combination of tweaks that would keep our biochemistry mostly working but change other things a bit? The effect I'm looking for is "hell": looks darker and redder & feels hotter, but not a lot different otherwise. $\endgroup$
    – Monty Wild
    Commented Jun 1, 2020 at 22:17
  • $\begingroup$ @MontyWild Yes, I think so. See revised answer. $\endgroup$
    – Nosajimiki
    Commented Jun 2, 2020 at 13:50

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