Question: What kind of things might exist? (e.g another universe, another planet with humans on)

Susan Cartwright answered on 14 Jun 2015:

This is a tricky one, because “might” is such a vague term. How improbable does something have to be for it not to count as something that “might” happen? I mean, according to quantum mechanics, I “might” be able to walk through my office wall – it’s just that the probability if my doing so is so infinitesimally small that I could have tried to do it 100 times a second for the entire age of the universe (if the wall and I had existed at the beginning of the universe!) and the probability of my having made it through would still be small compared to the chance of winning the lottery next week.
However, if what you mean is “what kind of things are permitted by the laws of physics, as we understand them?”, then I can at least attempt an answer, along the lines suggested by your examples.

The first thing to understand is that our universe is certainly extremely large, and is very possibly infinitely large – and that there is a huge difference between “extremely large” and “infinitely large”. If our universe is genuinely infinitely large (which is what the current observations suggest, though like all observations they do have a margin of error, which would permit merely “extremely large”), then anything which has a non-zero probability of existing will exist somewhere. Another planet with humans on has a non-zero probability of existing – some of the stages in our evolution may be extremely improbable, but none is impossible, or we would not be here – so, in a genuinely infinite universe, another planet with humans on exists. In fact, infinitely many planets with humans on exist. But they exist a very very very long way away from here, because the probability of exactly duplicating the steps that led to humans is very small indeed, so the number of such duplicates per billion billion billion cubic light years is also very small indeed – but not zero. There are also infinitely many planets with a copy of yourself – same DNA, same life history, same everything – but they’re even further apart, because the probability of a duplicate of you is very much smaller than merely a duplicate of the human species in general. (If our understanding of quantum mechanics is right, there are even an infinite number of planets on which I manage to walk through that wall – but they are so astounding far apart I can’t even begin to calculate the distance.)
None of this is true if the universe is merely extremely large, and not infinitely large. If there are only a finite number of planets out there, then very small probabilities are unlikely ever to come to pass (there are no planets in which a duplicate of me has just walked through my office wall). It then becomes important just what small probability you assign to any given event. For example, our own Galaxy contains about 100 billion stars. Observations over the past couple of decades suggest that a large fraction of these have planetary systems – say between 100 and 1000 billion planets in our Galaxy. Therefore, any form of planet which has a probability of more than 1 in 10 billion (say) probably does exist in our Galaxy, but any form of planet with a probability of less than 1 in 10000 billion probably doesn’t. We ourselves don’t count, because our Galaxy is defined to be the one that we live in, so the probability of our existing in our Galaxy is 1. What about other humans? Well, how many planets are sufficiently Earth-like to allow life to develop? We don’t really know, because planets like the Earth are too small to be easily detected: most of our sample of detected planets are larger and closer to their star than the Earth is, but that’s because such planets are easier to detect (if you go looking for insects in your garden, you will find the large, brightly-coloured ones more often than you find tiny camouflaged ones – not because the big bright ones are really more numerous, but because the tiny camouflaged ones are much harder to see). Let’s say perhaps 1 planet in 100, which gives between 1 and 10 billion possible abodes for life. Then you have to consider how likely it is that life would evolve on any suitable planet, how likely it is that life, once evolved, will develop into complex multicellular organisms (remember that for most of the history of life on Earth, only single-celled organisms existed), how likely it is that the evolution of multicellular organisms would progress as it did here, and so on. Some of the earlier stages in this chain of probabilities are measurable (we should soon have a reasonable idea of what fraction of planets are Earth-like, and within a few decades we should be able to look for signs of life, such as oxygen-rich atmospheres), but most of the later ones are not (it is hard to see how one could distinguish single-celled and multicellular organisms from 100 light years, and short of receiving a radio message from ET almost impossible to identify intelligent life-forms, let alone humanoid ones). My personal suspicion is that there are many life-bearing planets in the Galaxy, but few with complex life and very few or none with anything approaching an alien civilisation. I think the distance to the nearest planet with a species indistinguishable from humans is probably much larger than the radius of the observable universe: in other words, I think the probability of that is very small compared to 1 in 100 billion.
Other universes, surprisingly, are quite probable: both of our best current theories of the early universe (inflation, and extra dimensions), naturally lead to the likely existence of other universes (in inflation, our universe is a “bubble” where the initial exponential inflation stopped, replaced by the slower expansion we see now: in many inflation models, there would be other such bubbles, representing other universes; if our universe is a 3D structure, or “brane”, existing in a 4-dimensional space, or “bulk”, there could easily be many other such branes, like (approximately 2D) pages in a (3D) book). Unfortunately, it would be extremely difficult, probably impossible, to obtain direct evidence of this: however, if a theory which predicted the existence of other universes became experimentally established because all its other predictions were tested and found to be correct, then most scientists would accept this as indicating that the existence of other universes was at least highly likely. (In the same way, we have no *direct* evidence of the existence of black holes. However, the theory that predicts their existence – General Relativity – is well-tested in its other predictions (the behaviour of orbiting objects in strong gravitational fields, the slowing of clocks in gravitational fields, the bending of starlight by massive objects, the expansion of the universe, …), and all the astrophysical objects we have discovered that GR says ought to be black holes have properties that are consistent with their being black holes (their radii are consistent with those expected of black holes, they show no evidence of having a solid surface, etc.). So pretty much every astrophysicist is confident that black holes exist, even though we don’t have definitive proof.

So, what other kinds of things might exist? Well, it’s certainly possible that at least some of the imaginary animals of folktales and fantasy stories could exist: that is, I can’t see anything in the laws of physics that precludes, say, unicorns. Dragons don’t fit into Earth’s biosphere – all our large animals are based on a four-limbed body plan, and your traditional dragon has 6: four legs and a pair of wings – but I can’t see any reason why large animals on some other planet couldn’t have developed from a 6-limbed body plan, in which case winged quadrupeds would be a natural development (flying is clearly a good idea: in the history of terrestrial life it’s been invented at least 4 times – insects, pterosaurs, birds, and bats – so it would probably be invented by an extraterrestrial biosphere as well). Breathing fire is a bit of a stretch, but lots of animals do spit nasty chemicals: some sort of organic flamethrower isn’t totally impossible (e.g. have symbiotic bacteria that process some of your food into a flammable hydrocarbon, and an electric organ to ignite it with a spark as you spit it out: bacteria in ruminants do produce methane, and some fish can generate quite nasty electric fields, so the individual ingredients do exist even though no terrestrial life-form has put them together). They’d probably be a good bit smaller than fairytale dragons, but the largest pterosaurs were the size of a small plane, and perhaps the atmosphere of the hypothetical planet would more easily support flight (higher pressure perhaps, and higher oxygen content to allow faster metabolism). Vampires might work – there are animals that survive by drinking blood, e.g. leeches and vampire bats – and it’s possible to imagine that the metabolism of such a creature would produce chemicals that caused a severe reaction to sunlight (some people with genetic diseases have such reactions), though spontaneous combustion seems unlikely. But I don’t believe in vampires that don’t reflect in mirrors – not repealing the laws of physics, sorry! – or ones that transform into bats much smaller than themselves without going up in a spectacular nuclear explosion courtesy of E = mc^2.

I think I’ve wittered on long enough. Maybe someone else will offer more ideas! Also, it would be worth trying Brian Greene’s book “The Hidden Reality”, which discusses in detail various types of “parallel universe”.