This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Having just orbited our way through another summer solstice, it feels like time to let slip some more speculative ideas before the hot days of the northern hemisphere shorten too much again and rational thinking returns.
So, grasping a fruity alcoholic beverage in one hand, consider the following thought experiment.
The so-called ‘Fermi Paradox’ has become familiar fodder for speculations on the nature of life in the universe, so I’m not going to repeat it in any great detail here. Instead, take a look at this nice description by Adam Frank, and remember that the basic premise is: if life in the universe is not incredibly rare, it should have already shown up on our proverbial doorstep. The fact that is hasn’t is therefore interesting.
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But the universe is such a paltry thing. Hordes of physicists are telling us that our reality, our cosmos, may not be the only one – rather that we exist inside a multiverse.
This could be structured in a variety of forms: from pocket universes produced by cosmic inflation, to quantum mechanical diversions and ‘many-worlds’, to ‘branes’ in higher dimensional M-theory, and so on. Furthermore, all of these variants may not really be variants at all, they could all be mushed together into one stupendous array of realities. So many realities, in fact, that anything that can happen will (and must) happen, and will happen an enormous (dare I say, infinite) number of times.
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Other hordes of physicists (well, perhaps not hordes, but a significant and sober segment of the physicist species) roll their eyes and point out the whiff of ludicrousness in some of this talk. After all, they say, theories that can explain absolutely anything you throw at them by just saying ‘anything is possible,’ are not exactly theories according to the true scientific method, because they can’t be rationally falsified. Touché.
Now, before you harangue me on behalf of one or the other side of this argument, take another slurp of your fruity cocktail and consider the following.
Let’s suppose that the most liberal of multiverse ideas are true. In this case, a modern-day Fermi might find themselves making precisely the same statement that was made back in the 1950s: ‘Where is everyone?’
Except this time the question is not about where the interstellar travellers are, or why galactic civilizations haven’t been spotted. The new puzzle is ‘Where are all the pan-multiverse travellers and civilizations?’
Have another sip, and let’s unpack that question a little. If reality is actually composed of a vast, vast number of realities, and if ‘anything’ can, does, and must happen, and happen many, many, times, this presumably has to include the possibility of living things (whatever they’re composed of) skipping between universes willy-nilly. After all, just because physics in our universe makes that look kind of tricky, it doesn’t prevent the physics of a huge number of other universes from saying ‘sure, go right ahead!’
And there’s the rub. Discounting our all-to-human capacity for self-delusion, there is absolutely no hard evidence that we are being, or ever have been, visited by stuff from other realities. (And really, if you do feel inclined to comment and tell me I’m wrong about that, save your breath, sorry).
So what’s the answer? Why isn’t this happening?
It could be that traveling between parts of the multiverse is impossible (except it shouldn't be impossible everywhere, almost by definition) or very, very difficult.
It could be that no entity who reaches a stage where they could hop between universes actually wants to (except, there has to be someone somewhere who does. Again, almost by definition).
It could be that we're alone, the only form of life in any reality (except, yet again, almost by definition, a multiverse will contain other life).
It could also be, very simply, that there is no multiverse.
Go on, drink up, that’s what I’m doing.