It's actually hard to come up with any conceivable experiment to look for the existence of universes that don't interact with ours in any way... heck, we have a hard enough time with the 'cold, dark matter' which apparently constitutes up to 96% of our own universe.
But that 'any aspects shifted'/'sustain life by chance' idea is a little bit wrong. Yes, if the fundamental constants of the universe were different, we wouldn't see this universe... but that's like saying '2+3=5! If either 2 or 3 was changed, they wouldn't be able to equal 5 any more!'
In fact, it's a lot like that. There are two big holes in the idea:
1/ What makes 5 so special? If 3 turned into 4, you wouldn't make 5 - but you'd make 6, and that 6 would no doubt come up with a Weak Sixthropic Principle which claims that the universe was fine-tuned to make 6. The life we have now (or even the uranium we have now - why're we focussing on life when there's far more of other stuff? If the universe is balanced for anything, it's hydrogen and dark matter!) is hardly the only setup that an infinite number of universes could possible contain. Bind electrons more tightly to their atoms, and you could create life based on covalent bonds between metals.
2/ 2+3=5... but so does 1+4. You can 'compensate' for shifting one constant by shifting another to match. There's a lot more options for 'universe containing life' than you might think.
3/ Did I say two? I meant three. ^_^ The third is error bars. I rounded the actual values of 2.10526, 3.33983, and 5.44509 to get that lovely neat 2+3=5. Any values that round to 5 would've worked. Similarly, the 'fine tuning' of the universe is rather more coarse than you might imagine - the figures could vary by a long way and still give substantially the same universe.
There's a lovely example of this in one of the Science of Discworld books... actually, I think it's in three of the four, they enjoyed it so much! For your reading pleasure, I'll now spend twenty minutes copying it out from Google Books:
'The story of the carbon resonance can be found in many science books, because it creates a powerful impression of hidden order in the universe, and it seems to explain so much. [...]
'Here's how the story goes. Carbon [which living cells are based on] is created in red giant stars by a rather delicate process of nuclear synthesis, called the triple-alpha process. This involves the fusion of three helium nuclei. [...]
'All very well, but the odds of such a triple collision occuring inside a star are very small. Collisions of two helium nuclei are much more common, though still relatively rare. It is extremely rare for a third helium nucleus to crash into two that are just colliding. [...] This means that the synthesis of carbon has to take place in a series of steps rather than all at once, and the obvious way is for two helium nuclei to fuse, and then for a third helium nucleus to fuse with the result.
'The first step is easy, and the resulting nucleus has four protons and four neutrons: this is one form of the element beryllium. However, the lifetime of this particular form of beryllium is 10^-16 seconds, which gives that third helium nucleus a very small target to aim at. The change of hitting this target is incredibly small, and it turns out that the universe hasn't existed long enough for even a tiny fraction of its carbon to have been made in this way. So triple collisions are out, and carbon remains a puzzle.
'Unless... there is a loophole in the argument. And indeed there is. The fusion of beryllium and helium, leading to carbon, would occur much more rapidly, yielding a lot more carbon in a much shorter time, if the energy of carbon just happened to be close to the combined energies of beryllium and helium. This kind of near-equality of energies is called a
resonance. In the 1950s Fred Hoyle insisted that carbon has to come from somewhere, and predicted that there must therefore exist a resonant state of the carbon atom. It had to have a very specific energy, which he calculated must be about 7.6 MeV.
'Within a decade, it was discovered that there is a state with energy 7.6549 MeV. Unfortunately, it turns out that the combined energies of beryllium and helium are about 4 per cent higher than this. In nuclear physics, that's a huge error.
'Oops.
'Ah, but miraculously, that apparent discrepancy is just what we want. Why? Because the additional energy imparted by the temperature found in a red giant star is exactly what's needed to change the combined energy of beryllium and helium nuclei by that missing 4 per cent.
'Wow.
'It's a wonderful story, and it rightly earned Hoyle huge numbers of scientific brownie points. And it makes our existence look rather delicate. If the fundamental constants of the universe are changed, then so is that vital 7.6549. So it is tempting to conclude that our universe's constants are fine-tuned for carbon, making it very special indeed. And it is equally tempting to conclude that the reason for that fine-tuning is to ensure that complex life turns up. Hoyle didn't do that, but many other scientists have given into these temptations.
'Sounds good: what's wrong? [...] Another physicist, Craig Hogan, has put his finger on one of the weak points. The argument treats the temperature of the red giant and that 4 per cent discrepancy in energy levels as if they were independent. That is, it assumes that you can change the fundamental constants of physics without changing the way a red giant works. However, that's obvious nonsense. Hogan points out that 'the structure of stars includes a built-in thermostat that automatically adjusts the temperature to just the value needed to make the reaction go at the correct rate'. It's rather like being amazed that the temperature in a fire is just right to burn wood, when in fact that temperature is caused by the chemical reaction that burns the wood...'
When they revisit it in a later book, they talk about how much variance you can have in the figures before stars become impossible. From memory, the conclusion was that roughly 25% of possible universes obtained from varying the fundamental constants would look substantially like ours.
So yeah: even if there has only ever been one universe, created by a completely random process, with no tuning of any kind... you've still got around one chance in four that we'd turn up to marvel at how lucky we are. ;)
hS
(PS: Or comets. Maybe the universe is fine tuned for comets. There's certainly a lot more of them than there are people, and they look so pretty... has any considered that our solar system might be a Christmas decoration?)