Some physicists have long been searching for a “theory of everything”: a set of basic laws, or perhaps just a single equation, from which all the other principles of physics can be derived. But they have found there are more alternatives to choose from than there are fundamental particles in the known universe.
Many physicists who delve into these waters believe that an idea called string theory is the best candidate for a “final theory”. But the latest version offers a huge number of distinct solutions: 1 followed by 500 zeros. Each solution yields its own set of physical laws, and we have no obvious reason to prefer one over any other.
The inflationary multiverse relieves us of the need to choose at all. If parallel universes have been popping up in an inflating false vacuum for billions of years, each could have different physical laws, determined by one of these many solutions to string theory.
If that is true, it could help us explain a strange property of our own Universe. The fundamental constants of the laws of physics seem bizarrely fine-tuned to the values needed for life to exist.
For example, if the strength of the electromagnetic force were just a little different, atoms would not be stable. Just a 4% change would prevent all nuclear fusion in stars, the process that makes the carbon atoms our bodies are largely made of.
Similarly, there is a delicate balance between gravity, which pulls matter towards itself, and so-called dark energy, which does the opposite and makes the Universe expand ever faster. This is just what is needed to make stars possible while not collapsing the Universe on itself.
In this and several other ways, the Universe seems fine-tuned to host us. This has made some people suspect the hand of God.
Yet an inflationary multiverse, in which all conceivable physical laws operate somewhere, offers an alternative explanation.
In every universe set up in this life-friendly way, the argument goes, intelligent beings will be scratching their heads trying to understand their luck. In the far more numerous universes that are set up differently, there is no one to ask the question.
This is an example of the “anthropic principle”, which says that things have to be the way we find them: if they were not, we would not be here and the question would never arise.
For many physicists and philosophers, this argument is a cheat: a way to evade rather than explain the fine-tuning problem.
How can we test these assertions, they ask? Surely it is defeatist to accept that there is no reason why the laws of nature are what they are, and simply say that in other universes they are different?
The trouble is, unless you have some other explanation for fine-tuning, someone will assert that God must have set things up this way. The astrophysicist Bernard Carr has put it bluntly: “If you don’t want God, you’d better have a multiverse”.
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