One of the concepts that always crops up when people discuss fine-tuning is “naturalness”, which is basically akin to saying “what you’d have expected”.
For instance, in everyday life its natural that a mother would comfort her crying baby - if she didn’t, we might find that a bit strange, a bit unnatural (surprising, not what we’d have expected) and wonder why she might refrain from comforting her babe. We’d think this cries out for an explanation.
In the context of physics, likewise, the cosmological constant has a surprisingly small, nonzero value compared to what you’d naturally expect if calculating it from first principles - and yet the observed value is inexplicably conducive to complex chemistry and by extension life. Change it to fit the more “natural” calculated value and we wouldn’t be here to even ponder this due to the fact that in the possible range of hypothetical universes one can think of where this constant, which drives the expansion of the universe, were significantly more energetic (as you’d expect working from first principles), matter would instantly disperse and no stars and galaxies would be able to form.
The incredible delicacy of the cancellation involved here is particularly disturbing. This amazing sensitivity means that the properties of our universe have to be, very precisely, just as they are — like a radio that is set exactly to the frequency of a desired radio station, finely tuned.
Supercomputers have been able to demonstrate this with the right modelling software too.
Professor Matt Strassler, a leading particle physicist working at CERN, used the analogy of kids running around a table with a vase to explain this:
After nearly an hour with the kids playing nearby, where is the vase? On the table? Smashed on the floor? Or right at the edge? We’d all believe the first two before we’d believe the third — unless the third was carefully arranged.
You get three options. Choose the most plausible.
- The vase was exactly where mother left it, comfortably placed at the center of the table.
- The vase was smashed, and the flowers crushed, down on the floor.
- The vase was hanging off the table, right at the edge, within a millimeter of disaster.
Professor Strassler notes:
Well, the answer is #3. There it was, just hanging there.
I suspect you don’t believe me. Or at least, if you do believe me, you probably are assuming there must be some complicated explanation that I’m about to give you as to how this happened. It can’t possibly be that two young kids were playing wildly in the room and somehow managed to get the vase into this extremely precarious position just by accident, can it? For the vase to end up just so — not firmly on the table, not falling off the table, but just in between — that’s … that’s not natural!
There must (mustn’t there?) be an explanation.
Maybe there was glue on the side of the table and the vase stuck to it before falling off? Maybe one of the kids was hiding behind the table and holding the vase there as a practical joke on his mom? Maybe her husband had somehow tied a string around the vase and attached it to the table, or to the ceiling, so that the vase couldn’t fall off? Maybe the table and vase are both magnetized somehow…?
Something so unnatural as that can’t just end up that way on its own… especially not in a room with two young children playing rough and throwing things around.