Paradigm Shifts

One day, when I'm old and my hair is grey, and I'm sitting in a rocking chair stroking a cat on my lap when the neighbor's son comes with a book on the history of science, I want to say "Yes, I was there."

There are as many different motivations to become a physicist as there are physicists. But one of them is certainly the wish to be part of something greater, an event of historical importance. It's the wish to be there and have a say when our view of the world fundamentally changes; when a new picture comes into focus that will be passed on to future generations.

The change of our fundamental understanding of Nature, the emergence of a new way of thinking about the world is what is known as a "paradigm shift." It's a notion that occasionally creeps up in a discussion. It most often does so either as a means of defense, when a new proposal is widely rejected or when the speaker tries to make himself more interesting.

I was wondering the other day what paradigms there are that might be shifting today. In the 22nd century's textbooks, what of our today's understanding will appear in the historical appendix instead?

There's three such potential paradigm shifts that I've come across.

The first one is about the limits of reductionism. With the incredible success reductionism has had in physics in the first half of the century, there came the believe that one day we'll be able to explain everything by taking it apart into smaller and smaller pieces. This paradigm has by now pretty much shifted over in favor of acknowledging that emerging features might not be possible to explain, either in practice or in principle, by reduction to more elementary constituents (see also). This change on our perception came along with the rise of chaos theory and complexity, features that are both very common to natural systems and hard if not impossible to address by reductionist approaches. It is funny in fact how silently this shift seems to have taken place. You sometimes find today people in talk vigorously arguing that reductionism has limitations, just to find there's nobody actually disagreeing with them. Except for the old professor in the front row.

The second potential paradigm shift that has crossed my way is the multiverse. The multiverse I have in mind is the one forced upon you from the string theory landscape, a vast number of possible universes with different laws of Nature, versus the previously prevailing idea that our universe is unique and is so for a reason that we have to find. Various other sorts of multiverses seem to creep up from other considerations. The multiverse is presently a very hotly discussed topic, with strong defenders both for and against it. I have previously expressed my opinion that the multiverse isn't so much a new paradigm but a new way of thinking about an old paradigm. Instead of finding a way to derive the parameters of the standard model as a 'most optimal' configuration of some sort, one now searches for a measure in the multiverse by means of which our universe is (ideally) most likely. It's a watered-down version of the same game. In any case, I recall that Keith Dienes (the guy with the String Vacuum Project) spoke about the probabilistic attempt as a new way of thinking about "why" we have exactly these laws. And yes, I was thinking, maybe he's right and in some decades from now that will be how we think about our reality. That we're embedded in a vast number of different universes which different laws of Nature and our grandchildren will laugh about that we once thought we were unique.

The third potential paradigm shift is that spacetime might not be a fundamental entity. I think that everybody who works on quantum gravity (whatever sort of) is familiar with the idea. But I noticed on occasion, most recently when I was talking to Sophie about Verlinde's emergent gravity scenario, that the idea that space-time is only seemingly a smooth, continuous manifold with a metric on it and on small scales might be nothing like this is not very widely spread outside the community. While there are many approaches towards finding a more fundamental description of spacetime, they each suffer from their own problems. So I think it is pretty unclear presently whether this will turn out to be a true (and useful) description of Nature in some way. But it's certainly a thought hanging in the air. On the completely opposite side is the idea that space-time instead is the only fundamental entity and that matter indeed emerges from it (an idea that dates back at least to Kaluza and Klein). Or that neither is fundamental, but arises from something unified that's neither matter nor space-time.

These are all ideas that physicists have been chewing on for quite some while now. I am curious how people will be think about them in the future, if they will laugh about or foolishness or admire our imagination.