This essay is part of a series called The Big Ideas, in which writers respond to a single question: what is reality? You can read more by going to The Big Ideas series page†
Perhaps because of recent commentary from tech billionaires, perhaps because of intriguing advances in quantum mechanics, or perhaps because people are so unhappy with the state of the world today, questions of reality and meaning seem to be everywhere.
Such questions were never completely ignored, but given the increasingly fantastic answers that have been entertaining people lately, now seems like a good time to think about what we know, what we can know, and what we know that is impossible.
One way to answer these questions more systematically is by: invoke the concept of an effective theory, which is the deep but simple-sounding approach to reality that physicists have taken for years. Effective theories use what is measurable to create physical laws whose claim of applicability extends only to the regime that has been tested. Such theories make predictions and describe measurements, while recognizing the possibility of a more fundamental description of nature that could emerge as measurement equipment and measurements improve.
That’s okay, because the most basic description isn’t always the most enlightening. Particle physics states that elementary particles are the core ingredients of nature. string theory continues with this paradigm to say that particles arise as oscillating fundamental strings. Both agree that without the underlying presence of elementary particles, matter would not exist.
But even the most inveterate theoretical physicist would say that doesn’t mean we can easily interpret everything in terms of those basic ingredients. Knowing that music comes from oscillating atoms doesn’t tell us what music is. A music theorist would describe music very differently from an atomic physicist. They are both right, but they answer different questions that apply at different scales.
I find effective field theories reassuring. They say you don’t need to know all the answers to find meaning and make predictions to test. You don’t need to know about those more fundamental ingredients that underlie what we see if you can’t detect measurable effects.
This is one way of thinking about almost everything – after all, we can’t say much about things we don’t even know are there. But physics goes one step further: it tells us that a theory isn’t necessarily complete if predictions fail, even by a tiny bit. A new theory can take over on a fundamental scale; it does not deny the existing theory, but shows how that theory is an approximation – albeit often a very good one for the field of interest. Isaac Newton’s laws, for example, work well enough to send a human to the moon, but quantum mechanics and relativity are more fundamental laws of nature.
We must keep in mind effective theories because new ideas will conform to that framework. You don’t have to drop everything you thought you understood about the world. And physicists can still trust their predictions. They may be approximations, but they are accurate enough to describe what we can currently observe.
However, this leads to a great insight: that small inconsistencies can reveal huge underlying ideas. Those inconsistencies could be measurements that don’t quite match predictions, or they could be purely theoretical.
The fundamental description of reality is not necessarily just one thing. Effective theories in this context are indeed a source of comfort in our search for meaning. Even without knowing all the elemental components, and although life, our planet, the solar system and the universe as we know it are all ephemeral, we can still view the world as a relevant system and find meaning.
Indeed, let’s not forget that just as complex phenomena require fundamental particles for their existence, nearly eight billion people need one world – ours – to survive. Whatever other realities are possible, the work we do to create effective theories reminds us that we must cherish and preserve the beauty of reality that underlies our present existence.
Lisa Randall is a professor of physics at Harvard University and the author, most recently, of “Dark Matter and the Dinosaurs.”