Theories of Everything

by Mariel Pettee

A physics discovery requires immense patience and resilience, but every so often, the payoff is dazzling. Scientific results can incite irreversible shifts in our understandings of our universe and even our own personal realities. Through the lens of physics, for example, we now understand that time doesn’t tick by at the same pace for everyone, that the relics of black hole collisions from billions of years ago rippled through spacetime itself to reach us today, and that as far as we know, everything around us (including our own bodies) is composed of a handful of fundamental building blocks we call particles. Such mind-boggling results inevitably call for a human perspective, aside from all the plots and numbers, to help us process the discovery as part of our personal origin story. 

But many physicists, myself included, are reluctant to talk about what our results should mean to the world beyond a purely pragmatic level, or to tout physical results as a description of an experienced or objective “reality.” We’re focused on the process, and we need to keep the conversation tightly within the confines of what we can measure in order to keep that process churning efficiently. 

To venture outside of scientific shop-talk can be dangerous, too, since the vocabulary of a physicist is peppered with words that we’ve all seen before: energy, space, time, dark, information, measurement, decay, symmetry. It is hugely tempting for a non-scientist to take a physics result, with words they think they understand, and extrapolate it onto their ‘real’ human experience — in which case the physics is usually rendered nonsensical. 

Physics thrives in crisply-defined regions of possibility. Every major physics theory necessarily carves out its own ranges in size, energy, temperature, speed, etc. for which it is a valid description of the universe, such as “for lengths much longer than the diameter of an atom” or “for speeds far smaller than the speed of light”. For example: while physicists observe both particle-like and wave-like behaviors from matter at tiny length scales, the wave-particle duality concept just isn’t very useful when describing humans made up of those same particles. In fact, humans are demonstrably not quantum mechanical because we’re both way too big and way too hot to fall within the range at which quantum mechanical effects become important.

Many popular depictions of physics emphasize a search for a “Theory of Everything” that can one day describe the entire universe flawlessly, regardless of what range you're looking at. In my opinion, this misses the point. I don’t think we’ll ever approach a Theory of Everything. Instead, we can strive for a relatively seamless patchwork quilt of descriptions of “reality” at various scales. As a particle physics researcher at the Large Hadron Collider, I admit that this idea has been difficult to confront. I wish that understanding fundamental particles like the Higgs boson could unlock all the answers for us.

I’ve now come to terms with the fact that while it would be interesting if every behavior in the universe could be perfectly traced back to its origins at the smallest scales, our universe doesn’t appear to work that way. All too frequently, scientists find examples of so-called “emergent” behaviors that can’t be predicted or described by the physics that defines their constituent parts. A comprehensive knowledge of particle physics can’t tell us much about friction, biology, or whether it’ll rain today. It’s impossible to measure the temperature of a single particle, because

we commonly define temperature as an average property of a collection of particles. Given that the concept of temperature loses its meaning at a certain scale, some theorists have even proposed that the same could happen for spacetime itself.

The concept of emergence isn’t just a physical phenomenon — it pops up in math as well. In my study of quantum field theory (roughly speaking, a treatment of quantum mechanics combined with special relativity), I’ve learned that admitting the limits of our theories can actually enable us to do powerful calculations. The everyday work of quantum field theory can yield some nasty integrals whose answers end up being infinity. Usually, when physicists encounter an infinity in a calculation, that’s our cue to crumple up that sheet of paper and toss it in the trash. One creative tactic in quantum field theory, however, involves imposing an upper limit to our theory’s predictive power, and as a result, our answers actually become finite.

In doing so, we essentially demote the theory we’re working with from a theory of all of particle physics to a theory of a specific energy range of particle physics. This idea of defining a theory that’s only valid up to a certain point — an “effective” field theory — turns out to not just be a cute mathematical trick, but a cornerstone of modern particle physics. Quantum field theory has now unified our understandings of every fundamental force in the universe apart from gravity. Armed with the ability to understand where and why our theories can break down, physicists don’t have to try to understand everything about the universe all at once. Instead, we can (and must!) break this monolithic problem into bite-sized pieces.

Reality is right here, all around us, described by equations that can predict the trajectories of baseballs and children on swingsets. But zoom in or out enough in scale and you’ll find a realm that’s utterly foreign, with sometimes a completely new set of laws required to understand it. That’s reality, too, though it’s no more “real” than the one right in front of you, because the universe is best described thus far by a collage of many equally valid physical theories, not one overall Theory of Everything.

This is good news, and in my eyes, the heartbeat of science. Rather than discovering one fundamental reality, we can instead spend our days unlocking a series of doors, always knowing there will be something unexplainable on the other side.

Mariel Pettee is a PhD candidate in Physics at Yale University who researches a particle called the Higgs boson using data from the Large Hadron Collider at CERN. As a choreographer, writer, and performer, she also uses art to research audience activation, duration, authenticity, fear, and playfulness.