Unification

25
Nov

Is the Standard Model as Good as it Gets?

We all dream of finding the one: dependable, motivated, and beautiful, “the one” has it all. But can the search for perfection keep us from appreciating the good thing we already have?

I’m talking, of course, about the search for the one theory of everything, a theory of physics that works in all circumstances no matter how extreme, is motivated by observations, and can be expressed in a few elegant axioms. While some theorists devote their careers to finding the one, others believe that this ideal may be fundamentally unattainable. So we are left with the big question: Is the hope for the one theory of everything realistic, or should we be satisfied to settle down and grow alongside the theories we have?

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From the vastness of the cosmos to the inside of an atom: Can one theory accurately describe every layer of the cosmic onion? Image credit: Adapted from Brian Westin, ProLithic 3D & NASA/JPL-Caltech/T. Pyle (SSC/Caltech) by Greg Kestin.

We are currently living with a beautiful theory, a theory that almost has it all: the Standard Model. It is the most precise theory in human history. The Standard Model can make predictions that match experiments to one part in 10 billion. That is like measuring the width of the United States to the accuracy of a human hair. The Standard Model explains the Sun’s glow, the inner workings of computers, and every atom that makes up our bodies. This theory is in our hands, it’s reliable, and we’re pretty happy…but it isn’t perfect.

There is one huge, glaring omission in the Standard Model: It doesn’t explain gravity. Of the four forces in the universe—electromagnetism, the strong force (holding nuclei together), the weak force (governing radioactive decay), and gravity—gravity is the black sheep. Not only is it the runt of the forces, with a strength around a trillion trillion trillion times weaker than the typical strength of the other forces, but our current theory of gravity is completely separate from, and at odds with, the Standard Model. Until we reconcile the two, humanity’s understanding of the universe will be incomplete.

Some scientists believe that this reconciliation is just around the corner. Theoretical physicist Garrett Lisi, for one, thinks that extensions of his model, which aims to unify general relativity and the Standard Model within a single framework, can “reproduc[e] all known fields and dynamics through pure geometry.”

“The theory currently evolving from this observation is wonderfully complex and gives me hope that we might be getting close to the full picture,” says Lisi.

Others, like Dartmouth physicist Marcelo Gleiser, argue that we are stuck with at least partial ignorance. “As long as we can’t measure all there is in the natural world—and the point is that we simply can’t—we can’t have a theory of everything. As a consequence, any theory that we may have that purports to explain ‘all’ that we know of the world is also necessarily incomplete.”

Indeed, the more closely we examine the universe, the more levels of complexity we find. Will observing the world more deeply finally lead us to a theory of everything? Or will we be perpetually pulling layers from an infinite onion—a prospect that would make innumerable physicists cry?

If it is the latter, then we must be content with what physicists call an “effective field theory.” The idea is that one should describe the world with the same degree of complexity one wishes to understand. The smaller the details you aim to study, the greater the complexity you should expect from your theory. It is like looking at the sun. If we peer at it just for a moment, it seems to be a smooth, bright, glowing sphere (see on the left, below). This is an “effective theory” of the sun. But if you zoom in (on the right), there is more going on: solar flares, sunspots, and streams of hot plasma shooting into space.

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Image credit: NASA/SDO

In the same way, if you peer at a collision between two particles (say, electrons), then the effective theory would describe this as a simple bounce off each other (see on the left, below), but when you zoom in (on the right), there is more complexity: the electrons exchange other particles, causing them to repel each other and “bounce.”

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By looking closer, you realize your perfect, simple picture of what you may have thought was “the one” correct description is just an approximation of something more complicated and complete.

Indeed, every time we find a theory that seems to “have it all,” a closer look reveals gaps and errors in the theory. Yet from the time of Archimedes, who fathered the idea that we could describe all of nature from just a few axioms, to the modern era of the Standard Model, physicists have kept searching for the one, refusing to settle for “good enough” when flaws and omissions in their theories were revealed.

It is natural to wonder, then, is the Standard Model just an effective theory, the latest in a long line of close-but-not-quite ideas? The consensus among physicist is a resounding “yes,” leaving us with questions: Is there an infinite number of layers, or if we look closely enough can we find that there is one final center onion-core? And how does gravity fit into the onion?

There is reason to believe there may be a “core” to the onion. While historically physicists have peeled back the layers of the onion by “zooming in” on ever-smaller size scales, Heisenberg’s uncertainty principle may limit the ultimate resolution at which we can observe the universe. If you go small enough, then particles don’t have a definite position, you can’t tell where they are, and looking closer could not improve the resolution. The size at which this occurs is called the “Planck scale.”

At the tiny size where we lose particle resolution, gravity, once the runt of the forces, may intensify to a strength similar to that of the forces in the standard model. Gravity would no longer be the weak outcast, giving hope that gravity may “fit in” with the standard model, producing a theory of everything that unites quantum and gravitational phenomena.

Unfortunately, current experiments are far from being able to confirm such a unification. Physicists’ most powerful experiment for examining tiny-distance physics is the Large Hadron Collider (LHC), with its incredible smashing ability that can peel away layers of the onion. But to explore the physics of the Planck scale, we would need a machine more than a million billion times more powerful.

Despite experimental limitations, some of the greatest scientists have searched for a theory of everything. Einstein spent last decades of his life looking for a theory of everything. Unfortunately he passed away before he was able to find it. Stephen Hawking also searched for the theory of everything, before having a change of heart. “Some people will be very disappointed if there is not an ultimate theory that can be formulated as a finite number of principles. I used to belong to that camp, but I have changed my mind,” he has said. Richard Feynman, who is often considered “the best mind since Einstein” once said, “If it turns out there is a simple ultimate law which explains everything, so be it—that would be very nice to discover. If it turns out it’s like an onion with millions of layers… then that’s the way it is.”

So, while searching for a theory of everything is exciting, we may be well advised to take time to appreciate what we already have.

Go Deeper
Author’s suggestions for further reading

American Museum of Natural History: Isaac Asimov Memorial Debate: Theory of Everything
A panel of acclaimed physicists, including Lee Smolin, Brian Greene, and Janna Levin, debates whether it is possible to explain the universe with a single, unifying theory.

Godel and the End of Physics
In this lecture, Stephen Hawking asks whether it is possible to find a complete set of laws of nature.

The Island of Knowledge: The Limits of Science and the Search for Meaning
In his forthcoming book, Marcelo Gleiser asks if there are fundamental limits to how much science can explain.

NOVA: A Theory of Everything
In this essay, Brian Greene explores how string theory could unite quantum mechanics and general relativity.

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Greg Kestin

    Greg Kestin holds a faculty position at Harvard University, where he conducts theoretical physics research, teaches, and produces educational online content. He earned his physics Ph.D. from Harvard, as a member of The Center for the Fundamental Laws of Nature, focusing on theoretical particle physics and quantum field theory. Over his career, he has also conducted research in nuclear physics, fusion energy, and gravitational wave physics. For over a decade he has been involved with innovative educational outreach endeavors, bringing science to both students and members of the public through his writings, videos, lectures, and multimedia.