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?
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.
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."
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.
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.
In this video pencast, theorist Delia Schwartz-Perlov explains what physicists are really talking about when they talk about extra dimensions of space. Could our universe actually contain unseen dimensions, and could these extra dimensions help unify quantum theory and gravity?
Editor's picks for further reading
Cosmos: Carl Sagan: The 4th Dimension
In this scene from the classic "Cosmos" series, Carl Sagan imagines what happens when a three-dimensional character enters a two-dimensional world.
FQXi: Taking on String Theory’s 10-D Universe with 8-D Math
In this article, discover how theorists Tevian Dray and Corinne Manogue are using ten-dimensional math to describe subatomic particles.
NOVA: Imagining Other Dimensions
Journey from a two-dimensional "flatland" to the ten- (or more) dimensional world of superstring theory in this illustrated essay.
Conventional wisdom has it that putting the words “quantum gravity” and “experiment” in the same sentence is like bringing matter into contact with antimatter. All you get is a big explosion; the two just don’t go together. The distinctively quantum features of gravity only show up in extreme settings such as the belly of a black hole or the nascent universe, over distances too small and energies too large to reproduce in any laboratory. Even alien civilizations that command the energy resources of a whole galaxy probably couldn’t do it.
Physicists have never been much for conventional wisdom, though, and the dream of studying quantum gravity is too enthralling to give up. Right now, physicists don’t really know how gravity works—they have quantum theories for every force of nature except this one. And as Einstein showed, gravity is not just any old force, but a reflection of the structure of spacetime on which all else depends. In a quantum theory of gravity, all the principles that govern nature will come together. If physicists can observe some distinctively quantum feature of gravity, they will have glimpsed the underlying unity of the natural world.
Even if they can’t crank up their particle accelerators to the requisite energies, that hasn’t stopped them from devising indirect experiments—ones that don’t try to swallow the whole problem in one gulp, but nibble at it. My award-winning colleague Michael Moyer describes one in Scientific American's February cover story, and lots of others are burbling, too. Rather than matter and antimatter, “quantum gravity” and “experiment” are more like peanut butter and chocolate. They actually go together quite tastily.
An example came out at the American Astronomical Society meeting in Austin earlier this month. Robert Nemiroff of Michigan Technological University presented his team’s study of extremely high-energy, short-wavelength cosmic gamma rays. The idea, which goes back to the late 1990s, is that short-wavelength photons may be more sensitive than long-wavelength ones to the microscopic quantum structure of spacetime, just as a car with small tires rattles with road bumps that a monster truck doesn’t even feel. The effect might be slight, but if the photons travel for billions of years, even the minutest slowdown or speed-up can appreciably change their time of arrival. Nemiroff’s team focused on gamma-ray burst GRB 090510A, observed by the Fermi space telescope. It went off about 7 billion years ago, and photons of short and long wavelength arrived at almost the same time—no more than about 1 millisecond apart. Any speed difference was at most one part in 1020, implying that quantum gravity hardly waylaid these photons at all.
Theoretical physicists have long debated whether quantum gravity would alter photon speed, and most were not surprised by the negative result. But what’s important is the change of mindset. Experimenters and observers care less about what we should see than what we can see. These are people who love to build stuff. If they can build some gizmo that might bring gravity and quantum mechanics into contact, they’ll do it, whatever the theorists might say. They take an “if you build it, something will come” attitude. Historically, physics has been well-served by going out to look at nature with a minimum of prejudice.
The latest brainstorm is to apply techniques from quantum optics and related disciplines, which manipulate photons of light and other particles in order to build encrypted communications links, develop the components of a quantum computer, and study matter at extremely low temperatures. The tool of this trade is an interferometer, an apparatus that probes the wave nature of particles. It consists of a particle source, a particle detector, and two paths to get from one to the other. Being quantum, a particle goes both ways. That is to say, the wave corresponding to the particle splits in two, travels the distance, and fuses back together again. The relative length of the paths (or anything else that differentiates them) determines whether the waves will mutually reinforce or cancel and therefore what the detector will detect.
At first glance, these setups are the last place you’d go to look for quantum gravity. They are decidedly low-energy experiments, usually conducted on lab benches the size of dining-room tables. There is nary a gamma ray or accelerated particle to be found. But Moyer’s cover story describes how an interferometer can serve as an extremely precise ranging instrument. Any change in the paths’ relative length, as you might expect if spacetime is roiled by quantum fluctuations, will register at the detector.
Last spring, a team of physicists in Vienna led by Çaslav Brukner explored another use of interferometers: to see whether quantum particles truly obey gravity as Einstein conceived it. This isn’t quantum gravity, per se—the particles are quantum, but gravity behaves in a strictly classical way. Nonetheless, it is a fascinating case of how the two theories interact. You might think that the gravity on a single particle is way too feeble to measure, but an interferometer can manage it. You set it up so that the two paths are at different heights and therefore experience a different gravitational potential, which registers at the detector.
The Vienna team proposed sending not just any particle through the interferometer, but one that acts like a miniature clock—marking time by rotating or decaying. General relativity predicts that clocks run slower the deeper they get into a gravitational field, which, in this experiment, would act to wash away the wave nature of the particle altogether. The fading-away of the wave properties would be the unmistakable fingerprint of general relativity and a stepping-stone to quantum gravity. Current interferometers lack the necessary precision to look for this effect, but it is just a matter of time. (Sorry, couldn’t resist.) For more, see the authors’ own blog post and their paper in Nature Communications last fall.
It it also possible that quantum gravity could modify Heisenberg’s famous uncertainty principle. As Sabine Hossenfelder at Backreaction described last Wednesday, gravitational effects may set a minimum length that anything in nature could ever have, which means that no matter how much momentum imprecision you’re willing to accept, a position measurement could never be more precise than the minimum length. Experiments like this one could use tiny mirrors and springboards to pick up that effect.
Still another approach suggested by the ever-inventive Viennese is to define quantum gravitational ideas in concrete rather than abstract terms. Theorists think that quantum fluctuations in spacetime might make cause-effect sequences ambiguous, with the practical consequence of changing the types of correlations physicists observe in the lab. But the Viennese suggest thinking about it the other way round: Physicists observe certain types of correlations in the lab and, from these, draw conclusions about spacetime.
Some such correlations—those that muddle cause and effect—would be be inexplicable in ordinary physics. When quantum effects enter into play, “spacetime” loses some of the most basic features we associate with it, such as the notion that objects reside in certain places at certain times. In the Viennese scenario, you lose the ability to tell a story: One thing happened, then another, then another. It becomes a Dadaist jumble.
This approach hasn’t lent itself to a specific experiment yet, but is generally inspired by the experimentalist mindset. In this, it follows a trail blazed by Einstein himself, who developed his theories of relativity by thinking of abstract ideas in a concrete way. Even when experimenters can’t build actual experiments, their feet-on-the-ground mentality provides a fresh look at some of the hardest problems in modern science.
This post is adapted and reprinted from Scientific American; find the original here.
Editor's picks for further reading
FQXi: Journeying through the Quantum Froth
Are cosmic rays revealing the quantum nature of spacetime?
FQXi: Table-Top Tests for Quantum Gravity
In this podcast, discover how scientists are probing quantum gravity using quantum optics.
Perimeter Institute: Lectures on the experimental search for quantum gravity
Watch a series of scientific lectures on experimental probes of quantum gravity.