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Is the Standard Model as Good as it Gets?

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  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?
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<p>
  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 &amp; NASA/JPL-Caltech/T. Pyle (SSC/Caltech) by Greg Kestin.
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<p>
  We are currently living with a beautiful theory, a theory that
  <em>
   almost
  </em>
  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.
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<p>
  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.
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<p>
  Some scientists believe that this reconciliation is just around the corner. Theoretical physicist Garrett Lisi, for one, thinks that extensions of
  <a href="http://arxiv.org/abs/0711.0770">
   his model
  </a>
  , 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.”
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<p>
  “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.
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<p>
  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.”
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<p>
  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?
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<p>
  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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<p>
  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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<p>
  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.
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<p>
  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.
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<p>
  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?
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<p>
  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.”
 </p>
<p>
  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.
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<p>
  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.
 </p>
<p>
  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.”
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<p>
  So, while searching for a theory of everything is exciting, we may be well advised to take time to appreciate what we already have.
 </p>
<p>
<strong>
   Go Deeper
  </strong>
<br/>
<em>
   Author’s suggestions for further reading
  </em>
</p>
<p>
  American Museum of Natural History:
  <a href="http://www.youtube.com/watch?v=lYeN66CSQhg">
   Isaac Asimov Memorial Debate: Theory of Everything
  </a>
<br/>
  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.
 </p>
<p>
<a href="http://www.hawking.org.uk/godel-and-the-end-of-physics.html">
   Godel and the End of Physics
  </a>
<br/>
  In this lecture, Stephen Hawking asks whether it is possible to find a complete set of laws of nature.
 </p>
<p>
<a href="http://www.amazon.com/The-Island-Knowledge-Science-Meaning/dp/0465031714">
   The Island of Knowledge: The Limits of Science and the Search for Meaning
  </a>
<br/>
  In his forthcoming book, Marcelo Gleiser asks if there are fundamental limits to how much science can explain.
 </p>
<p>
  NOVA:
  <a href="http://www.pbs.org/wgbh/nova/physics/theory-of-everything.html">
   A Theory of Everything
  </a>
<br/>
  In this essay, Brian Greene explores how string theory could unite quantum mechanics and general relativity.
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How to See Quantum with the Naked Eye

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  How can you train yourself to be a quantum detector? Quantum interactions happen at impossibly small scales. But the life-size effects are all around you. You can detect quantum mechanics all over — if you know how to look for it.
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  In the video below, see how simply squinting at a lamp highlights the quantum nature of light.
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Do We Live in a Multiverse?

<p></p><p>Sometimes the world seems suspiciously perfect. There seems to be no rhyme or reason for why gravity is 10 36 times weaker than electromagnetism. Or why a neutron is 1.00137841870 times heavier than a proton. But these seemingly random, yet precise, values landed in the extremely narrow window that would allow life to exist.</p><p>How did we end up in such a precariously balanced universe? How did we get so lucky?</p><p>Some physicists think there could be many—maybe infinite—universes. Other universes could be empty—or they could be full of things we never imagined. Perhaps we just live in the only one that can support us.</p><p></p><p></p><div>
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<p></p><p></p><p><b>Go Deeper</b><br/> <i>Picks for further reading</i></p><p>Harper’s Magazine: <a href="https://harpers.org/archive/2011/12/the-accidental-universe/">The Accidental Universe: Science’s Crisis of Faith</a> Alan Lightman delves into the history of the fine-tuning problem—and makes a case for the multiverse.</p><p>The Nature of Reality: <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/03/is-the-universe-fine-tuned-for-life/">Is the Universe Fine-Tuned for Life?</a> Anil Ananthaswamy examines the scientific community’s support for and resistance against the possibility of a multiverse.</p>

5 of the Biggest Puzzles about the Universe

<p></p><p>The discovery that light gets redder as its source moves away from Earth led astronomers to uncover some of the universe’s strangest truths, including dark energy, dark matter, and the expansion of the universe.</p><p>How well can you read these five clues and decode the universe’s best kept secrets?</p>

Q&A with Greg: Black Holes, Consciousness, Greg's Research, and More!

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  In this episode of What the Physics?! we answer questions from our viewers. This session tackles questions like:
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  1. What happens when two black holes collide?
  <br/>
  2. Why do certain particles only become detectable at the high energies inside particle colliders?
  <br/>
  3. What’s it like to do research in theoretical physics?
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<p>
  If you have questions you’d like answered, comment below or tweet @WTPhysics.
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The Biggest Puzzle in Physics: Reconciling Quantum Mechanics and General Relativity

<p></p><p>Sometimes the biggest puzzle in physics seems like the worst relationship in the universe. Quantum mechanics and general relativity are the two best theories in physics, but they have never been able to get along.</p><p>Quantum mechanics successfully describes the world of the very small, where nothing is predictable and objects don’t have precise positions until they are observed.</p><p>General relativity does well with describing massive objects. It says that the world behaves in a precise, predictable way, whether or not it’s observed.</p><p>Neither one has ever failed an experimental test. But so far no experiment has been able to show which — if either — of the two theories will hold up in the places where the two converge, such as the beginning of the universe and the center of a black hole.</p>

What is Dark Matter? A New Clue

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   The goal was to find the first light that ever existed in the universe: the moment the first stars “turned on” and the universe went from dark to light. After a decade of searching, a team of radio astronomers finally found the signal—a dip in an absorption spectrum—set off by these first stars. This discovery, if confirmed by outside experiments, is momentous in itself.
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<p>
<span style="font-weight: 400">
   But there was something else.
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   The signal was more than double what models predicted, suggesting that gas at the dawn of the universe was colder than anyone expected. The culprit? Possibly dark matter—bumping against the gas, cooling it down.
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<p>
<span style="font-weight: 400">
   If true, this could be the first evidence that dark matter—which scientists know little about—is made of particles. Now, decades since dark matter was discovered through the effects of its gravity, this may help researchers figure out how to search for it.
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Greg Kestin

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