The Force of Symmetry

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It was well-known that certain quantities in nature are always conserved: the amount of energy including energy in the form of mass , the amount of electric charge, the amount of momentum. Noether showed that the symmetries of general relativity — its invariance under transformations between different reference frames — ensure that energy is always conserved. Noether and symmetry have both occupied center stage in physics ever since. Post Einstein, the pull of symmetry only became more powerful. It does. Soon after, Wolfgang Pauli, in an attempt to account for the energy that seemed to go missing during the disintegration of radioactive particles, speculated that perhaps the missing energy was carried away by some unknown, elusive particle.

It was, and that particle is the neutrino.

The Simple Idea Behind Einstein’s Greatest Discoveries

Gauge symmetries describe the internal structure of the system of particles that populates our world. They indicate all the ways physicists can shift, rotate, distort and generally mess with their equations without varying anything important. The result is a peek at the hidden scaffolding that supports the basic ingredients of nature.

Video : David Kaplan explains how the search for hidden symmetries leads to discoveries like the Higgs boson. Filming for this video by Petr Stepanek. Editing and motion graphics by Ryan Griffin. Music by Kevin MacLeod. The abstractness of gauge symmetries causes a certain unease in some quarters. To compound the problem, gauge symmetries produce a multitude of ways to describe a single physical system — a redundancy, as the physicist Mark Trodden of the University of Pennsylvania put it.

Where does all that complexity in the middle come from? And one possible answer to that is this redundancy of description that gauge symmetries give you. Such internal complexity is the opposite of what symmetry normally offers: simplicity. True, symmetry told physicists where to look for both the Higgs boson and gravitational waves — two momentous discoveries of the past decade.

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In some cases, symmetries present in the underlying laws of nature appear to be broken in reality. But if the energy of the Big Bang created matter and antimatter in equal amounts, they should have annihilated each other, leaving not a trace of matter behind.


Yet here we are. The perfect symmetry that should have existed in the early hot moments of the universe somehow got destroyed as it cooled down, just as a perfectly symmetrical drop of water loses some of its symmetry when it freezes into ice. A snowflake may look the same in six different orientations, but a melted snowflake looks the same in every direction. Clearly, some aspects of nature — like the orbits of the planets — are the result of history and accident, not symmetry.

Biological evolution is a combination of known mechanisms and chance. Certain aspects of physics will have to remain intact — causality for example. Other things almost certainly will not. The smooth fabric of space-time Einstein wove a century ago inevitably gets ripped to shreds inside black holes and at the moment of the Big Bang. Many researchers believe these long-distance links stitch space-time together. The high bar for new ideas is that they cannot contradict consistently reliable theories like quantum mechanics and relativity — including the symmetries that support them.

ISBN 10: 052145591x

Einstein once compared building a new theory to climbing a mountain. Get highlights of the most important news delivered to your email inbox. Abusive, profane, self-promotional, misleading, incoherent or off-topic comments will be rejected. Moderators are staffed during regular business hours New York time and can only accept comments written in English. Read Later. For example, sometimes you can change the sign of the charges on particles, sometimes you can run processes forward or backward in time , and sometimes you can run a mirror-image version of the process.

This last one, looking at a process in the mirror, is called the symmetry of parity. Most subatomic interactions in physics give you the exact same result whether they're done right in front of you or in the mirror. But some interactions violate this symmetry, like the weak nuclear force, especially when neutrinos are produced in interactions involving that force.

Ask Symmetry – How is the Force like dark matter?

Neutrinos always spin "backward" in other words, the axis of their spin points away from their direction of motion , while antineutrinos spin "forward" their axis of spin points straight ahead as they fly around. That means there are very subtle differences in the numbers of neutrinos and antineutrinos produced when you run a regular, versus a mirror-flipped experiment that relies on the weak nuclear force. Nature's Tiniest Particles Dissected ].

As far as we know, the weak nuclear force and the weak nuclear force alone violates the symmetry of parity. But maybe it's not alone.

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We know that physics beyond what we currently understand must exist. And some of those hypothetical ideas and concepts also violate the symmetry of parity. For example, some of these theories predict subtle asymmetries in otherwise-normal interactions that involve the kinds of particles the LHC typically examines.

Of course, these hypothetical ideas are exotic, complex and very hard to test. And in many cases, we're not exactly sure what we're looking for. The problem is that while we know that our current conception of the particle world, called the Standard Model, is incomplete, we don't know where to look for its replacement. Many physicists hoped that the LHC would reveal something — a new particle, a new interaction, anything at all — that would point us toward something new and exciting, but so far all those searches have failed.

Many of the former front-runner theories for what's beyond the Standard Model like supersymmetry are slowly being ruled out. This is where parity-symmetry violation might come in handy. Almost all common hypothetical extensions to the Standard Model include the limitation that only the weak nuclear force violates parity symmetry. This is baked into the fundamental mathematics of the models, in case you were wondering how this works. That means concepts like supersymmetry, axions and leptoquarks all keep this symmetry breaking exactly where it is, and nowhere else.

Symmetry making and symmetry breaking |

But look, folks, if these common extensions aren't panning out, maybe it's time to broaden our horizons. For that reason, a team of researchers searched for parity violations in a cache of data released by the Compact Muon Solenoid CMS experiment at the LHC; they detailed their results in a study published April 29 to the preprint server arXiv. This was a pretty tricky search, since the LHC isn't really set up to look for parity violations.

But the researchers cleverly figured out a way to do it by examining the leftovers in interactions between other particles. The result: No hints of parity violation were found.