The Higgs Boson, Symmetries And Unification : 13.7: Cosmos And Culture It's time to reconsider the grand quest for "unification" in modern physics, says commentator Marcelo Gleiser. Our unrelenting search for symmetry and perfection in the natural world leaves us blind to data that seems to depict an imperfect and asymmetric Universe.
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Imperfection Makes The Universe Beautiful

High-energy physics in action: an image of an event in CERN's CMS detector during the search for the Higgs boson. CERN hide caption

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High-energy physics in action: an image of an event in CERN's CMS detector during the search for the Higgs boson.


To celebrate the publication of the paperback edition of my book A Tear at the Edge of Creation, I want to go back to some of its themes, given that it's been three years and a lot has happened since then. From the discovery of the Higgs boson to the rampant absence of evidence for supersymmetry, it is fair to say that the predictions in Tear have held true.

Tear is a counterculture book where I present a critique of some of the mainstream ideas in theoretical physics, especially high-energy physics and cosmology. Its main point is that the notion of symmetry, useful as it is in the physical sciences, has been abused, elevated from a descriptive tool of how the world works to some sort of fundamental principle of Nature.

Instead, I argue that given the nature of science, where results are obtained from observations with instruments of limited precision, it is inconsistent to state that a symmetry in a system is exact. At most, symmetries are approximations, albeit some may hold to very high accuracy. Adjectives such as "perfect" or "exact" don't really apply in the physical sciences. Every model that we use to describe a natural phenomenon has a range of validity and accuracy.

In other words, to confuse models with reality is a serious mistake.

The notion that there is some kind of secret code underlying all of Nature is the intellectual heir of Platonic idealism, which presupposes that there is an abstract realm wherein lie eternal mathematical truths, expressions of the "mind of God." Scientists who subscribe to such idealism believe that mathematics is not an invention of the human mind but a discovery: able mathematicians pluck their fruit from this elusive Tree of Knowledge and, like prophets, translate their meaning in terms others can understand.

In physics, the highest aspiration of this Platonism is to find the "Theory of Everything," the theory that shows that all four known interactions that act on matter and radiation are but manifestations of a single force, the unified field.

There is no question that we identify patterns of order at all levels of the Universe, from the smallest chunks of matter to astrophysical objects. But let's look more closely at particle physics, where we say there are four fundamental forces that explain how particles interact with one another. We are familiar with the gravitational and electromagnetic forces; the other two act only within atomic nuclei, the strong and the weak nuclear forces.

We can say that everything that we know of how particles interact is described by these four forces; but we can't say if these are the only forces. There could be others, lurking beyond the range of our experiments. So, any unified theory, assuming it's possible — which is a huge assumption — is by necessity temporary, given that we can't be sure if other forces exist or not.

Let's then take the only current unification of the four forces, the electroweak unification. What does it mean? It means that the weak and the electromagnetic forces behave similarly above a certain energy.

Recall that in high-energy physics the properties of matter are explored through violent collisions between particles. The higher the energy of the collision, the more detail we can obtain, the deeper we can see.

In a limited sense, this is what happens if you throw an orange against a wall; the higher the impact velocity, the more of the internal structure of the orange you can see. (Although, in reality, more wonderful things happen in high-energy physics experiments as the colliding particles morph into new material particles through the E=mc2 relation.)

This "unification" was predicted in the 1960s and has been spectacularly confirmed. The last piece of the puzzle, the Higgs boson, was discovered last year at the European Organization for Nuclear Research, commonly known as CERN. But can we really say that the electroweak unification is a true unification?

A true unification, in the language of physicists, means that the two forces behave as one, that they are described by the same symmetries. That is not what happens in electroweak unification; the two forces maintain their identities all the way. (Physicists say that the theory has two coupling constants, one for each force. A true unification would have only one coupling.) So, even though the two forces do behave in similar ways at high energies — all particles, like the electron and the quarks, become massless — the electroweak model is not a unification in the pure sense that some would like.

Real unification would happen at much higher energies, when the strong force is also included. Such three-forces-into-one unification is known as "grand unification" or GUT (for grand unified theory). Sheldon Glashow and Howard Georgi from Harvard University proposed it in the mid 1970s. To date, none of its predictions has been verified. In fact, in order for the unification to really happen (that is, for the three forces to merge into one at high energies) a new symmetry of Nature has been invoked, the supersymmetry I mentioned at the outset of this post.

Supersymmetry predicts many new particles and most physicists (at least those who believe in supersymmetry) were eagerly expecting to see some evidence of their existence at the LHC, together with the Higgs. Alas, nothing has showed up, so far. At this point, most of the simpler versions of models with supersymmetry have been ruled out.

Of course, a die-hard could always argue that Nature doesn't care for our criteria of simplicity. Supersymmetry may be realized in some funky, complicated model. It's possible, but clearly not as elegant as one would have hoped when such symmetry was proposed.

Or, we could listen to the data and, after some 45 years of negative searches, begin to consider the possibility that the notion of grand unification, or of perfect unification, is more a product of our over-eager imagination than a model of how Nature works.

If anything, it seems that the imperfect and the asymmetric is what makes the world go. But that's a story for another time.

You can keep up with more of what Marcelo is thinking on Facebook and Twitter: @mgleiser