The search for the next frontier in physics
“China has an incredible opportunity to become the world leader here — don't waste it. A good example is to build the Great Collider that can lead high energy physics for the next fifty years.”
-Stephen Hawking, November 2016
The passing of Stephen Hawking in early 2018 deprives the particle physics community of one of its greatest and most respected scientists. It also occurred at a time when the future of particle physics looks very uncertain. Since the discovery of the Higgs Boson in 2012, the search for new physics has yielded very little, leading to concerns over what the next particle collider would even be looking for.
If you’ve seen the biopic of Hawking, The Theory of Everything, you might be left with the impression that the title of the film was what the scientist hoped to discover. That he, like Einstein, had a lifelong ambition of finding the great unifying theory of physics. The reality is that toward the end of his life, Hawking had given up on the idea that there could be any such thing.
His mind was changed by an obscure German mathematician called Kurt Gödel who wrote his first “incompleteness theorem” in the 1930s. The theorem was a refutation of the idea that the entire natural world could be explained based on a discoverable set of axioms. Popular opinion at the time held that like geometry, all mathematical areas contained a system that would explain all possible states of nature from now until the end of time. What Gödel showed in his theorem is that logical paradoxes such as “this statement is false” or “The proof for G is that it cannot be proven” can actually be described in mathematics. It proved that in certain situations axioms will always break down and lead to paradoxes, so that a theory of physics that describes all of nature is impossible.
Hawking described how it took him a long time to come to terms with the reality that a complete theory of physics may be unobtainable. His later optimistic take on this was “Gödel’s theorem ensured there would always be a job for mathematicians”. Some of his colleagues at CERN have other ideas, like physicist Michio Kaku, who said recently that humanity would prove “the theory of everything” by the end of this century.
Unifying Physics
Whether one sees the Large Hadron Collider at CERN as a success or a failure partly depends in which of the two camps one belongs. On the one hand, the LHC was specifically designed to test theories about the Higgs Boson, the so called “God Particle” that emits a Higgs field which allows particles to have mass. On these terms you’d have to admit the discovery of the Higgs in 2012 was a great success. But if you were one of those hoping that the LHC would find evidence of new physics beyond the standard model, you’d be less happy. Exotic things like new particles or signs of extra dimensions predicted by other theories were nowhere to be seen.
The standard model of particle physics which groups together the six quarks that make up atoms, the four forces (electromagnetic, gravitational, and the strong and weak forces), lepton particles, and all of their anti-matter counterparts, was nicely rounded off with the discovery of the Higgs Boson. But there are extremely large unsolved problems in particle physics that the standard model seems unable to address. These are namely:
1. The Quantum Gravity Problem
Quantum theory (the behaviour of the smallest known particles) breaks down mathematically over large distances and so cannot be reconciled with general relativity.
2. The Hierarchy Problem
Gravity is a lot weaker than the other three forces by many orders of magnitude and we don’t know why. If it weren’t, matter wouldn’t stay together.
3. The Charge Parity Violation Problem
Anti-matter particles do not always behave in exact opposite ways to their counter particles, a phenomenon known as symmetry breaking.
4. Neutrinos
How exactly neutrinos (tiny particles with no electric charge) work and the role they play in the weak interaction.
If all this weren’t enough, you have the additional problems of the cosmological observations that show the expansion of the universe is accelerating (the opposite of what the Big Bang would predict, winning the discoverers the 2011 Nobel Prize). And the fact that the matter we can observe in the universe isn’t enough to actually keep the galaxies in motion. For these problems the terms dark matter and dark energy are employed but physicists are still trying to determine what those terms actually mean.
Quantum gravity is the biggest puzzle on the list and there have been hopeful theories created to explain it for decades. The most famous and influential of those theories is called string theory.
Super-Split Super-Symmetry
The basic idea of string theory was first suggested in the 1960s to explain the mechanisms behind the strong nuclear force (which is essential in keeping particles together to create atoms). The problem was how to account for the fact that particles seemed to be able to decay into smaller particles or combine into larger particles at will, and the mathematics for this broke down if the particles were considered as solid points.
The string theory explanation was that rather than being solid points, the particles were actually made up of tiny loops of string, and the speed at which the strings vibrated determined what kind of particle they were.
Although the theory proved to be wrong in the case of the strong nuclear force, it was later used for a much more ambitious purpose, the combination of quantum theory and general relativity. In ordinary quantum field theory which describes how tiny particles move, adding gravity leads to infinite gravity spikes and black holes which clearly aren’t present in nature, but turning the particles into strings actually requires the existence of gravity in the exact way that Einstein described.
This was the first theory to provide a real solution to the quantum gravity problem, combining Einstein’s general relativity with quantum theory and has since been held as the best contender for unifying all of physics.
One of the drawbacks of the hypothesis, is that to remain stable the theory requires an entire extra table of symmetric particles. Anti-matter is a proven and established part of the standard model, and is essential for the initial formation of the universe after the big bang. String theory requires another layer of particles called “sparticles” with selectrons (symmetric electrons), squarks (symmetric quraks) and smuons (symmetric muons). This is what’s known as super-symmetry or “SUSY” and it’s what many scientists were hoping the Large Hadron Collider at CERN would find. At least some evidence of SUSY particles should have been visible at low energies.
The final, and most bizarre prediction of string theory is the existence of extra dimensions. According to Einstein’s general relativity there are four dimensions, the three that we are used to, and time. String theory, in order to work, requires a whole ten dimensions. The reason we don’t experience these extra dimensions, supposedly, is that they are folded in on themselves in an intricate structure called a Calabi-Yau manifold and so they rarely interact with our universe.
These extra dimensions somewhat fit into the idea of the multi-verse in physics. Though they wouldn’t lead to the scenario that popular culture seems to have picked up on. They would be universes where matter can’t actually exist, rather than alternate realities where dinosaurs have invented computers and the like, which rely on a rather outdated notion of quantum mechanics.
The modern form of string theory has developed into something different called m-theory. Formulated in the late 1990s, m-theory combined five types of string theories to suggest that actually the separate dimensions operate on vast webs of connected strings called membranes, which is where the “m” in the theory comes from. It suggests that there are also strands of strings that connect the different dimensions and posits an 11th dimensional super-gravity which solves the hierarchy problem and the quantum gravity problem at the same time.
While it sounds bizarre, it is currently the best hope that physicists today have of explaining the nature of the universe. Stephen Hawking wrote in his last book before his death that it was the best contender for a grand unified theory that had ever been devised, and the discovery of a SUSY particle would vindicate half a century’s work on string theory.
Waves in Space Time
This is the crux of the debate around the building of the next collider. The LHC should have been able to find at least some evidence of the SUSY particles that are necessary for string theory to work. The lack of evidence so far doesn’t kill string theory, it just means that the SUSY particles may be heavier than previously thought. But this confuses the elegant mathematics that made the theory so appealing in the first place.
Proponents of string theory naturally argue that to find the particles all we need is a more powerful collider. More skeptical scientists suggest that the reason they weren’t discovered is that they don’t exist, a difficult idea for many in the field to face given the lack of alternatives.
People who aren’t convinced by string theory often see a new collider as a waste of money that could be better spent. This is the case made by Sabine Hossenfelder at the Frankfurt Institute. She correctly points out that, based on current predictions, SUSY particles or extra dimensions wouldn’t be found by the next collider because the energy required would take a collider thousands of times more powerful than we are currently capable of building.
Others point to the developing research in cosmology to provide direction for particle physics. There are many experiments under-way such as DUNE and T2K that are trying to identify the exact properties of neutrinos, very small particles that are involved with the weak nuclear force, with some believing they may be the key to explaining dark matter. Two Scientists won the Nobel prize in 2015 for the discovery that neutrinos have mass and can oscillate between different states.
Another is the LIGO observatory, based on theories of gravity and space time developed in the 80s. The observatory successfully detected gravitational waves rippling through space caused by the collision of two black holes (and found no evidence of extra dimensions that were previously predicted by string theorists). The scientists involved won the 2017 Nobel prize for the finding that opened up a whole new area of astronomy. LIGO cost a fraction of what a new collider is scheduled to cost, and the results it could yield are possibly far more significant.
The arguments made by Hawking and others for building the collider were somewhat political as well as scientific. In 2016, when Hawking encouraged the Chinese to build the next collider, he referred to the example of the 1993 Texas collider that was cancelled because of political turmoil in American Congress. The scientist pointed out that if China were to take the lead in physics the United States and others would follow out of a need to not be out-competed.
The Future Circular Collider proposed by CERN is scheduled to cost 24 billion euros (£21.4B) over the next 30 years. That may seem like a colossal burden on the tax payer, but it amounts to actually €0.8 billion (£0.71B) a year, hardly substantial when compared to the €23.6 billion (£21B) that the European Union spent on administration costs alone in 2017.
The International Linear Collider (ILC) that was scheduled for planning permission in Japan has fallen by the wayside, partly because of lack of funds and partly because of the outdated technology of linear colliders. Earlier this year, the Chinese government announced plans to build the Circular Electron Positron Collider (CEPC) by 2022, reportedly to better understand the nature of Higgs Bosons and proving that Stephen Hawking’s suggestions did not fall on deaf ears.
The Projection Chamber
Though many in the particle physics community still have hope that the next collider will point them in the direction of a unified theory like SUSY, the evidence that it will is rather minimal.
It’s interesting that this debate is hardly considered within the particle physics community itself, most scientists there as revealed by the CERN strategy forum seem to simply assume that the next collider will be built just as straightforwardly as Apple releasing a new iPhone. Hossenfelder is a rare example in this sense, but this is precisely why this issue should be debated in public rather than behind closed doors.
While its true that the gravitational waves found at LIGO appear to have revolutionised our understanding of astrophysics, scientists still appear no closer to finding the particles that make up dark matter or explaining dark energy than they are to finding SUSY particles. All predictions of “axions” and “Weakly Interacting Massive Particles” that were previously predicted have also been nearly ruled out.
Physicists should make an honest case for the next particle collider. In the past, particle physics discoveries have almost always led to advancements in science and technology. Even seemingly useless particles like muons are now used in brain and body scanning technology and the penetrating properties of neutrinos may help develop technology for scanning deep into the earth’s surface. In the same way, a more accurate comprehension of Higgs Bosons and quark arrangements are worthwhile pursuits in themselves, without the added promise of grand unified theories.
New technology will always depend on a better understanding of the physical world. To that end, the next generation of particle colliders should be built. Though perhaps we should temper our expectations for what they will find.
