Medical News Axions may or may not exist – but we’re not just making things up

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Are we just making things up? | Everything theoretical physicists do is speculative, and likely wrong, except for the things we get right

Physics

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15 May 2019

ShutterstockBy Chanda Prescod-Weinstein
RECENTLY, I visited a prestigious physics department and gave a presentation about my research on a particle called the axion. Fifteen minutes in, a member of the department interrupted me to insist, “Isn’t the axion just a matter of speculation? Shouldn’t you say that?” I had been warned by the graduate students to beware this particular professor, who has a habit of rudely interrupting talks to ask female speakers unnecessary questions. “Yes,” I responded. “I have no idea if the axion is real. Everything theoretical physicists do is speculative, and likely wrong, except for the things we get right.”
I continued by reminding the audience of something that I had already noted: the axion is a particle that may be produced during what researchers like me call “the early universe”, when space-time as we traditionally speak of it was less than 1 second old.
The existence of the axion was originally hypothesised with a far from frivolous purpose: to prevent particles that we know to be real from developing properties that we are pretty sure they don’t have. It originates in the Peccei-Quinn mechanism, named after its inventors Roberto Peccei and Helen Quinn. Here, it exists to stop the standard model of particle physics – our best stab yet at explaining how material reality works – from endowing a particle in the centre of all atoms, the neutron, with properties that are inconsistent with our laboratory observations.
As its name suggests, the neutron has no overall electric charge. But it is made of smaller fundamental particles, called quarks, that do have a charge. The mystery that the standard model is hard-pressed to explain, and that the axion clears up, is why the distribution of positive and negative charges seems to be exactly the same – why it has precisely zero electric dipole moment, to use the jargon.
Professor Sceptic was right. This is all hypothetical. First of all, the neutron may have an electric dipole moment – just one so small we can’t detect it. But, as I told my audience, this would create new problems to address: why have a property that exists, but is so unnaturally small? Even if the electric dipole moment is zero, it is entirely possible that the axion still doesn’t exist. Maybe the Peccei-Quinn mechanism is a nice idea that doesn’t reflect how the universe works, so we need a different way of clearing up the mystery. In other words, I told the gathering, I wasn’t worried about running out of things to work on.
“Much like detectives in a mystery novel, we develop theories about what happened, then refine or alter them”
Professor Sceptic’s concern goes to the heart of what theoretical physicists who study the origins and evolution of the cosmos do: we ask questions about the nature of the evolution of space and time and everything that exists in the universe, starting billions of years before humans showed up asking questions.
This type of scientific work presents us with unique challenges. Unlike chemists in a laboratory, we can’t rerun the experiment. We have exactly one sample universe to work with, and it operates entirely beyond our control. The best we can do is collect information, by taking images of distant stars and galaxies with telescopes, by testing ideas about how particles interact with each other at facilities like the Large Hadron Collider and by detecting vibrations in the fabric of space-time, aka gravitational waves.
To interpret this data, we make some mathematical assumptions, and we simultaneously use the data to hone our mathematical assumptions. Much like detectives in a mystery novel, we develop ideas about what happened, and then we refine or radically alter those ideas, based on new evidence. Then, we try to convince ourselves and each other that our ideas are good, realistic models of the universe.
There is much we don’t know. The universe certainly doesn’t care if we figure it out. At the same time, it is a great pleasure to do the work of pairing speculative imagination with hard-won data to get to know the universe in this way. I came to study the axion not for its role in the dipole problem, but because it may also be a good candidate to solve a problem of missing, transparent matter – what has historically been called the “dark matter problem“. Axions still might not exist. But their demise would raise so many questions that it is hard to feel worried about that possibility. I say, bring it on.

Chanda Prescod-Weinstein is an assistant professor of physics and astronomy, and a core faculty member in women’s studies at the University of New Hampshire. Her research in theoretical physics focuses on cosmology, neutron stars and particles beyond the standard model

Chanda’s week
What are you reading?
I am enjoying Cosmological Koans by Anthony Aguirre, which comes out in the US later this month.
What are you watching?
The Desus & Mero talk show is making me laugh so hard.
What are you working on?
I am doing my usual: worrying about what axions get up to.

More on these topics:
particle physics

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