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Is the ‘new muon’ really a great scientific discovery? For now, I’m cautious

Carlo Rovelli
·5-min read

There is something curious about the great experiments and discoveries in fundamental physics from the past few decades. They have covered black holes, gravitational waves, the Higgs particle and quantum entanglement. They have led to Nobel prizes, reached the front pages of newspapers and made the scientific community proud. But they haven’t told us anything new: they have confirmed what we expected about the world. All these phenomena were in the university textbooks I studied almost half a century ago. Their existence was predicted by our best established theories. I do not mean to diminish the awe. On the contrary. It is amazing that the phenomena were observed, and even more amazing that they were figured out before we could see them. Their detection is a celebration of the power of scientific thinking to see into the unseen. Yet a malignant voice could have whispered in our ears at each step: “What’s the great surprise? We expected this.” Fundamental experimental physics has long been, in this sense, quite conservative. It has simply been confirming the best theories of last century over and over again.

Last week findings from Fermilab, the US’s particle physics and accelerator laboratory, appeared to contradict what we thought to be the case. The laboratory announced a new measurement of the “magnetic moment” of the muon – one of the universe’s elementary particles, a heavier brother of the electron. The measured value of the muon seems to disagree with the value predicted by the theory. It is an observation that does not complement our established theories; it clashes with them.

Physicists crave this type of result. To learn something unexpected about the fundamental laws of nature, we need to observe phenomena that do not fit the established frameworks: quantum theory, the standard model of particle physics and general relativity. There are reasons to expect that these frameworks are not the complete story. Hence, theoretical physicists spend their time trying to guess what happens beyond them. Nature has been conservative, but physicists love to think of themselves as radical. They want to walk in the shoes of Rutherford and Wu, Einstein and Heisenberg, the experimentalists and the theorists who opened up the knowledge of new levels of reality.

This is why at every minimal hint of the unexpected, physicists jump up with excitement. Over and over again, I have seen this thirst for novelty. New forces, new particles, discrepancies between data and predictions. Neutrinos faster than light. Anomalies in the data of the big particle physics machines. So far, each wave of enthusiasm has turned into disappointment. Sometimes it was a just statistical outlier. In many random variables, you always find some strange ones. Sometimes an experimental mistake is the cause, even a poorly connected plug (this is what turned out to be behind the neutrino-faster-than-light false alarm). Sometimes it’s a theoretical mistake.

Years ago, we got excited because the magnetic moment of the electron was not as predicted. It turned out to be down to a mistake in the theoretical calculations – two terms had been computed with a different convention about signs (plus or minus), something your first-year university professor tells you to be careful about. The history of supersymmetry – a theoreticians’ speculation – is particularly telling: I do not remember a time without some colleague talking about “hints” that new supersymmetric particles had been “nearly discovered”. Decades have passed, and they haven’t yet.

So when I hear my colleagues cry wolf, as they did last week, I can’t help but be cautious. Could the new muon result be the real thing? Maybe, maybe not. On the same day news of the measurement was greeted with enthusiasm, a paper appeared in the scientific journal Nature presenting the results of a theoretical calculation using supercomputers indicating that the previous theoretical estimates of the muon were slightly off. Taking this into account, the theoretical value may be closer to the value measured last week. There might be no contradiction after all.

I understand the excitement of my colleagues. Some of them spend their lives searching for the wolf. If they see a hint of the tail, they’ll be happy. But I also think that we scientists should be cautious. Journalists can be quick to translate a “could” into a “can” and a “can” into an “is”. The public may like to see us struggling, watching our excitement and our disappointment, but may also get bored by big announcements that then go nowhere. The risk is losing credibility.

What’s truly surprising about our understanding of nature that has been developed over the last century is how durable it appears to be. General relativity was long seen with suspicion, its predictions too outlandish. The standard model of the particle was initially considered a bad patchwork, and violations of its predictions were expected at every single run of the particle experiments. Quantum mechanics is so strange that people considered its predictions implausible. But they’re still the best we have. Do we now have hints of something really new? Perhaps. But if we cannot expect what we know about nature to be definitive, neither should we expect it to be so easily wrong.

  • Carlo Rovelli will be speaking to Guardian science editor, Ian Sample, about his new book Helgoland in a livestreamed Guardian Live event on Tuesday 4 May, 7pm BST | 8pm CEST | 11am PDT | 2pm EDT. Book tickets here