There is something curious in the great experiments and observations of recent decades in fundamental physics: black holes, gravitational waves, Higgs particles, entanglement quantum…
They brought in Nobel prizes, they made headlines, they made the scientific community proud and happy, but they didn’t teach us anything really new. They confirmed what we expected. All these phenomena were in the university textbooks on which I studied at the University of Bologna almost half a century ago. Their existence was predicted by our best theories.
I do not intend to diminish the importance of these findings. Rather. Amazing phenomena have been observed, and it is even more surprising that they were understood before they even saw them. Being able to observe them was a celebration of the power that has the scientific thought to see far. But an evil voice could have whispered at every step: what’s the surprise? We expected it. Basic experimental physics has long been rather conservative in this sense. It has only confirmed and reconfirmed the strange predictions of the best theories of the last century.
Last week, the lab Fermilab in the United States announced a new measure of the magnetic moment of the muon, one of the elementary particles, a heavier brother of the electron.
This result is of a different kind from those mentioned above. The measured value seems to disagree with the theory predicted value. It is an observation that does not confirm our established theories. It seems to contradict them. Physicists are hungry for such results.
To learn something new about the elementary laws of nature we must observe phenomena that escape established theories: quantum mechanics, the standard model of particle physics and general relativity. There are reasons to expect these to not be the complete story. So theoretical physicists spend their time trying to guess what might happen beyond these theories, and experimentalists long to find something that is not foreseen by these theories.
Nature was conservative, but physicists like to think of themselves as radicals. They want to walk in the shoes of Rutherford and Wu, Einstein and Heisenberg, the experimenters and theorists who have uncovered new levels of reality. This is why at every hint of the unexpected, physicists jump to their feet in excitement, hoping for a “here it is!”
Over and over, throughout my life, I have witnessed this excitement of novelty. New forces, new particles, discrepancies between data and predictions. Faster than light neutrinos. Anomalies in the data of large particle physics machines. So far, every wave of enthusiasm has turned to disappointment. Sometimes it was just an anomalous statistical value: in many random variables, one always finds one that is strange.
Sometimes an experimental mistake: even a badly connected plug (the false alarm of faster-than-light neutrinos).
Sometimes a theoretical mistake. Years ago, we got excited because the electron’s magnetic moment was not what we expected. But it was a mistake in the theoretical calculation: two terms were calculated with a different convention on signs (more or less): something that the first-year university professor tells you to be careful of.
The story of super-symmetry, a speculation of theorists, is particularly significant: I don’t remember a time without some colleague talking about “clues” that the new super-symmetrical particles were “almost discovered.” Decades have passed, they are not yet. Over and over again, in short, I heard a wolf cry.
Could the new muon result be the wolf? Maybe yes maybe no.
On the same day that the news of the measurement was enthusiastically received, it appeared on Nature, a leading scientific journal, an article with the results of a theoretical calculation obtained using supercomputers, indicating that previous theoretical estimates misjudged some small effects. Taking these into account, the theoretical value is closer to the measured value. The discrepancy becomes much less significant. In other words, this could prove to be yet another case of physicists crying wolf. It could be a repetition of what had happened for the electron: the measured value could be correct, without contradicting current theory. Who is right? I am unable to tell. Nor, I believe, is anyone today.
I understand the enthusiasm of my colleagues. Some of them have spent their lives looking for the wolf. If they see a hint of a tail, they are happy. I share the excitement. I also think that we scientists must be cautious. Journalists are ready to translate a ‘could’ into a ‘can’ and a ‘can’ into an ‘is’. I sincerely hope that this time the little tail glimpsed is the real wolf. But let’s be cautious. The public may like to see us panting, following our emotions and our disappointments, but they may also get tired of ads that are then rejected. The risk is to lose credibility.
The real surprise from reading the book of Nature over the past few decades is that Nature follows the basic theories we discovered last century much more precisely than we initially thought. General relativity has long been viewed with suspicion, its predictions too extravagant. The standard particle model was initially considered a bad patchwork, violations of its predictions were expected at every single run of the experiments. Quantum mechanics is so strange that many considered its predictions implausible.
Is it time for something truly new? Has the wolf arrived? Could be. But if we can’t expect what we know about nature to be definitive, we shouldn’t expect it to be easily wrong either.
April 14, 2021 (change April 15, 2021 | 09:38)
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