Planned particle accelerator

Von Michela Massimi

- 10:23

Artistic impression of a collision event in the planned Future Circular Collider. Bild: CERN

Who thinks that the LHC, today’s biggest particle accelerator, is a disappointment because expectations haven’t been met, works with outmoded notions of method and progress in science. A philosopher’s point of view.

Bitte melden Sie sich an, um diesen Artikel auf Ihrem Merkzettel zu speichern.

A new project has recently been proposed for Cern: the creation of a new collider called Future Circular Collider (FCC) that will continue and expand the work currently done at the Large Hadron Collider (LHC). The LHC hosts a number of large international experiments (including Atlas and CMS) that over the past years have explored the outcomes of proton-proton collisions at energies of 8 TeV (and 13 TeV in the second round).

The new Future Circular Collider will increase the energy of the collisions to 100 TeV. A more powerful collider will be able to explore energy regions that currently fall outside the sensitivity of the LHC and investigate a wider-range of collisions: not just proton-proton collisions (as in the LHC) or electron-positron collisions (as with the old LEP) but also proton-electron collisions. The FCC is an international collaboration of 150 universities and industrial partners and the plan is currently under consideration as part of the European Particle Physics Strategy.

In an era where climate change is becoming a dangerous emergency, and many other challenges face our society (including finding a cure for cancer and Alzheimer, among many others), the idea of investing money in the creation of a more powerful collider at Cern (indeed what promises to be the largest collider in the world) might strike some as blue sky thinking at best; otiose at worst. To the eye of the general public, that money should be spent looking for a new physics beyond the Standard Model (as FCC promises to do)[1] just is not the top of the priorities (maybe not even approaching the lower end of the priorities). And yet, I maintain, we should care (and care deeply) about the tremendous international scientific effort behind the FCC. In what follows, I am going to give three reasons for it.

Fundamental research and its benefits to society

The first (and more mundane) reason concerns the wider long-term benefits to society that come from developing a range of technologies associated with a project of this nature. It is not just the World Wide Web that was originally developed by British scientist Tim Berners-Lee while working at Cern. The very first PET scan, now routinely used for cancer diagnostics among other, took place at Cern in 1977 using technology originally developed for particle physics. And Cern is still actively involved in researching new therapies for cancer (such as proton beam therapy — see this recent workshop). More in general, Cern continues to play a pivotal educational role: 70% of the graduate students in physics that have trained at Cern do not remain at Cern but apply their knowledge and skills in a variety of technological and statistical fields in society. But leaving aside the practical reasons as to why investing in colliders is a way of investing in long-term benefits for society at large, let me come to some more philosophical and methodological considerations.

I am a philosopher of science and I spend most of my time studying what scientists do, what methods they choose to investigate particular phenomena, how they go about building models with the hope to find new particles, and so on. My job as a philosopher of science is to step back and take a long view on broader (philosophical) issues such as evidence, progress, and truth in science (among others). And I cannot help but feeling dismayed when I read in social media and other outlets comments (the New York Times among them) to the effect that we should not invest in a new collider because the predictions for a new physics Beyond the Standard Model (BSM) have proved false, and not enough progress has been made to justify further spending. Let me briefly make two considerations about prediction and progress, respectively.

It is a good rule of thumb that one should not take physics lesson from a philosopher, or, equally, lessons of philosophy of science from a physicist. In either case, the risk is that the discussion is not going to be well informed. Let me then break the news to FCC-'naysayers' physicists that science does not proceed by Popperian conjectures and refutations (if that is the most recent philosophy book that comes to mind). It is factually and philosophically inaccurate to say that predictions (about new promised particles, be they dark matter candidates or else) have turned out to be wrong.

You won't observe the Popperian method at Cern

It is factually inaccurate because it does not reflect the way in which a very complex scientific community such as Cern actually works. For my ERC-funded philosophy project I visited Cern and had the opportunity to speak to both theoretical physicists and experimentalists working both at Atlas and CMS searches for BSM (in particular Susy searches, where the promise of dark matter candidates is mostly, but not exclusively concentrated). And what is immediately clear to any visitor (even from the physical geography of how the theoretical division at Cern is confined to one long corridor of a building) is that neither the theoreticians nor the experimentalists see themselves as in the business of one coming up with hypotheses and predictions, and the other testing the predictions with proton-proton collision data. There is instead a very clear division of labour.

The theoreticians work on the foundations of the Standard Model (and try to find theoretical solutions for some of the pressing problems still open within the Standard Model). The experimentalists, by contrast, follow a more data-driven model-independent approach in collecting and interpreting the vast amount of data produced by proton-proton collisions. The theoretical models produced by the theoreticians (one of them told me that he can produce „one theoretical model a day“) are very different in nature from the more phenomenological and data-driven models that the experimentalists use in reading and interpreting the data coming from LHC. Where the two communities meet (via the use of effective field theories and simplified models; see here for an example) is where the really interesting conversations take place. The challenge of Big Data and the proliferation of theoretical models rival to the Standard Model has meant that particle physics community has long stopped (if ever did) following any Popperian method of hypotheses-testable predictions-falsification. So much for the breaking news.

It is then philosophically and methodologically more accurate to portray what goes on in contemporary HEP as an open-ended explorative kind of research (rather than a research that has been rail-roaded to test any particular kind of prediction about particle X rather than Y). This is evident in the sub-groups within both Atlas and CMS (see this recent one for example) that work on exotic particles for example, where the search concentrates on possible signatures for missing transverse momentum in a particular sector of the detector that might be evidence for a new BSM particle, without any heavy theoretical pre-conception about what can be found. In my work, I have presented this kind of exploratory searches at LHC as designed to carve out the space of what is objectively possible in nature. Thinking of HEP research as an exploration into the realm of physical possibility (rather than some mischievous hit-and-run testing process in the graveyard of once-believed-to-be-real-but-now-defunct BSM candidate particles) helps also assessing progress in this area.

Scientific progress also means excluding possibilities

Has enough progress being made in particle physics to warrant further spending into a new collider? Arguably a sense of frustration can be perceived among those who think of particle physics as in the business of coming up with testable hypotheses and predictions that so far have not been found. But let me conclude by suggesting a different image of how science grows. High-energy physics beautifully exemplifies a different way of thinking about progress, where progress is measured by ruling out live possibilities, by excluding with high confidence level (95%) certain physically conceivable scenarios and mapping in this way the space of what might be objectively possible in nature. 99.9% of the time this is how physics progresses and in the remaining time someone gets a Nobel Prize for discovering a new particle.

But it is not that 0.1% of time that alone defines whether enough progress has been made in particle physics. Equally important, progress should be assessed on the basis of the remaining 99.9% of the time that physicists spent ruling out live possibilities and carving out the space of what might be objectively real. This is progress enough in science and being able to convey it to the public (and Government officials) is also the task of philosophers of science to do, working alongside scientists, for a better public understanding of science.

This year we celebrate the 100th anniversary of Eddington’s solar eclipse expedition, which saw two teams going to Sobral (Brazil) and Island of Principe (West Africa) to observe the light bending predicted by Einstein’s relativity theory. A powerful reminder of the importance of international scientific collaborations for fundamental research at a time where the whole Europe was coming out of a bloody horrific war (1915-1918) and public spending was very thin. But we should not forget that this wonderful successful singular prediction of Einstein’s theory was preceded by century-long vain attempts at building mechanical models of the ether. Some of the best scientists of the time (including Maxwell and Hertz) engaged with the then open problems of electromagnetic theory by devising more and more ether models and experiments designed to detect the ether drag.

These problems were eventually solved when the foundations of electromagnetic theory and Newtonian mechanics were overhauled by the young Albert Einstein (and the ether proved eventually dispensable). All the same, the road to Einstein’s breakthrough meandered through half-century-long attempts at engineering models of the ether and testing for the ether (including the Michelson-Morley null experiment in 1887). This is another historical reminder that it is a fallacy to go from the premise “we have not found anything so far” to the conclusion “There is nothing to be discovered here”. Maybe the solution to some of the open problems within the Standard Model requires a revolution similar to the one behind relativity theory in rethinking the theoretical foundations for a new physics (see a recent study in this direction). All the same, it is the ongoing, unfailing, and indefatigable efforts of experimentalists at places like Cern that will equip us to better answer this question.

[1] It is worth specifying that the costs are spread among over 20 nations which are at the top of the GNP worldwide.

Michela Massimi is Professor of Philosophy of Science at the University of Edinburgh. In her ERC project „Perspectival Realism“ she studies, among other topics, the practice of scientific modelling within contemporary physics.