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Beliefs and proofs in particle physics March 21, 2007

Posted by dorigo in italian blogs, personal, physics, science.

Today Jeff posted a reply to a long thread originated by  my post on the new Tevatron average of the top quark mass , and I think he makes a few interesting points, which can be appreciated without reading all the thread. 

It all started with my comment “whether or not we are able to detect a particle with our technology does not affect its existence.”… Jeff replied that “A modern scientific theory has an obligation to furnish observables. We are obliged to detect the particle, at least produce a very tight set of quantitative observations that point to it. Otherwise an outsider would say “these physicists are no better than those that believe in supernatural ectoplasms”.”

And then he left an analysis of the issue:

Tommaso. Avoid bad philosophy and bad relations with outsiders. Bad philosophy can and IS used as an excuse by hostile outsiders to argue the emptiness and hence uselessness of the scientific enterprise.

Regards evidence and existence [of the Higgs boson – TD] you guys can do better than that! Maybe most of you are too young and haven’t read enough history or have forgot it. How about the neutrino! Far more paradigmatic than the recent top quark. From the mid 30s to mid 50s, the neutrino seemed to elude direct detection, BUT it was leaving finger prints everywhere! The neutrino was an essential ingredient to allow physicist to make sense of weak interactions. It allowed them to be quantitative and scientifically creative: data could be accounted for and new ideas and experiments could be conceived to mount further evidence that the theory was on the right track. Ultimately these further experiments pointed to further subtle and deeper problems.

Consilience: a network of loose evidence collectively furnishes a tight argument. Single experiments are not water tight proofs for any theory; it is the network of experiments and accounted phenomena that make modern science. It is “detection of existence” in the broad sense of the word. Most physicists were convinced of the existence of the neutrino before the “direct” Reines and Cowan 1956 experiment. The top quark was a replay. After the discovery of the bottom quark the evidence for the top mounted in a anticlimatic crescendo. Is the Higgs story another example? A replay of a story alread told. Do you really think it is?

I have left HEP for some years now so I am not updated (was I ever? No, just cog in a big machine.). I personally believe that physicist formulate effective theories – the Fermi point theory of weak interactions with neutrinos was the first modern example. I mentioned above consilience. Let me add that quantitative and profound qualitative changes may occur: a theory evolves to become signicantly more solid and profound when new unexpected phenomena become available as when energy thresholds are crossed. Higher energies required the Fermi effective theory be transformed to incorporate the heavy W and Z bosons and the theoretical and experimental work eventually led to the Standard Model.

In the past 100 years energy thresholds have uncovered surprises: existence of nucleus, of nucleons and nuclear structure, of nucleon substructure (partons and quarks), heavy intermediate bosons, jets, gluons, etc… To my knowledge NONE of these were predicted in any water tight way and when they were observed there was a great hustle of theory building and killing. I do find it remarkable that a consistent Standard Model emerged. It truely is a magnificent showpiece of how modern science works and it should be taught to outsiders emphasizing the hard work, the wrong turns, the great insights, the good and the bad philosophy, hoping to learn from mistakes, the predictables (hard work) and the unpredictables (the unexpected discoveries), but absolutely leaving out the the “gee-whiz”. That can sound like hocus-pocus to hostile outsiders.

From this point of view the real risk of accelerator physics is that the energy thresholds for uncovering new and welcomed unexpected phenomena are out of reach. But that is not to argue that the LHC should not have been built. There is no gain without pain. There is no thrill with a risk. There is the possibility that the Higgs might not make a show. Now that would be a discovery!!! Astrophysics and ultimately cosmology are probably the best realms for finding unaccountable and new phenomena.

I can only say I agree with most of what he said, and that I have always thought that the absence of the Higgs would indeed be a great discovery.


1. jeff - March 21, 2007

Thanks for putting my post up for comments. Let me just make a few “corrections” before I get bad comments.

For unexpected discoveries I was not clear in mentioning quarks and gluons. These were theoretical concepts that took hold in the wake of unexpected discoveries: deep-inelastic scattering and then jets were unexpected discoveries and the dynamical theory of quarks and gluons (QCD) was the theory that finally emerged once it proved it could account for scaling, then asymptotic freedom and finally hadronizations of quarks and gluon jets into observable particle jets.

Never forget that jets were unexpectedly discovered at CERN when someone had the great idea of putting detectors at large angles respect to intersecting beams. Until then physicists were interested in small angle stuff. There was no reason to expect hard scattering just as Thomson with his plum cake model of the atom would never have thought of looking for large angle scattering. It took an energetic alpha and a great intuition for Rutherford to produce and look for the right signal. Similarly the discovery of scaling in deep-inelastic scattering at the end stations of the linac of SLAC is another piece of wonderful scientici history. Then of course the discovery of charm was a gigantic surpise and to a more limited extent even the discovery of bottom was.

I was also not clear regards the intermediate vector Z and W bosons. Rubbia did not make an unexpected discovery! The bosons were sought for and they confirmed the Standard Model. But the need to have intermediate vector bosons in the first place became painfully clear many years earlier with a generation of physicts toiling with the oddities of the weak interactions.

Discoveries are unexpected things. Physics is fun because discoveries make it so. If there were no suprises then physics would be boring.

2. Fred - March 22, 2007

Hello Jeff,
I always learn something from this site. What are the reasons why that the energy thresholds for uncovering new and welcomed unexpected phenomena are out of reach? Is this a permanent condition and does it matter? How important is it to continue to financially support accelerator physics in the future?

3. jeff - March 22, 2007

Hi Fred
short answer:
I didn’t say that the next energy threshold is out of reach. I said it might be out of reach. I spoke of risk.

Here is a long answer:
Accelerators are a controlled way of perfoming experiments while astrophysics is not as the phenomena are already out there. Accelerator physics is active but the phenomena are artificial, in the good sense of the word: reproducible enough to be studied in great detail. In astrophysics the phenomena are natural but you are passive. Complex in the sense that there is a certain uniqueness of the object and it takes ability to see beyond the contingent to extract general laws. Passive the modern and active SENSE of the world: the difficult task is to learn how to search for phemonena and then extract as much information as is possible by designing new detectors and ways to extract information is a methodical way. Passivity is certainly not the best way to describe astrophysicists. They are very VERY busy! This information is used by the theorists that try to account for what is seen. That modern passive science is far from being passive just look at the history of the early years of PASSIVE cosmic-ray physics: from the 30s discovered new and unusual particles and started the particle revolution before accelerators out performed it in the late 50s.

Modern astrophysics is mature enough to tell what to look for, anticipating the characteristics of the objects seen in the sky. Suprises, discoveries, occur when unexpected objest are detected (pulsars! what a suprise that was) and the modern game of theory-vs-experiment starts rolling with new observations giving more and more information that theory has to account for, or vive versa a theory tells experimentalist to look for certain signal characteristics (time shape, energy spectrum, etc.). Astrophysics is in full bloom.

Instead in a controlled experiment you artificially create simple phenomena in a controlled context free of wild (natural) contingencies. This is how experimental science has always worked since Galileo. It is far from easy as you must first learn HOW to remove contingencies and recognize an isolated interesting phenomena. And then always be vigilant and ask whether you have simplified too much and whether what you learn is pertient to understanding natural phenomea on a general and wild (natural) scale. Its like a zoologist that observes an animal. He might learn basic things about the animal in a cage, but the same animal in the wild will behave in a far more complex way, in ways that will not be observable in cage-life. Banal? In the hans of irresponsbile people it can be banal. But if you spend some time to think about it you will realize how profound a problem it is. It is the essence of science.
Indeed this philosophical point is the delicate and hard part and ever since Galileo’s day is the direction from which most of the attacks to modern science come from. Science responds by showing how much the wild phenomena that nature presents us can be understood and anticipated on the basis of the laws we learned to recognize in controlled experiments. This approach works! Of course it has its limitations. A general law does not pretend to explain contingent phenomena. Those that attack science inevitably end up saying “But science does not explain everything!” But good Science does not claim to explain everything. Some things are unique and are due to history and anyone that says the contrary is just a foolish reductionist. A wise reductionist would never say such a thing. Theories of Everything (TOE)? Scientific hogwash and philosophical crap!

Suppose you do basic research by performing a controlled experiment: you might confirm your theories but you might also uncover anomalies. These are obervable only because you have invested so much in controlling the experiment. Only because you know the experiment like the insides of your pockets can the anomalies be recongnized as such! They then might be explained away by a better version of your theory or be a good reason to conclude your theories are not good enough or even wrong. But this is possible only once the experiment is controlled. Controlled experiments are the hallmark of modern science!

Accelerators fall into this tradition and I firmly believe they are to be used, funded and new ones contructed. But there must be good scientific reasons for it resources are limited. The public and their representatives should be addressed and informed and the scientisist should not automatically expect to be paid for for something they believe is right.

My argument in favor of funding accelerators is this:

Modern science is based on controlled experimentation and the interplay with theory. Accelerator physics is an important way to do basic physics. The recent breakthrus in astrophysics and cosmology started from what was learned by particle physicists that progressed once accelerators made controlled experiments possible. Modern science will always need controlled experiments and it is certainly going to make good of the observations of passive science. Controlled experiments have and will continue to foster the growth of active ways to do passive science. But never forget that passive science is complicated because nature is complicated. The natural phenomena, those in the wild, are not simple and are complicated by their histories. But also never forget that controlled science is artificial and to check that it is pertient it must confront nature in the wild. A mixture of controlled and modern passive science is healthy and works wonders if both work, like two legs: one outsteps the other to then be outstepped in turn (paraphrasing what was said about experiment and theory).

4. Fred - March 23, 2007

Thanks again, Jeff. Here’s a little piece of interesting news concerning natural phenomena released today: http://saturn.jpl.nasa.gov/home/index.cfm. Ever think about producing a documentary?

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