I'm currently reading the 2006 book Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions, by the brilliant theoretical physicist Lisa Randall. As you might imagine from the title, it's a provocative and mind-blowing read. And although it's written for laypeople, with most of the abstruse mathematics removed -- theoretical physics is, honestly, 99% math -- I must admit that a good chunk of it is going so far over my head that it doesn't even ruffle my hair.
The rest, though, is way cool.
The heart of the book is the consideration of superstring theory as a model for the way the universe is built. The idea -- at least at the level I understand it -- is that the fundamental building block of matter and energy is the string, a one-dimensional structure that can either be open-ended or a closed loop, and the various manifestations we see (particles, for instance) are the different vibrational modes of those strings. But deeply embedded in this model is the idea that the universe has fundamental symmetries, which unify seemingly disparate forces and allow you to make predictions about what exists but is as yet undiscovered based upon what might be necessary to complete the symmetry of the theory.
This search for underlying patterns in what we see around us drives a lot of theoretical physics. And certainly there are times the approach pays off. It was that mode of inquiry that allowed Sheldon Glashow, Abdus Salam, and Steven Weinberg to come up with electroweak theory, which showed that at high enough energy the electromagnetic and weak nuclear forces act as a single force. (It was later experimentally confirmed, and the three won the Nobel Prize in Physics in 1979 for the discovery.) Carrying this approach to its extreme are people like Garrett Lisi, whose eight-dimensional model of particle physics (based upon a mathematical structure called a Lie group) tries to unify everything we know from experimental results into a symmetrical whole based upon it seeming to fit into a pattern that is "too beautiful not to be true."
The superstring model, too, makes predictions of particles and forces, largely based upon arguments of symmetry and symmetry breaking. Each of the particles in the Standard Model should, the math tells us, have a "supersymmetric partner" -- each known fermion paired with a boson with the same charge and similar interactions, but a higher mass, and vice versa.
Experimental confirmation, of course, is the hill on which scientific theories live or die, and what the theorists need is hard evidence that these predicted particles exist. Randall's book is peppered with optimistic statements such as the following:
In a few years, CERN will be the nexus of some of the most exciting physics results. The Large Hadron Collider, which will be able to reach seven times the present energy of the Tevatron, will be located there, and any discoveries made at the LHC will almost inevitably be something qualitatively new. Experiments at the LHC will seek -- and very likely find -- the as yet unknown physics that underlies the Standard Model.
Randall's book was published in 2006; the LHC came online in 2008.
And in the sixteen years since then, not a single particle has been found confirming superstring theory -- no superpartners, no Kaluza-Klein particles, nothing. It did find the Higgs boson, which was a coup, but that was already predicted by the Standard Model, and didn't explain anything about the fundamental messiness of particle physics; why particles have the masses they do, forces have the strength they do, and (most vexing) why the extremely weak gravitational force seems to be irreconcilable with the other three.
But the fact is, every one of the efforts to get the Standard Model to fit into a more beautiful and elegant theoretical framework has failed. Physicist Sabine Hossenfelder, in a brilliant but stinging takedown of the current approach that you really should watch in its entirety, puts it this way: "If you follow news about particle physics, then you know that it comes in three types. It's either that they haven't found that thing they were looking for, or they've come up with something new to look for which they'll later report not having found, or it's something so boring you don't even finish reading the headline." Her opinion is that the entire driving force behind it -- research to try to find a theory based on beautiful mathematics -- is misguided. Maybe the actual universe simply is messy. Maybe a lot of the parameters of physics, such as particle masses and the values of constants, truly are arbitrary (i.e., they don't arise from any deeper theoretical reason; they simply are what they're measured to be, and that's that). In her wonderful book Lost in Math: How Beauty Leads Physics Astray, she describes how this century-long quest to unify physics with some ultra-elegant model has generated very close to nothing in the way of results, and maybe we should accept that the untidy Standard Model is just the way things are.
Because there's one thing that's undeniable: the Standard Model works. Just to give one recent example, a paper last year in Physical Review Letters described a set of experiments showing that a test of the Standard Model passed with a precision that beggars belief -- in this case, a measurement of the electron's magnetic moment that agreed with the predicted value to within 0.1 billionths of a percent.
This puts the Standard Model in the category of being one of the most thoroughly-tested and stunningly accurate models not only in all of physics, but in all of science. As mind-blowingly bizarre as quantum mechanics is, there's no doubt that it has passed enough tests that in just about any other field, the experimenters and the theoreticians would be high-fiving each other and heading off to the pub for a celebratory pint of beer. Instead, they keep at it, because so many of them feel that despite the unqualified successes of the Standard Model, there's something deeply unsatisfactory about it. Hossenfelder explains that this is a completely wrong-headed approach; that real discoveries in the field were made when there was some necessary modification of the model that needed to be made, not just because you think the model isn't pretty enough:
If you look at past predictions in the foundations of physics which turned out to be correct, and which did not simply confirm an existing theory, you find it was those that made a necessary change to the theory. The Higgs boson, for example, is necessary to make the Standard Model work. Antiparticles, predicted by Dirac, are necessary to make quantum mechanics compatible with special relativity. Neutrinos were necessary to explain observation [of beta radioactive decay]. Three generations of quarks were necessary to explain C-P violation. And so on... A good strategy is to focus on those changes that resolve an inconsistency with data, or an internal inconsistency.And the truth is, when the model you already have is predicting with an accuracy of 0.1 billionths of a percent, there just aren't a lot of inconsistencies there to resolve.
I have to admit that I get the particle physicists' yearning for something deeper. John Keats's famous line, "Beauty is truth, and truth beauty; that is all ye know on Earth, and all ye need to know" has a real resonance for me. But at the same time, it's hard to argue Hossenfelder's logic.
Maybe the cosmos really is kind of a mess, with lots of arbitrary parameters and empirically-determined constants. We may not like it, but as I've observed before, the universe is under no obligation to be structured in such a way as to make us comfortable. Or, as my grandma put it -- more simply, but no less accurately -- "I've found that wishin' don't make it so."
****************************************
 |
No comments:
Post a Comment