Particle physics experiments have stopped answering to grand theories

It's easier to get money to build a new collider if there's something specific to look for. "We're expecting to find the Higgs, but we need a better tool to look for it."

Trying to do the same with a data-driven model is a hard sell. "We haven't found anything, please give us a better machine so we might find something, but we don't know what"

I highly recommend Feynman's QED book [0], its easy to read and at the same time blows your mind off. You'll be proud of yourself after :)

[0] en.wikipedia.org/wiki/QED:_The_Strange_Theory_of_Light_and_Matter

In the context of politics, magister-dixit or top-down has a negative connotation, and democracy or grass-roots or bottom-up has a positive connotation.

In physics we should disregard the political connotation.

Yes historically most new physics were top-down observations, i.e. noticing small (unexplained) deviations, and first modeling the deviation, and then eventually realising the underlying cause. A good example is the discovery that some planet motions: while most did seem to move in perfect circles, one of them had a measurable deviation, and after exhaustively trying to fit "state of the art" mathematics (degree 4 polynomialss or ovals), because Kepler incorrectly thought others would have already tried the simpler conic sections (ellipses for closed orbits). Only after the ovals kept failing did he try the too-obvious-otherwise-it-would-already-have-been-discovered conic sections, and found a very precise match.

Examples of bottom-up physics are the early theories of statistical mechanics like Ludwig Boltzmann's, or the atomistic theory of chemistry: with little experimental evidence, they could derive physically realistic behaviours for large ensembles of particles, and only much later were molecules and atoms discovered. Bottom-up is postulating smaller particles and investigating how they would influennce the behaviour of bigger collections, and then trying to prescribe experiments to prove their predictions.

So when the author of this article talks about bottom-up he is actually describing the return to top-down, explain as you measure deviations, and when he talks about top-down he seems to actually refer to the bottom-up postulates of new physics with hopefully falsifiable tests...

Again, a good article, but its nomenclature of top-down and bottom-up seems reversed to me.

As John Mather put it, right now we're not even sure what the right questions to ask are... but we definitely to understand it yet.

Or can you do something like Google did with Tri Alpha Energy [1] to explore interesting parts of the state space, and just generate tons of data for people to chew on?

Or does reconfiguring it for human-directed experiments effectively explore the state space enough that there's data to chew on just fine?

Asking out of curiosity. I know the smartest people in the world are working on this stuff; not trying to “hey what if they just tried X…” on this :-)

[1] https://ai.googleblog.com/2017/07/so-there-i-was-firing-mega...

"All these challenges arise because of physics’ adherence to reductive unification. Admittedly, the method has a distinguished pedigree. During my PhD and early career in the 1990s, it was all the rage among theorists, and the fiendishly complex mathematics of string theory was its apogee. But none of our top-down efforts seem to be yielding fruit. One of the difficulties of trying to get at underlying principles is that it requires us to make a lot of theoretical presuppositions, any one of which could end up being wrong."

"Instead, many of us have switched from the old top-down style of working to a more humble, bottom-up approach. Instead of trying to drill down to the bedrock by coming up with a grand theory and testing it, now we’re just looking for any hints in the experimental data, and working bit by bit from there."

In practice, both ways to look at the evidence are needed. And in pure science, sometimes a lot has to be done in many different directions before some of non-obvious directions bear fruits. One of big dangers is inventing the experiments that will surely "confirm." The big insights come also when something expected by the most is not confirmed, like the famous

https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper...

Without these experiments Einstein wouldn't be able to invent General Relativity. The popular culture talks too much about Einstein but sadly doesn't understand and hardly even knows Michelson.

https://en.wikipedia.org/wiki/Albert_A._Michelson

His experiment, from the end of the 19th century, is also the basis of the LIGO experiments that just recently confirmed gravitational waves:

https://www.ligo.caltech.edu/page/ligos-ifo

"Although much more sophisticated, at their cores, LIGO's interferometers are fundamentally Michelson Interferometers, a device invented in the 1880's."

The 1880's experiments didn't "confirm" what was "expected" but they are one of the most impressive science success stories, looking from what we know now.

Of course, LIGO itself is a technological marvel compared to what was possible in 1880, and we should celebrate all the advanced experiments where our reach extends. Not "confirming" expected can be even much bigger story, even if it disappoints those whose "pet" "expected" theory is not confirmed.

> A test case for the bottom-up methodology is the bottom meson, a composite particle made of something called a bottom quark and another known as a lighter quark.

I thought mesons were just a quark + an antiquark, I suppose they meant another (anti)quark that's lighter than the bottom quark?

It is unfortunate that they refer to the 26 dimensions...

> [...] particles as tiny vibrating loops of string that exist in somewhere between 10 and 26 dimensions.

... of what is known as "Bosonic string theory". It is called bosonic because it only has bosons (e.g. photons) and no fermions (e.g. electrons). This is obviously not a realistic theory because of that reason, and it also suffers other serious problems. But, this was the first formulation of string theory, not meant as a fundamental theory of gravity even, and if you do open some of the famous text books in string theory, you do find that it starts with the bosonic string theory. This is because it is a more gentle introduction.

The "between 10 and 26" comment is also a bit unfortunate. String theory is ten dimensional (space-time, meaning I include time in there). A lot of physics is formulated in terms of perturbation theory, meaning you have that the full result is expressed as an infinite sum of smaller and smaller terms, and you can truncated this infinite series and get a approximate result. This holds if the parameter you are expanding indeed is ever-smaller, which it isn't necessarily. One of those parameters (string theory has two of them built in) is the string coupling "g_s". If you start taking this parameter large than one, so the perturbation breaks down, string theory (type IIA in particular) grows an extra dimension into a theory known as M-theory. Note that this theory has no strings, it only has other fundamental objects. Similarly, there is an F-theory that is in some sense 12D, which also describes non-perturbative physics.

So, if physics in our universe is described by this non-perturbative physics, then sure, it's 11D or so, but we do not know which parameter regime of string theory our universe is in ( yet ;) ). But it is not a choice willy-nilly.

Then regarding effective theories against fundamental ones. Effective theories, or models rather, are things like: the inflationary model, cosmological constant to explain dark energy, standard model, minimally-supersymmetric standard model, F(R) gravity, DBI gravity, and so on. The problem is that there are too many of them. Claudia de Rham had a talk a month or so back in which she said something along the lines of (this is how I remember it) "We are quite good at excluding effective gravitational models, but we are however better at constructing new ones.". We need some deeper understanding of what is allowed when it comes to model building, and even theory building. But the point is, for gravity for example, that there are several models out there that are consistent with observations, but we do not know which ones can be consistently included in a fundamental theory.

And theory gives us ideas of what to look for. In this thread "seeing extra dimensions" are discussed, but it is misrepresented a bit. There are potentially several ways that we could start seeing evidence for extra dimensions, at least in principle. For example, "compactification" which means that we make the extra dimensions small, hidden for us, comes with the so-called "Kaluza-Klein tower" of particles, in which particles are essentially separated in mass inversely to the size of the size of the extra dimensions (small extra dimensions -> high mass). So this is one indirect way of how one could in principle see them (then they may be very massive, and virtually undiscoverable, but space-time warping brings down these masses... so we don't know).

Some of the particle physics experiments are looking for, in a sense, "anything that deviates" from the standard model. Note that for such experiments, any fundamental theory would have the same "problem" as string theory: it must show new physics at higher energies than already explored. LHC results are often seen as a "string theory is wrong"-result, which is not true, but what it rather shows is how boring the universe is at those energies, independently of the theory. Hopefully theory can give predictions in other places as well (in addition to the predictions of susy, extra dimensions, etc), like of what gravitational waves can say something fundamental about black holes.

I also question whether thinking in terms of particles is even the right paradigm. We may be just attempting to mold observations into our own incorrect model.