
Particle physics experiments have stopped answering to grand theories - pseudolus
https://aeon.co/essays/has-the-quest-for-top-down-unification-of-physics-stalled
======
peterburkimsher
I see a human problem with an incremental experiment-driven model, instead of
a theoretical model.

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"

~~~
walrus1066
No chance of getting funding by justifying 'this $multi-billion collider may
or may not find new physics outside the Standard Model'.

The LHC only got funding because it was guaranteed to answer the question 'is
there a Higgs Boson'.

~~~
Cthulhu_
Your last statement needs a citation; I think that's the line mainly used to
sell it to the public in pop sci headlines.

~~~
sfifs
The people who funded it were politicians who cater to public who read the pop
sci magazines. I wonder if any significant number of people who actually made
the funding decision looked at anything beyond "this will make you look like
you support Science"

~~~
targafarian
Funding on large science projects that I've seen, at least how the NSF does
it, "filters up" where the proposal is passed through technically aware
committee before being passed up to less technically aware people. And there
is also the P5 committee which makes high-level recommendations about general
directions and projects that can more holistically drive "important" (or at
least what they deem to be so) science goals forward, where their advice tends
to guide many levels of the funding process pretty strongly (if your proposal
gets a low grade from them, you aren't getting funded, period). At the level
of really big science, this "filtering up" process will go all the way to
Congress, where of course you then have people with average-zero science
knowledge making a final decision. But to get to that point, I'd wager most if
not all science proposals will have passed many valid technical hurdles
presented by knowledgeable people.

------
auslander
Hats off for the author and physicists. Imagine a blind and deaf person
painting and writing songs.

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

~~~
dyukqu
There is a blind (since birth) painter who is considerably famous -
[https://www.youtube.com/watch?v=JTDQcSS809c](https://www.youtube.com/watch?v=JTDQcSS809c)

------
DoctorOetker
It's a good article, but I have one big complaint: the terminology of bottum-
up and top-down is reversed!

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.

~~~
ssivark
Not quite. When talking about bottom-up and top-down, one can talk about small
to large scales in size (microscopic to macroscopic) or or can talk about
small energies to large energies. The two descriptions are inversely related
because small length scales correspond to large energies, and vice versa
(roughly due to the Heisenberg uncertainty principle). Conventionally, when
particle physicists talk about top-down and bottom-up, they are talking about
energy scales: "top" is high energy, more fundamental, smaller distances and
"bottom" is low energy, more emergent, larger distances.

~~~
DoctorOetker
first, thank you very much for responding,

but still I have the impression that bottom refers to the more fundamental
perspective, and top refers to the more general perspective...

i.e. the fundamental equations for electromagnetism are the maxwell equations
with epsilon, mu and c for vacuum, and then one can use these as microscopic
equation for other media and generate effective equations for other media so
that the new maxwell equations are similar but modified, i.e. different scalar
epsilon, mu, c or perhaps for birefringent crystals the epsilon and mu are
genralized to matrices/tensors, or for nonlinear optics theres second and
third order tensors... and those new effective equations are top and the
fundamental is bottom... at least thats how I think most people would denote
top and bottom...

I kind of see what you mean, but I don't think top annd bottom on energy
scales is very significant, I think the derivabilitiy of new effective
constitutive equations/propeerties from fundamental equations/properties is
more top vs bottom...

but I agree, at a certain point its not a discussion of physics but just
dictionary wars...

------
rubidium
Next big movement in understanding the physical laws of the universe is going
to come from the areas we least understand. Right now, that's astrophysics.

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.

------
zellyn
A completely clueless question: do they dramatically reconfigure the LHC and
other colliders for each type of experiment?

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...](https://ai.googleblog.com/2017/07/so-there-i-was-firing-megawatt-
plasma.html)

~~~
ganzuul
The latter. They generate much more data than what they can even retain, so
they filter it in multiple stages.

~~~
DoctorOetker
I think your answer (with which I agree) is a confirmation of the former. The
questions-to-be-answered must be posed first, so that the low level filtering
can filter out what we aren't interested in, because we can't record all the
hits for each event and store them. (although I think one should consider it,
perhaps if a huge part of the budget had always gone to storage, we might have
petabyte databases for lower energies, which could now be stored on consumer
HDD's and new patterns in old experiments could be found, while at the time of
this old experiment this may have seemed senseless as no individual physicists
could back then hope to have a local copy. Similarily today it would generate
preposterous amounts of data, but if we record them now (even if expensive)
then physicists 20 years from now might be able to store them on their future
personal media...

------
acqq
The point is near the end:

"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...](https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment)

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](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](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.

~~~
lloeki
Kind of tangential but this reminds me of Feynman:

> It doesn't matter how beautiful your theory is, it doesn't matter how smart
> you are. If it doesn't agree with experiment, it's wrong.

~~~
gwern
The funny thing about that quote is that Feynman ignored it all the time. Back
during the post-WWII explosion, Feynman would get told about some exciting new
result and regularly said that the result was wrong because it contradicted
theory and was experimental error. And it usually was error. A lot of those
anecdotes in his oral history: [https://www.aip.org/history-programs/niels-
bohr-library/oral...](https://www.aip.org/history-programs/niels-bohr-
library/oral-histories/5020-1)

~~~
marcosdumay
Experimental data comes with confidence levels. There's no point in being
excited at low confidence extravagant data.

This does not detract from his phrase in any way.

~~~
gwern
Yes, it does. He didn't say, "it doesn't matter how beautiful your theory is,
unless of course, it's sufficiently beautiful that it has high posterior
probability such that you will dismiss an indefinite but potentially large
number of experiments showing it's wrong until enough experiments have been
done that you have to admit that it's wrong because it disagrees with them".

Of course, Feynman was right to dismiss those experiments. It's his quote
which is cute, oversimplified, and wrong. It's Duhem-Quine all over.

------
monster_group
Wow, an article explaining standard model and unification but also mentioning
"E=mc2 (energy equals the square of the mass)"!

~~~
gjkood
You just forced me to skim the article for that nugget!

------
ionathan
Minor point, but:

> 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?

~~~
InclinedPlane
Mesons have a wide variety. Some are quarks plus different anti-quarks (e.g.
up + anti-down). Neutral mesons are not just quarks plus their anti-quarks,
they are usually quantum superpositions of multiple quark/anti-quark pairs.
For example, the pi-0 meson is a superposition of up + anti-up _minus_ down +
anti-down, all divided by the square root of 2 (quantum mechanics is weird).
However, it gets even more complicated when spin comes into play, an up +
anti-down meson could be a charged pion with 0 spin and a mass of 139 MeV or
it could be a rho meson with a spin of 1 and a mass of 775 MeV.

------
rotorblade
Just have to add a comment on some things.

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.

------
XalvinX
If they are going to use up to 26 dimensions in their theories, of what use is
a machine that only senses the 4 dimensions we normally perceive? Maybe all
these missing particles are in some of the other 22? I'm sorry, but I don't
think solid answers to some of the big questions are ever going to be found
using these experiments.

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.

~~~
snowwrestler
It's fun to speculate, but it's worth pointing out that science progresses via
actual work.

If you know how to build a machine that senses more than 4 dimensions, please
build it and run some experiments! If you can't, then recognize the
limitations of critique. Anyone can sit back and say "hey, what if we're doing
things wrong?" Scientists already spend most of their time in this area of
thinking.

The operative question isn't "what use are our limited experiments?"
Scientists know their experiments are limited. The real question is, "how,
specifically, can we do new experiments?"

Or for theoreticians, it's not "is our model incorrect?" Scientists know the
theories aren't correct... not totally correct, anyway. The real question for
a theoretician is, "what new model can I propose that matches all known
evidence, and also opens the door for new understanding?"

~~~
XalvinX
I guess I was just thinking out loud, but then I did read the article and have
read many other like it over the last 30+ years.

I can see it must be fun to speculate, but also seemingly profitable for some
of these guys too, eh? I wonder how much a "theoretical physicist" gets paid,
anyways?

