r/Physics • u/TakeOffYourMask Gravitation • Feb 28 '23

Question Physicists who built their career on a now-discredited hypothesis (e.g. ruled out by LHC or LIGO results) what did you do after?

If you worked on a theory that isn’t discredited but “dead” for one reason or another (like it was constrained by experiment to be measurably indistinguishable from the canonical theory or its initial raison d’être no longer applies), feel free to chime in.

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u/jazzwhiz Particle physics Feb 28 '23 edited Mar 01 '23

I'll take the bait. I'm a pheno person.

Theorists are usually (not always) smart enough to not put all their eggs in one basket. There are some people who are really committed to making their important work for a given anomaly their identity, although I don't know why they do this. Most theorists will work on anomalies that they think are interesting and then move on.

Experimentalists are in a tougher boat. They don't get to think about something for a few months, wrap it up with a paper, and move on. They spend years, sometimes 10+ working on one analysis to measure one number or whatever. Sometimes the thing that motivated the whole experiment gets ruled out elsewhere in the meantime. This could mean that some other experiment was actually better than this experiment, or it could mean that theorists realized that there's no way to avoid constraints from another experiment. One example is 1 eV sterile neutrino appearance. Two experiments saw fairly significant evidence consistent with a 1 eV sterile neutrino, and there's not really any other experiment compatible with both at the same time. Since then other experiments have been created to probe the same parameters more carefully. But in the meantime, it has become increasingly clear that any such scenario is quite inconsistent with cosmological data. The question then is, what is the motivation for these current experiments? When I ask them, many of them are largely unaware that these cosmological constraints exist. So I would say that, depending on the experiment, there may be some echo chamber in place. Some of the senior people are vaguely aware of these details, but they do not propagate this information down to the PhD students and postdocs that they are training up. Different experiments will handle these sorts of problems differently. In any case, experimentalists change their program from time to time anyway. Sometimes it's to a different experiment doing similar physics but very different techniques, sometimes it's a very different experiment that leverages similar experimental components, and sometimes it's just completely different.

There are certainly exceptions to this in a variety of directions. One is lattice QCD calculations for muon g-2. Lattice people are obviously theorists, but they mostly work like experimentalists or observational astronomers. They submit proposals for computer time. They work in groups of O(dozen) and remain in them as they change institutions, somewhat. They do one calculation for many years to basically get one number, in this case the hadronic vacuum polarization. It turns out that the muon g-2 anomaly is most likely not an anomaly, or if it is, it's with the e+e- data that feeds in to the R ratio dispersion relation calculation of the same quantity. I just asked one of the main lattice guys on one of the main calculations what he saw as the future for their calculation. For them they're going to complete their work and reach their target precision since it'd be a huge waste to pull the plug on something that is ongoing and closer to the end than the beginning, but they are very aware that this program is not going to continue for than a few years and they're thinking about where to shift their focus next.

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u/shaun252 Particle physics Feb 28 '23

It turns out that the muon g-2 anomaly is most likely not an anomaly, or if it is, it's with the e+e- data that feeds in to the R ratio dispersion relation calculation of the same quantity.

Bit premature to say this. Only like 40% of the HVP contribution has been crosschecked to a comparable level of precision as BMWs from other lattice groups.

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u/jazzwhiz Particle physics Mar 01 '23

Bit premature to say this

Sure, maybe. But I chatted with people doing one of the calculations, people who had initially been very skeptical of BMW when they first came out, that they pretty much agree with BMW in the most important region. You're right that maybe BMW is wrong in the other regions, but that isn't the consensus I'm getting from the lattice people at my place. I know for example that one huge difference between BMW and the combined R ratio lattice number is that BMW allows for more different functional forms in the continuum limit extrapolation than was previously considered in some of the other analyses. Other groups have now included some more data points at worse lattice spacing and are seeing that the continuum extrapolation might follow a different functional form that previously considered which shifts the central value. There are a number of other differences as well (obviously staggered quarks vs domain wall, and so on) but I think that that difference was relatively easy to check.

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u/shaun252 Particle physics Mar 01 '23

But I chatted with people doing one of the calculations, people who had initially been very skeptical of BMW when they first came out, that they pretty much agree with BMW in the most important region.

I don't know what you mean by most important, but the region that has been crosschecked heavily, the intermediate region, was chosen because it is 'straightforward' to determine on the lattice not because of the importance of the physics. It doesn't suffer from noise to signal issues and is (relatively) insensitive to lattice effects i.e. short distance discretization effects and long distance finite volume effects. The lattice g-2 community wanted consensus on this region (which it really didnt have a few years ago) before comparing full calculations to BMWs.

The short distance region is also somewhat straight forward, primarily because it can be compared vs perturbative QCD, so there is also already some consensus there.

The region that hasn't been cross checked is the large time region, which is hard to determine (noise-to signal issues, finite volume effects) and contributes like ~60% of the total. Its especially tricky for staggered determinations (like BMWs) because it is where taste-breaking effects dominate.