Please check physics on this one.
https://www.bbc.com/news/uk-scotland-36597142
> Most of the fundamental theories of physics are based on symmetry. That symmetry dictates, among other things, that the nuclei of atoms can be one of just three shapes: spherical, discus or rugby ball.
> Dr Scheck says: “We’ve found these nuclei literally point towards a direction in space. This relates to a direction in time”
Um, no?
https://www.sciencealert.com/physicists-discover-brand-new-nucleus-shape-time-travel-not-possible
To be honest, much of this feels like very irresponsible journalism, partly on the part of the BBC and very much so on the part of Science alert.
If you’re looking for an accessible resource to what the paper does, the cover piece on APS Physics and the phys.org piece are much more sedate and, I think, much more commensurate with what’s actually reported in the paper.
The paper itself is very moderate in its claims and it restricts itself very well, from what I can tell, to what they found: that certain radium and barium nuclei appear to be pear-shaped. Finding pear-shaped nuclei is not new (a similar paper (eprint) made the news in 2013, and was discussed on this site here), though Bucher et al. seem to have found hints of a discrepancy with theory with regard to just how pear-like these nuclei look like. This is, however, not at the level of statistical significance that would require any rethinking of the theory at this time.
It is important to note that pear-shaped nuclei are indeed consistent with the Standard Model of particle physics. Pear-shaped nuclei are a bit of a problem because their shape has a direction, that is, you can draw a vector that starts at the flat end and points toward the pointy end (call this the pear vector P⃗ ). (The alternative, a rugby-ball-shaped nucleus, has an axis, but no preferred direction on this axis.) Because of symmetry considerations, this pear vector P⃗ needs to be on the same axis as the nucleus’ spin S⃗ , but these symmetries don’t tell you which way they have to point, so you get two different versions of the same nucleus:
In a theory of nuclear physics that is mirror-symmetric, then both of these nuclei need to be completely equivalent (and, particularly, have the same energy), because they are mirror images of each other. What this paper finds (and what Gaffney et al. found in 2013) is that there are nuclei where this is not true, and these two nuclear states have different energies: the ground state is the “pear” state, and not the “anti-pear” one.
Fortunately, this is not a problem: in fact, we’ve known since 1956 that nuclear physics is not mirror symmetric, i.e. it is not invariant under the parity operator P. Fortunately, though, there is a related symmetry that takes up the slack, and it is charge conjugation symmetry C, which takes matter to antimatter and vice versa. Much of the Standard Model, including a lot of nuclear physics experiments, is CP symmetric: if you take a mirror version of the experiment, and on top of that you swap out all particles for their antiparticles, then the physics is the same.
However, CP violations are still compatible with the Standard Model and have already been observed experimentally. On the other hand, the known CP violations are not really So how does the paper at hand relate to all these generalities? The authors have confirmed the existence of P
violations, already observed, and they have found hints that these violations – the peariness of the pear-shaped nucleus are stronger than the existing theory. However, they are not comparing against ab-initio theory (i.e. they compare against approximate theoretical models, so the fault could be in the approximations they made) and, to quote from the paper’s discussion, the large uncertainty on the present result does not allow one to elaborate further.
So how did we get from there to time travel? That’s where you need a large amount of journalistic ‘creativity’ for the joins to work. The APS Physics piece is clear and to the point, and it makes a good show of understanding the …