May 23, 2016 at 02:21AM

Today I Learned: 1) Flails aren't real, maybe. Probably. I should clarify. Military flails definitely exist, but somewhere between most and all of them are artistic re-creations built far after the time period in which they were supposedly used. There are also contemporary-to-the-period artistic renditions of flails in use... but the same artists that drew the flails also drew some ridiculous, obviously-fantasy equipment in similar contexts, so it's really hard to tell whether they were meant to be taken seriously. More details here: http://ift.tt/1qhbLmH. This is one rare article where it's also worth going through some of the top comments. My favorite bit: "I suspect the flail had religious overtones and was therefore more of a ceremonial or "for show" piece for the pious knight." -> "Maybe like a weaponized censer? A 400 year old precuser to the over the top religious images of Warhammer 40K perhaps?". Thanks to Sarah Seid for teaching me a little bit more about medieval weaponry! 2) There's a new sensor for ATP! It's called "QUEEN", and it's similar to GCaMP, if you're familiar with that -- it's a modified verison of GFP (Green Fluorescent Protein) that's been split open, fused to an ATP-binding subunit of another protein (a bacterial ATP synthase, for the curious). When ATP binds to that subunit, it causes a conformational change and a stiffening in the subunit, which brings the two halves of the split GFP together and allows it to function. There are actually two versions of QUEEN, tuned for different concentrations ranges of ATP. QUEEN-7µ is linearly responsive between roughly 5 µM and 500 µM, and can be used to measure "low" concentrations of ATP (100 µM is pretty high concentration! Someone correct me if my math is wrong, but I think that's roughly an (average) inter-molecule spacing of about 25 nanometers.). QUEEN-2m is responsive between 1 or 2 mM and 10 mM, which is roughly physiological concentrations. Details behind the (sigh) paywall here: http://ift.tt/1TpHks2 3) ...about Bohmian mechanics, an interpretation of quantum mechanics in competition with the Copenhagen Interpetation (though never as popular). Bohmian, first developed by de Broglie and later championed by one David Bohm, postulates that particles are real, have actual, defined positions that don't require measurement to exist, and act non-locally. The theory trades the "weirdness" of wave/particles without defined positions and momentums for the "weirdness" of non-locality, i.e. explicit spooky action at a distance. For most physicists, the former bullet is easier to bite than the latter, and a paper in 1992 claimed to show that Bohmian mechanics predicted some nonsensical predictions of possible particle paths. A recent paper claims to show that the 1992 paper (known as the "ESSW" paper) didn't use Bohmian mechanics correctly, and that the "surrealistic trajectories" predicted by that paper actually make perfect sense if you account for the theory's nonlocality. Like the Copenhagen Interpretation (and like Many Worlds), Bohmian mechanics postulates a wavefunction that evolves according to the Schrödinger equation and (unlike Many Worlds predicts) collapses under observation. However, Bohmian mechanics doesn't claim that the wavefunction *is* particles the way other theories do -- instead, particles are real, have defined, precise positions and momentums, and interact non-locally. The evolution of particles follows a so-called "pilot equation", which is a deterministic function of the wavefunction and the positions of all the particles in the universe. I feel this is the time and place for a brief overview of the various interpretations of QM, according to Sam. Note that (as far as I know), all of these interpretations make the same predictions, for the simple reason that any theory that wasn't in experimental accordance with any other interpretation would be immediately discarded as wrong, since QM as a whole is one of the most experimentally well-supported theories in physics. The differences between interpretations is one of, well, interpretation, and what the theory implies about how reality is structured (spoilers: none of them are intuitive). The Copenhagen Interpretation says that the wavefunction is a full description of an unobserved system. When a system is measured by an experimental aparatus or observed by an observer, it "collapses" probabilistically to a point function (in violation of the Schrödinger equation, mind you!), essentially "picking" one state of particles in defined positions from the wavefunction, where the probability of picking any particular state is proportional to the (square of the magnitude of) the value of the wavefunction at that state. As far as I can tell, this is an essentially supernatural, dualist interpretation of QM. The Copenhagen Interpretation is still quite popular among quantum physicists, though nowhere near as popular as it used to be. Many Worlds (my favorite interpretation simply because it is the one I understand best except the Copenhagen Interpretation, which I'm fairly sure is nonsense) takes the wavefunction at face value -- it rejects collapse and instead proposes that all of the states of the wavefunction exist simultaneously. Many Worlds trades the ridiculous notions of "observers" and "collapse of the wavefunction" for the ridiculous notion that there are infinitely many parallel versions of the world playing out in accordance to the wavefunction. This is too much for some scientists, who claim that Many Worlds vilates Occam's Razor by postulating tons of extra worlds. Personally, I would counter that Many Worlds is a *strictly simpler* theory than Copenhagen and any other collapse-based theory, it just doesn't pretend to not notice all the impliciations of the wavefunction... but I digress. Bohmian mechanics proposes a wavefunction much like the Copenhagen Interpretation, but claims that the wavefunction guides the real, deterministic, particle-like evolution of *acutal* stuff, albeit in a nonlocal way. Bohmian mechanics trades the wave/particle duality for non-locality. Consistent Histories claims... something about how particles can have many alternate histories, and experiments give different weights to these histories...? Something about lots of matrix math and tensors...? I would really like to understand consistent histories, but I don't. Ensemble interpretation basically states that QM only applies to statistical ensembles of particles. Not sure what else it says. Relational QM, transactional QM (Object-oriented QM, anyone?), stochastic mechanics, objective collapse, many minds, quantum logic, quantum information theory, modal interpretations... no clue what these are. That concludes today's mini-rant on QM. Expect another in about six months.