The dominant paradigm for darkish matter has lengthy been the weakly interacting huge particle (WIMP). WIMPs are hypothetical particles motivated by supersymmetry. That is well-posed scientific speculation insofar because it makes a testable prediction: the chilly darkish matter thought to dominate the cosmic mass finances needs to be composed of a particle with a mass within the neighborhood of 100 GeV that interacts through the weak nuclear power – therefore the identify.
That WIMPs couple to the weak nuclear power in addition to to gravity is what provides us a window to detect them within the laboratory. They need to scatter off of nuclei of comparable mass, albeit solely on the uncommon events dictated by the weak power. If we construct sufficiently big detectors, we must always see it occur. That is what an entire host of huge, underground experiments have been in search of. Thus far, these experiments have succeeded in failing to detect WIMPs: if WIMPs existed with the properties we predicted them to have, they’d have been detected by now.
The failure to seek out WIMPs has led to the consideration of a myriad of different potentialities. Few of those are as properly motivated as the unique WIMP. Some have nifty properties that may assist with the phenomenology of galaxies. Most are woefully uninformed by such astrophysical issues, as it’s laborious sufficient to do the particle physics with out violating some fundamental constraint.
One chance that almost all of us have been reluctant to ponder is a particle that doesn’t work together in any respect through robust, weak, or electromagnetic forces. We already know that darkish matter can not work together through electromagnetism, because it wouldn’t be darkish. It’s equally troublesome to cover a particle that responds to the robust power (although folks have in fact tried, with unusual nuggets within the ’80s and their fashionable reincarnation, the macro). However why ought to a particle must work together at the least by the weak power, as WIMPs do? No motive. So what if there’s a particle that has zero interplay with customary mannequin particles? It has mass and subsequently gravity, however in any other case interacts with the remainder of the universe by no means. Let’s name this the Angel Particle, as a result of it’ll by no means reveal itself, irrespective of how a lot we pray for divine intervention.
I first heard this concept mooted in a chat by Tom Shutt within the early teenagers. He’s a frontrunner within the seek for WIMPs, and has been for the reason that outset. So to recommend that the darkish matter is one thing that merely can’t be detected within the laboratory was anathema. A logical chance to be famous, however solely in passing with a shudder of existential dread: the legions of experimentalists in search of darkish matter are losing their time if there isn’t a conceivable sign to detect.
Flash ahead a decade, and what was anathema then appears affordable now that WIMPs stay AWOL. I hear some theorists saying “why not?” with a straight face. “Why shouldn’t there be a particle that doesn’t work together with the rest?”
One the one hand, it’s true. So long as we’re making up particles exterior the boundaries of recognized physics, I do know of nothing that precludes us from inventing one which has zero interactions. However, how would we ever know? We might simply hand over on laboratory searches, and settle for on religion that “gravitational detection” from astronomical proof is sufficient – and certainly, the one potential proof for invisible mass.
Experimentalists go house! Your providers aren’t required.
To me, this isn’t physics. There isn’t any technique to falsify this speculation, and even check it. I used to be already involved that WIMPs aren’t strictly falsifiable. They are often confirmed if discovered within the laboratory, but when they aren’t discovered, we are able to all the time tweak the prediction – all the way in which to this restrict of zero interplay, a state of affairs I’ve beforehand described because the categorical elevator to hell.
If there isn’t a technique to check a speculation to destruction, it’s metaphysics, not physics. Entertaining the existence of a particle with zero interplay cross-section is a logical chance, however it’s also a type of magical pondering. It supplies a technique to keep away from confronting the numerous issues with the present paradigm. Certainly, it supplies an excuse to by no means must take care of them. This manner lies insanity, and the top of scientific rationalism. We would simply as properly think about that angels are accountable for shifting objects about.
Certainly, the one advantage of this speculation that springs to thoughts is to deal with the age-old query: what number of angels can dance on the top of a pin? We all know from astronomical knowledge that the native density of angel particles have to be about 1/4 GeV cm-3. Let’s say the everyday pin head is a cylinder with a diameter of two.5 mm and a thickness of 1 mm, giving it a quantity of 10 mm3. Doing a number of unit conversions, this implies a darkish mass of 1 MeV* per pin head, so precisely one angel can occupy the top of a pin if the mass of the Angel particle is 1 MeV.
In fact, we do not know what the mass of the Angel particle is, so we’ve actually solely established a restrict: 1 MeV is the higher restrict for the mass of an angel that may match on the top of a pin. If it weighs greater than 1 MeV, the reply is zero: an angel is simply too fats to suit on the top of a pin. If angels weighs lower than 1 MeV, then they’ll match numbers in inverse proportion to their mass. Whether it is as small as 1 eV, then 1,000,000 angels can get together on the huge dance ground that’s the head of a pin.
So I suppose we nonetheless haven’t answered the age previous query, and it appears like we by no means will.
*An electron is about half an MeV, so it’s tempting to think about darkish matter composed of positronium. This doesn’t work for a lot of causes, not least of which is {that a} mass of 1 MeV is a coincidence of the amount of the top of a pin that I made up for ease of calculation with out bothering to measure the scale of an precise pin – to not point out that the scale of pins has nothing no matter to do with the darkish matter drawback. One more reason is that, being composed of an electron and its antiparticle the positron, positronium is unstable and self-annihilates into gamma rays in lower than a nanosecond – quite lower than the Hubble time that we require for darkish matter to nonetheless be round at this juncture. Consequently, this speculation is straight away off by an element of 1028, which is the type of factor that tends to occur if you attempt to assemble darkish matter from recognized particles – therefore the necessity to make up solely new stuff.
God forbid we ponder that possibly the power regulation may be damaged. How loopy would that be?
