For example, the mathematical framework of the model does not play well with general relativity – another “almost complete” theory – especially with the Big Bang or black holes. The model also fails to address many unsolved mysteries in physics related to neutrino oscillation, matter–antimatter symmetry, or widely accepted ideas such as dark matter and dark energy.
An entire field devoted to this very disparity, surprisingly called “Physics beyond the Standard Model”, focuses on the gaps in the theory while providing alternatives. Often, this means that physicists will come up with new particles to supplement those known to exist. These hypothetical particles are, well, hypothetical, but that doesn’t mean they’ll remain out of reach forever. After all, it was only in 1897 that humanity discovered the electron and the Higgs boson in late 2012.
Read on for some popular hypothetical particles that could fix physics – if they exist.
1. Squark and Sleipn
“Squark” and “slepton” refer to broader categories of “superpartners” of more familiar fundamental particles, such as quarks, electrons, or neutrinos. So an up quark’s superpartner would be a “super quark”, while an electron would be paired with a “selectron”.
These sound like jokes, but I promise they’re not jokes. They are based on complex predictions related to supersymmetry, which hypothesizes that all force-carrying particles (such as photons) and matter particles (such as electrons) come in pairs. If valid, the particle partners should have a separation of half a unit, which theoretically pushes them toward the same, very high energy that mathematically unifies the different forces – a coveted dream for physicists including Albert Einstein.
2. Gravity for quantum gravity
Another major, unresolved issue in physics is to reconcile general relativity and quantum mechanics, two successful, groundbreaking theories that famously do not match. This is such a serious issue that, arguably, a large part of string theory is devoted to resolving this conflict.
But before string theory, scientists like British theoretical physicist Paul Dirac had other ideas. He considered the possibility of quantized gravity, possibly in the form of a particle. Physicists later named this hypothetical particle “graviton”.
If gravity is real, it would mediate the forces of the gravitational interaction, indicating that “gravity is indeed a quantum force, which gives us a direct clue to building a theory of quantum gravity,” Daniel Whiteson, a particle physicist at CERN and the University of California, Irvine, told Gizmodo.
3. Particles to tame neutrino “misfits”
Neutrinos are nearly massless, fundamental particles that pervade almost every corner of the universe. According to Symmetry Magazine, these neutrinos are “Standard Model misfits”. To begin with, the Standard Model initially predicted that neutrinos were massless. They were not. In fact, experimental results showed that neutrinos (1) have mass, (2) come in three different “flavors” (electron, muon, and tau) that mix between mass states, and (3) oscillate between those types, possibly due to quantum mechanical effects.
All of this raises some serious red flags for the Standard Model, and it’s an anomaly that physicists have yet to resolve. As a solution, researchers have introduced certain hypothetical particles, such as the sterile neutrino (the neutrino’s very heavy relative), the majoron, and triplets of zero-spin particles, which would give the neutrino mass in the same way as the Higgs boson gives regular matter its mass.
4. Most Dark Matter Candidates
Dark matter is invisible matter that makes up about 85% of the matter in the universe. Therefore, it would be reasonable to assume that something that makes up such a large portion of the universe could be a particle. And indeed, this has been the prevailing belief among scientists looking for signs of dark matter.
If dark matter is indeed a particle, it would be a strange particle, which does not absorb, emit or reflect any light and barely interacts with other particles. None of the known Standard Model particles fit that bill, so physicists have come up with a few alternatives, one of the popular ones is a class of particles called weakly interacting massive particles, or WIMPs. But this represents one end of a broad spectrum. Another well-received hypothesis argues that primordial black holes could contain dark matter, for example.
Accordingly, different dark matter particle candidates “could be anywhere within several tens of orders of magnitude, and may or may not be coupled to Standard Model particles,” Andrea Thamm, a theoretical physicist at the University of Massachusetts at Amherst, explained to Gizmodo.
5. Strive for our evil twin
Many unsolved mysteries in physics are interconnected, meaning that the same hypothetical particle can wear many different hats. A well-known, prominent hypothetical particle is the axion, which was first proposed to explain the strong CP problem. Put very simply, the strong CP problem arises from the issue of charge-parity (CP) violation, in which the behavior of particles and their antimatter counterparts is not aligned. But this does not seem to be the case in systems with only the strong nuclear force, or the force that holds protons and neutrons together. The strong CP problem considers how to understand it externally.
The late Italian physicist Roberto Peccei and Australian physicist Helen Quinn proposed a well-known solution to this problem that involves the axes. According to Frank Wilczek’s column on the discovery, the light particle presumably “cleared” the strong CP problem through a new symmetry involving the directional properties of the system. (Wilczek, an American Nobel laureate, officially named Axion after a brand of laundry detergent). Incidentally, the properties of the axion make it a strong candidate for dark matter as well.
6. Particle X
Here’s the strangest idea of them all: The imaginary particle of humanity’s strangest dreams — the thing that solves all our problems in the most unexpected ways — might be something we’ve never even bothered to think about.
Again, the probability of this is probably low. After all, it seems that physicists have no shortage of ideas, devising all kinds of possible scenarios and theories (see the Wikipedia article on hypothetical particles to see what I mean). My list represents a fraction of that broader practice, which focuses on more mainstream issues. That said, there’s a lot on this list not covered. And with major upgrades to humanity’s most powerful colliders, who knows what we’ll find (or not find)?
“The great thing about particle colliders is that you don’t have to hope for new particles to find them,” Whiteson said. “And the best discoveries, the ones that teach you the most about the universe, are the surprises!”
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