5 Unanswered Questions in Physics
5 C That Will Keep Physicists Awake at Night
October 25, 2013 | 
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Physics is all about probing the most fundamental mysteries in nature, so it’s no surprise that physicists have some very basic questions about the universe on their minds. Recently,Symmetry Magazine (published by two U.S.-government funded physics labs) asked a group of particle physicists to name the open questions in physics they most want answers to. Here’s a sample of the quandaries they shared:
“What will be the fate of our universe?”
The poet Robert Frost famously asked whether the world would end in fire or ice, and physicists still can’t answer the question. The future of the universe—the question named by Steve Wimpenny of the University of California, Riverside—largely depends on dark energy, which at this point is an unknown entity. Dark energy is responsible for the accelerating expansion of the universe, but its origins are entirely mysterious. If dark energy is constant over time, we’re likely looking at a “big freeze” in the future, at which point the universe continues to expand faster and faster, and eventually galaxies are so spread out from each other that space seems like a vast wasteland. If dark energy increases, this expansion could be even more severe, so that not just the space between galaxies but the space within them expands, and galaxies themselves are ripped apart—a fate dubbed the “big rip.” Another option is that dark energy decreases so that it cannot counteract the inward-pulling force of gravity, causing the universe to fall back in on itself in a “big crunch.” So basically, whichever way it goes, we’re doomed. On the bright side, none of these eventualities should come to pass for billions or trillions of years—plenty of time to decide if we’re hoping for fire or ice.
The poet Robert Frost famously asked whether the world would end in fire or ice, and physicists still can’t answer the question. The future of the universe—the question named by Steve Wimpenny of the University of California, Riverside—largely depends on dark energy, which at this point is an unknown entity. Dark energy is responsible for the accelerating expansion of the universe, but its origins are entirely mysterious. If dark energy is constant over time, we’re likely looking at a “big freeze” in the future, at which point the universe continues to expand faster and faster, and eventually galaxies are so spread out from each other that space seems like a vast wasteland. If dark energy increases, this expansion could be even more severe, so that not just the space between galaxies but the space within them expands, and galaxies themselves are ripped apart—a fate dubbed the “big rip.” Another option is that dark energy decreases so that it cannot counteract the inward-pulling force of gravity, causing the universe to fall back in on itself in a “big crunch.” So basically, whichever way it goes, we’re doomed. On the bright side, none of these eventualities should come to pass for billions or trillions of years—plenty of time to decide if we’re hoping for fire or ice.
“The Higgs boson makes absolutely no sense. Why does it exist?”
The tone of this question was tongue in cheek, says its asker, Richard Ruiz of the University of Pittsburgh, but it points to a very real lack of understanding about the nature of the particle famously discovered last year at the Large Hadron Collider (LHC) in Europe. The Higgs boson helps explain how all other particles got their mass, yet it raises many other questions. For example, why does the Higgs boson interact with each particle differently—the top quark interacts much more strongly with the Higgs than the electron does, giving the top quark a much greater mass than the electron. “This is the only example of a ‘non-universal’ force in the Standard Model,” Ruiz says. Furthermore, the Higgs boson is the first fundamental particle found in nature with zero spin. “This is an entirely new sector in Standard Model particle physics,” Ruiz says. “How it comes about, we have no idea.”
The tone of this question was tongue in cheek, says its asker, Richard Ruiz of the University of Pittsburgh, but it points to a very real lack of understanding about the nature of the particle famously discovered last year at the Large Hadron Collider (LHC) in Europe. The Higgs boson helps explain how all other particles got their mass, yet it raises many other questions. For example, why does the Higgs boson interact with each particle differently—the top quark interacts much more strongly with the Higgs than the electron does, giving the top quark a much greater mass than the electron. “This is the only example of a ‘non-universal’ force in the Standard Model,” Ruiz says. Furthermore, the Higgs boson is the first fundamental particle found in nature with zero spin. “This is an entirely new sector in Standard Model particle physics,” Ruiz says. “How it comes about, we have no idea.”
“Why is the universe so exquisitely balanced such that life can exist?”
Based on the odds, we really shouldn’t be here. Galaxies, stars, planets and people are only possible in a universe that expanded at just the right speed during its early days. This expansion was governed by the outward push of dark energy warring with the inward gravitational pull of the universe’s mass, which is dominated by the invisible kind called dark matter. If these quantities were different—if dark energy had been just a tad stronger after the universe’s birth, for example, space would have expanded too fast for galaxies and stars to form. But a smidge less dark energy would have caused the universe to collapse in on itself. So why, asks Erik Ramberg of Fermilab in Batavia, Ill., are they so perfectly balanced to enable the universe we live in? “We don’t know of a fundamental reason why that balance should exist,” Ramberg says. “There’s no doubt that the amount of dark energy in the universe is the most exquisitely fine tuned number in the history of physics.”
Based on the odds, we really shouldn’t be here. Galaxies, stars, planets and people are only possible in a universe that expanded at just the right speed during its early days. This expansion was governed by the outward push of dark energy warring with the inward gravitational pull of the universe’s mass, which is dominated by the invisible kind called dark matter. If these quantities were different—if dark energy had been just a tad stronger after the universe’s birth, for example, space would have expanded too fast for galaxies and stars to form. But a smidge less dark energy would have caused the universe to collapse in on itself. So why, asks Erik Ramberg of Fermilab in Batavia, Ill., are they so perfectly balanced to enable the universe we live in? “We don’t know of a fundamental reason why that balance should exist,” Ramberg says. “There’s no doubt that the amount of dark energy in the universe is the most exquisitely fine tuned number in the history of physics.”
“Where do astrophysical neutrinos come from?”
Extremely high-energy neutrinos are predicted to result from the collisions of speedy charged particles called cosmic rays with light particles (photons) in the Cosmic Microwave Background radiation that pervades the universe. But what sets this process in motion, and how the cosmic rays are accelerated, are open questions. A leading idea is that matter falling into the hungry supermassive black holes at the centers of galaxies gives rise to cosmic rays—but there’s no proof of this hypothesis yet. The resulting neutrinos are thought to be traveling so fast that each teensy-weensy particle has as much energy inside it as a fast-pitched baseball (which has billions of billions of atoms). “We can’t even fathom where these things are coming from,” says Abigail Vieregg at the Kavli Institute for Cosmological Physics at the University of Chicago, who posed the question. “If we find out, we can learn about the sources that are accelerating these particles to extremely high energies.”
Extremely high-energy neutrinos are predicted to result from the collisions of speedy charged particles called cosmic rays with light particles (photons) in the Cosmic Microwave Background radiation that pervades the universe. But what sets this process in motion, and how the cosmic rays are accelerated, are open questions. A leading idea is that matter falling into the hungry supermassive black holes at the centers of galaxies gives rise to cosmic rays—but there’s no proof of this hypothesis yet. The resulting neutrinos are thought to be traveling so fast that each teensy-weensy particle has as much energy inside it as a fast-pitched baseball (which has billions of billions of atoms). “We can’t even fathom where these things are coming from,” says Abigail Vieregg at the Kavli Institute for Cosmological Physics at the University of Chicago, who posed the question. “If we find out, we can learn about the sources that are accelerating these particles to extremely high energies.”
“How come the universe is made of matter and not antimatter”
Antimatter is like matter on opposite day: it has the same properties as the stuff that makes up planets, stars and galaxies, but one vital piece is different—its charge. The universe supposedly started off with equal parts matter and antimatter, but somehow, matter won out, with most of both substances annihilating each other shortly after the big bang, leaving a small surplus of matter remaining. Why antimatter lost this tug of war is anyone’s guess. Scientists are busy searching for processes called charge-parity violations, where particles prefer to decay to matter and not antimatter, to explain the disparity. “We’re particularly interested in trying to see if neutrino oscillations are different between neutrinos and antineutrinos,” says Alysia Marino of the University of Colorado, who shared the question with Symmetry. “This is something that hasn’t been seen so far, but we hope the next generation of experiments will look at in more detail.”
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