The Week in Space and Physics
On neutrinos, model universes and the threat of hypersonic weapons
Of the dozens of subatomic particles, one stands out as especially mysterious. The neutrino – a particle so small and elusive that we detect far less than one in every trillion – has long puzzled physicists with its strange and volatile behaviour. There is, indeed, a fair bet that new physics – when it comes – will somehow involve these odd particles.
Part of the problem lies in how neutrinos pay little heed to the atoms that make up our everyday world. They interact only through the aptly named weak nuclear force – an interaction that can only occur when a neutrino strikes the core of an atom. Since atoms are mostly empty space – the core is a tiny fraction of the overall thing – neutrinos are free to travel through the entire planet – the entire galaxy, indeed – with little trouble.
There may, however, be an even more ghostly form of neutrinos. In theory, some neutrinos – known as sterile neutrinos - should be able to ignore the weak force too, rending them all but invisible to our world. True, they should still exert a gravitational pull – and might, therefore, explain dark matter - but gravity is weak, and neutrinos are very small. No detector on Earth could ever hope to pick up that signal.
They should, however, affect the way slightly more visible neutrinos behave. Physicists know that those neutrinos can be divided into three kinds: the electron, muon and tau neutrinos. An individual neutrino can change its type as it travels, flipping from electron to muon, or from muon to tau. The presence of sterile neutrinos may affect those flips - slightly favouring electron neutrinos.
One experiment – MiniBooNE – did indeed find signs there are more electron neutrinos than expected. At the time this was taken as possible evidence that the sterile neutrino really does exist. If so, that would be a breakthrough in physics - as it would hint at new, deeper laws of nature. Those results were, though, inconclusive, with a chance that the extra neutrinos were actually other particles.
To find out, researchers built a more advanced experiment: MicroBooNE. In results released last week, however, the mystery seemed to deepen. MicroBooNE found no sign at all of the extra electron neutrinos. That, probably, means that the particles seen by MiniBooNE were not neutrinos at all – and instead must be another unknown particle.
That is bad news for people hoping to find sterile neutrinos, as the MiniBooNE data was the strongest evidence in their favour. But it could be good news for other people hunting for new particles – such as those looking for signs of dark matter, dark energy or other deviations from the standard theories of physics. One thing is for sure: the results will keep physicists thinking for some time to come
Deep Space Neutrinos
A constant flood of neutrinos washes across our planet – a flood that might, if we could see it, open our eyes to a new form of astronomy. Unfortunately, of course, neutrinos are hard to spot. Would-be neutrino astronomers must build massive and sophisticated detectors, buried deep underground or far from civilization.
One of the world’s most powerful neutrino observatories sits close to the South Pole. There, buried in a cubic kilometre of ice, is an array of sensors, each watching for a faint flash of light – the sign of a neutrino colliding with an atom. Every day the detector – named IceCube – picks up a handful of such flashes.
In a paper published earlier this year, researchers at IceCube reported an unusual flash they picked up in 2016. It was, they say, extremely energetic – a sign of a particle racing across the cosmos.
After ruling out other explanations the team concluded that it was indeed a neutrino. And - more - that it had come far, far away, somewhere far beyond our galaxy. Such extragalactic neutrinos are rare. Almost all the ones we see come from the Sun, and only a handful have ever been confirmed to come from more distant sources.
When the neutrino arrived on Earth, it did so with enough energy to create an effect known as a Glashow resonance. This effect, though long predicted, had never been seen before – mostly because no particle accelerator on Earth can get close to the energy needed. It also, the researchers also note, confirms we actually saw an anti-neutrino – the antimatter partner of a regular neutrino.
That is interesting for two reasons. First, this is one of the only times we’ve ever been able to definitely say whether we saw a neutrino or an anti-neutrino. And second, as it shows that cosmic neutrinos could be used to probe high energy physics beyond the reach of our most powerful accelerators.
A Giant Model Universe
Cosmologists – those that study the long term evolution of the universe – have a good idea of how things started out. Sometime in the distant past – probably around fourteen billion years ago – the cosmos flashed into existence; an event known as the Big Bang. Ever since the laws of physics have ruled supreme: sculpting dust and gas into stars, galaxies and superclusters.
Working out precisely how the universe evolved, however, is a complicated problem. How things play out depends on how matter and energy spread across the cosmos – and on how mysterious things like dark matter and dark energy behave.
Cosmologists often build models, each slightly different, and then use computer simulations to see what comes out. By comparing the results of these simulations with the real universe they can work out which models seem most accurate. That then allows them to pin down the properties of dark matter and dark energy.
A problem, of course, is that the universe is both very big and old, and computers are limited in power. That means cosmologists must simplify the models. Instead of trying to simulate every star in the universe, they often clump millions of stars together as “particles”. The models then look at how those particles move around the virtual universe.
A paper last week reported on one of the most powerful such models ever run. In total it contained sixty trillion particles and modelled the evolution of the early universe over billions of years. The team behind it tried almost a hundred scenarios, each with slightly different properties.
The model, indeed, is more detailed than is really useful at the moment. Instead astronomers expect that new observations over the next few years – involving the mapping of tens of millions of galaxies – will support or rule out the models tried. That – assuming one of the models matches what they see – should teach us a bit more about the dark side of the universe.
Did China Test a Hypersonic Weapon?
Strategically speaking, the idea of space based weaponry makes perfect sense. A bomb waiting in orbit could hit anywhere on Earth, with very short notice and almost without warning. It would – since something falling from orbit moves incredibly fast – be all but impossible to defend against such a space missile.
That makes China’s rumoured test of an orbital glider worrying. According to reports, the test took place in July last year and involved a missile or glider sent into orbit. After looping around the planet, the glider re-entered at hypersonic speeds, steering towards a target. American generals reacted with alarm – going so far as to label it a Sputnik moment for America.
If the test really did happen, then it may signal the beginning of a new age of nuclear strategy. China and America could position dozens of nuclear warheads in orbit, each capable of a sudden attack on the other. The prospect is frightening. In a world ringed by hypersonic nuclear weaponry, global annihilation would never be more than a few minutes away.
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