The Week in Space and Physics
A giant comet from outer space, alien astronomers, new views of the universe and the fundamentals of quantum theory.
The edge of our Solar System is a still unknown region. Scientists are confident a ring of dwarf planets and comets lies roughly four billion miles from the Sun - something known as the Kuiper Belt. But beyond that, things get murky. Many astronomers think there should be a cloud of scattered, icy objects surrounding the Solar System and stretching far out into interstellar space.
Hard evidence for its existence has so far been lacking. The outer solar system is an area of unbroken darkness, with the light of the Sun no brighter than any other star. Out there, small comets and planets can spend an eternity drifting invisibly through the gloom.
A handful of objects have been discovered that may belong to the inner regions of this cloud. Certain comets, for example, originate incredibly far away, hundreds of billions of miles from the Sun. One small planet, Sedna, appears to drift between the Kuiper Belt and the clouds lying further out - by lucky coincidence it is currently close enough to the Sun for our telescopes to faintly pick it out.
Now, however, astronomers have discovered something that clearly does come from those outer reaches of space. The object, named 2014 UN271, was picked up in several images collected as part of the Dark Energy Survey, a project studying the evolution of galaxies. It appears to be a large comet - perhaps a few hundred miles across - and is hurtling towards our region of the Solar System.
The observations from the Dark Energy Survey show two other interesting things about this comet. First, it comes from extremely far away, more than half a light year from the Sun. Second, it will come extraordinarily close to Earth - almost as close as Saturn.
These two factors have gotten astronomers excited. The close approach of 2014 UN271 will offer them an unprecedented opportunity to study a big object from the outer solar system. That will probably be done by telescope - though we still have ten years until its arrival, that’s barely enough time to build and send a probe all the way to Saturn.
Even so, this is the biggest object we’ve so far discovered on the fringes of the Solar System. That we will soon get the chance to study it up close just makes the news even sweeter for astronomers.
When we think about the possibility of aliens, we often think about how we might spot them lurking in the night sky. We rarely seem to consider the reverse - how an alien astronomer might spot the Earth, and perhaps our own civilization.
Partly that is because we may not be easy to find. Though life has been on Earth for a long time, our modern civilization only formed a few thousand years ago - and for most of that time our impact on the planet was small enough to be missed. Alien astronomers might spot Earth as a potentially habitable world, then, but they may not realise a civilization exists here at all.
First, though, they would need to find the Earth. Astronomers often look for stars that display regular patterns of dimming - caused by a planet passing in front of the star - to find exoplanets. That approach, however, only works when the orbits of the Earth and the exoplanet line up in the right way.
Two American astronomers applied the technique in reverse, looking for star systems where the alignment would have made Earth detectable to alien astronomers. They identified over a thousand nearby stars which could have spotted the Earth sometime in the last five thousand years - within the period when human civilization was flourishing.
Several of those stars are already known to have exoplanets, some of which could be habitable. What’s more, seventy-five stars are close enough to have picked up the radio waves coming from our industrial civilization. Even if we haven’t spotted aliens yet, then, their astronomers may already be watching us.
For decades astronomy has relied on the electromagnetic spectrum - from visible light to radio waves - to watch the universe. Now that is starting to change. Recent years have seen several breakthrough advances in two new fields of astronomy: neutrino observatories and gravitational wave detectors.
These two techniques promise to give us a new view of the universe by revealing previously hidden information. But even more intriguing is the possibility of combining approaches to gather even more details about a single event - something known as multi-messenger astronomer.
Though still in its infancy, the idea has already seen some results. Back in 1987 the closest supernova in centuries erupted just beyond the Milky Way. Astronomers picked up faint signals of a neutrino blast - the first sign of an incoming shockwave from a supernova. Though the signal was weak, it was enough to give researchers clues to the final moments of a dying star.
Even more catastrophic events - such as black hole collisions - shake the fabric of the universe, sending gravitational waves rippling across space. Advanced detectors have already found a few examples of such events. In one, seen in 2017, astronomers picked up both light and gravitational waves from a collision between two neutron stars.
A recent paper detailed the ongoing search for a gravitational wave background - a kind of constant hum caused by colliding galaxies and spiralling black holes. By looking for slight distortions in the light from far away stars, the researchers hoped to spot this hum. Though they did not yet succeed, the results do place some limits on supermassive black holes in our cosmic neighbourhood.
Quantum Field Theory is perhaps the most successful theory of physics yet discovered. It describes the universe, or most of it, almost perfectly. Three of the four fundamental forces of nature - all except gravity - are encapsulated in its equations.
Even so, physicists feel the theory is not yet complete. Gravity is one glaring, and incompatible, problem. The mathematical foundations of the theory are weak or missing. Physicists themselves disagree about the correct approach towards the subject.
One giant of the field is Nathan Seiberg, a professor at the Institute for Advanced Study in Princeton. His ideas and theories have helped to develop the theory, especially placing the subject on a firmer mathematical ground. A recent interview with him in Quanta Magazine is worth reading, to hear his thoughts and feelings about the future of the theory.
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