The Week in Space and Physics #6
At first physicists thought the neutrino, like the photon, was a massless particle. That, as Einstein told us, allows it to move through the cosmos at the speed of light. Observations seemingly confirmed this fact. Neutrinos do indeed move very fast, close to the speed of light (and not beyond, as one experiment mistakenly found in 2011).
That should give neutrinos some rather unusual properties. According to relativity, time appears to slow the closer one travels to the speed of light. In the limit – at c itself – time ceases to flow altogether. Photons, when they travel across the universe, do not experience the passing of time – and, therefore, cannot change or decay as they do so.
Photons follow this rule quite happily. But neutrinos, as physicists discovered with some shock in 1998, do not. Indeed, neutrinos do appear to change as they move, changing from one type of neutrino into another. That means, unless something is wrong with relativity, that they must experience time – and therefore must have a mass.
Since neutrinos are literally everywhere – countless trillions are passing through you right now – this mass could add up to something quite substantial. Indeed, some even speculate that neutrinos make up almost everything, accounting for much of the missing “dark” matter that should fill the cosmos.
That would be an impressive achievement for a particle so fleeting it is often referred to as a ghost particle. Neutrinos rarely interact with the atoms that make up our world – passing right through the planet, through the entire galaxy, with ease. To them our world is little more than a shadow, a faint wisp in a universe filled with neutrinos.
The obvious question, then, is how much a neutrino weighs. The answer has long been elusive – we can only say they weigh close to nothing. One experiment, however, aims to answer the question. KATRIN, running in Germany, uses sensitive measurements of radioactive hydrogen to put an upper limit on neutrino masses. Every time a hydrogen atom decays, it spits out one electron and one anti-neutrino. Physicists cannot capture the neutrino – but they can trap the electron.
When they do, they measure its energy precisely. Simple subtraction can then reveal the energy carried by the neutrino – and then, through some more calculations, place a limit on its mass. Over time, as the experiment gathers more data, this limit should become ever more accurate.
Last week KATRIN reported that this mass must be below 0.8 electron volts – a unit used to measure the mass of subatomic particles. That compares to the mass of the electron – five hundred thousand electron volts – and the mass of the proton – nine hundred million electron volts.
Neutrinos, in other words, must be tiny. Quite how this is possible is unknown – our best theories of physics struggle to explain how such low masses are possible. When – or if – we manage to weigh one, then, the results should point physicists towards new theories. That could help resolve other long standing mysteries of physics: from the nature of dark matter to absence of antimatter in our universe.
The First Private Space Walk
Last year Jared Isaacman – an American billionaire – paid Elon Musk for a four day trip into space. Musk delivered, sending him and three other astronauts into orbit in his Crew Dragon capsule. It seems Isaacman enjoyed his trip: he has now purchased three more flights from SpaceX.
The three launches, together named Polaris, are deeply ambitious. The first, which could happen by the end of the year, will see the first private spacewalk. Flying higher than any other crewed mission, bar the Moon flights, Isaacman and his crew will don suits designed by SpaceX and enter the void, unprotected by their capsule.
That would be a big step forward for SpaceX. Spacewalks are dangerous, and NASA astronauts often train for years before undertaking one. The Dragon capsule, too, is not specifically designed for spacewalks. The entire capsule will have to be depressurised, thus exposing each crew member simultaneously to danger.
The bigger step, however, may come in the third flight. Isaacman says he has booked the first crewed flight on Starship – the mammoth new vessel under construction by SpaceX. Quite when that flight might take place is still unclear. Starship has not yet reached space, let alone orbit, and several successful test flights into space will be needed before humans dare to ride inside.
Still, the announcement is interesting news. When SpaceX tested the Dragon capsule for the first time, two experienced NASA astronauts took the controls. Handing Starship over to far less trained commercial astronauts is an interesting decision .
Musk has also sold a flight on Starship to Yusaku Maezawa, a Japanese billionaire. That flight – which was originally scheduled for next year – will fly around the Moon. Maezawa, like Isaacman, has already been to space: he paid Russia for a trip to the International Space Station last year.
How much has Isaacman paid for all this? No figures have been publicly announced. Isaacman did, however, say that his first flight last year cost less than two hundred million dollars. Three more launches, then, could have cost several hundred million. NASA, by contrast, pay around fifty million dollars for each astronaut SpaceX transports to the space station.
No Life in the Galactic Centre?
Alien hunters have, unfortunately, little hard data to go on. Only one intelligent civilization is known – our own – and despite decades of searching we’ve never spotted traces of another. In some ways that is odd: calculations suggest the galaxy – or at least one galaxy somewhere – should show signs of a hyper-advanced civilization; one millions of years more advanced than us.
If such a civilization does exist in our own galaxy, where might it be? The galactic centre, perhaps, is an obvious starting place. Millions of stars cluster there, swarming around the central black hole, and, in all likelihood, billions of planets must exist there too.
Such a concentration of stars does, however, raise some dangers. Supernovae are more frequent, as are close approaches between stars. Both could disrupt the formation of planets and harm the development of life. Studies confirm these dangers – but they also suggest that they should be outweighed by the sheer number of planets in the galactic centre. It should, therefore, be a good place to look for life.
To test these ideas, astronomers in Australia recently directed a radio telescope towards one hundred and forty stars in the galactic centre. They examined each for signs of unusual radio activity, hoping to spot traces of alien communications. Nothing, however, was found.
That doesn’t necessarily mean the galactic centre is devoid of life. Searching for radio waves is a somewhat naive way to look for aliens – based on an assumption advanced species will continue using a relatively primitive technology. Indeed, a sophisticated alien civilization may not look anything like we expect – and may, therefore, seem completely invisible to us.
First Light for the James Webb
The deployment of the James Webb continues to progress smoothly. At the end of January the telescope reached its target orbit, settling into a spot roughly one million miles from Earth. Operators have since started commissioning the telescope: turning on high capacity radio links and continuing to cool its instruments.
They have also started aligning the telescope’s mirrors, capturing the James Webb’s first images in the process. Eighteen separate segments make up the primary mirror; each must be carefully aligned for the telescope to work properly. The process of doing this began with a test shot: an observation of a single star in the sky.
The image created shows this star eighteen times – once for each segment. As operators bring the segments into alignment, however, these eighteen visions of the star will gradually coalesce into one: demonstrating for the first time the true power of the telescope.