The Week in Space and Physics: Gravitational Waves
The Week in Space and Physics: Gravitational Waves
On gravitational waves, pyramids and muons, DART and Rocket Lab
Like so many other strange things, gravitational waves are a consequence of Einstein’s theory of relativity. His theory predicts they appear when enormous objects accelerate: as they move, it says, they leave a ripple of disturbance behind them.
Such waves are constantly washing over the Earth, creating slight expansions and contractions. These normally pass unnoticed: even the most powerful waves create distortions far smaller than the size of a proton. Yet the waves are there, and with a suitable detector physicists can pick them up.
Still, the first such detection came only in 2015, when an American observatory known as LIGO spotted a wave sweeping through the Earth. LIGO uses laser beams fired over distances of four kilometres to look for the slight shifts caused by passing waves. The device is extraordinarily sensitive: it can pick up an expansion equivalent to one ten-thousandth the width of a proton.
The wave seen in 2015 came from two black holes spiralling towards each other. As they drew closer, they began to generate powerful disturbances in spacetime. At its peak, the collision briefly emitted more energy than every star in cosmos combined.
Since then, LIGO has gone on to spot several other gravitational waves coming from other colliding black holes or neutron stars. These, of course, are catastrophic events, giving out enormous amounts of energy. Yet theory predicts that smaller waves should be rippling across the universe. Even the Earth, as it orbits the Sun, should be emitting miniature waves of its own.
We can’t, of course, hope to spot waves as small as that. Yet Europe and America are working on a more advanced version of LIGO, named LISA, that should help find waves from other sources. LISA, when it launches in the 2030s, will consist of three spacecraft flying in a triangular formation around the Sun. As they fly, they will maintain a separation of two and a half million kilometres, a distance over which they will fire their lasers and look for signs of passing waves.
This ambitious observatory should be able to pick out gravitational waves from binary stars within our galaxy, or from pairs of supermassive black holes in others. Of particular interest are waves coming from smaller black holes. These are hard to spot at the moment, but an observatory like LISA would finally allow us to see them.
Ultimately LISA will help usher in a new era of astronomy: one based around new observing techniques unimaginable in the past. Until recently astronomy has focused on electromagnetic waves. Telescopes typically look only at visible light, radio waves or even gamma rays.
This approach, of course, has taught us much about the cosmos. But it has its drawbacks. Much of the universe is hidden from us, obscured by clouds of dust or by the bright heart of the Milky Way. The early universe, too, is shrouded in darkness: for the first four hundred thousand years it was filled with a “fog” of hydrogen.
Gravitational waves, along with neutrino astronomy, will allow us to penetrate that darkness. But astronomers are perhaps more excited by what they can learn through a combination of techniques. Supernovae, for example, shine brightly in visible light - but they also produce hordes of neutrinos and strong gravitational waves. By combining all three approaches, astronomers will one day be able to study them in greater depth than ever before.
Cosmic Rays Shed New Light on Ancient Egypt
Six years ago researchers uncovered signs of a hidden chamber within the Great Pyramid of Egypt. They found it through an exotic approach: using a rain of cosmic particles to peer through the pyramid’s thick walls. Now archaeologists have penetrated that chamber with a camera, revealing a space unseen by human eyes in thousands of years.
The discovery of the chamber relied on an approach known as “muon radiography”. Muons are particles rather like electrons, though somewhat heavier and more unstable. They are short-lived but made in large numbers as cosmic rays smash into the upper atmosphere. From there they constantly rain down on Earth.
When muons pass through a rocky material - like the walls of a pyramid - some of them will be absorbed. The result is a kind of muon shadow behind such structures; one that specialised detectors can spot. The darkness of the shadow will vary with the density and depth of the rock - just as dark and bright areas of an X-ray can reveal the varying densities of our bodies.
In the past scientists have used this approach to survey all kinds of structures, from volcanoes to pyramids. In 2019, for example, researchers used muon radiography to scan the interior of the Italian volcano Stromboli. They could then track the movements of magma and gas within the volcano’s chambers.
The 2017 muon study of the Great Pyramid of Egypt identified two mysterious voids within the structure. Precise details were hard to make out at the time. Last week, however, further muon studies showed the smaller void was likely a corridor about nine meters in length and two meters in height.
The corridor itself appears to be inaccessible without damaging the pyramid. Yet archaeologists were able to feed a small camera into it via a crack in the pyramid’s structure. It revealed an empty, vaulted corridor; one likely unseen by human eyes in thousands of years. The larger void remains, for now, unexplored. Its location - directly above the pyramid’s Grand Chamber - has led to much speculation about its purpose and contents.
Analysis Confirms DART’s Success
Six months ago, NASA crashed the spacecraft DART into an asteroid. The mission aimed to test asteroid redirection, thereby proving our ability to change the course of an incoming space rock.
Last week the science journal Nature published a set of five papers analysing the impact in detail. DART, they found, had been a smashing success: accelerating the orbit of Dimorphos by more than expected. Much of the extra change in velocity, they found, came from the vast cloud of debris thrown up by the impact. Over a thousand tons of dust and rock were blasted off the asteroid, forming a long tail that stretched for tens of thousands of kilometres behind it.
Interestingly, the impact also affected the appearance of the asteroid. Not only did it temporarily grow much brighter, but it also seems to have become bluer in colour. This is probably because the impact blasted off the outer layers of the asteroid, revealing an interior untouched by the effects of space.
Astronomers plan to keep monitoring the asteroid over the next few months, though it is now gradually moving further away from Earth. In a few years’ time, however, the European Space Agency will send a follow-up mission. This, named HERA, will survey the asteroid in detail, revealing more details about both the impact and its interior structure.
No More Helicopter Captures?
In November last year, Rocket Lab attempted to catch a falling rocket booster with a helicopter. The attempt failed, thanks, apparently, to a loss of telemetry data as the booster fell back to Earth. That left the booster to fall in the ocean, from where it was later recovered.
This was a bit of a blow to Rocket Lab, who had hoped to prove they could reuse their rockets just as SpaceX does. Ever since the assumption has been that Rocket Lab will, sooner or later, try again; especially as one earlier attempt had come close to success.
After checking the recovered booster, however, Rocket Lab seems to be considering giving up on the helicopter approach. Peter Beck, Rocket Lab’s CEO, said that the rocket had survived the ocean waters surprisingly well. In future they may simply focus on waterproofing their boosters, instead of trying to catch them with helicopters.