12 mins read 17 Mar 2021

Without Distant Radio Quasars, You Can’t Have Self-Driving Cars

For self-driving cars to be viable and safe, they’re going to need the most up to date mapping, along with positioning and navigation systems operating in real-time. So then, what does the ancient light, from distant supermassive black holes have to do with making sure we're all ready to let go of the wheel?

The Australian Telescope Compact Array (ATCA) located in Narrabri, NSW. Credit: CSIRO.

In the not too distant future, when self-driving vehicles are going to be fairly common, most people won’t realise that these vehicles have the ability to know exactly where they are going due to the assistance of distant supermassive black holes (called quasars) that help pin our coordinate reference frame here on Earth. 

That’s right - in the far-distant Universe, and spread across the sky at all locations, these extremely bright behemoths (in which the most powerful shine with the luminosities thousands of times greater than the entire Milky Way Galaxy) dictate where our computer-equipped vehicles will end up.  

It might sound like science fiction but in fact, several Australian radio telescopes help contribute to this extremely accurate record-keeping of our coordinate system on a regular basis - under a global program that keeps track of where each radio telescope is on Earth, down to the millimetre.

This is all achievable by linking these radio telescopes together (even if some of them are not on the same continent) to take precise measurements of the electromagnetic signals generated by distant quasars, in an intriguing method known as Very Long Baseline Interferometry or VLBI.

The extremely accurate measurements are able to determine where each continent - like Australia - is located on Earth’s surface, and how this positioning changes over time.

This becomes especially handy when you consider some of the current and future applications this might apply to, especially given Earth is a very dynamic system which is orbiting the Sun and the continents are always on the move through large-scale geological processes like plate tectonics.

Whilst future self-driving vehicles will need this handy information to navigate the streets without anyone behind the wheel (assisted through Global Navigation Satellite System (GNSS) technology), currently, the information is also used for position and navigation purposes, as well as assisting with natural disasters and helping model the impacts of climate change. 

This works because most GPS-linked devices, like cars, phones, aircraft, and more all take advantage of triangulation of signal arrival times from any of the GNSS constellations (e.g. GPS or Galileo). However, the GNSS network themselves (i.e. the satellites in orbit) need to also be told where they are in the first place, in reference to Earth’s orientation.

This is not a simple task to do - Earth’s orientation can be influenced by global air and oceanic currents, or even massive forces well below the surface in the interior or the planet. Not to mention, the whole system is always dynamically on the move (such as Earth’s rotation). 

And here's where the quasars and the VLBI networks come in. Due to their remoteness, their enormous distances from Earth and their brightness - they make the perfect ‘landmarks’ in space to orient the Earth, and thus, link to the GNSS networks that provide us with the accuracy and positioning we’ve come to expect. 

Across Australia, radio telescopes (operated by a number of institutions) as far south as Hobart in Tasmania, and as far north as Katherine in the Northern Territory, along with antennas out in Western Australia and the big telescopes located in New South Wales/Canberra can all contribute to the VLBI programs. 

Science Check: What are Quasars?

Structure of the Quasar 3C 273. Credit: B. Peterson/Nature.

Quasars are some of the most mind-boggling objects in all of astronomy to learn about. Discovered alongside the evolution of radio astronomy, quasars at first confused astronomers due to their bizarre behaviour, at a time when scientists really didn’t know what they were observing. 

Here were these objects, which were giving off enormous amounts of electromagnetic radiation across a range of frequencies but were only presenting as tiny, distant star-like point sources, or even more mysteriously, with no optical counterpart in some cases. 

These bright energetic objects also had their luminosity rapidly change, which helped scientists constrain an upper limit on their size - measuring them to be no larger than the Solar system. How could an object of this size, fluctuate in luminosity so much and yet, still be a small point-like object?

It took scientists some time to study the redshifts of optical counterparts to the radio emissions received from these bright sources to realise that these were not in fact stars, but instead the bright cores of very distant galaxies. They emit across the entire electromagnetic spectrum, but due to their large distances and the expansion of space over cosmological timescales, their photons are redshifted to longer wavelengths as they arrive here on Earth. 

Their energy output doesn’t come from the supermassive black hole itself, because no light can escape this - but instead, it is generated in the accretion of material as it orbits and falls into the black hole, being subjected to immense stresses and frictional forces.

Artist illustration of quasar J031343.84-180636.4 - the most distant known quasar. Credit: NOIR Lab/NSF/AURA/J. da Silva.

Over 750,000 quasars have been discovered so far, all of which are at great distances to Earth (at high redshifts). The furthest quasar discovered is the remarkable object J0313-1806, which resides at a redshift of z = 7.64, which means we’re seeing the light from this object as it was only 600 million years after the Big Bang. What’s even more astonishing, and really starting to get astronomers to have to re-think their models, is that its mass comes in at 1.6 billion solar masses - it is not yet understood how a supermassive black hole of this mass could have existed in the short amount of time after the Big Bang. 

To keep powering their enormous energy output, quasars are hungry beasts - with the supermassive black hole having to consume on average about 10 stars per year. The biggest quasar known is estimated to consume about 10 Earth masses per second. 

Because of this conversion of mass to energy, quasar emissions spectral lines have been measured in detail (and in particular, their redshifts due to their distance). These spectral features have shown elements heavier than helium (metals) which demonstrates that second-generation stars were present at such an early epoch of the Universe (i.e. the first generation of stars only contained hydrogen and helium). 

Because of these features (bright, small point-like sources, extremely distant to Earth) quasars then become excellent tools for us Earthlings to consider as a celestial reference point - from our perspective, they are the truly fixed points of light in the sky from which all other things can then be measured from.

So then, what's a VLBI?

A global network of radio telescopes, located on each continent. Credit: P. Boven./NASA Visible Earth.

In order to take advantage of the distant quasars as a celestial reference coordinate system, scientists use a particular technique in astronomy that combines the data from more than one radio telescope, known as interferometry.

In astronomy, objects are unimaginably far. As such, a lot of the finer structure detail we would want to see is often too small for a single telescope to define. This is known as resolution and is very much applicable and similar to the resolution that is achieved by the normal cameras we use on our mobile phones or digital cameras (that is, the higher the resolution – the finer the detail we can see).

Radio telescopes (and their instruments) are a form of a camera – except they collect radio light, rather than optical light. And radio light, unlike optical light, has much longer wavelengths – which is why radio telescope antennas and dishes are often excessively big. So instead of building even bigger (more expensive, harder to manage) radio dishes, astronomers have employed the interferometry technique to simulate a large single antenna by using multiple antennas, spread across a large distance.

It’s the equivalent of using a single eye to view something off in the distance, vs. using two or more eyes – with the latter option providing more detail about depth, features, and more. So by using two or more dishes to simultaneously collect data from a distant astronomical object, we can then use the distance between the antennas as the baseline of the simulated telescope.

The 64m Parkes radio telescope (‘Murriyang’) and a smaller foreground dish. Credit: R. Mandow.

The 70m DSS 43 radio telescope, located at the Canberra Deep Space Communication Complex. Credit: R. Mandow.

For example, the CSIRO Parkes radio telescope on its own has a 64m diameter, and in similar size, the biggest steerable dish in the southern hemisphere, DSS 43 located at the Canberra Deep Space Communication Complex (CDSCC) has a diameter of 70m. Individually, these two telescopes are limited in their own resolution, by the size of their dish diameters.

However, if we were to use both these telescopes simultaneously to observe an astronomical source, then the size of the ‘simulated’ radio telescope will be the difference of distance between them – roughly building a virtual dish antenna with a 348 km diameter baseline. This becomes a much more powerful telescope to look into deep space with.

But an even greater baseline can be developed by using telescopes across entire continents or even the world. In other words, we can simulate the size of a radio telescope to be as big as the entire East Coast of Australia or even the face of the Earth.

The VLBI method thus offers an even greater resolution to the astronomical source being studied. VLBI arrays exist in many countries like the USA, Australia, across Europe and through Russia/Asia, and are often linked to the work of international scientific collaborations.

VLBI arrays are also used for many things these days, but in particular, they provide the opportunity to generate radio astronomy images in high resolution of cosmic radio sources – like active galactic nuclei (AGN) at the centre of galaxies, distant quasars, and even imaging the surface of large, nearby stars (this technique was used to image the red supergiants Betelgeuse and Antares).

The VLBI arrangement of radio telescopes used as part of the Southern Hemisphere Long Baseline Array – with the baseline stretching from South Africa, across Australia and over to New Zealand. Note that the LBA also stretches north-south from Hobart to Katherine. Credit: CSIRO ATNF.

In Australia, the VLBI here is known as the Southern Hemisphere Long Baseline Array and consists of telescopes that stretch across the continents east-west dimension from Perth to Tidbinbilla, as well as a north-south aspect, running from Hobart to Narrabri. The LBA is operated by the CSIRO Australian Telescope National Facility (ATNF), the University of Tasmania and NASA’s Deep Space tracking stations. Additionally, radio telescopes in both South Africa and New Zealand are also incorporated into this network, extending the size of the baseline. 

This includes the CSIRO ATNF managed Compact Array (6 x 22m dishes), the Parkes radio telescope (Murriyang – 64m) and the Mopra telescope, located in Coonabarabran (22m). In addition to this, both the Hobart (26m) and Ceduna telescopes (30m), which are managed by the University of Tasmania and the 70m Dish (DSS 43) located at the CDSCC. 

The Importance of accuracy for future technology

Artist illustration of the view of the internal and external displays of a proposed self-driving vehicle. Credit: Endurance.

In this context, VLBI arrays are used as a geometric technique, with the exact arrival time of the radiation from a quasar signal being measured by two radio telescopes which are separated by some distance on Earth’s surface being able to be determined down to the picosecond (i.e., one-trillionth of a second). 

From this extremely accurate data, the position of the radio antennas (relative to each other) can be determined down to the millimetre, and the quasar position down to a small portion of a milliarcsecond. As such, VLBI arrays offer the highest resolution of any astronomical observation available. 

This then creates an interesting insight into which geologists and geophysicists can seize as an advantage. If the telescopes are fixed to the Earth, and able to be measured down to the millimetre, then when comparing the changes in the distances between each telescope - we should be able to track Earth’s tectonic plate movements. 

And here is where it becomes important for our future self-driving vehicles.

Illustration of Earth’s tectonic plates and their boundary direction of movement. Higher elevations are shaded white. Credit: Science Education Resource Centre, Carleton College.

Over time, as Earth’s plates and continents move - their positions will need to be continually updated in mapping software to ensure that the positioning of sensors placed in vehicles follows accurate mapping for all purposes, but in particular - safety. 

The tectonic plate that Australia sits on for example is moving about 5 - 7cm per year northward. This may not sound like a lot per annum, but 7cm per year for 20 years equates to a displacement of 1.4 metres - which is well outside the acceptable limits for self-driving vehicles (if we consider stopping at a crossing intersection, 30cm is outside this limit as well). 

This level of accuracy in our technology doesn’t need to wait for future applications though, even though self-driving vehicles do sound very cool. Right now, there are thousands of devices and applications across Australia that utilise high precision positioning and navigation within their automated systems. 

This can include military and defense applications like autonomous drones or missile guidance systems, agricultural/mining machinery - like automated tractors and diggers that are remotely operated from great distances, or civilian applications like ensuring fire fighting trucks can calculate the fastest route to the emergency call-out they just received. 

It’s thanks to the ongoing study of distant quasars by our institutions involved in VLBI programs that allow us to take advantage of this clever use of astronomical observations and astronomy, and apply some real-world benefits to our community. 

So when you finally get a chance to sit in a self-driving vehicle, years from now, think about the hungry, supermassive black holes - sprinkled across the Universe at vast distances and how their light plays an important part in your journey, here on Earth.