TL;DR – We pretty much derived a bunch of equations for circular motion, did some calculations and used the Small Radio Telescope to find data which suggested either Newton had incorrect equations or Dark Matter exists. This sentence does not do the entire week justice, and it says little about the huge amount of fun we all had. I’d personally like to thank Gregory Rowbotham and our professor, Matthew Young. I learnt so much about the type of work astrophysicists do! While learning physics and doing other maths stuff is what we do at school, learning how to learn about it is something we don’t get and it’s the part I enjoyed most out of this Work Experience. We were taken through the process of research and discovery, building our knowledge like researchers would – through experimentation. This process is incredibly rewarding, and I recommend it to pretty much anyone interested in anything space or physics related. But don’t just take my word for it, before me are many other reports of their ICRAR visits which (probably) say the same thing.
Upon arrival, we met Professor Matthew Young, Gregory Rowbotham and Tamara (who took us through all the tour stuff and safety procedures), we were taken into a conference room on the ground floor. Once all four work experience students introduced ourselves, we were posed a physics challenge by Matthew. We were asked to derive the formula for a body orbiting a point at fixed radius and speed, such that change in velocity was in terms of radius and time. After some maths (and many failed attempts), we managed to find that change in velocity was inversely proportional to the radius squared (inverse square law).
To test this equation, we grabbed values for orbital period and orbital radii for each planet from the internet and (with a bit of excel/math magic) we found the instantaneous change in velocity and plotted this against radii. It’s log-log plot showed us that radii had an exponent value of -2, which meant velocity is inversely proportional to radius squared. We did the same thing with Jupiter’s moons, which gave us the same results. This supported our formula.
After a bit more algebra (and approximating the mass of the Earth), we derived a formula for universal gravitation, and approximated the value of a constant (which was actually pretty close). We then ate lunch (and almost got lost).
Once we returned from lunch, we went on a walk around the campus with Matthew as he explained the bare basics of electrodynamics and a bit of quantum mechanics. He told us that electrons are supposed to fall into the nucleus according to Newton’s Laws of orbital mechanics and electrodynamics, however, this is not what is observed. Then he talked a bit about a guy named Louise De Broglie who posed the idea that electrons can act as waves. Neils Bohr built on this idea and determined that these electrons must be at quantised radii from the nucleus, as these electron waves must be standing waves (as they loop in on themselves). This pretty much concluded our first day.
Matthew told us on the second day that the force between electrically charged bodies also follow the inverse square law. After a bit more maths, algebra and substituting formulas for the Planck-Einstein relation, Mass-Energy Equivalence, Newton’s Second Law, Orbital acceleration, Coulomb’s Law, and a bunch more, we managed to derive an ugly-looking (long) equation to relate radii and number of waves in the electron’s orbit. We put formula into excel to compute each specific radii in which the electrons can exist at (according to the Bohr Model). We then computed the change in total mechanical energy as an electron moves from an orbital radius to the ground state (first orbital). This is known as the Lyman Series. Since this energy is emitted as a photon, we were then able to calculate the frequency and wavelength of these photons, which matched up exactly to what the values should be!
The professor then told us about hyperfine transitions, where an electron goes to a lower energy state when it’s already at ground level. This occurs because when both the proton and electron are spinning in the same direction, it’s at a higher energy state than when they’re in opposite directions. This transition from higher to lower energy state occurs at an average of about 11 million years. The wavelength of this emission is known very precisely, at 21cm. We were going to be measuring these photons with the Small Radio Telescope (SRT). While it’s very unlikely for a photon to undergo this transition as it’s being observed, since space is very big, it is likely that we will observe many of these transitions occurring as we look through space.
We were then shown a bit of fancy maths to find an equation for the total potential energy of a body with known radius (we would later use this to find the mass of a localised gas cloud).
Since the line of sight of recording is a straight line across the Milky Way, there would always be a hydrogen gas cloud in which its motion is parallel to the line of recording. Since its motion will be greatest in the direction we’re looking, its Doppler shift will make it the maximum or minimum frequency we observe. Therefore, we can find the velocity of the cloud and with some more maths.
We used the Small Radio Telescope to observe values for velocity of the body, recording further and further from the centre each time. According to Newton’s Laws of orbital mechanics, the velocity of a body should decrease as the radius increases, however, this is not what is observed. While the data we recorded were crude approximations from a graph, it showed us a general trend. The velocities we observed were way too high for what it should be. This suggested that either the equations were wrong or a mass with spherical symmetry exists outside the galaxy – Dark Matter. Dark matter isn’t really dark, this just means it doesn’t interact with electromagnetic waves. After getting our minds blown, we went home.
We arrived early on the third day to go to GinGin Gravity Discovery Centre where the SRT was located. We did some more precise approximations and then wrote a script to record a wide band of frequencies. Unfortunately, it looked like there was a lot of radio frequency interference (RFI) and we couldn’t get good data that day. We then left for the hour-long drive (where we continued to do more data analysis). Once we got there, we ate some lunch and travelled up the Leaning Tower of GinGin – a 45m tall tower which leans into a sandpit (the YouTube Channel, How Ridiculous do their drop tests there). We took some photos and met the professor at the Exhibit. Spencer and I went to the Space Capsule Exhibit, where you spin yourself inside a tiny capsule to see how much G-force you can withstand (Spencer and I got to 2.45G before I started to feel dizzy – astronauts have to withstand 3). We stumbled with the rest of the group the Cosmology Gallery. Matthew showed us the first gravitational wave detector in the Southern Hemisphere – some of the designs involved in making it was used in LIGO. We then went out to the back to find the SRT, which really was a small radio telescope. We brushed off the cobwebs which we had been looking through that week in the webcam and went to the computer controlling the SRT. The professor showed us the system and then we left. Thus concluded our 3rd day of work experience.
On the fourth day, we went to the Pawsey Supercomputing Centre, where a bunch of supercomputers are regularly used by researchers for data analysis. In fact, the computer “Galaxy” is used to process data collected in radio astronomy. Magnus is the biggest computer at the centre, and they use multithread processing to churn through data incredibly quickly. We then took apart an old cell from the supercomputers and then had some time to ask questions. After this, we went to the building where all the staff members worked. One of the staff members showed us a demo of the Mini Magnus – a bunch of Raspberry Pi 3Bs connected together to form a single computer with multiple cores. He launched a particle simulation program in which you can change the number of cores computing the simulation. Our group spent most of the time trying to bring its fps as low as possible (I brought it down to 10 fps, which was apparently the record – obviously my greatest achievement). After this we went to a conference room where we sat in a zoom call to talk about goals, resilience and the growth mindset for two hours. We went back to the initial building for a while and then caught an Uber back to UWA.
We spent the last day processing data (good data that our professor had previously collected). First, we made a graph which plotted Average Intensity (K) over Frequency. The graph returned a steady increase and a very sharp spike – this consisted of the HI emissions we were looking for. Then we fitted a polynomial to the curve (excluding the spike) and removed it from all data points – this returned a graph where the intensity was approximately 0 with a large spike for the HI emissions (we essentially just removed the curved part from the graph to make the data flatter on the graph). From this, we approximated the highest frequency just before the Intensity become 0 after the spike. We plugged this into an equation to find the velocity of the gas cloud travelling parallel to our line our recording (relative to the centre of the Milky Way). From this we found the mass of the gas cloud. While this process sounds short, it took an incredibly long time and consumed most of the day. We took some pictures with the professor and thus, concluded our final day of work experience. While we felt slightly tired, we all went home that day feeling incredibly enlightened (or at least I did). Writing this blog a few days later, I’m already missing that week of discovery and learning. I hope to experience this once again in the future.