Dr Chan Man-ho: Humbled by the study of dark matter and the universe

There is abundant observational data suggesting that an unknown matter called “dark matter” exists in our universe, with a property that resembles none of the existing particles we already know of. Arguably, dark matter is one of the most intriguing scientific mysteries that continues to captivate the attention of the wider world, as well as engage the analytical minds devoted to studying astrophysics and particle physics.

Dr Chan Man-ho, Associate Professor from the Department of Science and Environmental Studies, has studied the mysterious subject of dark matter for many years. Earlier this year, his research team has proven that there is a substantial amount of dark matter surrounding black holes. FLASS FORWARD interviewed Dr Chan about the origin and progress of research on dark matter over the last century. The astronomer also shared with us the significance of conducting fundamental scientific research, and what studies about the immense universe mean to him.

Dr Chan is currently an elected Fellow of the International Society for Science and Religion, Expert Advisor of The Hong Kong Space Museum, and a member of the International Astronomical Union (IAU). Dr Chan holds doctoral degrees in astrophysics and philosophy. His research interests cover astrophysics, cosmology, philosophy of science, philosophy of religion, dialogue between science and religion, and science and theology.

A: Dark matter is a type of matter that is believed to make up a large portion of all matter in the universe. Unlike normal or ordinary matter that has interactions with electromagnetic (EM) waves, dark matter doesn’t emit, absorb or reflect EM waves. As it has no interaction with light or other forms of electromagnetic radiation, scientists cannot make direct observations of dark matter through EM. That means it is invisible to our usual ways of observation; this is why it is called dark matter.

Even though we cannot spot dark matter directly, scientists believe that dark matter plays a crucial role in the formation and stabilisation of galaxies and other large celestial structures. Since the discovery of dark matter by indirect means in 1930s, scientists have spent extensive efforts to understand its existence and nature.

Scientists predict that around 95% of all matter/energy of the universe is either dark matter or dark energy, with the rest being ordinary or visible matter. Dark matter doesn’t exhibit any properties that resemble that of any existing particles we have discovered thus far. If dark matter exists, it must not be made of particles that we already know of.

A: In a galaxy of stars, every object revolves around the galaxy’s centre. In accordance with Newtonian mechanics, stars farther away from the galaxy’s centre should orbit at a lower velocity, if the estimation of the mass of a galaxy – calculated based on how much light the galaxy mass is emitting – is correct. However, in the 1930s, several astrophysicists observed that stars on the outside of some galaxy systems were moving faster than the speed predicted by Newtonian mechanics. In other words, the orbiting velocities of the outer stars had already exceeded the theoretical velocity calculated based on the total mass of all of the stars in the galaxy. That means the mass within the galaxy is greater than what scientists can predict according to the total amount of light the galaxy emits.

Similar observations at galaxy clusters prompted scientists to consider the existence of dark matter. In the universe, there are systems of galaxies held together by gravitational force. In a cluster of galaxies, galaxies orbit as if there is a centre within the cluster to orbit around. In the 1930s, some scientists observed some distant galaxy clusters. They found that the outer galaxies of the cluster moved around its centre at speeds noticeably faster than expected. Moving at such speeds should have resulted in the outer galaxies escaping from the cluster. If the outer galaxies weren’t flying away from the cluster at those speeds, scientists concluded that the cluster of galaxies must have much more mass than what had been computed based on the amount of electromagnetic radiation emitted from the cluster. This meant that there had to be invisible matter inside the cluster.

It would be like seeing 10 people being weighed on a scale, and reading 10,000 kg on the scale instead of the more likely reading of 1,000 kg. It is very strange. There must be something on the scale that nobody can actually see.

A: As dark matter does not interact with any form of EM, scientists cannot spot it through its interaction with EM radiation. This creates a great hurdle for any study on dark matter.

A majority of scientists believes dark matter has mass, and therefore still exerts a gravitational effect on other ordinary visible matter. Mainstream scientists believe that, due to the gravitational mass of dark matter, dark matter still follows the gravitational laws of physics. Therefore, scientists can still make indirect observations of dark matter through the gravitational effect it seems to have on other ordinary visible matter.

A: Though there is no direct evidence of its existence, scientists still want to find evidence of its existence through indirect means. There are several types of research that the scientific community is carrying out on dark matter.

Scientists believe dark matter is everywhere around us, and that it can collide with other particles, although the probability is extremely low. There are international collaborations that use several earth-based laboratories to observe and record collisions of dark matter with other known particles. Figuratively, these scientists try to use a physical space to capture dark matter. These are called direct-detection experiments.

Scientists also believe that during dark matter collisions/annihilation, EM waves are emitted, like gamma rays or high-energy electrons and positrons. My research tries to locate celestial systems in which dark matter has a higher density. We can then detect signals of gamma rays or positrons originating from dark matter annihilation from these systems and infer the properties of dark matter from these signals. If we can understand the properties of dark matter, we have a higher probability of capturing it.

The third type of research aims to create dark matter through particle acceleration and collision. The large laboratory operated by CERN on particle physics is working in that direction.

A: There are people who say that we should not allocate so much funding in space exploration and other basic research while there are millions of people suffering from different kinds of humanitarian crises. Instead, we should use the money spent on these fundamental scientific studies to save the lives of people facing poverty and humanitarian crises.

To answer your and their question, I’d say human beings do not live only for the purpose of eating and living. As people, we have an innate curiosity for the things that we don’t know. We want to make inquiries about the world we live in: the earth, the solar system, and the universe.

It is true that most astronomical studies and other fundamental scientific research do not contribute directly to economic or social developments of human society. Viewed in this way, most of our research is not useful. But we should not ignore the fact that these studies and research contribute greatly to human knowledge. They deepen our understanding of the physical world. And in the process of discovery, people will come across new things that might stimulate their reflection on their lives. The research findings might help people discover a higher purpose in life.

 

Q: So would you agree that such fundamental and theoretical research works do little to help human society?

A: This leads into the second point I’d like to raise. Discoveries that were useless at its time might turn out to have tremendous impacts on other scientific research works and inventions, and even on the people’s everyday lives, as the years roll on and more findings evolve. There are numerous examples I can cite to support this point. When Albert Einstein published the general theory of relativity in 1915, not many people thought it had any practical value. Riemannian geometry provided a mathematical framework for Einstein to establish his general theory of relativity. When Riemannian geometry was developed in the 19th century, it was purely a mathematical model that had no practical implications.

Today, almost everyone has heard of Einstein. In popular science, people know that the atomic bomb was invented based on Einstein’s mass-energy equivalence theory, which states that mass can convert into energy. And the rest is history. In this sense, the general theory of relativity has altered the fate of humankind. In today’s world, the general theory of relativity is essential for the high precision positioning needed for GPS. Riemannian geometry also occupies a central role in daily life for its application in facial recognition technology. To many mathematicians, discovering the properties of prime numbers was purely driven by their inquisitive minds. But today, prime number theory forms the core of cryptography.

I can continue to use other examples to prove my argument: fundamental research that appears to be useless at the time of discovery has the potential to become a powerful tool for other practical scientific and engineering inventions at a later stage. From this perspective, one can argue that basic theoretical research has much greater implications to human developments than applied research. We can even say that it is fundamental theoretical research that drives human developments.

A: I think the discovery of gravitational waves can shed some light on your question. As stated by Einstein’s theory of general relativity, the disturbance of spacetime caused by accelerated mass means that gravitational waves must exist. Although Einstein has predicted the existence of gravitational waves in 1916, little progress was made in detecting its existence for almost one century. It was as late as 2015 when scientists made the first direct capture of gravitational waves. The discovery opened up a totally unseen world of knowledge.

Can we say that the effort spent over the last 100 years to spot gravitational waves before the final discovery was a waste? I think we can’t. I believe new discoveries are based on what has been done before. It is a long process made from accumulated experiences, during which scientists learn lessons from what went wrong, and come up with new approaches that would work better.

Maybe, one day in future, we will make great leaps forward in understanding the properties of dark matter. The day may come when our knowledge about dark matter is rich enough for us to operate on it. Nobody knows when that moment will arrive. But when it comes, dark matter research may make a huge contribution to people’s lives.

A: We have to admit that in the first seventy or eighty years of research conducted on dark matter, little progress was made. In the past two decades, however, scientists have invented many new observation methods and technologies, like the James Webb Space Telescope, and new efforts have been made, including the international collaboration on direct detection. These new approaches have allowed our research in dark matter to take steps forward.

Over the same period, scientists are pushing the frontiers of big-data analytics and machine learning, which in turn have become powerful tools for dark matter research. With new observation tools and breakthroughs in AI technology, there is a stronger likelihood we can overcome big hurdles in astronomical research. Some scientists even say that the 21st century is the golden age of astronomy.

Admittedly, much of our study of dark matter remains clouded. There are still significant challenges to overcome before meaningful breakthroughs can be made in the field. We may still have a long way to walk before we see the light at the end of the tunnel; or we may be approaching the point of a substantial discovery. Nobody can say for sure.

A: Many research works conducted in western countries indicate that astronomy is one of the most interesting science subjects. I have given many talks on astronomy and popular science in Hong Kong, and I have received positive feedback from the audience. From what I have seen, there is no lack of Hong Kong people, many teenagers included, who are curious about the universe. But as students go on to higher grades and prepare for public examinations, they lose interest and lack the time to go further and try to understand things outside their syllabus.

Hong Kong may not have a supportive environment for students to study astronomy. That said, EdUHK still has a role to play. Through our teaching work, we can inspire more future teachers to have a passion in astronomical matters.

A: At the end of the 19th and the beginning of the 20th century, there was a belief that humanity had already mastered all the fundamental facts of the physical world. We had Newtonian mechanics predicting the motions of physical objects, the laws of thermodynamics describing the transfer of heat and the increase of randomness in nature, and Maxwell’s electromagnetic theory explaining all electromagnetic phenomena. People at that time believed that with these three great pillars of classical physics, they had already explored all the knowable physical world. But when relativistic mechanics and quantum mechanics were proposed in the early 20th century to fill in the gaps classical physics could not apply, people became much more humble about how much they knew about the physical world.

Today, we think that we have already grasped the general picture of the universe: for example, how big and how old it is, how many galaxies are there, and how space is expanding. But actually, a very large part of the universe is still unknown to us. As I said, 95% of our universe is made up of dark matter and dark energy, both of which we have little knowledge about. What human beings know only constitutes 5% of the whole knowable universe. The fact that we only know a small part of the universe reveals how mysterious the vast space above us is. It humbles us. At the same time, it tells us that there are still many challenges for human beings to face and overcome. There is a lot of room for scientists to explore.

I think that although human beings are very limited in terms of space and time, we still possess the intelligence to study the universe. Einstein once said, “The most incomprehensible thing about the universe is that it is comprehensible.” In the first half of 20th century, the famous American scientist Edwin Hubble made a lot of astonishing astronomical discoveries. He claimed that that we, as human beings, could understand the universe. I think words from these eminent people have encouraged us to continue to explore the unknown world.

After all these years studying astronomy in general, and dark matter in particular, I believe the universe is knowable – just like what Einstein and Hubble have said. Saying that, a huge part of the cosmos remains unknown to humankind. That reminds us that we need to continue to work hard and keep our humility. And this is the right attitude that we scientists should have in our continual quest for the truth.

Note: People interested in discussions about dark matter and the origin of the universe might also want to read the following stories: “RGC-funded SES research proves existence of dark matter surrounding black holes” and “Scholars from EdUHK and HKBU co-host platform to discuss the origin of the universe”.