Friday, January 8, 2021

Remembering Dr. Stephen Hawking: One of the greatest physicists of our times

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Remembering Dr. Stephen Hawking: One of the greatest physicists of our times

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Global Views 360

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January 8, 2021

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Graffiti art remembering Dr. Stephen Hawking

Graffiti art remembering Dr. Stephen Hawking | Source: duncan c via Flickr

The last 50 years have produced some of the most fascinating ideas from physics which have ever been known to us mere mortals. Whether it is the idea of string theory where the world is made of tiny strings smaller than whatever lengths we can possibly encounter or whether it is the astonishing revelations that we possibly do not understand 96% of what constitutes the Universe, all of these brilliant ideas have caught the attention of both professional physicists and the normal population alike. This has also shot loads of world class physicists to limelight, with the likes of Roger Penrose, Edward Witten, Juan Maldacena, Abhay Ashtekar and Erik Verlinde amongst a huge number of physicists who have achieved great public acclaim for their work on Gravitational theories while the likes of Alan Guth, Andrei Linde, Paul Steinhardt, Jim Peebles amongst others have become famous names for their groundbreaking work in Cosmology. But perhaps the best-known figure of theoretical physics in the last half century has been someone who, despite all kinds of odds stacked against him, has contributed deeply to both Gravitational Physics and Cosmology, and his name is Stephen Hawking!

The depth and the length of Hawking’s scientific discoveries can not possibly be described to their full glory in one single article and that speaks volumes of the kind of incredible physics he pursued throughout his life. But intriguingly enough, physics was not what a young Stephen was supposedly going to do in his life. Stephen was born into a family which placed a high value towards a good education, as his father, Frank, was a medical researcher while his mother, Isobel, (having read Philosophy at Oxford, where she met Hawking’s father) was a secretary at a medical institute. While Hawking was named “Einstein” in his school days, his father actually wanted him to also study medicine like him. However, the young Stephen was actually fond of mathematics and since Oxford - where he pursued his undergraduation - didn’t offer a Mathematics degree at the time, he decided to major in Physics instead. Slowly, he gained an incredible amount of interest towards Physics although he was a conventionally “lazy” student throughout his undergraduation. He would not study seriously as he found most of work really easy and interestingly enough, it was the boat club in his university which slowly propelled him towards putting efforts as a student.

When Hawking started his PhD in Cambridge, he was quite disappointed to have not been made a student of legendary astronomer Fred Hoyle, instead he was made a student of Dennis Sciama. This proved fortuitous however, as Sciama was incredibly knowledgeable about almost everything in Cosmology and eventually became a central figure in British Cosmology. It was through him that Hawking got to meet his life-long collaborator and recently awarded Nobel Prize Winner, Sir Roger Penrose.  The meeting with Penrose, who was then working on some bewildering properties of the Black Hole, proved to be a pivotal moment of Hawking’s career. Penrose had shown in a general way the existence of space-time singularities, which is a point inside the black hole where the known laws of Physics, like General Relativity, collapse. Hawking used Penrose’s theorem to show that if one completely rewinds the entire history of the universe, then one would reach exactly to the kind of point which Penrose had described for a black hole; a Space-Time or in this case the Big-Bang Singularity.

Dr. Stephen Hawking at official opening of the Weston Library, Oxford, England | Source: John Cairns via Wikimedia

This idea shows that the universe began from an infinitesimally small point of seemingly infinite density, and hence, Einstein’s seminal theory of General Relativity also fails to explain the properties of the Universe at the time of its creation. This work of Hawking came to be of an astounding magnitude, and this has propelled work on loads of theories both of the early universe and even towards considerations of modifying General Relativity itself! This excellent work got Stephen his doctorate degree at Cambridge, a fact made even more stupendously inspirational considering that he was diagnosed with the Motor Neuron Disease by this time which made him completely paralyzed. He was in a state of depression after being diagnosed with this disease with doctors claiming that he had not much time left to live. It was then through the support of his family and his girlfriend (who soon became his wife) that got him through a very dark realization and motivated him to again pursue physics to the best of his abilities.

After his great work on the Big Bang, Hawking shifted his attention quite literally towards Black Holes. He produced a number of incredible theorems regarding them with Sir Penrose, which are now known as “Penrose—Hawking singularity theorems”. He was also collaborating vigorously with James Bardeen and Brandon Carter at this time, and together they produced some excellent work which showed how Black Holes could lose energy. Around the same time Jacob Bekenstein (who was then a PhD Student at Princeton University) showed that there had to be the existence of some quantum mechanical effects which would lead to the Black Hole having a so-called “entropy” (which is the classical measure of the disorder of a physical system). On the basis of his work with Carter and Bardeen with considerations to Bekenstein’s ideas, Hawking then showed that Black Holes lose energy by radiating it away through a particular mechanism. Considering Einstein’s seminal idea of Mass-Energy equivalence through E=MC2, this incredible work of Hawking meant that Black Holes actually lose Mass by radiating it away in a process now fittingly known as “Hawking Radiation''. Hawking Radiation has become a central idea in studies of Black Holes, Quantum Gravity and the very early universe, and was the key idea which propelled the concept of “Primordial Black Holes”, which refers to the Black Holes which were created in the very early universe. Recently there has been a lot of work which points towards the realization that these primordial black holes may constitute a huge part, if not all, of the dark matter in the universe (which is a mysterious form of matter which forms approximately 23% of the universe). If it is indeed the case, then Hawking’s work will inadvertently be the propeller towards the understanding of dark matter.

Throughout the time in which Hawking did all the above-mentioned work, his research was up there with the finest (if not the finest itself!) on gravitational physics and cosmology in the world. In his later years, Hawking became fascinated with even more exotic ideas which ranged from understanding quantum gravity (the theory of gravity at the smallest scales) and the Multiverse (the idea of an infinite number of universes) to the prospect of Extraterrestrial life and Time Travel. He produced some really insightful work on Quantum Gravity, and his work on Hawking Radiation has fueled loads of work in quantum gravitational theories like String theory and Loop Quantum Gravity. He even hosted a party for time travelers and discussed in length about Aliens & the effects of AI on humans in his later life.

But let’s end this very brief note of his life with this anecdote. Somak Raychoudhary, the current director of IUCAA in India, reminisces how he once met Sir Penrose’s office during his PhD days in Oxford about the allowance to attend one of his classes. Penrose was discussing some work with another PhD student at that time and was startled when he heard Somak’s surname. He said “ Are you related to the Raychoudhary?”. Somak was startled by hearing this and asked who it was that Penrose referred to. Penrose then exclaimed that he was referring to Amal Kumar Raychaudhuri, the Indian astrophysicist who discovered a seminal equation known by his name as the “Raychaudhri Equation”. When Somak told that he had indeed taken classes from Professor Amal, Penrose was very happy and immediately granted him permission to attend his classes. At this, the quiet PhD Student sitting with Penrose said to Somak “ We (him and Penrose) are incredibly inspired by his work and wish to meet him once in person “. That PhD Student was none other than Stephen Hawking and goes to show, the incredibly high regard Raychaudhri’s work is held in, while the general Indian don’t know much about him.

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April 13, 2021 2:32 PM

Detecting The Ultra-High Energy Cosmic Rays With Smartphones

Smartphones have become the most commonplace objects in our daily lives. The unimaginable power that we hold in our hands is unrealized by most of us and, more importantly, untapped. Its creativity often gets misused but one can only hope that it’s fascinating abilities would be utilized. For example, did you know that the millions of phones around the globe can be connected to form a particle detector? The following article covers the CRAYFIS (Cosmic RAYs Found in Smartphones) phone-based application developed by the physicists from the University of California—Daniel Whiteson, Michael Mulhearn, and their team. CRAYFIS aims to take advantage of the large network of smartphones around the world and detect the cosmic or gamma rays bursts which enter the Earth’s atmosphere almost constantly.

What Are Cosmic Rays?

Cosmic rays are high velocity subatomic particles bombarding the Earth’s upper atmosphere continuously. Cosmic ray bursts have the highest energy compared to all forms of electro-magnetic radiation. When we say ultra-high energy particles (energy more than 10<sup>18</sup> eV), we mean two million times more energetic than the ones that can be produced by the particle colliders on Earth.  These rays are thought to be more powerful than typical supernovae and can release trillions of times more energy than the Sun. They are also highly unpredictable as they can enter Earth’s atmosphere from any direction and the bursts can last for any period of time ranging from a few thousand seconds to several minutes.

Despite many theoretical hypotheses, the sources of these ultra-high energy cosmic rays are still a mystery to us even after many decades of their discovery. These rays were initially discovered in the 1960’s by the U.S. military when they were doing background checks for gamma rays after nuclear weapon testing. Cosmologists suggest that these bursts could be the result of super massive stars collapsing - leading to hypernova; or can be retraced to collisions of black holes with other black holes or neutron stars.

How Do We Detect Them?

When the high-energy particles collide with the Earth’s atmosphere, the air and the gas molecules cause them to break apart and create massive showers of relatively low-energy particles. Aurora borealis i.e., the Northern and the Southern lights are the lights that are emitted when these cosmic rays interact with the Earth’s magnetic field. Currently, these particles are hitting the Earth at a rate of about one per square meter per second. The showers get scattered to a radius of one or two kilometers consisting mostly of high-energy photons, electrons, positrons and muons. But the fact that these particles can hit the Earth anytime and anywhere is where the problem arises. Since the Earth has a massive area, it is not possible to place a detector everywhere and catch them at the exact moment.

Energetic charged particles known as cosmic rays hit our atmosphere, where they collide with air molecules to produce a shower of secondary particle | Source: CERN

Detecting such a shower requires a very big telescope, which logically means a network of individual particle detectors distributed over a mile or two-wide radius and connected to each other. The Pierre Auger Observatory in South America is the only such arrangement where 1,600 particle detectors have been scattered on 3,000 square kilometers of land. But the construction cost of the same was about $100 million. Yet, only a few cosmic ray particles could be detected using this arrangement. How do we spread this network around the Earth?

In addition to being cost-effective, such a setup must also be feasible. The Earth’s surface cannot possibly be dotted with particle detectors which cost huge fortunes. This is where smartphones come into the picture.

Detecting The Particles Using Smartphones

Smartphones are the most appropriate devices required to solve the problem. They have planet wide coverage, are affordable by most people and are being actively used by more than 1.5 billion users around the planet. Individually, these devices are low and inefficient; but a considerably dense network of such devices can give us a chance to detect cosmic ray showers belonging to the highest energy range.

Previous research has shown that smartphones have the capability of detecting ionizing radiation. The camera is the most sensitive part of the smartphone and is just the device required to meet our expectations. A CMOS (Complementary Metal Oxide Semiconductor) device is present in the camera- in which silicon photodiode pixels produce electron-hole pairs when struck by visible photons (when photons are detected by the CMOS device, it leaves traces of weakly activated pixels). The incoming rays are also laced with other noises and interference from the surroundings.  Although these devices are made to detect visible light, they still have the capability of detecting higher-energy photons and also low-ionizing particles such as the muons.

A screenshot from the app which shows the exposure time, the events- the number of particles recorded and other properties

To avoid normal light, the CRAYFIS application is to be run during nighttime with the camera facing down. As the phone processor runs the application it collects data from its surroundings using a camera as its detector element. The megapixel images (i.e., the incoming particles) are scanned at a speed of 5 to 15 frames per second, depending on the frame-processing speed of the device. Scientists expect that signals from the cosmic rays would occur rarely, i.e., around one in 500 frames. Also, there is the job of removing background data. An algorithm was created to tune the incoming particle shower by setting a threshold frequency at around 0.1 frames per second. Frames containing pixels above the threshold are stored and passed to the second stage which examines the stored frames, saving only the pixels above a second, lower threshold.

The CRAYFIS app is designed to run when the phone is not being used and when it is connected to a power source. The actual performance would be widely affected by the geometry of the smartphone’s camera and the conditions in which the data is being collected. Further, once the application is installed and is in the operating mode, no participation is required from the user, which is required to achieve wide-scale participation. When a Wifi connection is available the collected data would be uploaded to the central server so that it could be interpreted.

There is much complicated math used to trace back the information collected from the application. The most important parameters for the app are the local density of incoming particles, the detection area of the phone and the particle identification efficiency. These parameters are used to find the mean number of candidates (photons or muons) being detected. Further, the probability that a phone will detect no candidates or the probability that a phone will detect one or more candidates is given by Poisson distribution. The density of the shower is directly proportional to the incident particle energy with a distribution in x and y sensitive to the direction in which the particle came from. An Unbinned Likelihood (it is the probability of obtaining a certain data- in this case the distribution of the cosmic rays including their energy and direction, the obtained data is arranged into bins which are very, very small) analysis is used to determine the incident particle energy and direction. To eliminate background interference, a benchmark requirement has been set that at least 5 phones must detect and register a hit to be considered as a candidate.

It is impossible to express just how mind-blowing this innovation is. As the days pass, Science and Technology around us keep on surprising us and challenge us to rack our brains for more and more unique ways to deal with complex problems. The CRAYFIS app is simply beautiful and it would be a dream-come-true to the scientists if the project works out and we are able to detect these high energy, super intimidating cosmic rays with smartphones from our backyard.

Further Reading

The paper by Daniel Whiteson and team can be found here.

An exciting book “We Have No Idea” by Daniel Whiteson and cartoonist Jorge Cham can be found here.

The CRAYFIS app can be found here.

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