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When scientists first discovered cosmic rays, they argued over whether these mysterious space rays were made of high-energy light (photons) or charged particles. Finding the answer meant measuring their intensity all over the Earth. An Indian physicist, Piara Singh Gill, played a crucial role in solving this puzzle alongside a Nobel laureate and later brought modern cosmic-ray research to India.
What is the latitude effect in cosmic rays?
Victor Hess won the 1936 Nobel Prize for discovering that radiation was coming from space (Wikipedia). Victor Hess and the balloon flights that found cosmic rays proved their existence, but their exact nature remained a hotly debated mystery among physicists.
Arthur Compton reasoned that if cosmic rays were charged particles, Earth's magnetic field would deflect them. A particle approaching the equator cuts across the magnetic field lines and experiences a strong magnetic force. A particle approaching the poles travels parallel to the field lines and feels little to no force.
Compton's global measurements proved that cosmic rays are 15% more intense at the Earth's poles than at the equator (Wikipedia). This latitude effect proved they were mostly positively charged particles, rather than neutral photons as some competing scientists had suggested. Since neutral photons would not be affected by a magnetic field, the latitude variation was the definitive proof needed to settle the debate.
How did Piara Singh Gill measure cosmic rays?
Piara Singh Gill travelled to the United States in 1929 and eventually went to complete his PhD under Compton at the University of Chicago. He spent years tracking the fine variations in cosmic-ray intensity to understand their origins and energy levels.
In 1939, Piara Singh Gill and William P. Jesse published their findings on the latitude effect for very large cosmic-ray bursts in the Physical Review (Crossref). That same year, Gill also published his solo research on the long-period variations of cosmic rays (Crossref). His careful measurements provided hard evidence for how cosmic particles interact with the atmosphere over time.
Visualising the 3D paths of charged particles in a magnetic field can be difficult when reading a textbook. The Fermi Simulation Lab lets you adjust the latitude, velocity and charge of a particle to see exactly how it curves, making these abstract ideas easier to see than to read.
Why build an observatory in Gulmarg?
After completing his PhD in 1940, Gill returned to India as a lecturer at Forman Christian College in Lahore. He continued his experiments, studying the azimuthal variations of cosmic rays—how their intensity changes depending on which compass direction you look. He published this work on azimuthal variations at Lahore in 1947 (Crossref).
Following independence, Gill became a key figure in Indian science, working closely with figures like Homi J. Bhabha and Jawaharlal Nehru (Wikipedia). To measure cosmic rays before they crashed into too many air molecules, scientists needed to get as high up as possible into the atmosphere. The High Altitude Research Laboratory in Gulmarg was established in 1954 to conduct atmospheric and cosmic-ray astrophysics research at high elevation (Wikipedia).
Under Gill's direction from 1951 to 1971, the Gulmarg observatory provided a clear window to study the cosmos without the heavy interference of the dense lower atmosphere. This work cemented India's role in the global study of high-energy particle physics.
How do you solve magnetic deflection problems?
The physics that Compton and Gill used to map cosmic rays is the same physics tested in the JEE exams. When tackling a problem about a charged particle in a magnetic field, use this three-step checklist:
1. Check the angle: Is the velocity parallel or perpendicular to the magnetic field?
2. Apply the force rule: Use F = qvB sin(θ) to find the magnitude of the force.
3. Find the path: A perpendicular velocity creates a circular path, while an angled velocity creates a helical path.
Try solving problems on magnetic deflection in the Fermi Problem Library to see how this translates into exam questions.
