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In school physics a photon is a packet of light: no mass, no charge, no parts. Yet at high energies a photon can behave as though it carries quarks and gluons, like a proton does. Rohini Godbole spent much of her career working out how to describe and measure that behaviour. This post covers her life, the physics in plain terms, a rough estimate you can do yourself, and what she did for women in science.
Who was Rohini Godbole?
A CERN Courier obituary records that she was born on 12 November 1952 and grew up in Pune. She graduated from IIT Bombay in 1972 and completed a PhD in particle physics at Stony Brook in 1979. After a temporary post at the Tata Institute of Fundamental Research she taught at the University of Bombay until 1997, then moved to the Centre for High Energy Physics at the Indian Institute of Science in Bengaluru, where she worked for the rest of her life. She died in Pune on 25 October 2024.
The obituary says she will probably be remembered above all for pioneering, with Manuel Drees, photon structure functions for use at future colliders. Her work also covered Higgs physics, top-quark physics and supersymmetry, on which she co-wrote a textbook with Drees and Probir Roy. India awarded her the Padma Shri in 2019, and France the Ordre National du Merite in 2021.
How can a photon have quarks inside it?
Quantum physics allows a photon, for a very short time, to turn into a quark and an antiquark and back again. Most of the time this does not matter. But when a photon strikes another particle at very high energy, it can hit during one of these moments. Then it behaves less like a point of light and more like a bag of quarks and gluons. Physicists call this a "resolved" photon.
Drees and Godbole worked out what this means for experiments. In a 1989 paper on the HERA collider in Hamburg, Measuring the parton content of the photon, they pointed out that production of two high-energy jets is dominated by events in which "the photon is resolved into quarks and gluons" for jet momenta up to about 40 GeV. In a 1990 paper on the Japanese collider TRISTAN, Probing the hadronic structure of the photon, they showed that collisions between two photons there "begin to be dominated by effects of the hadronic structure of the photon".
In 1994 they added a refinement in Virtual photon structure functions and the parton content of the electron: photons that are far from being real light particles carry fewer quarks and gluons, so the usual shortcut for counting photons from an electron beam must be modified.
Worked example: how long can a fluctuation last?
The energy-time uncertainty relation gives a rough answer: a fluctuation that borrows energy delta E can last a time of about h-bar / delta E. NIST gives h-bar as 6.582 x 10^-16 eV s.
Take delta E = 1 GeV = 10^9 eV.
Time = 6.582 x 10^-16 / 10^9 = 6.6 x 10^-25 seconds.
Distance light travels in that time = 3.0 x 10^8 x 6.6 x 10^-25, about 2 x 10^-16 metres.
That is a fifth of a femtometre, smaller than a proton. This is an order-of-magnitude estimate, not a precise result, but it shows why such fluctuations matter only in very high-energy collisions. A common mistake is to forget to convert GeV to eV, which makes the answer a billion times too large.
What did she work on in Higgs physics?
The Higgs boson, found at CERN in 2012, has properties that still need testing. One is how it behaves under CP, a combined mirror-and-swap symmetry of particle physics. In a 2008 paper in Physical Review Letters, Determining the CP properties of the Higgs boson, she and her colleagues showed that these properties could be "unambiguously assessed by measuring just the total cross section and the top polarization" when a Higgs is produced with a top quark pair in electron-positron collisions. Our post on the Higgs boson discovery gives the background.
Why did she campaign for women in science?
The CERN Courier notes that, having faced gender discrimination when young, she worked to make sure young women scientists got proper opportunities and recognition. She wrote two books on the work of Indian women scientists to give younger students role models, and she mentored a long line of graduate students and postdocs. Our post on girls' confidence in maths and physics looks at what research says about this for families.
What can a student take from how she worked?
Question the simple picture. "A photon has no parts" is true in most problems, and she found where it stops being true.
Connect theory to a measurement. Her papers asked what an experiment could actually count.
Build the community. She organised schools and workshops and made room for others.
When a topic like quantum ideas feels too abstract, the Fermi Concept Library has every concept in physics with its own lesson and its place in the map.
Try the estimate again with delta E = 10 GeV and check that the distance shrinks tenfold.
