Answered:
Dear Suvankar Majumder ,
As the photon leaves a gravitational potential well, it does not change its speed but changes its wavelength (λ) and frequency (f), resulting in energy (ΔE) expenditure or, it's infinitesimal wavelength (Δλ) and infinitesimal frequency (Δf) changes, specifying the equation v = λf. Photon energy is defined by Planck's energy-frequency equation. E = hf.
The ratio of Planck length and the Planck time (lp/tp), set the upper speed limit without requiring other constants such as the proportionality constant or universal gravitational constant, the reduced Planck constant, and the speed of light in vacuum. Photons crossing the gravitational potential well can change energy, called gravitational redshift. Photons are massless, so they always travel at speed of light
1. Therefore, according to the above statement, the speed of photon does not vary with time in free space.
A photon is an elementary particle. Elementary particles are either elementary fermions or elementary bosons. A photon is a gauge boson, the carrier of the electromagnetic force. Pair production often refers specifically to a photon producing an electron-positron pair near a nucleus. To produce a pair the photon's contributing energy must be above a threshold of the total rest mass energy of the two particles produced.
2. Therefore, photons can actually become electron and positron pairs, subejct to above said conditions.
Best regards,
Soumendra Nath Thakur
