Special & General Relativity Questions and Answers
Why do photons produce pressure even though they are massless?
Because photons carry energy by virtue of their frequency ( E = h x frequency where h = Planck's Constant), and from relativity, energy and mass are related by E = m c^2, so that photons can be described as particles that have an EFFECTIVE mass for dynamical purposes. They have zero rest mass, but have non-zero effective mass, so they are just as effective at carrying momentum ( P = h x frequency/speed of light) as ordinary matter.
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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.