Special & General Relativity Questions and Answers
Would the collision of two photons cause them to slow down as a result?
No. Photons do not interact with each other, except at very high energies and in the presence of other matter. Two photons by themselves in free space would just pass trough each other.
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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.