Why photon has zero rest mass?

Related questions

1. Why are photons affected by gravity?
2. What is inside a photon?
3. Can a photon decay?
4. Can you see a photon?
5. Can a photon be trapped?
6. Can two photons interfere?
7. Do photons decay?
8. Do photons exist forever?
9. Do photons go on forever?
10. Do photons die?

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2. #energy
3. #nature
4. #special
5. #pressure
6. #exert
7. #means
8. #always
9. #travels
10. #speed
11. #light
12. #though
13. #particle
14. #relativity
15. #according
16. #photons
17. #which