ArcSecs.com: Rethinking Spacetime

Do Photons Have Mass? Rethinking Gravity and Light

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Einstein’s general relativity attributes light bending near massive objects to spacetime curvature. But an alternative hypothesis posits that photons carry a tiny mass and are subject to Newtonian gravity—which can fully account for observed deflections without invoking curved spacetime [1]. If photons have mass, gravity would pull on them directly, producing the same lensing effects that GR describes geometrically.

Historical Perspectives

As early as the 18th century, John Michell suggested that if light had mass it would be influenced by gravity, predicting “dark stars” [2]. In 1801, Johann von Soldner calculated the Newtonian deflection of starlight grazing the Sun, assuming a photon mass, and arrived at half the value later measured [3]. Einstein’s 1911 semi-Newtonian approach reproduced Soldner’s result before GR doubled it by accounting for spacetime curvature.

Theoretical Frameworks

The Proca equations modify Maxwell’s laws by adding a mass term for the photon, predicting a finite photon range and three polarization states [4]. In quantum field theory, a nonzero photon mass would break gauge invariance unless restored by a Stueckelberg mechanism or a Higgs-like field [5]. Such models remain speculative but mathematically consistent.

Modern Experimental Limits

Laboratory and astrophysical tests have constrained photon rest mass to below ~10−52 kg. Coulomb’s law precision tests, geomagnetic field observations, and pulsar timing set the current best limit at 9.6×10−50 kg [6]. Fast radio bursts have tightened this further to ~10−46 kg [7].

Gravity vs. Curved Spacetime

Newtonian gravity acting on a massive photon yields a deflection angle θN = 2GM/(bc2), matching half of GR’s θGR = 4GM/(bc2). However, some argue that additional polarization states or higher-order interactions could raise the Newtonian result to match GR exactly [8]. Until such effects are ruled out, the possibility remains open.

Challenges and Outlook

Einstein’s curved-spacetime framework explains a broad range of phenomena—from perihelion precession to gravitational redshift—with unmatched precision. A massive-photon model must also reproduce time-delay measurements (Shapiro delay) and waveform dispersion tests from gamma-ray bursts [9]. So far, no deviations from c have been observed across wavelengths, pushing the photon mass bound ever lower [10].

Conclusion

While Einstein’s general relativity remains the most successful description of gravitation, revisiting the photon’s masslessness assumption encourages fresh experimental tests and theoretical insights. Should a nonzero photon mass ever be detected, it would require a paradigm shift as profound as the transition from Newton to Einstein.

References

  1. Einstein’s General Relativity Foundation (Einstein 1915)
  2. John Michell and the Concept of Dark Stars
  3. Soldner’s Newtonian Calculation of Light Bending (1801)
  4. Proca’s Equations for a Massive Photon (1936)
  5. Chang, Challenges to Curved Spacetime, 2023 (arXiv)
  6. Luo et al., Photon Mass Limit from Solar System Tests, Phys. Rev. D (2003)
  7. Lakes, Fast Radio Burst Constraints on Photon Mass, Phys. Rev. Lett. (1998)
  8. Particle Data Group, Photon Mass Review (2024)
  9. Davis, Gamma-Ray Burst Dispersion Tests, Phys. Rev. Lett. (1975)
  10. PNAS: Limits on Photon Mass from Time Delay Measurements