Special & General Relativity Questions and Answers
How do we know gravitational lenses aren't just refraction off of intergalactic clouds?
Because intergalactic space contains very little gas or clouds to provide a medium for refracting light. Because of its high frequency, light refraction requires either a gravitational field a la general relativity, or a very dense medium ( glass or a thick atmosphere like the Earth's) and you do not find such media in intergalactic space. When astronomers measured the bending of light near the Sun at visible light frequencies, the presence of the solar plasma was not important, and was essentially transparent. At radio frequencies, however, the solar corona does refract radio waves significantly, and this effect was modeled-out and removed from the data, leaving behind a confirmation of the general relativistic effect.
Return to the Special & General Relativity Questions and Answers page.
All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.