That has been true of the claim that dinosaur tracks and human tracks are found together at Paluxy River in Texas. It is also true of the claim that the speed of light slows over time. Scientists have compelling evidence that the universe is some 10-15 billion years old. In one instance in 1996, a creationist on the Internet's Astro listserv mailing list brought this up. But not only is there not a shred of evidence for this idea, scientists are so confident that the speed of light is invariant that today it is used as the standard of length measurement.
Light travels slower through other mediums, such as air, glass and water.
This happens because light is absorbed and re-radiated by the atoms that make up the medium. And it's not just light that we're concerned with. All electromagnetic radiation, which includes radio, microwave, infrared, light and X-rays travel at the speed of light. The history of speed of light measurements To see why the confidence in the invariance of the speed of light is so high, we need to look at the history of its measurement, and some of the foundations that lead to the development of Albert Einstein's theory of special relativity. The first attempt to measure the speed of light that was successful was made by Olaus Roemer in the late 1600s.