The universe’s ultimate speed limit seems set in stone. But there’s good reason to believe it might once have been faster – and may still be changing now.

By Stuart Clark

The speed of light in a vacuum is the ultimate cosmic speed limit. Just getting close to it causes problems: the weird distortions of Einstein’s relativity kick in, so time slows down, lengths go up, masses balloon and everything you thought was fixed changes. Only things that have no mass in the first place can reach light speed – photons of light being the classic example. Absolutely nothing can exceed this cosmic max.

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