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Letter to the editor: Oldest light?


Reader Bill Principe raises an interesting question about the headline of a recent symmetry article.

Dear symmetry,

I am not a physicist, so forgive me if I get my physics wrong.

The most recent issue has an article called “The oldest light in the universe.”

But I thought that Einstein’s [Special] Theory of Relativity says that, as an object moves faster and faster, time slows relative to everything else. If that is true, doesn’t that mean that time has stopped for light, traveling at the speed of light?

In other words, for all photons, has not time stopped? For photons, is it not, right now, the time of the big bang? If so, there is no such thing as the oldest light in the universe. All light is 13.8 billion years old, and all light is brand new.

I understand the point of the article. It is the information that the light carries that is old, so I am essentially mincing words. But I think my observation might make a fun counterpoint.


Bill Principe

Ayer, Massachusetts


Editor’s note: The reader is correct! According to theorist Lance Dixon at SLAC National Accelerator Laboratory: If a clock could somehow travel with a photon at the speed of light, it would show that no time had elapsed during the photon's journey.


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