Why doesn’t Light Age?

There’s a graduate student that I’m sort-of mentoring/working with at Arizona, named Xiaoying Xu (hi Xiao!). She’s bright and curious, and she asks some very good questions. She asked me one yesterday that’s pretty tough to wrap your head around:

How do I explain to someone why light doesn’t age?

Well, here on Earth, time progresses at a certain speed. That is, if I measure how many seconds tick by as the Earth revolves once around the Sun, I’ll get 31,556,926 seconds. (31,558,150 if I’m measuring a sidereal year.)

But let’s say in the course of that year, I put you on a rocket ship, and you come back exactly one Earth revolution later. I send you off at 12:00 AM on New Year’s day. Well, if you’re moving at typical rocket ship or satellite speeds (a few kilometers/second), your clock will be about one-hundredth of a second faster after a year due to the time dilation effect of special relativity, hardly noticeable.

Big deal. But what if you start moving fast? If you move at 10% the speed of light (30,000 km/s), your clock will say that it’s about 44 hours earlier than mine. While I ring in the New Year, you think it’s 4 AM on December 30th. If you get up to 90% the speed of light, I get a kiss and champagne while you think it’s early morning on June 9th. At 99.99% the speed of light, only 5 days will have passed for you while I’ve lived a whole year. And at 99.9999999% the speed of light, an entire year passes for me in just 23.5 minutes for you.

So as things move faster, time passes slower for them. Now, being made of matter (and having mass), we can never move at the speed of light. But things that do travel at the speed of light never have any time pass for them. Now as if that wasn’t neat enough, most things that we know of will decay eventually. Things that we like, such as neutrons. But if they move faster, they live longer. That’s how cosmic rays known as muons can actually reach the surface of the Earth, because they move at 99.999% or more the speed of light! But what about light? Well, that’s the one thing in the Universe that we know will never decay. Protons might decay (we know that if they do, their half life is over 1035 years), electrons might decay, but particles of light can’t. Because time doesn’t pass for them!

And that is why light doesn’t age.


  1. #1 Perched bird
    3400 ft elevation, on land
    February 18, 2014

    This has one glaring error, may be fun to correct without my saying what it is.

  2. #2 Michael Kelsey
    SLAC National Accelerator Laboratory
    February 18, 2014

    @Perched bird #1: Even more fun will be to see what you think is a glaring error, in resurrecting a six-year-old blog posting.

    The only “error” I noticed was a slight overestimate (an overestimate of 3% in the exponent) in the measured limit on proton lifetime.

  3. #3 Juan C Gaspar
    Puerto Rico
    April 25, 2015

    When it mention the time dilation effect on someone moving at faster speed in a rocket, it should say the clock in the rocket will go slower as from the reference point of someone standing on earth. So, the clock in the rocket would be 1/100th of a second delayed.

  4. #4 Diamond C St.Pierre
    631 Coquina Dr
    November 25, 2016

    Photons which are light particles radiating from the sun don’t age.
    Tachyons which travel faster than the velocity of light also do not age. Both experience time dilation but tachyons experience more and would by any chance warp through time.
    Every other particle in the basic laws of electrostatics ages.

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