Dear Cecil, a friend of mine recently asked me a question that kinda got me stumped. I’m a bit of a physics buff but I wasn’t sure how to answer him…here goes.
I’ve read that photons (the fundamental constituents of light) are essentially ageless, that is, a photon is the same “age” now as it was at the time of the Big Bang due to Special Relativity, which as you know, states if you travel at the speed of light, in a vacuum, you don’t age. Here’s the quandary, light travels at different rates through different mediums, does this affect it’s “age”, will it get “younger” or “older”? I hope you can shed some light (ha…get it?) on this. Thanks
Light does not travel at different speeds through different media. Or, at least, individual photons don’t. They always travel at c. The average speed of the many billion photons you see when you look at a beam traveling through, say, water is lower than c. This is due to the adsorption and re-emission of the individual photons by the molecules of the material the light is traveling through, which process takes some small time. But every individual photon is traveling at c from the moment of creation to the moment of annihilation. Also, remember that matter at that scale is mostly empty space, so the photons are flying unhindered through gobs of empty space before interacting with the small amount of matter.
It’s also not true that you don’t age if you travel near the speed of light (you cannot travel at the speed of light). You will age less as measured by someone travelling slower, but you yourself will experience time as you experience it in every reference frame.
Forgive my ignorance, it’s been about 10 years since my Physics and Relativity class, but are ALL photons the same age? Does a light bulb create new photons?
The photon from its own frame of refference exists for exactly zero time, existing at all points uppon its tragectory. As Mangetout says they don’t experience time at all. So even if a photon has been according to us travelling continuously from soon after the Big Bang, without ever being absorbed. This 14 Billion (if I remeember correctly) Year old photon, from its own frame of refference has existed for 0 time.