# So how old are photons? **URL:** \ **Category:** Factual Questions\ **Created:** [September 26, 2010, 8:19pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136 "2010-09-26T20:19:54Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![ChrisBooth12](https://avatars.discourse-cdn.com/v4/letter/c/e95f7d/32.png) [@ChrisBooth12](https://boards.straightdope.com/u/ChrisBooth12)\ **Post date:** [September 26, 2010, 8:19pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/1 "2010-09-26T20:19:54Z")
I was watching the first Episode of the Documentary The Universe, the first episode was about the sun. It mentioned as photons are created everything in the middle is so dense they bounce around for quite a while before actually breaking out and heading to earth. Now is a photon is moving at the speed of light, by definition 0 time has passed for it. But relative to us on earth are we really getting hit by photons from the sun that were created thousands of years ago? Another question about photons, when they “bounce” around hitting over partials and crap does the change in direction make it slow down at all? ---
**Author:** ![Morgenstern](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@Morgenstern](https://boards.straightdope.com/u/Morgenstern)\ **Post date:** [September 26, 2010, 8:32pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/2 "2010-09-26T20:32:27Z")
I’m not a scientist but I think the speed of light constant is the speed in a vacuum. If that’s the case, then it implies that the speed of light is slower in a non-vacuum. (I assume) Interesting question. I’m anxious to see the answer. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 26, 2010, 8:40pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/3 "2010-09-26T20:40:15Z")
This is actually a lot harder to answer than you’d think, since photons are indistinguishable. Whenever a photon interacts with matter, it can be interpreted as the photon being absorbed, and then a moment later a new photon is emitted. Or maybe it’s the same photon that’s being emitted. There’s really no way to say. ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [September 26, 2010, 9:25pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/4 "2010-09-26T21:25:59Z")
The random walk of radiation from the sun Lisa May Walker Physics Department, The College of Wooster, Wooster, Ohio 44691, USA (Dated: May 11, 2006) A simulation was run to find the average escape time of photons from the radiation zone of the Sun. An escape time of was found for a constant-density Sun and for a linear density gradient Sun by extrapolation from smaller radii. These values are respectively less than and greater than the expected value by an order of magnitude. [http://www3.wooster.edu/physics/jrIS/Files/Walker\_Web\_article.pdf](http://www3.wooster.edu/physics/jrIS/Files/Walker_Web_article.pdf) ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [September 26, 2010, 9:57pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/5 "2010-09-26T21:57:30Z")
> [@ChrisBooth12](#): > > Now is a photon is moving at the speed of light, by definition 0 time has passed for it. But relative to us on earth are we really getting hit by photons from the sun that were created thousands of years ago? A photon can’t really have a frame because light must move at c in all frames and light can’t move at c wrt light. But in our frame it ages just like anything else. > [@](#): > > Another question about photons, when they “bounce” around hitting over partials and crap does the change in direction make it slow down at all? No. photons always travel at c regardless of the medium. They only appear to slow down because absorption, pseudo absorption and emission takes time. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [September 26, 2010, 10:34pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/6 "2010-09-26T22:34:48Z")
> [@](#): > > No. photons always travel at c regardless of the medium. They only appear to slow down because absorption, pseudo absorption and emission takes time. I should have said _virtual_ absorption rather than _pseudo_ absorption. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [September 26, 2010, 11:14pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/7 "2010-09-26T23:14:52Z")
> [@Ring](#): > > No. photons always travel at c regardless of the medium. They only appear to slow down because absorption, pseudo absorption and emission takes time. Huh? Just when I thought I understood something. The slowing down of light in different media is usually described in terms of the wave-like properties of light and is called the refraction index. In fact it has been slowed to [38 mph](http://www.jupiterscientific.org/sciinfo/slowlight.html). Are you saying that it is better described as in a particle vernacular with absorpition and re-emission as the actual basis of the refractive index? ---
**Author:** ![ChrisBooth12](https://avatars.discourse-cdn.com/v4/letter/c/e95f7d/32.png) [@ChrisBooth12](https://boards.straightdope.com/u/ChrisBooth12)\ **Post date:** [September 26, 2010, 11:19pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/8 "2010-09-26T23:19:35Z")
Photons always travel at C? Would not the sheer density inside a star slow down a photon? After all it cannot occupy the same space as another photon right? ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [September 26, 2010, 11:59pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/9 "2010-09-26T23:59:35Z")
> [@ChrisBooth12](#): > > Photons always travel at C? Would not the sheer density inside a star slow down a photon? After all it cannot occupy the same space as another photon right? 1. Yes they can occupy the same space. 2. The density isn’t as high as you might think. The standard illustration is to take a really large stadium, lit so that the whole place is the equivalent of full sunlight. If you could stop time absolutely you would only find one photon in the whole stadium. Despite the size and how bright it is, photons travel so incredibly fast that at any given moment only one exists in a space that big and that well lit. The sun is a little brighter than that, but it’s also really big, so the actual “density” of photons probably isn’t that high. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [September 27, 2010, 1:40am UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/10 "2010-09-27T01:40:57Z")
> [@DSeid](#): > > > [@Ring](#): > > > > No. photons always travel at c regardless of the medium. They only appear to slow down because absorption, pseudo absorption and emission takes time. > > Huh? Just when I thought I understood something. > > The slowing down of light in different media is usually described in terms of the wave-like properties of light and is called the refraction index. In fact it has been slowed to [38 mph](http://www.jupiterscientific.org/sciinfo/slowlight.html). Are you saying that it is better described as in a particle vernacular with absorpition and re-emission as the actual basis of the refractive index? In classical mechanics and Maxwell’s Equations it’s described in terms of waves, but in order to describe certain phenomenon it’s necessary to use quantum mechanics and/or quantum field theory. And in QM light is an ensemble of photons (particles). It’s a little like General Relativity superseding Newton’s gravitation. Also photons are particles called bosons and they _can_ occupy the same space. ---
**Author:** ![ChrisBooth12](https://avatars.discourse-cdn.com/v4/letter/c/e95f7d/32.png) [@ChrisBooth12](https://boards.straightdope.com/u/ChrisBooth12)\ **Post date:** [September 27, 2010, 2:21am UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/11 "2010-09-27T02:21:21Z")
Occupy the same space? They have no mass? I need to watch a bunch of nova particle shows ---
**Author:** ![Baron\_Greenback](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/baron_greenback/32/48_2.png) [@Baron\_Greenback](https://boards.straightdope.com/u/Baron_Greenback)\ **Post date:** [September 27, 2010, 2:22am UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/12 "2010-09-27T02:22:46Z")
> [@Blake](#): > > 1. The density isn’t as high as you might think. The standard illustration is to take a really large stadium, lit so that the whole place is the equivalent of full sunlight. If you could stop time absolutely you would only find one photon in the whole stadium. Despite the size and how bright it is, photons travel so incredibly fast that at any given moment only one exists in a space that big and that well lit. Mind=blown. 🆒 ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [September 27, 2010, 2:41am UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/13 "2010-09-27T02:41:03Z")
In the interests of accuracy, I should note that I’ve seen that factoid repeated by a couple of reliable sources, but I make no promises as to its absolute veracity. Can’t be bohered to do the calculations myself. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 27, 2010, 2:51am UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/14 "2010-09-27T02:51:10Z")
Quoth **Blake** : > [@](#): > > 1. The density isn’t as high as you might think. The standard illustration is to take a really large stadium, lit so that the whole place is the equivalent of full sunlight. If you could stop time absolutely you would only find one photon in the whole stadium. Despite the size and how bright it is, photons travel so incredibly fast that at any given moment only one exists in a space that big and that well lit. That’s considerably exaggerated. Taking sunlight as 1000 W/m^2, and approximating sunlight as consisting entirely of 500 nm radiation, I find a density of [about 10[sup]13[/sup] photons per cubic meter](http://www.google.com/#hl=en&expIds=25657,26751&sugexp=ldymls&xhr=t&q=%28100+watts/m%5E2%29/%28c%5E2*h/500+nm%29&cp=22&pf=p&sclient=psy&safe=off&aq=f&aqi=&aql=&oq=%28100+watts%2Fm%5E2%29%2F%28c%5E2*h%2F500+nm%29&gs_rfai=&pbx=1&fp=8d9c50a61d5b9175). You’d be closer if you replaced the stadium with a closet, and “full sunlight” with “enough light to read by”, but that’s still probably a factor of a million or more off. ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [September 27, 2010, 5:39am UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/15 "2010-09-27T05:39:54Z")
> [@Ring](#): > > No. photons always travel at c regardless of the medium. They only appear to slow down because absorption, pseudo absorption and emission takes time. What about [refraction](http://en.wikipedia.org/wiki/Refractive_index)? ---
**Author:** ![DHMO](https://avatars.discourse-cdn.com/v4/letter/d/ecc23a/32.png) [@DHMO](https://boards.straightdope.com/u/DHMO)\ **Post date:** [September 27, 2010, 5:11pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/16 "2010-09-27T17:11:21Z")
> [@standingwave](#): > > > [@Ring](#): > > > > No. photons always travel at c regardless of the medium. They only appear to slow down because absorption, pseudo absorption and emission takes time. > > What about [refraction](http://en.wikipedia.org/wiki/Refractive_index)? As **Ring** was trying to make clear, the results of the experiment depend heavily on what, exactly, you are looking for. As a _wave_ experiment, refraction may be described as the velocity of light in a transparent medium being slowed. As a _particle_ (QM) experiment, the absorption and re-emission of photons, while taking an infinitessimal time, causes the same result; that is, the velocity of light is slowed in the transparent medium. The individual photons themselves, however, travel from one interaction to the next at a constant velocity of c. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 27, 2010, 5:35pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/17 "2010-09-27T17:35:08Z")
> [@DHMO](#): > > As **Ring** was trying to make clear, the results of the experiment depend heavily on what, exactly, you are looking for. As a _wave_ experiment, refraction may be described as the velocity of light in a transparent medium being slowed. As a _particle_ (QM) experiment, the absorption and re-emission of photons, while taking an infinitessimal time, causes the same result; that is, the velocity of light is slowed in the transparent medium. The individual photons themselves, however, travel from one interaction to the next at a constant velocity of c. So what is happening in a Bose-Einstein condensate that makes photons _appear_ to slow down? ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [September 27, 2010, 6:11pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/18 "2010-09-27T18:11:12Z")
> [@DHMO](#): > > The individual photons themselves, however, travel from one interaction to the next at a constant velocity of c. Thanks. I think I get it now. From the wiki article: \*At the microscale, an electromagnetic wave’s phase speed is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the permittivity of the medium. The charges will, in general, oscillate slightly out of phase with respect to the driving electric field. The charges thus radiate their own electromagnetic wave that is at the same frequency but with a phase delay. The macroscopic sum of all such contributions in the material is a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave’s phase speed. Most of the radiation from oscillating material charges will modify the incoming wave, changing its velocity. \* ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [September 27, 2010, 6:55pm UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/19 "2010-09-27T18:55:55Z")
> [@](#): > > Huh? Just when I thought I understood something. > > The slowing down of light in different media is usually described in terms of the wave-like properties of light and is called the refraction index. In fact it has been slowed to [38 mph](http://www.jupiterscientific.org/sciinfo/slowlight.html). Are you saying that it is better described as in a particle vernacular with absorpition and re-emission as the actual basis of the refractive index? There was a time when physicists thought that basic science was dead since everything had finally been figured out. Unfortunately for them, the _Photoelectric Effect_, the _Ultraviolate Catastrophe_ and the _electron not spiraling into the nucleus_ proved them wrong. The solution to these problems all involved the quantization of variables and was therefore called Quantum Mechanics. This didn’t mean that classical mechanics or Maxwell’s Equations were useless, it just meant that they were limited in their scope. I suggest you look up the above italicized items, and if you do, I think you’ll get a pretty good idea of why photons are necessary. ---
**Author:** ![jackdavinci](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackdavinci/32/3259_2.png) [@jackdavinci](https://boards.straightdope.com/u/jackdavinci)\ **Post date:** [September 28, 2010, 7:43am UTC](https://boards.straightdope.com/t/so-how-old-are-photons/555136/20 "2010-09-28T07:43:23Z")
OK. So initially, I was a big believer in, light travels at C, and refraction etc is just an artifact of photon absorption delay or whatever. But now I wonder… Maybe everything moves at C and some labyrinthine path detour makes other particle seem like they travel slower also? Like extra mass involves extra dimensional travel or…? [Next page](https://boards.straightdope.com/t/so-how-old-are-photons/555136.md?page=2)