View Full Version : Discussion: Where Does Visible Light Come From?

2005-Jan-25, 06:57 PM
SUMMARY: It's amazing thing but many amateur astronomers (and possibly the occasional professional as well!) don't have the big picture on where the bulk of the visible light in the universe comes from. "Sure" you say, "from the stars!" Ah but that's the easy answer. In fact the more you learn about light, the less straightforward such an answer becomes. In this article, Jeff Barbour probes a little deeper and the implications could light the way to an extraordinarily new appreciation for the "star stuff" seen all around us.

View full article (http://www.universetoday.com/am/publish/where_does_visible_light.html)

What do you think about this story? Post your comments below.

2005-Jan-25, 07:38 PM
Excellent read, I quite enjoyed that article.

The Near-Sighted Astronomer
2005-Jan-26, 02:57 PM
Hi All,

My thanks to Fraser for making it possible to post information using the UT Forum. It's great to be able to correct errata and post claritfications in real time and I'd would like to respond publically to an email I received on the article "Where Does Visible Light Come From?"

Here is part of my correspondent's (Janos') message:


I just read your article on the origin of
visible light over at Universe Today. Very nice work,
clear and easy to understand. I wish more physics
teachers had your skill. I just wanted to point out a
minor error in your essay:

"When light passes into matter, light slows - while
its frequency increases."

If I understand my physics correctly, frequency
remains unchanged when light (or any wave for that
matter) passes from one medium to another. It is the
WAVELENGTH which changes...

Which prompted the following response (in full):

Greetings Janos,

Thank you so very much about alerting me to the problems of phase shift within a transparent media. There are some very interesting issues here that need a full exploration.

1. AT the entrance transition-barrier, there is an apparent phase shift as
wavelengths shorten and later wavefronts slightly overtake their predecessors.

2. WITHIN the substance, wavelength shortens due to what I call "wave-front"
bucking BUT frequency remains constant due to the fact that the propagation
speed is reduced between any two wave-peaks.

3. AT the exit barrier, wavelengths lengthen but frequency remains the same due to translation back to the speed of light. This also gives the appearance of a phase-shift.

All these points assume that we move from low refractive index (impedence) to higher impedence and back again. It also treats light as a wavefront - not necessarily as individual photons. Nor does it treat of reflected waves (transitional and internal).

Now here are the puzzlers:

1. How do individual photons behave in a substance? Do all photons undergo
wavelength shifts? Do some of the photons undergo phase distortions while others do not? (I imagine the regularity of molecular structure, dopants etc are all a factor here.)

2. How precisely does molecular jostling (wavefront impedence - parasytic photonic capacitances) bleed off energy from the wavefront if no individual photons are absorbed (based on quantum absorption)?

Now you may note that my way of talking about "light" is more like that of a
student of electronics than of optics. And that's precisely the situation. I have
no formal training in physics and it is precisely for this reason that I write the kind
of "big picture" article like "Where Does VIsible Light Come From? - because
as far as I can tell the experts are too specialized and the average writer on these
matters is not synthetic enough in their thinking to treat such things in broad strokes.

Thanks Again for the Heads-Up,


Now I know we have some younger readers on these forums and our discussion is a little technical so let me speak simply by analogy:

If you drop a bunch of pebbles into a pool of water one at a time in perfect "1,2,3,4" rhythm (not rap!) each pebble hitting the water is slowed down at the surface and the next one seems to catch up slightly. This causes the wavelength to shorten but the rhythm of the pebbles stays the same. Once all the pebbles are in the pool. the distance between them is less but if you re-checked your rhythm as they hit the bottom of the pool it would still be the same. So in no case does the rhythm (frequency) change - only the physical distance between any two pebbles.

Or if you are not musically inclined, watch a Formula One road race some time and notice how the cars slow into a hairpin turn - they get closer together in space but the time between them at any given point (assuming they are all pretty much maintaining a similar driving style) is the same...

When you think about the pebbles - or the race cars - it makes sense to think of how the water or race course "heats up" as the pebbles or race cars slow down. This is the main idea behind the article. That heat is given off by those high energy photons from the core passing through the matter in its mantle and photosphere and it is the "heat" we see while it is very rare for the "pebble" or the "race car" to get through at all!

Carpe Noctem,


2005-Jan-27, 03:02 AM
Playing with Light and Color (http://www.physorg.com/news2833.html)
Article from PhysOrg.

I saw a doco recently about these 'first stars' hydrogen rich, exploding then condensing to form heavier elements. But this Promordial light sounds like it precedes the Cosmic Microwave Background, if it isn't a part of it??

APOD (http://antwrp.gsfc.nasa.gov/apod/ap031119.html)

The Near-Sighted Astronomer
2005-Jan-27, 11:50 PM
Hi Zephyr,

But this Promordial light sounds like it precedes the Cosmic Microwave Background, if it isn't a part of it??

I believe the current thinking is that about 360K years after the Big Bang the high energy "soup" ejected by the Big Bang separated out into light and matter. The matter then began to condense into stars - with much of it still in a primordial state (hydrogen gas) remain primordial even to this day. Meanwhile the light suffused an ever expanding space-time continuum. As that continuum continued to expand, primordial light "red-shifted" to the point that all that now remains of its original intensity and frequency is the cosmic background radiation - equivalent in energy level to a temperature of about 2 degrees above absolute zero.

To get a sense of how much red-shift has occurred during cosmic expansion imagine a piano with about 43 three sets of white and black keys on it (such a piano would be about 6 times longer than a standard Piano of a little more than 7 sets of keys). Now assuming that the Primordial light took the form of gamma rays and that these rays are the last highest 7 sets of keys on our "Cosmic Piano" then the notes of these primordial tones has now shifted almost 4 regular Piano widths down from where they started. Thats about 2/3rds the way down the electromagnetic scale almost into the radio-wave frequencies...

The Near-Sighted Astronomer
2005-Feb-18, 10:06 PM
Hi All,

I have since learned from a professional astronomer that the CMBR started out at a blackbody average color temperature comparable to the Sun's photosphere. As telescopes reach back further in space astronomers hope to resolve the gamma ray component which should have shifted to the visible light spectrum.

So the actual shift since the Big Bang is somewhere around 17 octaves (or sets of DO RE ME FA SO LA TI DO note scales).

As it stands now we can see back within 1 billion years of the Big Bang - but this takes weeks of light gathering to accomplish. Hopefully the Hubble will be replaced with a larger instrument in the next few years so we can actually capture the original Big Bang in all its glory.



2005-Feb-18, 10:37 PM
Originally posted by The Near-Sighted Astronomer@Feb 18 2005, 10:06 PM
Hopefully the Hubble will be replaced with a larger instrument in the next few years so we can actually capture the original Big Bang in all its glory.
Don't nail your hopes to that too much or you'll be in for a life of bitter disappointment.

We won't ever see photons from the first instant.

2005-Feb-19, 02:42 AM
i see one error of reasoning---it all hinges on the term 'visible'. as dr. wheeler and so many others have pointed out, photons, w/o some 'perceptor', may not be known. certainly they may exist with no observer, but then the issue of 'visible' light becomes moot. the entire concept of 'the universe' as an interpretive function of living sensory systems implies 'photons-to-galaxies' are more closely related to Life, as human sensory units, than ultimate fact---something we may never know, only interactivly interpret, and then assume as fact. my 1st philosophy prof asked our class if we considered the difference between 'things' and 'no-things'---the room was silent---nobody knew what-to-heck he was asking.

the 'inerpretive universe' is probably the oldest idea after sex, food, shelter---perception seems the major problem, not wave-length---while it may elevate humanity to god-like proportions---the ability to see stars creates them---this may be more common than understood.

2005-Feb-19, 03:10 AM
Deep vet22!

Very Heidegger, very fond of phenomenolgy myself.

I think, though, that seeing a star, may, as much as create a star, create a story of how it got there! Particularly in the past.

Relating new discoveries back to the accepted theory of the day is as much a case of this as any tribal interpretations.

To see the big bang, today, we would need to be as far from it again as we are now, and outside it, to my way of thinking.

But, according to the theory, we couldn't be there, because there is nothing there (outside the big bang), not even time.

One of the interesting articles that came out on light recently, mentioned how long a photon was, km if my memory serves me. And what I still can't swallow is that photons have no mass. If light can be affected by gravity, ie gravitational lensing, gravity is acting on something other than just mass, Time? Definately the time - space continuum, that doesn't exist outside of the big bang.

Well, I don't ever expect to understand it myself, but it is very intriguing. :)

The Near-Sighted Astronomer
2005-Mar-08, 03:51 PM
To see the big bang, today, we would need to be as far from it again as we are now, and outside it, to my way of thinking.

In fact we see the Big Bang even now - as it has worked out its fundamental principles over some 13.7 billion years of non-relativistic time. One way to understand this is to think of the Universe as a means to "model everything"

To anthropomorphize: Imagine "God" saying I wonder what would happen if I set such and such a parameter of electric charge to this, and such a parameter associated with gravity to that. Then after making a whole bunch of adjustments to a whole bunch of parameters associated with a potential universe he let's her rip and everything starts to play out.

In that sense we do see the Big Bang on the basis of all its consequent Universal Computer calculations.

But in another sense we can never see the Big Bang with our instruments. For one reason: The universe was opaque for some 380,000 years after the Big Bang. For another the two balloons of Matter and Energy have decoupled to some degree. Once the opaque period passed (many of the free electrons joined with atomic nuclei and photons were allowed to radiate freely) the space-time continuum expanded beyond the realm of matter to all directions. Meanwhile other light stayed bound to matter through re-radiation and in the composition of matter itself. Effectively most of the primordial light is now wizzing off into the hinterlands of space-time while the part that was constantly re-radiated is now seen by us as the CMBR.