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crosscountry
2006-Nov-07, 11:16 AM
I mean all of them. Some are picked up by dust or other matter, but there are a lot of photons that never do that and just keep going.

Some for nearly 13 Billion years, but eventually they must hit a wall or something right?

ozark1
2006-Nov-07, 12:12 PM
Number of walls in observable universe = 0

ozark1
2006-Nov-07, 12:23 PM
On a slightly more illuminating note...:)

Photons decay by redshift with a half life of 5-6 billion years. Photons gradually increase their wavelength and decrease their frequency with increasing time. Eventually there will be insufficient energy left to trigger a detector.

The short answer to the question is that photons fade to black. (eventually)

crosscountry
2006-Nov-07, 01:01 PM
that's a sad ending to a long life. :(

korjik
2006-Nov-07, 08:16 PM
The photon dosent notice it. To the photon, the universe just goes poof! and is gone.

Corgon
2006-Nov-07, 08:31 PM
Number of walls in observable universe = 0
*Corgon observes his room*
There may not be very many of them, and they may not intercept very many photons, but that's not quite the same thing ;)

trinitree88
2006-Nov-07, 10:32 PM
Number of walls in observable universe = 0

ozark. If there were obvious walls, than somebody would get a federal grant to apply wallpaper to them.
There are some subtle walls though. The Local Bubble is a region of space that has been largely swept clean of gas and dust. We live here in the solar system. It has a still-being-defined shape. The ultraviolet spectra of relatively nearby stars 10-180 light-years, says it is roughly that large. The missing ultraviolet spectra of more distant stars kind of defines the periphery of the Local Bubble wall.
It's possible for photons to strike that wall of gas and dust....and several things can happen to their energy. pete.

space cadet
2006-Nov-08, 01:48 AM
I never thought about that much. Funny it never crossed my mind while sitting in the bathtub.

I always come up with weird questions like that in the bathtub. Don't ask me why. Last time I tried to figure out what would happen to a black hole if they actually couldn't radiate away into space. Assuming that the big rip theory is correct, would the black hole get ripped apart?

BigDon
2006-Nov-08, 04:56 AM
On a slightly more illuminating note...:)

Photons decay by redshift with a half life of 5-6 billion years. Photons gradually increase their wavelength and decrease their frequency with increasing time. Eventually there will be insufficient energy left to trigger a detector.

The short answer to the question is that photons fade to black. (eventually)

Ozark, how do you explain the pictures of stuff that are 13 billion light years away?

Titana
2006-Nov-08, 05:14 AM
I mean all of them. Some are picked up by dust or other matter, but there are a lot of photons that never do that and just keep going.

Some for nearly 13 Billion years, but eventually they must hit a wall or something right?


The life and death of a photon (http://www.mira.org/museum/photon.htm)........:)


THIS (http://www.propermotion.com/jwreed/Essays/The%20Life%20of%20A%20Photon.htm) is another article I thought was pretty cool when I read it......



Titana

Attiyah Zahdeh
2006-Nov-08, 06:52 AM
Number of walls in observable universe = 0

The universe has 6 walls.

publius
2006-Nov-08, 06:58 AM
The universe has 6 walls.


No, the universe has no walls. This is Q & A, not ATM, and there's no place here for that kind of thing.

-Richard

Attiyah Zahdeh
2006-Nov-08, 07:09 AM
No, the universe has no walls. This is Q & A, not ATM, and there's no place here for that kind of thing.

-Richard

Hi Publius<

What is your evidence that the universe has no walls?

01101001
2006-Nov-08, 07:16 AM
What is your evidence that the universe has no walls?

Stop that. The onus is on you to support your claim that the universe has walls.

But, do it in the ATM Forum, please.

Attiyah Zahdeh
2006-Nov-08, 07:23 AM
Stop that. The onus is on you to support your claim that the universe has walls.

But, do it in the ATM Forum, please.
Hi Mr. zeros and ones,
Suppose that Publius has that evidence, why do you want to prevent me from being aware of it?

01101001
2006-Nov-08, 07:41 AM
Hi Mr. zeros and ones,
Suppose that Publius has that evidence, why do you want to prevent me from being aware of it?

Stop it. I'm not preventing it. I'm just asking please that you conduct that business where it belongs. Do it in the ATM Forum (http://www.bautforum.com/forumdisplay.php?f=17).

You've already started a thread there (http://www.bautforum.com/showthread.php?t=46055) for the purpose of asking that question -- actually advancing your claim. Get it unlocked and you can support your claim there. Thanks.

crosscountry
2006-Nov-08, 07:59 AM
The life and death of a photon (http://www.mira.org/museum/photon.htm)........:)


THIS (http://www.propermotion.com/jwreed/Essays/The%20Life%20of%20A%20Photon.htm) is another article I thought was pretty cool when I read it......



Titana


Thank you. But those articles describe things I already knew. What happens if the photon doesn't strike my eye or any absorptive matter at all?

gzhpcu
2006-Nov-08, 03:33 PM
The tired light theory, postulates that light travelling over vast distances loses energy, and red shifts as a consequence of that energy loss.
At present there is no way to test this theory.

So maybe, photons just keep on going indefinitely, unless they get absorbed by some solid object.

dhd40
2006-Nov-08, 03:44 PM
THIS (http://www.propermotion.com/jwreed/Essays/The%20Life%20of%20A%20Photon.htm) is another article I thought was pretty cool when I read it...... Titana

Nice article (THIS). And then you could ask how we can see Andromeda unblurred after all this complicated travelling

Ken G
2006-Nov-08, 05:43 PM
Actually, "tired light" is already a falsified model, insofar as it invokes physical mechanisms that are observationally refuted. Time to put it to bed. Of course, one could certainly call cosmological redshifting "tiring light", but that is not what the term is generally used for.

Also, there are several common errors in those articles cited in the thread, but the articles are written for a general audience so we'll let it go, except that I do want to comment on one thing that could really cause misconceptions. The photons we see from the Sun do not originate in the core, although the energy that is responsible for those photons certainly did get transformed from mass-energy to light energy in the core. First of all, the radiant energy released in the core balances what escapes from the surface, but the temperature is about two thousand times less at the surface. That means you have about two thousand visible photons leaving the surface for every X-ray photon released in the core. Where did the other 1,999 photons come from? They were created elsewhere in the star, as the temperature drops and the photons get downgraded. Secondly, the Sun has a convection zone which carries most of its luminosity through its outer layers in the form of rising hot gas, not photons. So basically, virtually all the photons we see from the Sun were created in the very outermost layers, called the photosphere, and most of them occured when an electron cosied up to a neutral hydrogen atom and got weakly captured in the dipole field of that neutral hydrogen (yes, there's a single bound state for an electron outside of a neutral hydrogen atom, forming a negative ion called the "H minus" ion). That's where the photons from a star like the Sun begin their journey to us, not in the core. Call it a technical correction, but it's important not to imagine that sunlight is actually diffusing out to us all the way from the core of the Sun. The energy that is responsible for sunlight, however, does use diffusion to get most of the way out, and convection to finish the job.

galacsi
2006-Nov-08, 07:46 PM
The very best thing in this tread is the Title , very inspired .

Nereid
2006-Nov-08, 08:21 PM
Thank you. But those articles describe things I already knew. What happens if the photon doesn't strike my eye or any absorptive matter at all?It goes on a wild journey around the universe, dipping down into gravity wells, and coming out of them again; being 'bent' by the mass that it passes near; ....

In addition to absorption and 'bending', light may undergo scattering ... for example, CMB photons scatter off hot electrons (in the IGM, for example) via the inverse-Compton effect ... and get boosted into the gamma region as a result. If they have the right energy, and 'encounter' a 'seed', they may vanish in a puff of matter-antimatter particles (well, pair actually).

I don't know if scattering is covered in your two cases ....

And here's another, related, question that points to 'travel forever' as the answer: other than being redshift and bent, what happened to the CMB photons before they were absorbed by the detectors in WMAP?

Other than being redshifted and (maybe) bent, what happened to the (NIR) photons from some z = ~7 galaxy, before they were absorbed by the HST's ACS detectors? In the latter case, you might like to consider Lyman alpha or Lyman limit photons (the ones that 'get through').

publius
2006-Nov-08, 08:26 PM
Hi Mr. zeros and ones,
Suppose that Publius has that evidence, why do you want to prevent me from being aware of it?


The question is what is *your evidence* that the universe has walls, whether 6 walls of a cube, or maybe 1 curved wall of a sphere, or 4 walls of a tetrahedron or whatever.

There is no observational evidence of any such boundary to space that can be seen as far as I know.

This Q&A section is not for giving wild ATM answers.

-Richard

RussT
2006-Nov-08, 08:35 PM
Actually, "tired light" is already a falsified model, insofar as it invokes physical mechanisms that are observationally refuted. Time to put it to bed. Of course, one could certainly call cosmological redshifting "tiring light", but that is not what the term is generally used for.

Also, there are several common errors in those articles cited in the thread, but the articles are written for a general audience so we'll let it go, except that I do want to comment on one thing that could really cause misconceptions. The photons we see from the Sun do not originate in the core, although the energy that is responsible for those photons certainly did get transformed from mass-energy to light energy in the core. First of all, the radiant energy released in the core balances what escapes from the surface, but the temperature is about two thousand times less at the surface. That means you have about two thousand visible photons leaving the surface for every X-ray photon released in the core. Where did the other 1,999 photons come from? They were created elsewhere in the star, as the temperature drops and the photons get downgraded. Secondly, the Sun has a convection zone which carries most of its luminosity through its outer layers in the form of rising hot gas, not photons. So basically, virtually all the photons we see from the Sun were created in the very outermost layers, called the photosphere, and most of them occured when an electron cosied up to a neutral hydrogen atom and got weakly captured in the dipole field of that neutral hydrogen (yes, there's a single bound state for an electron outside of a neutral hydrogen atom, forming a negative ion called the "H minus" ion). That's where the photons from a star like the Sun begin their journey to us, not in the core. Call it a technical correction, but it's important not to imagine that sunlight is actually diffusing out to us all the way from the core of the Sun. The energy that is responsible for sunlight, however, does use diffusion to get most of the way out, and convection to finish the job.


I have always had problems with the concept that the light (photons) we see from the sun took millions of years to reflect/deflect/re-emit their way out of the core to finally get re-emited to us! Thanks Ken G!

What I am not getting here though is...how can the sun have any 'neutral hydrogen' in it? Isn't neutral hydrogen HI (atomic hydrogen, which is not ionized). Isn't this what the 21cm Radio Telescopes are searching for when they are trying to find HI?

Which brings me to another question. The sun is releasing tons of hydrogen per sec/min/hr, right? Is this conidered HII or is the release of hydrogen from any star considered another type of hydrogen?

Which brings me to a final question. When jets are jetting material away from the accretion disc of a MBH, this must be jetting exhorbatant (sp) amounts
of hydrogen into the extragalactic medium (perpendicular to the plane of the galaxy). Is this HI, HII or other? Or is the fact that it is always being heated by all the other MBH's in the cluster that prevents cooling enough for star to form in the extra-galactic medium?
http://www.bautforum.com/showthread.php?t=48872

Ken G
2006-Nov-08, 11:37 PM
What I am not getting here though is...how can the sun have any 'neutral hydrogen' in it? Isn't neutral hydrogen HI (atomic hydrogen, which is not ionized). Isn't this what the 21cm Radio Telescopes are searching for when they are trying to find HI?Yes, it is, but that just means you can have neutral hydrogen in free space, and also in the Sun. It exists over a very wide temperature range, maybe 500 - 5000 K very roughly, colder at low density, warmer at high density.


Which brings me to another question. The sun is releasing tons of hydrogen per sec/min/hr, right? Is this conidered HII or is the release of hydrogen from any star considered another type of hydrogen?It depends on the star. The Sun emits HII because it gets ionized in the process of getting flung out. But other stars can emit neutral HI, because they use different mechanisms. It's a real zoo out there.


Which brings me to a final question. When jets are jetting material away from the accretion disc of a MBH, this must be jetting exhorbatant (sp) amounts
of hydrogen into the extragalactic medium (perpendicular to the plane of the galaxy). Is this HI, HII or other?Probably HII. Note that some jets aren't even hydrogen at all-- they may be electron/positron jets or even jets of pure electromagnetic energy.


Or is the fact that it is always being heated by all the other MBH's in the cluster that prevents cooling enough for star to form in the extra-galactic medium?
Some stars do form that way, but you are right that heating of the H tends to make it difficult. That process is very much on the frontier of research atm.

crosscountry
2006-Nov-08, 11:43 PM
The question is what is *your evidence* that the universe has walls, whether 6 walls of a cube, or maybe 1 curved wall of a sphere, or 4 walls of a tetrahedron or whatever.

There is no observational evidence of any such boundary to space that can be seen as far as I know.

This Q&A section is not for giving wild ATM answers.

-Richard



Even so that's the only answer so far given, correct or not. It is generally not accepted, but that's the reason I ask, is there an accepted answer? I seen Ken sidestepped it - he's usually the poster I read first. My impression is that's one we can't answer just yet.

I'm also reminded of black holes where light is bent inward so much that the photon never escapes, at least no in its original state. Maybe the photons in question never escape our universe due to the expansion?

Ken G
2006-Nov-09, 12:03 AM
Even so that's the only answer so far given, correct or not. Which question do you mean, where do the photons go or are there walls? For the first, most photons do indeed just keep on going, maybe forever-- perhaps the expansion is fast enough that most photons will never hit anything, that's an interesting question. As for the walls, we have seen as far as we ever will, and the only "walls" are the surface of last scattering. But they are everywhere, not in any particular location. They are walls in time, much like the Big Bang itself.

Maybe the photons in question never escape our universe due to the expansion?

They do escape our observable universe, across our horizon (observable horizon, not event horizon). What happens to them after that will never be known, but the simplest assumption is that it will be the same as what happens to the photons zooming near us.

Nereid
2006-Nov-09, 12:23 AM
Even so that's the only answer so far given, correct or not. It is generally not accepted, but that's the reason I ask, is there an accepted answer? I seen Ken sidestepped it - he's usually the poster I read first. My impression is that's one we can't answer just yet.

I'm also reminded of black holes where light is bent inward so much that the photon never escapes, at least no in its original state. Maybe the photons in question never escape our universe due to the expansion?Oh the pain! :cry:

Not only is Nereid's answer (http://www.bautforum.com/showpost.php?p=862492&postcount=22) relegated to "not even worthy of being read", but (by implication), what Nereid writes (in general) goes into the same round file!

And here I was, thinking I'd added at least two new aspects (gravitational bending, and scattering) ....

Attiyah Zahdeh
2006-Nov-09, 06:18 AM
It goes on a wild journey around the universe, dipping down into gravity wells, and coming out of them again; being 'bent' by the mass that it passes near; ....

In addition to absorption and 'bending', light may undergo scattering ... for example, CMB photons scatter off hot electrons (in the IGM, for example) via the inverse-Compton effect ... and get boosted into the gamma region as a result. If they have the right energy, and 'encounter' a 'seed', they may vanish in a puff of matter-antimatter particles (well, pair actually).

I don't know if scattering is covered in your two cases ....

And here's another, related, question that points to 'travel forever' as the answer: other than being redshift and bent, what happened to the CMB photons before they were absorbed by the detectors in WMAP?

Other than being redshifted and (maybe) bent, what happened to the (NIR) photons from some z = ~7 galaxy, before they were absorbed by the HST's ACS detectors? In the latter case, you might like to consider Lyman alpha or Lyman limit photons (the ones that 'get through').

Why not to consider that the universe as a whole is "a cubic black hole" such that the photons continue (for ever) their reflections from on its six walls, and refractions inside it?

Ken G
2006-Nov-09, 06:28 AM
And while we're at it, shall we consider that it is all balanced on the back of a turtle?

Attiyah Zahdeh
2006-Nov-09, 06:34 AM
And while we're at it, shall we consider that it is all balanced on the back of a turtle?

Not on the back of a turtle, but on AZ's back!
Have you any evidence that the universe as "a cubic black hole" is not on AZ's back?!

dhd40
2006-Nov-09, 10:34 AM
For the first, most photons do indeed just keep on going, maybe forever-- perhaps the expansion is fast enough that most photons will never hit anything, that's an interesting question.


If expansion is not fast enough ... shouldnīt they, in curved space-time, return to their birth place (provided, they donīt meet something to interact with)?
Itīs a shame that we cannot see moving photons, isnīt it?

crosscountry
2006-Nov-09, 02:54 PM
Which question do you mean, where do the photons go or are there walls? For the first, most photons do indeed just keep on going, maybe forever-- perhaps the expansion is fast enough that most photons will never hit anything, that's an interesting question. As for the walls, we have seen as far as we ever will, and the only "walls" are the surface of last scattering. But they are everywhere, not in any particular location. They are walls in time, much like the Big Bang itself.


They do escape our observable universe, across our horizon (observable horizon, not event horizon). What happens to them after that will never be known, but the simplest assumption is that it will be the same as what happens to the photons zooming near us.

That wall is a passed one and not what will stop a photon given no physical barrier.

The Horizon is what happens to a photon from our point of view. It leaves our capability to be seen. :clap:

But what about the last question in this post?



Oh the pain! :cry:

Not only is Nereid's answer (http://www.bautforum.com/showpost.php?p=862492&postcount=22) relegated to "not even worthy of being read", but (by implication), what Nereid writes (in general) goes into the same round file!

And here I was, thinking I'd added at least two new aspects (gravitational bending, and scattering) ....

While poetic your's wasn't an answer to the question. That only describes some of the path taken but not the end of a photon's "life"





Given no absorbtion how does a photon die?

peteshimmon
2006-Nov-09, 03:36 PM
Old photons never die, they only fade away! Mind I suppose the E and H field remains. Or do they provide it for new photons? Is space a vast energy gas of almost infinite numbers of ultra low energy EM waves? Or would you say there would still be destructive energy to matter? But then matter only exists with a field that keeps atoms jiggled up. Its a line of thought.

Nereid
2006-Nov-09, 03:39 PM
[snip]

While poetic your's wasn't an answer to the question. That only describes some of the path taken but not the end of a photon's "life"

Given no absorbtion how does a photon die?Ah yes, but my post did contain one little niggle ... just how similar is scattering to 'the end of a photon's "life"'?

Which in turn was getting at another thing implicit in the question - just how certain can we be of the unique identity, or even existence, of 'a photon'?

We've had this discussion before, in other guises - are the photons which make it through a transparent medium (a pane of glass, say, for 'yellow' photons) the same photons? Or are they different ones (absorbed and re-emitted)? Which we can tease apart into all kinds of interesting discussions about the 'true' nature of 'the photon'.

Or, if you prefer, perhaps the question itself is more about the theory in which it is (implicitly) couched than about 'reality' (whatever that is)?

Ken G
2006-Nov-09, 04:53 PM
Given no absorbtion how does a photon die?

It doesn't, it lives "forever", if there is such a thing. Either that, or the laws of physics as we know them change in some kind of "big rip" or who knows what, and all bets are off. Personally, I think extrapolating time too far forward runs into many of the problems of asking about "before" the Beginning-- it becomes entirely hypothetical and untestable even in principle. But a more straightforward question is-- given the known processes that can absorb CMB photons, and our current understanding of the expansion dynamics, will the average photon ever be absorbed? I don't know the answer to that, it requires a calculation, but my guess would be-- no.

crosscountry
2006-Nov-09, 04:55 PM
well, that's something. I realize some questions cannot be answered; just didn't think one so simple would be an example of that.

Ken G
2006-Nov-09, 05:03 PM
The simplest questions are often the hardest to answer.

Nereid
2006-Nov-09, 06:07 PM
It doesn't, it lives "forever", if there is such a thing. Either that, or the laws of physics as we know them change in some kind of "big rip" or who knows what, and all bets are off. Personally, I think extrapolating time too far forward runs into many of the problems of asking about "before" the Beginning-- it becomes entirely hypothetical and untestable even in principle. But a more straightforward question is-- given the known processes that can absorb CMB photons, and our current understanding of the expansion dynamics, will the average photon ever be absorbed? I don't know the answer to that, it requires a calculation, but my guess would be-- no.So let's start doing some 'what if?' thinking, shall we?

What could 'kill' CMB photons? Being 'absorbed' ... by dust, by rocks, by planets, by stars, by black holes, ...

What about being scattered (as in the SZE)? crosscountry hasn't said how he regards this 'fate'!

What about passing through a transparent medium (such as the Earth's atmosphere)? crosscountry hasn't said how he regards this 'fate'!

But why do we need answers to these questions? Because we can do calculations on how many CMB photons 'survive' passage through the IGM of a rich cluster (for example), and from that estimate the 'half-life' of a CMB photon (due to 'death by SZE'), or if you prefer 'half-length'.

And CMB photons certainly won't go 'forever' ... there will come a time when they're so redshifted that they can't propogate through space at all .... why?

Let's see who comes up with the 'correct answer' to this question first! :) (then we can work out how long it will be before they die, in this way).

crosscountry
2006-Nov-09, 06:39 PM
So let's start doing some 'what if?' thinking, shall we?

And CMB photons certainly won't go 'forever' ... there will come a time when they're so redshifted that they can't propogate through space at all .... why?

Let's see who comes up with the 'correct answer' to this question first! :) (then we can work out how long it will be before they die, in this way).

this is more of what I'm asking, not the other part.

Nereid
2006-Nov-09, 06:46 PM
this is more of what I'm asking, not the other part.Hint (http://scienceworld.wolfram.com/physics/PlasmaFrequency.html).

BigDon
2006-Nov-10, 10:38 AM
Nereid you seem to be using the word "scattering" in a way I'm unfamiliar with in regards to the extinction of light. (You mean its not consumed by dread Urkuk, The Darkness in Shadows, The Blackness Between The Stars?)

Do you have a dummied down explaination that an old furniture mover like myself can understand?

Ozzy
2006-Nov-10, 11:11 AM
I thought I read somewhere in this forum that photons dont redshift, it is we who observe the redshift due to the photon moving away from us. Or is it light that doesn't actually redshift, or are light and photons the same thing?

Please pardon and alleviate my ignorance.:o

Nereid
2006-Nov-10, 02:31 PM
Nereid you seem to be using the word "scattering" in a way I'm unfamiliar with in regards to the extinction of light. (You mean its not consumed by dread Urkuk, The Darkness in Shadows, The Blackness Between The Stars?)

Do you have a dummied down explaination that an old furniture mover like myself can understand?Sorry.

The relevant scattering I was focussing on is the Compton effect (http://en.wikipedia.org/wiki/Compton_scattering) (and the inverse Compton effect) - a 'collision' between a photon (viewed as a particle) and an electron.

In the SZE (Sunyaez-Zel'dovich effect (http://en.wikipedia.org/wiki/Sunyaev-Zeldovich_effect)), CMB (cosmic microwave background) photons 'scatter' off hot electrons in the IGM (inter-galactic medium) in the gas between galaxies in rich clusters; the 'scattering' is the inverse Compton effect.

Now crosscountry was initially unclear whether, in this kind of scattering, it was the 'same photon' that exited the 'collision', or a 'new photon'. If the former, then photons don't 'die' when scattered (at least, not via this kind of scattering) - and this is what, crosscountry later confirmed, was intended.

Note that this SZE scattering results in photons of a very different energy - gammas - than the 'incoming' (microwave) photons had.

The 'death' of photons which my hint link points to, can be (perhaps) best understood thinking of 'photons' as waves, not particles - further illustrating my comment about the question reflecting more (perhaps) about the theory implicit in it than 'reality'.

BTW, would anyone care to make an estimate of what the z (redshift) of the CMB needs to be when its photons start to get 'slaughtered' in huge numbers? For simplicity, assume slaughter in the local ISM (interstellar medium), with an electron density of 100,000 per cubic metre.

BigDon
2006-Nov-10, 08:20 PM
Thank you Nereid, succinct and informative, as always. :)

Ken G
2006-Nov-12, 03:10 AM
And CMB photons certainly won't go 'forever' ... there will come a time when they're so redshifted that they can't propogate through space at all .... why?


Well, the hint is clearly suggesting we are talking about the plasma frequency here, wherein the maximum wavelength that can propagate is proportional to the inverse square root of the density. That would have it scale like the scale factor to the 3/2, while the wavelength itself scales like the scale factor to the 1. But that doesn't cause a problem-- as the scale factor grows, the maximum wavelength gets longer faster than the photon wavelength does, so the CMB never exceeds the plasma frequency. This is true in general-- expansion always makes it easier for the photon to propagate, and so it is possible that if the expansion is fast enough, the photon will indeed live "forever", or until physics as we know it fails.

crosscountry
2006-Nov-12, 05:36 PM
or the universe collapses.

Nereid
2006-Nov-12, 10:53 PM
Well, the hint is clearly suggesting we are talking about the plasma frequency here, wherein the maximum wavelength that can propagate is proportional to the inverse square root of the density. That would have it scale like the scale factor to the 3/2, while the wavelength itself scales like the scale factor to the 1. But that doesn't cause a problem-- as the scale factor grows, the maximum wavelength gets longer faster than the photon wavelength does, so the CMB never exceeds the plasma frequency. This is true in general-- expansion always makes it easier for the photon to propagate, and so it is possible that if the expansion is fast enough, the photon will indeed live "forever", or until physics as we know it fails.Yeah ... I didn't take this into account properly :o

Plus, there are quite a few other things that I overlooked ... a more detailed post to follow ...

(notes to self:
- IGM; how big? how long?
- ISM; how will ne change?
- ISW; how important?
- ISZE; does it matter?
- injecting electrons into the IXM
- do DM particles, neutrinos, etc matter, in the long run?)

crosscountry
2006-Nov-13, 12:26 PM
a side note, does the initial energy of a photon relate to its lifetime?

also, can photons dephase?

Nereid
2006-Nov-13, 02:19 PM
Let's do some sums, on the backs of some old envelopes (i.e. OOM calculations) ...

If the ISM has an electron density of 10^5 /m^3, what must the z of the CMB be for it to be at the plasma frequency of the ISM?

Today the CMB's z is ~1100, and the plasma frequency of the ISM is ~20 kHz, so the answer is z ~1.6 x 10^10.

That's quite some time into the future! (anyone want to do an OOM calculation, to estimate just how far?)

Will the ISM have an electron density of ~10^5, that far into the future? Will there even be an ISM then??

But of course the CMB photons don't just travel on (Euclidean) straight lines ... they get scattered (SZE), they go up and down (gravitational) coasters (the Sachs-Wolfe effect), they pass 'near' big concentrations of mass (e.g. galaxies), ... In all their perambulations, how likely is it that they will hit a pocket of plasma which is (electron) dense enough that the photon would 'die'? And in all their scattering and frequency shifting (other than redshift), how far from the blackbody spectrum will the photons be moved?

We can get some clues to all these from things like the Hubble UDF (e.g. how much of the sky is not covered by galaxies?), and from the 'CMB' itself (e.g. how much of the sky is not covered by SZE footprints?)

closetgeek
2006-Nov-15, 04:42 PM
You know I have put substantial thought into why the best idea's come in the bathroom. I Know a lot of people who have the same phenomina. I have concluded that it is because the bathroom is the one place where we can loose all the rules of society and be completely natural. Nothing is getting in the way of our free flowing thoughts.


I never thought about that much. Funny it never crossed my mind while sitting in the bathtub.

I always come up with weird questions like that in the bathtub. Don't ask me why. Last time I tried to figure out what would happen to a black hole if they actually couldn't radiate away into space. Assuming that the big rip theory is correct, would the black hole get ripped apart?

Nereid
2006-Nov-15, 05:04 PM
Thinking more about this ... there is a principle in physics, called the ergodic principle, which in ordinary English may be summarised as "what is not forbidden is compulsory".

In astronomy, we are familiar with this, ironically, in the "forbidden lines (http://en.wikipedia.org/wiki/Forbidden_line)" that are so common in so many beautiful images (http://antwrp.gsfc.nasa.gov/apod/ap021108.html).

So this lead me to ask: what extremely low probability things may happen to a photon, in the real universe, that would ensure its demise? I mean, the CMB photons are only ~14 billion years' old, so they have had essentially no opportunity to undergo a rare collision. What might happen if there were 1014 billion years' old?

There was, many years ago I think, an interesting article in Sky and Telescope (I think; maybe it was Scientific American?), about the ultimate fate of the universe (assuming it didn't collapse). The authors used a logarithmic timescale, so all stars were dead within another 3 or so tick marks on the scale, and all black holes had evaporated not too many more away, and all protons were gone (the authors assumed the proton is unstable, with a half-life of 'only' some 1033 years).

So if we look deeply into known physics, what can we say would happen to photons, over a reallllly looooong period of time (in our real universe)?

Ken G
2006-Nov-15, 08:44 PM
That is indeed an interesting question-- I think that the accelerated expansion, assuming it's a cosmological constant, would cause the density to drop fast enough that the average CMB photon would just get very slowly redshifted into anonymity, but I don't know if other physics will kick in at some point or if I'm overlooking some process that does not depend on the density.

crosscountry
2006-Nov-16, 11:34 AM
Nereid, that is in fact THE question.


Red shifted into anonymity is a possible answer. Wouldn't they possibly dephase before that happened? I mean, they aren't infinite beams and have to dephase at some point even if their lifetime is long.

Nereid
2006-Nov-16, 01:32 PM
If we expand our time scope, then I don't think we can answer the question.

For example, dark energy: it seems, based on current understanding, to be constant .... but that only means 'we can rule out it varying by {insert &#177; % here}, over {insert cosmic time here}'. Even if {cosmic time} were 13 billion years, that is utterly trivial compared with 1013 billion years, so any (direct or indirect) effect of a time-varying dark energy would be impossible to rule out.

Another example: we have few constraints, today, on the nature of dark matter. If it turns out to be (primarily) the lowest mass supersymmetric particle, there's a vast number of possible, very low probability interactions that we will need to explore ... and how any of these affect the likely fate of photons over 1013 billion years is unknown (let alone 10^(13 billion^13 billion) years).

On the one hand we could invoke 'new physics' (there's little from the experimental or observational realm to constrain the behaviour of DM particles, for example); but on the other hand, we do know that both DM and DE 'exist'.

gzhpcu
2006-Nov-16, 04:12 PM
Photons travel at the speed of light, so for us it seems as if their reference frame clock is standing still. So we never see the photon "change", just like the image of a trapped space ship at an event horizon of a black hole.

So don't photons seem to us to "last forever"?

crosscountry
2006-Nov-16, 05:00 PM
That's another thought.

crosscountry
2007-Jan-22, 11:04 PM
anyone know the coherence length of a photon?

Thomas(believer)
2007-Jan-22, 11:26 PM
What happened with the photons in the very early universe, when the universe was very small? Could the photons collide with themelves?
Or maybe, was the universe so dense, that the photons would not come very far and that "speed of light" had no meaning then?

neilzero
2007-Feb-13, 01:51 AM
Another thread www.space.com perhaps supposes that Xray and gamma photons collide with visable photons.
I think it is presumed that half of all the photons ever emitted by photospheres of stars are still traveling at the speed of light, red shifted by the expansion of the Universe, some having traveled more than 13 billion light years. The half that colided with a bit of matter often resulted in the emission of one or more new photons, typically of longer wave length. It has been hypothesised that the Universe is awash with photons with wave lengths exceedining a lightyear and frequencies much lower than one hertz. Photons are thought not to decay, or die, but they do get diluted as the Universe expands.
There is no good reason to think that anything happens to a photon as it leaves the observable universe, but it is reasonable to suppose something happens when, and if, a photon travels beyond spacetime, if spacetime has an outer edge. Neil