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normdowling
2007-Oct-05, 12:39 PM
If there are some atoms in the vacuum of space,however tiny the amount, then wouldnt this have a significant effect on light travelling for very large distances. ????? eg- 10 billion light years????


Dont some photons get absorbed and re emitted ?????

If there were 2 atoms in every square cm of space, then, light travelling for 10 billion years @ 300,000 km per second, would move through a hell of a lot of cm,s , eventually running into some atoms.

Do astronomers have to take into account this possible dimming of light when calculating the distance or age of deep space objects???????

grant hutchison
2007-Oct-05, 02:51 PM
Do astronomers have to take into account this possible dimming of light when calculating the distance or age of deep space objects???????They do. Because light is scattered and absorbed in transit, distant objects are dimmer than one would predict from the simple inverse square law. They're also redder, because short wavelengths are affected more than long wavelengths.

Grant Hutchison

Saluki
2007-Oct-05, 02:57 PM
If there are some atoms in the vacuum of space,however tiny the amount, then wouldnt this have a significant effect on light travelling for very large distances. ????? eg- 10 billion light years????

It has an effect, but "significant" is not how I would describe that effect.



Dont some photons get absorbed and re emitted ?????

Yes.


If there were 2 atoms in every square cm of space, then, light travelling for 10 billion years @ 300,000 km per second, would move through a hell of a lot of cm,s , eventually running into some atoms.

There aren't. A quick google gave me numbers on the order of 1 atom per cubic meter for the density of the universe. Of course those numbers go up within a galaxy, but overall, the universe is a very empty place. Remember that only the nucleus of an atom would effect a photon. Most of an atom itself is empty space.

I did some rough calculations. If there is one hydrogen atom in every cubic meter the light passes through, there will be about 8x10-29 square meters of blocked space (area of a proton). 10 billion light years is 9.5x1022 meters. So, if we assume that the hydrogen atoms encountered are spread out so that none is directly in line with another, we have a total of 7.4x10-16 square meters of area blocked by protons out of each square meter of space. So, the percentage of photons effected would be about 0.00000000000000074%.

Again, this is very rough, and could be off by a few orders of magnitude, but the point remains that very nearly 100 % of photons make it through unaffected.


Do astronomers have to take into account this possible dimming of light when calculating the distance or age of deep space objects???????

I am not an astronomer, but I am sure they are well aware that there is matter in the universe, and are fully capable of accounting for its presence.

normdowling
2007-Oct-05, 03:17 PM
the question was 'do' they take this into account.?

Not, if they are capable of doing it.

Hornblower
2007-Oct-05, 04:09 PM
Remember that only the nucleus of an atom would effect a photon. Most of an atom itself is empty space.


That is not correct. Photons with just the right amount of energy are absorbed by electrons, with the energy going into quantum-mechanical potential of some sort. The electrons soon drop back to their ground states, re-emitting photons in all directions. The net result is scattering of the light. The spectrum of a star, when viewed through enough intervening gas, will have dark lines superimposed on it as a result. The gas would be virtually opaque at those wavelengths and highly transparent otherwise.

I do not know how visible light interacts with the nucleus, if at all. I would need to hear from an expert in nuclear physics.

Hornblower
2007-Oct-05, 04:11 PM
the question was 'do' they take this into account.?

Not, if they are capable of doing it.

Of course they take it into account, unless they know from experience that it is negligible for the particular task at hand. Let's give them some credit for being smart enough to check all well-known potential sources of error.

Saluki
2007-Oct-05, 04:21 PM
That is not correct. Photons with just the right amount of energy are absorbed by electrons, with the energy going into quantum-mechanical potential of some sort. The electrons soon drop back to their ground states, re-emitting photons in all directions. The net result is scattering of the light. The spectrum of a star, when viewed through enough intervening gas, will have dark lines superimposed on it as a result. The gas would be virtually opaque at those wavelengths and highly transparent otherwise.

I do not know how visible light interacts with the nucleus, if at all. I would need to hear from an expert in nuclear physics.


Does it matter for purposes of this question? An electron's diameter is 3 orders of magnitude smaller than a proton. Either (1) photons interact with both, and the electron's impact is lost in the lack of significant figures; or (2) photons interact only with the electron, which makes the relative target area even smaller. Niether impacts my basic point that there is a whole lot of empty space for the photons to pass through.

grant hutchison
2007-Oct-05, 05:01 PM
the question was 'do' they take this into account.?

Not, if they are capable of doing it.As I said in post #2 (http://www.bautforum.com/1082979-post2.html), they do.
The technical term for this effect is "extinction", and if you Google on "interstellar extinction", you'll find a great deal of information about how astronomers take extinction into account.


Does it matter for purposes of this question? An electron's diameter is 3 orders of magnitude smaller than a proton. Either (1) photons interact with both, and the electron's impact is lost in the lack of significant figures; or (2) photons interact only with the electron, which makes the relative target area even smaller. Niether impacts my basic point that there is a whole lot of empty space for the photons to pass through.The bulk of interstellar extinction is due to dust, not hydrogen, so your calculation isn't really targeted at the right problem. But the fact remains, as you say, that there's a lot of space and not much matter.

Grant Hutchison

galacsi
2007-Oct-05, 05:40 PM
Does it matter for purposes of this question? An electron's diameter is 3 orders of magnitude smaller than a proton. Either (1) photons interact with both, and the electron's impact is lost in the lack of significant figures; or (2) photons interact only with the electron, which makes the relative target area even smaller. Niether impacts my basic point that there is a whole lot of empty space for the photons to pass through.

In the case of the rayleight diffusion this is the atom which interact with the photon (The positive nucleus and the negative electronic cloud ) . IN this case the target is 10,000 times bigger than the nucleus and the cross section is 100 ,000, 000 times bigger that you say.

There is also the compton diffusion , but I believe it is important only with very energetic photon .And I am not sure about the cross section of this diffusion.

galacsi
2007-Oct-05, 06:00 PM
I did some rough calculations. If there is one hydrogen atom in every cubic meter the light passes through, there will be about 8x10-29 square meters of blocked space (area of a proton). 10 billion light years is 9.5x1022 meters. So, if we assume that the hydrogen atoms encountered are spread out so that none is directly in line with another, we have a total of 7.4x10-16 square meters of area blocked by protons out of each square meter of space.

Very rough calculations indeed :

If I accept your values for the surface of the proton and 10 billions light years in meters I cannot see how you got :
7.4x10-16 .

I get : 7.4x10-6

Anyway your distance is wrong :

One light is 9.5 thousand billions km = 9.5x1015 meters.

10 billions light years are :

9.5x1015 * 109 : 9.5x1024 meters :

And your cross section of photon /matter interaction is also very wrong as I showed in my previous post. So your point that "that very nearly 100 % of photons make it through unaffected." lack substance.

Hornblower
2007-Oct-05, 11:50 PM
Does it matter for purposes of this question? An electron's diameter is 3 orders of magnitude smaller than a proton. Either (1) photons interact with both, and the electron's impact is lost in the lack of significant figures; or (2) photons interact only with the electron, which makes the relative target area even smaller. Niether impacts my basic point that there is a whole lot of empty space for the photons to pass through.

Let's get out of the trap of trying to describe subatomic particles as if they were miniature opaque balls casting simple shadows in streams of passing photons. With quantum mechanics at this scale, all bets are off. Estimates of the size of a particle depend strongly on the methods used to perform these estimates.

I just Googled "Electron diameter" and found estimates all over the place. Depending on the method, they ranged from somewhat larger (and vastly less dense) than a proton down to a point with no definable measure of diameter.

In the case of light extinction, let me remind everyone again that the energy level of the photon is critical. For interstellar hydrogen in the ground state, ultraviolet photons in the Lyman series of energy levels are strongly absorbed by the electrons, as are those about 13.6 or more electron volts which ionize the hydrogen. In these cases the electrons give the illusion of being rather large balls. Other photons of less than about 13.6 electron volts go by virtually uninhibited. For all we know the latter might go through an electron uninhibited, though we must keep in mind that precise numerical statements about positions are meaningless at this scale in keeping with Heisenberg's uncertainty principle.

Do you have the foggiest idea what I am saying? If not, no shame in that. I was plenty bumfuzzled as a college freshman.

In a nutshell, electrons and nuclei are not simple opaque balls for the purpose of understanding how they block photons, or not as the case may be. A given amount of interstellar or intergalactic hydrogen can be opaque to some ultraviolet and almost perfectly transparent to visible light.

normdowling
2007-Oct-06, 02:37 AM
i do have some idea of what u r saying, . but u have gone off on a ridiculus tangent. a rough approximation will do fine for this discussion.

Fortunate
2007-Oct-06, 02:58 AM
normdowling,
Hi. I think grant hutchison's posts, #2 and #8, were succinct and seemed to me to address the crux of your question as I interpreted it. I suggest using them as a starting point.

normdowling
2007-Oct-06, 03:02 AM
the exact amount is of little consequence

normdowling
2007-Oct-06, 03:08 AM
Fortunate,

Thanks very much, Ill keep researching this topic . Maybe super nova google search will be interesting??????

normdowling
2007-Oct-06, 03:13 AM
[QUOTE=Saluki;1082983]It has an effect, but "significant" is not how I would describe that effect.





[PHP] There aren't. A quick google gave me numbers on the order of 1 atom per cubic meter for the density of the universe. Of course those numbers go up within a galaxy, but overall, the universe is a very empty place. Remember that only the nucleus of an atom would effect a photon. Most of an atom itself is empty space.


2 atoms was an approximation. 2 per cubic cm or 1 per cubic meter? Who cares?????

Nereid
2007-Oct-06, 03:39 AM
If there are some atoms in the vacuum of space,however tiny the amount, then wouldnt this have a significant effect on light travelling for very large distances. ????? eg- 10 billion light years????


Dont some photons get absorbed and re emitted ?????

If there were 2 atoms in every square cm of space, then, light travelling for 10 billion years @ 300,000 km per second, would move through a hell of a lot of cm,s , eventually running into some atoms.

Do astronomers have to take into account this possible dimming of light when calculating the distance or age of deep space objects???????In the case of line absorption - whether electron transitions or otherwise - astronomers are very keen to find any of such! :) After all, these tell us a very great deal about the composition of the space through which the photons have passed, so the tales they can tell are very well worth listening to.

Broadband absorption is somewhat more troublesome, but, at least in the case of dust in spiral galaxies, we have one of the world's foremost experts as a regular BAUT member, so perhaps he can give you the assurances you seek.

Various other kinds of such absorption are also most welcome by astronomers, and a great deal of effort is spent measuring it - the Sunyaev-Zel'dovich effect, for example, or the Gunn-Peterson trough, or Lyman-break (high-z) galaxies. One particularly interesting example is Compton (self-)scattering - a fascinating, very modern, area of extragalactic astronomy.

The most difficult of all would be 'grey dust' - stuff between us and some very distance source of photons which absorbed photons in a way that has no wavelength dependence, much less any lines.

I guess the best news here is that, AFAIK, no one has ever found any such dust (of a baryonic kind), either in any lab or in analysis of photons from any distant source. At least, not something grey from gammas to radio waves.

So, in a nutshell, far from not having considered it, the kinds of 'dimming' you refer to are the object of a great many astronomers' intense scrutiny.

normdowling
2007-Oct-06, 03:47 AM
Nereid,

very interesting, Thankyou.

you people are very well informed. Surely you must have studied Physics??????

Or r u just keen amateurs??????

normdowling
2007-Oct-06, 03:53 AM
These great answers lead me to question the accelerating universe theory.

Im sure the astronomers are very clever, im not saying they are all wrong, im just trying to understand why they might be RIGHT.

Cougar
2007-Oct-06, 04:49 PM
The acceleration of the expansion fundamentally rests on the finding that distant Type Ia supernovae reach maximum brightnesses approximately 25 percent fainter than the peak brightnesses they would attain if the expansion was not accelerating.

But how do we know this "25% fainter" isn't due to dimming from dust, etc?

Well, because, as previously mentioned, the differential absorption of different colors of light allows astronomers to recognize the effects of dust grains by spreading the light from a distant supernova into its detailed spectrum.

The cleverness of astronomers never ceases to amaze me. They know their physics, too.

Michael Noonan
2007-Oct-07, 12:11 AM
Certainly the case for direct hit on a proton is low but what about the passage of light between the overall gap between the proton and the volume that an electron could take in at any given time?

Surely on orders of magnitude if there was a reaction there then that would be far more statistically significant perhaps.

Michael Noonan
2007-Oct-07, 12:42 AM
Surely the improbability of a photon hitting anything even when in the abundance of light and matter, say from the sun or a beam of electrons from a CRT tube could be in question due to the tiny space that real matter occupies.

Has that ever been questioned?

Yet when light falls on an object we see it from the surface only. One could presumably think why not struggle to see the object at all or at best as translucent as photons passing into depth and being re-radiated outwards should theoretically give a view of any 'solid' object as being from where ever the photon interacts with the electron and is re-emitted.

Cheers :)

Hornblower
2007-Oct-07, 01:29 AM
Surely the improbability of a photon hitting anything even when in the abundance of light and matter, say from the sun or a beam of electrons from a CRT tube could be in question due to the tiny space that real matter occupies.

Has that ever been questioned?

Yet when light falls on an object we see it from the surface only. One could presumably think why not struggle to see the object at all or at best as translucent as photons passing into depth and being re-radiated outwards should theoretically give a view of any 'solid' object as being from where ever the photon interacts with the electron and is re-emitted.

Cheers :)
Once again, some of us are oversimplifying our mental pictures of the particles in question. The electrons are not merely opaque balls separated by some 100,000 times their diameter. They are bundles of energy with electrostatic and electromagnetic components that get transformed into something vastly different in metals in the solid or liquid states. This is not the place to go into great detail, even if I could remember all the details I studied in college close to 40 years ago. Let's just approximate it roughly by saying that the effective cross section for capturing or reflecting incoming photons is enormously increased, and the wave nature of the light is a big deal.

normdowling
2007-Oct-07, 12:02 PM
ok, Im going to attempt to calculate how many atoms a photon is likely to run into on its ten billion year journey.

I build houses for a living so Im going to need some help.

Firstly, can someone give me the size of a hydrogen atom.
Just make it a point like object,
Please dont talk about wave packets , partical/wave duality, or quantum theories. just come up with a size. No excuses.

my guess is "("less than a cubic meter")

My first physics calculation so be nice.

Michael Noonan
2007-Oct-07, 12:03 PM
Once again, some of us are oversimplifying our mental pictures of the particles in question. The electrons are not merely opaque balls separated by some 100,000 times their diameter. They are bundles of energy with electrostatic and electromagnetic components that get transformed into something vastly different in metals in the solid or liquid states. This is not the place to go into great detail, even if I could remember all the details I studied in college close to 40 years ago. Let's just approximate it roughly by saying that the effective cross section for capturing or reflecting incoming photons is enormously increased, and the wave nature of the light is a big deal.

Most curious as is the brightening effect that light traveling through expanding space is also most curious.

Looking at this phenomenon in my simple way of course it means that light intensifies as the space it travels through expands. For any other substance one needs to apply a compression to intensify or add brightness.

Since this clearly breaks the Law of Thermodynamics then there must be an unlimited and infinite energy potential in the free lunch of harvesting all the light that we will receive in ever greater abundance :)

Michael Noonan
2007-Oct-07, 12:29 PM
The big rip in the expanding universe is not an option if light brightens on the journey through expanding space.

Case in point the standard candles are getting brighter. Next given we exist in space and time. Space and time are curved and that is pure Einstein.

Then as space continues to increase expanding and causing light to intensify there will be a time dilation effect as expanded space expands time.

We will ever more rapidly get a brighter sky from reducing energy and ever less time to experience it it. On that basis we are looking at the ultimate free energy lunch that will have us burnt to a crisp long before the world falls apart due to gravitational failure.

I could be wrong here but to my mind heat death due to an accelerated expansion is to my way of thinking not quite logical.

I would appreciate if this could be better explained please.

normdowling
2007-Oct-07, 12:37 PM
what rubbish is that.

get on topic.

Michael Noonan
2007-Oct-07, 12:58 PM
what rubbish is that.

get on topic.

I appreciate what you are saying but all the points I have raised are mainstream science for once.

If you increase space you increase time otherwise you violate the speed of light.

The standard candles are getting brighter and open space has been confirmed by real scientists to freshen up light, make it brighter.

The universe is experiencing accelerated expansion and that is why we have teams looking at dark energy.

Unfortunately when you add those things together it equals free lunch or more scientifically 'heat death' energy being created from nothing.

normdowling
2007-Oct-07, 01:12 PM
it will end in ice .

How big is a hydrogen atom please

Michael Noonan
2007-Oct-07, 01:33 PM
This site here (http://www.jc-solarhomes.com/photovolt.htm) has a comparison

If we use an orange to represent a positive nucleus an electron would be the size of a dot and have a spherical orbit a mile from the orange.
And this (http://dl.clackamas.cc.or.us/ch104-07/atomic_size.htm) site gives the size in Angstroms

I hope that helps :)

normdowling
2007-Oct-07, 11:42 PM
With regard to the light through a vacuum question being blocked by atoms over long distances?

mybe i was forgetting the constant stream of light being emitted.?????

If a star was turned on then off like a light switch, would this light still make it to a telescope?????????

Kaptain K
2007-Oct-08, 12:13 AM
Yes.

normdowling
2007-Oct-08, 12:17 AM
Yes.

thanks for your straight forward answer, but im hungry for more info

normdowling
2007-Oct-08, 12:19 AM
thanks for your straight forward answer, but im hungry for more info

dont forget im a junior member, and a carpenter not a n astrophysics major

Nereid
2007-Oct-08, 12:27 AM
I appreciate what you are saying but all the points I have raised are mainstream science for once.

If you increase space you increase time otherwise you violate the speed of light.

The standard candles are getting brighter and open space has been confirmed by real scientists to freshen up light, make it brighter.

The universe is experiencing accelerated expansion and that is why we have teams looking at dark energy.

Unfortunately when you add those things together it equals free lunch or more scientifically 'heat death' energy being created from nothing.I'm sorry to say Michael Noonan that this post is so confused that I don't even know where to begin to try to sort it out.

Perhaps you could just ask one, simple, question?

Also, if you don't really know what the standard, modern physics theories - of direct relevance to the questions being asked - are, don't you think it would be better to ask questions, rather than trying to guess?

Perhaps just one thing: one of the common misconceptions about modern cosmology is that 'nothing can go faster than the speed of light'. This Scientific American article (http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf), by Lineweaver and Davis, may help clarify this misunderstanding (the link is to a PDF); here (http://arxiv.org/abs/astro-ph/0310808) is a link to the arXiv preprint of a paper that addresses this in a more technical fashion.

Nereid
2007-Oct-08, 12:42 AM
It may be interesting, and relatively intuitive, to think of atoms and photons having a physical size, and that how much light (photons) is blocked a simple matter of measuring the size, counting how many atoms are along any line that photons travel, and crunching the numbers in a very straight-forward way.

And this is, indeed, similar to a standard way to estimate how much light is blocked ... using something called 'collision cross-sections' (it also works for neutrinos, and much more).

But we've known for over a century that light and atoms don't work like this; an easy example: If you have a source of light, of equal intensity in all wavebands (colours) and wavelengths, shining through a long column of H atoms (not H molecules, nor a plasma of protons and electrons), the classical 'atoms block light' picture would predict that the amount of light blocked would be a nice, slow function of wavelength.

However, that's not what happens. Instead, there are some wavelengths ('lines') where a lot of the light doesn't get through, but it does for the rest (that 'rest' has the classical, slow attenuation function). It's as if the H atom is very much bigger at these few wavelengths than it is at all others.

This is well-understood, using quantum theory, one of two huge breaks with classical physics that was developed around a century ago (the other is relativity). Unfortunately, a great deal of quantum theory is very un-intuitive, so while some aspects of how light and atoms interact can be explained, relatively straight-forwardly, in a quasi-intuitive fashion, some details and consequences can really mess with your head ... so much so that several of the very best physicists who worked on quantum theory have stated that no one really understands it (of course, anyone can apply the equations, turn the handle, and get results which match experimental results to an astonishing extent).

normdowling
2007-Oct-08, 11:57 AM
Ok, OK , OK.

Ive worked it out.

photons and atoms are not point like objects.

Quantum physics is weird.

Light does make it through.

Accelerating unirverse astronomers are not 100 percent sure about these findings based on the light dimming only. they needed other evidence as well.




I dont know what other supporting evidence there is but it must be around.

now my head hurts even more.

THANK YOU..