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mkline55
2012-Sep-21, 01:37 PM
I know that question sounds ridiculously simple, but it's not a question about running out of energy like the tired light theories. It's a question about density. Clearly, I could point a light at the ground, and say it doesn't go very far. In space, though, light travels much farther. How far can it go before it strikes something that absorbs, reflects, refracts or does something else that prevents it from continuing merrily on its way? Never mind gravity in this discussion. I believe this is a statistical question, so choose any of the wavelengths produced by a hydrogen atom. What are the things that could affect the hydrogen light, and what is the maximum distance it could travel before 50% of the light is effectively blocked?

ritwik
2012-Sep-21, 02:06 PM
light needs energy to travel so it loses energy as to go further and it becomes so feeble that we cant detect it eventually it become a part of fabric of cosmos.

Cosmologist
2012-Sep-21, 02:15 PM
We can see Galaxies only a couple of billion years old. Currently light can only travel the distance in light years corresponding to the years the universe has existed. Would a future astronomer with a more advanced telescope be able to look across a trillion light years of space? No idea. At some point interstellar gas has got to soak up those dwindling photons.

antoniseb
2012-Sep-21, 02:34 PM
light needs energy to travel so it loses energy as to go further and it becomes so feeble that we cant detect it eventually it become a part of fabric of cosmos.
Depending on what ritwik means this is either wrong or very misleading.

mkline55
2012-Sep-21, 02:37 PM
At some point interstellar gas has got to soak up those dwindling photons.

That's the part I am interested in. What properties cause light to be effectively blocked by the interstellar medium? Is light from a hydrogen source more likely to be affected by hydrogen atoms? And, given enough distance, which I believe is aplenty in space, how many hydrogen atoms might a beam of light encounter before it was effectively blocked?

antoniseb
2012-Sep-21, 02:39 PM
... What are the things that could affect the hydrogen light, and what is the maximum distance it could travel before 50% of the light is effectively blocked?

Not sure what you mean by wavelengths produced by a Hydrogen Atom... the 21cm hyperfine transition radiation seems like it can pretty much go on forever with no obstacle absorbing a significant fraction of it. The Lyman series hard ultraviolet got blocked before z=12 (and sometimes as recently as z=6), but has been able to go mostly unabsorbed since then, and as the material of the universe gets thinner, that will only get easier to pass through.

mkline55
2012-Sep-21, 03:40 PM
Not sure what you mean by wavelengths produced by a Hydrogen Atom... the 21cm hyperfine transition radiation seems like it can pretty much go one forever with no obstacle absorbing a significant fraction of it. The Lyman series hard ultraviolet got blocked before z=12 (and sometimes as recently as z=6), but has been able to go mostly unabsorbed since then, and as the material of the universe gets thinner, that will only get easier to pass through.

Your response prompted me to read up on 21 cm hyperfine transition radiation and the Lyman series. Thanks. I believe it relates to the original question. Some of the discussions about the 21cm line indicate it is relatively rare, though with hydrogen being so common, it's not that hard to find. Is it rare because it only occurs in the transition between parallel and anti-parallel spins? And, if so, is it also rarely absorbed for the same reason?

ctcoker
2012-Sep-21, 05:32 PM
It's rare because the charasteric lifetime of the excited triplet hyperfine state is so long; IIRC it takes about one million years for your average hydrogen atom to undergo the transition and emit a 21 cm photon. However, because there's a lot of hydrogen in the universe, we see lots of 21 cm emission from HI on large scales. Since the lifetime is so long, the interaction probability between a hydrogen atom and a 21 cm photon is very low, so hydrogen is mostly transparent to 21 cm radiation, further contributing to its visibility.

However, neutral hydrogen is highly opaque to Lyman and Balmer series radiation; it is relatively difficult for ionizing radiation to escape galaxies due to the neutral gas and dust present. The universe as a whole is actually transparent to these wavelengths because the intergalactic gas density is very low and most of that gas is ionized.

As to the original question, the answer is highly dependent on the wavelength of the light, what's in the way, and how much of it there is. Dust is highly opaque to UV and optical light, but virtually transparent to the far IR (coincidentally where cold dust emits most of its radiation). We can see out of the galaxy in the UV and optical only because there isn't very much dust above and below the plane of the galaxy, so it does not completely absorb or scatter the light away. Radio, however, isn't heavily absorbed by much of anything in interstellar space, and can go a very long ways before finally being absorbed.

mkline55
2012-Sep-21, 06:03 PM
Thanks everyone. I appreciate the excellent, informative responses. I'm actually working on a concept, but I can see I have a lot of background work to do first. Now, if things were just a little simpler . . .

cjameshuff
2012-Sep-21, 07:03 PM
Not a simple question...the cosmic microwave background is light emitted at the time the density and temperature of the expanding universe dropped low enough for it to travel long distances. The comoving distance to the surface of last scattering is about 45.7 billion light years, no light has traveled farther than that because there hasn't been time for it to do so. The universe is continuing to expand (the expansion is accelerating, in fact), becoming lower density and cooler, shifting that background radiation to longer wavelengths that interact differently with matter. The cosmic background might eventually redshift into ranges blocked by sparse ionized gases, or those gases may become too sparse and cool to block it, in which case some of the photons emitted will never hit anything.

Jens
2012-Sep-22, 03:23 AM
That's the part I am interested in. What properties cause light to be effectively blocked by the interstellar medium? Is light from a hydrogen source more likely to be affected by hydrogen atoms? And, given enough distance, which I believe is aplenty in space, how many hydrogen atoms might a beam of light encounter before it was effectively blocked?

Others may correct me, but as a complicating factor, I think that when a photon strikes an atom, it is absorbed, but this causes an electron to gain energy and the electron will subsequently release a photon and drop back in energy level. So it's not really that the energy is being lost (energy has to be conserved).

Jens
2012-Sep-22, 03:26 AM
light needs energy to travel so it loses energy as to go further and it becomes so feeble that we cant detect it eventually it become a part of fabric of cosmos.

I'm thinking you might be talking about something else. I don't think light requires energy to travel. But you are probably talking about the background radiation. But I think that is because the photons are redshifted because they are emitted by something traveling away from us.

JohnD
2012-Sep-22, 10:59 AM
Anything will travel for ever unless something gets in its way. Newtons first law. A photon a brick an elephant a planet or a galaxy. No energy is used once an object has velocity. Energy is used to change that velocity or direction.

Is thequestioner wondering abot the black night sky that should shine with the light of an infinite number of stars from all distances to infinity in this infinite universe? I forget the name ofth is paradox that is resolved by the time light takes to get here. It just hasnt arrived yet from the most distant stars.

John

antoniseb
2012-Sep-22, 12:30 PM
... the black night sky that should shine with the light of an infinite number of stars from all distances to infinity in this infinite universe? I forget the name ofth is paradox ...
Olbers' Paradox (http://en.wikipedia.org/wiki/Olbers'_paradox)... but I don't think that's what the OP was asking.

mkline55
2012-Sep-24, 12:57 AM
Olbers' Paradox (http://en.wikipedia.org/wiki/Olbers'_paradox)... but I don't think that's what the OP was asking.

Right. I was asking about how the density of interstellar matter affects light. In other words, if a million photons leave a source at roughly the same time and in approximately the same direction, then how far could they get before half of them had been absorbed, reflected, or otherwise prevented from following a nearly straight line (ignoring gravity and other influences which might affect the path besides direct interaction with matter.)

Amber Robot
2012-Sep-24, 07:26 PM
Right. I was asking about how the density of interstellar matter affects light. In other words, if a million photons leave a source at roughly the same time and in approximately the same direction, then how far could they get before half of them had been absorbed, reflected, or otherwise prevented from following a nearly straight line (ignoring gravity and other influences which might affect the path besides direct interaction with matter.)

Depends on the wavelength of the light.

Jerry
2012-Sep-24, 08:00 PM
The best answer is we do not know. Lyman absorption is somewhat of a brick wall that closes a gateway in redshifted space; but there are at least three other poorly defined parameters: 1) gravitational lensing - we know these lenses reveal quasar and supernova events much deeper in space that we anticipated. 2) Radiation transfer - while most of the time, absorption leads to loss, there are a number of meta-stable states that can intensify; amplifying just as Lyman lines decay. 3) EM lensing - this can be fresnel or conventional lensing that occur randomly. These are currently thought to be trivial effects; but we don't know enough about the gas structure and volume of space to tightly constrain these parameters.

alwis
2012-Sep-27, 07:53 AM
yeah i always wondered how far can the light travel. when we aim the flash light at the night sky. how far can it travel and become feeble

Xibalba
2012-Sep-27, 12:50 PM
Well, theoretically it can travel for eternity, and reach infinite distances, given it has infinite time to do so, and nothing that blocks it.

JohnD
2012-Sep-29, 09:22 PM
As I said above, anything can travel for ever, unless another force acts on it.
Voyager 1 is approaching or has reached the heliopause, the boundary around the Sun where the particles of the Solar wind run out of puff against the combination of forces due to the Sun's gravity, and the movement of the Sun and it's wind through the interstellar gas. The 'pause is about 6x10^12 kilometers away.

Voyager has reached that vicinity in 35 years, a lot more than a photon would. Because it is heavier than a solar wind particle with more inertia it will be less affected by the above and keep on going.

JOhn

mkline55
2012-Oct-03, 06:06 PM
I've read all the responses and waited a few days in hopes of seeing more, but am still waiting for some useful information. If you're not sure what the question was, please read the entire first post. I am asking how far a beam of light can travel provided it does not strike anything massive before the dust in the ISM absorbs a significant portion of it. For some reason, I thought this would be a fairly straight-forward experiment. Aren't there any distance estimates that compare different measuring systems, such as parallax vs. Cepheid variables and color/magnitude or something else? Essentially, I am looking for a loss factor.

NEOWatcher
2012-Oct-03, 06:14 PM
I am asking how far a beam of light can travel provided it does not strike anything massive before the dust in the ISM absorbs a significant portion of it. For some reason, I thought this would be a fairly straight-forward experiment.
No; it's not straight forward. I think Amber Robot's answer should have clued you into refining your question.

ETA:
Here's a thought experiment for you. Why do different telescopes observe in different wavelengths?

Durakken
2012-Oct-03, 06:17 PM
Isn't there equations for the lifespan of particles? I would assume that a photon, which is simply energy/matter converted from some other type of energy/matter, would have a life span that can be figured out for how long a photon can exist before breaking down...

NEOWatcher
2012-Oct-03, 06:33 PM
I'm certainly far from an expert, but from the photon's point of view, no time passes, so how can it break down?
Although; we do know it's at minimum 13 billion years (from our point of view).

Shaula
2012-Oct-03, 07:29 PM
Isn't there equations for the lifespan of particles? I would assume that a photon, which is simply energy/matter converted from some other type of energy/matter, would have a life span that can be figured out for how long a photon can exist before breaking down...
It is not that simple. You have to take into account things like quantum number conservation and suitable decay paths. Upshot is photons don't decay. They can interact with matter and do stuff like pair production, but on their own they don't spontaneously decay in the same way a muon or pion does.

mkline55
2012-Oct-03, 07:35 PM
No; it's not straight forward. I think Amber Robot's answer should have clued you into refining your question.


Thanks for the response, NeoWatcher. The question I have is exactly as originally stated: Choose any of the wavelengths produced by a hydrogen atom. What are the things that could affect the hydrogen light, and what is the maximum distance it could travel before 50% of the light is effectively blocked?

Can you please expand on what you believe I should refine about the question?

Do I need to specify just one wavelength? I think not, because I believe all wavelengths would be absorbed in similar percentages. Do I need to include redshift? Again, I believe not, because in relation to nearby space, there is little or none. In relation to large distance, based on current theory, everything is redshifted, so the probability of a single photon striking just the right atom at just the right time is changed, but it is changed the same for all photons. Differences should only occur when shifts result in matching one original frequency redshifted so it matches a different atomic variation occurring in the distant space. Please excuse my use of incorrect terms, because though I understand the theory, I have not memorized the language.

mkline55
2012-Oct-03, 07:43 PM
Isn't there equations for the lifespan of particles? I would assume that a photon, which is simply energy/matter converted from some other type of energy/matter, would have a life span that can be figured out for how long a photon can exist before breaking down...

I believe that photons continue indefinitely as long as nothing else interferes with them, which is where my question comes in: how long before it's more likely than not that 50% of the photons in a beam of light will have been affected by something?

Durakken
2012-Oct-03, 07:49 PM
Thanks for the response, NeoWatcher. The question I have is exactly as originally stated: Choose any of the wavelengths produced by a hydrogen atom. What are the things that could affect the hydrogen light, and what is the maximum distance it could travel before 50% of the light is effectively blocked?


Wait... a photon can not be 50% blocked. It is either blocked or not. If you mean 50% of the photons from the radiant source then the answer is most likely 0 as most sources are spherical and more than half the light is being blocked, not seen, or reflected before it ever gets to you... Of course this probably isn't want you mean either so...

Do you mean something like... How far can a photon travel before it's "brightness" is halved... as in if you were to be able to have a source to eye transfer it would be 100%, how far back would you have to get before it is half that value? Then that is dependent on wavelength. White light has all wave visible wavelengths but I'd imagine that by the time light gets to you that it has lost 50% of it's "brightness" it is probably going to be yellow/red light rather than white because the red light has been lost, the blue has fallen to yellow, and the yellow to red or something like that... So... yes it does matter what wave length, because different wavelengths can travel different distances before they lose enough energy to be a different wavelength... i think.

NEOWatcher
2012-Oct-03, 07:59 PM
Do I need to specify just one wavelength?
That's a good start. If you look at the[URL=http://www.astronomyknowhow.com/hydrogen-alpha.htm] spectral lines even from a hydrogen atom[/QUOTE], you will see that there's not just one wavelength. And that is just considering the visible range.


I think not, because I believe all wavelengths would be absorbed in similar percentages.
It will depend on the makeup of the space between. Of course hydrogen and helium are good starts since there's so much of it. But; it's going to depend on the density. Like comparing passing through galaxies, where the odds are going to change depending on if you are in a galactic cluster or not.


Do I need to include redshift? Again, I believe not, because in relation to nearby space, there is little or none.
But; we see so much of the light in nearby space. I doubt there is much to absorb that light in nearby space (with the exception of within the galaxy where it will vary greatly)


In relation to large distance, based on current theory, everything is redshifted, so the probability of a single photon striking just the right atom at just the right time is changed, but it is changed the same for all photons.
True; but the change passes it through many different absorption lines as it shifts. So; theres going to be hills and valleys of probability depending on how red-shifted it is.
I wouldn't know how to calculate this, I only know that it needs to be considered.

cjameshuff
2012-Oct-03, 08:20 PM
Do I need to specify just one wavelength? I think not, because I believe all wavelengths would be absorbed in similar percentages.

Er, that's clearly not true. Did you mean to say something else?



Do I need to include redshift? Again, I believe not, because in relation to nearby space, there is little or none.

What do you mean by "in relation to nearby space"? Over the distances photons have been seen to travel, redshift is certainly not "little or none". The background radiation from when the universe cooled enough to become transparent has been redshifted all the way down to the microwave range, from a 3000 K black body (with a peak at the low end of the visible range) to a 2.7 K one, wavelengths being stretched by a factor of 1100.



In relation to large distance, based on current theory, everything is redshifted, so the probability of a single photon striking just the right atom at just the right time is changed, but it is changed the same for all photons. Differences should only occur when shifts result in matching one original frequency redshifted so it matches a different atomic variation occurring in the distant space. Please excuse my use of incorrect terms, because though I understand the theory, I have not memorized the language.

It's not at all clear to me why you think redshift doesn't matter.

Durakken
2012-Oct-03, 08:51 PM
After re-reading the OP.. another viable answer...
You're wrong... well somewhat wrong.
Let's say you take a laser and you somehow get in the middle of two galaxies and shine the lazer towards someone on the other side of the universe... the likelyhood of hitting another thing... well that depends on your definition of thing, but the likelyhood of the light going from point A to point B without 50% of the photons being blocked or whatever is precisely 0... or 100% as we ave seen that even the smallest section of sky that looks completely black to us turns out to be full of galaxies, which is inverse to the distance between atoms and particles so even if you it is impossible for it to hit anything unless you aim it exactly you more than likely wouldn't be able to distinguish it from the rest of the light from whatever path it's crossed...

On the other hand if you are on earth the answer is...i have to presume really small, specially compared to the previous thought... The answer is fairly simple to figure out the max...since the earth doesn't bend light, it's not strong enough to, to make it go around it, at least not much... pick a point on earth and pick a point in the atmosphere where beyond that point it is impossible for light to pass without being refracted or whatever, and then say about half of that because it is just as likely, if not more that light gets no where when it is radiated.

Then again the answer to your question could be the mixture of those two points, but then you have 0 distance and infinite distance... and I'm pretty sure you can't come up with an answer to that lol.

JohnD
2012-Oct-04, 12:01 PM
New Scientist has a regular pull-out feature "Instant Expert", and this week's issue is about cosmic rays.
It includes this, which may be relevant. "Protons of very high energies are unable to travel more than about 150 million light years before interacting with these photons" The photons being those of the Cosmic Microwave Background. This, I read is the GZK cut-off, after Greisen, Zatsepin and Kuz'min.

That is about cosmic ray protons, not photons, but raises the CRB. Those photons have been around since the Big Bang, and they are necessarily travelling at light speed, just like your visible light laser. The distance they have travelled must be immense, infinite by every measure we can make. So I return to Newton's First Law, an object will remain in motion unless acted on by an outside force.
Thus the question ceases to be one of distance or penetration, but of the statistical probabilty that a photon will either strike a solid body or be influenced by its gravity. This must be one for cosmology macromappers, and will entirely depend on where you aim it. Purely becasue of the different density of matter in that neighbourhood, heading for the Great Attractor of the Milky Way will be far more likely to stop it nearer and sooner than aimed, say towards the most distant galaxy so far seen, somewhere in the Hubble Deep Field. But either way, there will still be a small probability that the photon will strike right through the Milky Way or be stopped by a dust particle halfway to Jupiter.

John

Amber Robot
2012-Oct-04, 06:54 PM
Do I need to specify just one wavelength? I think not, because I believe all wavelengths would be absorbed in similar percentages.

This is not true. Interstellar dust has a characteristic wavelength dependence in its extinction properties. It's called an extinction curve. Ultraviolet light is far more heavily extincted than infrared light for the same amount of dust. Further, the shape of the extinction curve varies from sightline to sightline, so it really depends where you're looking. If you were to specify a wavelength, a dust density and an extinction curve shape then it will be easy to determine the distance light could travel before 50% is absorbed (and by that I assume you to mean when exp(-tau_lambda) = 0.5, where tau_lambda is the dust optical depth at wavelength lambda).

mkline55
2012-Oct-04, 08:20 PM
This is not true. Interstellar dust has a characteristic wavelength dependence in its extinction properties. It's called an extinction curve. Ultraviolet light is far more heavily extincted than infrared light for the same amount of dust. Further, the shape of the extinction curve varies from sightline to sightline, so it really depends where you're looking. If you were to specify a wavelength, a dust density and an extinction curve shape then it will be easy to determine the distance light could travel before 50% is absorbed (and by that I assume you to mean when exp(-tau_lambda) = 0.5, where tau_lambda is the dust optical depth at wavelength lambda).

Thanks, Amber Robot! That was just the lead I needed. A quick search turned up this link (http://astro.pas.rochester.edu/~aquillen/ast142/Lecture/Lecture12.pdf) which has an excellent description of exactly the kind of information I was seeking. Now, I'm off to better understand what the tau, lambda and those other thingies are doing.

Amber Robot
2012-Oct-04, 09:17 PM
Thanks, Amber Robot! That was just the lead I needed. A quick search turned up this link (http://astro.pas.rochester.edu/~aquillen/ast142/Lecture/Lecture12.pdf) which has an excellent description of exactly the kind of information I was seeking. Now, I'm off to better understand what the tau, lambda and those other thingies are doing.

One of the big areas of research in interstellar dust is to try to determine the composition of the dust from the extinction curve. If you think you know what elements are available in the dust phase and you think you know what the optical properties of various kinds of dust are, you can attempt to fit both the relative composition and dust size distributions for a given sightline based on it's measure extinction as a function of wavelength.

mkline55
2012-Oct-16, 01:30 PM
In further answer to my own question, and in case someone else is curious, this link (http://en.wikipedia.org/wiki/Interstellar_extinction) provides an overview of extinction.

WayneFrancis
2012-Oct-17, 12:48 AM
light needs energy to travel so it loses energy as to go further and it becomes so feeble that we cant detect it eventually it become a part of fabric of cosmos.

This is not correct. The reason things further away are dimmer isn't because the photons loose energy as much as the further away the object is the less photons will intersect your collector based off the inverse square law rule. Let us take something at the small scale where cosmic inflation does not have any appreciable effect. At a arbitrary distance of 1[insert unit of your choice] the sun seems to have a apparent brightness of 1. At a distance of 2 it has a apparent brightnessof .25. At a distance of 3 it has a apparent brightness of .11111111. At a distance of 4 it has a apparent brightness of .0625.

Now the individual photon at a distance of 1 has the same amount of energy as the individual photon at a distance of 4. It doesn't matter if your metric is in millimetres or a billion light years. The difference is in the number of photons you receive over your entire collection area. Want the apparent brightness to be what it would be at a distance of 4 as it would be at a distance of 1? Then your collection area must be 16x larger.

Cosmic expansion does cause photons to loose energy but this is because the photon is travelling through a medium who's metric is changing.

WayneFrancis
2012-Oct-17, 12:57 AM
To the OP. The furthest a photon can currently travel is ~14.75 billion light years. In the future it will be further in direct proportion to the age of the universe. The CMBR are photons from just a few hundred thousand years after the start of the universe. They fill the sky even with everything that is between where they where emitted and where we are now. Space is mostly empty even looking at something like lead. For example lead is almost completely transparent to neutrinos as a neutrino can easily pass through a light year worth of led without hitting anything. Photons from the CMBR are to weak to actually interact with electrons so that effect is simply off the table. When the universe is 30 billion years old there will be plenty of photons that would have travelled 30 billion light years.

WayneFrancis
2012-Oct-17, 01:12 AM
That's the part I am interested in. What properties cause light to be effectively blocked by the interstellar medium? Is light from a hydrogen source more likely to be affected by hydrogen atoms? And, given enough distance, which I believe is aplenty in space, how many hydrogen atoms might a beam of light encounter before it was effectively blocked?

Lets take a photon at 160.2 GHz. That photon will, as far as I'm aware. not interact with ANY of the electrons in a hydrogen atom, or any other atom, ever. The nucleus of the atom is prevented from absorbing the photon also but it can reflect the photon. But when you look at the area that atomic nuclei actually occupy even spanned over tens of billions of light years it amounts to VERY little.

So effectively as time goes on and the universe gets less and less dense the distance a photon can statistically travel before getting reflected actually goes up. IE the longer the universe is around the further photons can travel freely. Not very intuitive until you think that the photons have been dealing with the same amount of energy in the universe since the beginning and for the first ~300,000 years it was much to dense for photons to travel far at all. But after that the density hit a critical point where the universe basically became transparent.

WayneFrancis
2012-Oct-17, 01:20 AM
Right. I was asking about how the density of interstellar matter affects light. In other words, if a million photons leave a source at roughly the same time and in approximately the same direction, then how far could they get before half of them had been absorbed, reflected, or otherwise prevented from following a nearly straight line (ignoring gravity and other influences which might affect the path besides direct interaction with matter.)

Like others have said, it depends on the wave length. Photons that make up our CMBR? Well put yourself away from Earth where the sun is not right behind you and I'd guess that over 99% of the CMB photons that pass to your left and right will probably still be travelling unimpeded for effectively for ever.

WayneFrancis
2012-Oct-17, 01:28 AM
Thanks for the response, NeoWatcher. The question I have is exactly as originally stated: Choose any of the wavelengths produced by a hydrogen atom. What are the things that could affect the hydrogen light, and what is the maximum distance it could travel before 50% of the light is effectively blocked?

Can you please expand on what you believe I should refine about the question?

Do I need to specify just one wavelength? I think not, because I believe all wavelengths would be absorbed in similar percentages. Do I need to include redshift? Again, I believe not, because in relation to nearby space, there is little or none. In relation to large distance, based on current theory, everything is redshifted, so the probability of a single photon striking just the right atom at just the right time is changed, but it is changed the same for all photons. Differences should only occur when shifts result in matching one original frequency redshifted so it matches a different atomic variation occurring in the distant space. Please excuse my use of incorrect terms, because though I understand the theory, I have not memorized the language.

But there becomes a point where a photon doesn't have enough energy to interact with any bound electron and as I understand it free electrons that are not constrained, like they are in a mirror, would also not interact with the photon. This means your photon ends up only being effected by reflection of atomic nuclei an that isn't very much space. Take, again, the fact that a neutrino can pass through a light year of lead without interacting with anything.