<p>The light from the Sun spreads, at least initially, in a roughly isotropic fashion into the universe.</p>
<p>As it gets further from the Sun, <em>some</em> of that light will interact with the interstellar medium (ISM) and therefore <em>some</em> of the energy emitted by the Sun will be used to excite atoms and molecules or even ionise some atoms. This will be the fate of almost all the light which is emitted from the Sun in the direction of the plane of our Galaxy, which contains sufficient molecular gas and dust to block starlight travelling through it for any distance. We know this happens because we can &quot;see&quot; dark clouds in the Milky Way, that can be penetrated by longer wavelength radiation to reveal all the billions of Sun-like stars that lie behind them. Roughly speaking, about half the visible light from the Sun will be absorbed every 1000 light years when travelling in directions within <span class="math-container">$\pm 5$</span> degrees of the Galactic plane, so it is essentially all absorbed within a few thousand light years.</p>
<p>But most of the Sun's light is <em>not</em> travelling in the direction of the Galactic plane, and interstellar and intergalactic space has a <em>very</em> low density of gas and dust. The equivalent extinction number for the intergalactic medium is that light travels many billions of light years with almost no chance of being absorbed (see <a href="https://arxiv.org/abs/1005.4406" rel="noreferrer">Zu et al. 2010</a>).</p>
<p>This means that most of the light from the Sun will travel to cosmological distances (billions of light years) over the course of the next billions of years. Indeed, light emitted from the Sun shortly after its birth has already travelled 4.5 billion light years. That this happens is demonstrated by the fact that <em>we</em> can observe galaxies (the light from which is nothing more than the summation of light from many stars like the Sun) that are 4.5 billion (and more) light years away.</p>
<p>As the sunlight travels towards cosmological distances, its wavelength is &quot;stretched&quot; by the expansion of the universe, becoming redder and redder. We know this happens because distant galaxies have redshifted spectra. If the universe keeps expanding, then its matter density will continue to decrease and there is little to stop the radiation from the Sun travelling on forever, with a wavelength that scales as the scale factor, <span class="math-container">$a$</span>, of the universe.</p>
<p>We could consider a co-moving and co-expanding cube containing the Sun's radiation as the universe expands. The total radiative energy inside that cube diminishes as <span class="math-container">$a^{-1}$</span> - that is, the energy content of the universe in the form of radiation from stars (and other sources) becomes energetically less important as the universe expands and appears to be being superseded by the energy contained in the vacuum itself (a.k.a. dark energy).</p>
<p>In conclusion, most of the energy emitted by the Sun is not &quot;used&quot; for anything; it propagates into space, becoming more and more dilute and less energetically important as cosmic time marches on.</p>

CC BY-SA 3.0
2016-08-21T07:27:23.510Z
Am I incorrect to point out that light emitted 4.5 billion years ago has indeed (from its reference) traveled 4.5 billion lightyears, but is much more than 4.5 billion lightyears as measured in ours?
Nij
CC BY-SA 3.0
2016-08-21T08:13:58.257Z
@Nij yes I think you are correct, the "wavefront" ends up being more than 4.5 billion light years away because of the expansion of space.
ProfRob
CC BY-SA 3.0
2016-08-22T08:34:47.197Z
The force of gravity is infinite, but it get exponentially weaker. Wouldn't photons orbit everything else, and the universe eventually get a ring of photons much in the same way that Saturn has dust rings?
BlackCap
CC BY-SA 3.0
2016-08-22T12:40:27.513Z
It gets only quadratically weaker, not exponentially. Also, light can escape from most gravitational wells -- light leaves Earth just fine, and can go on forever! By definition, only black holes can confine light to orbit them. BUT all that's irrelevant: because there is no "center" of the universe! The matter is spread across infinitely in every direction, like an infinite chess board where 1/100th of the squares are filled up with matter and galaxies. The light just keeps seeing more expanding space, with matter inside, but there's no central force for it to orbit.
Alex Meiburg
CC BY-SA 3.0
2016-08-22T14:30:28.047Z
@Nij from lights reference it hasn't moved at all (length contraction) and no time has passed to get there
Steve Cox
CC BY-SA 3.0
2016-08-22T15:55:15.573Z
The sentence "we can observe galaxies ... that are 4.5 billion (and more) light years away" makes no sense to me. We can also observe galaxies which are closer, and the age of Sun has no influence on what we can observe.
Dmitry Grigoryev
CC BY-SA 3.0
2016-08-22T16:40:49.313Z
@DmitryGrigoryev It demonstrates that light emitted by stars can and does travel more than 4.5 billion (and more) light years without being absorbed by anything.
ProfRob
CC BY-SA 4.0
2016-08-20T17:35:20.077Z
2021-10-02T07:30:09.927Z
2021-10-02T07:30:09.927Z
ProfRob
<p>You want nature to be frugal and efficient. You want all the energy of the sun to have a purpose. However what you <strong>want</strong> nature to be like has no bearing on what it <strong>is</strong>. </p>

<p>The light from the sun is a colossal amount of energy in human terms, but very minor in comparison to the rest of the universe. The light that didn't fall onto anything left the solar system and was never "used".</p>

<p>The root of your misunderstanding is that you think that the sun has a purpose. <a href="http://zeus.colorado.edu/astr1040-toomre/Lectures/Lecture06--30jan14.pdf" rel="noreferrer">It is a ball of plasma that emits energy</a>. See for example Ernst Mayr's <a href="http://www.informationphilosopher.com/solutions/scientists/mayr/Teleology.pdf" rel="noreferrer">essay on Teleology</a></p>

CC BY-SA 3.0
2016-08-22T13:45:35.077Z
Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/44306/discussion-on-answer-by-james-k-what-happens-to-the-99-9-of-the-sun-rays-that).
called2voyage
CC BY-SA 3.0
2016-08-20T15:40:09.627Z
2016-08-22T18:04:07.847Z
2016-08-22T18:04:07.847Z
James K
<p>Entropy is a fundamental condition of our universe, and has been recognized as such since as long ago as Newton's laws of thermodynamics.</p>

<p><strong>Entropy:</strong> Order does not increase over time, it decreases, except locally with the expenditure of energy. This expenditure trades off increased disorder elsewhere for increased order locally, and the tradeoff is always negative: The amount of order gained is always smaller than the amount of disorder created.</p>

<p>Each emitted photon either keeps moving until the end of Time as we know it, slowly losing energy as the cosmos expands, or else, it interacts with other particles along the way. However, the notion that those interactions, or the lack of interactions, implies greater and lesser degrees of Utility or of Purpose or of Destiny is a metaphysical one, not an astronomy question or even a physics question.</p>

CC BY-SA 3.0
2016-08-21T19:16:05.400Z
2016-08-21T19:38:22.987Z
2016-08-21T19:38:22.987Z
Beanluc
<p>I assume that about 99.9% of the sun-rays that do not fall on any planet or any other celestial body keep on traveling farther and farther unto infinity. Apparently such rays get lost. Keeping in mind the colossal energy Sun has produced since 4.5 billion years I am somehow reluctant to  reconcile myself to the idea that Nature would have allowed wastage of so much energy produced by the Sun. Nonetheless I want to get enlightened whether it got really lost or got utilized. If it got utilized I want to know how it may have got utilized at all and whether any sustainable evidence is available in support of any such finding?  </p>

CC BY-SA 3.0
2016-08-22T13:13:54.603Z
Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/44304/discussion-on-question-by-s-c-sawhney-what-happens-to-the-99-9-of-the-sun-rays).
called2voyage
CC BY-SA 3.0
2016-08-22T14:44:37.260Z
I'd guess you can probably add a few more 9s to the end of that
David says Reinstate Monica
CC BY-SA 3.0
2016-08-22T15:52:46.877Z
What is your definition of "utilized?"  Is it merely that it causes a change of energy from light to some other form?  Do remember that every machine we have ever created to "utilize" energy increases entropy.
Cort Ammon
CC BY-SA 3.0
2016-08-22T22:34:20.213Z
Lots of good information here as well: https://en.wikipedia.org/wiki/Olbers%27_paradox
Rag
CC BY-SA 3.0
2016-08-22T23:25:58.963Z
Mother Nature don't care.
Dan Dascalescu
CC BY-SA 3.0
2016-08-23T06:56:54.607Z
Do not anthropomorphize nature. Nature does not have a mind, a will or a sense of "balance". Nature knows only one thing: the laws of nature, i.e. physics. In there you will find no such thing as a "Oh, must not waste anything!". Instead, the most final and definite law of nature is that of Entropy. Entropy means: every action generates waste. **Waste heat** to be exact. And in the end, the universe as we know it will be entirely dead because there is nothing but waste heat left, with no difference in energy potential left to utilize for things like sunlight, work or even **life**.
MichaelK
CC BY-SA 3.0
2016-08-23T07:13:24.347Z
@DavidGrinberg I'd probably just round it right up to 100% myself.
Williham Totland
CC BY-SA 3.0
2016-08-23T07:27:37.283Z
@MichaelKarnerfors If nature has a will, its one purpose is it spread things out as thinly as possible.
Aron
CC BY-SA 3.0
2016-08-23T07:30:38.290Z
@Aron To be pedantic and very boring I will translate that to: "If nature has a will, we cannot know it, but assuming that this will exists and assuming that this will is to spread things out as evenly as possible, those assumptions fit very well with observed data".
MichaelK
CC BY-SA 3.0
2016-11-09T17:09:05.563Z
@MichaelKarnerfors As a boring pedant, I'm surprised that you consider the statement that Nature doesn't care to be an anthropomorphization. The sentiment which DanDascalescu expressed is not contrary to your own. Now, if it had been said that "Nature cares about other things but not this..."
Beanluc
CC BY-SA 3.0
2016-11-09T17:10:30.270Z
@Beanluc The comment was to OP. :)
MichaelK
CC BY-SA 4.0
2021-07-12T02:19:17.367Z
Is the amount of sunlight that hits other stars comparable to the amount of starlight that hits the Sun?
Keith McClary
<the-sun><light><astrophysics>
What happens to the 99.9 % of the sun-rays that do not fall on any planets or any other celestial body?
S C Sawhney
# What happens to the 99.9 % of the sun-rays that do not fall on any planets or any other celestial body?

(Asked by: username_0 on 2016-08-20)

I assume that about 99.9% of the sun-rays that do not fall on any planet or any other celestial body keep on traveling farther and farther unto infinity. Apparently such rays get lost. Keeping in mind the colossal energy Sun has produced since 4.5 billion years I am somehow reluctant to reconcile myself to the idea that Nature would have allowed wastage of so much energy produced by the Sun. Nonetheless I want to get enlightened whether it got really lost or got utilized. If it got utilized I want to know how it may have got utilized at all and whether any sustainable evidence is available in support of any such finding?

**Tags:** the-sun, light, astrophysics

> **Comments:**
> **username_1**: Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/44304/discussion-on-question-by-s-c-sawhney-what-happens-to-the-99-9-of-the-sun-rays).  
> **username_2**: I'd guess you can probably add a few more 9s to the end of that  
> **username_3**: What is your definition of "utilized?"  Is it merely that it causes a change of energy from light to some other form?  Do remember that every machine we have ever created to "utilize" energy increases entropy.  
> **username_4**: Lots of good information here as well: https://en.wikipedia.org/wiki/Olbers%27_paradox  
> **username_5**: Mother Nature don't care.  
> **username_6**: Do not anthropomorphize nature. Nature does not have a mind, a will or a sense of "balance". Nature knows only one thing: the laws of nature, i.e. physics. In there you will find no such thing as a "Oh, must not waste anything!". Instead, the most final and definite law of nature is that of Entropy. Entropy means: every action generates waste. **Waste heat** to be exact. And in the end, the universe as we know it will be entirely dead because there is nothing but waste heat left, with no difference in energy potential left to utilize for things like sunlight, work or even **life**.  
> **username_7**: @DavidGrinberg I'd probably just round it right up to 100% myself.  
> **username_8**: @username_6arnerfors If nature has a will, its one purpose is it spread things out as thinly as possible.  
> **username_6**: @username_8 To be pedantic and very boring I will translate that to: "If nature has a will, we cannot know it, but assuming that this will exists and assuming that this will is to spread things out as evenly as possible, those assumptions fit very well with observed data".  
> **username_9**: @username_6arnerfors As a boring pedant, I'm surprised that you consider the statement that Nature doesn't care to be an anthropomorphization. The sentiment which DanDascalescu expressed is not contrary to your own. Now, if it had been said that "Nature cares about other things but not this..."  
> **username_6**: @username_9 The comment was to OP. :)  
> **username_10**: Is the amount of sunlight that hits other stars comparable to the amount of starlight that hits the Sun?  

---

## 3 answers

### Answer by username_11 on 2016-08-20. Score: 70

The light from the Sun spreads, at least initially, in a roughly isotropic fashion into the universe.

As it gets further from the Sun, *some* of that light will interact with the interstellar medium (ISM) and therefore *some* of the energy emitted by the Sun will be used to excite atoms and molecules or even ionise some atoms. This will be the fate of almost all the light which is emitted from the Sun in the direction of the plane of our Galaxy, which contains sufficient molecular gas and dust to block starlight travelling through it for any distance. We know this happens because we can "see" dark clouds in the Milky Way, that can be penetrated by longer wavelength radiation to reveal all the billions of Sun-like stars that lie behind them. Roughly speaking, about half the visible light from the Sun will be absorbed every 1000 light years when travelling in directions within $\pm 5$ degrees of the Galactic plane, so it is essentially all absorbed within a few thousand light years.

But most of the Sun's light is *not* travelling in the direction of the Galactic plane, and interstellar and intergalactic space has a *very* low density of gas and dust. The equivalent extinction number for the intergalactic medium is that light travels many billions of light years with almost no chance of being absorbed (see [Zu et al. 2010](https://arxiv.org/abs/1005.4406)).

This means that most of the light from the Sun will travel to cosmological distances (billions of light years) over the course of the next billions of years. Indeed, light emitted from the Sun shortly after its birth has already travelled 4.5 billion light years. That this happens is demonstrated by the fact that *we* can observe galaxies (the light from which is nothing more than the summation of light from many stars like the Sun) that are 4.5 billion (and more) light years away.

As the sunlight travels towards cosmological distances, its wavelength is "stretched" by the expansion of the universe, becoming redder and redder. We know this happens because distant galaxies have redshifted spectra. If the universe keeps expanding, then its matter density will continue to decrease and there is little to stop the radiation from the Sun travelling on forever, with a wavelength that scales as the scale factor, $a$, of the universe.

We could consider a co-moving and co-expanding cube containing the Sun's radiation as the universe expands. The total radiative energy inside that cube diminishes as $a^{-1}$ - that is, the energy content of the universe in the form of radiation from stars (and other sources) becomes energetically less important as the universe expands and appears to be being superseded by the energy contained in the vacuum itself (a.k.a. dark energy).

In conclusion, most of the energy emitted by the Sun is not "used" for anything; it propagates into space, becoming more and more dilute and less energetically important as cosmic time marches on.

> **Comments:**
> **username_12**: Am I incorrect to point out that light emitted 4.5 billion years ago has indeed (from its reference) traveled 4.5 billion lightyears, but is much more than 4.5 billion lightyears as measured in ours?  
> **username_11**: @username_12 yes I think you are correct, the "wavefront" ends up being more than 4.5 billion light years away because of the expansion of space.  
> **username_13**: The force of gravity is infinite, but it get exponentially weaker. Wouldn't photons orbit everything else, and the universe eventually get a ring of photons much in the same way that Saturn has dust rings?  
> **username_14**: It gets only quadratically weaker, not exponentially. Also, light can escape from most gravitational wells -- light leaves Earth just fine, and can go on forever! By definition, only black holes can confine light to orbit them. BUT all that's irrelevant: because there is no "center" of the universe! The matter is spread across infinitely in every direction, like an infinite chess board where 1/100th of the squares are filled up with matter and galaxies. The light just keeps seeing more expanding space, with matter inside, but there's no central force for it to orbit.  
> **username_15**: @username_12 from lights reference it hasn't moved at all (length contraction) and no time has passed to get there  
> **username_16**: The sentence "we can observe galaxies ... that are 4.5 billion (and more) light years away" makes no sense to me. We can also observe galaxies which are closer, and the age of Sun has no influence on what we can observe.  
> **username_11**: @DmitryGrigoryev It demonstrates that light emitted by stars can and does travel more than 4.5 billion (and more) light years without being absorbed by anything.  

---

### Answer by username_17 on 2016-08-20. Score: 39

You want nature to be frugal and efficient. You want all the energy of the sun to have a purpose. However what you **want** nature to be like has no bearing on what it **is**.

The light from the sun is a colossal amount of energy in human terms, but very minor in comparison to the rest of the universe. The light that didn't fall onto anything left the solar system and was never "used".

The root of your misunderstanding is that you think that the sun has a purpose. [It is a ball of plasma that emits energy](http://zeus.colorado.edu/astr1040-toomre/Lectures/Lecture06--30jan14.pdf). See for example Ernst Mayr's [essay on Teleology](http://www.informationphilosopher.com/solutions/scientists/mayr/Teleology.pdf)

> **Comments:**
> **username_1**: Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/44306/discussion-on-answer-by-james-k-what-happens-to-the-99-9-of-the-sun-rays-that).  

---

### Answer by username_9 on 2016-08-21. Score: 10

Entropy is a fundamental condition of our universe, and has been recognized as such since as long ago as Newton's laws of thermodynamics.

**Entropy:** Order does not increase over time, it decreases, except locally with the expenditure of energy. This expenditure trades off increased disorder elsewhere for increased order locally, and the tradeoff is always negative: The amount of order gained is always smaller than the amount of disorder created.

Each emitted photon either keeps moving until the end of Time as we know it, slowly losing energy as the cosmos expands, or else, it interacts with other particles along the way. However, the notion that those interactions, or the lack of interactions, implies greater and lesser degrees of Utility or of Purpose or of Destiny is a metaphysical one, not an astronomy question or even a physics question.


---

