<p>Your statement "most of the universe is composed of mostly empty space" is very vague. In reality, the Universe is embedded in diffuse background photon fields, from low energy (like the so-called cosmic microwave background which is relic from the big bang) to very high energy (from the extreme compact objects, like AGNs). Of course, there is also unknown dark matter. It seems that dark matter probably only takes part in weak interaction. Thus, it probably won't interact with photons (the same argument with cosmic background neutrinos). The chance of photon-photon interaction and photon-electromagnetic-fields interactions depends on the energy of the photons. For high energies, it is not low as you expected. Even if a photon gets lucky and doesn't interact with the other particles, still it won't reach the edge of the Universe. The reason is that the Universe is expanding.  </p>

CC BY-SA 3.0
2014-10-28T15:57:20.300Z
Correction : "...Universe is expanding." And some parts of it faster than light.
Yashbhatt
CC BY-SA 3.0
2014-10-28T16:46:49.653Z
@Yashbhatt, "some parts of it faster than light."?  Are you saying that some 'local region' of the universe is expanding faster than light?  Or that some objects are moving away from each other faster than light due to space expanding?  The latter is my understanding.  I haven't seen anything confirming the former.
Tracy Cramer
CC BY-SA 3.0
2014-10-28T19:13:08.233Z
I mean due to space eexpanding I.e. recession velocity.
Yashbhatt
CC BY-SA 3.0
2014-10-28T15:45:41.153Z
2014-10-29T13:45:26.970Z
2014-10-29T13:45:26.970Z
Mrk421
<p>Even vacuum is seething with virtual particle pairs, so photons <em>always</em> interact with something. Check Wikipedia on <a href="http://en.wikipedia.org/wiki/Scharnhorst_effect" rel="nofollow">the Scharnhorst effect</a> for a bit of an explanation:</p>

<blockquote>
  <p>Owing to Heisenberg's uncertainty principle, an empty space which appears to be a true vacuum is actually filled with virtual subatomic particles. These are called vacuum fluctuations. As a photon travels through a vacuum it interacts with these virtual particles, and is absorbed by them to give rise to a virtual electron-positron pair. This pair is unstable, and quickly annihilates to produce a photon like the one which was previously absorbed. The time the photon's energy spends as subluminal electron-positron pairs lowers the observed speed of light in a vacuum.</p>
</blockquote>

CC BY-SA 3.0
2014-10-28T17:18:13.803Z
2014-10-28T17:18:13.803Z
Wayfaring Stranger
<p>A photon like that will simply travel forever, since what it find around (other photons and non-baryonic matter) is hard to interact with. But since Universe is expanding, the photon frequency is decreasing.</p>

<p>Look at microwave cosmic background we observe today: those radio photons have been travelling for the entire Age of the Universe until they hit Earth, but they were gamma rays once and are microwaves today.</p>

CC BY-SA 3.0
2014-10-29T14:15:19.480Z
Interesting points but you don't describe a scenario were a photon lasts "forever". If photon frequency is decreasing, what happens as the frequency approaches zero? The known universe is old, but it's not yet *infinitely* old. Consider the analogy that white dwarfs have been cooling for "the entire Age of the Universe". That doesn't mean they will simply last forever.
Robert Cartaino
CC BY-SA 3.0
2014-10-29T21:03:57.187Z
Some simple maths show a photon frecuency approaches zero asintotically, so it will never reach that.
Envite
CC BY-SA 3.0
2014-10-28T16:01:40.797Z
2014-10-28T16:01:40.797Z
Envite
<p>According to absorber theory of Wheeler, photon  is a derived concept. Photon is the action between two charges. The action is called "action at a distance" i.e. aaad. Hence photon must start from an emitter ended on an absorber. Even the photon is just emitted from an emitter, it knowns its ending absorber which perhaps is an eye of a man who will bore in hundreds years latter.</p>

CC BY-SA 3.0
2017-06-23T02:46:54.967Z
2017-06-23T02:46:54.967Z
Ren
<p>Say a photon left one of the first stars ever created in our universe.  This photon has been traveling for 13+billion years through the vastness of space without ever hitting anything.  Sure, it's probably been tugged at by gravitational interactions, but it's still happily traveling at the speed of light without direct interaction with matter. </p>

<p>As we know, most of the universe is composed of mostly empty space.  The odds of any single photon hitting something (like the Earth as starlight, etc) would seem to be very small.</p>

<p>What will eventually happen to this photon?  Will it eventually reach the other end of the universe, and if so, what would happen to it there?</p>

CC BY-SA 3.0
2017-06-23T08:40:05.750Z
Related https://astronomy.stackexchange.com/questions/17990/what-happens-to-the-99-9-of-the-sun-rays-that-do-not-fall-on-any-planets-or-an/
ProfRob
CC BY-SA 3.0
2014-10-29T17:27:23.233Z
`"Will it eventually reach the other end of the universe"` The universe has no "edge". This may be strange and hard to grasp at first, but is quite interesting :) If you are curious about it, I suggest opening a question about the "end" of the universe in http://physics.stackexchange.com.
Geeky Guy
<universe><photons>
What happens to photons that don't interact with something?
Scottie
# What happens to photons that don't interact with something?

(Asked by: username_0 on 2014-10-28)

Say a photon left one of the first stars ever created in our universe. This photon has been traveling for 13+billion years through the vastness of space without ever hitting anything. Sure, it's probably been tugged at by gravitational interactions, but it's still happily traveling at the speed of light without direct interaction with matter.

As we know, most of the universe is composed of mostly empty space. The odds of any single photon hitting something (like the Earth as starlight, etc) would seem to be very small.

What will eventually happen to this photon? Will it eventually reach the other end of the universe, and if so, what would happen to it there?

**Tags:** universe, photons

> **Comments:**
> **username_2**: `"Will it eventually reach the other end of the universe"` The universe has no "edge". This may be strange and hard to grasp at first, but is quite interesting :) If you are curious about it, I suggest opening a question about the "end" of the universe in http://physics.stackexchange.com.  
> **username_1**: Related https://astronomy.stackexchange.com/questions/17990/what-happens-to-the-99-9-of-the-sun-rays-that-do-not-fall-on-any-planets-or-an/  

---

## 4 answers

### Answer by username_3 on 2014-10-28. Score: 2 | Accepted answer

Your statement "most of the universe is composed of mostly empty space" is very vague. In reality, the Universe is embedded in diffuse background photon fields, from low energy (like the so-called cosmic microwave background which is relic from the big bang) to very high energy (from the extreme compact objects, like AGNs). Of course, there is also unknown dark matter. It seems that dark matter probably only takes part in weak interaction. Thus, it probably won't interact with photons (the same argument with cosmic background neutrinos). The chance of photon-photon interaction and photon-electromagnetic-fields interactions depends on the energy of the photons. For high energies, it is not low as you expected. Even if a photon gets lucky and doesn't interact with the other particles, still it won't reach the edge of the Universe. The reason is that the Universe is expanding.

> **Comments:**
> **username_4**: Correction : "...Universe is expanding." And some parts of it faster than light.  
> **username_5**: @username_4, "some parts of it faster than light."?  Are you saying that some 'local region' of the universe is expanding faster than light?  Or that some objects are moving away from each other faster than light due to space expanding?  The latter is my understanding.  I haven't seen anything confirming the former.  
> **username_4**: I mean due to space eexpanding I.e. recession velocity.  

---

### Answer by username_6 on 2014-10-28. Score: 2

Even vacuum is seething with virtual particle pairs, so photons *always* interact with something. Check Wikipedia on [the Scharnhorst effect](http://en.wikipedia.org/wiki/Scharnhorst_effect) for a bit of an explanation:

> Owing to Heisenberg's uncertainty principle, an empty space which appears to be a true vacuum is actually filled with virtual subatomic particles. These are called vacuum fluctuations. As a photon travels through a vacuum it interacts with these virtual particles, and is absorbed by them to give rise to a virtual electron-positron pair. This pair is unstable, and quickly annihilates to produce a photon like the one which was previously absorbed. The time the photon's energy spends as subluminal electron-positron pairs lowers the observed speed of light in a vacuum.


---

### Answer by username_7 on 2014-10-28. Score: 1

A photon like that will simply travel forever, since what it find around (other photons and non-baryonic matter) is hard to interact with. But since Universe is expanding, the photon frequency is decreasing.

Look at microwave cosmic background we observe today: those radio photons have been travelling for the entire Age of the Universe until they hit Earth, but they were gamma rays once and are microwaves today.

> **Comments:**
> **username_8**: Interesting points but you don't describe a scenario were a photon lasts "forever". If photon frequency is decreasing, what happens as the frequency approaches zero? The known universe is old, but it's not yet *infinitely* old. Consider the analogy that white dwarfs have been cooling for "the entire Age of the Universe". That doesn't mean they will simply last forever.  
> **username_7**: Some simple maths show a photon frecuency approaches zero asintotically, so it will never reach that.  

---

### Answer by username_9 on 2017-06-23. Score: 1

According to absorber theory of Wheeler, photon is a derived concept. Photon is the action between two charges. The action is called "action at a distance" i.e. aaad. Hence photon must start from an emitter ended on an absorber. Even the photon is just emitted from an emitter, it knowns its ending absorber which perhaps is an eye of a man who will bore in hundreds years latter.


---

