# What happens to photons that don't interact with something?

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?

• "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 physics.stackexchange.com. – Geeky Guy Oct 29 '14 at 17:27
• – ProfRob Jun 23 '17 at 8:40

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.

• Correction : "...Universe is expanding." And some parts of it faster than light. – Yashbhatt Oct 28 '14 at 15:57
• @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 Oct 28 '14 at 16:46
• I mean due to space eexpanding I.e. recession velocity. – Yashbhatt Oct 28 '14 at 19:13

Even vacuum is seething with virtual particle pairs, so photons always interact with something. Check Wikipedia on the 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.

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.

• 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 Oct 29 '14 at 14:15
• Some simple maths show a photon frecuency approaches zero asintotically, so it will never reach that. – Envite Oct 29 '14 at 21:03

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.