A hypothetical universe where only photons exist?

Question

dear great minds out there, can you noodle this?

if the universe were comprised of photons only, would the photons interact with one another, and if so, how?

Is this question answerable? If yes, then can the answer be given in layman's terms?

Many thanks!

Answer 1

Photons are the particle representing the electromagnetic field. This means, your universe has no matter (yet), but a strong electromagnetic field.

Now, your universe is empty. But this means, it has a strong vacuum expectation value, creating relatively many matter-antimatter pairs. These short-lived particles can interact in many thinkable ways with your photons.

Finally, even without your energy it should be resulting in matter - after a process I can't really understand myself yet, but well, I still have hope.  

Answer 2

They would travel at the speed of light, in straight lines. They would not collide, because they are bosons.

Photons transmit the electromagnetic force. Just as electrons can interact by exchanging photons (which is the basis of quantum electrodynamics, QED, an excellent explanation of which can be found in the Feynman book of the same title), photons can interact by exchanging electrons. However, since electrons don't exist...

Photons would also not interact gravitationally in such a universe, at least I'm sure this would be the opinion of most leading experts in particle physics. A relativistic puritan would insist that the photons can still bend space by virtue of their energy, but most physicists believe that gravitons are needed to mediate the gravitational force.

It would be a pretty boring universe. The photons wouldn't even red-shift, as far as I know,  because that would violate the conservation of energy.

Answer 3

There was a time in the very early universe when there were only photons.  The confinement temperature of the quark is about 10.1 trillion degrees, so when the universe cooled below that point, matter began to phase change out of pure energy.  It would have been a smooth and homogeneous universe (no "clumping") up to that point, however, since photons don't interact directly with each other.  They do leave wave amplitude interference patterns, but they're only detectable after they are absorbed by matter.

Answer 4

What's emitting the photons?

And as they are essentially massless, why would they interact with each other, aside from a random collision?

Answer 5

the string and m theories tell of diferent universes one of which is ours which used to be all empty, and one full of quarks which collided with ours and Bang or rather Big Bang it also tells of millions of universes full of all kinds of diferent stuff, there is no reason to dissmis a pure photon universe.

Answer 6

Well, I guess we're assuming light would behave in the same manner in the hypothetical univrse as it does here, and that all laws (the speed of light, for example) still apply....

If that was the case, then they'd interact with each there there the same way they do here - interfering with each other on occasion, and continuing to travel in straight lines - only, without matter, they'd travel forever, never running into anything.  Without observers, and nothing to reflect off of, and no source to generate them - it would be a *very* dark universe.