0
$\begingroup$

If there was a Universe with nothing but photons, and these photons were all left-circularly polarized, would it stay like this?

(I know this is possible since in a lab you can create a beam of left-circularized light for use with 3D glasses).

But on the other hand we know that massless spin-1 particles have 2 quantum states (which is partly what makes them spin-1 and not spin-0) particles.

So I see an apparent contradiction. In that a masslesss spin-1 particle must come in two states, yet, it seems like it is consistent to have a Universe which only has one of those states.

Where is my mistake in my reasoning?

Improve this question
$\endgroup$

2 Answers 2

Reset to default
1
$\begingroup$

Your reasoning is sound.

One consequence of this is: a reflection cannot happen. Reflection in a mirror changes a left-hand polarized light beam to a right-hand beam (presumably, it amounts to absorbing the left-hand photons, and generating right-hand photons). The hypothetical universe would be a very strange place, indeed, if all photon/matter interactions didn't include backscatter.

Such a universe would be devoid of us humans (the glint in your eye is a reflection), and rather uninteresting for that reason. Still, since photon/photon interaction is very small (gravity and such), if it were populated only with left-handed photons to begin with, it could age without generating right-handed ones.

The possibility of both handedness of photons does not intrinsically mean that both are present (similarly, the antimatter possibility does not mean that our universe contains large amounts of that, either). It only says that a model of the photon as an EM wave, and the known photon/matter interactions, allows two spins, but not a zero-spin particle, for a photon (light quantum).

Improve this answer
$\endgroup$
1
  • $\begingroup$ One thing that confused me was a vector field is supposed to "transform as a vector". Yet if a photon existed only in one polarisation would it be a scalar field? Or would it still be represented as a vector? Or perhaps each polarisation is represented by a spinor separately, but together they can be represented by a vector? $\endgroup$
    – user84158
    Oct 21, 2020 at 16:39
0
$\begingroup$

You first stated that you wanted a universe setup so that there are only photons with left circular polarisation, but then you assumed that all these photons exist in other possible polarisation states then called this a contradiction.

Photons exist in either left or right circular polarisation or a superposition of these or they can exist as up-down or left-right polarisation or a superposition of these.

You can't state a system has photons in a definite particular state (and that these states do not evolve in time) then assume simultaneously they may exist in other states.

And if all the photons existed in one particular state, there is no reason why they will not evolve into other possible states later on.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.