# Does light wave never collaspe?

1. Observation means interaction in QM.
2. Photon doesn't interact with gravity or electromagnetic force.
3. Thus, light wave is never observed and will therefore maintain superposition.

Is this deduction correct?

## Answers and Replies

scilover89 said:
1. Observation means interaction in QM.
Theoretically considered
Observation in QM mechanics means that your state vector $\Psi$
collapses to one of the eigenvectors $\Phi_{n}$ of the observable (operator).
For example if you measure the energy of an electron in the hydrogen atom,
your state $\Psi$ collapses into one of the energy eigenstates
of the Hamilton-operator ($H \Phi_{n} = E_{n} \Phi_{n}$)
(That's an axiom of QM)

Ok the above example is not about the photon's superposition but it shows
you what measurement means in QM.

So here an example for photons:
An example would be polarizers for light. Say we have a polarizer at 45 degree to the original light polarization. Then there's a certain probability that the wavefunction will collaps into the state that can pass the polarizer.

scilover89 said:
2. Photon doesn't interact with gravity or electromagnetic force.
I am not sure about that. Physicists believe that photons are for example attracted by a black hole due to its great gravity.

scilover89 said:
3. Thus, light wave is never observed and will therefore maintain superposition.
Light is obviously observed because we can see it with our eyes..hmm..
or look at this site: http://www.hqrd.hitachi.co.jp/em/doubleslit.cfm [Broken]
You can see the photon 'dots' on the screen.

(see my answer to 1)

I think that you are asking yourself how exactly the photon is interacting
with matter, for example photons are scattered by electrons, they are absorbed and emitted by an electron in an atom and so on.

I think someone else can explain that to you with QED (unfortunately I can't tell you anything about QED).

-Edgardo

Last edited by a moderator:
scilover, you are confusing two different aspects of physics here. When we say light is of a wave nature, what we really mean is that it's momentum eigenstate has a phase dependence which repeats after $$2\pi$$. This is not the same as the wavefunction which, in general, could be of any shape. It is the wavefunction which collapses.

Secondly, the interaction due to gravity is a different kettle of fish since gravity manifests itself in terms of spacetime curvature. Since all objects inhabit spacetime, they all feel a gravitational "force".

dextercioby
Homework Helper
scilover89 said:
1. Observation means interaction in QM.
2. Photon doesn't interact with gravity or electromagnetic force.
3. Thus, light wave is never observed and will therefore maintain superposition.

Is this deduction correct?
No.The bolded statement is false.Photons interact with gravity.And certainly among themselves in QFT (for example QED,where they scatter one on another virtual electron-positron fields...)

The underlined statement is false as well.Light can be observed.As you've been given the example with the polarizers;helicity states can descrbe the quantum state of the EM field.And yes,these states collapse.

Daniel.