# How directional is a single photon?

## Main Question or Discussion Point

A photon's momentum vector points in the direction of its propagation but interacts with particles off its axis. How this directional preference is revealed by QM? Is there an ontological picture of the photon's propagation?

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bhobba
Mentor
A photon's momentum vector points in the direction of its propagation but interacts with particles off its axis.
Why do you say that?

Thanks
Bill

naima
Gold Member
A photon may have a definite momentum direction (plane wave) bur when it passes thru a pinhole, the wave becomes spherical.
Then every direction have the same probability.

Why do you say that?

Thanks
Bill
O.k., so this is not the case since the direction is defined at the moment of the interaction with the target (or if the recoiled source hits something) and the consequent simultenous definition of the recoil direction of the source by entaglement, right? My question remains though: how reflection is possible or how hits on one side of the mirror are privileged?
Hope this reply makes sense :)

bhobba
Mentor
O.k., so this is not the case since the direction is defined at the moment of the interaction with the target (or if the recoiled source hits something)
Why do you think you can extrapolate a direction from where it hits a target - think about the double slit - with both slits open you cant say which slit it went through or what direction it had - indeed even if it had the property of direction.

Why do you think a photon has the classical property of 'propagation'?

In QM its much better not to ascribe properties like propagation etc except when observed.

Thanks
Bill

bhobba
Mentor
My question remains though: how reflection is possible or how hits on one side of the mirror are privileged?
See Feynman - QED Strange Theory Of Light And Matter.

Thanks
Bill

Why do you think you can extrapolate a direction from where it hits a target - think about the double slit - with both slits open you cant say which slit it went through or what direction it had - indeed even if it had the property of direction.

Why do you think a photon has the classical property of 'propagation'?

In QM its much better not to ascribe properties like propagation etc except when observed.

Thanks
Bill
So you mean that, when observed, the received photon's momentum does not necessarily has to be aligned with the photon's origin position? It could be pointing for example at an angle of 90 degrees?

I suppose that for many photons most of received momentum is aligned so light sailing is possible.

Nugatory
Mentor
So you mean that, when observed, the received photon's momentum does not necessarily has to be aligned with the photon's origin position? It could be pointing for example at an angle of 90 degrees?
Did you follow through on Bhobba's suggestion that you find and read Feynman's book? If not, do so.

bhobba
Mentor
So you mean that, when observed, the received photon's momentum does not necessarily has to be aligned with the photon's origin position? It could be pointing for example at an angle of 90 degrees?
Momentum aligned with position?

They are independent variables.

Thanks
Bill

naima
Gold Member
I think that his question is about the conservation of the momentum.

bhobba
Mentor
I think that his question is about the conservation of the momentum.
The very essence of QM is all you can predict is averages and momentum conservation is preserved on the average.

Why exactly is that an issue?

Thanks
Bill