Degrees of freedom

  • Thread starter Lapidus
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  • #1
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Main Question or Discussion Point

They say that the two degrees of freedom of a photon are its two helicity states.

Why are the energy or the momentum of a photon not degrees of freedom of a photon? They can differ and they are Lorentz invariant.

thanks
 

Answers and Replies

  • #2
Bill_K
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For each value of the propagation vector there are two degrees of freedom.
 
  • #3
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thanks, Bill

Now I see it is a silly question. In mechanics, like a ball moving around, momentum is not a df, either. But the directions it can move are.
 
  • #4
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thanks, Bill

Now I see it is a silly question. In mechanics, like a ball moving around, momentum is not a df, either. But the directions it can move are.
Wouldn't a constraint on the directions it can move be a constraint on its momentum? I think the usual example of this type of constraint is something like an ice skate -- where the velocity / momentum can only be in one direction (or perhaps within a small range).

Maybe I'm confusing phase space coordinates with degrees of freedom...anyone feel like discussing the differences between the two, or expanding on what exactly a degree of freedom is?
 

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