Do Photons have shadows?

  • #1

Main Question or Discussion Point

...Do they? Are they too small or fast? If they are LIGHT particles, still, could they?

...That is all...
 

Answers and Replies

  • #2
Drakkith
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Hrmmm. Short simple answer is that I don't believe they have the sort of shadow we normally think about, if any at all. They are about 1 billionth the size of an atom, so they are VERY small, and dont always block light. And at that scale you can't try to compare everday things to the quantum world.

Edit: Looks like the correct size of a proton is around 145,000 times smaller than an atom of Hydrogen.

Edit 2: Scratch all that, i thought this was about PROTONS lol.
 
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  • #3
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You must ask what is a shadow? It is the absence of light simply. So when a body casts a shadow, this means it is something interacting on a very small level with light particles. For a light particle to interact with another light particle, is not really going to cast a shadow for two reasons: a broad light where distinguishing the shadow part from another part would not exist. both sides of a photon is made of light, so no matter what side you hit light off another particle of light, the other side is still made of light. (That was a little hard to explain, if no one understands it, just say). Also, light does not cast a shadow from a single quanta of light. You cannot hit one photon off another particle and expect it to cast a shadow, one quanta of light does not make up a broad enough ray.
 
  • #4
Drakkith
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You must ask what is a shadow? It is the absence of light simply. So when a body casts a shadow, this means it is something interacting on a very small level with light particles. For a light particle to interact with another light particle, is not really going to cast a shadow for two reasons: a broad light where distinguishing the shadow part from another part would not exist. both sides of a photon is made of light, so no matter what side you hit light off another particle of light, the other side is still made of light. (That was a little hard to explain, if no one understands it, just say). Also, light does not cast a shadow from a single quanta of light. You cannot hit one photon off another particle and expect it to cast a shadow, one quanta of light does not make up a broad enough ray.
I don't believe that light interacts with other light.
 
  • #5
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I don't believe that light interacts with other light.
It does, but only very weakly. I was going to originally say light does not interact with light but rather fall into the same quantum states because the equations describing them are symmetric... however, photons can interact, but only very weakly, so you need a lot of energy to do so. I can recite papers if you want. Sometimes you need a mediator, like an electron.
 
  • #6
Drakkith
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It does, but only very weakly. I was going to originally say light does not interact with light but rather fall into the same quantum states because the equations describing them are symmetric... however, photons can interact, but only very weakly, so you need a lot of energy to do so. I can recite papers if you want. Sometimes you need a mediator, like an electron.
Ah I see.
 
  • #7
They are about 1 billionth the size of an atom, so they are VERY small, and dont always block light.
They aren't small... nor large. Photons are particles in the QFT sense (elementary
excitations of the EM field). They aren't 'particles' in the sense you seem to be
implying.
 
  • #8
Drakkith
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They aren't small... nor large. Photons are particles in the QFT sense (elementary
excitations of the EM field). They aren't 'particles' in the sense you seem to be
implying.
Maybe so, but they still have a "size" associated with them.

Edit: Thought this was about Protons, my mistake. =)
 
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  • #9
Maybe so, but they still have a "size" associated with them.
Which would be... what?
 
  • #10
Drakkith
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Which would be... what?
I apologize, I didn't realize you said photons, I thought you said Protons. Wow, actually this entire thread i thought was saying Protons...i must need new glasses...scratch my other stuff i said above.
 
  • #11
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Even so, all particles are considered pointlike with no dimensions.

It's not very elluminating mind you. Just makes equations all the more simpler...
 
  • #12
Even so, all particles are considered pointlike with no dimensions.

It's not very elluminating mind you. Just makes equations all the more simpler...
Errr... no, that's not correct. They can be considered pointlike in MOMENTUM space,
meaning you can consider states arbitrarily close to an eigenstate of the momentum
operator. Most particles, in addition, have a position operator, so you can consider
states almost arbitrarily close to a position eigenstate (but never perfectly localised,
the particle's Compton wavelength giving you an lower limit). However, particles
with zero mass and spin higher than 1/2, like the photon, don't have a position
operator. Thus photons seem to be fundamentally nonlocalisable (though they
can be *aproximately* localisable).
 
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  • #13
I apologize, I didn't realize you said photons, I thought you said Protons. Wow, actually this entire thread i thought was saying Protons...i must need new glasses...scratch my other stuff i said above.
Oh, protons. Totally different beasts indeed. ;-)
 
  • #14
Drakkith
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In reply to the OP's original question, no. Photons do not have a shadow. They almost always just pass right through each other and keep on going.
 
  • #15
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Errr... no, that's not correct. They can be considered pointlike in MOMENTUM space,
meaning you can consider states arbitrarily close to an eigenstate of the momentum
operator. Most particles, in addition, have a position operator, so you can consider
states almost arbitrarily close to a position eigenstate (but never perfectly localised,
the particle's Compton wavelength giving you an lower limit). However, particles
with zero mass and spin higher than 1/2, like the photon, don't have a position
operator. Thus photons seem to be fundamentally nonlocalisable (though they
can be *aproximately* localisable).
No, I am right.

Are you saying an electron has a structure of some sort? If you argue they don't then they must be pointlike. All particles are pointlike. They are not extended objects in spacetime according to theory, unlike the strings of string theory.
 
  • #16
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  • #17
Drakkith
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There are fundamental reasons to consider them pointlike. I am a bit estranged by your reply. It's a well known topic in physics,

http://en.wikipedia.org/wiki/Point_particle
I think he was referring to Photons in his posts. I think I have accidently derailed this post into a confusing mess lol. Everyone blame this guy!
 
  • #18
Yes, photons interact quite a lot.

Photons exhibit positive and negative interference characteristics via point-wave duality definitions. From one viewpoint, canceling the presence of a light source via another light source could be thought of as "casting a shadow".

Does a proton cast a shadow? Of course it can cast a "shadow" if you mean blocking EM radiating at appropriate frequencies. Somewhere in the far X-ray spectrum the frequency approaches 1 angstrom, about the size of a proton.

Much excitement is brewing in materials science with the development of x-ray lasers and the ability to generate highly detailed holography imagery of the atomic structures of complex molecules.
 
  • #19
vanesch
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The OP might try to clarify: does he/she mean:

"does a photon CAST a shadow ?"

or does he mean:

"can a photon be "shadowed" by some screen or something ?"
 
  • #20
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I think he was referring to Photons in his posts. I think I have accidently derailed this post into a confusing mess lol. Everyone blame this guy!
Well, photon, proton, even a graviphoton, goldstone boson, even an electron, nuetrino, all of these are pointlike systems. Every particle in the standard model, which is just over 400 particles all in all are pointlike systems. To say that they are pointlike because of momentum space is just rubbish!

Now, Drakkith, in order not to derail any further, I recall back to the original answer I gave. Photons do not cast shadows if they are a single quanta of light. Light cannot cast a shadow, because a shadow by definition is the absence of light.
 
  • #21
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Shouldn't crisscrossing 2 laser beams answer this question?
 
  • #22
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Shouldn't crisscrossing 2 laser beams answer this question?
An accelerator is even better. And we have made two photons interact, as I said before. Physicists in 1997 managed to hit light off each other in a high energy interaction using electrons as mediators.
 
  • #23
No, I am right.

Are you saying an electron has a structure of some sort? If you argue they don't then they must be pointlike. All particles are pointlike. They are not extended objects in spacetime according to theory, unlike the strings of string theory.
No, I'm not saying an elementary particle has a structure (a bare particle anyway),
but I claim that the DENSITY corresponding to a particle in a proper (normalisable)
state is indeed always extended in spacetime. Depending on the particularities of
both the state and the characteristic length scale of the problem at hand, this density
can be concentrated in a volume with a radius much smaller than a characteristic
length (hence becoming well-localised), or it could be much larger (being then fairly
delocalised). But it can never be shrinked to a region smaller than a*lC3 (with lC the
particle's Compton wavelength and a a factor of order unit), let alone to a single point.
The energy necessary to shrink a single-particle wavepacket below lC3 would suffice to
excite new particle-antiparticle pairs out of the vacuum; the energy necessary to
shrink it to a single point would be infinite.

I think our disagreement stems from, or at least hints at, our considering differing
meanings for 'pointlike'.

A (bare, thus unphysical) pure, single-particle state can be constructed as a linear
combination of terms having the following general form (weird, LaTeX isn't working
for me today):

|x> = phi(x) |G>,

where phi(x) is the field operator (phi representing the corresponding adjoint field), and
|G> the (free) ground state.

Such a term can be interpreted as a 'bump' in the field, corresponding to an eigenstate
of the free number operator with eigenvalue 1, that's infinitely small and centered at
the point x. Thus, a truly pointlike elementary excitation (ie, particle).

However, just one term by itself cannot represent a proper state, because it wouldn't
be normalisable. You can use it as a base to construct more realistic states of the form

|Psi> = (integral for all x) d3x A(x) |x>.

In a non-relativistic scenario, A(x) would be the wavefunction corresponding to the
pure state |Psi>. The requirement that |Psi> be normalisable forbids the density
A(x)*A(x) to be a Dirac delta.

Hence, in this sense, an electron can never be truly pointlike.
 
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  • #24
Agreeing with Oudeis. Particles are not singularities.
They have energy and energy in a point is a black hole.

Disagreeing with Oudeis. Light is a particle.
What gives light particle properties is that you will never measure half a photon's energy. The true definition of particle, which is just a word anyways.
 
  • #25
Agreeing with Oudeis. Particles are not singularities.
They have energy and energy in a point is a black hole.

Disagreeing with Oudeis. Light is a particle.
What gives light particle properties is that you will never measure half a photon's energy. The true definition of particle, which is just a word anyways.
I agree. ;-) My 'photons aren't particles' statement was poorly worded, sorry for that.
They're particles, the discrete excitations of the EM field.
 

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