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Author Topic: Do out-of-phase photons cancel each other out?  (Read 6640 times)

Johann Mahne

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Do out-of-phase photons cancel each other out?
« on: 05/06/2011 17:30:03 »
Johann Mahne  asked the Naked Scientists:
   
Hi Chris,

If two waves approach each other, and they are:
1) in antiphase 2) Identical in frequency 3) At the same orientation

Then:

If the waves are in the ocean they will cancel out and there will be a calm section of ocean.

If the waves are in the air then there will be quite section of air with no sound present.

If the waves are running from both ends of a rope,then after they collide the rope will have no ripples.

But what if they are light waves in space? There is no medium to transport the waves, so what happens to the photons? Where do they go?

Regards
Johann

What do you think?
« Last Edit: 05/06/2011 17:30:03 by _system »


 

Offline lightarrow

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Do out-of-phase photons cancel each other out?
« Reply #1 on: 05/06/2011 19:08:46 »
*In the region where they interfere destructively* there are no photons. But if you start from two beams of light and you make them interfere (example: two opposed laser beams), there will be regions of destructive and regions of constructive interference, and the total number of photons won't vary.
 

Offline yor_on

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Do out-of-phase photons cancel each other out?
« Reply #2 on: 08/06/2011 01:24:23 »
weird stuff :)

I like your questions Johann.
 

Offline lightarrow

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Do out-of-phase photons cancel each other out?
« Reply #3 on: 08/06/2011 16:25:21 »
weird stuff :)

I like your questions Johann.
At high school this question was a real mistery for me. I could answer it only many years later, at univ.
 

Offline Soul Surfer

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Do out-of-phase photons cancel each other out?
« Reply #4 on: 09/06/2011 22:27:53 »
There is one very important additional point that most people forget in this discussion.  If they approach each other the MUST be on different tracks and can therefore only cancel each other out for a brief period of time at a specific location. 
This also applies of course if they were on the same track but in opposite directions At other times and locations they are both perfectly normal photons.  If they were on the exact same track and direction but at different times they could not approach each other because they would always be travelling at exactly the same speed.  The final case is if they were on the same track at exactly the same time they would always cancel each other out for all time and space and so might as well not exist.
 

Offline imatfaal

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Do out-of-phase photons cancel each other out?
« Reply #5 on: 09/06/2011 22:49:48 »
Surfer - I presume if they are on same track at same time - but different frequency/wavelength you would get beating to a greater or lesser extent
 

Offline wolfekeeper

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Do out-of-phase photons cancel each other out?
« Reply #6 on: 10/06/2011 00:35:56 »
This is a quantum mechanics question; photons are quantum mechanical particles.

The answer is that the photons' wavefunction will essentially bend around the points where the interference occurs.

This is actually almost exactly the same as the dual slit experiment.
 

Offline Geezer

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Do out-of-phase photons cancel each other out?
« Reply #7 on: 10/06/2011 06:13:09 »
This is probably a very silly question, but that does not seem to have stopped me in the past.

If photons could cancel each other out, where would their energies go?
 

Offline wolfekeeper

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Do out-of-phase photons cancel each other out?
« Reply #8 on: 10/06/2011 11:39:31 »
They can't cancel each other out, since that would violate conservation of energy. The photons just steer themselves around the interference by the magic that is quantum mechanics.
« Last Edit: 10/06/2011 11:41:06 by wolfekeeper »
 

Offline lightarrow

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Do out-of-phase photons cancel each other out?
« Reply #9 on: 11/06/2011 20:34:19 »
This is probably a very silly question, but that does not seem to have stopped me in the past.

If photons could cancel each other out, where would their energies go?
In other photons.
They don't have a localized identity, so you cannot say "the photon is here" before the detection. So you cannot even say that "two photons disappeared here and re-appeared there".
« Last Edit: 11/06/2011 20:36:42 by lightarrow »
 

Johann Mahne

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Do out-of-phase photons cancel each other out?
« Reply #10 on: 22/06/2011 15:52:56 »
*In the region where they interfere destructively* there are no photons. But if you start from two beams of light and you make them interfere (example: two opposed laser beams), there will be regions of destructive and regions of constructive interference, and the total number of photons won't vary.
Thanks for the reply.
If the waves consist of one wave length each,what will be the end result.Will the total number of photons be the same?Will the waves annihilate one another?
« Last Edit: 22/06/2011 15:56:04 by Johann Mahne »
 

Johann Mahne

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Do out-of-phase photons cancel each other out?
« Reply #11 on: 22/06/2011 15:53:50 »
This is a quantum mechanics question; photons are quantum mechanical particles.

The answer is that the photons' wavefunction will essentially bend around the points where the interference occurs.

This is actually almost exactly the same as the dual slit experiment.
Thanks for the reply,see my question to lightarrow
 

Offline wolfekeeper

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Do out-of-phase photons cancel each other out?
« Reply #12 on: 17/08/2011 17:40:21 »
*In the region where they interfere destructively* there are no photons. But if you start from two beams of light and you make them interfere (example: two opposed laser beams), there will be regions of destructive and regions of constructive interference, and the total number of photons won't vary.
Thanks for the reply.
If the waves consist of one wave length each,what will be the end result.Will the total number of photons be the same?Will the waves annihilate one another?
Two waves can only annihilate each other perfectly if they start at exactly the same point at exactly the same time with opposite phase. In the real world, there's no photons emitted in the first place like that.

If they're not emitted at the same place, then you will find the photons somewhere where their wavefunctions don't cancel.

Quantum mechanical particles like photons travel like waves, and appear as a lump where the wave doesn't cancel.
 

Johann Mahne

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Do out-of-phase photons cancel each other out?
« Reply #13 on: 18/08/2011 09:20:57 »
Quote
Two waves can only annihilate each other perfectly if they start at exactly the same point at exactly the same time with opposite phase. In the real world, there's no photons emitted in the first place like that.
  If two laser are pointing to each other and two photons are generated in antiphase at exactly the same time.Are you saying that this is not possible?
 

Offline wolfekeeper

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Do out-of-phase photons cancel each other out?
« Reply #14 on: 18/08/2011 16:40:34 »
They'll certainly cancel out in some places, but they won't cancel out in all places.

You get nodes and antinodes- the total energy is conserved.
 

Offline wolfekeeper

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Do out-of-phase photons cancel each other out?
« Reply #15 on: 18/08/2011 16:48:27 »
You can do it by absorbing photons though; either locally, or by surrounding the source with antiphase emitters that are less than one wavelength apart from each other or the source.

Absorbing photons is really, at bottom, done by creating an antiphase photon; the energy of the photon ends up in the antiphase transmitter.
 

Offline lightarrow

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Do out-of-phase photons cancel each other out?
« Reply #16 on: 18/08/2011 20:26:36 »
If two laser are pointing to each other and two photons are generated in antiphase at exactly the same time.Are you saying that this is not possible?
No because what you have in that case is a "standing wave":
http://www.slideshare.net/makadelhi/stationary-waves
http://en.wikipedia.org/wiki/Standing_wave

In a laser beam the wave is instead "propagating": its nodes are not still in space but moves.

 

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Do out-of-phase photons cancel each other out?
« Reply #16 on: 18/08/2011 20:26:36 »

 

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