Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Johann Mahne 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?

*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.

weird stuff :)
I like your questions Johann.

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.

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.

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

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.

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?

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.

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 reappeared there".

*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?

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

*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.

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?

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.

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.

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/stationarywaves
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.

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.
what if on the rope an ant was standing on the spot that which the two waves were about to collide and it was able to hold on to that point long enough to reach the instance the culmination of both negative Peak amplitude and positive Peak amplitude come into total cancellation..what would happen to the ant?

Johann Mahne asked the Naked Scientists:
...
If the waves are running from both ends of a rope,then after they collide the rope will have no ripples.
...
Regards
Johann
What do you think?
They will pass through each other in this case, as if nothing is coming from the other end.

They will pass through each other in this case, as if nothing is coming from the other end.
Hi Hamdani Yusuf, Thanks for your reply. So the ant would notice no perceived change in it's reality?, but would their reality change in an infinitesimal way, by dynamically altering the visualization of the spatial position? regardless of how miniscule .. if they could observe the waves interaction?