Phase cancellation of light - Is it possible to make light disappear?

[Claude Bile]

Can I make a general request that people do not talk in terms of "Photons interfering"? It can b e very confusing for someone new to the subject. The two concepts are from entirely different parts of Physics. Interference is a classical phenomenon and has to do with Waves. Photons are Quantum particles and are only relevant when discussing quantum phenomena. (But this is not really part of a discussion of 'stealth' methods).

Hi Sophiecentaur,

I can't agree with this statement I'm afraid. Interference is a key part of QM - perhaps the phrase "Photonic wavefunctions interfering" is better? In any case, particles interfering is quite an acceptable part of the QM lexicon :).

Anyway, to the OT at hand;

I find the idea quite interesting. Let's review how noise cancelling headphones work. They detect a sound (pressure) wave, then re-emit an inverted wave to cancel the sound. This is quite effective, even though the energy of the sound wave is being redistributed (as pointed out by many), it is being redistributed AWAY from you ear (i.e. your detector) and so the effect is quite convincing.

The main problem with doing this with light is that the phase of a light wave is much more difficult to detect and replicate - for two reasons;

- Light has a much higher bandwidth (i.e. it is faster) than electronic circuits, so you can't rely on electronics to "keep up" with the optical wave.

- Light is generally incoherent, meaning it doesn't have a well-defined phase and cannot easily be canceled out.

BUT, if you had a sufficiently fast detector, it might be possible? Certainly plausible enough for science fiction anyway...

Claude.

 

[sophiecentaur]

Hi Sophiecentaur,

I can't agree with this statement I'm afraid. Interference is a key part of QM - perhaps the phrase "Photonic wavefunctions interfering" is better? In any case, particles interfering is quite an acceptable part of the QM lexicon :).

Anyway, to the OT at hand;

I find the idea quite interesting. Let's review how noise cancelling headphones work. They detect a sound (pressure) wave, then re-emit an inverted wave to cancel the sound. This is quite effective, even though the energy of the sound wave is being redistributed (as pointed out by many), it is being redistributed AWAY from you ear (i.e. your detector) and so the effect is quite convincing.

The main problem with doing this with light is that the phase of a light wave is much more difficult to detect and replicate - for two reasons;

- Light has a much higher bandwidth (i.e. it is faster) than electronic circuits, so you can't rely on electronics to "keep up" with the optical wave.

- Light is generally incoherent, meaning it doesn't have a well-defined phase and cannot easily be canceled out.

BUT, if you had a sufficiently fast detector, it might be possible? Certainly plausible enough for science fiction anyway...

Claude.

Science Fiction that doesn't keep one foot on the ground can be a bit of a let-down. You just have to re-name it as fantasy fiction.
I still say that the concept of photons interfering is not meaningful - except in the very specialised conditions that Google throws up (in a very few relevant hits) and where the photons seem to have been generated by the same original source in the first place (afaics?) Whatever the facts are, about experimental evidence, it's certainly not 'interference as we know it'. All this (as ever) brings in the question about existence of photons except when they are actually detected. Where are the photons, for example, in a directional radio antenna array with multiple, independent but highly stable transmitters? If you want photons to exist as particles in their own right, I have to ask how they could differ from one another, according to their sources. Would they have different qualities that could be detected, for instance, in light received from a distant star? What extent / size would they have?
When you say that light has a "high bandwidth", I am not sure what you mean. There are many audio frequency sources with a wider (less well defined) bandwidth than the width of a spectral atomic emission line. A light source is usually made up of a large number of individual emitters (atoms) except in the case of a Laser, in which the stimulated emission causes the individual emitters to be phase coherent.

I think the notion of chasing individual photons and somehow producing another set of photons that could 'cancel them out' is really not worth considering - even in the context of SciFi.
I could also point out that any electronic circuitry which produces cancellation of audio signals will also involve Photons (just extremely low energy ones, associated with audio frequency AC signals). But it would be pretty futile to base any descriptions of the functions of an AC circuit at the 'photon level'.

Stealth techniques for eliminating radar reflections using active reflectors are perfectly reasonable - based on good-old classical wave techniques. Also, 'cloaking' techniques at optical frequencies come up in the press on occasions but they are usually over-cooked by the press afaics. There is a long way to go till Harry Potter stuff is available - if ever.

 

[Pgotthelf]

The concept of cloaking has already been demonstrated by bending light which is not the same as using destructive interference to eliminate light. However, Nikola Tesla demonstrated canceling light using a magnetic induction bulb all the way back in 1896. I doubt he canceled all the light in a defined space like a room, but he prove that the duel nature of light (wave and particle) did not foreclose destructive interference. For whatever it may be worth.

 

[Drakkith]

The concept of cloaking has already been demonstrated by bending light which is not the same as using destructive interference to eliminate light. However, Nikola Tesla demonstrated canceling light using a magnetic induction bulb all the way back in 1896. I doubt he canceled all the light in a defined space like a room, but he prove that the duel nature of light (wave and particle) did not foreclose destructive interference. For whatever it may be worth.

Please provide a reference for this.

 

[rumborak]

Many years back I remember asking a physics friend a similar question. The gedanken experiment was that you have two lasers, and you combine the output of the two lasers with (I guess?) a reverse beam splitter, but at pi phase offset. Now, the resulting beam would be no beam at all!
Apparently this was a common student exercise, and IIRC the outcome was that the energy would actually end up going backwards, essentially ending up inside the laser.

Many years ago, and many beers were involved. Mileage may vary.

 

[sophiecentaur]

Many years back I remember asking a physics friend a similar question. The gedanken experiment was that you have two lasers, and you combine the output of the two lasers with (I guess?) a reverse beam splitter, but at pi phase offset. Now, the resulting beam would be no beam at all!
Apparently this was a common student exercise, and IIRC the outcome was that the energy would actually end up going backwards, essentially ending up inside the laser.

Many years ago, and many beers were involved. Mileage may vary.

That's exactly what you would expect. It is easy to achieve cancellation of waves in one place or in one direction. Fact is that the energy never disappears, it just turns up in other places / directions. In your case, the beam splitting arrangement will end up directing all the power out of the 'other' port of your beam splitter. This effect of re-directing power happens with noise cancelling headphones. the extraneous sound level on the outside of the phones will be twice as high as when the phones are turned off.

 

[Science23413]

It's Possible. Light is an electromagnetic wave, which like radio waves can 'interfere' with corresponding waves. But the fact is that the speed and frequency of light makes it hard to not only cancel out, but to make a large differerence in the surrounding area. Light will only cancel out in the smallest fraction of a second, which can not be seen by human eyes.

 

[sophiecentaur]

It's Possible. Light is an electromagnetic wave, which like radio waves can 'interfere' with corresponding waves. But the fact is that the speed and frequency of light makes it hard to not only cancel out, but to make a large differerence in the surrounding area. Light will only cancel out in the smallest fraction of a second, which can not be seen by human eyes.

Cancellation happens in particular places and not at particular times. Interference of waves (light [EM], sound, water) produces 'fringes' where the waves augment and fringes where they cancel. You don't have to be 'quick enough' to spot it happening because the fringes stay where they are and are there all the time. The only thing about light that makes interference patterns 'hard' to produce is that the wavelength is short and the trains of waves from most sources are very short (many short bursts which are uncorrelated. I am not talking of Photons, here, BTW). You need coherent sources and a split laser beam is particularly suitable.
You should look up the basics of interference, which are well documented all over the place. 'Alternative' views are very confusing for questioners on PF.

 

[mfb]

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