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Invisible Laser

Two laser beams out of phase combined to make invisible laser.
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As you all know, laser is a combination of photons that are all in same phase but not necessarily same plane. To make invisible laser: 1. Produce some laser beam 2. Split it in two 3. Make one beam go an extra 1/2 wavelength distance 4. Now polarize the beams such that their planes are perpendicular 5. Recombine them

Based on the laws of interference the beams should cancel each other.

So, what do we do with invisible laser? We can burn someone's hair without being noticed, for example. Or could use it for energy transfer, which is why we polarized them. Filter one beam and you get one back. Terribly inefficient. May still exceed SAR limits. Still, this is invisible laser and one had to invent it.

Note: Invisible here means undetectable, unless filtered.

xkuntay, Jun 30 2014

Depends on the laser http://www.physicsf...thread.php?t=282195
Some lasers produce highly polarized light. [Vernon, Jun 30 2014]

r/askscience http://www.reddit.c...are_put_in_perfect/
When two beams of laserlight are put in perfect, destructive interference, where does their energy go? [xaviergisz, Jul 01 2014]

r/physics http://www.reddit.c...arized_light_beams/
Will two orthogonally polarized light beams interfere? [xaviergisz, Jul 01 2014]

Stack Exchange http://physics.stac...-of-polarized-light
Interference of polarized light [xaviergisz, Jul 01 2014]

beam splitter - phase shift http://en.m.wikiped...plitter#Phase_shift
[xaviergisz, Jul 01 2014]

double slit experiment with electrons http://physicsworld...ent-gets-a-makeover
[xaviergisz, Jul 04 2014]

Scientists build world’s first anti-laser http://cosmosmagazi...s-first-anti-laser/
[xaviergisz, Jul 05 2014]

[link]






       A graviton is a particle in some respects analgous, but not equal, to photons:   

       Like photons, gravitons:
- travel at lightspeed
- are constantly being emitted and absorbed between objects, like black-body radiation.
- affect objects they hit.
  

       Per the last one, gravitons have anti-mass properties, unlike photons' mass properties: while a photon will slightly push an object, a graviton will pull, having imparted a bit of anti-momentum. Thus "gravity".
FlyingToaster, Jun 30 2014
  

       What FT said
not_morrison_rm, Jul 01 2014
  

       [Max] is a poopy head...   

       I mean what [Max] will say.
Voice, Jul 01 2014
  

       What..oh forget it...
not_morrison_rm, Jul 01 2014
  

       I found the following comment from the 'Stack Exchange' link very interesting:   

       //As others have noted, you will not get any intensity modulation from the interference of two linearly polarized light beams with orthogonal polarizations. It's worth noting, though, that this does not mean that beams with perpendicular polarizations don't affect each other. In fact, a counter-propagating pair of beams with orthogonal linear polarizations-- the so-called "lin-perp-lin" configuration-- is the best system for understanding the Sisyphus cooling effect, the explanation of which was a big part of the 1997 Nobel Prize in Physics.   

       The superposition of two counter-propagating linearly polarized beams with orthogonal polarizations doesn't give you any modulation of intensity, but does create a polarization gradient. For the lin-perp-lin configuration, you get alternating regions of left- and right-circular polarization, and combined with optical pumping this lets you set up a scenario where you can cool atomic vapors to extremely low temepratures. This makes laser cooling vastly more useful than it would be otherwise, and allows all sorts of cool technologies like atomic fountain clocks.   

       It's not interference in the sense that is usually meant, but it is a cool phenomenon that results from overlapping beams with different polarizations. So you shouldn't think that just because it doesn't produce a pattern of bright and dark spots it's not interesting.   

       Chad Orzel//   

       This 'polarization gradient' means the polarization cycles from linear to circular to orthogonal linear to opposite circular in the space of only half a wavelength of light.
xaviergisz, Jul 01 2014
  

       All the sites I found say you can't cancel this way because you can't have the light originating from the same location sine you can't combine two light rays. But no one mentioned half silvered mirrors and partial reflection.   

       If one beam hits one side at 45 degrees and the other hits the other side at 45 degrees, then half of the light from one beam will overlap half of the light from the other beam on both sides of the mirror. At first I assumed this wouldn't actually work because a half silvered mirror is presumably has small spots of reflective area and small spots of transparent area, so once the spots get smaller than the wavelength of the light, they probably become transparent or something. So photons on on a trajectory that hit a reflective spot would be reflected and photons not hitting a relective spot would pass thorough, so out of phase photons could still never come from exactly the same direction.   

       But when I searched to verify this theory I found articles saying that a half silvered mirror can be used to create quantum entangled photons by basically splitting a photon. What that probably means is that even if you perfectly combined two out of phase beams using a half-silvered mirror, the quantum effects of the entangled photons would send you back in time so that you changed your mind and decided not to try it after all in order to maintain conservation of energy.
scad mientist, Jul 01 2014
  

       I feel left out. What [—scad mientist] and [—xaviergisz] said.   

       There now I feel better.
blissmiss, Jul 01 2014
  

       I think my first link (to a reddit thread) is a fairly good discussion on interfering out of phase (but same polarization) light beams. I'll attempt my own explanation:   

       Let's say you have two beams of light, I1 and I2, that are aimed at a beam splitter.   

       If the input light is non-coherent then one side of the beam splitter will output 1/2(I1 + I2) from each side.   

       If the input light is coherent and 180° out of phase then the beam splitter will output I1 + I2 from one side, while the other side will output nothing (due to destructive interference). This is because a half- silver mirror beam splitter will add a phase delay of 180° to reflected light.
xaviergisz, Jul 01 2014
  

       Hmm, how do you know if you've got one or not, if it's invisible?
not_morrison_rm, Jul 01 2014
  

       What [Voice] said.
MaxwellBuchanan, Jul 01 2014
  

       So, I think I agree with the reddit /askscience thread, which says this is not physically possible.   

       If it were possible, however, I'm pretty sure it would not transmit energy. While I can't speak to the exact mechanism, I'm going to assume it's identical to the same effect in water waves. If you cancel a water wave with an exact complement, the energy is conserved as chaotic vibration at the point of cancellation.   

       Finally, why bother? Laser beams are pretty much undetectable unless you're standing in line with them, anyway. The only thing that is detectable is light reflected of physical objects in the beam path, or, for higher power beams, fluorescent emissions of physical objects in the beam path. If this approach were able to transmit energy, both of these would still apply.
MechE, Jul 01 2014
  

       Invisible lasers are dangerous, because you can walk into them without seeing them. ssssst goes your eyeball.
popbottle, Jul 01 2014
  

       Hey wait, what's going on? The topic got hijacked by a bunch of physicists.
xkuntay, Jul 02 2014
  

       what [n_m_r] said - "I'd like to sell you this invisible laser..."
hippo, Jul 02 2014
  

       so what happens when two photons collide ?
FlyingToaster, Jul 02 2014
  

       Well, when a mommy photon and a daddy photon really like each other..
not_morrison_rm, Jul 02 2014
  

       //so what happens when two photons collide?//   

       If the energy of the photons is high enough (i.e. high energy gamma rays), then you can get an electron and positron pair popping out. Otherwise, they don't generally interact.
xaviergisz, Jul 02 2014
  

       //Otherwise, they don't generally interact.   

       That explains the low birth rate of electrons and positrons.
not_morrison_rm, Jul 02 2014
  

       Don't electrons come only once every 4 years?
AusCan531, Jul 03 2014
  

       My point exactly. And then the positrons bit the heads off the electrons, or is it the other way around?
not_morrison_rm, Jul 03 2014
  

       Since we started receiving some QM lessons, let me project another question: if two beams have to be identical in order to interfere, how do two electrons in double slit experiment interfere? Does that mean they have the same spin, same frequency etc.?
xkuntay, Jul 03 2014
  

       //how do two electrons in double slit experiment interfere? Does that mean they have the same spin, same frequency etc.?//   

       Do you mean photons? The answer is yes regardless.   

       The really neat thing is that even if you send one photon at a time, it will still interfere with itself.
xaviergisz, Jul 04 2014
  

       // will interfere with itself//. That I totally don't believe. How do they know they sent only one photon?? Also, if that were the case, and regarding that same claim is made for electrons, how come an electron in a hydrogen atom does not interfere with itself? How can something be in two places at same tome? QM alone does not warrant that. The cat can be alive or dead but it cannot kill itself.   

       // You mean photon?//. No, I mean electron. I want to know if same principle applies to fermions (photon being a boson). Bosons cam share states, maybe that's why they cannot be in opposite states at same space and time.
xkuntay, Jul 04 2014
  

       The internet can answer all your questions better than I can. Just search for "single photon interference" and "single electron interference".   

       I have linked to a recent paper that describes physicists doing the double slit experiement with one electron at a time.
xaviergisz, Jul 04 2014
  

       I have read "single electron interference" experiments before. I read the one in the link as well. Still, I do not want to believe this is just a diffraction pattern. If an electron has 50% possibility of being in point 1 and 50% in point 2, then its wave function going to be identical to that of 2 electrons? No way Jose.
xkuntay, Jul 04 2014
  

       I have come to the understanding that the individual particles do not interact with each other at all, that they interact only with themselves, and that the detected diffraction pattern is an emergent trait of numerous similarly-propertied particles with approximately equal wave functions interacting under the same environmental conditions (the slit(s)). The double slit experiment has been done with large molecules, each containing hundreds of atoms, and the same types of patterns appear.
LimpNotes, Jul 04 2014
  

       //individual particles do not interact with each other at all// You mean in the double slit experiment? Maybe. In fact, likely, since it is virtually impossible to send them at exactly same time.   

       But, where on the Wave Function does it say the electron can interfere with itself? There is a term for outside potentials, other electrons or protons, but there is no term on the wave function for "itself". How do you explain this with QM?   

       And what if there are 15 slits? Does that mean it will interact with itself like 15 electrons?   

       Back to original topic: You say that two out of phase beams cannot be superimposed since this is illegal.Then, how about we make them 99% out of phase such that they cancel 99% of each other rather than 100%. Would that be permissible? (Let me interfere with myself to answer: the beams will reduce to their resultant weak beam and the rest will get reflected like in the other case, right?)   

       Another challenger: What if two out-of-phase beams meet at an orthogonal angle? Will they bounce back the way they came? Or will there be a spot that is dark at the intersection of the two beams and they will continue their way thereafter?
xkuntay, Jul 04 2014
  

       //How do you explain this with QM?//

I don't, but I expect an examination of the Huygens–Fresnel principle should clear things up a bit.
LimpNotes, Jul 04 2014
  

       Ok I did my reading. Did you? The Huygens-Fresnel Principle says firstly that all points on a wave front act as sources for spherical waves and secondly that a wave function solution plus a wave function solution equals a wave function solution. So the wave function doesn't need a term for itself because the component solutions we are discussing are already included in the solution itself.

//Does that mean it will interact with itself like 15 electrons?//Yes, 15 suitably sized slits will cause an interference pattern akin to 15 electrons. As for the original topic, I'll have to think more about it, but my intuition tells me that quanta energy from cancelling phase beams would be superimposed at some point along the wave and thereby occupying the same space. So what [Flying Toaster] said.
LimpNotes, Jul 05 2014
  

       Yes I did. On another note, let us remind the readers that the wave function of an electron is not a continuous wave but more of a pulse made up of several wavelets that travel in time (changing the subject). Anyhow, that was a total red card for breaking Neyman's back. tsssk tssk
xkuntay, Jul 05 2014
  

       I just added a link to an anti-laser. Only tangentially related to this idea, but I thought it was cool. The most interesting thing about the anti-laser is that it could be used as an optical isolator using a completely different mechanism than the one currently used (i.e.the faraday effect).
xaviergisz, Jul 05 2014
  

       If all you're after is an invisible laser light, why not use infrared or ultraviolet?
csea, Jul 06 2014
  

       //.e.the faraday effect   

       That used to be prevalent in South America, people got to be El Presidente for a day, until the next coup...
not_morrison_rm, Jul 07 2014
  
      
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