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Invisible Laser
Two laser beams out of phase combined to make invisible laser. | |
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.
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]
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A graviton is a particle in some respects analgous, but not equal, to photons: |
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Like photons, gravitons:
- travel at lightspeed
- are constantly being emitted and absorbed between objects, like black-body radiation.
- affect objects they hit. |
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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". |
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I mean what [Max] will say. |
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I found the following comment from the 'Stack Exchange' link very interesting: |
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//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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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I feel left out. What [—scad mientist] and
[—xaviergisz] said. |
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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: |
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Let's say you have two beams of light, I1 and I2, that are aimed at a
beam splitter. |
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If the input light is non-coherent then one side of the beam splitter will
output 1/2(I1 + I2) from each side. |
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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. |
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Hmm, how do you know if you've got one or not, if it's invisible? |
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So, I think I agree with the reddit /askscience
thread, which says this is not physically possible. |
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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. |
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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. |
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Invisible lasers are dangerous, because you can walk into them without seeing them. ssssst goes your eyeball. |
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Hey wait, what's going on? The topic got hijacked by
a bunch of physicists. |
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what [n_m_r] said - "I'd like to sell you this invisible laser..." |
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so what happens when two photons collide ? |
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Well, when a mommy photon and a daddy photon really like each other.. |
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//so what happens when two photons collide?// |
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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. |
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//Otherwise, they don't generally interact. |
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That explains the low birth rate of electrons and positrons. |
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Don't electrons come only once every 4 years? |
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My point exactly. And then the positrons bit the heads off the electrons, or is it the other way around? |
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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.? |
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//how do two electrons in double slit experiment interfere? Does that mean they have the same spin, same frequency etc.?// |
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Do you mean photons? The answer is yes regardless. |
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The really neat thing is that even if you send one photon at a time, it will still interfere with itself. |
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// 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. |
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// 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. |
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The internet can answer all your questions better than I can. Just search for "single photon interference" and "single electron interference". |
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I have linked to a recent paper that describes physicists doing the double slit experiement with one electron at a time. |
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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. |
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//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. |
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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? |
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And what if there are 15 slits? Does that mean it
will interact with itself like 15 electrons? |
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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?) |
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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? |
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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 |
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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). |
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If all you're after is an invisible laser light, why not
use infrared or ultraviolet? |
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That used to be prevalent in South America, people got to be El Presidente for a day, until the next coup... |
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