Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 15/07/2012 10:47:30
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If red, white and blue laser light were combined, would it be possible to create a white laser? And would would be the practicalities of doing so?
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You could of course use three lasers to produce an approximation of white light but lasers are sources of single wavelengths of light while white such as you obtain from the sun or other thermal sources is a continuous spectrum of wavelengths.
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White light is the combination of all wavelengths there is right? So, following up on Chris idea, how many wavelengths are there? How small can we make the 'jumps' between each wavelength/frequency?
Or :)
What the he* is white light?
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Apparently it can be done ...
This laser kit allows you to mix your Red, Green and Blu-Ray (Violet) lasers to produce any color you like. Even white!
http://laserpointerforums.com/f64/fs-white-fusion-laser-mixing-kit-42013.html
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But could you make a white laser? Or would you need to fire the beams at a target where the reflected result looks white...?
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You would not get proper white light if you tried to use it for colour matching you would get a most un-satisfactory result.
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Chris - you cannot see light until it hits something. If you think you can see a light beam what you are actually seeing is light hitting motes of dust, smoke, water vapour etc.
Even with more normal lights this is the case - everyday reflected light is rarely monochromatic. Our eyes see everything by relative amounts of firing of three different colours - I cannot see how you couldn't make any colour; with enough hard work calculating and testing.
But that said - the laser is not white, its a mixture of colours. White is a completely human and subjective quality (ie there is no such thing as a white photon or a white wavelength) - and you could get it subjectively correct.
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"What the he* is white light?"
White light is light with a continuous distribution of frequencies such as you would get from a thermal source commonly specified as about 5000°K in no way could a combination of light from three monochromatic lasers be considered white this light illuminating and object that only reflected light of a frequency in a gap between the emissions of these three lasers would appear black whereas if it was illuminated with true white light it would show its natural colour.
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Ahhh, physics :)
Gotta love it-
Think you're right Syhprum. Although white light is a subjective definition for the direct observation it still has a strict definition physically. But I'm still wondering how many frequencies and wavelengths one can get, assuming black body radiation and the rest of the 'jumping' radiation does? Or am I bicycling in the great younder here? Never thought of that one actually but assuming discrete energies it seems to logically follow that you must have a limited number of 'energy levels', at least as directly measurable for us, and from that?
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But I'm still wondering how many frequencies and wavelengths one can get,
You'd need to have all of them :)
The frequency bands in a spectrum are a consequence of the atoms that produced the light, but there is no quantization of light frequencies.
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There is such a thing as a supercontinuum laser: http://en.wikipedia.org/wiki/Supercontinuum
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Thank you JP for drawing my attention to a field of research that I did not know existed, I do not think it has much relevance to the question asked by the original correspondent Who asked "Can I make myself a white laser?" who probably does not have access to 700Kw pico second lasers but it is an interesting field of research never the less.
PS Geezer, I believe that the energy levels of light photons are quantized but the steps are to small to be measured
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True, but, since the OP probably doesn't have a lot of rather odd science gear, the answer to "Can I make myself a white laser?" is simply "No" which is rather dull.
In principle you can get a laser to turn white by switching it on and off really quickly.
It might not behave much like a laser beam when you do that.
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PS Geezer, I believe that the energy levels of light photons are quantized but the steps are to small to be measured
Would that not also mean that radio frequencies are not continuous?
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It's the light quanta that are discrete, meaning that they have different 'jumps' of energy levels, but you can pack them as tight as you like in a beam. And if described as waves? I don't know? The main stream definition of a wave is dependent on what time is seen as, and if you think that time is a continuous process without 'breaks' then a wave should be one too. Weird stuff, and once again the particle/wave duality.
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Or am I wrong there?
Is light discrete when it comes to light quanta? And does discrete mean that they have different defined energies, and does it then mean that there is 'jumps' between those energy levels? I'm not sure?
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No, it should be correct. A light quanta is a discrete 'particle', but does it answer if there is jumps between their energy levels, or if you could imagine that there is a infinite procession of energy levels creating a 'smooth' procession?
Eh, it can't be smooth :)
Da*
Ah well.
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Or maybe it can?
It all depends on how you think of it, doesn't it?
Assume you superimpose photons, do the energy then jump in steps or do they gradually, ever so smoothly, ahem, increase their energy?
How about a laser?
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It would still be 'breaks' involved, wouldn't it? But how tight would it be between them? Plank scale?
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I better stop, I'm getting myself confused here :)
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Geezer
Although we do not think of radio frequency emissions as photons they must be although of much lower energy than the 1 to 2 ev of visable light IMHO the smallest increment of photon energy must be the 6.63x10-34 Joule-sec of Planck's constant.
Could the most high resolution spectrum analyser display this ? , not a hope but I wonder if we examined the the electromagnetic radiation of a rotating galaxy the discrete nature of the energy levels might be resolvable.
Bored Chemist
If you you switched your laser on and off rapidly you would be modulating it with a square wave and generating side bands although to generate white light your switching frequency would have to be quasi random.
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The reason that lasers tend to emit a very narrow range of frequencies is the way in which they work. A thermal source, such as the sun or an incandescent light bulb emits light when the electrons that are inside the heated material jiggle about fast (due to heat). Moving charged particles emit light, so these jiggling electrons emit radiation. This radiation has a continuous spectrum because the electrons can jiggle with a continuous range of motions.
Lasers on the other hand work in a completely different manner. You put two mirrors facing each other and bounce light back and forth between them. One of the mirrors is slightly less reflective, so a tiny amount of light leaks through it at each bounce. Then you put what's called a "gain medium" between the mirrors. This is a medium where you can pump energy into the electrons to raise many of them to a precise higher energy state. Because of the laws of quantum mechanics, when they drop back down to a given lower state,
they release a very precise amount of energy as a photon. This alone isn't enough to create a laser, however. The light bouncing between the two mirrors has to match that energy as well, so you have to place the mirrors just the right distance apart, so the wavelength of the light fits inside the space between the mirrors (actually it can be a full or a half wavelength). "Lasing" happens when the light bouncing back and forth interacts with the electrons in the gain medium, causing them to drop down and emit photons which are identical to the photons bouncing back and forth. The fact that they are identical is what gives lasers their very unique properties (known as coherence). This emission is called "stimulated emission," which is a quantum phenomenon, where you need to have a photon pass by the excited electron to cause it to emit another identical photon. Hence Light Amplification by Stimulated Emission of Radiation (LASER). As another aside, you have to continually pump energy into the gain medium to continually re-excite those electrons.
At any rate, if you followed all or part of that, the photons coming out have a very narrow band of frequencies, given by the matching of the mirror cavity length with the properties of the gain medium.
The problem with coming up with a way of broadening the spectrum of a laser is that you generally want to preserve much of the coherence properties of the light. The ways to preserve coherence while making a broader spectrum are to chop up your laser into short pulses (pulses by definition have a range of wavelengths) as BC said (the primary techniques are called Q-switching or mode locking) or to send it into a nonlinear medium, which can shift laser frequencies while maintaining coherence. The supercontinuum lasers I mentioned use this technique.
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I have struggled maintaining just the type of lasers you describe mercifully you don,t have to adjust the distance between the mirrors to the nearest half wavelength (which would be bloody nye impossible) the light frequency adjusts to suit the distance between the mirrors.
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Geezer
Although we do not think of radio frequency emissions as photons they must be although of much lower energy than the 1 to 2 ev of visable light IMHO the smallest increment of photon energy must be the 6.63x10-34 Joule-sec of Planck's constant.
Could the most high resolution spectrum analyser display this ? , not a hope but I wonder if we examined the the electromagnetic radiation of a rotating galaxy the discrete nature of the energy levels might be resolvable.
This is where frequency of light (EMR) might become a bit dodgy, so I suppose we need to talk in terms of wavelengths. In that case, does the wavelength of monochromatic light have to be an integer number of Planck lengths or something? If that's the case, it would set an upper limit on the shortest possible wavelength.
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It's the light quanta that are discrete, meaning that they have different 'jumps' of energy levels, but you can pack them as tight as you like in a beam. And if described as waves? I don't know? The main stream definition of a wave is dependent on what time is seen as, and if you think that time is a continuous process without 'breaks' then a wave should be one too. Weird stuff, and once again the particle/wave duality.
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Or am I wrong there?
Is light discrete when it comes to light quanta? And does discrete mean that they have different defined energies, and does it then mean that there is 'jumps' between those energy levels? I'm not sure?
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No, it should be correct. A light quanta is a discrete 'particle', but does it answer if there is jumps between their energy levels, or if you could imagine that there is a infinite procession of energy levels creating a 'smooth' procession?
Eh, it can't be smooth :)
Da*
Ah well.
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Or maybe it can?
It all depends on how you think of it, doesn't it?
Assume you superimpose photons, do the energy then jump in steps or do they gradually, ever so smoothly, ahem, increase their energy?
How about a laser?
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It would still be 'breaks' involved, wouldn't it? But how tight would it be between them? Plank scale?
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I better stop, I'm getting myself confused here :)
Aren't the jumps that we see with light a consequence of the energy level jumps in atoms? Light is EMR and you can produce any arbitrary EMR frequency so, in theory, you could produce light of any arbitrary frequency (with a very exotic radio transmitter).
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I am open to correction on both these points I do not know if it accepted that the frequency/energy levels of photons are quantitized but it seem logical to me that they should be, as you say this would put an upper limit on the energy that photons could have again this seems reasonable but I do not know if it is generaly accepted.
We must talk about photons in a general sense not only light a rotating galaxy with a magnetic field is emitting photons all be it of a very low frequency and energy 10^-31 Hz
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Geezer
Although we do not think of radio frequency emissions as photons they must be although of much lower energy than the 1 to 2 ev of visable light IMHO the smallest increment of photon energy must be the 6.63x10-34 Joule-sec of Planck's constant.
Could the most high resolution spectrum analyser display this ? , not a hope but I wonder if we examined the the electromagnetic radiation of a rotating galaxy the discrete nature of the energy levels might be resolvable.
Bored Chemist
If you you switched your laser on and off rapidly you would be modulating it with a square wave and generating side bands although to generate white light your switching frequency would have to be quasi random.
If I shorten the pulse far enough then the uncertainty relation will broaden the bandwidth to an extent that makes the light "white"
Also, in my not very humble opinion, you can have photons with any energy and any separation you want. Planck's constant isn't a limit here.
Apart from anything else the units don't tally up for the assertion that " the smallest increment of photon energy must be the 6.63x10-34 Joule-sec of Planck's constant"
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There isn't a low-frequency cutoff for photons. Even a constant background EM field is made up of photons. Photons are a way of writing how the energy of an EM field groups itself into packets on a quantum level. There are (relatively easy) rules for how the energy of a plane wave of a single fixed frequency splits up into photons. The rules are less easy when your EM field isn't a nice monochromatic plane wave, but they still exist.
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There isn't a low-frequency cutoff for photons.
I was wondering if there is a short-wavelength (high-frequency) cutoff?
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I don't think anyone knows. As I understand it, the theory breaks down at Planck energies because we would have to measure time so precisely that quantum gravity fluctuations in space-time would become significant. This would correspond to some Planck frequency for photons (E=hf). But we don't know if this breakdown signifies that nothing exists higher than that limit or if some new theory of quantum gravity will explain ultra-high-energy photons and other particles.
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Thanks! I'm not sure my Mickey and Donald calculator will let me calculate the energy in a photon with a wavelength of one Planck :)
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It's weird, to me a light quanta is a 'energy' (and a momentum). And the question I'm wondering about is if those energies are continuous or if they come in steps. And I don't know? I would, and will, assume that it (a light quanta's energy) can have any value, just because it's 'energy', although when we involve matter emitting radiation they will have defined energies?
It's weird, and I don't know if my assumption is right. The light quanta's we observe is as far as I know always produced in interactions with matter, without matter no interactions.
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The light quanta's we observe is as far as I know always produced in interactions with matter, without matter no interactions.
Not necessarily. Radio waves are a form of "light" that is produced without interaction with matter. You can even receive light with a very small antenna. I don't think there is a fundamental reason that makes it impossible to produce light of an arbitrary wavelength with an antenna.
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Not sure I follow there Geezer?
Are you thinking of the question itself?
If 'energy' is 'quantified' and then using radio waves to consider it?
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Geezer, do you mean that radio waves don't come from atomic transitions? As far as I know, you have to have a moving current to produce them (and indeed all EM radiation should have moving charges as a source).
Yor_on, photons are defined as having one particular energy and momentum (and spin/polarization). The field emitted by a moving electron doesn't have to only consist of one photon, however. A photon oscillating on an antenna doesn't have to emit only one energy of photons: it can emit a spectrum of them.
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Yes JP :)
What I'm thinking of here is actually what 'fields' might be, and using radiation as a base. Then it becomes interesting if the 'fields' flicker, sort of, energy wise :) The quantums may be quantums, or light quanta', but the 'energy' they contain, can it be a smooth phenomena in itself? Meaning that its 'energy' doesn't come in steps until it 'interacts' with matter, if that now makes sense?
A weird question I guess :) but it's in some way about dimensions we can measure in and, what 'degrees of freedom' something can have? As if you consider 'energy' as a smooth field permeating everything, quantizing in interactions.
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Not sure I follow there Geezer?
Are you thinking of the question itself?
If 'energy' is 'quantified' and then using radio waves to consider it?
Radio signals can be of any arbitrary frequency, but I think light sources are limited in the specific frequencies that they produce (that may be baloney!). So, I was suggesting that it should be possible to produce light of any frequency with an antenna.
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Geezer, do you mean that radio waves don't come from atomic transitions? As far as I know, you have to have a moving current to produce them (and indeed all EM radiation should have moving charges as a source).
Electron current is certainly required in an antenna, but I don't think the frequency of the current is a function of atomic transitions - or is it?
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Radio signals can be of any arbitrary frequency, but I think light sources are limited in the specific frequencies that they produce (that may be baloney!). So, I was suggesting that it should be possible to produce light of any frequency with an antenna.
I see :)
Yeah, you got a point there. So maybe 'energy' is a smooth phenomena?
It's been bothering me for some while, dimensions. Also called degrees of freedom. Actually I like 'degrees of freedom' better as a description than dimensions as it takes away a little of the mystery/fog a 'dimension' begets when people throw it around in SF etc :)
And then we have 'energy', as an expression of transformations. Not to be touched or hold in your hand otherwise, except when used as a description of something transforming as I think of it, but I'm not sure..
If we consider the Higgs field the Higgs boson is a 'excitation' of that field as I understands it. And if you then assume that the field as such is everywhere, always there, with our arrow allowing for interactions to happen at the proper 'energies'?
Just put it together, 'dimensions' or degrees of freedom and 'fields'. But it's important to me to remember that you need a arrow of time for a excitation to be defined.
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Geezer, do you mean that radio waves don't come from atomic transitions? As far as I know, you have to have a moving current to produce them (and indeed all EM radiation should have moving charges as a source).
Electron current is certainly required in an antenna, but I don't think the frequency of the current is a function of atomic transitions - or is it?
Yes. That's what I mean. Moving electrons generate EM radiation. Moving can include atomic transitions, but it can also include free electrons flowing (as in radio antenna).
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Radio signals can be of any arbitrary frequency, but I think light sources are limited in the specific frequencies that they produce (that may be baloney!). So, I was suggesting that it should be possible to produce light of any frequency with an antenna.
I see :)
Yeah, you got a point there. So maybe 'energy' is a smooth phenomena?
It's been bothering me for some while, dimensions. Also called degrees of freedom. Actually I like 'degrees of freedom' better as a description than dimensions as it takes away a little of the mystery/fog a 'dimension' begets when people throw it around in SF etc :)
And then we have 'energy', as an expression of transformations. Not to be touched or hold in your hand otherwise, except when used as a description of something transforming as I think of it, but I'm not sure..
If we consider the Higgs field the Higgs boson is a 'excitation' of that field as I understands it. And if you then assume that the field as such is everywhere, always there, with our arrow allowing for interactions to happen at the proper 'energies'?
Just put it together, 'dimensions' or degrees of freedom and 'fields'. But it's important to me to remember that you need a arrow of time for a excitation to be defined.
Not sure I follow there Geezer?
Are you thinking of the question itself?
If 'energy' is 'quantified' and then using radio waves to consider it?
Radio signals can be of any arbitrary frequency, but I think light sources are limited in the specific frequencies that they produce (that may be baloney!). So, I was suggesting that it should be possible to produce light of any frequency with an antenna.
That's baloney! :)
Thermal radiation includes light that has a broad spectrum because electrons can move in other ways than atomic transitions (or other quantized phenomena).
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Radio signals can be of any arbitrary frequency, but I think light sources are limited in the specific frequencies that they produce (that may be baloney!). So, I was suggesting that it should be possible to produce light of any frequency with an antenna.
That's baloney! :)
Thermal radiation includes light that has a broad spectrum because electrons can move in other ways than atomic transitions (or other quantized phenomena).
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What I was thinking of there was Geezers thought about frequencies and that they can be split infinitely? This is a very weird thought JP :) but ...
The point to me is what those 'degrees of freedom' implies. If you imagine a 'field' that only exist to us at some 'energy' of interaction, but still is there, what is its degree of freedom? One can define it such as we have four dimensions. Three that is the room and one that is the arrow, but a field seems somehow to have another degree of freedom to me, now ignoring our arrow? Because if you do ignore it then this field is static, still existing. Add a arrow and some rules and you get a orderly procession.
Like all 'energy' there is actually has a life of its own :) if you see how I think, and that's indeed a pretty weird thought.
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Maybe a better question would be, ignoring a arrow, how many degrees of freedom can exist? As I think only one, and that one is what I call 'static'.
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For a single photon you have: momentum and spin as free variables. (Momentum determines energy since E2=p2 for massless particles.) A general field can always be decomposed into photons, and I believe the number of degrees of freedom you have will be roughly proportional to the number of photons making up that field (multiplied by 4, since you can specify 3 momentum components over space and spin/polarization).
Dealing with photons without understanding the fundamentals is overly confusing, however.
The same principle holds for sound. A single frequency of sound technically goes on forever and has only one degree of freedom: frequency. A general sound wave has much more variation over space and time than a single note. It turns out (http://en.wikipedia.org/wiki/Fourier_analysis) that you can write any general sound wave as the sum of a bunch of single-frequency waves. If you want to create a general sound wave, you can specify how it varies over space and time, or you can alternatively tell me how much of each frequency component to add together. Both are legitimate and equivalent ways of writing the wave. The same holds true for photons/light: you can specify the wave or you can specify how much of different quantum photon states to add together to generate that wave.
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What was the question again? :)
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That is assuming 'virtuality' too JP? How would you handle it if you decided it was indeterministic instead? To me the difference is one between something 'there' as a 'particle' versus something representing a probability. If I want it to be 'virtual' I assume that I also have to allow it a 'time', even if unmeasurable?
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A field defined by Higgs is not there at all, if we by that mean that it is measurable by us, well, as long as you don't see inertia/gravity as that field? As far as I understand inertia/gravity then should be a result of it, if one trust Higgs fields. And, if you can't measure it, still expecting it to exist, do you define that as photons? Seems a big difference between measurable photons and this to me. I'm not saying it isn't bosons though.
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It is tricky.
I throw a ball at some other person. I feel a force pushing me backwards as I throw, I see the ball travel and then get caught by the other person, that now will feel a force transmitted as he catches it. But what transmitted that force? Photons? Is all fields then consisting of photons?
Then we have the definition of something, easily able to superimpose, as being 'scalar', now having definite positions in SpaceTime, and they should need too, shouldn't they? If we assume the 'force carriers' to consist of photons making up a field?
I'm not sure I understand what a field is.
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And to up it, it must also be observer dependent if relativity is correct. And that should mean that what you and I measure will differ, so to summarize, a scalar, but observer dependent 'field'. This is assuming that LorentzFitzGerald contractions exist naturally. That time dilations exist we already know and their complementary should be a LorentzFitzGerald contraction. I would be surprised if it wasn't.
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If you wanted to make a projection TV, you could modulate the output of red, green and blue light sources and project it in a raster pattern, forming a TV image. These light sources could be lasers.
By combining all three colours in different combinations, you can generate most of the colours that the human eye can see, including white (just like a normal TV).
But the sum of 3 coherent lasers of different wavelengths is not coherent white light, so it's not really a "white laser".
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I don't know if anyone has commercialized laser displays, but various organizations have worked on the technology. The idea works exactly as you say: red, green and blue lasers are combined and scanned across a display, where the amount of each at each pixel determines the color seen. (http://en.wikipedia.org/wiki/Laser_video_display)